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Generic RequirOptical ComponTelcordia Technologies Generic RequirementsGR-1209-COREIssue 3, March 2001

Comments Requested (See Preface)An SAIC Companyements for Passive ents

GR-1209-COREGeneric Requirements for Passive Optical Components Issue 3, March 2001Copyright Page Generic Requirements for Passive Optical Components

Prepared for Telcordia Technologies by: Systems Technology and Reliability Effectiveness Group The Physical Network, Product Integrity and Reliability Department

Related Documents: GR-357-CORE, GR-78-CORE, and GR-1221-CORE.

Technical contact: M. A. Ali, 1-732-758-3017

To obtain copies of this document, contact your companys document coordinator or your Telcordia account manager, or call 1-800-521-2673 (from the USA and Canada) or 1-732-699-5800 (worldwide), or visit our Web site at www.telcordia.com. Telcordia employees should call 1-732-699-5802.

Copyright 1994, 1998, 2001 Telcordia All rights reserved. This document may not be reproduced without the express written permission of Telcordia and any reproduction without written authorization is an infringement of the Telcordia copyright.

Project Funding year: 2000

Trademark AcknowledgmentsTelcordia is a trademark of Telcordia Technologies, Inc.ii

Issue 3, March 2001 Generic Requirements for Passive Optical Components

GR-1209-CORETelcordia Technologies Generic Requirements Notice Of Disclaimer

This Generic Requirements document (GR) is published by Telcordia Technologies to inform the industry of the Telcordia Technologies Forums view of proposed Generic Requirements for Passive Optical Components. The generic requirements contained herein are subject to review and change, and superseding generic requirements regarding this subject may differ from those in this document. Telcordia reserves the right to revise this document for any reason (consistent with applicable provisions of the Telecommunications Act of 1996 and applicable FCC rules).

Telcordia specifically advises the reader that this GR does not directly or indirectly address any Year-2000 (Y2K) issues that might be raised by the services, systems, equipment, specifications, descriptions, or interfaces addressed or referred to herein. As an example, and not a limitation, neither this GR nor Telcordia is directly or indirectly assessing or determining whether specific services, systems, or equipment, individually or together, in their current form or as they may be implemented, modified, or augmented in the future, will accurately process dates and date-related data within or between the twentieth and twenty-first centuries, in either direction, including elapsed time, time difference, and/or leap year calculations.

TELCORDIA AND THE FUNDING PARTICIPANTS IDENTIFIED IN THE PREFACE MAKE NO REPRESENTATION OR WARRANTY, EXPRESSED OR IMPLIED, WITH RESPECT TO THE SUFFICIENCY, ACCURACY, OR UTILITY OF ANY INFORMATION OR OPINION CONTAINED HEREIN.

TELCORDIA AND FUNDING PARTICIPANTS EXPRESSLY ADVISE THAT ANY USE OF OR RELIANCE UPON SAID INFORMATION OR OPINION IS AT THE RISK OF THE USER AND THAT NEITHER TELCORDIA NOR ANY FUNDING PARTICIPANT SHALL BE LIABLE FOR ANY DAMAGE OR INJURY INCURRED BY ANY PERSON ARISING OUT OF THE SUFFICIENCY, ACCURACY, OR UTILITY OF ANY INFORMATION OR OPINION CONTAINED HEREIN.

LOCAL CONDITIONS MAY GIVE RISE TO A NEED FOR ADDITIONAL PROFESSIONAL INVESTIGATIONS, MODIFICATIONS, OR SAFEGUARDS TO MEET SITE, EQUIPMENT, ENVIRONMENTAL SAFETY OR COMPANY-SPECIFIC REQUIREMENTS. IN NO EVENT IS THIS INFORMATION INTENDED TO REPLACE FEDERAL, STATE, LOCAL, OR OTHER APPLICABLE CODES, LAWS, OR REGULATIONS. SPECIFIC APPLICATIONS WILL CONTAIN VARIABLES UNKNOWN TO OR BEYOND THE CONTROL OF TELCORDIA. AS A RESULT, TELCORDIA CANNOT WARRANT THAT THE APPLICATION OF THIS INFORMATION WILL PRODUCE THE TECHNICAL RESULT OR SAFETY ORIGINALLY INTENDED.

This GR is not to be construed as a suggestion to anyone to modify or change any product or service, nor does this GR represent any commitment by anyone, including but not limited to Telcordia or any funder of this Telcordia GR, to purchase, manufacture, or sell any product with the described characteristics.iii

Generic Requirements for Passive Optical Components Issue 3, March 2001

GR-1209-COREReaders are specifically advised that any entity may have needs, specifications, or requirements different from the generic descriptions herein. Therefore, anyone wishing to know any entitys needs, specifications, or requirements should communicate directly with that entity.

Nothing contained herein shall be construed as conferring by implication, estoppel, or otherwise any license or right under any patent, whether or not the use of any information herein necessarily employs an invention of any existing or later issued patent.

TELCORDIA DOES NOT HEREBY RECOMMEND, APPROVE, CERTIFY, WARRANT, GUARANTEE, OR ENDORSE ANY PRODUCTS, PROCESSES, OR SERVICES, AND NOTHING CONTAINED HEREIN IS INTENDED OR SHOULD BE UNDERSTOOD AS ANY SUCH RECOMMENDATION, APPROVAL, CERTIFICATION, WARRANTY, GUARANTY, OR ENDORSEMENT TO ANYONE.

For general information about this or any other Telcordia documents, please contact:

Telcordia Technologies Customer ServicePiscataway, New Jersey 08854-41561-800-521-2673 (USA and Canada)1-732-699-5800 (worldwide)1-732-336-2559 (FAX)http://www.telcordia.comiv

Issue 3, March 2001 Generic Requirements for Passive Optical Components Contents

GR-1209-COREContents Contents

Preface .................................................................................................................Preface1

1. Introduction............................................................................................................. 111.1 Purpose and Scope of Document ...............................................................121.2 Requirements Terminology ......................................................................... 121.3 Requirement Labeling Conventions ........................................................... 13

1.3.1 Numbering of Requirement and Related Objects ....................... 131.3.2 Requirement, Conditional Requirement, and Objective ............ 14

1.4 Changes from GR-1209-CORE, Issue 2....................................................... 141.5 Organization .................................................................................................. 151.6 Order of Precedence..................................................................................... 151.7 Harmonization With GR-1221-CORE............................................................ 16

2. General Information............................................................................................... 212.1 Scope .............................................................................................................. 212.2 Passive Optical Components - Couplers and WDMs................................ 22

2.2.1 General Product Description ........................................................ 222.2.2 Coupler and WDM Classes ............................................................ 222.2.3 Coupler and WDM Technology ..................................................... 242.2.4 Coupler and WDM Applications ................................................... 26

2.2.4.1 Bidirectional Transmission ............................................ 272.2.4.2 HFC Architecture for Video and Telephony ................292.2.4.3 Fiber in the Loop Systems............................................ 212

2.3 Passive Optical Components - Filters ...................................................... 2142.3.1 General Product Information ...................................................... 2142.3.2 Filter Classes ................................................................................. 2142.3.3 Fiber Optic Filter Technology ..................................................... 215

2.3.3.1 Dielectric Interference Coatings ................................. 2152.3.3.2 Fiber Optic Bragg Grating ............................................ 2152.3.3.3 Fiber Fabry-Perot Filters.............................................. 216

2.3.4 Fiber Optic Filter Applications ................................................... 2162.3.4.1 Fiber Optic Filters in EDFAs ....................................... 2162.3.4.2 Fiber Optic Filters in Multiwavelength Networks.....2192.3.4.3 Wavelength Add-Drop Mutiplexers (WADMs)........... 2202.3.4.4 Fiber Optic Filters in Remote Fiber Test Systems.... 222

2.4 Passive Optical Components - Isolators and Circulators ...................... 2232.4.1 General Product Description ...................................................... 2232.4.2 Isolator and Circulator Classes ................................................... 225

2.4.2.1 Polarization/analyzer Isolators .................................... 2252.4.2.2 Walk-off Isolators .......................................................... 2262.4.2.3 Optical Circulators ........................................................ 228

2.4.3 Isolator Technology ......................................................................2302.4.4 Isolator and Circulator Applications .......................................... 231

2.4.4.1 Transmitter Protection ................................................. 2322.4.4.2 Optical Amplifiers ......................................................... 232v

Generic Requirements for Passive Optical Components Issue 3, March 2001Contents

GR-1209-CORE2.4.4.3 In-Line Noise Reduction ............................................... 2332.4.4.4 Circulator Applications................................................. 234

2.5 Passive Optical Modules ............................................................................ 237

3. General and Design Criteria .................................................................................. 313.1 Documentation.............................................................................................. 31

3.1.1 General Documentation ................................................................. 313.1.2 Workcenter Information Package ................................................ 32

3.2 Marking, Packaging, and Shipping.............................................................. 323.3 Physical Design Criteria...............................................................................33

3.3.1 General Physical Design Criteria .................................................. 333.3.2 Optical Fiber .................................................................................... 333.3.3 Optical Connectors ......................................................................... 353.3.4 Materials ..........................................................................................35

3.3.4.1 Toxicity............................................................................. 353.3.4.2 Corrosion Resistance......................................................353.3.4.3 Dissimilar Metals .............................................................363.3.4.4 Fungus Resistance........................................................... 363.3.4.5 Flammability .................................................................... 36

3.3.5 Safety ................................................................................................ 363.3.6 Mounting ..........................................................................................37

3.4 Passive Optical Component Qualification................................................. 383.5 Reliability Assurance.................................................................................... 393.6 Quality Technology Program..................................................................... 3103.7 Application Environments ......................................................................... 311

3.7.1 Background ................................................................................... 3113.7.2 Environmental Classifications ....................................................3123.7.3 Environmental Definitions .......................................................... 312

3.7.3.1 Controlled Environment (C) ........................................ 3123.7.3.2 Uncontrolled Environment (U) ................................... 3123.7.3.3 Summary of Environmental Definitions..................... 313

4. Optical Performance Criteria ................................................................................ 414.1 Optical Bandpass ..........................................................................................44

4.1.1 Optical Bandpass - General ........................................................... 444.1.2 Optical Bandpass Guidelines ........................................................ 444.1.3 Optical Bandpass Testing .............................................................. 484.1.4 DWDM Bandpass Testing .............................................................. 494.1.5 Center Wavelength Definition ..................................................... 4104.1.6 Central Frequency of DWDM Components Testing ................. 410

4.2 Insertion Loss .............................................................................................. 4104.2.1 Insertion Loss for All Products ................................................... 4104.2.2 Insertion Loss for Optical Isolators and Circulators ................4124.2.3 Insertion Loss for Fiber Optic Filters .........................................4134.2.4 Insertion Loss for Gain Flattening Filters ................................. 4134.2.5 Insertion Loss Testing .................................................................. 414

4.3 Uniformity.................................................................................................... 4154.3.1 Uniformity for All Products ......................................................... 415vi

Issue 3, March 2001 Generic Requirements for Passive Optical Components Contents

GR-1209-CORE4.3.2 Uniformity for Optical Isolators and Circulators ..................... 4164.3.3 Uniformity (Flatness) for Fiber Optic Filters ........................... 4174.3.4 Uniformity Testing ........................................................................ 417

4.4 Isolation ....................................................................................................... 4184.4.1 WDM Wavelength Isolation or Far-End Crosstalk (FEXT) .....418

4.4.1.1 WDM Wavelength Isolation or Far-End Crosstalk (FEXT) Testing ............................................................................ 419

4.4.1.2 DWDM Wavelength Isolation or Adjacent Channel Crosstalk Testing...........................................................419

4.4.2 Isolation for Optical Isolators and Circulators .........................4194.4.3 Wavelength Isolation (Optical Crosstalk) for Filters ............... 420

4.5 Directivity .................................................................................................... 4214.5.1 Directivity for All Products ......................................................... 4214.5.2 Directivity for Optical Isolators and Circulators ...................... 4224.5.3 Directivity for Fiber Optic Filters ............................................... 4224.5.4 Directivity Testing ........................................................................ 422

4.6 Return Loss.................................................................................................. 4224.6.1 Return Loss for All Products ....................................................... 4224.6.2 Return Loss for Optical Isolators and Circulators .................... 4234.6.3 Return Loss for Fiber Optic Filters ............................................ 423

4.7 Polarization-Dependent Loss (PDL)......................................................... 4244.7.1 PDL for All Products ....................................................................4244.7.2 PDL for Optical Isolators and Circulators ................................. 4244.7.3 PDL for Fiber Optic Filters .......................................................... 4254.7.4 Polarization-Dependent Loss (PDL) Testing ............................. 425

4.8 Polarization Dependent Wavelength (PDW) ...........................................4264.8.1 PDW for All Products ................................................................... 4264.8.2 PDW for Optical Isolators and Circulators ................................4274.8.3 PDW for Fiber Optic Filters ........................................................ 4274.8.4 Polarization Dependent Wavelength (PDW) Testing ............... 427

4.9 Polarization-Mode Dispersion (PMD) ...................................................... 4274.9.1 PMD for All Products ................................................................... 4274.9.2 PMD for Optical Isolators and Circulators ................................4284.9.3 PMD for Fiber Optic Filters ......................................................... 4284.9.4 Polarization-Mode Dispersion (PMD) Testing .......................... 428

4.10 Temperature Effects on DWDM Components. ....................................... 4294.11 Other Fiber Optic Filter Specific Criteria ................................................ 429

4.11.1 Center Wavelength Misalignment ............................................... 4294.11.2 Polarization Sensitivity ................................................................ 430

5. Environmental and Mechanical Performance Criteria ...................................... 515.1 Performance Criteria ....................................................................................... 515.2 Standard Conditions of Test........................................................................515.3 Operating Environments .................................................................................525.4 Environmental and Mechanical Criteria ....................................................52

5.4.1 Transportation and Handling Criteria .......................................... 555.4.1.1 Temperature-Humidity Aging ........................................ 55vii

Generic Requirements for Passive Optical Components Issue 3, March 2001Contents

GR-1209-CORE5.4.1.2 Vibration ........................................................................... 555.4.1.3 Component Impact.......................................................... 565.4.1.4 Module Impact ................................................................. 565.4.1.5 Temperature Cycling....................................................... 57

5.4.2 Operational Performance Criteria ................................................ 575.4.2.1 Temperature-Humidity Cycle: Controlled

Environment .................................................................... 575.4.2.2 Temperature-Humidity Cycle: Uncontrolled

Environment .................................................................... 585.4.2.3 Water Immersion ...........................................................5105.4.2.4 Controlled DWDM Temperature Effects .................... 5105.4.2.5 Uncontrolled DWDM Temperature Effects ............... 510

5.4.3 Fiber Integrity Criteria ................................................................. 5115.4.3.1 Fiber Flex ....................................................................... 5115.4.3.2 Fiber Twist ..................................................................... 5115.4.3.3 Fiber Side Pull................................................................ 5125.4.3.4 Fiber and Cable Retention (Straight Pull).................. 512

5.5 Optical Measurement Facilities ................................................................ 513

Appendix A: The Transfer Matrix .............................................................................. A1A.1 Definition of Terms...................................................................................... A1A.2 Example ........................................................................................................ A3

References.................................................................................................... References1

Acronyms ....................................................................................................... Acronyms1

Glossary.............................................................................................................Glossary1

Requirement-Object Index ...................................................................................... ROI1viii

Issue 3, March 2001 Generic Requirements for Passive Optical Components List of Figures

GR-1209-COREList of Figures Figures

Figure 2-1. Symbols for Passive Components: (a) Star Coupler, (b) Tree Coupler, Splitter or Combiner, (c) WDM ............................................................ 22

Figure 2-2. Example of Common Coupler and WDM Packages .......................... 23Figure 2-3. Example of Insertion Loss versus Wavelength for 3 Types of 1x2

Couplers .................................................................................................. 24Figure 2-4. Three Splitters as Examples of Fabrication Technologies ............... 25Figure 2-5. Arrayed Waveguide ................................................................................ 26Figure 2-6. 1x1 Protected a) Unidirectional Transmission and

b) Bidirectional Transmission Using Couplers or WDMs ................. 27Figure 2-7. N-Channel Bidirectional WDM Transmission..................................... 28Figure 2-8. A Sample Frequency Allocation Spectrum for Video and POTS

Services.................................................................................................... 29Figure 2-9. An Example of Hybrid Fiber/Coax (HFC) Architecture With 1x4

Splitting.................................................................................................. 210Figure 2-10. HFC Optical Link Reference Model With 1x4 Splitting ................... 210Figure 2-11. A Point-to-Multipoint FITL System With 1x8 Splitting .................... 212Figure 2-12. Point-to-Multipoint ODN Reference Model....................................... 213Figure 2-13. Block Diagram of a Fiber Optic Filter ............................................... 214Figure 2-14. Forming a Grating Structure in an Optical Fiber ............................. 216Figure 2-15. 1480 nm Pump Filter in an EDFA....................................................... 217Figure 2-16. Pump Power Suppression Using a Longpass Filter at the Amplifier

Output .................................................................................................... 218Figure 2-17. Fiber Bragg-Grating Filter for Pump Power Reflection .................. 218Figure 2-18. Example of Wideband Uniform Gain Spectrum............................... 219Figure 2-19. Broadcast-and-Select WDM Star Network........................................ 220Figure 2-20. (a) Wavelength-Routing WDM Network

and (b) Associated Wavelength Assignment Table.......................... 221Figure 2-21. WADM Using Optical Circulators....................................................... 222Figure 2-22. WADM Using Dielectric Thin Film Bandpass Filters ...................... 222Figure 2-23. Symbols for (a) an Optical Isolator, (b) a Three-Port Optical

Circulator .............................................................................................. 223Figure 2-24. Schematic Showing Operation of a Magneto-Optical Isolator .......224Figure 2-25. An Example of Temperature Dependence of the Backward Loss in

Single-Stage and Double-Stage Isolators...........................................224Figure 2-26. Configuration of a Cascaded Optical Isolator ..................................225Figure 2-27. Operating Principle of a Reciprocal Isolator .................................... 226Figure 2-28. Operating Principle for a Polarization Independent Walk-Off

Isolator................................................................................................... 227Figure 2-29. Schematic for a Polarization Independent Quasi-Circulator.......... 228Figure 2-30. Positions and Polarization of Beam 2 and Beam 2 on Some Element

Surfaces (a) A A, (b) B B, (c) C C, and (d) D D in Figure 2-29............................................................................................. 229

Figure 2-31. Isolation Bandwidths for Isolators Made using Yttrium Iron Garnet (YIG) and Bismuth Iron Garnet (BIG) at 1550 nm ........................... 231ix

Generic Requirements for Passive Optical Components Issue 3, March 2001List of Figures

GR-1209-COREFigure 2-32. A Schematic Configuration of the DFB Integrated Laser Module . 232Figure 2-33. OFA Gain Module Followed (a) or Preceded (b) by an Isolator.... 233Figure 2-34. Three Discrete Reflections between a Transmitter and Receiver . 234Figure 2-35. Bidirectional Transmission Using Circulators ................................. 234Figure 2-36. A Reflective Double-Pass Fiber Amplifier Using an Optical

Circulator .............................................................................................. 235Figure 2-37. Dispersion Compensation Using an Optical Circulator .................. 236Figure 2-38. A Schematic for an Optical Add/Drop Multiplexer Using Optical

Circulators............................................................................................. 236Figure 4-1. Center Wavelength, Offset, and Bandwidth for a Demultiplexer..... 45Figure 4-2. DWDM Reflection Spectrum................................................................. 48Figure 4-3. Gain Flattening Filter Spectrum......................................................... 414Figure 4-4. A Uniformity Measurement Example for a 1x2 Coupler................. 417Figure 4-5. Configuration for Measuring Polarization Dependent Loss ........... 426Figure 4-6. Polarization-Affected Wavelength Offset .......................................... 426Figure 5-1. Controlled Temperature Cycling Test Profile ....................................58Figure 5-2. Uncontrolled Temperature Cycling Test Thermal Profile ................59Figure 5-3. Example of Transmission Measurement Facility............................. 513x

Issue 3, March 2001 Generic Requirements for Passive Optical Components List of Tables

GR-1209-COREList of Tables Tables

Table 2-1. Sample HFC Power Budget for 1x4 Splitting.................................... 211Table 3-1. Summary of Environmental Definitions............................................ 313Table 4-1. Optical Performance Criteria Summary.............................................. 41Table 4-2. Nominal Optical Bandpass Ranges ......................................................44Table 4-3. Grid Central Frequencies ......................................................................46Table 4-4. Insertion Loss Criteria for Digital and AM-VSB Transmission ...... 411Table 4-5. Insertion Loss Criteria for 1xN and 2xN Couplers........................... 412Table 4-6. Uniformity Criteria and Effective Minimum Loss ............................ 416Table 4-7. PDL Requirement for MxN Couplers ................................................. 424Table 4-8. Polarization Mode Dispersion of Passive Optical Components .....428Table 5-1. Summary of Environmental Definitions.............................................. 52Table 5-2. Environmental and Mechanical Requirements................................... 53Table 5-3. Classifications Guidelines for Impact Testing....................................57Table 5-4. Fiber Ribbon Cable Retention Loads................................................. 513xi

Generic Requirements for Passive Optical Components Issue 3, March 2001List of Tables

GR-1209-CORExii

Issue 3, March 2001 Generic Requirements for Passive Optical Components Preface

GR-1209-COREPreface Preface

This Preface contains important information about the Telcordia Technologies GR process in general, as well as important information about this document.

The Telcordia Technologies GR Process

Generic Requirements documents (GRs) provide the Telcordia Technologies Forums view of generic criteria for telecommunications equipment, systems, or services, and involve a wide variety of factors, including interoperability, network integrity, funding participant expressed needs, and other input.

The Telcordia GR process implements Telecommunications Act of 1996 directives relative to the development of industry-wide generic requirements relating to telecommunications equipment, including integral software and customer premises equipment. Pursuant to that Act, Telcordia invites members of the industry to fund and participate in the development process for such GRs. Invitations to fund and participate are issued monthly in the Telcordia Digest of Technical Information, and posted on the Telcordia web site at http://www.telcordia.com/digest.

At the conclusion of the GR development process, Telcordia publishes the GR, which is available by subscription. The subscription price entitles the purchaser to receive that issue of the GR (GR-CORE) along with any Issues List Report (GR-ILR) and revisions, if any are released under that GR project. ILRs contain any technical issues that arise during GR development that Telcordia and the funding participants would like further industry interaction on. The ILR may present issues for discussion, with or without proposed resolutions, and may describe proposed resolutions that lead to changes to the GR. Significant changes or additional material may be released as a revision to the GR-CORE.

Telcordia may also solicit general industry nonproprietary input regarding such GR material at the time of its publication, or through a special Industry Interaction Notice appearing in the Telcordia Digest of Technical Information. While unsolicited comments are welcome, any subsequent work by Telcordia regarding such comments will depend on funding support for such GR work. Telcordia will acknowledge receipt of comments and will provide a status to the submitting company. Preface1

Generic Requirements for Passive Optical Components Issue 3, March 2001Preface

GR-1209-COREAbout GR-1209-CORE

A. Funders and technical contacts of GR-1209-CORE, are

ADC: Doug Atwill, Craig Robilliard

Ameritech Network Services: Mark Curtis

Tyco Electronics, Inc.: David Fisher, Michael Kadar-Kallen

Corning Inc.: Rob Johnson, Jon Pesansky, Steve Swanson

JDS Uniphase Corporation: Rick Scholes.

Agere Systems: Al Walcheski, Mike Musky

Optical Coating Laboratories, Inc.: Dan Roberts

B. Relative Maturity Level

This technology area is ready for deployment and implementation.

C. GR-1209-CORE Plans

Telcordia plans to update this document in 2003.

To Submit Comments

When submitting comments, please include the GR document number, and cite any pertinent section and requirement number. In responding to an ILR, please identify the pertinent Issue ID number. Please provide the name and address of the contact person in your company for further discussion.

Comments may be submitted at any time.

Send comments to:

Telcordia GR-1209-CORE Mark A. AliManager, Systems Technology and Reliability Effectiveness331 Newman Springs Road, Room 3Z-279Red Bank, NJ 07701-5699

Phone: 1-732-758-3017FAX: 1-732-758-5972E-Mail: [email protected]

Issue 3, March 2001 Generic Requirements for Passive Optical Components Introduction

GR-1209-CORE1. Introduction

This Generic Requirements document (GR) provides the Telcordia Technologies Forums view of generic criteria for single-mode, passive optical components1 (in this context this includes branching devices (WDM), isolators, circulators, and filters). These are intended for use in Telcordia client company2 interoffice, loop feeder, Fiber In The Loop (FITL), Hybrid Fiber-Coaxial Cable (HFC) systems, Synchronous Optical Network (SONET) and Synchronous Digital Hierarchy (SDH) systems and other telecommunications networks, employing optical communications systems that utilize Optical Fiber Amplifiers (OFAs), Wavelength Division Multiplexer (DWDM) systems.

The broad variety of passive optical components applications includes multi-channel transmission, distribution, optical taps for monitoring, pump combiners for fiber amplifiers, bit-rate limiters, optical connects, route diversity, polarization diversity, interferometers, and conherent communication. This document is focused on transmission and distribution applications.

Wavelength-Division Multiplexers/Demultiplexers (WDMs) are optical components in which power is split or combined based on the wavelength composition of the optical signal. Dense Wavelength Division Multiplexers (DWDMs) are optical components that split power over at least four wavelengths. Wavelength insensitive couplers are passive optical components in which power is split or combined independently of the wavelength composition of the optical signal. A given component may combine and divide optical signals simultaneously, as in bidirectional (duplex) transmission over a single fiber. Passive optical components are data format transparent, combining and dividing optical power in some predetermined ratio (coupling ratio) regardless of the information content of the signals. WDMs can be thought of as wavelength splitters and combiners. Wavelength insensitive couplers can be thought of as power splitters and combiners.

An optical isolator is a two-port passive component that allows light (in a given wavelength range) to pass through with low attenuation in one direction, while isolating (providing a high attenuation for) light propagating in the reverse direction. Isolators are used as both integral and in-line components in laser diode modules, optical amplifiers, and to reduce noise cause by multi-path reflection in high bit-rate and analog transmission systems.

An optical circulator operates in a similar way to an optical isolator, except that the reverse propagating lightwave is directed to a third port for output, instead of being lost. An optical circulator can be used for bidirectional transmission, as a type of branching component that distributes (and isolates) optical power among fibers, based on the direction of the lightwave propagation.

1. First time usage of a glossary term is italicized.

2. Client company, as used in this document, means any Network Operator. 11

Generic Requirements for Passive Optical Components Issue 3, March 2001Introduction

GR-1209-COREA fiber optic filter is a component with two or more ports that provides wavelength sensitive loss, isolation and/or return loss. Fiber optic filters are in-line, wavelength selective, components that allow a specific range of wavelengths to pass through (or reflect) with low attenuation (see Section 2.3.2 for classification of filter types).

1.1 Purpose and Scope of Document

This document contains generic criteria for passive optical components to help promote the satisfactory operation of such components in Network Operators single-mode fiber transmission networks and to permit economical planning and engineering of interoffice, loop feeder, FITL, HFC, and SONET Systems. This document is not intended to offer the definition for a specific product design.

Transmission is assumed to be over single-mode fiber networks. Passive optical components with multimode outputs are considered because they may be used near a receiver that has a multimode fiber-coupled detector.

The specified performance tests are intended to reflect a composite picture of various conditions. The generic criteria, desired features, and test methods may be subject to change. Network Operators may alter generic criteria to meet their individual needs, and to complement their short- and long-term deployment strategies. Since each Client Company may have needs and requirements differing from the generic requirements set forth in this document, suppliers should communicate directly with each Network Operator or other user to ascertain that company's individual requirements.

The criteria fall into three broad categories. The first consists of general requirements common to most outside plant equipment. The second category consists of functional design criteria. These are features or characteristics of device design that are compared to the stated criteria. The third category consists of performance criteria. These criteria are used to verify the device's performance under various conditions and to demonstrate its ability to perform over its expected lifetime.

GR-1221-CORE, Generic Requirements Assurance Requirements for Passive Optical Components, addresses the long-term reliability of passive optical components. It is part of the overall reliability program described in GR-357-CORE, Generic Requirements for Assuring the Reliability of Components used in Telecommunications Equipment.

1.2 Requirements Terminology

The following requirements terminology is used throughout this document:

Requirement Feature or function that, in the Telcordia view, is necessary to satisfy the needs of a typical Network Operator. Failure to meet a requirement may cause application restrictions, result in improper functioning of the 12


GR-1209-COREproduct, or hinder operations. A Requirement contains the words shall or must and is flagged by the letter R.

Conditional Requirement Feature or function that, in the Telcordia view, is necessary in specific Network Operator applications. If a Network Operator identifies a Conditional Requirement as necessary, it shall be treated as a requirement for the application(s). Conditions that may cause the Conditional Requirement to apply include, but are not limited to, certain Network Operator application environments, network elements, or other requirements, etc. A Conditional Requirement is flagged by the letters CR.

Objective Feature or function that, in the Telcordia view, is desirable and may be required by a Network Operator. An Objective represents a goal to be achieved. An Objective may be reclassified as a Requirement at a specified date. An objective is flagged by the letter O and includes the word should.

Conditional Objective Feature or function that, in the Telcordia view, is desirable in specific Network Operator applications and may be required by a Network Operator. It represents a goal to be achieved in the specified Condition(s). If a Client Company identifies a Conditional Objective as necessary, it shall be treated as a requirement for the application(s). A Conditional Objective is flagged by the letters CO.

Condition The circumstances that, in the Telcordia view, will cause a Conditional Requirement or Conditional Objective to apply. A Condition is flagged by the letters Cn.

1.3 Requirement Labeling Conventions

As part of the Telcordia GR Process, proposed requirements and objectives are labeled using conventions that are explained in the following two sections.

1.3.1 Numbering of Requirement and Related ObjectsEach Requirement, Objective, Condition, Conditional Requirement, and Conditional Objective object is identified by both a local and an absolute number. The local number consists of the object's document section number and its sequence number in the section (e.g., R3-1 is the first Requirement in Section 3). The local number appears in the margin to the left of the Requirement. A Requirement object's local number may change in subsequent issues of a document if other Requirements are added to the section or deleted.

The absolute number is a permanently assigned number that will remain for the life of the Requirement; it will not change with new issues of the document. The absolute number is presented in brackets (e.g., [2]) at the beginning of the requirement text.13


GR-1209-CORENeither the local nor the absolute number of a Conditional Requirement or Conditional Objective depends on the number of the related Condition(s). If there is any ambiguity about which Conditions apply, the specific Condition(s) will be referred to by number in the text of the Conditional Requirement or Conditional Objective.

References to Requirements, Objectives, or Conditions published in other Generic Requirements documents will include both the document number and the Requirement objects absolute number. For example, R2345-12 refers to Requirement [12] in GR2345CORE.

1.3.2 Requirement, Conditional Requirement, and ObjectiveA Requirement object may have numerous elements (paragraphs, lists, tables, equations, etc.). To aid the reader in identifying each part of the requirement, an ellipsis character (...) appears in the margin to the left of all elements of the Requirement.

1.4 Changes from GR-1209-CORE, Issue 2

This section summarizes changes in requirements since the previous issue. The section numbers below refer to GR-1209-CORE, Issue 2.

Section 1 In addition to passive optical components which were contained in GR-1209-CORE, Issue 2, criteria from GR-2882-CORE, Generic Requirements for Optical Isolators and Circulators, and GR-2883-CORE, Generic Requirements for Fiber Optic Filters, has been added.

Section 2 In addition to passive optical components which were contained in GR-1209, Issue 2, criteria from GR-2882-CORE, Generic Requirements for Optical Isolators and Circulators, and GR-2883-CORE, Generic Requirements for Fiber Optic Filters, has been added.

Section 3 Some editorial changes and inclusion of Performance Verification, Test Methods and Application Environments and Standard Conditions of Test sections.

Section 4 Now contains only Optical Performance Criteria for passive optical components and is now expanded to include criteria for other passive optical component families, e.g., Isolators and Circulators, Filters, etc.

Section 5 Now contains Environmental and Mechanical Performance Criteria for all passive optical component families. It also contains Performance Verification and Test Procedures.

Section 6 Passive Optical Component Code information deleted (entire section deleted).14


GR-1209-CORE1.5 Organization

The information contained within this document is divided into the following sections:

Section 1 -- Introduction, summary of document scope, requirements terminology and labeling conventions, and changes from previous Issue.

Section 2 -- General information including passive optical component descriptions and applications.

Section 3 -- Physical and functional design and documentation requirements; guidance procedures are outlined for quality assurance purposes.

Section 4 -- Optical performance criteria.

Section 5 -- Environmental and Mechanical performance criteria Test procedures used by Telcordia and suggested for manufacturers to analyze conformance to the criteria in Section 4.

Appendix A --Defining optical power distribution through a passive optical component in terms of a Transfer Matrix.

References --Telcordia and other references, and how to obtain references.

Acronyms -- List of acronyms used within this document.

Glossary -- List of technical terms used within this document.

Sections 4 and 5 are presented topically by subsections in the same order, to facilitate the cross-referencing of criteria and their associated test procedures.

1.6 Order of Precedence

This document forms part of a requirements document hierarchy. The order of precedence of these documents is listed below. The requirements in any of the listed documents can be overridden by those requirements contained in a document listed at a higher level, i.e., the purchase order has the highest order of precedence and overrides any requirement in a document at a lower level.

The purchase order

The equipment/device manufacturer agreed detail specification

The device manufacturer detail specification

This Telcordia generic requirements specification

Other Telcordia generic requirements specification

Other national or international specifications.15


GR-1209-CORE1.7 Harmonization With GR-1221-CORE

Wherever possible, the environmental and mechanical tests specified in this document to demonstrate the short term operational performance of passive optical components have been chosen to be compatible with those specified in GR-1221-CORE, which is used to demonstrate the long term reliability of components. This is in order to minimize the use two different sets of samples to accomplish short term performance and long term reliability evaluations.16

Issue 3, March 2001 Generic Requirements for Passive Optical Components General Information

GR-1209-CORE2. General InformationThis section provides product descriptions and applications for passive optical components.

2.1 Scope

This document includes the following passive optical components:

Passive components used in the interoffice and subscriber loop plant environment

Passive components used for analog or digital transmission using lasers and LEDs

Passive components with single-mode optical fiber-coupled ports

Passive components with single-mode input and multimode output ports

Unidirectional and bidirectional passive components

Uniform and non-uniform (taps) passive components; having equal and unequal coupling ratios for all ports, respectively (criteria for coupling ratio are not defined because they are application dependent)

Wavelength-division-multiplexing and demultiplexing passive components

Dense Wavelength-division-multiplexing and demultiplexing passive components

Star, tree, and access couplers

Reflective star passive components

Filters

Isolators and Circulators.

The following passive optical components are not included in this document:

Tunable passive components (passive components having a variable coupling ratio)

Active passive components, those using optoelectronic components or optical amplifiers (but those may be considered in the future)

Passive optical integrated circuits that have a higher degree of functionality or that integrate several passive components on the same chip.21

Generic Requirements for Passive Optical Components Issue 3, March 2001General Information

GR-1209-CORE2.2 Passive Optical Components - Couplers and WDMs

2.2.1 General Product Description

Figure 2-1 shows the symbols for three types of couplers, where (b) and (c) are special cases of (a). In (c), the symbol can be substituted for WDM.

In the MxN (can also be denoted M:N or M-by-N) star coupler, power at any of M ports, PI = (P1, P2, ..., PM), is distributed to any of N ports or vice versa, with powers PO = (PM+1, PM+2, ..., PM+N). Ideally, all of the input power is totally transferred to the output ports (no excess loss). Similarly, in the ideal case, the optical inputs are totally isolated from each other (high directivity). For non-WDM couplers, the optical outputs are not isolated from each other with respect to wavelength (no wavelength isolation).

In WDM passive components, however, the optical outputs are isolated from each other with respect to wavelength (high wavelength isolation). WDM port i transfers power in optical channel i only. In general, the designation of input and output ports is somewhat artificial in that light may be launched into any of the M + N ports and light may propagate through the passive component in both directions simultaneously. A WDM demultiplexer would typically have M = 1 input port demultiplexed to N output ports.

Optical terms are discussed in more detail in Section 4, the Appendix, and Glossary. Figure 2-2 shows some examples of common passive optical passive component packages.

2.2.2 Coupler and WDM Classes

Coupler and WDM classes are generally distinguished as follows:

Splitter typically has one input and two or more outputs. It can be used for bidirectional transmission or to distribute a signal to two or more service points.

Combiner typically has one output and two or more inputs. It can be used for bidirectional transmission or to combine signals from two or more service points.

Figure 2-1. Symbols for Passive Components: (a) Star Coupler, (b) Tree Coupler, Splitter or Combiner, (c) WDM

M N

12

M

M+1M+2

M+N

(a) (b) (c)

WDM22


GR-1209-CORE Star Coupler has two or more input ports and two or more output ports, and often has a large number of ports (e.g., 32). It is used to distribute and combine signals between multiple originating points and multiple receiving points.

Tree Coupler has a single input distributed among several outputs or vice versa. It is used to distribute signals from a single node to multiple service points or vice versa.

Access Coupler or Tap1 is a 3 or 4 port passive component that permits an add/drop function, usually of some small fraction of optical power, at a node (i.e., highly non-uniform coupling ratio). It can be used for coupling nodes to bus networks, for signal line status monitoring, and for add/drop multiplexing.

Non-Intrusive Tap2 connects to the network in-span, without introducing a discontinuity in the fiber (not covered in this document).

Wide Band Coupler, Wavelength Insensitive Coupler, or Broadband Coupler (BBC) operate in dual bandpasses, usually encompassing both the 1310 nm and 1550 nm windows. This document requires all passive components to be of this type, assuring transmission at all telecommunications wavelengths (see Section 4.2.1).

Wavelength-Flattened Coupler is designed to be insensitive to variations in wavelength within a single window. This feature is desirable for all passive components to minimize variations in component insertion loss for any transmission wavelength within the window.

Figure 2-2. Example of Common Coupler and WDM Packages

1. For additional generic mechanical criteria on some types of taps, refer to GR-1009-CORE, Generic Requirements for Fiber Optic Clip-On Test Sets.

2. Coupling ratio criteria for taps are not proposed. 23


GR-1209-CORE Wavelength-Division-Multiplexer or Demultiplexer (WDM) distributes light based on its wavelength composition. It is used to transmit multiple signals, each at a different wavelength, on a single fiber.

Dense Wavelength-Division-Multiplexer or Demultiplexer (DWDM) enables multiple closely-spaced wavelengths (optical frequencies) to be distinguished within the 1550 nm window, facilitating multi-channel transmission at low attenuation wavelengths and the use of optical amplifiers.

To clarify the wavelength dependence of different couplers, Figure 2-3 shows the insertion loss versus wavelength for 3 types.

The loss for all 3 is the same, 3.9 dB at 1310 nm, but differs for other wavelengths. The Single-Window coupler's loss has a large rate of change with respect to wavelength. It is intended for use over a narrow wavelength range such as 1310 20 nm. The Wavelength-Flattened coupler has a low loss in the entire 1310 nm region (1310 50 nm). The Wideband coupler has low loss in both the 1310 nm and 1550 nm regions. These couplers are required if dual window operation is planned.

2.2.3 Coupler and WDM Technology

Manufacturing technology is not considered in detail here, other than to describe some of the methods used to produce couplers and WDMs. In the Fused Biconic Taper (FBT) method, bare or etched fibers are brought into contact (alone or encapsulated in glass), stretched, possibly twisted, and fused so that a coupling of modes is achieved along an interaction length. GRIN-rod lenses, micro-optic lenses and mirrors have been used to fabricate passive components with as many as 1000

Figure 2-3. Example of Insertion Loss versus Wavelength for 3 Types of 1x2 Couplers24


GR-1209-COREports. Planar waveguide passive components are made photolithographically, using parallel processing techniques. To produce the refractive index profile, ions are diffused into a substrate, such as a glass, semiconductor, or polymer. Alternatively, doped silica glass is fabricated by flame hydrolysis deposition and consolidation. The waveguide and Y branch are defined by photolithographic masking techniques followed by ion diffusion and/or etching. Another technology (currently in research) for manufacturing passive components, utilizing the planar approach, is Chemical Vapor Deposition (CVD).

Figure 2-4 shows 3 splitters to exemplify the different fabrication technologies: a) 1x8 Planar Waveguide splitter, b) 1x6 Inter-fused Fiber splitter, and c) 1x16 concatenated splitter with 1x2 splitters.

To manufacture the planar waveguide device, fiber leads are epoxied to the substrate in such a way as to avoid low return loss. The inter-fused device is made by fusing 7 fibers together simultaneously. Large passive component configurations can also be made by concatenating together smaller passive components. After fusing, the bare fiber is thin and fragile, and must be sealed and reinforced.

Non-intrusive optical taps can be considered extrinsic passive components in that they may operate by local injection and detection of modes radiating out of the fiber at a bend. Such devices may be installed without the need for conventional fiber termination and splicing. More advanced methods based on electro-optically active waveguide materials such as LiNbO3, allowing electronic control of the coupling ratio, have been used to make passive components, switches and modulators.

Figure 2-4. Three Splitters as Examples of Fabrication Technologies

1

16

Common25


GR-1209-COREArrayed waveguide or multiplexers represent an integrated approach where fabrication is done on a polymer substrate such as single SiO2, Al2O3 or LiNbO3 slabs, see Figure 2-5. Light from the input is imaged onto the input waveguide and it diverges into the arrayed waveguides. The optical path lengths of sequential waveguides are an integer multiple of the central design wavelength of the demultiplexer. The phase and intensity distribution of the input light are reintroduced into the second slab waveguide resulting in light focusing on the receiver plane on the far side of the slab. The difference in path length results in a phase front that will place the focal spot on successive output waveguides dependent upon wavelength.

Polarization independence of these devices is achieved by several methods. These include non-birefringent waveguide design, overlap of transmission peaks of orthogonal polarizations of different orders, and a reflective quarterwave plate placed midway in the waveguide array span which converts TE modes to TM modes and vice-versa.

2.2.4 Coupler and WDM Applications

Couplers and WDMs have applications in a number of telecommunications architectures discussed in the following sections. They are also extensively used in fiber optic equipment.

Equipment applications will not be discussed in detail here. In optical transmitters and stabilized sources, couplers provide feedback for stabilizing output power. In Optical Time Domain Reflectometers (OTDRs), 2x1 couplers launch light onto the fiber and guide light that is backscattered and reflected along the fiber to the detector.

Figure 2-5. Arrayed Waveguide26


GR-1209-CORE2.2.4.1 Bidirectional Transmission

Conventional unidirectional point-to-point transmission requires 4 fibers for 1x1 protected service (Figure 2-6a). Bidirectional transmission3 using couplers, or wavelength duplexing using WDMs, requires only 2 fibers (Figure 2-6b).4

Figure 2-6. 1x1 Protected a) Unidirectional Transmission andb) Bidirectional Transmission Using Couplers or WDMs

In bidirectional transmission, two optical channels propagate simultaneously, at the same wavelength, in opposite directions, along the same fiber.

Bidirectional transmission systems are more sensitive to reflections than their unidirectional counterparts. A typical fiber facility installation may include interconnect or cross-connect panel connectors and access splices (Figure 2-7).

Light reflected at these components, as well as light originating from poor passive component directivity, and from a fiber-cut, can end up at the wrong receiver. The receiver can lock on to the wrong signal and suppress an Loss-of-Signal (LOS) alarm. This is referred to as receiver sync-up. Another impairment, which is less catastrophic than sync-up, but that can cause errors is intersymbol interference.

For dual wavelength bidirectional transmission, the insertion loss due to couplers can be reduced by replacing them with WDMs. To explain this, consider the following example: If the pair of 1x2 couplers in Figure 2-6 meet the insertion loss

3. Also referred to as duplex transmission.

4. Couplers here means a 1x2 wavelength-insensitive 3 dB coupler.27


GR-1209-CORErequirement in Section 4.2.2, their combined loss is at most 8.0 dB. The equivalent loss requirement for a pair of 1x2 WDMs is only 3.0 dB, in both wavelength regions. Replacing the couplers with WDMs results in a loss reduction of 5.0 dB.

Figure 2-7 shows an N-channel, bidirectional fiber optic WDM transmission system in which N independent channels are modulated onto optical carriers at N different wavelengths, 1, 2, ..., N.

The optical transmitters, Tx1, Tx2, ..., TxN, and the optical receivers, Rx1, Rx2, ..., RxN, are fiber-coupled to corresponding WDM channels. Both WDMs function as multiplexer and demultiplexer to combine and separate signals onto a single fiber, through which the light propagates between remote locations (e.g., CO, subscriber terminal). Transmission can occur over any number of channels N, in any combination of directions, not just the directions shown, to accommodate network traffic. As was the case for Figure 2-7, these passive components require high return loss and high directivity (low crosstalk), because bidirectional transmission systems are sensitive to reflections. However, bidirectional transmission using WDM is not as sensitive to reflections as using couplers because the WDM wavelength isolation serves to isolate the receivers from reflected light in the reverse channel.

2.2.4.1.1 Steps for Converting to Bidirectional Interoffice Transmission

This section describes how to convert an existing 4-fiber 1x1 protected unidirectional fiber optic transmission system (as in Figure 2-6a) into a 2-fiber 1x1 protected bidirectional transmission system (Figure 2-6b).

Figure 2-7. N-Channel Bidirectional WDM Transmission28


GR-1209-CORE1. Mount the 1x2 coupler hardware on shelves in local and remote office and run fiber jumpers to transmission equipment. Measure the end-to-end insertion loss, span return loss, and discrete reflectances.

2. Convert the existing unidirectional protect path to bidirectional by inserting couplers.

3. Switch service (carried over a pair of fibers) from main to protect mode (now carried over a single fiber).

4. Convert the existing unidirectional main path to bidirectional by inserting couplers.

5. Switch service from protect mode back to main.

The above procedure will vacate 2 fibers for each converted span.

2.2.4.2 HFC Architecture for Video and Telephony

Vestigial side-band amplitude modulation (AM-VSB) is a signal format for encoding the color, picture, sound and synchronization information for one video channel into a 6 MHz bandwidth. Interactive video channels are digitally compressed using either QAM or digital-VSB signal formats. The analog and digitally compressed video channels are subcarrier multiplexed onto 6 MHz subcarriers according to a frequency allocation plan, an example of which is shown in Figure 2-8.

Figure 2-8. A Sample Frequency Allocation Spectrum for Video and POTS Services

The number of compressed digital video channels that occupy each 6-MHz subcarrier will depend on the digital modulation format chosen (e.g., 16-QAM, 64-QAM, 256-QAM, 16-VSB, etc.).

In the Hybrid Fiber/Coax (HFC) architecture, an example of which with 1x4 splitting is shown in Figure 2-9, frequency-division multiplexed (FDM) carriers (see sample frequency allocation spectrum above) combine analog video, digital video and POTS.

A linear laser analog transmitter (Analog Tx) is used to modulate the resultant signal onto an optical carrier, usually at a wavelength of 1310 20 nm. The optical signal is distributed through (up to about 4 miles of) optical fiber and splitters to a remote optical node (O/E Node). From there, coaxial transmission cable (coax),

UpstreamPOTS, etc.

DownstreamPOTS, etc.

AnalogBroadcast

Video

DigitalBroadcast

Switched Video? ?

42 545 ? ? 750 MHz29


GR-1209-COREamplifiers (two-way amp) and taps are used to deliver the signal to living units. Optical splitting serves to distribute the signal economically by reducing the amount of fiber and the number of expensive laser transmitters required to serve a particular O/E node size. For this reason, the use of splitters for analog video distribution is becoming widespread.

A reference model for an HFC link with 1x4 splitting5 is shown in Figure 2-10 and its associated power budget is shown in Table 2-1.

Figure 2-9. An Example of Hybrid Fiber/Coax (HFC) Architecture With 1x4 Splitting

5. The splitting ratio depends on the power budget of the optical link and on the bandwidth available for POTS and other switched services.

Figure 2-10. HFC Optical Link Reference Model With 1x4 Splitting

Tap

Tap

Tap

O/E

two-wayamp

O/E Node (ONU) serving100-500 LUs

Tap

Analog Video

Digital Video AnalogModulatorsPOTS

AnalogTx

fiber

coax210


GR-1209-COREThis model allows for two connectors and 10 splices as follows: C2 and C3 represent a fully cross-connected FDF. S1 represents a single splice in the cable vault. S2-5 represent OSP splices in the feeder backbone. S6-9 represent splices in the splitter closure and its stub cable to feeder/access cable splices. And, S10 represents a splice at the O/E remote node. The 0.07 dB is considered to be a typically achievable splice loss (0.10 dB should be the worst-case splice loss).

Since HFC power budgets are small (~10 to 15 dB) compared with those for purely digital transmission, and since the splitter consumes a considerably large portion of the power budget, the required splitter insertion loss and uniformity are tighter than for a correspondingly configured splitter used in digital telecommunication.

Uniformity serves to limit the minimum coupler loss, which is necessary to limit the maximum receivable power level. A 1x4 coupler uniformity of 0.8 dB means that the minimum loss is 5.4 dB6. Also, since CNR (~50 dB) is much higher than for digital transmission, the required reflectance (55 dB) is also much lower. The bandpass for analog video transmission (1310 20 nm) is narrower than for most digital transmission.

The requirements in Section 4 for AM-VSB splitters are intended to cover all of the conditions described above. The AM-VSB requirements in Section 4 should allow generic splitters to be used for most AM-VSB transmission.

In couplers, abrupt transitions in the rate of change of insertion loss as a function of wavelength are believed to result in distortion of AM video transmission. Such distortion is expected to occur when the transmitters chirped laser linewidth and the abrupt transition overlap. A new requirement for insertion loss slope0.1 dB/nm is being considered. The exact value and test methods are under study and industry comments are invited.

Table 2-1. Sample HFC Power Budget for 1x4 SplittingItem Unit Loss Number Units Power

Launched Power - - +10.0 dBmFiber 0.4 dB/km 4 -1.6 dB

Splices (S1-10) 0.07 dB 10 -0.7 dBConnectors (C1-4) 0.2 dB 2 -0.4 dB

1x4 Coupler 6.9 dB 1 -6.9 dBReceived Power - - +0.4 dBm

6. A 5.4 dB is derived by assuming a 0 dB excess loss and 3 ports coupled at a 23.8% (6.23 dB) coupling ratio. By conservation of energy, the 4th port must then be coupled at 28.6% (5.43 dB). See section on uniformity for method of calculation.211


GR-1209-CORE2.2.4.3 Fiber in the Loop Systems

FITL systems are expected to be widely deployed for loop distribution of existing telecommunications services7 and future broadband/video services. FITL systems are meant to serve contiguous groupings of residential and small business customers and are fundamentally different from high capacity point-to-point fiber optic systems employed in the interoffice and loop feeder networks. A prime objective for FITL systems is that they be both cost-competitive with todays metallic-based distribution and accommodate economic upgrades for high-bandwidth capabilities. Passive optical components are one of the key elements for determining the technological readiness and cost-effectiveness of point-to-multipoint systems. For a more complete discussion of FITL systems, see GR-909-CORE, Generic Criteria for Fiber In The Loop Systems.

A generic FITL system consists of a Host Digital Terminal for narrowband (HDT) and broadband (BHDT) services, and subtending Optical Network Units (ONUs) that are managed by the HDT and BHDT in a master-slave relationship. In the point-to-multipoint architecture, a splitter in the Optical Distribution Network (ODN) connects ONUs to the HDT, providing optical pathways by which they communicate (see Figure 2-11).

The HDT interfaces the FITL system to the Client Company transmission and operations network. The ONU delivers telecommunications services to residential and small business customers. See GR-909-CORE for details.

The ODN contains passive optical components, including single-mode optical fiber cables, splices, connectors, attenuators, couplers, and WDMs. It could contain active components such as optical amplifiers, but no O/E or E/O devices. The ODN may be implemented in either a point-to-point configuration, where a unique fiber facility extends from an optical line unit at the HDT to an ONU, or a point-to-multipoint configuration, where a splitter distributes the fiber facility from the HDT optical line unit to several ONUs. In either case, the fiber link between the HDT or coupler/splitter and the ONUs could consist of either one or two fibers per ONU.

7. Existing telecommunications services refers to the full menu of service capabilities, such as Plain Old Telephone Service (POTS), voice frequency specials, digital data, and basic rate Integrated Services Digital Network (ISDN) services, as defined by existing electrical interfaces.

Figure 2-11. A Point-to-Multipoint FITL System With 1x8 Splitting

ONU

NetworkInterface

POTS

POTS

1x8splitter

closure

ODN

ONU

ONU

HDT212


GR-1209-COREThe splitter could be located anywhere within the ODN. The value of N in the coupler/splitter is typically 32.Figure 2-12 shows a Point-to-Multipoint ODN Reference Model for FITL systems with two fibers per ONU. For illustrative purposes, the WDM upgrade is shown only on the downstream (top) fiber. The WDM upgrade permits transmission of a second optical channel on the same fiber. Note that the upgrade passive components may or may not be connectorized whereas the point-to-multipoint splitters are generally (not always) spliced.

If the transmitter (Tx) and receiver (Rx) are considered to represent transceivers comprising integrated, bidirectional splitters, then this model also represents the one fiber per ONU case. However, in this case the splitter upgrade option is not viable.

Operators may choose to deploy systems with a longer reach than the traditional 12 kft (3.6 km) Carrier Serving Area (CSA) range. The requirements specified in Section 4 are applicable for both traditional and extended-reach 24 kft (7.2 km) systems. Refer to GR-909-CORE for the link loss values for point-to-point and point-to-multipoint configurations.

Passive components will be expected to perform reliably8 with other ODN components purchased from any combination of acceptable suppliers. Worst-case assumptions are made, and end-of-life values of component parameters, including environmental ranges, are utilized for transmission design and are specified in this document. The requirements specified in Section 4 are intended to cover all of the configurations described above.

Figure 2-12. Point-to-Multipoint ODN Reference Model

8. See GR-1221-CORE, Generic Reliability Assurance Requirements for Fiber Optic Passive Components, Issue 2, for reliability criteria.213


GR-1209-CORE2.3 Passive Optical Components - Filters

2.3.1 General Product Information

As noted in Section 1, this document covers fiber optic filters with two or more ports, one port for input and one or more output ports. Figure 2-13 shows the block diagram of a two port filter where Pi (l) is an input signal, Po (l) is an output signal and l denotes wavelength.

2.3.2 Filter Classes

Single-mode optical filter classes are generally distinguished as follows:

Short-pass filter isolates longer wavelengths from the input optical signal so that the output optical signal spectrum contains shorter wavelengths.

Long-pass filter isolates shorter wavelengths from the input signal so that the output optical signal spectrum contains longer wavelengths.

Fixed Bandpass filter transmit a band of wavelengths around a fixed central wavelength of an input signal while blocking other wavelengths.

Tunable Bandpass filter transmits a band of wavelengths around a central wavelength that can be tuned over a given wavelength range of an input signal, while blocking other wavelengths. Tunable filters can be active devices.

Bandstop (or notch) filter blocks a band of wavelengths of an input signal while transmitting other wavelengths. In some bandstop filters the blocked wavelength band is reflected, as in Bragg-grating devices, as opposed to being discarded.

Gain Flattening filter - Gain Flattening filter - equalizes optical power within the wavelength region of interest. Gain flattening filters are used in conjunction with optical fiber amplifiers to "flatten" the wavelength-dependent gain of the amplifier.

Figure 2-13. Block Diagram of a Fiber Optic Filter

Pi () Po () Fiber Optic Filter 214


GR-1209-CORE2.3.3 Fiber Optic Filter Technology

Various technologies such as dielectric interference coating, fused fiber couplers, acousto-optics, liquid crystals, fiber Fabry-Perot cavities, and fiber Bragg-grating are being used to fabricate fiber optic filters. A few of these technologies are described below.

2.3.3.1 Dielectric Interference Coatings

This type of filter consists of substrate (usually glass) coated with multilayer dielectric thin films. These layers consist of a stack of alternating high and low refractive index materials. Usually, the thickness of each layer is such that the optical path length through the layer is a quarter of the central wavelength to be transmitted. By selecting the number of layers and changing the refractive index as well as the thickness of some of the layers, desired spectral characteristics of such filters are achieved. The use of thin film planar process technology allows for the production of low-cost, high reliability filters. It also provides design flexibility to produce two-port shortpass, longpass, and fixed or tunable bandpass filters.

When an interference filter is used as a bandpass tunable filter, the tuning of the central wavelength of the passband may be provided by varying the angle of incident light. In this scheme, the central wavelength decreases with the increasing incidence angle relative to normal incidence.

Another method of providing the tuning is to vary the optical path length of the multilayer film during the film fabrication process. By presenting a varying optical length to the transmitted beam, the central wavelength of the passband can be tuned without changing the angle of incidence.

2.3.3.2 Fiber Optic Bragg Grating

Bragg gratings, fabricated in a single-mode optical fiber, promise to be useful in a wide variety of applications, including channel selecting optical filtering in WDM systems and providing various wavelength dependent loss mechanisms in Erbium-doped fiber amplifiers.

The formation of a grating in fibers is based on their photosensitivity. As shown in Figure 2-14, when the fiber core is exposed to an interference fringe pattern of a UV laser over a several mm length, an intensity dependent change in the refractive index results in a spatial modulation of the refractive index of the core.

Each modulation period of the refractive index acts as a small reflector. These multiple waves constructively interfere around the Bragg wavelength, for which the optical path difference between two consecutive periods of the grating is a half wavelength.215


GR-1209-CORE2.3.3.3 Fiber Fabry-Perot Filters

Fiber Fabry-Perot filters are tunable bandpass filters that utilize interference between multiple reflections in a cavity formed by two highly reflective mirrors. Such mirrors are either deposited directly on the fiber endfaces forming the cavity or separate mirrors, combined with suitable collimating and focusing lenses, are used to form the cavity. The spectral response of the filter consists of multiple transmission peaks. The wavelength separation between two successive peaks is known as Free Spectral Range (FSR). It should be chosen to be large enough to prevent selecting more than one optical signal. A voltage applied to a piezo-electric (PZT) actuator is used to vary the cavity spacing and tune the transmission wavelength. To address problems of hysteresis, temperature changes, and source wavelength drifts due to aging or temperature, a feedback circuit may be used to track the signal. The response time of such a filter varies according to the complexities of the feedback circuit which depends on the filter application.

2.3.4 Fiber Optic Filter Applications

Fiber optic filters find applications in both single-wavelength and multiwavelength optical networks, in fiber optic equipment such as spectral analyzers, and in devices such as tunable lasers. They may also find use in the developing technology of wavelength converters utilizing four-wave mixing, and for frequency stabilization of semiconductor lasers. In the following sections, a few of the key current and future applications of these filters are discussed.

2.3.4.1 Fiber Optic Filters in EDFAs

Fiber optic filters are used in EDFAs to improve amplifier performance through noise reduction, gain optimization, and gain shaping. These applications are described in Sections 2.3.4.1.1 through 2.3.4.1.4.

Figure 2-14. Forming a Grating Structure in an Optical Fiber

UV Radiation

Optical Fiber

Core

InterferencePattern216


GR-1209-CORE2.3.4.1.1 Optical Pump Filters for EDFAs

Both the 1480 nm and 980 nm pump bands are widely used in EDFAs, but the 1480 nm pump band has a more mature pump laser diode technology. This band is necessary for applications such as distributed or remote pumped amplifiers, where a low pump power attenuation is required.

High power (typically + 15 to +20 dBm peak) multimode 1480 nm pump lasers have a spontaneous emission spectrum in the OFA gain band that is typically ~40 to50 dB below the peak pump output power. Such spontaneous emission also gets amplified while passing through the Erdoped fiber, and such amplified spontaneous emission (ASE) increases noise.

A shortpass filter, passing the pump power within the absorption band (1480 nm pump band) while removing the pump power within the gain band of the EDFA, is used in the pump power path immediately following the 1480 nm pump laser as shown in Figure 2-15. Such a filter reduces the ASE noise and isolates the pump laser from the 1550 nm signal band.

For 980-nm pump bands, such a filter is not necessary because this band has high attenuation over conventional single-mode fiber, which is optimized for 1300 nm and 1550 nm transmission. Such a filter is also not necessary for a preamplifier (just before a receiver) because an ASE-suppression filter (discussed in Section 2.3.4.1.3) will filter out this pump wavelength band.

2.3.4.1.2 EDFA Pump Power Suppression/Recycling

A significant portion of pump power remains unabsorbed in the EDFA, and this residual pump power may create a problem in an optical communication system. For example, the receiver performance may be adversely affected by the residual pump power in a preamplifier because the pump power may cause excess noise and receiver saturation. Two different types of filters may be used to improve system performance.

Figure 2-15. 1480 nm Pump Filter in an EDFA1480 nm Pump

Signal

Shortpass Filter

Er-doped fiber217


GR-1209-COREIn the first approach (Figure 2-16), the longpass filter at the amplifier output passes the amplified signal and prevents the residual pump power from reaching the receiver.

In the second approach (Figure 2-17), a fiber Bragg grating filter used to pass the amplified signal and reflect the pump power back to the Erbium-doped fiber to be reabsorbed, results in signal gain and noise reduction.

Both approaches are useful in single and multiwavelength systems.

2.3.4.1.3 ASE Suppression in EDFAs

ASE has a spectrum which closely emulates the gain spectrum of the amplifier. When multiple amplifiers are cascaded, accumulation of ASE noise power can degrade system performance. ASE can also degrade the noise figure of preamplifiers.

A bandpass filter at the output of an EDFA can be used to pass the signal while rejecting most of the ASE. Generally, such ASE-suppression filters are used within-line amplifiers and preamplifiers.

The choice between the use of fixed versus tunable filters depends on:

1. The signal laser wavelength stability

2. The required bandwidth of the filter, which depends on the required SNR

3. The transmission bit-rate

4. And for multiwavelength transmission, the spacing between each laser central wavelength.

Figure 2-16. Pump Power Suppression Using a Longpass Filter at the Amplifier Output

Figure 2-17. Fiber Bragg-Grating Filter for Pump Power Reflection

Pump

SignalEr-doped fiber

Longpass filter

Pump

SignalEr-doped fiber

Bragg gratingfilter

ReflectedPump Light

AmplifiedSignal218


GR-1209-CORE2.3.4.1.4 Gain Flattening in EDFAs

EDFAs have a non-uniform gain spectrum with a sharp peak around 1535 nm and a broad band with reduced gain between 1540 nm and 1560 nm. In WDM systems, if the amplifier is operated away from the gain peak, increased noise and possible lasing at the gain peak can occur. In AM systems, distortion can occur due to the gain-slope.

A fiber optic filter, providing a wavelength selective loss, can be used to achieve a wideband, uniform gain spectrum (see Figure 2-18). A fiber Bragg grating filter, acting as a notch filter to suppress the gain peak, can be used to provide such a wavelength selective loss.

2.3.4.2 Fiber Optic Filters in Multiwavelength Networks

Various approaches are being explored for mulitwavelength optical networks using a variety of filters. With the great interest in multimedia communication and other services, there is an associated demand for huge bandwidth. This demand can be met by increasing the signal rate via time-division multiplexing (TDM) on each link, and by wavelength-division multiplexing (WDM) multiple optical channels per fiber. The availability of reasonably inexpensive passive components such as star couplers and wavelength-sensitive optical switching and routing elements has made it possible to consider the use of wavelength as another dimension in network and

Figure 2-18. Example of Wideband Uniform Gain Spectrum

Loss (dB)

Gain (dB)

GFF Respon

Composite Response

EDFA Response

Wavelength 219


GR-1209-COREswitch design. Thus, to realize WDM optical networks, the availability of inexpensive and high performance fiber optic filters is of paramount importance.

The two general architectural forms that have been commonly used in WDM networks are broadcast-and-select networks and wavelength routing networks. A broadcast-and-select approach is shown in Figure 2-19.

The star coupler combines various wavelength channels and then splits the power of the combined multiwavelength signal, broadcasting all the wavelengths to all output ports. A tunable optical filter selects the appropriate wavelength channel for the receiver.

In wavelength routing networks (Figure 2-20) the path that a signal takes is uniquely determined by the wavelength of the signal and the port through which it enters the network.

Thus, in the example shown in Figure 2-20, the wavelength to go from input port S1 to output port R3 is 1.There are many other examples of architectures which utilize these basic approaches. However, wavelength selective devices (fiber optic filters) play an indispensable role in all WDM networks.

2.3.4.3 Wavelength Add-Drop Mutiplexers (WADMs)Multichannel WDM systems are being deployed as a complement to Time Division Multiplexed (TDM) higher bit-rates. They enable more efficient utilization of the fiber plant before additional cable is installed. Criteria for such systems are covered in GR-2899-CORE, Generic Criteria for SONET Two-Channel (1310/1550-nm) WDM Systems. DWDM applications are treated in GR-2918-CORE, DWDM Network Transport Systems with Digital Tributaries for Use in Metropolitan Area Applications: Common Generic Criteria.

Figure 2-19. Broadcast-and-Select WDM Star Network

1

2

N X NStar Coupler

T1

T2

Ti

TN

R1

R2

Ri1

r1

rN

TransmittersReceiversTunable

Array of fixed-tunedreceivers

TunableFilter220


GR-1209-COREWDMs combined with other components such as optical amplifiers, wavelength adapters, switches and switch modules, and dispersion compensators, result in various network elements such as wavelength add-drop multiplexers, wavelength routers, wavelength terminals and cross-connects. Fiber optic filters are used to realize WDMs. Two such applications of filters are described below.

Figure 2-21 shows a bit-rate independent wavelength add-drop multplexer, in which a reflective filter and two circulators are used to drop and add a wavelength channel.

An incoming multiwavelength signal from Port 3 is routed to the reflective filter by the top circulator. The reflective filter reflects all wavelength channels except channel i, which it transmits. The reflected channels are directed by the top circulator to output at Port 4. The transmitted channel i is dropped off to Port 1 by the bottom circulator for detection by an optical line terminating multiplexer (OLTM). New incoming information from Port 2 on channel i is transmitted through the reflective filter and is added to Port 4, to join the other optical channels.

Figure 2-20. (a) Wavelength-Routing WDM Network and (b) Associated Wavelength Assignment Table

2

2

2

1

3

3

3

1

1

Inputs OutputsS1

S2

S3

R1

R2

R3

WDM WDM

(a)Inputs Outputs

S1S2S3

R1 R2 R31 23

33

11

22

(b)

(wavelength selection including filters)221


GR-1209-COREIn another type of WADM (Figure 2-22), a dielectric thin film filter transmits wavelength channel 1 and reflects all other channels 2 through N. To separate the reflected channels from a incoming multiwavelength signal, the filter is oriented so that the incoming signal forms a small angle with respect to normal signal and the filter surface. Similarly, a filter with the directions of the arrows reversed could be used to add channel 1 to the stream of all other channels 2 through N.

2.3.4.4 Fiber Optic Filters in Remote Fiber Test Systems

Low-cost optical subscriber systems and effective operations and maintenance are critical for deployment of fiber in local subscriber loops. Various non-intrusive Remote Fiber Test Systems (RFTSs) architectures have been developed that will not require personnel to perform field testing. The RFTSs quickly locate and diagnose problems in the optical cable plant before dispatching a crew to resolve the problem. For more detailed information, see GR-1295-CORE, Generic Requirements for Remote Fiber Testing Systems (RFTSs).

Figure 2-21. WADM Using Optical Circulators

Figure 2-22. WADM Using Dielectric Thin Film Bandpass Filters

Filter

Port 2 Port 1

Port 3 Port 4

new i from OLTM old i to OLTM

all s all s old i + new i

new i

old i

all s old i + new i

all s

new i

from OLTM

old i to OLTM Port 1

Port 4 Port 2

Port 3 222


GR-1209-CORE2.4 Passive Optical Components - Isolators and Circulators

2.4.1 General Product Description