fiber-optic communications james n. downing. chapter 7 fiber-optic devices

Fiber-Optic Communications

James N. Downing

Chapter 7

Fiber-Optic Devices

7.1 Optical Amplifiers

• Repeaters and Regenerators– Repeater

• An optical receiver converts the light to an electrical signal.

• An amplifier increases the signal.• The transmitter converts the electrical signal back to

an optical signal.

– Regenerator• Removes the noise from the digital signal and

regenerates the clean signal for transmission

7.1 Optical Amplifiers

• Erbium-Doped Fiber Amplifier– Consists of:

• Coupling device• Fiber: Highly doped with erbium• Two isolators: Suppresses reflection at the ends of the

fiber • Pump laser: Excites the erbium ions so they can be

stimulated by incoming signal photons

7.1 Optical Amplifiers

• Erbium-Doped Fiber Amplifier– Advantages

• Simultaneously amplifies a wide wavelength region with high output powers

• Gain is relatively flat across the spectrum• Power transfer efficiency of about 50%• Large dynamic range• Low noise figure• Polarization independent

7.1 Optical Amplifiers

• Erbium-Doped Fiber Amplifier– Disadvantages

• Long fiber lengths make them difficult to integrate with other devices

• Pump laser creates spontaneous noise even without light• Crosstalk• Gain saturation

7.1 Optical Amplifiers

• Semiconductor Optical Amplifier– Amplification is achieved by inserting a diode between two

fibers– Advantages

• Ability to be integrated with other semiconductors

• Wide spectral range

– Disadvantages• Higher noise figure due to coupling

• Changing light intensity causes gain changes

7.1 Optical Amplifiers

• Raman Amplifier– Based on principle of nonlinear Raman scattering– Discrete

• Packaged in a box with a pump laser.• Actual transmission fiber becomes the amplifier.• Amplifier is coupled to the receiver end directed in the

opposite direction of the signal. The pump transfers energy to the weak incoming signal.

• Signal is amplified as it decays due to fiber losses.

7.1 Optical Amplifiers

• Raman Amplifier– Advantages

• Increases transmission length by a factor of four• Lower power signal can be transmitted• Improvement in noise performance• Denser channel counts• Faster transmission speeds

– Disadvantages• High power and long fiber lengths required• Thermal controls and safety issues

7.2 Couplers

• Types– Tree coupler: Distributes incoming light evenly

between the output ports– Star coupler: Many input ports coupled to many

output ports– Tee couplers: Three ports—input, output, and

monitoring

7.2 Couplers

• Manufacturing Methods– Fused biconical tapered coupler– Used for star, tee, and general coupling– Four-port directional coupler

• Two bare fibers are pulled and melted together

7.2 Couplers

• Loss– Insertion loss– Excess loss – Directional loss (splitting)

7.3 Modulators

• Direct Modulation– The amount of drive current can be controlled by

simply turning it on and off—pulses.– Small signal modulation or pulse code modulation

is more practical for communications.– Limited response time– Large wavelength chirp– High bias currents

7.3 Modulators

• Indirect Modulation– Devices are inserted into the optical path of the

source to implement modulation optically.– Major Devices

• Electro-optic—process by which the refractive index of a material is changed through the application of an electric field. May be amplitude, phase, or frequency types.

7.3 Modulators

• Indirect Modulation– Major Devices

• Electro-Absorption Modulators are efficient with low chirp and small drive voltage.

• Operate at frequencies greater than 40 GHz• Can be integrated on the same chip as a laser diode and

other transmitters• Future modulator of choice

7.4 Multiplexers and Demultiplexers

• Multiplexers– Combine optical signals by wavelength division– Add-drop multiplexers may use gratings or filters– Channel spacing can be widened to limit loss

• Demultiplexers– Single wavelengths can be picked off without

demultiplexing whole signal

7.4 Multiplexers and Demultiplexers

• Optical Filters– Allow certain light frequencies to pass– May transmit or reflect wide range of wavelengths– Interference filters

• Used for multiple channel separation

– Wavelength locker• Tunes a wavelength through a narrow passband

7.4 Multiplexers and Demultiplexers

• Optical Filters– Mach-Zehnder filter

• Separates wavelengths channels by using interference of two beams traveling different pathlengths

• Used as an interleaver to separate odd and even optical channels

– Fiber Bragg gratings• Allow wider channel bandwidth• Used as add-drop multiplexers

7.4 Multiplexers and Demultiplexers

• Optical Add-Drop Multiplexers (OADM)– Several different optical devices used together to allow

single wavelengths to be retrieved or added to the multiplexed signal.

• Regenerative OADM– Performs the electrical-to-optical conversion required for

regeneration

• Reconfigurable OADM– Can be electronically reconfigured to add or drop specific

wavelengths

7.5 Switches

• Near Future– Optical networks will be mesh-based WDM nodes

with multi-wavelength switching capabilities.– Optical cross connects– ROADMs to establish fast reconfiguration– Transport all types of communications protocols

7.5 Switches

• Optical Cross Connects (OXC)– Switch data from any input port to any output port– Types of optical functionality

• Transparent: entirely optic• Opaque: part electronic, part optic• Electronic: all electronics

7.5 Switches

• MEMS Switching– Micro-electromechanical systems– Miniature devices that contain mirrors that have

one or two dimensional motion– Mirrors are controlled digitally to move into or out

of the light beam to redirect the channel.

7.6 Integrated Optical Devices

• Placement of optical communication devices on a single chip

• Will reduce cost• Will improve system performance• Will provide versatile modules• Two methods of connectorization of components

– Free space– Planar