Laser Systems and Applications

Cristina Masoller Research group on Dynamics, Nonlinear Optics and Lasers (DONLL)

Departament de Física i Enginyeria Nuclear Universitat Politècnica de Catalunya

cristina.masoller@upc.edu www.fisica.edu.uy/~cris

MSc in Photonics & Europhotonics

mailto:cristina.masoller@upc.eduhttp://www.fisica.edu.uy/~cris

Outline

• Applications of low-power semiconductor lasers:

telecom, data storage, others.

• High-power semiconductor lasers.

• Photonic Integrated Circuits (PICs) and nanolasers.

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Optical communications

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No diode laser No internet!

Source: Infinera

Optical data storage vs communications

• The use of laser-based personal optical storage media (CDs,

DVDs, Blu-rays) is decreasing due to Flash drives, streaming

video, the Cloud, and the preference for lightweight portable

computers.

• But the opposite scenario holds for lasers used in

datacenters and telecommunications. Recent advances in

laser diodes allow Google to build energy efficient, low-cost

datacenters.

• Recent advances in telecom lasers (and signal processing)

allow internet providers to upgrade to a new technology: 100

Gbit/s.

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Wavelengths for information storage & telecom

29/01/2015 5 Source: SUEMATSU & IGA: SEMICONDUCTOR LASERS IN PHOTONICS,

JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 26, 1132, 2008

Short wavelength

Non telecommunications

Long wavelength

Telecommunications

Optical communications: a along way from the beginning

• The first optical transmission system operated over 11 km of

fiber at 45 Mbit/s: in May 1977 optical fibers were used to

connect three telephone central offices in downtown Chicago.

• In the late 1970s, indium gallium arsenide phosphide

(InGaAsP) lasers operating at longer wavelengths were

demonstrated, enabling systems to transmit data at higher

speeds and over longer distances.

• In the 1980s: wavelength-division multiplexing (WDM). A

multiplexer at the transmitter is used to join signals together,

and a demultiplexer at the receiver, to split them apart.

6 Source: Nature Photonics 4, 287 (2010)

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Wavelength-division multiplexing

• Channels need to be separated in frequency far enough such that

the modulation sidebands of “neighboring channels” don’t overlap.

• The faster the modulation, the more difficult this becomes. Souce: D. Natelson, Rice University

Erbium-doped fiber amplifiers (EDFAs)

• In the 1990s the development of EDFAs enabled long

transmission distances.

• Diode lasers are efficient pump sources for EDFAs.

• In 1996: 5 Gbit/s transoceanic systems spanning more than

6,000 km without the need for any optical-to-electronic

conversion.

• By 2010: single fibers that carried signals at hundreds of

different wavelengths could transmit terabits/s of information.

• The rapid growth of Internet traffic, driven by new applications

(video and music sharing), the single-fiber bandwidth

capacity is rapidly becoming fully utilized.

29/01/2015 8 Source: Nature Photonics 4, 287 (2010)

• “Access networks”: links between users and their telecom or datacom

service provider—in other words, the link to your Internet provider, cable-

television supplier or phone company ( “fiber to the home” or FTTH).

29/01/2015 9 Source: OPN march 2010

Networks, lasers, and modulation schemes

VCSELs for communications

• VCSELs can be used at all levels of the optical

communication network, except for very long distance

transmission, where externally modulated DFBs are used to

meet requirements of high power and low frequency chirp.

• VCSELs emitting in the 1310 and 1550 nm bands are used

for medium distance communication (metro and access

networks, which are based on single-mode optical fibers).

• Such VCSELs have to be single mode to enable efficient

laser–fiber coupling and to prevent pulse broadening.

29/01/2015 10 Source: A. Larsson, JSTQE 2011

Advantages: high-speed modulation and array integration

High speed modulation

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The maximum single

channel (per wavelength)

speed is at 10 Gb/s

(2011).

As the demand for

higher data rates

increases, 25 Gb/s are

required for a 4 × 25 Gb/s

100G solution.

Source: A. Larsson, JSTQE 2011

VCSELs in computing: parallel optical links

• 5000 optical links (2.5Gb/s/ch 850nm VCSELs)

29/01/2015 12 Source: R. Michalzik, VCSELs

By November 2008, the top 3 supercomputers and the world’s first 2

petaFLOP machines (1015 FLoating-point Operations Per Second) used

VCSEL-based parallel optical links for high speed input/output (I/O).

Reaching the fiber capacity limit

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The dispersion problem • Modal dispersion arises from differences in light propagation times

between different fiber modes; it can be avoided by using single-mode

fibers.

• Chromatic dispersion arises from refractive-index variation as a function of

wavelength. A fiber's refractive-index profile can be specially tailored to

fabricate dispersion-compensating fibers to offset those of standard

transmission fibers.

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• Polarization-mode dispersion (PMD)

arises from fiber birefringence, which

delays one polarization mode with respect

to the other. Birefringence in standard

transmission fibers is small, so PMD went

unnoticed until data rates reached gigabits

per second.

Dispersion management

• Optical dispersion has been managed by assembling transmission systems

from two or more types of fibers with different characteristic dispersion to

keep total dispersion low and uniform across the operating wavelengths.

• That delicate balancing act could manage chromatic dispersion for WDM

systems using narrow-line lasers at optical channel rates of 2.5 or 10 Gbit/s.

• However, transmitting at higher rates requires much tighter control of

chromatic dispersion + management of PMD.

• Electronic digital signal processing has replaced optics in dispersion

management: the replacement of in-line optical dispersion compensation

with digital signal processing (DSP) in special-purpose chips has been

key to the success of coherent fiber-optic transmission at rates of 100 Gbit/s

and up.

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Electronic dispersion compensation

• Electronic dispersion compensation was first demonstrated in the early

1990s. The compensators used the Viterbi algorithm—a standard signal-

reconstruction technique—and application-specific integrated circuits

(ASICs) to regenerate the original signal.

• Nowadays, electronic pre-compensation at the transmitter and post-

compensation at the receiver are being used in 40 Gbit/s systems and is

replacing optical compensation at 10 Gbit/s.

• Impressive demonstrations of the hybrid approach (electronics for

signal processing and optics for signal transmission):

– August 2013, Sprint (Overland, KS) and Ciena (Hanover, MD) reported

field tests of 400 Gb/s transmission on cables carrying live traffic in

Silicon Valley.

– August 2014: DANTE/Infinera 1Tb/s Budapest--‐Bratislava (loopback 510 km).

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Evolution of information storage

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Evolution of optical data storage systems

First generation (1980s): CDs

• The information is in a 2D surface of a

recording medium and occupies less

than 0.01 % of the volume.

• =780 nm

• Due to the limitation of the recording

wavelength and the numerical aperture

(NA) of the recording lens, the storage

capacity was 650-750 MB.

29/01/2015 18 Source: Optics and Photonics News July/August 2010

The following generations

• Digital versatile disks (DVDs, 1995)

• Blue DVDs (Blu-rays, 2000)

29/01/2015 19 Source: Optics and Photonics News July/August 2010

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What is next in optical data storage systems?

• Multi-dimensional systems (via two-photon absorption

to decrease depth of field for more layers, or via the

polarization of the laser beam),

• shorter wavelengths (via nonlinear optics: frequency

doubling),

• super-resolution (stimulated emission depletion STED),

• holographic data storage.

5D data storage

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The multiplexed information can be individually

addressed by using the appropriate polarization

state and wavelength.

Other applications: sensing, spectroscopy

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LI-curve of 1.85 μm VCSEL

used for direct absorption

measurements. The absorption

dips of water vapor are clearly

resolved.

VCSELs typically exhibit an

electro-thermal current

tuning rate of 0.3–0.6 nm/mA.

Source: R. Michalzik, VCSELs

VCSELs are also used in optical mice.

High power diode lasers Two approaches:

• Make individual diode lasers bigger (broad-

area lasers).

• Grouping many lasers together

– Multi-stripe monolithic laser array

– Multiple arrays in linear bar

– Stacks of multiple arrays

Advantages

– High power – to kWs

– Low cost (compared to non-diode high

power lasers)

Drawbacks

– Cooling required

– Poor bean quality, low coherence

– Fiber coupling

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Source: Jeff Hecht, Laser Focus World

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• The laser is bonded onto a

heat sink that has a

coefficient of thermal

expansion (CTE) matched to

that of GaAs.

• Lateral wide w 100 m;

cavity length L 4 – 6 mm

• multimode emission: 100-200

longitudinal modes, each

with 20 – 30 lateral modes.

• Typical output power >20 W.

Source: Optics and Photonics News, October 2010

Broad-area edge-emitting diode lasers

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Stacked bar

(100s of Ws to KWs)

Monolithic edge-emitting diode array (up to 10 Ws)

Source: Jeff Hecht, Laser Focus World

Diode bar (10s of Ws up)

VCSELs

• Single VCSEL power in

the mW range

• Arrays of several

hundreds: tens of watts

• Electrically or optically

pumped external-cavity

VCSELs (VECSELs)

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Thousands of single-mode

emitters (976nm) in array:

the output from 400-μm,

0.46 NA fiber was 40 W

Source: Jeff Hecht, Laser Focus World

29/01/2015 27 Source: Laser Focus World, December 2014

29/01/2015 28 Source: R. Michalzik, VCSELs

Two-dimensional 8 × 4 VCSEL arrays at 780 nm wavelength enable 2,400 dots

per inch resolution, high printing speed, and reduced power consumption.

Many applications

• Optical pumping

– Solid-state lasers

– Fiber lasers and amplifiers

• Industrial applications

– Soldering

– Cutting

– Thermal ablation

– etc

• Electrically pumped VECSELs:

powerful sources of single-mode

blue and green light for projection

displays.

Integrated circuit

From electronics to photonics

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Vacuum tubes Transistor

In electronics:

In photonics:

Diode laser Photonic Integrated Circuit (PIC) First diode lasers

Moving photons, rather than electrons,

requires much less power.

29/01/2015 30 Source: OPN set. 2014

Big data

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• Silicon is optically transparent at telecom wavelengths

(1,310 and 1,550 nm), so it can be used to create

waveguides.

• But silicon lacks the necessary physical properties for

active devices:

– the direct bandgap needed for light emission and

– the electro-optic effect used for modulation of light.

• The temperatures at which high-quality GaAs layers grow

tend to be so high (700 C) that they damage conventional

CMOS circuitry.

Silicon photonics

29/01/2015 32 Source: Intel

29/01/2015 33 Source: Optics and Photonics News, March 2011

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The key is monolithic integration: all optical functions on a single chip

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Source: Infinera, Laser Focus World webcast 12/2014

Arrayed waveguide

gratings

Can’t we mix InP and Silicon on the chip?

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Source: Infinera, Laser Focus World webcast 12/2014

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Various ways of fabricating

lasers on silicon are discussed.

Bonding: keep the optical mode in the

silicon waveguide and amplify the

evanescent tail in a III-V gain region

Recent developments

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• Nearly 180 mW maximum CW output power at 20C.

• CW lasing up to 119C.

• Not yet adequate for real applications.

Source: J. Bowers (UCSB)

How small can a laser be?

• Diffraction is the ultimate limit: the need to fit an optical

mode inside a cavity the cavity cannot be smaller than

1/2 wavelength of the emitted light.

• However, nanolasers (first demonstrated in 2007) instead

of confining optical energy in a cavity in the form of a

conventional optical mode, it is confined with the help of

surface-plasmons — free-electron oscillations that are

bound to the interface between a metal and a dielectric.

• The great advantage of plasmons is not only that they can

store optical energy on a very small dimension but they

are easy to excite by light and convert back into light.

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Noginov, M.A. et al.Nature 460, 1110–1112 (2009). Oulton, R.F. et al. Nature doi:10.1038/nature08364

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• Like a whispering gallery mode, the light

propagates around the edges of the

pillars, but in a helix rather than a circle.

• Although light propagating downward is

absorbed by the substrate, there was

enough gain in the upward propagating

light to provide lasing.

• The semiconductor cavity mode alone

provides enough confinement (no need

of metal cavity).

• sub-wavelength lasing: the pillars are

smaller on a side than the wavelength

they emit.

See also OPN May 2011

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Optically pumped by using a mode-locked Ti:sapphire laser

Summary

• Novel semiconductor lasers are nowadays actively being developed to meet the requirements of faster, and more energy-efficient optical communications.

• A lot of efforts are focused in developing silicon-compatible lasers.

• Integration is essential. • With PICs, microprocessor chips will use light,

rather than electrons, to move data, and will result in faster and more energy-efficient datacenters and super-computes.

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TF

Long-wavelength VCSELs are used for short and

medium distance optical communication links.

By increasing the diode laser wavelength, an huge

increase in the capacity of optical storage systems has

been achieved.

The high-temperatures required to grow III-V

semiconductor materials is the main problem for

integrating lasers into silicon chips.

High-power broad-area semiconductor lasers emit a

single-mode output.

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THANK YOU FOR YOUR ATTENTION !

Universitat Politecnica de Catalunya

http://www.fisica.edu.uy/~cris/