Department of Information Technology and Media Electronic Simulation Group [email protected] 1...

Department of Information Technology and Media

Electronic Simulation Group [email protected]

1

Devices• Couplers• Isolators and Circulators • Multiplexers and Filters• Lasers and LEDs• Detectors• Amplifiers• Switches



2

Couplers

• When two fibers are placed in proximity to each other, the signal are coupled from one fiber to another.

• The coupler are treated mathematically in exactly the same manner as a electrical RF-coupler with scattering parameters.

• Bild s84



3

Isolators and Circulators

• Isolators couples the signals in one direction and block the transmission in the other direction

• A circulator couples the signal to another port in a circular manner

– Insertion loss is the power loss in the coupled direction (As low as possible)

– Isolation is the loss in the blocked direction (As high as possible)



4

Isolator

• A isolator is using a combination of polarization filters and polarization rotators.– A single polarization

isolator is simple.– A polarization

independent isolator are using polarization splitters, a rotator and a /2-plate.

• Bild s88• Bild s 89:1



5

Circulator

• A circulator have similar operation as an isolator with more than two ports.

• The signal are coupled from port 1 to 2, 2 to 3 and so on.

Bild s 89:2



6

Multiplexers and Filters

• In the optical domain there exist– Filters– Multiplexers and

Demultiplexers– Wavelength Routers

Bild s 91



7

Filters

• Filters are usually designed with Bragg grating

• Any periodic perturbation in the propagating medium serve as a Bragg grating (Usually refractive index).– The wavelength corresponding to the

Bragg grating frequency are reflected while all other are transmitted.

– The side lobes can be reduced by having smaller refractive index changes near the edges of the filter.

Bild s 98



8

Fabry-Perot Filters• Fabry-Perot Filter (Etalon) is

fabricated by a cavity surrounded by two mirrors

• The transfer function of an etalon filter is periodical due to the multiple of standing waves in the cavity.

• Etalon is very simple and cheap.

Bild s 103,105



9

Multi Layer Filters

• A single Etalon filter is very narrow banded.

• The bandwidth can be increased by using multiple cavities.

Bild s 106,107:1



10

Add/Drop elements• Add/Drop elements can

be realized by a fiber gratings (isolator), a circulator and a coupler. Bild s 100



11

Multiplexers and Demultiplexers

• Static multiplexers can be realized using one multi-layer filters for each wavelength.

• The same device can be used as a multiplexer as well.

Bild s 107:2



12

Wavelength Routers

• A Static wave-length router can be realized by using a few multiplexers and demultiplexers Bild s 92



13

Planar Lasers

• An amplifying medium is surrounded by two mirrors (one semi-transparent)

• The amplifying medium is mostly a quantum well

Bild Agrawal s 96, 98



14

Planar Lasers• The Laser must Be

confined in one direction• Gain Guided Laser

– Oxide Strip– Junction Strip

• Index Guided Laser– Ridge wave guide

structure– Etched mesa

structure

Bild Agrawal s 99,100



15

Lasers

• The semiconductor amplifier is wide-banded

• The monochromatic lasing wavelength is determined by the cavity surrounding the amplifier

• For low power, high reflectivity is required for lasing operation.– Cavity Laser (Fabry Perot Cavity)– Distributed Feedback Reflector

(DFR)– Distributed Bragg Reflector (DBR)

Bild Agrawal s 105, 107



16

Vertical Cavity Surface Emitting Lasers (VCSEL)

• The cavity is made of epitaxial layers.

• Possible to make very small devices

• Devices can be tested before assembly

Bild Pessa 22:2



17

Pulse lasers

• For fast communication it is necessary to modulate the laser signal quickly.

Bild Oleg 1-2



18

Saturable Absorber• A saturable absorber

change the absorption very quickly.

• By population inversion the absorption decreases.

• For short decay time the life-time of the absorber must be reduced– Introduce a trap level in

the band-gap

Bild Oleg 3-4



19

Pulse AmplifierMode Lock Laser• Short Pulse amplifier

– The gain in the amplifier is constant

– At the pulse the absorber bleaches, giving a net gain

– Prior and after the pulse the absorber giving a net loss.

• Colliding Pulse Mode-Lock Laser– Two pulses together have

enough energy to saturate the absorber.

Bild Oleg 5-6



20

Light Emitting Diodes (LEDs)

• Much Simpler and cheaper compared with lasers

• For many applications with short distances and low data rates LEDs are sufficient.

• A LED is a forward biased pn-junction, where the injected minority carriers recombine by spontaneous emission of light



21

LEDs

Telecommunication LEDs can either be surface or edge emitting.



22

Detectors

• Telecommunication detectors are traditional pn-detectors

• PIN diodes are used to increase the efficiency• Avalanche photodiodes are also used to increase the

signal– To high amplification reduces noise performance



23

Semiconductor Optical Amplifiers (SOAs)

• A semiconductor amplifiers is realized as a semiconductor laser without mirrors

• Very short compared with fiber amplifiers

Bild Reale



24

SOAs

• For high amplification and large bandwidth a very good antireflective coatings are necessary.– Difficult to achieve.

• The antireflectivity can be improved by:– Tilted stripe structure– Window faced structure

Bild s 370, 369



25

SOAs

• SOAs are polarization dependent

• Multiple SOAs can be used to realize a polarization independent amplifier.

Bild s 372



26

Electro-Optic Switch

• Electro-optic directional coupler switch

• Semiconductor Optical Amplifier switch– An optical amplifier

where the amplifier bias switches the signal

Bild s 155



27

All-Optical Regeneration• Nonlinear Loop Mirror

– Without control pulse: Reshaping

– With control pulse: Retiming and reshaping

• A saturable absorber can be used as an optical gate for retiming and reshaping.

Bild Oleg 7-8



28

All Optic SwitchLoop Mirror

– If the two signals are equal the signal are coupled to the input.

– If the two signals experiences different absorption or index the signal are fully coupled to the output.

• The control signal saturates the SOA for a short moment.

• The two pulses reaches the SOA with a time difference, one where the amplified is saturated.

• The control pulse are filtered awayThe two other are based on Mach-

Zehnder Interferometers.

Bild Toliver



29

All Optic Switch

• Each data-pulse induces an non-linear refractive index change in the SOAs.

• Each clock pulse is split in two parts and passing the SOAs.

• The two pulses are interfering either destructively or constructively depending on the clock pulse arrival.

Bild Nakamura, 2xUeno,



30

All-Optical Pulse Regeneration

• Electro optical timing and reshaping– All Optical signal path– Electrical signal for timing signal.

• For all Optical Pulse Regeneration only the clock recovery is missing

Bild Oleg 5-6



31

Optical Clock Recovery

• For Optical Clock recovery a lasers which are able to create short pulses are required– Self Pulsating Laser

– Mode Lock Laser • Optical Clock recovery

up to 40 Gbit/s have

been achieved



32

Large Switches

• A multi-port switches can easily be realized using several 2:2 switches in a matrix. Bild s158



33

Wavelength converters

• Opto-electric approach• Cross Gain Modulation in

a SOABild s162, 164



34

Solitons

• Introduction

• Robustness

• Sliding-Frequency Filter

• Tapered Fibers

• Dispersion Managed Fibers

• Pulse-to-Pulse Interaction



35

Introduction

• Narrow pulses with high peak power and special shape.

• A soliton is not affected by dispersion.– The dispersion is exactly compensated by

nonlinear effects in the fiber.



36

Robustness

• A soliton is, when once created very robust.• A pulse where nonlinear effects not exactly

compensate the dispersion is shifted towards this case.

• Solitons ”feels” only average parameters (fiber dispersion, fiber mode area, pulse energy) as long as the variations are faster than the soliton dispersion length. – Stable in systems with lumped amplifiers (Lamp< zdisp).– Slow variation can be used for pulse reshaping

(compression and broadening)



37

Sliding-Frequency Filter

• Etalon Filter can be used due to the narrow bandwidth of the soliton– Cheap and simple,

compared with Gaussian filters

– Same filter can be used for multiple channels

• Bild 17:1



38


• The soliton adapts to successive slowly sliding-frequency shifting filters and moves in frequency– The noise cannot be

moved in frequency, which is removed efficiently

• Bild 19:2



39

Sliding-Frequency FilterNoise Reduction

• Bild 20:2 • Bild 21:1



40


Transfer Function• In a long transmission

line the transfer function looks like a step function.– Removing small

signals and noise– Large signals are all

given the same energy

24:2

25:1



41

Wavelength Division Multiplexing (WDM)

In WDM, Solitons of different channels overtake and pass thought (Collide with) each other, which results in.



42

Tapered Fibers

• Some of the problems can be corrected by using a tapered fibers, where the dispersion decays exactly as the intensity.– Behaves like loss less,

constant D-fiber

• The exponential dispersion can be replaced with an 3-step approximation.

• Bild 35:2



43

Gain Flatness• Optical amplifiers have not

a flat gain over several channels.

• After multiple amplifiers the signal power differs very much between channels.

• In Soliton transmission with filters the amplitude can be kept relative constant between the different channels.

• Bild 39:1



44

Gain Flatness

• Bild 40:2• Bild 39:2



45

Dispersion Managed Fibers

• Fibers with alternating dispersion where the pulses are true solitons only in a few points along the fiber.

– All advantages of ’classical solitons’– Power enhancement– Inexpensive and flexible design– High stability range– WDM



46

Pulse-to-Pulse Interaction

• The pulse-to-pulse interaction depends on the pulse width-spacing ratio.• The interaction increases

as the pulses starts overlap

• At large overlap the interaction vanishes

• Bild 60:1



47

Pulse overlapped dispersion managed Fibers

• Use the non-pulse interaction regime just at the transmitter and receiver

• Move quickly across the partially overlapped regime

• Spend most of the time in /t >10 regime

• Bild 61:2



48

State of The Art

• 320 Gbit/s Transmission over 200 km.• 10 GHz Clock Recovery from a 160 Gbit/s stream.• 168 Gbit/s Demultiplexing• 84 Gbit/s All Optical 3R Regeneration (No clock

recovery)• 40 Gbit/s All Optical Clock Recovery

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