Department of Information Technology and Media

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 891

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 892

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 106,1071

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 1072

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 222

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 Switch

• Loop 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 away
• The 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 22 switches in a matrix.

• Bild s158

33
Wavelength converters

• Opto-electric approach
• Cross Gain Modulation in a SOA

• Bild 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 (Lamplt
  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 171

38
Sliding-Frequency Filter

• 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 192

39
Sliding-Frequency Filter

• Noise Reduction

• Bild 202

• Bild 211

40
Sliding-Frequency Filter

• 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

• 242
• 251

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 352

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 391

44
Gain Flatness

• Bild 402

• Bild 392

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 601

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 gt10 regime

• Bild 612

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