Supporting Technologies : Active and
Passive Elements
Οπτικά Δίκτυα Επικοινωνιών
Two questions, one answer…
Either long haul communication or data center
interconnects, the main building block are the same.
Same active/passive components are used for
transmitting, driving, routing, modulating and receiving
light.
Fiber Bragg Grating (1/2)
Fiber Bragg Grating (FBG) is a simple and low-cost filter
built into the core of a wavelength-specific fiber cable
FBGs are used as inline optical filters to block certain
wavelengths, or as wavelength-specific reflectors
FBGs improve optical signal quality and are key enabler
to fiber optic construction
FBGs also maybe used to stabilize laser output
Uniform FBG: uniform grating periods are used
The grating parameters are: the length and the strength
of the grating, the refractive index
The different types of FBGs are: chirped, blazed, phase
shifted, long-period in-fiber
Fiber Bragg Grating (2/2)
Principle of operation
Periodic variations of the refraction index in the fiber
optic core determine the reflection of the guided light
at a specific λbragg
The relationship between the wavelength and the
period Λ of the grating is: λbragg = 2neff Λ where neff is
the effective index of the fiber
A variation of the period of the grating in the fiber
causes a shift of the reflected peak wavelength
Applications
Used for flattening the response of an EDFA
Filters
Fiber optic sensing- Optical sensors
Spectroscopy
Chromatic dispersion compensation
Optical ring resonator (1/3)
Valuable building blocks for SOI-based systems
Passive operations: Filtering and multiplexing
Active functions: electro-optic, thermo-optic, all optical
switching/modulation
Typical add-drop ring resonator design
Single all pass ring resonator design
Optical ring resonators (2/3)
Resonant frequency
Principle of operation
The concepts of ORR is the same as those behind
whispering galleries
Light obeys the properties behind total internal reflection
Constructive interference: circulating optical intensity is
built up to a higher value than that initially injected.
Resonators delay incoming signals via the temporary
storage of optical energy within the resonator.
Wavelength Dependent
Complementary transfer function of output ports
TF of a drop port
TF of a through port
Optical ring resonator (3/3)
Crucial parameters
Free Spectral Range (FSR): Distance between resonance
peaks
Full Width at Half Maximum (FWHM): The width of
between points that the resonance reaches half its
maximum intensity
Quality factor: Sharpness of the resonance
Extinction Ratio (ER): The ratio of the intensity on
resonance to the intensity off resonance.
Arrayed Waveguide Grating (AWG)
 AWGs are used to multiplex channels
of several wavelengths onto a single
optical fiber at the transmission end.
 Also used as de-multiplexers to
retrieve individual channels of different
wavelengths at the receiving end of an
optical communication network.
 Increase the transmission capacity of
optical networks considerably.
AWG Operating principles (1/2)
1: Input Waveguide
2: Free Propagation Region (FPR)
3: Arrayed Waveguides
4: Free Propagation Region (FPR)
5: Output Waveguides
1 -> 5 : De-multiplexer functionality
5 -> 1 : Multiplexer Functionality
AWG Operating principles (2/2)
 Light is coupled into the device via
an optical fiber connected to the input
port.
 The incoming light traverses a free
space region and then propagates
through the arrayed waveguides which
act as a discrete phase shifter.
 There is constant path difference DL
between waveguides.
 The light interferes at the entries of
the output waveguides in such a way
that each output channel receives only
light of a certain wavelength.
AWG for high channel count de-multiplexer
architectures
 Larger number of channels
requires the use of multiple
de-multiplexing stages
 A two-stage de-multiplexing approach using bands.
 First stage: the set of wavelengths is de-multiplexed into bands.
 Second stage: the bands are de-multiplexed and individual wavelengths are extracted.
 A 32-channel de-multiplexer is realized using four bands of 8 channels each.
Semiconductor Lasers (1/2)
 Semiconductor laser is today one of the most important types of lasers with its very important application in
fiber optic communication.
 These lasers use semiconductors as the lasing medium and are characterized by specific advantages such as
the capability of direct modulation in the gigahertz region, small size and low cost.
 The basic mechanism responsible for light emission from a semiconductor is the recombination of electrons
and holes at a p-n junction when a current is passed through a diode.
Semiconductor Lasers (2/2)
Principle of operation
 An electron in the valence band can absorb the incident radiation and be excited to the
conduction band leading to the generation of eletron-hole pair.
 When a current is passed through a p-n junction under forward bias, the injected
electrons and holes will increase the density of electrons in the conduction band.
 A stimulated emission may occur in which the incident radiation stimulates an electron in
the conduction band to make a transition to the valence band and in the process emit
radiation.
 The stimulated emission rate will exceed the absorption rate and amplification will occur
at some value of current due to holes in valence band.
 As the current is further increased, at threshold value of the current, the amplification
will overcome the losses in the cavity and the laser will begin to emit coherent radiation.
To convert the amplifying medium into a laser
Optical feedback should be provided
Done by cleaving or polishing the ends of the p-n
junction diode at right angles to the junction.
Vertical Cavity Surface Emitting Laser
Key Advantages:
 Low cost
 No noise
 No frequency interruptions
 Less power consumption
 Higher performance of transceivers fro
metro area networks
 High modulation bandwidth
Beam characteristics:
 The divergence of a laser beam is inversely proportional to the beam size at the source
 The reflectivity required for low threshold currents is greater than 99.9%, Distributed Bragg Reflectors (DBRs) are needed for this reflectivity.
 DBRs are formed by laying down alternating layers of semiconductor or dielectric materials with a difference in refractive index.
 The emitted laser can be controlled by selecting the number and thickness of mirror layers
 The cavity is along the vertical direction, with a very short length, typically 1-3 wavelengths of the emitted light
Advantages of VCSEL vs. Edge Emitting Diode Lasers
Edge emitting laser structure
VCSEL emitting structure
 The VCSEL is cheaper to manufacture in quantity
 Easier to test on wafer
 More efficient
 The VCSEL requires less electrical current to produce a given coherent energy output
 The VCSEL emits a narrow, more nearly circular beam than traditional edge emitters
(used in optical fiber) with low power dissipation and significantly lower operating currents
 Efficiency and speed of data transfer is improved for fiber optic communications
Optical modulation
An optical modulator is responsible to imprint the electrical data on the optical domain.
Concepts of optical modulation:
 Direct modulation
 External modulation
 Electro-absorption modulators
 Electro-optic modulators
Direct modulation
Modulation of the Laser’s electrical current.
+ Low complexity
+ Low cost
+ Low driving requirements
- Only M-PAM optical modulation
- High chirp values to the optical output
- Only for short reach links
- Limited by the bandwidth of the laser
- Baud-rates > 10Gb/s are challenging
Mainly used in Access networks
(DML,VCSELs & in datacom (VCSELs) due to
cost constrains
Direct modulation scheme
External modulation
 Dominate in most of the optical
Communications applications
+ Superior signal quality
+ Chirp free output
+ High Electroptical bandwidth (~100GHz)
+ Baud-rates > 100Gb/s
+ High stability
+ Optical amplitude and/or phase modulation
- Higher cost
- Rather high integration form factor
- Higher driving voltage requirements
(RF amplifiers are needed)
- More complex electrical circuitry
External Modulation scheme
Mainly used in Long Haul networks where
cost/bit is much lower
Types of external modulator
Electro-absorption
Absorption coefficient variation
under an electric field
Electro-optic
Refractive index variation
under an electric field
Phase modulation
Interferometer
Intensity modulation
Intensity modulation
Electro-absorption modulator (1/2)
Principle of operation
 The effective bandgap Eg of a semiconductor
material decreases when an external voltage is
applied
 An incoming light wave with energy E < Eg
travels along a material and no voltage is applied,
then the material is transparent
 When an external voltage is applied, the
effective bandgap will be reduced and the light
wave will be absorbed (E > Eg)
 By properly selecting the signal wavelength so
that it experiences a significant change in
absorption when the voltage is applied, it
becomes possible to achieve optical modulation
controlled by an electrical signal.
Electro-absorption modulator schematic
Electro-absorption modulator (2/2)
Advantages of the Electro-absorption modulator
 Zero biasing voltage
 Low driving voltage
 Low/negative chirp
 High speed
 Lesser polarization dependence
 Integration with DFB laser
Electro-optic modulator
Principle of operation
 Modify refractive index of the material by applying an external
electric field through the linear electro-optic effect
 The phase shift experienced by a light wave of wavelength λ
propagating through a length L of a medium is proportional to the
refractive index variation
 Τhe applied voltage will modulate the refractive index of the
waveguide material, hence the phase shift experienced by a light
wave propagating along the waveguide
 Τhe phase modulation induced by the electro-optic effect have to
be transformed to intensity modulation using an interferometric
structure (Mach-Zehnder interferometer) .
Principle of operation of Mach-Zehnder modulator
Photodetector
 Convert an optical signal into an electrical signal
 Photodetectors made of semiconductor materials absorb incident photons and produces electrons.
 If electric field imposed on photodetector an electric current (photocurrent) is produced.
 Materials of semiconductor photodiodes: silicon, germanium, GaAs, InGaAs, etc.
Photodetector requirements
Basic requirements of a photodetector
 Sensitivity at the required wavelength
 Efficient conversion of photons to electrons
 Fast response to operate at high frequencies
 Low noise for reduced errors
 High reliability
 Low cost
P-i-N Photodiode (1/2)
Schematic diagrams of pin photodiodes
 A p-i-n photodiode is electrically reversed biased.
 It consists of an intrinsic region sandwiched between heavily doped p+ and n+ regions. The depletion layer is almost
completely defined by the intrinsic region.
 The photo-response of a photodiode results from the photo-generation of electron-hole pairs through optical
absorption in the depletion region.
P-i-N Photodiode (2/2)
 Reverse-biased p-i-n photodiode
 p-i-n energy-band diagram
 Photo-generated electrons and holes in the depletion layer are subject to the local electric field within that layer. The electron/hole carriers drift
in opposite directions. This transport process induces an electric current in the external circuit.
 In the depletion layer, the internal electric field sweeps the photo-generated electron to the n side and the photo-generated hole to the p side.
 A drift current that flows in the reverse direction from the n side (cathode) to the p side (anode).
 Within one of the diffusion regions at the edges of the depletion layer, the photo-generated minority carrier (hole in the n side and electron in
the p side) can reach the depletion layer by diffusion and then be swept to the other side by the internal field.
 A diffusion current that also flows in the reverse direction.
Avalanche photodiode
Reach-Through Avalanche Photodiode (RAPD)
 High reverse-bias voltage enhances
the field in the depletion layer
 Electrons and holes excited by the
photons are accelerated in the strong
field generated by the reverse bias.
 Collisions causing impact-ionization
of more electron-hole pairs, thus
contributing to the gain of the
junction.
P-i-N versus Avalanche photodiode
PiN Photodiode
AvalanchePhotodiode
 Photon detection represented in a space-time
diagram.
 Avalanche multiplication illustrated in a space-time
diagram.
 Photon absorption creates an electron-hole pair.
 The primary electron on the left starts a chain of
impact-ionization events.
 The oppositely charged particles drift in opposite
directions under the influence of the electric field in the
vicinity of the reversed-biased p-n junction
 The solid arrows depict electron trajectories.
 The dashed arrows depict hole trajectories
Photo-Receiver
Receiver: Photodiode + Transimpedance Amplifier
Incoming light
Photodiode
 Photodiode (PD) :
Receives optical signal
and converts it to current
Receiver
Transimpedance
Amplifier
 Transimpedance amplifier (TIA)
: Is needed to convert the current
into an amplified voltage
 TIA is characterized by several
parameters:
 Transimpedance Gain
 Input Referred Noise
 Bandwidth
Wavelength Selective Switches (1/2)
Key enabler for flexible spectrum networks
WSS is becoming the central part of ROADMReconfigurable Optical Add Drop Multiplexer.
WSS product from Finisar
1xN WSS Function schematics
Wavelength Selective Switches (2/2)
Dispersive element
(grating)
Collimation Optics
Switching elements
Focusing optics
Light of different wavelengths is expanded and
collimated by the lens
A dispersive element i.e. a conventional grating is
used to spatially separate the multiple
wavelengths in different angles.
Fiber array
Projected by another lens in the switching
element in order to direct different wavelengths
to different output parts
Switching element can be liquid crystal on silicon,
Micro-Electro Mechanical Systems (MEMS) etc.
Generic Design of a WSS
ROADMs (1/4)
Enables remote configuration of wavelengths at any
node
It is software-provisionable so that a network operator
can choose whether a wavelength is added, dropped or
passed through the node.
Problem! The adding of new light paths with red and blue
wavelengths requires changes in node configurations
ROADMs (2/4)
Solution: Reconfigurability!
Offers the ability to select the desired wavelengths to
be dropped and added “on the fly”
Reduced truck rolls
No need to plan ahead and deploy appropriate
equipment
Allows light paths to be set up and taken down
dynamically as needed between network nodes
Provides total flexibility in the routing of light paths
ROADMs (3/4)
ROADMs are used in bus and ring networks
Enable flexible add/drop of wavelengths
Enable “hitless” expansion where wavelengths can be
added without interruptions of traffic on adjacent
channels
Benefits the operator wanting to adapt to changing
subscriber requirements.
Increases network availability by simplifying protection
switching and restoration of light paths
Reduce the risk of manual error
Simplified planning, reduces the effects of inaccurate
traffic forecasting
Better bandwidth utilization
Simplified and reliable network engineering
ROADMs (4/4)
ROADM units can be designed around mux/demux and
optical switches
The most common architecture makes use of a 1xN
wavelength selective switch (WSS) that individually can
switch the wavelengths on its inputs to its output
The incoming wavelengths from “west” are split via an
optical coupler and made individually available locally
via a demux when using the 1x2 ROADM plug in unit
Local wavelengths to be added are multiplexed and
added to the incoming signal from “east” in the 2x1
WSS
Each of the incoming WSS ports is set to accept one or
several wavelengths. Thus, the WSS can for each
wavelength decide if it should be taken from line “east”
or be locally added
Next Generation ROADMs
Colorless: Any wavelength can be dynamically
added/dropped without having to re-fiber a tranceiver
Directionless: A wavelength can be dynamically
added/dropped from any direction without having to
re-fiber a tranceiver
Contentionless: A wavelength can be re-used on all
directions without any restrictions
Directions
A
B
C
D
CDC ROADM
(module)
CDC drop ports
CDC add ports
Ideal Colorless-Directionless-Contentionless ROADM
Silicon Photonics (1/2)
Low cost materials
Take advantage of CMOS technology
High index contrast-strong light confinement
Transparent in 1.3-1.6 region
Devices with sub-wavelength dimensions feasible
Applications in:
Interconnects
Optical Telecommunications
Data centers
Silicon Photonics (2/2)
Three different approaches:
1.
Hybrid silicon photonic integrated circuit technology:
Integration of III-V semiconductors onto silicon-oninsulator substrates based on bonding techniques(1)
Heterogeneous integration technology: 3D packaging,
wafer level packaging, parallel processing of materials(2)
Silicon Germanium technology(3)
2.
3.