Download Lect 4 - Components - Sonoma State University

OPTICAL COMPONENTS
9/20/11
Applications
• See notes
Optical Devices
• Optical Devices
• Active
• Passive (reciprocal & non-reciprocal)
• Wavelength Selectivity
• Fixed
• Tunable
Impacting the system:
- Error-free
- Selectivity
- # of channels that can be supported
- Interferences
• Parameters
• Temperature dependency
• Insertion loss (inputoutput loss)
• Inter-channel cross-talks
• Manufacturability
• Fast tunability
• Stability and polarization dependency
Spectral Width
Spectral content of a channel
Passive Devices
• Reciprocal (input/outputs act the same way)
• Couplers
• Half-wavelength plates
• Non-reciprocal
• Circulators
• Rotators
• Insulators
Couplers
• Structure
• NxN (e.g., 2x2)
• α is proportional to l (α is coupling ratio, l is coupling length)
• Parameters of interest
• Coupling ratio
• Coupling length
• Excess loss (beyond α)
• Type
• WL dependent (α has WL-dependency)
• WL independent
• Splitting ratio
• 3dB (splitting the power evenly) - α=0.5
• Taps (e.g., α ∼ 1 – thus, a very small portion is dropped)
Couplers
• They can combine or separate different wavelengths
• The lights (different wavelengths) are coupled together
• Example: 8x8 3-dB couplers
1310 (signal)
1550 nm
(pump)
Amplified
Signal
Half-Wavelength Plates
• Passive reciprocal devices
• They maintain the polarization but rotate the orientation of
polarization is rotated by by ΔΦ=2πR; (R=+/-0.25 for λ/4)
• Note d= Rλ/Δn; d is the thickness of the birefringent plate
– assuming mica or quartz plate
Passive Non-Reciprocal Devices
• Types
• Isolators
• Faraday Rotators
• Circulators
Isolators
• Transmit in one direction only
• Avoid reflection of laser – or any reflection
• One input, one output or multiple ports
• Key parameters are insertion loss and excess loss
• Example of circulators:
Operation of Isolators
Only Ex exists
State of polarization is fixed (SOP)
Rotator rotates by 45 degree
Operation of Isolators – more realistic
Polarization Independent Isolator
Half-wavelength plates are used to rotate 45 degree
The Spatial-walk-off polarizer splits the signal into two orthogonally polarized signals
Prism
Spectral-Shape Parameters
Cascaded filters  narrower passband
We desire broad passband at the end of the cascade
Thus, each filer must have a flat passband (accommodating for small changes in WL)
The flatness of the filer is measure by 1-dB bandwidth
Components
Gratings
• Describe a device involving interference among multiple
optical signals coming from the same source but having
difference phase shift
• There are a number of gratings
• Reflective
• Transmission
• Diffraction
• Stimax (same as reflection but integrate with concave mirrors
Gratings
--- Transmission
• The incident light is transmitted through the
slits
• Due to diffraction (narrow slits) the light is
transmitted in all direction
• Each Slit becomes a secondary source of
light
• A constructive interference will be created on
the image plane only for specific WLs that
are in phase  high light intensity
• Narrow slits are placed next to each other
• The spacing determines the pitch of the
gratings
• Angles are due to phase shift
Diffraction Gratings
• It is an arrayed slit device
• It reflects wavelengths in different directions
Bragg Grating Structure (notes)
• Arrangement of parallel semi-reflecting plates
•
Fiber Bragg Gratings
• Widely used in Fiber communication systems
• Bragg gratings are written in wavelengths
• As a result the index of refraction varies periodically along the length of
the fiber
• Variation of “n” constitutes discontinuities  Bragg structure
• Periodic variation of “n” is occurred by exposing the core to an intense
UV interference pattern
• The periodicity of the pattern depends on the periodicity of the pattern
Optical Add/Drop Using Fiber Bragg
Grating
FBG has very low loss (0.1 dB)
Temperature dependent  change of fiber length
The are very useful for WDM systems
They can be used with 3-port Circulators
Optical Add/Drop Using Fiber Bragg
Grating
Fiber Bragg Chirped Grading
• Fiber Bragg grating with linear variable pitch
compensates for chromatic dispersion
• Known as chirped FBG
• Due to chirps (pitches) wavelengths are reflected back
• Each WL reflection has a different phase (depth of grating)
•  compensating for time variation  compensating for chromatic
dispersion
Fabry-Perot Filters
• A cavity with highly reflective mirrors parallel to each other
(Bragg structure)
• Acts like a resonator
• Also called FP Interferometer
• Also called etalon
Fabry-Perot Filters (notes)
Power Transfer Function
• Periodic in terms of f
• Peaks are called the passbands of the transfer function
occurring at f (fτ=k/2)
• R is the coefficient of reflection or reflectivity
• A is the absorption loss
FSR and Finesse
• Free spectral range (FSR) is the spacing in optical frequency or wavelength between
two successive reflected or transmitted optical intensity maxima or minima
• An indication of how many wavelength (or frequency) channels can simultaneously
pass without severe interference among them is known as the finesse
Transfer function is half
Tunability of Fabry-Perot
• Changing the cavity length
• Changing the refractive index within the cavity
• Mechanical placement of mirrors
• Not very reliable
• Using piezoelectric material within the cavity
• Thermal instability
Multilayer Dielectric Thin Film
• Dielectric thin-film (DTF) interference filters consist of alternating
quarter-wavelength thick layers of high refractive index and low
refractive index
• each layer is a quarter-waveleng th thick.
• The primary considerations in DTF design are:
• Low-pass-band loss
« 0.3 dB)
• Good channel spacing (> 10 nm)
• Low interchannel cross-talk (> -28 dB)
Thin-Film Resonant Multicavity Filter
• Two or more cavities separated by reflective dielectric thin-film layers
• Higher number of cavities leads to a flatter passband
• Lower number of cavities results in sharper stop band
Thin-Film Resonant Multicavity Filter
• A wavelength multiplexer/demultiplexer
Mach-Zehnder
Interferometer
• Uses two couplers
• The coupling ratio can be different
• A phase difference between two optical paths may be artificially induced
• Adjusting ΔL changes the phase of the received signal
• Because of the path difference, the two waves arrive at coupler 2 with
a phase difference
• At coupler 2, the two waves recombine and are directed to two output
ports
• each output port supports the one of the two wavelengths that satisfies a certain
phase condition
• Note:
• Δf=C/2nΔL
• ΔΦ=2πf.ΔL.(n/c)
Tunability
• Can be achieved by altering n or L
Absorption Filter
• Using the Mach-Zehnder Interferometer
• consist of a thin film made of a material (e.g., germanium) that
exhibits high absorption at a specific wavelength region