OPTICAL COMPONENTS 9/20/11

Applications See notes

Optical Devices Active Passive (reciprocal & non-reciprocal) Wavelength Selectivity Fixed Tunable Parameters Temperature dependency Insertion loss (input  output loss) Inter-channel cross-talks Manufacturability Fast tunability Stability and polarization dependency Impacting the system: -Error-free -Selectivity -# of channels that can be supported -Interferences

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