Optical Components Ajmal Muhammad, Robert Forchheimer Information Coding Group ISY Department Outline Types of optical components Passive (reciprocal & non-reciprocal) Lens, couplers, isolators, circulators, filters, multiplexer, demultiplexer Active Modulator, switch, optical amplifier, wavelength converter, gain equalizer Wavelength Selectivity Fixed Tunable Parameters Temperature dependency, insertion loss (inputoutput loss) inter-channel cross-talks, fast tunability, stability and polarization dependency Requirements Bandwidth Low insertion loss (inputoutput loss) High return loss (outputinput loss) Polarization insensitivity Low crosstalk High extinction ratio Temperature insensitivity Low control power Small size Cost Optical Component Platform Micro-optic components The assembly of discrete elements are placed together in an optical component. The elements require precise optical alignment to maximize the performance. Integrated optics components Uses planar manufacturing techniques to develop devices such as Array Waveguide Grating (AWG), Variable Optical Attenuators (VOA), Electro-optic Modulators, etc. Fiber based optical components Devices made of fibers such as fused optical couplers, fused WDMs, FiberBragg gratings (FBG), etc. Hybrid type Passive Components Coupler: versatile device used as a building block for several other optical devices Isolator: used in systems at the output of amplifiers and lasers to prevent reflections Filter: to multiplex and demultiplex wavelengths in a WDM system, and to provide equalization of the gain and filtering of noise in optical amplifier MUX & DEMUX: MUX combines signals at different wavelengths on its input ports onto a common output port, DEMUX performs the opposite function Couplers Structure Coupling ratio Coupling length Excess loss (beyond α) Type NxN (e.g., 2x2) α is proportional to l (α is coupling ratio, l is coupling length) Parameters of interest Couplers Wavelength dependent (α has wavelength-dependency) Wavelength independent (wavelength flat) Splitting ratio 3dB (splitting the power evenly) - α=0.5 Taps (e.g., α ∼ 1 – thus, a very small portion is dropped) Couplers - Passive Reciprocal Device 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 Wavelength-dependent coupler Multiple signals combined and broadcast to many outputs Couplers 1x2 coupler 6x6 coupler Isolators - Passive Non-Reciprocal Device 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: circulator Operation of Isolators Isolators Passive Components Coupler: versatile device used as a building block for several other optical devices Isolator: used in systems at the output of amplifiers and lasers to prevent reflections Filter: Variety of technologies are available 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 gratings The incident light is transmitted through the slits Due to diffraction (narrow slits) the light is Transmission transmitted in all direction gratings 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 Reflective gratings Diffraction gratings Fiber Bragg Gratings Any periodic perturbation in the propagation medium serves as a Bragg gratings Diffractive optical element Optical Add/Drop Using Fiber Bragg Grating Fabry-Perot Filters A cavity with highly reflective mirrors parallel to each other (Bragg structure) Acts like a resonator Also called FP Interferometer Used in lasers Tunability of Fabry-Perot Changing the cavity length (l) Varying the refractive index (n) within the cavity Mechanical placement of mirrors Not very reliable Using piezoelectric material within the cavity Thermal instability Multilayer Dielectric Thin Film Filters 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-wavelength thick. The primary considerations in DTF design are: Low-pass-band loss (« 0.3 dB) Good channel spacing (> 10 nm) Low inter-channel cross-talk (> -28 dB) DTF filters MUX/DEMUX using DTF filters Mach-Zehnder Interferometer Uses two couplers 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 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 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 Arrayed Waveguide Grating (AWG) AWG is a generalization of the Mach-Zehnder interferometer AWG as DEMUX and CrossConnect Input coupler Arrayed guides Output coupler Static Wavelength Cross-connect Multiplexer/Demultiplexer Multiplexer/Demultiplexer Active Components Modulator, switch, and router Optical amplifier (fiber amplifier, semiconductor amplifier) Wavelength converter Gain equalizer Optical switch can be used for: 1) Light modulation(phase & intensity) 2) Routing optical data Type of Optical Modulators/Switches Micro-Electro-Mechanical (MEMS) Switch Electro-Optic Modulator Need material with high electro-optic effect Electro-optic: refractive index change is proportional to applied electric field Wavelength Converter Different types of Wavelength Converter OE/EO regeneration SOA-based Cross-gain modulation Cross-phase modulation Four-Wave mixing Fiber-based Cross-phase modulation Four-Wave mixing OE/EO Cross-gain Cross-phase Four-Wave mixing Gain Equalizers Gain/Power Equalizers Gain/Power Equalizers
