Optical Components Ajmal Muhammad, Robert Forchheimer Information Coding Group ISY Department

Outline Types of optical components Wavelength Selectivity Parameters 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 (inputoutput loss) inter-channel cross-talks, fast tunability, stability and polarization dependency

Requirements Bandwidth Low insertion loss (inputoutput loss) High return loss (outputinput 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, Fiber-Bragg 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 Couplers Structure Parameters of interest Type 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 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) Amplified Signal 1550 nm (pump) 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 Filter: Variety of technologies are available 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 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 Transmission gratings Reflective gratings Diffraction gratings

Diffractive optical element 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 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

Arrayed Waveguide Grating (AWG) AWG is a generalization of the Mach-Zehnder interferometer

AWG as DEMUX and Cross-Connect 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 OE/EO Cross-gain Cross-phase Four-Wave mixing

Gain Equalizers

Gain/Power Equalizers

Gain/Power Equalizers