Optical Components

Ajmal Muhammad, Robert ForchheimerInformation Coding Group

ISY Department

Outline

Types of optical components Passive (reciprocal & non-reciprocal)Lens, couplers, isolators, circulators, filters, multiplexer, demultiplexer ActiveModulator, 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 componentsThe assembly of discrete elements are placed together in an optical

component. The elements require precise optical alignment to maximize the performance.

Integrated optics componentsUses planar manufacturing techniques to develop devices such as Array

Waveguide Grating (AWG), Variable Optical Attenuators (VOA), Electro-optic Modulators, etc.

Fiber based optical componentsDevices 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 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 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

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

6x6 coupler1x2 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 gratingsThe 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 intensityNarrow slits are placed next to each other The spacing determines the pitch of the gratingsAngles are due to phase shift

Transmission gratings

Reflective gratings

Diffraction gratings

Fiber Bragg GratingsAny 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)

MUX/DEMUX using DTF filtersDTF 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 effectElectro-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

Cross-gain

Cross-phase

Four-Wave mixing

OE/EO

Gain Equalizers

Gain/Power Equalizers

Gain/Power Equalizers