Optical Fiber Communications Chapter 10 WDM Concepts and Components Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Overview – Chapter 10 10.1 Overview of WDM 10.2 Passive Optical Couplers 10.3 Isolators and Circulators 10.4 Fiber Grating Filters 10.5 Dielectric Thin-Film Filters 10.6 Phased-Array-Based Devices 10.7 Diffraction Gratings 10.8 Active Optical Components 10.9 Tunable Light Sources

Overview of WDM A characteristic of WDM is that the discrete wavelengths form an orthogonal set of carriers that can be separated, routed, and switched without interfering with each other. WDM networks require a variety of passive and active devices to combine, distribute, isolate, and amplify optical power at different wavelengths.

WDM Spectral Bands Many independent narrowband regions in the O- through L-bands can be used simultaneously. These regions are designated either in terms of spectral width or optical bandwidth. The optical bandwidth Δν related to a particular spectral width Δλ is found by differentiating c = λν; for Δλ << λ2

WDM Standards ITU-T Recommendation G.694.1 specifies DWDM operation in the S-, C-, and L-bands for frequency spacing of 100 to 12.5 GHz (or, equivalently, 0.8 to 0.1 nm at 1550 nm). The number NM is used by ITU-T to designate a specific 19N.M-THz C-band 100-GHz channel, e.g., the frequency 194.3 THz is ITU channel 43.

10.2 Passive Optical Couplers Passive devices operate completely in the optical domain to split and combine light streams. They include N  N couplers (with N ≥ 2), power splitters, power taps, and star couplers. They can be fabricated either from optical fibers or by means of planar optical waveguides using material such as LiNbO3, InP, silica, silicon oxynitride, or various polymers.

The 2  2 Fiber Coupler P0 is the input power, P1 is the throughout power, and P2 is the power coupled into the second fiber. P3 and P4 are extremely low signal levels (-50 to -70 dB below the input level) resulting from backward reflections and scattering in the device The evanescent tail from one fiber core couples into another closely spaced fiber core Optical power coupling

Performance of an Optical Coupler 3-dB coupler: P1 = P2 = 0.5 P0 Tap coupler: P2 = 0.005 P0 (- 23 dB)

Example Coupler Performance

Star Couplers In general, an N  M coupler has N inputs and M outputs

N  N Star Coupler Can construct star couplers by cascading 3-dB couplers The number of 3-dB couplers needed to construct an N  N star is

Mach-Zehnder Interferometer Multiplexers By splitting the input beam and introducing a phase shift in one of the paths, the recombined signals will interfere constructively at one output and destructively at the other. In the central region, when the signals in the two arms come from the same light source, the outputs from these two guides have a phase difference Δφ (below) where ΔL=c/(2neffΔν)

Optical Isolators Optical isolators allow light to pass in only one direction. This prevents scattered or reflected light from traveling in the reverse direction. E.g., can keep backward-traveling light from entering a laser diode and possibly causing instabilities in the optical output.

Optical Circulators An optical circulator is a nonreciprocal multiport passive device that directs light sequentially from port to port in only one direction. In the 3–port example, an input on port 1 is sent out on port 2, an input on port 2 is sent out on port 3, and an input on port 3 is sent out on port 1.

Isolator and Circulator Parameters

Fiber Bragg Grating (FBG) Example formation: Two ultraviolet beams will create a permanent interference pattern in a GeO2-doped silica fiber to form a periodic index variation along the axis. Operating Principle: Incident optical wave at l0 will be reflected back if the following grating condition is met: l0 = 2neffL, where neff is average weighting of n1 and n2 and L = grating period (periodicity of index variation) n2 Incident l0 Reflected l0 n1

Fiber Bragg Grating Application Demultiplexing (wavelength dropping) process: Consider 4 wavelengths entering a circulator at port 1. All wavelengths exit from port 2. The fiber Bragg grating is designed to reflect λ2 and pass all other wavelengths. After reflection, λ2 enters port 2 and comes out of port 3.

Multiplexing of Four Wavelengths One needs to cascade N-1 FBGs and N-1 circulators for combining or separating N wavelengths. Example for multiplexing four wavelengths using three FBGs and three circulators (labeled C2, C3, and C4). The fiber grating filters labeled FBG2, FBG3, and FBG4 are constructed to reflect wavelengths λ2, λ3, and λ4, respectively, and to pass all others.

Optical filters Optical filters are key components in WDM networks. They allow the manipulation of wavelengths just like time slots are manipulated in time division multiplexed networks. Several optical filtering technologies are available, all utilizing the property of interference between optical waves. In addition, some filters utilize the diffraction property of light, which is the property by which light from a source tends to spread to all directions. The technologies include: Gratings Bragg gratings Fiber gratings Fiber Bragg gratings Fabry Perot filters Multilayer dielectric thin-film filters Mach-Zehnder interferometers Arrayed waveguide gratings Acousto-optic tunable filters

Fabry-Perot filters A Fabry-Perot filter is a cavity formed by two highly reflective parallel mirrors. Light enters the cavity and After a number of reflections, those wavelengths for which the cavity length is an integral multiple of half the wavelength (so that the round trip through the cavity is an integral multiple of the wavelength) add in phase.

These wavelengths, called the resonant wavelengths, propagate through the cavity, while the remaining wavelengths destructively interfere. This can be done by mechanically tuning the cavity length. A major drawback of the filter is that the tuning time is in the order of tens of milliseconds.

Etalon Theory A dielectric thin-film filter (TFF) is used as an optical bandpass flter. It allows a very narrow wavelength band to pass straight through it and reflects all other wavelengths. The basis of these devices is a reflective mirror surfaces called a Fabry-Perot interferometer or an etalon. The periodicity of the device is called the free spectral range or FSR

Arrayed Waveguide Grating At (4): DF = 2p neff DL / lc neff = effective index lc = center wavelength 4 3 Lens region Lens region 2 5 At (6): DlFSR = lc2/ (DL neff) l1, l2, l3, l4 l1 l2 FSR-A FSR-B 1 6 l3 l1A l4A l1B l4B ·· ·· l4 l4 l The input waveguides (1) enter a lens region (2) (2) divides the power among the different waveguides in the grating array (3) Each grating waveguide has a precise length difference DL with its neighbors Light in each waveguide emerges with different phase delays DF at (4) The second lens region (5) refocuses the light from all array waveguides onto the output waveguide array (6) [DlFSR = free spectral range = AWG periodicity] Each wavelength is focused into a different output waveguide in region (6)

FSR Example The FSR specifies the spectral width that will be separated across the output waveguides of an AWG

Diffraction-Grating Couplers Diffraction gratings spatially separate s in a beam Reflection gratings are ruled or etched fine parallel lines on a reflective surface Transmission gratings have periodic index variations Each wavelength will reflect or refract at a different angle