Components for WDM Networks Xavier Fernando ADROIT Group Ryerson University

Passive Devices These operate completely in the optical domain (no O/E conversion) and does not need electrical power Split/combine light stream Ex: N X N couplers, power splitters, power taps and star couplers Technologies: - Fiber based or Optical waveguides based Micro (Nano) optics based Fabricated using optical fiber or waveguide (with special material like InP, LiNbO3)

10.2 Passive Components Operate completely in optical domain N x N couplers, power splitters, power taps, star couplers etc.

Fig. 10-3: Basic Star Coupler May have N inputs and M outputs Can be wavelength selective/nonselective Up to N =M = 64, typically N, M < 10 3

Fig. 10-4: Fused-fiber coupler / Directional coupler P3, P4 extremely low ( -70 dB below Po) Coupling / Splitting Ratio = P2/(P1+P2) If P1=P2  It is called 3-dB coupler 4

Definitions Try Ex. 10.2

Coupler characteristics : Coupling Coefficient 5

Coupler Characteristics By adjusting the draw length of a simple fused fiber coupler, power ratio can be changed Can be made wavelength selective

Wavelength Selective Devices These perform their operation on the incoming optical signal as a function of the wavelength Examples: Wavelength add/drop multiplexers Wavelength selective optical combiners/splitters Wavelength selective switches and routers

Filter, Multiplexer and Router

A Static Wavelength Router

Fig. 10-11: Fused-fiber star coupler Splitting Loss = -10 Log(1/N) dB Excess Loss = 10 Log (Total Pin/Total Pout) Fused couplers have high excess loss 11

Fig. 10-12: 8x8 bi-directional star coupler by cascading 3 stages of 3-dB Couplers 1, 2 1, 2 5, 6 1, 2 3, 4 7, 8 (12 = 4 X 3) Try Ex. 10.5 12

Fiber Bragg Grating This is invented at Communication Research Center, Ottawa, Canada The FBG has changed the way optical filtering is done The FBG has so many applications The FBG changes a single mode fiber (all pass filter) into a wavelength selective filter

Fiber Brag Grating (FBG) Basic FBG is an in-fiber passive optical band reject filter FBG is created by imprinting a periodic perturbation in the fiber core The spacing between two adjacent slits is called the pitch Grating play an important role in: Wavelength filtering Dispersion compensation Optical sensing EDFA Gain flattening and many more areas

Fig. 10-16: Bragg grating formation

FBG Theory Exposure to the high intensity UV radiation, the refractive index of the fiber core (n) permanently changes to a periodic function of z z: Distance measured along fiber core axis : Pitch of the grating ncore: Core refractive index

Reflection at FBG

Fig. 10-17: Simple de-multiplexing function Peak Reflectivity Rmax = tanh2(kL)

Wavelength Selective DEMUX

Dispersion Compensation using FBG Longer wavelengths take more time Reverse the operation of dispersive fiber Shorter wavelengths take more time

ADD/DROP MUX FBG Reflects in both directions; it is bidirectional

Fig. 10-27: Extended add/drop Mux

Advanced Grating Profiles

FBG Properties Advantages Easy to manufacture, low cost, ease of coupling Minimal insertion losses – approx. 0.1 db or less Passive devices Disadvantages Sensitive to temperature and strain. Any change in temperature or strain in a FBG causes the grating period and/or the effective refractive index to change, which causes the Bragg wavelength to change.

Interferometers

Interferometer An interferometric device uses 2 interfering paths of different lengths to resolve wavelengths Typical configuration: two 3-dB directional couplers connected with 2 paths having different lengths Applications: — wideband filters (coarse WDM) separate signals at1300 nm from those at 1550 nm — narrowband filters: filter bandwidth depends on the number of cascades (i.e. the number of 3-dB couplers connected)

Fig. 10-13: Basic Mach-Zehnder interferometer Phase shift of the propagating wave increases with L, Constructive or destructive interference depending on L

Mach-Zehnder interferometer Phase shift at the output due to the propagation path length difference: If the power from both inputs (at different wavelengths) to be added at output port 2, then, Try Ex. 10-6

Mach-Zehnder interferometer

Fig. 10-14: Four-channel wavelength multiplexer

Mach-Zehnder interferometer

Mach-Zehnder interferometer

MZI- Demux Example

Fiber Grating Filters Grating is a periodic structure or perturbation in a material Transmitting or Reflecting gratings The spacing between two adjacent slits is called the pitch Grating play an important role in: Wavelength filtering Dispersion compensation EDFA Gain flattening and many more areas

Different wavelength can be separated/added Reflection grating Different wavelength can be separated/added

Arrayed wave guide grating

Phase Array Based WDM Devices The arrayed waveguide is a generalization of 2x2 MZI multiplexer The lengths of adjacent waveguides differ by a constant L Different wavelengths get multiplexed (multi-inputs one output) or de-multiplexed (one input multi output) For wavelength routing applications multi-input multi-output routers are available

Diffraction gratings source impinges on a diffraction grating ,each wavelength is diffracted at a different angle Using a lens, these wavelengths can be focused onto individual fibers. Less channel isolation between closely spaced wavelengths.

Arrayed Waveguide Grating -- good performance -- more cost effective -- quicker design cycle time --- higher channel count

Multi wavelength sources Series of discrete DFB lasers Straight forward, but expensive stable sources Wavelength tunable lasers By changing the temperature (0.1 nm/OC) By altering the injection current (0.006 nm/mA) Multi-wavelength laser array Integrated on the same substrate Multiple quantum wells for better optical and carrier confinement Spectral slicing – LED source and comb filters

Tunable Filters At least one branch of the coupler has its length or ref. index altered by a control mechanism Parameters: tuning range (depends on amplifier bandwidth), channel spacing (to minimize crosstalk), maximum number of channels (N) and tuning speed

Fig. 10-23: Tunable optical filter

Fig. 10-21: Tunable laser characteristics Typically, tuning range 10-15 nm, Channel spacing = 10 X Channel width

Summary DWDM plays an important role in high capacity optical networks Theoretically enormous capacity is possible Practically wavelength selective (optical signal processing) components decide it Passive signal processing elements are attractive Optical amplifications is imperative to realize DWDM networks