Components for WDM Networks Xavier Fernando ADROIT Group

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 In. P, Li. Nb. O 3)

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

Fig. 10 -4: Fused-fiber coupler / Directional coupler • P 3, P 4 extremely low ( -70 d. B below Po) • Coupling / Splitting Ratio = P 2/(P 1+P 2) • If P 1=P 2 It is called 3 -d. B coupler

Definitions Try Ex. 10. 2

Coupler characteristics : Coupling Coefficient

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) d. B Excess Loss = 10 Log (Total Pin/Total Pout) Fused couplers have high excess loss

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

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 = tanh 2(k. L)

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 -d. B directional couplers connected with 2 paths having different lengths Applications: — wideband filters (coarse WDM) separate signals at 1300 nm from those at 1550 nm — narrowband filters: filter bandwidth depends on the number of cascades (i. e. the number of 3 -d. B 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

Reflection grating Different wavelength can be separated/added

Arrayed wave guide grating

Phase Array Based WDM Devices • The arrayed waveguide is a generalization of 2 x 2 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 multiinput 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 -- quicker design cycle time -- more cost effective --- 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/m. A) • 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