EE 230: Optical Fiber Communication Lecture 15

From the movieWarriors of the Net

WDM Components

ITU Grid

• Wavelengths for CWDM and frequencies for DWDM defined by International Telecommunication Union, a part of the United Nations located in Geneva

• Central frequency is 193.1 THz, equivalent to 1552.52 nm

• Frequencies for 50 GHz channel spacings are thus defined as 193.1 + 0.05n THz where n is a positive or negative integer

Active vs. Passive Devices

• Passive: requires no electrical power and transfer function cannot be modified by user

• Active: allows user to manipulate what it does to light pulses. Requires power.

Platforms for WDM components

• Discrete optics: thin-film filters, microelectromechanical systems (MEMS), isolators, circulators

• All-fiber components: couplers, Mach-Zehnder interferometers

• Planar lightwave circuits (PLC): arrayed-waveguide gratings (AWG), couplers, MZs, etc.

Coupler parameters

Splitting ratio: P2/(P1+P2)

Excess loss: 10 log (P0/P1+P2)

Insertion loss: 10 log (Pin/Pout)

Crosstalk: 10 log (P3/P0)

Coupling as function of length

zezPP 202 sin

Mach-Zehnder Interferometer

where neff is determined from the Pcore/P graphs

Ln

c

eff

2

Multiplexing/demultiplexing criterion

where L is the path length difference between the two arms

Lneff

21

112

Wavelength dependence of MZ output

For wavelengths 1 entering at input port 1, and 2 entering at input port 2,

2

2

1

21 cossin

LnLn

PO

Wavelength adjustment (“trim”)

• Coarse adjustment possible with fiber MZs by heating and pulling shorter arm to increase channel spacing

• Fine adjustment for both fiber and PLCs done with UV irradiation to line transmission peaks up with ITU grid

Example

To multiplex four wavelengths separated by 50 GHz (0.4 nm)

How many stages needed?

2. (log2 W). How many total MZs?

3. Two in one stage, one in the next.

What is L for each stage?

Example, continued

If first frequency is ITU center, what are other three, and their wavelengths?

193.10, 193.15, 193.20, and 193.25 THz

1552.52, 1552.12, 1551.72, and 1551.32 nm

If neff=1.45, determine L values

Example, continued

• First stages have 100 GHz channel spacing, one for even-numbered wavelengths and one for odd. L equals c/2n(100x109)=1.0 mm

• Second stages have 50 GHz channel spacing. L =c/2n(50x109)=2.1 mm

• As channel spacing gets smaller, it gets easier to make MZs (larger L)!

General MZ expression

For a multiplexer or demultiplexer with N wavelengths, you need n=log2N stages where the path length difference for stage i is

n

cL

ini 2

Arrayed-Waveguide Grating

AWG channel spacing

where ns=input/output waveguide index, nc=central waveguide array index, and

gfc

cs

nLm

ncdxn2

d

dnnn ccg

Tuning an AWG

Each input waveguide corresponds to a different center wavelength and channel spacing. Several waveguides around the center one will correspond to the correct channel spacing within the tolerance, and the peak wavelengths will vary from one waveguide to another.

WDM Muxes and Demuxes

Grating Based Demultiplexer

Optical Filters

Interference Filter Based WDM

Thermal drift in waveguide devices

n/T for silica=7.5x10-6 per degree for silicon=2.63 ppm per degree

d/dT = 12 pm per degree (red shift)

2/3 due to thermooptic effect, 1/3 to CTE

T

n

nT

n

nT

L

LT

nL

T

Ln

nLT

nL

nLT

111

Effect of thermal drift

Channel spacing=100 GHz=0.8 nm=800 pm

DWDM device completely transparent every 800 pm, opaque between

Silica-on-silicon drifts 12 pm/Device becomes a beam stop if temperature

changes by ?

33! “Passive” devices routinely T stabilized; customers unhappy

Athermalization Techniques

• Mechanical compensation: flex entire chip, adjust point at which signal injected into device

• Materials compensation: design waveguide to be inherently athermal