Dynamic Dispersion Compensator

Christi Madsen, James Walker, Joseph Ford, Keith Goossen, David Neilson, Gadi Lenz

References:"Micromechanical fiber-optic attenuator with 3 microsecond response"   J. Ford, J. Walker, D. Greywall and K. Goossen, IEEE J.of Lightwave Tech. 16(9), 1663-1670, September 1998 "A tunable dispersion compensating MEMS all-pass filter"    Madsen, Walker, Ford. Goossen, Nielson, Lenz, IEEE Photonics Tech. Lett. 12(6), pp. 651-653, June 2000.

Chromatic dispersion in long-distance telecom

Is that OK? Depends on data rate B and length L:(relation for 1 dB power penalty; Tigye Li, Proc. IEEE, 1993)

B2DL ~ 105 ps/nm (Gb/s2)

V. Srikant (Corning) OFC 2001

But residual dispersion at 3000 km D = 1050 ps/nm

63 ps/nm @ 40 Gb/s CRITICAL

1 km100 km500 km1500 km

Fiber core index depends (slightly) on Any modulated signal has nonzero linewidth

Chromatic dispersion is the result: Spread in arrival time after signal transmission

Fiber core index depends (slightly) on Any modulated signal has nonzero linewidth

Chromatic dispersion is the result: Spread in arrival time after signal transmission

Fiber core index depends (slightly) on Any modulated signal has nonzero linewidth

Chromatic dispersion is the result: Spread in arrival time after signal transmission

Fiber core index depends (slightly) on Any modulated signal has nonzero linewidth

Chromatic dispersion is the result: Spread in arrival time after signal transmission

Fiber spans are “dispersion compensated”

DC

F

DC

F

DC

F

DC

F

DC

F

Cumulative dispersion budget: MARGINAL1000 ps/nm @ 10 Gb/s

Dynamic chromatic dispersion compensation

Uncompensated Compensated

EqualizerI Dispersion

Compensator

BER feedback

Phase-only “all-pass” filter

• For a lossless filter, magnitude response = 1 (allpass!)• Periodic Gaussian dispersion feature (DCF requires linear chirp)• Approximately linear dispersion over a limited bandwidth

Madsen, Walker, Ford, Goossen & Lenz, ECOC 1999; see also recent IEEE LEOS article

1 1 0 1

L / 2

cLnT g /Round Trip Delay

TFSR /1Free Spectral Range

Gires-Tournois Interferometer Periodic spectral phase response

Multi-stage Filter Dispersion

Ripple = dev. from ideal linear response

1 2 3 4

Madsen, Walker, Ford, Goossen & Lenz, ECOC 1999

Increases passband width and total dispersion

The “MARS” resonant MEMS modulatorMARS (Membrane Anti-Reflection Switch) analog optical modulator /4 Silicon Nitride “drumhead” suspended over a Silicon substrate

0 < Vdrive < 30V3/4 < gap < /2

input

/4 SiNx

Silicon

PSG

reflect

transmit

Vdrive

0 < Vdrive < 30V3/4 < gap < /2

input

/4 SiNx

Silicon

PSG

reflect

transmit

VdriveVoltage Response

theory

measured

Drive voltage (V)

Ford, Walker, Greywall & Goossen, IEEE J. Lightwave Tech. 16, 1998Greywall, Busch & Walker, Sensors & Actuators A A72, 1999.Goossen, Arney & Walker, IEEE Phot. Tech. Lett. 6, 1994

MARS All-Pass Filter

2 control parameters per stage:MEMS voltage controls front mirror reflectivity (phase feature amplitude)Substrate temperature controls free spectral range (phase feature location)

L/2

100% Reflector(dielectric enhanced gold mirror)Tunable

PartialReflector

SubstrateV

0 70%2

R

R

Madsen, Walker, Ford Goossen, Neilson & Lenz, IEEE Phot. Tech. Lett. 12, 2000

Double polysilicon MEMS structure(flat response, no charging)

411 um thick Silicon (100 GHz FSR)

$

hermetic MEMS VOA package

Fiber-coupling package

optical breadboard package

Key optical package parametersLens focal length f = 3 mmFiber separation d = 125 umIlluminated diameter D = 600 umMEMS device diameter 1250 umSubstrate thickness t = 411 umPackage loss (mirror at device plane) 0.4 dB

Vmirror TTEC controller

devicecollimatorferrule

inpu

tou

tput f f

d D

0

1

2

3

4

5

6

7

8

9

10

1 2 3 4 5 6 7 8 9 10

Round trip number

Lo

ss

in d

B

Coupling

Defocus

Shift

Scatter

Reflection

Absorption

Cavity round-trip loss

Absorption = (e-L)N

Shift = 10-0.434(NdT/nF)2

Defocus = f(N)

Scatter = ( )NAwindow-Afeatures

Awindow

Reflection = (Rmirror)N

Coupling = To

= 10-4/cm

R

T

T

y / 600 um

devi

cepa

ckag

e

fmembrane < 444 mm (20 um / pass)

Single filter response

1

Madsen, Walker, Ford Goossen, Neilson & Lenz, IEEE Phot. Tech. Lett. 12, 2000

Measured Phase & Amplitude Wideband (30 nm) Transmission

2-stage DCF results

Design: Dispersion goal = +/-104 ps/nm; predicted ripple of +/- 2.5 psResult: Set at +/- 102 ps/nm, yielded ripple of +/- 2.5 ps

Negative Positive

Tuned for 50 GHz bandwidth and 100 GHz (0.8 nm) FSR

1 2

Madsen, Walker, Ford Goossen, Neilson & Lenz, IEEE Phot. Tech. Lett. 12, 2000

2-stage DCF results (continued)

200 ps/nm range, 1.5 ps ripple(further improvement in loss uniformity required)

Madsen, Walker, Ford Goossen, Neilson & Lenz, IEEE Phot. Tech. Lett. 12, 2000

2x dispersion for 30 GHz bandwidth and 100 GHz (0.8 nm) FSR

1 2

Current status: Still R&D!

Optical performance (loss uniformity) needs to be improvedControl algorithms need more developmentDispersion compensation not critical until 40 Gb/s deployed