Transcript Optical Switch Fabrics: What is their value, and When will

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
Fiber core index depends (slightly) on l
Any modulated signal has nonzero linewidth
1 km
100
km
1500
km
500
km
Chromatic dispersion is the result:
Spread in arrival time after signal transmission
But residual dispersion at 3000 km D = 1050 ps/nm
DCF
DCF
DCF
DCF
DCF
Fiber spans are “dispersion compensated”
V. Srikant (Corning) OFC 2001
Is that OK? Depends on data rate B and length L: B2DL ~ 105 ps/nm (Gb/s2)
(relation for 1 dB power penalty; Tigye Li, Proc. IEEE, 1993)
Cumulative dispersion budget: 1000 ps/nm @ 10 Gb/s
63 ps/nm @ 40 Gb/s
MARGINAL
CRITICAL
Dynamic chromatic dispersion compensation
Equalizer
I
Dispersion
Compensator
BER feedback
Uncompensated
Compensated
Phase-only “all-pass” filter
Gires-Tournois Interferometer
Periodic spectral phase response
L/2
1  1
Round Trip Delay
T  ng L / c
0  1
Free Spectral Range
FSR  1 / T
• 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
Multi-stage Filter Dispersion
1
2
Increases passband width and total dispersion
3
4
Ripple = dev. from ideal linear response
Madsen, Walker, Ford, Goossen & Lenz, ECOC 1999
The “MARS” resonant MEMS modulator
MARS (Membrane Anti-Reflection Switch) analog optical modulator
l/4 Silicon Nitride “drumhead” suspended over a Silicon substrate
input
reflect
l/4 SiNx
PSG
Vdrive
Voltage Response
measured
Silicon
theory
0 < Vdrive < 30V
3l/4 < gap < l/2
transmit
Drive voltage (V)
Ford, Walker, Greywall & Goossen, IEEE J. Lightwave Tech. 16, 1998
Greywall, Busch & Walker, Sensors & Actuators A A72, 1999.
Goossen, Arney & Walker, IEEE Phot. Tech. Lett. 6, 1994
MARS All-Pass Filter
0  R  70%
R 
L/2
2
Double polysilicon MEMS structure
(flat l response, no charging)
411 um thick Silicon (100 GHz FSR)
100% Reflector
(dielectric enhanced gold mirror)
Tunable
Partial
Reflector
V
Substrate
2 control parameters per stage:
MEMS voltage controls front mirror reflectivity (phase feature amplitude)
Substrate temperature controls free spectral range (phase feature location)
Madsen, Walker, Ford Goossen, Neilson & Lenz, IEEE Phot. Tech. Lett. 12, 2000
Fiber-coupling package
optical breadboard package
input
Vmirror TTEC controller
d
output
D
f
ferrule
$
f
collimator
device
hermetic MEMS VOA package
Key optical package parameters
Lens focal length f = 3 mm
Fiber separation d = 125 um
Illuminated diameter D = 600 um
MEMS device diameter 1250 um
Substrate thickness t = 411 um
Package loss (mirror at device plane) 0.4 dB
Cavity round-trip loss
Absorption = (e-aL)N
device
a=
10-4/cm
Reflection = (Rmirror)N
R  98.5%
Scatter
= ( Awindow-Afeatures )N
Awindow
T  99.3%
Shift = 10-0.434(NdT/nF)
2
package
Dy  5 / 600 um
Defocus = f(Nf)
fmembrane < 444 mm (20 um / pass)
Coupling = To
T  93.3%
Loss in dB
10
9
Coupling
8
Shift
7
Scatter
Defocus
Reflection
6
Absorption
5
4
3
2
1
0
1
2
3
4
5
6
7
Round trip number
8
9
10
Single filter response
1
Measured Phase & Amplitude
Wideband (30 nm) Transmission
Madsen, Walker, Ford Goossen, Neilson & Lenz, IEEE Phot. Tech. Lett. 12, 2000
2-stage DCF results
Tuned for 50 GHz bandwidth and 100 GHz (0.8 nm) FSR
1
Negative
2
Positive
Design: Dispersion goal = +/-104 ps/nm; predicted ripple of +/- 2.5 ps
Result: Set at +/- 102 ps/nm, yielded ripple of +/- 2.5 ps
Madsen, Walker, Ford Goossen, Neilson & Lenz, IEEE Phot. Tech. Lett. 12, 2000
2-stage DCF results (continued)
2x dispersion for 30 GHz bandwidth and 100 GHz (0.8 nm) FSR
1
2
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
Current status: Still R&D!
Optical performance (loss uniformity) needs to be improved
Control algorithms need more development
Dispersion compensation not critical until 40 Gb/s deployed