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), , September 1998 "A tunable dispersion compensating MEMS all-pass filter" Madsen, Walker, Ford. Goossen, Nielson, Lenz, IEEE Photonics Tech. Lett. 12(6), pp , 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) B 2 DL ~ 10 5 ps/nm (Gb/s 2 ) V. Srikant (Corning) OFC 2001 But residual dispersion at 3000 km D = 1050 ps/nm 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” DCF Cumulative dispersion budget: MARGINAL Gb/s

Dynamic chromatic dispersion compensation UncompensatedCompensated Equalizer I 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 L / 2 Round Trip Delay Free Spectral Range Gires-Tournois InterferometerPeriodic spectral phase response

Multi-stage Filter Dispersion Ripple = dev. from ideal linear response 1234 Madsen, Walker, Ford, Goossen & Lenz, ECOC 1999 Increases passband width and total dispersion

The “MARS” resonant MEMS modulator MARS (Membrane Anti-Reflection Switch) analog optical modulator /4 Silicon Nitride “drumhead” suspended over a Silicon substrate 0 < V drive < 30V 3 /4 < gap < /2 input /4 SiN x Silicon PSG reflect transmit V drive 0 < V drive < 30V 3 /4 < gap < /2 input /4 SiN x Silicon PSG reflect transmit V drive Voltage Response theory measured Drive voltage (V) Ford, Walker, Greywall & Goossen, IEEE J. Lightwave Tech. 16, 1998 Greywall, Busch & Walker, Sensors & Actuators A A72, 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 Partial Reflector SubstrateV 070% 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 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 V mirror T TEC controller devicecollimatorferrule input output ff d D

Cavity round-trip loss Absorption = (e -  L ) N Shift = (NdT/nF) 2 Defocus = f(N  ) Scatter = ( ) N A window -A features A window Reflection = (R mirror ) N Coupling = T o  = /cm R  T  T   y  / 600 um device package f membrane < 444 mm (20 um / pass)

Single filter response 1 Madsen, Walker, Ford Goossen, Neilson & Lenz, IEEE Phot. Tech. Lett. 12, 2000 Measured Phase & AmplitudeWideband (30 nm) Transmission

2-stage DCF results Design: Dispersion goal = +/-104 ps/nm; predicted ripple of +/- 2.5 ps Result: 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 12 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, x dispersion for 30 GHz bandwidth and 100 GHz (0.8 nm) FSR 12

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