Enabling Devices using MicroElectroMechanical System (MEMS) Technology for Optical Networking December 17, 2007 Workshop on Optical Communications Tel Aviv University Dan Marom Applied Physics Department The Selim and Rachel Benin School of Engineering and Computer Science Hebrew University http://aph.huji.ac.il/marom
Outline From Optical Communication Systems to Optical Networking Exemplary Optical MEMS Devices for Optical Networking Switching WDM Channels in Optical Mesh Networks Wavelength-Insensitive Switching Wavelength-Selective Switches (WSS) Hybrid Free-Space & Guided-Wave WSS Chromatic Dispersion Trimming for Performance Enhancement Conclusions 2
Optical Networking The Old View Traffic demand is met by optical networks built with high-capacity WDM line systems, optical add/drop nodes, and digital cross- connects. Disadvantages: Many O/E/O conversions. Long provisioning time. Inventory. High OpEx. 3
Optical Networking The New View Transparent optical mesh network with no end terminals. Optical path switching and reconfigurable optical add/drop. Integrated electrical switches for add/drop interface, grooming, and electrical regeneration. Wavelength services. Challenges: Wavelength management Wavelength blocking Performance monitoring 4
Optical cross-connect connect with O/E/O conversion Resolves the challenges of transparent optical networking (wavelength ength management, blocking, and performance monitoring). However, solution is inefficient: Lose effectiveness of ultra-long long haul transmission. High cost due to o/e/o conversions. Full cost incurred on deployment. N number of WDM channels K number of I/O line systems K N switch fabric size 5
Large Transparent Optical Cross-Connect Connect Eliminate O/E/O conversions in optical cross-connect connect to lower deployment cost and introduce transparency to network. Disadvantages: Switch fabric overkill / complexity. Demux and mux required. Full cost on deployment. Supported switching states: (K N) 2 Allowable switching states: K (K N) 6
Wavelength-Modular Cross-Connect Connect Solution Solution to lower deployment cost: modular cross-connect connect architecture. Wavelength-modular cross-connects connects use an array of small cross- connects, each dedicated to a specific WDM channel: Disadvantages: Demux and mux required. Operational expenses. When using fixed channel transmitters, need to install additional switch module at each network node on channel turn on. Using tunable transmitters, lose advantage of wavelength routing due to partially deployed switch modules. 7
MEMS Switch Fabrics Input Fiber array Output Fiber array 8
Port-Modular Cross-Connect Connect Solution Port-modular cross-connects connects can be constructed using wavelength-selective 1 K1 switches. These switches distribute the N WDM channels across the K output ports. Free-spaced based WSS provide wide passbands, again facilitating cascadability. Broadcast capability enabled. Least hardware requirement. 9
Wavelength-Selective 1 K 1 K Switch: Architecture (1 input, K outputs) Performs functions of demultiplexing, switching, and multiplexing g in one low loss unit. Mirror tilt angle determines selected output port 10
Fringe Field Activated SOI Micromirrors Monolithic No snapdown No contact No stiction No charging 11
64 channel wavelength-selective 4 14 1 switch 12
Switch Characterization Typical voltage response: Spectral response: Switch provides switching, 10 db dynamic equalization range, and wavelength blocking. Losses inclusive of isolators on input ports. 13
Mirror Actuation Options: Are They Equivalent? MEMS mirrors tilting in the dispersion direction (DD) MEMS mirrors tilting in the direction orthogonal to dispersion (DOD) 14
Problems Due to Rotation in Dispersion Direction -5 0 db Transmission (db) -10-15 -20-5 db -10 db -15 db 1542.5 1543 1543.5 1544 1544.5 1545 Interchannel spikes in coupling efficiency Wavelength (nm) Dynamic equalization by detuning mirror angle, results in Rabbit Ears Mirror edge diffraction results in coupling to adjacent channels and rabbit ear phenomena 15
Mirror Curvature Effect on WSS Reflected beam will strike diffraction grating at a displaced position, resulting in a different optical path length. Δλ Path length changes as a function of wavelength offset i.e., chromatic dispersion: CD = 8f tan 2 2 Rcλ 0 ( φ ) 16
MEMS Channelized Dispersion Compensator MEMS Mirrors with variable curvature Grating Fold Mirror Lens φ 400 ps/nm 0 ma 27 ma 24 ma 28 ma Z ( μ m) 3 2 1 0 R= -160mm R= 3.9 mm 0.2 0.4 0.6 0.8 1.0 1.2 X (mm) 25 ma 26 ma 0 ps/nm -400 ps/nm 29 ma 30 ma 17
WSS + Dispersion Compensator An ideal device would provide not only wavelength-selective switching, but also add the ability to perform dispersion trimming. This required a tilting mirror with adjustable curvature; not easy! An alternative to MEMS technology, LCoS,, can perform this trick. +/- 60 ps/nm dispersion 18
Is LCoS an All-Capable MEMS Replacement? A LCoS device replaces a continuous mirror with a diffractive version, comprised of many independent phase-controlled pixels. Routing to different fiber ports requires setting a phase ramp. Dispersion compensation requires setting a quadratic phase distribution. LCoS performance is primarily challenged in diffraction efficiency dependence on tilt angle and crosstalk to adjacent ports. 19
Hybrid Integration of Planar Lightwave Circuits with Free-Space Optics Optical system consists of multiple elements, increasing cost and assembly difficulty. 20
Planar Lightwave Circuit Designed for Generating Angular Dispersion Waveguide array with length increasing by ΔL. ΔL=m λ 0 /n Grating order Design wavelength Accumulated phase: K ΔL = 2πnΔL/λ = 2πmλ 0 /λ Output facet phase: λ > λ 0 λ 0 λ < λ 0 21
Wavelength-Selective 1 21 2 Switch Input PLC 1 and PLC 2 Port 1 Port 2 Cylindrical collimators Fourier lens MEMS micromirror array PLC s designed for telecom C-band (1530-1560 nm), and support 40 channels at 100 GHz separation. MEMS micromirror array was reused from previous WSS, therefore mirror pitch did not match AWG parameters. 22
Switching of WDM Channels Switching with bulk mirror Switching with micromirror array Switch to Port 1 Switch to Port 2 Port 1 signal Port 2 signal 23
Hybrid Wavelength-Selective 1 31 3 WSS with Integrated Mux/Demux Functionality Advantages: Less discrete components Lower losses from input to drop Maintain multiplexed path switching ports Transmissivity [db] 0-5 -10-15 -20-25 -30-35 -40-45 -50 Out 1 Out 2 Out 3 1528 1533 1538 1543 1548 1553 1558 1563 Wavelength [nm] 24
Hybrid Wavelength-Selective 1 31 3 WSS with Integrated Mux/Demux Functionality Spatially demultiplexed spectrum ~125 mm Spatially demultiplexed spectrum Matched spatial dispersions Channel demux PLC input 25
Tunable Dispersion Compensation (TDC) with a Deformable Mirror TDC combining PLC and deformable mirror with ±500 ps/nm tuning range for all channels on 100 GHz grid. Single knob tuning Compact design Low-power consumption CD = 8f tan 2 2 Rcλ 0 ( φ ) 26
Testing TDC at 42.7 Gb/s Data Rate Eye diagram of 42.7 Gb/s CSRZ signal with 425 ps/nm dispersion. Signal compensated. 27
Conclusions Photonic devices based on optical MEMS actuators can meet the functional requirements of optical networking. The efforts behind telecom optical MEMS in the last 8 years has resulted in significant progress in: MEMS device fabrication Micro-optics Packaging Control To successfully compete in the telecom device space, MEMS-based components are introducing higher degrees of integration into more compact and robust packages such as planar lightwave circuits. 28