Free-Space MEMS Tunable Optical Filter in (110) Silicon Ariel Lipson & Eric M. Yeatman Optical & Semiconductor Group
Outline Device - Optical Filter Optical analysis Fabrication Schematic Fabricated 2
Device Application Optical Communication Network Broadcast System Lasers Fibre 400 channels Tunable receiver Spectrometer Sensor 3
Device Configuration 1D Photonic band gap Two alternating materials Dimensions are in the order of ¼ of a wavelength (or odd multiples) Multiple reflections and phase matching create a wavelength selective mirror Two mirrors form a Fabry-Perot filter 4
Device Configuration 1D PBG structure 3 silicon bar mirrors and a cavity Fabry Perot Dimensions ~2µm thick x 25µm deep Lensed Fiber Light beam 150µm 5 λ 2 4n L n 21 4n H λ 2 L Oxide λ Silicon Air Lensed Fiber Collimating Lensed fibers 9µm radius Gaussian beam 200µm Alignment grooves & springs created with an additional etch step Tuning by varying the centre cavity width Alignment Springs Filter 5
Optical Analysis The ideal case Transfer matrix formulation M ( λφ, ).. φ.. nh. dl... nl =...... nh........ nl..... ( λφ, ) ( λφ, ), ( λφ, ) M r t Gaussian beam input Plane wave expansion of Gaussian beam using spatial Fourier transform ( ) ( ) ixk kx E λφ, = E λ, x e dx, sinφ= k Mode overlap integral with the exit fiber 2 E out, E fiber η ( λ ) =, E1, E2 E1E2 dφ E, E E, E = φ out out fiber fiber n L r 1,t 1 (λ,φ) n H m air ¼λ c m gap ½λ c m si ¼λ c 6
Optical Analysis The ideal case Collimation decreases losses Loss [db] 0-10 -20 Gaussian Beam Width 4 µm 9 µm 31 µm Thin layers m=[1 1 1] Thick layers m=[21 5 2] Layer thickness effects losses and pass band Optical communication systems need narrow pass bands ~100GHz 4 si layers -30 m=[1 1 1] 193.1 193.3 193.5 193.7 Freq [THz] 7
Optical Analysis The non-ideal case Due to imperfect deep etching, angles are introduced Fabry-Perot cavity analysis for a wedged cavity with a PBG mirror on either side ~ E out ~ p 1 2 p2 [ 2 q2 1 q1 ] i klp 2 i k ( Lq 2 + Lq1 ) ( λ, φ) = E ( λ, φ) t ( λ, φ) t ( λ, φ ) e r ( λ, φ ) r ( λ, φ ) e in 1 p= 0 q= 0 r 1,t 1 (λ,φ) ϑ r 2,t 2 (λ,φ) L 01 φ L p2 L p1 m air ¼λ c m gap ½λ c m si ¼λ c 8
Optical Analysis The non-ideal case Simulation for an 9µm Gaussian beam with different etching angles 0-10 Angle [deg] 0 0.001 0.005 Ripples appear at high frequencies Loss [db] -20-30 0.01 Above >0.001 deg we get excess loss and pass band widening -40 192.4 192.6 192.8 193 193.2 Freq [THz] vertical etching! 9
Fabrication Deep Reactive Ion Etching (DRIE): Suitable for all silicon orientations Scalloping and 1-2 deg sidewall angle. KOH etching in (110) silicon (Kendall 1979): Vertical etching, but limited by etch ratio between (111):(110) Smooth surfaces Not suitable for non (111) planes Depth limited by width and length DRIE followed by short KOH etching in (110) silicon: Vertical etching Smooth surfaces If kept short, does not effect too much other orientations 10
Fabrication 2nd DRIE etch DRIE (111) 1st etch + KOH smoothing KOH wet etching Initial scalloping DRIE + KOH 11
Fabrication Process Flow 1. Oxide + Photoresist 5. Thick Photoresist Photo resist Oxide Silicon Gold 2 DRIE 6. DRIE 3. Quick KOH dip 7. Sputter Gold 4. Oxidation + RIE 8. HF Release Fiber 12
Fabrication Results 0.65nm pass band, -10.5dB loss and a 200Ghz channel spacing Loss [db] -5-15 -25 Angle [deg] 0.005 0.01 Exp. MEMS tuning mechanisms <0.01 deg verticality -35 192.4 192.6 192.8 193 193.2 Freq [THz] 13
0 Filter Tunability +150 nm 0nm -150 nm Fabrication Tunability Loss [db] -5-10 -15-20 Mechanical movement large tuning range for the whole C & L bands ( λ=~70nm) -25 1510 1530 1550 1570 1590 Wavelength [nm] Use of BSOI to release moving structures: HF etched or Back etch release Downside are speed and settling time But proper design can reach µs timescales. 14
Future Work Complete a tunable device Work on designs with fast response time and lower loss Reshape the band pass by passing the light twice through the cavity Loss [db] 0 1 pass 3-layers -5 1 pass 2-layers 2 pass 2-layers -10-15 -20-25 -30 193 193.2 193.4 193.6 193.8 194 Freq [THz] 15
Summary 1D PBG filter was fabricated for DWDM networks with -10.5 db fiber to fiber losses, 200 Ghz channel spacing. Combined DRIE + KOH wet etching on (110) silicon wafers for very vertical (<0.01 ) and smooth surfaces. On going work on tunability using MEMS techniques. 16
Acknowledgments We would like to thank the UPC, Lambdax and Coventor for their support. Thanks to Dr. John Stagg, Dr. Munir Ahmad and Michael Larsson for their kind help. Further information: alipson@ic.ac.uk 17