Photonics Systems Integration Lab University of California San Diego Jacobs School of Engineering Fast, Two-Dimensional Optical Beamscanning by Wavelength Switching T. K. Chan, E. Myslivets, J. E. Ford 1 Photo: Kevin Walsh, OLR
Introduction Free Space Optical Communications: Dynamic connections: platform and environment Require fast, active alignment and tracking Retro-reflecting modulators Single sided alignment MEMS (Chan et al, J. Light. Tech, 24(1), 2006) MQW (Rabinovich et al. CLEO 2003, 2003) Scanning/Tracking Challenges: Fast (<<1 ms switching) Accurate and repeatable Wide angle range ± 5, (± 60 ideally) Physically small & robust Deformable MEMS mirror Flat mirrors 2
Existing Scanning Technologies Speed Range Aperture Key limitation Galvanometric KHz ~ 30 ~10mm Bulk, power, reliability MEMS mirror KHz ~ 5 ~1mm Aperture, power handling Acousto-optic KHz ~ 1 ~10mm Angle range Liquid Crystals 100 Hz ~ 60 >100mm Speed, environmental constraints Electro-optic MHz ~ 1 ~10mm Drive current, angle range Question: How to decouple fast response from other performance parameters? Field of View tradeoff Speed Aperture Accuracy 3
Wavelength Scanning Fast λ-tuning Laser source Fixed collimator and diffraction grating Vertical angle Random-access scan Far-field distribution δθ y H=kλ δθ x Θ y Diffract wavelength to angle: Decouples aperture from speed How fast? Grating-assisted codirectional coupler with rear sampled reflector (GCSR) lasers Simsarian, J. E. et al, IEEE Phot. Tech. Let. 15 (8) p1038, 2003. < 50 ns switching times in 40 nm scanning range > 1.5 dbm per channel What about 2D scanning? 4
Concept: 2D Wavelength Scanning λ 1,1 λ 1,2 λ 2,3 High-order grating Arrayed waveguide grating (AWG) VIPA free space echelon grating Low-order grating Blazed reflection grating Holographic transmission grating λ 2,4 Insertion Loss (db) 0-5 -10-15 -20-25 Diffraction order m: 198 197 196 195 194 193 192 191 190 189 FSR = 7.7 nm (0.998 THz) 1520 1530 1540 1550 1560 1570 1580 1590 1600 Wavelength (nm) W avelength (nm ) Channel 1 Channels 2-8 FSR = 8.5 nm (0.998 THz) 5
2D Integrated Optics Demux Hybrid wavelength demultiplexer T. K. Chan et al, J. Light. Tech. 25(3) 2007 Combines a 1x40 channel AWG and a free space grating demultiplexer Fourier-Transform Lens focal length = f Blazed Grating line spacing = d AWG Demultiplexer Demultiplexed plane (optoelectronic / MEMS device) 6
2-D Single mode fiber demux 40 AWG Outputs 1x40 AWG + 50 lines/mm grating 600 nm wavelength range 7-15 db insertion loss into SMF 0.1 db power penalty @ 10 Gb/s 1x40 input array Lens Output fiber Grating 1092 channels (39 x 28 grid) 7
2D Beamscanner Modifications: (1) Substitute JSDU 1x8 AWG to increase # of diff. orders (2) Increased grating frequency to cover a greater angle range (3) Add a mirror and short focal length objective for beamscanning Tunable Source JDSU 1x8 AWG Mirror Lens f = 100 mm Grating 3 rd order 300 lp/mm Source Options: Tunable Laser Broadband noise source + Tunable Filter V-groove array 635 um pitch NA determines aperture 8x Microscope Objective f = 25 mm Focal length determines angular range 8
2D Beamscanning Demonstration Tunable laser 1535 1590 nm sweep Microscope objective Free-space reflection grating AWG V-Groove fiber array 9
2D Beamscanner Demonstration C-Band ASE illumination 1545.0 nm 1586.4 nm Angular Output (degrees) Calculated Directions 8 4 0-4 10.3 1547.0 nm 1588.3 nm 11.0-8 -8-4 0 4 8 Angular Output (degrees) Gaussian Output Beam Profile Coherent illumination Numerical aperture = 0.12 Lens focal length = 25 mm 1/e 2 diameter = 6 mm For a telephoto lens Lens focal length = 100 mm 1/e 2 diameter = 24 mm 1/e 2 diameter 6 mm 10
Fast tuning ASE Source CoreTek Tunable Filter JDSU 1x8 AWG Mirror Lens f = 100 mm Grating 3 rd order 300 lp/mm Optical Amplifier V-groove array 635 um pitch Coretek/Nortel MEMS Tunable Filter 8x Microscope Objective f = 25 mm 80 nm span tunable etalon filter ~100 µs sweep times Channel bandwidth = 0.47nm res Microelectromechanical tuneable filters with 0.47 nm linewidth and 70 nm tuning range, Tayebati, et al, Electronics Letters 34(1) 1998. 11
Fast Sweeping w/ Tunable Filter 1578.2 nm 1546.7 nm 183 µs switching time Insertion Loss (db) Filter Passband 0-5 AWG channels -10-15 -20 1531 1532 1533 1534 Wavelength (nm) AWG channel pitch = 50 GHz Narrow bandwidth source is desired. higher dispersive device more diffraction orders over the same bandwidth! 12
Virtually Imaged Phased Array: VIPA Virtual line sources are created by multiple reflections Large spatial offset between source origins create high-order echelle grating Free-space optics equivalent to planar arrayed waveguide grating r = 100% r = 95% VIPA echelle grating concept M. Shirasaki, Fujitsu Sci. Tech. J., 35(1), 1999. 2D Dispersion using a VIPA S. Xiao and A. M. Weiner, Optics Express 12 (13), p.2895-2902, 2004 Multi-order VIPA + free space grating 41 Channels (~4x10) 13
Future directions: Planar integration High-resolution 2-D scanning possible Grating VIPA Tunable Source VIPA design parameters - 100 µm slab with n = 1.5, 2.5 tilt Transmission grating: 500 lp/mm Scan Output: Scan area = 5.4 x 8.1 Wavelength Range = 1400 1600 nm Number of Rows = 26 Caution: tight alignment tolerances required 14
Conclusion 2D beamscanning can be achieved by combining 2 dispersive elements orthogonally Direction is wavelength dependant via raster scanning Speed is determined by wavelength tuning source, not the optical deflectors Combined an AWG with a free-space grating Demonstrated 183 µs switching using off the shelf parts Discrete 6x8 directional array 11.0 by 10.3 direction range More desirable to combine a VIPA with a free-space grating Continuous scanning in one direction Very dispersive (more diffraction orders over the wavelength range) Wavelength tuning determines sweep speeds ~10s ns wavelength sweeps are commercially available 15