2-D Array Wavelength Demultiplexing by Hybrid Waveguide and Free-Space Optics

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Dwain Matthews
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1 2-D Array Wavelength Demultiplexing by Hybrid Waveguide and Free-Space Optics Trevor K. Chan, Maxim Abashin and Joseph E. Ford UCSD Jacobs School of Engineering Photonics Systems Integration Lab: PSI-Lab PHOTONIC SYSTEMS INTEGRATION LABORATORY UCSD JACOBS SCHOOL OF ENGINEERING Photo: Kevin Walsh, OLR 1

2 PHOTONIC SYSTEMS INTEGRATION LABORATORY UCSD JACOBS SCHOOL OF ENGINEERING 2 Motivation: Ultra-large channel count WDM Long Term Goal: channel λ-mux for access networks using Ultra-Dense WDM Direct Approach: Large 1xN Linear AWG or Free-Space Optical MUX 512 channel AWG: K. Takada et al. Phot Tech Lett, 13 (11), p1182, Challenges with single stage-approach Yield for large-area waveguides (70x80 mm for AWG above, despite folding) Connectorizing 1xN output fibers (125µm pitch: 8 fibers/mm) 64 channels: 8 mm 125 µm 1 micron alignment tolerance over full length N x mm Solution: Hierarchical demultiplexing into 2-D channel array 1024 channels: meter!

3 Hierarchical Multiplexing Alternatives Bandwise Broad, tight-spaced bands then DWDM Interleaved Interleaved WDM then CWDM Band DWDM Interleaved CWDM NTT demonstrated Bandwise Hierarchical DeMUX using many cascaded AWGs 1010 DeMUX (1x10 then 1x160 AWGs): K. Takada et al, Phot. Tech Lett. P577, June DeMUX: K. Takada et al., Electronics Lett, 38 (12), 572-3, Used 21 AWGs! Our approach: Combine secondary multiplexers into single free-space optics component PHOTONIC SYSTEMS INTEGRATION LABORATORY UCSD JACOBS SCHOOL OF ENGINEERING 3

4 PHOTONIC SYSTEMS INTEGRATION LABORATORY UCSD JACOBS SCHOOL OF ENGINEERING 4 Wraparound Property of AWG Multiple AWG orders from wideband illumination creates an interleaved output = m FSR λ m 1 1 c Gaussian profile among AWG output channels Characteristic AWG Output (measured) 0-5 Channel 1 Channels 2-8 Diffraction order m: Insertion L Insertion Loss (db) FSR = 7.7 nm (0.998 THz) Wavelength (nm) FSR = 8.5 nm (0.998 THz)

Array-parallel surface-normal optics Prior art free space mux Surface-Normal Devices: - Spectral equalizer LC or MEMS VOA -Add/drop MEMS tilt mirrors - Dispersion compensator

5 Array-parallel surface-normal optics Prior art free space mux Surface-Normal Devices: - Spectral equalizer LC or MEMS VOA -Add/drop MEMS tilt mirrors - Dispersion compensator MEMS mirror - Channel monitor OE power monitors - WDM Transceivers VCSELS MQW modulators OE Receivers PHOTONIC SYSTEMS INTEGRATION LABORATORY UCSD JACOBS SCHOOL OF ENGINEERING 5

6 PHOTONIC SYSTEMS INTEGRATION LABORATORY UCSD JACOBS SCHOOL OF ENGINEERING 6 Hybrid Waveguide and Free Space Demultiplexing Multi-order primary + Free Space Optics Single system for secondary demux Multi-order AWG + free-space grating Proposed 1999 but not demonstrated Dragone & Ford, US Patent #6,263,127 Published 2-D demux demo S. Xiao and A. M. Weiner, Optics Express 12 (13), p , 2004 Multi-order VIPA + free space grating (Virtual Image Phased Array) 41 Channels (~4x10), 17 db loss

Limits to U-DWDM channel count For extremely large channel count access networks: Potentially 2.5Gb/s per channel data at 5Ghz (0.04 nm) pitch Optimized AWG No dead space between FSR K. Takada et al.

7 Limits to U-DWDM channel count For extremely large channel count access networks: Potentially 2.5Gb/s per channel data at 5Ghz (0.04 nm) pitch Optimized AWG No dead space between FSR K. Takada et al. Phot Tech Lett, 13 (11), p1182, Coupling output from 2D slab area directly into free space optics D. M. Marom et al, IEEE Optical MEMS 2004 Fine channel pitch (1 GHz) K. Takada et al., IEEE J. Lightwave Tech., 20 (5), 850-3, Optimize free space grating demultiplexer Low dispersion grating Short focal length, wide field Fourier lens Achievable insertion loss AWG <5dB Free space grating ~ 2 db AWG Demultiplexer 288 Channel, 5 ghz Fourier-Transform Lens (f = 5 cm) Theoretical channel count Optimize grating/awg resolution 6,400 channels from nm 32,000 channels w/ 1GHz pitch Practical constraints AWG and grating bandwidth range 6.3 cm 3.6 cm 10 cm Blazed Grating 25 lines/mm PHOTONIC SYSTEMS INTEGRATION LABORATORY UCSD JACOBS SCHOOL OF ENGINEERING 7

8 Experimental Setup Concept Waveguide Grating Fourier-Transform Lens Initial Setup (published summary) Blazed Grating Polarization beamsplitter Fiber ribbon V-groove array Improved Setup (for lower insertion loss) ¼ wave plate Gold mirror IR Camera SM Fiber OSA PHOTONIC SYSTEMS INTEGRATION LABORATORY UCSD JACOBS SCHOOL OF ENGINEERING 8

9 Experimental system (cont) AWG Free Space Assembly InGaAs Camera Imaged 2D Array Grating 1x8 AWG 50 GHz channel pitch ~8 nm FSR 9 orders (189 th to 198 th ) Free-Space Optics 300 lp/mm Grating 100 mm lens focal length ASE spectral range = um PHOTONIC SYSTEMS INTEGRATION LABORATORY UCSD JACOBS SCHOOL OF ENGINEERING 9

10 PHOTONIC SYSTEMS INTEGRATION LABORATORY UCSD JACOBS SCHOOL OF ENGINEERING 10 Initial Experimental Results: 8x9 demux Spot Position Map (scanning SMF) Non-normalized, linear spot intensity profile Wavelengths of peak ASE power nm nm nm = m FSR λ m 1 1 c nm M-4 M-3 M-2 M-1 M M+1 Noise from M+2 PBS surface reflections M nm (multi-spectral) M+4 ASE Spectrum Spot positions matched calculated locations; wavelengths matched expected (chirped) values Intensity (uw) Wavelength (nm)

Initial Results: Spectral Measurements Superimposed Output Spectrum Representative Channel Passbands -10-15 -20 1531.94 nm 1578.94 nm 1595.68 nm -25 Insertion Loss (db) -30-35 -40-45 -50-55 -60-0.

11 Initial Results: Spectral Measurements Superimposed Output Spectrum Representative Channel Passbands nm nm nm -25 Insertion Loss (db) Spectral Detuning From Peak (nm) Insertion loss far too high: Problem was optical aberrations created by polarizing cube beamsplitter PHOTONIC SYSTEMS INTEGRATION LABORATORY UCSD JACOBS SCHOOL OF ENGINEERING 11

camera image of 8x9 array Normalized (& defocused) perspective plot

12 PHOTONIC SYSTEMS INTEGRATION LABORATORY UCSD JACOBS SCHOOL OF ENGINEERING 12 Updated Experimental Results: Spot Profile Raw IR camera image of 8x9 array Normalized (& defocused) perspective plot nm nm nm nm Insertion losses vary from db

13 Updated Experimental Results Representative Passbands Insertion Loss Breakdown Insertion Loss (db) nm nm nm nm Insertion Loss (db) 0-5 AWG output FSO output Spectral Detuning From Peak (nm) Wavelength (nm) Insertion loss 8 db 5.5 db loss from AWG FWHM = 0.71 nm 2.0 db loss from grating Crosstalk < 50 db 0.5 db loss from connectors Low PDL (note: ASE source) 3 db Uniformity Bandwidth narrowing not relevant to double-pass systems PHOTONIC SYSTEMS INTEGRATION LABORATORY UCSD JACOBS SCHOOL OF ENGINEERING 13

14 Conclusion Investigating 2D hierarchical spectral demultiplexing: Combining multiple-order primary w/ free space grating secondary Projected capacity up to 32,000 channels(!) Experimental demonstration 72 channels over 75 nm 8 db insertion loss 0.71 nm FWHM channel bandwidth Ultimate applications using surface-normal optical device arrays Wavelength add/drop switches Spectral equalizers WDM Transcievers Acknowledgements K. Okamoto of NTT: Provided bare 128 channel AWG for next expt stage C. Dragone of Bell Labs: For useful discussions PHOTONIC SYSTEMS INTEGRATION LABORATORY UCSD JACOBS SCHOOL OF ENGINEERING 14

Deep-etched fused silica grating as a (de)multiplexer for DWDM application at the wavelength of 1.55µm

Deep-etched fused silica grating as a (de)multiplexer for DWDM application at the wavelength of 1.55µm Yanyan Zhang*, Changhe Zhou, Huayi Ru, Shunquan Wang Shanghai Institute of Optics and Fine Mechanics,