11/1/2007 PHOTONIC SYSTEMS INTEGRATION LABORATORY – UCSD JACOBS SCHOOL OF ENGINEERING

UCSD Photonics

1Photo: Kevin Walsh, OLR

Photonics Systems Integration Lab

University of California San DiegoJacobs School of Engineering

Photonics Systems Integration Lab

University of California San DiegoJacobs School of Engineering

Fast, Two-Dimensional Optical Beamscanning by Wavelength Switching

Fast, Two-Dimensional Optical Beamscanning by Wavelength Switching

T. K. Chan, E. Myslivets, J. E. FordT. K. Chan, E. Myslivets, J. E. Ford

11/1/2007 PHOTONIC SYSTEMS INTEGRATION LABORATORY – UCSD JACOBS SCHOOL OF ENGINEERING

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Flat mirrors

Deformable MEMS mirror

• Free Space Optical Communications:

• Dynamic connections: platform and environment– Require fast, active alignment and tracking

• Retro-reflecting modulators Single sided alignmentMEMS (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

Introduction

11/1/2007 PHOTONIC SYSTEMS INTEGRATION LABORATORY – UCSD JACOBS SCHOOL OF ENGINEERING

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Existing Scanning Technologies

Bulk, power, reliability~10mm~ 30°KHzGalvanometric

Drive current, angle range~10mm~ 1°MHzElectro-optic

Speed, environmental constraints>100mm~ 60°100 HzLiquid Crystals

Angle range~10mm~ 1°KHz Acousto-optic

Aperture, power handling~1mm~ 5°KHzMEMS mirror

Key limitationAperture RangeSpeed

Aperture Accuracy

Field of View

Speedtradeoff

Question: How to decouple fast response from other performance parameters?

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Diffract wavelength to angle: Decouples aperture from speed

Wavelength ScanningFast λ-tuning Laser source

Fixed collimator and diffraction grating

Vertical angleRandom-access scan

δθx

Far-field distribution

Θy

δθy

H=kλ

How fast?• Grating-assisted codirectionalcoupler 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?

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Concept: 2D Wavelength Scanning

- 2 5

- 2 0

- 1 5

- 1 0

- 5

0

1 5 2 0 1 5 3 0 1 5 4 0 1 5 5 0 1 5 6 0 1 5 7 0 1 5 8 0 1 5 9 0 1 6 0 0

W a v e le n g t h ( n m )

Inse

rtio

n Lo

ss (d

B)

FSR = 7.7 nm(0.998 THz)

Diffraction order m:198 197 196 195 194 193 192 191 190 189

Channel 1

Channels 2 - 8

FSR = 8.5 nm(0.998 THz)

Wavelength (nm)

High-order gratingArrayed waveguide grating (AWG)VIPA free space echelon grating

Low-order gratingBlazed reflection gratingHolographic transmission grating

λ1,1

λ1,2

λ2,3

λ2,4

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2D Integrated Optics Demux

• Hybrid wavelength demultiplexerT. 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 Gratingline spacing = d

AWG Demultiplexer

Demultiplexed plane(optoelectronic / MEMS device)

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2-D Single mode fiber demux

• 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

Grating

Lens

40 AWG

Outputs

1x40 input array

Outputfiber

1092 channels (39 x 28 grid)

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2D Beamscanner

JDSU 1x8AWGTunable Source

Grating3rd order

300 lp/mm

Mirror

8x Microscope Objectivef = 25 mm

Lensf = 100 mm

V-groove array635 um pitch

Source Options:• Tunable Laser• Broadband noise source

+ Tunable Filter

Focal length determines angular range

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

NA determines aperture

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2D Beamscanning Demonstration

Tunable laser1535 – 1590 nm

sweep

AWG V-Groove fiber array

Microscopeobjective Free-space

reflection grating

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2D Beamscanner Demonstration

Angular Output (degrees)

Ang

ular

Out

put (

degr

ees)

-8

-4

0

4

8

-8 -4 0 4 8

11.0 °

10.3 °

1545.0 nm 1586.4 nm

1547.0 nm 1588.3 nm

Calculated Directions

C-Band ASE illumination

Gaussian Output Beam ProfileCoherent illumination

1/e2 diameter6 mm

Numerical aperture = 0.12 Lens focal length = 25 mm

1/e2 diameter = 6 mm

For a telephoto lens Lens focal length = 100 mm1/e2 diameter = 24 mm

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Fast tuning

“Microelectromechanical tuneable filters with 0.47 nm linewidth and 70 nm tuning range,” Tayebati, et al, Electronics Letters 34(1) 1998.

• 80 nm span tunable etalon filter• ~100 µs sweep times• Channel bandwidth = 0.47nm res

Coretek/Nortel MEMS Tunable Filter

JDSU 1x8AWG

Grating3rd order

300 lp/mm

Mirror

8x Microscope Objectivef = 25 mm

Lensf = 100 mm

V-groove array635 um pitch

OpticalAmplifier

CoreTek TunableFilter

ASE Source

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Fast Sweeping w/ Tunable Filter

• AWG channel pitch = 50 GHz

• Narrow bandwidth source is desired.

• higher dispersive device more diffraction orders over the same bandwidth!

1546.7 nm

1578.2 nm183 µs

switching time

-20

-15

-10

-5

0

1531 1532 1533 1534

Wavelength (nm)

Inse

rtio

n Lo

ss (d

B) Filter Passband

AWG channels

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Virtually Imaged Phased Array: VIPA

r = 95%

VIPA echelle grating conceptM. Shirasaki, Fujitsu Sci. Tech. J., 35(1), 1999.

• 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%

2D Dispersion using a VIPAS. Xiao and A. M. Weiner, Optics Express 12 (13), p.2895-2902, 2004Multi-order VIPA + free space grating 41 Channels (~4x10)

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Future directions: Planar integration

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

VIPA

High-resolution 2-D scanning possible

Grating

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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