Coherent Beam Combining of 21 Semiconductor Gain Elements in a - - PowerPoint PPT Presentation

Coherent Beam Combining of 21 Semiconductor Gain Elements in a Common Cavity*

SSDLTR 2012

12-SSDLTR-029

Wenqian Huang (Ronny), Juan Montoya, Steven Augst, Kevin Creedon, Jan Kansky, T.Y. Fan, Antonio Sanchez-Rubio MIT Lincoln Laboratory

June 12, 2012

*This work is sponsored by High Energy Laser Joint Technology Office under Air Force Contract FA8721-05-C-0002. Opinions, interpretations, conclusions, and recommendations are those of the author and are not necessarily endorsed by the United States Government.

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• Number of elements (3 - 6)
• Combining efficiency (70% - 80%)
• Power ~10 mW per element

Overview

• Array of gain elements inside a common optical cavity – an
• ld concept for scaling with diffraction limited beam quality
• Scalability in earlier proof-of principle demos was hampered

by the need to maintain phase across the array In this work: Active control of the phase allows for scaling to 21 elements with excellent beam quality.

• J. R. Leger, G. J. Swanson, W. Veldkamp,

“Coherent laser addition using binary phase gratings,” Appl. Opt. 26, 4391 (1987)

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Master Oscillator Power Amplifier (MOPA) Configuration

• MOPA configuration requires multiple amplification stages with isolators

and mode matching optics, but has been a successful platform for coherent beam combining.

MITLL has demonstrated coherent beam combining (CBC) of 218 semiconductor amplifier elements.

• S. Redmond et al, Active Coherent beam combining of diode lasers,

Optics Letters, Vol. 36, No. 6, 2011

Amplifier Array Beam Sampler DOE Output Beam Transform Lens Losses Phase controller Detector Seed source 1xn Splitter Isolator, Mode-matching, Pre-amplifier

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Power-Oscillator Configuration

Laser Array DOE Output Beam Transform Lens Losses Output Coupler

A power-oscillator is simpler and more compact than a MOPA implementation

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Number Scaling of Passive CBC Cavities

• Efficient CBC in passive cavities (no phase control of

individual elements) does not scale well above ~ 8 elements – Arbitrary arm lengths cause random phase relationships

BC efficiency for Random Phasing Experiment ●○ Theory ̶ ̶

Kouznetsov, et. al,

• Opt. Rev. 12, 445 (2005)

Scaling to higher number of elements can be achieved using active phasing

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Active Phase-Control Power Oscillator

Laser Array DOE Output Beam Beam Sampler Transform Lens Losses Output Coupler Detector Phase controller

Active phase-control allows for scaling beyond passive limits

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• 1.5
• 1.4
• 1.3
• 1.2
• 1.1

1 2 3 4 5 6 Time(s) On-Axis Intensity (arb. u.)

Stochastic Parallel Gradient Descent (SPGD) Phase-Control Algorithm

• SPGD has enabled multiple CBC

demonstrations at MIT LL

• SPGD is a hill climbing algorithm
• Does not require a reference beam or

phase knowledge

• Optimizes zero order output of DOE

Amplifier Array Beam Sampler DOE Output Beam Transform Lens Losses Phase controller Detector Seed source

dither channels ct

f N t 4 »

1xn Splitter

SPGD Convergence

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Diode Arrays with Individually Addressable Elements

• Stackable high density arrays were developed to demonstrate

coherent combination of semiconductor amplifiers:

– 21 individually addressable gain elements – 200-µm spacing – Precise position tolerances – Back facet HR-coated and front facet AR-coated

• Each array is collimated with a spherical microlens array to

increase the fill factor

Flex print cable Array on cooler Connector Coolant lines Low profile flex print cable Single bar cooler (SBC) SCOWL array CuW bus AlN subcarrier Flex cable traces SCOWL – Slab Coupled Optical Waveguide Laser

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21 Diode Array Combining Efficiency Measurement and Diagnostics

• Cavity output ports allow for efficiency measurement (ƞ=P0/PT)
• Spatial filter (slit) prevents feedback from higher DOE orders
• Intracavity diagnostics include:

– Near-Field Spectrometer, Far-Field Camera, Power monitors

Cavity designed with diagnostics for proof-of-principle

1% Beam Sampler Near Field Spectrometer Grating 50/50 21 Element Laser Array Slit Slit SPGD Detector f=500mm

f1 f2

1x21 DOE Power Monitor f=500mm f1=300mm

f2=75mm

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SPGD Control Loop Phase-Locks 21-Element Array

Random phase – no active control* With active phase control*

*Color scales normalized to show peaks, horizontal cross sections (line profiles) are not normalized

Far-field images

• Random-phase combining efficiency is ~ 5%, consistent with

incoherent beam combining of 21 beams

Combining efficiency = 5% Combining efficiency = 81%

Active phase-control enables scaling to large number of elements

x position (mm) x position (mm)

Combining efficiency = 5%

Intensity (a.u.) Intensity (a.u.) 0.9mm 7mm 0.9mm

Combining efficiency = 81%

7mm

• 3
• 2
• 1

1 2 3

• 3
• 2
• 1

1 2 3 50 100 150 200 10 20 30

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Near-Field Spectrometer Results

Random Phase Active Phase Control

• Near field spectrometer illustrates that all elements operate at the

same wavelength when SPGD is activated

SPGD adjusts optical path lengths (phase) of each emitter to coherently combine the beams

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Combined Power/ Efficiency

Loss Mechanism Efficiency Penalty (%) Cumulative Max Efficiency (%) DOE splitting efficiency 10 90 Pointing Error 3 87 SPGD Dither 1 86 Aberrations 1 85 Amplitude Variations 4 81

21-Element Combined Output Power P0 = 2.5 W, M2=1.11

Efficiency Estimates

Achieved record combining efficiencies of 81% for 21 semiconductor elements. Cavity not optimized to produce high power.

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SPGD Convergence and Long-Term Stability

Long-Term Stability with SPGD Frozen at Converged Values SPGD Convergence

1.4 1.2 1.0 0.8 0.6 0.4 0.2 0 0 20 40 60 80 time (minutes) 8

• 40

6 4 2

• 20

20 40 SPGD Signal (rel. units) SPGD Signal (rel. units) time (ms)

• Experimentally observed convergence time ~ 4 ms
• Once CBC is established and phases are held fixed at optimum values, the

active phase control may be turned off and efficiency is self-sustaining

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Summary

• Power oscillators are more compact than MOPA lasers
• Diode and bulk solid-state lasers are well-suited to power
• scillator configurations

– We have successfully demonstrated 21 diode element CBC in a power oscillator with an 81% combining efficiency Acknowledgements:

• George Turner, Leo Missaggia for diode arrays
• Shawn Redmond for SPGD discussions
• SCOWL array and DOE development funded by DARPA MTO