ligo-g09xxxxx-v1 form f0900043-v1 development of a low noise external cavity diode laser in the...
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LIGO-G09xxxxx-v1
Form F0900043-v1
Development of a Low Noise External Cavity Diode Laser in the
Littrow Configuration
Chloe Ling
LIGO SURF 2013
Mentors: Rana Adhikari and Tara Chalermsongsak
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BACKGROUND
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Semiconductor Basics
Can increase conductivity of semiconductor materials by doping
Doping = adding impurities which create extra mobile electrons or moving “holes” of positive charge» N-type: more electrons than holes
» P-type: more holes than electrons
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Laser Diodes (LDs)
Semiconductor diodes formed by putting p-type and n-type semiconductors next to each other
Laser diodes work by reverse biasing the material
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Laser Diodes (LDs)
When electrons and holes recombine, emit light (spontaneous emission)
Photons can cause other annihilations, cascading effect (stimulated emission)
Front/back of diode chip form optical cavity for light to resonate
This method has high noise and large linewidth → poor choice for high precision optics
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Diffraction Gratings
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External Cavity Diode Lasers (ECDLs)
Lock LD to external cavity formed by a diffraction grating at appropriate angle
Generates optical feedback Reduces noise levels significantly!
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Littrow vs. Littman-Metcalf Configurations
Littrow has higher output power Littrow will be more straightforward to tune for correct
wavelength
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Figure courtesy of RP Photonics Online Encyclopedia
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Goals of Project
Determine noise requirements for various LIGO experiments, select experiment ECDL can be used for
Design 1064 nm ECDL to meet requirements Build/assemble ECDL Test ECDL to see if these requirements are met
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MODELING NOISE REQUIREMENTS TO SELECT COMPONENTS
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Estimate Noise of Bare LD
Current noise: fluctuations in injection current will affect output frequency
– Libbrecht and Hall (1993) current driver
Temperature noise: temperature of LD changes output frequency
– Intrinsic noise due to temperature fluctuations
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Noise Requirements
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Noise Suppression of LD with External Cavity
Estimate parameter X, which indicates how noise level is suppressed (Saito, et al.)
Estimate noise suppression after the external cavity
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Effect of Different Components on Noise Suppression
Laser diode » Choice of lasing material
» Length of lasing material cavity
Diffraction grating» Efficiency of effective reflectivity of grating
Length of external cavity
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Compare Diodes
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Chose Thorlabs M9-A64-0200 200 mW GaAs diode
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Compare Diffraction Gratings
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Chose Thorlabs GR13-1210 1200/mm, 1 um blaze, 12.7 x 12.7 x 6 mm
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Compare Cavity Lengths
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Chose to have cavity length between 6-10 cm
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Additional Noise Reduction
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DESIGN OF MECHANICAL COMPONENTS
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Collimating Lens
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Beam profile at about 2 cm away from LD
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LD
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LD
Collimating lens
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LD
Collimating lens
Diffraction grating
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LD
Grating mount
Collimating lens
Diffraction grating Fine adjustment
screw
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LD
Grating mount
Collimating lens
Diffraction grating Fine adjustment
screw
Window mount
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Control Methods
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TEC
Heat sink
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Control Methods
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TEC Low-noise current driver
Heat sink
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Control Methods
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TEC Low-noise current driver
PZTHeat sink
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ASSEMBLY
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Assembly Progress
Working so far… TEC is wired up and the
PID gain has been tuned to reach correct temperature quickly
Current driver is wired to laser diode socket, can detect the output beam
Bare LD noise measurements taken
Still waiting on… Some ordered parts have
not yet arrived (collimating lens, PZT, window)
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NOISE MEASUREMENTS
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Free Running Noise
Michelson interferometer with different arm lengths
Measure output voltage of photodiode with spectrum analyzer
Convert noise on output voltage to noise in frequency
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Free Running Noise
Output voltage will be sinusoidal with differential arm length
Will want to measure at a differential arm length with greatest signal response (greatest slope β)
Convert from voltage noise to frequency noise:
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V
LD
slope = β
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Free Running Noise
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Future Work…
Continue with assembly Get entire ECDL setup working Examine the effects of changing different parameters
on final noise levels Improve upon design (current driver, etc.) to reduce
noise levels further
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Acknowledgements
Rana Adhikari and Tara Chalermsongsak for funding me, working with me, and offering help whenever needed
SFP/LIGO office for funding for this project
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