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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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