LIGO-G09xxxxx-v1 Form F 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 LIGO Laboratory1

LIGO-G09xxxxx-v1 Form F v1 BACKGROUND LIGO Laboratory2

LIGO-G09xxxxx-v1 Form F v1 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 LIGO Laboratory3

LIGO-G09xxxxx-v1 Form F v1 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 LIGO Laboratory4

LIGO-G09xxxxx-v1 Form F v1 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 LIGO Laboratory5

LIGO-G09xxxxx-v1 Form F v1 Diffraction Gratings LIGO Laboratory6

LIGO-G09xxxxx-v1 Form F v1 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! LIGO Laboratory7

LIGO-G09xxxxx-v1 Form F v1 Littrow vs. Littman-Metcalf Configurations Littrow has higher output power Littrow will be more straightforward to tune for correct wavelength LIGO Laboratory8 Figure courtesy of RP Photonics Online Encyclopedia

LIGO-G09xxxxx-v1 Form F v1 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 LIGO Laboratory9

LIGO-G09xxxxx-v1 Form F v1 MODELING NOISE REQUIREMENTS TO SELECT COMPONENTS LIGO Laboratory10

LIGO-G09xxxxx-v1 Form F v1 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 LIGO Laboratory11

LIGO-G09xxxxx-v1 Form F v1 Noise Requirements LIGO Laboratory12

LIGO-G09xxxxx-v1 Form F v1 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 LIGO Laboratory13

LIGO-G09xxxxx-v1 Form F v1 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 LIGO Laboratory14

LIGO-G09xxxxx-v1 Form F v1 Compare Diodes LIGO Laboratory15 Chose Thorlabs M9-A mW GaAs diode

LIGO-G09xxxxx-v1 Form F v1 Compare Diffraction Gratings LIGO Laboratory16 Chose Thorlabs GR /mm, 1 um blaze, 12.7 x 12.7 x 6 mm

LIGO-G09xxxxx-v1 Form F v1 Compare Cavity Lengths LIGO Laboratory17 Chose to have cavity length between 6-10 cm

LIGO-G09xxxxx-v1 Form F v1 Additional Noise Reduction LIGO Laboratory18

LIGO-G09xxxxx-v1 Form F v1 DESIGN OF MECHANICAL COMPONENTS LIGO Laboratory19

LIGO-G09xxxxx-v1 Form F v1 Collimating Lens LIGO Laboratory20 Beam profile at about 2 cm away from LD

LIGO-G09xxxxx-v1 Form F v1 LIGO Laboratory21 LD

LIGO-G09xxxxx-v1 Form F v1 LIGO Laboratory22 LD Collimating lens

LIGO-G09xxxxx-v1 Form F v1 LIGO Laboratory23 LD Collimating lens Diffraction grating

LIGO-G09xxxxx-v1 Form F v1 LIGO Laboratory24 LD Grating mount Collimating lens Diffraction grating Fine adjustment screw

LIGO-G09xxxxx-v1 Form F v1 LIGO Laboratory25 LD Grating mount Collimating lens Diffraction grating Fine adjustment screw Window mount

LIGO-G09xxxxx-v1 Form F v1 Control Methods LIGO Laboratory26 TEC Heat sink

LIGO-G09xxxxx-v1 Form F v1 Control Methods LIGO Laboratory27 TEC Low-noise current driver Heat sink

LIGO-G09xxxxx-v1 Form F v1 Control Methods LIGO Laboratory28 TEC Low-noise current driver PZT Heat sink

LIGO-G09xxxxx-v1 Form F v1 ASSEMBLY LIGO Laboratory29

LIGO-G09xxxxx-v1 Form F v1 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) LIGO Laboratory30

LIGO-G09xxxxx-v1 Form F v1 NOISE MEASUREMENTS LIGO Laboratory31

LIGO-G09xxxxx-v1 Form F v1 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 LIGO Laboratory32

LIGO-G09xxxxx-v1 Form F v1 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: LIGO Laboratory33 V LDLD slope = β

LIGO-G09xxxxx-v1 Form F v1 Free Running Noise LIGO Laboratory34

LIGO-G09xxxxx-v1 Form F v1 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 LIGO Laboratory35

LIGO-G09xxxxx-v1 Form F v1 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 LIGO Laboratory36