Frequency Offset Locking (FOL) for Dual Wavelength Laser Stabilisation

Overview

In order for the Arm Length Stabilization (ALS) system to be effective in its purpose, the beat frequency between the AUX lasers and the PSL must be within the efficient working range of ALS (< 50 MHz). Thus, our purpose in Frequency Offset Locking (FOL) is to design a feedback-control loop that will keep this beat frequency well within the working range of ALS, so that manual tuning of auxiliary (AUX) laser frequencies may be avoided.

Frequency offset locking works by sampling light from each laser source, PSL and AUX laser, and combining them to produce a beat note which corresponds to the difference between the frequencies of either laser.

The combined PSL and AUX light is sensed at an RF photodiode and an RF frequency counter is used to measure the beat frequency. A digital PID control loop compares the detected beat frequency with the desired beat frequency to produce an error signal. The error signal is then converted back to an analog signal, to actuate on the temperature of the crystal in the AUX laser to keep its frequency within the desired range.

Schematic

A schematic of the setup for FOL is shown below:

Project Tasks

Tasks	Who	Status	Last updated
General tasks
Make procurements list and purchase stuff	MT	Done	Oct 9
Find out the fiber mode with collimator	MT	Waist of output beam = 210um	Oct 16
Y End laser fiber setup
Waist of beam from doubling crystal	MT	Elog 10287 --> Waist = 40.2um	Oct 9
Designate space for the setup at end tables	MT	~30cm of propagation distance available	Oct 13
Design telescopes for coupling light into the fiber	MT	converging lens f=12cm	Oct 13
Assemble end telescope and couple light into fiber	MT	Status - elog 10638	Oct 24
Layout the fiber along 40m arms in insulated tubes	MT+help	Laid out	Nov 5
PSL fiber setup
Waist of beam from doubling crystal	MT	Elog 10287 --> Waist = 43.5um	Oct 23
Designate space on PSL table	MT	Lot of space available	Oct 23
Find out the fiber mode with collimator	MT	Waist of output beam = 176um	Oct 23
Design telescopes for coupling light into the fiber	MT	f_1=75mm@z=10.4cm+f_2=-1m@z=32.3cm with target @z=40cm	Oct 23
Assemble end telescope and couple light into the fiber	MT	Temporary telescope in place	Nov 5
X End laser fiber setup
Waist of beam from doubling crystal	MT
Designate space for the setup at end tables	MT
Design telescopes for coupling light into the fiber	MT
Assemble end telescope and couple light into fiber	MT
Layout the fiber along 40m arms in insulated tubes	MT+help	Laid out	Nov 21
Beat note setup
Make fiber chassis layout	MT	Done	Nov 21
Setup fibers in the fiber chassis	MT
Couple fiber to fiber	MT
Combine PSL and end laser with appropriate polarization (50/50 --> 90/10)	MT
Detecting beatnotes at the PD	MT
Frequency counter setup
Hook up frequency counter	MT
Check FC acquisition scripts	MT
Read beat frequency on StripTool	MT
Mount FC module on IOO rack	MT+help
End laser thermal actuator characterization
Digital servo
MISC tasks
Behavior of PM fiber to temperature fluctuations
Polarization controller characterization

Procurements list

Component	Quantity needed/procured	Part number	Order status
70m PM fiber	4+1/4+1	Corning PM98-U25A	Procured
6 axis fiber mount	4/4	Thorlabs K6XS	Procured
Fiber insulating tubes	1/1	-	Procured
Polarization controller	1/1	Acrobat PCM4102-333	Procured
Fiber coupled PD	2/2	Thorlabs Menlo FPD310	Procured
Frequency counter	4/2	Minicircuits UFC6000	Procured
OPTICS & OPTOMECHANICS
Fiber mounting adapters	4/4	Thorlabs SM1FCA	Procured
Fiber collimators	6/6	Thorlabs CFC-2X-C	Procured
Fiber collimator mounts	6/6	Thorlabs AD9.5F	Procured
HR1064 mirrors	used mirrors in stock	CVI Y1 mirrors	Procured
Lenses for telescopes	used lenses in stock	?	Procured
Fiber BS for splitting PSL light between the arms(PM = PM+PM in 50-50)	1/1	Afw Tech POBC-64--C-(s)PM-7-2-25 dB	Procured
PM fiber mating sleeves PM FC/APC & (FC/APC)	6,4/0	Thorlabs ADAFCPMB2&ADAFCB3	Procured
PM and SM fiber patch cables	multiple lengths	Thorlabs	Procured
Fiber BC for beat note between arm and PSL (PM+PM=SM)	2/2	Afw tech POBC-64-C-1-7-2-25 dB	Procured
Fiber BS for splitting combined light between PD and pick off (SM 10-90)	2/2	Afw Tech FOSC-1-64-10-C-(s)3m-H64-2	Procured
Fiber chassis	1/1	Front Panel Express	Procured

Notes

The transimpedance gain for the Menlo FPD 310 is hard to find. After a few emails with the company's technical team, the information we have now is that it has 50ohm RF transimpedance, and an RF amplifier with a gain of +42 dB. Assuming an InGaAs sensitivity of 0.65A/W @ 1064nm, this means that we get 4092 V/W at the nominal gain setting. If the attenuation setting on the PD is set to 20dB, the number becomes 409 V/W. This PD is actually obsolete as per the Thorlabs catalog. The manual FPD310-Manual.pdf I found from the Thorlabs website is in the Attachments section of this page. The more modern version of the manual,MENLO_FPD310-D-EN_2015-02_3w.pdf , is also attached. According to it, the maximum output swing of the PD is 1Vpp which corresponds to a signal level of ~4dBm. What this means for the optical power incident on the PD is that the optical beat amplitude should be <122 uW in the high gain setting, and 1.2 mW in the lower gain setting.