Differences between revisions 31 and 32

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

End laser fiber setup

Find out the fiber mode
Designate space for the setup at end tables
Design telescopes for coupling light into the fiber
Layout the fiber along 40m arms in insulated tubes

PSL fiber setup

Designate space on PSL table
Find the fiber beam splitter(FBS) mode
Design a telescope to couple light into the FBS mode

Beat note setup

Couple fiber to fiber
Combine PSL and end laser with appropriate polarization (50/50 --> 90/10)
Detecting beatnote at the PD

Frequency counter setup

Hook up frequency counter
Read beat frequency on StripTool

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	As of
70m PM fiber	4/4	Corning PM98-U25A	Procured	Oct 7
6 axis fiber mount	6/4	Thorlabs K6XS	?	Oct 7
Fiber mounting adapters	6/4	Thorlabs SM1FCA	Need to rder more	Oct 7
Fiber collimators	6/3	Thorlabs CFC-2X-C	Need to order	Oct 7
HR1064 mirrors	?/plenty	CVI Y1 mirrors	Procured	Oct 7
Lenses for telescopes	?/1064nm lens kit available	?	?	Oct 7
Fiber insulating tubes	2/2	-	Procured	Oct 7
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	Oct 7
PM fiber mating sleeves (FC/APC)	?/0	?	?	Oct 7
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	Oct 7
Polarization controller	?/1	Acrobat PCM4102-333	Procured	Oct 7
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	Oct 7
Fiber coupled PD	2/1	Thorlabs FPD310	Procured	Oct7
Frequency counter	2/2	Minicircuits UFC6000	Procured	Oct 7

-  ⇤ ← Revision 31 as of 2014-10-08 23:10:23 → 
  Size: 3415
  Editor: ManasadevithirugnanasambandamATligoDOTorg
  Comment:
+   ← Revision 32 as of 2014-10-09 19:52:54 → ⇥
  Size: 3415
  Editor: ManasadevithirugnanasambandamATligoDOTorg
  Comment:
-Deletions are marked like this.
+Additions are marked like this.
 Line 51:
-||Fiber mounting adapters||6/2||Thorlabs SM1FCA||Need to rder more||Oct 7||
+||Fiber mounting adapters||6/4||Thorlabs SM1FCA||Need to rder more||Oct 7||