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:

Tasks

Step 1

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

Step 2

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

Step 3

Beat note setup

• Couple fiber to fiber

• Combine PSL and end laser with appropriate polarization (50/50 --> 90/10)

• Detecting beatnote at the PD

Step 4

Frequency counter setup

• Hook up frequency counter

• Read beat frequency on StripTool

Step 5

End laser thermal actuator characterization

Step 6

Digital servo

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Other links

\Frequency Counter Characterization