FREQUENCY OFFSET LOCKING

Overview

In order for Absolute Length Measurement to be effective, the differential frequency (beat frequency) between prestabilized laser (PSL) and Auxiliary Laser (AUX) must remain constant.

However, over time, the AUX frequency tends to wander due to things like temperature change. Thus, we employ our frequency offset locking (FOL) system.

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

This light is then fed into a photodiode, the signal from which is digitized, and sent into a digital PID control loop. The PID Controller uses this signal as the current state of the system, which is further analyzed to produce an error signal.

The error is then converted back to an analog signal, which actuates upon the frequency of the AUX laser, keeping it within the desired range.

The actuator is a temperature control, which controls the dimensions of the crystal resonator within the NPRO via thermal expansion.

Through this scheme, we hope to effectively achieve a constant frequency offset between the AUX and PSL.

Optics -- Project Plan

\Fiber characterization

\Fiber coupling

\Beat Note Setup