The scope of this project is to demonstrate a highly stable laser feedback control system used to lock the frequency of a laser. This is accomplished by using a thermal-stabilized reference interferometer in conjunction with a high-speed servo controller. The problem our system addresses is the short-term wavelength stability issue found in narrow-linewidth lasers.
The interferometer is made up of two fiber optic paths of differing lengths immersed in a thermally stabilized ice-water solution. The outputs of the interferometer are converted to electrical signal and detected using a balanced homodyne photo-detector. The photo-detected output depends sinusoidally on the frequency of the laser, allowing the laser frequency to be measured with an uncertainty below tens of kiloHertz as opposed to being inferred by the scan voltage, which is how laser wavelength is normally ascertained (with an uncertainty of tens of MegaHertz). The adoption of the balanced detector cancels the laser intensity noise that may otherwise appear as noises to the detected signal. This ultra-accurate wavelength measurement is used in a feedback control (servo) implemented to actively lock the laser to a specific wavelength. This locked state is a small window of stability which is easily lost due to any large system disturbance. To improve the robustness of the system and allow the locking state to last longer, a Labview program was written to monitor the output of the servo controller and make small adjustments to the laser piezoelectric transducer.