Stabilization of an optical frequency comb to an external cavity

Abstract: The subject of this master's thesis is stabilizing a frequency comb laser to an external cavity using a couple of servo controllers. The aim of the project was to build a pair of servo controllers, replacing parts of the existing commercial and proprietary solution already in use. The system under control is an optical frequency comb, which is locked to an external cavity and is used for trace gas detection and spectroscopy. The comb is a broadband light source and needs to be locked to the external cavity in order to achieve maximum transmission through the cavity. The goal was to replace two of the original controllers and try to improve the locking capabilities of the system. The controllers were also supposed to be customizable and for that reason the control system with all its components was built on breadboards and confined in an aluminium box. Control circuits were built for the purpose, one for controlling the comb offset frequency by modulating the pump diode current, the other for controlling the repetition rate of the comb laser by altering the length of the laser cavity using a piezo-electric transducer (PZT). A commercial and proprietary servo controller was also in the system, controlling an intra-cavity electro-optic modulator. It was kept for controlling the higher frequency region, for which the PZT no longer worked. In order to simulate and design the system, Matlab was used with functions described by both theoretically and experimentally obtained mathematical equations. The controllers were tested thoroughly in order to make sure they acted according to the intended design, before they were tested with the laser. After an initial lock was obtained, the controllers were optimized further using both experimental and theoretical methods until the lock was optimized and the transmission through the cavity was maximized. The error signals that were used for controlling the system were monitored with both an oscilloscope and a spectrum analyser, the latter producing a spectrum with the power ratio plotted versus frequency. The transmission intensity through the cavity was measured when a good lock had been achieved and the results were analysed by applying a Fourier transform to the measured data. This was done with both the old controllers and the new controllers and the resulting plots were compared. Analysis showed that the new control system yielded a transmission signal with a slightly reduced noise level compared to the signal resulting from using the old controllers. The results from the spectrum analyser also showed slightly reduced error signals for the new controllers compared to those of the old controllers. When summarising this work it can be concluded that the goals set up at the start were achieved with results living up to the expectations. The results also verified that such a control system can be built for locking an optical frequency comb to an external cavity with simple and rather cheap components and with good results.