Abstract
Stabilization of optical sources are becoming more important in high precision measurements. Whispering gallery mode resonators are an ideal candidate for a cavity to stabilize laser diodes. Imperfections in the resonator act as scattering centers and reflect resonant modes, which allows the self-injection locking of the laser diode.
The work presented here studies the effect of optical feedback on distributed feedback and Fabry-Perot lasers by looking at the linewidth and stability. The resonator was fabricated from a lithium niobate crystal, which the quality factor was measured at 6.92 × 107. Linewidth was measured with two different self-heterodyne methods, both using an acoustooptic modulator of 100 MHz to induce a frequency shift. The first regime was the traditional method where the unmodified beam is delayed by a significant path length. The second was a modified method which recycles a portion of the heterodyne through the delay and acousto-optic modulator. This effectively increases the delay length by creating integer multiples of the beat frequency. The linewidth of the distributed feedback laser was measured has decreased from 63.9 kHz to 10 kHz with the feedback.
Stability of the Fabry-Perot laser was measured by calculating the Allan deviation. The Fabry-Perot laser was mixed with a reference ECDL with some offset frequency and the beat signal from the photodector was analysed by an oscilloscope. The Allan deviation was caculated by performing a fast Fourier transform at different averaging times. The Allan deviation of the Fabry-Perot laser decreased from 6022 Hz to 4089 Hz at the averaging time of 0.1 ms. Furthermore, the linewidth was calculated at the same averaging time, which decreased from 37.3 kHz to 33.2 kHz.