A narrow linewidth laser is a necessary element in high resolution spectroscopy. Especially, a probe laser of nearly 10 kHz linewidth is needed in our future experiments for measuring the emission linewidth of a single-atom laser, where the linewidth is expected to be several tens of kHz. In our previous study, a tunable diode laser(Vortex from New focus) of 780.24 nm wavelength was frequency-stabilized by a servo circuit on a reference cavity made of BK-7 material. However, the resulting 100 kHz linewidth did not meet our requirement.
In the present study, we introduced an ultra low expansion(ULE) cavity as a frequency reference. As a result, the laser linewidth was reduced from its original linewidth of 540 kHz down to a narrow linewidth of 70 kHz for 10 s sampling time.
The frequency instability of the laser was reduced from 150 kHz to 2 kHz of root Allan variance for the sampling time longer than 100 ms. An acousto optic modulator(AOM) was used to reduce the remaining noise from the frequency stabilization using the ULE reference cavity. The resulting laser frequency was analyzed by another Fabry-Perot cavity made of BK-7 material. The result of the AOM correction was a linewidth of 20 kHz and the root Allan variance less than 4 kHz. We then further improved the servo circuit by introducing an integrator with 0.1 s integration time, an amplifier with a bandwidth of DC-40 kHz and a differential amplifier with 50 kHz cutoff frequency. As a result, we obtained a linewidth of 13 kHz and the root Allan variance less than 500 Hz. Throughout the experiments, the sampling time for the root Allan variance was fixed at 100 ms and the two cavities were placed in a vacuum chamber.
