  We demonstrate laser frequency stabilization using a high-Q MgF2 crystalline
whispering gallery mode resonator coupled with a tapered fiber. We discovered
that the tapered fiber, acting as a microcantilever, exhibits mechanical
resonance characteristics that is capable of transmitting acoustic
perturbations to the frequency locking loop. Both experimental and theoretical
investigations into the influence of external acoustic waves on the coupling
system were conducted. After acoustic isolation, the locked laser exhibits a
minimum frequency noise of 0.4Hz2/Hz at 7kHz and an integral linewidth of 68Hz
(0.1s integration time). Benefiting from the ultralow frequency noise of the
stabilized laser, it achieves a minimum noise equivalent acoustic signal level
of 4.76*10-4 Pa/Hz1/2. Our results not only facilitate the realization of
ultralow noise lasers but also serves as a novel and sensitive photoacoustic
detector.
