Laser-frequency locking to a whispering-gallery-mode cavity by spatial interference of scattered light

Abstract

We present a simple and effective method for frequency locking a laser source to a free-space-coupled whispering-gallery-mode cavity. The scheme relies on the interference of spatial modes contained in the light scattered by the cavity, where low- and high-order modes are simultaneously excited. A dispersion-shaped signal proportional to the imaginary component of the resonant optical field is simply generated by spatial filtering of the scattered light. Locking of a diode laser to the equatorial modes of a liquid droplet resonator is demonstrated using this scheme, and its performance is compared to the Pound-Drever-Hall technique. This new approach makes laser-frequency locking straightforward and shows a number of advantages, including robustness, low cost, and no need for sophisticated optical and electronic components.

Fundamental limits in high-Q droplet microresonators

“…However, no correlated change was observed in the photon lifetime. Next, we have frequency-locked the laser on one of the narrowest resonances using an all-optical feedback loop 13 and we have calculated the fast-Fourier transform (FFT) of the transmission and scattering signals. From the FFT noise spectra only an overall increase in the amplitude noise is found on resonance without any noticeable spectral feature associated with high-frequency vibrations or surface waves.…”

“…For this purpose, a servo amplifier is used as a feedback element to control the laser wavelength. A suitable frequency discriminator to drive the servo input can easily be obtained from the far-field spatial mode interference between scattered WGMs without resorting to conventional radio-frequency modulation schemes 13 . To observe the cavity ringing signals ( Fig.…”

“…On the other hand, droplets are extremely appealing as opto-mechanical oscillators as they are among the simplest optofluidic devices and are made of materials 10 6 times softer than glasses, which increase the amplitude of Brownian fluctuations and capillary waves 8 9 . Excitation of high-quality WGMs in droplets have been recently demonstrated and advanced techniques from the realm of standard cavities have been extended to liquids for lasing, spectroscopy, sensing and manipulation 10 11 12 13 14 15 16 . Analogously to solid microcavities, the quality factor is the primary figure of merit, which carries information on the photon lifetime within the system.…”

Liquid Droplet Microresonators

“…Actually, it has also been shown that a suitable frequency discriminator for laser-to-microcavity locking can be derived from the interference of multiple cavity modes or, in the free-space excited droplets, from the scattered WGMs [41]. Thus, the light reflected by the cavity contains the interference pattern among wavefronts of various modes (see inset of Figure 3).…”

“…By spatial selection of the cavity scattered light, the interference term between a narrow WGM and a secondary mode is recovered, the latter acting as a phase reference. This selection can be carried out by a diaphragm, located just in front of the photodetector PD2, whose position must be carefully adjusted until an odd-symmetrical profile is obtained [41]. The resulting signal exhibits a dispersive lineshape with a symmetry point located at the main resonance center and an amplitude proportional to the imaginary component of the resonant optical field.…”