Anatoliy A. Savchenkov

Tunability and synthetic lineshapes in high-Q optical whispering gallery modes

We demonstrate novel techniques to manipulate spectral properties of high quality factor whispering-gallery modes (WGM) in optical dielectric microresonators. These include permanent frequency trimming of WGM frequencies by means of UV photosensitivity of germanium doped silica resonators; electro-optical tuning of WGM in lithium niobate resonators, and cascading of microresonators for obtaining second-order filtering function. We present theoretical interpretation of experimental results, and examples of applications of these techniques for photonic microwave filtering.

Tunable filters and time delays with coupled whispering gallery mode resonators

We demonstrate tunable optical filters fabricated from metalized lithium niobate disk resonators and discuss further applications of the resonator chains for tunable photonic delay lines.

High-efficiency microwave and millimeter-wave electro-optical modulation with whispering-gallery resonators

We present electro-optic modulator based on toroidal lithium niobate cavity with whispering-gallery modes, superimposed with stripline resonator. With microwave resonance (quality-factor Q ~ 102) tuned to the free spectral range of optical modes (Q ~ 5x106), controlling power ú10mW is achieved in 9GHz prototype, and preliminary results with 33GHz prototype are obtained. Further efficiency improvement will enable various applications in microwave photonics.

Compact tunable kHz-linewidth semiconductor laser stabilized with a whispering-gallery mode microresonator

We report on the realization of a compact-package (44x27x14mm) narrow linewidth laser based on self-injection locking of a distributed feedback semiconductor diode laser to a high-Q whispering gallery mode resonator fabricated with electro-optic material. The packaged device operates at 1,550 nm and offers instantaneous spectral linewidth performance smaller than 1.8 kHz for 3 mW of output power. We are able to tune the laser frequency by applying voltage to the resonator. This suggest that the technology enables fabrication of ultra narrow linewidth semiconductor lasers in a broad wavelength range of 390 nm to 2,900 nm. The laser source in a compact footprint enables a multitude of sensing, monitoring, and metrology applications where high resolution and precision and absolute accuracy are required.

RF-induced change of optical refractive index in strontium barium niobate

It is well known that light is able to modify properties of solid state media. Photorefractivity is one of the brightest demonstrations of such an ability. The phenomenon is related to the change of refractive index resulting from light-mediated redistribution of charges within the material.1 This redistribution is particularly pronounced in a selected class of optical materials. The magnitude of photorefractivity depends on the energy of photons that induce the charge redistribution, and thus is generally not observed with infrared light. In this work we demonstrate experimentally that not only light, but also low power radio-frequency (RF) electromagnetic radiation results in a significant modification of the refractive index of strontium barium niobate (SBN), one of the widely used photorefractive material. To our knowledge, the observed effect cannot be explained using existing theories of photorefractivity in bulk material. We expect that the effect originates from the influence of the boundary of the material on the space charge distribution as well as RF field induced pyroelectric effect; however a more detailed study is required to completely unveil the origin of the phenomenon.

Whispering gallery mode lithium niobate microresonators for photonics applications

We review various photonics applications of whispering gallery mode (WGM) dielectric resonators and focus on the capability of generating trains of short optical pulses using WGM lithium niobate cavities. We introduce schemes of optical frequency comb generators, actively mode-locked lasers, and coupled opto-electronic oscillators where WGM cavities are utilized for the light amplification and modulation.

Surface acoustic wave frequency comb

We investigate opto-mechanical oscillation (OMO) and subsequent generation of acoustic wave frequency combs in monolithic crystalline whispering gallery mode (WGM) resonators. The OMO is observed in resonators made of electro-optic (lithium tantalate), non-electro-optic birefringent (magnesium fluoride), and non-birefringent (calcium fluoride) materials. The phenomenon manifests itself as generation of optical harmonics separated by the eigenfrequency of a surface acoustic wave (SAW) mechanical mode of the same WGM resonator. We show that the light escaping the resonator and demodulated on a fast photodiode produces a spectrally pure radio frequency (RF) signal. For instance, we demonstrate generation of 200 MHz signals with instantaneous linewidth of 0.2 Hz.

Photonic front-end for millimeter wave applications

We propose theoretically and demonstrate experimentally both a direct (quadratic) and a coherent millimeter wave photonic receivers operating at 35 GHz carrier frequency. These devices are made based on lithium niobate or lithium tantalite or quartz optical whispering gallery mode resonators coupled to a millimeter wave signal with a strip line. The quadratic receiver has sensitivity exceeding -70 dBm in a 3 MHz frequency band. It also has more than 40 dB of dynamic range. The coherent receiver can have a dynamic range in excess of 100 dB in a 5 MHz band, as well as a much higher sensitivity. The coherent operation of the receiver is achieved using a millimeter wave local oscillator fed into the strip line resonator along with the millimeter wave signal. We also discuss possible photonic methods for the generation of spectrally pure local oscillator signals suitable for the application in the coherent receiver.

Slow light in vertically coupled whispering gallery mode resonators

We theoretically study propagation of light in a vertically coupled whispering gallery mode resonator (WGMR) waveguide consisting of a chain of disc WGMRs etched on the surface of a cylinder made of an optically transparent material. The waveguide is capable of reducing the group velocity of light by as much as a factor of a billion, is much more efficient than usual coupled resonator optical waveguides, and compete with slow light atomic systems. We discuss practical as well as fundamental advantages and disadvantages of the resonator and atomic delay lines.

Crystalline micro-resonators: status and applications

We report on fabrication of new ultrahigh Q crystalline microcavities. Optical Q factor of (4.4±1.2)×108 is achieved for Vacuum UV grade CaF2 cavity with 100 μm in diameter. It is shown that if excimer grade crystal is used, Q factor of 5.5 mm cavity can be as high as (5.31±0.04)×1010 at laser wavelength of 1064 nm. We discuss nonlinear properties of these cavities such as Raman lasing with threshold of less than a few microwatts. Possible application in cavity quantum electrodynamics is analyzed.