Generation of Kerr combs centered at 4.5 µm in crystalline microresonators pumped with quantum-cascade lasers.

We report on the generation of mid-infrared Kerr frequency combs in high-finesse CaF2 and MgF2 whispering-gallery-mode resonators pumped with continuous-wave room-temperature quantum cascade lasers. The combs were centered at 4.5 µm, the longest wavelength to date. A frequency comb wider than one half of an octave was demonstrated when approximately 20 mW of pump power was coupled to an MgF2 resonator characterized with quality factor exceeding 10(8).

Extended ultrahigh-Q-cavity diode laser.

We report on a study of a 698 nm extended cavity semiconductor laser with intracavity narrowband optical feedback from a whispering gallery mode resonator. This laser comprises an ultrahigh-Q (>10(10)) resonator supporting stimulated Rayleigh scattering, a diffraction grating wavelength preselector, and a reflective semiconductor amplifier. Single longitudinal mode lasing is characterized with sub-kilohertz linewidth and a 9 nm coarse tuning range. The laser has a potential application for integration with the 1S0-3P0 strontium transition to create compact precision atomic clocks.

Compact stabilized semiconductor laser for frequency metrology.

We report on the development of a frequency modulatable 795 nm semiconductor laser based on self-injection locking to a high-quality-factor whispering-gallery-mode microresonator. The laser is characterized by residual amplitude modulation below -80 dB and frequency noise better than 300 Hz/Hz(1/2) at offset frequencies ranging from 100 Hz to 10 MHz. The frequency modulation speed and span of the laser exceed 1 MHz and 4 GHz, respectively. Locking of the laser to the Doppler-free saturated absorption resonance of the (87)Rb D1 line is demonstrated and relative frequency stability better than 10(-12) is measured for integration time spanning from 1 s to 1 day. The architecture demonstrated in this study is suitable for the realization of frequency modulatable lasers at any wavelength.

Generation of a coherent near-infrared Kerr frequency comb in a monolithic microresonator with normal GVD.

We demonstrate experimentally, and describe theoretically, generation of a wide, fundamentally phase-locked Kerr frequency comb in a nonlinear resonator with a normal group velocity dispersion (GVD). A magnesium fluoride whispering-gallery mode resonator characterized with 10 GHz free spectral range and pumped either at 780 or 795 nm is used in the experiment. The envelope of the observed frequency comb differs significantly from the Kerr frequency comb spectra reported previously. We show via numerical simulation that, while the frequency comb does not correspond to generation of short optical pulses, the relative phase of the generated harmonics are fixed.

Tunable optical frequency comb with a crystalline whispering gallery mode resonator.

We report on the experimental demonstration of a tunable monolithic optical frequency comb generator. The device is based on four-wave mixing in a crystalline calcium fluoride whispering gallery mode resonator. The frequency spacing of the comb is given by an integer number of the free spectral range of the resonator. We select the desired number by tuning the frequency of the pumping laser with respect to the corresponding resonator mode. We also observe a rich variety of optical combs and high-frequency hyperparametric oscillation, depending on the experimental conditions. A potential application of the comb for generating tunable narrow band frequency microwave signals is demonstrated.

Crystal quartz optical whispering-gallery resonators.

A quality factor exceeding 5x10(9) is obtained in whispering-gallery mode (WGM) resonators fabricated of crystalline quartz. We observe significant electrical tunability of WGMs in x-cut resonators and demonstrate an electro-optic modulator with a submegahertz passband at 12 GHz. We discuss other photonics applications of the crystal quartz WGM resonators in narrowband agile tunable filters, compact narrow linewidth lasers, and microwave and millimeter wave oscillators.

Phase noise of whispering gallery photonic hyper-parametric microwave oscillators.

We report on the experimental study of phase noise properties of a high frequency photonic microwave oscillator based on four wave mixing in calcium fluoride whispering gallery mode resonators. Specifically, the oscillator generates approximately 8.5 GHz signals with -120 dBc/Hz at 100 kHz from the carrier. The floor of the phase noise is limited by the shot noise of the signal received at the photodetector. We argue that the performance of the oscillator can be significantly improved if one uses extremely high finesse resonators, increases the input optical power, supersaturates the oscillator, and suppresses the residual stimulated Raman scattering in the resonator. We also disclose a method of extremely sensitive measurement of the integral dispersion of millimeter scale dielectric resonators.

Efficient generation of truncated Bessel beams using cylindrical waveguides.

In this paper we address efficient conversion between a Gaussian beam (a truncated plane wave) and a truncated Bessel beam of a given order, using cylindrical optical waveguides and whispering gallery mode resonators. Utilizing a generator based on waveguides combined with whispering gallery mode resonators, we have realized Bessel beams of the order of 200 with a conversion efficiency exceeding 10 %.

Optical resonators with ten million finesse.

We report a significant improvement of the finesse and quality factor of a calcium fluoride whispering gallery mode optical resonator achieved with iterative thermal annealing. The initial and final values of the resonator finesse are F(i) approximately 5 x 10(5) and F(f) >/= 10(7) respectively. The measurements are conducted at room temperature in the laboratory atmosphere.

Measurement of the far field intensity distribution of a bent and cleaved fiber taper.

We have theoretically predicted and experimentally demonstrated mode conversion in fiber tapers subject to large adiabatic bending. The far field intensity distribution of the taper mode is imaged in part by cleaving the taper at the position of minimum diameter.

On fundamental quantum noises of whispering gallery mode electro-optic modulators.

Using the example of whispering gallery mode (WGM) electro-optic modulator (EOM) we show that the majority of phase EOMs, particularly the resonant types, introduce additional quantum noise to the modulated light. The noise power grows quadratically with the optical power and results from the unavoidable spontaneous emission process originating from the strongly nondegenerate parametric interaction. This latter process is the physical basis for modulation.

Morphology-dependent photonic circuit elements.

We theoretically propose and experimentally demonstrate the design of a novel one-dimensional ringlike macroscopic optical circuit element. The similarity between morphologies of an optical planar waveguide and a whispering-gallery axially symmetric solid-state resonator is used.

Nonlinear optics and crystalline whispering gallery mode cavities.

We demonstrate parametric frequency doubling in a whispering gallery cavity made of periodically poled lithium niobate. This demonstration is an example of utility of such crystalline optical whispering gallery resonators with very high Q factors, which we have fabricated.

Low threshold optical oscillations in a whispering gallery mode CaF(2) resonator.

We have observed low-threshold optical hyperparametric oscillations in a high-Q fluorite whispering gallery mode resonator. The oscillations result from the resonantly enhanced four-wave mixing occurring due to Kerr nonlinearity of the material. We demonstrate that, because of the narrow bandwidth of the resonator modes as well as the high efficiency of the resonant frequency conversion, the oscillations produce stable narrow-band beat-note of the pump, signal, and idler waves. A theoretical model for this process is described.

Dispersion compensation in whispering-gallery modes.

We show that manipulation by a spatial profile of the refractive index of a circularly symmetric dielectric cavity results in a novel way of fine tuning frequency separations as well as spatial localizations of high-Q whispering-gallery modes excited in the cavity. The method permits dispersion compensation in the modes (spectrum equalization), diminishes the quality-factor limitation by surface roughness and contamination, and allows critical coupling to ultra-high-Q modes without maintaining an air gap with evanescent couplers.