W-Band Photonic Receiver for Compact Cloud Radars.

We introduce an RF-photonics receiver concept enabling the next generation of ultra-compact millimeter wave radars suitable for cloud and precipitation profiling, planetary boundary layer observations, altimetry and surface scattering measurements. The RF-photonics receiver architecture offers some compelling advantages over traditional electronic implementations, including a reduced number of components and interfaces, leading to reduced size, weight and power (SWaP), as well as lower system noise, leading to improved sensitivity. Low instrument SWaP with increased sensitivity makes this approach particularly attractive for compact space-borne radars. We study the photonic receiver front-end both analytically and numerically and predict the feasibility of the greater than unity photonic gain and lower than ambient effective noise temperature of the device. The receiver design is optimized for W-band (94 GHz) radars, which are generally assessed to be the primary means for observing clouds in the free troposphere as well as planetary boundary layer from space.

On Sagnac frequency splitting in a solid-state ring Raman laser.

We report on an accurate measurement of the frequency splitting of an optical rotating ring microcavity made out of calcium fluoride. By measuring the frequencies of the clockwise and counter-clockwise coherent Raman emissions confined in the cavity modes, we show that the frequency splitting is inversely proportional to the refractive index of the cavity host material. The measurement has an accuracy of 1% and unambiguously confirms the classical theoretical prediction based on special theory of relativity. This Letter also demonstrates the usefulness of the ring Raman microlaser for rotation measurements.

Chasing the thermodynamical noise limit in whispering-gallery-mode resonators for ultrastable laser frequency stabilization.

Ultrastable high-spectral-purity lasers have served as the cornerstone behind optical atomic clocks, quantum measurements, precision optical microwave generation, high-resolution optical spectroscopy, and sensing. Hertz-level lasers stabilized to high-finesse Fabry-Pérot cavities are typically used for these studies, which are large and fragile and remain laboratory instruments. There is a clear demand for rugged miniaturized lasers with stabilities comparable to those of bulk lasers. Over the past decade, ultrahigh-Q optical whispering-gallery-mode resonators have served as a platform for low-noise microlasers but have not yet reached the stabilities defined by their fundamental noise. Here, we show the noise characteristics of whispering-gallery-mode resonators and demonstrate a resonator-stabilized laser at this limit by compensating the intrinsic thermal expansion, allowing a sub-25 Hz linewidth and a 32 Hz Allan deviation. We also reveal the environmental sensitivities of the resonator at the thermodynamical noise limit and long-term frequency drifts governed by random-walk-noise statistics.High-quality optical resonators have the potential to provide a miniaturized frequency reference for metrology and sensing but they often lack stability. Here, Lim et al. experimentally characterize the stability of whispering-gallery resonators at their fundamental noise limits.

Ultrahigh Q whispering gallery mode electro-optic resonators on a silicon photonic chip.

Crystalline whispering gallery mode (WGM) electro-optic resonators made of LiNbO3 and LiTaO3 are critical for a wide range of applications in nonlinear and quantum optics, as well as RF photonics, due to their remarkably ultrahigh Q(>108) and large electro-optic coefficient. Achieving efficient coupling of these resonators to planar on-chip optical waveguides is essential for any high-yield and robust practical applications. However, it has been very challenging to demonstrate such coupling while preserving the ultrahigh Q properties of the resonators. Here, we show how the silicon photonic platform can overcome this long-standing challenge. Silicon waveguides with appropriate designs enable efficient and strong coupling to these WGM electro-optic resonators. We discuss various integration architectures of these resonators onto a silicon chip and experimentally demonstrate critical coupling of a planar Si waveguide and an ultrahigh QLiTaO3 resonator (Q∼108). Our results show a promising path for widespread and practical applications of these resonators on a silicon photonic platform.

Tunable Microcavity-Stabilized Quantum Cascade Laser for Mid-IR High-Resolution Spectroscopy and Sensing.

The need for highly performing and stable methods for mid-IR molecular sensing and metrology pushes towards the development of more and more compact and robust systems. Among the innovative solutions aimed at answering the need for stable mid-IR references are crystalline microresonators, which have recently shown excellent capabilities for frequency stabilization and linewidth narrowing of quantum cascade lasers with compact setups. In this work, we report on the first system for mid-IR high-resolution spectroscopy based on a quantum cascade laser locked to a CaF2 microresonator. Electronic locking narrows the laser linewidth by one order of magnitude and guarantees good stability over long timescales, allowing, at the same time, an easy way for finely tuning the laser frequency over the molecular absorption line. Improvements in terms of resolution and frequency stability of the source are demonstrated by direct sub-Doppler recording of a molecular line.

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.

Calligraphic poling of Lithium Niobate.

We demonstrate a novel technique for instituting complex and arbitrary shaped micron-scale domain patterns in LiNbO3 at room temperature. Fabrication of continuous domains as narrow as 2 microm across and hexagonal patterns of the same order accompanied by real time visualization of the poling process are presented.

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.