High-Q whispering gallery mode resonators have been mostly studied in the visible and near-IR wavelength regions for optical frequency comb and spectroscopy. With crystalline materials, their use can be extended to the mid-IR beyond 2 μm where molecular gases not only have very rich characteristic spectral lines but also very large absorption cross sections. In this paper, we describe our continued efforts of pushing whispering gallery mode resonator applications in the mid-IR wavelength region for spectroscopic applications including Kerr comb generation and molecular absorption spectroscopy. With a variety of mid-IR transmitting crystalline materials, we investigate their Q and limiting factors, dispersion and spectral engineering, parametric oscillation and comb generation. We have also explored the utility and limitation of using high Q resonators for ringdown molecular absorption measurements.
Optical frequency combs, typically produced by mode locked lasers, have revolutionized many applications in science and technology. Frequency combs were recently generated by micro resonators through nonlinear Kerr processes. However, the comb span from micro resonators was found to be limited by resonator dispersion and mode spectrum. While dispersion engineering has been reported in on-chip devices, monolithic crystalline resonators offer an advantage of high optical quality factor. Moreover, most resonators used for comb generation support many mode families, leading to unavoidable crossings in resonator spectrum. Such crossings strongly influence comb dynamics and may prevent stable coherent mode-locking and soliton states. We report a new crystalline resonator approach supporting dispersion control and single mode spectrum while maintaining high quality factor. Dispersion engineering by waveguide micro-structuring is used to flatten the dispersion in our MgF2 resonator. Both absolute magnitude of dispersion and its slopes can be altered over a wavelength span exceeding an octave. Dispersion flattening leads to generation of an octave-spanning frequency comb with repetition rate of 46 GHz and coupled pump power below 100 mW. We also demonstrate that the micro- structuring dispersion engineering approach can be used to achieve flattened and anomalous dispersion in a CaF2 resonator near 1550 nm wavelength. In addition, we describe observation of discrete steps between the modulation instability states of the primary comb and on the three-stage comb unfolding dynamics. The micro-structured resonators may enable efficient low repetition rate coherent octave spanning frequency combs without external broadening, ideal for applications in optical frequency synthesis, metrology, spectroscopy, and communications.
We experimentally study the factors that influence the span in frequency combs derived from the crystalline whispering gallery mode resonators. We observe that cavity dispersion plays an important role in generation of combs by a cascaded four-wave mixing process. We observed combs from the resonators with anomalous dispersion and nearly zero dispersion at the pump wavelength. In addition, the comb generation efficiency is found to be affected by the crossing of modes of different families. The influence of Raman gain is discussed as well as the roles of the cavity diameter and pump power.
We present studies of using high Q whispering gallery mode resonators in Mid-IR laser spectroscopy. Several crystalline materials have high transparency in Mid-IR wavelength region and can be made into high Q optical resonators. We report recent measurements of Q values of greater than 1x108 in the wavelength region longer than 3 μm using one of these materials, Magnesium Fluoride (MgF2). These resonators are being used for cavity ring-down measurements, optical frequency comb generation, and their applications in Mid-IR spectroscopy
We report an investigation on optical whispering gallery mode (WGM) resonators made from non z-cut beta barium
borate (BBO) crystals. We first fabricated high quality (Q) factor WGM resonators made of an angle-cut BBO crystal.
Q factors of 1×108 level have been demonstrated at various wavelengths including UV. They led to new upper bounds for the absorption coefficients of BBO at 1560 nm, 980 nm and 370 nm. We observed only one set of ordinarily polarized WGMs with polarization rotating along the resonator circumference. We also fabricated xy-cut BBO WGM resonators, in which the optic axis is parallel to the resonator plane. In that case, two WGM families with different polarization exist, one with constant the other with oscillatory phase velocity. This enables a novel way of broadband phase matching in WGM resonators with cyclic gain. We experimentally demonstrated efficient second harmonic generation (SHG) to a wide harmonic wavelength range from 780 nm at near infrared to 317 nm in UV. It is also the first reported direct UV SHG in a high-Q WGM resonator. This work lays a foundation for further investigations of WGM properties of non-z cut birefringent resonators and their applications in nonlinear optics.
Crystalline whispering gallery mode (WGM) resonators are small and structurally monolithic, yet capable of ultra-high
quality factors and dramatically enhanced optical nonlinearities. These properties can be exploited in developing
miniature optical clocks. Several studies have explored using WGM resonators as frequency reference cavities, and there
also exists great research interest in using WGMs for frequency comb generation. In this paper, we will describe our
efforts in pursuing laser stabilization using WGM reference cavities with both passive and active temperature
stabilization schemes. We will also present our latest analysis and experimental results in frequency comb generation in
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.
Thermal nonlinearity can produce oscillatory instability in
optical microspheres. We analyze theoretically the conditions of
observations of this regime and demonstrate it experimentally. The
observed curves are well compared with results of numerical
modelling. In pure fused silica with low absorption thermal
oscillations are suppressed due to concurrency with Kerr
nonlinearity. We also describe for the first time experimentally
observed slow and irreversible thermooptical processes in
We present the results of measurements of thermal fluctuations in microspheres. Experimental noise spectra are in good agreement with the theoretical model of recently predicted thermorefractive noise.