Optical combs generated by a monolithic resonator with Kerr-medium can be used in a number of applications, including orbital clocks and frequency standards of extremely high accuracy, such as astronomy, molecular spectroscopy, and the like. The main difficulty of this approach is the relatively high pump power that has to be used in such devices, causing undesired thermorefractive effects, as well as stimulated Raman scattering, and limiting the optical comb quality and utility.
In order to overcome this problem, this innovation uses a different approach to excitation of the nonlinear oscillations in a Kerr-nonlinear whispering gallery mode (WGM) resonator and generation of the optical comb. By coupling to the resonator two optical pump frequencies instead of just one, the efficiency of the comb source can be increased considerably. It therefore can operate in a lower-power regime where the undesirable effects are not present. This process does not have a power threshold; therefore, the new optical component can easily be made strong enough to generate further components, making the optical comb spread in a cascade fashion. Additionally, the comb spacing can be made in an arbitrary number of the resonator free spectral ranges (FSR).
The experimental setup for this innovation used a fluorite resonator with Ω = 13.56 GHz. This material has very low dispersion at the wavelength of 1.5 microns, so the resonator spectrum around this wavelength is highly equidistant. Light was coupled in and out of the resonator using two optical fibers polished at the optimal coupling angle. The gap between the resonator and the fibers, affecting the light coupling and the resonator loading, was controlled by piezo positioners. The light from the input fiber that did not go into the resonator reflected off of its rim, and was collected by a photodetector. This enabled observation and measurement of the (absorption) spectrum of the resonator.
The input fiber combined light from two lasers centered at around 1,560 nanometers. Both laser frequencies were simultaneously scanned around the selected WGMs of the same family. However, they were separated by one, two, three, or ten FSRs. This was achieved by fine-tuning each laser frequency offset until the selected resonances overlap on the oscilloscope screen. The resonator quality factor Q = 7 × 107 was relatively low to increase the linewidth and, therefore, the duty cycle of both lasers simultaneously coupled into their WGMs. The optical spectrum analyzer (OSA) connected to the output fiber was continuously acquiring data, asynchronously with the laser scan. The instrument was set to retain the peak power values; therefore, a trace recorded for a sufficiently long period of time reflected the situation with both lasers maximally coupled to the WGMs.
This work was done by Dmitry V. Strekalov and Nan Yu of Caltech and Andrey B. Matsko of OEwaves for NASA’s Jet Propulsion Laboratory. For more information, download the Technical Support Package (free white paper) at www.techbriefs.com/tsp under the Physical Sciences category. NPO-46253
This Brief includes a Technical Support Package (TSP).
Generation of Optical Combs in a WGM Resonator From a Bichromatic Pump
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Overview
The document titled "Generation of Optical Combs in a WGM Resonator From a Bichromatic Pump" (NPO-46253) discusses advancements in the generation of optical frequency combs using a whispering gallery mode (WGM) resonator. Optical combs are crucial for applications such as optical clocks, frequency standards, astronomy, and molecular spectroscopy. However, traditional methods of generating these combs often require high pump power, which can lead to undesirable effects like thermorefractive noise and stimulated Raman scattering, ultimately limiting the quality and utility of the optical combs.
The proposed solution in this document involves a novel approach that utilizes two optical pump frequencies instead of one. This bichromatic pumping method significantly enhances the efficiency of the comb generation process, allowing it to operate at lower power levels where the negative effects associated with high pump power are minimized. The technique eliminates the need for a power threshold, enabling the generation of a strong new optical component that can cascade into further components, thus creating a robust optical comb.
A key innovation of this method is the ability to regulate the spacing of the optical comb. Unlike previous approaches where the comb spacing was fixed to a single free spectral range (FSR) of the resonator, this new technique allows for arbitrary comb spacing. This feature is particularly beneficial for the calibration of optical instruments, providing greater flexibility and precision in various applications.
The document highlights two main novelties of this approach: the no-threshold operation, which facilitates low-power applications free from thermorefractive and stimulated Raman scattering noise, and the regulated comb spacing, which is advantageous for optical instrument calibration.
The research is presented by Dmitry V. Strekalov and Nan Yu and published in the American Physical Society's journal, Phys. Rev. A. The findings are part of NASA's Commercial Technology Program, aimed at disseminating aerospace-related developments with broader technological, scientific, or commercial implications.
Overall, this document outlines a significant advancement in optical comb generation technology, promising improved performance and wider applicability in high-precision scientific and commercial fields.
