For many quantum information applications, quantum optics or spectroscopy, the ideal light source has narrow linewidth, tunable wavelength and it is at the same time stable and small. These requirements are met by the recently reported new class of optical parametric oscillator (OPO): whispering gallery OPOs. This article reports the results of pumping at 488 nm (blue laser) enabling signal wavelengths ranging from visible red to the near infrared. The resonator consists of a spheroidally-shaped monolithic cavity made of a z-cut stoichiometric lithium niobate crystal doped with magnesium oxide. The light is coupled into the structure with a rutile prism and guided in the crystal through total internal reflections from the curved crystal surfaces. The resonator then presents minimal reflection losses over the whole tunable wavelength range at all of the used wavelengths, pump, signal and idler. This constitutes a great advantage over resonators with dielectric coatings and makes it possible to build a high-Q cavity with great tunability, resonating at all three wavelengths. The losses and limits to the wavelength scaling in the resonator are thus mainly set by the material absorption. The low incoming pump power OPO threshold of 160 µW is achieved due to the tiny mode size in the millimeter size resonator, the three resonating waves and propagation inside the nonlinear medium. The authors compare this value to the one given by the theory. Furthermore the monolithic resonator structure offers excellent stability. In many applications, the needed narrow linewidth can be achieved with this high-Q cavity and the use of a high quality pump laser. The pump and output coupling is done by using a prism whose distance to the resonator tunes the coupling efficiency and also the spectral linewidth of the output.
The conservation of momentum in the nonlinear medium in this spherical geometry results in the conservation of orbital angular momentum. Together with energy conservation, the phase-matching condition is set for the geometry of the resonator. These conditions give the possible output wavelengths of the resonator. The output wavelengths as a function of the temperature of the nonlinear crystal are presented in the article. The paper also discusses the possibilities of tuning the signal wavelength further to cover more of the visible wavelength range.
The article presents a blue pumped whispering gallery OPO that allows signal wavelength from visible red to near infrared ranges tunable over hundreds of nanometers. This laser source has a number of great properties making it promising concept for many applications and research in the future.
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