Laser-active adiabatic frequency converters

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In this project, we aim for realizing and investigating optical resonators that can store light for time scales in the millisecond region and beyond. Based on these resonators, we plan a proof-of-concept of a mechanically driven optical frequency converter having the potential of tuning the frequency of laser light by 10 THz (50 nm in the near infrared) with 100 % internal efficiency. Typically, high-quality optical resonators have decay times of a few hundreds of nanoseconds. We plan to increase this value by a factor of ten thousand, i.e. into the millisecond range. This shall be achieved by doping the resonator material with laser-active ions and optical pumping: The losses for the circulating light having the frequency, we want to change, are compensated by optical amplification. Ideally, this scheme shall allow for arbitrarily long decay times. This would come along with arbitrarily small linewidths and arbitrarily large power enhancement. The basic idea of compensating the loss for light circulating in an optical resonator by laser amplification is already around for some time. However, so far, there is no systematic study regarding the laser-activity induced increase of the decay time and the fundamental limits of this scheme. The actual tuning of the frequency of laser light in this project is based on adiabatic frequency tuning. Its acoustic analog is well known: If one plugs a guitar string and varies its length during the ring-down time, the pitch of the tone changes accordingly. This concept can be transferred to optics: Here, light is coupled into a resonator and the optical size of the latter is changed during the ring-down time. The frequency of the circulating light strictly follows the one of the varying resonance frequency. This scheme for optical frequency conversion has significant advantages over conventional methods. All intracavity photons are converted, i.e. the internal efficiency is 100 %, independent of the light intensity. This process works without taking care of phase matching and without any mode hops. With the help of resonators having millisecond decay times, mechanical tuning of the resonator length becomes an option for adiabatic frequency tuning for the first time. This will enable frequency tunings outnumbering the state of the art by two orders of magnitude. We expect that this project will strongly inspire the scientific investigation, the technical development and the application of optical resonators. With decay times in the millisecond range and beyond, they provide new possibilities for other fields of research as well. Due to the ultra-small linewidth of less than 1 kHz which comes along with the large decay time, they are for example interesting for sensing applications, where the detection limit is given by the linewidth.