Adiabatic frequency conversion in microresonators for multi-wavelength holography
Adiabatic frequency conversion (AFC) in microresonators comes without phasematching restrictions and does not depend on light intensity, i.e. it can reach 100 % conversion efficiency even at the single-photon level. The AFC is experimentally achieved in various configurations since 2007. However, compared with their nonlinear-optical counterparts, they still lead a life on the edge of obscurity. Despite of some impressive proof-of-concept demonstrations, there seems to be only little interest to employ adiabatic frequency converters for real-world applications. We demonstrate an electro-optically driven adiabatic frequency converter based on a millimeter-sized whispering gallery resonator made out of a lithium niobate crystal. The electric field is applied with a self-built ultra-fast high-voltage pulse generator. It consists of a push-pull stage with two fast-switching 600-V GaN power transistors and a control unit. This enables us to generate pulses with voltages of up to 600 V, slew rates of up to 150 V/ns and repetition rates reaching 1 MHz. Considering 100 µm resonator thickness, this enables electrically-controlled frequency shifts of up to 100 GHz. We combine this frequency converter with a system for multi-wavelength digital holography. Here, interferograms are recorded at slightly different laser frequencies. Calculating the difference phase of the interferograms numerically, interferograms at the beat frequency of the respective wavelength pairs can be created that correspond to phase data at the difference frequency. Cascading this process, a large unambiguity range paired with a high axial resolution becomes possible. A single laser combined with an adiabatic frequency converter is very appealing to provide sequentially the many, exactly spaced laser frequencies needed here, replacing a series of stabilized fixed-frequency lasers.