In this presentation, we will present a light source based on adiabatic frequency conversion in millimeter-sized whispering gallery resonators (WGR) made from lithium niobate crystals, that was specifically developed to meet the stringent requirements of a digital holographic measurement system. Further on, first holographic measurements using this light source will be presented. Over 0.4 mW of average output power with 150 ns pulses and a central emission wavelength of 775 nm could be generated by the light source, with a linear tuning behavior of 66 MHz per Volt applied to the WGR.
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.
A rather unknown method to perform optical frequency tuning is the adiabatic frequency conversion. But this method has some appealing advantages compared to conventional frequency conversion schemes, i. e. nonlinear- optical based ones: The internal conversion efficiency can reach unity even on a single-photon level. No threshold and no phase-matching conditions need to be fulfilled. Previous realizations of adiabatic frequency conversion suffer from short photon lifetimes, limited tuning range and challenging experimental setups. Here, we employ the Pockels effect for adiabatic frequency conversion (AFC) in a non-centrosymmetric ultrahigh-Q microresonator made out of lithium niobate. With a 70-μm-thick resonator we observe frequency shifts of more than 5 GHz by applying a moderate voltage of 20V. In contrast to former schemes our setup is considerably simplified and provides a linear electric-to-optical link that enables us to generate also arbitrary waveforms of frequency shifts. Furthermore, our presented conversion scheme is well-suited for on-chip fabrication. Volume fabrication and application of larger electric fields for reasonable voltages become possible. By doing this, it is feasible to achieve tuning on the order of hundreds of GHz.
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