Broadband frequency-doubling of a swept-source laser from 1550 nm to 775 nm using a fan-out crystal and application in 2kHz LiDAR ranging.

Swept-source lasers have achieved significant success in sensing, imaging and microscopy. A special type of these lasers is the Fourier-domain mode-locked (FDML) laser, which operates at sweep rates in the megahertz (MHz) range while maintaining high instantaneous monochromaticity. By combining an FDML with an electro optical modulator (EOM) and master oscillator power amplifier (MOPA), a fast wavelength-swept pulsed laser with pulse peak powers in the kilowatt range can be constructed. An extension from the typical near-infrared (NIR) spectral range into the visible (VIS) range could enable a much wider field of applications. A direct approach is not feasible due to the lack of a suitable amplifier medium for MOPAs. A proven alternative is frequency-doubling through phase matching or quasi-phase matching (QPM) from the NIR. However, efficient frequency-doubling of low to mid-power lasers requires longer crystal lengths, limiting the input spectral bandwidth and hence is typically not feasible for broadband swept-source lasers. To overcome this limitation, here we describe a new approach using a periodically poled lithium niobate (PPLN) crystal with a special fan-out QPM structure. Spatial separation of the spectrally distinct pulses using an optical grating is used to obtain ideal QPM conditions for all wavelengths. This new concept allows broadband frequency-doubling and thus the generation of efficient swept laser sources in the NIR and VIS range. In this study we present the frequency-doubling of a 1550 nm FDML-MOPA laser to 775 nm with a pulse peak power of up to 35 W and a spectral span of 10 nm around 775 nm. We show application in ultrafast time-stretch LiDAR with 3D acquisitions of 2000 scans per second. This new laser technology opens up new possibilities for high speed and high bandwidth imaging and spectroscopy.