PhD Thesis Defenses

Pulse Generation and Detection through Intra-Cavity Up-Conversion


The work presented in this thesis concerns two main categories: pulse detection and pulse generation, however both utilise intra-cavity up-conversion. The work regarding pulse detection deals with up-conversion LIDAR and was split into two smaller projects, the first one focused on the resolution in measurements along one line with multiple reflections, and the second focused on measurements further into the MIR, as well as performing 3D imaging. The work on pulse generation was also split into two projects. In the first a solid-state laser was demonstrated that generated dark pulses, while the second project resulted in new way to achieve mode-locking and synchronously produced bright and dark pulses at two different wavelengths.

The first LIDAR project utilised up-conversion detection 2.4 μm. The system had a temporal response of 42 ps (FWHM) and was able to detect two microscope slides separated by a few millimetres. In the second LIDAR project we pushed the LIDAR wavelength to slightly above 3 μm, and it was the longest wavelength that photon counting LIDAR had been performed at. The system could clearly resolve 1 mm deep features on a target and produce 3D images. Both the LIDAR systems used intra-cavity up-conversion with periodically poled rubidium doped KTiOPO4 (PPRKTP) in Nd:YVO4 lasers operating at 1064 nm.

The dark pulse laser was based on sum-frequency generation (SFG) between a 1064 nm Nd:YVO4 laser and a mode-locked Yb-laser operating at 1040 nm. The mode-locked laser was focused into a PPRKTP placed inside the 1064 nm laser. By matching the cavity round-trip time of the 1064 nm laser to the repetition rate of the 1040 nm laser, the intensity dip produced by the SFG was enhanced for each round trip. The system could thereby produce dark pulses with a modulation depth of 90 % and a pulse width of 10 ps.

In the last project bright pulses were produced by matching the cavity length of a 1064 nm Nd:YVO4 laser with that of a 1342 nm Nd:YVO4 laser. The two lasers formed a y-cavity, with a shared leg where a PPRKTP crystal was placed for phase-matched SFG between the two lasers. When the cavity lengths were matched, one laser produced bright mode-locked pulses and the other one dark pulses, synchronously. When the lasers enter this regime the SFG induced loss is significantly reduced due to the temporal overlap between the bright and dark pulse. The mode-locked pulses had a pulse width of 240 ps.