This work demonstrates an up-conversion imaging system using silicon sensors and commercial optics. A 1550 nm laser and Nd:YVO4 laser mix in a PPKTP crystal, achieving a 61 mrad FOV and 0.96 mrad resolution.

In this work, the development of a novel ultra-short laser system is presented, building upon previous research in passive mode-locking using cross-amplitude modulation (XAM). By combining XAM with cascaded second-order nonlinearity mode-locking (CSM) the system produced a stable bright-dark two-color output with picosecond pulses and a repetition rate of 275 MHz with an average output power of 100 mW for an 808 nm pump power of 4 W. The experimental setup involved two Nd: YVO4 lasers operating at 1064 nm and 1342 nm, where the two cavities were interconnected with a dichroic mirror allowing for a shared section where a periodically poled KTiOPO4 (PPKTP) was introduced. In the separate sections, the independently diode-pumped laser crystals were placed. The enhanced intra-cavity intensity achieved through XAM enabled effective pulse compression via CSM. The results demonstrate the system's ability to generate near-transform-limited pulses as short as 14 ps, offering potential for applications such as medical imaging and LIDAR.

Backwards Wave Optical Parametric Oscillators (BWOPOs) eliminate the need for a resonant cavity, offering a compact and robust alternative to traditional OPOs. In this study, a BWOPO using a PPKTP crystal with a 765 nm poling period was pumped by a Q-switched laser at 1030 nm, delivering 40 mu J pulses. The backward wave generated at 2.8 mu m exhibited a narrow spectral bandwidth of 1.75 GHz, despite the pump's broad 360 GHz bandwidth. The extracted pulse energy was around 5 mu J. These results highlight the BWOPO's potential for applications requiring precise wavelength control and stability.

The first demonstration of a 2.7 µm CO2 gas sensing source exploiting a backward wave optical parametric oscillator (BWOPO). Transmission measurements of the backward wave are demonstrated through air with good agreement with simulations.

In this work, a novel 2.7 µm source used for CO2 and H2O vapor spectroscopy using the backward propagating wave of a backward wave optical parametric oscillator (BWOPO) is demonstrated for the first time to our knowledge. The unique properties of BWOPOs eliminate the need for additional spectral narrowing or wavelength stabilization, enabling the use of a multi-longitudinal mode Q-switched pump laser centered around 1030 nm. A full characterization of the source is presented, revealing a central output at 2712 nm, showcasing a temperature tuning of -1.77 GHz/K, and achieving an output pulse energy of 2.3 µJ. Novel methods are introduced for measuring the linewidth and wavelength stability using the ambient laboratory air. These approaches demonstrate a narrow output of 43 pm and establish an upper limit of stability at 65 MHz, with no active means of stabilization. These findings underscore the potential of BWOPOs as a robust platform for future differential absorption lidar (DIAL) systems.

A new technique for mode-locking is demonstrated based on two lasers sharing one leg for sum-frequency generation. When the two lasers had equal round trip time one will produce bright pulses and the other dark pulses. Both lasers used Nd:YVO4 as the gain material, but operated at different wavelengths, namely 1064 nm and 1342 nm. In the present configuration, sub-250 ps pulses were generated at a repetition rate of 276 MHz with an output power of 70 mW. With appropriate choice of round trip loss at the two wavelengths it was possible to choose which laser was generating the bright pulses.

A new technique for mode-locking through sum-frequency generation is presented. The technique produced 200 ps long 1064 nm pulses in a coupled-cavity system with two Nd:YVO 4 crystals and a PPRKTP phase-matched for sum-frequency generation.

Ultrafast lasers have proven to be a great asset in many fields such as biological imaging [1], high-precision machining [2] and nonlinear spectroscopy [3].This has resulted in a great interest in the further development of passive mode-locked sources.The most common passive mode-locking techniques today rely on semiconductor saturable absorbers (SESAM) [4] or other artificial saturable absorbers, such as Kerr lensing [5].While these methods are well established, they still have issues with the durability of the SESAM or the need to reach sufficient intensities for reliable operation of the Kerr lens mechanism.In this work we present a new mechanism for passively mode-locking solid-state lasers using intra-cavity sumfrequency mixing (SFM).The underlying idea is to have two laser cavities with a shared section in which a nonlinear crystal is placed.The nonlinear medium phase-matches the SFM between the two operating wavelengths.By matching the roundtrip time of the two cavities, the same temporal part of the light in the two cavities will always interact.This forces one of the lasers to form a dark pulse and the other a bright pulse.Advantages of this approach is that it works for high repetition rates, can be used for any wavelengths in the transparency window for the nonlinear material and is easy to setup.Moreover, the phase-mismatched frequency doubling in the same crystal might infer cascaded F (2) :F (2) Kerr mechanism which could lead to spectral broadening and solitary mode-locking regime.The setup is shown in Fig. 1, where two Nd:YVO4 lasers resonate in a folded y-cavity, one operating at 1064 nm and the other at 1342 nm.In the shared section of the cavity a periodically poled nonlinear RKTP crystal is placed which is quasi-phase matched (QPM) for SFM between the two lasing wavelengths.

A midinfrared single-photon-counting lidar at 3 µm is presented. The 3 µm photons were upconverted to 790 nm in a periodically poled rubidium-doped KTiOPO4 crystal through intracavity mixing inside a 1064 nm Nd:YVO4 laser and detected using a conventional silicon single-photon avalanche detector (SPAD). The lidar system could distinguish 1 mm deep features on a diffusely reflecting target, limited by the SPAD and time-tagging electronics. This technique could easily be extended to longer wavelengths within the transparency of the nonlinear crystal.

A Nd:YVO4 laser operating at 1064 nm generating a stable mode-locked train of 10 ps-long dark pulses with a 211 MHz repetition rate is presented. The mode-locking relies on a periodic loss modulation produced by intra-cavity sum-frequency mixing with a synchronous bright-pulse train from a mode-locked femtosecond Yb:KYW laser at 1040 nm. A modulation depth of 90% was achieved for the dark pulses, confirmed by cross-correlation measurements. The ultrafast loss modulation injects power into the Nd:YVO4 laser cavity modes beyond the laser gain bandwidth. At proper laser cavity length, the detuning interaction of these modes with the lasing modes leads to the generation of periodic ultrafast transients at frequencies above 1.5 THz.

Dark pulses, a continuous wave (CW) background with brief dips of low intensity, are not studied nearly to the same extent as their bright counterparts. While dark pulse sources have been previously demonstrated in fibers [1] , [2] , its free-space counterpart has not. Here the first, to the best of our knowledge, free-space dark pulsed source is presented.

The principles behind up-conversion time correlated single photon counting (TCSPC) lidar are presented. This technique can easily extend the detection range of conventional SPADs to reach into the MIR by utilising nonlinear processes to up-convert the wavelength of the MIR to the visible or NIR. The up-converted light can then be detected by a standard Si-SPAD. This allows for high resolution and sensitivity to be achieved while operating the system at room temperature. The technique is demonstrated through a short-range lidar system measuring off diffusely reflecting targets. The system was operating at 3 μm with pulse energies around 3 nJ and a pulse width of 300 fs. The up-conversion was performed inside a PPRKTP crystal placed within a Nd:YVO4 laser cavity operating at 1064 nm, which generated pulses at 790 nm. The temporal FWHM of the lidar response was 76 ps, and the system showed sub-mm range accuracy, demonstrated by scanning 3D targets.

A single-photon-counting mid-infrared LIDAR is presented. 2.4 µm mid-infrared photons were up-converted to 737 nm by intra-cavity mixing in a periodically poled rubidium-doped K T i O P O 4 crystal inside a N d : Y V O 4 laser. The up-converted photons were detected by a Si single-photon avalanche photodiode (SPAD). A temporal resolution of 42 ps and a dark count rate of 500 Hz were achieved, limited by the SPAD and ambient light leakage. It allowed for detection of two targets separated by only a few millimeters. This technique is easily extendable to longer wavelengths, limited primarily by the nonlinear crystal transparency.

Up-conversion of 2.4 µm pulses to 737 µm was performed which allowed for range determination measurements with conventional Si-based detectors. Temporal resolution of 42 ps was achieved, allowing distinguishability between targets separated by few millimeters.

We present the fabrication of channel waveguides in PPRKTP crystals for QPM SHG in the blue region. Our waveguides reached a normalized conversion efficiency of 119 %/Wcm2 and showed low loss (0.51dB/cm).

Light in the mid-infrared (MIR) has a higher transmission through certain media, such as fog and smoke [1], than visible/near-infrared due to the lower scattering probability.Therefore it is of interest to have LIDARs operating in this spectral region, specifically utilizing photon counting, which yields high sensitivity.However, most photon counting LIDAR systems operate in the near infrared [2][3][4].This is due to a lack of good detectors in the MIR.By up-converting the MIR photons to the working regime of established Si-detectors, it is possible to reap the benefits from both the MIR and good detectors.This technique has previously successfully demonstrated detection of MIR both through a single pass configuration [5] or when the nonlinear crystal was inside a laser cavity [6].This experiment was performed to extend the wavelength of the up-conversion MIR LIDAR to 2.4 µm and achieve high spatial resolution compared to previous up-conversion photon counting LIDARs [7,8].In this work the nonlinear up-conversion was done inside a Nd:YVO4 cavity lasing at 1064 nm generating light at 737 nm.The LIDAR was performed with the time of flight technique.To do this there were three main components of the setup, the MIR pulses, the conversion cavity and the detector for the up-converted light.The MIR pulses were reflected off a microscope slide, up-converted in the conversion cavity, spectrally filtered and detected.The MIR pulses had a temporal width below 100 fs with a spectral bandwidth of roughly 300 nm centered around 2.4 µm with a repetition rate of 1 kHz and pulse energy in the pJ regime.A 10 mm long (9 mm grating length) periodically-poled rubidium-doped KTiOPO4 (PPRKTP) with a 23.5 µm period was fabricated in-house and used for the up-conversion.The crystal was inserted into a straight Nd:YVO4 laser cavity pumped by a 2.5 W 808 nm laser diode.The PPRKTP crystal was uncoated which resulted in a relatively low intra-cavity power of 5 W at 1064 nm.A conventional silicon single-photon avalanche photodiode (Si-SPAD) with a detection efficiency of 23 % at 737 nm was used in combination with a time tagging unit with bin widths down to 4 ps.To test the resolution of the system LIDAR traces were collected for multiple separations between two microscope slides varying from 0.4 mm to 8.4 mm of air and 1 mm of glass (thickness of the microscope slide).The trace from a single reflection, as well as a Gaussian fit, can be seen in Fig 2 (a).The full width half maximum (FWHM) of a single reflection was only 42 ps.In Fig 2 (b) the estimated, through LIDAR, and the measured, by a caliper, air gap between the two microscope slides can be seen.An air gap separation of merely 0.4 mm (1.9 mm optical thickness) yielded a FWHM of 55 ps, which is 30 % broader than for a single target.

In range resolved differential absorption LIDAR for gas concentration measurement, an attractive solution would be mid-infrared (MIR) micro-pulse LIDAR with photon counting detection. Single-photon detection in MIR is, however, difficult because of the low photon energy. Both superconducting nanowire detectors and low bandgap APDs need cryogenic cooling and are still limited to wavelengths shorter than approximately 2.5 μm.