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.

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, 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 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 9050 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 ultra-fast transients at frequencies above 1.5 THz.

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:YVO4 crystals and a PPRKTP phase-matched for sum-frequency generation. 

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:YVO4 crystals and a PPRKTP phase-matched for sum-frequency generation.

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.

Denna avhandling kan delas in i två huvudkategorier: pulsdetektion och pulsgenerering i det infraröda spektralområdet. Gemensamt för båda är utnyttjandet av intrakavitär uppkonvertering. Pulsdetektionsarbetet består i sin tur av två mindre projekt, ett som fokuserade på upplösning av svaga signaler (reflektioner) längs en linje, och ett som använde längre våglängder där vi kunde göra 3D avbildningar. Även pulsgenereringen bestod av två projekt. I det första demonstrerade vi en fastatillståndslaser som genererar mörka pulser. Det andra projektet resulterade i ett nytt tillvägagångssätt för att generera modlåsta pulser och samtidigt skapa synkrona ljusa och mörka pulser vid två olika våglängder.

Det första LIDAR projektet använde uppkonvertingsdetektion för att genomföra åstadkomma avståndsmätning vid 2,4 µms våglängd. Systemet hade en tidsrespons på 42 ps FWHM (eng: Full Width Half Maximum), och kunde urskilja två mikroskopglas som var separerade med endast ett fåtal millimeter. I det andra projektet användes en våglängd strax över 3 µm med vilket vi kunde upplösa 1 mm djupa detaljer samt skapa 3D avbildningar. Detta är den längsta våglängden som fotonräknande avbildning genomförts vid. Båda systemen baserades på Nd:YVO$_4$ lasrar och använde periodiskt polade rubidiumdopad KTiOPO₄ (PPRKTP) kristaller placerade inuti kaviteterna för att åstadkomma uppkonverteringen.

Mörkpuls-lasern bygger på summafrekvensgenerering (SFG) mellan en 1064 nm Nd:YVO₄ laser och en 1040 nm modlåst Yb-laser. Den modlåsta lasern fokuserades in i en PPRKTP som var placerad inuti Nd:YVO₄ kaviteten. Genom att säkerställa att tiden mellan de modlåsta pulserna och kavitetsrundturstiden var densamma så kunde intensitetsdipparna förstärkas vid varje passage genom kristallen. Lasern producerade då mörka pulser med ett modulationsdjup på 90 % och en pulsbredd på 10 ps.

I det sista projektet genererade vi modlåsta pulser genom att matcha längderna på två Nd:YVO₄ lasrar. Den ena lasrade vid 1064 nm och den andra vid 1342 nm. De två lasrarna formade en y-kavitet där majoriteten av kaviteten var gemensamma. I den gemensamma delen placerades en PPRKTP kristall som var fas-matchad för SFG mellan de två våglängderna. När kavitetslängderna var densamma så började den ena lasern att producera ljusa pulser medan den andra producerade mörka pulser synkront. De ljusa pulser hade en pulsbredd på 240 ps. Fenomen uppstod spontant eftersom det leder till minskade SFG förluster.

A midinfrared single-photon-counting lidar at 3 mu m is presented. The 3 mu m 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. Published by Optica Publishing Group under the terms of the Creative Commons Attribution 4.0 License.

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.