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.

Femtosecond laser pulses are essential tools in modern science and technology, yet generating pulses that are both extremely short and experimentally accessible remains a persistent challenge. Conventional post-compression methods rely on the Kerr effect, a third-order nonlinear effect, but these approaches often require complex setups and a large laboratory footprint. This thesis explores a fundamentally different route to pulse compression by utilising the coherently driven transversal optical phonon-polariton modes in Potassium Titanyl Phosphate (KTP). Rather than relying on Kerr-based spectral broadening and subsequent dispersive compression, the method exploits strong second-order nonlinearities in KTP to generate polaritons by optical rectification so that their electric fields may be used for efficient Stokes sideband generation by electro-optic interaction. The resulting interplay with normal dispersion in the nonlinear crystal results in an order of magnitude shorter self-compressed pulses generated in a simple setup. Such pulses demand advanced characterisation methods. Building on insights from applied mathematics and optimisation theory, a new retrieval algorithm for Frequency-Resolved Optical Gating (FROG) measurements is introduced, which we call the Line-Search FROG (LSF) algorithm, that decouples the measurement data from the reconstruction process. This greatly improves the performance of the pulse retrieval fidelity in the presence of large amounts of noise. The LSF algorithm is highly versatile and applicable to all FROG geometries, including the so-called double-blind FROG, with which we managed to measure the phase of a mode-locked dark pulse for the first time. Other pulse measurement techniques such as dispersion scan could benefit as well as the underlying optimisation problem is similar. To further improve the performance of pulse characterisation techniques, we also present the Sigma Check, which is a general algorithmic step that aims to reduce the likelihood of stagnating at local minima. This is achieved by performing an image-recognition step that correctly identifies and counteracts local minimums.