Novel solid-state laser systems with engineered nonlinear materials

QC 2024-11-19

Abstract

This thesis presents the development of novel solid-state laser sources, divided into two primary areas: a new mode-locking scheme demonstrated using Nd:YVO₄ lasers with an intra-cavity nonlinear medium, producing bright and dark mode-locking, and a laser source based on a backward wave optical parametric oscillator (BWOPO) for CO₂ absorption measurements. The mode-locking research is detailed in three papers. The initial work demonstrates the first realization of dark pulse mode-locking in a solid-state laser. A periodically poled KTiOPO₄ (PPKTP) crystal placed within a continuous-wave (CW) Nd:YVO₄ laser operating at 1064 nm where a Yb:KYW femtosecond laser at 1040 nm was focused in to the nonlinear crystal. The phase matched sum-frequency mixing (SFM) in the crystal acted as a time dependent loss. When the CW laser’s cavity round trip time matched the femtosecond repetition rate, stable dark pulses with a 10 ps duration and 90% modulation depth were formed, minimizing SFM loss. In the second study, it was shown that the bright-dark pulse configuration could be passively generated by replacing the femtosecond laser with a CW laser at 1342 nm. This setup enabled interaction through cross-amplitude modulation (XAM) in the PPKTP crystal. A dichroic mirror allowed both lasers to share the cavity section containing the nonlinear medium, while the independently diode-pumped laser crystals were placed in separate sections. By matching the cavity round trip times, synchronous bright-dark pulses were formed: 250 ps bright pulses at 1064 nm and dark pulses at 1342 nm, effectively minimizing SFM losses. The third work further refined this setup by modifying the cavity to increase intra-cavity intensity, leading to cascaded second-order nonlinearity Kerr lens mode-locking. The increased intensity from XAM enabled significant pulse compression for the 1064 nm pulses. The 1064 nm cavity produced 14 ps bright pulses, with correspondingly compressed dark pulses. Lastly, this thesis explores the use of BWOPO as a platform for gas sensing. By employing a commercial pulsed multi-longitudinal mode laser, the BWOPO generated a narrow-bandwidth backward wave at 2.7 μm. This source was used to successfully measure CO₂ absorption in the lab, with temperature tuning enabling detailed characterization of the absorption spectrum. Further analysis of the ambient air provided a characterization of the source, revealing a tuning rate of -1.79 GHz/K, a bandwidth of 43 pm (1.75 GHz), and an upper limit of center wavelength stability of 65 MHz, all achieved passively by the BWOPO.

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