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Gain characteristics in single-mode pumped diamond Raman lasers
thesisposted on 2022-03-29, 01:09 authored by Soumya Sarang
Single longitudinal mode (SLM) lasers are significant for applications in spectroscopy, precision measurements, and high-resolution interferometry. The techniques for achieving SLM in inversion lasers are well-developed, as are also frequency extension by frequency conversion using harmonic crystals and optical parametric oscillators. In contrast, Raman lasers, which are well-known for having advantages for diversifying wavelength range and producing high-quality output beams, are much more immature in terms of SLM operation. This thesis presents an experimental approach to demonstrate SLM operation in a standing wave crystalline Raman laser. Theoretical concepts are developed to elucidate the gain saturation mechanisms and mode competition in Raman lasers. It is shown that gain saturation is homogeneous for narrow linewidth pumping and that the absence of spatial hole burning in a Raman gain medium leads to the possibility of intrinsically stable SLM operation.This concept is implemented in narrow-line width pumped external cavity Raman lasers in the continuous wave regime. Two Raman crystals –diamond and potassium yttrium tungstate (KYW) are investigated to understand the effects of material properties on longitudinal mode structure. In the case of KYW, it is seen that the laser output spectrum and efficiency were greatly influenced by a lesser-known low-frequency phonon mode (87 cm–1), which is shown to havehigh gain coefficient higher than the mode currently accepted as the primary mode. Owing to the complexity of the laser output spectrum due to the participation of this low-frequency mode in the Raman frequency conversion process, investigation of the mode spectrum was challenging. However, the low-frequency mode at 87 cm–1 is advantageous for applications requiring multi-wavelength laser sources. As the quantum defect is much lower than for the conventional phonon modes in Raman crystals, it could be used for beam enhancement and/or beam combination. Diamond is seen as a much better option for investigating SLM as it has a "pure" single phonon mode, in addition to its high Raman gain and superior thermal conductivity. Despite having 35 longitudinal modes in the diamond gain bandwidth (45 GHz), SLM operation is demonstrated at 1240 nm up to 4 W of Stokes output power for the free-running diamond Raman laser. Moreover, this SLM demonstration is significant as the gain medium wa spositioned at the midpoint of the standing-wave cavity, which would normally be unsuited to SLM operation in the case of an inversion laser. The pumping level and output power of this SLM operation are higher than the reported SLM inversion lasers. The coupling between the intracavity Stokes power and the optical cavity length via the thermo-optic effect and thermal expansion of diamond was found to be responsible for the multimode operation at higher powers. Extension to longer wavelengths through second Stokes generation was also investigated. The aggravated thermal effects in diamond and cascading effect on pump depletion led to the multimode operation at low output powers. As a result, a volume Bragg grating in a coupled cavity design was used to increase the SLM power. Up to 0.5W SLM power at 1485 nm wasdemonstrated with a frequency stability of the order of the pump frequency fluctuations. The feasibility of the DRL as a LIDAR transmitter was investigated through a demonstration of environmental water vapour detection. Finally, active stabilization of the cavity length was investigated to extend the range of power and to increase frequency stability. A novel variant of Hänsch-Couillaud (HC) technique for cavity locking was developed that is proposed to be based on polarisation dependent pump depletion. This successfully increased the maximum output power to 7.2 W, which is believed to be one of, if not the highest SLM power reported for a solid-state SLM oscillator. Stimulated Brillouin scattering is observed as parasitic effect but provides a novel platform for the future development of diamond Brillouin lasers.