Thesis

Low-noise narrow-linewidth optical parametric oscillator intra-cavity pumped by a continuous-wave VECSEL

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Awarding institution
  • University of Strathclyde
Date of award
  • 2026
Thesis identifier
  • T18187
Person Identifier (Local)
  • 201974368
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Qualification Name
Department, School or Faculty
Abstract
  • This thesis demonstrates a unique type of continuous-wave (CW) optical parametric oscillator (OPO), called an intracavity singly resonant OPO (ICSRO). This configuration exploits the high circulating optical field within a laser cavity to efficiently overcome the otherwise high oscillation threshold associated with externally-pumped SROs. The fundamental nonlinear process associated with OPOs is optical parametric down-conversion (PDC), which results in the simultaneous creation of two distinguishable photons from a single higher-energy pump photon. The parametrically generated signal and idler photon pairs are intrinsically correlated in energy, momentum, and polarization and, in the ideal lossless case, exhibit complete suppression of intensity-difference noise. An ICSRO based on 5%-mol magnesium-oxide-doped periodically-poled LiNbO3 (5%-MgO:PPLN) was implemented and fully characterised. The nonlinear crystal was integrated within a four-mirror, optically-pumped, vertical-external-cavity surface emitting laser (VECSEL). Optimised for low-oscillation threshold and in the absence of output coupling (OC), the system exhibited a SRO threshold at an input external pump power of 1.78 W, delivering 14.2 mW (one-way) of extractable non-resonant signal power at 2.96 W of input pump power, corresponding to a total down-conversion efficiency of 72.1%. With the introduction of a 1.7% OC at the resonant idler wavelength, the SRO produced 26.2 mW of signal and 5.6 mW of one-way idler output power. This corresponds to a down-conversion efficiency of 70.2% and an extraction efficiency of 43%. Thermal tuning of the 5%-MgO:PPLN crystal enabled continuous wavelength tuning over 1155–1300 nm (for the signal) and 1474–1718 nm (for the idler), limited solely by the bandwidth of the dielectric mirror coatings. For quantum applications, where balanced photodetectors exhibit high quantum efficiency at telecom wavelengths, emission was targeted near the telecom O-band (1.3 μm) and C-band (1.55 μm), achieved with a fixed VECSEL fundamental wavelength of 690.6 nm, a 5%-MgO:PPLN grating period of 14.55 μm, and a stabilised crystal temperature of 114◦C. Furthermore, the short-term power stability of the VECSEL fundamental was characterised both above and below the SRO threshold, demonstrating high amplitude stability in both regimes, in agreement with the Class-A dynamics of the VECSEL. Long-term RMS stability was also measured, yielding 0.4%, 1.8%, and 2.3% for the VECSEL fundamental, signal, and idler, respectively, with all three interacting waves oscillating in a single longitudinal mode. Due to the stable, single-frequency operation of the ICSRO VECSEL, further development of the system was demonstrated through the implementation of active stabilisation schemes. Under dual-locking conditions, where the VECSEL fundamental and the intracavity resonant idler were stabilised using Pound–Drever–Hall (PDH) and side-offringe (SOF) locking, respectively, relative intensity noise (RIN) was reduced by more than 20 dB for each interacting wave at low Fourier frequencies (∼10 Hz). Narrow integrated linewidths of 6.21(1) kHz for the VECSEL fundamental and 151(1) Hz for the resonant idler were achieved over integration times as long as 4s, illustrating the transfer of noise from the VECSEL fundamental to the OPO non-resonant signal. Even under PDH-locking alone, the non-resonant signal linewidth remained narrow at 3.48(1) kHz. In conclusion, the superior noise performance – particularly in terms of both intensity and frequency noise – demonstrates that the dual-locked ICSRO VECSEL system is a promising high-quality coherent light source for future quantum technologies, especially in quantum optics experiments and the generation of squeezed states of light. Future work may focus on achieving further noise reduction through additional active and passive stabilisation techniques, as well as exploring more complex OPO architectures.
Advisor / supervisor
  • Hastie, Jennifer
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DOI

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