David James Spence

We present the remote measurement of temperature vs length profiles in up to 50 m of clear (swimming pool) water. A pair of blue gallium-nitride laser diodes, with wavelengths separated by 4 nm, was multiplexed in time as the excitation source. Elastic and Raman returns were collected using two detector channels. The ratio of the Raman measurements can be calibrated to retrieve temperature as a function of depth, while the ratio of the elastic measurements can be used to correct for differential attenuation between the wavelengths. The temperature accuracy decreases with range and is typically better than 0.5 °C at 10 m and 1 °C at 50 m with depth resolution of 0.5-4 m, respectively, using 5 ns excitation pulses.

Raman lasers are emerging as promising sources of optical frequencies with low phase noise. Studies so far have centered on diamond as the Raman gain medium, a material well suited to stable single longitudinal mode operation. Here we report a single longitudinal mode GdVO ₄ Raman laser based on a doubly resonant cavity stabilized to the pump frequency using Pound–Drever–Hall locking and providing output wavelengths at 1164 nm and 1174 nm. Frequency noise measurements at high Fourier frequencies indicate a Lorentzian linewidth below 3 Hz, with the observed value limited by technical noise in the laser cavity and the linewidth measurement system. To our knowledge, this represents the narrowest linewidth reported for any Raman laser and demonstrates that non-diamond Raman materials can support narrow linewidth operation and a greater diversity of output wavelengths.

Modern tunable lasers are indispensable instruments in fundamental science and optical frequency applications. With the growing interest in structured light, there’s a need for tunable structured lasers with high transverse electromagnetic (TEM) modal purity across a broad spatial spectrum, but no commercial solutions exist. Here, we present the first tunable structured laser that can emit all possible single TEM modes with extremely high purity over a wide range. This is achieved by the collaborative use of intracavity pump geometry and astigmatic detuning to selectively gain a desired mode while blockading others. Our astigmatic oscillator can tunably generate any TEM mode within the spatial bandwidth, achieving over 40,000 orthogonal Hermite-Gauss modes in the experiment, and each can be further expanded into numerous Hermite-Laguerre-Gauss modes via unitary transformation. This approach does not require extra intracavity beam shaping components, offering a promising design for commercially available structured lasers with full spatial spectrum tunability.

We investigate key factors affecting the power output of a diamond Raman laser with double-pass intracavity second harmonic generation. Using experiment and model, we investigate laser power and efficiency as the temperature of the frequency doubling crystal is scanned through its phase matching condition. It is found that the inclusion of the Gouy phase shift in the calculation of round-trip phase offset for the second-harmonic, making its second pass through the frequency doubling material, is required to adequately account for the observed power behaviour. The experiments also reveal “drop-outs” in laser power at regular temperature intervals, which we attribute to flipping of the mode polarisation state around null points in the net retardance of the diamond and doubling crystal. Temperature scanning provides a simple technique for determining cavity conditions that optimise phase offset, avoid polarisation “drop-outs”, and lead to optimal mode sizes.

Modern tunable lasers are indispensable instruments in fundamental science and optical frequency applications. With the growing interest in structured light, there's a need for tunable structured lasers with high transverse electromagnetic (TEM) modal purity across a broad spatial spectrum, but no commercial solutions exist. Here, we present the first tunable structured laser that can emit all possible single TEM modes with extremely high purity over a wide range. This is achieved by the collaborative use of intracavity pump geometry and astigmatic detuning to selectively gain a desired mode while blockading others. Our astigmatic oscillator can tunable generate any TEM mode within the spatial bandwidth, achieving over 40,000 orthogonal Hermite-Gauss modes in the experiment and each can be further expanded into numerous Hermite-Laguerre-Gauss modes via unitary transformation. This approach does not require extra intracavity beam shaping components, offering a promising design for commercially available structured lasers with full spatial spectrum tunability.

Raman lasers are well known for wavelength shifting and beam clean up, but less so for linewidth narrowing. Recent advances in design and material choice have enabled stable single frequency operation and the opportunity to investigate their narrow linewidth properties. Here, we use modeling and experiments to investigate linewidth and frequency noise as a function of pump noise. The model reveals that the picosecond-scale phonon dephasing time, typical of most Raman materials, suppresses pump frequency noise by factors up to 108 at Fourier frequencies up to 1011 Hz. To support the model, frequency noise spectra were measured for the input and output beams of a singly-resonant Raman laser operating at 1178 nm, revealing noise suppression by more than 104 at offset frequencies above 1 MHz. The results show that Raman lasers offer orders-of-magnitude greater linewidth-narrowing than Brillouin lasers, even for cavities of moderate Q factor. This makes them a promising route toward ultra-narrow linewidth lasers across the spectrum with adaptability to a wide range of pumps.

Gallium nitride (GaN) laser diodes are shown to offer a viable alternative to the solid-state lasers typically used in photon counting ocean LiDAR. With their ability to operate at various blue wavelengths, coupled with compactness and efficiency, they offer some considerable advantages over conventional solid-state sources.

We report an intra-cavity frequency doubled diamond Raman laser operating at 607 nm. A z-fold cavity design was configured to prevent back reflections into the fiber amplifier, which avoided the use of isolators in the pump beam path. A maximum output power of 60 W was generated in two output beams at an optical conversion efficiency of 28% from the 1045 nm pump. A maximum single-beam output power of 40 W was obtained using a highly reflecting end mirror for the visible. Output power and single frequency stability are negatively impacted by an increasing role of stimulated Brillouin scattering at output powers above 10 W.

We present a method to simultaneously determine water temperature and salinity, which uses a pulsed excitation laser and a three-channel Raman spectrometer. The method relies on the systematic dependence of the Raman OH stretching band on temperature and salinity, and is compatible with LiDAR techniques. We have measured the variation of the OH stretching band in two seawater samples and a NaCl solution, and constructed a linear mapping between signal ratios derived from the three spectral channels and the temperature and salinity of each sample. For the natural seawater this approach has been determined by cross-validation to have a predictive accuracy of ±1.6 PSU and ±0.5 °C.

We present the retrieval of depth-resolved temperature measurements in water using Raman LiDAR. Using a 5 m pipe of laboratory water, we recover non-homogeneous temperature profiles with a temperature accuracy ranging between 0.35°C and 0.85°C, and a position resolution of 28 cm.