Mingjun Chi

Widely separated optical parametric oscillation (OPO) represents a powerful method for coherent wavelength conversion across infrared and visible spectra. While such generation has been demonstrated in material platforms such as silicon nitride and lithium niobate, 4H‐SiC remains unexplored despite offering combined strong second‐order and third‐order nonlinearities with ultralow material loss. Here we demonstrate tunable, widely separated OPO generation in 4H‐SiC microresonators through dispersion engineering. By optimizing the resonator geometry to achieve normal dispersion at telecommunication wavelengths and pumping at around 1550 nm, a pair of signal and idler spanning nearly an octave is generated, which represents the first demonstration of widely separated OPO in 4H‐SiC. The frequency separation is tuned by varying the pump wavelength, with measured signal and idler wavelengths aligning well with phase‐matching predictions. Leveraging the non‐centrosymmetric crystal structure of 4H‐SiC, the generated OPO signal undergoes cascaded second‐harmonic generation (SHG) and sum‐frequency generation (SFG) with the pump, yielding coherent visible light at wavelengths below 700 nm. This cascaded upconversion of widely separated OPO signals represents a novel pathway for visible light generation. These results establish 4H‐SiC as a promising platform for nonlinear wavelength conversion spanning from visible to 2 µm region.

This study explores the application of fluorescence hyperspectral imaging (FHSI) for marine ecosystem monitoring, combining laser-induced fluorescence (LIF) with hyperspectral imaging (HSI), an approach rarely applied in marine ecology. Photopigments such as chlorophylls, carotenoids, and phycobiliproteins emit distinct fluorescence signals when excited at specific wavelengths. Species-discriminating spectral signatures can then address persistent challenges in benthic monitoring, including overlapping optical properties among species and complex underwater conditions. To analyse these subtle interspecific pigmentation differences, we examined their continuous emission spectrum using an inexpensive 450 nm broad-area diode laser as the light source, instead of a broad-spectrum light, in combination with a push-broom hyperspectral camera and a one-dimensional convolutional neural network (1D-CNN). We classified brown, red, and green macroalgae, eelgrass, and blue mussels in laboratory and field experiments in Danish coastal waters, achieving macro-F1 scores of 80.2% and 79.6%, respectively. We show that the method was highly accurate for photopigmented organisms but limited for dark-coloured, low-fluorescence species such as mussels. Field tests identified sunlight and turbidity as the main constraints on in situ performance, while fluorescence fingerprints remained detectable under shaded conditions. These findings establish FHSI as a promising tool for automated benthic habitat analysis and remote-sensing-based marine ecosystem monitoring.

Raman lasing in microresonators has been observed in various material platforms, such as silicon, silica, lithium niobate, diamond, and silicon carbide. Frequency matching between cavity resonances and a Raman gain profile enables a lasing threshold as low as a milliwatt level with a continuous-wave pump. However, the distinct thermal responses of the cavity resonances and the Raman phonon frequency can induce frequency mismatch and lead to unstable Raman lasing with varying temperatures. This temperature-dependent Raman lasing has not been sufficiently investigated to date. Here, we characterize two prominent Raman phonons at 265 and 776 cm−1 in 4H-silicon carbide (SiC) microresonators and analyze their thermal evolution. We observe that the Raman signal near 776 cm−1 exhibits a thermal shift rate closely matching that of the cavity resonance, enabling stable Raman lasing for a temperature range exceeding 88 K. In contrast, Raman lasing near 265 cm−1 behaves in an unstable manner, due to the narrow Raman gain bandwidth and the large mismatch between the thermal response of the Raman gain and the cavity resonance. These findings demonstrate the stable lasing behavior of the dominant Raman signal at ∼776 cm−1 of 4H-SiC and establish crucial guidelines for achieving thermally robust Raman lasing in microresonators.

We investigate microsecond pulse-mode operation of a micro-integrated high-power diode laser based on volume Bragg grating external-cavity feedback around 808 nm. The laser system contains a tapered amplifier consisting of a ridge-waveguide section and a tapered section with separated electrical contacts. Thus, the diode laser system can be pulsed by modulating the injected current either to the ridge waveguide section ( I R W ) or to the tapered amplifier section ( I T A ). With a trigger signal of a 50 µs pulse width and a 10 kHz repetition rate, comparing the modulation depth, peak output power, beam propagation factor, and spectral bandwidth, we conclude that the pulse-mode operation achieved by modulating the I T A gives better results than by modulating the I R W due to the decreased thermal effect. At a constant I R W of 0.2 A and a modulated I T A of 6.0 A, 4.3 W of peak output power is obtained with an emission spectral bandwidth with an upper bound of 0.2 nm, and a beam propagation factor in the slow axis, M s l o w 2 , of 2.6 ( 1 / e 2 ). The modulation depth is almost 100%. The results show that the tapered diode laser system may be a good candidate for microsecond pulse-mode solid-state lasers.

A novel compact micro-integrated high-power narrow-linewidth external-cavity diode laser around 808 nm is demonstrated. The laser system contains a tapered amplifier consisting of a ridge-waveguide section and a tapered section with separated electrical contacts. Thus, the injection currents to both sections can be controlled independently. An external volume Bragg grating is utilized for spectral narrowing and stabilization. The diode laser system is integrated on a 5 m m × 13 m m aluminum nitride micro-optical bench on a conduction cooled package mount with a footprint of 25 m m × 25 m m . The diode laser system is characterized by measuring the output power and spectrum with the injection currents to the ridge-waveguide section ( I RW ) and tapered amplifier section ( I TA ) changed in steps of 25 and 50 mA, respectively. At I RW = 200 m A and I TA = 6.0 A , 3.5 W of output power is obtained with an emission spectral linewidth with an upper bound of 6 pm, and a beam propagation factor in the slow axis, M 2 , of 2.6 ( 1 / e 2 ). The characterization of the temperature stabilization of the laser system shows an increase of the wavelength at a rate of 6.5 pm/K, typical for the applied volume Bragg grating.

In this chapter, both blue and green high-power tunable diode laser systems based on GaN broad-area diode laser (BAL) in Littrow external cavity are demonstrated. For blue diode laser system, for high-power application, an output power around 530 mW over a 1.4 nm tunable range is obtained; for wide tunable range application, an output power around 80 mW over a 6.0 nm tunable range is obtained. For the green diode laser system, for high-power application, an output power around 480 mW with a tunable range of 2.1 nm is achieved; for wide tunable range application, an output power of 50 mW with a tunable range of 9.2 nm is achieved. The tuning range and output power optimization of an external-cavity diode laser system is investigated based on the experimental results obtained in the blue and green external-cavity GaN diode laser systems. The obtained results can be used as a guide for selecting gratings for external-cavity diode lasers for different requirements. The temporal dynamics of the green diode laser system is studied experimentally, and pulse package oscillation is observed, for the first time to our knowledge, in a BAL with an external-cavity grating feedback.

The results of photoluminescence (PL) imaging performed with a laser line scan in a prototype setup are presented in this work. Here we show the PL images with the photovoltaic cell positioned at different distances from the laser line source, therefore with the beam at different intensities and line characteristics. This study is motivated by the time demanding process of acquiring luminescence images for accurate PV inspections in the field, usually made with electroluminescence imaging. The PL imaging induced by a laser line has the potential to be compact enough to be portable and carried out by drones for field inspections.

Although external-cavity feedback with a grating is widely used to achieve a tunable high-power narrow-linewidth broad-area diode laser (BAL) system, the dynamics of such a system is seldom studied. In this paper, the temporal dynamics of a tunable high-power green external-cavity diode laser system based on a GaN BAL and Littrow external-cavity is investigated experimentally. The regular pulse package oscillation (PPO) is observed just above the threshold. The oscillating period of the pulse package decreases with the increasing injected current. As the current increases further, the pulse package oscillates irregularly, and finally changes to a chaotic state. The PPO is observed, for the first time to our knowledge, in a BAL with an external-cavity grating feedback.

Diffuse optical tomography (DOT) is a reliable and widespread technique for monitoring qualitative changes in absorption inside highly scattering media. It has been shown, however, that acousto-optic (AO) imaging can provide significantly more qualitative information without the need for inversion algorithms due to the spatial resolution afforded by ultrasound probing. In this Letter, we show how, by using multiple-wavelength AO imaging, it is also possible to perform quantitative measurements of absorber concentration inside scattering media.