Common methods of ocean remote sensing and seafloor surveying are mainly carried out by airborne and spaceborne hyperspectral imagers. However, the water column hinders the propagation of sunlight to deeper areas, thus limiting the scope of observation. As an emerging technology, underwater hyperspectral imaging (UHI) is an extension of hyperspectral imaging technology in air conditions, and is undergoing rapid development for applications in shallow and deep-sea environments. It is a close-range, high-resolution approach for detecting and mapping the seafloor. In this paper, we focus on the concepts of UHI technology, covering imaging systems and the correction methods of eliminating the water column’s influence. The current applications of UHI, such as deep-sea mineral exploration, benthic habitat mapping, and underwater archaeology, are highlighted to show the potential of this technology. This review can provide an introduction and overview for those working in the field and offer a reference for those searching for literature on UHI technology.

A stable passively mode-locked Er-doped silica fiber laser with a fundamental repetition rate of up to 5 GHz is demonstrated, which, to the best of our knowledge, is the highest repetition rate for 1.5 μm semiconductor saturable absorber mirror (SESAM) mode-locked Er-doped silica fiber (EDF) lasers. A segment of commercially available EDF with a net gain coefficient of 1 dB/cm is employed as gain medium. The compact Fabry-Pérot (FP) cavity features a fiber mirror, namely multiple-layer dielectric films (DFs) directly coated on end facet of a passive fiber ferrule, enabling a short cavity length of 2 cm configured. The mode-locked oscillator operates at 1561.0 nm with a signal-to-noise ratio (SNR) of 62.1 dB, whose average power is boosted to 27 mW by a single-mode Er-doped fiber amplifier (EDFA) and spectral bandwidth is broadened form 0.69 nm to 1.16 nm with a pulse width of 3.86 ps. The fiber laser shows excellent spectral stability without conspicuous wavelength drifting for 3 hours. Moreover, the basic guidelines of selecting SESAM for high repetition rate passively mode-locked fiber lasers is given.

A high-efficiency ultrafast laser amplifier based on thin-rod Yb:YAG was demonstrated, featuring a 940- and 969-nm brightness-maintaining wavelength-multiplexed laser diode (LD)-pumping method. Two high-brightness LDs (940 nm and 969 nm) were spectrally combined into one beam spatially with a dichroic mirror, thus enabling twice pump power while maintaining high brightness. A maximum signal power of 240 W was obtained at a repetition rate of 1 MHz, with a power extraction efficiency (PEE) of ∼51%. To the best of the authors’ knowledge, this is the first report of >50% efficiency as well as the highest average power operating at the fundamental mode for thin-rod Yb:YAG amplifiers. The beam quality factors (M²) of the amplified signal were measured to be 1.72 and 1.12 for the horizontal and vertical directions, respectively. A preliminary pulse compression was conducted at a signal power of 80 W with a chirped volume Bragg grating (CVBG) compressor. The compressed pulse duration was 744 fs with an average power of 66.5 W, corresponding to a compression efficiency of 83.1%.

We describe a setup of time-, spin-, and angle-resolved photoemission spectroscopy (tr-SARPES) employing a 10.7 eV (λ = 115.6 nm) pulse laser at a 1 MHz repetition rate as a probe photon source. This equipment effectively combines the technologies of a high-power Yb:fiber laser, ultraviolet-driven harmonic generation in Xe gas, and a SARPES apparatus equipped with very-low-energy-electron-diffraction spin detectors. A high repetition rate (1 MHz) of the probe laser allows experiments with the photoemission space-charge effects significantly reduced, despite a high flux of 1013 photons/s on the sample. The relatively high photon energy (10.7 eV) also brings the capability of observing a wide momentum range that covers the entire Brillouin zone of many materials while ensuring high momentum resolution. The experimental setup overcomes the low efficiency of spin-resolved measurements, which gets even more severe for the pump-probed unoccupied states, and affords the opportunity to investigate ultrafast electron and spin dynamics of modern quantum materials with energy and time resolutions of 25 meV and 360 fs, respectively.

The Yb:fiber based high-power and high-energy ultrafast lasers usually feature central wavelength of 1030 nm instead of the more common one of 1064 nm, thus limiting some potential wavelength-dependent applications. In this work, a hectowatt 1064-nm femtosecond chirped pulse amplification (CPA) system based on double-stage rod-type photonic crystal fiber (PCF) amplifier was demonstrated, with a pulse energy of 212-μJ and a repetition rate of 0.5 MHz. Compared with our previous work [Opt. Express 30(6), 3611, 2022], two key measures were employed. Firstly, a Yb:YAG crystal was intentionally introduced as a spectral filter for avoiding 1030-nm amplified spontaneous emission (ASE) oscillation, which hence improved the signal-to-noise ratio (SNR) by 15 dB. Secondly, an additional stage of rod-type PCF amplifier was adopted for further power scaling, achieving an average power of 127 W. Finally, the pulse was compressed to ∼460 fs, with an average power of 106 W and a compression efficiency of ∼83%. The beam quality factors (M ² ) were measured to be M2 X = 1.12 and M2 Y = 1.09. To the best of our knowledge, this is the highest pulse energy among 1064-nm hectowatt-level femtosecond lasers, thus providing a high-quality source for wavelength-dependent applications. For example, it can be used as a driver for high-repetition-rate vacuum ultraviolet (VUV) sources about 10.5 eV, with an expected conversion efficiency of up to 1%.

The underwater hyperspectral imager enables the detection and identification of targets on the seafloor by collecting high-resolution spectral images. The distance between the hyperspectral imager and the targets cannot be consistent in real operation by factors such as motion and fluctuating terrain, resulting in unfocused images and negative effects on the identification. In this paper, we developed a novel integrated underwater hyperspectral imaging system for deep sea surveys and proposed an autofocus strategy based on liquid lens focusing transfer. The calibration tests provided a clear focus result for hyperspectral transects and a global spectral resolution of less than 7 nm in spectral range from 400 to 800 nm. The prototype was used to obtain spectrum and image information of manganese nodules and four other rocks in a laboratory environment. The classification of the five kinds of minerals was successfully realized by using a support vector machine. We tested the UHI prototype in the deep sea and observed a Psychropotidae specimen on the sediment from the in situ hyperspectral images. The results show that the prototype developed here can accurately and stably obtain hyperspectral data and has potential applications for in situ deep-sea exploration.