A widely tunable terahertz (THz)-wave generation that has a high repetition rate and a narrow line width is demonstrated in this paper by injection-seeded THz-wave parametric generation (is-TPG) in a MgO:LiNbO3 crystal. By pumping the crystal using a passively Q-switched neodymium-doped yttrium vanadate microchip laser with a time duration of 140 ps and an average power of up to 5 W, a THz-wave output with an average output power of 30 μW, a peak power of 4 W, a pulse duration of 73 ps, and a pulse-repetition frequency of 100 kHz is obtained. To prevent laser damage and photorefractive damage to the crystal, the constraints on the pumping condition of the MgO:LiNbO3 crystal are experimentally studied by changing the pumping parameters. As a result, we achieved the stable generation of the THz-wave signal in the 100 kHz regime. Moreover, we performed THz-wave imaging by using the developed is-TPG source. The obtained THz image indicated that the developed system has a good stability over long periods of time.

An injection-seeded terahertz (THz)-wave parametric generator (is-TPG) with a footprint the size of an A3 paper is presented. We improved the measurement performance of the is-TPG source for nondestructive inspection applications. A high pulse repetition rate up to 70 kHz and a low pulse timing jitter of a few tens of picoseconds, which is approximately one ten-thousandth of the conventional is-TPG, were achieved. THz waves exhibited excellent performance with a maximum average output power of 20 µW, a monochromatic spectrum linewidth of ∼20 GHz, and a frequency tuning range of 1.7–3.0 THz. This was achieved by designing the entire system configuration from the pump laser source to THz-wave generation. A new double-pass all-solid-state optical amplifier was developed with a high gain and low noise using an externally pulse-modulated laser diode (LD) as the master oscillator source. An achromatic optical injection system was developed for the is-TPG with a 40% reduction in the conventional path length. They were housed in a single enclosure in two layers. LDs and optical fiber amplifiers could be rack-mounted, and the outputs were delivered to the housing via optical fibers. The developed THz-wave source performed nondestructive imaging of a human hair sample fixed with Scotch tape on a test pattern in an envelope by irradiating 2.1 THz waves. A clearly recognizable THz-wave image of an enclosed hair with a spatial resolution close to the THz wavelength was obtained.

High lateral resolution ( ∼ 5 µ m ) optical coherence tomography (OCT) that employs a variable cross-cylinder (VCC) to compensate for astigmatism is presented for visualizing minute structures of the human retina. The VCC and its sensorless optimization process enable ocular astigmatism correction of up to − 5.0 diopter within a few seconds. VCC correction has been proven to increase the signal-to-noise ratio and lateral resolution using a model eye. This process is also validated using the human eye by visualizing the capillary network and human cone mosaic. The proposed method is applicable to existing OCT, making high lateral resolution OCT practical in clinical settings.