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%.

A sub-Nyquist random single-pulse sampling method based compressive sensing technique is proposed firstly to reconstruct signals under test in phase-sensitive optical time domain reflectometry (φ-OTDR). The key information of Rayleigh backscattering curves can be recognized after sparse transformation, which can be successfully reconstructed with far smaller number of measurements by orthogonal matching pursuit algorithm. Unlike the uniform sampling in conventional φ-OTDR system, pulse intervals are randomly modulated to generate a random single-pulse sequence in the proposed method in order to achieve random single-pulse sampling along every sensing point of the fiber. Experimental results verify that pulse sequences with average pulse repetition rate less than 2-kHz can be efficiently utilized to reconstruct an 8-kHz signal under test and obtain its frequency information, as well as to realize the detection of high frequency signals at different locations along the sensing fiber. It is demonstrated that the sampling rate is reduced greatly without any hardware modification compared to that required in the conventional method. This method can break the limits of the traditional Shannon-Nyquist sampling theorem to reconstruct signal, which is of great significance for the sensor performance enhanced in traditional φ-OTDR system.

A widely wavelength tunable mode-locked Yb-doped fiber oscillator based on nonlinear amplifier loop mirror (NALM) is reported, in which only a piece of short (∼0.5 m) single-mode polarization-maintaining (PM) Yb-doped fiber is employed, instead of the frequently used long (a few meters) double cladding (DC) fiber in previous papers. Experimentally, the center wavelength can be consecutively tuned from 1015 to 1105 nm by tilting the silver mirror, corresponding to a tuning range of 90 nm. To the best of our knowledge, this is the broadest consecutive tuning range in Yb:fiber mode-locked fiber oscillator. In addition, the mechanism of wavelength tuning is tentatively analyzed and attributed to the combined action of the spatial dispersion induced by a tilting silver mirror and the limited aperture in the system. Specific to the wavelength of 1045 nm, the output pulses with 13-nm spectral bandwidth can be compressed to 154 fs.

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%.

A 520 nm laser-pumped 100 MHz LBO-based femtosecond optical parametric oscillator (fs-OPO) with high stability of spectra and power is presented, which is enabled by the side-of-fringe locking technique and whose signal wavelength can be tuned from 650 nm to 980 nm. The average powers are higher than 1 W within the nearly whole wavelength range (700–980 nm). Especially, the average power goes up to 1.8 W at 740 nm, with a conversion efficiency of 27.3%. To demonstrate the universality, three wavelengths of 740 nm, 800 nm, and 920 nm, which are called for by biophotonics, are advisedly chosen to verify the capability of the side-of-fringe locking technique for both spectral and power stabilizations. As a result, power fluctuations (in RMS) of 1.6%, 0.5%, and 1.6%, and central wavelength drifts (in SD) of 0.44 nm, 0.13 nm, and 0.19 nm, are obtained for those three wavelengths, respectively. In addition, pulse durations are measured to be 630 fs, 139 fs, and 182 fs. Those combined performances leave the possibility for long-time data acquirement in imaging experiments.

A method for maintaining a fixed phase relationship between the driving signal of acousto-optic modulator (AOM) and the original mode-locked seed laser is proposed and realized, which stabilizes the amplitude of diffracted signal output from the AOM for subsequent amplification. A field-programmable gate array (FPGA), combined with external summing amplifiers, is used to directly synthesize high-timing-precision driving signals that are synchronized with the seed laser pulses, and the accuracy of signal timing control reaches 160 ps. Using this driver, the standard deviation of the diffracted signal output from the AOM is significantly decreased from 0.52% to 0.18%. This pulse-picking solution also includes a control system that can flexibly control the frequency, gating width, etc., of the driving signal, which makes it more convenient for subsequent laser amplification and makes it suitable for a variety of mode-locked lasers.