High repetition tunable femtosecond light sources are widely used in time-resolved spectroscopy, nonlinear spectroscopy, and bioimaging, and other fields. Optical parametric oscillators (OPO) and optical parametric amplifiers (OPA) are the most frequent equipment to generate tunable femtosecond pulses and the high pump energy in single pass amplification. The threshold of OPA is limited by the minimum pulse energy to create a supercontinuum seed. OPA is usually applied in low repetition and high-energy laser systems. For high repetition system, a high average power pump laser source is required. OPO is applied in high repetition systems, where the threshold is much lower. However, the complex cavity and poor stability constrain its application. OPA with an extremely low threshold is a possible solution for a stable high repetition tunable ultrafast laser source. In earlier studies, various techniques were used to lower the OPA threshold. These techniques fall into two categories: lowering the threshold of seed generation through supercontinuum generation in highly nonlinear optical fibers, and replacing the gain medium with crystals with high nonlinear coefficients like PPLN. The cost of these OPAs is much higher, and the stability is also poor. The article reports a femtosecond OPA design with an extremely low threshold and excellent stability. The threshold of our setup is only 300 nJ at 1030 nm and the long-time stability for 6 h is approximately 0.22%. Our design is possible to replace femtosecond OPOs.

To reduce the OPA threshold, a YVO₄crystal, the nonlinear refractive index of which is much higher than that of common crystals, is used as the medium of supercontinuum generation. The calculated threshold of the YVO₄crystal is 0.54 MW at 1030 nm. Details of the setup of OPA are shown in Fig. 1. The pump laser source is a femtosecond fiber laser with a central wavelength of 1030 nm, a repetition rate of 1 MHz, an average power of 10 W, and a pulse width of 300 fs. The 0.3 W power is used to pump the OPA. The pump laser is split by a half wave plate and a thin film polarizer. Approximately, the 180 nJ energy is focused in the YVO₄crystal to generate the supercontinuum seed. The residue pulse is frequency doubled in a barium borate crystal (polar angle θ=23.4°, azimuth angle φ=0°, and thickness t=1 mm) to generate the pump pulse and the conversion efficiency is approximately 50%. The pump pulse at 515 nm and the supercontinuum seed are combined by a dichromic mirror and focused into another barium borate crystal (θ=22.3°, φ=0°, and t=5 mm). A wavelength separator separates the amplified signal and idler pulse, and the reflected signal is compressed by a pair of Brewster angle prisms.

The supercontinuous white light is generated in the YVO₄crystal using 1030 nm light with energy of about 180 nJ. The spectrum of supercontinuum white light generated in the YVO₄ crystal is shown in Fig. 3. The amplified signal is observed when the delay between the seed and pump pulse is carefully adjusted. The maximum output energy of the signal is 18.5 nJ at 690 nm. The transmission efficiency of the compressor is 93%. The compressed pulse width at 690 nm is 59 fs and the minimum pulse width is 35 fs at 870 nm. The wavelength of tunable pulses is 650-950 nm, which is limited by the spectrum of the supercontinuum seed. The time-bandwidth of the output signal is far from the Fourier transfer limit because the higher order dispersion is not compensated. The compressed pulse width is shorter than the pulse width of the reported OPO. The spectra of output signals are shown in Fig. 5. The energy and the minimum compressed pulse width are shown in Fig. 6. Also, the 6 h stability test reveals an excellent result. The power stability is 0.22%. This design's framework is straightforward, the optical distance is less, and the seed is only amplified once. These are the primary causes of the output power's stability. Due to the single-stage architecture, the input pump pulse energy of this OPA could not be so high. Herein, the highest pump pulse energy is 10 μJ, which is limited by the fiber laser. Under the highest pump energy, the output of the OPA is still stable.

In this article, a new design of single-stage OPA is reported. A YVO₄ crystal is used for supercontinuum seed generation to achieve an extremely low threshold, and the threshold is down to 300 nJ at 1030 nm. The tunable output is acheived at 650-950 nm, and the minimum compressed pulse width is shorter than 35 fs. This design also exhibits excellent long-time stability, and the stability of the output power for 6 h is only 0.22%. It is possible to replace OPO with a pump laser at the repetition rate of dozens of MHz. This OPA design has great potential in time-resolved spectroscopy, nonlinear spectroscopy, bioimaging, etc.