Ultrafast optical parametric amplifiers (OPA) providing femtosecond laser pulses with a tunable wavelength have broad applications in ultrafast sciences, high-field physics, generation of table-top X-ray, or extreme ultraviolet radiations. In a standard optical parametric amplification scheme, a microjoule laser pulse propagates in a solid transparent nonlinear medium to generate supercontinuum seed light and a specific red-shifted infrared spectral range is selected for further stages of amplifications. However, the conversion efficiency of the red-shifted component is considerably lower than that of the blue-shifted component because of the nonlinear optical effects, such as self-steepening and electron-hole plasma generation during supercontinuum generation, requiring a high-input laser intensity close to the damage threshold to obtain red-shifted spectral components with a wavelength of approximately 1600 nm. Thus, the solid nonlinear medium is vulnerable to laser damage and the system reliability is degraded. To solve this problem, we propose taking advantage of the high conversion efficiency of the blue-shifted spectral component in the supercontinuum and preamplifying it with the second harmonic generation of the pump laser pulse for infrared tunable idler wave, which is further amplified in the following power amplification stages. In this scheme, the requirement of the input laser intensity for supercontinuum seed light generation is relaxed, reducing the possibility of solid nonlinear medium damage and improving the system reliability.
In this study, the ultrafast OPA-based on second harmonic generation pumped-preamplification has three stages: the white light supercontinuum generation seed, second harmonic generation pumped-preamplification, and fundamental wave pumped power amplification stages. The incident laser pulse (1 kHz repetition rate, 800 nm wavelength, 37 fs pulse duration, and 295 μJ pulse energy) is split using beam splitters, and approximately 10%, 20%, and 70% of the pulse energy are coupled into the seed, preamplification, and power amplification stages, respectively. In the seed stage, a 5 mm thick c-cut sapphire crystal is used for white light supercontinuum generation. In the preamplification stage, a 2 mm thick, type-I (cutting angle of 28.5°) barium metaborate (BBO) crystal converts the fundamental pump pulse to its second harmonic. Then, the frequency-doubled pump is further chirped through a 2 mm thick SF11 glass medium to reduce group velocity mismatch between the pump and signal pulses. The 400 nm pump pulse preamplifies the blue-shifted supercontinuum seed light in a 3 mm type-I (cutting angle of 28.5°) BBO crystal for infrared idler beam. In the power amplification stage, the fundamental (800 nm) laser pulse amplifies the infrared idler output of the preamplification with a 2 mm type-Ⅱ (angle of 27.3°) BBO crystal.
The input pulse power for the supercontinuum seed stage is less than 1 mW, yielding a blue-shifted plateau region (500-700 nm) in the output spectrum. The white light supercontinuum generated by the sapphire crystal forms a stable plateau region in the range of 500-700 nm (Fig. 2). The blue-shifted spectral component has a conversion efficiency of ~5%, and the power is less than 50 μW. In the preamplification stage, the second harmonic (400 nm) pump pulse energy is 7 μJ, which is converted from 36 μJ of the 800 nm fundamental pulse. The wavelength tuning is achieved by controlling the temporal delay between the second harmonic pump light and the blue-shifted supercontinuum seed. Then, different spectral components (520-610 nm) of the supercontinuum seed are amplified as the signal beam and the wavelength of the corresponding idler beam ranges from 1800 to 1200 nm. The gain in the preamplification stage is approximately 2000 times (Fig. 3) and homogeneous for different phase-matching conditions. Using the idler output of the preamplification stage as the seed light for the power amplification stage, the signal pulse in the spectral range of 1160-1800 nm and the idler pulse in the range of 1440-2500 nm are obtained (Fig. 5(a)-(d)). The beam quality factors M2 of the final output signal and idler pulses are 1.91 (X-direction), 1.71 (Y-direction), 1.92 (X-direction), and 1.69 (Y-direction), respectively (Fig. 5(e)-(f)). For the optimized wavelengths of the signal pulse (1450 nm) and idler pulse (1785 nm), corresponding to the crystal cutting angle, the maximum conversion efficiency is 26.6% at 205 μJ pump energy (Fig. 6) and the output power stability (RMS) is approximately 1.8%. We have measured the pulse durations of the output signal (53.2±1.3) fs and idler (58.7±1.5) fs pulses from the power amplification stage (Fig. 7) using a home-built scanning autocorrelator, retaining the ultrashort pulse width of the pump pulse.
We have experimentally demonstrated an ultrafast optical parametric amplification system based on a second harmonic pumped-preamplification stage, realizing the wavelength tunability of the infrared signal pulse in the range of 1160-1800 nm and the idler pulse in the range of 1440-2500 nm. The energy conversion efficiency is approximately 26%, and the pulse width is 50-60 fs. The benefits of this scheme are three folds. First, it reduces the damage of the supercontinuum generation solid medium, improving the reliability of the entire system. Second, it leads to a uniform spectrum of the infrared idler output in the preamplification stage, thus improving the system wavelength tunability in the power amplification stage. Finally, this scheme can provide carrier-envelope phase stabilized-infrared idler output over a broadened spectral range from the power amplification stage.
