Significance Optical parametric chirped-pulse amplifier (OPCPA), which relies on the optical parametric process existing in nonlinear crystals to realize pulse amplification, can further enhance a yielded peak power by avoiding the gain narrowing and thermal effects that usually exist in a chirped-pulse amplifier. The generated ultrashort pulse with high peak power from OPCPA can considerably extend the ultrafast pulse to X-ray and infrared regimes, thereby bringing a new revolution to ultrafast science. OPCPA system pumped ultrafast X-ray desktop light source has potential applications in medicine, biology, and materials science. In this article, we review the development of OPCPA systems, including its main characteristics and research progresses in different wavelength regimes (~0.8, ~1.5, ~2, ~3, and >4 μm).

Progress In ~0.8-μm OPCPA systems, the pumping laser pulse (0.515 μm) is usually generated via the second harmonic generation process from a Yb-doped laser amplifier. The seed pulses can be generated using a mode-locking few-cycle Ti: Sapphire oscillator directly or via the supercontinuum generation process. The energy/power of seed pulses is usually scaled up inside the BBO nonlinear crystals. The dispersion compensation for the amplified seed pulses is realized by the chirped mirror or glass block. Currently, the highest pulse repetition rate of 11.5 MHz, shortest pulse duration of 5 fs, maximum average output power of 112 W, largest pulse energy of 54 mJ, and highest peak power of 5.5 TW have been realized in the ~0.8-μm OPCPA systems ( Fig. 6).

In ~1.5-μm OPCPA systems, the pumping source is a Ti: Sapphire laser amplifier or Yb-doped laser amplifier. The seed pulses are generated via the supercontinuum generation process in noble gas or transparent medium, which is stimulated by the Ti: Sapphire laser oscillator and Er- or Yb-doped fiber/solid-state laser oscillators. Different crystals, such as BIBO, DSTMS (organic crystal), KTA, and LBO, have been employed to amplify the seed pulses. The dispersion of the amplified seed pulses is compensated by the fused quartz, Si, or chirped mirrors. A 350-kHz pulse repetition rate, 6-fs pulse duration, 106-W average output power, 3-mJ pulse energy, and 263-GW peak power have been achieved in the ~1.5-μm OPCPA systems (Fig. 7).

For ~2-μm OPCPA systems, the Yb-doped disk laser or Ti: Sapphire laser amplifiers are mainly used as the pumping sources. The seed pulses are generated via the difference frequency generation after the supercontinuum generation process. The seed pulse energy/power is enhanced in a crystal, such as PPLN, LiNbO₃, BIBO, or YCOB. The amplified seed pulses are compressed by a high-transmittance crystal, such as Si, ZnSe, or quartz. The optimal output parameters achieved from the ~2-μm OPCPA systems are 100-kHz pulse repetition rate, 10.5-fs pulse duration, 33-W average output power, 3.3-mJ pulse energy, and 132-GW peak power (Fig. 8).

A ~3-μm OPCPA system is usually pumped with the Ti: Sapphire laser or Yb-doped lasers. A 3-μm ultrafast pulse can be directly amplified using the ~3-μm OPCPA system or can be the idler pulse from a ~1.5-μm OPCPA system. The employed nonlinear crystals for amplification are PPLN, KNbO_3, and MgO∶LiNbNO₃. The pulse is compressed with grating pairs or solid medium (Si). The highest pulse repetition rate of 160 kHz, shortest pulse duration of 20 fs, highest average output power of 21 W, largest pulse energy of 5.8 mJ, and highest peak power of 290 GW have been reported from ~3-μm OPCPA systems (Fig. 9).

In OPCPA systems operating beyond 4-μm wavelength region, the pump lasers are 1-μm Yb-doped or 2-μm Ho-doped laser amplifiers. In particular, the Ho-doped laser amplifiers are beneficial to realize a high-efficient long wavelength from the OPCPA system. The seed pulses realized from the difference frequency process are amplified by the ZGP, KTA, or LGS(@1 μm). The dispersion management is performed by CaF₂, Ge, or grating pair. At present, OPCPA systems can deliver a laser pulse with the longest wavelength of up to 9 μm (Table 2).

Conclusion and Prospect Although the performances of OPCPA systems in different wavelength regimes have remarkably improved, there is still scope for further improvement. With the progress of high-power pump laser sources and high-quality nonlinear crystals, the OPCPA system is heading toward achieving shorter pulse duration, larger pulse energy, higher peak power, and longer output wavelength than the existing ones.