Significance Since the first demonstration of the Kerr-lens-mode-locked Ti:sapphire laser, femtosecond laser technology has attracted tremendous research interest and evolved very rapidly. Thanks to the properties of short pulse duration, broadband spectrum, and high peak power, femtosecond laser pulses can probe the high-resolution dynamics in both time and spatial dimensions, and explore new regimes of light-matter interaction. Contributing to these advantages, femtosecond laser systems could serve as powerful and reliable platforms for many cutting-edge applications, such as material processing, frequency comb generation, metrology, microscopy, spectroscopy, and nanooptics. Apart from many application fields, femtosecond laser technology has led to many breakthroughs in fundamental research fields, including attoscience, femtochemistry, and nonlinear optics. Developments in pump diodes, gain media, and saturable absorber mechanisms advance the frontiers of pulse duration and output power. Up to now, extremely short duration of pulses down to a few-optical-cycles can be achieved both directly from the oscillator and nonlinear processes outside the cavity. On the other hand, the output power level of the femtosecond laser system can reach several hundred watts. In recognition of the role of the femtosecond laser technique, Mourou and Strickland won the Nobel Prize in 2018 for chirped-pulse amplification. Apart from advancement to shorter pulse duration and higher output power, more and more research focuses are placed on ongoing efforts to expand the frequency coverage to promote femtosecond laser systems into more widespread practical applications. However, the mode-locked spectral width of femtosecond laser output is limited by the effective laser gain bandwidth due to the relatively fixed energy levels of the gain medium, which hinders its large-scale application.

Nonlinear frequency conversion techniques can provide the possibility to achieve effectively tunable laser sources in a wide spectral region. Up to date, the optical parametric oscillator (OPO) has emerged as a compelling alternative to generate broadband tunable radiation, which can expand the spectral region from the UV to infrared. Among them, OPOs pumped by femtosecond fiber lasers have been recognized as ideal platforms providing tunable ultrafast pulses with formidable performance, such as high repetition rate, high output power, and broad wavelength coverage. To this end, femtosecond OPOs are appealing for numerous applications, including quantum information, laser processing, optical frequency comb generation, and biophotonics. Recent power scaling of the Yb-fiber lasers and the development of new nonlinear crystals advance the frontiers of femtosecond OPOs.

Progress To fulfill more widespread applications, there remains a strong motivation to expand the spectral tuning possibilities of OPOs. The development of birefringent crystals such as BIBO, BBO, and LBO, combined with a powerful femtosecond fiber laser source, enables the generation of tunable UV radiation on an ultrafast time scale (Fig. 1). Alternatively, thanks to the unique material properties of mid-infrared materials, i.e., CSP, OP-GAP, and ZGP, the operation of femtosecond OPOs in the far-infrared at 8 μm can be realized (Fig. 3).

Kerr-lens-mode-locked Ti: sapphire lasers are the most commonly used pump sources for OPOs; however, these systems suffer a limitation in terms of power scaling mainly owing to unavoidable heat load in the laser crystal. In recent years, the rapid development of a high-power Yb-laser system allows a new power scaling potential for OPOs, and W level signal output can be achieved. However, it is rather difficult for OPOs to achieve a few-cycle pulse duration directly from a femtosecond fiber laser owing to the gain bandwidth limitation and complex nonlinear control. To access even shorter pulses from OPOs pumped by a fiber laser system, chirped-pulse optical parametric oscillators and self-compressed MIR OPOs have been demonstrated by researchers in Huazhong University of Science & Technology and Tianjin University, respectively (Fig. 7 and Fig. 9).

For high-speed electrooptic sampling or future optical communication applications, moving operation regime of OPOs into the gigahertz pulse repetition rate regime has advantages. OPOs operating at GHz repetition rates have been reported using both synchronous and harmonic pumping schemes.

Light emission with space-variant polarization and phase distribution has become a popular topic for the research community. The development of methods to create wavelength-tunable, space-variant polarization light beams will be a very interesting topic. Hu’s research group in Tianjin University has demonstrated novel femtosecond OPOs that deliver high-order Poincaré sphere beams, cylindrical vector beams, and vortex beams (Figs. 12--14).

Conclusions and Prospect In this paper, we start with the progress in Yb-doped fiber laser-pumped femtosecond OPOs in recent years. Then, we present a variety of advanced designs of fiber laser-pumped OPOs, which are categorized into widely tunable OPOs, GHz repetition rate OPOs, few-cycle optical pulse OPOs, and structured beam OPOs. Finally, the applications of femtosecond OPOs in the fields of nanophotonics and Raman spectroscopy are introduced. With further development in nonlinear materials, combined with advances in pump laser technology, as well as new design concepts, femtosecond OPOs with wider spectral coverage, higher power, higher repetition rate, and shorter pulse duration are achievable in the near future. With the growth of novel femtosecond OPOs, completely new areas in application fields will arise.