Femtosecond optical parametric oscillators (OPOs) are efficient and flexible wavelength-conversion devices capable of generating ultra-short optical pulses at wave regions not directly addressable by conventional laser gain medium. Over the past few decades, substantial progress has been made in femtosecond OPOs for generating emission with high output power, high pulse energy, high repetition rate, and wide wavelength-tuning range. Ultrashort-pulsed lasers with a broad bandwidth have unique advantages and are important for many emerging applications, including multicomponent trace gas detection, optical coherence tomography, and multispectral imaging. Many technical schemes focus on producing ultrashort optical pulses with a broad bandwidth, for instance, via laser mode locking, supercontinuum generation, or differential frequency generation. In contrast, femtosecond OPOs, which use nonlinear media as the gain materials, have inherent advantages in wavelength tuning and conversion efficiency. Moreover, extremely wide gain bandwidth of a nonlinear gain medium can be achieved through artificial tailoring with phase-matching techniques; therefore, femtosecond OPOs exhibit significant potential for generating ultrashort optical pulses with a broad bandwidth.
However, a femtosecond OPO is usually pumped by an ultrashort pulse train. To achieve an efficient parametric transfer in an OPO, the duration of the oscillating OPO pulses should be similar to that of the pump pulses. Indeed, in a synchronously pumped OPO, the duration of the pump pulse imposes a lower limit on the pulse width of the oscillating signal wave. Therefore, in traditional femtosecond OPOs, a pump source with very short pulse width is required for generating signal pulses with a broad bandwidth. The possibility of generating optical pulses with considerably wider bandwidths than the pump bandwidth in femtosecond OPOs is quite intriguing.
Intracavity spectral combinations can be used to enhance the instantaneous bandwidth of the output idler light in synchronous-pumped OPOs. Idler light with an instantaneous bandwidth that is significantly larger than that of the pump bandwidth can potentially be obtained using a nonlinear crystal with a wide phase-matching bandwidth and by implementing a multichannel configuration (Fig. 1). This scheme transforms conventional synchronously pumped OPOs into devices capable of generating idler light with a very broad instantaneous bandwidth.
Chirped-pulse optical parametric oscillators (CPOPOs) allow the generation of ultrashort optical pulses with an instantaneous bandwidth that is considerably wider than the pump bandwidth. Chirped-pulse formation can be achieved by inserting a material with a large nonlinear index coefficient into an OPO cavity or using an aperiodic QPM crystal. By properly managing the pulse dynamics by optimizing the intracavity dispersion and spectral broadening, optical pulses with an octave-spanning instantaneous bandwidth can be obtained.
Moreover, spectral broadening in CPOPOs can be enhanced to generate spectrum with an instantaneous bandwidth significantly wider than the parametric gain bandwidth of nonlinear crystals. Our study shows that a relatively high residual second-order-dispersion inside the OPO cavity is required for achieving the maximum signal bandwidth that exceeds the parametric gain bandwidth from a CPOPO (Figs. 2 and 3). In addition, the parametric
Pulse-compression in cavity length-detuned OPOs can be used to generate high-quality, soliton-like optical pulses with a considerably shorter duration than the pump pulse duration. This can be achieved in a femtosecond OPO via positive cavity-length detuning (Fig. 6). At a relatively high pump rate, the resonating signal wave can evolve into multiple pulses. However, we show that single-pulse operations can be recovered by simply increasing the level of cavity-length detuning.
In this article, we review recent progress on the generation of optical pulses with very broad bandwidths from femtosecond OPOs. We summarize the progress of our work performed at the Huazhong University of Science and Technology. Furthermore, we show that spectral broadening in OPOs can be achieved via intracavity spectral beam combination, by configuring a chirped pulsed OPO, or by simply detuning the cavity length. Ultrashort optical pulses with a broad instantaneous bandwidth may be beneficial for many applications, including optical coherence tomography, ultrashort pulse synthesis, and spectroscopy.
