# Near to Mid-Infrared (1.3-5 um) Widely Tunable, High Power Picosecond Pulsed Laser
## 1 Introduction  
Near-infrared (1.3-2 um) and mid-infrared (2.2-5 um) wavelength tunable, short-pulse lasers have been applied to numerous applications across a diversity of fields. By generating laser emission at near-infrared wavelengths (and in particular, wavelengths in the eye-safe region), and with short-pulses, the Raman scattering microscopy has seen increasing use in cutting-edge applications in biology [1; 2; 3]. Lasers radiating in the mid-infrared wavelength range with picosecond pulse durations have found applications in ultrafast processes, time-domain spectroscopy and laser micromachining, this being due to wavelengths in this spectral band corresponding with the characteristic vibrational absorption of many organic materials [4; 5]. This is also an important characteristic for material processing methods, including resonant infrared pulsed laser deposition [6; 7]. There is hence great interest in expanding the range of novel lasers, which can further advance these applications. Nonlinear frequency conversion is perhaps the most commonly used method of wavelength converting laser sources. Some of the most utilized processes include second harmonic generation [8], sum frequency generation [9], OPO [10; 11; 12; 13], optical parametric amplification (OPA) [14] and Raman scattering [15]. For the generation of long-wavelength laser fields, OPOs and OPAs are the most commonly used techniques. Here, a short-wavelength pump field is used to generate two longer-wavelength fields. OPOs can be employed to generate widely tunable emissions in the near to mid-infrared wavelength region, with high conversion efficiency, and across both continuous-wave and pulsed regimes. Furthermore, utilization of an OPA with injection seeding can yield narrow linewidth, broadly tunable near to mid-IR laser emission [16; 17; 18]. Synchronously pumped optical parametric oscillators (SPOPOs) can be used to generate ultrashort laser pulses in new spectral regions. SPOPOs are typically used with lasers operating in the picosecond (ps) pulse regime and provide a wide wavelength tuning range while maintaining an all-solid-state design. They are useful for a variety of applications which require high power, short pulse durations and a narrow spectral bandwidth [19; 20]. A key issue in the design of SPOPOs is the choice of the nonlinear conversion medium. For instance, there are several reports detailing the generation of near- and mid-infrared outputs based on the birefringent-phase-matching (BPM) crystals, including KTiOPO\({}_{4}\) (KTP) [12], KTiOAsO\({}_{4}\) (KTA) [21], and ZnGeP\({}_{2}\) (ZGP) [22]. However, large walk-off effectsand the low nonlinear coefficient of these crystals impact the optical conversion efficiency and limit the generation of excellent beam quality and high power output. Recent improvements in nonlinear materials have allowed highly efficient and high beam quality laser radiation in the near- and mid-infrared wavelength ranges. Compared with birefringent crystals, crystals which utilize quasi phase matching (QPM) have been widely employed in the generation of near- and mid-infrared lasers based on optical parametric oscillationtors due to appealing characteristics of large effective nonlinearity, large acceptance angles, lower walk-off effects, and low susceptibility to photorefractive damage. Several reports are available concerning near- or mid-infrared OPOs utilizing quasi phase matched crystals. These include works based on periodically poled KTiOPO\({}_{4}\) (PPKTP) crystals with a fan-out grating design. From these systems, wavelength tunability in (741-922 nm) and (1258-1884 nm) were achieved with the output powers of 150 mW and 400 mW [23]. [PERSON] et al. achieved 1.2 W emission from an idler field at 3.8 \(\upmu\)m and 3 W emission from a signal field at 1.48 \(\upmu\)m from a periodically poled LiTaO\({}_{3}\) (PPLT) crystal-based OPO [24].