# Near to Mid-Infrared (1.3-5 um) Widely Tunable, High Power Picosecond Pulsed Laser
## 2 Experimental Setup  
A schematic of the experimental system was depicted in Figure 1. The pump laser was an all-solid-state Nd:YVO\({}_{4}\) laser with a central wavelength of 1064 nm, a pulse duration of \(\sim\)15 ps at a repetition rate of 120 MHz, a maximum power of 19 W, and a TEM\({}_{00}\) spatial mode. The pump beam was injected into the OPO, and its power was regulated by utilizing a half-wave plate (HWP) and a thin film polarizor (TFP). Another HWP in the setup was used to control the polarization of the pump beam so as to optimize the type-0 phase matching (e\(\rightarrow\)e + e) among the pump, signal, and the idler outputs within the nonlinear crystal. The pump field was focused with a radius of 60 \(\upmu\)m in the center of the MgO:PPLN crystal by using a focusing lens (f = 125 mm).  
The nonlinear crystal used in the SPOPO was a MgO doped periodically polled lithium niobate (MgO:PPLN) crystal. It had a fan-grating with a period which varied from 26-32 \(\upmu\)m and dimensions of 50 \(\times\) 16 \(\times\) 2 mm\({}^{3}\). Compared to MgO:PPLN crystals with ordinary gratings, a fan-shaped MgO:PPLN crystal ensures an output spectrum without wavelength gaps, which provides continuous wavelength tunability of the parametric outputs. The MgO:PPLN was placed in an oven and heated in a tuning range from 25 \({}^{\circ}\)C to 200 \({}^{\circ}\)C for phase matching. By varying both the phase matching temperature and the spatial position of the pump field within the MgO:PPLN crystal (so as to access different periods of the polled crystal), wide wavelength tunability of the OPO outputs could be obtained. The two end faces of the MgO:PPLN crystal were anti-reflection coated for the pump (1.064 \(\upmu\)m), signal (wavelengths of 1.4-1.75 \(\upmu\)m), and idler (wavelengths of 2.5-5 \(\upmu\)m).  
The OPO was constructed to be singly resonant for the signal field and comprised a Z-shaped standing wave cavity consisting of five reflective mirrors. Plane-concave mirrors M2 (R = 150 mm), and M3 (R = 150 mm) were positioned at a distance of 90 mm from the two end faces of the MgO:PPLN crystal. A plane-concave mirror M4 (R = 500 mm) and flat mirrors M1 and MS also formed the OPO cavity. The cavity mirrors M1-M4 were high-reflecting coated for the signal field (1.4-1.75 \(\upmu\)m, R \(>\) 99.9%) and anti-reflecting coated for the pump and idler fields (1064 nm, R \(<\) 0.5%; 2.5-5 \(\upmu\)m, R \(<\) 2%). The cavity mirror M5 was coated partially transmitting for the signal field (1.4-1.75 \(\upmu\)m, R\(-\)80%) to ensure the signal field was singly resonant with a low Q factor. Precise control of the cavity length was required in order to realize effective synchronous pumping of the OPO. It was also a significant factor for improvement of the broadband wavelength tunability of the system. In comparison to the X-shaped cavity, the Z-shaped cavity serves as a standing wave cavity, which has the advantages of simple operation and easy adjustment, and the fundamental mode spot size on the nonlinear crystal was also easy to control. The cavity mirror M1 was placed on a high-precision translation stage to control the cavity length. The round-trip optical path length of the cavity was set to \(\sim\)1250 mm and then fine-tuned to achieve synchronization with the 120 Mhz of the pump beam. In order to achieve good overlap between the pump and the signal in the cavity, we set the spot size of the signal to \(\sim\)65 \(\upmu\)m. The idler output beam was separated from the pump and signal beams using a wavelength separator (S), then measured using a pyroelectric camera. The signal beam was extracted from the reflecting mirror (M5) and imaged using a camera.  
Figure 1: Schematic of the picosecond pulsed synchronously pumped, widely wavelength tunable optical parametric oscillator. \(\uplambda\)/2: Half-wave plate, M1–M5: Cavity mirrors.