# Near to Mid-Infrared (1.3-5 um) Widely Tunable, High Power Picosecond Pulsed Laser
## 3 Results and Discussion
### Wavelength Tunability  
We measured the wavelength tunability of the picosecond OPO, which was constructed from the 80% output coupler, by controlling the grating period and heating of the MgO:PPLN crystal. In this OPO, the quasi phase matching condition was fulfilled when the phase mismatching wavenumber was equal to zero. This can be described as follows:  
\[\Delta k=2\pi\Bigg{(}\frac{n_{e}\big{(}\lambda_{p},T\big{)}}{\lambda_{p}}-\frac {n_{e}(\lambda_{i},T)}{\lambda_{i}}-\frac{n_{e}(\lambda_{s},T)}{\lambda_{s}}- \frac{1}{\Lambda}\Bigg{)}=0 \tag{1}\]  
where \(\lambda_{p}\), \(\lambda_{s}\), and \(\lambda_{i}\) are wavelengths of the pump, signal, and idler outputs, and \(\Lambda\) is the grating period. Additionally, \(n_{e}\) is the refractive index of extraordinary polarized waves in the MgO:PPLN crystal. This depends on the temperature and wavelength of the crystal and can be expressed using the Sellmeier equation [33]  
\[n_{e}^{2}=a_{1}+b_{1}f+\frac{a_{2}+b_{2}f}{\lambda^{2}-(a_{3}+b_{3}f)^{2}}+ \frac{a_{4}+b_{4}f}{\lambda^{2}-a_{5}^{2}}-(a_{6}+b_{5}f)\lambda^{2} \tag{2}\]  
where \(f=(T-24.5\ ^{\circ}\mathrm{C})(T+570.82)\) is called the temperature parameter. The corresponding parameters \(a_{i}\) and \(b_{i}\) can be found in [33]. Therefore, when the wavelength of the incident pump beam is known, the wavelength tuning characteristics of the signal and the idler fields can be determined by the crystal period and the crystal temperature. Figure 3a shows the wavelength tuning range obtained by varying the crystal period when the crystal temperature is fixed at 25 \({}^{\circ}\mathrm{C}\) where the points indicate the experimental values and the curves indicate the simulated theoretical curves. Figure 3b shows the experimental and simulated tuning curves of the output wavelengths of the signal and idler obtained by varying the crystal temperature when the grating period is ~32 \(\upmu\)m. It can be observed from the figure that the tuning curves calculated using the Sellmeier equation are in good agreement with the experimental values. Figure 2: Images showing the spatial distribution of (**a**-**c**) signal fields and (**d**–**f**) idler fields at different output wavelengths. Wavelength tuning of the OPO output fields generated from the picosecond OPO was investigated by tuning the period that the OPO fields interacted with and the temperature of the crystal, as shown in Figure 4. The wavelengths of the generated OPO output fields were collected with a SpectraPro HRS-500 spectrometer, and the representative spectra are shown in Figure 4. The wide tuning range of the signal (1352-1738 nm) and the idler (2743-4992 nm) output could be achieved with the MgO:PPLN crystal at room temperature (25 \({}^{\circ}\)C) and by varying the interaction period of the grating. The wide tuning range of the OPO outputs could be further extended by temperature heating the MgO:PPLN. By increasing the temperature of the MgO:PPLN to 190 \({}^{\circ}\)C, the signal field wavelength could reach 2015 nm, the idler field wavelength could reach 2254 nm, and the total signal and idler wavelength tuning was in the range of 3401 nm. As a result, by tuning both the temperature and period of the phase matching process, the total near to mid-infrared tuning range that could be achieved from the system was 1.3-5 \(\upmu\)m. Figure 4 shows the full width at half maxima (FWHM) spectra of the near and mid-infrared laser outputs at different center wavelengths. The shown near infrared laser center wavelengths are 1352 nm, 1579 nm, and 2015 nm, and their half-height widths are \(\Delta\lambda\approx 1.45\) nm, \(\Delta\lambda\approx 2.09\) nm, \(\Delta\lambda\approx 2.50\) nm, respectively. The shown mid-infrared laser center wavelengths are 2254 nm, 3262 nm, and 2992 nm, while their half-height widths are \(\Delta\lambda\approx 2.38\) nm, \(\Delta\lambda\approx 1.42\) nm, \(\Delta\lambda\approx 1.30\) nm, respectively.