# Near to Mid-Infrared (1.3-5 um) Widely Tunable, High Power Picosecond Pulsed Laser
## 3 Results and Discussion
### Wavelength Tunability
High output powers (signal and idler) were obtained in present synchronously pumped OPO by using an output coupling mirror (M5) with 20% transmittance for the signal field. This Z-shaped standing wave cavity design enabled easy control of the cavity spot size on the MgO:PPLN, which in-turn improved the overlap efficiency of the pump and signal, and therefore the optical conversion efficiency of the OPO. The power of OPO output fields were measured simultaneously over the entire wavelength tuning range of the system. As depicted in Figure 5, representative power-transfer curves at a number of output wavelengths are shown. When the incident pump power was 19 W, the signal field output power achieved was 8.67 W at 1.74 \(\upmu\)m, and the corresponding power of the idler field was 5.61 W at 2.73 \(\upmu\)m. The total optical conversion efficiencies of the system were 75.15% (with the signal field constituting 45.63% and the idler field 29.52%). The idler field output at 3.262 \(\upmu\)m had a passive rms power stability of 3% over 2 h. As can be seen from the curves, the output power of the signal and the idler fields increases and then decreases as the wavelength increases. There are a number of reasons for this characteristic. Figure 3: (**a**) Experimentally measured wavelengths and theoretically calculated tuning curves for different grating periods of the crystal at a MgO:PPLN temperature of 25 \({}^{\circ}\)C. (**b**) The mid-infrared wavelength plots at various temperature of the MgO:PPLN. The points indicate experimental, and curves show the simulated theoretical curves. These include quantum loss and a reduction in the wavelength conversion efficiency as the difference in wavelength between the pump field and that of the signal and idler fields increases, prompting an increase in photon absorption within the MgO:PPLN crystal for increasingly longer wavelengths. The output power of the signal field hence increases and then decreases as its wavelength gets longer, this being consistent with conservation of energy. We also investigated the change in the OPO field output powers as functions of output wavelength at a set input power of 19 W; these plots can be seen in Figure 6. The curves in the figure indicate that the incident power was fixed at 19 W, before the output power of the OPO fields increased and then decreased with increasing wavelengths, with these results being consistent with the data presented in Figure 5. We believe that the low output powers when the OPO outputs were close to degeneracy was largely due to the characteristics of the coatings applied to the cavity mirrors (M1-M4). These mirrors were only high-reflecting in the range of 1.4-1.75 \(\upmu\)m, so, as the wavelength of the signal field became longer than 1.75 \(\upmu\)m, the output power gradually decreased. Figure 4: Plots showing the range of experimentally measured spectra of (**a**) signal and (**b**) idler fields. Insets show the signal and idler output spectral bandwidths from the synchronously pumped OPO. Figure 5: Plots showing the power transfer characteristics of (**a**) the signal field; and (**b**) the idler field for a range of output wavelengths. The high overall output powers achieved from this system can be attributed to a number of factors. These include the nonlinear gain of the resonant beam (in this case, the signal field) which is heavily influenced by the spatial overlapping efficiency of the interacting fields. The calculated beam radius of the signal field was \(\sim\)65 \(\upmu\)m (using MatrixLaser), and this overlapped well with the pump field inside the crystal. Also, the output mirror M5 had a transmission of 20% for most of the signal field tuning range, and this struck a fine balance between the cavity Q factor and out-coupling to produce high-power picosecond laser output. In the SPOPO, the spatial overlap efficiency (\(\eta\)) of the pump and signal beams determined the nonlinear gain of the resonant signal beam during the three-wave interaction.