Infrared tunable 5%MgO:PPLN OPO pumped by a 1-kHz sub-nanosecond microchip laser

. We built and studied a singly resonant optical parametric oscillator using a 5%MgO:PPLN partial cylinder pumped by a sub-nanosecond microlaser emitting 1064 nm at a repetition rate of 1kHz. It is continuously tunable from 1410 nm up to 4330 nm by rotating the cylinder and a total energy of several micro Joules is emitted with a beam quality factor M² lower than 3.


The challenge of the sub-nanosecond temporal regime OPO operation
An Optical Parametric Oscillator (OPO) is well controlled when the parametric amplification of the generated signal and idler waves is achieved within a single pump pulse with a low repetition rate, at typically 10 Hertz.The pulse duration is then comparable to that of several round trips in a cavity of several centimeter size, allowing a temporal overlap between the generated and pump waves.The other temporal regimes well mastered for OPOs are the picosecond and femtosecond regimes thanks to synchronous pumping that is made possible by a high repetition frequency around 100 Megahertz [1].In this case the signal or idler pulses can be amplified by successive pump pulses as long as the time between two pump pulses exactly matches the duration of the signal or idler round trip in a cavity of several meter size.In the present paper we describe the design and the spectral, spatial and energy characterizations of an OPO singly resonant on the signal that is based on a cylindrical 5%MgO:PPLN pumped with a Passively Q-Switched (PQS) microchip laser.This pump source produced and commercialized by Teem Photonics emits at   = 1064 nm an energy per pulse of 85 µJ over a pulse duration of 490 ps with a repetition rate of 1 kHz.Then we address here an intermediate regime, i.e. the subnanosecond range, between the two favourable situations described above for the implementation of an OPO.It is then very challenging because the pulse duration is too short to operate many round trips in a centimetric cavity, and the repetition rate is too low to consider a synchronous pumping.As a consequence, very few of such OPOs have been described [2] and to our knowledge none are commercially available.

The 5%MgO:PPLN partial cylinder interest
Optical Parametric Oscillators using 5%MgO:PPLN crystals cut as polished cylinders have already been demonstrated.They are interesting because they provide a better spatial beam quality and pointing stability, with a continuous infinite angular tunability [3,4].Such OPOs have been already implemented in the nanosecond regime [5,6].The present cylindrical OPO based on 5%MgO:PPLN in the sub-nanosecond regime allowed us to achieve a continuous and wide spectral range tunability between 1410 nm and 4335 nm over a rotation of about 30° a shown in the following figure [7].

Performances
We realized the measurements of various performances like the signal spectral bandwidth, the oscillation threshold and slope efficiency at different generated wavelengths.
The OPO spectrum consists in the signal plus idler output power measured as a function of the AQPM angle  for a pump energy of 70 µJ per pulse is shown Fig. 2.a) at the corresponding wavelengths values [7].The average generated power varies from 0.2 mW to 9.0 mW, with an absorption peak around 2900 nm corresponding to the absorption of the idler wave by OH - impurities present in the crystal.
The measurement of the transverse and longitudinal profiles of the signal beam at the exit of the OPO with the knife method.An example at   = 1493 nm is given in Fig. 2.b) [7].The interpolation of the experimental data of Fig. 2.b) allowed us to determine that in the cylindricity plane (horizontal)  2 =2.60 and  2 =2.75 along the transverse axis (vertical).Then it clearly appears that the astigmatism is very low, which can be attributed to a suited focusing configuration of the pump.We determined the OPO threshold and conversion efficiency at three given wavelengths, that is to say for three different values of the rotation angle, by varying the input pump power as shown in Fig. 3 [7].Note that the spectral range of the idler beam is attenuated around 2900 nm by the atmosphere and above this wavelength by BK7 glass optics.

Conclusion
We have shown that it is possible to realize a microchip laserpumped OPO in the sub-nanosecond temporal regime, which was a real challenge.Indeed, the short pulse duration and the low repetition rate of the microchip laser prevent a synchronous pumping configuration.We have also shown that the linear cavity OPO architecture composed of two planar mirrors and a cylinder-shaped 5%MgO:PPLN crystal inserted in the middle, leads to satisfactory performances that are comparable to those in the nanosecond regime.This compact OPO allows us to cover the infrared range from 1410 nm to 4335 nm with an energy per pulse in the microjoule range.

Fig. 2 .
Fig. 2. (a) Average output power of the cylindrical 5%MgO:PPLN OPO as a function of the rotation angle for an average pump power of 70 mW.The dashed line is a guide for the eye and arrows are for associated signal and idler wavelengths.(b) Spatial profiles and associated quality factors M 2 of the signal beam at   = 1493 nm emitted by the cylindrical 5%MgO:PPLN OPO.

Fig. 3 .
Fig. 3. Signal plus idler average power at the output of the cylindrical 5%MgO:PPLN OPO as a function of pump power for different signal wavelengths.The dashed lines correspond to the interpolation of experimental data.