Stimulated Raman scattering in an optical parametric oscillator based on periodically poled MgO-doped stoichiometric LiTaO3

The evolution versus pump power of the spectrum of a singly resonant optical parametric oscillator based on an MgO-doped periodically poled stoichiometric lithium tantalate crystal is observed. The onset of cascade Raman lasing due to stimulated Raman scattering in the nonlinear crystal is analyzed. Spurious frequency doubling and sum-frequency generation phenomena are observed and understood. A strong reduction of the intracavity Raman scattering is obtained by a careful adjustment of the cavity losses.


I. INTRODUCTION
Continuous wave optical parametric oscillators (OPOs) are important sources of high power, widely tunable, narrowband radiation, for applications in spectroscopy and sensing. Such sources, when based on periodically poled MgO-doped stoichiometric LiTaO 3 (MgO-PPSLT), are very promising because of the increased resistance of this crystal to the photorefractive effect, its large optical nonlinearity, and its high optical damage threshold [1,2,3,4]. In cw Singly Resonant OPOs (SROPOs) developed to date, oscillation of a single frequency has been observed at low pumping levels (up to 5 times threshold) due to the relatively high threshold of SROPOs (a few Watts level). Exploring theoretically much larger values of the relative pumping ratio in cw SROPOs, Kreuzer has predicted that single frequency oscillation should occur at moderate pumping ratios, while multimode oscillation must occur above a critical value of the pumping ratio [5]. This has been experimentally demonstrated in the case of SROPOs based on periodically poled LiNbO 3 [6,7,8] in which the ratio of pump power to oscillation threshold could reach to 16 times. Moreover, in the same papers [6,7,8], it has been shown that further increasing the pump power can lead to the outbreak of new frequencies due to Raman scattering in the PPLN crystal. But till now, no demonstration of such effects has been performed in the case of PPSLT.
Moreover, in recents years, in order to reach wavelengths inaccessible to commercial solidstate lasers, cw intracavity-frequency-doubled SROPOs based on PPSLT crystals have been developed [9,10]. To be efficient, such systems require a high intracavity signal or idler power. However, such a requirement may change the spectral properties of these systems.
So, for further scaling of their power while retaining a good spectral purity, it is important to investigate the effects induced by strong intracavity powers in such systems. In particular in this paper, we report on simultaneous parametric oscillation and stimulated Raman scattering in a MgO-PPSLT based SROPO. We also describe the spectral characteristics observed at high (up to 15 times threshold) pumping ratios.

II. EXPERIMENTAL SETUP
Our experimental setup is shown in Fig. 1. As a pump source, we use a cw Verdi laser that produces 10 W of single-frequency radiation at 532 nm. The nonlinear crystal is a 30-

III. RAMAN LASER: RESULTS ET DISCUSSIONS
We obtain the oscillation of the SROPO with a threshold pump power of 500 mW at a crystal temperature of 103 • C (see Fig. 2). This threshold value is at least five times lower than the one previously demonstrated in similar cw OPO systems based on PPSLT crystal [3]. This is due to the low round trip losses in our optical cavity, and also to the rather good mode matching of the pump and idler beams. With 7.6 W of available pump power, we are thus able to pump the OPO at 15 times its oscillation threshold. The evolution of the The threshold of the Raman laser effect corresponds to an intracavity idler power of 30 W, which is equivalent to an intensity of the order of 0.7 MW/cm 2 in the PPSLT crystal. If we suppose that the Raman gain in the crystal is equivalent to the one found in the literature [13] for LiTaO 3 , i. e., 4.4 cm/GW, this corresponds to a gain of about 0.9% per round-trip at the Raman shifted wavelength. This is consistent with the order of magnitude of the round-trip cavity losses between 1200 and 1400 nm.
Investigation of the OPO output spectrum also revealed unusual peaks in the visible. Fig. 4 shows the infrared and visible parts of the output spectrum for a pump power of 5  Fig. 4(b)]. The infrared wavelengths λ 1 and λ 2 are taken from Fig. 4 W. The visible spectrum was measured using an AvaSpec-2048-2 spectrometer. The origin of these visible wavelengths is summarized in Table I. They all come from the second order nonlinearity of the PPSLT crystal, which mixes the infrared frequencies, either by sum frequency generation or by second harmonic generation. In such a situation, rather than emitting just the idler wavelength, one can see that the OPO emits the idler frequency, four Stokes Raman shifted wavelengths, four anti-Stokes Raman shifted wavelengths, and nine different wavelengths in the visible. This is too complicated for applications.

IV. SINGLE-FREQUENCY OPERATION
Indeed, for the applications mentioned in the introduction, we need a source operating in single frequency regime. We thus have to avoid the Raman laser effect. We then reduce the intracavity idler power and also increased the cavity losses at Raman wavelengths by changing mirror M 4 for a mirror with a transmission of the order of 8×10 −4 around 1200 nm.
With this mirror, the SROPO threshold is increased to 1.0 W, showing that we have roughly multiplied the losses for one round-trip in the cavity at the idler wavelength by a factor of 2. With this mirror, the evolutions of the signal and idler output powers are displayed in Fig. 5(a). The Raman laser threshold is now measured to correspond to a pump power of 5 W, which corresponds to an idler output power of about 50 mW. This is equivalent to an intracavity idler power of about 60 W. This shows that, in terms of idler power the Raman laser threshold is now twice larger than with the preceding cavity. This is consistent with the fact that the cavity losses are twice larger than before and that the Raman gain is proportional to the pump laser intensity [13]. The good thing for applications is that even at higher pump powers, the Raman effect remains very weak and can hardly be detected [see for example the spectrum of Fig. 5(b), obtained for a pump power of 7.5 W], even though the system is again very efficient, with a pump depletion measured to be equal to 90%.
If now we focus on the position and width of the idler frequency peak, some effects occurring at high pump powers can also be observed. Indeed, in the case of the cavity built with four highly reflecting mirrors, we also observe significant shifts in idler wavelength as a function of the pump power. This effect, which arises from the heating of the PPSLT crystal, has been previously observed in PPLN [8,14] and also more recently in PPSLT [9]. Fig. 6(a) reproduces a zoom on the idler frequency spectra at different pump powers.
At a pump power of 7W (respectively 540 mW), we measure an idler wavelength of 1221 nm (respectively 1208.5 nm). Using the Sellmeier equation for stoichiometric LiTaO 3 [15], we can extract the crystal temperature these wavelengthes. Fig. 6(b) displays the result of this calculation and also the intracavity idler power versus pump power. It indicates a rise of 6 • C of the crystal temperature with increased pump power. Since the green-induced infrared absorption is expected to be low in MgO-PPSLT [16], this effect may be due to the absorption of the intracavity idler, and also absorption of the pump and signal in the MgO-PPSLT crystal. Morever, as the pump power increases, Fig. 6(a) shows that the idler spectrum broadens. This effect has also been observed in the case of the PPLN crystal [7].
Here, we find that, unlike in PPLN, the spectrum showed a symmetric pattern of side modes at all pumping ratios, up to 15 times threshold.

V. CONCLUSION
In conclusion, we have observed the onset of lasing due stimulated Raman scattering in a cw singly resonant optical parametric oscillator pumped in the green and based on MgO-PPSLT, with a sub-Watt pump power threshold. This effect has been shown to be due to the high intracavity idler power and the high finesse of cavity at the Raman shifted wavelengths.
We have shown that single wavelength operation of the SROPO can be obtained while keeping several Watts of signal power and 90% pump depletion, by optimizing the cavity losses for the idler wavelength. Effects of the crystal heating due to the high intracavity power have also been observed.