1. Introduction

The frequency shift of lasers operated from subpicosecond pulse to cw generation by stimulated Raman scattering (SRS) is now a comprehensively researched phenomenon. A great number of Raman shifters have been developed for more than a 40-year period since the discovery of stimulated Raman scattering. They are broadly used in fundamental research and applications. However, severe difficulties caused by competing nonlinear effects arise at Raman conversion of laser pulses shorter than 100 fs. Mainly such effects are self-phase modulation and self-focusing [1,2].

SRS of laser pulses shorter than 100 fs is usually a highly transient regime of stimulated scattering when the pulse duration is much shorter than the dephasing time T₂ of active medium. A transient regime threshold of SRS is determined by the laser pulse energy rather than laser radiation intensity as at steady-state SRS. Therefore the shorter the laser pulse the higher laser intensity will be reached in the interaction region at transient SRS. The spectrum broadening due to self-phase modulation is observed for sub-100 fs laser pulses with energy more than the threshold energy for all commonly used Raman media. So the self-phase modulation completely suppresses the process of SRS of sub-100 fs laser pulses [1,2,3].

To overcome these problems the technique based on SRS of chirped laser pulses was developed [4]. The pulse lengthening by frequency chirping process allows significant decreasing of laser intensity in the Raman active medium and avoiding of self-focusing and self-phase modulation. It was shown [4,5] that the frequency chirp of the Stokes pulse replicates the frequency chirp of the pump laser pulse. So the recompression of the Stokes pulse can be carried out. By using this technique the Stokes pulses with duration of 120 fs were generated [4]. However, this technique has a considerable drawback connected with the Stokes pulse lengthening. The Stokes pulse duration after compression is approximately 2–3 times longer than the initial laser pulse. It does not allow getting Stokes pulses shorter than 100 fs. This results from the Stokes spectrum narrowing with respect to the pump spectrum because of nonlinearity of SRS process. This effect is stronger for transient regime of SRS when the leading part of the chirped pump pulse comprising the threshold energy is not converted. Moreover second Stokes generation and pump depletion at transient regime result in complex temporal form of first Stokes pulse which is not reproduced from pulse to pulse [6]. This is one more reason for the broadening of a Stokes pulse after compression. For example, the duration of the compressed Stokes pulse was 190 fs at initial laser pulse of 80 fs [5].

In the given paper we have developed a scheme of the Raman shifter which allows overcoming the above difficulties and gives possibility of Raman generation of Stokes pulses with duration close to a pump pulse. In other words, it allows shifting the spectrum of the laser pulse with duration shorter than 100 fs to red side.

The suggested scheme of Raman shifter is based on the technique of Raman scattering of a laser pulse with the energy below the threshold one (weak pump) on the preliminary created coherent phonon wave by a high energy laser pulse (strong pump). This technique is known as biharmonic pumping and is widely used for Raman shifting of a low intensity laser radiation. For example, it was employed for mid-IR radiation generation by SRS of CO₂ laser pulse in hydrogen [7,8]. The frequencies of weak and strong pump are usually different. So the effect of phase-mismatching had strong influence on the conversion efficiency especially in dense active medium. We propose the new scheme with the same frequencies of weak and strong pump but with orthogonal polarization of radiation. It is applicable at scalar type of SRS, when a nonlinear susceptibility χ ³ is a scalar value [9]. Vibrational SRS in gas media is related to this type of scattering. The weak and strong Stokes pulses can be separated by a polarizer because their polarization is also orthogonal. It is expected that the conversion efficiency of the weak chirped pulse will not depend on time in such scheme. It will allow avoiding the Stokes spectrum narrowing and getting the Stokes pulse with the duration close to the pump pulse after compression. The experimental research of the suggested scheme of Raman shifter is presented in this paper.

2. Experimental results and discussion

The scheme of the experiment is shown in Fig. 1. The laser used in our experiment is a commercial chirped-pulse-amplified (CPA) Ti:Sapphire laser system (Avesta Project Ltd., RAPOP-50) running at 2 kHz repetition rate, producing 0.2 mJ, 50 fs pulses with a central wavelength at 810 nm. Additional Pockels cell is used for reducing the repetition rate to 100 Hz to avoid gas heating. To split the initial laser pulse into two pulses with orthogonal polarizations we used the optical scheme including λ/2 plate, thin film polarizes and delay line formed by four mirrors. The energy ratio between two pulses was varied by rotating the λ/2 plate. Hereinafter we will call the light pulse with electric field vector lying in the figure plane as p-polarized and the one perpendicular to the plane as s-polarized. The laser beam was focused by a 60 cm focus lens in the centre of a 120cm-long cell with inner diameter of 1 cm filled with pressurized methane.

 

Fig. 1. The schematic diagram of experimental setup. P: thin film polarizer, λ/2: half-wave plate, M: high-reflected mirror, L: lenses (f=60 cm), black line: laser radiation at the wavelength of 810 nm, red line: first Stokes radiation at the wavelength of 1060 nm.

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The preliminary experiment on first Stokes generation was carried out by using only one p-polarized laser pulse. The energy of s-polarized pulse was lower than 1 µJ in this case. To achieve the maximum energy conversion efficiency to the first Stokes component we varied the methane pressure and laser pulse duration. In agreement with previous results [1,2,3] we did not observe the first Stokes generation at shortest laser pulse of 50 fs and methane pressure up to 120 atm. As we have noted earlier the reason is SRS suppression by nonlinear phase-self modulation [3]. To overcome this effect the laser pulse duration should be increased. It was done by changing grating separation in the pulse compressor set after the regenerative amplifier.

It was found that for single pulse pumping the more efficient first Stokes generation is observed at methane pressure of 60 atm and duration of the chirped pump pulse of (1–1.5) ps. Therefore all following experiments were carried out at the pressure of 60 atm. The energy conversion efficiency to first Stokes component on the pump pulse energy is shown in Fig. 2. The maximum conversion efficiency reached 20%. However, as in previous works [5], the Stokes spectrum narrowing with respect to the pump spectrum was observed. The spectral width of the Stokes radiation at the wavelength of 1060 nm was only 10 nm (Fig. 3(c)), whereas the pump spectral width was 18 nm (Fig. 3(a)). Here we used the pump pulse with negative spectral chirp. It means that the short wavelength spectral components come in to Raman cell first. Because of the nonlinear character of SRS, more efficient conversion should be observed for the central spectral components which have the higher intensity. It is confirmed by measurement of the spectrum of the output depleted pulse pump (Fig. 3(b)).

The chirped Stokes pulse with 10 nm spectral bandwidth can be compressed down to ~170 fs even by using a perfect pulse compressor. It is approximately 3.5 times longer than the initial pump pulse. It is also worth to note that the spectrum of the Stokes pulse is very unstable and non-reproducible from pulse to pulse. Such features of the Stokes spectrum are typical and inherent for high efficiency conversion of transient SRS [6].

 

Fig. 2. The dependence of energy conversion efficiency to the first Stokes component on the pump pulse energy at single pulse pumping.

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Fig. 3. Spectra of input (a) and output (b) laser radiation, first Stokes component (c) at pumping only by p-polarized pulse.

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The implementation of a double pulse scheme allows overcoming the noted disadvantages. In this case the p-polarized pump pulse with the energy exceeding SRS threshold excites the coherent phonon wave in a Raman medium. Then the delayed s-polarized pump pulse is scattered on that wave without distortion of the spectrum shape. It is clear that the time delay should be less than the dephasing time T₂ of Raman medium. T₂ is 11 ps for methane pressure of 60 atm [10]. We found that there is very weak dependence of the energy conversion of s-polarized pulse to the first Stokes on delay time within (2–10) ps. The experimental results presented below were obtained at the time delay of 5 ps.

The spectrum of the first Stokes of s-polarized pulse is shown in Fig. 4(a). It is seen that it is 2.8 times broader than the spectrum of p-polarized Stokes (Fig. 3(c)). This spectrum was recorded at 56 µJ and 43 µJ input energy of p-polarized and s-polarized pump pulse, correspondently. The output energy of the s-polarized Stokes pulse was 20 µJ. It gives 20.2% energy conversion efficiency of the laser radiation (p-polarized + s-polarized) to the s-polarized broadband Stokes component. As for energy conversion efficiency of s-polarized pump to s-polarized Stokes it reached 47 %. It corresponds to 62 % photon conversion efficiency.

We observed slight changing of spectrum of s-polarized Stokes radiation and energy conversion efficiency when varying the energy of s-polarized pump from 0 to about 40 µJ. The total pump energy was fixed at 100 µJ. (40 µJ pulse energy is the single pulse threshold energy of SRS (Fig. 2)).

We carried out the pulse compression of s-polarized Stokes pulse which was separated from p-polarized Stokes beam by the polarizer. The s-polarized Stokes pulse with negative frequency chirp was compressed by passing through the glass rod with the length of 32 cm. The compressed pulse duration was measured by a single-shot autocorrelator (Avesta Project Ltd., model ASF-20). To get maximum compression ratio of Stokes pulse the frequency chirp of the Ti:sapphire amplified pulse was varied by changing grating separation in the pulse compressor set after the regenerative amplifier. The autocorrelation trace of the shortest compressed Stokes pulse is shown in Fig. 4(b). The corresponding Stokes pulse duration is 68 fs. The time-bandwidth product for the Stokes pulse is 0.501. It is 1.6 times more than for sech²-pulse with the same duration. The possible reason of time-bandwidth product increasing is imperfect Stokes pulse compression in the glass rod. However a more comprehensive research is required to explain this result.

 

Fig. 4. The spectrum of s-polarized first Stokes component at two-pulse excitation (a) and autocorrelation trace of the compressed Stokes pulse (b). The red line in (b) is sech² fit.

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3. Conclusion

In conclusion, we developed the Raman laser which allows generating the Stokes pulse with 20% energy conversion efficiency and duration close to the pump one. 68 fs Stokes pulse was generated by using 50 fs pump pulses. The developed technique of Raman conversion has no principal restriction on pump pulse duration and can be used for Raman frequency shifter of laser pulses with duration down to 10 fs.