Mode-locked and Q-switched carbon monoxide laser system
Introduction
In mid-IR spectral range there are several atmospheric transparency windows which can be used for transportation of laser beam in the atmosphere. A carbon monoxide laser (CO laser) stands out among gas lasers due to high laser efficiency and rich laser spectrum in mid-IR [1]. A frequency conversion of CO laser in a nonlinear crystal can significantly extend and enrich CO laser spectrum. In [2], [3] two-stage (sum- and difference-) frequency generation was obtained in a single ZnGeP2 crystal pumped by a multi-line Q-switched CO laser with pulse duration ~1 μs. As a result a broadband lasing was observed simultaneously on ~670 spectral lines in the wavelength range from 2.5 to 8.3 μm (more than one and half octaves). The internal efficiency (taking into account optical losses due to Fresnel reflection on crystal facets) was 2% for sum-frequency generation (first stage) and 0.5% for difference-frequency generation (second stage).
In order to improve conversion efficiency one should increase laser intensity on a nonlinear crystal (and, hence, peak power of the laser pulse) and diminish the crystal heating. The two procedures can be fulfilled by decreasing the laser pulses duration. We developed elsewhere [4] a CO laser emitting a nanosecond pulse train (NPT) with a train duration from 0.1 to 0.5 ms (hereinafter, a long NPT). The peak laser power was about 70 kW in a single-line mode (lasing on a single ro-vibrational transition of CO molecule) and up to 120 kW in a multi-line mode [4]. Later in [5] a CO laser system was developed as a master oscillator (MO)-power amplifier (PA) CO laser system with peak power up to 0.1 MW in single-line mode and up to 0.4 MW in multi-line mode. When applying the MOPA CO laser system with long NPT for second harmonic generation in a single ZnGeP2 crystal, an internal efficiency increased up to 25% [6].
The latter was obtained for the whole NPT with a long low-power tail. Obviously, elimination of this tail by shortening the laser pulse train by Q-switching would enhance the conversion efficiency. Moreover, if one would like to mix different CO laser lines corresponding to low and high vibrational levels, he would face with one more problem. In different vibrational bands CO laser pulse starts with different time delay after an electric discharge pump pulse. Typically, the time delay increases with the vibrational quantum number due to vibrational kinetics of the CO laser medium [1]. Simultaneous CO lasing on two or more ro-vibrational transitions from different vibrational bands can be also obtained by Q-switching.
In this work we developed CO laser with concurrent mode-locking by an acousto-optical modulator and Q-switching by a rotating mirror. Duration of NPT was about 1 μs (hereinafter, a short NPT). We studied the MOPA CO laser system with short NPT in single-line, two-line (with ro-vibrational lines from different vibrational bands), or multi-line modes of operation.
Section snippets
Experimental setup
In this study we applied the cryogenically cooled pulsed e-beam sustained discharge (EBSD) laser facility described in detail in [7]. The optical scheme of the CO laser emitting a short NPT is shown in Fig. 1. The length of the laser gain medium 1 was 1.2 m. The laser resonator of the MO with the optical length of 15 m was formed by output mirror 2 (reflectivity of 75%) and flat mirrors 3 and 4 for single-line or two-line mode of operation. A spectral selection was due to the diffraction grating 5
Single-line mode
The time behavior of CO laser operating in single-line mode on ro-vibrational transition 10→9P(8) (λ=5.32 μm) is presented in Fig. 2. Hereinafter, the time t was measured from the beginning of EBSD pulse. Time delay between the beginning of EBSD pulse and Q-switching was 54 μs.
A short NPT of the CO laser (with train duration of ~1 μs) consisted of several spikes with duration of 10 ns (FWHM). The spikes followed with period of 100 ns. The laser power P(t) was calculated on the basis of measured
Two-line mode
The CO laser can operate simultaneously on many ro-vibrational transitions. For instance, spectrum of Q-switched CO laser consisted of 80 spectral lines in [2] and of 150 lines in [3]. However, as was mentioned above, for a pulsed CO laser it is difficult to get the synchronous lasing in vibrational bands spectrally far separated from each other. It is due to different time for the excitation to get to the different vibrational levels because of vibrational VV-exchange kinetics [1]. So, before
Multi-line mode
For CO lasing with short NPT in nonselective multi-line mode, diffraction grating 5 (Fig. 1) was replaced by a flat mirror. Fig. 7 shows the peak power at entrance and exit of the laser amplifier depending on time delay at specific input energy Qin=265 J/(l Amagat). The peak power of the multi-line CO laser system with short NPT reached its maximum of 0.55 MW at the time delay of 60 μs.
The peak power of the multi-line CO laser system reached its maximum of 0.8 MW at specific energy input Qin=360 J/(l
Conclusions
The MOPA CO laser system emitting a short train of nanosecond pulses with duration of ~1 μs was developed. CO lasing with short train was obtained in a single-line mode (on a single ro-vibrational line), two-line mode (synchronous lasing on two ro-vibrational lines from spectrally far separated vibrational bands), or multi-line mode (lasing on more than 10 ro-vibrational lines in the wavelength range from 5.0 to 5.8 μm). The peak power of CO laser was up to 0.1 MW in single-line mode, 70 kW in
Acknowledgments
This work was partially supported by the Russian Foundation for Basic Research (RFBR) with Grant nos. (RFBR 12-02-01085, 13-02-01135, 13-05-98074, and 15-02-08037) and the LPI Educational-Scientific Complex.
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