Optics Communications 282 (2009) 294–299
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Carbon monoxide laser emitting nanosecond pulses with 10 MHz repetition rate Andrey A. Ionin *, Yuriy M. Klimachev, Andrey A. Kotkov, Andrey Yu. Kozlov, Leonid V. Seleznev, Dmitriy V. Sinitsyn P.N. Lebedev Physical Institute of Russian Academy of Sciences, 53 Leninsky Prospect, 119991 Moscow, Russia
a r t i c l e
i n f o
Article history: Received 25 July 2008 Accepted 26 September 2008
Keywords: CO laser Active mode-locking Nanosecond spike Single-line operation NO molecule Relativistic heavy ion collider
a b s t r a c t Actively mode-locked electron-beam-sustained-discharge CO laser producing a train of 5–15 ns (FWHM) spikes following with repetition rate 10 MHz for both single-line and multiline mode of operation in the mid-IR range of 5 lm was experimentally studied. Total laser pulse duration was 0.5 ms for both mode-locked and free-running laser. Specific output energy in multiline CO laser mode of operation was up to 20 Jl1 Amagat1 and the laser efficiency up to 3.5%. The active mode-locking was achieved for single-line CO laser mode of operation in spectral range 5.2–5.3 lm. This sort of radiation can be used for pumping an optical parametric amplifier for optical stochastic cooling in relativistic heavy ion collider, for laser ablation, and for studying vibrational and rotational relaxation of CO and NO molecules. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction A carbon monoxide CO laser radiation was proposed in [1] to be applied for optical stochastic cooling of relativistic heavy ions in a ring collider such as relativistic heavy ion collider (RHIC) at the Brookhaven National Lab, USA. The idea consists in a wide-band amplification of 12 lm radiation emitted by relativistic heavy ions (79Au with energy 100 GeV per nucleon) in a pickup undulator. This amplified radiation can be used for cooling of ions in a kicker undulator. For this purpose a single pass optical parametric amplifier with a gain of 2 105 is required. This amplifier can be based on nonlinear optical crystals CdGeAs2 which can be optically pumped by high power laser pulses at wavelength 5 lm. A possibility of using a second harmonic of a CO2 laser for this optical pumping was discussed in [1], although as was pointed out in the same paper, a direct output of a CO laser is more attractive alternative for this purpose. To study this alternative, we modified our CO laser facility [2] in order to produce laser pulses required for optical stochastic cooling. In Ref. [1] the following parameters of the pumping laser were specified: (1) a narrow-band laser emission should be in the wavelength range from 5.3 to 5.6 lm; (2) a laser should be actively mode-locked in order to synchronize a train of pumping pulses with a pickup radiation; (3) a peak power of optical pumping should be 40 kW to produce peak intensity 20 MW cm2; (4) a single pulse duration s should be closer to 2 ns for average intensity to be less than a damage threshold of a
* Corresponding author. Tel./fax: +7 499 783 3690. E-mail address:
[email protected] (A.A. Ionin). 0030-4018/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2008.09.089
nonlinear crystal; (5) a pulse repetition rate (PRR) in a train of spikes should be equal to 10 MHz which corresponds to 114 bunches per ring of the RHIC with 3.834 km circumference. The sequence of short ns spikes following with the 10 MHz PRR can be produced in a mode-locked CO laser with 15 m long laser resonator. However, the possibilities of a mode-locked CO laser have not been much studied. There are only two papers [3,4] in which an actively mode-locked CO laser operating both in multiline [3] and single-line [4] mode was launched. The both papers [3,4] describe experiments which were carried out with the same laser facility operated at room temperature of an active medium and pumped by a pulsed self sustained TEA discharge. This facility previously had operated as a CO2 laser and after some modification it operated as a CO laser. It should be pointed out that this laser facility was not intended for efficient operation as a CO laser, because the maximum specific output energy and laser efficiency for a pulsed CO laser have been obtained at cryogenic temperature 100 K of its active medium pumped by non-self-sustained discharge, for instance, e-beam sustained discharge (EBSD) [5]. The objective of our paper consists in studying of cryogenically cooled actively mode-locked EBSD CO laser emitting short ns spikes on vibrational–rotational transitions of CO molecule in the spectral range 5 lm with 10 MHz PRR.
2. Experimental setup and techniques Our experiments were carried out with a pulsed EBSD laser facility described in detail in [2]. The optical scheme of the experiments is presented in Fig. 1. The active medium length was 1.2 m.
A.A. Ionin et al. / Optics Communications 282 (2009) 294–299
3 2
3 AOM 1 Diffraction grating CaF2
CO laser Photodetector
Spectrograph
Fig. 1. Optical scheme of the experiments 1 – flat output mirror; 2 – spherical mirror; 3 – turning mirrors.
The optical cavity of frequency selective CO laser with optical length 15 m consisted of a diffraction grating (240 grooves/mm) installed under near Littrow configuration and a flat output mirror 1 with reflectivity 75% for the wavelength range 5.0–5.7 lm. In the case of multiline operation the grating was substituted by a totally reflecting flat mirror, whereas the output mirror 1 was substituted by a flat mirror with reflectivity 50% for wavelengths 5.0–5.7 lm. A concave spherical mirror 2 with radius of curvature 9 m was situated in a middle of the laser cavity for providing a stable laser resonator and two passes through the active medium. An acousto-optical modulator (AOM) made of germanium with 8 mm aperture and 40 mm length was situated close to mirror 1. Its facets had antireflection coating for the wavelengths 4.5– 5.5 lm. A RF driver with a quarts-stabilized frequency 5 MHz was used to excite a standing acoustic wave in the AOM with piezoelectric transducer. The output power of the RF driver was 6 W. The frequency of intracavity optical losses created by the AOM was specified as 10.0 MHz ± 1.25%. The laser spectrum was detected by ‘‘CO Spectrum Analyzer Model 16C”. A part of the laser beam was directed to a Hg–Cd– Zn–Te photodetector PEM-L-3 with a response time 0.5 ns which signals were recorded by oscilloscope Tektronix TDS 5052. Our experiments were carried out under following conditions: gas mixture CO:N2 = 1:9; gas density – Ng = 0.2 Amagat (0.2 of the gas density under normal conditions: 273 K, 1.013 105 Pa); an initial (before an EBSD pump pulse) gas temperature T = 120 K; EBSD pulse duration 35 ls; an optical volume in the active medium was 1 l. In our experiments a specific input energy (a specific energy loaded into an EBSD) Qin changed from 50 to 500 Jl1 Amagat1 that corresponds to the range from 1.1 to 11 kJ mol1.
3. Experimental results The output of multiline and single-line CO laser operating both in mode-locked and free-running mode was experimentally studied. A switching between the two modes of operation was simply obtained by switching on or switching off the RF driver feeding the AOM. 3.1. Multiline CO laser Time behavior of the mode-locked EBSD CO laser emitting multiline radiation is presented in Fig. 2. There were 20 vibrational–rotational lines within spectral range 5.0–5.6 lm in the laser spectrum. Specific input energy was Qin = 340 Jl1 Amagat1. The laser pulse started after the beginning of EBSD pump pulse with time delay 16 ls. Fig. 2a demonstrates time behavior of mode-locked multiline CO laser radiation intensity at the begin-
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ning of laser pulse. The laser pulse consists of a train of spikes following with 10 MHz PRR. Such kind of short spikes was observed (see, for instance Fig. 2b) during the whole CO laser pulse which duration was more than 0.8 ms (Fig. 2d). Average spike duration was 15 ns FWHM (full width at half maximum). It should be pointed out that some distortion of the spike shape was regularly observed during the whole laser pulse like double peaks in Fig. 2c, for instance. It means that spike duration could be diminished. The output energy and spectrum of the CO laser did not change when switching off the RF driver feeding the AOM, whereas the time behavior of the laser output changed drastically. The free-running CO laser pulse had periodical structure with axial period of 100 ns (Fig. 3) due to an intracavity mode beating. It should be pointed out that laser power dropped down to zero between maxima of intensity (Fig. 3b). Such time behavior for free-running CO laser was previously observed elsewhere [6]. The mode beating was observed during the whole laser pulse, with duration of radiation spikes 70 ns that is a little bit longer than a half of axial mode period. Output energy was 2 J for both mode-locked and free-running CO laser. For the free-running CO laser the mean value of the laser power P1 averaged over the first 0.1 ms was 10 kW, and the maximum power Pmax was 12 kW. For the mode-locked CO laser the average power P1 was 15 kW, whereas the maximum power Pmax reached 120 kW. Fig. 4 demonstrates the specific output energy Qlas and laser efficiency dependences versus the specific input energy Qin for the mode-locked EBSD multiline CO laser. Maximum laser efficiency comes up to 3.5%, maximum specific output energy 12 Jl1 Amagat1. When increasing the specific input energy higher than 340 Jl1 Amagat1, the laser efficiency dropped down, which could be connected with a laser medium overheating. 3.2. Single-line CO laser The single-line CO laser operated on vibrational–rotational transition 9–8 P(11) with wavelength k = 5.285 lm at the specific input energy Qin = 350 Jl1 Amagat1 for both mode-locked and free-running EBSD CO laser. A spectral line width of this vibrational–rotational transition under experimental conditions was estimated as 0.42 GHz. Time behavior of the mode-locked CO laser pulse is presented in Fig. 5. Laser pulse consisted of a train of spikes. Average spike duration was 10 ns, whereas minimal spike duration was 5 ns being three times shorter as compared to multiline lasing 15 ns. Total output energy of the mode-locked single-line CO laser was 0.4 J, maximum laser efficiency being 0.7%. For the mode-locked CO laser the average power P1 was 2 kW, whereas the peak power Pmax reached 70 kW. When switching the CO laser to free-running single-line mode of operation, its output energy did not change and came to 0.4 J. Time behavior of the free-running CO laser pulse is presented in Fig. 6. There are chaotic spikes with increasing intensity at the beginning of the free-running CO laser pulse (Fig. 6a). A little bit later after that, intense spikes following with PRR 10 MHz corresponding to the frequency of mode beating begins to separate themselves from chaotic radiation (Fig. 6b) becoming more and more pronounced (Fig. 6c). The duration of such spikes was 50 ns FWHM. For the free-running CO laser the average power P1 was 2 kW and the peak power Pmax reached 10 kW. Following from the time patterns in Fig. 6 one can conclude that axial mode competition was going on finally leaving about two modes, which resulted in about sinusoidal time pattern with about 100% modulation depth. It means that the spectral width of the free-running CO laser line was 10 MHz being far less than that of vibrational–rotational transition 420 MHz.
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Fig. 2. Time behavior of mode-locked multiline CO laser radiation intensity at the beginning of laser pulse on time interval t from 16.4 to 17.4 ls (a), on time intervals from 18.3 to 18.9 ls (b), from 24.0 to 24.4 ls (c) and the whole CO laser pulse (d). Qin = 340 Jl1 Amagat1.
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Fig. 3. Time behavior of free-running EBSD multiline CO laser radiation intensity at the beginning of laser pulse on time interval t from 16.4 to 17.4 ls (a) and from 26.9 to 27.7 ls (b). Qin = 340 J l1 Amagat1.
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Fig. 4. Specific output energy Qlas (a) and laser efficiency (b) of mode-locked EBSD multiline CO laser versus specific input energy Qin.
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Fig. 5. Time behavior of mode-locked single-line CO laser radiation intensity on spectral line 9–8 P(11) at the beginning of laser pulse on time interval t from 22.4 to 23.4 ls (a) and on time interval from 26.4 to 27.2 ls (b). Qin = 350 J l1 Amagat1.
3.3. Duration of the mode-locked CO laser spikes Two procedures were used to measure the mode-locked CO laser spike duration over the whole laser pulse. The first one was a
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direct measurement of FWHM spike duration. The second one consisted in an analysis of a Fourier transform for the laser intensity. In the first procedure the FWHM spike duration was measured for each spike. Spike duration s averaged over a set of 20 adjacent spikes, is presented in Fig. 7 by diamond marks for the multiline (Fig. 7a) and the single-line (Fig. 7b and 7c) CO laser. The time duration of the mode-locked CO laser spikes was not a constant over the whole CO laser pulse. The linear trend was a decrease of s with time over the laser pulse which envelope is also presented in the above figures. The most notable trend was observed for the multiline laser pulse (Fig. 7a), for which the spike duration s dropped down by 3 ns for the first 0.1 ms. The spike duration averaged over the first 0.3 ms was hsi = 9.5 ns with the root-meansquare (rms) deviation of 1.5 ns. It should be pointed out that the parameter ds/dt for the multiline laser pulse did not change as Qin increased from 170 to 400 Jl1 Amagat1, although the laser output energy was enhanced as much as 2.6 times. The spike duration of the single-line CO laser decreased with the relatively low parameter ds/dt, which one can see in Fig. 7b for the CO laser running on vibrational–rotational line 10 ? 9 P(7) k = 5.313 lm at When Qin increased up to Qin = 170 Jl1 Amagat1. 340 Jl1 Amagat1 (Fig. 7c), which resulted in an increase of output energy as much as three times, the absolute value of the parameter ds/dt increased as much as 1.5 times. The Fourier transforms for the laser pulse intensity If of the single-line and multiline CO laser together with a Gauss function fittings to its envelopes are presented in Fig. 8a and b, respectively. Approximately eight axial modes (80 MHz FWHM) were locked. Since a FWHM of a Fourier transform is connected with spike duration, a second procedure was as follows. The whole laser pulse duration was divided into a set of time intervals with 2 ls duration and the Fourier transform was obtained for each time interval. Spike duration s was estimated from FWHM of the Fourier transform. The spike durations as a function of time measured by the two procedures are presented in Fig. 9 for the multiline (Fig. 9a)
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Fig. 6. Time behavior of free-running single-line CO laser radiation on spectral line 9–8 P(11) at the beginning of the laser pulse on time interval t from on 22.4 to 23.1 ls (a), from 24.0 to 25.2 ls (b) and from 31.2 to 32.4 ls (c). Qin = 350 Jl1 Amagat1.
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Fig. 7. FWHM spike duration s (1) and output intensity envelope I (arb.un.) (2) versus time under following conditions: (a) multiline mode-locked CO laser. Qin = 340 Jl1 Amagat1; (b) single-line mode-locked CO laser operating on vibrational–rotational transition 10 ? 9 P(7). Qin = 170 Jl1 Amagat1; (c) single-line mode-locked CO laser operating on vibrational–rotational transition 10 ? 9P(7). Qin = 340 Jl1 Amagat1.
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Fig. 8. The Fourier transform If of intensity (bars) and the Gauss function fit (curve) for the multiline mode-locked CO laser (a); for the single-line mode-locked CO laser (b) operated on vibrational–rotational transition 10 ? 9P(7). Qin = 340 Jl1 Amagat1.
and single-line (Fig. 9b) CO laser by diamond marks for the direct procedure and by triangle marks for the Fourier transform procedure. The second procedure resulted in a bit more spike duration that can be connected with a spike position jitter over the axial period resulting in a distortion of the Fourier transform. A difference Ds between two values obtained by the two procedures was equal to 1.5 ns. An analysis of a spike location in the time domain demonstrated that the rms deviation of a spike position was also 1.5 ns. Thus the parameter Ds describes the rms of a spike jitter. The cause of this jitter seems to be related to the width of the modulation band of optical losses in the AOM specified as much as 1.25% of its modulation frequency which is close to the relative deviation of a spike position of 1.5% of the axial period. The spike duration in a mode-locked pulsed CO laser decreases with time. The reason of this fact seems to be due to a competition
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t, μs
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Fig. 9. FWHM spike duration s obtained directly by averaging over 20 adjacent spikes (1) and by Fourier transform (2). Solid lines designate the trend of a spike duration decrease. (a) multiline mode-locked CO laser; (b) single-line mode-locked CO laser operating on vibrational–rotational transition 10 ? 9 P(7). Qin = 340 Jl1 Amagat1.
of two processes. One of them is a recurrent amplification of ns spikes resulting in their length increase. Another process is an increase of number of axial modes because of modulation of optical losses resulting in a spike duration decrease. The latter process prevailing over the former one. In a ‘‘master oscillator-laser amplifier” system the shortest ns spike that can be amplified in a laser amplifier should be selected from the tail of a CO laser pulse. Besides that, for some applications of a mode-locked CO laser, for instance, in heavy ions stochastic cooling, one has to take into the consideration casual a spike position jitter (1.5 ns in these experiments), which can be diminished by a decrease of a width of modulation band for optical losses in an acousto-optical modulator.
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4. Conclusions Mode-locked EBSD CO laser operated with gas mixture CO:N2 = 1:9 at gas density 0.2 Amagat demonstrated effective lasing both in multiline and single-line mode. This CO laser emitted a train of ns spikes at 10 MHz pulse repetition rate. Averaged spike duration was 10 ns FWHM for the mode-locked single-line 5.3 lm CO laser, whereas the minimal spike duration was 5 ns FWHM. For the mode-locked multiline CO laser average spike duration was 15 ns FWHM. Maximum peak power 120 kW was obtained for the mode-locked multiline CO laser. In the case of the mode-locked single-line 5.3 lm CO laser the peak power reached 70 kW. It should be pointed out that the nanosecond pulses can be easily amplified in wide-aperture CO laser amplifier [7]. Thus a mode-locked CO laser emitting ns spikes can be successfully applied for pumping an optical parametric amplifier for optical stochastic cooling in relativistic heavy ion collider [1], for laser ablation [8], and for studying vibrational and rotational relaxation of some molecules, for instance CO and NO [9]. Acknowledgments The authors are grateful to Marcus Babzien, Ilan Ben-Zvi, Igor Pavlishin, Igor V. Pogorelsky and Vitaly E. Yakimenko of BNL,
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who initiated this research, for helpful discussions. This research was supported by the Russian Foundation for Basic Research (Grant# 07-02-01400) and the Educational and Research Complex of the P.N. Lebedev Physical Institute. References [1] I. Babzien, I. Ben-Zvi, I. Pavlishin, et al., Phys. Rev. ST Accel. Beams 7 (2004) 012801. [2] S.V. Vetoshkin, A.A. Ionin, M. Klimachev Yu, et al., J. Russian Laser Res. 27 (2006) 33. [3] A.V. Nurmikko, J. Appl. Phys. 26 (8) (1974) 465. [4] A.V. Nurmikko, J. Appl. Phys. 46 (5) (1975) 2153. [5] A.A. Ionin, Electric Discharge CO Lasers, in: M. Endo, R.F. Walter (Eds.), In Gas Lasers, CRC Press, Taylor & Francis Group, Boca Raton, 2007. [6] C. Beairsto, R. Walter, A.A. Ionin, A.A. Kotkov, et al., Quant. Electron. 27 (1997) 614. [7] S.V. Vetoshkin, A.A. Ionin, M. Klimachev Yu, et al., Quant. Electron. 37 (2) (2007) 111. [8] P. Eliseev, A. Ionin, Yu. Klimachev, et al., Proc. SPIE. 143 (2002) 4760. [9] R.P. Andrusenko, A.A. Ionin, M. Klimachev Yu, et al., Proc. SPIE. 6729 (2007) 672923.
