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Cascaded carbon monoxide laser frequency conversion into the 4.3–4.9 μm range in a single ZnGeP2 crystal A. A. Ionin,1,* I. O. Kinyaevskiy,1 Yu. M. Klimachev,1 A. A. Kotkov,1 A. Yu. Kozlov,1 Yu. M. Andreev,2 G. V. Lanskii,2 A. V. Shaiduko,2 and A. V. Soluyanov3 1 2
P.N. Lebedev Physical Institute of the Russian Academy of Sciences, 119991 Moscow, Russia
Institute of Monitoring of Climatic and Ecological Systems of Siberian Branch of the Russian Academy of Sciences, 634055, Tomsk, Russia 3 National Research Nuclear University “MEPhI,” 115409 Moscow, Russia *Corresponding author:
[email protected] Received April 17, 2012; revised May 21, 2012; accepted May 22, 2012; posted May 22, 2012 (Doc. ID 166882); published July 10, 2012 Collinear cascaded mid-IR frequency conversion in a single nonlinear optical crystal was accomplished. Concurrent collinear generation of the sum frequency of multiline fundamental band carbon monoxide laser radiation as a first frequency conversion cascade resulted in collinear difference frequency generation within the 4.3 to 4.9 μm spectral range when mixing this sum frequency radiation with the fundamental one as the second cascade in the same ZnGeP2 nonlinear optical crystal. © 2012 Optical Society of America OCIS codes: 190.2620, 140.3070.
The carbon monoxide (CO) laser has an extremely broad emission spectrum consisting of about a thousand spectral lines in fundamental (wavelength range Δλ ≈ 4.7–8.2 μm) [1] and first-overtone (Δλ ≈ 2.5–4.2 μm) [2] vibration bands. This laser can run on both a selected single line and in a multiline mode. Due to rich emission spectrum and narrow lines, the CO laser can be successfully applied; e.g., in a multicomponent gas analysis [3]. However, there is 4.2 to 4.7 μm spectral gap between the fundamental band (FB) and first-overtone band in which the CO laser does not run. This spectral gap is of special interest in remote sensing due to its overlapping with one of the main mid-IR atmospheric transparency window. That is why laser sources operating in this range are being developed [4,5]. On the other hand, there is an opportunity of frequency shifting of CO laser spectrum into a 4.2 to 4.7 μm range by nonlinear optics techniques. In [6] we proposed to achieve a difference frequency (DF) generation (DFG) by nonlinear mixing of FB and first-overtone CO laser radiation in a nonlinear optical crystal (NOC). It should be pointed out that just in the same [6] efficient second harmonic (SH) generation (SHG) of FB CO laser radiation with internal efficiency of 25% was observed in ZnGeP2 NOC due to application of mode-locked FB CO laser emitting high-power nanosecond pulses. Then, in [7] we proposed to mix CO laser FB and SH radiation in the same ZnGeP2 NOC for getting DFG in a broad spectral range. In this paper, we observed a concurrent two-cascade frequency conversion of CO laser radiation in the same ZnGeP2 NOC that enabled us to cover the main part of the 4.2 to 4.7 μm spectral gap between fundamental and first-overtone CO laser emission bands. Up to now, three concurrent frequency conversion processes (optical parametric oscillation, SHG, and DFG) and many other cascaded processes have been already demonstrated in one NOC but just for near-IR pump lasers [8–10]. Such sort of frequency conversion of near-IR laser radiation into mid-IR was demonstrated in [8]. However, even though sum frequency (SF) generation (SFG) and SHG (that might be referred to as degenerated SFG) 0146-9592/12/142838-03$15.00/0
of CO laser radiation was earlier observed [11,12], no mid-IR laser radiation has ever been converted into a shorter wavelength range by cascaded processes in one NOC. In our study, collinear cascaded mid-IR frequency conversion in one NOC was realized. Concurrent collinear generation of SF and SH of multiline FB CO laser radiation as a first frequency conversion cascade resulted in collinear DFG within a 4.3 to 4.9 μm spectral range when mixing SF and SH radiation with FB one as the second cascade in the same ZnGeP2 NOC. To maximize efficiency of DFG of SH (or SF) and FB CO laser radiation in one ZnGeP2 NOC, it is necessary to accomplish approximately the same phase-matching conditions for both SHG (or SFG) and DFG processes. In this paper we consider only collinear forward phase-matching (PM). Dispersion equations reported for ZnGeP2 in [13] were used in calculations of SHG (or SFG) and DFG PM angles from the following relations: nSH 2nFB ; SHG; λSH λFB
(1)
nSH nFB nDF − ; DFG; λSH λFB λDF
(2)
where λFB , λSH , and λDF are wavelengths of fundamental, second harmonic, and difference frequency lines; and nFB , nSH , and nDF are refractive indices at those wavelengths, respectively. Note that Eqs. (1) and (2) are also valid for SFG if one substitutes nSH and λSH by index and wavelength of the SFG spectral range that coincides with the SHG one [11]. The SFG does co-exist with SHG because of following factors: a noncritical frequency matching in ZnGeP2 NOC with spectral width of ∼200 cm−1 takes place both for SF and SH of CO laser spectral lines, and the angular bandwidth of this phase-matching is ∼1.8 deg for frequency conversion of the whole spectrum of a CO laser [11]. Therefore, further results are valid not only for SH, but also for SF radiation. PM diagrams calculated for oe-e type interaction of CO laser SH (o-wave) and FB (e-wave) radiation are presented in Fig. 1. © 2012 Optical Society of America
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Fig. 3. Optical layout of the experiment: (1) CO laser, (2) CaF2 lens, (3) NOC ZnGeP2 , (4) flat CaF2 beam splitter, (5) spherical mirror, (6) photodetector, (7) laser power meter, (8) spherical mirror, (9) spectrometer, (10) fused silica filter, and (11) laser power meter; EeFB , EoSH , EoSF , and EeDF —polarization vectors of FB, SH, SF, and DF waves, respectively.
The diagrams in Fig. 1 correspond, respectively, to DFG phase-matching of SH lines within spectral range ΔλSH 2.5–2.9 μm and FB line at λFB 5.0 μm, ΔλSH 2.5–3.1 μm, λFB 5.6 μm, ΔλSH 2.5–3.2 μm, and λFB 6.2 μm. It is seen in Fig. 1 that the phase-matched DFG of the SH line at 2.5 μm and the FB line at 5.6 μm exists at the PM angle θDF 48 deg and yields an emission line at 4.5 μm. Besides, just the same PM angle corresponds to the SHG PM angle θSH under the 5 μm pump (Fig. 2). Therefore, the ee-o type SHG under the 5.0 μm FB CO laser line pump mixing with the FB 5.6 μm line in ZnGeP2 is simultaneously followed by the oe-e type DFG at 4.5 μm as a result of mixing 2.5 μm and 5.6 μm lines. It was theoretically found that the DFG of CO laser SH in the vicinity of 2.5 μm and FB CO laser lines within the 5.0 to 5.6 μm at a PM angle of 48 deg can result in the broadband range DFG in the vicinity of 4.5 μm. Taking into account DFG of all SF, SH, and FB CO laser lines, one could expect filling in the spectral gap between first overtone and FB CO laser spectrum. Experiments were carried out at the Gas Lasers Laboratory of the Lebedev Physical Institute of RAS. An optical schematic diagram of our experiments is
presented in Fig. 3. The homemade low-pressure cryogenic FB CO laser 1 pumped by the DC discharge was Q-switched by a rotating mirror and used in the experimental study. Nonselected (multiline) laser radiation was focused by the CaF2 lens 2 with a focal length of 115 mm onto high-quality ZnGeP2 crystal 3 (length of 17 mm, absorption less than 0.04 cm−1 at 2.3–8.4 μm) with a beam spot of 0.5 mm in diameter and peak intensity up to 2 MW∕cm2 . This crystal was installed at the SHG PM angle. To measure time and energy parameters of CO laser radiation, part of the laser beam (∼5%) was split off by flat CaF2 beam splitter 4 and then directed by spherical mirror 5 onto photo-detector PEM-L-3 6 and laser power meter Ophir-12A 7. FB, SF, SH, and DF radiation coming out of the NOC was collimated by spherical mirror 8 and directed to IR spectrometer 9 (IKS-31, LOMO PLC) equipped by a low-noise cryogenic Ge:Au photo-detector with a broad dynamic range of about four orders of magnitude. Measured FB, SFG, SHG, and DFG spectra were displayed by oscilloscope Tektronix TDS5052B. To select and measure the power of SF and SH pulses, fused silica filter 10 and laser power meter 11 Ophir-12A were installed in front of the spectrometer 9. Polarization of FB, SH, SF, and DF waves are labeled in Fig. 3, respectively, by vectors EeFB , EoSH , EoSF , and EeDF . The FB CO laser emitted a sequence of 0.5 to 1.0 μs pulses with a pulse repetition rate of 50 to 150 Hz depending on the rotation speed of the Q-switching mirror with an average output power of 0.2 W. Peak power of laser pulses was up to 4 kW. The laser spectrum (Fig. 4) consisted of ∼80 e-polarized spectral lines within the
Fig. 2. Type ee-o phase-matching for SHG of IR laser radiation within the 4 to 7 μm range in ZnGeP2 .
Fig. 4. (Color online) Spectra of fundamental band (FB), second harmonic (SH), sum frequency (SF), difference frequency (DF) radiation, and transmission spectrum of the laboratory air.
Fig. 1. (Color online) Type oe-e phase-matching for the difference frequency generation (DFG) of second harmonic (SH) and fundamental band (FB) carbon monoxide (CO) laser radiation in ZnGeP2 ; spectral gap on the curves between bold points is 0.1 μm.
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4.9 to 6.3 μm range. The laser was used as a pump source for ZnGeP2 NOC. Because of nonlinear frequency conversion, radiation within spectral ranges of 2.45 to 2.85 μm and 4.3 to 4.9 μm was detected. The spectrum of the former radiation consisted of ∼110 spectral lines, maximum power being for wavelengths around 2.5 μm (Fig. 4). Peak power distribution over emission lines within an interval of 2.45 to 2.85 μm in Fig. 4 was normalized in such a way that the total sum over all lines gave us the measured peak power of 40 W. These spectral lines outnumbered FB ones due to the fact that together with SHG, a nonlinear process of frequency summation of various pairs of FB spectral lines (i.e., SFG) took place. Under the pump of CO laser radiation, which was well under the optical damage threshold, a maximum SHG SFG external efficiency of 1.8% in ZnGeP2 NOC corresponding to the internal efficiency of 3.4% was observed at PM angle θSH 47 deg. (It should be pointed out that this efficiency was far lower than that of [6] for SHG because the laser power was much lower than that of CO laser emitting ns pulses). In further experiments, the crystal was installed under the PM angle of 48 deg, which resulted in maximum power of SH SF radiation with a wavelength near 2.5 μm, with lower external efficiency of ∼1%. SH and SF radiation had o-polarization. Following our above-mentioned expectations, apart from the SF and SH spectral lines, more than 80 DF lines were clearly observed within the spectral range of 4.3 to 4.9 μm (Fig. 4) corresponding to frequency mixing of SF, SH, and FB lines. Even though the longest awaited DF line wavelength was ∼5.1 μm, DF lines at wavelengths longer than 4.9 μm were not certainly identified because of severe interference by the very intense FB lines. Peak power of DFG pulses was determined by direct comparison of amplitudes of DF and FB lines. The external and internal efficiency of frequency conversion of FB lines into DF lines within the 4.3–4.9 μm range was 0.25% and 0.48%, respectively. One can see in Fig. 4 that the DF spectrum structure reflects the FB CO laser spectrum structure well. The lack of DF lines below 4.3 μm can be related to lower intensity of FB lines around 6 μm that are involved in the DFG process and to absorption by atmospheric carbon dioxide. Atmospheric transmission T for the lab conditions was recorded by a Fourier spectrometer AF-3 (manufactured by Scientific and Technological Center of Unique Instrumentation of RAS) with a spectral resolution of Δν 1 cm−1 and also presented in Fig. 4. Collinear cascaded mid-IR frequency conversion in ZnGeP2 NOC was realized. Concurrent collinear generation of SF and SH of multiline FB CO laser radiation resulting in concurrent collinear DFG within the 4.3 to 4.9 μm range when mixing SF and SH radiation with FB one in the same NOC was observed. More than 80 FB spectral lines of the CO laser used in the experiments were within the 4.9 to 6.3 μm range. Concurrent SFG and SHG resulted in ∼110 SF and SH lines within the 2.45 to 2.85 μm range. Maximal external efficiency of this first cascade frequency conversion process
amounted to 1.8%. Taking into account Fresnel optical losses on nonantireflection-coated faces of the crystal, internal efficiency is estimated as of 3.4%. The spectrum of the second cascade frequency conversion process— DFG within the 4.3 to 4.9 μm range contained ∼80 emission lines. External and internal DFG efficiency was 0.25% and 0.48%, respectively. An enhancement of DFG efficiency and full cover of the spectral gap of 4.2 to 4.7 μm between the fundamental and first-overtone bands of the CO laser seems to be possible by application of high-power mode-locked ns CO laser pulses. Taking into account an opportunity for a CO laser to run from 2.5 to 4.2 μm and from 4.7 to 8.2 μm and also run within the 4.3 to 4.9 μm range with NOC, one can draw the conclusion that a single CO laser with a nonlinear frequency converter can cover the spectral range from 2.5 up to 8.2 μm. Note that analogous concurrent SHG (or SFG) and DFG cascaded frequency conversion in one NOC can be realized in other mid-IR NOCs that meet the requirement of phase-matching for both SHG (or SFG) and DFG processes, such as gallium selenide. This research was partially supported by an RF Presidential Grant NS-512.2012.2, RFBR Project No. 12-08-0042, and IIP SB RAS No.46 of 2012. References 1. A. A. Ionin, in Gas Lasers, M. Endo and R. F. Walter, eds. (CRC Press, 2007), p. 201. 2. N. G. Basov, G. D. Hager, A. A. Ionin, A. A. Kotkov, A. K. Kurnosov, J. E. McCord, A. P. Napartovich, L. V. Seleznev, and N. G. Turkin, IEEE J. Quant. Electron. 36, 810 (2000). 3. O. G. Buzykin, S. V. Ivanov, A. A. Ionin, A. A. Kotkov, and A. Yu. Kozlov, J. Russian Laser Res. 26, 402 (2005). 4. M. E. Doroshenko, H. Jelínková, J. Šulc, M. Jelínek, M. Němec, T. T. Basiev, Y. A. Zagoruiko, N. O. Kovalenko, A. S. Gerasimenko, and V. M. Puzikov, Laser Phys. Lett. 9, 301 (2012). 5. V. A. Akimov, A. A. Voronov, V. I. Kozlovskii, Yu. V. Korostelin, A. I. Landman, Yu. P. Podmar’kov, and M. P. Frolov, Quant. Electron. 36, 299 (2006). 6. A. A. Ionin, J. Guo, L.-M. Zhang, J.-J. Xie, Yu. M. Andreev, I. O. Kinyaevsky, Yu. M. Klimachev, A. Yu. Kozlov, A. A. Kotkov, G. V. Lanskii, A. N. Morozov, V. V. Zuev, A. Yu. Gerasimov, and S. M. Grigoryants, Laser Phys. Lett. 8, 723 (2011). 7. A. Ionin, I. Kinyaevskiy, Yu. Klimachev, A. Kotkov, and A. Kozlov, SPIE Newsroom, doi: 10.1117/2.1201112.004016 (6 January 2012). 8. J. M. Yarborough and E. O. Ammann, Appl. Phys. Lett. 18, 145 (1971). 9. B. V. Bokut’, N. S. Kazak, A. S. Lugina, E. M. Miklavskaya, A. V. Nadenenko, V. A. Orlovich, V. K. Pavlenko, and Yu. A. Sannikov, Zhurnal Prikladnoi Spektroskopii 47, 293 (1987). 10. S. M. Saltiel and Y. S Kivshar, Bulg. J. Phys. 27, 57 (2000). 11. Yu. M. Andreev, S. N. Bovdey, P. P. Geiko, A. I. Gribenyukov, V. A. Gurashvili, V. V. Zuev, and S. V. Izyumov, Optika Atmosfery 1, 124 (1988). 12. Yu. M. Andreev, V. V. Zuev, G. V. Lanskii, A. V. Shaiduko, A. A. Ionin, I. O. Kinyaevskii, Yu. M. Klimachev, A. Yu. Kozlov, and A. A. Kotkov, J. Opt. Technol. 78, 102 (2011). 13. G. C. Bhar, L. K. Samanta, D. K. Ghosh, and S. Das, Sov. J. Quantum Electronics 17, 860 (1987).
