Difference frequency generation under mixing of repetitively-pulsed CO- and CO2-laser radiation in AgGaSe2, BaGa2GeSe6 and PbIn6Te10 nonlinear crystals was studied. Efficiency and refractive indices were examined for this frequency conversion into the long-wave domain of ∼12-20 µm in the mid-IR. The highest frequency conversion efficiency of 10−4 was obtained for a relatively new PbIn6Te10 crystal, which is an order of magnitude higher than previous results.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
The mid-infrared (IR) wavelength range of 2–20 µm (known as the molecular “fingerprint” region) hold a great promise for many important applications such as chemical and biological sensing, free-space communications and infrared-countermeasure applications. Therefore, different sorts of mid-IR lasers have been actively developed .
Solid state mid-IR lasers based on transition metal doped II–VI crystals, fiber lasers, optical parametric oscillators have successfully operated within a short-wave part of the mid-IR range up to ∼5 µm wavelength . In the mid-IR middle (5-12 µm wavelengths) an impressive progress has been done in developing of quantum cascade lasers [1,2]. Also, a number of laser devices based on two-stage frequency conversion (optical parametric oscillators combined with difference frequency generation (DFG) in an additional crystal) were developed. One of them, for instance, produced MW picosecond pulses around 5 µm wavelength with ∼1% efficiency of DFG stage in a GaSe crystal .
Despite the progress, a lack of mid-IR lasers operating in the long-wave domain (wavelengths 12-20 µm) still exists. In this spectral domain gas lasers based on pure rotational transitions can operate , but these are very rarely used devices. There are several quantum cascade lasers operating in the long-wave domain: Sb-based heterostructures or InGaAs/AlInAs system [1,2]. However, their long-wave lasing is limited by phonon absorption bands of the materials and a main spectral range of quantum cascade lasers is 5-12 µm, see e.g., Hamamatsu photonics products. DFG in a GaSe crystal of near-IR solid state lasers with auxiliary optical parametric oscillators can cover the mid-IR up to 28 µm wavelength, but its efficiency sharply decreases down to 10−4 for 10 µm emission and down to 10−6 for ∼28 µm emission . It should be pointed out that the DFG technique for mid- and far-IR generation has been studied in many papers with different lasers and nonlinear crystals. However, energetic, spectral and operational parameters of such laser systems did not altogether achieve a level needed for widespread applications. Therefore, various crystals, lasers and frequency mixing schemes have been still studied to accomplish it (see, for example [5,6].
In this paper we consider high-power molecular gas lasers for pumping nonlinear crystals, because they operate deep in the mid-IR, namely carbon monoxide (4.7-8.7 µm) [7,8] and carbon dioxide (9-11 µm)  lasers. The advantage of this approach is higher Manley–Rowe relation as compared to near-IR lasers. For example, kW- and MW- peak power of THz radiation was obtained for DFG of CO2 laser pulses with nanosecond  and sub-nanosecond  duration, respectively. Frequency conversion of CO and CO2 laser radiation into the long-wave domain of the mid-IR was for the first time used in [11,12]. The main problem of this technique was spatial and temporal synchronization of the lasers. Timing issue was successfully solved by synchronous Q-switching of the lasers with a single shared rotating mirror and the DFG of CO and CO2 lasers was demonstrated with wavelength up to 16.6 µm .
In the current DFG study, the system based on slab RF discharge synchronously Q-switched CO and CO2 lasers was developed and implemented. This design affords compactness, low-voltage power supply and similar power level as compared to elongated DC discharge gas lasers (that is why such lasers have been developed by different teams [14–16]).
2. Nonlinear crystals
One of the first experimental demonstrations of CO and CO2 gas lasers frequency mixing was performed with a CdGeAs2 crystal . Potentially, CdGeAs2 is a very attractive crystal for frequency conversion in the mid-IR. It is transparent up to 18 µm wavelength and has very high nonlinear coefficient (282 pm/V) . But it has high absorption and is rarely used in practice. A great number of researches studying DFG in 12-20 µm wavelength range (as well as in the THz range) was performed with the GaSe crystal. However, its efficiency for frequency conversion of nano- and microsecond pulses was quite low due to a spatial walk-off effect [13,18].
For DFG under mixing of CO and CO2 laser radiation into 12-20 µm wavelength range, we considered AgGaSe2 (AGSe), BaGa2GeSe6 (BGGSe) and PbIn6Te10 (PIT) nonlinear crystals. The main crystal properties are presented in Table 1. A selection of proper crystal properties is not a simple problem. For example, handbook  presents 9 references on dispersion equations for AGSe crystal and gives recommendation of equations from . However, the equations  provided significant deviation from the experimental results presented below. The best agreement for AGSe crystal was obtained with the combination of dispersion equations: one from  for pump radiation (wavelength below 11 µm) and one from  for DFG radiation (wavelength above 11 µm). Dispersion equations of PIT crystal presented in a few papers [21,22] were not analyzed for the long-wave domain of the mid-IR. The dispersion equations of BGGSe crystal was presented in  and corrected in  for wavelengths up to 10.59 µm. Table 1 presents dispersion equations which were the most adequate ones with our experimental data presented below.
Also, Table 1 demonstrates AGSe crystal to have moderate nonlinear coefficient, optical damage threshold, transparency range and other properties that makes this conventional crystal widely used for different applications in the mid-IR. BGGSe is a quite new promising mid-IR nonlinear crystal . In experimental study , the efficiency of a new BGGSe crystal was twice as higher as that of AGSe crystal for DFG with wavelength near 8 µm. PIT is a relatively new nonlinear crystal with unique broad transparency range up to ∼30 µm  which makes this crystal very attractive for frequency conversion into the mid-IR longwave domain.
Figure of merit (FOM), spectral (Δλ) and angular (Δθ) acceptance of these crystals (for 1 cm length) for CO and CO2 laser down-conversion were calculated and presented in Fig. 1. Also, phase-matching angles for type-I mixing were calculated and compared with measured ones (see Fig. 5 in the experimental part below). These calculations are “standard” procedures which can be found in handbook . FOM for type-I mixing were calculated by following expressions: dooe=d36sin(θ), deeo=d11cos2(θ) and deeo=d11cos2(θ) for AGSe, BGGSe and PIT crystals, respectively.
Figure 1 demonstrates for BGGSe crystal to have high FOM within 12-20 µm wavelength range, but its spectral and angular acceptance were the lowest ones. AGSe crystal has high FOM and the largest angular acceptance, but it is applicable only up ∼17 µm wavelength. PIT crystal has moderate FOM and the broadest spectral acceptance. While all these parameters including optical damage threshold and absorption significantly affected frequency conversion efficiency, it was quite difficult to expect which crystal would be the most efficient in the experiment.
3. Experimental set-up
The original laser system for DFG study presented in Fig. 2 was developed on the basis of two repetitively-pulsed slab RF discharge laser installations.
The first installation of 40 cm active medium length operated as a CO laser. CO laser cavity of 1.8 m length was formed by output mirror OM1 (Ge flat plate with reflectance of ∼50%) and rear flat mirror M1. Spherical mirror SM1 with 180 cm radius of curvature was situated inside of the CO laser discharge chamber in the cavity middle. Active medium composition was CO:Air:He = 1:1.5:8.8 at total gas pressure of 30 Torr. The second installation of 30 cm active medium length operated as a CO2 laser. The CO2 laser cavity of 2.35 m length was formed by output mirror OM2 (dielectric mirror with reflectance of ∼75% at 10 µm wavelength) and rear spherical mirror SM2 with 140 cm radius of curvature. Spherical mirror SM3 with 120 cm radius of curvature was placed into the CO2 laser discharge chamber. It divided the laser cavity into two arms with length ratio of 1:2.6. Active medium composition was as follows: CO2:Xe:He = 1:0.3:16 at total gas pressure of 75 Torr.
The CO and CO2 lasers operated in Q-switching mode with 100 Hz pulse repetition rate provided by a single shared rotating mirror RM. RF discharge pump pulses for CO and CO2 lasers was optimized and coordinated through a sync-pulse from the rotating mirror. Rectangular RF discharge pump pulses of 0.6 ms and 0.4 ms duration for CO and CO2 lasers, respectively, had about the same peak power of ∼0.8 kW for the both installations. The FWHM pulse durations measured with photodetector PEM-L-3 (VIGO system S.A.) were ∼ 0.25 µs for the CO2 laser and 0.75 µs for the CO laser Fig. 3(a).
The lasers operated in multi-line spectral mode. The CO2 laser operated simultaneously on 4 transitions of 0001→0200 band: P(14), P(26), R(18), R(34) Fig. 3(b). CO2 lasing in this band was preferable and took place due to a BaF2 Brewster window of the laser chamber. The CO2 laser total peak power was up to 0.3 kW. The CO laser typically emitted on ∼ 100 spectral lines in the wavelength range from 5.0 to 6.5 µm. However, low vibrational CO laser transitions were not required for frequency conversion to the long wavelength mid-IR domain. Therefore, CO laser radiation was spectrally modified by optical filter SP-5665 (Spectrogon). The filter was transparent for radiation with wavelength shorter than 5.6 µm and totally reflecting for longer ones Fig. 3(c). The spectrum of filtered CO laser radiation consisted of about 30 narrow spectral lines in the wavelength interval of 5.5-6.5 µm Fig. 3(c). Peak power integrated over the whole spectrum was ∼0.5 kW. It should be noted that different CO laser rotational–vibrational spectral components may be separated in time due to a cascading mechanism of lasing. It can reduce frequency conversion efficiency. However, our paper  demonstrated for the strongest lines of multiline Q-switched CO laser (which gives a major contribution to the frequency conversion) to have almost simultaneous lasing. Therefore, the decline of frequency conversion efficiency due to cascade CO lasing was insignificant.
Spatial overlapping of CO and CO2 laser beams was performed by a polarizer. It was transparent for CO laser radiation and totally reflecting for CO2 laser radiation, because CO and CO2 laser pulses had linear mutually perpendicular polarizations. Perpendicular polarizations were required for DFG phase-matching in nonlinear crystals. To control laser system parameters, its output beam was split by flat BaF2 plates and directed into power meter (12A-SH, Ophir Optronics Solutions Ltd) and photodetector (PEM-L-3, VIGO system S.A.), respectively.
The laser system emission consisting of spatially and temporally combined CO and CO2 laser pulses was focused into a nonlinear crystal by spherical mirror SM4 with the focal length of 15 cm. Afterwards, it was collimated by spherical mirror SM5 and directed into IR-spectrometer based on diffractive monochromator MDR-2 (LOMO systems). To measure waveform of DFG radiation integrated over its spectrum, laser beam was focused into photodetector installed in front of the IR spectrometer. We used cryogenic MCT photodetector J15D22 (Teledyne Judson Technologies) applicable for radiation with wavelength up to 22 µm. To separate the DFG radiation from the pump one, we applied a set of long-wave pass filters: two filters LP-11450 (Spectrogon) (its transmittance is below in Fig. 4(d)) with filters of IR-spectrometer IKS-31 (LOMO systems). It allowed us to attenuate the pump radiation by more than 108 times.
For DFG study of CO and CO2 laser radiation into 12-20 µm wavelength range, we applied following crystals samples: AGSe crystal of 10.1 mm length, polar angle θ=60° and azimuthal angle φ=45°; BGGSe crystal of 12.7 mm length, θ=30° and φ=30°; PIT crystal of 13 mm length, θ=33° and φ=30°. The crystals were produced in the High Technologies Laboratory of the Kuban State University, Russia.
4. Experimental results
In the first experiments we measured DFG power integrated over its spectrum. Dependencies of DFG peak power versus phase-matching angles for the crystals under consideration are presented in Figs. 4(a)–4(c). The dependencies have several spikes due to a discrete spectral structure of pump radiation.
The DFG output was calibrated with CO2 laser which was in good agreement with photodetector sensitivity (150 V/W). The highest DFG peak power was obtained for the PIT crystal and reached 37 ± 4 mW, that corresponded to internal conversion efficiency (taking into account Fresnel losses on uncoated crystal facets) ∼10−4. The conversion efficiency was an order of magnitude higher than our previous result [13,26]. DFG peak power for AGSe crystal was up to 32 ± 4 mW. The higher PIT efficiency as compared to that of AGSe is associated with its higher spectral acceptance Fig. 1(c). The efficiency of BGGSe crystal was about an order of magnitude less, probably due to absorption of DFG radiation. It should be noted that the conversion efficiency for some individual spectral lines was higher than efficiency integrated over the whole spectrum. For example, peak power of the strongest DFG line at ∼13.5 µm was about 8.5 mw (∼7·10−4 efficiency). An application of single line CO and CO2 lasers with similar total peak power can increase single line DFG power up to ∼1 W with ∼0.1% efficiency. Further laser system improving up to multi-kW DFG power at 20 µm can be performed with MW nanosecond CO and CO2 lasers as in [9,18].
Afterwards, DFG spectra in the long-wave domain of the mid-IR were measured. We recorded 25 narrow DFG spectral lines with wavelengths from 12 to 19.3 µm by tuning a phase-matching angle of the PIT crystal from 33 to 38 degree Fig. 4(d). Detector sensitivity, atmosphere absorption, diffraction grating reflectance and other factors affecting the spectrum measurement were not taken into account. The longest DFG line that could have been obtained in the experiment was 21.6 µm corresponding to DFG between pump radiation lines with wavelengths of 6.45 µm and 9.2 µm. However, the longest recorded DFG wavelength was 19.3 µm. It was associated with low peak power of the longest CO laser line and a drop of photodetector sensitivity for radiation with wavelength longer than 18 µm. Nevertheless, DFG spectrum was significantly extended as compared to previous results . To advance DFG spectrum into the longer-wave domain of the mid-IR towards the far-IR (THz), CO lasing on high vibrational levels with wavelength longer than 6.4 µm should be applied.
The refractive index dispersion equations for nonlinear crystals in the long-wave mid-IR domain were examined by means of comparison of calculated and measured phase-matching angles for type-I mixing Fig. 5. Experimental uncertainty for the data in Fig. 5 corresponds to a circle size. The calculations were performed for two CO2 laser wavelengths: 9.3 µm and 9.6 µm, CO laser wavelength was tuned from 5.0 µm to 6.5 µm. Also, transmittance (Tr) of the crystal samples (in absolute units) was measured by IR Fourier spectrometer and presented in Fig. 5. It should be noted that measured crystals transmission is in a good agreement with previous data (see Table 1).
A comparison of the calculated and measured phase-matching angles demonstrates the correctness of refractive index dispersion equations  for the PIT crystal. The best agreement for the AGSe crystal was obtained with a combination of dispersion equations: one from  for pump radiation (wavelength below 11 µm) and one from  for DFG radiation (wavelength above 11 µm). The accurate measurement for the BGGSe crystal was difficult due to low DFG power. Even so, experimental phase-matching angle at 14 µm wavelength has the notable deviation from calculated one. It should be noted that the deviation for the equations taken from  was higher. Therefore, the dispersion equations from  should be further modified for the long-wave domain. The best suitable dispersion equations were added into Table 1 together with other main crystal properties.
Thus, Fig. 5 also shows that BGGSe applicability for DFG in 12-20 µm range is limited by radiation absorption. AGSe crystal has high transparency up to 18 µm, but phase-matched DFG is possible only up to ∼17 µm due to its low birefringence. PIT crystal has the broadest phase-matching and transparence range that makes it the most favorable for DFG in the long-wave domain, especially taking into account high experimental conversion efficiency and the highest spectral acceptance Fig. 1(c).
The laser system based on combined multi-line repetitively-pulsed slab RF-discharge CO and CO2 lasers with synchronous Q-switching was applied for difference frequency generation of µs pulse duration in the long-wave domain of ∼12 - 20 µm in the mid-IR. The highest output power and broadest spectral range of frequency converted radiation was obtained for PbIn6Te10 crystal. The radiation with wavelength up to 19.3 µm was detected which significantly elongated the longest wavelength obtainable with this technique. A comparison of the calculated and measured phase-matching angles demonstrates a correctness of refractive index dispersion equations [19,20] for AGSe crystal and ones  for PIT crystal up to ∼20 µm. Dispersion equations  for BGGSe crystal need a further improvement for wavelengths longer than 10.6 µm.
Frequency conversion efficiency integrated over the whole spectrum was up to 10−4 which is an order of magnitude higher than our previous results [13,26] also obtained with µs pulses and comparable with result  obtained with DFG of picosecond pulses of solid state laser system in GaSe. The main reasons for obtaining better results are application of relatively new PbIn6Te10 crystal and suppression of CO lasing on low-vibrational transitions that is not required for frequency conversion but may cause crystal damage. Further significant DFG power and conversion efficiency enhancement can be achieved with MW nanosecond single- or multiline CO and CO2 lasers with synchronous mode-locking.
Russian Science Foundation (16-19-10619).
The authors declare that there are no conflicts of interest related to this article.
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