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We demonstrated a Q-switched, Tm:YAG-laser-pumped electronically tuned Cr:ZnSe laser, which was equipped with an acousto-optic tunable filter as a wavelength-tuning element. A tuning range from 2.17 to 2.71 μm and a maximum output energy of 7.9 mJ at 2.41 μm were realized. The energy conversion efficiency reached 34.1% at 2.41 μm. In addition, the Cr:ZnSe laser produced a high-quality beam in the TEM00 mode.
© 2015 Optical Society of America
1. Introduction
A tunable mid-infrared (IR) pulsed laser in the 2 to 3 µm region is useful for environmental remote sensing by means of lidar and as a pump light source for generating mid-IR waves of wavelength longer than 5 µm in optical parametric amplification and oscillation. Several molecules, CO, CO2, NO, N2O, CH4, and NH3, which are considered as air pollutants, are detectable utilizing laser spectroscopy technology in this tuning range [1]. Moreover, coherent light sources with a wavelength exceeding 5 µm are also attractive for scientific applications based on spectroscopy.
Cr:ZnSe is a suitable laser material for efficient oscillation in the range from 2 to 3 µm because of its large stimulated emission cross section and wide fluorescence range [2–4]. The realization of wide tunability, high-speed tuning, and high energy in a Cr:ZnSe laser allows the long-distance, real-time detection, and identification of multiple pollutants existing in the air.
Mirov et al. have demonstrated a gain-switched nanosecond Cr:ZnSe laser. The laser was pumped with a 1.906 μm coherent light source, which was realized by stimulated Raman scattering of a Q-switched Nd:YAG laser in H2, and produced an output energy of 20 mJ [5]. Although they also reported a 1 J/pulse Cr:ZnSe laser pumped with an Er-glass laser, the temporal pulse was 7 ms [5,6]. Millisecond pulses are not suitable for remote sensing even at a high energy. Here, these Cr:ZnSe lasers were not equipped with wavelength-tuning elements, such as a prism, grating, and birefringence filter, thus a broad spectral width was observed.
To realize rapid tunability in Cr:ZnSe lasers, an acousto-optic tunable filter (AOTF) [7–9] is an effective tuning element. Zakel et al. have reported a high-speed tunable Cr:ZnSe laser using an AOTF as a wavelength-tuning element [10]. The Cr:ZnSe laser was pumped with a Tm:YALO laser, which operated at 1.94 μm with a repetition rate of 5 kHz. The nanosecond Cr:ZnSe laser produced a maximum output energy of 0.7 mJ and had a tuning range from 2.04 to 2.74 μm. The AOTF enabled the Cr:ZnSe laser to electronically tune every pulse shot in the tuning range. However, the output energy was restricted to below 1 mJ. A Cr:ZnSe laser with the both of high energy and rapid tunability has not yet been realized.
Previously we applied a noncollinear AOTF to a Ti:Al2O3 laser as a wavelength-tuning element and we demonstrated electronic wavelength tuning in the range from 743 to 853 nm in the Ti:Al2O3 laser [11]. This is, to the best of our knowledge, the first proof of high-speed continuous or random-access tuning over a broad tuning range using the combination of a solid-state tunable laser and a noncollinear AOTF. The use of our technique allows a Cr:ZnSe laser to exhibit rapid tunability in the range from 2 to 3 μm. Furthermore, we have succeeded in developing a high-energy Q-switched Tm:YAG laser [12]. This Tm:YAG laser has high potential as a pumping source for Cr:ZnSe lasers. In the past, Tm:YAG lasers have seldom been used as pump sources for Cr:ZnSe owing to their low pulse energy. The use of our Tm:YAG laser as a pump source shows significant progress in increasing the pulse energy of Cr:ZnSe lasers.
In this paper, we report an electronically tuned Cr:ZnSe laser pumped with a Q-switched Tm:YAG laser using an AOTF for electronic wavelength tuning. Using the AOTF and the Tm:YAG laser, we achieved a high energy and rapid tunability for Cr:ZnSe laser. The maximum output energy of the Cr:ZnSe laser reached 7.9 mJ at 2.41 μm. This is, to the best of our knowledge, the highest output energy with nanosecond duration ever reported for a Cr:ZnSe laser with an AOTF. Moreover, using the AOTF, we demonstrated electronic wavelength tuning of the Cr:ZnSe laser from 2.17 to 2.71 μm while maintaining high beam quality. For the first time, we also report details of the filter tuning curve of the AOTF, the output energy as a function of the pumping energy and the radio frequency (RF) powers fed into the AOTF during the operation of the electronically tuned Cr:ZnSe laser.
2. Design of electronically tuned Cr:ZnSe laser cavity
A schematic diagram of the electronically tuned Cr:ZnSe laser is shown in Fig. 1. We prepared a Q-switched Tm:YAG laser operating at a repetition rate of 10 Hz as the pump source. The Tm:YAG laser produced a maximum output energy of 24 mJ and had a pulse duration of 300 ns at a wavelength of 2.01 μm. The Tm:YAG laser output passed through a half-wave plate and a thin film polarizer (TFP), so that the output energy could be controlled without changing the beam dimension and the pulse width by simply rotating the half-wave plate. We used a 7.5-mm-long, Brewster-cut Cr:ZnSe (IPG Photonics, Inc.) as the laser medium. The Brewster angle is approximately 66.7° for the refractive index of 2.44 of Cr:ZnSe. The Cr:ZnSe was grown by a diffusion doping method [13,14] and the doping concentration of Cr2+ was approximately 5.5 × 1018 cm−3. The Cr:ZnSe laser cavity was constructed as a Z-fold configuration using the Cr:ZnSe, mirrors M1-M4, and an AOTF (Gooch & Housego). Mirror M1 was a planar total reflector with a reflectivity of 99.5% in the range from 2.1 to 2.7 μm. Mirrors M2 and M3 were concave mirrors with a radius of curvature of 1000 mm. The concave mirrors were antireflection (AR)-coated for the pump wavelength and high reflection (HR)-coated for the region from 2.1 to 2.7 μm. Mirror M4 was an output coupler with a transmittance of 40% at 2.4 μm. The Cr:ZnSe was placed between M2 and M3. Here, total cavity length is 45 cm, and the distance of M1-M2 and M3-M4 were set to 16 cm.
The AOTF was inserted into one side of the arm of the cavity as a wavelength-tuning element. The AOTF consisted of a TeO2 crystal and a transducer. The ipunt and output surface of the TeO2 were AR-coated for the region from 2.0 to 2.7 μm. When an RF was fed into the crystal through the transducer, an acoustic wave propagated in the crystal and the selected wavelength was diffracted on the basis of the acousto-optic effect. Diffraction efficiency reched moere than 90% in the region from 2.0 to 2.7 μm and filter-bandpass width of the AOTF was approximately 480 GHz at 2.4 μm. The electronically tuned Cr:ZnSe laser cavity was composed for the diffracted wavelength. Thus, wavelength tuning was realized by simply changing the RF without mechanical control of the optical elements. The RFs and their powers were controlled and adjusted by a scanning program to enable high-speed switching of the wavelength at every pulse shot at the repetition rate. In our system, the RF can be controlled from 36 to 50 MHz and the RF power was controlled to less than 5 W.
3. Experimental results
The filter tuning curve and the tuning range of the electronically tuned Cr:ZnSe laser are shown in Fig. 2. The lasing wavelength of the Cr:ZnSe laser was measured by changing the RFs fed into the AOTF. The lasing wavelength was measured using a wavemeter (IR-III WS6-200, HighFinesse). A tuning range from 2.17 to 2.71 μm was achieved by changing the RFs between 36.4 and 46.2 MHz. The dashed line represents a theoretical curve obtained from a model of a filter tuning curve [9, 15]. The result shows good agreement between the experimental and simulated theoretical curves. The filter tuning curve using the AOTF is given by [15]
where Δn is the birefringence of the TeO2 crystal, fa is the frequency of the RF, ν(α) is the acoustic wave velocity in the TeO2 crystal, C11 C12, and C44 are elastic constants, ρ is the crystal density, α is the angle between the acoustic wave and the (001) axis, and θ is the angle between the incident beam and the (110) axis of the TeO2 crystal. The theoretical curve was simulated using Eqs. (1) and (2), and the values of α and θ were estimated to be 17° and 27°, respectively, by fitting. A maximum output energy of 7.9 mJ was obtained at 2.40 μm when a pump energy of 23.2 mJ was input to the Cr:ZnSe. The optimal RF power is different for each wavelength, because each wavelength has a different optimum RF power for achieving the highest diffraction efficiency [16]. Thus, the RF power was tuned from 0 to 5 W and controlled to produce the maximum output energy at each wavelength. Rapid wavelength tuning was accomplished using the AOTF in the range from 2.17 to 2.71 μm under a switching rate of 10 Hz.
Fig. 2 Filter tuning curve and tuning range of the electronically tuned Cr:ZnSe laser. The dashed line represents the theoretical curve obtained from a model of a filter tuning curve [15]. When the RF was tuned from 36.4 to 46.2 MHz, the output wavelength was tuned from 2.17 to 2.71 μm.
The output energy of the electronically tuned Cr:ZnSe laser as a function of pump energy is shown in Fig. 3. We measured the output energy at wavelengths of 2.20, 2.31, 2.41, 2.52, and 2.63 μm by feeding RFs of 45.0, 43.0, 41.0, 39.0, and 37.5 MHz, respectively. The output energy at each wavelength linearly increased up to a pump energy of 23.2 mJ. A maximum output energy of 7.9 mJ was obtained at 2.41 μm with a pump energy of 23.2 mJ. The conversion efficiency reached 34.1% and the oscillation threshold was obtained at a pump energy of approximately 1.4 mJ. Figure 4 shows the beam profiles at wavelengths of 2.20, 2.31, 2.41, 2.52, and 2.63 μm, which were measured using a beam profiler (Pyrocam III, Spiricon). The beam profiles in the TEM00 mode were observed at 2.20, 2.31, 2.41, and 2.52μm, and the beam quality was maintained while changing the pump energy. However, we observed deterioration of the beam quality in the transverse direction at 2.63 μm. The diffraction angle in the AOTF was different at every diffracted wavelength, which was caused by wavelength dispersion in the TeO2, thus misalignment of the Cr:ZnSe laser cavity was induced by changing the lasing wavelength. We previously described a prism for compensating the wavelength dispersion in [11]. By utilizing such a prism in the Cr:ZnSe laser cavity, we can obtain high-quality beam profiles in the entire tuning range.
Fig. 3 Output energy of the electronically tuned Cr:ZnSe laser as a function of the pump energy of the electronically tuned Cr:ZnSe laser.
Fig. 4 Beam profiles of the electronically tuned Cr:ZnSe laser. (a), (b), (c), (d), and (e) show beam profiles at wavelengths of 2.20, 2.31, 2.41, 2.52, and 2.63 μm, respectively.
The RF power fed into the AOTF should be optimized to obtain the maximum output energy so that the output energy is varied by changing the RF power fed into the AOTF [16]. The output energy of the Cr:ZnSe laser as a function of the RF power fed into the AOTF is shown in Fig. 5. We measured the output energy at wavelengths of 2.20, 2.31, 2.41, 2.52, and 2.63 μm, while controlling the RF power to between 0 and 5 W. The optimal RF powers giving the maximum output energy at the above wavelengths were 1.63, 1.70, 1.90, 2.27, and 2.30 W, respectively. This shows that the output energy of the Cr:ZnSe laser equipped with the AOTF could be controlled by adjusting the RF power fed into the AOTF.
Figure 6 shows temporal profiles of the Cr:ZnSe laser pulses and the Tm:YAG laser pulse. The Tm:YAG laser pulse was Gaussian with a pulse duration of approximately 300 ns (FWHM). The temporal profiles of the Cr:ZnSe pulses were measured at a wavelength of 2.43 μm by feeding an RF of 40.8 MHz and RF powers of 0.7, 1.1, and 1.9 W. The temporal pulse profiles were modulated by relaxation oscillations in the Cr:ZnSe laser, and these were included in the pump pulse duration. As the RF power was increased, the build-up time decreased and the secondary peak shifted towards the primary peak. A build-up time of 260 ns and a primary peak pulse width of 30 ns were observed by feeding an RF power of 1.9 W, which was the optimum power for obtaining the maximum pulse energy at 2.43 μm.
4. Discussion
In this study, the Cr:ZnSe was pumped with a high pulse energy at a wavelength of 2.01 µm. Under high-energy laser operation, damage to the Cr:ZnSe surface becomes a serious problem. We thus measured the damage threshold of the Cr:ZnSe surface using the Tm:YAG laser employed to pump the Cr:ZnSe. Damage tests were performed using the S-on-1 method. We used a 7-mm-long Cr:ZnSe without an optical coating as the sample. The input energy fluence was controlled from 0 to 13 J/cm2. Six hundred shots applied incident to the sample and the radiation time was 60 s. Here, the repetition rate of the Tm:YAG laser was 10 Hz. We performed six damage tests and the average damage threshold was found to be ~10 J/cm2 at a wavelength of 2.01 µm. In this study, we controlled the pump fluence on the Cr:ZnSe surface to about 2 J/cm2 to ensure damage-free operation, thus damage to the Cr:ZnSe surface is not expected to be a serious problem.
Next, we discuss the diffraction angle in the AOTF. The diffraction angle depends on the diffracted wavelength, thus the variation of the diffraction angle in the AOTF induces misalignment in the Cr:ZnSe laser cavity. This limits the range of tunability. In some cases, a prism to compensate the diffracted beam angle is required to obtain broad tunability, such as for the electronically tuned Ti:Al2O3 laser reported in [11]. However, the broad tunability of the Cr:ZnSe laser was realized without a prism. The main reason for this difference was that the wavelength dispersion of the refractive index in TeO2 was almost constant at wavelengths of longer than 2 μm compared with in the near-IR region, where corresponds to the tuning range of the Ti:Al2O3 laser. Here we estimated the diffracted beam angle, θd, using the following equation [15]
where no and ne are the ordinary and extraordinary refractive indices of TeO2, respectively. We used the Sellmeier equation for TeO2 reported in [17].The deviations of the diffracted beam angle in the tuning ranges of the Cr:ZnSe laser and Ti:Al2O3 laser are shown in Fig. 7. The deviations were 0.005° and 0.1° in the tuning ranges of the Cr:ZnSe laser and Ti:Al2O3 laser, respectively. The low deviation of the diffracted angle can be obtain in the Cr:ZnSelaser, because the variation of the refractive index in the region from 2 to 3 μm was less than one-tenth in the region from 0.7 to 1 μm. For this reason, the Cr:ZnSe laser can realize a broad tuning range without using a prism in the laser cavity for compensation. Using the AOTF, linewidth of the Cr:ZnSe laser emission depends on the filter-bandpass width of the AOTF and cavity round trips during laser oscillation [9]. The linewidth was estimated to be approximately 50~100 GHz in the tuning range.Fig. 7 Deviations of the diffracted beam angle in the tuning ranges of the Cr:ZnSe laser and Ti:Al2O3 laser.
5. Conclusion
In this study, we have demonstrated a Q-switched Tm:YAG-laser-pumped, electronically tuned Cr:ZnSe laser using an AOTF as a wavelength-tuning element. The tuning range of the Cr:ZnSe laser was from 2.17 to 2.71 μm, which was realized by tuning the RFs between 36.4 and 46.2 MHz. A maximum output energy of 7.9 mJ was obtained at 2.41 μm with a pump energy of 23.2 mJ, giving an energy conversion efficiency of 34.1%. Beam profiles in the TEM00 mode were observed in a broad wavelength region. The typical temporal profiles were observed to be gain-switched temporal profiles. We have provided detailed data of for an electronically tuned Cr:ZnSe laser and reported its high pulse energy and rapid tunability in the mid-IR region. In addition, we have shown the suitability of Tm:YAG lasers as a pump source for high-pulse-energy Cr:ZnSe lasers.
Acknowledgment
This research is supported by “R&D Program for Implementation of Anti-Crime and Anti-Terrorism Technologies for a Safe and Secure Society”, Strategic Founds for the Promotion of Science and Technology of the Ministry of Education, Culture, Sports, Science and Technology, Japan.
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