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Abstract
We report a laser operation from a Fe:CdTe single crystal, pumped by 40-ns pulses of a 4.12-µm Fe:ZnSe laser. The maximum output energy of 5.8 mJ was produced at 5.4 µm with 30% absorbed energy slope efficiency. A record 2300-nm-wide smooth and continuous wavelength tunability over 4.5-6.8 µm range was demonstrated, being the longest wavelength tuning achieved for Fe2+-doped chalcogenide lasers. We also discuss the features of the oscillation spectra.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Mid-infrared tunable lasers are of great interest for a variety of scientific, industrial and medical applications. In this connection laser active media based on transition-metal divalent-ions-doped chalcogenide compounds [1] attract special attention. High efficiency and broad gain band enabled a number of successes to be demonstrated with these materials. In particular, these include mid-IR femtosecond lasers [2–5], applications for optical frequency metrology [6,7] and for high sensitivity spectroscopy [8–10].
Lasers, based on Fe2+-doped telluride crystals of the second group, are the longest-wave mid-infrared solid-state lasers [1,11]. It is assumed that these tunable lasers can cover the spectral range from 5 to 7 µm, which is relevant for the spectroscopy of complex molecules, and it may be possible to move even further into the long-wave region using the CdHgTe crystal matrix. However, there are certain technological problems of growing Zn and Cd tellurides associated with a significant deviation of the composition of these crystals from stoichiometric towards excess tellurium [12,13]. Excess tellurium, falling out in the second phase, leads to an increase in internal losses for IR radiation scattering, and also, as an acceptor, increases absorption on free holes. In addition, free holes can reduce the concentration of Fe2+ ions, converting them to the Fe3+charge state.
One of the ways to solve this technological problem is to abandon binary compounds in favor of solid solutions of the Fe:Cd1-xMnxTe type, which are easier to obtain with less deviation from the stoichiometric composition [14–16]. Thus, 810 mW of average power at 5.223 µm was demonstrated from a liquid-nitrogen-cooled Fe:Cd0.85Mn0.15Te laser [15]. Room temperature, mirrorless, random lasing of iron doped Zn0.5Cd0.5Te powders at 5.9 µm was reported in [11]. Room temperature laser properties of Bridgeman technique synthesized Fe:Cd1-xMnxTe crystals with various concentrations of Mn were investigated under pumping by 4.1-µm 200-ns pulsed Fe:ZnSe laser [16]. The laser central oscillation wavelength was varied in the range from 5.4 up to 6.0 µm. The highest output energy of 30 µJ was achieved with slope efficiency of 2.3%.
The use of binary compounds is more promising for achievement of high average output power compared to solid solutions due to the significantly lower thermal conductivity of the latter. By using the vapor-phase growth technology with simultaneous Fe2+ ions doping, we succeeded to obtain higher-quality Fe:ZnTe [17] and Fe:CdTe [18] crystals and for the first time realized laser generation in the ranges of 4.35-5.45 and 5.1-6.3 µm, respectively. A maximum output energy of 2 mJ was obtained at room temperature from Fe:CdTe crystal, pumped by a Q-switched 2.94-µm Er:YAG laser. However, the wavelength of 2.94 µm is strongly shifted relative to the maximum of the Fe:CdTe absorption band (3.7 µm) which is why the pumping efficiency did not exceed 50% even at a high concentration of Fe2+.
In this work, the Fe:CdTe crystal was pumped by 40-ns 4.12-µm Fe:ZnSe laser, which allowed us to increase both the efficiency and the output energy of the Fe:CdTe laser. To extend the tuning range, an anti-reflection coating was implemented to one face of the active element, which prevented the occurrence of parasitic generation on the crystal faces limiting the tuning region. Features of the laser oscillation spectrum are also discussed.
2. Experimental setup
The experimental setup is depicted in Fig. 1. A 5.5 mm thick and 20 mm in a diameter laser element was cut from the Fe:CdTe single crystal boule grown from a vapor phase using a seeded physical vapor transport technique in He atmosphere of the initial materials CdTe and FeTe to a single-crystal seed in a quartz reactor with a “cold end” placed in the temperature gradient of an electric furnace [18]. The growth temperature was 1260 K. During the cooling of the furnace, the crystal was annealed for 24 hours at a temperature of 1050 K. The work faces of the active element were mechanically polished to an optical finish and were parallel to each other within 30”. The initial absorption of the Fe:CdTe crystal at the maximum of absorption band (3.65 µm) was measured to be 98%. Initially, experiments were conducted using uncoated active element. However, the occurrence of parasitic generation in the resonator formed by uncoated faces of the active element (Fresnel reflection of 21%) significantly limited the laser tuning range. Therefore, 6.0-µm-centered AR coating was deposited onto one of the surfaces.
Fig. 1. Experimental setup. M1, M3, M4, M5: gold mirrors; M2: output coupler of Fe:ZnSe laser; OC: output coupler of Fe:CdTe laser; HR: gold high reflector of Fe:CdTe laser; BS: beam splitter; PD: photodetector.
A 185-mm-long Fe:CdTe laser cavity was formed by a gold spherical (R=200 mm) high reflector HR and uncoated surface of the crystal. For tuning experiments, a Brewster cut CaF2 prism was inserted in the modified laser cavity. An additional output coupler OC was also used to extend the tuning range
A 4.12-µm room-temperature Fe:ZnSe laser provided an excitation for the Fe:CdTe laser experiments. A 16.7 mm thick plane-parallel single-crystal Fe:ZnSe laser element with Fe2+ ions concentration of 1.1×1018 cm-3 was installed into the 165-mm-long cavity formed by gold spherical (R=400 mm) high reflector M1 and output coupler M2, transmitting 80% at 4.12 µm and reflecting 99% at 2.94 µm. The Fe:ZnSe uncoated crystal was placed near M2 with working faces oriented normally to the cavity axis. The Fe:ZnSe laser was pumped by 2.94-µm Er:YAG (Er3+concentration of 50%) laser system consisting of Q-switched master oscillator and amplifier. The master oscillator contained 60 mm long and 4 mm in a diameter Er:YAG laser rod which was pumped by the Xe linear flash lamp. The passive Q-switching was obtained by the Fe:ZnSe single-crystal plate installed in the resonator at the Brewster angle. The amplifier 120 mm long and 6 mm in a diameter Er:YAG laser rod was pumped by two Xe linear flash lamps. The system was capable to produce up to ∼90 mJ of output in single pulse mode operation with pulse duration of 40 ns. The Er:YAG laser beam was directed onto the Fe:ZnSe crystal by a spherical gold mirror M3 at the angle of 3° to the cavity axis. The pump spot diameter at the Fe:ZnSe crystal was estimated of 2 mm. To increase the pumping efficiency, pump beams reflected from the crystal face and passed through the crystal were returned to the active area by mirrors M4 and M2, respectively.
The Fe:CdTe crystal was pumped at the angle of 2° to the cavity axis. The Fe:ZnSe laser beam was focused by a spherical mirror M5 onto the Fe:CdTe crystal with a spot size of approximately 1.5 mm. During laser experiments, ∼95% of incoming pump was absorbed by the Fe:CdTe crystal.
The temporal profiles of the Er:YAG, Fe:ZnSe and Fe:CdTe pulse emission were recorded by LN-cooled Au-doped Ge photodetectors, which provided a temporal resolution of ∼5 ns. The laser energies were measured with Ophir-Spiricon energy meters. Emission spectra were analyzed using two home-made grating spectrographs with spectral resolution of 2 nm and 0.25 nm. The spectra and beam profiles were recorded using a Pyrocam IIIHR (Spiricon) pyroelectric array camera.
3. Experimental results and discussion
The temporal behavior of Er:YAG, Fe:ZnSe and Fe:CdSe laser pulses at Er:YAG laser energy of 45 mJ is shown in Fig. 2. The Fe:ZnSe pulse is delayed with respect to Er:YAG pulse, and the Fe:CdTe pulse is delayed with respect to Fe:ZnSe pulse. The pulse lengths of Er:YAG and Fe:ZnSe lasers are approximately 40 ns (FWHM). The Fe:CdTe pulse length is about 30 ns. The delays and durations were dependent on the pump level.
Fig. 2. Temporal profiles of the Er:YAG, Fe:ZnSe and Fe:CdTe laser pulses at the Er:YAG laser energy of 45 mJ.
Output-input characteristics of Fe:ZnSe and Fe:CdTe lasers are depicted in Fig. 3. The Fe:ZnSe laser produced maximum output energy of 30 mJ with 35% slope efficiency with respect to the total Er:YAG laser energy (Fig. 3(a)).
Fig. 3. Fe:ZnSe laser output versus total Er:YAG laser energy (a), and Fe:CdTe laser output versus absorbed Fe:ZnSe laser energy (b). Slope efficiencies are also indicated.
The maximum Fe:CdTe laser energy of 5.8 mJ was achieved in free-running cavity (no prism) without an additional OC (Fig. 3(b)). The emission spectrum was centered at 5.4 µm. The observed wavelength is not consistent with the Fe:CdTe gain spectrum with a maximum at 5.94 µm represented in [1], since in free-running lasers with overlapping gain and absorption bands the output spectrum is usually shifted from the gain maximum to the long-wave side due to the reabsorption of emission in the absorption band. The threshold absorbed pump energy and the slope efficiency with respect to absorbed pump energy were 2.3 mJ and 30%, respectively. The demonstrated energy and efficiency are significantly higher than earlier reported values of 2 mJ and 16% [18]. The quantum slope efficiency was increased to 39% compared to the previously obtained value of 30% [18].
When using an additional OC with a transmission of approximately 28% (see Fig. 5) the threshold absorbed pump energy is noticeably reduced to 1.3 mJ, while the slope efficiency falls by about 3 times. This indicates a high internal loss in the crystal, probably due to an excess of Te [18].
Spectral studies of the Fe:CdTe laser emission were performed using the Fe:CdTe crystal with AR coating deposited on the crystal surface facing the HR. Note that the uncoated Fe:CdTe crystal and the crystal with AR coating demonstrated the same output laser characteristics.
Output spectra of the Fe:CdTe laser were recorded both in free-running mode (Fig. 4(a)) and with an intracavity prism (Fig. 4(b), (c), (d)). The spectra shown in Fig. 4 were recorded using the cavity without the additional OC. Spectral resolution was 2 nm (Fig. 4(a), (b), (c)) and 0.25 nm (Fig. 4(d)). Some of single-pulse spectra are represented by the blue curves, while the red curves show the spectral distribution of laser emission averaged over 16 pulses. In Fig. 4 the atmospheric air transmission spectra calculated using the HITRAN2012 database for a 1-m thick air layer at a partial water vapor pressure of 5 Torr are also depicted (green).
Fig. 4. Emission spectra of individual pulses (blue) and of 16 superimposed pulses (red) of the Fe:CdTe laser recorded in the free-running cavity (a) and with the dispersion prism in the cavity (b, c, d). Spectral resolution was 2 nm (a, b, c), and 0.25 nm (d). Transmission spectra of air, demonstrating atmospheric-water absorption lines, are also presented (green).
The observed spectra indicated that the laser operated on many longitudinal modes of the cavity simultaneously. A free-running laser bandwidth was found to be of the order of 200 nm (Fig. 4(a)). An intracavity prism reduced the spectrum bandwidth down to approximately 50 nm (Figs. 4(b) and (c)).
It can be seen that the single-pulse spectra show complicated line structure that is not reproduced from pulse to pulse. One of the reasons for the observed structure may be the absorption lines of atmospheric water vapor. Really, the averaging significantly smooths the random structure, revealing the intracavity absorption spectrum (see the red curves in Fig. 4). Traditionally, this approach is implemented for recording weak absorption spectra by the method of intracavity laser spectroscopy [9,10,19–21] based on multimode pulsed lasers.
A random structure occurs due to the fact that the fluctuation of the photon number in the individual mode has the same order as the photon number in the individual mode [19,20]. At a low spectrograph resolution, this appears as a pulse-to-pulse random spectrum fluctuations (Fig. 4(a), (b), (c)).
The role of fluctuations is shown more clearly when using a high-resolution spectrograph. Figure 4(d) presents a series of spectra recorded with a resolution of 0.25 nm. Although individual longitudinal modes of the resonator (0.098 nm interval) are not resolved the intensity of these modes is modulated with a period of 1.22 nm, that is consistent with the inter-mode distance of Fabry-Perot etalon formed by the facets of the Fe:CdTe crystal. It can be seen that the amplitude fluctuations of the crystal modes and therefore of cavity modes is comparable with mode amplitudes.
The tuning curves of the Fe:CdTe laser with an intracavity prism are shown in Fig. 5 for the two cases. In the first case, the laser operating in the cavity without additional OC was tuned from 4.60 to 6.35 µm (blue curve). Note that with an intracavity prism the laser, tuned to 5.4 µm (the wavelength of free-running operation), produced the same laser pulse energy as without prism at the same pump energy. The observed range is significantly extended to the short-wave side compared to 5.1-6.3 µm range demonstrated earlier using an uncoated crystal [18]. The total expansion of tuning spectral range is associated with an increase in the threshold of parasitic generation on the crystal faces due to the AR coating of one of them, and the shift of the tuning curve to the short-wave side is associated with a decrease in the reabsorption of laser radiation in a crystal due to a lower concentration of Fe2+ ions.
Fig. 5. Tuning curves of Fe:CdTe laser (blue and red), measured using the crystal with one AR-coated face. The blue curve was measured in the cavity without OC, and the red curve was measured in the cavity with an additional OC. Absorbed pump energy was about 15 mJ. The OC transmission curve (green) and AR coating reflection curve (black) are also shown.
In the second case, an additional OC was also used. The reflection spectrum of AR coating and OC transmission spectrum are shown in Fig. 5 (black and green curves, respectively). With OC the tuning was carried out in the range of 4.5-6.8 microns (red curve). One can see considerable broadening of tuning range compared to the cavity without OC. The less significant expansion into the short-wave range is probably due to the reabsorption of laser emission by the crystal.
It is known that the output spectrum of the lasers with wide strongly overlapping absorption and gain bands shifts to the long-wave region with increasing the cavity Q-factor. This is due to a faster decrease in the absorption coefficient compared to a decrease in the gain with increasing wavelength [22]. Thereby, reducing OC transmittance should shift the laser wavelength to the longer ones. However, in reality the intracavity losses limit this shift. Since intrinsic crystal losses give the main contribution to intracavity losses, a further improvement of crystal quality may lead to an additional expansion of tuning region to the long-wave side.
To properly measure the laser beam quality factor (M2), the output of the Fe:CdTe laser was focused using a spherical mirror (R=492 mm) and the 1/e2 beam diameter was measured at different propagation distances. Beam profiles measurements were taken using a Pyrocam IIIHR pyroelectric array camera. Figure 6 shows the beam profiles along x- and y-directions perpendicular to the beam axis, measured using the cavity with additional OC. The laser beam quality was measured to be M2x=2.78 and M2y=2.08. The insert is a picture of the beam at a distance of 650 mm from the OC.
Fig. 6. A picture of the Fe:CdTe laser beam at a distance of 650 mm from OC (insert), and the beam profile at the output energy of 0.83 mJ.
4. Conclusion
We obtained, what is to our knowledge, record 2300-nm-wide smooth and continuous tuning from 4.5 to 6.8 µm of a room-temperature Fe:CdTe laser. Pumped by 4.12-µm Fe:ZnSe laser Fe:CdTe single crystal produced as much as 5.8 mJ of output at 5.4 µm with absorbed pump energy slope efficiency of 30%.
Funding
Russian Academy of Sciences.
Acknowledgments
The authors acknowledge the support from Presidium RAS Program No. 5 “Photonic technologies in the sensing of inhomogeneous media and biological objects.”
Disclosures
The authors declare no conflicts of interest.
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