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Abstract
In this paper, we demonstrate the laser characterization of Cr:ZnS/Se polycrystalline gain media in non-selective unpolarized, linearly polarized, and twisted mode cavities. Lasers were based on post-growth diffusion-doped, commercially available antireflective-coated Cr:ZnSe and Cr:ZnS polycrystals with a length of 9 mm. The spectral output of lasers based on these gain elements in non-selective unpolarized and linearly polarized cavities was measured to be broadened to ∼20-50 nm due to the spatial hole burning (SHB) effect. SHB alleviation in the same crystals was realized in the “twisted mode” cavity, with linewidth narrowing to ∼80-90 pm. Both broadened and narrow-line oscillations were captured by adjusting the orientation of intracavity waveplates with respect to facilitated polarization.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
The development of mid-IR solid-state lasers is in great demand for many scientific and practical photonics applications, including high-peak power laser physics, medicine [1], molecular spectroscopy [2,3], material processing [4], eye-safe laser radars [5], monitoring of air pollution [6], and many others. The first mid-IR solid-state lasers based on transition metal-doped II-VI semiconductors (TM:II-VI) were proposed and demonstrated in 1996 by the research team from the Lawrence Livermore National Laboratory [7,8]. Among TM:II-VI lasers, those based on ZnSe or ZnS crystals doped with Cr and Fe ions demonstrated the most outstanding characteristics. Recent progress in the field is summarized in reviews and references therein [9,10]. Continuous-wave (CW) lasers based on Cr:ZnS and Cr:ZnSe active media pumped by high-power Tm and Er fiber lasers have been demonstrated to cover a 1.9–3.3 µm spectral range with a maximum output power of more than 140 W [10]. Early work demonstrated that single-crystal and polycrystalline gain elements feature similar spectroscopic and laser characteristics [8]. Considering the technical problems of growing large-size, high-optical-quality Cr:ZnS/ZnSe single crystals, the most commonly used fabrication method for Cr:ZnS/ZnSe gain elements is post-growth diffusion TM doping of polycrystalline II-VI materials [11]. In several publications [12,13], the authors reported broad (∼ 50 nm) oscillation bandwidth in a non-selective cavity with gain elements fabricated by post-growth diffusion doping. The authors suggested that strong spectral broadening results from the inhomogeneous broadening of the amplification band in the gain element [12,14]. The authors also suggested adding hot isostatic pressure (HIP) treatment to remove inhomogeneous broadening. However, no spectroscopic evidence exists for strong inhomogeneous broadening in polycrystalline samples. Moreover, it is established that the dominant spectral broadening in Cr:ZnSe/ZnS materials at room temperature (RT) is homogeneous broadening due to electron-phonon interactions [15]. Low temperature zero phonon lines (ZPL) of twin samples of Cr:ZnS, one HIPed (2000 atm at 1300 C for 100 hours), and one unHIPed, were measured in [15]. The ZPL bandwidths were practically identical for HIPed and un-HIPed crystals and had a FHWM upper limit of ∼5 nm. This indicates that the HIP process does not affect the inhomogeneous broadening in these samples and cannot be the reason for 20-50 nm laser linewidths in non-selective CW cavities of Cr:ZnSe/S lasers.
An alternative explanation of the broadband oscillation of TM:ZnSe/ZnS lasers in a non-selective cavity could be the spatial hole-burning effect in the gain element [16]. Spectral narrowing in lasers with homogeneously broadened amplification band results from “mode competition” in the laser cavity when the cavity mode with the highest round-trip gain wins the mode competition and consumes almost all of the inversion energy. It leads to a narrow line or even a single-frequency regime of laser oscillation. However, when laser oscillation forms a standing wave in the gain element, the oscillation does not interact with the gain in the nodes of standing waves [17]. This effect could remove “mode-competition” and result in a spectral broadening of laser oscillation even in a homogeneously broadened amplification band. The spatial hole burning effect could be suppressed using a special cavity design (a twisted mode cavity) where circular polarization of the laser oscillation does not form nodes in the gain element [18,19], or it could be achieved in a unidirectional ring cavity, where only a wave traveling in one direction exists and there is no standing wave, thereby eliminating the SHB effect [20].
In the current work, we studied the influence of the spatial hole burning effect on the spectral broadening of oscillation in CW Cr:ZnSe and Cr:ZnS lasers with polycrystalline gain elements fabricated by post-growth diffusion doping of Cr ions.
2. Experimental setup
The Cr:ZnSe/ZnS lasers investigated here were based on commercial AR-coated 3x3x9 mm Cr:ZnSe and 1.5x4x9 mm Cr:ZnS polycrystals fabricated using the post-growth diffusion doping method by IPG Photonics Corporation with chromium doping concentration NCr∼5 × 1018 cm-3. Doping was accomplished in vacuum-sealed ampoules using post-growth thermal diffusion of chromium from Cr metal films deposited on the facets of polycrystalline ZnSe/ZnS substrates grown by a chemical vapor transport technique. The schematic experimental setup is shown in Fig. 1.
Fig. 1. Experimental setup: Cr:ZnS/Se is used as an active medium. F1 and F2 are 50-mm lenses with AR coating. M1, M2, and HR are plane mirrors with high reflectivity around 2.4 µm and high transmission around 1567 nm. OC is an output coupler with 50% transmission around 2.4 µm. λ/4 are quarter-wave plates centered at 2.3 µm with AR-coating. Brewster Plate, made of ZnSe, was used as a polarizer. Er-fiber CW laser was used as a pump source. Output light was analyzed with a spectrum analyzer.
Both Cr:ZnS and Cr:ZnSe-based lasers were built using the same cavity by changing only the active media and removing some of the intracavity elements for the different experiments shown below. They were pumped by the randomly polarized radiation of an IPG Photonics ELR-10-1567 CW 1567 nm Er-fiber laser. The gain element (Fig. 1, Cr:ZnS/Se) was installed at normal incidence to the pumping radiation between two AR-coated lenses with focal lengths F = 50 mm (Fig. 1, F1 and F2). Two mirrors used for folding the cavity (Fig. 1, M1 and M2) and the back mirror (Fig. 1, HR) were transparent to pumping radiation with < 5% reflectivity at 1567 nm and highly reflective to laser oscillation of Cr:ZnSe/S with > 99% reflectivity over the 1900–3200 nm range. The output coupler (Fig. 1, OC) has 50% reflectivity near 2400 nm. A plate of ZnSe with a 3 mm thickness was installed at Brewster's angle after the HR mirror and used as a polarizing element (Fig. 1, Brewster Plate) to set the linear polarization of oscillating light. To produce mode twisting, two quarter-wave plates (Fig. 1, λ/4, QWP) for the 2000–2600 nm wavelength range, centered at 2300 nm, were installed on either side of the gain medium. Quarter-wave plates had zero order retardation and were AR-coated for 2000–2600 nm with < 0.5% reflectivity.
The total cavity length was approximately 47.5 cm. A laser wavelength meter, the EXFO WA-1500-IR, together with the EXFO WA-600 laser optical spectrum analyzer (OSA), were used to capture and analyze the output spectra with absolute accuracy ±2 ppm (Fig. 1, Spectrum Analyzer).
3. Results and discussion
In the initial experiments, to characterize gain elements, we measured the input-output characteristics of the ZnSe and ZnS lasers in the cavity shown in Fig. 1, but without a polarizer and the waveplates. Figure 2 demonstrates the input-output characteristics of lasers by plots with circle symbols. Both gain elements in the non-selective cavity without a polarizer and waveplates show a low pumping threshold of 0.90 W for Cr:ZnS and 0.51 W for Cr:ZnSe lasers. The slope efficiencies were measured to be 42% and 40% for Cr:ZnSe and Cr:ZnS for a 1567 nm Er-fiber laser CW pumping source, respectively. After these experiments, we installed only polarizers into the cavities (no waveplates were installed). Figure 2 depicts the input-output characteristics in these configurations by plots with rhomb symbols. As one can see from Fig. 2, an installation of the Brewster plate into the lasers cavity results in a slight increase of the lasers thresholds and a slight decrease of the slope efficiencies for both gain elements. In the polarized cavities, slope efficiencies were measured to be 37% for both gain elements with thresholds of 0.64 W for Cr:ZnSe and 1.04 W for Cr:ZnS.
Fig. 2. Input-output characteristics of Cr:ZnSe and Cr:ZnS lasers. The laser cavity without polarizer and waveplates (circle symbols). The laser cavity with a polarizer and without waveplates (rhomb symbols). The dashed line shows a linear fit.
In the next experiments, we installed waveplates as shown in Fig. 1. Initially, twisted mode operation was turned off by quarter waveplate ‘axes’ orientation adjustment, such that they are parallel to the axis of linear polarization oscillating inside the cavity. This orientation of the axes of quarter-wave plates does not affect the polarization.
Figure 3 shows output spectra measured for this orientation of the waveplates. As one can see, the broadband output spectra with linewidth ∼ 20 nm were measured for both gain elements. It should be noted that similar spectra broadenings were demonstrated for both crystals in the cavity configuration when Brewster's plate and waveplates were completely removed from the cavity.
Fig. 3. Emission spectra of Cr:ZnSe and Cr:ZnS lasers in a standing-wave linearly polarized cavity, all the elements shown in the experimental setup (Fig. 1) are installed, quarter-wave plates orientation does not affect the polarization of oscillating light.
A similar spectral broadening was also reported in [12–14], where authors attributed it to the inhomogeneous broadening in the gain elements fabricated by the thermal diffusion method and suggested the HIP process to mitigate it. However, we believe that the SHB effect is the major reason for the spectral broadening in these lasers. To show that, we now adjust QWPs to obtain a twisted-mode regime, thus eliminating the SHB effect. To enforce twisted mode-cavity oscillation, we rotated the optical axis of the waveplates to 45 degrees with respect to the intracavity polarizer and enforced circular polarization in gain elements. The output spectra of Cr:ZnS/Cr:ZnSe lasers are depicted in Fig. 4. Figure 4 shows a linewidth narrowing to 82-83 pm using the same crystals demonstrated in Cr:ZnSe/Cr:ZnS lasers. It should be noted that measured linewidth is limited by OSA resolution at this wavelength. The figure inserts show an overview of the same oscillation spectra, demonstrating narrow line oscillation in the twisted mode cavity.
Fig. 4. Output spectra of the Cr:ZnS/Se lasers with twisted mode enabled on a 0.5 nm scale. Insert – the same on a 10 nm scale.
Fig. 5. Input-output characteristics of Cr:ZnSe and Cr:ZnS lasers in a fully assembled linearly polarized cavity (circle symbols) and the narrow-linewidth operation regime with activated twisted mode (rhomb symbols). The dashed lines show a linear fit.
Figure 5 compares the input-output characteristics of the Cr:ZnSe/Cr:ZnS lasers with two different orientations of the waveplates: the broadband oscillation (plots with circular symbols) and the narrow line oscillation (plots with rhombic symbols). Figure 5 also shows that the installation of two waveplates while maintaining linear polarization in the gain medium (broadband oscillation) slightly decreases slope efficiencies and increases the threshold to 33% and 0.65 W for the Cr:ZnSe laser as well as to 35% and 1.29 W for the Cr:ZnS laser in comparison with the linearly polarized cavity shown in Fig. 2. The output power and the slope efficiency of the twisted-mode Cr:ZnSe laser (narrow line oscillation) were close to the slope efficiency and laser threshold of the broadband oscillation. The narrow line oscillation of the Cr:ZnS laser reveals slightly smaller slope efficiency. One of the possible reasons for this small drop in laser efficiency could be the influence of thermally induced birefringence in the gain element. As a result, the optimum orientation of the waveplates for the SHB suppression will not compensate for polarization rotation after the double pass over the waveplates, and it will create some losses at the Brewster element.
Table 1 summarizes all power efficiencies for different cavity configurations used in this study.
Table 1. Slope efficiencies and pumping thresholds for all cavity configurationsa used for this study
As demonstrated, both Cr:ZnSe and Cr:ZnS lasers show narrow linewidth operation in twisted mode cavities without frequency selective elements or intracavity etalon. This fact could be evidence of the predominant homogeneous broadening of the emission lines in Cr:ZnS and Cr:ZnSe polycrystalline materials. This result is also supported by [19], which reports a single-frequency emission spectrum from a unidirectional ring Cr:ZnSe laser where the absence of a standing wave excludes the SHB effect. Hence, the suggestion of authors [12], [14] that spectral broadening of the Cr:ZnS/Se laser oscillation results from the inhomogeneous broadening of the 5E-5T2 transition of Cr2+ ions due to defects obtained during post-growth diffusion doping contradicts the results shown in the current laser experiment.
4. Conclusion
We have shown that the spectral output of lasers based on polycrystalline Cr:ZnS/Se in non-selective unpolarized and linearly polarized cavities is broadened to ∼20-50 nm due to spatial hole burning in the gain elements. Mitigating the SHB effect in the same cavities via “mode twisting” results in narrow line oscillation with linewidth narrowing to ∼80 pm, corresponding to the resolution limit at ∼2.4 µm of the OSA used for this measurement. Switching between broadened and narrow line oscillations was demonstrated by carefully aligning the orientation of intracavity quarter-wave plates with respect to Brewster's plate-facilitated polarization. The narrow-linewidth operation was achieved without using frequency-selective intracavity elements in lasers based on commercial Cr:ZnS/Se polycrystalline gain media fabricated by the traditional post-growth diffusion method. These results confirm that Cr2+ spectral broadening is predominantly homogeneous and is not strongly perturbed by possible structural defects appearing in ZnSe/S crystals during post-growth chromium doping.
Funding
U.S. Department of Energy (DE-SC0018378); National Institute of Environmental Health Sciences (P42ES027723).
Disclosures
The work reported here partially involves intellectual property developed at the University of Alabama at Birmingham. This intellectual property has been licensed to the IPG Photonics Corporation. Drs. Fedorov, Martyshkin, and Mirov declare competing financial interests.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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