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Abstract
An extensive study of a novel room-temperature mid-infrared Ce3+-doped Ge20Sb10Ga5Se65 glass laser is reported. An influence of output-coupler transmission on laser efficiency and emission spectra is investigated. Pumped by a pulsed Fe:ZnSe laser at 4.1 µm, a maximum output energy of 35 mJ is demonstrated at 5.2 µm, with a laser threshold of about 60 mJ and a slope efficiency of 21%. The tuning range of a mid-infrared Ce:glass laser is reported for the first time: with an intracavity prism, the laser is continuously tunable in the spectral range of 4.5–5.6 µm. The internal losses are determined to be below 9% per roundtrip.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The development of laser sources in the mid-infrared (MIR) spectral range is of particular interest for numerous research and application fields [1]. Although quantum cascade lasers provide access to a large part of the MIR, their fabrication is still expensive, while the output powers/energies are orders of magnitude below the solid-state lasers’ (SSL) performance. Therefore, intensive research on MIR SSLs is currently carried out, with a strong focus on chalcogenide host materials (i.e. based on S, Se and Te). This choice is primarily dictated by the low phonon energies of these materials and their high transmittance in the long-wave MIR range [2]. Current research directions include (poly)crystalline [3–8], ceramic [9,10] and glass [11–14] chalcogenide host matrices in various geometries (bulk, fiber [15–20], thin film [21–23]), suitable rare-earth [24–30] and transition-metal [3,10] dopants, as well as promising graphene-based detector technologies [31,32] that are particularly interesting for integrated photonics.
Recently, we have reported the first MIR laser action of Ce3+, obtained in the Ge20Sb10Ga5Se65 glass [29]. With an outcoupler transmission of TOC = 3%, we obtained up to 0.5 mJ of output energy, at an absorbed pump energy of 270 mJ and a laser threshold of about 50 mJ. The laser emission was centered at 5.3 µm. In this work we present results of an extensive laser-performance study of the Ce3+-doped selenide glass. In an optimized resonator configuration, the influence of output-coupler transmission on laser efficiency and emission spectra is investigated. Also, the tuning range of the MIR Ce:glass laser is reported for the first time.
Although Ce3+-ions are directly in-band pumped at a rather unorthodox wavelength near 4.1 µm, an appropriate co-doping with Dy3+ and associated energy-transfer processes could be exploited to enable pumping at more convenient wavelengths around 1.3 or 1.7 µm, as demonstrated recently [27]. Therefore, the current work can be regarded as a significant step towards the realization of compact high-performance Ce3+-doped lasers emitting in the 4–6 µm spectral range.
2. Experimental details
The experimental scheme of the laser system is depicted in Fig. 1.
Fig. 1. Experimental setup. HR: high reflector; OC: output coupler; PD: photodetectors; EM: energy meters; BS: beam splitters; Pyrocam: pyroelectric camera.
The cylindrical high-purity Ge20Sb10Ga5Se65 glass rod used in this work, 12 mm in diameter, 24 mm in length and a Ce3+-doping concentration of 3×1019 cm−3, has been synthesized at the Devyatykh Institute of Chemistry of High-Purity Substances of the Russian Academy of Sciences using the technology described in [33,34]. The common problem of chalcogenide materials is the impurity with SeH- and GeH-groups leading to high absorption losses at the lasing wavelengths and luminescence quenching of the active ions. However, recent progress in the fabrication of high-purity chalcogenide glass resulted in absorption losses in best undoped samples of only 6×10−4 cm−1 at the peak extinction of SeH groups at 4.5 µm, and 6×10−3 cm−1 for GeH groups peaking at 4.9 µm [25].
The uncoated active element is located in the 145-mm long laser cavity defined by the highly-reflective concave mirror HR (r = 200 mm) and a plane-parallel output coupler (OC). The working surfaces of the active element, parallel to each other with an accuracy of 15 arcseconds, are oriented perpendicular to the optical axis of the resonator. In this work, several OCs with transmissions between 5 and 74% are utilized. In contrast to our previous work [29], here the active element is placed close (9 mm) to the plane-parallel OC, resulting in a better overlap of the pump and laser beams. As a pump source we use a homemade liquid-nitrogen cooled Fe:ZnSe laser emitting 250-µs pulses around λP = 4.1 µm with a maximum pulse energy of 500 mJ. A detailed description of a similar laser can be found elsewhere [35]. The pump-laser light is focused onto the Ce:glass with a spot size of about 2 mm. The angle between the resonator axis and the pump beam is about 0.06 rad.
The absorption and emission characteristics of the Ce3+-doped glass sample in the spectral range of 2.5–6.5 µm are shown in Fig. 2. Details of spectral measurements of the absorption cross-section (Fig. 2, bottom) and fluorescence (Fig. 2, top, grey) are explained in [27]. The calculated emission cross-sections from the experimental absorption spectrum (using the McCumber relation [36]) and from fluorescence of Ce3+ (Fig. 2, top) show good agreement. The dip around 4.2 µm originates from atmospheric absorption of CO2.
Fig. 2. Bottom: Ce3+ absorption cross-section in the glass rod. The pump wavelength is indicated by the vertical line. Top: the Ce3+ emission cross-section profiles calculated from absorption (red) and fluorescence (grey) show good agreement. The dip around 4.2 µm is due to atmospheric CO2 absorption.
The pump and Ce:glass laser pulse-shapes and energies are measured using photo detectors (PD in Fig. 1) and energy meters (EM) that have been described earlier [37]. A CaF2 prism (70°) is used for spectral tuning of the laser. Emission spectra of the Ce:glass laser are recorded using a homemade grating spectrometer (spectral resolution of about 8 nm) and the pyroelectric array camera Pyrocam IIIHR (MKS/Ophir, 160×160 pixels, 12.8×12.8 mm2 active area). The pump laser is synchronized with the pyroelectric camera using a pulse generator (not shown in Fig. 1).
3. Laser performance
3.1 Output parameters
Laser experiments have been performed in the free-running mode (i.e. without the prism in the resonator) and with different OC transmissions TOC of 5, 22, 43 and 74% at the expected laser wavelength around 5.2 µm. To avoid damage of the active element in this series of experiments, the incident pump energy Ein has been limited to 330 mJ, which corresponds to an absorbed energy of Eabs ≈ 160 mJ. The results are shown in Fig. 3. Dots are experimental results, while lines are linear fits.
Fig. 3. Output energy of the Ce:glass laser as a function of the absorbed pump energy with different OCs. The inset shows the beam profile at TOC = 74%, with M2 ≈ 6.
As expected, the laser threshold increases with increasing OC transmission: from 36 mJ at TOC = 5%, to about 90 mJ at TOC = 74%. The maximum slope efficiency (with respect to the absorbed pump energy) of about η = 21% is reached with TOC = 43% (red in Fig. 3) and the maximum output energy of Eout = 16 mJ is obtained at Eabs = 140 mJ. The beam profile recorded with TOC = 74% at Eout = 10 mJ is shown in the lower left corner of Fig. 3. The beam quality is determined to be about M2 = 6. This high value can be explained by poor mode-matching of the pump spot (2 mm in diameter) and the fundamental laser mode (0.8 mm in diameter inside the gain medium) leading to transversal multi-mode emission. The beam quality can be optimized in future experiments.
Figure 4 shows laser emission spectra measured with different OCs.
Fig. 4. Emission spectra of the Ce:glass laser (colored) recoded using different OCs in free-running mode (no prism in the cavity) at an absorbed pump energy of 160 mJ. A calculated transmission spectrum through 1 m of laboratory air is shown for comparison (grey, right axis).
As can be seen, with smaller OC transmission, the central emission wavelength shifts towards longer wavelengths: from 5.1 µm at TOC = 74% to 5.4 µm at TOC = 5%. This shift is typical for lasers with broadband and strongly overlapping absorption and gain spectra [38]. Note that the emission bandwidths reach several hundreds of nanometers, which is advantageous for broadband spectroscopic applications, e.g. for intracavity absorption spectroscopy (ICAS) [39]. A calculated spectrum of atmospheric transmission is shown on top of Fig. 4 (grey) for comparison. The calculation has been performed for 1 m of absorption path using the HITRAN database [40], under consideration of the experimental spectral resolution of 8 nm. As can be seen, laser action takes place predominantly in atmospheric transmission maxima.
In the next step, additional measurements have been performed using the OC providing the highest efficiency (TOC = 43%) and with an increased maximum incident pump energy of up to 450 mJ, corresponding to an absorbed energy of Eabs = 225 mJ. The results are shown in Fig. 5 (red). As can be seen, the output energy continues the linear growth with the increased absorbed pump energy. A maximum output energy of about 35 mJ is reached at Eabs = 225 mJ. The inset in Fig. 5 shows typical oscilloscope traces of the pump (green) and Ce:glass (brown) lasers recorded at an absorbed pump energy of 160 mJ. A spiking structure of the Ce3+ laser emission can be observed.
Fig. 5. Input-output characteristics of the Ce:glass laser with TOC = 43% (red) and without an OC (blue). The inset shows oscilloscope traces of the pump (green) and laser (brown) pulses, recorded at a pump energy of 160 mJ.
Another important result presented in Fig. 5 is the lasing obtained when no OC is used (blue), i.e. with the 19% Fresnel reflection of the Ce:glass element’s end facet determining the outcoupling. In this case, the threshold absorbed pump energy is Eth = 161 mJ, while the slope efficiency is η = 3.6%.
3.2 Spectral tuning
The tuning curve of the Ce:glass laser, recorded with TOC = 22% is shown in Fig. 6 (blue).
Fig. 6. Tuning curve of Ce:glass laser recorded with the intracavity prism and TOC = 22%, at an absorbed pump energy of 140 mJ. The transmission profile of the used OC is shown in red (right axis).
Spectral tuning is performed by using an intracavity CaF2 prism and incrementally tilting the HR mirror. The incorporation of the prism leads to an insignificant narrowing of spectral emission widths of individual laser spectra compared to the free-running case (cf. Figure 4). Blue dots correspond to central wavelengths of the recorded spectra. The Ce:glass laser is continuously tunable in the spectral range of 4.5–5.6 µm. The transmission profile of the used OC (red line, right axis) indicates that further extension of the tuning range is possible with a flattened transmission profile.
3.3 Internal-loss analysis
To quantify the passive losses of our active element, we generated a Caird plot [41]. This is done by analyzing the inverse slope efficiency 1/η as a function of the resonator transmission using the relation
The result of the Caird-plot analysis is shown in Fig. 7, where dots are experimental values and the linear fit is performed using Eq. (1).
Fig. 7. Caird plot: dependence of the inverse slope efficiency on the transmission of the Fabry-Perot interferometer formed by the OC and active element’s end facet.
As a result, we obtain η0 = 23%, ηP = 29% and L = 9%. It should be noted that the experimentally obtained slope efficiency of 21% when using TOC = 43% is close to the maximum available η0. The low pumping efficiency ηP is primarily due to the large pump-spot size and non-coaxial pumping. It should be noted that the passive roundtrip losses L in Eq. (1) are considered as wavelength-independent, whereas in our case some selective losses due to e.g. SeH- and GeH-groups absorption are present. Nevertheless, the obtained global value of L = 9% appears to be a good approximation.
4. Conclusion
In this work an extensive laser-performance study of a novel room-temperature mid-infrared Ce3+-doped Ge20Sb10Ga5Se65 glass laser is presented. In particular, output-coupler transmissions between 5 and 74% and their influence on laser efficiency and emission spectra are investigated. When pumped by a pulsed Fe:ZnSe laser at 4.1 µm, a maximum output energy of 35 mJ is demonstrated at 5.2 µm, with a laser threshold of about 60 mJ and a slope efficiency of 21%. Also, lasing is achieved without an output coupler, i.e. with the end facet’s 19% Fresnel reflection acting as the OC. In that case, the laser threshold is about 160 mJ and the slope efficiency is 3.6%. Furthermore, the tuning range of a MIR Ce:glass laser is reported for the first time: the laser is continuously tunable in the spectral range of 4.5–5.6 µm. The internal losses are determined to be 9% per round-trip. Although in this work the Ce3+-ions are directly in-band pumped at an unorthodox wavelength of 4.1 µm, an appropriate co-doping with Dy3+ is expected to enable pumping at more convenient wavelengths around 1.3 or 1.7 µm. With this, the present work is as a significant step towards the realization of high-performance Ce3+-doped lasers emitting in the 4–6 µm spectral range.
Funding
Russian Science Foundation (20-79-00155); Russian Academy of Sciences (Program No. 5); Russian Foundation for Basic Research (18-29-20079).
Acknowledgments
P. Fjodorow acknowledges the support by the PRIME program of the German Academic Exchange Service (DAAD), funded by the German Federal Ministry of Education and Research (BMBF). S.O. Leonov acknowledges the Russian Science Foundation for the support in the measurements of laser spectra. We acknowledge the support by the Open Access Publication Fund of the University of Duisburg-Essen.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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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