1 Introduction

Lasers operating in the 4.5–5.5 μm spectral range attract attention because on the one hand, the Earth’s atmosphere is transparent enough for these wavelengths. On the other hand - this range contains a large number of molecular absorption peaks and such lasers are of interest for remote detection of trace amounts of impurities in industry, medicine, environment control, etc. There are several approaches for creation of coherent light sources emitting in this range, such as quantum cascade lasers [1], supercontinuum emitters [2], parametric oscillators [3], gas-filled hollow-core fiber lasers [4] and short phonon spectra crystals with rare-earth [5, 6] or transition metal [7] doping. A few years ago, we showed the fundamental possibility to reach the ~ 5 μm lasing threshold in bulk selenide glass doped with Tb3+ ions [8]. The lasing effect was observed at room temperature in a cavity formed by a pair of highly reflecting dielectric mirrors. In this very first experiment the output energy was small (in the order of tens of microjoules that was close to the energy meter sensitivity), and the lasing threshold was also rather high - about 300 mJ at pump beam diameter 2.5–3 mm. In the subsequent works [9, 10] we’ve concentrated on fiber configuration of Tb-doped chalcogenide glasses and have managed to obtain up to 150 mW of CW output power. Worth noting, that the pump source used in [8] was a pulsed 2.94 μm Er: YAG laser (see the energy level scheme in Fig. 1) while in the fiber experiments the pumping was provided by CW Tm3+ fiber lasers emitting at about 2 μm.

The advantages of 2.94 μm pumping over ~ 2 μm one are the smaller Stokes shift and also a significantly higher absorption coefficient. A possible disadvantage of 2.94 μm pumping, as it can be seen from the scheme in Fig. 1, may be the excited state absorption (ESA) of the pump radiation from the 7F5 upper laser level to 7F0 state with further rapid relaxation back to the metastable level 7F5.

Fig. 1
figure 1

Energy levels of Tb3+ ions and the processes in the laser medium. MPR- multiphonon excitation relaxation to the upper laser level

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Since pulsed 2.94 μm flashlamp-pumped Er: YAG lasers are significantly more common than analogous thulium lasers, the former are of greater practical interest for pulsed pumping of bulk Tb-doped selenide glass elements. The goal of this investigation was to analyze the luminescent and lasing properties of Tb-doped selenide glass under 2.94 μm pumping. The experiments were held at room temperature (RT) and at the temperature of liquid nitrogen (LN). The practical motivation was to answer the question whether it is realistic to obtain significant output characteristics in a laser based on bulk terbium-doped selenide glass. LN cooling is a means that may significantly reduce the lasing threshold by sufficient reduction of the absorption from the thermally excited Stark components of the ground 7F6 state at the lasing wavelengths. Another positive effect of lowered temperatures is the population increase of low-lying Stark components of the upper laser level 7F5. Such an increase should lead to the growth of the emission cross-section in the long-wave wing of the emission band. From the other hand, LN cooling should inevitably slow to some extent the rate of multiphonon excitations relaxation from the pumped level 7F4 to the upper laser level 7F5. It is also a question whether this factor can lower the laser efficiency.

2 Sample characterization

The lasing experiment was held with Ge20Ga5Sb10Se65 glass co-doped with 4.3 × 1019 cm-3 of Tb3+ ions. Its synthesis methods are described in [11]. The laser element (Ø12 × 29 mm rod) had excellent optical quality with no visible striae or heterogeneous inclusions. Its uncoated optical surfaces were polished with parallelism not worse than 15”. Figure 2 presents the transmission spectra of the laser element at RT and at LN temperature. The spectra are normalized by unit (Fresnel reflection losses are excluded).

Fig. 2
figure 2

Normalized transmittance spectra of the laser element at room temperature (RT) and at liquid nitrogen temperature (LN). The levels at which the ground state absorption occurs are indicated near each absorption band

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There are substantial temperature changes of the absorption spectrum. As one can see, all the absorption bands are narrowed and blue shifted at LN temperature while their intensities can either increase or decrease. Changes at the pump and lasing wavelengths are of most importance for us. LN cooling causes noticeable absorption increase at the pump wavelength 2.94 μm: the transmission is reduced from ~ 29% to ~ 16%. Such absorption increase is a favorable factor for lasing. But the most important temperature change is the very substantial (several times) transmission increase in the spectral band 4.9–5.5 μm where the laser action was observed in [1]. In fact, LN cooling turns the 3-level lasing scheme of Tb3+ ions into almost 4-level that should be characterized with much lower lasing threshold. Of course, it would also be useful to compare the emission spectra of the sample at RT and at LN temperature. The RT emission spectrum of Tb3+ - doped selenide glass is presented in [8]. Alas, but insufficient sensitivity of our spectral equipment did not allow us to register the luminescence spectrum of the sample in the LN cooled cryostat.

The room temperature transmittance spectrum was processed by the RELIC software [12] implementing the Judd–Ofelt method. The results are shown in Table 1.

An important conclusion can be drawn from the Judd-Ofelt analysis. As Tb3+ energy level structure (column 2 in Table 1) shows, the 2.94 μm (3400 cm-1) pump radiation is almost in resonance with 7F0 → 7F5 transition and may be absorbed from the upper laser level. However, both electric-dipole and magnetic-dipole optical transitions between these levels are prohibited. Thus no substantial ESA is expected for 2.94 μm pump radiation. As for the ESA from 7F5 to 7F1, it may be actual only for wavelengths longer than 3 μm.

Table 1 Calculated radiative decay rates for the electric-dipole (AEDrad) and the magnetic-dipole (AMDrad) transitions. Their sum is indicated as Atotalrad. E is the energy gap between corresponding levels. The branching ratios of the transitions are also shown
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Another important question is whether the excitations from the pumped level 7F4 would reach the upper laser level 7F5 at LN temperature as rapidly and efficiently as they do at RT.

In order to evaluate the multiphonon relaxation rates of 7F4 level we have compared the buildup curves of ~ 5 μm luminescence from the 7F5 level at room and LN temperatures (Fig. 3). The measurements were held using a LN filled cryostat with aerodynamic window. A thin (~ 2 mm) Tb-doped glass sample was excited by a Q-switched flashlamp-pumped Er: YAG laser. The laser emitted ~ 50 ns 8–10 mJ pulses. The luminescent signals were recorded with a LN cooled InSb photodetector.

Fig. 3
figure 3

Normalized ~ 5 μm luminescence buildup functions at room (RT) and liquid nitrogen (LN) temperatures

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At RT the luminescence buildup curve was close to exponential with e-fold time of 10 µs. At LN temperature the e-fold buildup time became close to 35 µs. Both values are at least two orders of magnitude shorter than the luminescent lifetime of 7F5 level (3.7-4 ms in the test sample). In other words, on the lifetime scale of the upper laser level, the pump energy transportation to this level remains almost instant.

3 Laser experiment

The laser experiment arrangement is shown in Fig. 4.

Fig. 4
figure 4

Laser experiment setup. HR − highly reflective mirror; OC − output coupler; BS − beam splitter; PD – photodetectors

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The Tb: glass laser rod was mounted on a cold finger in a cryostat with CaF2 windows and could be cooled to LN temperature. The polished faces of the rod and uncoated plane-parallel CaF2 cryostat windows were aligned perpendicular to the laser cavity optical axis. The 19-cm long laser cavity was formed by a spherical (radius of curvature 30 cm) golden mirror (HR) and a flat dielectric output coupler (OC). Laser experiments were performed with different OC transmissions of 1.5, 6, 12, 28, 57 and 75% at the expected laser wavelength around 5.2 μm. The pump source was a multimode free-running flashlamp pumped Er: YAG laser emitting 200 µs pulses at λ = 2.94 μm. Using a concave golden mirror with 50 cm radius of curvature, the end of the of Er: YAG laser crystal (having 4 mm in diameter) was imaged on the input face of the Tb: glass rod. The diameter of the pump beam inside the glass rod was 5 mm. The output energy of the pump laser was limited to ~ 1 J (which corresponds to ~ 5 J/cm2 energy density at the Tb: glass element) to avoid its optical damage observed at 7 ÷ 10 J/cm2 in [8]. The pumping energy supplied to the Tb3+ laser rod was controlled using an attenuator (a set of calibrated optical filters). The laser rod was pumped at the angle of ~ 3° to the cavity axis. Tb3+ laser action was registered using a LN cooled Ge: Au photodetector behind an appropriate pump cutoff filter and a digital oscilloscope. Pump pulse profile was recorded using a similar photodetector. The temporal response of both photodetectors was about 50 ns. The lasing wavelength was measured using a grating monochromator. Spectral tuning was performed by using an intracavity Brewster angle cut CaF2 prism and incrementally tilting the HR mirror. Worth noting, that in these conditions the maximal pump energy density was ~ 30% lower, than the RT lasing threshold observed in [8]. Thus we could not expect lasing at room temperature, and actually it didn’t take place. The situation has changed radically at LN temperature. Laser action was easily obtained with any of the available outcouplers. Figure 5 presents the Tb3+: glass laser output energy versus the pump energy entering the laser element for the six different outcouplers.

Fig. 5
figure 5

Tb3+: glass laser output energy versus pump energy entering the laser element. Six different outcouplers (OC) are used. The slope efficiency η and threshold pump energy Eth are also presented

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The highest slope (7.4%) and total (4.6%) efficiencies with respect to pump energy entering the laser element were obtained with 28% outcoupler. The minimal threshold energy (140 mJ) was observed with 1.5% output coupler. The lasing spectra in the absence of the tuning prism in the cavity were centered close to 5.25 μm.

Figure 6 shows the typical oscillograms of the pump and Tb3+: glass lasing pulses. The Tb3+: glass lasing pulses demonstrates spike structure typical for free-running multimode solid-state lasers with a long lifetime of the upper laser level. It is interesting to note, that the last spikes took place 25–50 µs after the end of the pump pulse. This effect is especially clear at the threshold pump level (Fig. 6, right).

Fig. 6
figure 6

Oscillograms of the Tb3+: glass laser pulse (bottom) and pump pulses (top). Output coupler 6%, pump pulse energy: left − 760 mJ, right – 200 mJ

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This peculiarity is caused by the finite excitations relaxation rate from the pumped level 7F4 to the upper laser level 7F5. The observed delay is in good accordance with the datum in Fig. 3 (35 µs).

The results of spectral tuning with an intracavity prism are shown in Fig. 7.

Fig. 7
figure 7

The laser tuning curve with 6% outcoupling

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The experiment was held at the maximal pump energy with 6% output coupler. The demonstrated tuning range was 5.05–5.55 μm.

4 Conclusion

A laser based on Tb3+-doped selenide glass rod was investigated under pumping with a pulsed 2.94 μm Er: YAG laser. Cooling of the laser element to the temperature of liquid nitrogen has enabled to obtain practically significant output characteristics. The output energy up to 36 mJ at 5.25 μm and wavelength tuning within 5.05–5.55 μm spectral range were demonstrated.

The Judd–Ofelt analysis showed that terbium in selenide glass should not exhibit excited state absorption of 2.94 μm Er: YAG laser pump radiation. This statement is also valid for 2.8 μm Er: ZBLAN fiber lasers that can be used for pumping of terbium doped chalcogenide glass fibers.