1. Introduction

The erbium (Er³⁺) ion-based lasers, operating on the transition ⁴I_11/2 → ⁴I_13/2, allow to generate radiation directly in the mid-IR spectral region close to 3 μm. This wavelength is attractive for many medical applications, e.g. dentistry, dermatology, urology, and bone surgery [1–3]. Except medical applications, these lasers can be used in spectroscopy or as pumping sources for optical parametric oscillators [1, 4]. Nowadays, coherently pumped systems based on Er-doped crystalline oxides (Er:YAG, Er:YSGG, Er:YVO₄, Er:GGG) [5–9] are well known. Nevertheless, these materials have high phonon energy, thus fluorescence decay time at upper laser level (⁴I_11/2) is very short (up to 60 times shorter) in comparison with lower laser level (⁴I_13/2) [10, 11]. As consequence the self-termination phenomenon occurs in these laser systems. However, this effect could be suppressed by using high concentration of Er³⁺ ions or using cascade lasing. The 0.5 % Er:YAG used in cascade laser was successfully tested and the laser action was obtained at 2.7 μm at liquid nitrogen (78 K) temperature [8]. So, the laser action in Er³⁺ doped crystalline oxides matrix is limited by the non-radiative transition [5, 8, 11–13]. On the other hand, there are the sesquioxides doped by erbium, e.g. Er:Y₂O₃ and Er:Lu₂O₃ that possess the low-phonon energy and their material parameters outstrip commonly used crystals (YAG,YSGG, GGG, YVO₄, etc.). Nevertheless, the fabrication of these active laser materials is complicated and expensive mainly due to high melting temperature [14–17]. From this reason the low-phonon energy fluoride matrices such as Er:YLF and also the Er:CaF₂ and Er:SrF₂ crystals or ceramics were tested [11, 18–20]. The CaF₂ and SrF₂ crystals and ceramics doped by Er³⁺ ions are very promising materials, possessing low phonon energy ∼322 cm⁻¹ and ∼280 cm⁻¹, respectively [13], and their fabrication is not so complex as sesquioxides [21]. In such materials the probability of non-radiative transitions is lower than in crystalline oxides, therefore fluorescence decay time at ⁴I_11/2 is higher which brings benefits for CW laser operation [11, 13, 22].

The first lasing operation of Er:CaF₂ laser at 2.7 μm based on a stepwise up-conversion pumping scheme under Xe-flashlamp excitation was reported by S. A. Pollack et al [23]. Single crystal Er:SrF₂ lasing at 2.7 μm under diode pumping was realized by T.T. Basiev et al [22]. Recently, lasing in the near- and mid-infrared spectral range of Er³⁺ ions in laser quality fluoride crystal has been successfully demonstrated under laser diode pumping [11–13, 24–26]. It was found, that for laser generation in mid-infrared region (2.7 – 3 μm) it is not necessary to use matrix with high amount of Er³⁺ ions for room temperature lasing [12, 13, 26]. Due to broadband emission spectrum the Er:SrF₂ crystal is interesting for possible ultra-short pulse generation [13] and laser tuning. The laser tunability depends on several aspects - the laser linewidth, gain cross-section, fluorescence decay time and broadband emission spectrum [21, 27]. According to Moulton [27] the tunable laser systems with large gain cross-section and linewidth demand short fluorescence decay time. The broadband absorption and emission spectra of Er:SrF₂ are given by crystal structure [13, 21, 22]. The SrF₂ doped with Er³⁺ ions requires charge compensation if the trivalent rare-earth ions are used for doping [21, 22], and it should be also noted that the Er³⁺ ions form clusters, which is advantageous for energy transfer [13]. The Er:SrF₂ comply with all necessary requirements for tunable laser confirmed by results mentioned below and in literature [13, 22, 24, 28]. Thus the aim of this paper is to present tunable crystalline Er:SrF₂ active medium for the first time tuned over 100 nm at room temperature.

2. Experimental setup

The tested Er:SrF₂ crystal had a form of rectangular block, it was plane-parallel (10 mm long), face-polished (3 × 3 mm), see Fig. 1. The crystal does not have anti-reflection coatings. Since the tested sample was placed close to the pumping mirror (distance 2 mm) and the faces of the sample was plan-parallel the Fresnel losses could be neglected. The Er:SrF₂ contained 3 at. % of Er³⁺. During all measurement, the tested sample was placed in cooper holder without active cooling. The Shimadzu spectrophotometer type UV - 3600 with spectral resolution ±0.2 nm in visible and ultraviolet region, and ±0.8 nm in mid-infrared was used for measuring the transmission spectra. Fluorescence spectra and laser emission were measured by Oriel monochromator 77250 (grating 77300) with Thorlab photodiode PDA 30G-EC (PbS, 1.0 – 2.9 μm).

 

Fig. 1 Er:SrF₂ crystal sample photographs.

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The fiber-coupled (core diameter 100 μm, NA = 0.22) laser diode (LIMO35-F100-DL970-EX2082, LIMO) was used for longitudinal pumping of the crystal sample. The laser diode operated in the pulsed regime (frequency 10 Hz, pulse duration 5 ms). The pumping pulse duration was chosen to be close to fluorescence decay time at the upper laser level. Because of possible problems with a thermal lens and the overheating of the tested sample the duty cycle of 5 % was chosen, from this reason the pumping frequency of 10 Hz was used. Using temperature tuning, the emission wavelength of the laser diode was set at 969 nm. The Er:SrF₂ was placed inside the hemispherical optical resonator 145 mm long formed by a flat pumping dichroic mirror (PM) and a concave output coupler (OC, radius of curvature – 150 mm). The PM was highly transparent (T ∼ 94 %) in pumping range at 960 – 980 nm and at the same time highly reflective (R = 99 %) within the spectral range of 2.65 − 2.95 μm. The output coupler (OC) with reflectivity 95 % @ 2.65 − 2.95 μm was used. The pumping radiation was focused into the tested crystal by two achromatic doublet lenses with a focal length of f₁ = 75 mm and f₂ = 150 mm, thus the pumping beam diameter was ∼ 200 μm. The wavelength tuning of the erbium laser was obtained by using a birefringent filter (single MgF₂ plate 2 mm thick) placed at the Brewster angle inside the optical resonator between the output coupler and laser active medium. The laser layout is shown in Fig. 2. The mean output power of the laser was measured using a broadband high-sensitivity thermopile probe PM19 (Molectron) with a germanium plate as a filter (to separate pumping radiation). The temporal structure of the generated radiation was observed by an InAs/InAsSbP photodiode (model PD36-05, peak wavelength sensitivity 2.55 – 3.45 μm, IBSG Co, Ltd.) connected to the oscilloscope (Tektronix TDS 3052B, 500 MHz, 5 GS/s). The IR sensitive camera Pyrocam III (Spiricon) was used to investigate the laser beam spatial structure.

 

Fig. 2 Diode-pumped Er:SrF₂ laser: PM – HT @ 969 nm, HR @ 2.65 − 2.95 μm; OC – R = 95 % @ 2.65 − 2.95 μm, r = 150 mm, BF – birefringent MgF₂ plate 2 mm thick.

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3. Results and discussion

The absorption spectrum of the Er:SrF₂ in laser pumping spectral region is shown in Fig. 3(a). The maximum in absorption spectrum from Fig. 3(a) corresponds with wavelength of 969 nm and absorption coefficient is 1.62 cm⁻¹, the FWHM of absorption band is 17 nm (967 – 984 nm). During laser experiments, the ∼74 % of pumped power was absorbed in laser active medium. From the fluorescence spectrum shown in Fig. 3(b) it can be seen that Er:SrF₂ possesses broadband emission spectra. As evident, the mid-IR fluorescence of Er³⁺ ions corresponding to laser transition ⁴I_11/2 → ⁴I_13/2 has a maximum at region 2.71 μm. The fluorescence decay curves of Er³⁺ ions in SrF₂ crystal are presented in Fig. 4, the transition ⁴I_11/2 →⁴I_13/2 is self-terminated since the fluorescence decay time of the lower ⁴I_13/2 level (13 ms) is approximately 1.7 times longer than that for the upper ⁴I_11/2 level (7.3 ms).

 

Fig. 3 Absorption spectrum of Er:SrF₂ crystal (a) and fluorescence spectrum of Er:SrF₂ crystal (b).

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Fig. 4 Fluroescence decay time at lower ⁴I_13/2 (a) and at upper ⁴I_11/2 (b) laser level .

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The described laser system based on the Er-doped SrF₂ crystal was tested without the birefringent filter at first. The generated pulse energy and power amplitude were estimated from the mean output power, using the known pulse repetition rate and duration. Dependence of the laser output power amplitude on the absorbed power amplitude is presented in Fig. 5. From the results it is seen that in the described configuration with Er:SrF₂ it is possible to generate power amplitude up to 1.3 W with slope efficiency of 9.2 % with respect to the absorbed power amplitude. This result was reached for duty cycle 5 % and the corresponding mean output power was 58 mW. The generated laser beam profile for maximum output power is shown in Fig. 5, it can be seen that the beam profile is not clear fundamental mode TEM₀₀. The M² of the laser beam in x- and y-axis was calculated to be 1.9 and 1.8, respectively. This data was obtained as a ratio of measured and calculated laser beam divergence. The emitted laser radiation line was 2.75 μm with linewidth 9 nm (FWHM). The emitted wavelength is affected by series of up-conversion processes, the excited state absorption and energy transfer, that are described in detail in [13].

 

Fig. 5 Output characteristic of diode-pumped Er:SrF₂ pulsed laser with beam profile for maximum laser output power; duty cycle – DC, pumping pulse length – Δt, slope efficiency with respect to absorbed energy – σ, laser threshold – P_th, pumping wavelength – λ_pump, emission wavelength λ_laser, resonator length – L_res, pumping mirror transmission and reflectivity – T_PM and R_PM, output coupler reflectivity and radius – R_OC and r_OC.

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Using the MgF₂ intra-cavity birefringent filter inside the resonator, a continuously tunable output was achieved. The tuning curve of the laser is shown in Fig. 6. The birefringent filter causes the additional losses in resonator, so the laser threshold rose and maximal output power was decreased, particular values are mentioned in text below. The obtained tuning curves were not smooth due to numerous water absorption lines [29] contained in the air in this spectral region. The slow modulation of measured tuning curve correlates well with observed fluorescence spectrum (Fig. 3(b)). The fast modulation could be mostly explained by strong absorption on the atmosphere in the laser resonator. Also water or other substance adsorption on any optical surface present in the laser resonator could influence the laser tuning significantly and in different way that the air. Tuning range of 123 nm extended from 2690 nm to 2813 nm (crossing zero) was obtained for Er:SrF₂. The maximum output mean power obtained with MgF₂ intra-cavity birefringent filter was 21 mW @ 2.75 μm.

 

Fig. 6 Tuning curve of Er:SrF₂ measured for maximal pumping shown together with air absorption lines, excitation pulse length 5 ms, frequency 10 Hz, pumping wavelength 969 nm.

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4. Conclusion

The SrF₂ crystal doped with 3 at. % of Er³⁺ was tested and the spectroscopic and laser properties were investigated at room temperature. Under pulsed 969 nm laser diode pumping the mid infrared radiation at 2.75 μm was obtained with slope efficiency up to 9.2 %. The Er:SrF₂ laser emitted radiation with maximal output power amplitude of 1.3 W. Using the MgF₂ birefringent filter the new maximal tunability from 2690 nm to 2813 nm was reached. The obtained tunability range significantly exceed results (2720 – 2760 nm, tuning range 40 nm, tuning element quartz plate) on Er:SrF₂ published in [22].The broader tuning range was probably reached due to using MgF₂ instead of the quartz plate. Also, the lower laser threshold and lower losses in laser resonator helped to obtain the broader tuning range in comparison with [22]. The demonstrated broad gain bandwidth of ∼ 4 THz (FWHM) makes this laser potentially attractive for ultra-short pulse generation in the 2.7 μm region (corresponding Fourier limited Gaussian pulse width ∼ 110 fs). Regarding the tuning curve width, our results can be compared with data obtained using lasers based on transition metals, like Cr:ZnSe, Cr:ZnS or Cr:CdSe. These materials possess significantly broader tuning range (2100–3100 [30]) covering also the Er:SrF₂ laser emission. On the contrary, an efficient and powerful pumping of these lasers is usually based on systems like Er- or Tm-fiber lasers witch increase the overall price and complexity of final laser. In such a case Er:SrF₂ laser could be a good option if the desired emission wavelength corresponds to its tuning range. Moreover, the lifetime of Cr²⁺ upper laser level is much shorter (∼4 μs) comparing to Er³⁺ in SrF₂ (∼7.3 ms), so the Q-switch pulse generation or laser radiation amplification will be much more difficult with Cr-doped materials.

As was mentioned above the Er:SrF₂ is interesting active medium because of low phonon energy that strongly affected self-termination process and improve the laser action. Moreover, the fabrication of this laser material is not so complex and expensive as the low phonon sesquioxide. All these features makes from Er:SrF₂ promising candidate for medical and spectroscopic applications or for further non-linear conversion deeper to infra-red region.