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An efficient Er:SrF2 crystal, lightly Er-doped to a concentration of 4at.%, was successfully grown by the traditional Bridgman method and displayed excellent spectral properties. A diode-end-pumped passively Q-switched dual-wavelength laser, operating at 2.79 μm wavelength, was demonstrated with this crystal by using black phosphorus as the saturable absorber (BP-SA). In the compact passively Q-switched Er:SrF2 laser, the maximum average output power of 180 mW was achieved at an absorbed pump power of 2.47 W, with a pulse duration of 702 ns and a repetition rate of 77.03 kHz. To the best of our knowledge, this is the first reported application of BP-SA to dual-wavelength pulse laser operation in the mid-infrared region.
© 2016 Optical Society of America
1. Introduction
The unique features of the mid-infrared domain make it very attractive for many applications. It has several atmospheric transparency windows and is strongly absorbed by water and biological tissues [1,2]. Many applications (e.g. laser surgery, trace-gas monitoring, remote sensing, and nonlinear spectroscopy) require coherent light radiation such as that emitted by a laser source. Mid-infrared laser light is generated mainly in two ways. The first involves directly stimulated emission [3–5], and the second involves an optical parametric oscillator (OPO) [6]. Compared with an OPO, solid-state lasers directly pumped by a laser diode (LD) are effective for achieving a high efficiency in the mid-infrared region. Their low cost, simple structure, and low loss are the significant advantages. In addition, many important applications, such as detection, laser displays, medicine, lidar, and fine laser spectroscopy, require the simultaneously output of two or more waves. Direct solid-state lasers operating at 3 μm wavelength in the continuous-wave (CW) or pulsed regimes are currently of great interest, especially for dual-wavelength applications. Efficient emissions ranging from 2.7 to 3.0 μm wavelengths by erbium-doped lasers, involving the transition from the 4I11/2 to the 4I13/2 state in Er3+, make them valuable for the above potential applications. Despite the interest they have generated, erbium-doped laser materials suffer from the self-terminating effect that results from the much longer lifetime of 4I13/2 compared to that of 4I11/2 [5]. This problem can be solved, on the one hand, by improving the erbium concentration (which is beneficial for the population inversion), and on the other hand, by co-doping sensitive ions into the Er materials (an effective method for decreasing the 4I13/2 level lifetime of Er3+ by depleting the 4I13/2 population through energy transfer) [7,8]. However, a key advantage of SrF2 crystals is that rare-earth ions tend to form clusters, even at low doping concentrations. In addition, an SrF2 crystal with a low phonon energy of ~280 cm−1 is more suitable for a mid-infrared laser, owning to the smaller probability of nonradiative transitions [9]. An LD side-pumped laser was produced using a 5at.% Er3+:SrF2 crystal operating near 2.75 μm [10], and a 1at.% Er:SrF2 crystal was grown by the Czochralski method for a diode-pumped 2.72-μm-wavelength laser with a slope efficiency of 0.42% [11]. However, to our knowledge, no dual-wavelength diode-end-pumped Er:SrF2 laser has been reported to date.
Two-dimensional nanomaterials that have recently attracted much attention for their outstanding physical and chemical properties include graphene [12,13], hexagonal boron nitride (hBN) [14], and transition-metal dichalcogenides (TMDs) [15]. Black phosphorus (BP) monolayers, another attractive two-dimensional nanomaterial, have a direct bandgap [16], high carrier mobility (~1000 cm2V−1s−1) [17,18], and large on/off ratios (>105) [19,20] at the room temperature. Furthermore, BP has a layer-dependent direct bandgap that ranges from 0.3 (in the bulk) to 2.0 eV (for a single layer) [21–25]. Unlike indirect bandgap materials such as MoS2, the BP bandgap is always direct, depending on the numbers of layers, which is a significant advantage for opto-electronic applications [26]. These characteristics make BP a promising ideal broadband saturable absorber (SA) for mid-infrared wavelengths [27–30].
This present study, to the best of our knowledge, is the first to demonstrated a mid-infrared dual-wavelength passively Q-switched Er:SrF2 laser, made with a BP-SA, formed in a compact linear resonator. Several aspects of passively Q-switched lasers, including the output characteristics, and the variation of the pulse width and repetition rate with the absorbed pump power, have all been investigated in detail.
2. Experiment
An excellent 4at.% Er:SrF2 crystal was successfully grown by the traditional Bridgman method, using high purity (>99.995%) ErF3 and SrF2 crystalline powders as raw materials. The raw materials were first grinded and mixed, then sealed with additional deoxidant in platinum crucibles during the entire growth process. The crystal samples were cut and then polished to form pieces of size 10 × 3 × 3 mm3. The absorption and the emission spectra of Er:SrF2 are shown in Figs. 1(a) and 1(b). The absorption spectrum was measured using a Cary 5000 UV/VIS/NIR spectrophotometer at room temperature (T = 300k).
The BP bulk crystal was exfoliated and dispersed into an N-methylpyrrolidone(NMP) solvent, and a few-layer BP-SA was fabricated by the liquid-phase exfoliation technique, following the preparation described in [26]. The BP-SA was fabricated by drop-casting the collected BP dispersion onto a silicon substrate, before drying in a vacuum drying oven. Figure 2(a) shows the BP dispersion after sonication. Figure 2(b) compares the Raman spectrum of the fabricated BP nanoflakes with standard silicon Raman peak at 520.7 cm−1, and three obvious peaks located at 361, 437, and 465 cm−1, corresponding, respectively, to the characteristic modes A1g, B2g, and A2g of the few-layer BP. Compared with the different thickness of BP in [31], the three Raman peaks displayed the layer-dependent shift. In the experiment, the atomic force microscopy (AFM) measurement of the BP nano-platelets were performed in a 1.5 μm × 1.5 μm region, Fig. 2(c) shows that the multilayer BP is in the form of a layered structure approximately 200 nm in diameter. The BP-sample thickness was analyzed at four different locations, indicated in Fig. 2(c). Figure 2(d) shows the thickness ranging from 3 to 9 nm. Because the ultrafast recovery time of BP is 24 ± 2 fs and the nonlinear absorption coefficient β2 is (−0.15 ± 0.09) × 10−3 cm/GW at 1550 nm, as measured with a femtosecond laser system [32], a femtosecond or sub-picosecond laser source is therefore more appropriate for characterizing a BP-SA [33]. Given the limitation of the experimental conditions, we did not have a femtosecond laser source operating at 2.79 μm available, and therefore the exact optical properties of BP (such as the nonlinear transmission with the indicated modulation depth and saturation fluence) could not be measured. In our team, laser operation in the 2 μm regime was successfully achieved by utilizing this BP absorber [34].
Fig. 2 Parameters of multilayer BP: (a) Photograph of multilayer BP dispersed in NMP; (b) Raman spectrum; (c) AFM scan image; (d) Height profiles.
The pump source in such a laser is a fiber coupled CW laser diode operating at 972 nm with a core diameter of 105 μm and a numerical aperture (NA) of 0.22. The pump beam was expanded into the gain medium by a coupling system of 1:2. The uncoated Er3+:SrF2 crystal, lightly Er-doped at 4at.%, was mounted in a copper block stabilized at 10 °C by a cooling system. The humidity of the laboratory was controlled at 26% using a dehumidifier. The passively Q-switching laser is outlined in Fig. 3. The cavity consisted of a plane mirror M1 and a concave mirror M2 with the cavity length of 24 mm was designed. The input plane mirror M1 was anti-reflection (AR) coated for 974 nm and high-reflection coated for 2.7-2.95 μm. The output coupler (OC) M2 with a 50 mm radius of curvature, had a transmission of 1% over 2.7-2.95 μm. The CW laser was achieved by adjusting the position of the mirrors and the crystal. To study the Q-switched properties further, the BP-SA was inserted into the cavity located close to the crystal. By adjusting the position and the angle of BP carefully, the stable passively Q-switched pulses were obtained.
3. Results and discussion
The CW output performance was first measured at a low threshold power of 0.108 W (based on the absorbed pump power), which was measured using a power meter (30A-SH-V1, Israel). A maximum output power of 356 mW was achieved with an absorbed pump power of 2.47 W (the incident pump power was 3.05 W), corresponding to an optical-to-optical conversion efficiency of 12.6% and a slope efficiency of 15.5%, which is higher than the slope efficiency of 11% in [10]. Compared with the 15 at.% Er-doped Lu2O3 ceramic reported in [35], the 4 at.% Er-doped SrF2 crystal is more efficient in slope efficiency of laser. To protect the crystal from being destroyed, the maximum incident pump power was limited to 3.05 W. We then inserted the BP-SA into the cavity, as shown in the laser setup in Fig. 3. The Q-switched pulse appeared when the absorbed pump power approached 1.1 W. The average output power depended on the absorbed pump power, and Q-switched pulse trains appeared as shown in Figs. 4(a) and 4(b). When the maximum output power of 180 mW was achieved with a slope efficiency of 7.9%, the single pulse energy was 2.34 μJ and the peak power was 3.3 W.
Fig. 4 (a) Average output power of CW Er3+:SrF2 laser. (b) Average output power of Q-switched Er3+:SrF2 laser.
The pulse duration, repetition rate, single pulse energy, and peak power of the Q-switched operation are plotted as functions of the absorbed pump power in Figs. 5(a) and 5(b). At the maximum pumping energy, the laser-beam profile and the 3D light-intensity distribution, as recorded by a detector (PH00435, Model No: NS2-Pyro/9/5-PRO, Photon), are plotted in Figs. 5(c) and 5(d), respectively. These results indicate that the generated beam had an excellent TEM00 transverse profile. As the absorbed pump power increased, the repetition rate increased while the pulse duration decreased gradually. Using an infrared detector with a rise time of 3 ns (VIGO System PVM-2TE-10.6/MIPAC 250M, Poland), the Q-switched pulses were detected and displayed with a digital oscilloscope (Tektronix DPO4104, 1 GHz bandwidth, 5 G samples/s). Typical pulses trains are shown in Fig. 6, captured at the maximum average output power. The minimum pulse duration was 702 ns at the repetition rate of 77.03 kHz. The results are also better than those reported in [29]. Figure 7 shows the reliability of the laser emission spectrum recorded with an optical spectrum analyzer (MS3504i, made in Belarus). It clearly shows that the central wavelengths are 2789.3 and 2790.3 nm for the CW laser, and 2790.1 and 2790.9 nm for the Q-switching laser.
Fig. 5 (a) Pulse repetition rate and pulse duration versus the absorbed pump power. (b) Single pulse energy and peak power dependences on the absorbed pump power. (c) Laser beam profile. (d) 3D light-intensity distribution.
Fig. 6 Oscilloscope display of Q-switched pulse trains, at 1 and 100 μs /div resolutions, captured at the maximum output power of 180 mW.
4. Conclusion
In conclusion, we presented the first experimental demonstration of a 2.79 μm dual-wavelength Q-switched solid-state laser with a BP-SA. The maximum average output power was 180 mW with a pulse energy of 2.34 μJ, peak power of 3.3 W, pulse duration of 702 ns, and repetition rate of 77.03 kHz. The results confirm the BP-SA as a potential optical modulator for use in mid-infrared pulsed lasers. The easy fabrication, low cost, and variable bandgap of BP are promising features for applications in the mid-infrared region.
Funding
National Natural Science Foundation of China (NSFC) (Nos. 61475089, 61435010, 61422511 and 51432007). The Science and Technology Innovation Commission of Shenzhen (KQTD2015032416270385 and JCYJ20150625103619275).
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