A Diode Pumped Middle Infrared Laser Based on Ho: GdVO₄ Crystal

Abstract

Ho: GdVO₄ crystal is the host material for the production of laser working in the middle infrared range. In this contribution, the characteristic parameters of the Ho: GdVO₄ crystal were measured, and the material was used as a gain medium to build a diode-pumped laser for the first time, to reach a laser output at 2047.9 nm. The output beam quality factor M² was measured to be 1.4 and 1.3 in x-direction and y-direction, respectively. In addition, the influence of the transmittance of the output mirror on the generation of laser was obtained through exploration. The results showed that the laser wavelength blue-shifted as the output transmittance increased.

1. Introduction

Lasers emitting in the 2 μm spectral region have increasingly gained attention from scientific researchers for plentiful applications in medical detection, atmospheric sensing, molecular spectroscopy, optical pumping of longer wavelength solid-state lasers, etc. [1,2,3,4]. Holmium (Ho) is the main doping material in the gain medium that produces lasers working in the 2 μm wavelength range. According to related reports, one of the methods to achieve high-efficiency Ho³⁺ laser emission is co-doping with Tm³⁺, and the other one is the direct resonant pumping of singly Ho-doped lasers [5,6,7,8,9,10]. The 2 μm laser radiation obtained through the latter approach has high conversion efficiency, is operable at room temperature, and has minimal thermal loading. In recent years, the search for new subject materials for efficient Ho³⁺ laser operation is focusing on gadolinium vanadate (GdVO₄) crystal. The tetragonal uniaxial GdVO₄ crystal was previously proved to be an extremely practical laser material for Tm³⁺ ions [11]. GdVO₄, as a host material for thulium and holmium, has a significant feature for diode-pump lasers [12]. First of all, it has a giant thermal conductivity, which is conducive to the rapid cooling of the crystal during application. What’s more, its thermal conductivity is at least twice higher than that of YVO₄, and even higher than that of YAG crystals, a widely known material with high thermal conductivity. A lot of studies have reported on diode-pumped Tm: GdVO₄ lasers [13,14,15]. Additionally, both continuous-wave (CW) and Q-switched Ho: GdVO₄ lasers pumped by Tm-fiber were also reported [16]. However, we have yet to find a report on diode-pumped Ho: GdVO₄ laser.

In this contribution, for the first time, we measured the laser characteristics of Ho: GdVO₄ crystal and reported a diode-pumped Ho: GdVO₄ laser. A considerable output power 1.8 W was obtained in the laser, with a slope efficiency of 39.5%. The center wavelength was located at 2047.9 nm, and, moreover, the lasing wavelength red-shifted to 2048.2 nm at lower output transmittance. In addition, we obtained the beam quality factor M² of the laser in the x- and y-direction to be approximately 1.4 and 1.3, respectively.

2. Crystals and Lasers

2.1. Characteristics of Ho: GdVO4 Crystal

As a common vanadate crystal, Ho: GdVO₄ has a large birefringence, good mechanical properties, and stable chemical properties. An a-cut Ho: GdVO₄ crystal (Company: Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai, China) with dopant concentration of 1.0 at.% was used in our experiment, which has a dimension of 4 mm (width) × 4 mm (height) × 20 mm (length).

The upper-level fluorescence lifetime is an important indicator for evaluating the energy storage performance of laser gain media. The longer the upper-level lifetime, the easier it is to form population inversion. The gain medium is also easier to store energy. The experimental set up for measuring the fluorescence lifetime of the upper-level is shown in Figure 1. The pump source is a 2 μm pulsed Ho:YLF laser (pulse repetition frequency: 500 Hz, pulse width: 25 ns). After the excited fluorescence of the crystal passes through the WDM1-3 grating monochromator, the fast response InGaAs detector (the shortest response time can reach 100 ns) is used to detect the fluorescence attenuation signal. Finally, the wavejet 332 digital oscilloscope (bandwidth: 350 MHz) produced by Lecroy Company records the fluorescence decay curve. Figure 2 shows the measured fluorescence decay curve of the Ho: GdVO₄ crystal energy band. According to the exponential decay of e, the fluorescence lifetime of the crystal can be obtained by fitting Figure 2 to 2.012 ms.

The absorption spectrum of the crystal is an important basis for determining the pump wavelength and polarization mode. It can be seen from Figure 3 that the strongest absorption peak of Ho: GdVO₄ crystal in σ polarization direction is near 1942 nm, the position with weaker absorption intensity is near 1958 nm, and the corresponding absorption cross sections are 2.76 × 10⁻²⁰ and 2.50 × 10⁻²⁰ cm², respectively. In π polarization direction, the strongest absorption peak is located near 1940 nm, and the corresponding absorption cross section is 2.44 × 10⁻²⁰ cm². In order to obtain better laser performance, a laser with a wavelength of 1.94 μm will be selected as the pump source of the Ho: GdVO₄ crystal.

In order to obtain the emission cross section of the laser gain medium, we used the fluorescence spectrum measurement system shown in Figure 4 to record the fluorescence spectrum of Ho: GdVO₄ crystal at room temperature. After processing by the simulation software, the polarization-stimulated emission cross section of the Ho: GdVO₄ crystal is shown in Figure 5. There are two obvious emission peaks in π polarization direction of Ho: GdVO₄ crystal, which are located near 2038 nm and 2048 nm, and the corresponding emission cross sections are 3.64 × 10⁻²⁰ cm² and 3.61 × 10⁻²⁰ cm², respectively. In σ polarization direction, the largest emission peak is located near 2005 nm, and the corresponding emission cross-section is 1.90 × 10⁻²⁰ cm². In addition, there are four emission peaks with relatively weak intensity, respectively near 2025 nm, 2045 nm, 2059 nm, and 2096 nm. In summary, it can be seen that the stimulated emission cross section of the strongest emission peak of Ho: GdVO₄ crystal in the 2 μm waveband is on the order of 10⁻²⁰ cm², and the fluorescence branching ratio and the upper-level fluorescence lifetime (ms-level) are also relatively large. It is sufficient to prove that Ho: GdVO₄ crystal can provide excellent mid-infrared laser performance.

2.2. Experimental Setup

The experimental setup of the diode-pumped Ho: GdVO₄ laser is schematically illustrated in Figure 6. The laser adopted a linear cavity design with a cavity physical length of about 30 mm. The above Ho: GdVO₄ crystal was used as the gain medium, and both ends of the crystal were coated with anti-reflection coatings, resulting in the reflectivity of the end face being less than 0.5% at 1.94 μm and less than 0.3% at 2 μm. At the same time, the crystal needed to be sandwiched between two indium foil water-cooled copper heat sinks (thickness: 0.1 mm, cooling water temperature: 18 °C). A fiber-coupled laser diode (LD) (output power: 20 W, wavelength: 1.94 μm) was employed to pump the Ho: GdVO₄ crystal. Here, the core diameter of the partial fiber was 600 μm, and its numerical aperture was 0.22. Similarly, LD also required a water-cooled system to control the temperature at 18 °C. The output spectral characteristics of LD were monitored with an optical spectrum analyzer (Bristol 721A). The measurement showed that central wavelength of the LD was at 1940 nm with an FWHM linewidth of approximately 7.9 nm. Moreover, we also observed that the wavelength shift of the LD from the threshold to 20 W is about 6 nm. It is worth noting that the red shift of the emission wavelength of the 1.94 μm LD here is caused by the significant thermal loading of LD under large current driving. At the same time, the pump power absorption of Ho: GdVO₄ crystal after a single cycle of the laser cavity increases from 68–75%. The addition of lens F1(focal length: 25 mm) and F2(focal length: 25 mm) was beneficial to adjust the energy density of the pump beam incident on the crystal. Therefore, the pump beam radius after convergence was about 300 μm. The input mirror M1 was flat, which had high transmission at 1.94 μm, and high reflective at 2.05 μm. Plano-concave mirror M2(curvature radius: 500 mm) served as the output mirror of the laser. M3 was a dichroic mirror with a 45° beam splitting, which had high transmission at 2.05 μm and high reflection at 1.94 μm.

3. Experimental Results and Discussion

In our experiment, the influence of different output transmittances on M2 of diode-pumped Ho: GdVO₄ laser has been investigated, as shown in Figure 7. Through the comparison of different groups of the slope efficiency, an optimal result was reached by using the output couple with the output transmittance of 50%. When the pump power was 15 W, the laser obtained the maximum output power of 1.8 W, with a slope efficiency of 39.5%. It is worth noting that the slope efficiency here is for the absorbed pump power. In the case of transmittance of 30 and 10%, the output power dropped to 1.6 and 1.1 W, and the slope efficiencies at this time were 33.8 and 19.7%, respectively. With optical spectrum analyzer, the output spectra of diode-pumped Ho: GdVO₄ laser were recorded under different transmittances. It can be seen from Figure 8 that, when the transmittance was 10%, the center wavelength of the laser was located at 2048.2 nm. As the transmittance increases, the center of the spectrum moves to the shortwave direction to 2047.9 nm. The blue shift of output wavelength is mainly caused by absorption loss in quasi-three-level system of Ho laser. No obvious wavelength fluctuation was observed with the three output couplers used in the above experiment.

To further determine the beam quality factor M², we used a lens (focal length: 150 mm) to converge the laser beam. With the maximum output power, the output beam radii were measured at several positions through the 90/10 knife-edge technique. Then, the data were fitted by the Gaussian beam standard expression. The fitting results are shown in Figure 9. It can be seen from Figure 9 that the beam quality M² factors in x- and y-directions are 1.4 and 1.3, respectively.

4. Conclusions

In conclusion, we measured that the Ho: GdVO4 crystal has a large upper state fluorescence lifetime, and its polarization emission cross section was in a 2 μm waveband. It proves that Ho: GdVO₄ crystal is an excellent material for obtaining a middle infrared laser. Furthermore, an LD-pumped Ho: GdVO4 laser working at 2.05 μm was demonstrated. This is the first all solid-state Ho: GdVO4 laser. The laser obtained a maximum output power of 1.8 W at 2047.9 nm, and the slope efficiency corresponding to absorbed pump power is 39.5%. In addition, the beam quality factor M² was determined to be 1.4 and 1.3 in the x- and y-directions, respectively. The experimental results indicate that the LD-pumped Ho: GdVO4 laser is a candidate for an efficient compact all solid-state Ho laser. We believe that its output characteristics can be improved with the increasing of pump power and optimizing of the setup parametric.