1. Introduction

High power and widely tunable narrow line-width solid-state lasers operating at 2.1 μm wavelength region are promising sources for coherent Doppler wind detection lidar and remote sensing of water vapor in the atmosphere [1–3]. In addition, high power 2.1 μm laser could be employed as pump source for optical parametric oscillators operating in the mid-infrared region [4]. With the advent of 1.9 μm high power lasers such as Tm:fiber lasers [5], Tm:YLF solid-state lasers [6] and semiconductor InGaAsP/GaSb diode lasers [7], resonantly pumped singly Ho³⁺-doped crystal lasers have obviated the energy transfer loss between Tm³⁺ and Ho³⁺ ions in Tm-Ho system and demonstrated high quantum efficiency and reduced upconversion losses. Tunable laser operation around 2.1 μm has been achieved in Tm, Ho:YLF [8] and Ho:YAG [9] lasers with tens of nanometers tuning range and spectrum line-width of a few nanometers.

In general, tunable operation of solid-state lasers can be realized by inserting spectrum discrimination filters such as etalon [10], dispersive prism [11] and birefringent filter [12] into the cavity. However, these techniques always induce additional cavity losses and the selected laser spectrum is usually with line-width of several nanometers. Volume Bragg gratings (VBGs) are ideal spectrum selectors with high diffraction efficiency, low insertion loss and narrow spectrum line-width on sub-nanometer scale [13,14]. Meanwhile, they have high damage threshold and excellent thermal stability in high power operations. So far, VBGs have been successfully used as wavelength selective elements in widely tunable, narrow line-width and single-frequency Er³⁺/Tm³⁺ doped solid-stated lasers [15–18].

Recently, CaYAlO₄ (CYA) has attracted much attention as a new laser crystalline host. Due to the randomly distribution of Ca²⁺ and Y³⁺ ions in the crystal structure, lanthanide ions embedded in this compound could offer extremely broad absorption and emission bands. High optical quality Ho:CYA crystal was first developed and characterized in 2013 [19]. Owing to the broad and smooth fluorescence spectrum, Ho:CYA crystal facilitates widely tunable laser operation around 2.1 μm. Moreover, it should be promising for ultrafast pulse generation in pico- or even femtosecond scale. By exploiting the birefringent nature of Ho:CYA crystal, linearly polarized laser output could be generated with no extra polarizer needed in the cavity, which simplifies laser resonator design and obviates the depolarization loss of isotropy crystals such as Ho:YAG.

In this paper, we demonstrate high power and widely tunable operation of a Ho:CYA laser resonantly pumped with a home-constructed Tm:fiber laser at ~1922 nm. Using a VBG as the wavelength selective element, the operating wavelength could be tuned from 2055 to 2100 nm with a total tuning range of 45 nm and line-width of < 0.1 nm. The laser yields 7.4 W of output power at 2095 nm for narrow line-width operation and 11.3 W of output power at ~2085 nm for free-running mode.

2. Experimental setup

The laser configuration used in wavelength tuning and free-running operation is shown schematically in Figs. 1(a) and 1(b) respectively. A high power home-made Tm:fiber laser is employed as pump source and the operating wavelength was tuned to match the absorption peak of Ho:CYA crystal at 1922 nm. It comprised a 4.73 m double clad fiber with 25 μm diameter Tm-doped alumina-silicate core (0.17 NA) and 300 μm diameter pure silica inner-cladding (0.46 NA). The beam quality factor (M²) of the pump light was measured to be ~2.6 by a beam profiler (NanoScan, Photon Inc.). Laser emitted from the 25 μm fiber core was collimated by a 25 mm focal length plano-convex lens and subsequently focused to a beam of ~200 μm diameter at the center of the Ho:CYA crystal with a 200 mm focal length lens, providing a confocal parameter of ~14 mm. A Ho:CYA crystal of 1 at.% doping concentration was used as the gain medium in our experiment. The polarized absorption and emission spectra of the crystal has been reported in Ref [19]. It was a-cut in a dimension of 3 × 3 × 22.5 mm³ with both end facets antireflection-coated in the 1800-2150 nm wavelength range. The crystal sample was wrapped with indium foil (0.1 mm in thickness) and mounted on a water-cooled copper heat-sink maintained at ~15°C to ensure efficient heat removal. Single-pass absorption at 1922 nm for small signal pumping was measured to be 85% under non-lasing condition, while it decreased to 78% for incident pump power of 1.2 W due to the ground-state bleaching.

 

Fig. 1 a) Schematic diagram of the wavelength tuning experimental setup, DM: HT@1850-1960 nm, HR@2-2.25 μm. b) Schematic diagram of the free-running experimental setup.

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A high-reflective VBG (OptiGrate Corporation) was employed as the wavelength selective element. It has a dimension of 8 mm × 6 mm × 4.61 mm and a central wavelength of 2104 nm at normal incidence with a spectral line-width of ~1 nm. To avoid the instabilities in laser performance caused by surface Fresnel reflections, the grating was titled by a slant angle of 0.2° with respect to the glass surface and the facets were broadband anti-reflection coated. The diffraction efficiency at normal incidence is over 98% and decreased with increasing incident angle. That is caused by narrowing of angular acceptance of the VBG. In order to minimize the divergence angle of beam incident on the VBG and maintain good mode matching between the pump beam and laser mode, a z-shaped cavity was designed for wavelength tuning as shown in Fig. 1(a). The plane pump input coupler (IC) was with high reflectivity at the lasing wavelength and high transmittance at 1850-1960 nm and the folding mirror with a 200 mm radius-of-curvature was highly reflective at 1900-2250 nm. A step wise wavelength tuning was realized by rotating the VBG and realigning the output coupler (OC) according to the relationship λ_B = λ₀cos θ, where λ₀ is the wavelength at normal incidence and θ is the internal incident angle. The resonant mode diameter on the gain medium and VBG was ~260 μm and 900 μm respectively. The total cavity length was 508 mm and a small incident angle on the curve mirror was taken to minimize the astigmatism. A high reflected mirror at 1900-2150 nm was used to retro-reflect the unabsorbed pump light with the 2.1 μm leaked laser beam filtered out by a dichroic mirror (DM). In free-running mode, the laser cavity configuration was identical to that in wavelength tuning operation and the VBG was replaced by a plane mirror high reflected at 1900-2150 nm (see Fig. 1(b)).

3. Results and discussion

Free-running laser performance of the Ho:CYA crystal was first investigated with output couplers of 5%, 10% and 20% transmittance using the configuration in Fig. 1(b). Laser performance of the folded cavity in terms of maximum output power and lasing slope efficiency is comparable to that of a simple two-mirror resonator. As shown in Fig. 2, 6.2 W of output power has been obtained with the 20% output coupler under 22 W of incident pump power, corresponding to a slope efficiency of 32.9%. The output power and slope efficiency decreased for output couplers of 5% and 10% transmittance. As a result, the output coupler of 20% transmittance was used in wavelength tuning operation.

 

Fig. 2 Free-running output power versus incident pump power with output couplers of 5%, 10% and 20% transmittance.

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Figure 3 illustrates the narrow line-width laser output power as a function of operating wavelength under 22 W of incident pump power. The wavelength could be tuned from 2055 to 2100 nm, corresponding to a total tuning range of 45 nm. The tuning curve was determined by the spectral gain of the Ho:CYA crystal and the diffraction efficiency of the VBG. Over 4 W of output power could be maintained within the wavelength tuning range of 2076-2100 nm. Further tuning in operating wavelength to less than 2076 nm leaded to a decrease in output power, which can be attributed to the increasing absorption cross section and decreasing emission cross section of the Ho:CYA crystal as shown in Ref [19]. In addition, angular acceptance of the VBG narrowed at shorter wavelength due to the increasing of diffraction angle (12.4° for 2055 nm), resulting in higher cavity loss and hence reduced output power and slope efficiency. According to the emission spectrum of the Ho:CYA crystal, further extension of the tuning range should be possible by using VBGs with shorter and longer central wavelength such as 2050 nm and 2150 nm at normal incidence.

 

Fig. 3 Output power versus tuned operating wavelength at 22 W incident pump power.

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High power lasing characteristics of the free-running and narrow line-width Ho:CYA crystal laser was investigated with a 20% output coupler. In free-running mode, the laser reached threshold at ~4 W of incident pump power and yielded 11.3 W of output power for 40 W of incident power, corresponding to a slope efficiency of 30.7%. The linearly increase of output power indicated that further power scaling should be achieved by simply increasing the incident pump power. For narrow line-width operation with a VBG, over 7.4 W of output power has been obtained at 2095 nm under an incident pump power of 32 W. Unlike the free-running occasion, saturation in the output power appeared at higher incident pump powers, which was probably resulted from the feedback towards the Tm:fiber pump source induced by the retro-reflected mirror for double-pass pump. The problem could be solved by using single-pass pump scheme and increasing the crystal length to achieve sufficient pump absorption. Spectrum of the Ho:CYA crystal laser was analyzed by an optical spectrum analyzer (AQ6375, Yokogawa) with a resolution of 0.1 nm for the free-running mode and 0.05 nm for the narrow line-width operation. (as shown in the inset of Fig. 4). In free-running mode, the laser emission wavelength was centered at 2084 nm with a line-width of ~1 nm. After the laser wavelength was locked by a VBG, spectrum line-width of 0.1 nm has been achieved, which was dramatically decreased in comparison with the VBG bandwidth (~1nm). That is attributed to gain competition induced by the decrease of reflectivity with deviation from the Bragg wavelength within the VBG bandwidth.

 

Fig. 4 Free-running and narrow line-width output power versus incident pump power with 20% output coupler.

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The laser spectra of the tunable Ho:CYA crystal laser are shown in Fig. 5 with intervals around 5 nm. Spectrum line-width of ~0.1 nm was maintained over the whole tuning range. The laser output was measured to be linearly polarized with a polarization extinction ratio of 1:100. The beam quality of the narrow line-width Ho:CYA crystal laser was measured at 2095 nm under 22 W of incident pump power. Figure 6 depicts the beam radii along the z-axis and the inset shows the profile of the laser beam near the focus. Fitting the measured data with a hyperbolic curve, the beam quality factors in x and y-axis were calculated to be 1.45 and 1.49 respectively.

 

Fig. 5 Typical spectrum for the tunable Ho:CYA laser with a VBG.

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Fig. 6 Beam quality of the narrow line-width Ho:CYA crystal laser.

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4. Conclusion

We have demonstrated a high power and widely tunable, narrow line-width Ho:CYA crystal laser by using a VBG as the wavelength selective element. The laser wavelength could be tuned from 2055 to 2100 nm with the spectrum line-width smaller than 0.1 nm over the whole tuning range. The laser yielded 7.4 W of output power at 2095 nm for narrow linewidth operation and 11.3 W of output power at ~2085 nm for free-running mode. Further extending in wavelength tuning range and increase in output power should be possible by using a VBG of proper center wavelength and improved resonator design to avoid feedback towards the Tm:fiber pump source.