1. Introduction

Some of the applications of 1.9 µm Tm:YLF lasers are in surgery [1] and to pump high energy Ho:YLF slab amplifiers [2], which can be used for defense related applications [3]. Slab Tm:YLF crystal geometries allow the amount of pump, and therefore output power, to be significantly scaled, by spreading out the heat load in the horizontal direction [4]. We have previously demonstrated high average powers (~200 W) from such a system at ~1.9 µm [5]. However, efficient pumping of Ho:YLF requires that the Tm:YLF output wavelength reasonably well match one of Ho:YLF’s two strong absorption peaks at either 1890 nm or 1940 nm. This is usually accomplished by polarization and output-coupler selection using threshold calculations. Volume Bragg Grating (VBG) mirrors offer a more precise and sure way to select a specific wavelength. A VBG mirror has a periodic variation of the refractive index and is transparent at most wavelengths. It has a high reflectivity at a certain wavelength which fulfills the Bragg condition [6,7]. VBG’s are used to control and stabilize the wavelength of diode lasers [8], fibre lasers [9], and lasers where they can also facilitate single mode operation [10]. Dergachev [11], first reported a side-pumped, double Tm:YLF crystal, 3-pass laser resonator which used a VBG output-coupler to wavelength select it to 1940 nm. The output of this laser exceeded 30 W when pumped with 170 W of total diode power and had a slope efficiency of 30%. Recently Duan [12], reported a 1908 nm VBG wavelength selected dual Tm:YLF rod laser pumped with four laser diodes which delivered over 40 W of output power. In this paper we report on using a VBG as a back-reflector in a Tm:YLF slab laser. The slab crystal was pumped with a 300 W, 792 nm diode-stack using a pump reproducing scheme. It delivered over 80 W of output power at 1890 nm and had a similar beam quality compared to other slab laser systems [13].

2. Experimental setup

A single Tm:YLF slab crystal was pumped with a 300 W diode-stack using a pump pass configuration which reproduced the pump beam after each pass (Fig. 1 ). The slab crystal had a doping of 2.5%, dimensions of 1.5 × 11 × 15.5 (a × c × a) and was a-cut with the c-axis in the horizontal plane. A 300 W diode-stack from nLight (Model # NL-VSA-05-300-792-F900D) was used as a pump source. It had fast axis collimation lenses installed by the supplier while in the slow horizontal axis it was collimated with an f = 150 mm cylindrical lens. A polarizing cube reflected 90% of the pump light towards the crystal. The remaining 10% horizontally polarized pump light was transmitted by the cube and used to monitor the laser diode performance on a power meter.

 

Fig. 1 Layout of the double-pass pumped, VBG wavelength selected, Tm:YLF laser.

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The pump beam was then focused with a f = 100 mm spherical lens (L₂) through a 45° High Transmission (HT) 792 nm, High Reflecting (HR) 1900 nm mirror (45° laser mirror M₁) to a radius of w_x = 2.6 mm in the horizontal plane and w_y = 0.22 mm in the vertical plane, in the centre of the of the slab crystal. It then diverged through another 45° laser mirror (M₂) before it was collimated by a second f = 100 mm spherical lens (L₃). The pump beam was then reflected back to the crystal and its polarisation rotated to the horizontal by a λ/4 wave plate, mirror combination. The pump beam was subsequently focused by L₃ in the centre of the slab crystal to roughly the same focus size as the first pump pass. This could be done because the thermal lensing along the Tm:YLF slab’s a-axis was weakly positive due to bulging of the pump faces (despite the small negative dn_o/dT) [14], so that the pump beam was not drastically focused by the first pump pass. The remaining pump light that was not absorbed by the second pass was collimated by L₂ and transmitted by the polarizing cube after which it was dumped (which only amounted to ~5% of the incident pump power). The pump was designed to propagate as if it oscillated in a resonator when adding an additional pump mirror M₅ (with the exception that is coupled out to the diode after four passes). Another two pump passes could therefore be added if the absorption is insufficient after two passes.

A compact Z-shaped resonator of physical length 90 mm was built around the Tm:YLF slab crystal. A VBG mirror from OptiGrate was used as the resonator back-reflector. The VBG was designed to be HR at 1890.3 nm and had a clear aperture of 12 × 5 mm and a thickness of 8.4 mm. The VBG was mounted in a water-cooled mount to stabilize its temperature at 25°C. A R = 90% @ 1900 nm, r = 100 mm mirror (M₄) was used as the resonator output-coupler. The resonator was designed to be stable for a wide range of thermal lens dioptric powers, implying that the resonator could oscillate on either of the laser polarizations (which have opposite signs and different magnitudes of thermal lens dioptric powers). This is because large negative dioptric powers are associated with the π-polarization and small positive dioptric powers with the σ-polarization. During initial experiments spiking due to water absorption was found to be a problem at higher output powers, even with the wavelength stabilization. The entire setup was therefore enclosed in a box flushed with dry air. The output beam was also found to be strongly divergent (due to the curvature of the output-coupler) and was collimated with two spherical lenses of focal lengths f = 105 mm and f = 350 mm. The resonator optics, slab crystal, VBG mirror and collimating lenses are shown in Fig. 2 .

 

Fig. 2 Photo of the compact VBG stabilized resonator. The glowing crystal can be seen in the middle, the VBG in its water-cooled mount to the left and the output-coupler and collimating lenses to the right.

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3. Experimental results

It was confirmed that the laser operated at ~1890 nm with a monochromator (Thermo Jarrel Ash Model # 82-497). The laser output power was measured (Gentec model # UPN25N-100H-H9-DO) and is plotted in Fig. 3 (on the left axis) as a function of crystal incident pump power. The laser had a threshold power of 30 W, a maximum output power in excess of 80 W and a slope efficiency of 37%. The slope efficiency and threshold were comparable to previously demonstrated values of end-pumped Tm:YLF slab lasers [4,13].

 

Fig. 3 1890 nm output power as a function of incident 792 nm diode pump power.

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The divergence of the laser increased significantly from low to high power. The slight roll-off in measured output power at maximum pump power was later confirmed to be due to slight clipping of the divergent beam on the power meter. The oscillator was found to be horizontally polarized over its entire output range and therefore operated on the π-polarization which had a strong negative thermal lens. From previous results [5], we are confident that this output power can still be significantly scaled by increasing the amount of diode pump power and adding a second crystal. The total percentage of 792 nm diode pump power that is absorbed after both pump passes is also plotted in Fig. 3 on the right axis. It shows that the total absorption increased marginally from 92 to 95% as the pump power was increased. The high absorption over the entire range of pump powers led to the decision not to add third and fourth pump passes.

The long term laser output power was measured over the course of roughly half an hour and is presented in Fig. 4 . The output power decreased slightly over time to ~77.5 W, and was due to the system thermally heating up. This was confirmed by the fact that the output power again reached levels higher than 80 W once the laser was allowed to cool down.

 

Fig. 4 Long term output power stability of the VBG wavelength selected, Tm:YLF laser.

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The output beam at maximum output power was spatially characterized by scanning it with a knife edge in both transverse directions through the focus of an f = 105 mm spherical lens. The results are given in Fig. 5 along with a two-dimensional intensity beam profile recorded with a pyro-electric camera (Pyrocam III from Spiricon). The beam was clearly astigmatic with a high divergence and elongation in the horizontal direction as well as a high M² value of ~182 (calculated using ISO 11146). In the vertical direction the beam had a lower divergence and much lower M² value of ~2.5, which was due to the laser resonator mode better matching the pump beam size in this direction. The output beam properties are similar to previous results from a two diode, double pumped system [13].

 

Fig. 5 Propagation of the 1890 nm output beam in both transverse directions after an f = 105 mm spherical lens and an intensity beam profile at maximum output power (inset).

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4. Conclusion

We have demonstrated a Tm:YLF laser with an output power in excess of 80 W, which was wavelength selected to 1890 nm with a Volume Bragg Grating mirror. This laser is ideal to be used as pump laser for a Ho:YLF system which has an absorption peak at 1890 nm. The architecture can be scaled to higher output powers, which is the subject of future work. This includes adding another slab crystal and diode-stack.