1. Introduction

Corner-pumping is a new diode-pumped scheme for slab lasers first demonstrated by our research group [1–3]. It is different from traditional ones, such as edge-pumped, end-pumped and side-pumped configuration. The principle of corner-pumping is simple and practical. The slab is chamfered at one corner; therefore, one plane at the corner is formed, from which the pump light is coupled into the slab. The multi-pass absorption is realized by the total internal reflection of the pump light in the slab, which enhances the absorption path and benefits the extraction of the energy of the pump light and the rejection of heat of the laser medium. The corner-pumping method can offer a simple pump structure, a high pump efficiency and a uniform high pump power density. Furthermore, combined with the thermal-bonding technique, a composite slab with center doped and two sides undoped may result in a higher pump power density, a better pump uniformity and mode matching because the absorption region of the pump light is confined in the center area of the slab. In the corner-pumped configuration, the pump face and laser output face are separated to save the cost of dichroic mirror. In addition, the corner-pumped configuration has great potential for power scaling. By now, a maximum CW output power of 1050 W from a Yb:YAG laser has been reported [4]

Recently, solid-state lasers with low or medium output powers have broader application prospects. Corner-pumping, as a new diode-pumping method, not only has the merits of high pump efficiency and simple cooling for the slab crystal, which the end-pumped type also has [5–7] but also has the merits of high pump uniformity and easiness of enlarging the output power, which the side-pumped type also possesses [8–10]. Therefore, it is very important to carry out a study of high efficient solid-state lasers with low or medium output powers based on the corner-pumping method.

An efficient corner-pumped Nd:YAG/YAG composite slab CW 1064 nm laser with TEM₀₀ mode operation has been reported by our group. The maximum output power is 11.94W with an optical-to-optical conversion efficiency of 26% and the M² factors of beam quality at the width and thickness directions of the Nd:YAG/YAG composite slab are1.18 and 1.34, respectively [11]. We also have reported some theoretical study results, including the effects of the geometry parameters and doping content of the composite slab on the pump efficiency and pump uniformity, the effects of the diode temperature drift on the pumping characteristics, the coupling system of the corner-pumped type and the design of laser cavities [12–14].

Besides the well-known diode-pumped solid-state 1064 nm, 1319 nm or 946 nm lasers, efficient and compact diode-pumped solid-state lasers operating at 1.1 µm wavelength regions have also attracted increasing attention recently [15–23]. The lasers emitting at 1123 nm are applied in differential absorption lidar to allow remote monitoring of atmospheric water vapor concentration, and used as pump source for thulium upconversion fiber lasers to generate blue laser emission at 481 nm [16,17]. And the yellow-green laser at 561 nm from the frequency-doubled 1123 nm laser can play important roles in medical and dermatology applications, bio-fluorescence experiments, and holographic storage [18,19]. Besides, another yellow-green laser at 556 nm from the frequency-doubled 1112 nm laser is very close to the 555 nm laser that is the most sensitive wavelength to the human eyes [20].

A corner-pumped Nd:YAG/YAG composite slab CW 1.1 µm laser is presented and intracavity frequency doubling of this laser to the yellow-green spectral region using LBO is also demonstrated. The maximal CW output power from the 1.1 µm laser is up to 12.06 W with an optical-to-optical conversion efficiency of 24%. With a LBO crystal inserted in the cavity, a corner-pumped Nd:YAG/YAG LBO intracavity frequency doubling CW yellow-green laser with an output power of 841 mW is obtained. These results suggest that corner pumping is a feasible approach for diode-pumped solid-state 1.1 µm lasers with low or medium output powers.

2. Corner-pumped Nd:YAG/YAG composite slab CW 1.1 µm laser

First, a corner-pumped Nd:YAG/YAG composite slab CW laser operating at 1.1 µm is studied. A simple linear two-mirror cavity configuration used in the experiment is shown in Fig. 1 . The cavity length is 22 mm.

 

Fig. 1 Experimental setup of a corner-pumped Nd:YAG/YAG composite slab CW 1.1 µm laser.

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The detailed sizes of the composite slab are illustrated as Fig. 2 . The slab is a diffusion-bonded composite crystal of undoped YAG and 1.0 at.% Nd:YAG. The Nd:YAG crystal measures 1mm in thickness, 0.8 mm in width, and 14 mm in length. Two pieces of 1 mm × 3.5 mm × 14 mm undoped YAG are thermally bonded to the Nd:YAG crystal at each 1 mm × 14 mm face. After bonding, the slab is polished and one corner of the slab is chamfered with an angle of 45°; one 3 mm × 1 mm plane is formed, from which the pump light is incident into the slab. The two end surfaces of the Nd:YAG slab are well polished and coated with a coating antireflective at 1064 nm but not at 1.1 µm. Because the transition line at 1064nm is very close to that at 1.1 µm, the antireflective coating at the two end surfaces of the Nd:YAG slab also has a high enough transmission ratio at 1.1 µm. A coating with a high transmission ratio (T>99.8%) at 808 nm is coated on the incident plane of the Nd:YAG/YAG composite slab. Because the thermal-bonding technique has been mature in China, the cost of the Nd:YAG composite slab is only a little higher than that of the traditional Nd:YAG slab with the same size.

 

Fig. 2 Schematic of the sizes (in mm) of the composite slab.

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A CW LD bar with a maximum 808 nm output power of 50 W is used in the experiment. The rectangular chamfer of the slab is chosen to be 3 mm × 1 mm. For 10 mm × 0.7 mm of the light-emitting area of the LD bar and a certain angle of divergence of the pump light, an optical coupling system including three cylindrical lenses is employed to shape the pump light emitted from the LD Bar. One with focal length of 25 mm focusing the pump beam in the fast axis direction and the other two with focal length of 12.7 mm focusing the pumping beam in the slow axis direction are chosen according to the simulated results of ray-trace algorithm. Figure 3 shows the simulated facular shape of the pumping light on the incident plane of the slab crystal and its light distribution along the horizontal and vertical directions through the optical coupler, and the corresponding facular sizes are about 2.1 mm in width and 0.25 mm in thickness. In the corner-pumped configuration, the whole optical coupler is composed of only three lenses, which decreases the cost of the laser and makes the laser compact and easy operating.

 

Fig. 3 Facular shape of the pumping light on the incident plane of the slab crystal through the optical coupler.

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Considering that the strong laser oscillations at 1064 nm, 1319 nm and 946 nm must be suppressed in the cavity, special films at two mirrors are coated. The high reflector is a planar mirror with a high reflectivity coating at 1123 nm (R>99.8%) and high transmission coatings at 1064 nm (T>74%), 1319 nm (T>79%) and 946 nm (T>80%). The output coupler is also a planar mirror with a partial transmission at 1123 nm (T = 2%) and high transmission coatings at 1064 nm (T>73%), 1319 nm (T>34%) and 946 nm (T>80%). The experimental results show that the laser oscillations at 1064 nm, 1319 nm and 946 nm are all suppressed and multi-wavelength operation at 1.1 µm is obtained successfully. The relative performances of the three laser lines at 1112 nm, 1116nm and 1123nm and their stimulated emission cross sections are close to each other; therefore, the three laser lines easily oscillate simultaneously in the cavity under a certain pumping power. The simultaneous multi-wavelength laser operation is also observed during the experiment. In order to obtain a high output power of the CW 1.1 µm laser, the cavity length should be as short as possible to reduce the effect of the thermal lens on the laser mode under a high pumping power condition. The temperature of the pumping source is maintained at 25°C by water cooling. The larger surfaces of the composite slab are wrapped by two pieces of indium and cooled through thermal conduction by two heat sinks made of red copper. The temperature of the slab is kept at 13°C by water cooling.

The curve for the output power versus the pumping power is shown in Fig. 4 . The threshold pumping power is 4.2 W. When the pumping power is increased to 50.3 W, the output power goes up to 12.06 W with a slope efficiency of 27.12% and an optical-to-optical conversion efficiency of 24%. As is seen from Fig. 4, the output power is not saturated with the increase of the pumping power. If we continue increasing the pumping power, it is possible to obtain a higher output power and conversion efficiency. When the pumping power is 48.1 W, the short-term instability of the output power is measured and the result is shown in Fig. 5 . We record an output power every minute using a calibrated thermal power meter head from OPHIR NOVA II, and in a time period of fifteen minutes, the instability of the output power can be obtained from the following formula:

 

Fig. 4 Output power of the CW 1.1 µm laser versus pumping power

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Fig. 5 Short-term instability of the output power when the pumping power is 48.1 W

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During the experiment, the lasing lines at 1064 nm, 1319 nm and 946 nm are all observed to be successfully suppressed. Figure 6 shows the spectrum from 1060 nm to 1160 nm at an output power of 12.06 W. The three laser lines at 1112 nm, 1116 nm and 1123 nm oscillate simultaneously, and their spectral line widths are 0.101 nm, 0.089 nm and 0.081 nm, respectively, as are shown in Fig. 7 . Because of the absence of a certain etalon or other frequency selective component in the laser cavity, the laser is difficult to operate with a single wavelength output. We plan to insert a birefringent filter plate in the cavity and this should allow us to operate at any of the three wavelengths by rotating the birefringent filter plate.

 

Fig. 6 Spectrum from 1060 nm to 1160 nm at an output power of 12.06 W

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Fig. 7 Spectra of (a) 1112 nm; (b) 1116 nm; (c) 1123 nm laser lines

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The beam quality factor M² is measured by Spiricon M²-200 beam propagation analyzer. When the output power is 10.3 W, the measured M² factors of beam quality at width and thickness directions are 7.71 and 2.44, respectively, and the corresponding beam propagation behavior is shown in Fig. 8 . Because the simple linear short cavity is employed in the experiment, the laser operates with a multi-mode output at high pump powers.

 

Fig. 8 Propagation behavior of the output beam at an output power of 10.3 W

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3. Corner-pumped Nd:YAG/YAG composite slab LBO intracavity frequency doubling CW yellow-green laser

Based on the above results, we carry out the experiment of a corner-pumped Nd:YAG/YAG composite slab intracavity frequency doubling CW yellow-green laser achieved by type-I critical phase matching in a LBO crystal. The experimental arrangement of the system is illustrated in Fig. 9 . The cavity length is 45 mm. The pumping source, the optical couplers, and the Nd:YAG/YAG composite slab are not changed. A LBO crystal is used as the nonlinear crystal for intracavity frequency doubling due to its high damage threshold and low absorption at both the fundamental wave and second harmonic of the Nd:YAG laser operating at 1.1 µm. The LBO crystal is cut for type-I critical phase matching of second harmonic generation (SHG) ($\theta ={90}^0$,$\phi ={7.5}^0$) and its size is 3 mm × 3 mm × 15 mm. Both end faces of the LBO crystal are coated with high transmission coatings at 1123 nm and 561 nm. Because the permissible temperature of the LBO crystal is only 7.13 $K\cdot cm$, a temperature controller is used to maintain the temperature of the LBO crystal within an accuracy of up to ± 0.1°C. The reflecting mirror is a planar coupler with high reflection coatings at 1123 nm and 561 nm. The output mirror is also a planar coupler with a high transmission coating at 561 nm and a high reflection coating at 1123 nm.

 

Fig. 9 Experimental setup of a corner-pumped Nd:YAG/YAG LBO intracavity frequency doubling yellow-green laser

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The curve for the CW yellow-green laser output power versus the pumping power is shown in Fig. 10 . The threshold of the pumping power is 4.64 W, and when the pump power is increased to 38.3 W, the maximum output power of 841 mW of the yellow-green laser is obtained with an optical-to-optical conversion efficiency of 2.2%. Because of the absence of a certain frequency selective component in the cavity, it is difficult for the laser to operate at one wavelength only. Therefore, it is inevitable that the three wavelengths of 1112 nm, 1116 nm and 1123 nm oscillate simultaneously in the cavity. Simultaneous oscillations of the three fundamental wavelengths result in high competitions among their frequency doubling lasers and then make the output power of the yellow-green laser unstable. By inserting a certain frequency selective component, we expect that the instability of the output power of the yellow-green laser and the corresponding optical-to-optical conversion efficiency will be improved greatly.

 

Fig. 10 Output power of the CW yellow-green laser versus pumping power

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4. Conclusions

In summary, a novel pumping method for all-solid-state slab lasers with low or medium output powers are demonstrated. Compared with other pumping schemes, corner pumping has particular merits not only for high-average-power Yb:YAG/YAG slab lasers, but also for Nd:YAG/YAG slab lasers with low or medium output powers. A compact corner-pumped Nd:YAG/YAG composite slab CW 1.1 µm multi-wavelength laser is reported. When the cavity length is 22 mm, the maximal output power of 12.06 W is obtained with a slope efficiency of 27.12% and an optical-to-optical conversion efficiency of 24%. The corresponding spectral line widths of the 1112 nm laser, 1116 nm laser and 1123 nm laser are 0.101 nm, 0.089 nm and 0.081 nm, respectively. When the pumping power is 48.1 W, the short-term instability of the output power is better than 0.7%. When the laser output power is 10.3 W, the corresponding beam quality M² factors at width and thickness directions are 7.71 and 2.44, respectively. To the best of our knowledge, it is the highest CW output power in the reported 1.1 µm multi-wavelength lasers. Compared with other pumping methods, the corner-pumped type uses a LD bar to directly pump the slab crystal and adopts a simple optical coupler, which can reduce the cost of the laser effectively. If we compare this laser with other lasers under the output power and cost of the same level, this laser with the corner-pumped type is one of the most attractive lasers. Because others used two laser modules [22,23]or a ceramic laser crystal [17] in the laser to gain the high output power or high efficiency, the costs of their lasers are much higher than those of the laser we report. The optical-to-optical efficiency of the laser reported by C. Li et al. [22,23] is lower than that of the laser we report. Among the lasers with different pumping methods and the output powers of the same level, especially the low or medium output powers, the laser with the corner-pumped type will be very competitive due to its low cost and relatively high efficiency. Based on the above experiments, a corner-pumped Nd:YAG/YAG composite slab intracavity frequency doubling CW yellow-green laser is achieved by type-I critical phase matching in a LBO crystal. The highest CW output power of the yellow-green laser is 841 mW with an optical-to-optical conversion efficiency of 2.2%. Simultaneous oscillations of the three fundamental wavelengths result in high competitions among their frequency doubling lasers and then make the output power of the yellow-green laser unstable. In future work, by inserting a birefringent filter plate or an etalon in the cavity, the laser can operate at a single wavelength by adjusting the frequency selective component and the instability of the output power of the yellow-green laser and the corresponding frequency doubling conversion efficiency will be improved greatly.