We report on a novel amplifier configuration concept for a 10 kW laser system using a zig-zag optical path based on a cryogenic Yb:YAG Total-Reflection Active-Mirror (TRAM) laser. The laser material is a compact composite ceramic, in which three Yb:YAG TRAMs are combined in series to increase the output power. Output powers of up to 214 W with a slope efficiency of 63% have been demonstrated for CW operation, even at a quite low pump intensity of less than 170 W/cm2. Further scaling could achieve output powers of more than 10 kW.
©2011 Optical Society of America
Diode-pumped solid-state lasers with high average powers of more than 10 kW are interesting candidates for novel industrial applications, such as material processing. Nowadays 16 kW CW and 400 W Q-switched Yb:YAG thin-disk lasers are commercially available . 50 kW multi-mode and 10 kW single-mode fiber lasers were developed . In addition, a laser system with 105 kW in CW has been demonstrated by a coherent beam combining technology with seven Nd:YAG slab amplifiers .
The thin-disk laser concept is one of the most promising designs for high average power lasers. As the disk is thinner than 200 μm, a temperature rise of the disk is small. With a high diameter-thickness aspect ratio and a uni-directional heat flow along the disk axis, the radial temperature gradient is ideally negligible, reducing the wavefront distortion dramatically. Also, power scaling can be achieved by enlarging the beam aperture size , but is ultimately limited by Amplified Spontaneous Emission (ASE) effects. The complicated optics for the multi-pass pump is, however, indispensable to absorb the pump power efficiently. The necessary applied high-reflection (HR) coating on the back side of the thin-disk introduces an additional thermal insulation, which can increase the experienced thermal stress within the gain medium.
To confront these problems, we applied two approaches . One was to use an Yb:YAG gain medium at low temperature. Thermal properties of Yb:YAG (such as thermal conductivity k, thermal expansion (1/L) dL/dT, and the thermo-optic effect dn/dT) are considerably improved at low temperature [6–8]. These improvements allow the use of a thicker Yb:YAG disk to absorb most of the pump power using simple pumping optics. An additional advantage is the four-level laser system at low temperature compared to the quasi-three-level system at room temperature. Also, the emission cross section increases at low temperature. A high laser gain is therefore obtainable even at a lower pump intensity [9,10]. By using a cryogenic Yb:YAG, high average power operation has been extensively researched [11–14]. The other approach was to use a Total-Reflection Active-Mirror (TRAM) configuration . The TRAM uses total reflection on the bottom surface instead of the HR-coating. The laser gain material can be directly cooled by a coolant without additional temperature rise at a HR-coating. In addition, a spatial separation of input and output surfaces improves the optically induced damage threshold in pulse operation by reducing a locally increased electric field due to optical interference.
In our previous work on the cryogenic Yb:YAG TRAM , the laser oscillator generated a high output power of 273 W with an optical efficiency of 65% and a slope efficiency of 72%. Enlarging the laser aperture can increase the output power, similar to the thin-disk laser. Another way for power-scaling is to increase the pump power (pump intensity). These two ways are finally limited by ASE loss and the temperature rise of the active layer of the TRAM. For further power-scaling, serial amplification with multiple TRAM units is reasonable.
In this paper, based on the multiple TRAMs concept as a laser amplifier, we demonstrate a laser oscillator. A long, monolithic Yb:YAG/YAG composite ceramics, combining three TRAMs in series, is used. Both pump and laser beams propagate along a zig-zag optical path in the ceramics, consequently the composite ceramics is called ZiZa-AM (Zig-Zag Active-Mirror). Although our laser diode power limited the pump power to 470 W, a high output power of 214 W and a high optical efficiency of 50% are obtained.
2. Zig-Zag Active-Mirror for 10 kW
Figure 1 shows a ZiZa-AM configuration to achieve a 10 kW output power, which was fabricated by Konoshima Chemical Co., Ltd. The ZiZa-AM is a composite ceramics with an elongated trapezoidal YAG prism and three Yb:YAG layers. Top and bottom faces are 68 x 34 and 85 x 34 mm2, respectively. The height of the prism is 14.7 mm. Two slope surfaces at a skew-angle of 60 degrees were antireflection (AR) coated at the laser wavelength of 1030 nm. Three 9.8 at.% Yb:YAG layers were attached to the YAG prism, each with cross section of 34 x 34 mm2. The pump and laser beams are incident at right angles on the slope of the YAG prism, and reflected at the outer surfaces of the Yb:YAG layers, forming a zig-zag optical path. In the ZiZa-AM, the thickness of the layers were designed to satisfy the following requirements: (1) each temperature rise is almost same for all layers and is less than 15 K at the total absorbed pump power of 15 kW, (2) more than 95% of the pump beam can be totally absorbed. The absorption can be estimated by the equation of exp(−2αd/cosθ), where α is the absorption coefficient, d is the Yb:YAG thickness, and θ is the slope angle. The temperature rise in a Yb:YAG layer along the heat flow was roughly evaluated by the equation of ΔT = ηI abs d/k, where η is the rate of the generated heat power against the absorbed pump power, I abs is the absorbed pump intensity, and k is the thermal conductivity. We used the following parameters in the calculations: α = 12.7 cm−1, θ = 60 degrees, η = 10%, and k = 20 W/mK. The pump beam diameter is 15 mm.
In Table 1 , thickness and pump condition for each layer are summarized for a 10 kW laser system. Yb1, Yb2 and Yb3 denote the first, second and third Yb:YAG layers from the incident slope face, respectively. There are small differences in the corresponding layer thickness compared the designed and manufactured ZiZa-AM. In this design, a gain loss due to the ASE was not considered because of the limited doping concentration of 9.8 at.%. To suppress the ASE loss, optimization of doping concentration and/or absorbing cladding (e.g. Cr4+:YAG) would be necessary.
3. Experimental and discussion
Figure 2 shows the experimental setup for the ZiZa-AM laser oscillator. A photograph of the manufactured ZiZa-AM sample is also shown in Fig. 2. The sample was put into a liquid-nitrogen cryostat equipped with two AR-coated windows. Indium wires were used as sealant between the liquid nitrogen and vacuum. A 500 W, 940 nm fiber-coupled laser diode (LD) was used as pump source. Both pump and laser beams are totally reflected on the bottom faces of Yb:YAG layers. They reveal elliptical dimension onto Yb:YAG layers due to the 60 degrees angle of incidence. The pumping beam was focused onto the central Yb:YAG layer (Yb2) and the pump beam diameters on Yb1, Yb2 and Yb3 were approximately 9 mm, 8 mm and 9 mm, respectively. The laser cavity was V-shaped with a flat dichroic mirror (DM), flat output coupler (OC) and a lens. The focal length of the lens was f = 1000 mm. To estimate the laser gain, several output couplers with the reflectivity of 93.6, 84.6, 72.0, 63.5, and 42.5% were used. From a transmission experiment, the total absorption of incident pump power was evaluated to be about 94.8%. By considering the transmittance of optics, the total absorbed pump power of this ZiZa-AM can be estimated to be 431 W.
The output power P out as a function of absorbed pump power P abs is shown in Fig. 3 . The spatial mode of the output was multi-mode with a circular profile. Maximum output power of 214 W was obtained using an OC reflectivity of 72%. The optical efficiency and slope efficiency against the absorbed pump power are 49.7% and 62.7%, respectively. These results are lower compared with our previous experimental results with a single TRAM laser. This is because the absorbed pump intensity is quite low. The absorbed pump powers for each Yb:YAG layer are estimated to be about 181, 170, and 80 W. The absorbed pump intensities are evaluated respectively to be 142, 169 and 63 W/cm2. The optimum slope efficiency can be found experimentally by varying the output coupler reflectivities over wide range. This is done with a set of different output couplers. However, as the cavity length is approximately 1.5 times longer than in our previous experiments, diffraction losses can be accounted for a reduction of slope efficiency.
The laser output power shows a linear increase even at the maximum pump power, which is limited by the laser diode. Therefore, the temperature rise of Yb:YAG layers is estimated to be low. Both the laser power and the optical efficiency could be improved with a further increase in pump power. Using this ZiZa-AM, one can expect that the maximum pump power can reach 15 kW with a pump beam diameter of 15 mm, as shown in Table 1. In Fig. 3, the lasing threshold is also shown as a function of the mirror reflectivity to evaluate the laser gain and the resonator loss based on the Findlay and Clay method . This measurement reveals a loss in the resonator of δ = 3.6%, and the slope of the fitting line of 0.004. Thus, we can obtain the relation 2g 0 l = 0.004 x P abs (W−1). The small signal gain G = exp(g 0 l) can be estimated to be 2.36 when P abs is 431 W.
To verify the obtained gain of the ZiZa-AM amplifier, we performed direct measurements of the Small Signal Gain (SSG), shown in Fig. 4 . We used a 1 W CW, linearly polarized single-mode fiber laser as a seed source (1029.4-nm center wavelength, 0.2 nm FWHM). A dichroic mirror DM1 (R max@1030 nm, T max@940 nm) between the cryostat and the focusing optics combines the seed and pump beams. After amplification, DM2 and DM3 were used to separate the amplified laser beam from transmitted pump beam, and the output power was measured using a power meter. In Fig. 5 , the obtained SSG is shown as a function of the absorbed pump power together with calculations. The circles and red dotted line represent the experimental and theoretical results, respectively.
In the calculation at 77 K, an emission cross section of σ emi = 1.3 x 10−19 cm2, a fluorescence lifetime of τ f = 1 ms, and an absorption coefficient of α = 12.7 cm−1 were used. A detailed discussion of the gain can be found in Ref . As seen in Fig. 5, there is a good agreement between the experimental results and the calculations when P abs < 200 W. This shows that the temperatures of Yb:YAG layers do not rise so high to transform to a quasi-three level laser system. By using the inclination of the dotted line in Fig. 3(b), the SSG at P abs = 200 W is evaluated to be 1.49 (g 0 l = 0.4), which is consistent with Fig. 5.
However, there is a discrepancy between the experimental results and the calculation for P abs > 200 W. The temperature rise in the Yb:YAG layers can be considered small, as the layers are thin and the absorbed pump intensity in each of the layers is low. Therefore, we attribute the observed reduced SSG values to ASE and parasitic oscillations in radial direction on the Yb layer, rather than to thermal effects. In fact, the ratio between the ASE length l ASE = D/cosθ and the thickness of the Yb layer d is very large, where D is the pump beam diameter and θ is the angle of incidence. Then the ASE gain g 0 l ASE for each layer at P abs = 200 W can be calculated to be 7.28 (Yb1), 4.02 (Yb2), and 1.12 (Yb3), although the spatially averaged gain along the laser pass g 0 l is 0.16, 0.19, and 0.07, respectively. For simplicity we assumed the gain of Yb1 to be constant for P abs > 200 W, as indicated by the blue dashed line in Fig. 5, as it is the highest value of all the g 0 l ASE. Using this approximation, we obtained a reasonably good agreement with the experiments. The calculations will be more close to the experimental results by investigating the ASE condition in detail and considering the gain suppression within the other layers.
To investigate the thermal effect on the beam profile, such as thermal lensing and thermal birefringence of the ZiZa-AM laser, we measured the spatial beam quality and the degree of polarization (DOP) at maximum pump power. Figure 6(a) shows the near-field (NFP) and far-field patterns (FFP) of the amplified laser beam together with M 2-fit data in the x and y transverse dimensions at P abs = 400 W. NFP was observed by relaying the beam image on the DM2 to a charge-coupled device. The spatial beam propagation factor of the laser was characterized by the beam propagation method. The beam was focused using a 500-mm lens, and the beam radii were measured around the waist using a delay stage. Then, beam propagation factor was evaluated by fitting a hyperbola to the measured data using the least squares method. The beam diameters (1/e 2) of NFP and FFP were about 5.0 mm and 180 μm, respectively. A typical fit of beam diameter data to a calculated hyperbola with M 2 = 1.0 is also shown in Fig. 6(b).
As can be seen in Fig. 6(b), hyperbolic fits were quite good for measuring the M 2 factor in both directions when P abs = 400 W although the ZiZa-AM laser oscillator generated multi-transverse mode. Inasmuch as the position of the beam waist at P abs = 400 W was nearly the same as that with no pumping action, the thermal lens effect is negligible.
The DOP was measured using a λ/2 plate and a Glan laser polarizer. The DOP is determined by DOP = P t / (P t + P r) relation, where P t and P r are transmitted and reflected output powers from the polarizer, respectively. Figure 7 shows the DOP as a function of absorbed pump power. As shown in Fig. 7, the DOP of the ZiZa-AM laser maintains the linear polarization state with increasing absorbed pump power. At P abs = 400 W a maximum DOP of ~98% has been measured. Therefore, thermally induced birefringence in ZiZa-AM can be neglected.
In the beam profile and polarization measurements, we found that the thermal effects in the ZiZa-AM are very low. This result suggests that the ZiZa-AM laser is a compact high average power amplifier possessing a good beam quality at several hundred watt of output power. In the case of an amplifier, however, the effect of ASE and parasitic lasing in radial direction on the Yb:YAG is a key issue. For higher power operation, it is necessary to avoid this impact.
To suppress the parasitic lasing and minimize ASE while maintaining a larger aperture, a lower gain coefficient (lower doping concentration) is required. In our future efforts to develop a more advanced laser source, we will optimize not only the thickness but also the Yb concentration of each Yb:YAG disk. The maximum amplified power can be further increased by optimizing the amplifier specifications, as enlarging the size and increasing the number of disks inside the ZiZa-AM for symmetrical pumping from both sides. A conceptual design about this is shown in Fig. 8 . The entire surface of the Yb:YAG disks (6.8 x 6.8 cm2) is pumped, therefore, the ASE length is the diagonal size of the disk (l ASE = 9.6 cm). For example, the total pump power of about 40 kW from both ends of the slab may be possible with low ASE gain (g 0 l ASE < 3) when the doping concentration and thickness of Yb1 are 0.3 at.% and 2.6 mm. We expect that for an amplifier system with more than 10 kW output power, a high efficiency and a good beam quality will be achieved out of this ZiZa-AM laser concept.
In conclusion, we presented a multiple-TRAM laser using a zig-zag optical path in a monolithic composite ceramic, called “ZiZa-AM”-laser. Using this concept as a CW oscillator, we demonstrated 214 W output power with a 50% optical efficiency and 62% slope efficiency, although the pump intensities were quite low. We have estimated that by using a more powerful pump source and sufficient cooling, a maximum output power of 10 kW would be possible. As a cooling method we consider a liquid-nitrogen re-circulating flow system.
We studied the small signal gain both experimentally and theoretically. Reasonably good agreement between theoretical predictions and experimental results was obtained. The amplified beam profile showed a very good beam quality. The thermal lensing and thermal birefringence effects could be neglected under present experimental conditions. We believe that over a 10 kW amplifier system with high efficiency and good beam quality will be also achieved from this ZiZa-AM laser concept.
The authors wish to express their appreciation and thank to Dr. Daniel Albach and Dr. Haik Chosrowjan for reading and valuable comment for the manuscript. They also wish to thank to Dr. Shinji Motokoshi for coating the laser material.
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