Techniques for wavefront improvement in an end-pumped Nd:YAG zigzag slab laser amplifier were proposed and demonstrated experimentally. First, a study on the contact materials was conducted to improve the heat transfer between the slab and cooling blocks and to increase the cooling uniformity. Among many attempts, only the use of silicon oil showed an improvement in the wavefront. Thus, the appropriate silicone oil was applied to the amplifier as a contact material. In addition, the wavefront compensation method using a glass rod array was also applied to the amplifier. A very low wavefront distortion was obtained through the use of a silicone-oil contact and glass rod array. The variance of the optical path difference for the entire beam height was 3.87 μm at a pump power of 10.6 kW, and that for the 80% section was 1.69 μm. The output power from the oscillator was 3.88 kW, which means the maximum output extracted from the amplifier at a pump power of 10.6 kW.
© 2017 Optical Society of America
Zigzag slab lasers have been widely used to obtain high beam qualities in high-power solid-state lasers [1–19]. In zigzag slab geometry, the laser beam propagates along the zigzag path bouncing the slab of the gain medium. It averages over the thermal gradient so that a high-quality beam can be consequently achieved . Zigzag propagation occurs along the width of the slab. Thus, the wavefront distortion for the width axis of the amplified beam is very low. Along the height axis of the slab, however, the beam propagates in a straightforward manner. Therefore, the wavefront distortion occurs according to the temperature distribution of the slab height axis. For a high-power operation of over kW, a large-size slab of gain medium with a high aspect ratio is required to increase the output power while maintaining high cooling efficiency. Thus, it is very difficult to obtain a uniform temperature distribution along the height axis. Nevertheless, some researchers have shown quite good results by overcoming this problem with their own techniques in a high-power operation of more than kW [2–11].
Although the zigzag slab laser itself may show a low wavefront distortion, it is difficult to obtain a high beam quality close to the diffraction limited beam. Therefore, adoptive optical (AO) systems with deformable mirrors have usually been applied to achieve such a high beam quality [6–11,20–24]. Conventional AO systems include Shack-Hartmann sensors for measuring the wavefront distortion. However, the amount of distortion is easy to exceed the sensing range in the application of high-power zigzag slab lasers. Thus, some researchers developed wavefront sensor-less AO systems with stochastic parallel gradient descent (SPGD) optimization algorithms [22–24]. In addition, a double-deformable-mirror AO system was also applied to increase the controllable range of wavefront compensation . However, these methods do not solve the fundamental problems. If an improvement is expected through the use of the AO system, the wavefront distortion of the zigzag slab laser itself should be low enough to fall within the controllable range. Otherwise, the wavefront will deteriorate as expected. For this reason, there has been an attempt to minimize the wavefront distortion by optimizing the designs of the pump beam delivering optical system and the slab structure [25,26]. In a real situation, too many other factors exist that can cause wavefront distortion in addition to design errors of the optical system.
First, non-uniform cooling is the main cause of wavefront distortion. A typical diode-pumped zigzag slab laser uses a conduction cooling method with water-cooled micro-channel cooling blocks. A micro-channel cooling block not only has a high heat transfer coefficient to effectively cool the gain medium but also uniformly transfer heat over a large cooling area. The main cause of the cooling non-uniformity is the imperfect thermal contact between the gain medium and the cooling block. Many researchers may have developed technology to worry about this problem and increase the degree of thermal contact. However, no published methods improving the thermal contact have yet been found. In the case of using an indium sheet as a contact material, which is commonly used for improving the thermal contact, uniform cooling was not expected, and the reflectance to the laser beam was high, possibly causing parasitic oscillation. Therefore, in this work, a new method to improve the thermal contact was proposed, and the effect of this method was experimentally demonstrated in an end-pumped Nd:YAG slab laser.
In addition to non-uniform cooling, wavefront distortion occurs owing to many other factors, as mentioned above. For example, wavefront distortion owing to non-uniform pump power distribution generated in the laser diode module itself, which cannot be compensated by the optical design, also occurs. In addition, non-uniform pump beam absorption owing to inhomogeneous doping in the gain medium may also occur. An optical misalignment may also increase the amount of wavefront distortion. To remove these elements and acquire a high beam quality without an additional compensation, all of the components should be ideally fabricated to achieve a perfect performance, and no misalignments are allowed. It is practically impossible to satisfy such ideal conditions. G. D. Goodno et al. proposed a wavefront compensating method with additional pump laser diode modules generating spatially modulating power coupled into the gain medium by active control of each emitter . However, this technique is too complex to be introduced into a high power laser system including many optical components such as mirrors and image relays. Thus, in this work, a new wavefront compensation technique using a glass rod array was proposed and applied to an end-pumped Nd:YAG slab laser. This technique was very simple and did not take up much space, which is advantageous for application to complex high-power laser systems. Using this technique, it was demonstrated that the wavefront of the laser output beam can be effectively improved in the experiment.
2. End-pumped Nd:YAG zigzag slab laser amplifier
Figure 1 shows the conceptual scheme of the end-pumped Nd:YAG zigzag slab laser amplifier used in the experiment. This laser amplifier was composed using the pump sources, the pump light delivering optical system, and the gain medium with cooling blocks. The entire system was designed to accommodate a slab with a size of 2.5 mm (width) × 30 mm (height) × 120 mm (length). Detailed setups of each assembly will be described in the following paragraphs.
Four stacked laser diode (LD) modules with a wavelength of 805 nm were used as the pump sources. Two LD modules were located on the left side of the slab, and the others are located on the right side. The LDs on each side were assembled at a right angle and located at upper and lower positions. The pump lights from the two LDs were combined by a 45 degree polarizing beam splitter while rotating the polarization of the upper LD using an attached half wave plate. Each stacked LD module consists of 35 LD bars with 100 W output. For the fast axis of the LD emitter, the light had a large divergence angle of ~28 degrees. It was initially collimated by an attached micro-lens so that the light traveled in parallel. For the slow axis of the LD emitter, the light propagated with a relatively small divergence angle of less than 5 degrees. Therefore, the pump light was operated without collimating micro-lenses for the slow axis. The maximum power of each stacked LD module was 3.5 kW. Thus, the total available pump power for all LD modules was 14 kW.
The pump light of each side was delivered by two cylindrical shaped lenses into the slab. One was a fast-axis lens and the other was a slow-axis lens. An acylindrical lens with a focal length of 180 mm was used as a fast-axis lens, which was designed for the collimated pump light to be focused well within the width of the slab entrance. A cylindrical lens with an 18-mm radius of curvature, which was designed for the pump light to fill the height of the slab entrance uniformly, was used as a slow-axis lens. The position of the slow-axis lens was adjusted for the image plane to be located outside of the slab before the entrance so that the pattern of the emitter arrays was sufficiently blurred at the slab entrance. If the image plane was located in the slab and the periodical pattern of the emitter arrays was relayed, serious wavefront distortion took place and lowered the beam quality by spatially fluctuating pump light absorption along the height of the gain medium. To construct a compact amplifier, two 45 degree mirrors with high reflectivity at 805 nm were used for deflection of the pump light.
In the laser operation, the seed beam was incident on the slab and amplified with a zigzag propagation. The laser slab was sandwiched by cooling blocks and located at the center of the amplifier. As mentioned above, the size of the slab was 2.5 mm (width) × 30 mm (height) × 120 mm (length). Nd:YAG as the gain medium was located at the center of the slab with a doping rate of 0.15 at.%. At both ends of the 100-mm long Nd:YAG slab, 10-mm long pure YAGs were diffusion-bonded for absorbed heat dissipation. The entrance and exit of the slab were cut by 45 degrees and anti-reflection coated at 1064 nm. Both sides of the slab were anti-reflection coated at 805 nm as well as total internal reflection (TIR) coated for zigzag propagation. The water-cooled cooling blocks had 25-μm micro-channels with a cooling area of 33 mm (height) × 113 mm (length), which is a little bit larger than the size of the gain medium. The thermal resistance of the cooling surface was 0.03 K∙cm2/W for a water flux of 1 L/min/cm2. The cooling surface was polished with a high flatness of less than 2 μm for a fine thermal contact between the slab and cooling blocks, and blackened with an arithmetic roughness of 0.05 μm by CuO to avoid a parasitic oscillation. In addition to cooling the slab, the cooling blocks served as beam dumpers for the pump light that did not enter the slab.
3. Performance of the amplifier
A multimode oscillator was constructed to evaluate the initial performance of the end-pumped Nd:YAG zigzag slab laser amplifier. And the output power from the oscillator was measured, which means the maximum power of that can be extracted from the amplifier. In addition, a single-pass wavefront distortion at the corresponding output power was also obtained by observing the interference pattern using a He-Ne laser with a wavelength of 632.8 nm between the reference beam and the beam passing through the slab with a zigzag propagation.
Figure 2 shows the experimental setup of the laser oscillator for a performance test containing wavefront measuring components. The resonator consisted of a rear mirror and an output coupler. The rear mirror was a concave mirror with a radius of curvature of 500 mm, and the output coupler was a plane mirror with a reflectivity of 50%. Both the rear mirror and the output coupler were aligned so that the laser beam incident on the slab and the length axis of the slab had an angle of 22 degrees for the total reflection. In this case, the laser beam was bounced about 30 times inside the slab as a single pass. An 45-degree 1064-nm mirror (M3) was used for deflection of the oscillating beam onto the rear mirror so that the He-Ne laser beam effectively entered the slab without additional distortion passing through the rear mirror with a spherical surface. In addition, an additional 1064 nm mirror (M4) was applied to reflect the output beam from the oscillator and to measure the output power by a power meter (PM) without blocking the He-Ne laser beam. The total optical length of the resonator was set to less than 500 mm for a stable laser oscillator.
The He-Ne laser beam used for the measurement of wavefront distortion was initially expanded by a combination of a spherical lens and a cylindrical lens, in order to fill a slightly larger area that the slab entrance area. It was split into a reference beam and a slab-transmitted beam by a beam splitter (BS1). The reference beam was reflected by a silver mirror (SM2). The slab-transmitted beam passed the same path as the oscillating laser beam after reflection at a sliver mirror (SM1). The slab-transmitted beam met the reference beam after reflection at another beam splitter (BS2). Finally, the interference pattern was generated and the CCD camera measured the size-reduced pattern through the image relay with the spherical lenses (L1 and L2). The wavefront distortion was quantitatively measured by observing the distortion of the interference pattern. Moreover, the increase in the optical path length in the slab corresponding to the amount of absorbed heat was measured by counting the number of movements of the interference pattern. In this way, the cooling performances for various conditions were able to be compared with each other.
Figure 3 shows the experimental results on the initial performance of the amplifier. Figure 3(a) shows the output power from the oscillator against the pump power. The output power was measured to be 1.93 kW at 7.0 kW pump power, and the optical-to-optical efficiency was 27.6%. The slope efficiency was 38.7% up to a pump power of 5.2 kW, but decreased to 23.0% when the pump power was increased to 7.0 kW. This was due to the fact that the degree of wavefront distortion that occurred when passing through the slab increased sharply as the pump power exceeded 5.2 kW. Because the wavefront distortion caused a loss in the resonator, the increase in output power was reduced. As a result, a corresponding higher output power could not be expected even with more pump power of over 7.0 kW.
Figures 3(b) and 3(c) show the results of wavefront distortion at a pump power of 7.0 kW. Figure 3(b) shows a He-Ne interference pattern and 3(c) shows the corresponding optical path difference (OPD) along the beam height axis. For convenience, the OPD value at the point having the minimum optical path length (OPL) was set to zero. The ΔOPD value, which means the difference between the maximum and minimum values of OPD, was measured for the entire beam height and for the 80% section located at the center of the beam height except for the edge portion showing a large distortion. The ΔOPD for the entire beam height was 6.45 μm, and that for the 80% section was 6.29 μm. The wavefront for the edge of the beam height can be improved by adjusting the position of the slow-axis lens to change the pumping condition. However, the wavefront of the center did not change significantly for the conditions of the pump light delivering optical system.
4. Techniques for improving the wavefront
4.1 Improvement of the thermal contact
In the experiment described in Section 3, the wavefront distortion of the output beam was quite large, as shown in Fig. 3(c). This was mainly caused by the imperfect contact between the slab and cooling blocks. Although the surfaces of the cooling blocks were polished well and showed good roughness, an air gap still existed that obstructed the heat transfer and generated non-uniform thermal distribution in the slab. Therefore, an appropriate contact material was required to fill in the air gap to improve the heat transfer. For this reason, there have been many attempts to improve the thermal contact using thermal paste, ceramic resin, and other materials. However, the wavefront distortion was worse for most attempts than the simple contact. The only method showing improvement in the wavefront was the use of silicone oil. Silicone oil has a lower thermal conductivity of 0.15 W/m∙K compared with thermal paste, but can fill the air gaps with a very thin thickness of less than 1 μm. As a result, it showed the best heat transfer.
Table 1 lists the test results when in contact with various contact materials. ΔOPD indicates the difference between the maximum and minimum values of OPD for the entire beam height, and ΔOPL is the amount of optical path length variation by counting the number of movements of the interference pattern when the pump power was increased from zero to the corresponding value. In other words, ΔOPD indicates how uniform the thermal contact was, and ΔOPL indicates how well the heat transfer was. Different slabs and cooling blocks were used for each case. In Case 1, some types of resins, thermal pastes, and graphene sheet with resins were tested. For all contact materials in Case 1, the ΔOPDs were much larger than the simple contact and more wavefront distortion occurred. Thus, the ΔOPLs were not measured unnecessarily. In Case 2, a silicone oil (KF-96-30CS, Shin-Etsu Chemical Co., Ltd.) was used as a contact material. The ΔOPDs at pump powers of both 5.2 kW and 7.0 kW were lower than the simple contact. In addition, the ΔOPLs were also lower than the simple contact. Consequently, both more uniform cooling and better heat transfer were achieved, resulting in an improvement of the wavefront. In Case 3, another silicone oil (HIVAC-F-4, Shin-Etsu Chemical Co., Ltd.) was used as a contact material. The result also shows that both the ΔOPDs and the ΔOPLs were lower than the simple contact, resulting in an improvement of the wavefront.
As the effect of improving the wavefront of the silicon oil was experimentally proved, the silicone oil of Product No. HIVAC-F-4, which was suitable in terms of viscosity and heat resistance, was finally applied to the amplifier for contact between the slab and cooling block. Figure 4 shows the results of the wavefront distortion at a pump power of 7.0 kW. Figures 4(a) and 4(b) show the He-Ne interference patterns when the slab and cooling blocks were in simple contact and in contact with the silicone oil, respectively. The improvement of the wavefront was easily determined by comparing the interference patterns with each other, when applying the silicone oil. In the simple contact case, as analyzed in Section 2, the ΔOPD for the entire beam height was 6.45 μm, and that for the 80% section was 6.29 μm. When silicone oil was applied, on the other hand, ΔOPD for the entire beam height was 3.76 μm and that for the 80% section was 2.41 μm.
4.2 Wavefront compensation by glass rod arrays
As shown in Section 4.1, to improve the contact between the slab and cooling blocks, silicone oil was used as the contact material to achieve a wavefront improvement. However, even when silicone oil was applied, the degree of wavefront distortion was still quite large. If the pump power was increased by more than 7.0 kW, the degree of wavefront distortion became more severe. The thermal contact was not able to be ideally perfect even if the silicone oil was applied. In addition, as mentioned in Section 1, the wavefront distortion was caused by various factors, not only non-uniform cooling due to incomplete contact, but also non-uniformity of the pump light source itself, and inhomogeneous doping in the gain medium, among other aspects. Therefore, an additional wavefront compensation method using a glass rod array was proposed.
Figure 5(a) shows the principle of the wavefront compensation technique using a glass rod array. In this method, a glass rod array was equipped in the pump light delivering optical system. It spread a part of the pump light and reduced the intensity of the pump light entering the slab for the area blocked by the glass rods. By properly adjusting the degree of blindness of each glass rod, the intensity of the pump light distribution for the slab height axis could be controlled. In addition, the temperature distribution originating from the absorbed power in the slab was able to be changed, and consequently the wavefront of the output beam could be controlled. Therefore, if the glass rod array was adjusted in the direction of reducing the wavefront distortion, it might be able to obtain a better wavefront of the output beam. For this reason, this technique was applied to the amplifier. Figure 5(b) shows the glass rod array installed in the amplifier. It was mounted at the residual pump light shield plate, which was used for blocking the transmitted pump light passing through the slab from the LDs on the other side. As a glass rod, a silica glass fiber of 0.8 mm in diameter was used after cutting into a suitable length.
Figure 6 shows the He-Ne interference patterns with the slab assembly in contact with the silicone and with wavefront compensation by the glass rod array at a pump power of 7.0, 8.8, and 10.6 kW. The improvement of the wavefront was easily determined by comparing the interference patterns with each other, when applying additional wavefront compensation by the glass rod array. The wavefront was flat except for a little ruggedness at a pump power of 7.0 kW when the glass rod array was applied. When the pump power was further increased, a more rugged wavefront occurred, but much better wavefront was obtained than with the silicone oil alone. Figure 7 shows the corresponding OPDs along the beam height axis with the slab assembly in contact with silicone oil, and using additional wavefront compensation by the glass rod array at various pump powers. When using the silicone oil without application of the glass rod array, the ΔOPDs for the entire beam height were 3.76, 4.60, and 6.12 μm at 7.0, 8.8, and 10.6 kW pump powers, respectively. In addition, those for the 80% section were 2.41 μ, 3.04, and 4.06 μm at each pump power. With application of the glass rod array, the ΔOPDs for the entire beam height was 0.993, 2.52, and 3.87 μm at a pump power of 7.0, 8.8, and 10.6 kW, respectively. In addition, those for the 80% section were 0.537, 0.873, and 1.69 μm at each pump power.
The output powers from the oscillator against the pump power were measured, as shown in Fig. 8, with the slab assemblies in simple contact, and in contact with silicone oil, and with the slab assembly in contact with silicone oil when applying additional wavefront compensation by the glass rod array. With the silicone-oil contact, the output power increased almost linearly with a slope efficiency of 43.3%, while the increase in output power was reduced when the pump power exceeded 5.2 kW in the case of a simple contact. This was because the wavefront was improved by the silicone-oil contact so that a large amount of oscillating laser beam was survived in the resonator. At a pump power of 10.6 kW, the measured output power was 4.00 kW corresponding to an optical-to-optical efficiency of 37.8%. When applying an additional wavefront compensation technique by the glass array, the measured output power was reduced to 3.88 kW at a pump power of 10.6 kW, corresponding to an optical-to-optical efficiency of 36.6%. This was due to the reduction of the pump power entering into the slab. Nevertheless, it corresponded to 96.9% of the use of the silicone oil without a glass rod array, and the output loss by the glass rod was only 3.1%.
In conclusion, the techniques for improvement of the wavefront distortion in an end-pumped Nd:YAG zigzag slab laser amplifier were proposed. In addition, the multimode oscillator was constructed to evaluate the performance of the amplifier and demonstrate the effectiveness of these techniques. First, a study on the contact materials was conducted to improve the heat transfer between the slab and cooling blocks and to increase the cooling uniformity. In this study, many attempts were performed to improve the thermal contact using thermal paste, ceramic resin, and so on. Among them, the only method to show improvement in the wavefront was the use of silicone oil. Therefore, the appropriate silicone oil was applied as a contact material. In addition to the silicone-oil contact, the wavefront compensation method using a glass rod array was also proposed. With the use of the silicone-oil contact and glass rod array, the ΔOPD for the entire beam height was 3.87 μm at a pump power of 10.6 kW, and that for the 80% section was 1.69 μm. In this case, the measured output power from the oscillator was 3.88 kW, which means the maximum output extracted from the amplifier at a pump power of 10.6 kW. This work is meaningful as it demonstrated that the laser output with a high power of 3.88 kW and low wavefront distortion of a few micrometers can be obtained at the same time only with a simple device without a complex active control system.
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