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Abstract
A 253 J with 26 ns at 0.2 Hz laser performance was demonstrated using a LD pumped cryogenically cooled Yb:YAG ceramics laser amplifier. A high energy storage of 344 J was achieved with a stored energy density of 0.58 J/cm3 using a 1 kJ output multidirectional-pumping system. High energy-extraction efficiency of 56.5% was achieved with high energy fluence of 4.63 J /cm2. To the best of our knowledge, this is the highest output energy obtained with a repetitive nanosecond pulse by LD pumped solid-state laser. This paper presented a design of 1 kJ amplifier based on experimentally proven numerical data.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
The generation of a high-energy laser pulse with a single beam has been a major area of focus in laser development. Innovative high-energy laser pulses, of over 15 kJ with a pulse duration of a few 10 ns at 1053 nm, have been investigated at the Lawrence Livermore National Laboratory since 2009 [1]. Furthermore, many other high-energy pulsed lasers, based on flashlamp pumped Nd:glass lasers, have been used for research on laser fusion, neutron/gamma-ray generation, material science, and electron/ion acceleration [2,3]. Although many studies are expected to progress the development of high-energy lasers, the low repetition rate of flashlamp-pumped lasers is a significant limitation. A diode-pumped solid-state laser (DPSSL), replacing the flashlamp-pumped system, has been developed as a high-energy laser with a high pulse-repetition rate [4–8]. A laser pulse of 21.3 J at 10 Hz was demonstrated in 2008, and 102 J at 3.3 Hz was reported in 2017, using a laser diode (LD)-pumped Nd:glass laser [4,5]. In parallel with Nd:glass laser developments, LD-pumped Yb:YAG ceramic lasers with a 100 J class pulse energy are being rapidly developed. The first demonstration of a 100 J laser pulse with a 10 Hz repetition rate was achieved in 2016 with an LD-pumped cryogenically cooled Yb:YAG ceramics laser [6]. Other studies have demonstrated higher energies of 117 J, 153 J in 2020 [7,8] and higher average power of 1.5 kW in 2021 [9]. Cryogenic Yb:YAG ceramics have emerged as a candidate for laser amplifiers to achieve the high-repetition rate of 1 kJ class lasers, because of their capabilities of large size and good thermal properties [10,11]. Previous studies have reported the performance of 100 J class lasers, without quantitatively verifying the feasibility of the stored energy and stored energy density in high energy regions, required to realize an LD-pumped 1 kJ laser amplifier. This study aims to achieve a 1 kJ output LD-pumped cryogenically cooled Yb:YAG ceramics laser by investigating a novel quarter-scale laser amplifier. A 1 kJ amplifier is presented, based on key design parameters.
2. Experimental setup
When designing an LD-pumped solid-state laser amplifier, estimating the feasible range of input and output fluence, saturation fluence, and small-signal gain (SSG) are the most important parameters. These parameters strongly depend on the pumping characteristics of a laser medium and its temperature. In particular, for a 1 kJ class high-energy laser amplifier, the LD-pumping technology has technical difficulties, as a high-power and uniform irradiation are required over a large spatial area. It is necessary to precisely design the optical layout for more than 100,000 LD emitters, which have different divergences in the slow and fast axes. This study developed a compact LD module, including approximately 10,000 LD emitters. This LD module can irradiate the designed large area of 8 × 8 cm with an energy of 125 J and an intensity uniformity of over 80%. Typical spectral characteristics are 935 nm in central wavelength with 6 nm linewidth at operating condition with 1 ms pulse duration, 10 Hz repetition rate and 125 kW peak power. This compact LD module is suitable for multidirectional-pumping to achieve intensity homogenization and high-power irradiation, by superimposing the output patterns from the LD modules. Another notable feature is the scalability of the pump energy by increasing the number of LD modules and directions.
A schematic of a 250 J amplifier with multidirectional-pumping is shown in Fig. 1(a). This laser amplifier is a part of a 250 J laser system (HELIA: high energy laser for industrial application). A laser chamber with Yb:YAG ceramics (Konoshima Chemical Co., Ltd.) was irradiated by eight LD modules (Hamamatsu Photonics K.K.) from eight directions. LD modules are arranged with angle of incidence with 15 degree in horizontal and 12 degree in vertical direction. A seed pulse propagated between the upper four and lower four LD modules. A cross-sectional diagram is shown in Fig. 1(b). 10 Yb:YAG ceramics disks were installed in a pressure vessel located in the laser chamber. Helium gas flowed between all Yb:YAG ceramics. The mass-flow rate (MFR) of helium gas was 23 g/s with a pressure of 0.45 MPa and a temperature of 175 K. A vacuum chamber surrounded the pressure vessel as thermal isolation to prevent condensation. This laser amplifier realized a pump energy over 1 kJ with the 10 Yb:YAG ceramic disks. The helium gas circulation device has a maximum MFR of 300 g/s, which is sufficient for 10 Hz operation. In this experiment, the MFR of 23 g/s was limited due to leakage of helium gas from the pressure vessel. However, latest experiments have shown the over 80 g/s MFR by improving the sealing technology.
3. Experimental results
3.1 Fluorescence pattern
Figure 2(a) shows a photograph of the Yb:YAG ceramics, (b) shows the typical irradiation pattern of the LD module, and (c) shows the typical fluorescence pattern of the 250 J amplifier. The Yb:YAG ceramic disks had a 12 × 12 cm square shape with 1 cm thickness. Ten Yb:YAG ceramics include two each of five doping levels of 0.35%, 0.45%, 0.5%, 0.7% and 1.0%. And the Yb:YAG ceramics were installed so that the closer to the center, the higher the doping level. They were surrounded by Cr:YAG ceramics cladding on four sides to suppress the stored energy loss by amplification of spontaneous emission (ASE) or parasitic oscillation. The Cr:YAG ceramics cladding has transmissivity of less than 2% at 1030 nm with thickness of 7 mm. The fluorescence pattern with Yb:YAG ceramics was measured at a temperature of 225 K and a pumping energy of 500 J. The dashed lines in each figure indicate the fluorescence profile for the full width at half maximum (FWHM). The fluorescence pattern size was 8.9 cm in width and 8.5 cm in height. The size of the irradiation pattern of the LD modules was approximately 8 × 8 cm in FWHM. The fluorescence pattern size was slightly larger than the irradiation size of the LD module because of the angular eight direction pumping. The Filling factor, which is defined as the ratio of the average intensity to the peak intensity (defined in Eq. (1)) in the FWHM area was improved from 66.3% of the LD module pattern to 82.2% for the fluorescence pattern. This intensity smoothing was obtained by the superimposing effect.
Fig. 2. Yb:YAG ceramics cladded by Cr:YAG ceramics (a). Typical irradiation pattern of LD module (b). Fluorescence pattern of Yb:YAG ceramics pumped by eight-direction LD pumping (c).
3.2 Small-signal gain
Experimental setup for the SSG measurement of the 250 J amplifier was shown in Fig. 3. The SSG characteristics were evaluated by a large-aperture probe pulse with a size of 7.7 × 7.7 cm. Specifications of probe laser are wavelength of around 1029.4 nm, pulse width of about 40 ns, and less than 100 µJ pulse energy. For the analysis of the laser characteristics, a region of interest (ROI) volume 592.9 cm3 was defined with dimensions of 7.7 × 7.7 × 10 cm (length), based on the SSG probe size and thickness of 10 Yb:YAG ceramics. The helium gas temperature and repetition rate of LD-pumping for the SSG measurement were 175 K and 1 Hz, respectively.
Used equations for the SSG calculation are shown in Eqs. (2), (3) and (4).
The experimental and calculated results are shown in Fig. 4. An SSG of 7.4 was obtained at a pumping energy of 1,019 J. The stored energy density was calculated to 0.58 J/cm3 from Eq. (2) by using SSG 7.4, Es 2.89 J/cm2 and l 10 cm. And the stored energy of 344 J was calculated from Est and ROI volume 592.9 cm3. From Fig. 4, the calculation results were consistent with the experimental results. Here, σem of 6.7 × 10−20 cm2 used in this calculation was larger than 5.5 × 10−20 cm2 in another report [13]. Although this paper presents one result that agrees well with experimental and calculated results, more deep consideration for accuracy of the stimulated emission cross-section value is required.
Fig. 4. SSG characteristics of the 250 J amplifier at a cooling temperature of 175 K and repetition rate of 1 Hz.
The “g0×L” value is a well-known factor that determines the limitation of energy storage in a laser medium, where g0 is the SSG coefficient and L is the maximum optical path length with gain inside the laser medium. A “g0×L” value greater than 3 means that the efficiency of energy storage becomes low due to ASE growth. In this SSG experiment, “g0×L” was evaluated as 3.17 with a maximum gain length of 15.9 cm in the pumped volume in Yb:YAG ceramics. The obtained “g0×L” value was kept around 3 without significant stored energy loss. However, when the pumping energy exceeds 800 J, discrepancies between the calculation and the experiment was observed due to the influence of ASE with approaching the “g0×L” to 3.
3.3 Energy extraction performance
The output energy characteristics of the 250 J amplifier were measured. Experimental setup of the energy extraction experiment is shown in Fig. 5. The pump energy for the 250 J amplifier was fixed to 890 J. A seed pulse propagated the Yb:YAG ceramics twice at different angles. The angles of incidence are 6 degree at first pass and 2 degree at second pass. The energy of the seed pulse was adjusted from 40 mJ to 80 J. To evaluate the energy extraction characteristics of the 250 J amplifier in a condition without thermal effects, it was pumped at a 0.2 Hz repetition rate. Here, it was confirmed in advance that the change in SSG characteristics is negligible at 0.2 Hz and 1 Hz. Another reason for conducting the extraction experiment at 0.2 Hz was to slow down the expansion of the damage when optical damage occurs. The cooling temperature of the Yb:YAG ceramics was 175 K. Input and output surfaces on the Yb:YAG ceramics are coated with plasma assisted deposition (coated by Baikowski Japan co. Ltd.) for anti-reflection to 1030 ± 1 nm and 935 ± 10 nm. Designed reflection is less than 0.2%. Wavelength of input seed pulse was around 1029.4 nm. The results are shown in Fig. 6. At a SSG of 6.1, the stored energy of the 250 J amplifier at 890 J pumping in the ROI was estimated to be 311 J, and the stored energy density was 0.53 J/cm3. As an experimental result, an output energy of 253.6 J was obtained for an input energy 78 J. Temporal pulse shapes of input and output pulse were shown in the inset in Fig. 6. These pulse shapes were measured by silicon photo detector DET10A/M (Thorlabs, Inc.) with 1 ns rise time specification. Pulse duration of input pulse and output pulse in FWHM were evaluated to 38.3 ns shown in gray line and 26.8 ns shown in blue line, respectively. To the best of the authors’ knowledge, this is the highest output energy obtained with a repetitive ns output pulse by a DPSSL. The extracted energy and extraction efficiency in the ROI were 175.6 J and 56.5%, respectively. The optical-to-optical (O-O) conversion efficiency, defined as the ratio of pump energy to output energy, was 28.5%. A rate-equation-based calculation was performed. The result, shown by a solid line in Fig. 6, was consistent with the experimental results. Slight differences between the experimental and calculation results were observed in the region with an output energy exceeding 150 J. However, additional investigations are required to determine the cause of this discrepancy. A possible reason is the amplification of the high-energy pulses reflected at the anti-reflection coatings on the Yb:YAG ceramics. Some reflected pulses were observed in the form of plasma at pinholes of the telescopes in the 250 J amplifier system.
Fig. 6. Output energy characteristics as a function of the input energy at 0.2 Hz pulse repetition rate. The temporal pulse shape at an input (gray) and a 250 J output (blue) are shown in the inset.
3.4 Near-field pattern and far-field pattern
Figure 7 shows the typical near-field pattern (NFP) and far-field pattern (FFP) obtained at an output energy of 250 J. The size of the NFP in Fig. 7(a) was 7.7 cm (horizontal) × 7.1 cm (vertical). The coupling efficiency, evaluated as the ratio of the fluorescence pattern size area and NFP beam size, was 72.3%. The Filling factor of the NFP in 7.7 cm by 7.1 cm rectangle area was evaluated to 53.5% from Eq. (1) using average intensity and peak intensity measured by CCD. This is the highest fluence in cryogenic helium gas cooled disk lasers and is more than twice the previously reported value [4–8]. The peak fluence, calculated by dividing the average fluence of 4.63 J/cm2 by the Filling factor of 53.6%, was estimated to be 8.65 J/cm2. No optical damage of the 250 J laser amplifier was observed during 50 shots with over 245 J output energy. Short-term stability in 5 minutes during around 250 J energy output was 1.2%. This high fluence, at 1.6 times the saturation fluence of the Yb:YAG ceramics at 175 K, supports the finding from the experimental results of a high extraction efficiency of 56.5% within the ROI. Intensity modulation in the NFP is due to the seed pulse, which has a similar intensity modulation. A five times of diffraction-limit (TDL) area by focusing with 1350 mm focal length concave lens was described with dot line as 180 µm in X axis and 195 µm in Y axis in Fig. 7(b). From the FFP, an encircled energy ratio inside the 5 TDL area was evaluated as 33%. The deformable-mirror system, installed in the 250 J amplifier, effectively corrected wavefront distortion during energy amplification. However, more precise wavefront correction technology for large-aperture lasers is required to achieve a diffraction-limit beam quality.
Fig. 7. NFP (a) and FFP (b) obtained at an energy output of over 250 J with 0.2 Hz pulse repetition rate.
4. Conceptual design of 1 kJ laser amplifier
A 1 kJ laser amplifier was designed using the numerical data obtained in the experiment. This design has high feasibility because it is based on experimental results demonstrating quarter-scale energy output by scalable laser amplifier with a multidirectional LD-pumped, gas-cooled disk laser scheme. Table 1 shows a summary of the 250 J amplifier characteristics and specifications for the numerical design of a 1 kJ amplifier. From the energy extraction experiment of the 250 J amplifier, an output pulse fluence of a 4.63 J/cm2 can be obtained from Yb:YAG ceramics pumped to achieve an SSG of 6.1 by injecting an input pulse fluence of 1.43 J/cm2. The saturation fluence of Yb:YAG ceramics at 175 K was 2.89 J/cm2. Using these parameters based on the experiment, a laser amplifier with an input energy of 308 J and output energy of 1 kJ was numerically designed. Based on the pumping efficiency of 34.9%, which was estimated from the ratio of the stored energy in the ROI to the pump energy in the 250 J experiment, the required pump energy was estimated as 3,509 J. This low pumping efficiency is attributed to the low coupling efficiency of 72.3%, due to the difference in the pumping size and the seed beam size. It can be easily improved by increasing the seed beam size.
Table 1. Summary of the 250 J amplifier characteristics and specifications for the numerical design of a 1 kJ amplifier.
The actual design of the 1 kJ amplifier, based on the numerical design, is shown in Fig. 8. The beam size of the seed pulse, defined based on the output and input fluences, was 14.7 × 14.7 cm. Based on the coupling efficiency, the pumping size was 17.3 × 17.3 cm. The size of the ROI to characterize the 1 kJ amplifier performance was 15.3 × 15.3 cm. An SSG of 6.1 was obtained with a gain length of 24 cm. A “g0×L” value of 2.6 was obtained at the maximum optical path gain length (34.3 cm) inside the pumped region. This “g0×L” value is below the maximum value of 2.97 obtained in the SSG experiment, which is reasonably feasible. For the design of 3.5 kJ pumping, irradiation in 16 directions by LD modules with outputs over 200 J may be considered. The aperture of the Yb:YAG ceramics must be at least 20 × 20 cm. This design indicates the target value to overcome technical challenges related to key components, such as increasing the brightness of LD modules and upsizing the Yb:YAG ceramics. This design can be further developed based on the findings of this research. For an advanced design, based on the extraction efficiency of 56.5% demonstrated in the experiment, it is possible to increase the O-O conversion efficiency by over 35% by increasing the coupling efficiency from 72.3% to 90%. Furthermore, it is suggested that laser amplification with a very high extraction efficiency can be achieved by increasing the NFP Filling factor from obtained 53.5% to 80%, which is a feasible flat top pattern. If the Filling factor of NFP can be increased, high fluence amplification with suppressing optical damage becomes possible. This allows for more efficient energy extraction from laser amplifier. One concern in this design is reduction of the extraction efficiency and generation of optical damage due to the temperature increase and transmitted wavefront distortion at the Yb:YAG ceramics at high-repetition rate operation. To solve this problem, it is effective to increase the MFR of helium gas. Therefore, it is important to develop a technique for sealing high pressure helium gas at low temperatures. Thus, presented conceptual design based on the experimental results helps clarify the technical goals necessary for the advanced design of 1 kJ lasers.
5. Conclusion
A 250 J output LD-pumped cryogenically cooled Yb:YAG ceramics laser amplifier was developed, with energy scalability. A 253.6 J ns pulse output at a repetition rate of 0.2 Hz with a beam fluence of 4.6 J/cm2 was achieved, which is an important milestone towards the realization of a 1 kJ DPSSL. A 1 kJ laser amplifier was conceptually designed, supported by the results of the 250 J laser amplification. It was shown that the realization of a 1 kJ laser amplifier by LD-pumped cryogenic helium gas cooled Yb:YAG ceramics is possible. The significant increase in the volume of experimental data for applied research by high-repetition rate, high-energy lasers contribute not only to the improvement of the accuracy of experimental data, through statistical processing, but also to the promotion of data-driven research, based on AI technology.
Funding
New Energy and Industrial Technology Development Organization.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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