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Abstract
The growth and laser amplifier performance of a large-aperture Nd:LuAG ceramic are reported. Using the vacuum sintering and high-temperature insostatic pressing (HIP) methods, three pieces of a 50 mm-aperture Nd:LuAG ceramic are fabricated and used as the gain medium in a diode-pumped nanosecond distributed active mirror amplifier chain (DAMAC). The energy storage capacity of large-aperture Nd:LuAG is investigated and compared with that of Nd:YAG. Energy amplification up to 10.3 J at 10 Hz is achieved, which, to the best of our knowledge, produces the highest peak power (1 GW) using Nd:LuAG. The excellent energy storage and extraction performance confirm the great potential of Nd:LuAG in high-energy scaling.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
High-energy lasers with the pulse energy up to multi-joule level, good beam quality and high conversion efficiency are urgently required in a wide range of applications, such as inertial confinement fusion, particle acceleration and material processing. To obtain a laser source with such superior performance, Nd3+-doped materials have advantages of operating in room-temperature without complicated pump coupling systems, and have been used as the gain medium in several joule-level nanosecond laser systems [1–4]. To disperse the overall gain and thermal deposition, a novel configuration called as distributed active mirror amplifier chain (DAMAC) [4] is demonstrated by our group in 2017 using Nd:YAG crystals, in which the pump energy and heat load are distributed over multiple gain modules. However, to achieve efficient scaling, the number of gain modules needs careful control to decrease the passive loss, thus favoring higher energy stored in each gain module.
Nd:LuAG, firstly grown by the Czochralski method in 2009 [5], has shown great potential as the gain medium in high energy amplifier with strong capacity of energy storage, due to its moderate emission cross section (9.67 × 10−20 cm2 [6]), one-third of that of Nd:YAG, the most commonly used Nd3+-doped laser material. However, most of Nd:LuAG lasers reported so far are oscillators, working in the continuous-wave (CW), passively mode-locked or Q-switched mode operation. The CW lasing of Nd:LuAG crystal and ceramic were first demonstrated by Xu et al. [5] and Ye et al. [7] in 2009 and 2015 respectively. In 2017, Yan et al. reported an eye-safe Nd:LuAG CW ceramic laser producing 1.37 W at 1.4 μm [8]. Mode-locked Nd:LuAG laser at 1064 nm and 1338 nm were reported by Xu et al. [9] and Feng et al. [10] in 2012 and 2016 respectively. Q-switching performance of Nd:LuAG were also well investigated operating in active [11–13], passive [14–18] and self Q-switching [19] mode.
As for the research on the Nd:LuAG amplifier in terms of high energy scaling, we reported in 2016 a diode-pumped active mirror (AM) amplifier chain, using two Nd:LuAG crystal slabs in the amplifier, producing an output energy of 1.52 J at 10 Hz in a 10 ns Q-switched pulse, with the peak power of 1.52 MW [3]. Also, Ma et al. [13] reported in 2017 a 10.3 mJ amplified pulse using the Nd:LuAG ceramic seeded by a 5.2 mJ Nd:YAG Q-switched oscillator, corresponding to a pulse peak power of 1.47 MW. Investigation of Nd:LuAG amplifiers on higher power level was severely limited by the available growth size of gain medium. The maximum dimensions of Nd:LuAG crystal [3] and ceramic [13] ever reported are 4 × 3.6 × 12 mm2 and 15 × 10 × 7 mm2 respectively.
In this paper, the growth and laser performance of 50 mm-level aperture Nd:LuAG ceramic are reported. Using the vacuum sintering and high temperature insostatic pressing (HIP) methods, two pieces of 58 × 40 × 5.5 mm3 Nd:LuAG ceramic and one piece of 52 × 39 × 7.5 mm3 Nd:LuAG ceramic are successfully fabricated. By adopting a diode-pumped Nd:YAG-Nd:LuAG hybrid DAMAC system, a pulse energy of 10.3 J is obtained at the repetition rate of 10 Hz and the pulse width of 10 ns, corresponding to a peak power as high as 1 GW. To the best of our knowledge, this is the highest pulse peak power reported so far using Nd:LuAG crystal or ceramic as the amplifier medium. The beam quality of the 10-J amplified output is measured as β = 1.97. The energy storage capacity of Nd:LuAG and the necessity of using large-aperture ceramic are investigated in detail. The excellent laser performance shows great potential of energy scaling using Nd:LuAG ceramic as the gain medium.
2. Preparation and characterization of large-aperture Nd:LuAG ceramic
The composition of Nd (0.8 at.%):LuAG laser ceramics is (Lu0.992Nd0.008)3Al5O12. The samples are prepared by solid-state sintering of high-purity Al2O3 (99.99%), Nd2O3 (99.99%), and Lu2O3 (99.99%) powders. Commercially available α phase alumina (α-Al2O3, 99.99%, 200 nm), yttria (Y2O3, 99.99%, 50 nm) and neodymia (Nd2O3, 99.99%, 50 nm) are used as raw materials in a ratio according to the formula of Nd (0.8 at.%):LuAG. TEOS + MgO (mixture of TEOS with 0.4 wt% of all the powders and MgO with 0.1 wt% of all the powders) and TEOS (0.4 wt% of all the powders) are added as the sintering additives. The raw materials are mixed uniformly by planetary milling with alcohol (99.99%) as a milling assistant for 24 h. After milling, the resulted mixture slurry is dried at 80°C for 12 h, and then milled by mortar and pestle to obtain the mixture powders. The mixture powders are sieved though the 200 mesh sieve and then presintered at 900°C for 6 h to remove the residual organic matter. The monodispersed mixture powders are uniaxially pressed into disks, followed by an isostatic cold pressing under 200 MPa to obtain the green bodies, the size of which is 80 × 60 × 8 mm. The Nd:LuAG green bodies are firstly vacuum sintered at 1750°C for 5 h under a vacuum of 10−3 Pa. To ensure the success fabrication of this size of laser ceramics, the rate of heating up is very slow such as 20°C/h. After the sintering in vacuum, HIP sintered at 1750°C under 200 MPa in an argon atmosphere for 2h are used as the after treatment method. The fabricated ceramic samples are mirror-polished on both surfaces. The morphologies of the Nd (0.8 at.%):LuAG ceramics are characterized by the field emission scanning electron microscope (FESEM, Hitachi SU8200). Absorption spectra are studied using UV/VIS/NIR spectrometry (Lambda 750, PerkinElmer).
Figure 1 shows the FESEM micrograph of the mirror-polished and thermally etched surfaces of fabricated Nd (0.8 at.%):LuAG transparent ceramics. It can be seen that the ceramics fabricated from the powder reach a high relative density, and no obvious micro-pores are found in the ceramic samples. Transmission spectrum (200-1200 nm) of the prepared Nd:LuAG ceramics with a thickness of 5.5 mm is shown in Fig. 2. The transmittance at the lasing wavelength region (1064 nm) is about 84% level, which is very close to the theoretical transmission.
Fig. 2 Transmission spectrum of 0.8 at.% Nd:LuAG ceramics ranging between 200 and 1200 nm. (Sample thickness of 5.5 mm with optical polished surfaces).
3. Experimental setup
The hybrid DAMAC system consists of the oscillator, booster amplifier I, booster amplifier II, and the Nd:LuAG main amplifier, as shown in Fig. 3.
Fig. 3 Laser experimental configuration. FI, Faraday isolator; HR, high-reflection mirror; PBS, polarization beam splitter; QWP, quarter-wave plate; HWP, half-wave; BE, beam expander; DM: deformable mirror; SA, serrated aperture.
Same as our previous setup introduced in [4], the Q-switched Nd:YAG oscillator produces an output energy of 125 mJ at the repetition of 10 Hz and the pulse duration of 13 ns. The double-pass booster amplifier I scales the seed energy to 2.2 J, consisting of four AM modules, a quarter-wave plate and a high-reflection mirror. After being expanded and apodized by a serrated aperture, the beam enters the booster amplifier II with the size of 32 mm × 32 mm. The configuration of booster amplifier II is similar to amplifier I, except that the high-reflection mirror is replaced by a beam expander with the magnification of 2 and a deformable mirror to correct the wavefront distortion and improve the beam quality. The deformable mirror has a continuous mirror plate (92 mm × 92 mm full size, 80 mm × 80 mm clear aperture) and a 116-actuator hexagonally distributed stack piezoelectric (PZT) actuator array (8 mm pitch). Three pieces of Nd:YAG crystal slab are used in this stage, with the dimension of 60 × 40 × 8 mm3.
The Nd:LuAG main amplifier stage contains three single-pass AM modules. Both Nd:LuAG ceramic slab #1, #2 are 0.8 at.% doped, having the thickness of 5.5 mm and the transverse aperture size of 58 mm × 40 mm. Nd:LuAG ceramic #3 has the same doping density, but a different dimension of 52 × 39 × 7.5 mm3. The extracting beam goes through the ceramic twice in one passage at the incident angle of 45°. Each gain module in both the Nd:LuAG main amplifier and the booster amplifier II is pumped by the same type of LD array, consisting of 201 bars in three rows and providing a highest pump energy of 13.3 J with the maximum pump duration of 300 μs. The pump surface of each Nd:LuAG ceramic is high-reflection (HR) coated at 1064 nm and anti-reflection (AR) coated at 808 nm to water, while the front surface is AR coated at both 808 nm and 1064 nm. The pump area at the pump surface of the Nd:LuAG ceramic has the dimension of 52 × 36 mm2.
4. Results and discussion
First, small-signal gain of each Nd:LuAG ceramic module is measured, using the seed beam from the Nd:YAG oscillator with the size of 32 × 32 mm2 and the pulse energy of 8.8 mJ. Note that as we discussed in previous work [3], there is a gain reduction of around 10% caused by the weak mismatch of the central lasing wavelength between the Nd:YAG seed and the Nd:LuAG amplifier in the hybrid MOPA system.
Using the measured small-signal gain, the stored energy in the gain medium can be calculated as
where Esat is the saturation fluence, Sp is the pump area, θ is the beam refractive angle within the gain medium, g0 is the measured small signal gain factor per unit length and L is the effective gain length, which for the AM configuration is defined aswhere d is the thickness of the gain medium.Figure 4 describes the dependence of stored energy of three Nd:LuAG modules on the peak pump power respectively, calculated based on the measured small-signal gain and compared with that of the Nd:YAG module in booster amplifier II. It is shown in Fig. 4 that the stored energy at full pump power within Nd:LuAG #1, #2, #3 and Nd:YAG are 5.24 J, 5.29 J, 5.75 J and 4.49 J, respectively, using the experimental parameters that the pump area is 52 × 36 mm2 and the full peak pump power is 44.3 kW for each gain module. The passive loss of the Nd:LuAG gain module is measured as 4%.
Next, the saturated scaling performance of Nd:LuAG gain module versus the pump energy is investigated. As shown in Fig. 5, a laser beam of 4.7 J pulsed energy generated by the two stages of booster amplifiers goes through the three Nd:LuAG AM gain modules. The output energy is measured by an energy meter (Standa) at the exit of the whole hybrid DAMAC system.
At the total pump energy of 40 J of the main Nd:LuAG stage, a 10.3 J output energy at the repetition rate of 10 Hz is obtained. 1.7 J, 1.8 J and 2.1 J of pulsed energy is individually extracted from the Nd:LuAG ceramic gain module #1~#3, corresponding to an optical-to-optical efficiency of 12.8%, 13.5% and 15.6%, respectively. The peak power of the amplified output reaches to as high as 1 GW, which is, to the best of our knowledge, the highest peak power generated using Nd:LuAG crystal or ceramic as the amplifier medium. The total optical-to-optical efficiency, and the total extraction efficiency of stored energy of the Nd:LuAG main amplifier stage are 14.0% and 34.4% individually, both of which are expected to be highly enhanced by increasing the input energy fluence, and further decreasing the passive loss of the gain medium. The root mean square (RMS) stability at the output energy of 10.3 J at 10 Hz is measured as 1.0%.
The beam quality factor is measured at the exit of the hybrid DAMAC system. Using the deformable mirror located at booster amplifier II to correct the wavefront distortion caused by the optical components and thermal effects, the measured β is 1.67 before the beam is scaled by the Nd:LuAG main amplifier, and 1.97 after the amplification at 10 Hz. The far-field profile of the Nd:LuAG main amplifier output is shown in Fig. 6.
Furthermore, we analyze the energy storage limit of Nd:LuAG. Assuming the extracting beam fills the whole volume V and matches well with the pump light, Eq. (1) can be expressed as
The energy storage limit is estimated with the consideration of the ASE effect. To suppress the depumping effect caused by ASE effect, g0 is usually constrained by the ASE threshold of g0LASE = 3, where LASE is the maximum path length that ASE rays can travel inside the gain module and is set as the diagonal length of the medium. Taking the ASE threshold into account, the maximum allowed energy stored within a single AM module for the cases of Nd:LuAG and Nd:YAG are calculated respectively versus different transverse size, as shown in Fig. 7. In the calculation, the saturation fluences used for Nd:LuAG and Nd:YAG are 1.93 J/cm2 and 0.62 J/cm2, respectively, while the thickness of gain medium is fixed at 8 mm.
It is shown in Fig. 7 that for a certain gain medium and a given ASE threshold, the energy storage capability is dominated by the saturation fluence of the gain medium. With the same gain level and slab dimension, Nd:LuAG could store more than three times pump energy as Nd:YAG (e.g. 16.5 J vs. 5.3 J at the transverse size of 8 × 4 cm2), demonstrating its great advantage over Nd:YAG in the case of high energy saturated scaling (most of the stored energy can be extracted).
It should be noted that in the saturated scaling experiment introduced above, the g0LASE of Nd:LuAG #1(#2), Nd:LuAG #3, and the Nd:YAG module in the booster amplifier II are 1.50, 1.10 and 2.80 respectively. The experimental results reveal that Nd:LuAG module is operating far below the ASE threshold and thus the stored energy (5~6 J) has not yet reached the energy storage limit (approximately 15 J according to Fig. 7), showing a large room of improvement. In comparison, the Nd:YAG module is operating near the ASE threshold and thus its energy storage limit (4.49 J stored vs. the limit of 4.9 J).
Figure 7 also shows that the maximum allowed stored energy is not sensitive to the enlargement of slab transverse size. However, increase of gain medium aperture in the AM amplifier configuration is critical, as analyzed in the following.
For a slab configuration, the maximum temperature difference inside the medium can be expressed as
where Kt is the thermal conductivity of the medium, given as 10 W/mK and 14 W/mK for Nd:LuAG and Nd:YAG respectively, and q is the average thermal power density, defined aswhere Ppump is the peak pump power, F is the repetition rate, τp is the pump duration, and ηt is the portion of heat generation, set as 30%.By setting the maximum allowed temperature difference as 80 K, the peak pump intensity needed at the pump surface and the maximum allowed repetition rate for the case of energy storage limit is shown in Fig. 8.
Fig. 8 Pump intensity needed and maximum allowed repetition rate at respective energy storage limit: (a) Nd:LuAG; (b) Nd:YAG.
Figure 8 demonstrates the requirement for large-aperture Nd:LuAG ceramic. At the energy storage limit, larger-aperture medium corresponds to lower pump intensity, which thus avoids using the large-size pump coupling system and pushing the peak intensity of diode arrays far beyond the commercial available level. Large-aperture medium also helps to reduce the heat load density in each module, crucial in supporting stable operation at higher repetition rate at room temperature. Moreover, the risk of laser damage to optical components is reduced with large clear aperture.
5. Conclusion
In this paper, GW-level nanosecond laser pulse using the Nd:LuAG ceramic as the amplification gain medium is reported for the first time. Using the vacuum sintering and HIP method, large-aperture Nd:LuAG with the size up to 50-mm level is successfully prepared. With three Nd:LuAG gain modules in a hybrid DAMAC system, a pulsed output of 10.3 J at the repetition rate of 10 Hz is achieved, having 5.6 J extracted from the Nd:LuAG amplifier stage with an optical-optical efficiency of 14.0% and an extraction efficiency of stored energy of 34.4%. Beam quality is measured as β = 1.97 at the maximum output. The energy storage capacity of Nd:LuAG as well as the necessity of using large-aperture gain medium are analyzed. Further investigations include growing larger size of Nd:LuAG ceramic, increasing the extracting efficiency, decreasing the passive loss of the gain medium and so on. The Nd3+-doped DAMAC configuration holds the promise of energy scaling to tens of joules level.
Funding
National Key Research and Development Program of China (2017YFB1104500); Natural Science Foundation of Beijing Municipality (4172030); Beijing Young Talents Support Project (2017000020124G044).
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