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Abstract
A novel bidirectional ring amplifier with twin pulses for high-power lasers is proposed, and the performances on output energy capability and extraction efficiency are comprehended with detailed simulation. The results show that an extraction efficiency of 62.3% and the output energy of 13.4 kJ per pulse at the B integral limit can be obtained at low average fluence of 10.3 J/cm2 and the low injection energy of 3.9 mJ in the bidirectional ring amplifier. Compared with the multi-pass amplifier, the bidirectional ring amplifier is more compact and the extraction efficiency is much higher at low injection energy and low laser fluence operation, which is beneficial to simplify the preamplifier system and reduce the effects of nonlinear phase shift.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
High-energy, high-power laser system for inertial confinement fusion (ICF), such as the Nova [1–3], National Ignition Facility (NIF) [4–6], Laser Mégajoule (LMJ) [7–9] and Shenguang-III (SG-III) [10,11], is large in size and expensive in construction. The multi-pass amplifier (MA) with the large aperture amplifiers is widely used in high-power laser systems, such as ICF drivers. The systems with MA usually have four features: square beam, single aperture, single pulse and unidirectional propagation, and the expensive preamplifier systems are required to compensate for the limited gain of the main amplifiers. The gain and the extraction efficiency are limited in part by the number of passes that the beam can make through the amplifiers. Besides, the laser system in the multi-pass amplifier should operate at much higher fluence to achieve high extraction efficiency, which results in a technical challenge in the damage of optical components under high-power lasers irradiation.
In 2013, a bidirectional amplifier (BA) with twin pulses was proposed by X. M. Zhang [12] in China Academy of Engineering Physics. The simulation results showed that an extraction efficiency of 61% can be obtained in the BA architecture with the output energy of 13.1 kJ per pulse and the injection energy of 24 mJ at the B integral limit, but this architecture was still based on the traditional multi-pass amplifier and the laser system was still large. Furthermore, the traditional regenerative amplifier for the preamplifier system can only amplify the seed pulse to ~10 mJ [13–15], and more stages of preamplifiers are required in the BA architecture. In 2014, a patent about ring amplifier with single pulse for compact, multi-pass pulsed laser amplifiers was proposed by A. C. Erlandson [16] at the Lawrence Livermore National Laboratory (LLNL). Although this ring amplifier was more compact than that of MA, it is difficult to extract the stored energy of amplifiers efficiently and uniformly due to the single pulse injected into the amplifier. Besides, the relevant characteristics of this architecture have not been discussed in this patent.
To efficiently extract the stored energy with low injection energy at low laser fluence operation and make the system compact, a bidirectional ring amplifier (BRA) with twin pulses is proposed and discussed in this paper. The structure of the bidirectional ring amplifier is described. The characteristics of the bidirectional ring amplifier on extraction efficiency and output energy capability are simulated and discussed. The simulation results show that an extraction efficiency of 62.3% and the output energy of 13.4 kJ per pulse at the B integral limit can be obtained at low average fluence of 10.3 J/cm2 and the low injection energy of 3.9 mJ in the bidirectional ring amplifier. Compared with the multi-pass amplifier, the bidirectional ring amplifier is more compact and the extraction efficiency is much higher at low laser fluence operation, which is beneficial to reduce the effects of nonlinear phase shift. Furthermore, the preamplifier system for the bidirectional ring amplifier is simple, only a fiber oscillator and a regenerative amplifier can work.
2. Principle of bidirectional ring amplifier with twin pulses
The design of multi-pass amplifier for ICF, such as NIF, is typically separated into the main amplifier block and the booster amplifier block [13,14]. The earlier passes through the main amplifier block provide high gain in the system, and the final pass through the booster amplifier block provides the main output energy of the beam. Therefore, the laser energy extracted from the booster amplifier is much larger than that from the main amplifier.
The schematic diagram of bidirectional ring amplifier with twin pulses is shown in Fig. 1 including four polarizers, four amplifier blocks, two plasma electrode Pockels cells, two mirrors and two spatial filters with two spherical lenses and a pinhole at the common focus. Compared with the multi-pass amplifier, the bidirectional ring amplifier is separated into four amplifier blocks and shared by two pulses with the same energy and temporal pulse shape.
Fig. 1 Schematic diagram of the bidirectional ring amplifier with twin pulses. P1, P2, P3, P4: polarizers; AMP1, AMP2, AMP3, AMP4: amplifiers; PEPC1, PEPC2: plasma-electrode Pockels cells; L1, L2, L3, L4: spherical lens; PA1, PA2: pinholes; M1, M2: mirrors.
The bidirectional ring amplifier works as follows: the pulse-1 with p polarization is injected through the 1st polarizer (P1) whose transmission axis is aligned with p polarization. Then, the pulse-1 is amplified by the 1st amplifier block (AMP1), and incident to the 1st plasma-electrode Pockels cell (PEPC1). At this time, the PEPC1 with the high voltage rotates the polarization of the pulse-1 from p polarization to s polarization. Then, the pulse-1 passes through the 1st spatial filter and is amplified by the 2nd amplifier block (AMP2). Since the transmission axes of 2nd and 3rd polarizers (P2 and P3) are both aligned with the p polarization, the pulse-1 can be effectively reflected by these polarizers and mirror (M1). After transmitting through the 3rd amplifier block (AMP3), the pulse-1 passes through the 2nd spatial filter. Then, the pulse-1 is incident to the 2nd plasma-electrode Pockels cell (PEPC2) with zero voltage, and passes through the 4th amplifier block (AMP4). The applications of Pockels cells with high and zero voltages can be considered that the Pockels cells operate in on and off states, respectively. The transmission axis of 4th polarizer (P4) is aligned with the p polarization, and the pulse-1 can be effectively reflected by it. If the Pockels cells work in off state, the pulse can be trapped and amplified in this closed loop cavity. Once the pulse reaches the predetermined number of round trips through the closed loop, the operating state of Pockels cells (PEPC1) is changed into the on state, and the pulse with polarization rotated 90° can be exported from the 2nd polarizer (P2) directly.
Two spatial filters between the amplifier blocks are used to improve the uniformity of the beam and suppress the nonlinear effects. To avoid diffraction effects, the spatial filters reimage the beam at the first mirror (M1) to the second mirror (M2), and vice versa. Besides, the mirrors can be replaced by the adaptive optics systems to compensate for wavefront errors, such as those produced by gravity and temperature variations in large mirrors and supporting structure [17–19]. The amplifier blocks consist of any transmission-type amplifiers which can transmit and amplify the pulses with both s and p polarization efficiently, such as normal incidence slab amplifiers [16,20], gas-cooled slab amplifiers [21–23], zig-zag amplifiers [24–26] and rod amplifier [1–3].
Since the ring amplifier is symmetric, the propagation and amplification sequence of the pulse-2 in the twin pulses is similar to that of the pulse-1, but they have the opposite propagating directions. The two pulses are synchronously injected into the ring amplifier, and the Pockels cells are fired in on and off states to control the polarization of the pulses. The sequence diagrams of 1st and 2nd plasma-electrode Pockels cells are shown in Fig. 2.
3. Simulation of the bidirectional ring amplifier with twin pulses
According to the amplification theory [27,28], the propagation and amplification of twin pulses in the bidirectional ring amplifier is simulated and compared with that of the multi-pass amplifier. Schematic diagram of the multi-pass amplifier is shown in Fig. 3. Both the bidirectional ring amplifier and multi-pass amplifier consist of 18 amplifier slabs. For the bidirectional ring amplifier, 3 slabs are in the 1st and 4th amplifier blocks and 6 slabs are in the 2nd and 3rd amplifier blocks. For the multi-pass amplifier, 11 slabs are in the main amplifier blocks and 7 slabs are in the boost amplifier blocks. The relevant parameters in simulation are listed in Table 1.
Fig. 3 Schematic diagram of the multi-pass amplifier. P1: polarizer; AMP1, AMP2: amplifiers; PEPC1: plasma-electrode Pockels cells; L1, L2, L3, L4: spherical lens; PA1, PA2: pinhole; M1, M2: mirrors; CM1, CM2: cavity mirrors.
Table 1. Parameters Used in Simulation
The dependence of output energy and extraction efficiency with the injection energy is shown in Fig. 4. For high-power ICF laser drivers, the B integral is often required pinhole-to-pinhole B integral (ΔB)≤1.8 rad and total B integral (ΣB)≤3.5 rad [29,30]. Additionally, the pulse duration used for this simulation is 5 ns [12]. At this condition, the extraction efficiency of amplifiers can be achieved 62.3% at the injection energy of 3.9 mJ in the bidirectional ring amplifier and the output energy of 13.4 kJ can be obtained for each pulse, which corresponds to the average operation fluence of 10.3 J/cm2. In contrast, the multi-pass amplifier only provides an extraction efficiency of 38.2% and output laser energy of 16.5 kJ at the injection energy of 0.254 J, which corresponds to the average operation fluence of 12.7 J/cm2. Therefore, the bidirectional ring amplifier with twin pulses can achieve higher extraction efficiency at lower injection energy and lower output fluence than that of the multi-pass amplifier. Besides, the traditional regenerative amplifier for the preamplifier system can amplify the seed pulse to ~10 mJ, which is enough for the requirement of the injection energy for the bidirectional ring amplifier.
Fig. 4 The dependence of output energy and extraction efficiency with the injection energy for (a) the multi-pass amplifier and (b) the bidirectional ring amplifier (single pulse).
To reveal the process of energy conversion in two amplifiers, further simulations are performed to investigate the amplification process at the B integral limit of ΔB≤1.8 rad and ΣB≤3.5 rad. The pulse energy in the amplification process in multi-pass amplifier and bidirectional ring amplifier is shown in Fig. 5. The variation of pulse energy in the amplification process for the two amplifiers is similar: the pulse energy increases exponentially with the number of slabs that the pulse has passed through under the saturation fluence, and the pulse energy increases linearly with the number of slabs that the pulse has passed through above the saturation fluence. There are some discontinuity points a, b and c in Fig. 5, which resulted from the pinhole-passing efficiency or the non-ideal transmittance and reflectance of optics in the amplifier chain. As shown in Fig. 5(b), most of the stored energy in the amplifier has already been extracted in the 4th round, which results in obvious gradient change of the curve in the 5th round.
Fig. 5 Variation of pulse energy in the amplification process for (a) the multi-pass amplifier and (b) the bidirectional ring amplifier (single pulse). The dashed pink line shows the beam energy corresponding to the saturation. The discontinuity points a, b and c are resulted from the pinhole-passing efficiency or the non-ideal transmittance and reflectance of optics in the amplifier chain.
The dependence of the extracted energy with the slab number is shown in Fig. 6. Due to the fluence of pulse is high enough, most of energy can be extracted in the 4th pass from the multi-pass amplifier and in final two rounds from the bidirectional ring amplifier, respectively. For the multi-pass amplifier, the extracted energy increases with the slab number in the final pass, and the maximum energy of 1.23 kJ is extracted from the 18th slab and the minimum energy of 0.38 kJ extracted from the 1st slab. As a contrast, the bidirectional ring amplifier has a more symmetrical and uniform distribution of energy extraction, as shown in Fig. 6(b). It is indicated that the extracted energy in the 5th round is lower than that in the 4th round because the pulse has only propagated half ring in the final round.
Fig. 6 Extracted energy as a function of the slab number for (a) the multi-pass amplifier and (b) the bidirectional ring amplifier.
The extraction efficiency of each slab for the two amplifiers is shown in Fig. 7. For the multi-pass amplifier, 50~70% stored energy in slabs of the main amplifier block (1~11 slabs) cannot be sufficiently extracted. The earlier passes through the main amplifier block provide high gain with low fluence in the multi-pass amplifier, and the final pass through the booster amplifier block provides the main output energy of the beam, so the slabs in the booster block (12~18 slabs) can be more sufficiently extracted at high operation fluence than that in the main block, still retaining about 50% stored energy. In contrast, the extraction efficiency of slabs in the bidirectional ring amplifier is higher than 73%. Moreover, it also provides a uniform extraction amplifying architecture, in which the insufficient extraction in slabs are compensated by the twin pulses with opposite directions.
Fig. 7 Allocation of stored energy versus the slab number for (a) the multi-pass amplifier and (b) the bidirectional ring amplifier.
The B integral is one of parameters to determine the effects of nonlinear phase shift in the high-power laser system [29,30]. The B integral of the two architectures is shown in Fig. 8. The maximum ΔB of the multi-pass amplifier is about 1.8 rad, and the output energy is 16.5 kJ. In contrast, the maximum ΔB of the bidirectional ring amplifier is about 1.43 rad, and the output energy is 13.4 kJ for each pulse. The maximum ΔB of the bidirectional ring amplifier is much lower than that of the multi-pass amplifier due to lower operation fluence in the bidirectional ring amplifier, which is beneficial to reduce the effects of nonlinear phase shift. Moreover, the bidirectional ring amplifier can achieve more uniform and higher extraction efficiency at lower operation fluence, which can greatly increase the reliability and reduce the cost of system.
4. Conclusion
In conclusion, a novel architecture for the bidirectional ring amplifier with twin pulses is proposed. The performance on output energy capability and extraction efficiency is discussed with detailed simulation. Compared with the extraction efficiency of about 38.2% with the average fluence of 12.7 J/cm2 and injection energy of 0.254 J for the multi-pass amplifier, simulation results show that the extraction efficiency of 62.3% and the output energy of 13.4 kJ per pulse can be achieved at low average fluence of 10.3 J/cm2 and the low injection energy of 3.9 mJ in the bidirectional ring amplifier at the B integral limit. The proposed bidirectional ring amplifier with twin pulses provides uniform and efficient extraction amplifier architecture at a low fluence operation, which is beneficial to reduce the effects of nonlinear phase shift and make the system reliable. Besides, the preamplifier system for the bidirectional ring amplifier is simple, only a fiber oscillator and a regenerative amplifier. This work is expected to provide a solution for the next generation of high-power laser amplifier for ICF drivers and other laser systems.
Funding
National Major Scientific Instruments and Equipments Development Project of China (2016YFF0100900, 2016YFF0100903); National Natural Science Foundation of China (NSFC) (61775153, 61705153, 11504255); Natural Science Foundation of Jiangsu Province (BK20141232); Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
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