Stimulated Brillouin scattering (SBS) is a nonlinear optical process that generates backward scattered phase conjugate wave (Zel’dovich et al., 1972; Zel’dovich et al., 1985; Damzen et al., 2003; Brignon & Huignard, 2004). A device that generates the phase conjugate wave by the SBS process is called SBS phase conjugate mirror (PCM). An SBS-PCM can compensate wavefront distortion induced by a phase aberrator, such as a laser gain medium; hence, it is widely used in high-energy laser systems to obtain a high-quality beam. Efficient heat dissipation is a major issue in high-energy laser systems, particularly with regard to the high repetition rate. The combination of beams from small laser systems is a constructive approach to this issue. Of the various beam combined systems using SBS-PCMs, the cross-type beam combined system has many outstanding advantages, such as perfect isolation of leak beam, compensation of thermal birefringence, easy alignment and convenient maintenance (Kong et al., 1997, 2005a). Since the SBS wave generates from a thermal noise, it naturally has a random phase with respect to the incident beam. Therefore the phase controlling of the SBS wave is a key technology in the realization of a coherent beam combined system. For this reason, the self-phase control method was proposed and has been developed by Kong et al. (2005a, 2005b, 2005c), which can control the phase of the SBS wave with the simplest composition as well as ease of alignment, no limitations on the number of combined beams, and excellent phase conjugation. Furthermore, the active phase control with a piezoelectric translator (PZT) enables long-term phase stabilization (Kong et al., 2006, 2008). In addition to a random phase characteristic, the distortion that generally occurs in a pulse waveform of an SBS wave is another negative characteristic in terms of the beam combination. Kong et al. (2005d) has overcome this problem with the SBS waveform preservation technique, which is called the prepulse injection method. These works are expected to boost the development of laser systems in term of a high level of energy and power, a high-quality beam, and a high repetition rate. A laser system with these characteristics has tremendous potential in the fields such as laser machining, particle accelerators, neutron or proton generators, military weapons, and laser fusion drivers.
2. Stimulated Brillouin scattering phase conjugate mirror
2.1. Optical phase conjugation
Optical phase conjugation is a nonlinear optical phenomenon that accurately reverses the propagation direction and phase variation of an incoming light beam. A nonlinear optical device that produces the phase conjugate reflection is called a PCM (Zel’dovich et al., 1972; Eichler et al., 2001). Fig. 1 compares a PCM with a conventional mirror. In case of a conventional mirror, the wavefront is distorted twice when passing through an aberrating medium twice; however, there is no distortion with the PCM. The use of a PCM therefore eliminates phase distortions in optical systems (Andreev et al., 1992; Seidel et al., 1997). For example, in a solid-state laser amplifier, phase distortions arise from thermal refractive index changes in the laser crystal. If a PCM is used to make the incoming beam pass the laser crystal twice, these distortions disappear. Hence, PCMs are widely used in the high-energy laser systems.
2.2. Stimulated Brillouin scattering
The most commonly used way of producing optical phase conjugation is SBS (Boyd, 1992; Damzen et al., 2003; Shen, 2003). SBS is normally achieved by focusing a laser beam into a SBS medium. In this medium, spontaneous scattering from a sound wave generates a wave that travels in the opposite direction and that wave interferes with the incoming wave, thereby inducing density modulations. Because the induced density modulations have the same frequency as the initial sound wave, they are amplified and reinforce the backscattering. The phase conjugated backscattered part is dominant because the amplification depends strongly on the extension of the interference area. As a result, there is an exponential rise of the reflected phase conjugated signal. The acoustic grating seems to be a self-adapting mirror because the wavefronts of the acoustic grating match the wavefronts of the incoming beam. If the incident wavefront has any disturbance, it results in a self-adapting mirror curvature with a response time in the nanosecond range. In addition, SBS lowers the frequency of the phase conjugated wave as much as the sound wave frequency in accordance with the energy conservation.
3. Reflectivity of an SBS-PCM
The SBS reflectivity is almost equal to that of an ordinary mirror when the pump bandwidth, Δ
|Carbon tetrachloride (CCl 4 )||528||3.8||5.9||0.4||1.7|
|Carbon disulfide (CS 2 )||50||68||122||0.020||0.1|
The experimental setup for measuring the reflectivity of the SBS-PCMs is shown in Fig. 2. The pump laser is a Q-switched Nd:YAG laser and, using its single longitudinal mode injection seeder, it can be operated in the single-mode or the multimode. The laser linewidth is approximately 0.09 GHz in the single-mode and approximately 30 GHz in the multimode. Thus, the linewidth of the multimode case is much larger than the Brillouin linewidth of the liquids used in this experiment, listed in Table 1. (Kmetik et al., 1998; Yoshida et al., 1997; Erokhin et al., 1986; Sutherland, 1996). The focal length of the lens used for the SBS-PCM is 15 cm. This length corresponds to a Rayleigh range,
The SBS materials used in this experiment are Fluorinert FC-75, carbon tetrachloride (CCl4), acetone, and carbon disulfide (CS2). The SBS properties and the nonlinear refractive index,
Brillouin linewidth, ranging from 50 MHz to 528 MHz. The breakdown threshold,
Figs. 3(a) and 3(b) show the SBS reflectivity for the single-mode and the multimode cases of CCl4 and Fluorinert FC-75 as a function of the pump energy. Note that CCl4 and FC-75 have a very similar SBS gain and Brillouin linewidth (see Table 1), which results in a similar reflectivity curve of typical nonlinear variation for the single-mode pump. For the multimode pump, the SBS reflectivity is different for each of the liquids. The fact that the SBS threshold for both pump modes is approximately the same regardless of the liquids indicates that the SBS gain for the multimode pump is as high as that for the single-mode pump in both liquids The peak reflectivity is 30% in CCl4 and more than 65% in FC-75, though the reflectivity decreases as the pump energy increases. Note also that even though the single-mode pump generally has a higher SBS gain (Valley, 1986; Arecchi, 1972), the SBS reflectivity in CCl4 is slightly higher for the multimode pump than for the single-mode pump near the SBS threshold of the single-mode case. For FC-75, on the other hand, the behavior is exactly the opposite.
Several factors appear to contribute to the reflectivity difference of the multimode pump. We interpret the SBS reflectivity for the multimode pump in terms of the temporal intensity spikes of the multimode pulse, which are absent in the single-mode pulse. A beating between a large numbers of longitudinal mode brings the intensity spikes to rise, and the intensity spikes have enough power to induce nonlinear effects, such as self-focusing and the optical breakdown. The self-focusing that is caused by the intensity spikes is likely to lead to an anomalously high reflectivity of the multimode pump near the SBS threshold in CCl4. The critical power of the self-focusing is given by
The self-focusing seems to be deleterious for SBS because it can enhance the optical breakdown. The experimental results in Table 1 confirm that the optical breakdown starts at an
In addition to the breakdown due to self-focusing, an intensity spike can easily generate an optical breakdown by itself because it has a very steep rising edge. For efficient SBS to occur, temporal fluctuations in the pump must be slow in relation to the acoustic phonon lifetime. If the temporal fluctuations are fast in relation to the acoustic phonon lifetime, the acoustic waves have insufficient time to build up. Hence, the intensity spikes with the steep rising edge and energy levels that exceed the breakdown threshold can reach the focal area without losing their energy since they don’t provoke a backward reflection. They can therefore generate an optical breakdown and reduce the SBS reflectivity even at low energy. For the single-mode case, the region of SBS reflection moves fast in the opposite direction to the pump pulse; and the pump pulse is reflected before the focal area from a region in which the optical intensity is too small to induce an optical breakdown (Hon, 1980). Thus, even if the pump energy is large, no optical breakdown is generated for the single-mode pump.
Fig. 3(c) shows the SBS reflectivity for acetone. The multimode pump provides higher reflectivity than the single-mode pump near the SBS threshold, which is very similar to the results of CCl4. Table 1 shows that acetone has approximately the same nonlinear refractive index as CCl4. Thus, the SBS reflectivity of the multimode pump with a large number of longitudinal modes is significantly affected by self-focusing induced by the high intensity spikes. The reflectivity for the multimode pump increases as the energy rises to 6 mJ and then decreases strongly because of the severity of the breakdown; in contrast, the reflectivity for the single-mode pump increases monotonically. Fig. 3(d) shows the measured reflectivity of CS2. Of all the four liquids examined, CS2 has the lowest SBS threshold energy (approximately 0.3 mJ) and the highest reflectivity (approximately 95%) for the single-mode pump because it has the highest steady state SBS gain (Table 1). On the other hand, the SBS reflectivity for the multimode pump is the lowest and almost zero throughout the entire region. The critical power (20 KW) for the self-focusing is about half the SBS threshold (40 KW). Furthermore, CS2 has the longest acoustic lifetime (6 ns) of the liquids used (Erokhin et al., 1986), and this life-span is comparable to the pulse-width of the pump beam. As already mentioned, if the temporal fluctuations of the pump pulse are fast in relation to the acoustic phonon lifetime, the acoustic waves lack sufficient time to build up. As a result, CS2 has a lower breakdown threshold (approximately 0.1 mJ) than the SBS threshold, and this very low threshold can account for the almost zero reflectivity observed. We observed that the optical breakdown produces a filament if the pump energy is almost as weak as the SBS threshold. Note that the stimulated Raman scattering (SRS) may be also responsible for the low reflectivity. CS2 has a high SRS gain. The very short response time of the SRS process (10-11 s) implies that the SRS response to the intensity spikes of the multimode pulse is better than that of the SBS process (Linde et al., 1969).
4. The cross-type double-pass laser amplifier with symmetric SBS-PCMs
Kong et al. proposed a cross-type double-pass laser amplifier with symmetric SBS-PCMs. (Kong et al., 1998; Kong et al., 2001) Fig. 5 shows the conceptual layout. In this section, the noteworthy advantages of the cross-type amplifier with SBS-PCMs will be examined, particularly the use of the threshold of SBS-PCM to attain perfect isolation of the leak beam, the compensation of thermally induced birefringence (TIB), and the alignment-free property.
4.1. Optical isolation by the threshold characteristic of SBS
As shown in Fig. 5, the proposed amplifier with an SBS-PCM is cross-shaped and consists of an amplifier and an isolator. The SBS-PCM in the right arm works as an optical isolator. It isolates the master oscillator from the leak beam perfectly if the optical path length, L, between the SBS-PCM and the post-stage is longer than the spatial pulse length. When the energy of the leak beam is lower than the SBS threshold, the leak beam cannot be reflected by the SBS-PCM and consequently the oscillator is isolated from the leak beam (Kong et al., 1998; Kong et al., 2001). Fig. 6(a) shows the experimental setup for testing the isolation when the SBS-PCM is used. An amplifier in the left arm is omitted and a mirror, M1, is in place for the experiment. Fig. 6(b) shows the optical isolator that uses the SBS-PCM can completely cut off the leak beam and the backward propagating beam such as the amplified spontaneous emission from the post-stages.
4.2. Alignment-free optical system
The principle of the isolator that uses SBS-PCMs was explained in the previous section. However, the fact that an additional arm uses SBS-PCMs implies that the system has an alignment-free characteristic, which is one of the advantages of the cross-type amplifier system with SBS-PCMs. As shown in Fig. 7, although other systems such as the systems shown in Figs. 7(b) and 7(c) change the beam pointing due to the misalignment of the polarizing beam splitter, the suggested system of Fig. 7(a) has no effect on the beam direction or the beam position. Thus, the beam pointing of the output has the same level as the input (the master oscillator). The insensitivity to the misalignment and tilting of any optical components relies on the fact that the beam reflected by SBS-PCM follows exactly the same path as the incoming beam on account of the phase-conjugate characteristics.
4.3. Compensation for the thermal birefringence in the amplifier medium
Diminishment of the thermal load and the related thermal effects, particularly the compensation for TIB, is a traditional and important topic in the high-energy laser field. As shown in Fig. 8, the amplifying part of a cross-type amplifier with an SBS-PCM has three components: an amplifier rod, a polarization rotator (such as a Faraday rotator or a quarter-wave-plate), and an SBS-PCM (SBS cell and focusing optics). The total system has four available configurations, which are shown in Figs. 8(a) to 8(d). As in similar situations (Han & Kong, 1995), the theoretical prediction of TIB compensation can be given by Jones matrices. When we use the matrix elements of the Faraday rotator,
In the setup of Fig. 8(b), the output polarization is represented by
In the setup of Fig. 8(c), the output polarization is represented by
In the set up of Fig. 8(d), the output polarization is represented by
Eq. (5) shows that the setup of Fig. 8(d) gives a perfect 90 rotated output and compensates the TIB.
Fig. 9(a) shows the corresponding leak beam patterns for the four possible optical schemes of Fig. 8. This experimental result shows typical shape for each case. And Fig. 9(b) shows the depolarization ratio versus electrical input energy. The experimental result for the setup of Fig. 8(d) shows that the depolarization ratio is maintained at the low value as the electrical input energy increases, while the results for other setups (Fig. 8(a) - Fig. 8(c)) shows the depolarization ratio rises as the electrical input energy increases (Fig. 9(b)).
5. Waveform preservation of SBS waves via prepulse injection
There are difficulties in a laser system with SBS, particularly when multiple SBS cells are used in series for a high-power laser system. As the pulse is reflected from the SBS cell, the temporal pulse shape is deformed so that the reflected SBS wave has a steep rising edge (Shen, 2003). If SBS cells are used in series, the rising edge of the pulse becomes steeper and can cause an optical breakdown in the optical components. For the SBS-PCM, the steep rising edge leads to low reflectivity and low fidelity of the phase conjugated wave in the SBS medium (Dane et al., 1992). Thus, a suitable technique is needed to preserve the temporal waveform of the reflected SBS wave (Kong et al., 2005d).
The loss of the front part of the pumping energy to create the acoustic Brillouin grating is one of the main causes of the deformation. As a solution, the prepulse technique can be used to maintain the temporal waveform. In this scheme, the incident wave is divided into two pulses, the prepulse and the main pulse, and the prepulse is sent to the SBS medium before the main pulse with some delay. When the prepulse is injected before the main pulse, the main pulse can be reflected by means of a preexisting acoustic grating and the reflected pulse waveform can be preserved.
The scheme of the proposed setup for the temporal waveform preservation is presented in Fig. 10(a). A single longitudinal mode Nd:YAG laser oscillator is used as a pump source. It has a pulse width of 7~8ns and a repetition rate of 10 Hz. A Pockels cell (PC) is used to adjust the proper ratio of the prepulse energy and the main pulse energy, and the adjustment is made by adapting the high voltage that is applied to a PC for 10 ns, which is the time it takes for an incoming pulse to pass through the PC. The PC is in the off state when the pulse returns. The incident wave is split into two paths after PBS3, namely path 1 (prepulse) and path 2 (main pulse). The prepulse, which is initially s polarized, is reflected when it reaches PBS2 after the SBS process because the PC is in the off state when the pulse returns. The main pulse, which initially has p polarization, follows a process that is very similar to the process of the prepulse and consequently has the p polarization needed to pass through PBS2. There is another variation that uses no active optics. In Fig. 10(b), HWP2 and the Faraday rotator (45 rotator) are used instead of the PC in Fig. 10(a). HWP2 is used to adjust the ratio of the prepulse energy and the main pulse energy. The measurement is taken on path 2. A wedge plate is inserted to monitor the shape of the reflected main pulse and the incoming main pulse waveform and the reflected SBS waveform are obtained. The delay is modulated by the movable mirror, and the FC-75 fluid is used as the SBS medium.
Let us define Emain as the energy of the main pulse, Epre as the energy of the prepulse, and Tdelay as the delay between the prepulse and the main pulse. Fig. 11 shows the waveform measured for values of Tdelay = 8 ns and Emain = 10 mJ. As Epre increases, the temporal waveforms of the reflected wave become similar to that of the incident wave. When Epre exceeds 3 mJ, the experimental data have very similar aspects as the case of Epre = 3 mJ. This similarity implies that if we set the prepulse energy equal to or larger than 3 mJ with a delay of 8 ns, the main pulse need not consume its own energy to build the acoustic grating.
Fig. 12 shows the minimum prepulse energy required to preserve the waveform of reflected pulse for various Tdelay (Yoon et al., 2009). For small Tdelay, the main pulse arrives so early that a part of the main pulse energy can play a role in building the acoustic grating, because the integrated energy of the prepulse is insufficient to generate the grating before the main pulse arrives. Therefore the energy required to preserve the waveform of the main pulse is higher than the moderate Tdelay. For large Tdelay, most of the acoustic grating disappears before the main pulse arrives at the SBS interaction region so that more energy is required to preserve the waveform.
A theoretical calculation that describes these experimental results was formulated using a simple model. If the pump pulse is focused in the SBS medium, acoustic phonons are generated and then accumulated in the focal area. Considering the phonon decay, the pump pulse energy transferred to acoustic phonons and accumulated by time
To instigate the stimulated process, an amount of acoustic phonons over the required threshold is required. The accumulated phonon energy needed for SBS ignition, called the critical energy
6. Coherent beam combined laser system with phase stabilized SBS-PCMs
To achieve a high repetition rate in a high-power laser, many researchers have widely investigated several methods, such as a beam combination technique with SBS-PCMs, a diode-pumped laser system with gas cooling, an electron beam–pumped gas laser, and a large ceramic crystal (Lu et al., 2002; Kong et al., 1997, 2005a, 2005b; Rockwell & Giuliano, 1986; Loree et al., 1987; Moyer et al., 1988). The beam combination technique seems to be one of the most practical of these techniques. The laser beam is first divided into several sub-beams and then recombined after separate amplification. With this technique there is no need for a large gain medium; hence, regardless of the output energy, this type of laser can operate at a repetition rate exceeding 10 Hz and can be easily adapted to modern laser technology. However, with conventional SBS-PCMs, the SBS waves have random initial phases because they are generated by noises. For this reason, the phase locking of the SBS wave is strongly required for the output of a coherent beam combination.
6.1. Phase control of the SBS wave by means of the self-generated density modulation
There have been several successful works in the history of the phase locking of SBS waves (Rockwell & Giuliano, 1986; Loree et al., 1987; Moyer et al., 1988). Although these works show good phase locking effects, they have some problems in terms of the practical application of a multiple beam combination. In the overlapping method, all the beams are focused on one common point. The energy scaling is therefore limited to avoid an optical breakdown, and the optical alignment is also difficult. In the back-seeding beam method, the phase conjugation is incomplete if the injected Stokes beam is not completely correlated. Kong et al. (2005a, 2005b, 2005c) proposed a new phase control technique involving self-generated density modulation. In this method, which is simply called the self-phase control method, a simple optical composition is used with a single concave mirror behind the SBS cell; furthermore, each beam phase can be independently and easily controlled without destruction of the phase conjugation. Thus, the phase control method obviates the need for any structural limitation on the energy scaling.scheme for phase control of the SBS wave by means of the self-generated density modulation: M1,M2&M3, mirrors; W1,W2,W3&W4, wedges; L1&L2, cylindrical lenses: L3,L4,L5&L6, focusing lenses, CM1,CM2,CM3&CM4, concave mirrors; H1&H2, half wave-plates; PBS1&PBS2, polarizing beam splitters.
The wavefront division scheme, which spatially divides the beam, is used to demonstrate the phase control effect with the self-phase control method in the first experiment (Kong et al., 2005a, 2005b, 2005c). The experimental setup is shown schematically in Fig. 13(a). A 1064 nm Nd:YAG laser is used as a pump beam for the SBS generation. The pulse width is 7 ns to 8 ns, and the repetition rate is 10 Hz. The laser beam from the oscillator passes through a 2 cylindrical telescope and is divided into two parts by a prism, which has a high reflection coating for an incident angle of 45 . The two parts of the divided beam pass through separate wedges and are focused into SBS-PCMs. The wedges reflect part of the backward Stokes beams so that they are overlapped onto a CCD camera. Then, the interference pattern of them is generated. The degree of the fluctuation of the relative phase difference between the SBS waves is quantitatively analyzed by measuring the movement of the peaks in the interference pattern.
For the case of the wavefront division, the divided sub-pump beams get fluctuating energies for every shot due to the beam pointing effect of the laser source, which seems to generate the fluctuation of the relative phase difference between the SBS waves, because the phase of the SBS wave depends on the pump energy. This beam pointing problem can be overcome by using an amplitude division method, whereby the sub-pump beams have almost the same level of energy (Lee et al., 2005). The experimental setup of the amplitude division scheme is shown in Fig. 13(b). In the amplitude division scheme, the laser beam from an oscillator is divided into two sub-beams by a beam splitter (BS).
Fig. 14 shows the experimental schematic and experimental results for the unlocked case. Each point in Fig. 14(c) represents one of 160 laser pulses. As expected, has random value for every laser pulse. Fig. 14(b) shows the intensity profile of the 160 horizontal lines selected from each interference pattern. The profile also represents the random fluctuation.
Fig 15 shows phase control experimental results in the wavefront division scheme. Fig. 15(a) shows the schematic and the experimental result of the concentric-type self-phase control. A small amount of the pump pulse is reflected by an uncoated concave mirror and then injected into the SBS cell. The standard deviation of the measured relative phase difference is ~ 0.165. Moreover, 88% of the data points are contained within a range of 0.25 (90). This result demonstrates that the self-generated density modulation can fix the phase of the backward SBS wave. Fig. 15(b) shows the schematic and the experimental result of the confocal-type self-phase control, where the pump beams are backward focused by a concave mirror coated with high reflectivity. The standard deviation of the measured relative phase difference is ~ 0.135. Furthermore, 96% of the data points are contained in a range of 0.25.
Fig. 16 shows phase control experimental results in the amplitude division scheme. Fig. 16(a) shows the schematic and the experimental result of the concentric-type self-phase control. The standard deviation of the measured relative phase difference is ~ 0.0366. And Fig. 16(b) shows the schematic and the experimental result of the confocal-type self-phase control. The standard deviation of the measured relative phase difference is ~ 0.0275. By employing the amplitude division scheme, the relative phase difference is remarkably stabilized compared with the wavefront dividing scheme.
6.2. Theoretical modeling on the phase control of SBS waves
In the previous section, the experimental results demonstrate the effect of the self-phase control method. On the basis of the phase control experiments, we present in this section the theoretical model suggested by Kong et al. to explain the principle of the self-phase control (Ostermeyer et al., 2008). Given that the pump wave propagates towards the positive z direction in the SBS medium, the pump wave, EP, and the Stokes wave, ES, can be expressed as
where A and B are the amplitudes of EP and ES; ω, k and
Only the final term of Eq. (12) can contribute to the acoustic wave because the first two terms are DC components and the third term denotes the fast oscillating components. The acoustic wave can be also expressed as
where ρ0 is the mean value of the medium density and Ω, ka, and
If the acoustic wave is assumed to be initially generated at time t0 and position z0, the acoustic wave can be rewritten as
The phase of the acoustic wave is then given by
In conventional SBS generation, t0 and z0 have random values as the SBS wave is generated from a thermal acoustic noise. However, t0 and z0 can be locked effectively by the proposed self-phase control method.
Fig. 17 describes the concept of the self-phase control method. The weak periodic density modulation is generated at the focal point due to the electrostriction by an electromagnetic standing wave that arises from the interference between the main beam, EP, and the low intensity counter-propagating beam, rEP. In the suggested theoretical model, the weak density modulation from the standing wave is assumed to act as an imprint for the ignition of the Brillouin grating. Hence, the initial position, z0, is no longer random but fixed to one of the nodal points of the density modulation. However, there are many candidates of the nodal points in the Rayleigh range because the Rayleigh length, lR, is much larger than the period of the stationary density modulation, λP/2, where λP is the wavelength of the pump wave. The phase differences between the acoustic waves generated at different nodal points have the values of
The initial time, t0, when the acoustic wave is determined should be known. In the research on the preservation of the SBS waveform (Kong et al., 2005d), the front part of the pump energy is consumed to create the acoustic Brillouin grating of the SBS process. This consumed energy is regarded as the SBS threshold energy. The critical time, tc, when the SBS is initiated can then be determined by the following equation:
where Eth is the SBS threshold energy of the SBS medium and
if we assume that z0 is fixed;
Fig. 18 shows the calculated critical time, tc, as a function of the pump energy, which ranges from the SBS threshold of FC-75 (2 mJ) to 100 mJ.
When two beams are combined by the SBS-PCM, energy fluctuations of the each input beam give the shot-to-shot change on the critical time difference. Fig. 19 shows the calculated results and the experimental results of the phase fluctuation. Using the measured energy fluction of the each input beam, the phase fluctuation of Fig. 19(a) is simulated. The experimental investigation is conducted for the cases of E1= 10mJ, 30mJ, 50mJ, and 70mJ with several E2 values. In both graphs, the standard deviation of the relative phase fluctuation is shown. The shapes of the graphs are similar but the vertical scales are different.
6.3. Long-term phase stabilization of SBS wave
The self-phase control method ensures the SBS wave is well stabilized for several hundred shots. However, a thermally induced long-term phase fluctuation occurs when the number of laser shots increases (Kong et al., 2006, 2008). This slowly varying phase fluctuation can be easily compensated through the active control of PZTs attached to one concave mirror of the SBS-PCM. Figs. 20 and 21 show the phase control experimental results for the cases with PZT control and without PZT control, respectively, in a two-beam combination system. The phase difference and the output energy are measured during 2500 laser shots (250 s) for a pump energy level of Ep1,2≈50 mJ. The case without PZT control showed long-term phase and output energy fluctuations. In the case with the PZT control, the phase difference between the SBS beams is well stabilized with a fluctuation of 0.0214λ(=λ/46.8) by standard deviation; furthermore, the output energy is stabilized with a fluctuation of 4.66%.
6.4. Coherent beam combined laser system for high energy, high power, high beam quality, and high repetition rate output
Figs. 22(a) and 22(b) show the conceptual schemes of the coherent beam combination laser system for high energy, high power, high beam quality, and a high repetition rate (Kong et al., 1997, 2005a, 2005b). Fig. 22(a) shows the wavefront division scheme, and Fig. 22(b) shows the amplitude division scheme. In this beam combination laser system, the main beam is divided into many sub-beams for separate amplification; the beam is divided either by prisms in the wavefront division scheme or by polarizing beam splitters in the amplitude scheme. Both schemes include a series of cross-type amplifier stages. Each cross-type amplifier has SBS-PCMs on both sides and is insensitive to the misalignments of the optical components because the reflected phase conjugate waves return to exactly the same path as the incident beam. As a result, the cross-type beam combination system is highly beneficial in terms of alignment, maintenance, and repair. The SBS-PCMs on the right-hand side of each cross-type amplifier stage perform as optical isolators. On the left-hand side of each cross-type amplifier stage, the array amplifier can increase the beam’s energy with double pass optical amplification when it is divided by some of the sub-beams. For the reflectors in the array amplifier, we used SBS-PCMs instead of conventional mirrors. The SBS-PCMs can compensate for the thermally induced wavefront distortions, and self-focusing can occur in the active media with the generation of phase conjugate beams. A diffraction-limited high quality beam can therefore be obtained at the output stage. The divided sub-beams are recombined again after the double-pass amplification and become the input beam of the next amplifier stage. By using many amplifier stages of beam combination, we can obtain a high-energy laser output for the fusion. In the array amplifier, Faraday rotators are located on the amplification beam lines to compensate for the thermally induced birefringence, and phase-controlled SBS-PCMs are used with the self phase control method for coherent output.
In this chapter, a high-energy, high-power amplifier system using SBS-PCMs is introduced. The system, which is constructed systematically with a cross-type amplifier and SBS-PCMs as a basic unit, has many advantages: for example, it has freely scalable energy and a perfectly isolated leak beam; it also compensates for the thermally induced optical distortion and it has misalignment insensitiveness. For the coherent output of the combined beam, a new phase control method of the SBS wave with self-density modulation has been developed. The principle of this phase control method in the experiments for the wavefront and amplitude division schemes has been also explained and successfully demonstrated, as well as in theoretical modeling, and in the active control of the long-term phase fluctuation. In conclusion, the proposed beam combination laser system with SBS-PCMs, which is based on the cross-type amplifier, contributes to the realization of the a high energy, high power laser that can operate with a repetition rate higher than 10 Hz, even for a huge output energy in excess of several MJ.
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