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Abstract
We have theoretically and experimentally investigated the evolution of the temporal contrast in a 10-PW-level Ti:sapphire laser in the Shanghai Superintense Ultrafast Laser Facility (SULF). The effects induced by the grism pair, spectral shaping filter, and increase in gain on the temporal contrast were investigated. First, it was found that the energy loss of clean seed pulses in the grism pair is a major factor in contrast degradation. Because of the low transmission efficiency of the grism pair (~10%), the temporal contrast is degraded by one order of magnitude. Second, the spectral shaping filter in the regenerative amplifier degrades the temporal contrast by increasing the intracavity loss. Finally, as the amplified spontaneous emission pedestal experiences gain more than the main pulse in Ti:sapphire amplifiers, particularly during saturated amplification, the temporal contrast will further deteriorate as the gain increases in multi-stage Ti:sapphire amplifiers. In addition, the effect on the temporal contrast induced by the extraction during pumping technique in large-aperture Ti:sapphire amplifiers has been considered. According to the investigations described above, the design of the SULF can be further improved. It is predicted that a temporal contrast of over 10−11 can be achieved at a peak power of 10 PW following the improvements. The investigations conducted in this study can provide guidelines for improving the temporal contrast in ultrahigh-peak-power Ti:sapphire lasers.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Since the invention of the chirped pulse amplification (CPA) [1] and optical parametric chirped pulse amplification (OPCPA) [2] techniques, the peak power of ultraintense femtosecond laser pulses has been boosted to petawatt levels [3–7]. The corresponding focused peak intensity has already reached 1022 W/cm2 in petawatt lasers [6,7]. Furthermore, with the development of 10-PW lasers [3,8,9], the focused intensity will reach 1023 W/cm2 in the near future. Such a high focused laser intensity has potential applications for the acceleration of charged particles (electrons, protons, and heavier ions) [10] and the generation of coherent or incoherent high-energy radiation [11]. Among these applications, proton acceleration has attracted significant attention for its potential applications in proton therapy and proton imaging [12]. However, for these applications a prepulse or amplified spontaneous emission (ASE) with an intensity level of over 1011 W/cm2 can produce preplasma and modify the target before the arrival of the main pulse [13]. For a laser intensity of 1022 W/cm2, the temporal contrast, defined as the ratio of the intensity of the ASE background to the peak intensity of the main pulse, should be better than 10−11 to restrict destructive preplasma dynamics. Furthermore, the required temporal contrast increases as the focused intensity increases. Therefore, achieving a high temporal contrast is a key issue in high-peak-power laser systems. Note that the terminology “high temporal contrast” corresponds to a low value of contrast ratio in this work.
Owing to the importance of the temporal contrast, considerable effort has been invested in this matter. In recent decades, many pulse cleaning techniques have been developed to improve the temporal contrast based on a double-CPA scheme [14], such as saturable absorber [15], cross-polarized wave generation [16], and optical parametric amplification [17]. In addition, plasma mirror is also a solution for improvement of temporal contrast [18]. By utilizing these techniques, a temporal contrast level of 10−11 can be achieved in high-peak-power femtosecond laser systems [7,19,20]. Although considerable progress has been achieved in research on the temporal contrast, some problems remain. For example, few studies have shown the temporal contrast evolution in high-peak-power femtosecond laser systems. Furthermore, limited by measurement methods, the temporal contrast for PW or multi-PW laser pulses is generally obtained at low energy levels. The actual temporal contrast at full peak power remains a puzzle for many laser systems. To illuminate the evolution of the temporal contrast and find solutions for further improving it, it is important to study the mechanisms of temporal contrast degradation in a chain of laser amplifiers.
The Shanghai Superintense Ultrafast Laser Facility (SULF) is a large-scale project that aims to deliver 10 PW laser pulses [5]. The designed focused laser intensity will be over 1022 W/cm2, which requires a high temporal contrast of 10−11 at full peak power. In our previous work, an ultrahigh-contrast front-end was developed and applied in the SULF [19]. A single-shot contrast level of 10−10 was achieved under an amplified energy of 50 J, which is still insufficient for the future focused intensity of 1022 W/cm2. As the contrast degradation results mainly from the chain of laser amplifiers in our case, we must investigate the evolution of the temporal contrast in the SULF-10PW laser, and identify the mechanisms of temporal contrast degradation.
In this study, the evolution of the pulse temporal contrast in the SULF-10PW laser was investigated both theoretically and experimentally. The effects induced by the grism pair, spectral shaping filter, and increase in gain on the temporal contrast were investigated. First, the energy loss of clean seed pulses in the grism pair is a major factor in contrast degradation. Owing to the low transmission efficiency of the grism pair (~10%), the temporal contrast is degraded by one order of magnitude. Second, the spectral shaping filter in the regenerative amplifier (RA) degrades the temporal contrast by increasing the intra-cavity loss. Finally, as the ASE pedestal experiences more gain than the main pulse in Ti:sapphire amplifiers, particularly during saturated amplification, the temporal contrast will deteriorate further as the gain increases. In addition, the effect on the temporal contrast of the extraction during pumping (EDP) technique in large-aperture Ti:sapphire amplifiers was considered. According to our theoretical analysis and experimental measurements described above, the design of the SULF can be further improved. For example, an acousto-optic programmable dispersive filter (Dazzler, Fastlite) can be introduced as a substitution for the grism pair and spectral shaping filter, and a multi-pass preamplifier can be employed as a substitute for the RA. Furthermore, the saturated amplification process can be optimized. It is predicted that a temporal contrast of over 10−11 can be achieved at an output peak power of 10 PW. The investigations described above can provide guidelines for improving the temporal contrast in ultrahigh-peak-power Ti:sapphire lasers, including but not limited to the SULF-10PW laser.
2. System characteristics of the SULF-10PW laser
The SULF-10PW laser has been proposed by the Shanghai Institute of Optics and Fine Mechanics, and is still under construction. In 2017, pulses with 5.4 PW peak power were reported in the prototype facility [5]. A simplified layout of the laser system is presented in Fig. 1. The system is based on a double-CPA scheme [14]. A pulse from a commercial Ti:sapphire CPA laser passes through a nonlinear pulse cleaner, and is then stretched to approximately 2 ns by an Öffner stretcher. The nonlinear pulse cleaner, which links the two CPA stages, combines cross-polarized wave generation with femtosecond optical parametric amplification techniques [19]. A grism pair is inserted between the Ӧffner stretcher and the RA, and is utilized to reduce the high-order dispersion up to the fourth order [21]. After passing through the grism pair, the stretched pulse is amplified by an RA. A spectral shaping filter is inserted into the RA to control the spectral shape and suppress gain narrowing. After the RA, three multi-pass amplifiers amplify the signal beam to 7 J with a 1 Hz repetition rate. The signal beam is further amplified by a power amplifier and a final booster amplifier, achieving a maximum energy of 202.8 J. Finally, the vacuum compressor compresses the pulse duration to 24 fs with a 64% throughput efficiency. The corresponding maximum pulse peak power can reach 5.4 PW.
Here, the temporal contrast evolution in the above laser amplifier chain has been thoroughly investigated. We find that the grism pair, spectral shaping filter, and total gain have a significant influence on the temporal contrast. Therefore, the degradation of the temporal contrast induced by these three factors is analyzed in detail in the following sections.
3. Degradation of the temporal contrast induced by the grism pair
The grism pair plays an important role in the 10-PW-level laser system, because it can shorten the compressed pulse duration from 30 fs to 24 fs. However, owing to the poor quality of its coating, the transmission efficiency of the grism pair is only 10% in a double-pass configuration. Therefore, the effective seed energy is reduced by one order of magnitude. As noted in previous work [22–24], the temporal contrast is inversely proportional to the effective seed energy, which can be expressed as
where A is a constant determined by a particular amplifier, Eeff is the effective seed energy, and C is the temporal contrast. Note that a decrease of the seed pulse energy will lead to degradation of the temporal contrast. Therefore, the energy loss induced by the grism pair will degrade the temporal contrast by one order of magnitude.We have measured the temporal contrast when the grism pair is inserted or removed [19] by a third-order cross-correlator (Sequoia, Amplitude) under a similar pulse energy in the measurement setup. The energy of the seed pulse is amplified to 7 J at 1 Hz in the above two cases. The experimental results are illustrated in Fig. 2. The difference between the ASE pedestals for the two curves is approximately one order of magnitude, in agreement with the theoretical results. This demonstrates that the grism pair can drastically degrade the temporal contrast by causing a large energy loss. Therefore, increasing the transmission efficiency of the optical component or enhancing the initial seed energy represents an alternative method of improving the temporal contrast of a laser system.
4. Degradation of the temporal contrast induced by the spectral shaping filter
To control gain narrowing and gain redshift, one of the most commonly employed methods is to shape the spectrum in the RA by utilizing a spectral shaping filter [5,25]. By rotating the filter angle relative to the beam polarization, the attenuation for different wavelengths can be adjusted. Therefore, we can obtain different output spectrum shapes after the RA. However, this method will introduce a large intra-cavity loss. For the RA, Eq. (1) can be rewritten as [26]
where‾Isat is the averaged saturation intensity over the spectral bandwidth, KΔΩ describes the spatio-angular acceptance, KΔν describes the spectral acceptance, Kp describes the probability of an emitted photon to match the polarization direction of the amplifier,‾g0 is the averaged overall small signal gain of the amplifier, τ is the final compressed pulse duration, and‾L is the intra-cavity loss in the RA. Equation (2) shows that the temporal contrast depends on the intra-cavity loss. Inserting the spectral shaping filter will increase the intra-cavity loss, thus causing a degradation of the temporal contrast.A numerical simulation has been performed to analyze the evolution of the temporal contrast in the RA. This simulation is based on the model described in [27], which is derived from the Franz–Nodvik equations [28,29]. In our simulation, two typical shapes are considered for the output spectrum. Figure 3(a) illustrates the two spectrums: a blueshift spectrum (red line) and a narrow spectrum (blue line). The blueshift spectrum is generally adopted in high-peak-power Ti:sapphire lasers and can pre-compensate the gain red-shift in the laser system [30]. In the following discussion, for the sake of simplicity, the blueshift spectrum and narrow spectrum are referred as Blueshift and Narrow, respectively. Blueshift is modulated by the spectral shaping filter, while for Narrow the filter is moved outside the RA. The Fourier-transform-limited pulse durations for Narrow and Blueshift are 34 fs and 23 fs, respectively.
Fig. 3 Two typical shapes of the spectrum (a) and the corresponding output energies and temporal contrast (b) after the RA in the simulation.
Before the numerical simulation, some initial parameters of the model must be determined. First, owing to the attenuation of the stretcher, grism pair, Pockels cell, and Farady, the input seed energy of the RA is reduced to 0.8 µJ. The pump energy of the RA is set to 20 mJ. According to the expression of spontaneous emission power in [26,31],
the calculated initial spontaneous emission powers in the RA are 15 µW and 22 µW for Narrow and Blueshift, respectively. Second, considering the transmission of the Pockels cell, the reflectivities of the thin-film polarizer and cavity mirrors, and the transmission of the spectral shaping filter, the intra-cavity losses of Narrow and Blueshift are estimated to be 15% and 32% per pass, respectively.After the RA, the output energies of Narrow and Blueshift are 2.3 mJ and 0.28 mJ, respectively. The final compressed pulse duration is assumed to be near Fourier-transform-limited. The ASE noise generated by the RA mainly results from two aspects: (i) the fluorescence emitted during each pass and (ii) the amplification and integration of the fluorescence obtained by (i) over time. The calculated ASE energies are 0.41 nJ and 0.28 nJ for Narrow and Blueshift, respectively, considering an ASE duration of 2 ns. Therefore, the temporal contrasts after the RA are calculated to be 3.0 × 10−12 and 1.1 × 10−11 for Narrow and Blueshift, respectively. The simulation results are illustrated in Fig. 3(b), which shows that the temporal contrast can be deteriorated by approximately a factor of four by utilizing the spectral shaping filter. Furthermore, compared with the input temporal contrast, the output temporal contrast is deteriorated by over one order of magnitude by the RA. To solve the problem, one solution is to further enhance the output energy of the high-contrast front-end. Another solution is to substitute an OPCPA scheme for the RA. OPCPA can simultaneously support a better temporal contrast than a classical RA and overcome the gain-narrowing problem [23].
To develop a deep understanding of the evolution of the temporal contrast in the RA, the gain of the noise and signal in each round trip is calculated and plotted in Fig. 4. The initial ASE noise of the seed pulse itself is also taken into consideration in the calculation. In the early round trips, the noise has a higher gain than the main pulse, owing to the contribution of the newly generated fluorescence. Correspondingly, the temporal contrast deteriorates significantly in the early round trips, especially the first round trip. However, with the increase in round trips, the relative contribution of the newly generated fluorescence becomes insignificant. Then, the noise gain decreases to nearly the same value as the signal gain. As a result, the degradation of the temporal contrast becomes trivial. Meanwhile, in the case of Narrow, the signal gain is higher. The gain difference between the signal and noise is smaller in the initial round trips for the Narrow case, which will reduce the required number of round trips and help to improve the temporal contrast.
Fig. 4 Signal gain (red upside-down triangles), noise gain (black squares), and temporal contrast (blue triangles) per pass in the RA for Blueshift (a) and Narrow (b).
A proof-of-principle experiment was also conducted. The experimental parameters were almost the same as the simulation parameters. The output energies after the RA were 2.2 mJ and 0.26 mJ for Narrow and Blueshift, respectively, which agree well with the simulated values. The temporal contrast after the RA was measured by Sequoia. The results are presented in Fig. 5. As shown in the inset of Fig. 5, the mean ASE levels are 3.4 × 10−12 and 1.2 × 10−11 for Narrow and Blueshift, respectively. Therefore, the utilization of a spectral shaping filter in the RA will cause a deterioration of the temporal contrast by a factor of three-four. The experimental results validate the theoretical analysis. Both the theoretical and experimental investigations demonstrate that the RA is a major cause of degradation in the temporal contrast, especially with the introduction of a spectral shaping filter.
Fig. 5 Measured temporal profile. The inset is an enlarged drawing of the pulse profile within the temporal window from −300 ps to −250 ps.
5. Degradation of the temporal contrast induced by an increase in gain
In Ti:sapphire amplifiers, saturated amplification enhances the level of ASE [32]. In this section, the influence of the total gain on the temporal contrast is thoroughly analyzed. A numerical simulation is performed, where we consider the influence of the EDP technique [5,33]. The EDP technique has been applied in the booster amplifier of the SULF-10PW laser, which was proposed to suppress the parasitic lasing (PL) and transverse amplified spontaneous emission (TASE) [4,34]. The PL and TASE effects are not considered in our simulation. Furthermore, the newly generated spontaneous emission in the following cascaded amplifiers is neglected.
The evolution of the temporal contrast in the chain of Ti:sapphire amplifiers is illustrated in Fig. 6. The horizontal axis represents the amplifiers, and stages 1–6 represent the RA, first multi-pass amplifier, second multi-pass amplifier, third multi-pass amplifier, power amplifier, and final booster amplifier, respectively. The output energy of the RA is 2 mJ, and the temporal contrast is 1.6 × 10−11. After the five cascaded multi-pass amplifiers (stages 2–6), the energy increases to approximately 200 J. The output energy of each amplifier is illustrated in Fig. 6(a). Correspondingly, according to the numerical simulation, the temporal contrast will be degraded to 5.5 × 10−10 when the energy reaches 200 J. The variation of the temporal contrast in each amplifier is also illustrated in Fig. 6(a). The deterioration of the temporal contrast exceeds one order of magnitude in this amplification process. Therefore, in ultra-high-power laser systems, the large gain of the signal will significantly affect the temporal contrast of the laser pulses.
Fig. 6 (a) Output energies and corresponding temporal contrast, and (b) the conversion efficiency and corresponding deterioration coefficient of temporal contrast at different amplification stages.
The deterioration coefficient in each amplifier, defined as the ratio of the output temporal contrast to the input temporal contrast, is plotted in Fig. 6(b). The deterioration coefficient increases rapidly, especially in the power amplifier (stage 5) and final booster amplifier (stage 6). The left axis in Fig. 6(b) represents the conversion efficiency in each amplifier. Note that the conversion efficiencies in the power amplifier and final booster amplifier are considerably higher than in the other amplifiers. This is because these two amplifiers are running in saturation. Correspondingly, they suffer from larger deterioration coefficients than the other amplifiers. The significant saturation is responsible for the high deterioration coefficients in the power amplifier and final booster amplifier. However, in the third multi-pass amplifier (stage 4), although the conversion efficiency is high, the deterioration coefficient is small, in fact being even smaller than that of the second multi-pass amplifier (stage 3). In fact, the output temporal contrast depends not only on the conversion efficiency, but also on the small signal gain [26]. The small deterioration coefficient in the third multi-pass amplifier (stage 4) is a result of its low small signal gain. Therefore, to obtain a better temporal contrast in ultrahigh peak power Ti:sapphire laser systems, decreasing the small signal gain and avoiding saturated amplification in the multi-pass amplifiers should be helpful.
In the above numerical simulation, we have considered the EDP technique in the final booster amplifier. To theoretically analyze the effect on the temporal contrast of the EDP technique, we have also considered the case when the final booster amplifier is pumped without the EDP technique. The results are compared in Fig. 7. In the case of utilizing the EDP technique, a three-pass amplifier is necessary to make full use of the pump energy, reaching an output energy of over 200 J. When pumping without the EDP technique, the pump energy can be fully utilized after two amplification passes. The signal pulses quickly become saturated, and a two-pass amplifier is available to support the similar output energy. However, in the EDP case, the gains for both the main pulse and ASE noise in each amplification pass are smaller. Therefore, although more amplification passes are required in the EDP case, the total gains for both the main pulse and ASE noise are almost the same for the two different pumping schemes. The numerical simulation demonstrates that the effects on the temporal contrast are almost the same for pumping with and without the EDP technique. However, in practice pumping without the EDP technique results in serious PL and TASE effects in large-aperture Ti:sapphire amplifiers, which will restrict the signal energy extraction. Therefore, the EDP technique is preferred in large-aperture amplifiers.
In addition to our simulation, we also investigated the influence of the increase in gain on the temporal contrast experimentally. The measured results are shown in Fig. 8. As illustrated in Fig. 8(b), with the seed pulses amplified from stage 1 to stage 6, the measured output energy reaches 203 J. The output energies measured after stages 4 and 5 are 7 J and 45 J, respectively. The corresponding temporal contrast was measured, and is illustrated in Fig. 8(a) within a temporal window from −90 ps to 5 ps. For simplicity, a sampled laser beam with a small size and a low energy was used for the measurement. Serrated apertures and beam splitters were used to reduce beam size and beam energy, respectively. The temporal contrast after stage 1 (red line) and stage 4 (black line) was measured by Sequoia, while the temporal contrast for stage 5 (blue line) was measured by a single-shot cross-correlator [35]. Pre-pulses before the main pulse are generally attributed to artifacts, which are generated by multiple reflections at the optical components in the amplifiers and the measurement setup. However, real pre-pulses can also be generated by nonlinear mixing of post-pulses [36]. The ascending pedestal from −60 ps to 0 ps is generally generated by the scattering noise in the stretcher [37], the residual high-order dispersion in the compressor [38], and can also be reproduced by the post-pedestal during amplification [39]. Investigations on prepulses and ascending pedestal are still necessary and a detailed work will be presented in our future work. The pedestal at times beyond −75 ps is regarded as the ASE background. The mean ASE level was calculated as 2.2 × 10−10, 6.6 × 10−11, and 1.6 × 10−11 for stages 5, 4, and 1, respectively. This confirms that the temporal contrast deteriorates during the amplification process, while this deterioration can exceed one order of magnitude. A comparison between the experimental results and numerical results is presented in Fig. 8(b). The experimental results coincide well with the simulation results, which demonstrate the validity of our model. According to the simulation, a temporal contrast of 5.5 × 10−10 is predicted under full-amplified energy after stage 6.
Fig. 8 (a) Measured pulse profiles for different output energies. The inset shows an enlarged drawing of the pulse profile within the temporal window from −90 ps to −75 ps. (b) Measured output energies and corresponding temporal contrast.
According to the above theoretical and experimental investigations, the signal pulse cannot avoid deterioration of its temporal contrast during the amplification process owing to the increase in gain and saturation effects. When working in the saturation regime, the ASE will receive a higher gain than the main pulse. However, when working in the unsaturation regime, the conversion efficiency and energy stability will cause problems for high-peak-power laser systems. Therefore, there should be a tradeoff between the efficiency and temporal contrast in a laser design.
6. Temporal contrast improvement in the current SULF-10PW laser
In 2018, an amplified energy of 339 J was achieved by adding a final booster to the original 5.4 PW laser system, which can support a peak power of 10.3 PW with a compressed pulse duration of 21 fs [33]. As the designed focused laser intensity of the SULF-10PW laser is above 1022 W/cm2, a high temporal contrast of 10−11 is required at full peak power. However, the output temporal contrast under this scheme is far from the application requirements. From the above investigations, it can be concluded that the grism pair, RA (including the spectral shaping filter), and saturated amplification will lead to degradation of the temporal contrast. Therefore, compared with the previous SULF-10PW laser [33], in the current SULF-10PW laser the grism pair and RA are removed from the laser system. Instead, a Dazzler and a multi-pass preamplifier are inserted. Also, a new stretcher with broadband dielectric coating is introduced to further reduce the energy loss of the seed pulse. Moreover, to avoid contrast degradation induced by saturated amplification, the energy conversion efficiency of high-energy amplifiers is also optimized, especially in the final two Ti:sapphire amplifiers. The simplified layout of the current SULF-10PW laser is illustrated in Fig. 9(a).
Fig. 9 (a) Layout of the current SULF-10PW laser. (b) Evolution of the temporal contrast. MA: multi-pass amplifier, PA: power amplifier, BA: booster amplifier.
The transmission efficiency of the new stretcher is as high as 40%, which is twice the previous one. The Dazzler can simultaneously pre-compensate gain narrowing and compensate for high-order phase distortions at a higher transmission efficiency (~17%), which makes it an ideal substitute for the grism pair and spectral shaping filter. Utilization of the new stretcher and Dazzler can significantly decrease the energy loss of the high-contrast seed pulse. The RA is replaced by a multi-pass preamplifier, which suffers a smaller energy loss and material dispersion and can support a higher temporal contrast during amplification. Subsequently, the signal pulses are amplified by four multi-pass amplifiers, two power amplifiers, and one booster amplifier. The first power amplifier is pumped via a traditional double-end pump technique, while the second power amplifier and final booster amplifier are pumped via the EDP technique based on [5,33]. At present, the 1kHz Ti:sapphire CPA laser, the new stretcher, the Dazzler and MA1-MA4 have already been implemented in the SULF-10PW laser, while the other Ti:sapphire amplifiers are still under construction. In addition, high-energy repetitive pump lasers for PA1, PA2, and BA are being prepared. The repetition rate of the SULF-10PW laser will be enhanced to one shot per minute in the near future.
The evolution of the temporal contrast in the current SULF-10PW laser has been investigated theoretically, and is illustrated in Fig. 9(b). After replacing the stretcher, the grism pair and RA in the laser system, a higher temporal can be achieved. Meanwhile, after improving the energy conversion in the high-energy amplifiers, the evolution of the temporal contrast exhibits slow growth. The dramatic contrast degradation owing to the RA and saturated amplification has disappeared. A temporal contrast of 3.0 × 10−12 at 100 ps before the main pulse is predicted after the final booster amplifier, which can satisfy the requirements in the physical experiments. Therefore, it has been demonstrated that pulses with a temporal contrast of over 10−11 at an output peak power of 10 PW can be obtained in the current SULF-10PW laser.
Recently, amplified energy of 8 J at 1 Hz repetition rate has been obtained after MA4. Compressed pulse duration of ~22 fs has been be achieved with a sampled laser beam after MA4. The measured contrast ratio (−50 ps before the main pulse) is about 1.7 × 10−12, which is limited by the dynamic range of the third-order cross-correlator. In our previous experiments, the contrast ratio of amplifier stage 4 is only 6.6 × 10−11, which is shown in Fig. 8(a). Therefore, the contrast enhancement is more than one order of magnitude after improving the design of the 10-PW-level laser system. The recent progress of the SULF-10PW laser has demonstrated that the investigations and the solutions above is meaningful and feasible for the improvement of temporal contrast. The detailed measurement results will be presented after the upgradation of the SULF-10PW laser.
7. Conclusion
We have thoroughly investigated the temporal contrast evolution in the SULF-10PW laser both theoretically and experimentally. The effects induced by the grism pair, spectral shaping filter, and increase in gain on the temporal contrast were investigated. First, the energy loss of clean seed pulses in the grism pair was found to be a major factor in contrast degradation. Because of the low transmission efficiency of the grism pair (~10%), the contrast ratio is degraded by one order of magnitude. Second, the spectral shaping filter in the RA will degrade the temporal contrast by increasing the intra-cavity loss. Finally, because the ASE pedestal experiences more gain than the main pulse in Ti:sapphire amplifiers, particularly during saturated amplification, the temporal contrast is further deteriorated as the gain increases. The final temporal contrast is deteriorated by over one order of magnitude in the cascaded multi-pass Ti:sapphire amplifiers. In addition, the effect on the temporal contrast of the EDP technique in large-aperture Ti:sapphire amplifiers was also considered. A numerical simulation showed that the effects on the temporal contrast are almost the same when pumping with or without the EDP technique. In the current SULF-10PW laser, the RA and grism pair will be removed, and the saturated amplification will be optimized. Following this improvement, it has been demonstrated that pulses with a temporal contrast of over 10−11 can be obtained at an output peak power of 10 PW. The above investigations can provide guidelines for improving the temporal contrast in ultrahigh-peak-power Ti:sapphire lasers, including but not limited to the SULF-10PW laser.
Funding
National Natural Science Foundation of China (11127901, 61521093, and 61505234); International S&T Cooperation of China Program (2016YFE0119300); Strategic Priority Research Program of the Chinese Academy of Sciences (XDB160301); Science and Technology Commission of Shanghai Municipality (2017SHZDZX02); Youth Innovation Promotion Association of the Chinese Academy of Sciences.
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