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Abstract
By combining cross-polarized wave generation and femtosecond optical parametric amplification, a high-contrast front end featuring ultrahigh contrast, a broadband spectrum, an excellent beam profile, and good stability is built for a 10-PW-level Ti:sapphire laser in the Shanghai Superintense Ultrafast Laser Facility (SULF-10PW laser). The front end can deliver a cleaned pulse with a 110 μJ energy at 1 kHz, and the bandwidth of the cleaned pulse exceeds 60 nm (FWHM), which can support a 17 fs compressed pulse duration. The measured output energy fluctuation in one hour is <1.8% in rms value. The measurement-limited contrast is 10−10 at 3 ps before the main pulse. Utilizing the high-contrast front end, single-shot contrast at 10−10 level has been demonstrated in the SULF-10PW laser at a 24 fs pulse duration.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
With the remarkable progress in the chirped pulse amplification (CPA) technique [1] and optical parametric chirped pulse amplification technique [2], many petawatt-level femtosecond laser systems have been built worldwide [3–8]. Currently, some laboratories are trying to develop as high as 10-PW-level femtosecond laser systems, such as ELI-10PW [9], Vulcan-10PW [10], the 10 PW laser in SIOM [11], and PEARL-10PW [7]. The corresponding focused peak intensity of the femtosecond laser pulse has now reached 1021–1022 W/cm2 [12], and will reach 1023 W/cm2 in the near future. This high focused laser intensity creates many exciting opportunities for laser–matter interactions in the relativistic and even ultra-relativistic regime. For these plasma-creating experiments driven by ultra-intense femtosecond laser pulses, the higher temporal contrast is required to prevent intense prepulses and amplified spontaneous emission (ASE). Prepulses and ASE with intensities at the 1011 W/cm2 level can generate unwanted low-density plasma in advance of the main pulse and thus significantly influence laser–plasma interactions [13]. Therefore, for laser intensity at 1022 W/cm2, the temporal contrast should be better than 10−11 to limit destructive pre-plasma dynamics. Higher temporal contrast is necessary as the laser intensity increases further to 1023 W/cm2.
To improve the temporal contrast of high-peak-power femtosecond laser systems, many pulse cleaning techniques have been developed, such as plasma mirror (PM) [14], saturable absorber (SA) [15, 16], self-diffraction (SD) [17], nonlinear ellipse rotation (NER) [18, 19], cross-polarized wave generation (XPWG) [20, 21], and optical parametric amplification (OPA) [22–24]. Using these techniques, a temporal contrast of 10−9-10−11 can be achieved in high-peak-power femtosecond laser systems. However, these techniques are still more or less limited by some problems. For example, the PM technique can improve the temporal contrast by only 2-3 orders of magnitude, and it generally introduces an obvious pulse energy loss; the SA technique can enhance the temporal contrast by only 1-2 orders of magnitude. The SD technique, which is affected by noise from scattering in the bulk medium, can improve the temporal contrast by only 4 orders of magnitude. Moreover, the angular dispersion of the clean pulse in the SD technique is also a problem. The NER and XPWG techniques, which are limited by the extinction ratio of the polarizers, can improve the temporal contrast by only 4-5 orders of magnitude [18–21]. In addition, NER and XPWG are based on third-order nonlinear processes. When working at a high output energy, the high-intensity incident pulse can damage the nonlinear crystal and thus degrade the reliability of long-term operation. Femtosecond OPA (around 800 nm) technique, which is limited by low gain, can improve the temporal contrast by only 3-4 orders of magnitude [22]. Moreover, the lack of a suitable broadband seed for 800 nm femtosecond OPA is another problem [25]. Thanks to a high gain, picosecond OPA can improve the temporal contrast by more than 5 orders of magnitude [23]. However, the requirement for an extra picosecond pump source obviously increases the cost and complexity of the laser system. In addition, the parametric fluorescence within the pump time window (picoseconds to tens of picoseconds) will degrade the temporal contrast on the picosecond scale [24]. In our previous work, a novel pulse cleaning technique with ultrahigh contrast was experimentally demonstrated. However, the spectral width of the clean pulse after the OPA and second-harmonic generation (SHG) processes is still a bottleneck for achieving a short pulse duration (below 30 fs) [26]. All of the problems mentioned above will influence or limit the application of these pulse cleaning techniques.
An ideal pulse cleaning technique should simultaneously possess ultrahigh contrast, a broadband spectrum, good beam quality and good stability. Further, it should be inexpensive and easy to implement. By combining XPWG with femtosecond OPA, a novel nonlinear pulse cleaning technique that possesses all the above features is proposed in this work. In this new technique, XPWG in air can generate a high-contrast, broadband seed pulse for the femtosecond OPA. In addition, the low output energy (several microjoules) of XPWG can ensure the reliability of long-term operation. The femtosecond OPA that works in type I phase matching at degeneracy can support a broad gain bandwidth and further promote the seed pulse energy and temporal contrast simultaneously. Moreover, the initial incident pulse for XPWG and the pump pulse for OPA are produced by a common 1 kHz Ti:sapphire CPA laser, which can reduce the cost and simplify the design. The combination of the XPWG and femtosecond OPA techniques can break through the technical bottlenecks that exist in the sole XPWG technique or the sole OPA technique, which enables the generation of high-quality seed pulse for high-peak-power femtosecond laser systems.
In our experimental investigations, a high-contrast front end based on the XPWG and OPA techniques can run steadily day to day. It can generate clean pulses with an excellent beam profile and a 110 µJ energy at 1 kHz. The measured output energy fluctuation in one hour is about 1.8% in rms value. The spectral width of the clean pulse exceeds 60 nm (full width at half-maximum, FWHM), which can support a 17 fs pulse duration. Limited by the dynamic range of the measurement, the measured contrast ratio of the clean pulse is 10−10 at 3 ps before the main pulse. In the framework of the Shanghai Superintense Ultrafast Laser Facility (SULF), single-shot contrast at 10−10 level has been demonstrated at a 24 fs pulse duration, indicating that the front end can serve as a good seed source for the high-peak-power femtosecond laser. In addition, the simplicity of the front end makes the miniaturization possible.
2. Experimental setup of the front end
The experiment is performed using a commercial Ti:sapphire CPA laser (Coherent, Astrella), which can deliver sub-40-fs laser pulses with a 6 mJ energy at a 1 kHz repetition rate. A linearly polarized initial pulse with an energy of only ~3.9 mJ is injected into the nonlinear pulse cleaner. After a beam splitter, the incident pulse is divided into two laser beams. The transmitted laser pulse with an ~80 μJ energy is the initial seed pulse for the cross-polarized wave (XPW) filter, and the reflected laser pulse with a 3.82 mJ energy is the pump pulse for the femtosecond OPA. The experimental scheme of the front end is presented in Fig. 1.
Fig. 1 Experimental scheme of the front end based on XPWG and OPA techniques. BS, beam splitter; CM, chirped mirror pair; P, polarizer; HWP, half-wave plate; TDL, time delay line; DM, dichroic mirror.
The XPW filter consists mainly of two sets of chirped mirrors, two crossed polarizers, two telescopes, two nonlinear crystals (BaF2), and an achromatic λ/2 plate. The first set of chirped mirrors (CM1) can compensate for the dispersion introduced by the beam splitter and the first polarizer (P1). The extinction ratio of the polarizers (P1 and P2) used in the XPW filter is better than 105. After the first polarizer, the laser pulse is focused by an all-reflective telescope, which consists of a concave mirror (M1) and a convex mirror (M2). Two BaF2 crystals ([011]-cut orientation, 1.5 mm in thickness) are placed in air. At the cost of conversion efficiency, the positions of the two crystals are optimized to ensure good beam quality and long-term stability in the XPW filter. After the BaF2 crystals, the second polarizer (P2) is used to discriminate the fundamental pulse and the XPW signal. The output XPW signal with a 9 µJ energy is up-collimated by the second telescope (formed by M3 and M4). Subsequently, the other set of chirped mirrors (CM2) is used to compensate for the residual chirp of the XPW signal. After the chirped mirrors, an achromatic λ/2 plate is used to horizontally polarize the XPW signal. The XPW signal serves as the seed pulse for the subsequent femtosecond OPA.
The OPA filter consists mainly of two sets of time delay lines, three dichroic mirrors, an SHG crystal, a telescope, and an OPA crystal. The first time delay line (TDL1) is used to delay the pump pulse around the 800 nm central wavelength. After the first time delay line, the laser pulse is frequency-doubled by a 1-mm-thick beta-barium borate (BBO) crystal. The conversion efficiency of the SHG process (type-I phase matching) is approximately 47%. The thickness of the BBO crystal is optimized to ensure a good conversion efficiency and to avoid excess nonlinear effects such as self-phase modulation and self-focusing, which can degrade the temporal and spatial quality of the SHG pulse. A dichroic mirror (DM1) is installed after the SHG crystal to separate the fundamental pulse and the SHG pulse. The SHG pulse reflected by the dichroic mirror is then down-collimated by an all-reflective telescope (formed by M9 and M10). After the telescope, the other time delay line (TD2) is used to accurately adjust the delay between the SHG pump pulse and the signal pulse in the OPA process. After the second time delay line, another dichroic mirror (DM2) is used to combine the SHG pump pulse and the signal pulse. Owing to the loss of the optical components, the energy of the signal pulse after the second dichroic mirror decreases to ~5 µJ. The total energy of the SHG pump is about 1.8 mJ. The angle between the pump pulse and the signal pulse is about 1°. Both the SHG pump pulse and the signal pulse are injected into a BBO crystal. The BBO crystal in the OPA process works at type-I phase matching; the thickness of the crystal is 0.5 mm. After the OPA crystal, a third dichroic mirror (DM3) is used to separate the residual SHG pump pulse and the signal pulse. The signal pulse is amplified from ~5 µJ to ~110 µJ in the OPA process; the energy gain is more than 20 times. Consequently, the temporal contrast of the signal pulse is further promoted by more than 20 times. The B-integral value for the front end is approximately 0.57. Although an ultrahigh-contrast idler pulse with an energy above 100 µJ is also obtained in this front end, the signal pulse of the OPA is still used as the clean seed pulse in this work.
The entire experimental setup was installed compactly on a bench. Because it does not require complicated equipment such as a hollow-core waveguide, the front end is simple, which makes miniaturization possible. By further upgrading the system, for example, using parabolic mirrors as the focusing and collimating optics in the XPW filter, the size of the front end can be further reduced.
3. Characterization of the front end
In the experiments, we examined the characteristics relevant to application of the front end to high-peak-power femtosecond laser systems. They are the pulse energy, energy stability, beam quality, spectrum, and temporal contrast.
The energy of the clean seed pulse is a key parameter for high-peak-power, high-contrast femtosecond laser systems. Our previous work demonstrated the effect of the seed pulse energy on the temporal contrast [27]. The energy of the clean pulse is approximately 110 µJ, which is generally sufficient for suppression of ASE in the subsequent amplification. The conversion efficiency of the nonlinear pulse cleaner is approximately 2.8% for the 3.9 mJ initial incident pulse. Owing to the lack of a suitable telescope, the beam size of the SHG pump pulse is larger than that of the signal pulse in our experiment. Although the total energy of the SHG pump pulse reaches 1.75 mJ, the effective pump energy for the OPA is measured to be only 1.3 mJ. Therefore, a higher conversion efficiency and larger output energy can be obtained by improving the spatial matching between the SHG pump pulse and the signal pulse. The front end can run steadily day to day. The output energy stability is also measured. The measurement shows that the energy fluctuation in one hour is as low as 1.8% (rms). The good energy stability results from the stable operation of the commercial 1 kHz Ti:sapphire CPA laser and the optimization of the XPWG process. The energy and energy stability of the clean pulse are shown in Fig. 2(a).
Beam quality is also important for the practical application of the clean pulse. In the front end, the self-focusing effect in the bulk medium is a major reason for the degradation of the beam quality. By using a nonlinear crystal of suitable thickness and controlling the incident pulse intensity on the nonlinear crystal in both the XPWG and OPA processes, a cleaned pulse with a good beam profile can be obtained. In addition, the spatial filtering effect in the XPWG process is also very helpful for improving the beam quality of the front end [28]. A clean pulse with a smooth beam profile can be obtained in our experiment. The beam profile of the clean pulse is shown in Fig. 2(b).
The spectrum of the clean pulse is another crucial parameter for high-peak-power femtosecond laser systems. A broadband spectrum is required to obtain a short pulse duration and thus to increase the pulse peak power. Figure 3 shows the spectral evolution in the front end, including the spectrum of the initial pulse from the commercial 1 kHz Ti:sapphire CPA laser (gray curve), the spectrum of the XPW seed pulse (blue curve), and the spectrum of the final clean pulse (red curve). Significant spectral broadening effects in the XPWG process can produce a seed pulse with a smooth, broad spectrum for the subsequent femtosecond OPA. The OPA working at type-I phase matching at degeneracy can support a very broad gain bandwidth. In addition, the spectrum-dependent gain in the OPA process will reshape the spectrum of the injected XPW seed pulse. Then, after the OPA, a slightly red-shifted spectrum with a total bandwidth exceeding 60 nm (FWHM) is obtained. The bandwidth of the clean pulse is nearly twice that of the initial pulse, which corresponds to a Fourier-transform-limited (FTL) pulse duration of 17 fs. Because the SULF is designed as a 20-fs-level laser system, the output spectrum of the front end completely satisfies the design specification.
Fig. 3 Spectral evolution in the front end. Gray, initial pulse from commercial Ti:sapphire CPA laser; blue, XPW seed pulse; red, final cleaned pulse.
Temporal contrast enhancement is the most important task of the front end. A commercial third-order cross-correlator (Amplitude Technology, Sequoia) is used to examine the temporal contrast of the clean pulse. The measured temporal contrast curves are presented in Fig. 4. The measured contrast ratio of the initial pulse from the commercial Ti:sapphire CPA laser is approximately 2 × 10−8 at tens of picoseconds before the main pulse (black curve). Unfortunately, the dynamic range of the third-order cross-correlator depends strongly on the incident pulse energy. Limited by the dynamic range of the measurement, the contrast ratio of the clean pulse is only ~10−10 at several picoseconds before the main pulse (red curve). However, according to the extinction ratio of the polarizer (better than 105:1) in the XPWG process and the gain (~22 times) in the OPA process [29, 30], the temporal contrast enhancement in the front end should be about 6 orders of magnitude in theory. Correspondingly, the estimated temporal contrast of the clean pulse should be 10−13~10−14 if there is no misalignment of polarizers in the XPWG process. Further, the inset in Fig. 4 indicates excellent coherent contrast (~10−10 at 3 ps before the main pulse) of the clean pulse.
Fig. 4 Measured temporal contrast of the commercial Ti:sapphire CPA laser (black curve) and the front end (red curve). The inset shows the measured temporal contrast on the 15 ps scale.
4. Contrast measurement in the SULF
The front end has been applied in the SULF [31, 32]. After injection of the clean pulse into the system, a high-peak-power and high-contrast laser pulse can be obtained. The temporal contrast is measured under two conditions: with or without insertion of a grism pair. The grism is a combination of a grating and a prism, which is utilized to reduce the high-order dispersion up to the forth order [32]. The grism pair has played an important role in the 10-PW-level laser system, as it can shorten the compressed pulse duration from ~30 fs to 24 fs [32]. However, owing to the poor quality of the coating in the grism pair, the transmission efficiency of the grism pair is only 10% in a double-pass configuration. The energy loss in the grism pair will significantly degrade the temporal contrast of the laser system. In addition, the spectral filter in the regenerative amplifier also contributes to degradation of the temporal contrast [33].
The contrast measurements are first conducted under a low repetition frequency and low output energy. The energy of the clean pulse is amplified to 7 J at 1 Hz. The experimental results are presented in Fig. 5. The black curve in Fig. 5 is the temporal contrast curve with the grism pair inserted. The measured contrast ratio of the laser system is better than 10−10 at 100 ps before the main pulse. The red curve is the temporal contrast curve without the grism pair inserted. The measured contrast ratio is much better than 10−11 at 100 ps before the main pulse. The difference between the results of the two measurements is caused by the energy loss in the grism pair. The experimental results demonstrate that the temporal contrast is significantly affected by the seed pulse energy, which is consistent with our previous work [27]. The experimental result in Fig. 5 also demonstrates that the actual temporal contrast of the clean seed pulse should be much better than the measured value (the red curve in Fig. 4). Compared with Fig. 4, the dynamic range shown in Fig. 5 can reach ~10−12. This is because the energy of the amplified laser pulse is large enough to support the maximal dynamic range of the measurement.
A single-shot third-order cross-correlator [34, 35] is also applied to measure the temporal contrast of the 10-PW-level Ti:sapphire laser system. By decreasing the pump energy of the single-shot amplifiers, an amplified energy of about 50 J is obtained before the compressor, which supports a peak power of 1.5 PW. The temporal profile after the compressor is shown in Fig. 6. The typical pulse duration shown in the inset of Fig. 6 is 24 fs (FWHM), which is obtained by the application of the grism pair. The temporal contrast reaches 10−10 level at −50 ps before the main pulse, which is the maximal dynamic range of the single-shot third-order cross-correlator. Prepulses 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, as real prepulses can also be generated by the nonlinear mixing of postpulses [36], a detailed work on investigation of prepulses and postpulses are still necessary and will be present in our future work.
Fig. 6 Temporal profile after the compressor measured by a single-shot third-order cross-correlator. The inset shows the measured autocorrelation trace of the compressed pulses in single-shot mode.
5. Conclusion
In conclusion, we demonstrated that a front end based on the XPWG and femtosecond OPA techniques can produce cleaned pulses with high temporal-spatial quality (10−10 contrast near 3 ps before main pulse). The successful application in the SULF demonstrated that the front end can serve as a good seed source for high-peak-power femtosecond laser systems. Further, the simplicity of the front end makes the miniaturization possible. We believe that by driving the nonlinear pulse cleaner with the full energy (~6 mJ) of the 1kHz Ti:sapphire CPA laser, cleaned pulses with higher contrast and larger energy can be obtained. Further, an OPCPA pre-amplifier is being developed to substitute for the existed Ti:sapphire regenerative amplifier in the SULF. That can further improve the temporal contrast of the 10-PW-level Ti:sapphire laser, especially for the contrast within 100 ps temporal window before the main pulse.
Funding
National Natural Science Foundation of China (Nos. 11127901, 61521093, and 61505234); International S&T Cooperation of China Program (No. 2016YFE0119300); Strategic Priority Research Program of the Chinese Academy of Sciences (No. XDB160301); Youth Innovation Promotion Association, CAS.
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