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Abstract
We demonstrate the simultaneous temporal contrast enhancement and spectral broadening via nonlinear elliptical polarization rotation in a solid thin plate. The efficiency, temporal contrast enhancement, spectral broadening, pulse compression and power stability are experimentally investigated. With this simple and efficient scheme, the temporal cleaned pulses with energy of 325 µJ and total efficiency of 30% are obtained. The temporal contrast and spectral bandwidth of the filtered pulse are 1011 and 104 nm, respectively. The pulse compressed from 180 fs to 45.8 fs is realized by utilizing chirped mirrors, corresponding to a compression factor of 3.93. With stable output power, presented scheme could be implemented in the ultra-intense femtosecond laser facilities.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Profiting from the invention of the chirped pulse amplification (CPA) [1] and optical parametric chirped pulse amplification (OPCPA) [2] technology, the peak power of the femtosecond laser pulses increases dramatically in the last few years. And the focusing intensity of the femtosecond pulses reaching 1022-1023 W/cm2 has been demonstrated [3–5], which becomes powerful tools for research on the heavy-ion and proton generation [6], laser wake field acceleration [7], laser fast ignition [8] and X-ray generation [9]. The temporal contrast is a crucial parameter for the laser matter interaction experiments mentioned above, as 1013 W/cm2 focusing intensity of the pre-pulses or amplified spontaneous emission (ASE) will ionize the target materials and generate pre-plasma [10], which deteriorates the experimental results. So, it is all-important to enhance the contrast of the high peak power laser pulses to eliminate the negative impact of the pre-pulses and ASE.
Several techniques for temporal pulse cleaning have been proposed and performed, such as high-energy and high contrast seed injection technology [11], cross polarized wave generation (XPW) [12–14], optical parameter amplification (OPA) [15–18], second harmonic generation (SHG) [19,20], saturable absorbers (SA) [21], self-diffraction (SD) [22–24], plasma mirror (PM) [25,26] and nonlinear elliptical polarization rotation (NER) [27–31]. Among them, the NER could enhance the contrast with serval orders of magnitude and broaden the spectrum several times simultaneously. Therefore, the NER technology is beneficial to improve the peak power of amplified laser pulses. So far, the scheme of integrating NER into gas filled hollow core fiber (HCF) to achieve high internal efficiency temporal pulse cleaning and spectra broadening has been widely investigated [27,30,31]. Nevertheless, HCF suffers deleterious effects of low transmittance, poor long-term stability and interminable alignment for daily operation [32].
In 2004, Aurelie Jullien et al demonstrated a convenient, stable, low-cost and easy to align temporal cleaning scheme, which refers to NER occurring directly in air. The transmittance of their setup was 25% and the pedestal of the input pulse was cleaned effectively [28]. In 2005, M. P. Kalashnikov et al applied NER with the air as the nonlinear medium to a double CPA system, and the temporal contrast was improved with three orders of magnitude [29]. Nevertheless, no spectral broadening occurred in the two reports above due to low nonlinear refractive index (n2) of air (n2 = 5.57×10−23m2/W) [33] and insufficient input intensity. The n2 of the solid mediums is typically three orders of magnitude larger than air. Therefore, the efficient temporal pulse cleaning and spectral broadening could appear simultaneously via NER in solid materials. In 2008, C. Zhang et al demonstrated the contrast enhancement by NER in a BK7 glass plate, and the contrast was improved by two orders of magnitude. With the increase of input pulse energy, a deep saddle-shaped spectrum generated by self-phase modulation (SPM). Nevertheless, broader output spectrum failed to appear due to the insufficient input intensity [34].
In this work, the efficiency of the NER occurring in air is enhanced by adding a fused silica plate as the Kerr medium, even though nearly 10% of the input power is lost by the uncoated plate. Driven by a Yb-doped solid state femtosecond amplifier, the temporal cleaned pulse with a total efficiency of 30%, spectral bandwidth of 104 nm (-20 dB), pulse energy of 325 µJ, stable power of RMS 0.105% is obtained. The temporal contrast of the filtered pulse within the delay range of ±40 ps reaches 1011, corresponding to a contrast enhancement of 4 orders of magnitude, and the pulse shortened to 45.8 fs with a compression factor of 3.93 is realized.
2. Experimental setup
The schematic of the experimental setup is shown in the Fig. 1. A solid state Yb-doped femtosecond laser with 2 mJ pulse energy at 1 kHz repetition rate is employed as the driving laser. The driving laser has a spectral full width at half maximum (FWHM) bandwidth of 9.7 nm at the central wavelength of 1036.5 nm and the pulse duration of 180 fs. A half wave plate (HWP) is employed for rotating the polarization of the input laser from horizontal to vertical. The beam diameter is reduced to 3.4 mm by a pair of lenses L1 and L2 with 400 mm and -200 mm focal length. Then the polarization of the laser beam is transformed into ellipse through a quarter wave plate (QWP1). Using a lens L3, the elliptically polarized laser is focused into a 1 mm thick uncoated fused silica plate. The plate is positioned after the focal point to suppress the self-focusing effect. Behind the plate, the laser beam is re-collimated by utilizing the lens L4. With the pulse propagation through the air and fused silica plate, the NER and spectral broadening happen at the same time. The orientation of the QWP2 is orthogonal to the QWP1 to rotate the polarization of the weaker portion to the original vertical direction. After that, the pre-pulses and ASE are filtered by a high extinction ratio (<5 × 10−6) Glan prism (GL). The intense portion around the main pulse propagates through the GL. And then it is temporally compressed by the chirped mirrors.
Fig. 1. Experimental setup, HWP, half wave plate, L1-L4, lenses, QWP1-QWP2, quarter wave plate, FS, fused silica plate, GL, Glan prism.
3. Experimental results
To begin with, the efficiency of the NER in air as a function of θNER, which is the angle between the polarization of the input laser and the optical axis of the QWP1, is investigated directly without inserting the fused silica plate for an input pulse energy of 566 µJ. The power passing through the GL increases firstly and then decreases with rotating the θNER from 0° to 44° at an interval of 2°, as shown in Fig. 2(a) (red line). The maximum transmission power is 76.5 mW at θNER = 24°, corresponding to a global optimum efficiency of 13.5%. Meanwhile, the laser power reflected by the GL is 423 mW, so the internal efficiency is 15.3% given by the equation η=P///(P⊥+P//), where P// refers to the power passing through the GL (horizontal polarization) and P⊥ denotes the power reflected by the GL (vertical polarization). The loss introduced by the GL is 11.7% given by the equation l = 1-(P⊥+P//)/P, where P denotes the power before the GL. Higher total efficiency is probably acquired by replacing a higher transmittance GL. Then a 1 mm thick uncoated fused silica plate with 25.4 mm diameter is placed behind the focal point for further NER efficiency scaling. The power lost by the plate is 54 mW. The power passing through the GL with rotation θNER is also investigated, the results shown in Fig. 2(a) (blue line). Obviously, the transmission power with plate increases when the θNER is in the range of 10° to 38°. And the maximum filtered power is 115 mW at θNER = 22°, corresponding to a total efficiency of 20.3%, while the power reflected by GL is 311 mW, which yields an internal efficiency of 27%. It is obvious that the maximal total efficiency and internal efficiency with plate increases 6.8% and 11.7%, respectively. Nevertheless, the efficiency with plate decreases when the θNER is less than 10° and larger than 38°. This result is attributed to the power lost by the uncoated plate. Figure 2(b) shows the measured spectrum (HR4000, Ocean Optics) in front of the GL at optimum total efficiency. Without fused silica plate, the spectrum is broadened slightly with a spectral bandwidth of 41.4 nm at the intensity of -20 dB (red line). With plate, the spectral bandwidth is extended to 67 nm (blue line). Both spectra feature intensity modulation, which attributes to SPM effect.
Fig. 2. (a) The filtered power as a function of θNER when the input pulse energy is 566 µJ. Red line, without fused silica plate. Blue line, with fused silica plate. (b) Spectra measured in front of the GL. Gray area, input spectrum. Red line, Without fused silica plate. Blue line, with fused silica plate.
Then we investigate the efficiency at different input pulse energy. With the increase input pulse energy, the spatial profile of the temporal cleaned laser beam appears a macroscopic ring structure arising from the self-focusing effect. To eliminate this negative phenomenon, the fused silica plate is moved back to enlarge the beam size on it. When the input pulse energy is 660 µJ, 764 µJ, 880 µJ, 990 µJ and 1.08 mJ, the plate is moved back by 3.8 cm, 7.7 cm, 8 cm, 11.3 cm and 11.3 cm, respectively. And the beam size at the position of plate is measured by a beam analyzer (WinCamD-LCM, DataRay), the results shown in the Fig. 3. And the profile of the beam are depicted in the inset of the Fig. 3. The beam diameter at the intensity of 1/e2 is enlarged from 574.8 µm × 635 µm to 802.4 µm × 821.5 µm. The beam profile has no significant intensity distortion over all input pulse energy. But at low input pulse energy (<800 µJ), the beam profiles feature an obvious elliptic shape.
Fig. 3. The beam size at different input pulse energy at the position of fused silica plate. Inset, the beam profiles at the position of fused silica plate. Blue line, major width. Red line, minor width.
Figure 4(a) presents the variation of the total efficiency under the circumstance of having fused silica plate or not. During the experiments, we found that the θNER corresponding to the optimum efficiency shifted with the increase input pulse energy. Therefore, the QWP1 and QWP2 are rotated to maximize total efficiency. Without plate, the total efficiency varies from 13.5% to 28% (red bar), but the total efficiency increases from 20.3% to 30% (blue bar) when adding the fused silica plate. And the optimum total efficiency is at the input pulse energy of 1.08 mJ. So, the differences of the total efficiency gradually decrease from 6.8% to 2%. The internal efficiency varies from 15.3% to 33% without the fused silica plate, as shown in the Fig. 4(b) (red bar), while the internal efficiency with plate is in the range of 28% to 36.4%, as depicted in the Fig. 4(b) (blue bar). And the optimum internal efficiency is at the input pulse energy of 880 µJ.
Fig. 4. (a) Total efficiency at different input pulse energy, without fused silica plate (red bar), with fused silica plate (blue bar). (b) Internal efficiency at different input pulse energy, without fused silica plate (red bar), with fused silica plate (blue bar).
Figure 5 describes the spectra of the temporal cleaned pulse at different input pulse energy. All spectra are recorded at optimum total efficiency. The θNER corresponding to the optimum efficiency is marked in the upper left corner of Fig. 5(a)-(f). It can be seen that the spectrum broadens gradually with increasing input energy whether there is fused silica plate or not. But with plate, stronger spectral broadening will happen. The spectra are extended to blue side and red side simultaneously, which indicates the spectral broadening is dominated by SPM and no efficient ionization or Raman scattering occurs [35,36]. Compared with spectra in front of the GL (eg. Figure 2(b)), the spectra after GL are smoother, and this phenomenon is similar with Ref [31]. The spectral bandwidths at different input pulse energy are summarized in the Fig. 6(a). With fused silica plate, the spectra at the intensity of -20 dB are broadened from 65.1 nm to 104 nm (red line). Without plate, the spectra are just broadened from 47.3 nm to 61.1 nm (black line). Therefore, we infer that the spectral broadening mainly occurs in the fused silica plate. The calculated FTL pulse duration with plate is shortened from 70.2 fs to 44 fs, as shown in Fig. 6(b) (black line). At different input pulse energy, the pulse duration is compressed by chirped mirrors and characterized by a home-built second harmonic frequency-resolved optical gating (SHG-FROG), the results shown in the Fig. 6(b) (red line). We clearly see that the compressed pulse is close to the FTL over all input pulse energy. And the shortest pulse of 45.8 fs is retrieved when the input pulse energy is 1.08 mJ, corresponding to a compression factor of 3.93. The results are shown in the Fig. 7.
Fig. 5. The measured spectra with and without fused silica plate behind GL. Black lines, without fused silica plate. Red lines, with fused silica plate.
Fig. 6. (a) Spectral bandwidth at different input pulse energy. Black line, without fused silica plate, red line, with fused silica plate. (b) The calculated FTL (black curve) and compressed pulse duration (red curve) at different input pulse energy.
Fig. 7. (a) Spectrum (red line) and spectral phase (blue line). (b) Input pulse duration (green line), retrieved pulse duration (red line), calculated FTL pulse duration (black line) and phase profile (blue line).
To evaluate the spatial quality of the temporal cleaned pulse, the beam profiles behind GL are investigated at different input pulse energy, the results shown in Fig. 8. And the pictures of the beam shape are presented in the inset of Fig. 8. With the increase input pulse energy, the beam diameter at 1/e2 intensity reduces from 3.19 mm × 3.14 mm to 1.73 mm × 1.7 mm due to self-focusing effect, while more energy is concentrated in the central part of the beam, which causes the beam intensity distributions gradually deviating from Gaussian profile.
Fig. 8. Beam size at 1/e2 intensity as a function of input pulse energy. Red line: X-direction. Blue line: Y-direction. Insets, output beam profiles at different input pulse energy.
Figure 9 shows the measured temporal contrast of the laser pulse within the delay range of ±40 ps by utilizing a commercial third order correlator (Sequoia, Amplitude Technology). The red curve presents the temporal contrast of the temporal cleaned pulse, which is 1011. And the blue curve depicts the temporal contrast of the input pulse, which is about 107. This yields a temporal contrast improvement of nearly four orders of magnitude. Meanwhile, the contrast of the pre-pulse (a) before the main pulse 23.5 ps is improved five orders of magnitude, and the temporal contrast of the pre-pulse (b) before the main pulse 11.6 ps is improved nearly four orders of magnitude. A newly generated pre-pulse (c) before the main pulse 9.6 ps is the artifact from the relatively strong post-pulse (d) behind the main pulse 9.6 ps, which originated from the 1 mm thick uncoated fused silica plate. When the plate is removed, the pre-pulse (c) and post pulse (d) will disappear.
To evaluate the reliability and robustness of our temporal contrast enhancement device, the power stability of the temporal cleaned laser is characterized, which exhibits a root-mean-square (RMS) stability of 0.105%, as shown in the Fig. 10 (red line). And the RMS of the input laser is also shown in the Fig. 10 (black line), which is 0.088%. Therefore, the laser power fluctuation is barely deteriorated by the solid thin plate-based NER device.
Fig. 10. Power stability of the Yb-doped femtosecond laser (black curve) and NER filtered laser (red curve).
3. Summary
In conclusion, high efficiency temporal pulse cleaning and spectral broadening are performed simultaneously by integrating NER into a fused silica plate. The total efficiency up to 30% is achieved when the input pulse energy is 1.08 mJ. With the enhancement nearly four orders of magnitude, the temporal contrast of the filtered pulse within the delay range of ±40 ps reaches 1011. Meanwhile, the spectral bandwidth of the filtered main pulse is 104 nm at the intensity of -20 dB. In addition, the pulse duration is compressed to 45.8 fs with a compression factor of 3.93. The power stability of the filtered laser is good enough. Such a contrast enhancement scheme with high efficiency, simple configuration and low cost has great potential to be applied in ultra-intense femtosecond laser facilities.
Funding
National Key Research and Development Program of China (2017YFE0123700); National Natural Science Foundation of China (61925507); Shanghai Rising-Star Program (21QA1410200); National Natural Science Foundation of China (62075227); Youth Innovation Promotion Association of the Chinese Academy of Sciences (2020248).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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