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Abstract
We present an optical parametric chirped-pulse amplification (OPCPA) based on mixed cascaded crystals, taking advantage of the unique parametric phase-matching of lithium triborate (LiB3O5, LBO) and yttrium calcium oxyborate ((YCa4O(BO3)3, YCOB) crystals. The OPCPA properties of LBO at 880 nm and YCOB at 750 nm are studied respectively. After amplification by two LBO and two YCOB crystals, a total signal gain of 108 and spectral bandwidth close to 400 nm is obtained. After accurate dispersion compensation with a grating-pair compressor and chirped mirror compensator, a pulse duration of 9.4 fs is obtained by a SHG-frequency-resolved optical grating (FROG). This approach will be of great significance in high energy amplifier for high peak power few-cycle laser sources.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Ultra-intense, ultra-short lasers are of great importance for various physical applications [1], and also have broad applications in other fields, ranging from laser technology, materials science, biomedicine, and quantum information processing [2–5]. High peak power lasers, a significant segment of laser science and technology, is continually advancing towards higher power, stronger intensity, shorter pulses, and broader spectrum [6]. Amplification technology in ultra-broadband spectrum ranging from the near-infrared (NIR) to the mid-infrared (MIR), is particularly demanded and extensively explored for the pulses generation of few cycle duration [7–10]. Optical parametric amplification (OPA) as well as chirped pulse OPA (OPCPA) have been widely adopted in laser facilities since proposed [11]. The OPCPA takes the advantages of chirped pulse amplification (CPA) and OPA, is characterized by high gain in small thickness crystal, minimal thermal loading, excellent wavelength flexibility and intrinsically broad gain bandwidth [7]. It has consequently become the primary technology for associated laser devices.
Larger gain bandwidth in amplifier is essentially required to obtain shorter pulse duration. Consequently, obtaining a larger bandwidth via OPCPA technology has received more attention. Many researches have concentrated on the utilization of new nonlinear crystals to enhance the OPCPA bandwidth and gain. Currently, beta-barium borate (β-BaB2O4, BBO), potassium dideuterium phosphate (KD2PO4, DKDP), LBO, and YCOB are commonly used for OPCPA systems. BBO has high nonlinear coefficient but low damage threshold, the available crystal aperture is limited to 20 mm [12,13]. On the contrary, DKDP offers an available aperture of up to 400 mm and high damage threshold but low nonlinear coefficient, making it particularly suitable for master amplifiers of high energy and low gain [14,15]. What’s more, the deuteration level can be adjusted to suit different matches [16,17]. LBO and YCOB crystals, on the other hand, have much higher nonlinear coefficient, 5 times of DKDP crystal and half of BBO. They also have high damage threshold and excellent thermal properties. Their currently available sizes exceed 100 mm, making them suitable for high energy and high gain amplification systems, which has been experimentally verified in PW-scale laser facilities [18–20].
Other researches have focused on the exploration of new amplification techniques and structural designs to meet requirements on bandwidth and gain. Multiple-color pumped OPCPA has obtained a spectral bandwidth of 440 nm with the amplified signal energy of 101 mJ [21]. Multiple-beam pumped OPCPA based on LBO crystals has been adopted in the frontend of the Station of Extreme Light (SEL) facility, chirped pulse with energy of 5.26 J and spectral bandwidth of 200 nm were obtained near the center wavelength of 925 nm at a repetition rate 0.1 Hz [22]. Wide-angle noncollinear OPCPA (WNOPCPA) was proposed and phase-matching bandwidth of 470 nm can be expected in simulation [23]. However, these structural designs have been plagued by issues such as moderate conversion efficiency and complexity of the system. Quasi-parametric amplification (QPA), which is based on a nonlinear crystal of high absorption on the newly generated idler and high transmittance for the pump and signal, has experimentally demonstrated a conversion efficiency of 85% pump depletion [24,25]. However, the method is restricted for broader utilization because there are very few nonlinear crystals that meet the requirements for coupled equations in QPA.
In this paper, a new scheme for segmental amplification is proposed by taking advantage of the parametric matching of different nonlinear crystals in the complementary middle-wave and two ends-wave spectral domains. Compared to F. Krausz’s scheme, this scheme employs mixed cascaded crystal and pump pulse of only one wavelength to realize an amplified broadband chirped laser pulse [21]. Theoretically, it is expected to obtain a gain bandwidth of several hundreds of nanometers and a million-scale gain by mixed cascaded scheme. It provides a new technological route for the generation of high energy few-cycle light sources. Experimentally, an OPCPA system based on two LBO crystals and two YCOB crystals was explored. A total gain of 108 and spectrum ranging from 696 to 1087 nm have achieved. The amplified signal energy is approximately 20 mJ. Finally, a pulse duration of approximately 9.4 fs is measured by a SHG-FROG after a grating-pair compressor and chirped mirror compensator.
2. Experimental setup and design
2.1 Experimental setup
In this section, we describe the overall structure of the mixed cascaded amplification OPCPA system based on LBO and YCOB, the detail schematic is shown in Fig. 1.
Fig. 1. Schematic of the experimental setup. HWP: half-wave plate, SCG-800: supercontinuum generation, CW-SLM: continuous wave single longitudinal mode, A-O: acousto-optic, EOM: electro-optical modulator, AWG: arbitrary waveform generator, SHG: second harmonic generation, GPC: grating-pair compressor, CM: chirped mirror compensator, SHG-FROG: second harmonic generation-frequency-resolved optical grating
The seed pulses are produced by a commercial Ti: sapphire mode-locked oscillator that delivers a 500 mW, 6 fs pulse train at 85 MHz repetition rate (Synergy Pro 500). The pulse spectrum after the oscillator is measured by a spectrometer (HR4000+, Ocean Optics) and depicted by black-line in Fig. 2(a). It ranges from 690 to 888 nm. The seed pulse is delivered into a Faraday isolator to prevent the backward-reflected laser beam of the successive optical components from coupling into the oscillator and disturbing mode-locking conditions. After that, the seed passes through a half wave plate and a Glan-Laser polarizer, from which the seed power is capable to be regulated by attenuating the half wave plate. Then, the regulated seed pulse is coupled into a photonic crystal fiber (PCF) (SCG-800, Newport Corp.) via a 40× objective lens for supercontinuum (SC) generation, and 20× objective lens is used to collimate the output beam. The power as well as spectrum of the newly generated SC is optimized by attenuating the injected seed power. A SC spectrum ranging from 498 to 1140 nm is achieved, as shown by blue line in Fig. 2(a). Since the measurement upper limitation of the spectrometer is 1140 nm, infrared spectrum beyond 1140 nm cannot be measured. The SC near field is measured by CCD after collimation. As shown in Fig. 2(b), the PCF provides SC signal pulses with high beam quality and the beam diameter is ∼6 mm. The collimated seed pulse is temporally expanded to 2 ns with a chirped ratio of 5 ps/nm by an Öffner stretcher, which keeps a transmission bandwidth around 400 nm. As the signal injected into OPCPA stages, pulses after stretcher are of a smaller spectrum bandwidth because of spectral clipping in the optical components of finite size. It ranges from 683 nm to 1140 nm, shown by green line in Fig. 2(a).
Fig. 2. (a) Normalized spectrums of pulses after oscillator (black), PCF (blue) and stretcher (green), (b) near-field of supercontinuum
The seed of pump laser derives from a continuous wave single longitudinal mode (CW-SLM) laser operating at 1064 nm. After an acousto-optic chopper and electro-optic modulator, laser pulses with 1 nJ energy are obtained with an adjustable rising edge to realize the temporal pre-compensation for the gain saturation effect in the subsequent amplifiers at a repetition rate from 1 to 10 Hz. The pump pulse is then amplified to approximately 1 mJ in a regenerative amplifier, and injected into main amplifiers for high energy extraction. Three stages of Nd: YAG-based amplifiers are designed for 5 J pulses generation at fundamental frequency (1ω). The laser beam diameter and temporal pulse width are 25 mm and 2 ns at the last stage. A piece of KTiOPO4 (KTP) crystal is adopted for second harmonic generation (SHG) with type-II phase-matching. Finally, a 3 J output pump pulse at 532 nm (2ω) with super-Gaussian spatial-temporal profiles is expected, corresponding to SHG conversion efficiency ∼60%. At present, the pump laser has not been completely installed and the delivery capacity is restrained ∼0.5 J SHG pulses. The temporal profile of the pump pulse and the near-field distribution of the pump beam are depicted in Fig. 3(a) and (b). A small portion of the pulse chain is photo-electrically converted and used as a trigger for the electronic synchronization system, which ensures synchronization accuracy of 4.7 ps root mean square (RMS) for OPCPA system.
2.2 Experimental design
To verify the ultra-broadband amplification capability of this OPCPA system, two LBO crystals and two YCOB crystals are utilized in our experiment and aligned as shown in Fig. 1. Coupling waves in optical process are type-I phase-matched in XOY principal plane with an angle of θ=90°, φ=13.429° and an internal non-collinear angle of 1.266° for LBO. And they are type-I phase-matched in XOZ principal plane with an angle of θ=28.345°, φ=180° and the internal non-collinear angle 2.5° for YCOB. The lengths of two LBO and YCOB crystals are 20 mm and 25 mm, respectively. Figure 4(a) and (b) present the phase-matching angle and phase-mismatching factor (Δφ=Δk × Lc) for LBO (blue) and YCOB (green), respectively. It can be observed in Fig. 4 that the matching angles differ by no more than 0.4° and the phase-mismatching factors keep small for spectrum from 770 nm to 1065 nm in LBO crystal. By contrast, the green line depicts the phase-matching condition of YCOB crystal, revealing differences less than 0.2° in matching angles between 704-766 nm and 1066-1090 nm, which indicates the phase-mismatching factors around [-π, π]. In summary, the numerical analysis on phase-matching angles and phase-mismatching factors for LBO and YCOB crystals indicates a potential amplification capacity for spectrum region of 704-1090 nm by combining the two kinds of the crystals. Taking the damage threshold of the LBO and YCOB crystals into account, pump intensity of 1.5 GW/cm2 for LBO and 1.2 GW/cm2 for YCOB are set respectively. The beam diameter is decreased to 3 mm for both branches, corresponding to pump pulse energy of 212 mJ and 170 mJ, respectively. As shown in Fig. 4(c), the gain bandwidth of LBO can reach 295 nm, primarily ranging from 770 nm to 1065 nm. By contrast, the spectral region of YCOB with high gain is only at the short-wave and long-wave ends: 704-766 nm and 1066-1090 nm. Consequently, a broad spectrum ranging from 704 to 1090 nm with a gain exceeding 104 can be expected by a single mixed LBO and YCOB crystal.
Fig. 4. (a) phase-matching of LBO (blue) and YCOB (green), (b) phase-mismatching factors for 20 mm LBO crystal (blue) and 25 mm YCOB crystal (green), (c) OPA gain based on 20 mm LBO and 25 mm YCOB under 1.5 GW/cm2 (blue) and 1.2 GW/cm2 (green), respectively
The amplified pulses are sampled to measure the spectrum, energy, pulse width, and near-field distribution. Spectrums are measured by a spectrometer (HR4000+, Ocean Optics), energy by an energy meter (PE50BF-C, OPHIR) and temporal profiles by a photodiode (Thorlabs, 5 GHz) and an oscilloscope (Keysight, DSOX6004A). The near-field distribution is captured via a CCD camera. The precision control of pulse width compression and dispersion is achieved through a grating-pair compressor and chirped mirror compensator. Subsequently, the pulse temporal waveform and phase as well as the spectrum and spectrum phase after compression are measured by a SHG-FROG [26,27].
3. Experimental results
In this experiment, OPCPA design with cascaded crystals mixed alignment is adopted to take advantage of the complementary parametric phase-matching characteristics at different spectral regions with various nonlinear crystals. LBO crystal is used for phase-matching in the middle spectral region, while YCOB for the regions on both ends, as shown in Fig. 1.
The optical parametric fluorescence (OPF) spectrum can indicate gain characteristics for an OPA amplifier when the signal is blocked [24]. In this experiment, OPF spectrum after the two-stage LBO crystals and without the amplification of the YCOB crystal, is provided by the blue line in Fig. 5(a), and a spectrum ranging from 760 nm to 1048 nm with a bottom bandwidth of ∼288 nm is measured. On the other hand, the OPF spectrum after two-stage YCOB crystals without the amplification of the LBO crystal, shown by the green line in Fig. 5(a). It is primarily distributed on both sides in spectral domains: 696 nm to 760 nm at the short-wave side and 1048 nm to 1087 nm at long-wave side, with a total bottom bandwidth ∼100 nm. The measured OPF spectrums are essentially consistent with those in simulation depicted by Fig. 4(c). The signal pulses are injected into the OPCPA chain with four crystals, which are accurately adjusted for an optimal delivery energy and spectrum. The normalized signal spectrum is presented in Fig. 5(b), and it ranges from 696 nm to 1087 nm with a bottom bandwidth 391 nm.
Fig. 5. (a) Normalized spectrums of optical parametric fluorescence (OPF) after two LBO crystals (blue) and YCOB crystals (green) respectively, (b) normalized spectrum of signal pulses after the whole amplification system
Furthermore, the temporal profile is measured by a photodiode and an oscilloscope, as depicted in Fig. 6(a). The duration of the signal pulse is approximately 1.8 ns. The energy of signal pulses after amplification was measured approximately 20 mJ with a total gain of 108. The shot-to-shot output energy of the pulses in 5 minutes indicates a stability of 3% RMS. The near-field distribution of the amplified beam was captured by a CCD, as illustrated in Fig. 6(c). The beam quality of the signal was not deteriorated.
Finally, the amplified pulse was delivered into a grating-pair compressor and chirped mirrors for accurate dispersion compensation. The compressed pulse duration is measured by a SHG-FROG. The retrieved spectrum as well as the spectral phase is shown in Fig. 7(a), corresponding to the retrieved temporal shape and temporal phase displayed in Fig. 7(b). The Fourier-transform-limited (FTL) pulse is shown by the red line in Fig. 7(b) with duration ∼3 fs (1 cycle at 900 nm central wavelength). The retrieved pulse exhibits a duration of 9.4 fs, which is 3.1 times larger than that of FTL pulse.
Fig. 7. Temporal compression of amplified pulses (a) normalized retrieved spectrum (black) and spectral phase (blue), (b) retrieved temporal pulse shape (black), retrieved temporal pulse phase (blue) and Fourier-transform-limited (FTL) pulse by calculation (red)
4. Conclusion
In summary, we demonstrate a new OPCPA scheme that employs mixed cascaded crystals with a single wavelength pump pulse. Ultra-broad gain bandwidth of 400 nm-scale was realized with an energy of 20 mJ and total gain of 108. This scheme is capable to offer a superior broadband output compared to the traditional OPCPA that only uses one kind nonlinear crystal. The alternating amplification of LBO and YCOB crystals effectively addresses the issue of underutilization of pump pulse. The OPCPA with mixed cascaded LBO and YCOB crystals makes feasible the design of more compact table-top OPCPA systems. Considering the presently available size (∼100 mm scale) of the LBO and YCOB crystals, it will be of great significance in hundred-Joule-level few-cycle laser sources, and indicating a potential of output peak power up to 10 PW.
Funding
Science and Technology Commission of Shanghai Municipality (20ZR1464400, 22YF1455300); Key projects of intergovernmental international scientific and technological innovation cooperation (2021YFE0116700); National Natural Science Foundation of China (12074399).
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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