Ultrashort and ultraintense laser systems contribute to many significant applications, such as laser-plasma electron and ion accelerations^[1,2], fast ignition^[3], and astrophysics in laboratory^[4]. With the invention of the chirped pulse amplification (CPA) technique^[5], many ultrashort and ultraintense laser systems have been built worldwide. In recent years, several 10-PW-level high-power laser systems are under construction^[6], such as the Shanghai Superintense Ultrafast Laser Facility (SULF) 10 PW at the Shanghai Institute of Optics and Fine Mechanics (SIOM)^[7,8], the high-power laser system that delivers two 10 PW laser beams at the Extreme Light Infrastructure Nuclear Physics (ELI-NP)^[9], and Apollon 10 PW laser^[10], which are all based on the Ti:sapphire (Ti:S) amplifier. For these 10-PW-level Ti:S laser systems, Ti:S crystals with more than 200 mm diameter aperture are needed. However, when the aperture of the Ti:S crystal is more than 200 mm, the crystal becomes expensive and good-optical-quality crystals become extremely difficult to grow. Meanwhile, for >200 mm Ti:S crystals with high pump energy, the parasitic laser (PL) and the transverse amplified spontaneous emission (TASE) severely limit the signal gain and energy extraction efficiency. A series of techniques for suppressing PL and TASE have been reported, such as using the matched index liquid and lightly doped crystal and the temporal dual-pulse pump technique^[11–13]. However, the existing Ti:S crystals and the techniques for suppressing PL and TASE cannot meet the requirements for future ultrashort and ultraintense laser systems of more than 10 PW. Moreover, coherent beam combination (CBC) has been regarded as a technique to improve the output power of laser systems with several small crystals^[14,15]. But, accurate CBC in the time domain and space domain for femtosecond-class pulse duration and micron-class focused spot is very difficult and complex. Until now, the CBC technique has not been used in 10-TW-level laser systems^[16].

In recent years, optical tiling technology has been used in many research activities, such as larger-aperture telescopes (tiling several small mirrors together to make a large mirror achieve the same properties as a single mirror)^[17], tiling large-aperture nonlinear optical crystals^[18], and tiling gratings^[19]. The tiled Ti:S technique, known as one of the CBC techniques, is easier and simpler for controlling dispersion, synchronization, and focused spot. In a previous study, the tiled Ti:S crystal technology based on the CPA laser system had been studied^[20], and the synchronization feasibility had been previously demonstrated with lower energy. Therefore, for petawatt (PW)-level Ti:S CPA laser systems, the amplifier based on tiled Ti:S crystal technology should be a potential way to achieve higher peak power output and suppress PL with small crystals.

In this study, we experimentally demonstrated the feasibility of a new type of tiled Ti:S four-pass amplifier based on a 10-TW-level CPA laser. An amplified output energy of 1.18 J with pump energy of 2.75 J corresponding to an energy conversion efficiency of 35.3% was obtained. After the compressor, a pulse laser of the maximum peak power with 28 TW from the tiled Ti:S amplifier was finally achieved. Moreover, the effect of beam gap on the far-field spatial profile was discussed.

The overall experimental setup is shown in Fig. 1. The femtosecond Ti:S laser oscillator generated 12 fs pulses at 80 MHz repetition rate. Then, the pulse duration was stretched to 2.4 ns via an all-reflective Öffner-type stretcher with output energy of 5 nJ. After a Ti:S regenerative amplifier and two multipass Ti:S amplifiers, a soft-edge double-array square diaphragm was used to spatially acquire the double-array square beam to fit the mounting structure of crystals [Figs. 2(a) and 2(b)]. Therefore, the initial parameters of the two signal beams should be the same, including the relative time delay and phase. The whole energy of the double-array square beams was 209 mJ with a double size of 10 mm× 10 mm before the four-pass tiled Ti:S amplifier, which was pumped by 2.75 J pumping lasers. Finally, a grating compressor was used to compress the amplified chirped pulses simultaneously.

The simplified layout of the tiled Ti:S four-pass amplifier is presented in Fig. 2. Since the redesign of the tiled Ti:S four-pass amplifier, the pump beams and signal beams were situated on a different optical plane, and the tiled Ti:S crystals were installed in the vertical array direction (upper and lower), as shown in Fig. 2(b). The pump beam was shaped into a square having a 10 mm× 10 mm size to fit the appearance and aperture of the tiled Ti:S crystals using a spatial filter. Then, the pump pulse was divided into two lasers with approximately the same energy using a beam splitter and injected into one side of the tiled Ti:S crystals. Meanwhile, similar pump lasers were injected into another side of the tiled Ti:S crystals. Even more remarkably, the two tiled Ti:S crystals had the same length, width, and thickness, having dimensions of 14mm×14mm×25mm (the crystals were processed together, including cutting, orientation of the optical axis, polishing, and coating), and the optical axis error of the two Ti:S crystals should be <1° to avoid possible issues including spectral modulation^[20]. In addition, the gap of the tiled crystals was ∼6 mm. To weaken the diffraction effect from the square aperture of the Ti:S crystals, a soft-edge square diaphragm with a double size of 12mm×12mm was used to attach to one side of the mounting rack of the Ti:S crystals.

The following Frantz–Nodvik (F–N) function was used to simulate the output energy of the tiled Ti:S four-pass amplifier^[21,22]:ΔN=NT{1−11−exp(−αpz)[1−exp(Jp/Js)]},(1)G0=exp(Jsto/Js),(2)Jout=Jsln{1+G0[exp(Jin/Js)−1]},(3)where ΔN is the inverted population density, NT is the Ti3+ particle concentration, αp is the pump absorption coefficient, Jp is the pump energy density, Js is the saturation energy flux, Jsto is the stored energy per unit volume, G0 is the small-signal gain, and Jout is the output energy flux.

As shown in Fig. 3, the experimental and simulated results of the output energy coincide well with the same input energy. Due to the energy limitation of the pump and the input signal laser, the amplifier was not saturated finally. A maximum amplified output energy of 1.18 J with an input signal energy of 209 mJ and a pump energy of 2.75 J was obtained, corresponding to approximately five times gain and 35.3% energy conversion efficiency, which was similar to a single Ti:S amplifier.

To prove that the tiled Ti:S amplifier would not introduce the spectral modulation, the spectrum obtained in Fig. 4 illustrates the laser spectral evolution of the input and output of the tiled Ti:S amplifier. It clearly shows that the spectral modulation in the upper and lower output spectra hardly exists. Moreover, the spectral width of the upper and lower Ti:S crystals could be identified as uniform to support a 22.5 fs transform-limited (TL) pulse.

The amplified pulse was compressed through the optimized compressor consisting of two 1480 grooves/mm gold-coated reflective gratings. As shown in Fig. 5, the duration of the compressed pulse with the 28.7 fs FWHM was obtained by a Wizzler device (Fastlite). Meanwhile, the residual high-order dispersion brings the aberration for the spectral phase and pulse profile. Due to the 70% compressor efficiency, the output energy of compressed pulse was 820 mJ, corresponding to the maximum peak power of 28 TW.

The near-field spatial profile of output beam is presented in Fig. 6(a). The diffraction effect of the square aperture of the Ti:S crystals can cause slight modulation in the near-field spatial profile, which could be decreased by further enlarging the scale of the crystals. Furthermore, a convex lens with a focal length of 1 m was used to obtain the far-field spatial profile of the output beam after the compressor, and the pictures are the experimental (upper) and simulated (lower) results, as shown in Figs. 6(b) and 6(c). According to the expression of the combining efficiency^[23], in the practical experiment, the combining efficiency of the two beams was 78.2%.

It is noted that the side peak intensity of the far-field spatial profile of the output beams is about 50% of the main peak intensity, as shown in Fig. 6(b), and that this will decrease the focused intensity and combining efficiency. This phenomenon appears because the output beam gap (the Ti:S crystals have the effective optical aperture of 70% loading the output beam gap of ∼10 mm) indicated in Fig. 6(a) is too large compared with the 10 mm beam size. According to the numerical simulation, the main peak intensity and combining efficiency will increase when the output beam gap is reduced. Therefore, we can design an adjusting scheme to decrease the beam gap and use the spatial filter to remove the side lobe of the far-field spot in future practical applications. In addition, due to the adoption of the same crystal thickness and beam transmission systems, such as mirrors and gratings, the relative spatial phase, wavefront, and combining efficiency are controllable in the tiled Ti:S technique. However, the time jitter problem of the conventional CBC technique can cause wavefront jitter^[24]. Therefore, the tiled Ti:S technique is more beneficial to the compensation of the wavefront than the conventional CBC technique.

We first demonstrated the feasibility of a new type of tiled Ti:S crystal four-pass joule-level amplifier based on a 10-TW-level Ti:S CPA laser. Meanwhile, the redesign of the amplifier structure and the Ti:S crystal installation were used to solve some potential problems, including spectral modulation by slightly adjusting the crystal angle. We obtained the maximum output energy of 1.18 J with pump energy of 2.75 J, corresponding to the energy conversion efficiency of 35.3%. Ultimately, after the compressor, a pulse laser with a maximum peak power of 28 TW and pulse duration of 28.7 fs FWHM was achieved, and the combining efficiency of the two beams was 78.2% in a practical experiment. Due to similar energy conversion efficiency of the tiled Ti:S amplifier with that of the conventional single Ti:S amplifier, it could be used to replace the conventional Ti:S amplifier as a boost amplifier.

The tiled Ti:S crystals technology could suppress the PL of large-aperture Ti:S crystals, and the decrease of beam gap could further increase the focused intensity and combining efficiency. We will study the large-aperture tiled Ti:S amplifier to achieve PW-level or even higher peak power output and suppression of PL in the further experiments. Also, the tiling technology based on Ti:S crystals can be used to combine the tiling grating technology to further update the peak power and focused intensity in the CPA Ti:S lasers.