**4. Gain bandwidth engineering**

The bandwidth of the amplified spectra, as discussed in the previous sections, can be extended further by using different crystals or a crystal with different phase-matching angles. The combination of BBO and LiB3O5 (LBO) crystals can be used to extend the amplified spectrum to longer frequencies, as the BBO crystal does not support amplification for spectral compo‐ nents above 1.1 μm. The amplified spectrum in both crystals supports near-single-cycle pulses, which is not unobtainable with solely either of them.

To this end, the broadened seed spectrum generated in the HCF subsequently focused on a 4 mm Y3Al5O12 (YAG) crystal to extend the spectrum to 1400 nm and is amplified in an OPCPA chain similar to the system described in Section 2. Combinations of LBO and BBO crystals at different OPCPA stages are used to increase the amplification bandwidth. The first OPCPA stage was optimized to amplify a broad spectral range from 750 to 1400 nm up to 50 μJ energy in a 2 mm LBO crystal. In the second stage, a 2 mm BBO crystal was employed. The amplified spectrum measured in this stage, using an Si-based spectrometer, spans from 670 to 1100 nm and contained 1.1 mJ energy. Finally at the third stage, 1.8 mJ energy was obtained in a 3 mm LBO crystal.

The amplified spectra at each OPCPA stage, normalized to their energy, are shown in **Figure 7**. The amplified spectrum obtained after the third stage supports 4.3 fs transformlimited pulses (FWHM). The preliminary pulse compression was performed by using a set of chirped-mirror compressor designed for spectral wavelength of 700–1300 nm. **Figure 7(b)** shows the pulse compression to 9 fs measured with an SH-FROG containing a 10 μm BBO crystal. The retrieved spectrum from the FROG measurement is in a good agreement with a spectrum measured after the third OPCPA stage. Pulse compression to its Fourier transform limit would require a specially designed chirped-mirror compressor for compensating the higher-order chirp.

The conversion efficiency of the system can be optimized further by using a longer crystal in the last OPCPA stage without relinquishing the amplified spectral bandwidth.

As shown in this section, the utilization of different well-selected nonlinear crystals extends the OPCPA gain bandwidth substantially. The realized three-stage OPCPA system, using one BBO and two LBO crystals, delivers 1.8 mJ pulses with a Fourier transform limit of 4.5 fs. The system supports shorter pulse duration than an all-LBO three-stage OPCPA system with 5.3 fs (FWHM) pulses. The reported extension of the amplified spectral bandwidth is crucial for experiments that rely on high-energy, single-cycle pulses.

**Figure 7.** (a) Amplified spectra in a three-stage OPCPA system. A 2 mm LBO crystal is used to amplify the spectral components from 750 to 1400 nm in the first OPCPA stage. In the second stage, the spectral components from 680 to 1100 nm were amplified in a 2 mm BBO crystal and finally in the last stage a 3 mm LBO crystal is used to boost the amplification to 1.8 mJ. (b) Measured and retrieved SH-FROG traces (top) and the retrieved spectrum and temporal profile of the pulses (bottom) of the OPCPA system [7].

#### **5. Summary**

uniform due to the even amplification gain. The red wing of the spectrum carries more energy in the stacked OPCPA compared to the amplified spectrum achieved from the system discussed in Section 2. Moreover, the spectral spike at 780 nm is heavily suppressed. **Fig‐ ure 6(c)** shows amplified spectrum for a similar system but with a larger stretching factor. Here, after stretching the seed pulses using a 12 mm thick SF57 plate, 5 W of the average power was obtained. It can be seen that the amplified spectrum contains a hole, leaving the 8 mm

Temporal stretching of the signal pulses to twice the pump-pulse duration, demonstrated in this section experimentally, did not show further increase in the OPCPA conversion efficiency compared to the scheme realized in Section 2. This similarity in the conversion efficiency is due to the fact that the spatiotemporal quality of the residual pump pulses after one amplifi‐

However, it is shown analytically that the further temporal stretching of seed pulses results in the increase in the conversion efficiency, as the spatiotemporal quality of the remaining pump pulses after two amplification stages is preserved just for the case of heavily chirped input

The bandwidth of the amplified spectra, as discussed in the previous sections, can be extended further by using different crystals or a crystal with different phase-matching angles. The combination of BBO and LiB3O5 (LBO) crystals can be used to extend the amplified spectrum to longer frequencies, as the BBO crystal does not support amplification for spectral compo‐ nents above 1.1 μm. The amplified spectrum in both crystals supports near-single-cycle pulses,

To this end, the broadened seed spectrum generated in the HCF subsequently focused on a 4 mm Y3Al5O12 (YAG) crystal to extend the spectrum to 1400 nm and is amplified in an OPCPA chain similar to the system described in Section 2. Combinations of LBO and BBO crystals at different OPCPA stages are used to increase the amplification bandwidth. The first OPCPA stage was optimized to amplify a broad spectral range from 750 to 1400 nm up to 50 μJ energy in a 2 mm LBO crystal. In the second stage, a 2 mm BBO crystal was employed. The amplified spectrum measured in this stage, using an Si-based spectrometer, spans from 670 to 1100 nm and contained 1.1 mJ energy. Finally at the third stage, 1.8 mJ energy was obtained in a 3 mm

The amplified spectra at each OPCPA stage, normalized to their energy, are shown in **Figure 7**. The amplified spectrum obtained after the third stage supports 4.3 fs transformlimited pulses (FWHM). The preliminary pulse compression was performed by using a set of chirped-mirror compressor designed for spectral wavelength of 700–1300 nm. **Figure 7(b)** shows the pulse compression to 9 fs measured with an SH-FROG containing a 10 μm BBO crystal. The retrieved spectrum from the FROG measurement is in a good agreement with a

thick SF57 plate, the optimum thickness for temporal stretching of the seed pulses.

cation stage is preserved for both cases.

66 High Energy and Short Pulse Lasers

**4. Gain bandwidth engineering**

which is not unobtainable with solely either of them.

seed.

LBO crystal.

In this chapter, three few-cycle OPCPA systems operating at the near-infrared spectral range and pumped by the second harmonic generation of a Yb:YAG thin-disk amplifier were reviewed. The feasibility of increasing the conversion efficiency of the system by reusing the pump energy after each amplification stage, in the subsequent OPCPA stages, was demon‐ strated. It was shown that by controlled deposition of pump energy in different parts of the seed spectrum, high conversion efficiency along with a smooth amplified spectrum can be achieved. Furthermore, the feasibility of TW-level monocycle OPCPA systems was studied by using different crystals in different amplification stages. In addition to the presented systems, different harmonics of the Yb:YAG amplifier can be used to pump few-cycle pulses in visible or mid-infrared spectral range [16].
