
- High Power Laser Science and Engineering
- Vol. 8, Issue 4, 04000e35 (2020)
Abstract
1 Introduction
Ultra-intense, ultrafast lasers[
Ytterbium (Yb)-doped solid-state lasers have attracted attention in the last two decades. The Yb-ion has a long storage time, which is suitable for adopting a diode laser as a pump. Ytterbium-doped lasers have been developed to exhibit high pulse energy and power[
The Petawatt Optical Laser Amplifier for Radiation Intensive Experiments (POLARIS) project based on the Yb:glass amplifier, which aimed to achieve a petawatt peak power output with a sub-hertz repetition rate, was started in 1999, and has demonstrated 54 J pulse energy[
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Figure 1.Conceptual design of the kilohertz ultra-intense ultra-short Yb:KGW-based CPA system.
We developed a diode-pumped Yb:KGW regenerative amplifier as the front end of the kilohertz terawatt laser system. Broad bandwidth pulses were obtained by applying dual crystals with different orientations of their optical axes[
2 System design
Figure 2.Laser system design. HW: half-wave plate; QW: quarter-wave plate; FR: Faraday rotator; TFP: thin-film polarizer; PC: Pockels cell; M1, M2, M6 and M7: cavity mirrors; M3 and M4: dichroic mirrors.
2.1 Gain-narrowing effect compensation
The Yb:KGW laser system has a quasi-three-level behavior. The net gain of the Yb:KGW crystal was not high: several tens of roundtrips were needed to amplify the nanojoule seed to a millijoule level. Thus, the gain-narrowing effect was inevitable in Yb:KGW regenerative amplifiers.
Several methods have been adopted to solve this problem. An acousto-optical programmable dispersive filter can be utilized to modulate the seed’s spectrum to suppress the gain-narrowing effect[
Figure 3.(a) Illustration of gain-narrowing effect compensation under the dual-crystal configuration; (b) spectrum of
The combined gain is complex and should be measured under different pumping conditions because of the quasi-three-level system behavior of the Yb-doped gain medium. The blueshift, with one peak from 1037 nm under the same crystal placement orientation to 1035 nm under the orthogonal crystal placement orientation, could result from the reabsorption losses at a long wavelength in the crystal where the laser polarization is parallel to its Nm-axis, making the gain at 1035 nm higher than that at 1037 nm under orthogonal orientation.
2.2 Thermal lensing effect compensation
The thermal lensing effect can be estimated as[
Previous work showed that the dual-crystal configuration can be used to compensate for the thermal lensing effect[
Figure 4.Beam radius variation on the two crystals while changing the focal length of the thermal lensing effect.
Figure 5.Output characteristic of the cavity working under QCW pumping (1/3 duty cycle at 1 kHz) with a pump peak power up to 80 W. PC: Pockels cell.
2.3 Dispersion management
Figure 6.Ray-tracing model of our stretcher and compressor. TG: transmission grating; P1, P2: periscope; M1: broadband high reflection mirror at 0°; M2–M6: broadband high reflection mirrors at 45°; L1, L2: lenses; S1: translation stage where M2 and M3 were fixed.
Figure 7.Calculated compressed pulse output (blue solid line) with 150 fs pulse input (dark dashed line) by our compact stretcher and compressor.
3 Results and discussion
Figure 8.Intra-cavity pulse amplification process monitored by an oscilloscope. The pulse underwent an unsaturated amplification because of the mirror damage limitation.
The two-pump laser could offer up to 120 W power. Approximately 60% of the maximum pump power was applied in our experiment considering the low performance of the dichroic mirrors. Under this dual-crystal configuration, our cavity supports a 100 W pump power input, as discussed in
Figure 9.(a) Input seed spectrum (black line) and output pulse spectrum (red line) delivered by the regenerative amplifier. (b) Compressed pulse duration (black line) and its Gaussian fit (red line) showing a pulse duration of approximately 227 fs. (c)
Due to the high transparency of our transmission grating, the laser pulses were compressed to 1.2 mJ with a beam diameter of 4 mm corresponding to a total compression efficiency of 80%. Thus, with a further improvement in our regenerative amplifier, 2 mJ compressed pulses can be expected, without damaging any component inside our compressor.
The compressed pulse duration was 227 fs (
4 Conclusions
This paper reports on the Yb:KGW dual-crystal cavity. The regenerative amplifier boosted the seed from sub-0.2 nJ to 1.5 mJ. The laser pulses were compressed to 1.2 mJ with a pulse duration of 227 fs. The output laser beam is near the diffraction limit with M2 ~1.1, benefiting from the dual-crystal configuration. Moreover, the Q-switched output spectrum of the regenerative amplifier supports a sub-160 fs pulse amplification. Therefore, pulses with energies of up to 2 mJ and duration below 200 fs can be obtained using higher-quality dichroic mirrors and a broader-spectrum laser seed. This regenerative amplifier works as a good pre-amplifier for further laser amplification.
In the future, an investigation on the spectral gain curve of Yb:KGW in the main amplifier will be performed under cryogenic temperature evolution from 77 K to 300 K. The working temperature will be controlled to realize broader gain bandwidth and less shift at the central wavelength of the main amplifier. We plan to boost the pulse energy to above 40 mJ in two-stage multi-pass amplifiers, corresponding to a gain factor of 20. In each amplifier, we will also adopt two cascaded crystals placed under orthogonal orientation to reduce the thermal load on each crystal and compensate for the gain-narrowing effect.
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