Splicing technology of Ti:sapphire crystals for a high-energy chirped pulse amplifier laser system Download: 841次
1. Introduction
Several high-power femtosecond laser systems have been recently built because of the urgency in developing ultra-intense and ultra-short lasers for high-field physics[1–5]. A petawatt (PW) level peak power laser pulse has even been achieved based on the Ti:sapphire chirped pulse amplifier (CPA) concept.
Although Ti:sapphire crystals have been used widely in PW-level ultra-intense and ultra-short lasers because of their advantages, such as broad gain bandwidth for supporting 10 fs-level pulse duration and excellent optical and mechanical properties, Ti:sapphire crystals and the related CPA scheme have not been considered as a suitable gain medium and laser amplification scheme for the even higher 10 PW-level peak power laser. The critical problem is that parasitic lasing (PL)[6], which results from transverse amplified spontaneous emission (ASE), exists in larger-sized Ti:sapphire crystal amplifiers for higher energy laser pulse amplification. When the chirped laser pulses are amplified to near 100 J energy, further energy amplification would be difficult, and the chirped pulse beam quality and energy gain decrease quickly because of the parasitic lasing in a Ti:sapphire CPA laser system[1]. However, large-diameter Ti:sapphire crystal growth is complex and expensive, and will take a long period. Thus, optical parametric chirped pulse amplification (OPCPA)[5] based on a large-size nonlinear crystal has to be used in developing a 10 PW even EW laser,because this technology can avoid the influence of the PL[6], although there is higher efficiency and more reliability and stability for a CPA laser systems based on Ti:sapphire. The possibility of using many pieces of Ti:sapphire crystals to compose a larger splicing Ti:sapphire crystal for a high-energy CPA amplifier was considered several years ago. However, further research has not been carried out to our knowledge[2].
In this paper, we develop a Ti:sapphire crystal splicing technology, and demonstrate its feasibility in a CPA laser. The theoretical investigation demonstrates that PL can be suppressed efficiently with a larger aperture Ti:sapphire CPA amplifier. Moreover, compared with a single Ti:sapphire crystal amplifier, the splicing Ti:sapphire crystal amplifier have the similar results of the theoretical and experimental investigations, such as, the spectrum, the pulse duration, and the beam spatial distribution in the far field.
2. Theory
For a high-energy CPA system, a large aperture of the laser crystal is necessary for saturable pumping or damage flux existing. Unfortunately, PL will be obtained more readily with the enlarging of the aperture, thereby reducing the amplification efficiency of the system. In this section, the principal condition of parasitic lasing is discussed, and it is shown that splicing technology can suppress the parasitic lasing theoretically. The possible issues resulting from the splicing Ti:sapphire crystal technology, and the influences of these issues, such as spectral modulation, the temporal profile of the compressed pulse, and the spatial distribution, are also discussed.
The threshold of parasitic lasing depends on the gain of the crystal. The gain of the crystal based on the amplifier (which in this case is Ti:sapphire) can be calculated with inverted population density, which can be obtained by using the Frantz–Nodvik (F–N) equation[7]:
The result is shown by the dashed curve in the Figure
Fig. 1. Theoretic relationship between the radius and the threshold of pump energy with a single crystal (dashed line), splicing crystals (dot line), and splicing crystals (solid line). The absorption coefficient is 0.94.
The dashed curve from Figure
Meanwhile, the transverse gain can be decreased and the threshold can be increased significantly when the optical path length of the ASE can be cut off, which can be achieved with splicing technology. Figure
Figure
In the period of the CPA system running, the accurate threshold of the PL, in our experience, is always lower than the results previously shown, and the calculation process is also more complicated because of the influence of some uncertain factors, such as the beam homogeneity, and the delay jitter between the pump laser and the extraction laser. However, these factors can hardly affect the conclusion of this section, because the splicing crystals have similar effects to those of the single crystal. For example, the threshold of PL with the single crystal decreased from to , while the threshold of the splicing crystals decreased from to under the same conditions.
According to the analysis above, crystal splicing technology has the potential to increase the threshold of PL in a CPA system. However, as previously mentioned, possible issues should be considered, including spectral modulation, stretching or splitting of temporal profile, and the sidelobe in spatial domain which may worsen the consistency of the focusing spot in the spatial domain or time domain compared with the single crystal amplifier. Furthermore, the solving of these issues is limited to inevitable errors in the process of crystal splicing, such as the different thicknesses of the crystals, angle errors of the optical axis, and the gap of the splicing crystal. Thus, details of these issues will be discussed theoretically in what follows.
(i) Spectral modulation and the temporal profile of the compressed pulse.
Unlike the single crystal amplifier, the errors mentioned before can be hardly corrected by manual adjustment during the period of the system running. For example, as the splicing crystals have four crystals and four optical axes correspondingly, if one of the crystals is adjusted and its optical axis is corrected, the errors of the other crystals may be worsened, which would generate spectral modulation and a sidelobe pulse in the time domain as well as thickness differences. For these reasons, these kinds of influence should be calculated.
The Ti:sapphire crystal is a uniaxial crystal, and the grating in the compressor system is sensitive to the laser pulse polarization[9]. Therefore, a Ti:sapphire crystal and grating are equivalent to a birefringent plate and a polarizer, and the angle errors (Figure
Equation (
Fig. 4. (a) Situation of spectral modulation (single-pass) at , ; the thickness of the crystal is 10 mm. (b) Sidelobes generated by spectral modulation.
As we know, the spectral modulation changes the temporal profile of the compressed pulse. Thus, using Fourier transformation, the temporal profile from the spectral modulation can be calculated, as shown in Figure
Another inevitable error that can influence the temporal profile of the compressed pulse is the thickness difference, show in Figure
(ii) The spatial distribution of the focused point.
The existence of a gap for the compression of parasitic lasing is inevitable, which may affect the spatial distribution. This phenomenon can be explained easily by diffraction theory[13]. In fact, the result can be easily obtained through FFT (fast Fourier transformation) using Matlab. Figure
3. Experiment
As we discussed before, some inevitable errors in the processing of crystal splicing generate possible issues, and the issues are solved in two aspects [(i) and (ii)]. However, this study focuses on the influence of these possible issues, that is, whether these inevitable errors can be limited to be small enough to be ignored using existing crystal processing technology. Thus, in this study, some experiments have been carried out to test the influence of these issues with the two aspects in theory, and the parameters used below are processed with ordinary accuracy. Notably, the experiment is limited to the aperture of the splicing crystals (10 mm); an experiment about suppressing PL has not been carried out.
Fig. 6. Theoretical relationship between the spot in the near field and the spot in the far field.
The experiment was carried out in a typical CPA system based on Ti:sapphire. The scheme is shown in Figure
Fig. 7. Scheme of experiment: (a) measurement of the energy, spot in the near and the far fields, and spectra; (b) measurement of the autocorrelator trace; (c) photo of the splicing crystals.
The diameters of the extraction and pump beams are 10 mm; the extraction beam is 0.5 mJ and the pump is 1.7 J; we obtained 1.1 mJ output for a single pass and 3.1 mJ for a double pass. Considering that the absorptivity of the crystal without coating is 80% for the pump, this result is in accordance with the theoretically expected result.
Fig. 8. Spectrum of the seed: the dashed curve is the original spectrum; the dotted curve is the spectrum amplified by a single pass; the solid curve is the spectrum amplified by a single pass.
(i) Experiment of the spectral modulation and the temporal profile.
The calculation result shows that the angle errors generate spectral modulation. However, when the angle errors are less than (ordinary accuracy), this kind of spectral modulation can be ignored. Figure
Fig. 9. Spectrum of the beam passed through the smaller crystals of the splicing crystals.
In Figure
The temporal profile of the compressed pulse can also be measured in the experiment, and the result is shown in Figure
The result in Figure
Fig. 10. Autocorrelation trace in the experiment: (a) original trace; (b) the trace of the splicing crystals.
(ii) Experiment of the spatial distribution of the focused point.
There are two problems about the spatial distribution: the diffraction effect in the near field and the spatial sidelobes in the far field. Figure
Fig. 11. Spot of the extraction beam in the near field: (a) with soft-edge aperture; (b) without edge aperture, and (c) schematic of the soft-edge aperture.
Fig. 12. Spot of the extraction beam in the far field: (a) did not pass through the crystal, (b) passed through the crystal without the soft-edge aperture, and (c) passed through the crystal with the soft-edge aperture.
The spatial distribution in the far field is also measured. Figure
4. Conclusion
We have developed a splicing technology of Ti:sapphire crystals for chirped pulse amplifier (CPA) laser systems. Theoretical and experimental investigations on the amplifier with four splicing Ti:sapphire crystals, such as amplification efficiency, spectral modulation, temporal profile of the compressed pulse, and the beam spatial distribution (near field and far field), have been carried out. The theoretical result shows that splicing technology of the Ti:sapphire crystal can be a possible scheme to enlarge the aperture of the crystal and suppress PL in the CPA systems. Some experiments were carried out to demonstrate that the possible issues generated by the inevitable errors can be ignored with ordinary accuracy.
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
Article Outline
Yanqi Liu, Yuxin Leng, Xiaoming Lu, Yi Xu, Cheng Wang. Splicing technology of Ti:sapphire crystals for a high-energy chirped pulse amplifier laser system[J]. High Power Laser Science and Engineering, 2014, 2(2): 02000e11.