- High Power Laser Science and Engineering
- Vol. 8, Issue 3, 03000e29 (2020)
Abstract
Keywords
1 Introduction
Ultra-short and broadband laser sources are formidable tools for a wide range of scientific areas. In the field of ultrafast science, laser pulses lasting only a few optical cycles are used to generate secondary sources, such as soft X rays via high harmonic generation[
Over the past three decades, Ti:sapphire- and Nd:glass-based laser systems have been the regular workhorses for reliably generating energetic ultra-short pulses, supporting sub-100 fs durations and energies up to hundreds of joules[
A widely-used amplification technique that allows the overcoming of the tunability and spectral bandwidth issues faced by conventional laser amplification is optical parametric chirped pulse amplification (OPCPA)[
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For low-energy amplification stages (such as the front-end of high-energy systems), beta-barium borate (BBO)[
Broadband operation using BBO pumped by tens of microjoules with kilohertz-to-megahertz repetition rate pulses was shown by Andersen et al. and other groups[
Meanwhile, for high-energy stages there are only a few crystals that can be implemented in view of the large apertures and tens of centimeter length required[
In this work, we evaluate the performance of two alternative crystals for both low- and high-energy, few-cycle OPCPA: bismuth borate (BiB3O6 or BiBO) and yttrium calcium oxyborate (YCa4O(BO3)3 or YCOB). In particular, we have developed a noncollinear optical parametric amplification (NOPA) setup, operating in the near-infrared (IR) region, in order to test the performance of each crystal. We obtained for a 5 mm YCOB crystal an amplified bandwidth of 200 nm and a gain of ~102, and for a 2.5 mm BiBO crystal an amplified bandwidth of 240 nm and a gain of ~103. These results are analyzed in the context of numerical simulations using an in-house developed code[
To the best of our knowledge, this is the first demonstration of both ultra-broadband OPA in BiBO in the near-IR wavelength range and the broadest bandwidth obtained using YCOB.
These results are therefore of high interest to the laser developer community, highlighting promising alternatives to the commonly used nonlinear crystals.
Due to its size, YCOB can be implemented in high-energy, long-pulse (nanosecond) regime NOPA stages with comparable performance, both in bandwidth and efficiency, to the commonly used LBO and KDP crystals in the near-IR regime where high-power laser systems usually operate.
Meanwhile, BiBO can be advantageously adopted in low-energy, high repetition rate, few-cycle or femtosecond/picosecond stages (within high-power systems), showing superior performance when compared to BBO or LBO crystals in the near-IR regime. This capability is also critical for multi-stage systems where preservation of the ultra-broad bandwidths involved is essential for further efficient high-power amplification.
In the past two decades, several crystals have emerged as promising nonlinear media for ultra-broadband OPA. In this section we review the properties of two of those, namely YCOB and BiBO.
1.1 Yttrium calcium oxyborate
Yttrium calcium oxyborate was first identified in 2000, and extensive studies have been performed[
1.2 Bismuth borate
Bismuth borate is a biaxial nonlinear crystal with unique optical properties for frequency conversion at the visible and ultraviolet frequencies[
2 Ultra-broadband OPA stage design
To amplify a substantial spectral portion of the super-continuum, a noncollinear interaction geometry is required. We used in-house developed software[
A | B | C | D | |
---|---|---|---|---|
nx | 2.7697 | 0.02034 | 0.01779 | 0.00643 |
ny | 2.8741 | 0.02213 | 0.01871 | 0.01078 |
nz | 2.9107 | 0.02232 | 0.01887 | 0.01256 |
nx | 3.0740 | 0.0323 | 0.0316 | 0.01337 |
ny | 3.1685 | 0.0373 | 0.0346 | 0.01750 |
nz | 3.6545 | 0.0511 | 0.0371 | 0.02260 |
Table 1. Sellmeier equation () coefficients for YCOB and BiBO[27,35].
In the case of YCOB, interaction geometries outside of the principal planes are used to also maximize the nonlinear coefficient deff[
The simulations rely on solving the three-wave mixing coupled equations for optical parametric processes. In the software that we developed, the system of equations is solved for the envelope of the fields in a plane wave scenario, and it does not take into account spatial effects like diffraction or birefringence, nor high-order temporal effects such as cascaded nonlinear effects. The only spatial effect taken into account is the walk-off, which, although being a 2D effect, may be inserted as a lossy term[
The system of equations[
Using our code we performed a parametric scan regarding the signal central wavelength (
Figure 1.Representation of noncollinear phase-matching condition. o.a. is the crystal optic axis.
Figure 2.Parametric scan for the BiBO nonlinear crystal. (a) Simulated phase-matched wavelength (
Taking into consideration the results of the parametric scan for the BiBO crystal and the theoretical study performed by Pires et al. [
NL crystal | YCOB | BiBO |
---|---|---|
Axis plane | xz | yz |
Interaction | oo-e | ee-o |
Central λ | @850 | @850 |
θC (°) | 55 | 12 |
ϕ (°) | 62 | 90 |
deff (pm/V) | 1.41 | 3.02 |
θNC (°) | 3.75 | 2.9 |
LC (mm) | 5 | 2.5 |
LC (mm) | 7.5 | / |
LC (mm) | 15 | / |
Table 2. Parameters for the nonlinear crystals BiBO and YCOB to obtain broadband amplification. θC (°) and ϕ (°) are the crystal angles for perfect phase matching, deff (pm/V) is the nonlinear efficiency, θNC (°) is the noncollinear angle and LC (mm) is the crystal length.
3 Experimental setup
This work was performed in the framework of the development of an OPCPA laser chain installed at the Laboratory for Intense Lasers (L2I) at Instituto de Plasmas e Fusão Nuclear (IPFN) in Lisbon. In this context the optimal performance of OPCPA crystals aiming to provide ultra-broadband pulses at the sub-millijoule level was studied. Focus was given to BiBO due to the previously observed very high efficiencies[
The experimental setup (
4 Results
An extraction beam optical path was set at the output of the sapphire crystal to measure the spectral bandwidth of the supercontinuum (WLG). The measured WLG spectrum of the beam, acting as an ultra-short broadband seed of the designed noncollinear OPA stage, is shown in
The fringes observed in the measured spectrum could be linked to multiple filament generation, leading to an unstable spectral phase. To mitigate this effect, we implemented a soft aperture on the beam to ensure an ideal trade-off between the single filament and broadband operation. The fringed spectral phase would be worrisome for pulse compression. It should be noted, however, that these features are due to the seed signal used and not due to the amplification process.
4.1 YCOB NOPA performance
The 5 mm YCOB crystal was pumped at 515 nm with an intensity of ~50 GW/cm2. A 200 nm amplified bandwidth (1/e2 width) is observed, ranging from 780 nm to 980 nm (
We have performed measurements with the seed beam blocked, and no gain was observed. Ten consecutive shots are plotted to show the NOPA stability in terms of bandwidth. The visible shot-to-shot fluctuations are a result of the nonlinear supercontinuum generation process (namely, multi-filament operations and unstable spectral phase) and are not affecting the amplified bandwidth.
The results from the simulated data (shaded curve) agree well with the measured experimental spectra.
In order to study the influence of the crystal thickness on the amplified bandwidth and the gain, in
4.2 BiBO NOPA performance
In this case only one crystal length was available, so the performance was optimized by slightly readjusting the noncollinear angle obtained from the simulations (
Figure 3.Simulated amplification spectrum for a 5 mm YCOB crystal pumped at 515 nm with an intensity of ~50 GW/cm2.
Figure 4.Simulated noncollinear angular dependence of the BiBO amplified spectrum. Crystal thickness is 2.5 mm and the pump intensity is ~50 GW/cm2.
Figure 5.Schematic of the OPCPA chain used for crystal comparison. SHG, second harmonic generation; WLG, white light generation.
Figure 6.Noncollinear OPA stage seed: supercontinuum generation.
There is a noticeable soft cut-off in the longer wavelength region of the amplified spectrum resulting from the 1030 nm filter mentioned in
5 Result comments
By comparing the performance of the two crystals at similar pump intensities (~50 GW/cm2) in this low-energy regime, a number of conclusions can be drawn.System.Xml.XmlElementSystem.Xml.XmlElementSystem.Xml.XmlElement
From this we can infer that BiBO can play a major role in OPA in the low-energy, few-cycle regime because of the broadband, high gain capability presented. One simple way to remove the spectral dip and lead to a more homogeneous spectrum is through the implementation of a second NOPA stage with a different noncollinear angle, in order to shift the amplified spectral region. Concerning YCOB, and based on the recently demonstrated large aperture growth capability, the high nonlinear coefficient and the broadband flat spectrum supported, it can efficiently replace the more common crystals (such as LBO and DKDP) used in the high-energy, few-cycle regime.
In order to place these results in a wider context and understand their relevance better, we performed similar simulations for the supported bandwidth of BBO and LBO stages, comparing their outputs with the significant experimental results present in the literature.
We conclude that the BiBO nonlinear crystal shows a great potential to replace the commonly used BBO thanks to its broader supported bandwidth (
6 Conclusions
The experimental results presented, benchmarked with simulations, show that YCOB and BiBO are able to support ultra-broad bandwidth, high gain operation, with comparable (in the case of YCOB) or even better (in the case of BiBO) results in comparison with the commonly used nonlinear crystals. We also showed that BiBO is a perfect candidate for low-energy OPA stages, surpassing BBO/LBO in both conversion efficiency and supported bandwidth (reaching up to 240 nm). Bismuth borate presents a higher gain although a less homogeneously amplified spectrum. For the high-energy stages, YCOB is a good candidate with its capability of being grown as large as LBO and KDP, and its large supported bandwidth, while maintaining a high gain. Yttrium calcium oxyborate, in fact, can even be used for low-energy stages where it has similar performance to LBO (surpassed only by BBO and BiBO), but is best suited for high-energy stages such as those to be implemented in future multi-petawatt systems, where it allows ultra-short pulses to be scaled to high energies.
Figure 7.Experimental results for the YCOB NOPA stage compared to theoretical analysis. Amplified spectra for (a) 5 mm, (b) 7.5 mm and (c) 15 mm crystals. The shadowed curve is the numerically calculated amplified spectrum for the following parameters:
Figure 8.Experimental results for the BiBO NOPA stage: amplified spectrum for a 2.5 mm crystal. Different noncollinear angles are plotted to show the influence of
Figure 9.Theoretical amplified spectrum for LBO and BBO crystals in a noncollinear geometry to maximize the bandwidth.
In summary, we have described the broadband operation of these two nonlinear crystals and evaluated their performance. They can be implemented in low- or high-energy NOPA stages, competing or surpassing the performance of the more well-known BBO and LBO. We expect these results to be useful for the community of high-power laser developers at large.
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