Compression Gratings Research Articles

Spatiotemporal aberrations due to the groove density mismatching of compression gratings in ultra-intense femtosecond lasers

The groove density mismatching of compression gratings, an often-neglected key issue, can induce significant spatiotemporal aberrations especially for super-intense femtosecond lasers. We mainly investigate the angular chirp and the consequent degradation of the effective focused intensity introduced by the groove density mismatching of compression gratings in ultra-intense femtosecond lasers. The results indicate that the tolerances of grating groove density mismatching will rapidly decrease with the beam aperture or spectral bandwidth increases. For our 100PW laser under construction, the grating groove density mismatching should be as small as 0.001 gr/mm if the drop of effective focused intensity has to be controlled below 15%. More importantly, new angular chirp compensation schemes are proposed for both double-grating and four-grating compressors. This work reveals the importance of groove density matching of compression gratings, and can provide helpful guidelines for the design of ultra-intense femtosecond lasers.

Viability verification of asymmetric four-grating compressor in SEL-100 PW frontend

The laser-induced damage observed in optics has rendered compressor grating the most limited component in petawatt (PW) lasers, owing to the considerable challenges associated with manufacturing gratings possessing both a sufficiently high damage threshold and large dimensions. An asymmetric four-grating compressor (AFGC) based multistep pulse compressor method was proposed recently to alleviate the requirement of such gratings. The viability of AFGC is further investigated experimentally in SEL-100 PW frontend with spectral bandwidth of over 200 nm in this study. It is verified that the AFGC has the ability of beam-smoothing resulting from the introduced spatial dispersion. Meanwhile, the spatial dispersion scarcely impacts temporal contrast and focal spot of the output beam. By reducing the laser spatial intensity modulations from 2.0 to 1.3, the pulse energy capacity of the AFGC can be increased by more than 1.5 times through simple translation. The results mark a breakthrough toward experimentally renovating the grating compressor, and pave the way for a reliable and efficient constructions of 100 s PW lasers.

Detrimental effects of period-chirped gratings in pulse compressors.

We present a comprehensive simulative and experimental investigation of how period-chirped pulse compression gratings affect the compressed pulses. A specifically developed ray-tracing tool was used for the simulative investigations. It is shown that the chirp creates a characteristic spatio-spectral error pattern, which leads to a degradation of the beam quality and an increase of the pulse duration. The experimental investigations, for which both a narrow-bandwidth continuous-wave and a pulsed laser beam were guided through a Treacy-compressor comprised of period-chirped gratings, confirm the simulation results and present methods on how to identify the chirp's characteristic error pattern in practice.

Specifications and control of spatial frequency errors of components in two-beam laser static holographic exposure for pulse compression grating fabrication

Abstract The large-aperture pulse compression grating (PCG) is a critical component in generating an ultra-high-intensity, ultra-short-pulse laser; however, the size of the PCG manufactured by transmission holographic exposure is limited to large-scale high-quality materials. The reflective method is a potential way for solving the size limitation, but there is still no successful precedent due to the lack of scientific specifications and advanced processing technology of exposure mirrors. In this paper, an analytical model is developed to clarify the specifications of components, and advanced processing technology is adopted to control the spatial frequency errors. Hereafter, we have successfully fabricated a multilayer dielectric grating of 200 mm × 150 mm by using an off-axis reflective exposure system with Φ300 mm. This demonstration proves that PCGs can be manufactured by using the reflection holographic exposure method and shows the potential for manufacturing the meter-level gratings used in 100 petawatt class high-power lasers.

Impact of the multilayer dielectric design on the laser-induced damage threshold of pulse compression gratings for petawatt-class lasers.

The peak-power of petawatt-class lasers is limited by laser-induced damage to final optical components, especially on the pulse compression gratings. Multilayer dielectric (MLD) gratings are widely used in compressor systems because they exhibit a high diffraction efficiency and high damage threshold. It is now well established that the etching profile plays a key role in the electric field distribution, which influences the laser damage resistance of MLD gratings. However, less attention has been devoted to the influence of the multilayer design on the laser damage resistance of MLD gratings. In this Letter, we numerically and experimentally evidence the impact of the dielectric stack design on the electric field intensity (EFI) and the laser-induced damage threshold (LIDT). Three different MLD gratings are designed and manufactured to perform laser damage tests. On the basis of the expected EFIs and diffraction efficiencies, the measured LIDTs show how the multilayer design influences the laser resistance of the MLD gratings. This result highlights the impact of the multilayer dielectric design on the electric field distribution and shows how to further improve the laser-induced damage threshold of pulse compression gratings.

Simple spatially resolved period measurement of chirped pulse compression gratings.

We present an easy-to-implement and low-cost setup for the precise measurement of the period chirp of diffraction gratings offering a resolution of 15 pm and reasonable scan speeds of 2 seconds per measurement point. The principle of the measurement is illustrated on the example of two different pulse compression gratings, one fabricated by laser interference lithography (LIL) and the other by scanning beam interference lithography (SBIL). A period chirp of 0.22 pm/mm2 at a nominal period of 610 nm was measured for the grating fabricated with LIL, whereas no chirp was observed for the grating fabricated by SBIL, which had a nominal period of 586.2 nm.

Ultrashort X-ray Free Electron Laser Pulse Manipulation by Optical Matrix

Free electron laser (FEL) is capable of producing ultra-short X-ray pulses. The estimation of X-ray pulse propagation is the key process of X-ray FEL beamline design. By using the Kostenbauder matrix approach, the evolution of an ultra-short pulse in a beamline system can be calculated. Therefore, it is of significant importance to investigate the Kostenbauder matrices of different kinds of X-ray optics. In this work, we derive a unified 6 × 6 optical matrix to describe various kinds of X-ray optical elements, including varied-line-spacing (VLS) toroidal grating, VLS spherical grating, VLS cylindrical grating, VLS plane grating, toroidal grating, spherical grating, cylindrical grating, plane grating, toroidal mirror, spherical mirror, cylindrical mirror, and plane mirror. These optics are usually adopted in soft X-ray regime. We apply this method to describe the transverse focusing, pulse front tilt, and pulse stretching after an X-ray pulse going through a VLS plane grating monochromator (VLS-PGM). We also use this approach to simulate a grating compressor which can be used to compress chirped soft X-ray pulse. This work is helpful in the design and optimization of X-ray beamline systems.

Advantages of out-of-plane pulse compression gratings.

Diffraction gratings for pulse compression generally perform best at the Littrow angle, but reflection gratings require a nonzero deviation angle to separate the incident and diffracted beams, so they cannot be used at the Littrow angle. In this paper, we show both theoretically and experimentally that most practical multilayer dielectric (MLD) and gold reflection grating designs can be used with quite substantial beam-deviation angles-as large as even 30°-by mounting the grating out-of-plane and choosing the optimal polarization. The impact of polarization when mounting out-of-plane is explained and quantified.

Monitoring and characterization of particle contamination in the pulse compression chamber of the OMEGA EP laser system

Abstract The laser-damage performance of optics is known to be negatively affected by microscale particle contamination induced by the operational environment. This work investigates the properties of particles accumulating in various locations near critical optics inside the OMEGA EP grating compressor chamber during quarterly operational periods over a 2-year duration. The particles found were characterized using optical microscopy, scanning electron microscopy and energy dispersive X-ray spectroscopy. The analysis indicates significant concentrations of micrometer- to nanometer-scale particles inside the vacuum chamber, with higher values observed near the port leading to the OMEGA EP target chamber. The distribution of the chemical composition of these particles varies between collection periods. Although understanding of the mechanisms of particle generation and transport remains uncertain, the hypothesis is that this particle load represents a risk for contaminating the surfaces of high-value optics located inside the chamber, including the compression gratings and deformable mirrors, and therefore affecting their laser-damage resistance and overall operational lifetime.

Research on post-processing polishing technology of pulse compression gratings with high laser damage threshold

Research on post-processing polishing technology of pulse compression gratings with high laser damage threshold

Manufacturing-induced contamination in common multilayerdielectric gratings.

Contamination of pulse compression gratings during the manufacturing process is known to give rise to reduced laser damage performance and represents an issue that has not yet been adequately resolved. The present work demonstrates that the currently used etching methods introduce carbon contamination inside the etched region extending to a 50- to 80-nm layer below the surface. This study was executed using custom samples prepared in both, a laboratory setting and by established commercial vendors, showing results that are very similar. The laser-induced-damage performance of the etched and unetched regions in the grating-like samples suggest that contaminants introduced by etching process are contributing to the reduction of the laser-induced damage threshold.

An Ultra-Precision Absolute-Type Multi-Degree-of-Freedom Grating Encoder.

An absolute-type four-degree-of-freedom (four-DOF) grating encoder that can simultaneously measure the three-axis pose (θx, θy, θz) and one-axis out-of-plane position (Z) of an object with high accuracy is demonstrated for the first time in this research. This grating encoder is composed of a stationary reading head and a movable grating reflector. A light beam from the reading head is projected onto the grating, and three diffracted beams (0th-, +1st-, and -1st-order) are generated, collimated, and received by three separate quadrant photodetectors (QPDs). The information of θx, θy, θz, and Z is coded into spot positions of these three diffracted beams on the QPDs. Thus, the modeling and decoupling algorithms were investigated, and an independent calculation of these four-DOF absolute positions was theoretically guaranteed. A prototype was then designed, constructed, and evaluated. Experimental results verified that the proposed grating encoder could achieve the absolute measurement of four-DOF θx, θy, θz, and Z with an accuracy of sub-arcseconds and sub-micrometers. To the best of our knowledge, the proposed encoder in this research is the first one to achieve absolute simultaneous measurements of four-DOF position and pose with a large measurement range. The success of this new grating encoder can benefit various multi-DOF positioning applications, especially for large-scale synthetic aperture optics (SAO), including stitching off-axis parabolic mirrors and pulse compression grating.

Experimental study on the fluctuation–magnification effect of chirped volume Bragg grating compressor in high-energy, ultrafast, thin-disk chirped pulse amplification system

The fluctuation–magnification effect on the peak intensity of a laser pulse caused by the nonlinear Kerr effect in the chirped volume Bragg grating (CVBG) compressor is investigated experimentally for a high-energy, thin-disk, chirped pulse amplification system. The nonlinear Kerr effect occurs at the blue end, and the accumulated nonlinear phase shift (B-integral) in the CVBG rises with the increase of laser pulse energy. Experiments show that small fluctuations in peak power of uncompressed pulses cause increasing of the temporal fluctuation and spatial fluctuation due to high Kerr-nonlinearity in the CVBG when B-integral is larger than π. Thus the initial fluctuation would be magnified by the CVBG compressor.

Compact optical grating compressor.

A novel design of a grating-based optical pulse compressor is proposed. The proposed compressor provides a large group delay dispersion while keeping the compressor linear size small. The design of the proposed compressor is based on a traditional Treacy compressor with a straightforward modification of inserting two lenses between the compressor's gratings. This simple alternation aims to substantially increase group delay dispersion of the compressor or alternatively to decrease the compressor size while maintaining its group delay dispersion. A theoretical description of the enhanced compressor has been developed in the paraxial approximation. A detailed numerical model has been built to calculate the compressor parameters more accurately. These theoretical studies have revealed that the enhanced optical compressor provides a significant increase in the group delay dispersion compared to a standard Treacy compressor.

In-situ measurement and compensation of complex spatio-temporal couplings in ultra-intense lasers

Diffraction wavefront errors of large-scale Treacy compressors may induce complex spatio-temporal couplings (CSTCs) in chirped pulse amplification and degrade the quality of the pulsed beam in both near and far fields. This problem is particularly pronounced in petawatt laser facilities that require tiled compressor gratings. Here, to the best of our knowledge, we present the first in-situ measurement method named pseudo lateral shearing interferometry (PLSI) for the CSTC wavefront error and manufacture a continuous phase plate (CPP) according to the measurement to reduce the distortion. A Shark-Hartmann wavefront sensor is used to characterize wavefronts of the entire system at two different wavelengths, and the CSTC wavefront error can be reconstructed similarly to lateral shearing interferometry (LSI). Our approach strongly reduces systematic errors and the impact of other optics. The root-mean-square (RMS) value of the resulting error is 40 nm in a single measurement, and the day-to-day repeatability is 55 nm. The employed CPP reduces the peak-to-valley (PV) value of the CSTC wavefront error from 675 to 312 nm, and the focused peak intensity is improved by 51%.

Impact of compression grating phase modulations on beam over-intensities and downstream optics on PETAL facility

Chirped pulse amplification has been widely implemented in high power laser chains. It consists of a set of diffraction gratings used to stretch and compress short laser pulses. In the case of high power laser chains, the compression stage is followed by the transport mirror in order to carry the laser beam to its final target. In such laser chains, laser beams propagate over a complex set of optical components and understanding the propagation of phase noise turns out to be of crucial importance. Phase modulation can induce laser damage on the final optical components. Here, we study the impact of phase modulation induced by the different diffraction gratings of the Petawatt Aquitaine Laser (PETAL) compressor on the downstream over-intensities, in particular on the transport mirror. This work allows us to quantify the impact of phase modulation for every single grating element in the compression stage, and to estimate the quantity of laser induced damage sites on transport optics for a specific laser shot.

Full-aperture chirped-pulse grating compression with a non-uniform beam

A novel chirped-pulse compression scheme is proposed to pursue higher pulse peak power of chirped-pulse amplifiers based on state-of-the-art gratings. It is characterized by the spectral non-uniformity of the beam cross section as well as by the uniform size of the beam projections on the four compression gratings. The spatial distribution of spectra is well designed so that the beam can remain generally collimated in the compressor and the gratings can be employed with a full work area. The maximum possible output pulse energy can therefore be increased within the damage threshold limit. A detailed design is developed for a scheduled 50-PW optical parametric chirped-pulse amplifier. Results of the numerical simulation on the focused output show that the spot size and pulse duration in the focal plane are close to those of conventional grating compression. Further, it is a promising method for upgrading existing chirped-pulse amplifiers.

Laser induced damage threshold and incubation effects of high-power laser system optics

We present a laser induced damage threshold (LIDT) measurement technique where a mm-diameter non-Gaussian laser beam is used. This allows both a large number of measurements points and a large range of fluence to be sampled with a single measure. The method is used in-situ, inside the laser radiation – matter interaction vacuum vessel used for high power experiments with a 100 TW-class laser system. With our 2.5-Hz repetition rate laser system, the well-known incubation effect is observed where the laser damage threshold on the optics decreases with increasing number of laser shots. The incubation effect is studied with 22-fs laser pulses at a laser 2.5-Hz nominal repetition rate with several optics like dielectric mirrors and gold compression gratings irradiated by up to 9 × 103 laser shots.

Nanosecond laser damage characteristics of multilayer dielectric films and gratings

Multilayer dielectric gratings (MLDGs) have been widely used as pulse compression grating (PCG) in chirped pulse amplification (CPA) technology due to their high laser induced damage thresholds (LIDTs). The quest for MLDGs LIDTs improvement is endless. As one of the core components of CPA process, MLDGs will encounter laser irradiation of nanosecond, picosecond and femtosecond. Therefore, the damage characteristics of MLDGs should be studied at various pulse widths. We performed the LIDTs test on a Nd:YAG laser system with a wavelength of 1064 nm and a pulse width of 8 ns. Damage characteristics of both MLDFs and MLDGs were investigated. MLDFs were deposited on the substrates cleaned by hand wipe or ultrasonic cleaning. The results show that the LIDTs of MLDGs are approximately 60% of MLDFs. Besides, LIDTs of MLDFs with HfO2 top layer will not be affected by the methods of substrates cleaning due to its surface damage characteristic related to the non-zero EFI on the surface material. However, for the MLDFs with top layer of Ta2O5, LIDTs of MLDFs deposited on substrates cleaned by hand wipe are higher than those deposited on the ultrasonically cleaned substrates.

Review of pulse compression gratings for chirped pulse amplification system

The ultrashort, ultrahigh intensity pulse laser has been fully developed in past three decades. Chirped pulse amplification (CPA) system plays an important role in the generation of the ultrashort, ultrahigh intensity pulse laser. Pulse compression gratings (PCGs) are the key element of CPA system and determine the performance and lifetime of the whole system. We introduce the principle of CPA system and the performance requirements of PCGs. Then the development status of PCGs, including Au-coated grating, multilayer dielectric grating (MDG), and metal MDG, is fully reviewed. Finally, the development prospect of PCGs in the future is presented. Our study is helpful for comprehensive understanding of PCGs.