Large-scale (180 × 60 × 1 mm(3)) transmission gratings with groove densities of 1250 and 1740 lines/mm have been developed, resulting in diffraction efficiencies above 95%. The throughput of a folded pulse compressor with two large-scale transmission gratings was approximately 80% in a 20-fs Ti:sapphire chirped-pulse amplification (CPA) laser. The parabolic bending of the transmission grating due to anti-reflection (AR) coating was minimized to 2.9 λ at 633 nm by improving the evaporation process. By a simple analysis, we explain why this level of bending does not induce a wavefront distortion through the transmission grating near the Littrow condition while the wavefront from a reflection grating is distorted to nearly twice the bending of the grating. The calculation based on the measured bending shows that both the group delay difference relative to the ideally flat grating from 750 to 850 nm and the spatial pulse front distortion over a 60-mm-diameter input beam are negligible, even when the dispersive beam covers ~140 mm on the grating. The spatial pulse front distortion measured after the compressor was less than the measurement limit (1.5 fs) for a 20-mm-diameter beam, where the beam size in the dispersive direction on the grating was 85 mm.

Wavefront Analysis of High-Efficiency, Large-Scale, Thin Transmission Gratings

A swing ion-beam etching method to fabricate convex blazed gratings used in shortwave infrared hyperspectral imaging spectrometers is presented. This method solves the consistency problem of blaze angles by swing etching through the meridian direction of the gratings. The mathematical relationship of the curvature, aperture, and diffraction efficiency of convex gratings is studied to demonstrate the limitation of conventional translational lithography and the necessity of swing etching. A geometric model is built to analyze the influence of swinging speed and beam slit width on groove evolution. Convex gratings with a 45.5 gr/mm groove density, 67 mm aperture, 156.88 mm radius of curvature, and 2.2° blaze angle have been fabricated and measured where the peak and average diffraction efficiency in the shortwave infrared band reach 90% and 70%, respectively. Experimental results validate that high-efficiency convex gratings of small blaze angle and high groove consistency can be produced by swing etching, which satisfy the requirements for high spectral resolution and miniaturization of imaging spectrometers.

Convex blazed grating of high diffraction efficiency fabricated by swing ion-beam etching method.

In this paper, our simplified model for grating formation in photopolymers is extended to describe the effects of uniform post-exposure on an existing hologram, and further to present a new model of holographic multiplexing for calculating the exposure schedule for multiplexed gratings with uniform diffraction efficiency. It is experimentally verified that the refractive-index modulation of the existing gratings continues to increase when a current grating is being recorded. Twenty gratings were multiplexed using the exposure schedule calculated with the uniform post-exposure model, and comparison with the result from traditional method confirmed the validity of this model.

Investigation on mechanism of multiple holographic recording with uniform diffraction efficiency in photopolymers.

Beam sampling gratings (BSGs) employed in high-power laser systems usually have large aperture so that the adequate uniformity of diffraction efficiency is difficult to obtain. We proposed a deterministic method using controllable non-uniform etch to improve the efficiency uniformity of large-aperture BSGs. During the ion beam etching (IBE) process, etch depths are finely adjusted by the dynamic leaf. The motion trajectory of the dynamic leaf is calculated using the fine adjustment algorithm. Simulations are conducted on the basis of a typical example. The simulation predictions show that the cumulative error is 0.067 nm and about 99.1% of depth differences are in the range of the required etch depth tolerance, which suggests that the diffraction efficiency uniformity of BSG is expected to be effectively improved and thus can meet the requirement of a RMS of 5%. As a cost-effective solution, it also has a broad prospect in many optical fabrication fields, especially for the fabrication of large optics.

http://iopscience.iop.org/article/10.1088/2040-8978/17/3/035401/pdf

Algorithms for finely adjusting etch depths to improve the diffraction efficiency uniformity of large-aperture BSG

We designed and fabricated two-dimensional cylindrical subwavelength gratings (SWGs) on fused silica for use in 1064 nm laser system. The transmittance and laser-induced damage threshold (LIDT) under the irradiation of 1064 nm pulses were performed, and a lower LIDT compared to blank fused silica was obtained. To understand damage mechanism quantitatively, macro-temperature on the SWGs integrated fused silica and micro-Electric field, micro-thermal-stress distribution on SWGs during the laser irradiation process were investigated by Finite Element Analysis, and the comparison between theoretical and experimental research indicated the presence of absorption centers and the non-uniform thermal mechanical distribution of SWGs contributed to the LIDT reduction on SWGs integrated fused silica.

Theoretical and experimental research on laser-induced damage of cylindrical subwavelength grating.

Design and fabrication of 1300-line/mm polarization-independent reflection gratings for spectral beam combining

The convex grating is one of the key elements in Offner hyperspectral imaging spectrometers. In this paper the design of
the holographic convex grating focuses on the optimization of its recording parameters for aberration correction.
Meantime, the diffraction efficiency of the convex grating is discussed by using rigorous coupled-wave theory. The
results show: within the wavelength range from 0.4μm to 0.78μm, the diffraction efficiency can be over 20% and up to
40% through controlling the groove depth and duty cycle of rectangular grating; the diffraction efficiency can be over
35% through controlling the blaze degree of the blazed grating. The rectangular surface-relief convex grating with the
period of 5μm in the center, and the ruled area - a convex with its radius 100mm and aperture 40mm has been fabricated
by holographic-ion beam etching. A prototype blazed convex grating will be fabricated in late 2010.

Study on convex grating in hyperspectral imaging spectrometers

The invention discloses a method for manufacturing a grating with a low diffracted wave aberration on a grating substrate having aberrations. The method comprises the following steps of: forming a holographic record interference light field by a parallel light writing light beam and the other parallel light writing light beam with an adjustable wave surface; performing holographic exposure on a holographic record substrate; and performing development to obtain the needed grating. The method is characterized in that: a distorting lens is put into a divergent light path of one of the writing light beams, and the holographic record interference light field used for compensating the base aberration of the holographic grating is obtained by controlling the distorting lens. In the process, the distorting lens, a distorting lens controller and an interferometer form a closed-loop control and detection system of the holographic record interference light field; and the holographic record interference light field obtained after adjusting the distorting lens is used for exposing the holographic record substrate coated with a photosensitive material, recording the holographic grating, and performing the development to finish the manufacturing of the needed grating with the low diffracted wave aberration.

Method for manufacturing holographic grating

Large scale and broadband pulse compression grating which is used in the femtosecond Ti: sapphire laser is studied. The
line density of gold coated pulse compression grating is 1740lp/mm. The working wavelength of gold coated grating is
from 700nm to 900nm and its deviation angle is 16.6 degree(@800nm). By using rigorous coupled wave theory, a series
of calculation results with different profiles of gold coated pulse compression grating, such as rectangular, sinusoid,
semi-sinusoid, are discussed in this paper. In order to get high diffraction efficiency throughout the whole working band,
the duty cycle of grating should be more than 0.5 for the rectangular or semi-sinusoidal profile of groove. Duty cycle of
grating causes the diffraction efficiency to change quickly at the short wavelength. By using holographic recording
method, the gold coated pulse compression gratings with the aperture of 200mm×400mm have been fabricated. The
gratings exhibit -1st order diffraction efficiency in excess of 94%(TM@808nm).

Design and fabricate large scale gold coated pulse compression grating for Ti: sapphire laser

A antireflection film with adjustable refractive index from 1.17 to 1.41 was prepared at room temperature via a template-free sol–gel method, using tetraethylorthosilicate(TEOS) as co-precursor. In this study, the regulation of the refractive index is achieved by mixing different volumes of acid-catalyzed silica sol having a refractive index of 1.41 with a base-catalyzed silica sol having a refractive index of 1.17. The silica coating, with a refractive index of 1.2332, prepared by this base/acid two step catalysis sol-gel process can increase the transmittance of K9 glass by about 3.5%.

Chaoming Li

Papers

Design and fabrication of 1300-line/mm polarization-independent reflection gratings for spectral beam combining

Study on convex grating in hyperspectral imaging spectrometers

Method for manufacturing holographic grating

Design and fabricate large scale gold coated pulse compression grating for Ti: sapphire laser

Template-free sol-gel preparation of antireflection films with adjustable refractive index