Separation detection and correction of mosaic errors in mosaic gratings based on two detection lights with the same diffraction order and different incident angles

https://doi.org/10.1016/j.optlaseng.2020.106281Get rights and content

Highlights

  • A new method for separation detection and correction of mosaic errors in mosaic gratings is proposed.

  • The mosaic error separation detection system is designed.

  • The correction steps for mosaic errors are summarized.

  • The mosaic accuracy of the grating wavefront is analyzed.

Abstract

Grating tiling is an important fabrication technology for large-size gratings. However, when using grating tiling technology to form large-size echelle gratings, the interferometer cannot detect the grating's zero-order diffraction wavefront because the zero-order diffraction light of the echelle grating is weak. This prevents use of the zero-order and non-zero-order diffraction light of the grating to realize separation detection and correction of mosaic errors. To solve this problem, a new method for separation detection and correction of mosaic errors in mosaic gratings based on two detection lights with the same diffraction order but different incident angles is proposed and a mosaic error detection system is designed. Then, the correction steps for mosaic errors are summarized and the error in mosaic error detection system is analyzed. Finally, the measurement uncertainty in detecting the wavefront of the mosaic grating and the mosaic accuracy of the grating wavefront are analyzed. The uncertainty is 0.008λ (λ=632.8 nm) and the mosaic accuracy of the peak-to-valley wavefront is 0.069λ, which shows that high-precision measurements of the wavefront and high-precision mosaic of the wavefront were successfully achieved. The proposed method can be used for the mosaic of all blazed gratings.

Introduction

Diffraction gratings have been widely used as optical elements in applications including spectrometers [1], [2], [3], lasers [4], [5], [6], and couplers [7], [8], [9]. Among these applications, astronomical spectrometers and nuclear fusion laser systems need to be equipped with large-size diffraction gratings to meet the spectrometer's requirements for high resolution and the laser's requirements for high energy. Spectrometers such as the GMT-Consortium Large Earth Finder (G-CLEF) [10] and ESPRESSO [11] and laser systems such as OMEGA EP [12] and PETAL [13] are all equipped with large-size diffraction gratings. In light of the difficulty of fabricating single large-size diffraction gratings, grating tiling technology was proposed. Grating tiling technology involves placing two or more relatively small-sized gratings together, adjusting their attitudes and their relative positional relationships, and then rectifying the five dimensional errors until their error tolerance requirements are met. The core process of the grating tiling technique is the separation detection and correction of mosaic errors.

In 2007, based on the far-field diffraction principle, Yang et al. realized the separation detection and correction of rotation errors using zero-order and diffraction-order light of a single wavelength and also realized the separation detection and correction of translation errors using a Michelson interferometer [14]. Zeng et al. realized the separation detection and correction of rotation errors using zero-order and diffraction-order light of a single wavelength and realized the separation detection and correction of translation errors using diffraction-order light with dual wavelengths [15], [16], [17]. However, Qiao et al. analyzed the far-field pattern and the near-field diffraction wavefront and concluded that there was aberration in the far-field imaging system for large-aperture beams. This aberration would lead to inconsistency between the mosaic errors reflected by the mosaic wavefront and the measured far-field pattern. The optimal far-field pattern quality thus did not correspond to the optimal mosaic state. The mosaic quality is better reflected by all mosaicked grating wavefronts in the near field [18].

Therefore, in 2016, based on the interference principle, Lu et al. realized the separation detection and correction of the five dimensional mosaic errors using zero-order diffraction light and non-zero-order diffraction light of a single wavelength [19]. However, based on the interference principle, the mosaic error is detected and corrected using the zero-order and non-zero-order diffraction light of the grating, which is only suitable for a mosaic of ordinary blazed gratings and is not suitable for a mosaic of echelle gratings with weak zero-order diffraction light. Therefore, to solve the problem that the zero-order diffraction light of the echelle grating cannot be used to complete the separation detection and correction of the mosaic errors, in 2018, Cong et al. proposed reservation of a specific area of the aluminum film on the edge of the mosaic grating; they then used the light reflected by the aluminum film rather than the zero-order diffraction light of the echelle grating at the same angle of incidence to detect and correct mosaic errors [20,21]. However, the effective area of the mosaic gratings is much smaller than that of the larger aluminum film. Additionally, the mosaic errors will not be completely corrected if a smaller aluminum film is used because the surface shape of the aluminum film area is insufficient to characterize the surface shape of the grating area. Therefore, the separation detection of mosaic errors should mainly focus on detection of the grating area.

To solve the problem of error separation detection and correction in mosaicking echelle gratings, we propose a new five dimensional mosaic error separation detection and correction method for all blazed gratings. In this paper, Section 2 introduces the method for separation detection and correction of mosaic errors. Section 3 introduces the mosaic error separation detection system and the correction of the initial errors Δθx and Δz. Section 4 introduces the results and discussion.

Section snippets

Description of mosaic error

There are six dimensional errors in the mosaicking of two gratings as shown in Fig. 1. The six dimensional mosaic errors are listed as follows: Δθx, which is the error of rotation around the x-axis (grating vector direction), as shown in Fig. 1(a); Δθy, which is the error of rotation around the y-axis (grating line direction), as shown in Fig. 1(b); Δθz, which is the error of rotation around the z-axis (grating normal direction), as shown in Fig. 1(c); Δx, which is the error of translation

Mosaic error detection system

To cause the interference fringes of mosaic gratings in the α1 and α2 interference fields to appear on the same detector simultaneously and improve the correction accuracy of mosaic errors, a prism is introduced into the error detection optical path to generate a second incident angle for the detection beam in the same error detection optical path, as shown in Fig. 2.

Fig. 2 shows the experimental optical path diagram of the separation detection system for the mosaic error based on the same

Interference fringe adjustment process

Fig. 4 shows the correction process for mosaic errors based on the interference fringes of mosaic gratings. In each interference fringe pattern, the first row is the interference fringe of a static grating in the α1 interference field, the second row is the interference fringe of the moving grating in the α1 interference field, the third row is the interference fringe of the static grating in the α2 interference field, and the fourth row is the interference fringe of the moving grating in the α

Summary

This study presented a new method for separation detection and correction of mosaic errors based on use of detection beams of the same diffraction order but with different incident angles. This method solves the problem where the interferometer cannot detect the zero-order wavefront of an echelle grating because of the weak zero-order diffraction light, which means that the zero-order and non-zero-order diffraction light of the echelle grating cannot be used to complete separation detection and

CRediT authorship contribution statement

Guojun Yang: Conceptualization, Methodology, Validation, Investigation, Data curation, Writing - original draft, Writing - review & editing, Visualization. Xiangdong Qi: Conceptualization, Resources, Supervision. Xiaotao Mi: Conceptualization, Writing - review & editing, Funding acquisition. Shanwen Zhang: Funding acquisition. Hongzhu Yu: Resources. Haili Yu: Resources. Xiaotian Li: Funding acquisition.

Declaration of Competing Interest

The authors declare no conflicts of interest.

Acknowledgments

The authors acknowledge supports from National Key R&D Program of China (2016YFF0102006); National Natural Science Foundation of China (NSFC) (61975255, 61605204, 61505204); Key Technological Research Project of Jilin Province (20190302047GX); Bethune Medical Engineering and Instrument Center Project (BQEGCZX2019017); National Youth Program Foundation of China (61805233); Jilin Province Outstanding Youth Project in China (20180520190JH); Jilin Province Science and Technology Development Plan

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