Fabrication of a high-quality, small blaze angle grating for visible short-wave infrared hyperspectral cameras.

This study outlines the development of a low line density, small blaze angle grating, optimized for a visible to short-wave infrared hyperspectral camera. An analysis of grating specifications was conducted to meet the precise requirements of this application, particularly focusing on the stringent tolerance limits for the blaze angle. A specialized ruling tool adjustment device was designed to adhere to these exacting blaze angle tolerances. The grating groove shape was examined using atomic force microscopy (AFM), and the theoretical diffraction efficiency of the grating was calculated based on these observations. Additionally, laser-based methods were employed to measure the actual diffraction efficiency of the grating, while interferometry was used to assess the grating's diffraction wavefront. The test results demonstrate our capability to fabricate high-quality gratings with a low line density and small blaze angles that are suitable for advanced hyperspectral imaging applications.

Detection of microplastics via a confocal-microscope spatial-heterodyne Raman spectrometer with echelle gratings.

Microplastic pollution has become a global environmental problem that cannot be ignored. Raman spectroscopy has been widely used for microplastics detection because it can be performed in real-time and is non-destructive. Conventional detection techniques have had weak signals and low signal-to-noise ratios (SNR). Here, an efficient and reliable detection method is demonstrated. Specifically, a confocal microscope combined with an echelle-grating spatial-heterodyne Raman spectrometer (CM-ESHRS) was constructed. The confocal microscopy and the characteristics of the echelle grating enabled high optical throughput, high SNR, high spectral resolution, and a wide spectral detection band. After spectral calibration, the resolution approached 0.67 cm-1, moreover, the spectral detection range for a single order was 1372.16 cm-1. We detected and analyzed nineteen kinds of microplastics, such as polyamide, polypropylene, and polymethylmethacrylate, and the main vibrational spectral bands were categorized. Compared with commercial dispersive spectrometers, CM-ESHRS has a higher optical throughput. In addition, we examined microplastics with various particle sizes, microplastics mixed in flour, and microplastic particles of different materials under mixed conditions, all of which yielded complete spectral information. Overall, CM-ESHRS exhibits good potential applications for the detection of microplastics.

Differential Confocal Optical Probes with Optimized Detection Efficiency and Pearson Correlation Coefficient Strategy Based on the Peak-Clustering Algorithm.

Quantifying free-form surfaces using differential confocal microscopy can be challenging, as it requires balancing accuracy and efficiency. When the axial scanning mechanism involves sloshing and the measured surface has a finite slope, traditional linear fitting can introduce significant errors. This study introduces a compensation strategy based on Pearson's correlation coefficient to effectively reduce measurement errors. Additionally, a fast-matching algorithm based on peak clustering was proposed to meet real-time requirements for non-contact probes. To validate the effectiveness of the compensation strategy and matching algorithm, detailed simulations and physical experiments were conducted. The results showed that for a numerical aperture of 0.4 and a depth of slope < 12°, the measurement error was <10 nm, improving the speed of the traditional algorithm system by 83.37%. Furthermore, repeatability and anti-disturbance experiments demonstrated that the proposed compensation strategy is simple, efficient, and robust. Overall, the proposed method has significant potential for application in the realization of high-speed measurements of free-form surfaces.

A Differential Confocal Sensor for Simultaneous Position and Slope Acquisitions Based on a Zero-Crossing Prediction Algorithm.

A new sensor type is proposed to accurately detect the surface profiles of three-dimensional (3D) free-form surfaces. This sensor is based on the single-exposure, zero-crossing method and is used to measure position and angle simultaneously. First, the field intensity distribution in the posterior focal plane of the confocal microscope's objective was modeled accurately. Second, because the camera needs to trigger acquisition when the surface (to be measured) reaches the focal position of the sensor, a zero-crossing prediction method based on a sliding window was proposed. Third, a fast, spatially convergent, peak-extraction algorithm was proposed to improve the accuracy and efficiency of peak extraction. This scheme reduces system installation and adjustment difficulties, and the single-exposure, zero-crossing method achieves high-speed, real-time image acquisitions. The experimental results indicate that the average error of the zero-crossing prediction system was 17.63 nm, the average error of the tilt degree measurement was 0.011° in the range of 0-8°, and the prediction error of the tilt direction measurement was 0.089° in the range of 0-360°. The sensor can measure the slope and can be potentially used for 3D surface precision detection.

Precise Two-Dimensional Tilt Measurement Sensor with Double-Cylindrical Mirror Structure and Modified Mean-Shift Algorithm for a Confocal Microscopy System.

To improve the accuracy of three-dimensional (3D) surface contour measurements of freeform optics, a two-dimensional (2D) tilt measurement sensor for confocal microscopy (CM) systems is proposed based on a double-cylindrical mirror structure. First, the proposed system is accurately modeled. Second, we introduce a modified mean-shift-based peak-extraction algorithm with a novel kernel function (MSN) because the reflectivity of the measured object and fluctuation of the light source affect the measurement accuracy. Third, a partition fitting (PF) strategy is proposed to reduce the fitting error and improve the measurement accuracy. Simulations and experiments reveal that the robustness, speed, and angular prediction accuracy of the system effectively improved as a function of MSN and PF. The developed sensor can measure the 2D tilt, where each tilt is a composition of two separate dimensions, and the mean prediction errors in the 2D plane from -10°-+10° are 0.0134° (0.067% full scale (F.S)) and 0.0142° (0.071% F.S). The sensor enables the optical probe of a traditional CM to obtain accurate and simultaneous estimates of the 2D inclination angle and spatial position coordinates of the measured surface. The proposed sensor has potential in 3D topographic reconstruction and dynamic sampling rate optimization for 3D contour detection.

High-quality and large-area echelle grating ruled on 500-mm ruling engine for the fiber array solar optical telescope (FASOT).

A 300 mm×500 mm large-area echelle grating with groove density of 79 grooves/mm is fabricated for the spectrometer of the fiber array solar optical telescope (FASOT). This paper focusses on measurement methods of the grating performance. We present a method to evaluate the grating's stray light intensity, which is measured to a level of 10-4. The directly measured grating efficiency is approximately 90% of the designed value, and an indirect measurement method based on the grating groove profile is proposed. Based on the Rayleigh criterion and the grating diffraction wavefront, a physical optics method and a geometric grating method are proposed and are used to calculate the actual grating resolving power; the calculated results exceed 95% of the grating's theoretical resolving power. These results show that the CIOMP-6 ruling engine has sufficient precision to fabricate high-quality, large-area echelle gratings.

Design and analysis of broadband guided-mode resonant reflectors with coated triangular and trapezoidal profiles in TE polarization.

A low-refractive-index grating layer with symmetrical triangular/trapezoidal grooves covered with a high-refractive-index Si layer is used to design a broadband guided-mode resonant reflector. Software Rsoft is used to simulate the reflection and transmission spectra as well as the internal electric field distribution at the resonant wavelength. It is discovered that the interaction between resonant modes promotes the formation of a wideband spectrum. The reflector has been proven to provide wideband (Δλ > 450 nm) and high reflectivity (R > 98.4%) spectra over a wide range of base angles from 44° to 72°, and the maximum high reflectivity (R > 99%) spectral range in transverse electric polarization is 458 nm, spanning 1422 to 1880nm. The results not only demonstrate excellent tolerance to the base angle and grating depth but also provide more possibilities for the design of broadband reflectors.

Effect of the measuring mirror surface shape error on ruled groove straightness and the grating diffraction wavefront.

Measuring mirror requirements and their impact on groove errors are related to the error compensation strategy for a ruling engine. We analyze why the measuring mirror of the CIOMP-6 engine affects the groove straightness and the grating diffraction wavefront. We study a theoretical model of the relationship between the measuring mirror's surface shape error and the grating wavefront, propose a requirement for the measuring mirror surface shape error, and reprocess the measuring mirror. Comparative ruling experiments prove that the grating's wavefront quality at the diffraction order along the groove direction improved significantly after reprocessing of the measuring mirror.

Numerical calculation of the mosaic error between mosaic gratings.

The theoretical calculation model for a mosaic error was established based on the plane equation for a grating surface and the relationship equation for a mosaic grating surface. A mosaic grating was obtained based on this model. In the experiment, the mosaic error was calculated based on the diffraction wavefronts of two groups of mosaic gratings that were obtained simultaneously with a Zygo interferometer. The difference between the wavefront of the mosaic grating and the average wavefront of the mosaic grating element was 0.031λ. The maximum far-field intensity of the mosaic grating was 90% of that without an error. This model provides a theoretical basis for the numerical mosaic between gratings. In addition, the mosaic error can be calculated with this model, and the quality of the mosaic grating can be evaluated.

Ruling engine using adjustable diamond and interferometric control for high-quality gratings and large echelles.

A concept for position adjustment of the diamond in a ruling engine to enable nanoscale groove positioning is proposed. Based on this concept, we fabricate a diamond carriage, design an optical path, and propose an appropriate control method. Several high-quality gratings are ruled for spectrometer or interferometer applications. After implementation of the improvements, the maximum intensity of ghosts and scattered light of the echelle grating with 79 grooves/mm is reduced to half of that before the improvement, and the highest achievable groove density is increased from 6000 grooves/mm to 8000 grooves/mm. The grating ruling results indicate that the proposed concept and the related improvements to the engine significantly improve the accuracy of the CIOMP-6 ruling engine.

Method for calculation and analysis of the weights of grating mosaic errors.

A method to calculate the weight of grating mosaic errors is proposed based on an analytical hierarchy process. An accurate mosaic error tolerance calculation formula is also presented that is useful for large-size grating fabrication by the mosaic method. The grating mosaic error weights and tolerances are analyzed for mosaic echelle gratings. The analytical error weight and tolerance results agree well with the experimental results. The mosaic grating's far-field intensity is 95.8%, which is higher than the 94% value from the simulation results.

Analysis and removal of five-dimensional mosaicking errors in mosaic grating.

Combining the mathematical relationships between the grating wavefront and surfaces with the spatial relationships between the two grating wavefront, a mathematical model of the mosaicking errors is established to mosaic gratings. The five-dimensional mosaicking errors will respectively be calculated and then removed by the adjustment mechanisms. Mosaicking experiments are performed by using two gratings. First, by using zeroth order, the longitudinal offset is calculated and removed. Second, by using the diffraction order, the in-plane angle and grating spacing are calculated and removed, while the tip and tilt angles are calculated and removed. And then a mosaic grating is obtained.

Broadband transmission Raman measurements using a field-widened spatial heterodyne Raman spectrometer with mosaic grating structure.

A field-widened spatial heterodyne Raman spectrometer with a mosaic grating structure is developed for the simultaneous sensitivity enhancement and broadband transmission Raman measurements. We optimize the etendue to maximize the signals collected from the samples by using field-widening prisms and employ two mosaic gratings to achieve broadband operation, covering 5638 cm-1 with 2.865 cm-1 spectral resolution. The signal-to-noise ratios are improved by a factor of more than 11 and show a good stability and fair repeatability. We investigate the effects of the sample thickness and outer layer depth and observe liquids, solids, mixed targets, and anti-Stokes shifts. The instrument exhibits good performance for wide-field, high-resolution broadband transmission Raman measurements.

Development of a spatial heterodyne Raman spectrometer with echelle-mirror structure.

Spatial heterodyne Raman spectroscopy is a spectroscopic detection technique that is particularly suitable for Raman measurements. The spectral range of traditional spatial heterodyne Raman spectrometer (SHRS) is limited by its spectral resolution and the number of detector elements. We propose an SHRS with an echelle-mirror structure that employs multiple diffraction orders to achieve a broad spectral coverage and high spectral resolution simultaneously. This SHRS is used to obtain the Raman spectra of organic liquids, inorganic solid targets, and mixed targets. Observations of aqueous solutions, and minerals are presented. In addition, anti-Stokes Raman shifts are also presented. The proposed SHRS technique shows good performance for broadband, high-resolution Raman measurements.

Using a unique mirror to minimize the effect of ruling engine cosine error on grating performance.

The cosine error of the diffraction grating ruling engine is a systematic error caused by the measurement system. This error affects grating performance directly. To reduce the effects of the cosine error on grating performance, we analyze the cause of the cosine error, establish a mathematical model based on the error and the related grating indicators, and propose a method to reduce the error. To validate the proposed method, we present the results of grating ruling experiments performed before and after cosine error correction, which show that the method effectively reduces the cosine error effects.

Correcting groove error in gratings ruled on a 500-mm ruling engine using interferometric control.

Groove error is one of the most important factors affecting grating quality and spectral performance. To reduce groove error, we propose a new ruling-tool carriage system based on aerostatic guideways. We design a new blank carriage system with double piezoelectric actuators. We also propose a completely closed-loop servo-control system with a new optical measurement system that can control the position of the diamond relative to the blank. To evaluate our proposed methods, we produced several gratings, including an echelle grating with 79 grooves/mm, a grating with 768 grooves/mm, and a high-density grating with 6000 grooves/mm. The results show that our methods effectively reduce groove error in ruled gratings.