Jinchao Pang

Holographic fabrication of large-constant concave gratings for wide-range flat-field spectrometers with the addition of a concave lens

We present a new design for the fabrication of concave gratings with large grating constants for flat-field miniature spectrometers with a wide spectral band. In this new design, one of the two optical paths for the holographic lithography of a curved grating structure with variable line spacing is modified by adding a concave lens in front of the point source. The addition of the concave lens allows the real point source, as well as the spatial filter for generating this point source, to be moved back. In this manner, the two spatial filters for generating two point sources are separated. Avoiding the physical conflict between these two spatial filters reduces the difficulty of fabricating large-constant concave gratings. Experimental results verify the feasibility of the proposed design in fabricating concave gratings with large grating constants. The resolution of a spectrometer using the fabricated concave grating is evaluated and found to be better than 1.1 nm across a spectral band ranging from 360 nm to 825 nm.

Improving the spectral resolution of flat-field concave grating miniature spectrometers by dividing a wide spectral band into two narrow ones

In this study, a new flat-field concave grating miniature spectrometer is proposed with improved resolution across a wide spectral band. A mirror is added to a conventional concave grating spectrometer and placed near the existing detector array, allowing a wide spectral band to be divided into two adjacent subspectral bands. One of these bands is directly detected by the detector, and the other is indirectly analyzed by the same detector after being reflected by the mirror. These two subspectral bands share the same entrance slit, concave grating, and detector, which allows for a compact size, while maintaining an improved spectral resolution across the entire spectral band. The positions of the mirror and other parameters of the spectrometer are designed by a computer procedure and the optical design software ZEMAX. Simulation results show that the resolution of this kind of flat-field concave grating miniature spectrometer is better than 1.6 nm across a spectral band of 700 nm. Experiments based on three laser sources reveal that the measured resolutions are comparable to the simulated ones, with a maximum relative error between them of less than 19%.

Design of a variable-line-spacing grating pattern for spectrometers based on a grating Fresnel device.

In this Letter, we propose a variable-line-spacing (VLS) grating pattern for a hybrid diffractive device termed a grating Fresnel (G-Fresnel) lens, which is used in spectrometers to improve spectral resolution over a wide spectral range. The VLS grating pattern disperses light of specific wavelengths with a different angle and position such that the aberration caused by the Fresnel surface can be compensated for. In this manner, high resolution can be achieved over a relatively wide spectral range. The VLS grating pattern is designed based on the least wave-change principle and simulated by ZEMAX. Results reveal that the VLS G-Fresnel device allows a subnanometer resolution over a spectral range of 200 nm.

Fabrication of a concave grating with a large line spacing via a novel dual-beam interference lithography method

We introduce a novel dual-beam interference lithography (IL) method that makes it possible to fabricate a concave grating with a large line spacing. A concave lens is placed between two point sources for spatial interference and a concave substrate to produce the grating pattern. The original positions of the two point sources are separated by the concave lens, which permits the IL method to fabricate a concave grating that bypasses the line spacing limitation of the conventional IL system. A concave grating with a line spacing of about 3.8 μm was fabricated and fitted inside a miniature spectrometer. The enlarged line spacing reduces the detector length by 66.5%, while keeping the resolution better than 1.5 nm over a wide spectral band (360 - 825 nm).

Improved master-replica separation process for fabrication of a blazed concave grating by using a combination-type convex grating

An imprinting process that enables fabrication of blazed concave gratings with small radius and large curvature is presented. In this process, a combination-type convex grating substrate is used as a master to replace the single part used in the traditional process. The two parts of the combined convex grating are independently separated from the resist layer. In this manner, the concave blazed grating pattern in the resist can maintain maximum consistency with that on the combined convex grating. Experiments are conducted to demonstrate the feasibility of a two-step fabrication technology.

A portable flat-field concave grating spectrometer with high resolution

The resolution of spectrometer can be increased by using multiple detectors. But for the portable spectrometer, the relatively wide edge that used to fix the detector can seriously influence the imaging quality. And the multiple detectors will increase the difficulty of the circuit part. In this paper, a novel method is introduced to increase the resolution by using one detector. The whole waveband is divided into two wavebands in this new structure. And the long waveband will be reflected by a mirror to the definitely location in which the short wave is located. This structure will not only solve the influence of the detector’s edge but also lower the cost of circuit part. By the simulation of the ZEMAX, the resolution of the spectrometer using the new method is better than the current works using one detector in the whole waveband.

A method of fabricating wide spectrum flat-field concave grating with compensation concave lens

Flat-field concave diffraction grating is the key component of a portable grating spectrometer, which integrates three optical properties: dispersion, imaging, and flat spectral image in a single device. In recent years, fabricating concave grating has attracted much attention. However, the distance between two exposure light sources in the fabrication light system are often short, which makes it difficult to build the fabrication structure, sometimes even impossible. In order to solve this problem, two methods have been adopted. One is using special microscope objectives, but it reduces the system’s ability of eliminating of aberrations. The other way is building spatial filters respectively, but this adds the difficulty of the system installing and adjustment, furthermore, it makes it impossible to fabricate wide spectrum FCDG. In this paper, a method to increase the distance between the two exposure light sources is proposed by using a compensation mirror. The use of compensation mirror can not only reduce the difficulty of fabricating grating, but also achieve results almost as better as that of the two original exposure points. We calculated the two new exposure points position with Matlab, and then performed system modeling, simulation and optimization in the Zemax software. A theoretical analysis is given to show that the proposed method can ensure the spectral image quality and greatly reduce the fabrication difficulty.

Demonstrating and optimizing the dual dispersion and focusing functionality of grating-Fresnel lens

As optical spectroscopy plays a vital role in many of modern science and engineering, there is a growing need for developing an inexpensive and miniature spectrometers. Many attempts have been tried to solve the issue. Grating-Fresnel is a hybrid device that fuses the functions of a grating and Fresnel lens into a single device. In this paper, we try to simulate reflection type and transmission type G-Fresnel device in ZAMAX. And with the aids of ZEMAX, we try to optimize the Fresnel lens, grating pattern. A better alignment for the CCD detector could also improve sensitivity of the system as well. In order to improve the resolution and sensitivity, the length between Fresnel lens and gratings will be optimized.