1. Introduction to Gratings
Definition of Diffraction Grating: A diffraction grating is an optical element that can modulate the amplitude or phase of incident light waves in a spatially periodic manner, or modulate both amplitude and phase simultaneously in a spatially periodic manner.
Spectral Dispersion of Gratings: The Fraunhofer diffraction pattern of a diffraction grating, also known as a grating spectrum, consists of fine and bright lines on the focal plane. The positions of these lines vary with the illumination wavelength. When polychromatic light passes through the grating, each wavelength forms its own set of lines, separated by certain distances, allowing the spectral composition of the illuminating light to be distinguished.
Types of Diffraction Gratings: Gratings can be classified in different ways. According to the working method, they can be divided into transmission and reflection types; according to the manufacturing method, they can be classified into ruled gratings, replica gratings, and holographic gratings. In fiber optic spectrometers, the main types of gratings used include reflective blazed ruled gratings, reflective holographic gratings, and volume phase holographic gratings (VPH).
2. Reflective Blazed Ruled Gratings
In a reflective blazed ruled grating, the grooves are not parallel to the grating surface; there is an angle γ (known as the blaze angle) between them. This design separates the central maximum of single-slit diffraction and the zero-order principal maximum of slit interference, transferring and concentrating the light energy into a specific order spectrum (usually the first order), achieving the blazing effect. Blazed gratings have higher diffraction efficiency than ordinary ruled gratings.
Product Recommendation:
The Jinsp SR series fiber optic spectrometers, which include UV spectrometers, visible spectrometers, and near-infrared spectrometers, predominantly use reflective blazed ruled gratings for diffraction efficiencies in the required diffraction orders, providing high sensitivity.
3. Reflective Holographic Gratings
Reflective holographic gratings are typically produced by exposing photosensitive materials to two interfering laser beams. A layer of photoresist or other photosensitive material with a specified thickness is coated onto an optically stable flat glass substrate. The laser beams create a series of uniform interference fringes on the coating, exposing the photosensitive material. The exposed areas are then etched away with a special solvent, leaving behind the holographic image of the interference fringes. Although blazed gratings have high efficiency at design wavelengths, they can have periodic errors such as ghosting and higher scattered light, which may negatively impact sensitive measurements. Holographic gratings are specifically designed to reduce or eliminate these errors.
Product Recommendation:
The Jinsp SR series fiber optic spectrometers, which include UV spectrometers, visible spectrometers, and near-infrared spectrometers, predominantly use reflective holographic gratings for diffraction orders above 1800 lines/mm. These gratings offer high resolution, no ghost lines, and low stray light, eliminating the inherent drawbacks of ruled gratings.
4. Volume Phase Holographic Gratings (VPH)
Volume phase holographic gratings (VPH) are created by holographically recording a periodic structure of high and low refractive index regions within a dichromated gelatin (DCG) layer, which is sealed between two optical windows. Each grating is original, not replicated, greatly enhancing product consistency. The DCG film, sealed between two glass substrates, prevents material degradation and protects the grating from external damage, resulting in high durability and long life. VPH gratings offer flat diffraction efficiency over a bandwidth range of 200–300 nm, achieving over 80% diffraction efficiency and up to 99% at a single wavelength, and are not sensitive to polarization. The high diffraction efficiency of VPH gratings provides spectrometers with ultra-high sensitivity.
Product Recommendation:
The Jinsp ST series transmission spectrometers, including 532/785/830/1064 nm Raman spectrometers and OCT series spectrometers, all use VPH gratings. Paired with scientific-grade cameras, these spectrometers offer ultra-high sensitivity.
5. Summary
Spectrometers using different types of gratings have distinct characteristics, mainly in terms of diffraction efficiency, blaze wavelength, stray light, resolution, and polarization dependency. For more details on spectrometers, please visit: Fiber Optic Spectrometers - JINSP Company Limited (jinsptech.com)
About Us
Jinsp has exceptional expertise in the field of spectrometer technology. With high-precision measurement equipment, advanced analysis techniques, and extensive industry experience, we provide comprehensive and reliable spectral analysis solutions for customers. Our mature product line includes near-infrared spectrometers, miniature fiber optic spectrometers, transmission spectrometers, high-sensitivity fiber optic spectrometers, and OCT spectrometers. Whether for scientific research, industrial applications, or environmental monitoring, Jinsp is a trustworthy partner.
Post time: Jun-13-2024