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English| Launch：2019-08-13 |
Optical fiber spectrometer
Optical fiber spectrometer usually USES optical fiber as signal coupler, coupling the measured light to the spectrometer for spectral analysis. Because of the convenience of optical fiber, users can set up the spectrum acquisition system very flexibly. The advantages of fiber optic spectrometer are modularity and flexibility of measurement system. The tiny fiber-optic spectrometer at MUT in Germany is fast enough to be used for online analysis. And because of the low cost universal detector, it reduces the cost of the spectrometer, and therefore the cost of the whole measurement system. The basic configuration of the fiber optic spectrometer includes a grating, a slit, and a detector. The parameters of these components are usually used as signal couplers in the purchase of optical fiber spectrometer, and the measured light is coupled to the spectrometer for spectral analysis. Because of the convenience of optical fiber, users can set up the spectrum acquisition system very flexibly.
The advantages of fiber optic spectrometer are modularity and flexibility of measurement system.
The basic configuration of the fiber optic spectrometer includes a grating, a slit, and a detector. The parameters of these components must be specified when purchasing a spectrometer. The performance of the spectrometer depends on the precise combination and calibration of these components. After the calibration of the optical fiber spectrometer, in principle, these components cannot be changed at all.
Grating: grating selection depends on spectral range and resolution requirements. For optical fiber spectrometer, the spectral range is usually between 200nm-2200nm. It is difficult to get a wide spectral range because of high resolution. At the same time, the higher the resolution requirement, the less luminous flux. For low resolution and wide spectral range, 300 line /mm gratings are the usual choice. If a high spectral resolution is required, it can be achieved by selecting a 3600 line /mm grating or a detector with more pixel resolution.
Slit: narrow slit can improve the resolution, but the luminous flux is small; A wider slit, on the other hand, increases sensitivity but loses resolution. Appropriate slit widths are selected to optimize the overall test results for different application requirements.
Detector: the detector determines the resolution and sensitivity of optical fiber spectrometer in some aspects. The sensitive area of light on the detector is limited in principle. It is divided into many small pixels for high resolution or smaller but larger pixels for high sensitivity. Generally, CCD detectors with back sensitivity are more sensitive, so they can achieve better resolution to some extent without sensitivity. Near infrared InGaAs detector
Due to the high sensitivity and thermal noise, the SNR of the system can be improved by refrigeration.
Filter: because of the multilevel diffraction effect of the spectrum itself, the use of filter can reduce the interference of multilevel diffraction. Different from conventional spectrometers, optical fiber spectrometers are realized by coating the detector, and this part of function needs to be installed in the factory. At the same time, the coating has the function of anti-reflection and improves the SNR of the system. The performance of spectrometer is mainly determined by spectral range, optical resolution and sensitivity. Changes to one of these parameters will generally affect the performance of the others.
Resolution: optical resolution is an important parameter to measure the spectral capability. It depends on the bandwidth of the monochromatic light being detected by the thermal sensor. Three components affect resolution: incident slit, grating and detector pixel size. Smaller slits provide better resolution but lower sensitivity. High-delineated gratings increase resolution but decrease spectral range. Smaller detector pixel sizes increase resolution but decrease sensitivity.
Spectral range: a spectrometer with a smaller spectral range can usually give detailed spectral information, while a larger spectral range has a wider visual range. Therefore, spectral range of spectrometer is one of the important parameters that must be clearly specified.
The main challenge for the spectrometer is not to maximize all parameters at the time of manufacture, but to make the spectrometer's technical specifications meet the performance requirements for different applications in this three-dimensional selection. This strategy enables the spectrometer to meet customers' requirements for maximum return on minimum investment. The size of the cube depends on the technical requirements of the spectrometer and is related to the complexity of the spectrometer and the price of its products.
The main factors affecting the spectral range are grating and detector, according to different requirements to choose the corresponding grating and detector.
Imaging principle
Each optical fiber at the collecting end and the detecting end corresponds to each other. This arrangement makes the two spatial dimensions of the image compressed into one. The light emitted from the sample thus generates spectral dispersion and is imprinted once by the CCD detector. That is, each pixel (or region) on the CCD contains the location information for the three-dimensional (X/Y/ lambda) image. 3D data cube reconstruction requires simply mapping the position of the optical fiber in the linear arrangement to the position of the circular receiving end, after which specific image processing can be performed by any image processing software.
FIC's greatest advantage over tunable filter (TF) and linear scan (LS) imaging methods is that it does not require repeated scans to construct a spectral image. In addition, different from TF imaging methods, FIC provides a complete spectrum of all the points in the field of view samples at one time, rather than the spectrum of a specific wavelength. The spatial resolution of FIC method is determined by the image that is imitated to the end face of FIC fiber beam after microamplification, that is, the optical diffraction limit of the system, which is consistent with TF and LS methods. FIC also has a strict limit on image resolution, that is, the number of pixels involved in imaging, which is determined by the number of single fibers in the fiber bundle. This constraint is, in turn, specified by the height of the CCD detector and the resolution of the spectral image, because the CCD receiving surface must image all the optical fibers at the end of all FIC fiber bundles.
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