Spectrometer

definition:
A device for recording spectra.

A spectrometer is an instrument used to record the spectrum of a light source. The equation that measures the power spectral density (PSD) of light as a function of wavelength or frequency. Not all spectrometers can provide calibrated PSDs; Usually, intensity is not calibrated, and the calibration factor (responsivity) is related to wavelength.  
The use of spectral interference method can not only obtain power spectral density but also spectral phase.  

catalogue

1. Spectrometer using diffraction grating and prism
2. Interference spectrometer
3. Spectrometer records spectral details
4. Extreme Spectral Region Spectrometer

Spectrometer using diffraction grating and prism
Many spectrometers use one or more diffraction gratings to obtain wavelength dependent diffraction effects, or one or more prisms to obtain wavelength dependent refraction. The incident light needs to be collimated before entering the grating or prism. After passing through the dispersion device, the propagation direction of components with different wavelengths is slightly different. Then they pass through some other optical devices and finally enter the photodetector.  

In a scanning spectrometer, the detector can be a photodiode or a photomultiplier tube, placed behind a narrow slit, so that only a very narrow wavelength range of light can reach the detector at a time. It is possible to change the position of the narrow slit or the direction of the grating or prism to scan light within a specific wavelength range, assuming that the PSD of the incident light remains constant during that time. At this point, the function of the optical device is similar to that of a tunable monochromator. Figure 1 is a design diagram of a common Czerny Turner monochromator. If the spectrum is wide and the scanning resolution is high, and if the detector does not respond quickly enough, the entire spectrum acquisition time will be long.  

  
 

Figure 1: Design diagram of Czerny Turner monochromator. The light entering the narrow slit is collimated by a curved mirror, then deflected by a diffraction grating that varies with wavelength, and then refocused by another curved mirror. For a diffraction grating pointing towards a certain direction, only light with a narrow range of wavelengths can pass through the exit slit. The rays in the figure correspond to this wavelength range. The entire device is placed in a box, including additional apertures and a black casing to minimize stray light.  

If a non scanning spectrometer is used, the spectral acquisition time will be greatly reduced. In this case, a spatially resolved detector is used, which can simultaneously detect the wavelength components used. For example, the detector can be a CCD camera chip.  
Some grating spectrometers are very small, with a width of only a few centimeters. But to achieve good performance, that is, high resolution and sensitivity, larger devices are required.  

Interference spectrometer 
Interference spectrometers typically have high spectral resolution, but a narrow spectral range:

1. Some devices use Fabry Perot interferometers and piezoelectric actuators to scan the spacing between mirrors and record the transmitted optical power. The available spectral range is called the free spectral range, which is determined by the spacing between the mirrors; Usually ranging from 0.1 GHz to 10 GHz, the value expressed in nm is very small. The resolution bandwidth is equal to the free spectral range divided by the precision, which is determined by the reflectivity of the mirror. When the distance between the mirrors is large, higher resolution is obtained, but the free spectral range becomes narrower.  
2. Some Fourier transform spectrometers use Michelson interferometers, in which one arm of the interferometer performs mechanical scanning over a long range (mm, cm, or even larger). The function of the detection signal obtained over time within the entire scanning range needs to be Fourier transformed to obtain the spectrum. Another simple method is to use a wavelength meter to measure only the wavelength of the laser source, rather than recording the entire spectrum.  
3. Array waveguide gratings can be used in very small-sized spectrometers. Utilizing the interference effect of small waveguide structures.  

Spectrometer records spectral details
According to the spectrometer used, different quantities need to be observed: - The incident light needs to enter a variable width incident slit. In order to achieve the highest spectral resolution, the narrow slit needs to be narrow enough, but this will reduce the transmission power, thus increasing noise and increasing acquisition time, especially when the light source brightness is relatively low. Some spectrometers use fiber optic input light, which can be multimode fiber or single-mode fiber. Multimode fibers are easy to collect light, but single-mode fibers can achieve the best spectrometer performance. Diffraction gratings usually use first-order diffraction, but sometimes advanced diffraction is required to obtain higher spectral resolution. Regardless of the situation, there is always the issue of the influence of other orders of diffraction. If you encounter spectral properties that are difficult to explain, you can check if they come from this problem. The response of the spectrometer is polarization dependent, as the diffraction efficiency of the grating or the reflection loss of the prism are both polarization dependent. The user needs to calibrate the spectrometer. When calibrating wavelengths, discharge lamps that emit a determined wavelength spectrum can be used. It is difficult to calibrate responsivity across the entire wavelength range. Incandescent lamps with known filament temperature or calibrated spectra can be used.  

Extreme Spectral Region Spectrometer
Typically, spectrometers operate in the visible light range, infrared light, or possibly even ultraviolet light range. Some spectrometers can operate in extreme spectral regions, such as extreme ultraviolet (EUV) or X-ray regions, where the wavelength is only a few nm. These spectrometers may use diffraction gratings with very small spacing, or even single crystals in the X-ray region, utilizing atomic sized periodic structures. And for photodetectors, X-ray CCD cameras or multi-channel detectors (MCP) can be used (see photomultiplier tubes).