(Design of Detector Array) Spectrometers
Various principles for the separation of individual wavelength are developed.
Detector array based grating spectrometers have the advantage that no light is filtered or blocked, for the whole spectrum is captured in parallel at the same time. (Polychromator Design)
The use of a detector array limits either resolution or spectral range*. At a given detector array the pixel dispersion is Range/Number of Pixels, the resolution about 2.5 to 3 times that value. A spectrometer is the more sensitive the lower the resolution (higher the resolution value) is.
The detector array limits also the effective spot size for the incoming light . The larger the pixel size the higher the larger the entrance slit can be, the higher the throughput will be.
The entrance slit width should be matched to the pixel width and resolution. As a rule of thumb the slit width should be about 2.5 to 3 times larger than the pixel width. (assuming a 1:1 imaging setup)
The output focal positions of the different wavelength across the spectrum have to be as good as possible on a flat plane, for the detector array is flat. If not in focal position the picture blurs sacrificing the resolution. The blue curves in the graphic below show the two focal plane curves (focal position in dep. of the wavelength) of a flat-field corrected grating.
To fix the optical axis, fibers can be permanently added to a spectrometer input. To improve the throughput* multiple fibers arranged as a cross-section converter* can be used.
Second-order blocking is required if the spectrum spans more than one octave (double the wavelength). Such a filter can be placed in front of the detector or directly coated on the chip. The window on a chip is protecting the device from moisture and mechanical impact. Depending on the angle of light incidence ghost spectra may occur due to multiple reflections at the window boundaries.
Material selection for the components, the design (number of components and their arrangement) as well as the mounting of the components influence the mechanical and thermal stability.