Spectroscopy Focus How to Choose the Right Miniature Spectrometer


Choosing the right miniature spectrometer is very application-dependent, so there are a few questions that need to be asked. What are you trying to measure and why? How fast do you need measurements? Where is the sample and who will be operating the instrument? The design also involves money, so it is important to have a cost target for the system in mind as well.


To configure a spectrometer, there are some important choices that need to be made, such as the choice of wavelength range, optical resolution and system sensitivity. The optical bench directs broadband (white) light through a narrow entrance slit onto a diffraction grating and focuses the spectrum onto a detector array. The wavelength range will depend on the groove density of the grating and the details of the bench and detector.


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... the gratings in most miniature spectrometers are fixed in place to ensure long-term performance and stability


Author Details:


Jason Eichenholz (CTO) and Nick Barnett (UK Country Manager) Tel: +44 1865 263 180 Email: nick.barnett@oceanoptics.eu


WAVELENGTH RANGE AND GRATINGS


The wavelength range is determined by the grating. With each grating, you consider its groove density (which helps determine the resolution), its spectral range (which helps determine the wavelength range) and its blaze wavelength (which helps determine the most efficient range). Instead of the gratings rotating as they do in instruments such as scanning monochromators, the gratings in most miniature spectrometers are fixed in place to ensure long-term performance and stability. A grating must be specified for each spectrometer.


CHOOSING THE RIGHT GRATING


The groove density (mm-1) of a grating determines its dispersion, while the angle of the groove determines the most efficient region of the spectrum. The rule of thumb is this: The greater the groove density, the better the optical resolution possible, but the smaller the spectral range. The spectral range is the dispersion of the grating across the linear array. The spectral range (bandwidth) is a function of the groove density and does not change.


out over twice the angle, so it is half as bright and the signal will be half as great. With everything else held constant, the optical resolution would be twice as fine.


CHOOSING A LARGER BENCH FOR HIGHER RESOLUTION


If you choose a larger bench, you can also get higher resolution and less wavelength range. The ‘USB’ bench is a 42mm focal length design. The ‘HR’ bench (Figure 2) is a 102mm focal length design. In the ‘HR’ bench, the same 600 line mm-1 grating would yield a range of ~430nm and resolution that is 66% finer. Of course, the signal is lower because of the higher resolution. The other trade-off with the longer focal length is size and weight.


DETERMINING OPTICAL RESOLUTION


The optical resolution, measured as Full Width Half Maximum (FWHM) of miniature spectrometers depends on the groove density of the grating and the width of the entrance aperture (slit width or fibre diameter). In selecting these components, consider two trade-offs: First, the optical resolution improves as the groove density of the grating increases, but at the expense of spectral range and signal strength. Second, the resolution improves as the slit width or diameter of the fibre decreases, but at the expense of signal strength. For example, in an Ocean Optics HR4000 Spectrometer, a 100 micron slit will have an optical resolution of 14.0 pixels FWHM. Decreasing the slit width to 50 microns will improve the optical resolution to 7.4 pixels FWHM, but with the trade-off that there will be ~50% as much light. As the slit width decreases it approaches the diffraction limits and the improvement in resolution begins to diminish. So, going from a 50 micron slit to a 5 micron slit in the HR bench only improves resolution by a factor of 3.7 (instead of 10X).


Figure 1: USB Bench


FIRST AND SECOND ORDER EFFECTS


Light striking the grating is diffracted into multiple angles. The angles are whole-number multiples or orders that vary with wavelength. So, for example, first order light at 200 nm will be diffracted at angle #1, and also at angle #2, which is exactly twice as large as #1. Angle #2 is also the same as the first order angle taken by light at exactly twice the wavelength, or 400nm. If the application requires measurements at 400nm, and 200nm light is also present,


Figure 2: HR Bench


For example, in the ‘USB’ bench (Figure 1) in an Ocean Optics USB4000 Spetrometer, a 600 line mm-1 grating will cast a ~650nm spectra across the active area of the detector. So, for example, you can choose a spectral range from 200-850nm, or a spectra from 300-950nm. If you use a 1200 line mm-1 grating, then the light is diffracted at twice the angle and the detector will intercept half the wavelength range. The light is spread


Figure 3: Linear Variable Filters


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