Miniature Spectrometers Detecting the Correct Wavelength

A grating is the wavelength dispersive element in most mini-spectrometers. Other than the detection of primary wavelengths (λ0), gratings also produce wavelengths at higher orders n, which are fractions of the primary wavelength (λn0/n with n=[2,3,4, ...]).

These higher orders should be 'sorted out' or eliminated before they reach the detector. For those spectrometers covering less than one octave (ratio of upper to lower wavelength is less than two), this can be conveniently achieved by using a long wave pass filter with a fixed cut-on wavelength. It may be noted that the cut-on wavelength should just be a little smaller than the lower wavelength of the spectrometer.

When the spectrometer covers more than one octave, it is necessary to use a second or third long wave pass filter with a different cut-on wavelength. For instance, three different order sorting filters are needed for a typical spectrometer that ranges from 190 nm to 1100 nm. Irrespective of whether these different filters are mechanically assembled or directly coated onto one substrate, a transition region will always exist where the detector will collect meaningless signals.

Suppressing Higher Orders

Continuously Variable Order Sorting Filters (CVOSFs) developed by Delta Optical Thin Film can help resolve this situation, as the coating is continuous and the cut-on wavelength changes continuously along the filter.

Moreover, a CVOSF can subdue more stray light, as the cut-on wavelength moves together with the dispersion of the grating. In the case of discrete filters, the distance from the cut-on wavelength to the 1st order one wants to transmit gradually increases, and thus, allows transmission of more stray light through the filter.

Delta Optical’s LVOSFs are coated on UV-grade fused silica substrates. These LVOSFs can be supplied in different sizes and with different dispersions matched to a specific detector. Such LVOSFs can be manufactured either with a section allowing UV light to pass through or with a coating covering the full length of the filter.

Principle of a Mini-Spectrometer

(Graphic used by courtesy of Avantes)

1. SMA-connector
2. Collimating mirror
3. Grating
4. Focusing mirror
5. Continuously Variable Order Sorting Filter
6. Collection lens
7. Line scan detector

This information has been sourced, reviewed and adapted from materials provided by DELTA Optical Thin Film.

For more information on this source, please visit DELTA Optical Thin Film.

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