Detecting the right wavelength

The wavelength dispersive element in most mini-spectrometers is a grating. Besides the primary wavelengths (λ0) that are to be detected gratings also generate wavelengths at higher orders n. These higher order wavelengths are fractions of the primary wavelength (λn=λ0/n with n=[2,3,4, ...]). These higher orders need to be removed or 'sorted out' before they reach the detector. For spectrometers that cover less than one octave (ratio of upper to lower wavelength is less than two) this can easily be done by a long wave pass filter with a fixed cut-on wavelength. In this case the cut-on wavelength must just be a little smaller than the lower wavelength of the spectrometer.

Read the article Filtering advances widen spectroscopy options (Electro Optics April 2020).

Download the white paper The rise of continuously variable filters (Electro Optics April 2020).

Overlapping orders in a spectrometer (source:

However, when the spectrometer covers more than one octave a second or third long wave pass filter with a different cut-on wavelength is needed. A typical spectrometer for example that ranges from 190 nm to 1100 nm needs three different order sorting filters. No matter if these three different filters are mechanically assembled or directly coated onto one substrate there will always be a transition region where the detector will collect meaningless signals.


Suppressing higher orders

Delta Optical Thin Film’s Continuously Variable Order Sorting Filters remedy this situation because the coating is continuous and the cut-on wavelength changes continuously along the filter. Furthermore, an CVOSF suppresses more stray light because the cut-on wavelength moves together with the  dispersion of the grating while with discrete filters the distance from the cut-on wavelength to the 1st order one wants to transmit gets gradually bigger and allows more stray light to pass through the filter.

Comparison of traditional order sorting filter (red curve) and continuously variable order sorting filter (brown curve)

Delta Optical Thin Film’s LVOSF are coated on UV grade fused silica substrates and can be supplied with different dispersions matched to a specific detector and in different sizes. They can be manufactured either with a coating that covers the whole length of the filter or with a section that allows UV light to pass.

Contact us for an CVOSF specific to your grating.

Order an CVOSF for testing.

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

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