Increasing Accuracy with Stray Light Filtering

Stray-light is a radiation of unwanted wavelengths that trigger a signal at a detector element. Some of the sources of stray-light can be order overlap, ambient light, scattering light from imperfect optical components, or reflections of non-optical components.

Avantes Instruments

Avantes has developed a symmetrical Czerny-Turner optical bench that favors stray-light rejection in relation to crossed designs.

In addition, Avantes Ultra-Low Stray-light (AvaSpec-ULS) spectrometer has several internal measures to minimize stray-light from backscattering and zero order.

Detection Limit

When working at the spectrometer’s limit of detection, the level of the stray-light from the focusing mirrors, grating, and optical bench will define the ultimate detection limit. Holographic gratings are often used as they have a low level of stray-light. Stray-light measurements are performed using long-pass or band-pass filters and a halogen light source.

Standard stray-light performance for the AvaSpec-ULS and a B-type grating is <0.04% at 250-500nm. Second order effects can play a key role for gratings having low groove frequency and hence a wide wavelength range. These effects are often caused by gratings’ 2nd order diffracted beam. Although the effects of these higher orders can be overlooked, they sometimes need to be addressed by means of filtering.

Order sorting window in holder

Figure 1. Order sorting window in holder

The plan is to limit the light to the spectra region, where order overlap does not occur. Second order effects can be filtered out by means of an order-sorting coating on a window in front of the detector (figure 1), or a permanently installed long-pass optical filter in the SMA entrance connector.

Depending on the range and type of the selected grating, the order-sorting coatings on the window include two long-pass filters (350nm and 600nm) and one long-pass filter (600nm).

Table 1. Filters installed in AvaSpec spectrometer series

OSF-305-3 Permanently installed 3 mm order sorting filter @ 305 nm
OSF-385-3 Permanently installed 3 mm order sorting filter @ 385 nm
OSF-475-3 Permanently installed 3 mm order sorting filter @ 475 nm
OSF-515-3 Permanently installed 3 mm order sorting filter @ 515 nm
OSF-550-3 Permanently installed 3 mm order sorting filter @ 550 nm
OSF-600-3 Permanently installed 3 mm order sorting filter @ 600 nm
OSF-850-3 Permanently installed 3 mm order sorting filter @ 850 nm
OSC Order sorting coating with 600 nm long pass filter for VA, BB (>350nm) and VB gratings in AvaSpec-ULS2048(L)/3648/2048x16/64/XL
OSC-UA Order sorting coating with 350 and 600 nm long pass filter for UA gratings in AvaSpec-ULS2048(L)/3648/2048x16/64/XL
OSC-UB Order sorting coating with 350 and 600 nm long pass filter for UB or BB (<350nm) gratings in AvaSpec-ULS2048(L)/3648/2048x16/64/XL
OSC-HS500 Order-sorting coating with 350 and 600 nm long-pass filter for HS500 gratings in AvaSpec-HS
OSC-HS900 Order-sorting coating with 600 nm long-pass filter for HS900 gratings in AvaSpec-HS
OSC-HS1000 Order-sorting coating with 350 nm long-pass filter for HS1000 gratings in AvaSpec-HS
OSC-NIR
 
Order-sorting coating with 1400 nm long-pass filter for NIR100-2.5 and NIR150-2.0 gratings in AvaSpec-NIR256/512-2.2/2.5TEC

The above table shows a range of optical filters for installation in the optical bench. The filter types that are 3mm thick offer better 2nd order reduction in contrast to the 1mm filters. To this end, the following long-pass filters can be used:

  • OSF-600-3 for grating IB
  • OSF-515-3/550-3 for grating NB
  • OSF-475-3 for grating NB and NC

For 2048XL and 1024x58/122 back-thinned detectors, an OSF-305 filter can be used when the starting wavelength is 300nm and higher. Besides the order-sorting coatings, partial DUV coatings are also applied on the Sony 2048 detectors so as to prevent second-order effects from UV response and to improve sensitivity and reduce noise in the visible range.

This partial DUV coating is performed automatically for two grating types: UA for 200-1100nm, DUV400, only first 400 pixels are coated; and UB for 200-700nm, DUV800, only first 800 pixels are coated.

Conclusion

Avantes’ AvaSpec-ULS spectrometers have several internal measures to reduce stray-light from zero order and backscattering, and can effectively eliminate unwanted wavelengths that set off signals at a detector element.

This information has been sourced, reviewed and adapted from materials provided by Avantes BV.

For more information on this source, please visit Avantes BV.

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