New Tunable Bandpass Filters with Wide Out-of-Band Blocking for Photonic Instruments
Tunable Bandpass filters with wide deep out-of-band blocking
Tunable bandpass filters have many applications in photonic instruments. Some common examples are exciter and emission filters of spectrofluorometers and tunable light sources.
Traditionally, tunable filters are constructed based on a rotating diffraction grating or prism using a precise angular motor to tune the desired wavelength and a variable slit to select the bandwidth. However, such systems are very sensitive to environmental conditions like temperature and vibration as well as wear in the motor parts.
Recently, a new kind of tunable bandpass filter has been introduced that utilize a set of Continuously Variable Interference Filters (CVFs) – also known as Linear Variable Filters. This construction is simpler and more stable than the rotating grating type since it only requires linear translations along one axis.
For many applications, it is desirably to cover the full wavelength range of silicon-based detectors (350 – 1100 nm). Constructing CVFs with high out-of-band blocking for such a wide wavelength range is a challenge. However, in this technical note we will describe how tunable bandpass filters with wide and deep out-of-band blocking can be built using CVFs combined with homogeneous blocking filters.
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Continuously Variable Filters
A Continuously Variable Filter is an optical component where the filter edge varies continuously along one physical dimension of the filter as illustrated on Figure 1 for a Continuously Variable Long Wave Pass edge Filter (CVLWP). At position 1 the filter edge is in the blue wavelength region and basically the entire spectrum is transmitted through the filter. At position 2, the edge is in the green wavelength range and thereby wavelengths longer than the green are transmitted only. Finally, at position 3, the edge is in the red wavelength region letting only the red wavelengths of the spectrum through.
It is worth noting that the function of the filter also depends on the beam spot size as well the Angle-of-Incidence on the filter. This is explained in a separate technical Note.
Tunable Bandpass filters constructed using Continuously Variable Filter sets
By combining a Continuously Variable Long-Wave Pass (CVLWP) filter with a Continuously Variable Short-Wave Pass (CVSWP) filter and sliding the two, it is possible to construct a tunable bandpass filter.
Figure 2 shows how the bandwidth of the filter can be adjusted by sliding the two filters relative to each other
Figure 3 shows how the center wavelength of a bandpass filter with fixed bandwidth can be tuned by sliding the two filters in parallel.
Wideband blocking using homogenous blocking filters
As explained above, it may not be practical or economically viable to produce CVFs with a working range covering the entire 350 – 1100 nm range where Silicon detectors are sensitive.
A simple way to extend the blocking range of a tunable bandpass filter is to slide in a small, fixed (homogeneous) wide-band bandpass filter as illustrated on Figure 6. The bandpass filter (BP) transmits almost 100% in the tuning range and blocks typically OD5-6 outside the tuning range.
From Figure 5 it can be seen that it is not possible to choose a single wide-band fixed filter to cover the complete tuning range from 400 – 1000 nm. At short center wavelengths, say 400 – 600 nm, we obviously need transmission below 600 nm and blocking above 600 nm. But, for longer center wavelengths, say 600 – 1000 nm, we need blocking below 600 nm and transmission above 600 nm.
For a practical implementation, it is therefore beneficial to use a range of blocking filters that we use one at a time dependent on the center wavelength of the tunable filter. The transmission ranges of the different blocking filters overlap to enable both narrow and wide bandwidths.
Delta Optical Thin Film has manufactured several Filter sets for design of widely tunable bandpass filters with high out-out band blocking. Figure 7 shows an example of a filter set for the 400 – 1000 nm range with 4 different blocking filters. With this filter set transmission can be above 90% at the center wavelength and blocking is typically better than OD5 outside the filter bandwidth.
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Dive into our groundbreaking filters
The table below lists an example of a filter set that Delta Optical Thin Film manufactures. This filter set consist of 6 filter, a continuously variable long wave pass filters, a continuously variable short wave pass filters and 4 homogeneous blocking filters for various wavelength regions. CVLWP 383-1000 (LF104550) can be combined with CVSWP 386-1005 (LF104555) to make a continuously variable bandpass filter for the range 385 nm to 1000 nm with a tunable bandwidth between 10 – 75 nm.
| Product Number | Product Type | 50% Edge Min. (nm) | 50% Edge Max. (nm) | Max size | Data sheets |
|---|---|---|---|---|---|
| LF104555 | CVLWP | 383 | 1000 | 110 mm x 17 mm x 2 mm |  |
| LF104550 | CVSWP | 386 | 1005 | 110 mm x 17 mm x 2 mm | |
| LF104551 | Blocking | 370 | 590 | 80 mm x 55 mm x 1 mm | |
| LF104552 | Blocking | 410 | 710 | 80 mm x 55 mm x 1 mm | |
| LF104553 | Blocking | 490 | 850 | 80 mm x 55 mm x 1 mm | |
| LF104554 | Blocking | 615 | 1040 | 80 mm x 55 mm x 1 mm | |
You can find more details about this filter set on our web-page here:
If our range of filter sets does not match your exact requirements, you can always contact Delta Optical Thin Film if you need a customized design.
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