Wood birefringent filters

Wood birefringent filter is suitable as a separator of two near spectral lines, for example by the Sodium discharge lamp, which radiates in doublet 589.0 and 589.6 nm. Rotation of one polarizer from parallel to crossed position can change the transmitted wavelength.

Wood birefringent filter consists of two polarizers and a crystalline quartz plate cutted parallel with crystal axis. The thickness of the plate depends on wavelengths we want to separate. For already given example of Sodium doublet it gives the thickness of approximatelly 31.8 mm (depends also on operated temperature). These types of filters are very exact optical devices and it is necessary to hold very exact manufacture thickness tolerances.

Lyot birefringent filters

This filter is in fact constructed from several Wood filters serially lined up. The thickness 'd' of the first plate is such that transmits requested wavelength and provides requested performance of the filter. Each next birefringent plate has a double width of the previous one. That provides two facts; firstly, the requested wavelength is transmitted and secondly, the unwanted transmitted wavelengths of a previous birefringent plate are filtered out. Such a cascade of birefringent plates sandwiched between polarizers provides high performance filter with a half-width in order of 1/100 nanometers.

There is a disadvantage of the Lyot filter in a low transmittance of the filter that is caused by all polarizers placed inside the filter. The overall transmittance achieve several percent.

This filter is designed for desired working temperature that has to be strictly adhered. The filter must lay in massive tube with heat assembly and must be thermo-controlled. The birefringence is temperature-dependent and even a small change in temperature can cause a wavelength shift especially by high-performance filters with narrow bandpass.

The accurate setting of filter in working wavelength is possible with temperature control or with the small rotating of optical axis.

Solc birefringent filters

This type of filter is called a birefringent chain filter. It is composed of a series of birefringent plates that are not devided by polarizers. The polarizers are only in front and behind the birefringent plates. The thickness of plates is always the same because of the desired transmitted wavelength but the azimuth of each plate varies. In the azimuth variation consist the physical principle of the Solc filter. After the light polarization at entrance polarizer the beam entres the first plate where gets divided into two beams - extraordinary ('e') and ordinary ('o') - because of birefringent material. These two beams then entre the second plate that is azimuth shifted with respect to the first plate. Therefore these two beams get divided into four beams (e-e, e-o, o-e and o-o) that are all four phase-shifted with respect to each other. After all 'N' plates it exists 2*N phase-shifted beams that after passing through the polarizer are summed together and socalled channel spectrum is observed (see the figure below).

The desired wavelength is often isolated by use of a dielectric interference filter, or by two or three chain filters, which are combined together to fulfill the condition for the desired wavelength transmittion.

Solc filter is designed for desired working temperature that has to be strictly adhered. The filter must lay in massive tube with heat assembly and must be thermo-controlled. The birefringence is temperature-dependent and even a small change in temperature can cause a wavelength shift especially by high-performance filters with narrow bandpass.

We produce for example the filter for observing of Sun protuberances with coronograph. This filter has optical diameter 30 mm, working wavelength is Halpha spectral line 656.28 nm and halfwidth 0.8nm, working temperature 38°C. The thermocontroller can be mounted into the mechanical tube of filter or can be make as a special instrument.