Tunable Optical Filter Design SpaceKey Features
- High peak transmission
- Replaces thousands of interference filters
- Stable performance over wide ambient temperature range
A Tunable Optical Filter consists of multiple liquid crystal variable retarders and polarizers housed in a temperature-controlled housing. Temperature control is important since the birefringence of the liquid crystal variable retarders is a function of temperature as well as voltage. Each liquid crystal cell that goes into the Tunable Filter is made to be uniform in retardance using our propriety manufacturing processes, which produce the best uniformity of transmission wavelength across the clear aperture.
The electronics controller supplies the appropriate voltages to each of the liquid crystal variable retarders and maintains the temperature of the housing. All of the calibration data for each variable retarder is stored into memory in the electronics controller. When commands are issued to the controller via USB and serial port; the calibration data are accessed and new voltages to each of the variable retarders are initiated simultaneously.
Our FilterDRIVE software allows a user to input a desired wavelength value using a keyboard or mouse, which are transferred to the controller via USB or serial interface. Firmware serial commands are provided in the user manual to control tunable filters using custom software.Custom Tunable Optical Filters
Meadowlark Optics excels at providing customized Tunable Filters for all types of applications. Whether you are doing astronomical imaging, microscopy, or remote imaging, we can develop a solution that meets your needs. Since all components are made in-house, a wide range of customization is possible, including a larger clear aperture, narrower pass band, wider field of view, variable pass band, or more, we can do it. When deciding on specifications for a custom filter, it is important to remember that not all specs can be chosen completely independently. The most important spec to consider is the finesse, f, which is defined as is the ratio between the free spectral range (FSR) of the filter and the full width at half maximum (FWHM) of the pass band. A higher finess is achieved by adding more stages and thus more optics, which leads to a more expensive filter and to a lower peak transmission.
Peak transmission is another important specification and is defined as the maximum transmission of the pass band curve for linearly polarized input light. If unpolarized light is incident on the filter then the peak transmission will be one-half of the polarized peak transmission. The peak transmission is a function of the center wavelength of the pass band as well as the number of optics in the Tunable Filter. This means that the peak transmission is highly dependent on the optical finesse, since higher finesses have more optics and hence more absorptive elements.
Other specifications include field of view, which defines the applicable half-cone angle from the optical axis for which the filter performance is guaranteed. Thus, the rays entering the Tunable Filter should be kept less than this angle to avoid a decrease in the out-of-band rejection. Out-of-band rejection is the measure of how much light, which is not within the pass band, gets through the filter. Additional specifications which can be customized include tuning accuracy of the peak transmission.