Optical Filters
UV Raman Edge Filters
We are currently pushing the limits of optical thin-film technology to produce environmentally stable UV Raman edge filters using ALPHA technology. These filters exhibit steep edge slopes, deep blocking of the laser line, and high throughput of the Raman signal. The performance features are superior to low throughput monochromators, at far less cost. Beyond the obvious cost advantages, these coated windows are compact and exhibit very high contrast between the Rayleigh and Raman transmission.
Raman edge filters are constructed of an exposed, front surface hard oxide coating on an optical quality substrate. Using ion assist technology produces durable filters with no detectable wavelength shifting under different moisture conditions. We are capable of producing filters for laser wavelengths as low as 229nm, and we are currently exploring the options of pushing this limit to lower wavelengths.
In the UV wavelength region, we can currently produce edge filters with 5-decade slope factors as low as 3%. The 5-decade slope factor (c5) is defined by:
where l OD=5 is the wavelength at which optical density 5 is achieved, and l OD=0.3 is the wavelength at which the optical density is 0.3, or equivalently, the transmission is 50%. The filter in the example above is designed to block the 248nm laser line at OD=5, and transmit 50% at 256nm.
Angle-tuning the filter (up to about 15° from normal incidence) will blue-shift the transmission curve and allow Raman signals closer to the laser line to pass through the filter, at some expense to blocking at the laser line. Another option for achieving higher transmission at low Raman shifts is to use two filters in series, each designed to block the laser line at OD=2-3 levels. Used in combination, the blocking at the laser line is additive (OD=4-6) and the 5-decade slope factor is effectively decreased from 3% to as low as 1.5%.
See also Raman Filters