Longpass & Shortpass Filters
Longpass and shortpass
filters transmit only above or below a certain "cut-on" or
"cut-off" wavelength and continue to transmit a wide energy
band. They are used in diverse applications such as detection of
fluorescence emission, colour separation in TV and photography, and
spectral band isolators. These filters can be designed for use in
reflection and transmission mode, and can be used as beamsplitters or combiners.
Cut-On (CF) filters
typical of the industry standard for these filters, the cut-on filter
series provides peak transmission of 85 to 90% while blocking a
region typically 99.9%. These filters have a shallow edge slope
between reflection and transmission.
Edge (EF) filters
provide a high-performance longpass or shortpass filter that
complements the DF series Bandpass filters. These filters feature
very steep edge slopes and a narrow (30 - 80nm) band of attenuation
by reflection. Typical configuration includes sharp cut-on absorption
glass substrates to attenuate a greater wavelength range.
Edge (REF) filters are optimized for deep attenuation (> OD
6) and are available for custom applications.
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