Laser Line Filters, Laser Edge Filters, Laser Rejection Filters
In the fast growing category of applications and instrumentation which utilize laser sources—such as Raman Spectroscopy, Confocal and Multiphoton Microscopy, and Flow Cytometry—it is critical to eliminate all unwanted laser background, scatter, and plasma in order to optimize signal-to-noise. Laser line and shortpass edge filters can be used to "clean-up" the signal at the laser source. Longpass edge and laser rejection filters can be used for "rejecting" unwanted noise at the detector. A combination of laser filter solutions can provide performance superior to Holographic Notch filters at significantly less cost.
All laser filters are designed with high laser damage thresholds of up to 1 watt/cm2.
At the laser source, while output is typically thought of as monochromatic and is described by a prominent line and a single output wavelength, there are often lower level transitions, plasma, and "glows" all of which create background errors. In addition, laser sources can shift in wavelength depending on power, temperature, and even manufacturing tolerances. Transmitting pure excitation energy requires a laser "clean-up" filter to control the unwanted energy.
Laser line filters are narrow bandpass filters centered on the resonance of the laser, which attenuate the background plasma and secondary emissions that often result in erroneous signals. In the case of diode lasers and LEDs, these filters can be used to make the light output more monochromatic. In the case of gas lasers, these filters can eliminate plasma in the deep blue wavelength region. Laser line filters provide 60 - 90% throughput (except UV) with spectral control from 0.85 to 1.15 of the CWL. An accessory blocker can be ordered to control a much wider spectral range from the deep UV to the IR. This additional blocker results in a minor loss of throughput (<20%).
At the detector, both desired signal and unwanted scatter will be present, with the signal orders of magnitude lower than the scatter. Scatter is the result of minor irregularities and characteristics of the system optics and application, including uncontrolled light from the sample and holder. To improve signal-to-noise, both Edge filters and Laser Rejection filters can be used to attenuate, or block, the scattered energy from reaching the detector.
Longpass Edge filters are an excellent laser rejection solution when used in a collimated light path on the detector side of the system. They attenuate shorter wavelengths to ~ 0.7 l edge or to the deep UV for l < ~ 500nm, and exhibit steep slopes (< 3% 5-Decade slope factor), deep blocking of the laser line (OD > 5), and high throughput of the Raman signal. Edge filters will transmit ~ 85% of Stokes or anti-Stokes Raman or fluorescence signal and exhibit very high contrast between the Rayleigh and Raman transmission. They are manufactured with edges defined as 1.03 x laser wavelength. Angle tuning is required for optimal performance. Edge filter performance is superior to low throughput monochromators and Holographic Notch filters at far less cost.
Angle Tuning Edge Filters
All Edge filters can be angle-tuned to achieve optimal signal-to-noise. Angle-tuning the filter 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. The filter can be oriented up to about 15 degrees from normal incidence.
At a 15 degree angle of incidence, the cut-on wavelength of the longpass edge filter will shift blue approximately 1% of the cut-on value at normal incidence. So a filter that cuts on at 600nm with normal orientation will cut on at 594nm when tipped to 15 degrees. A consequence of this blue shift is that the blocking at the laser line will decrease approximately 2 optical densities.
A secondary feature of angle-tuning is that reflected energy is redirected from the optical axis. For Longpass Edge filters, select a filter with an edge that is to the red of the desired cut-off, and adjust the filter angle until optimal performance is achieved.
Another option for achieving higher transmission at small 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%.
At the detector, both signal and scatter will be present, with the scatter orders of magnitude higher than the signal. To improve signal-to-noise, both Laser Rejection and Edge filters can be used to attenuate, or block, the scattered energy from reaching the detector.
Laser Rejection filters are designed to block more than 99.9% of light in a 15 to 40nm bandwidth. The average transmission outside the stopband is 75% except in those spectral regions where higher and lower harmonics cause relatively high reflection. Specially designed Rejection Band filters reflect more than one spectral band, or perform at off-normal angles of incidence. Rejection, or Notch, filters provide the ability to measure both Stokes and anti-Stokes signals simultaneously and have tunability for variable laser lines. Edge filters can also be used for laser rejection, providing deeper blocking of the laser line and steeper edges, for small Stokes shifted applications.