Dichroic beamsplitters are longpass and shortpass filters that are utilized at non-normal angles of incidence. The filters can efficiently reflect at a specified spectral region and simultaneously transmit at another spectral region.
Dichroic beamsplitters are available in all ranges of wavelengths, with reflectivity usually reaching higher than 98% values and transmission often reaching greater than 85% values.
Since dichroic beamsplitters are non-absorbing, the entire light that is not transmitted is specularly reflected. In most applications, the dichroic beamsplitters are oriented at an angle of 45°, allowing the reflected light to produce an angle of 90° to the incident light, although it is possible to achieve any angle less than 100°.
Longpass angled filter will cause the longer wavelengths to be transmitted through the filter lying parallel to the path of the incident and original light beam, while causing the shorter wavelengths to be reflected in an off-axis direction, hence the term dichroic, meaning two colors.
Upon using dichroic beamsplitters at angles that are different from the normal angle of incidence (AOI), the polarization of the incident beam becomes important. P-polarized light and S-polarized light interact differently with the thin-film stack. This often results in a step in the %T or %R measurements at above 20° AOI when a standard setup is utilized.
If the polarization state of the incoming light is known, users can make the measurements using either S-polarization or P-polarization to reflect their experimental setup more accurately. Although many customers desire to decrease the difference between P-polarized and S-polarized responses, they can also manipulate the different spectral properties to separate the P-plane and S-plane of the polarization states.
Dichroic beamsplitters are defined by:
- Cut-on edge (LP) — refers to the edge wavelength at 50% of the highest transmission of the filter
- Cut-off edge (SP) — refers to the edge wavelength at 50% of the highest transmission of the filter
- Reflection range — refers to the spectral region where the filter does not transmit
- Transmission range — refers to the spectral region where the filter transmits
- Edge steepness — refers to the edge slope between reflection and transmission
- Polarization — refers to the level of spectral separation induced by polarization
Wide regions of both transmission and reflection are provided by dichroic filters. These filters show a high degree of polarization, along with a somewhat shallow transition slope.
Dichroic Mirror (DM)
A broad reflection region, a restricted transmission region, a steep transition slope, and reduced polarization effects are included in the dichroic mirrors.
Dichroic Reflector (DR)
A broad transmission region, a restricted reflection region, a steep transition slope, and low polarization are included in the dichroic reflectors.
Cube beamsplitters offer highly efficient polarization of the incident beam. These beamsplitters also avoid the undesirable lateral displacement of the transmitted beam and even ghost images of the reflected beam.
Cube beamsplitters are produced by first coating the hypotenuse of a right-angle prism and then cementing this prism to the hypotenuse of another right-angle prism. Omega Optical offers a product range that includes all kinds of filters available in a polarizing cube configuration.
Mirrors and Partial Mirrors
Mirrors and partial mirrors reflect uniformly over a broad spectral range. Partial mirrors work at off-normal angles of incidence, reflecting only a part of the incident light and transmitting the remaining portion of the incident light.
Beamsplitter filters can be developed to work over ranges as wide as 700 nm. The performance can be located anywhere in the spectral region ranging from 240 to 20,000 nm. In addition, the ratio of the transmitted light to the reflected light (%T/%R) can be anywhere between 99/1 and 1/99, with regular products including ratios of 70/30, 50/50, and 30/70.
Total Reflector Mirrors
Total reflector mirrors are capable of reflecting 92% to 99% over their main spectral region. Such mirrors are created with a broad range of metal coatings, like aluminum, gold, silver, and unique types of alloys. A protective thin film is applied over these coatings.
Dichroic Reflector Mirrors
Dichroic reflector mirrors can reflect greater than 99% over a somewhat narrower spectral region. They are produced with various layers of dielectric materials and absorb virtually none of the light inside the spectral region of the design.