DICHROIC BEAMSPLITTERS

OVERVIEW

Dichroic beamsplitters are longpass and shortpass filters that are used at non-normal angles of incidence. They highly reflect one specified spectral region while optimally transmitting another. Dichroics are available at all wavelengths, with reflectivity typically reaching values greater than 98% and transmission typically exceeding 85%. Dichroics are non-absorbing so that all light that is not transmitted is specularly reflected.

In most applications, the dichroic beamsplitter is oriented at 45º so that the reflected light makes an angle of 90° to the incident light, although any angle less than about 100° is possible. If the angled filter is a longpass, then the longer wavelengths will be transmitted through the filter parallel to the path of the original, incident light beam, while the shorter wavelengths will be reflected in an off-axis direction. Hence the term dichroic—two colors.

When used at angles differing from normal AOI (angle of incidence), the polarization of the incident beam becomes important. S- and p- polarized light interact with the thin-film stack differently, often resulting in a step in the %T or %R measurements at AOI above about 20 degrees when using a standard setup. If the polarization state of the incoming light is known, measurements can be made using either s- or p- polarization to more accurately reflect the customer's experimental setup. While most customers want to minimize the difference between s- and p- polarized responses, the differing spectral characteristics can also be exploited to separate the P-plane and S-plane of polarization states.


FILTER DESCRIPTION

Dichroic beamsplitters are defined by:

  • Cut-On Edge (LP)—the wavelength of the edge at 50% of the maximum transmission of the filter;
  • Cut-Off Edge (SP)—the wavelength of the edge at 50% of the maximum transmission of the filter;
  • Edge Steepness—the slope of the edge between reflection and transmission;
  • Reflection Range—the spectral region in which the filter does not transmit;
  • Transmission Range—the spectral region in which the filter transmits;
  • Polarization—the degree of spectral separation due to polarization

FILTER TYPES

Dichroic (DC)

Provide wide regions of both transmission and reflection. They exhibit a high degree of polarization along with a somewhat shallow transition slope.

Dichroic Mirror (DM)

Feature a steep transition slope, minimal polarization effects, a wide region of reflection, and a limited region of transmission.

Dichroic Reflector (DR)

Provide a steep transition slope, low polarization, a wide region of transmission, and a limited region of reflection.

Cube beamsplitters

 

Cube beamsplitters provide highly efficient polarization of the incident beam. They also eliminate undesired ghost images of the reflected beam and the lateral displacement of the transmitted beam. Cube beamsplitters are made by coating the hypotenuse of a right angle prism and then cementing it to the hypotenuse of another right angle prism. Any type of filter in our product line is available in a polarizing cube configuration.

MIRRORS AND PARTIAL MIRRORS

OVERVIEW

Mirrors and partial mirrors reflect uniformly over a broad spectral range. Partial mirrors work at off-normal angles of incidence reflecting only a portion of the incident light while transmitting the remainder.

Beamsplitter Filters

Can be produced to perform across ranges as wide as 700nm. Performance can be located anywhere in the spectral region from 240nm–20,000nm. The ratio of the transmitted light to reflected light (%T/%R) can be any value between 99/1 to 1/99, with typical products including ratios of 50/50, 70/30, and 30/70.

Total Reflector Mirrors

Reflect 92–99% over their primary spectral region. They are made with a variety of metal coatings—including aluminum, gold, silver, and special alloys—that are over- coated with a protective thin film.

Dichroic Reflector Mirrors

Reflect greater than 99% over a somewhat narrower spectral region. Made with multiple layers of dielectric materials, they absorb virtually none of the light within the spectral region of design.