Optical Filter Design

In addition to being deposited on the surface of an optical component, optical coatings can also be incorporated into a single element through deposition on one or many substrates followed by a lamination process. These elements are referred to as optical filters and are used to carefully control or filter the transmission of light. The process of filtering may be accomplished by using coatings that absorb, reflect, or transmit certain portions of the incident radiation. Optical filters can be manufactured in many physical forms and can involve single or multiple elements. An individual filter can be fabricated by applying soft coatings to separate substrate plates that are then laminated together or hard coatings deposited on different components that are then mechanically assembled together. The choice of the method of filtering, the physical form of the filter, the method of assembly of the filter, and the coating technology used to coat the optical filter substrates, will depend upon the constraints imposed by the filter specifications. Some of the most important parameters associated with the specifications of an optical filter are environmental operating conditions, physical dimensions, durability, optical surface quality, and, most importantly, the spectral filter parameters.

Dichroic Filters

Dichroic filters separate a broad spectrum of light into two components: a reflected component and a transmitted component. These filters provide the ability to select different bands from a spectrum and direct those bands to where they can either be used or discarded. Figure 1 shows two examples of dichroic filters: a heat control dichroic filter can be used to remove heat associated with NIR light from an optical system and a dichroic laser beam combiner efficiently combines or separates multiple laser beams at a 45° angle of incidence.
Two applications of dichroic filters: hot mirror reflecting IR and UV wavelengths while transmitting VIS wavelengths and laser beam combining or separating using multiple filters
Figure 1. Two applications of dichroic filters: hot mirror reflecting IR and UV wavelengths while transmitting VIS wavelengths (left) and laser beam combining or separating using multiple filters (right). A transmission spectrum associated with a specific filter from the schematic is shown below it.

Bandpass Filters

Another type of filter is the bandpass filter which possesses a specifically defined region of transmittance, bounded on both sides by regions of spectral rejection. Bandpass filters isolate a band of wavelengths from the total spectrum by providing a band of high transmission and bands of high rejection of spectral energy on the long and/or short wavelength sides of the transmission band. Figure 2 shows two examples of bandpass filters, which show that a very narrow bandpass or a wide passband can be achieved. Bandpass filters can utilize dielectric interference coatings or absorptive coatings. One type of absorptive bandpass filter is a neutral density filter which is a wavelength insensitive filter that is used to suppress the transmission of light by a prescribed amount.
Transmission spectra of two types of bandpass filters
Figure 2. Transmission spectra of two types of bandpass filters: a narrowband laser-line interference filter (left) and an absorptive longpass filter (right).

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