Fiber optic filters are designed to connect into a fiber optic system. They pass specific wavelengths and reject others. Fiber optic filters can be either low-pass or high-pass filters. A low-pass fiber optic filter allows only shorter wavelengths of light to pass through the filter, while a high-pass fiber optic filter allows only the longer wavelengths to pass through. Fiber optics filters can be fine-tuned to select very narrow wavelength ranges. Fiber optic filters are used extensively in optical telecommunications networks and aerospace applications.
Tunable bandpass filters are fiber optic filters used for high-performance applications that require wavelength equalization, switching or tuning. A tunable bandpass filter is able to narrow a wavelength down to a very small and specific wavelength of light. A tunable bandpass filter may also use a microprocessor to select a wavelength and control amplitude. A tunable optic filter is used in tunable laser applications, measurement applications, and to control the interference of extraneous photons in a system using an erbium-doped fiber amplifier (EDFA). Other types of tunable fiber optic filters include acousto-optic filters that use anisotropic crystals for tuning of laser wavelengths. Such an acousto-optic tunable filter may also be coupled with a fiber optic system or cable using a polarization-maintaining fiber, expanding the capability of fiber optic based instruments such as communications equipment, spectroscopes, and other remote sensing equipment.
Fiber optic filters may also use fiber optic grating systems to filter or scatter particular wavelengths of light. A fiber grating system typically uses a specially-treated optical fiber which has a core with a specific refractive index that varies along the length of the fiber. Other fiber optics filters are used with fiber optic coupling devices, which either combine the input signal from two or more optical fibers into one output, or distribute the signal from one input fiber into two or more outputs. A fiber optic coupling filter selects for a specific incoming or outgoing wavelength, depending upon the configuration of the coupler.