Cutoff wavelength is the lowest frequency or longest wavelength of an optical signal transmitted in an optical fiber. Below the cutoff wavelength (longer wavelengths), optical signals cannot propagate through an optical fiber and are absorbed or scattered. Above the cutoff wavelength (shorter wavelengths), optical signals can be efficiently transmitted in an optical fiber.
The cutoff wavelength is determined by the refractive index distribution of the fiber and the geometry of the fiber. The cutoff wavelength of an optical fiber is related to the transmission mode of the fiber, of which the most common modes are singlemode and multimode.
For single-mode fibers, the cutoff wavelength is the lowest frequency or longest wavelength of the optical signal that can only propagate in the fundamental mode. Below the cutoff wavelength, only the fundamental mode can be transmitted in the fiber, while higher-order modes are cut off. Typically, the cutoff wavelength for single-mode fibers is between 1.25 microns (μm) and 1.35 μm.
For multimode fibers, the cutoff wavelength is the lowest frequency or longest wavelength at which only the lower-order modes can be transmitted in the fiber. Below the cutoff wavelength, optical signals can be transmitted in multiple modes in the fiber, but above the cutoff wavelength, higher-order modes are cut off. The cutoff wavelength of multimode fibers typically ranges from 0.8 microns (μm) to 1.0 μm, depending on the design and manufacturing parameters of the fiber.
Knowing the cutoff wavelength of an optical fiber is important for proper selection of the appropriate light source and fiber. Selecting a light source with a wavelength lower than the cutoff wavelength ensures single-mode transmission in the fiber, avoiding mode distortion and loss caused by multimode transmission. For multimode fibers, choosing a light source with a wavelength higher than the cutoff wavelength avoids distortion and loss caused by transmitting in higher-order modes.
