Single mode optical cable is a kind of optical fiber transmission equipment.
It is mainly composed of fiber core, cladding and coating layer; the fiber core is made of highly transparent material; the refractive index of cladding layer is slightly smaller than that of fiber core, resulting in an optical waveguide effect, so that most of the electromagnetic field is bound in the fiber core for transmission; the function of coating layer is to protect the fiber from water vapor erosion and mechanical scratch, while increasing the flexibility of the fiber. In the coating, often add a plastic jacket.
According to the raw materials of optical fiber, the optical fiber can be divided into the following types:
(1) Quartz fiber
(2) Multicomponent glass fiber
(3) Plastic clad optical fiber
(4) All plastic optical fiber
According to the refractive index distribution on the cross section of optical fiber, optical fiber can be divided into step index type and gradual index type (also known as gradient index type). For step index fiber, the refractive index distribution is uniform in the core, and the refractive index changes suddenly at the interface between the core and cladding; for graded index fiber, the refractive index changes continuously in the core.
N1 > N2 (refractive index of N1 core and refractive index of N2 cladding) is a necessary condition for optical fiber to guide light wave transmission in the core. For step index fiber, it can make light wave form total reflection at the interface between core and cladding and guide light wave to propagate forward along the core; For the graded index fiber, it can make the light wave refract continuously in the fiber core, form the light ray similar to the sine wave passing through the fiber axis, and guide the light wave to propagate forward along the fiber core.
According to the number of transmission modes in optical fiber, optical fiber can be divided into multimode fiber and single-mode fiber. At a certain wavelength. Multimode fibers are dielectric waveguides that can transmit many modes, while single-mode fibers only transmit fundamental modes.
Multi mode fiber can adopt step index distribution or gradient index distribution, while single mode fiber adopts step index distribution. Therefore, generally speaking, quartz fiber can be divided into three types: multi-mode step index fiber, multi-mode graded index fiber and single-mode step index fiber.
The propagation of electromagnetic wave in optical fiber belongs to dielectric circular waveguide. When the light is totally reflected at the interface of the medium, the electromagnetic wave is limited in the medium, which is called guided wave or guided mode. Given the guided wave and its working wavelength, there are many kinds of incident cases satisfying the total reflection condition, which are called different modes of guided wave.
The transmission mode is divided into multimode fiber and single-mode fiber. Multimode fiber can transmit several modes, while single-mode fiber can only transmit one mode for a given wavelength.
When the normalized frequency v of the optical fiber is less than its normalized cut-off frequency VC, the single-mode transmission can be realized, that is, only the fundamental mode is transmitting in the optical fiber, and all the other high-order modes are cut off.
That is to say, in addition to the fiber parameters such as core radius, numerical aperture must meet certain conditions, in order to realize single-mode transmission, the optical wave must grow to a certain value, that is, λ≥ λ C, which is called the cut-off wavelength of single-mode fiber.
Cut off wavelength (λ C) means the minimum working wavelength that can make the fiber realize single-mode transmission. That is to say, although other conditions are satisfied, if the wavelength is not greater than the cut-off wavelength of single-mode fiber, single-mode transmission is still impossible.