Characteristic parameters of single-mode optical fiber

① Attenuation coefficient a: its provisions and physical meanings are identical to those of multimode optical fiber, which will not be described here.

② Dispersion coefficient D（ λ)： As we have known, the dispersion of optical fiber can be divided into three parts, namely mode dispersion, material dispersion and waveguide dispersion. For single-mode fiber, there is no problem of mode dispersion due to the realization of single-mode transmission, so its dispersion is mainly represented by material dispersion and waveguide dispersion (collectively called intra mode dispersion). Comprehensively consider the material dispersion and waveguide dispersion of single-mode fiber, which are collectively called dispersion coefficient. The dispersion coefficient can be understood as the pulse broadening value caused by the unit spectral width of the fiber per kilometer. Therefore, the pulse broadening value caused by dispersion of Lkm optical fiber is: σ=δλ· D（ λ）· L (2.17) where: δλ Is the spectral width of the light source σ Is the root mean square broadening value. The smaller the dispersion coefficient, the better. The smaller the dispersion coefficient of the optical fiber, the larger the bandwidth coefficient, that is, the larger the transmission capacity. For example, CCITT recommends that the dispersion coefficient of single-mode fiber at 1.31 μ m wavelength should be less than 3.5ps/km nm。 Through calculation, its bandwidth coefficient is more than 25,000 MHz · km, more than 60 times that of multimode fiber (the bandwidth coefficient of multimode fiber is generally less than 1000 MHz · km).

③ Mode field diameter d: the mode field diameter indicates the degree of concentrated light energy in single-mode fiber. Since only the fundamental mode is transmitting in the single-mode fiber, roughly speaking, the mode field diameter is the diameter of the fundamental mode spot on the receiving end face of the single-mode fiber (in fact, the fundamental mode spot has no obvious boundary). It can be considered very roughly (very loosely) that the mode field diameter d is similar to the core diameter of a single-mode fiber.

④ Cut-off wavelength λ c: We know that when the normalized frequency V of the optical fiber is less than its normalized cut-off frequency Vc, single-mode transmission can be realized, that is, only the basic mode is transmitted in the optical fiber, and all the other higher modes are cut off. That is to say, in addition to the parameters of the fiber, such as the core radius and the numerical aperture, which must meet certain conditions, the optical wave must also grow to a certain value in order to achieve single-mode transmission, namely λ ≥ λ c. This value is called the cut-off wavelength of single-mode fiber. Therefore, the cut-off wavelength λ C means the minimum working light wave length that can enable optical fiber to achieve single-mode transmission. That is to say, although other conditions are met, if the optical wavelength is not greater than the cut-off wavelength of single-mode fiber, single-mode transmission is still impossible.

⑤. Return loss: also known as return loss, it refers to the decibels of the ratio of the optical end and the backward reflected light to the input light. The greater the return loss, the better, so as to reduce the impact of the reflected light on the light source and system.

The optical device used by single-mode transmission equipment is LD, which can be generally divided into 1310nm and 1550nm according to wavelength, and can be divided into ordinary LD, high power LD, DFB-LD (distributed feedback optical device) according to output power. G.652 is the most common optical fiber used for single-mode optical fiber transmission, and its wire diameter is 9 microns.