Optical signal transmission is different from electrical signal transmission. For electrical signals, as long as the output end of the amplifier is connected to the transmission cable, the signal can be transmitted normally. But for optical communication, the light incident on the end face of the optical fiber, Some of them cannot enter the optical fiber, and the light entering the end face of the optical fiber may not necessarily be transmitted in the optical fiber. Only the light that meets a certain condition can be totally reflected in the optical fiber and transmitted.
As shown in the figure above, a beam of light, no matter how large the angle range is, can only be transmitted normally when it enters the fiber along a specific light cone angle (red part). The sine value of this angle θ is called the fiber numerical aperture NA .
NA: Numerical Apeture
One of the important optical parameters of optical fiber
dimensionless, without units
(n: refractive index of medium; θ: angle between light and fiber axis, usually called half angle)
It can be seen from the formula that only light with a numerical aperture NA≦nsinθ can be coupled into the optical fiber for transmission, and the numerical aperture NA represents the ability of the optical fiber to receive light.
Relative refractive index difference △
The light can be transmitted in the optical fiber because the light will be refracted when it enters the low refractive index medium from the high refractive index medium, and the refractive index difference will be controlled to achieve total reflection. Therefore, the numerical aperture NA and the relative ratio between the core and the cladding related to the refractive index.
Core refractive index n1, cladding refractive index n2
How to choose the numerical aperture NA correctly?
From a physical point of view, the numerical aperture of an optical fiber represents the ability of the optical fiber to receive incident light.
The larger the NA, the stronger the ability of the optical fiber to receive light. From the perspective of increasing the optical power entering the optical fiber, the larger the NA, the better, because a larger numerical aperture of the optical fiber is beneficial to the butt joint of the optical fiber.
However, when the NA is too large, the mode distortion of the fiber will increase, which will affect the bandwidth of the fiber. Therefore, in the optical fiber communication system, there are certain requirements on the numerical aperture of the optical fiber.
Usually, in order to inject light into the optical fiber most effectively, a lens whose numerical aperture is the same as that of the optical fiber should be used for light collection.