How to select a right optical fiber?
The basic requirement when selecting the optical fiber is that the optical power coupled into the optical fiber from the emitting light source should be large enough, and the distortion generated after the optical signal is transmitted through the optical fiber is minimal.
Low transmission attenuation
In a particular wavelength range, the attenuation must be small enough to allow the relay distance to be as long as possible while meeting the optical power required by the receiver. When designing the system, consider the insertion loss of connectors, connectors, and couplers, and the amount of headroom required for system operation. For this reason, the correct choice of operating wavelength and fiber type.
Low coupling loss
Coupling losses include light source coupling loss and detector coupling loss. When the core size and numerical aperture are large, the coupling loss of the light source can be reduced, but the detector coupling loss increases. In order to reduce the coupling loss with the detector, it is required that the core size and the numerical aperture be sufficiently small so that the outgoing light of the optical fiber completely falls on the receiving surface of the detector. In order to improve the response speed of the receiver and reduce the noise, the area of the detector is required to be small, so the method of increasing the photo-sensitive surface of the detector cannot be used to reduce the coupling loss. Fibers with a large core size and numerical aperture have a small transmission bandwidth and are suitable for systems using LEDs.
Low connection loss
Connection losses include loss of connectors and connectors. The core diameter tolerance, out-of-roundness, and core and cladding concentricity errors are as small as possible to obtain the minimum connection loss. To increase the geometrical accuracy of the optical fiber, it is necessary to increase the manufacturing cost. Increasing the size of the core and the numerical aperture can reduce the adverse effect of the geometrical tolerance on the connection loss, but it is inconsistent with the increase of the bandwidth and a comprehensive consideration must be taken for the compromise.
Dispersion and bandwidth
In order for the modulated optical signal to pass through the full length of the optical fiber with minimal distortion, the dispersion of the optical fiber is required to be sufficiently small. In order to reduce fiber dispersion, the refractive index distribution index and the zero dispersion wavelength must be strictly controlled. For a specific system, the fiber type and working wavelength should be selected correctly. For example, a long-distance, high-rate submarine cable system should use a G.654 fiber with a zero-pigment shift of 1.55 μm. The wavelength-division multiplexing system should choose a small dispersion system. However, the non-zero but G.655 fiber reduces the four-wave mixing but has an adverse effect. The DWDM system used in the metropolitan area network must be a full-wave fiber with an extremely wide wavelength range and a wide range of available wavelengths. Systems using LEDs must fully consider the effects of material dispersion and other factors. The table above shows the typical parameters of a zero-cable single-mode fiber for reference when selecting the fiber.