In the purchase of the optical module, we will pay attention to the selected effective transmission distance of the optical module. Of different optical modules, the effective transmission distance is different. In general, the optical module and supporting the use of single mode fiber transmission distance. Supporting the multi-mode fiber used in the optical transmission module near distance.
The optical transmission module, the light entering from the end of the fiber, and then emitted from the other end, the transmission process will inevitably produce fiber loss, i.e. the fiber attenuation. Fiber loss factors, optical loss can be divided into internal and external optical loss. Different loss single-mode fiber and multimode fiber at a different wavelength intervals caused. G1 single-mode fiber and multimode fiber at the wavelength range 1300nm to 1500nm range lower fiber loss. Multimode optical fiber at a wavelength of 850nm range lower fiber loss. Thus, most of the value of a single mode fiber operating wavelength around 1300nm optical module, the value at the operating wavelength of about 850nm multimode fiber optical module. In general, in the same band, low loss single mode fiber than multimode fiber. This also explains why the optical modules and single-mode fiber transmission distance farther supporting the use of this issue.
When the optical fiber connector module, the need to receive the optical fiber transmission, optical attenuation can cause this problem during transmission. So light the attenuation of the optical module can not be received? Become a key constraint working distance optical modules. Transmission power minus the fiber access we take the light in the power loss, the power output terminal is obtained, then let this power versus minimum specification value of the power acceptable optical module, the optical module can be calculated the effective transmission distance. 
Suppose a single-mode optical module, the operating wavelength is 1310nm. The maximum optical power which access is acceptable -4dbm - 0dbm, minimum -22dbm --- 3dbm. It marked the farthest distance on the specifications of the optical module 40KM, the data on the Will specifications are correct?
A: First, we can see from the figure, the wavelength is 1310nm single mode fiber loss 0.35dbm / km. Maximum optical power of the access -4dbm - 0dbm, minimum -22dbm --- 3dbm. Here we -4 - (- 22) = 18dbm. That is the optical module of acceptable fiber loss 18dbm. Loss per km for the optical module 0.35dbm, 18➗0.35 = 51.4. That 51.4km fiber loss of approximately 18dbm. Therefore, the maximum transmission distance of the optical module 51.4km. The maximum transmission distance indicated on the specifications for the 40KM. So it is wrong specifications on?
Fiber loss including loss of intrinsic and extrinsic losses, losses external to the optical module (including bending loss, splice loss, loss of connection or coupling, the insertion loss of the connector and connector) is not counted. So to get rid of the external part of the loss, 40KM maximum transmission distance on the specifications are correct. Optical module transmission distance of the farthest simple algorithm, this calculation method is relatively simple, it can only estimate the approximate optical module transmission distance, hoping to help you.