We know that the transmission distance of an optical module is determined by many factors, such as wavelength, rate, transmit optical power, receive sensitivity and a series of indicators, all of which affect the transmission rate of the optical module, which we have explained in detail in previous articles.

Recently a customer asked, in the case of the specified rate and wavelength, whether the target transmission distance can be achieved? Then this question I think should be the optical device of the transmitter optical power and receiver sensitivity to determine. The good or bad degree of the optical device determines the transmitting optical power and receiving sensitivity, which also determines the transmission distance of the gigabit optical module. First of all, let's explain the parameters of transmit power and receive sensitivity.

Transmit optical power refers to the optical intensity of the transmitter of the optical module, and is measured in dBm.

Receiving sensitivity refers to a certain rate, BER case of optical modules, the minimum received optical power, in dBm as a unit.

Optical module transmission process, the light is from one end of the optical fiber into the other end, and then issued by the other end, the entire transmission process will inevitably produce fiber optic loss, so the fiber attenuation is another key factor in the transmission distance of the optical module. 850nm wavelength is generally used for multimode communications, in the international standard, the fiber optic transmission loss of about 2.5bd/km; 1310nm wavelength of 0.4db/km; 1550nm wavelength is 0.4db/km; 1550nm wavelength is 0.4db/km. In international standards, fiber optic transmission loss is about 2.5bd/km for 1310nm wavelength, 0.4db/km for 1310nm wavelength, and 0.2db/km for 1550nm wavelength.

Therefore, before the optical module is put into practical applications, we can calculate the transmit optical power and receive sensitivity of the module can transmit the farthest distance according to the above data. There is a formula below that can be simply calculated:

Transmission distance = |minimum transmit optical power - receive sensitivity| / optical attenuation coefficient

Take 2.5G 1310 wavelength single mode optical module as an example:

The optical module adopts 1310 wavelength, ideally, the fiber transmission loss is 0.4db/km, the output optical power is -2~3, the conventional receiving sensitivity is -24, but this customer requires the receiving sensitivity to be -28.5, after confirming that our company can do the highest -27. Then, we can get the following results by calculating with -27:

(-2-(-27))/0.4=62.5

Then based on the reception sensitivity of -27, our 2.5G 1310nm can transmit up to 62.5km.

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