Fiber Dispersion and Broadband Measurements

Precise Measurement of Fiber Dispersion and Fiber Broadband

The dispersion characteristic of optical fiber is a key factor that affects the transmission volume and relay spacing of optical fiber communication. In the network signal communication, if the dispersion is large, the optical single pulse broadening is more serious, and the single pulse broadening will cause the overlapping of adjacent single pulses at the receiving end, and then a bit error will occur. In order to prevent such a thing, the symbol spacing can only be increased, or the transmission spacing can be decreased.

Obviously, this will reduce the transmission volume and shorten the distance between the relay offices, which is not expected by everyone. In the simulated analog transmission, because of the large dispersion, the simulated analog data signals of different frequencies have different frequency bands, and the digital signal will be seriously lost at the receiving end. In order to prevent such a thing from happening, it can only reduce the bandwidth of the transmission simulation simulation network, or reduce the transmission distance, which is not expected by everyone.

Therefore, in the optical fiber communication system of high bit rate and optical fiber wide bandwidth digital signal, the dispersion of optical fiber should also be carefully considered. As mentioned above, due to fiber dispersion, the wavelength of a single pulse of light is broadened. This is a situation that is analyzed in the time domain. If it is viewed in the frequency domain, if the fiber has dispersion, it means that the fiber has a certain transmission network bandwidth. Therefore, monopulse broadening and network bandwidth are two closely related parameters that describe the characteristics of optical fiber transmission from different perspectives.

From the measurement method, there are also two ways to correspond to this. One is based on the time domain perspective to accurately measure the broadening of a single light pulse; the other is based on the frequency domain perspective to accurately measure the total width of the baseband chip of the optical fiber.

Using Frequency Domain Method to Accurately Measure Optical Fiber Network Bandwidth

The frequency domain method is accurate measurement, that is to say, the sine data signal with continuous frequency change caused by a swept oscillator is used to adjust the laser generator, and then the transmission capacity of the data signal adjusted by the optical fiber for different frequencies is studied. Practically speaking, it means to try to measure the phase-frequency characteristics of the deployed microwave transmitted by the optical fiber. After obtaining the phase-frequency characteristics, the network bandwidth of the fiber can be calculated in a general way.

Let Pin(f) be the relationship between the optical power of the input fiber under test and the frequency f of the deployment. Pout(f) is the correlation between the optical power output by the tested fiber and the deployment frequency f. Then the phase-frequency characteristic H(f) of the tested fiber is H(f)=Pout(f)/Pin(f). (f)/Pin(f)]=10lg1/2=-3dB. fc calls it the 3dB optical network bandwidth of the fiber.

Because the phase-frequency characteristics of the optical fiber are accurately measured, the optical power characteristics of the input fiber and the optical power output from the optical fiber must be measured, that is, two data signals must be obtained, so the output optical power of a short optical fiber is used to replace the measured optical fiber. type of optical power. A sine data signal whose frequency is continuously adjustable is output by the sweep frequency counter. This data signal is used to adjust the compressive strength of the data signal of the laser generator, and then the variable light data signal is coupled into the optical switch, and the optical switch sends out two data signals successively, and one data signal enters the The short fiber is sent to the spectrometer through the optical detector behind the short fiber.

Use the output data signal of the short fiber to replace the input data signal of the tested fiber (because the fiber is short, the data signal does not change much after transmission, so it can be considered as the input data signal). The other data signal is sent into the fiber under test through the optical switch, and the data signal prepared by the continuous sine waveform is transmitted through the fiber, with the reflection of the data signal with different frequencies of the fiber under test, and is output from the fiber through optical detection. into the spectrum analyzer. In this way, the input and output data signals of the fiber under test can be obtained in the spectrum analyzer, so the phase-frequency characteristics of the fiber under test can be obtained, and the network bandwidth of the fiber can be measured.

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