Measurement of optical fiber and fiber optic communication systems

In this paper, the measurement methods of optical fiber and optical fiber communication system are briefly introduced, this paper mainly introduces optical fiber measurement: single-mode fiber Mode field diameter, optical fiber loss, fiber dispersion and bandwidth measurement, optical fiber communication system measurement: Optical transmitter transmit light power, light source extinction ratio, optical receiver sensitivity, optical receiver dynamic range, eye map measurement.

Optical fiber communication technology is an emerging technology in recent 20 years, it is an important symbol of new technological revolution in the world, and the main transmission tool of high-speed information network in the future information society. The optical fiber communication has become the mainstream of optical communications because of its excellent transmission performance. In the existing and designed optical fiber communication system, we must measure it to determine whether the existing and designed optical fiber communication system can meet the requirements of the system. The measurement of optical fiber communication should include the measurement of fibre itself and the measurement of optical fiber communication system.

Measurement of optical fiber parameters

1. Measurement of Mode field diameter of single mode fiber

Theoretically speaking, only the basic mode (LP0L) transmission in Single-mode fiber, the existence of the base mode field strength in the fiber cross section is related to the structure of the optical fiber, and the Mode field diameter is the physical quantity that measures the range of a certain field strength in the fiber mode section. For the homogeneous single-mode fiber, the base mode field strength is approximately Gaussian distribution in the fiber cross section, and the width of the maximum 1/e field of the core is usually defined as the mode diameter. In short, it describes the distribution of optical power along the fiber radius, or the degree of concentration of light energy transmitted by the optical fiber. So the core of the measurement of Single-mode fiber mode field diameter is to measure the distribution.

The method of measuring mode field diameter of single mode fiber is: transverse displacement method and transmission power method. The transmission power method is described below.

Taking a 2-meter-long optical fiber, the end-face treatment is put into the measuring system, and the measuring system consists of a photoelectric detector which can rotate the light source and angle. The input end of the optical fiber should be aligned with the light source. In addition, in order to ensure that only the main mode (LP01) and no high die, in the system added a filter, the easiest way is to make the fiber a 60mm diameter small circle. When the light is emitted by the optical fiber, at the end of the fiber to get far-field radiation map, using the detector along the polar coordinates to measure the relationship between the output light power and scanning angle, p-θ line as shown in Figure 2. Then, by inputting the p and Theta values according to the definition formula of the Mode field diameter, the Model field diameter is calculated by the computer according to the calculation program.

2. Measurement of optical fiber loss

Optical fiber loss is an important transmission parameter of optical fiber. Because of the attenuation of optical fiber, the optical power in the optical fiber decreases exponentially with the distance. However, the multimode fiber attenuation coefficient A for single-mode fiber or approximate steady state mode distribution is a position-independent constant. If you set P (Z1) as the optical power of the Z=Z1, the input light power. If you set P (Z2) as the optical power of the Z2, that is, the output power of the optical fiber. Therefore, the attenuation coefficient of a fiber is defined as

Therefore, as long as the optical fiber length z2-z1 and Z2, Z1 light power P (Z1), P (Z2), we can calculate the attenuation coefficient a of this fiber. There are many ways to measure the loss of optical fiber, and only two of them are described below.

1) Truncation method

Truncation is the best measure of precision, but its disadvantage is to truncate the fiber. The measurement box for this measurement is shown in Figure 3.

The optical power P (Z2) of the point is measured using a long optical fiber access measurement system and a power meter at the "2" point position of the graph. Then, the input state of the light source is kept unchanged, and the optical Power P (Z1) at the "L" point is measured at "1" points near the input end of the measured fiber. This measurement is equal to the input optical power P (Z1) and the Output light power P (Z2) of the optical fiber between the 1~2 two points, and the distance z2-2l between "1" and "2" points, thus substituting these values into

The average attenuation coefficient of the optical fiber can be calculated.

In the measuring block diagram, the chopper (also called the truncated light) is a device capable of periodic intermittent beams. For example, a turntable with a radial slit. It transforms the DC light signal into an alternating light signal, which is sent to the phase-locked amplifier as a reference light signal, and is locked by the optical signal of the measured fiber to overcome the effects of DC drift and dark current to ensure the measurement accuracy.

2) Backscattering method

Measurement principle. The principle of measuring optical fiber loss by backscattering method is similar to the principle of radar detection target. When the input of the measured fiber is injected into a strong light pulse, the light pulse is transmitted inside the fiber, because of the inhomogeneity inside the fiber will produce Rayleigh scattering (of course, the connector and the breakpoint of the fiber will produce more strong reflection). A portion of this scattering light travels along the optical fiber to the input end, and the continuous transmission of the scattered light to the input is called the backscattering light. From the physical concept, this backscatter light sends the "information" of the dots on the optical fiber back to the input end. Near the input of light wave transmission loss, so scattered back signal is strong, far from the input side of the optical transmission loss of large, scattered back to the weak signal. The loss of optical fibers is measured by the backscattering from the "information" of each point of the fiber. This measuring instrument is called the optical Time-domain Reflector and is abbreviated to OTDR (optical time domain reflectometer). A representative measurement curve is shown in Figure 4.

A and D two strong echoes on the curve correspond to the reflection caused by the input and output faces of the fiber. The curve b point corresponds to a scattering echo caused by a fiber optic connector. The C point may correspond to a scattering echo caused by a bubble in the optical fiber. How to use the Rayleigh scattering of optical fiber to measure the optical fiber is discussed from the quantitative point of view. Since the OTDR machine is now used to measure the loss of optical fiber link, it can directly read the required data from the OTDR machine, so there is no quantitative discussion here.

The principle block diagram of the optical time domain reflector, as shown in Figure 5. The working principle of this instrument is: first, the pulse generator is used to modulate a light source to produce narrow pulse light, and the optical system is coupled into the fiber. Light waves in the transmission of optical fiber in the scattering, scattering light along the fiber return, the way through the optical fiber directional coupler input photoelectric detector, through the photoelectric detector into electrical signals, and then amplified and signal processing into the display. The reason for the signal processing is that the backscattering light is very faint and submerged in a noise, therefore, the sampling integrator is used to sample and sum the faint scattering signal at a certain time interval. In this process, because the noise is random, the summation is cancelled out, thus the scattering signal is taken out. In addition to measuring the loss of optical fiber, OTDR can also observe the loss of optical fiber along the line, as well as the device of a sudden change of loss, the insertion loss of fiber connector. OTDR also has an important engineering utility that allows easy identification of optical fiber breakpoints. Now measuring optical fiber loss with OTDR is one of the most common methods. The utility model has the advantages of nondestructive measurement, multiple functions and convenient use. However, there is always a blind spot when using. In addition, the attenuation values measured from the two ends of the optical fiber are different, usually the average value is OTDR.