Working principle of optical modulation diffuser measurement technology

Working principle of optical modulation diffuser measurement technology

The working principle of the light modulation diffuser measurement technology is: by placing a very small diffuser on the measured field point, the field value of the point can be measured and scanned to obtain the entire field distribution on the scanning path. By reciprocity theorem, we can know that the scattering signal generated by the source S at the point P is scattered by the diffuser and returns to S. The scattering signal is proportional to the field value at the point P. In order to separate the weak scattered signal from other reflected signals of the same frequency, a photodiode is connected in the middle of the diffuser, and the impedance characteristic of the entire diffuser is changed by controlling the photodiode to achieve the purpose of modulating the scattered signal.

At the receiving end, the signal scattered back by the diffuser can be obtained by demodulation. Therefore, light modulation diffuser measurement has many advantages over traditional measurement methods. First, the probe can be made very small, so the probe's disturbance to the measured field is also very small. Secondly, there is no metal connected to the probe, so the disturbance of the connection to the field is also reduced to very small. At the same time, due to the application of the principle of reciprocity, even if there is a complex path between the diffuser and the transmit / receive antenna, the measured value is accurate. If two diffusers of different lengths are used for joint measurement, the influence of multiple reflections between the diffuser and the environment on the measurement can also be eliminated.

Diffuser design The diffuser is a small dipole, the length is usually made less than / 2λ, and the middle is connected with a photodiode. The laser source is irradiated onto the photodiode through the optical fiber to make it turn on and off at a certain frequency, so that The impedance of the dipole also changes with the corresponding frequency. When the length of the diffuser is designed to be relatively long, high sensitivity can be obtained, but the effect between the diffuser and the measurement environment will be correspondingly enhanced, and the error in this respect will be increased accordingly; when the design is relatively short, the High spatial resolution and small errors, but at the same time its sensitivity will be reduced. In the design, both should be considered.
Since the frequency of the measured field is 8 to 12 GHz, the length of the dipole is designed to be 10 mm, as shown in Figure 1. The light modulation unit in the middle part of the diffuser is 4 photodiodes, the total length is 1.4 mm, the diameter of the metal wire connected to it is 0.5 mm, and the photodiode and the metal wire are pasted together with an insulating substance with a dielectric constant close to 1. The photodiode is connected to the laser source through 4 ordinary multimode optical fibers. If the middle part of the light modulation device adopts GaAs light guide switch, it can achieve a higher modulation frequency and make the measurement error smaller. In view of the cost factors for manufacturing GaAs photoconductive switches, a photodiode is used, and its operating frequency is 10 kHz. Figure 2 shows the actual light modulation diffuser designed.