Dual probe optical quadrature sensing method based on a single multi-port optical Michelson interferometer

Abstract

Most of the 3×3 optical fiber coupler interferometer uses only 2 port of 3 port and uses it as one probe. Therefore, to acquire signals at two points on the surface of the sample at the same time, an interferometer composed of two 3×3 optical fiber coupler had to be designed or one probe had to be scanned. Both methods have the disadvantage of requiring double components, and disadvantage of having measurement instability because the probe moves during the scan. Therefore, method of using an unused port as another probe is proposed. For this purpose, a situation with one reference arm and two sample arms was simply configured, but this caused a problem in that the inherent phase difference of the 3×3 optical fiber coupler was lost due to the coupling of the interference signal. To overcome this, the reference of 3×3 optical fiber coupler was divided into two and the system was configured so that each pair interfered. In the system, a signal was applied to two probes respectively, but when the signals of each probe entered the detector at the same time. The inherent phase difference was not maintained again due to the overlap of the signals. When the signals having time interval were applied to each probe, the two signals were detected with time difference. By separating this signal with respect to time, it was possible to reconstruct the signal generated by each probe. Next, a different frequency was applied to the probe simultaneously. Although the signal acquired from the detector did not maintain the initial phase difference, it was possible to separate only the components corresponding to each probe by performing frequency filtering through Fourier Spectrum. The reconstruction result shows a form like vibration generated by applied voltage. Through this, a system having two probes was constructed with only single 3×3 optical fiber coupler, and it was confirmed that the signal applied to each probe could be reconstructed.