Phase Modulation Of Laser Self-mixing Interference In The Measurement Of Micro And Nano Technology

When a portion of light emitted from a laser source is reflected or scattered back into the laser cavity by an external target, the reflected light will mix with the light inside the cavity, causing a modulation of the laser output power. This phenomenon is called as self-mixing interference (SMI) , in which the reflected light carries some information of the external target. The self-mixing interference comes from the optical feedback effect. People always tried to eliminate the optical feedback effect previously. Gradually people began to make use of it actively to measure some physical quantities, thus self-mixing interference technology was brought forth.In order to improve the displacement measurement accuracy of the self-mixing interference, high accuracy phase measurement method is introduced into the SMI system. Two methods have been reported: injection current modulation and external cavity length modulation. Considering the unavoidable measurement error caused by the disadvantages of the two methods, a new external cavity modulation method: phase modulation technique is proposed in our paper to improve the measurement accuracy of the SMI system. An electro-optic modulator in the external cavity is used to modulate the phase of the laser beam with high modulation accuracy and frequency. Based on the phase modulation, some analysis method of the SMI signal is proposed: linear phase modulation and temporal carrier phase shifting technique, sinusoidal phase modulation and four-bucket integration technique, sinusoidal phase modulation and FFT analysis method. Theoretical analysis is presented respectively. Error sources during the measurement and its influence on the measurement accuracy is simulated. Experimentally, He-Ne laser and laser diode SMI system with phase modulation have been set up to reconstruct micro-displacement of the external target and a measurement accuracy of a few nanometers can be obtained. Study results show that the sinusoidal phase modulation and FFT analysis method is more immune to various noises and the measurement range is more larger.In addition, a new absolute distance measurement method based on double modulation technique using SMI system is proposed. Wavelength modulation of the laser beam is obtained by modulating the injection current of the laser diode. Phase modulation of the laser beam is obtained by EOM in the external cavity. Absolute distance of the external target is determined by Fourier analysis method. Experimental results show that a resolution of±0.3mm can be achieved for absolute distance ranging from 277mm to 477mm. In the same measurement range, the resolution we obtained is better than other absolute distance measurement system proposed based on self-mixing interference.