Research On Frequency Modulation Laser Techniques In Synthetic Aperture Ladar System

The spatial resolution of imaging radar can be greatly improved by using synthetic aperture techniques. However, the resolution of the imaging radar is limited by the wavelength of the carrier, it can be hardly promoted in the microwave region. So with the development of the synthetic aperture techniques, scientists are trying to obtain higher spatial resolution by using the carrier in optical wave region. Then it is especially important to study the freaquency modulation laser techniques in synthetic aperture ladar system.According to the features of the signal in synthetic aperture ladar system, the frequency modulation laser techniques is presented in this paper. The method to modulate the laser frequency linearly by tuning resonant cavity is proposed, and the PZT micro-positioner is used as the tuning device. On one hand, the mechanical and electrical properties of the piezoelectric material is discussed to understand the piezoelectric constant. To prevent too high output voltage of the electric driver, a PZT stack is used, and the displacement of the PZT stack can be as large as several microns. On the other hand, the response characteristics of the PZT stack drived by AC voltage are analysed.To meet the requirement of high measurement precision, a method using double-beam interferometry and machine vision technique was proposed. Firstly, the basic components of a machine vision system are introduced; secondly, a digital image processing algorithm has been put forward in reference to the measuring condition; finally, the experimental results of the PZT displacement measurement show that a 10 nm accuracy is obtained and this method can also be used in other practical cases.In order to design the frequency modulation laser system, it is also very important to obtain the actual curve of the displacement of PZT driven by AC voltage. Because the variation of the laser frequency can be reflected by this curve. So a measuring system using laser Doppler effect is presented. Furthermore combined with Fourier analysis method the actual curve is obtained. It is shown that the measuring system has high efficiency.Based on a semi-external cavity He-Ne laser, a frequency modulation laser system is designed and carried out. The structure of the semi-external cavity He-Ne laser is introduced at first. Then a single longitudinal mode linear frequency modulation continuous laser system is assembled. Through careful design of driving circuit in the experiment, the net power rate of the frequency modulation continuous laser is improved.Since the FM laser bandwidth can hardly be measured directly, a novel method, in which the bandwidth of FM laser can be measured effectively, is proposed. The experimental results show that the method can work well when measuring the FM laser with any chirp rate in synthetic aperture ladar system.