Di Feng

Narrow-linewidth single-polarization frequency-modulated Er-doped fiber ring laser

We demonstrate a 1550-nm narrow-linewidth fiber ring laser with stable single polarization by using single-mode Er-doped fiber as active fiber and saturable absorber. A polarization-maintaining circulator is used to acquire single-polarization laser light with the degree of polarization of 99.8%-99.9%. The linewidth measured using a delayed self-heterodyne method is less than 0.5 kHz. Frequency of the fiber laser can be modulated by driving the waveguide phase modulator with proper voltage. A Mach-Zehnder interferometer with the optical path difference between two arms of about 36 km is used to study the long-distance coherent detection of the fiber laser for frequency-modulated continuous-wave application.

Generation of an extended depth of focus using diffractive micro-lenses with binary structures in the non-paraxial domain

Diffractive micro-lenses (DMLs) with binary structures are analyzed and designed to generate an extended depth of focus (DOF) based on rigorous electromagnetic theory in the non-paraxial domain. The DOF can be extended by setting a positive preset DOF and by setting a lower height for the binary profile of the DML, respectively. Compared with the paraxial approximation (scalar theory), the focal characteristics of the designed DMLs, such as the actual DOF, the ratio between the DOF of the designed DML and the DOF of the conventional DML, and the spot size of the central lobe at the actual focal plane, for different f-numbers and for different heights of the binary profile, have been studied by using the rigorous vector method (FDTD method). Numerical and graphic results show that the designed DMLs indeed have an extended DOF while maintaining high transverse resolution by modulating the distribution and height of the binary structures of a DMLs profile. The phenomenon that the variation of the height of the patterns can improve the DOF is also given an initial interpretation.

Analysis of low F-number dual micro-axilens array with binary structures by rigorous electromagnetic theory

We investigate a two-dimensional low F-number dual micro-axilens array with binary structures based on a rigorous electromagnetic theory. The focal characteristics of a binary dual micro-axilens array (BDMA), including axial performances (focal depth and focal shift) and transverse performances (focal spot size and diffraction efficiency), have been analyzed in detail for different F-numbers, different incident polarization (TE and TM) waves, and different distances between micro-axilens. Numerical results reveal that the interference effect of a BDMA is not very evident, which is useful for building a BDMA with a high fill factor, and the focal characteristics of a BDMA are sensitive to the polarization of an incident wave. The comparative results have also shown that the diffraction efficiency of a BDMA will increase and the focal spot size of a BDMS will decrease when the F-number increases, for both TE polarization and TM polarization, respectively. It is expected that this investigation will provide useful insight into the design of micro-optical elements with high integration.

Laser Doppler echo signal detection method

The coherent lidar system for speed measurement and the characteristics of the echo signal are discussed. And the principle and the advantages of the digital quadrature transform are analyzed. A digital quadrature detection scheme based on polyphase filter for coherent lidar system for speed measurement is presented by using the idea of software radio. Firstly the amplified analog signal is sampled to digital signal, and then is re-sampled with parity, time-aligned by delay filtering, finally the signal is divided into in-phase component and quadrature component. The scheme can be used to decrease the hardware complexity of system to a large extent, to avoid aliasing between the positive and the negative spectrums, to improve the SNR, and to obtain the signal in the quadrature form so as to fit the follow-up processing. Simulations and experiments indicate that the scheme is capable of resisting the noise compared with the traditional methods and is prone to real-time implementation, which, in turn, makes a contribution to greatly improve the performance of coherent Doppler lidar system for speed measurement and to orient the direction of a moving target.