Ultra-narrow-band Birefringent Filter

In the detecting system of laser illumination and laser guide star (LGS), the application of optical filter is an important approach to reduce background noise and improve signal-to-noise ratio (SNR). Detecting capacity could be increased by utilizing the optical filter with bandwidth narrow enough. In this paper, for the need of beacon detection in daylight, the contrast between signal and noise which was received by detecting system in different bandwidth was calculated. Some characteristics of main optical filters were analyzed, and the application of ultra-narrowband filter in laser illumination and laser guide star was discussed. The peak transmissivity of narrowband interference filter is low (generally, the peak transmissivities correspond to the bandwidth with 10nm and 1nm are 55% and 30% respectively), and it couldn't satisfy the need of laser illumination and laser guide star detecting system perfectly. Moreover, there are some technical difficulties in making bandwidth of filter less than lnm. For exploring application of ultra-narrowband filter in laser illumination and laser guide star system, the accurate design theory of the ultra-narrowband filter was studied. The birefringent filter and its experimental equipment for detecting signal with narrowband were designed. The application of ultra-narrowband filter in detecting system was explored.In this paper, an incident coordinates was set up on the basis of the incident direction and normal direction on crystal surface. Making use of the inverse surface equation of wave normals and the boundary condition about the continuity of electromagnetic field tangential component at the surface of crystal, expressions about wave normal and ray direction, wave normal and ray refractive indexes were obtained. Based on the ray direction and index of extraordinary light, an expression of optical path difference between ordinary and extraordinary lights was derived, when a ray of light with arbitrary incident direction passed through uniaxial crystal with arbitrary crystal axis direction. And the precision of the bandwidth and peak transmissivity of ultra-narrowband birefringent filter is increased. The filtering characteristic of birefringent filter was studied.The continuous temperature coefficient was got by fitting the crystal temperature coefficient. The field of view of birefringent filter was analyzed. The center wavelength of filter vs incident angle was researched. As there is not only one signal wavelength in laser illumination and laser guide star, the center wavelength should be tuned. Taking laser guide star for instance, the tuning way of center wavelength was carried out (from the wavelength of Na beacon to the wavelength of Rayleigh beacon). The birefringent filter consisted of three Lyot filters was designed, and the characteristic of transmissivity of the filters vs tuning angle of crystal was derived from the experiment. Comparing the experimental result with the theoretical result, the accurate optical path difference was proved. The tuning of the birefringent filter was studied. Corresponding to the tuning angles calculated, the tuning approach of wavelength from 589.00nm to 532.00 was verified in the experiment. Based on the simulation under the conditions of strong background noise and dim signal light, an imaging experiment was carried out. The results indicate that the signal contrast of image is increased with the birefringent filter. The feasibility of improving SNR in laser illumination and laser guide star detecting system by ultra-narrowband filter was validated. And it provides for engineering application of ultra-narrowband filter in narrow band signal detecting system.