薄膜截止滤光片的消偏振设计

薄膜截止滤光片在倾斜入射时不可避免地会产生s和p二个偏振分量的分离，因而在许多应用，特别是光通讯的应用中成为一个棘手的难题。提出了一种新的设计方法，对最常用的45°入射角，实现了长波通和短波通两种截止滤光片的完全消偏振, 在透射率为50%处,其偏振分离分别为0.3 nm和0.1 nm。基本的设计方法是采用宽带法布里珀罗薄膜干涉滤光片中心波长两侧的干涉带作为长波通或短波通截止滤光片的初始膜系,然后经过适当的优化以提高透射带的透射率。宽带干涉滤光片的间隔层常由半波长厚度的高、低折射率混合膜层组成，如2H2L2H或2L2H2L。由于这种设计的截止区和透射带带宽常嫌不足，故提出了展宽截止区和透射带的方法。对一个典型的短波通截止滤光片，在波长1550 nm,截止区和透射带宽均达到了200 nm。这种设计方法不仅简单、性能优良，而且膜厚控制容差较大，故易于制造。

Design of Depolarization Thin Film Cutoff Filters

The separation between s- and p-polarization components is an invariable effect in thin film edge filters used for tilted incidence. So it is difficult for many applications, especially for optical communication. A novel design method is put forward and the cutoff filters with depolarization at incidence angle of 45° are achieved. The polarization separation at transmittance 50% for a long-wave-pass filter and a short-wave-pass filter is 0.3 nm and 0.1 nm respectively. The design method is based on a broad band Fabry-Pérot thin film interference filter in which higher or lower interference band in both sides of main transmittance peak can be used for an initial design of long-wave-pass filter or short-wave-pass filter and then can be refined to reduce the transmittance rippers. The spacer 2H2L2H or 2L2H2L of the interference filter is usually taken. Moreover, the method for expanding bandwidth of rejection and transmission is explained. The bandwidth of 200 nm for both rejection region and transmission band is obtained at wavelength 1550 nm. In this way, the long-wave-pass and short-wave-pass edge filters with zero separation between two polarization components can easily be gotten and fabricated.