고차 구조 광섬유 다파장 필터의 스펙트럼 특성 및특성 저하 요인에 대한 연구

Abstract

In this paper, to investigate the spectral characteristics of high-order optical fiber multiwavelength filters, various high-order birefringent filters based on a polarization-diversity loop structure (PDLS) were fabricated and theoretically analyzed. And the spectral deviation of experimentally observed output spectra from theoretically predicted ones in the 2nd-order birefringent filter was also investigated. First, high-order filters based on the PDLS were constructed by using a polarization beam splitter (PBS). In the birefringence loop, 1st- and 2nd-order Solc type birefringence combinations or Lyot type ones were formed using polarization-maintaining fibers (PMFs), those spectral properties were compared with the conventional structure composed of a single PMF. It was found that the passband bandwidths of the filter with Solc type birefringence combination increased with the number of PMF senctions. In case of the Lyot type filter, the spectral bandwidth in a flat-top band mode spectrum showed a sharper contrast between a passband and a stopband than the conventional one. Another flat-top band mode spectrum in the Lyot type structrure, which has an insertion loss of ~4 dB, has a passband bandwidth as wide as that of the 2nd Solc type filter using 3 PMF sections. Next, optical factors that might cause the spectral degradation of experimentally observed output spectra in the 2nd-order filter were investigated. We regarded single-mode fiber (SMF) used for connecting optical components comprising the filter as low-birefringent fiber with an arbitrary small birefringence and also modelled that the phase retardation difference of a half-wave plate (HWP) for wavelength switching was not achromatic, i.e., wavelength-dependent. It was found from the theoretical analysis that the inherent birefringence of the SMF rather than the wavelength-dependency of the HWP strongly affected the spectral degradation. In particular, it was also found that the spectral deviation induced from degradation factors, especially the inherent birefringence of the SMF, could not be compensated by simply controlling the orientation angles of the HWPs comprising the filter, and they should be considered as practical constraints in the design or the fabrication of high-order optical fiber filters.