A novel copolymer optical fiber with high azobenzene concentration is reported (more than 3.2 mol %). The orientation process of the preform was fitted with corrected bi-exponential equation. Compared with the doped one, the photosensitivity of this kind of copolymer optical fiber preform is analyzed. The influence of azobenzene concentrations and write conditions on photosensitivity of copolymerized PMMA was analyzed. Then, long-period of 120um birefringent grating was fabricated in the single mode fiber with core refractive index of 1.485 (at wavelength of 1.5um), and relative index difference delta of 0.008. The duty cycle is 50%, and the refractive index change in the exposed area is about 4*10<sup>-4</sup> for the ordinary or extraordinary ray.
Inexpensive PMMA based Polymer Optical Fiber (POF) has the feature of a large core diameter, high numerical aperture and great flexibility, thus allow low connection cost and cheap LED source. These advantages make it a promising candidate for short distance communication. In this article, coarse wavelength division multiplexing (CWDM) test was performed with commercially available POF using its low loss transmission window. Light of two different wavelengths (650nm and 530nm) were sending on a single POF. Here 650nm red light was used for duplex IP data digital signal transmission and 530nm green light was used for voice signal transmission. Light sources are LEDs. A POF Coupler (Splitter) of 1:1 ratio was employed as multiplexer and prisms were used for demultiplexing. The channel isolation and insert loss of both channels were measured, for 650nm channel they are 20.5dB and 17.65dB, for 530nm channel they are 19.16dB and 20.55dB.
To find whether a set of reduced density matrixes come from a common multi-party state is a hard and important problem. In this paper, (1) we introduce a method to find out some polytopes in one-party eigenvalue-space which are sufficient conditions of this problem. (2) We point out that there are some relations between the compatible conditions and the entanglement of pure states. And we show this idea more clearly in the three-qubit case. (3) We investigate the relations between the compatibility problem and the invariants of a matrix-set under some groups. Furthermore, we show that it is one of the reasons why the compatibility problems which involve the multi-party density matrixes are much more difficult than the one-party case.