To lower the polarization dependent loss(PDL) and to shorten the coupler length by previously reducing the outside diameter of optical fiber to a specific range, clamping the optical fiber in tight contact with each other, specifying the heating range, fusing the fiber and stretching the fiber at a specific speed.
The two optical fibers 1, 2 are preferably reduced in the outside diameter to ≤120μm and are clamped in clamping parts 5 of a moving stage 3. The fibers are heated and fused to a range of ≥8000μm by using a burner 4 and are stretched by moving the moving stage 3 at a speed of ≤100μm/sec. At this time, the monitor light from an LD light source 6 of a wavelength 1560nm is made incident on one end of the optical fiber 1. This monitor light is received by photodetectors 9, 10 from two port parts 7, 8 on the opposite side of the optical fibers. The stretching is stopped when the monitor light attains a desired branching ratio. As a result, the PDL value of the coupler is lowered at the respective wavelengths and the PDL 0.2dB band width exhibits a large value. The low PDL is thus confirmed.
SHIROTA MASAAKI
KAMIYA KAZUO
