Single mode fiber coupler

Single mode fiber couplers are the most widely used. two × 2 single mode fiber coupler is typical, and its composition is shown in the figure. It has two inputs and two outputs

End, with a coupling region in the middle. Figure 2 (a) is made by melting and stretching process. The stretching part forms a conical coupling region. Stretching makes the light energy in the core area expand outside the core; At the same time, the two cores are close to each other, which enhances the coupling. The optical signal input by 1 or 2 is distributed through the conical coupling area and output by 3 and 4 terminals. The proportion allocated is called the coupling ratio, which is controlled by the length of the stretching zone. In figure (b), part of the cladding of the optical fiber embedded in the quartz block with a certain curvature is removed by grinding and polishing, and then the two such quartz blocks are pasted closely to make the two fiber cores close to each other to form an optical fiber coupler. The coupling ratio is determined by the proximity of the fiber core and the curvature radius of the fiber in the quartz block. The coupling ratio can be selected between 1% and 99%, and the insertion loss is less than 0.5dB. The optical signal coupled by the input light in the opposite direction is very weak, so the optical fiber coupler is often called a directional coupler.