# Research on Improving Optical Field Distribution of TOSA Based on 3D Defocusing Coupling
## 3 Design of 3D Defocusing Coupling Model  
Based on the 2D coupling model in Figure 3, the changing of coupling efficiency between 2D coupling and defocusing coupling is simulated, as shown in Figure 7. The light source in the simulation model is 10 G DFB TO from Mitsubishi, ML768K42T-91. The TO uses 1.5 mm ball lens, and the material is BK-7, \(n=1.5\). The fiber length in the receptacle is 4 mm, whose refractive index is 1.468, and the core diameter is 8.2 m with NA (Numerical Aperture) \(\sim\)0.14. For 1310 nm, the mode field diameter is approximately 9.2 \(\upmu\)m. The end-face angle of fiber is 8\({}^{\circ}\). Figure 7a shows that the coupling efficiency decreases rapidly when the TOSA uses the 2D coupling model, which can help to achieve target optical power within the specification fast. The X-axis represents the distance of the receptacle moving up from the optics axis along the X-axis, and the Y-axis represents the optical power loss with the receptacle moving up along the X-axis. Figure 7b shows that the coupling efficiency decreases slowly when defocusing coupling along the Z-axis. The X-axis represents the distance of the receptacle moving up along  
Figure 6: The distribution of optical field with 8\({}^{\circ}\) fiber moving up along X/Y-axis. (**a**) is the optical fiber offset 4 \(\upmu\)m along the negative X-axis, namely the coordinate (\(-\)0.004,0,0); (**b**) is the optical fiber offset 4 \(\upmu\)m along the negative X-axis, namely the coordinate (\(-\)0.002,0,0); (**c**) is the optical fiber offset 2 \(\upmu\)m along the positive X-axis, that is, the coordinate (0.002,0,0); (**d**) is the optical fiber offset 4 \(\upmu\)m along the positive X-axis, namely the coordinate (0.004,0,0); (**e**) is the optical fiber offset 4 along the negative Y-axis, that is, the coordinate (0,\(-\)0.004,0); (**f**) is the optical fiber offset 2 \(\upmu\)m along the negative Y-axis, that is, the coordinate (0,\(-\)0.002,0); (**g**) is the optical fiber offset 2 along the positive Y-axis, that is, the coordinate (0,0.002,0); (**h**) is the optical fiber’s positive Y-axis deviation of 4 \(\upmu\)m, that is, the coordinate (0,0.004,0). the optics axis (positive Z-axis), and the Y-axis represents the optical power loss with the receptacle moving up along the optics axis. From the above changing of the coupling efficiency, the 2D coupling model can improve the production efficiency of the TOSA. Therefore, the 2D coupling is favored by the designer and manufacturer of the TOSA. Even Figure 7b shows the 3D structure, the actual coupling process is 2D coupling, also. So, the distribution of the optical field in the fiber cannot be guaranteed, so that the power difference at four directions of the TOSA will be large, and the power difference tested by jumpers with different concentricity is relatively large also. Figure 7: Changing of coupling efficiency along X-axis and defocusing coupling. (**a**) is the changing tendency curve of optical loss based on 2D coupling model. The horizontal axis is the distance the end-face moving from (0,0) along positive direction of X-axis, and the vertical axis is the power loss that the power at focal point minus the power at X-axis; (**b**) is the changing tendency curve of optical loss based on defocusing coupling along Z-axis. The horizontal axis is the distance the end-face moving from (0,0) along positive direction of Z-axis, and the vertical axis is the power loss that the power at focal point minus the power at Z-axis. According to Equation (6), when the center of the laser beam light field coincides with the center of the fiber core, the optical field in the fiber is approximately a Gaussian distribution with the central line of the fiber core.