# Research on Improving Optical Field Distribution of TOSA Based on 3D Defocusing Coupling
## 4 Verification of 3D Defocusing Coupling Model
### Data Analysis  
A Class B jumper with a 1.12 um concentricity and a Class C jumper with 1.46 um concentricity are used to test the four-direction performance of the TOSA, respectively. The TOSA is fixed, and the jumper is rotated along the directions of 0\({}^{\circ}\), 90\({}^{\circ}\), 180\({}^{\circ}\), and 270\({}^{\circ}\) to test the optical power. The difference between the maximum optical power and the minimum optical power is the four-direction performance, and the unit is dB. Figure 11a shows the four-direction performance of the TOSA with a 2D coupling model, which is shown in Figure 10a. During coupling, the optical power changes by moving up the receptacle in the X/Y plane. The four-direction performance of TOSA samples is tested with the Class B and Class C jumper, respectively. The blue curve represents four-direction performance tested with the Class B jumper, and the red curve represents the four-direction performance tested with the Class C jumper. From the distribution of data, the minimum value is 0.3 dB and the maximum is 3.1 dB. The distribution range is relatively large, and the four-direction performance tested with the Class B jumper is not significantly better than that tested with the Class C jumper. It indicates that the light in the core and cladding of a TOSA with 2D coupling model is not uniform or in approximate Gaussian distribution, but in discrete distribution. When a jumper with a different concentricity is used to test the four-direction performance, the light in the core and cladding of the TOSA cannot completely enter the core and cladding of the jumper at different direction, resulting in a large difference of optical power. Figure 11b shows the four-direction performance of the TOSA with a 3D defocusing coupling model, as shown in Figure 3b. The four-direction performance is tested with Class B and Class C jumpers as mentioned above. The blue curve represents the result with the Class B jumper, and the red curve represents the result with the Class C jumper. From the data distribution, there is a small difference in the four-direction performance between the Class B jumper and Class C jumper, and the difference is <0.5 dB. It is deduced that the light in the core and cladding of the TOSA is near Gaussian distribution, and most of the light concentrates in and around the core. Even though the concentricity of the jumper is different, the light enters into the core, so that the four-direction performance is good. Furthermore, these samples are tested at 0\({}^{\circ}\) direction with 2 pcs Class B jumpers, whose concentricity is 0.42 um and 0.97 um, and also with 2 pcs Class C jumpers, whose concentricity is 1.38 um and 1.42 um. The difference between maximum and minimum optical power was compared. Figure 12a shows the difference of the optical power tested with Class B and Class C jumpers when the TOSA has 2D coupling. The blue curve represents the distribution of the optical power difference tested with 2 pcs Class B jumpers, and the average difference is approximately 0.5 dB. The red curve represents the distribution of the optical power differences tested with 2 pcs Class C jumpers, and the maximum difference is approximately 1.8 dB. Comparing the distribution, the difference of optical power tested with the Class C jumper is larger than that tested with the Class B jumper. Figure 12b shows the difference of the optical power tested with Class B and Class C jumpers, and these samples use 3D defocusing coupling. The blue curve represents the distribution of the optical power difference tested with 2 pcs Class B jumpers, and the red curve represents the distribution of the optical power difference tested with 2 pcs Class C jumpers. Both curves show that the difference of optical power and fluctuation between Class B and Class C jumper is small, and the difference of optical power tested with the Class C jumper is less than 0.4 dB.