# Research on Improving Optical Field Distribution of TOSA Based on 3D Defocusing Coupling
## 1 Introduction  
Optical fiber communication technology has the characteristics of large capacity, high transmission quality, and good security. With the rapid development of optical fiber communication technology, it has become the irreplaceable communication technology in 4G and 5G networks and data center construction. As the core component in optical fiber communication networks, the optical module is mainly composed of optoelectronic components, functional circuits, and optical interfaces. Among them, optoelectronic devices include an optical emitting assembly (TOSA, transmitter optical sub-assembly) and an optical receiving assembly (ROSA, receiver optical sub-assembly), which performs the role of photoelectric conversion and electro-optical conversion. The TOSA converts the received electrical signal into the optical signal, and the optical signal is transmitted by the optical fiber. The TOSA is composed of a semiconductor laser diode (LD) and an optical fiber component through active coupling, and the ROSA is composed of a photodiode and an optical fiber component through active coupling [1].  
The LD as a light source is used in a TOSA, and the beam emitted by the LD is shaped with a spherical or aspheric lens, and then coupled into the optical fiber. Usually, the appropriate coupling mode is selected according to the application requirements of the product and the requirements of coupling efficiency. Active coupling directly detects the optical power coupled into the optical fiber, but it cannot directly identify the distribution of the optical field in the optical fiber. If the distribution of the optical field is not uniform when the TOSA is tested with a jumper at different directions (four-direction performance) or tested with jumpers with different concentricity, the difference of optical power is large. Eventually, because of out of specification issues, the TOSA is not used as good product, which directly leads to product scrap and economic waste. The performance of the TOSA is affected by the coupling model, but there is little research on the coupling process at present [2; 3; 4; 5; 6; 7; 8]. Therefore, based on the study of factors affecting the distribution and optical simulation of the optical field in the fiber, this paper explores a coupling modelthat can improve the uniform distribution of the optical field. The three-dimensional (3D) defocusing coupling model entails the fiber moving up at the X/Y plane and coupling the maximum optical power, then moving up along the Z-axis with a step to change the optical power. At each step along the Z-axis, the fiber must couple the maximum optical power. When the optical power is the maximum, the optical energy in the core of fiber is the highest, and the energy distribution is the most uniform in the fiber core and cladding. The verification shows the optical power difference at four directions of the TOSA with the 3D defocusing coupling model within 1 dB, and the optical power difference tested by the jumper with different concentricity within 0.4 dB, which is better than the performance using two-dimensional (2D) coupling and 3D coupling. So, the 3D defocusing coupling model is a new coupling model for improving manufacturing efficiency and yield. This paper has important technical guiding significance and economic value as follows:  
1. 3D defocusing coupling can guarantee the Gaussian distribution in the fiber without visual real-time monitoring.
2. The difference of a TOSA's performance tested with different jumpers is small, which not only improves the yield of the TOSA, but also guarantee the performance of the optical module and client. At the same time, it reduces the technical requirements of the jumpers and directly reduces the cost of jumpers.
3. The 3D defocusing coupling model presented in this paper can be applied to other active-coupling products. In multi-mode coupling, the performance of circular flux in fiber can be improved, and the transmission performance of multi-mode products can be improved.