Optical fiber array; FA; U-groove; V-groove; error analysis; reliability

At present, there are several FA (fiber Array) positioning technology schools in the industry. Starting from the structural characteristics of various optical fiber positioning methods in the production of optical fiber arrays, this paper focuses on the characteristics of two mainstream optical fiber positioning methods, U-shaped groove and V-shaped groove, including the processing characteristics of the two positioning grooves, positioning structural characteristics, Accuracy comparison, analysis of reliability characteristics of fiber arrays, etc., put forward the cognition based on the reliability point of view of fiber array devices.

In recent years, with the development of optical communication from communication trunk lines to FTTH direction, various integrated optical devices such as planar waveguide devices, micro optical switches, and laser array chips in AWG optical communication networks have moved from the development stage to the large-scale practical stage.

As the core of all kinds of integrated optical devices – fiber array components, its fiber positioning accuracy, lateral error, fiber parallelism, reliability and cost directly affect the performance of these optical products, and thus affect the development of communication networks.

This paper firstly introduces the fabrication principle of fiber array, analyzes the characteristics of various fiber positioning structures, and analyzes the influence of various structures from the perspective of device reliability requirements. The research results show that V-groove positioning technology has unique advantages in fiber arrays .

Fiber array components involve input or output parallel coupling interfaces of various one-dimensional and two-dimensional array devices, and need to achieve high-precision positioning of several fibers. The methods for realizing optical fiber positioning in the industry include drilling method, optical channel close-packing method and groove positioning method. Among them, the groove positioning method can be divided into V-shaped and U-shaped according to the end face of the positioning fiber.

As shown in Figure 1, an array of positioning holes is made on a substrate with a certain thickness, and the optical fibers are inserted into the small holes, and then glue is dispensed and cured. Since injection molding is generally used for drilling molding, there is no change in the degree of freedom of the hole spacing. And because the performance of the polymer material used for injection molding and the silicon substrate used for integrated optical devices is very different, the hole spacing error is large. Moreover because the diameter of the inner hole is larger than the diameter of the optical fiber, the optical fiber position deviation is caused, which is not suitable for mass production.

Fig. 1 Drilling method

As shown in Figure 2, the optical channel close-packed method is to arrange and fix the optical fibers closely in the grooves with high flatness. This method has good scalability, but the spacing of fiber channels cannot be adjusted arbitrarily, and it is only suitable for making densely arranged fiber arrays. In addition, the groove width is larger than the accumulated outer diameter of the fiber, resulting in a large error, and it has not been widely promoted in practical use.

Fig. 2 Optical channel close-packed method

The U-groove uses the inner wall of the groove with the same diameter as the fiber to locate the fiber. Since the outer diameter of the optical fiber has a tolerance of ±0.3 microns, in order to ensure that the optical fiber can enter the groove, the inner diameter of the circular groove must be larger than the outer diameter of the optical fiber. This causes the uncertainty of the fiber position, which is undoubtedly the main source of coupling loss for the single-mode fiber that is widely used in FTTH and is very sensitive to the fiber position accuracy.

In order to solve the uncertainty of the position of the optical fiber in the U-shaped groove, there is also a method of designing the U-shaped groove with a groove width smaller than the diameter of the optical fiber. At this time, it is necessary to rely on the ridges on both sides of the groove on the plane to locate the optical fiber. After adding the cover plate , to achieve three-point positioning fiber, as shown in Figure 3(b). Because this positioning structure can realize the determination of the position of the optical fiber, it is used in large-scale production.

Fig. 3 U-shaped groove

When a V-shaped groove is used to fix the optical fiber, there must be two points of the optical fiber cross-section in contact with the side wall of the groove at the same time to ensure the uniqueness of the radial position of the optical fiber. The V-groove relies on the two planes in the groove to locate the optical fiber. After adding the cover, the three-point positioning of the optical fiber is shown in Figure 4. Since this positioning structure can easily insert the optical fiber into the groove and realize the determination of the position of the optical fiber when assembling the optical fiber array, it is widely used in the optical fiber array at present.

Fig. 4 V-groove

With the development of MEMS, various ultra-precision machining methods have been introduced, which meet the requirements of the optical communication industry for device positioning accuracy to varying degrees. However, based on the different principles and implementation methods of various processing methods, the difference in accuracy is also sufficient to affect the product performance difference caused by the accuracy of various positioning grooves in FA.

As a traditional physical processing method, ultra-precision grinding has become the main method in the processing of V-groove. The servo system with feedback signal can achieve sub-micron position accuracy and shape accuracy. The controllable processing process also ensures the stability of batch production, and it has the advantage of adjusting the number of channels and channel spacing at any time. The dominance of the grinding method in the production of V-groove.

In microfabrication, the commonly used deep reactive ion etching method overcomes the disadvantage that the groove sidewall is 57.4° in conventional wet etching, but the anisotropy of the etching method itself makes the sidewall accuracy only 2 microns; Moreover, due to the limitation of the etching depth, the sharp ridges on the plane become the best choice for supporting the optical fiber, such as the U-groove shown in Figure 3(b). Although this processing method is similar to the planar waveguide chip process, in the FA process, the precise location of the positioning groove determines the performance of the device, and the similarity of the process has no substantial help.

On the side, the center of the fiber is below the plane. At this time, the area occupied by the glue above the fiber tangent point and the area occupied by the glue below the tangent point are 0.65-0.9. Generally, an appropriate fiber burial depth is selected, and the area ratio can be obtained.

Fig. 5 Glue area ratio

During the fiber preparation process, there are many tiny cracks in the core of the fiber. These cracks can meet the system life requirements in normal use. However, when the fiber is subjected to uneven or excessive force, the tiny cracks will become the fatal defect of fiber cracks. In the structure of U-shaped and V-shaped positioning optical fibers, since the U-shaped positioning uses the ridge line on the plane as the positioning point, the optical fiber is subjected to glue shrinkage, temperature changes, and other external forces during use through these two. The ridge line acts on the fiber, and this concentrated stress acts on the fiber with sharp edges and corners, which is bound to greatly increase the possibility of fiber cracks. According to the reliability analysis of BELCORE experimental requirements, the proportion of visible fiber cracks caused by stress changes in the FA prepared with U-shaped grooves in the experiment is as high as 30%, which demonstrates the risk of U-shaped ridge line positioning the fiber from a practical point of view.

For the device with V-shaped positioning fiber, there is no fiber crack at the contact point when the same stress experiment is carried out. It is this structure that locates the optical fiber on three sides, avoids stress concentration caused by sharp ridges, and avoids uneven distribution of glue, so the reliability of the device can be guaranteed from the structural aspect.

As two mainstream optical fiber positioning methods, U-groove and V-groove can theoretically achieve accurate three-point fiber positioning. The V-groove is structurally positioned by the sidewall of the groove, and is superior to the U-groove structure in terms of stability and fiber fission. In terms of reliability, the ratio of the volume occupied by the upper and lower glues of the optical fibers in the two structures also determines the higher reliability of the V-groove.