How to make an High Accuracy Linear Fiber Array?

Linear fiber array; silicon V-groove; error analysis; adhesive

Summary

This paper discusses the feasibility of fabricating high-precision linear optical fiber arrays by silicon V-groove method, introduces the fabrication mechanism of silicon V-groove, analyzes the main factors affecting device accuracy, and gives full consideration to device design and fabrication. According to the reliability requirements of the device, the characteristics of the adhesive used for optical fiber bonding are analyzed, and the experimental comparison between the UV-curable adhesive and the infrared adhesive is carried out. The results show that the Norland UV-curable adhesive and the 353ND infrared adhesive is the best choice. Silicon V-grooves were fabricated by anisotropic wet etching technology, fiber alignment, positioning and end face treatment were performed, and high-precision linear fiber arrays were fabricated. After testing the end face shape error and surface roughness, the results show that the longitudinal position error of the end face of the fiber array is less than or equal to 324 nm, and the root mean square value of the surface roughness is less than 3.85 nm.

1. Introduction

Fiber arrays are widely used in integrated optical devices, optical imaging and detection systems. In these applications, the coupling accuracy of the device is very strict, so the coupling between the fiber array and the chip or with the lens array is a big problem.

At home and abroad, researches have been carried out on low-channel-count fiber arrays in the field of optical communication, and image beams in optical imaging and detection systems, respectively, and many achievements have been made in improving device performance and reliability. But so far, there are still some technical problems in the fabrication of long-line densely packed fiber arrays. In this paper, a long-line fiber array made of silicon V-grooves is proposed.

The array has the advantages of accurate positioning, small lateral cumulative error, high fiber parallelism, and small longitudinal and height position errors of the fiber array end face, which can greatly reduce the coupling alignment. loss, to meet the application requirements of fiber arrays in many fields.

This paper firstly introduces the fabrication mechanism of silicon V-groove, analyzes the main factors that affect the precision of the device, and gives full consideration to the design and fabrication of the device. According to the reliability requirements of the device, the characteristics of the adhesive used for optical fiber bonding are analyzed, and the experimental comparison between the UV-curable adhesive and the infrared adhesive is carried out. The optical fiber arrangement, positioning and end face treatment were carried out, and a high-precision linear optical fiber array was produced, and the end face type error and surface roughness were tested.

The research shows that the long-line-column fiber array based on silicon V-shaped groove has the characteristics of low loss, wide operating temperature range and high device reliability.

2. Structural Design

2.1 Basic principle of image transmission

The one-to-one fiber array at both ends is the image beam. The image-transmitting bundle is a fiber-optic element that regularly gathers multiple optical fibers of a certain length into a bundle to achieve the ability to transmit images. Each fiber has good optical insulation, and its independent light-transmitting surface is not affected by other adjacent fibers. In this way, each light transmission surface (ie, the fiber core layer) can be regarded as a sampling hole, carrying a pixel in the independent light transmission process, where the size of the pixel is the size of the sampling hole, and the number of pixels is equal to the number of fibers on the end face.

The two ends of the image transmission beam must be in a one-to-one correspondence, that is, the geometric positions of each optical fiber on the incident end face and the exit end face of the image transmission beam are required to be exactly the same, so the outgoing image and the incoming image should be basically the same.

2.2 Selection of preparation method

At present, the methods of developing optical fiber image transmission bundles at home and abroad include drum winding method, acid solution method, V-groove method and so on.

Usually, a laterally moving drum is used for winding and arranging, and what is produced is a densely arranged low-fiber-diameter optical fiber array. This technology is relatively mature, but because it relies on the fiber diameter accuracy and the feed accuracy of the special machine tool to control the fiber position, the structural accuracy is limited.

The acid-dissolving method is to arrange the double-clad single optical fibers with a certain length closely to each other, fasten the two ends, and then place them on a special drawing equipment, and heat and draw from one end to make a composite wire. At this time, the acid-soluble glass of the single optical fiber in the composite wire is melted and condensed together to form a hard image bundle. Cut the hard image beam into the required length, protect both ends with acid-resistant material, soak it in an acid solution with a certain concentration, and dissolve the unprotected middle part of the acid-soluble glass to make an optical fiber image transmission. bundle. This method cannot make linear fiber arrays.

The optical fibers in the long-line optical fiber imaging bundle have two methods: close arrangement and V-groove positioning. Since the drawing of the optical fiber is not uniform, the accumulated string length error is relatively large for the close arrangement. This problem is solved by using V-groove positioning. The V-groove is made by bulk silicon technology, and the precision is very high.

In addition, considering the influence of the phase matching between the input signal and the optical fiber on the modulation transfer function (MTF) in the optical fiber image transmission beam, the average MTF of the optical fiber image transmission beam in the V-groove arrangement and the close arrangement (shown as Fig. 1) is analyzed. The results show that the difference between the average MTF value of the former and the ideal value is small, so the image quality of the V-groove arrangement is better than the tight arrangement. This experiment adopts the V-groove method.

Fig. 1 Two arrangements of optical fibers

2.3 V-groove structure design

The relationship between the wall width of the V-groove and the fiber radius and the position of the fiber and the V-groove as shown in formula (1) can be obtained by calculation (as shown in Figure 2), where d is the distance between adjacent V-grooves , h is the distance from the center of the fiber to the V-groove, and r is the radius of the fiber.

formula (1)

In the actual production of the V-groove, after determining the fiber radius and the opening width of the V-groove, there is a minimum value x of the groove depth. When the groove depth is greater than or equal to x, the fiber position can be uniquely determined, so results in:

formula (2)

Fig. 2 Positional relationship between optical fiber and V-groove

3. Preparation of the device

3.1 Fabrication of silicon V-groove

The process of making silicon V-groove in this paper is as follows:

(1) Cleaning, cleaning the (100) silicon wafer after grinding and polishing;

(2) Prepare a mask, and use SiO2/Si3N4 as the masking layer of the etching window on the surface of the silicon wafer;

(3) Photolithography, coating photoresist, and aligning, exposing and developing the photoresist pattern with the photolithography plate with the pattern of the optical fiber array positioning groove;

(4) Open the window and remove the glue, use the photoresist as the masking layer to dry etch SiO2/Si3N4, and remove the photoresist by wet method;

(5) Under the mask of silicon dioxide and silicon nitride, etch the silicon V-groove with 30% KOH etching solution at 70 °C;

(6) The silicon dioxide and silicon nitride films are removed, and the V-shaped groove is detected to complete the fabrication of the V-shaped optical fiber groove array.

3.2 Fiber Positioning

First, ultrasonically treat the V-groove and the optical fiber, then put the clean optical fiber into the silicon V-groove in parallel, inject UV or infrared adhesive, then cover the glass cover and press it tightly, and use a UV curing lamp or heat the device to cure the adhesive. During this operation, attention should be paid to the cleanliness of the environment, otherwise the fine dust in the air will affect the accuracy of the device.

The performance of the binder is directly related to the life of the device. In order to enhance the reliability of the device, the adhesive should meet the following aspects:
(1) Good adhesion with glass;
(2) Low viscosity, good fluidity and strong permeability;
(3) It has a certain toughness after curing, which is convenient for the completion of the subsequent cutting process;
(4) High hardness and strength, capable of grinding and polishing;
(5) Good long-term stability and aging resistance;
(6) Moderate curing temperature, too high temperature will affect the quality of the whole device. For the above reasons, Norland UV curing adhesive or 353ND infrared adhesive is selected.

In most fiber arrays, the fibers are displaced from their ideal positions during fabrication, resulting in a reduction in device contrast because the displaced fibers pass light from regions of extreme intensity to regions of extreme intensity, It is important to keep the fiber in a precise position during the curing process.

3.3 Packaging

In the experiment, the following two encapsulation schemes are proposed: Si-fiber-Si and Si-fiber-glass, as shown in Figure 3. For the Si-fiber-Si package, when the diameter of the fiber is very small, the unpolished upper cover Si-V groove cannot be accurately aligned with the arranged fiber and the Si base. Therefore, this solution is only suitable for For larger diameter fibers. In addition, a UV-transparent substrate or cover is required when using UV-curable adhesives, while Si is opaque, but heat-curable adhesives can be used. Therefore, this scheme can fabricate linear fiber arrays of single-mode fibers. The second scheme has no special requirements on the thickness of the optical fiber and the curing glue, so this project adopts this scheme to make a linear optical fiber image transmission beam.

Fig. 3 Schematic diagram of two packages

3.4 Grinding and polishing

Grinding and polishing is a very important link in the process of making high-precision linear fiber arrays, which has a great impact on the coupling accuracy of the device. The purpose of grinding and polishing is to remove the damage and deformation on the surface of the optical fiber, making it flat, smooth and with high-precision geometric dimensions. In this experiment, the mechanical polishing method was used to realize the end face polishing of the long-line densely packed fiber array. The quality of the grinding and polishing process of the two end faces of the optical fiber image transmission bundle has a great influence on the quality of the device. The roughness of the polished rear surface will directly affect the transmission quality of the optical fiber image transmission bundle, and whether the processed optical fiber end faces are perpendicular to The center axis of the fiber also affects the quality of the image transmission. If the fiber end face is not perpendicular to the axis of the fiber, the situation shown in Figure 4 will occur.

Fig. 4 Tilt diagram of fiber end face

At the incident end, when the incident direction of the light and the normal direction of the inclined section are on the same side of the light axis, the numerical aperture is:

formula (3)

In the formula:
n1 and n2 are the refractive indices of the core and skin, respectively, and
α and β are the acceptance angle and tilt angle, respectively.

It can be seen from formula (1) that if the light with the incident angle β is to be accepted, the numerical aperture should be larger than that of the normal end face. When the incident direction of the ray and the normal of the inclined end face are at both ends of the central axis of the ray, the situation is just the opposite of (1). Likewise, the inclination of the light outgoing end face will also cause a change in the angle of the outgoing light, so that the outgoing light cone of the light is deflected. Therefore, when grinding and polishing the end face of the optical fiber image beam, it is necessary to ensure that the end face is perpendicular to the central axis of the optical fiber.

4. Experimental results and discussion

The longitudinal position error of the end face of the final sample is less than or equal to 324 nm, and the root mean square value of the surface roughness is at most 3.844 nm. The clear state of the sample is shown in Figure 5.

Fig. 5 The light-transmitting state of the sample

The high-precision linear optical fiber array fabricated by the silicon V-groove method inevitably has various errors, such as:

(1) Silicon V-groove errors, including material errors and process errors. Material error mainly refers to the curvature, warpage, flatness and thickness of single crystal silicon material; process error mainly refers to the production error of lithography, crystal orientation in lithography, exposure and development conditions and masking layer. Errors introduced in the etching process and etching uniformity errors of anisotropically etched V-shaped grooves on silicon wafers. These factors directly determine the positional accuracy of the fiber array, such as fiber spacing, vertical position, etc. The technology of silicon V-groove is relatively mature, and the error is within the sub-micron level;
(2) Optical fiber diameter and cross-sectional shape errors, which are caused by the inhomogeneity of materials and processes during the optical fiber drawing process;
(3) Adhesive thickness error;
(4) The process error in the grinding and polishing process of the end face of the optical fiber array, which will affect the position of the end face of the optical fiber array, the angle of the optical fiber and the surface roughness, etc. In order to reduce errors, the production of high-precision linear fiber arrays requires detailed exploration in material preparation, process route and process condition selection, and the establishment of perfect detection methods (see Figure 6 and Figure 7).

Fig. 6 Surface roughness test chart

Fig. 7 End face type test chart

5. Conclusion

In this paper, a scheme for preparing high-precision long-line densely-arranged linear optical fiber arrays is proposed, and several key issues that need to be paid attention to in the preparation process are given:

(1) Fabrication of silicon V-groove;
(2) a clean environment;
(3) Guarantee of the accuracy of the optical fiber position;
(4) The importance of grinding and polishing.

After the comparison of various adhesives, the best adhesives Nor-land UV curing adhesive and 353ND infrared adhesive were selected, and a long-line densely arranged optical fiber array with precise positioning was prepared. The longitudinal position error of the end face of the fiber array was measured to be less than or equal to 324 nm, and the root mean square value of the surface roughness was less than 3.85 nm. This fabrication method can be applied to a variety of optical fiber devices, and the entire fabrication process is easy to operate and has good repeatability. The next step is to further improve the accuracy and conduct reliability studies.

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