Connecting Optical Fibers

28.05.2024

The use of fiber optic technology across telecommunications networks is becoming increasingly popular due to the high bandwidth it enables. Technical advances in this area result into lower prices for optical equipment, making fiber optic solutions more affordable for telecommunications providers and local operators.

Optical fiber is a core component of fiber optic networks, linking stations with subscriber equipment. However, the reliability of fiber optic lines largely depends on the quality of the cable routing and installation. Given that the cost of associated construction and installation works largely exceeds the cost of optical fiber, proper installation operations are essential to ensure the stability and reliability of the network.

Page Contents

Introducing Fiber Fusion Splicing

This brings to the forefront the issue of choosing cost-effective and efficient technology for connecting and splicing optical fibers. Fiber splicing is typically performed using mechanical connectors such as Fibrlok or CoreLink or electric arc splicing.

The former technology is used more often for temporary reconnection of lines or in cases where only a handful of connections are needed. However, this technology may cause significant insertion and reflection losses, especially when splicing different types of fiber. Moreover, this type of splice requires special splice holders or custom splice trays, which may prove highly inconvenient and expensive.

The mechanical splicing technology involves the use of V-groove mechanical splice-on connectors and a splicing feeder blocks for installing the fibers into the connector. Obviously, this technique isn’t the cheapest option available. It’s clear that if you only have to connect two or four fibers, there is no point in purchasing a fusion splicer. However, if you are planning to build even a small subscriber network, the cost of fiber installation during construction and subsequent operation will generally be lower when using splicing equipment.

The most widespread technology for joining fibers is fusion splicing, which is by far the most reliable way to ensure minimal loss and light reflection from the spliced juncture point. Most importantly, fusion splicing ensures the stable mechanical and optical performance of the resultant connection.

Impact of Fiber Cleave on Fusion Splicing Quality

Optical fibers are spliced using a fusion splicer. However, the success of this operation doesn’t entirely depend on the device itself. Crucial importance is attached to the quality of the cleave of the end surface of the optical fiber. The quality of the cleave has a direct impact on the quality and time required for fusion splicing, as many fusion splicers automatically detect the cleave angle and fiber end condition before proceeding with fusion splicing.

Whenever the fiber’s properties don’t meet preset specifications, the fusion splicer may abort the splicing operation. In this case, a repeated cleaving of splice termination will be required until such specifications are met, which will require additional time. Some fusion splicers may carry through with the splicing operation even in the event of a poor quality cleave. However, this may affect the quality of the resulting splice adversely.

Impact of Fiber Cleave on Fusion Splicing Quality Fig. 1. Impact of poor cleave quality on splicing outcomes (a) before splicing, during alignment, and (b) after splicing. The cleave angle of the right fiber is approximately 5°. The geometric displacement of the core resulted in increased splicing losses (0.25 dB at 1,550 nm).

Given the impact of fiber cleaving on the quality of the splice connection, it’s essential to use high-quality precision chippers for this purpose. Typically, a cleave is produced by notching the side surface of the fiber and then applying a bending force to this point, which causes the fiber to spall.

A distinctive peculiarity of cleaves obtained by using mechanical fiber cleavers is the mark of the notch applied to the fiber’s metal surface. This defect doesn’t affect the quality of metal splicing and may be disregarded. A fiber cleave without a notch mark may only be obtained with an ultrasonic fiber cleaver.

Fiber cleave end face Fig. 2. Fiber cleave end face

Optical Fiber Splicing Technology

Nowadays, joining optical fibers by fusion splicing is the most widespread technology for obtaining permanent connections. This technology, improved over the years, allows for high-quality connections with low signal loss (up to 0.1 dB for different models). The advantages of fusion splicing include the speed and efficiency of the connection, its security, high mechanical performance, and reliability.

Optical fiber splicing involves melting the ends of fiber optic light conductors by placing them in a field of a powerful heat transfer source, such as an electric arc. When splicing optical fibers in an electric arc field, the following technological steps are executed in a sequence:

  1. putting a protective heat-shrinkable tube on one of the fibers;
  2. preparing the end surfaces of the optical fibers being spliced;
  3. securing prepared fiber ends in the rail mounted splice box of the fusion splicer;
  4. alignment of the optical fibers in two planes;
  5. actual splicing of the optical fibers;
  6. initial evaluation of splice quality;
  7. securing the splice area with a heat-shrinkable tube.

Advanced fiber splicers may be subdivided by several criteria: by fiber end alignment method (based on the geometric dimensions of fiber ends or light signal power loss at the splicing point) or by the number of optical fibers that can be spliced at a time (single and multi-fiber fusion splicers).

There are various methods of fiber convergence, with three of them being the most widespread. Most advanced fusion splicers use the method of aligning optical fiber cores or jackets by their geometric dimensions (Profile Alignment System (PAS)), which is based on side illumination of the ends of the fibers being spliced.

During this operation, the convergence of the fibers illuminated from the side is verified by analyzing the image processed by the microprocessor of a computational video camera installed on the opposite end of the splicing point. Analysis of the light passing through the illuminated fiber provides information about its structure and, accordingly, its core. This technology was developed and patented by Fujikura.

Profile Alignment System (PAS) Fig. 3. Generation of fiber image in the PAS system.

Yet another technology is based on aligning fiber cores using optical emission transmitted through the juncture to minimize the loss of the test light signal. This technology, known as the LID (Light Injection and Detection) method, involves the injection and reception of optical emission through fiber bending points on special mandrels. However, the drawback of this method is that it doesn’t address the physical properties of the fiber after fusion, which may lead to increased losses in fibers with complex refractive index profiles.

Both of the described methods cover active fiber alignment based on the analysis of fiber images or light energy passing through the splice juncture and controlled by the device's microprocessor.

The technology we’re going to cover next is passive. In it, fiber ends are aligned along their jackets using fixed V-shaped grooves. This method involves precise positioning of the grooves opposite each other, with rather demanding standards applied to the geometric specifications of the fibers. Normally, this alignment method would be used where high tolerances of splicing losses are acceptable, as it results in somewhat higher losses than the active methods discussed earlier.

Optical Fiber Fusion Splicing Operations

Optical fiber splicing involves both fibers being melted simultaneously so as to avoid the overheating of either of the fibers, which may cause it to become thinner at the juncture. Melting and splicing operations are usually automated. Advanced automatic fusion splicers have a splicing point warm-up mode to help relieve mechanical stress after the splicing operation is complete. It is known as the “relaxation mode.”

The splicing cycle for different types of optical fibers may differ from one fusion splicer to another.

Optical Fiber Fusion Splicing Operations Fig. 4. Optical fiber splicing cycle of the Fujikura automatic fusion splicer: A – Pre-arc power; B – Main arc power; C – Relaxation arc power; D – Cleaning arc; E – Pre-arc duration; F – Consolidation; G – Main arc duration; H – Relaxation arc duration; I – Pause; J – Duration of a sequence of relaxation pulses; K – Pause; L – Fiber distribution; M – Fiber distribution value; N – Repeat arc.

In addition to the above methods of quality control of the splicing juncture, some fusion splicers also use a tensile test to avoid damage to the connection resulting from manipulations during insertion into the tray and subsequent operation.

The connected optical fiber is firmly fixed in the mounting boxes (used during the alignment). Controlled by a microprocessor, after the splicing stage is completed, such mounting boxes move in opposite directions, creating a strictly standardized longitudinal tensile force (400 g) applied to the splicing point.

Takeaways

As the quality of splicing equipment and splicing technology improves, the ability to produce high-quality spliced optical fiber joints is increasing. Losses at splicing points depend on a number of factors, such as personnel experience, geometric errors of the optical fibers being spliced, and the materials from which the fibers are made.

Challenges often arise when splicing optical fibers from different manufacturers due to differences in their manufacturing processes. Even if the optical and mechanical properties of optical fibers specified in the manufacturers’ specifications are slightly different, quartz glass, the material optical fiber is made of, may not be the identical in fibers of different origin.

Toolboom Team

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