Optical couplers are used to branch or combine optical signals. They are used in public and private fiber optic networks to provide passive distribution and point joining for optical data transmission (telephone, cable TV, etc.).
Other applications for optical couplers include measuring equipment, measuring instruments, and sensor technology.
Properties
Fusion couplers are characterized by the following properties:
- All fiber components have low insertion loss.
- High return loss and directivity.
-
Good performance in wavelength-selective or broadband applications.
- High thermal and mechanical stability
. - Free choice of coupling ratio (1....50%) and bifurcation level.
- Manufactured to meet customer specifications.
Manufacturing Process
: Fusion couplers are manufactured using a method called FBT (Fused Biconical Taper), in which the coupling zones are created by simultaneously fusing and bonding the optical fibers. See Figure 1.
The basic material is a single-mode fiber with an acrylate protective layer (primary layer, typical diameter 250 mm). To produce a coupler with two outputs, the first step is to strip a small portion of the protective layer from both optical fibers. The stripped areas are carefully cleaned, aligned in parallel, and secured. The subsequent process of simultaneous fusion and bonding allows light to pass through, or couple, from one core into the other. Thus, the two fibers form a single glass connection that is not interrupted by the splicing, as would occur in the case of an adhesive splice. The simultaneous fusion and merging process is controlled by sophisticated measuring equipment that allows for simultaneous measurements. This enables the pulling process to be interrupted at any point, thus allowing the degree of coupling of the input signal within the second fiber, or the coupling ratio, to be controlled (Figure 2).
After the merging process, the coupler is impregnated with a quartz substrate using a special adhesive (Figure 3). The substrate with the coupler is inserted into a metal tube, and the ends of the tube are sealed with silicone. After this encapsulation, the described coupler has two input fibers and two output fibers (= 2x2 coupler). By terminating the second input fiber without reflection, we would have a 1x2 coupler.
Fusion couplers and coupler modules with a large number of ports are actually manufactured as fusion couplers or coupler modules. Fusion couplers are actually manufactured by simultaneously fusing and merging up to four fibers (= 1x4 coupler). The coupler modules are formed with a high number of simple couplers. Using these simple couplers and specially developed boxes, a cascade structure is created that provides coupling modules of up to 132 ports (Figure 4).
Types of Couplers
By manipulating the bonding process and special pretreatment of the fibers to be fused, couplers with different transmission and coupling properties can be manufactured. The coupling properties of the different types in relation to wavelength are represented in Figures 5 to 10.
There are couplers for applications in the wavelength range of
telecommunications from 1200 nm to 1700 nm, as well as couplers for 
wavelengths
of shorter wavelength. For example, in the visual range. (Figure 11).
POWERSPLITING
SSC (Standard Singlemode Couplers)
single-mode couplers
For a wavelength with minimal deviation. For example, 1310 ± 5 nm (Figure 5).
WFC (Wavelength Flattened Couplers).
WFC couplers are single-window couplers optimized
for a range of wavelengths, ensuring constant branching power across a wide bandwidth. For example, 1310 ±40 nm (Figure 6).
WIC (Wavelength Independent Couplers).
WIC couplers are dual-window couplers that guarantee constant branching power in the second and third windows (1310±40 and 1550±40 nm) (Figure 7).
EIC (Extended Wavelength Independent Couplers).
EIC couplers are dual-window couplers that guarantee constant branching power in the second extended and third windows (1310±50 and 1550±75 nm). (Figure 8).
FIC (Full Range Wavelength Independent Couplers)FIC couplers are triple window couplers that guarantee constant branching power in the second, third and fourth windows (1310±50 and 1550±100 nm) (figure 9).
Wavelength
Division Multiplexer
WDM couplers are used to separate or combine two or more wavelengths (Figure 10).
Single-mode
SHORT WAVELENGTH
(Short Wavelength Couplers)
SWC couplers are passive optical elements that allow the distribution and combination of optical signals with wavelengths below the wavelengths of the telecommunications range, i.e. 532±5 nm (Figure 11).
Basic Technology
As can be seen in Figure 12, the coupling ratio depends on the joining length and the operating wavelength. If the joining process stops at a specific point, a specific coupling ratio is achieved at that wavelength.
Point A marks a standard coupler with a 50% coupling ratio at 1550 nm. If this coupler operates at a wavelength of 1310 nm, the coupling ratio is approximately 20%. Point B marks
a standard coupler with a symmetrical coupling ratio at 1310 nm.
Point C marks a coupler operating in a single window at 1550 nm. At this inflection point, the coupler is highly insensitive to changes in wavelength. However, this characteristic is not required to occur at
100% coupling, so one of the fibers is pretreated by chemical etching or pre-joining. This allows the inflection point to be reduced to 50% (Figure 12, point C).
Point D shows a double-window coupler. At this intersection, the coupling is the same for both wavelengths, but asymmetric (approximately 10%). Here, too, an intersection with an asymmetric coupling ratio has been achieved by fusing two fibers that have been pretreated differently (Figure 12, point D).
Point E shows a specific coupler, the well-known wavelength division multiplexer. At this point, 100% of the signal at 1550 nm and 0% of the signal at 1310 nm are coupled. This means that a WDM, like a filter, can separate two wavelengths so that each output carries only one wavelength.
Quality Assurance:
Each coupler can be identified by its serial number, which also allows for the identification of manufacturing parameters, the work location, and the materials used in each coupler. Each component undergoes fatigue testing to ensure long-term stability. Additionally, Bellcore-based long-term tests, such as storage in humid and dry heat, temperature cycling, and mechanical stress, were successfully completed.
All couplers are designed to pass Telcordia GR-1209-CORE and GR-1221-CORE tests and IEC specifications.
Table I presents various definitions, calculations, and schematics related to the parameters for couplers, coupler modules, and wavelength division multiplexers.
Technological Development and In-House Manufacturing:
FOC couplers are manufactured using Fused Biconical Taper technology. FOC's manufacturing technology principles date back to 1985. At that time, FOC was one of the few companies with its own in-house development and manufacturing technology. The development of manufacturing technology, tooling, automation, and measuring equipment was viewed as a fully integrated process from the outset. FOC possessed comprehensive knowledge and was able to continuously develop its future technology in line with market demands.
In recent years, a significant number of new components have been created. FOC was the first company to introduce some of these to the market and offer them to its customers. Production at FOC is computerized and highly automated. On the one hand, this allows for the manufacture of high-quality, highly repeatable standard products. On the other hand, FOC's technological expertise enables the company to manufacture customized components and then organize production to meet each customer's specific requirements.
Since 1991, a large number of fusion couplers have been produced under industrial conditions using a core technology that has remained virtually unchanged. Over more than 10 years of use, this manufacturing technology has demonstrated a high degree of long-term stability in numerous projects for various customers. Among others, the majority of couplers installed as part of Deutsche Telekom's OPAL projects between 1993 and 1995 were manufactured using FOC technology (then known as Krone AG's coupler production).
Couplers based on FOC technology have been successfully tested on four continents under a wide range of operating conditions. Network operators and other customers in more than 20 countries are using FOC couplers.
This excellent track record is due to compliance with the criteria specified in Bellcore/Telcordia GR-1221-CORE and Bellcore/Telcordia GR-1209-CORE.
These criteria are always given top priority in the development and production of FOC couplers.
Quality and Test Data:
From the very beginning, extensive testing for quality monitoring and certification has been conducted in parallel with manufacturing. FOC offers individual testing and customer launches. The first performance and efficiency tests carried out on FOC couplers date back to before 1992. The most recent published test is from 2006. These also include a detailed summary of the results in the Qualification and Test Report (QTR). FOC provides this test report to all its customers.
Over the past 10 years, requirements, products, and testing methods have evolved. To meet these requirements, FOC has designed its tests to comply with the internationally recognized IEC Standard from the outset. This allows customers to make simple and logical comparisons.
Today, FOC's manufacturing is based on a robust ISO quality management system, which was established when the company was part of its parent company, Diamond GmbH. This enables FOC to maintain a high level of quality and trace manufacturing parameters, materials, and production processes, drawing on 10 years of experience.
of
products. Thus, almost all types of couplers are available from one supplier:
Single-mode and multimode couplers.
Wavelength-dependent and wavelength-independent couplers.
Power-splitting couplers and multiplexers (WDM).
Symmetrical and asymmetrical couplers.
Direct-fused couplers with 2, 3, and 4 fibers and coupler modules.
Couplers for telecom wavelengths and couplers for special wavelengths (short-wavelength couplers).
Attenuators.
Couplers for signal transmission and power transmission.
However, FOC's product range is not limited to components based on coupler technology. FOC also assembles connectors in line with all required market standards and manufactures and sells distribution equipment and enclosures. Thus, FOC can supply all passive lines, transmission equipment, and related components.
FOC components offer additional advantages such as:
Reduced component insertion losses by optimizing the adjustment of the thermal expansion coefficient of all materials used to match the expansion coefficient of the optical fiber.
Prevention of premature failure by exposing all components to thermal shock for 24 hours before taking measurements.
Verification of measurements and guarantee of optical characteristics at the wavelengths specified by the customer.
High mechanical strength and thermal capacity through strict adherence to predetermined breaking points, achieved using a fiber-adapted manufacturing process and in-line component manufacturing, i.e., without couplings, intermediate components, or similar components.
In the unlikely event of a FOC defect, FOC additionally guarantees immediate service and quick replacement of the guaranteed parts and offers fast delivery.
José Céspedes, Commercial Director of CENVALSA
