Standards are defined with customers thanks to cross-disciplinary measuring technology

Optical fiber assemblies are used in different areas: The demands placed on multimode assemblies for sensor technology, medical technology, and industrial laser power transmission are different but just as high as those placed on single-mode patch cables for communication technologies.

The greater the coupled power density, the more important product properties such as resilience, centricity between connector and fiber, and the reduction of back reflections become in multimode assemblies. Elaborate assemblies require not only an experienced team but also state-of-the-art measurement technology to manufacture fiber optic cables for demanding tasks.
Many measurement methods are standardized in single-mode technology: this is how the power level (dB) of fiber assemblies and cables is checked. The end face geometry is measured interferometrically in order to check the apex offset, for example. Finally, the cleanliness is tested according to the IEC standard. When it comes to the packaging of multimode fibers, standards are not set, or they only come into play when the undercuts of the fibers are measured.

At Laser Components, important parameters such as the centricity from ferrule to fiber core and cleanliness are precisely checked, measured and, if required, documented individually. For FSMA connectors, which are mainly used for laser applications in medicine and industry, the connection dimension is also checked.

Multi-mode Assemblies
Among other things, Laser Components specializes in the assembly of optical fibers for the transmission of high laser power: a classic field of multimode fibers with core diameters from 100 µm to 1 mm, which are suitable for wavelengths from UV to the far IR in a wide variety of forms. The following fibers are generally processed: primarily step-index fibers but gradient index fibers with a quartz glass core as well, hollow-core fibers for the transmission of CO₂ wavelengths (10.6 µm), and doped fibers for the ultraviolet range.

Processing of Single-mode Fibers for Diode Laser Modules
Depending on the application (e.g., pumping, spectroscopy, or biotech applications) with fiber-coupled laser diode modules, either multimode or single-mode fibers are coupled to the module.

The challenge in processing single-mode fibers for short wavelengths (e.g., 480 nm) lies in the very small dimensions of the fiber cores. With diameters of only 4 to 5 µm, they are about twenty times thinner than a hair. Power in the mW range causes high power densities at the transition surfaces of the plug connections. Precise core adjustment and high cleanliness are an absolute prerequisite for excellent transmission characteristics. In order to ensure the safe operation of the modules, special techniques that require a high level of expertise must be used in the manufacture of the fibers:
Over the years, Laser Components and its experienced team have developed their own assembly technologies and connectors. This has led to a high connector assembly quality and thus guarantees the desired operational reliability. One example mentioned here includes connectors with mode-strip capability.

Endcaps for High-Power Output Coupling from the Fiber
So-called endcaps are one possible solution for high power densities at connector ends: These are cylindrical pure quartz rods that are spliced onto the optical fiber and, if required, can also be integrated into standard connectors such as FC/PC. The diameter of the end cap is larger than that of the fiber core. The light leaves the core according to its numerical aperture; its power density is reduced by the end cap. This prevents burning of the fiber or performance-related damage to the end faces. There are different types of endcaps that are used for both single-mode and multimode fibers. The insertion of the end cap at a defined length into the ferrule of a single-mode connector is a special technology.

More than Twenty Years of Experience in Production
In order to achieve the greatest transmission efficiency from the assembled fibers, it is necessary to understand, apply, and optimize the material properties, bonding technologies, connector assemblies, and assembly technologies. Our measuring and processing equipment is always state of the art. New technologies are constantly being added, such as fiber processing using the ring-of-fire technology. In this way, we can also offer customized solutions for demanding projects.

State-of-the-Art Testing Equipment for Reliable Products
Laser Components unites different production facilities under one roof. The cross-disciplinary R&D team has access to all available technologies and measuring equipment.

Determining the Quality of Multi-mode Optical Fibers
Optical fibers are subject to the same physical laws as glass. There are hardly any standards for determining the quality of step-index multimode fibers. Therefore, we derive our knowledge from the production of laser optics and develop standards with our customers.

AQL Recommendation
In cooperation with customers, we have thus derived the existing optical standard to the small surfaces of the fiber facets and are able to document the defect density and make recommendations. These are recorded in the acceptance quality limit (AQL). This is how we manage to reduce the incoming goods inspection at the customer’s premises to 10%.

Parameters for High-Quality ConnectorsThe distribution of laser power in the fiber core, cladding, or coating can be one reason that the fiber facet or the assembled connector is heated up. Even imperfect end faces, irregular gluing, the wrong choice of adhesive, and poor connector design have an influence on the power transmission, cause “hot spots,” or in a worst case scenario damage the end face.

The surface roughness of glass surfaces (RMS value) can be determined with a white light interferometer. We mainly use this measurement technique to determine the surface quality of laser optics. However, it can also be used to test critical fiber end faces and to evaluate and document their quality. In precision optics, the roughness of the surface can be influenced by the polish. We also apply this knowledge to fibers and fiber coatings.

We use interferometric methods to measure fiber displacement or define the limits for undercutting or protrusion.

AR Coatings on Fiber End Faces
Power losses occur at every air/glass interface. The derivation of the Fresnel equations results in a reflection loss of 4% for each transition with vertical light irradiation. This can be minimized by so-called anti-reflection coatings.

We coat preassembled fiber end faces to minimize the power losses during input and output coupling of light and to avoid back reflections, which can lead to the destruction of pigtailed laser diodes.

Centricity
The centricity of the assembled multimode fiber optic cable in the free-standing connector plays a decisive role. In order to couple light as loss-free as possible and to prevent the destruction of the connector, the fiber must be located as close as possible to the center of the ferrule.
Trained specialists can precisely position the cores: Currently, centricities below 5 µm are possible. A measurement log displays the exact values. In practice, exactly centered fibers mean less adjustment effort and therefore a more efficient installation of the applications.

Further product information:
Fiber Assemblies and Connectors

Contact:

Contact Person:	Dr. Andreas Hornsteiner
Company:	Laser Components Germany GmbH
Address:	Werner-von-Siemens-Str. 15
ZIP / City:	82140 Olching
Phone:	+49 (0) 8142 2864-82
Fax:	+49 (0) 8142 2864-11
Email:	andreas.hornsteiner@lasercomponents.com