Overview
Many specialty fibers, such as active laser fibers, polarization maintaining fibers, or sensing fibers are made by chemical vapor deposition of individual layers inside a substrate tube to create a core rod or preform with specific functions. This can be either MCVD (modified chemical vapor deposition, PCVD (plasma chemical vapor deposition) or FCVD (furnace chemical vapor deposition). During a second step, these core rods are sleeved with a jacket tube to form the final preform that is ready for fiber drawing. Because the characteristics of the final fiber are fixed during the preform manufacturing process, the starting tubes used to manufacture or jacket the core rod influence the performance of the fiber. The key elements are the material grade, purity, and geometrical precision of the tubes.
Material Grades and Purity
Impurities in the material can increase the absorption of the fiber, especially fast diffusing elements like copper. Other particles of high melting elements like tungsten can cause fiber breakage during fiber drawing. Therefore, high purity is essential to achieve a high performance fiber. Since the 1990s it has been known that only synthetic fused silica can meet these requirements and it therefore has completely replaced natural materials.
Our Fluosil® F300 tubes are the standard for all types of CVD processes. The material is produced by a gas phase in a flame hydrolysis process to the highest purity with a typical OH content of less than 0.1 ppm. For more demanding products, we can offer materials with even lower OH content or customized refractive index profile.
Typical Material Parameters
F110 | Spectrosil | F300 | F500 | F310 | F320 | F520 | |||
Refractive Index* [x10-3] | - 0.1 … 0 | - 0.16 … - 0.25 | + 0.35 … + 0.5 | + 0.35 … + 0.5 | 0 | - 0.10% - 0.20% - 0.30% - 0.40% | - 1.17 … - 0.59 - 3.28 … - 2.56 - 4.29 … - 3.36 - 6.34 … - 5.39 | - 0.10% - 0.20% - 0.30% - 0.40% | - 1.17 … - 0.59 - 3.28 … - 2.56 - 4.29 … - 3.36 - 6.34 … - 5.39 |
OH [ppm] specified typical | - 400 | - 1,000 | ≤ 1 ≤ 0.2 | ≤ 0.1 ≤ 0.02 |
200 | ≤ 1 ≤ 0.2 | ≤ 0.1 ≤ 0.02 | ||
CI-content [ppm] typical | 200 … 300 | < 0.15 | 800 … 2,000 | 800 … 2,000 | < 0.2 | - 0.10% - 0.20% - 0.30% - 0.40% | 200 … 300 200 … 300 ~ 1,000 - | - 0.10% - 0.20% - 0.30% - 0.40% | 200 … 300 200 … 300 ~ 1,000 - |
F-content [ppm] | - | - | - | - | - | - 0.10% - 0.20% - 0.30% - 0.40% | ~ 3,200 ~ 6,800 ~ 10,800 ~ 16,000 | - 0.10% - 0.20% - 0.30% - 0.40% | ~ 3,200 ~ 6,800 ~ 10,800 ~ 16,000 |
Trace impurities | Below detection limit of ICP-MS | ||||||||
Viscosity dPas [1150°C] | 12.7 | 11.9 | 13.1 | 13.1 | 13.1 | - 0.10% - 0.20% - 0.30% - 0.40% | 11.8 11.5 11.2 10.6 | - 0.10% - 0.20% - 0.30% - 0.40% | 11.8 11.5 11.2 10.6 |
* Difference to un-doped silica (Heraeus standard) | |||||||||
Trace impurities in all our materials are below the detection limit. To maintain this purity, we avoid contact with metal parts in all process steps wherever possible. For example, our tubes are drawn contact free without the use of forming dyes.
Geometry
In addition to purity, another major influencing factor on the later performance is geometry. Fluctuations in outer diameter or wall thickness change the deposition conditions inside the tube. Ovality will cause non-round core shapes and wall thickness variances can cause core eccentricity errors.
With our tight tolerances you can prevent such failures in your CVD process which is key to produce a high performing fiber.
To achieve these tight tolerances, we have developed our sophisticated HP drawing process and the metrology equipment to evaluate the starting material, both at the drawing towers and during quality inspection.
