Most electronic systems default to copper wire as the transmission medium, and for good reason: it’s highly conductive, readily machinable, and cost-effective. While copper’s applications are nearly universal, its performance can suffer at microwave frequencies and speeds well into the GHz range, where the skin effect causes current to concentrate towards the surface of the copper solid. Current crowding is energy inefficient and causes material aging due to the excess heat generated. Fiber optic cables provide an alternate option with many superior transmission characteristics for systems whose transmission lines only need to communicate data. Fiber optic cable manufacturing is more involved and expensive than traditional copper cable manufacturing, but an experienced cable manufacturer can walk designers through it and system integration concerns.
Standard Fiber Optic Cable Assembly Tolerances |
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| Cable length (feet) | Tolerance |
| ≤ 5 | +/- 1.5 inches |
| 5 < length ≤ 50 | +5% / -0 |
| 50 < length ≤ 100 | +4% / -0 |
| 100 < length ≤ 250 | +3% / -0 |
| 250 < length ≤ 500 | +2% / -0 |
| ≥ 500 | +1% / -0 |
The Fiber Optic Cable Manufacturing Process
Fiber optic cable manufacturing is more involved than copper or other conductive metals because of the construction of glass rods (a preform) from quartz substrate tubes. At the same time, this process grants a high degree of customization to the customer.
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Fiber optic cable manufacturing begins with high-quality materials relatively free from defects and contaminants, with water concentration below <5 ppm to limit outgassing that can imperil the structural integrity of the glass fiber cable.
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A burner heats a rotating quartz substrate tube to 1600℃/2900℉. Silicon and germanium halides, such as silicon tetrachloride (SiCl4) and germanium tetrachloride (GeCl4), form oxidized salts in the presence of high heat and thermophoresis distributes these molecularly heavy particles away from the heat source. As these salts form a sooty deposition, the heat source passes over them to create a glass layer. This process repeats successively, with the concentration of silicon/germanium tetrachloride increasing closer to the core, establishing the requisite refractive index for the transmission medium.
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After the deposition, the quartz substrate tube requires collapse and sleeving processes that create the necessary diameter and volume for cost-effective preform drawing.
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A furnace at 2100℃/3800℉ heats the preform to draw down to a diameter of 125 µm/5 mils. A laminar flow of inert gases (typically argon) and extensive filtration prevents reaction or contamination of the glass fiber, which could promote defects and eventual cracking during manufacturing, testing, or use.
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A cladding or coating of the glass fiber guides light through the cable with a refractive index less than that of the glass core, maintaining total internal reflection that significantly inhibits loss during transmission.
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Fiber Optic Cable Testing
Optical fibers must withstand mechanical tests exceeding expected operating conditions to assure the cable’s integrity during field performance. These tests include:
- Environmental testing – Fiber optic cables undergo exposure to high- and low-temperature extremes to determine when failure occurs and the effect on temperature-dependent material properties.
- Crush test – Technicians apply a compressive load to the fiber optic cable to study how the cable recovers from the applied weight. Interpretation of the results can include measuring the attenuation of the signal during and after the applied load, only measuring the attenuation after load removal, or measuring fiber continuity.
- Weight tolerance – Similar to the crush test, the fiber optic cable submits to a tensile loading to check the change in performance during and after bearing a standard weight. Most common test metrics measure attenuation throughout the process as an impact of loading.
- Bending – A fiber optic cable endures torsion forces under tension to track the effects on the cable’s attenuation. Additionally, neither the sheath nor the fiber itself can express any traces of physical damage during the test, i.e., all deformation must remain temporary and reversible.
Further testing will focus on aging (due to temperature cycling and moisture ingress) to determine the sheath’s general resilience and the ability to keep the gap-filling gel confined within the cable. Softening, cracking, discoloration, and other markers of embrittlement stipulate a poor material match for the application environment under consideration.
Customizing Fiber Optic Cables for System Assembly
When ordering fiber optic cables for sub-system assembly, product designers should be aware that there are multiple definitions for the length of the cabling system. Consider which description best fits the installation:
A. For a single connector to a single connector, measure ferrule tip to ferrule tip.
B. For a multi-channel connector to connector fanout, measure the front face of the connector to the ferrule tip.
C. For a multi-channel connector to connector, measure from the front faces of the connectors.
E. When specifying breakout length for cables, 12 inches is typical, as is a +2.0/-0.0 inch tolerance—this distance measures from the edge of the heat-shrink tubing to the ferrule tip.
F. For breakout length behind a connector, 6 inches is typical with a +0.0/-1.0 inch tolerance. Measure from the back of the connector to the edge of the heat-shrink tubing.
There are multiple ways to customize the overall design of the fiber optic cable.
Varying Aspects of Fiber Optic Cable Assembly |
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| Tubing/Wrap | Cable type | Connector type |
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See the Glass Half-Full With Your Contract Manufacturer
Fiber optic cable manufacturing encounters more difficulties than traditional metal conductor cable assembly, but a well-manufactured cable enjoys exceptional longevity and superior transmission qualities. Fiber optics are irreplaceable for performance at high speeds – just like a cable manufacturer with a proven record of customer satisfaction. Here at VSE, we’re a team of engineers committed to building electronics for our customers, including complex sub-system cable assemblies and wire harnesses. We’ve been building life-saving and life-changing products for over forty years with our manufacturing partners.
