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Light You Can't See

DATE��2014/4/11 20:56:04

SpecialChem | Donal McNally - May 20, 2013
Fiber Optic Data Cables with Glass Fibers
Plastic Materials Benefit IR Optics


There is probably a hermit in a cave someone who has not yet heard the words "fiber optics", but thanks to massive expenditures on technology, installation and marketing by telecommunications companies, most of us are certainly familiar with the phrase. What we likely think of right away are decorative objects, toys and other things that sprout an array of thin fibers that glow at the ends when the device is switched on. Beyond consumer items, such fiber arrays are used in "light pipes" in medical, dental and engineering applications where it is necessary to illuminate something in a space too small to allow use of an in situ light. Very large arrays of optical fiber are even used to deliver daylight to an internal space that has no windows.

What makes all this possible is a phenomenon called total internal reflection. This results from the fact that light, indeed all electromagnetic radiation, travels at different speeds in different media. As we learned in high school, the speed of light in a vacuum is about 300,000 kilometers per second (~186,000 miles/sec). It's a little bit less in air and much slower in water. The ratio of the speed in vacuum to that in some medium is n, the refractive index, so that n = c/v where c is the speed in vacuum and v is the speed in the medium.

When a beam of light travels from a medium of higher into a medium of lower refractive index, it speeds up and this causes the beam to bend further away from the vertical to the surface between the media. If the incident beam is at a big enough angle off the vertical, it never emerges into the lower n medium but instead is totally reflected back into the higher n medium. While optical fiber depending on total internal reflection is used in consumer novelties, endoscopes, borescopes, generic light piping and other applications, by far its most important and most highly technical use is in data transmission. In the United States there are more at least 175 million miles of fiber optic cable, with some 19 million miles having been installed in 2011 alone.

TopFiber Optic Data Cables with Glass Fibers

Unlike the other application areas, if you were to look at the ends of any of these data cables you would not see any thing because data transmission does not use visible light. Neither does it use continuous light, but instead employs extremely rapidly pulsed infra-red (IR) lasers to provide digital signals. Most important, fiber optic data cabling uses glass fibers. Not the kind of glass fibers used in insulation or composite reinforcement, but precisely drawn fibers made from chemically pure silica-based glasses. There are several reasons for this, the most important of which are low signal attenuation and acceptable cost. In early work with glass fibers, signal attenuation due to impurities severely limited transmission efficiency. Over time the degree of signal attenuation was reduced to 4dB/km making fiber optic cabling far more efficient than electrical copper cables and enabling long-haul fiber connections with repeater distances of 70 – 150 kilometers (43 – 93 miles).

The other important aspect of fiber optic cabling is the use of invisible IR light for signaling. Silica has an attenuation minimum around a wavelength of 1600 nanometers which is well into the near IR. The combination of available high purity silicon to make the glass with this low signal loss has enabled millions of miles of high performance optical fiber to be installed at a reasonable cost per mile. The following table describes telecommunication and other uses of IR transmission and sensing.

NameRange, nmSome Applications
Near IR780 – 1400Telecommunications, image intensifiers
Short-wavelength IR 1,400 – 3,000Long-distance telecommunications
Mid-wavelength IR3,000 – 8,000Heat-seeking missiles use the 3,000 – 5,000 range
Long-wavelength IR8,000 – 15,000Thermal imaging, forward-looking IR sensors

Table 1: Uses of IR Transmission and Sensing

The silica glass used to make optical fibers is also employed in the lenses, light pipes and waveguides that are needed to create, manage and sense the precise wavelengths used in telecommunications; and in the optical systems of detection and sensing equipment. But, just like glass used in the visible spectrum, it is not always the perfect solution. The inexorable march of technology toward smaller, faster, and more powerful applications that are both more integrated and more distributed presents challenges to the use of glass.

TopPlastic Materials Benefit IR Optics

Among the challenges are cost, fragility, quantity production and precise replication, parts integration, and exposure to on-board wave soldering conditions. Again, just like visible light applications, IR optics can benefit from the use of suitable plastic materials. In the visible region, resins such as poly (methyl methacrylate) (PMMA), cyclic olefin polymer and copolymer (COC), polycarbonate (PC) and even polystyrene (PS) all find appropriate uses. Grinding and polishing of glass optics is costly and time consuming whereas molding of precision polymer optics is relatively inexpensive and offers distinctive advantages in terms of design capability and flexibility. Mounting features can be integrated with optical components, significantly simplifying assembly and reducing manufacturing cost. Single-cycle injection molding of several integrated optical components and mounting features is common in current injection molding design and production. The main reasons for using polymer optics in a variety of devices and applications include:
  • Light transmittance comparable to high-grade crown glasses

  • Can be used just like glass for refractive, reflective and diffractive substrates,

  • Low cost polymer material and low cost volume manufacturing

  • High precision and fast repeatable production process;

  • High impact resistance — plastics do not splinter like glass

  • Low density makes products lighter by 2.5 to 5 times

  • Suitable for difficult and sophisticated aspheric, cylindrical and/or free form shapes

  • Parts integration capability

Because optical plastics have high IR transparency up to an absorption peak around 1700 nm, they are also very useful in the IR region. Compared with glass, their main drawback is limited temperature capability, both for operating in a hot environment and for supporting wave soldering temperatures encountered during printed circuit board construction. Other differences include higher coefficient of thermal expansion, lower refractive index and higher dn/dT (change in refractive index with change in temperature). Here is where some high temperature resins come into play. Most high temperature resins such as polysulfone (PSU), polyphenylene sulfide (PPS), thermoplastic polyimide (TPI) and polyetherimide (PEI) are colored or opaque in the visible region, but some, particularly PEI and TPI, have good IR transparency. These last two resins have IR transmission values approaching 90% in the near IR region. The thermal capability of these resins is shown by their heat deflection temperatures under a load of 264 psi and their maximum continuous use temperature ratings, as given in the table below.

Resin TypeHDT @ 264 psi, °CMax Continuous Service Temp., °C
Polysulfone (PSU)175140 – 160
Polyetherimide (PEI)201170
Thermoplastic Polyimide (TPI)237 – 254180 – 260
Polymethyl methacrylate (PMMA)9260 – 70
Cyclic Olefin70 – 150130

Table 2: Thermal Capability of Resins


Of the four high temperature resins, PEI and TPI have the highest temperature capability, with TPI being particularly suited for reflow soldering applications using lead-free solder in the range from 240°C to 260°C. In addition to high temperature tolerance, EXTEM™ (TPI) resins from SABIC offer superior impact resistance and excellent stiffness and strength as well as good chemical resistance — in short, all the advantages of glass for IR transmission without the disadvantages. A special grade that acts like a high-pass optical filter is also available. IR-transparent black color technology enables this grade to provide a sharp cut-off in the visible region, with essentially zero transmission below 700 nm. Further, the cut-off wavelength can be tuned to meet the specific requirement of the application. This technology significantly reduces visible light interference in fiber-optic lens applications and can help reduce heat build-up compared to standard pigment black parts.

Light attenuation control is possible in SABIC's ULTEM™ (PEI) resins where calibrated color management enables selected levels of transmission to be achieved with a tolerance of ± 2%. This facilitates multi-channel signal encoding to increase available bandwidth. Though of slightly lower heat tolerance than EXTEM resins, ULTEM resin grades still exceed the high temperature performance of competitive materials and provide excellent mechanical properties, significantly exceeding the Rockwell hardness of other optical polymers.

Both EXTEM and ULTEM resins have higher refractive indices and lower coefficients of thermal expansion than lower-performing optical plastics. This means that comparable refraction can be achieved with thinner lenses and that lens geometry will be less affected by temperature changes. In addition to possessing a high refractive index, a stable and low dn/dT value is required for a fiber-optic lens to focus properly and provide enough power to transmit/receive the signal.

Save Cost and Improve Performance vs. Glass With EXTEM and ULTEM
2 Success Stories In Optical Applications
Very high heat resistant , IR transparent and lead-free soldering capabilities
IR Transparent and excellent data transmission properties at reduced cost in high heat environment

The temperature dependence of refractive index is an important parameter in the design of high-end optical lenses for high temperature environments. As Aditya Narayanan, scientist at SABIC's Innovative Plastics business explains, lower dn/dt numbers mean that the refractive index of the material remains relatively stable across a given temperature range. This is important in the design of a lens, as it allows a collimated beam of light to focus accurately and reliably at the desired point and thus achieve a high signal-to-noise (S/N) ratio, essential for high efficiency communication. Conversely, a lens made from a material with a high dn/dT number (refractive index changes significantly with temperature) will shift the beam focus more with a temperature change. As a result, delivered power may be insufficient for signal transmission (low S/N ratio) over changes in operating temperature. ULTEM and EXTEM resins exhibit high RI with low and stable dn/dT values of about 11 x 10-5 over the measured temperature range (30-120°C) at 850 nm. These low and stable dn/dT values are maintained even at the higher telecommunication wavelengths of 1310 nm and 1550 nm.

The unique features of injection-moldable ULTEM and EXTEM resins such as exceptional dimensional stability, high melt flow, near-IR optical transparency and high temperature resistance helps to drive down system costs. These characteristics enable the tight tolerances required to manufacture high performance precision optical components such as optical lenses and connectors and provide design freedom to allow complex features to be integrated into the parts.

Overall, when one considers cost and performance, the case for using glass in IR optics is limited to very high service temperatures and small production volumes over which the cost of plastics tooling cannot be adequately amortized.

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