ACES Int'l

Secrets & Tips

for Fiber Optics

Welcome to the first edition of ACES Int'l Secrets & Tips for Fiber Optics.  Be sure to bookmark this page!  ACES Int'l presents what we believe to be the most in-depth and enlightening articles we can publish involving the field of Fiber Optics.  

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  I’d like to take you on a field trip, so to speak, to an "electronics greenhouse" where technicians "grow" high-tech electronics components in "clean rooms" and wear containment suits much like astronaut space suits. You might ask if this is NASA, or a super secret government agency, but in truth, it might be run by the same folks that manufacture everything from TVs and stereos for your home to Fiber Optic transceivers and LAN gear for multi-national businesses. This place, with the bright lights and ultra scientific setting, is at the cutting edge in electronics development...where the race is not just for space travel or super-smart military weaponry, but to develop the fastest and most advanced electronics components which ultimately change the way we live and work. So get naked, scub up, and put on your Biocontainment Suit and follow me into the plexiglas room where some rather interesting things are made.

  Let’s look at one of the components being developed here now...Light emitting diodes (LEDs). LEDs are well known for their long lifetimes. Although 105 to 106 -hour lifetimes are considered infinite compared to other electronic components, manufacturers are constantly seeking ways to improve the length of usable life for these highly useful semiconductors. What causes the LEDs to eventually fail is a distribution of non-radiactive sites, such as crystal lattice defects and impurities in manufactured and manufacturer-constructed materials. In the highly advanced field of photonics, the LED is a cornerstone of the highly efficient multi-mode fiber optics systems used for data transference. Increasing the lifetime and efficiency of LEDs is a never ending task for manufacturers and involves state-of-the-art manufacturing methods and precise design techniques that are a challenge to the most experienced technicians that develop them.

  Figure 1 shows two of the simplest LED structures...homojunction epitaxially-grown and homojunction single-diffused LED devices.

The epitaxially-grown LED is usually constructed of silicon-doped gallium-arsenide. A combined melt of elemental gallium containing arsenic and silicon dopant is brought in contact at high temperature with the surface of an n-type gallium-arsenide wafer. At the initial growth temperature the silicon atoms in the dopant replaces some of the gallium atoms in the crystal lattice. At that point, they add an excess electron to the bond which results in the grown layer becoming n-type. During the growth, the temperature is systematically reduced until, at a critical temperature, the silicon atoms begin to replace some of the atoms in the crystal. This removes an electron from the bond which results in the formation of a p-type layer...as a finished diode, the entire surface (as well as the four sides) radiate light. This non-directionality (also called a lambertian emission pattern), along with the relatively slow turn-on and turn-off times of 150 ns makes it unsuitable for use with optical fibers.

  Planar diffused LEDs (also shown in Figure 1) are formed by controlled diffusions of zinc into the center portion of a tellurium-doped n-gallium-arsenide wafer. Turn-on and turn-off times are quicker (15-20 ns), but the emission pattern is similar to the grown junction LED (lambertian) which again makes it unsuitable for use with optical fibers. (I’d like to point out at this time that the "growing" of layers for LEDs takes place on the micron level, with very precise scientific methods and procedures...so don’t try it at home...)

  The two basic structures of LEDs used with optical fibers are surface emitting and edge emitting. The surface emitting LEDs are further broken down into planar heterojunction and etched-well, with the etched-well now being the preferred LED among the surface emitters for use with optical fiber, while the most preferred LED for use with optical fiber is the edge emitting LED. The most common material used to construct these devices is the ternary crystal aluminum-gallium-arsenide (AlGaAs). Aluminum-gallium-arsenide is used extensively because it results in very efficient devices and has a characteristic wavelength of around 850 nm, at which wavelength many optical fibers give the lowest attenuation. (Many fibers are even better around 1300 nm, but the materials technology for LEDs at this wavelength---InGaAsP---is still on the front end of the learning curve, making these devices very expensive to produce.) 

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