Fiber Optics



Introduction

Fiber optic technology has become increasingly more popular in recent years, and is used primarily for real time data communications. Before fiber optics came along, the primary means of communicating data was in the form of electronic signals transmitted on copper wire, or electromagnetic radio waves that traveled through the air. Although these long proven technologies are effective and still widely used today, advancements in fiber optics has proven this technology to be a very reliable and appealing alternative to traditional means of data transmission. Some of the advantages of fiber optic technology include:

  • Larger information carrying capacity (bandwidth)
  • Greater transmission distance
  • Immunity to EMI and RFI interference
  • Much thinner and lighter
  • Lower attenuation (signal loss)

Of course, as is true of any technology, fiber optic technology does have a few disadvantages. For instance, installation can be more expensive, and the cable is more fragile and harder to split than copper. Nevertheless, the advantages offered by this technology far outweigh these installation challenges. So much so that telephone companies have steadily been replacing their traditional telephone lines with fiber optic cabling. This is also true of cable companies, enterprise networks and even many LANs (local area networks). In fact, many believe that almost all communications in the future will employ fiber optics. How, though, does fiber optic technology work?

How Fiber Works

Fiber optic technology is based on the use of light energy to transmit data. Basically, the encoded data is converted from electrical signals to optical light pulses and then transmitted through the medium to its destination, where it is then converted back. From this, we can see that there are basically three main elements in any fiber optic data link: a transmitter, an optical cable (the transmission medium), and a receiver. The transmitter handles the conversion from electrical to light energy, the optical cable carries the light waves, and the receiver handles the conversion from light pulses back to the original electrical format.

After translating the electrical signals, the transmitter uses either a light emitting diode (LED) or an injection laser diode (ILD) to generate the light pulses. Using a lens, this light energy is then sent down the fiber optic cable. The principle that makes this possible is referred to as total internal reflection. According to John Huber in an article in R&D Magazine, “this principle of total internal reflection states that when the angle of incidence exceeds a critical value, light cannot get out of the glass; instead, the light bounces back in” (Huber 115). This happens when two materials with different refractive indices cause the angle of incidence to be too large for refraction (bending) of light to take place. Since the light cannot be bent and exit the material, this means that 100 percent is reflected back. Thus, when a fiber optic cable, which consists of a glass or plastic core surrounded by a cladding with a lower refractive index, receives a light ray, the light ray is confined and travels down the core to the receiving end. Simply put, the difference in the materials used for the core and the cladding make an extremely reflective surface at the point where they interface, which makes the principle of total internal reflection possible. This is the fundamental concept behind all fiber optic transmissions. (See Figure 1 below.)

Figure 1 (Source: Adva Optical Networking)

In addition to the core and the cladding, a fiber optic cable also has an outer jacket that protects it from abrasion and other forces. Most high end cabling will also have a protective buffer and strength material between the cladding and the outer jacket. These outer layers are added to help protect the fragile core and cladding from damage. There are two common types of cabling used for most fiber optic applications: single-mode and multi-mode. Single-mode fiber is generally used for long distance communications. It has a narrower core diameter, generally 8-10 microns, with a 125-micron cladding. Single-mode optical fiber only allows one mode of light to travel down its core. On the other hand, multi-mode fiber generally has a 62.5-micron core diameter, with a 125-micron cladding. Multi-mode optical fiber allows multiple rays of light to travel down the core simultaneously; however, it can only be used where transmission distances are less than two kilometers, whereas single-mode can reach distances of three kilometers. Although it may appear that multi-mode cabling would offer a higher bandwidth, the opposite is actually true. Because single-mode retains the integrity of each light pulse, it has a larger information carrying capacity. (See Figure 2 below.)

Figure 2 (Source: Lascomm Fiber Optics)

In order to receive the signal and then convert it back to its original format, a fiber optic receiver uses a phototransistor to convert the light energy into an electrical current. This current is then sent into an amplifier in order to boost the electrical signal back to its original level, and then a digitizer circuit is used to convert the signal into the appropriate digital voltage levels to be used by the external logic. At this point, the electronic signal is ready to be received by the communications device, whether it is a switch, router, computer, etc.

Fiber Optics Lab

In order to better understand how this works, I performed a fiber optics communications lab. To accomplish this, I used the Radiant Energy in Action Educational Communication Kit by Industrial Fiber Optics, Inc. The lab consisted of the following items:

  • Printed wiring board, w/ both transmitter and receiver (see Figure 3)
  • All necessary electronic circuit components
  • 1 meter of 1000-micron plastic fiber (980-micron core/10-micron cladding)

Figure 3

After making sure that everything on the parts list was in the kit, I began construction of the printed wiring board. First, I soldered the resistors, transistors, capacitors, ICs, LED and phototransistor components to the circuit board. It was also necessary to bridge the +5V and GND connections between the Transmitter and Receiver sections. This would allow the circuit to be powered by a single power supply, instead of one for each side of the circuit. Then I prepared the fiber optic cable by cutting both ends to make sure that it had a clean edge, and then I polished the ends using the 600 grit polishing paper. This was done to ensure that the fiber would transmit light effectively.

After accomplishing all of the above, it was time to hook up the circuit and test it. In order to do this, it was necessary to have the following equipment: an oscilloscope, function generator, and DC power supply.

The DC power supply was set to +5V, and connected to the positive and negative terminals of the Transmitter section only (the bridged connections powered the Receiver section). Next, the Function Generator was set to output square wave on its TTL output, and it was then attached to the EXT and EN connections. I set the initial frequency to about 100 Hz. Lastly, I attached a probe to each channel on the oscilloscope (dual-trace) and then set the horizontal time scales to .2 ms/division and the vertical scales to 2V/division. The probe on channel 1 was attached to TP1 on the transmitter circuit and the probe on channel 2 was attached to TP3 on the receiver circuit.

At this point, a signal could be seen on the oscilloscope. Channel 1 (TP1) displayed a square wave signal that measured about 5V in amplitude and had a frequency of about 100 Hz. The square wave on channel 2 (TP3) was approximately the same in amplitude and frequency. This demonstrated that the circuit was operating correctly, and that the signal being transmitted through the fiber optic cable was being received properly and accurately.

Next, I began increasing the frequency output of the generator to find the lowest and highest acceptable range for this circuit. I started at about 100 Hz and gradually increased the frequency, adjusting the horizontal and vertical divisions on the scope as necessary in order to view the signal. The output and received signals maintained accuracy up to about 3 kHz. After that, the signal received at TP3 (channel 2 on the scope) began to suffer. Not only did the amplitude begin to decrease, but also the waveform on TP3 no longer resembled the square wave on TP1. From this, I drew the conclusion that I was no longer within the acceptable frequency range for this particular circuit, and as a result, the signal being received at TP3 was no longer accurate. I continued to observe this up to about 25 kHz, at which point the signal being received on TP3 was nothing more than a flat line, although TP1 was still outputting a solid 5V square wave. I plotted the voltage at TP3 from 104.6 Hz to 25 kHz, and as can be seen in from the chart, the voltage received gradually tapered off to the point, at 25 kHz, where a signal was no longer being received at all. This demonstrates that it is necessary for the transmitter to output light energy that is within the acceptable specifications of the receiver; otherwise, the signal will not be accurately received. The wavelength of light is measured in nanometers, which relates to frequency. A longer wavelength means a lower frequency, and a shorter wavelength means a higher frequency. The ideal wavelengths that are commonly used for most multi-mode fiber communications is either 850 or 1300 nm, and for single-mode fiber it is either 1300 or 1550 nm.

Conclusion

As the need for more and more bandwidth and for communications to span even greater distances, it is obvious that fiber optic communications offer many advantages over traditional copper cabling. In addition to the telephone and cable companies, it is very likely that even smaller companies will employ fiber optics in the future. It is not so much a choice of whether or not to convert to fiber optics, but rather it is a question of when. Although wireless communications will also continue to play a large role, when it comes to a physical medium for transmitting data, fiber optics are obviously superior to traditional copper cabling. Understanding how fiber optics works provides the necessary foundation for us to embrace and implement this leading edge technology.

ComTest Technologies can assist you with your Fiber Optic needs. Just visit some of the Additional Resources below, or call (808) 831-0601.


Additional Resources

Tutorials on WDM, FSP @ Adva Optical
More white papers at Larscom
FAQ at IMC Networks

References

This page was written by Will Twiggs, an associate with ComTest Technologies, Inc. For more information or if you have questions about this material, you can contact the author at william@comtest.com

“Fiber Optic Technology,” Adva Optical, http://www.advaoptical.com.

“Fiber Optics Basics,” Lascomm Fiber Optics Tutorial, http://www.lascomm.com/tutorial.htm.

Huber, John C., "Getting Down to the Basics of Fiber-Optic Transmission," R&D Magazine, February 1994, pp. 115 - 118.

Macy, Terry, “Fiber Optics Basics,” http://oak.cats.ohiou.edu/~sl302186/fiber.html.


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Last updated on 02/20/2001