The EMPF is working on a project to replace shipboard Ethernet cables with a high speed fiber optic network. Rather than point-to-point connections from one shipboard location to another, all the Ethernet communication signals from each location will be combined onto one fiber and transmitted throughout the ship. This will eliminate a large number of cables and their associated weight and costs. The fiber optic network will be easier to install, maintain, and will provide higher bandwidth, growth capability, and reconfiguration flexibility.

This project is focused on the manufacturing challenges of combining both electrical and optical components on printed circuit board (PCB) assemblies associated with wavelength-division multiplexing (WDM) technology. Current manufacturing processes for PCB assemblies with optical components are costly and labor intensive due to the attached fiber pigtails. Manufacturing methods are being developed to automate the attachment of these electro-optical components and the soldering of all electrical and optical components on the PCB. These manufacturing developments will significantly reduce assembly time and increase reliability of the electro-optical assemblies.

Fiber optics is the technology of transmitting information down thin strands of transparent fiber using pulses of light. It began in the 1970s in R&D laboratories, but by the early 1980s, major cities across the country were connected with a network of fiber.

In general, fiber optic communication systems have many advantages over copper. In copper based Ethernet networks, loss increases with signal frequency (Figure 1-1). Higher data rates increase power loss and therefore decrease transmission distances. Optical fiber signal loss does not change with signal frequency. Data can be transmitted much faster and much further in fiber optic networks than in copper. Fiber cable is also much smaller in diameter (Figure 1-2) and weighs less than a similar copper cable.

Since fiber signals are optical rather than electronic, they are not affected by electromagnetic interference (EMI) or radio frequency interference (RFI). This allows information transmission with less noise, error, and crosstalk and makes them ideal for placement near electronic devices that can cause RFI and EMI disruption in copper networks. They also offer a higher degree of security since fiber optics do not radiate electromagnetic signals.

Fiber optics are safe for high voltage areas. Since they are insulators, devices can be connected of different electrical potentials without arcing.

The biggest advantage is that more information can be carried over longer distances in the least time with fiber optics than any other system. Using fewer cables, fewer repeaters, less power, and less maintenance, fiber optics is the most cost effective choice for data transmission.

Multiplexing

Fiber optic communication occurs by first converting an electrical signal into a modulated light beam. This is done by either directly modulating the input power of a laser or light emitting diode (LED) or by changing the intensity of the beam after leaving the source. Next, the signal is relayed along the optical fiber while maintaining signal strength and accuracy. Finally, the signal is received and converted back to an electrical signal.

Similar to other communication systems, optical signals are often combined or multiplexed to take advantage of the huge capacity of the fiber. Three types of multiplexing can be used: directional, time-division, and wavelength-division.

While signals can be sent in opposite directions in the same fiber, directional multiplexing works best when different wavelengths are used to reduce interference.

By combining data from several different signals into a timed sequence, a time-division multiplexed signal can be formed carrying interleaved data. As shown in Figure 1-3, four 10 Mbit/s signals can be combined into a single channel carrying 40 Mbit/s of data. By decreasing the width of the incoming pulses more signals can share the output fiber, increasing the information carrying capacity (bandwidth) of the fiber. The receiving end of the fiber uses a demultiplexer to sort the samples into their original form so the information can be recovered.

Analogous to the frequency-division multiplexing used to electronically combine many television channels onto one coaxial cable, wavelength-division multiplexing combines optical signals of different wavelengths onto a single fiber. Figure 1-4 shows how inputs from four fibers can be combined using multiple wavelengths. In this example, 10 nanometer spacing was used, but using a dense wavelength-division multiplexing (DWDM) system design, wavelengths can be as close as a few nanometers before interference occurs. Typically, 100 GHz frequency spacing is used between channels or 0.8 nm wavelength difference. Starting at 1550.0 nm, the next two channels would be 1549.2 nm and 1548.4 nm. The relation between wavelength and frequency is determined by the following formula.

wavelength = c / frequency
l = c / n

where: l = wavelength in meters
c = the speed of light = 3x108 m/s
n = frequency in Hertz (cycles per sec)

In a WDM system, each wavelength is modulated separately with its own transmitter and receiver. The example shown in Figure 1-4 would require four transmitters and four receivers which can be packaged together into a single WDM transmitter and a single WDM receiver (using a single fiber between them). The speed of the modulation (the on or off cycle of a light pulse) determines the data rate. The light intensity can be varied by directly driving an LED or laser output. To achieve higher speeds (10 to 40 Gbits/sec), an external modulator is used to essentially chop a steady light beam using an interference phenomenon.

By using a combination of time-division multiplexing and wavelength-division multiplexing, a variety of signals of different data rates can be combined onto a single fiber for high speed transmission lines. The EMPF is working on a project to replace shipboard Ethernet cables with a high speed fiber optic network. Rather than point-to-point connections from one shipboard location to another, all the Ethernet communication signals from each location will be combined onto one fiber and transmitted throughout the ship. This will eliminate a large number of cables and their associated weight and costs. The fiber optic network will be easier to install, maintain, and will provide higher bandwidth, growth capability, and reconfiguration flexibility.

For more information on wavelength-division multiplexing and electro-optic assembly, please contact the EMPF at 610.362.1320, via email at helpline@empf.org or visit the website at www.empf.org.

References

• Hecht, Jeff. Understanding Fiber Optics. Upper Saddle River, NJ: Prentice Hall, 1999. Print.
• "The FOA Reference For Fiber Optics - Fiber To The Home." The Fiber Optic Association. Web. http://www.thefoa.org/tech/ref/appln/FTTH.html.
• Crisp, John. Introduction to Fiber Optics. Oxford: Newnes, 1996. Print. References