| System Design Procedure |
System Analysis
The system designers must proceed through the following five steps in order to develop a fiber optic communication system:
1. |
Specify the system's operational requirements. |
2. |
Describe the physical and environmental requirements. |
3. |
Compute the signal optical power budget. |
4. |
Perform a signal bandwidth analysis. |
5. |
Review the system design. |
Important considerations in these steps of the design process are detailed below.
Figure 18 - Considerations in developing a fiber optic system.
System Operational Requirements (Step 1 System Operational Requirements (Step 1 )
The system design process begins with a determination of the signal-to-noise ratio which depends on the bandwidth or data rate for an application. This implies a choice of signal types, either analog or digital, since even a simple point-to-point link will employ appropriate hardware. The goal is to establish what optical power level will be required at the optical detector inside the receiver unit.
Figure 19 - Fiber Optic system for analog or digital transmission.
Fiber can handle either analog or digital transmission and it offers the additional option of future upgrading by simply changing the electronics hardware at the transmitter and receiver ends. For this reason most fiber system designers specify more fiber bandwidth capacity than is minimally required.
Analog Signals
Analog signals such as video and audio can directly modulate optical output by causing the optical emitter to brighten and dim. This is called intensity modulation and is a simple and straightforward method of encoding lightwave signals.
Improvements in both signal-to-noise and linearity can be obtained by the use of frequency modulation (FM) techniques.
Here the information source is used to frequency modulate a subcarrier, then this signal is used to intensity modulate an LED or a laser. Because of material and intermodal dispersion factors, FM links normally require fibers with bandwidths of 200 MHz-km and higher. Short unrepeatered links are occasionally analog modulated. However most lightwave applications today use digital transmission with simple on-off modulation.
Digital Signals
In fiber optics, a digital pulse can be formed by turning the source "on" for a brief instant. The time of optical radiation emission is the pulse. A binary "1" state can be used to represent optical power turned "on", while a binary "0" state is used to represent "off". These two states represent binary signals. Digital signals consist of a series of bits that result in the emitter being "on" or "off".
Figure 20 - Each pulse represents a bit in digital transmission and the rise and fall times of a series becomes the bit rate.
The time it takes for a pulse to reach full amplitude is the rise time. Faster rise and fall times allow more pulses per second, consequently more bits of information can be transmitted.
In digital systems one parameter for system performance is bit error rate (BER). The majority of digital systems achieve a BER of 1 X 109 (1 error in 109 bits = 1 error in 1,000,000,000 bits).
There is a length dependence with digital systems because the farther a pulse has to travel down a fiber the more distortion occurs. The resulting optical power level required at the detector is a function of the data rate or bandwidth. These levels for digital and analog signals are indicated for silicon detectors at 850 nm below.
Figure 21 - Average optical power required by digital or analog systems.
Once the application (TV, telephone, or computer), the type of signals (analog, digital), and the data rate have been determined, the next step is to describe the physical layout and environmental requirements.
System Layout (Step 2)
To determine the components necessary to complete a fiber optic system requires detailing run lengths and determining system operating environments.
A simple point-to-point system or a more elaborate local area network involving telephone, data, video, control and alarm functions are both common-place installations for fiber optic cable. Current fiber optic technology employs a separate fiber to transmit the signals in one direction.
Figure 22 - View of cable assembly hook-ups in a simple fiber optic link.
Figure 23 - A wiring diagram for a fiber system incorporating telephone, computer and video links.
Therefore most point-to-point systems will require at least two fibers for duplex communications. Higher fiber count cables are also readily available.
The system designer should develop a layout schematic similar to the one shown below and use the resulting information on the worksheets at the back of this section.
Figure 24 - A fiber optic layout should detail distances between each fiber segment.
Signal Optical Power Budget (Step Signal Optical Power Budget (Step 3)
With the system layout and components known, it's now possible for the designer to compute expected losses at each point in the system.
Figure 25 - An Optical Link Power Analysis is done for each T/R pair.
Every component including fiber has a range of optical loss due to variations in manufacturer. An LED device, for example, will be specified with a minimum, average, and maximum optical output power. The range may be as much as 4 dB (60%).
Detectors also have sensitivity ranges. It is up to the system designer to determine the optical power necessary at the detector surface from information supplied by the manufacturer.
Once the receiver and transmitter power levels have been established it is possible to consider the power transmitted by various cable lengths. This can be seen by plotting the power on a diagram.
Figure 26 - Use stated receiver.
Figure27 - Fiber distances and transmitter ranges to determine compared for two core sizes: fiber distances (e.g. 10Mb/s transmitter 50 micron and 100 micron at 850 nm with 100 micron core fiber). 850 nm wavelength and 10 Mb/s.
In the example shown, a fiber with a 100 micron core has been analyzed for use with a 10 Mb/s transmitter at the 850 nanometer wavelength. Both the best and the worst case curves are shown with the average expected range in between.
The detector sensitivity upper and lower limits are also shown. This figure indicates that a transmission distance of about 1.4 km is maximum.
Starting power levels vary due to the emitter launch range. When taps and splices are included, their values can be considered as part of the launch loss, or displayed where they might occur in the system.
Figure 28 - Typical optical power level in system with a tap and a splice.
Use either peak or average optical power values for determining attenuation throughout the system. Be consistent in your choice throughout the system analysis.
Power coupled to various fiber types by a few typical source emitters is detailed in Figure 29. Allow approximately 4 to 6 dB to account for thermal variations in the optical fiber, repair of damaged cables, and source degradation over time.
Figure 29 - Optical source-to-fiver power coupling chart for various emitters.
Fiber selection
The various fiber properties such as attenuation, numerical aperture (NA), core diameter have all been covered earlier in this section. NA and core diameter must be considered for launch conditions. All fibers can be compared over one kilometer lengths for fiber properties and relative optical power.
Figure 30 - Operating properties for various fiber types.
| Bandwidth | Relative Collection Factor (dB)¹ |
Relative Optical Power (dB) at 1 km² |
|||
Material Structure |
Type |
Core Dia. Micron (µm) |
Numerical Aperture |
||
Silica |
Single Mode |
10µm |
0.11 |
-28.4 |
-23.9 |
Silica |
Multimode |
50 |
0.20 |
-9.25 |
-7.25 |
Silica |
Multimode |
62.5 |
0.275 |
-4.54 |
-2.55 |
Silica |
Multimode |
100 |
0.29 |
0.0 |
0.0 |
PCS* |
Multimode |
200 |
0.27 |
+5.4 |
+3.4 |
*Plastic-Clad Silica
¹ Relative amount of radiation coupled to fiber based on 1 km length NA value. Shorter lengths may have higher values.
² Based on the difference in transmission over a 1 km length of cable using the 100 micron core fiber at 5 db/km (850 nm) as the basis for normalization.
³ Primary use at 1300 or 1550 nm.
Table 7 - Optical power comparison for various fiber types.
Certain fiber types have proven suitable for special applications.
Choices for most LAN or data systems, for example, currently centers on the all-silica fibers. Here various core/cladding constructions are available with tradeoffs in performance, cost, and standardization. Currently 3 sizes are most often considered:
| Core |
Cladding |
Bandwidth |
||
850 |
1300 |
|||
50 |
125 |
500 |
500 |
|
62.5 |
125 |
160 |
500 |
|
100 |
140 |
100 |
200 |
|
Video and CATV systems often employ 50/125 and single mode fibers because of their high bandwidth and low loss performance characteristics. Modern intercity telephone trunks also employ single mode fibers.
Fibers may be selected in a variety of bandwidths and attenuation, in either one or two window versions. Again, attenuation of optical fibers will vary depending on the source wavelength of the transmitter. A fiber cable loss table for Belden products is shown below.
Material Structure |
Core Dia. Micron (µm) |
Numerical Aperture |
Attn.* dB/km |
Bandwidth MHz/km |
Silica |
50 |
0.200 |
3.0 |
500 |
Silica |
62.5 |
0.275 |
3.5 |
160 |
Silica |
100 |
0.290 |
5.0 |
100 |
PCS |
200 |
0.270 |
7.0 |
20 |
* Values for 850nm wavelength.
Table 8 - Belden Optical Fiber Cable Performance.
Bandwidth Analysis (Step 4)
While attenuation is one major determinant in fiber optic system performance, bandwidth is the other. Here the goal is to assure that all components have sufficient bandwidth to transmit the required signal. Local area networks typically require 20 to 600 MHz-km fiber bandwidth. On the other hand, long-haul telephone systems employ large distances between repeaters and require the 100,000 MHz-km fiber bandwidths associated with single mode fiber.
A fiber has a 3dB (half power) optical signal magnitude decrease at the bandwidth specified for that fiber. Conversion between electrical and optical bandwidth for the system or any component such as a fiber, receiver, or transmitter unit is performed by using:
BW optical = 1.41 BW electrical
In some cases a receiver or transmitter manufacturer will specify risetimes. The electrical bandwidth (BW in MHz) for a component is related to its 10%--90% risetime (t in nanoseconds) by:
BW = 350/t
And the total system electrical bandwidth is obtained from individual component bandwidth by:
where BWR, BWC and BWT are the electrical bandwidth of the receiver, cable and transmitter respectively.
For digital systems the system bandwidth will depend on the data rate (R in bits per second) and the coding format according to:
BW system = R/K
Where K equals 1.4 for a non-return-to-zero (NRZ) coding format and 1.0 for a return-to-zero (RZ) format.
The system bandwidth is limited by the lowest bandwidth component in the link. When high bandwidth fiber is used, for example, the system frequency response may be more influenced by the terminal equipment than the fiber.
A general guideline in selecting the terminal equipment is to choose a receiver with a bandwidth equal to or greater than the required system bandwidth. The transmitter and optical fiber should then have bandwidths about 1.5 to 2 times greater than the receiver.
Again, systems are usually more cost effective at higher data rates. And allowing for more fiber bandwidth than is minimally required, for example, allows system capacity to be upgraded later. Care should be taken in estimating the optical bandwidth in MHz-km of series connected cable runs with lengths greater than a kilometer.
The approximate relationship between the total cable bandwidth (BWCo) and one kilometer section fiber bandwidth (BWf)
BWf = BWCo (L) x
L is the fiber length in kilometers. The x equals 1.0 for cable run lengths (L) of one kilometer or less. And x equals 0.75 for fiber in cable run lengths greater than 1 kilometer.
Here the 
terms are
individually calculated and then combined in a series of steps to yield the total system
bandwidth.
System Review (Step 5)
Now is the time for the system designer to review all of the pieces to determine that all work together to deliver the right signal to the right place at the right time.
The number of fibers or a cable depends on the number of channels or signal carrying capacity desired. Cables employing fibers with special high bandwidths are available as custom products.
The complete cable structure can be established using the following criteria:
Specific materials and multi-fiber construction have resulted in numerous cable designs
which incorporate a variety of fibers to meet specific applications. Hybrid designs having
both optical fibers and metallic conductors are also part of the Belden
Hopefully this guide will permit the identification and description of a useful fiber optic system. Due to advancing technology and extensive tradeoffs, system design is constantly changing. This guide is based on currently available components. To keep abreast of changes, ask questions, or to request design assistance, contact Belden's local sales representative or the regional offices.