The first step in any fiber optic system design requires making careful decisions based on operating parameters that apply for each component of a fiber optic transmission system. The main questions, given in the table below, involve data rates and bit error rates in digital systems, bandwidth, linearity, and signal-to-noise ratios in analog systems, and in all systems, transmission distances. These questions of how far, how good, and how fast define the basic application constraints.
| Table 1 - System Design Considerations | |
| System Factor | Considerations/Choices |
| Transmission Distance | System Complexity Increases with Transmission Distance |
| Types of Optical Fiber | Single-mode or Multimode |
| Dispersion | Incorporate Signal Regenerators or Dispersion Compensation |
| Fiber Nonlinearities | Fiber Characteristics, Wavelengths, and Transmitter Power |
| Operating Wavelength | 780, 850, 1310, 1550, and 1625 nm Typical |
| Transmitter Power | Typically Expressed in dBm |
| Source Type | LED or Laser |
| Receiver Sensitivity/Overload Characteristics | Typically Expressed in dBm |
| Detector Type | PIN Diode, APD, or IDP |
| Modulation Code | AM, FM, PCM, or Digital |
| Bit Error Rate (Digital Systems Only) | 10-9, 10-12 Typical |
| Signal-to-Noise Ratio | Specified in Decibels (dB) |
| Number of Connectors or Splices in the System | Signal Loss Increases with the Number of Connectors or Splices |
| Environmental Requirements & Limitations | Humidity, Temperature, Exposure for Sunlight |
| Mechanical Requirements | Flammability, Indoor/Outdoor Application |
All of these considerations are inter-related, and transmission distance is the predominant consideration. Transmission distance affects the strength of the transmitter output, which dictates the type of light source used. It impacts fiber type, as single-mode fiber is better suited to long distance transmission. Transmitter and fiber type dictate receiver type and sensitivity. Transmission distance also dictates the modulation scheme as some are better for longer distances than others. While designing a system can be complex, several techniques simplify this process. One such technique is used to determine the link's optical loss budget, which evaluates the transmitter output power, the operating wavelength, fiber attenuation, fiber bandwidth, and receiver optical sensitivity. This process is described at length in the article, Optical Link Loss Budget. Another technique determines the link's rise time budget, which describes the transmission device's ability to turn on and off fast enough. A sensitivity analysis determines the minimum optical power that must be received in order to achieve the required system performance. The receiver sensitivity can be affected by source intensity noise, inherent to the light source being used, fiber noise, inherent to the optical fiber, receiver noise, inherent in the detector used, time jitter, intersymbol interference, and bit error rate. Environmental considerations must be made. Temperature affects the performance of LEDs and lasers as well as the optical fiber itself. Building installations will generally require safety testing for fire safety, EMI radiation, or other parameter specific to the application environment. Certain environments present more hazards for fiber optic systems than others, which may impact the type of cable that can be specified. A good system design must consider these factors. The cost of a fiber optic transmission system can also be a critical consideration. Component considerations such as light emitter type, emitter wavelength, connector type, fiber type, and detector type will have an impact on both the cost and performance of a system. Common sense goes a long way in designing the most cost-effective system to meet an application's requirements. A properly engineered system is one that meets the required performance limits and margins with little extra. Excess performance capability often means the system costs too much for the specific application. Fortunately, there's no need to design a system on your own. Once you've determined your need for fiber and the basic system requirements, a sales engineer or applications engineer can step you through the technical details. Some common questions you'll be expected to answer include: 1. What is the fiber loss for your system? This is not the same as optical loss; it refers to the bandwidth•distance product which describes how much optical attenuation occurs over a certain length of fiber. If the system is previously installed and is being upgraded, this information is probably readily available. If the installation is new, knowing the transmission distance (i.e. the distance between the transmitter and the receiver) can help an applications engineer calculate the fiber loss. The fiber loss will determine transmitter optical output requirements and/or the inclusion of regenerators in the fiber path. 2. What type of signals do you wish to transmit? This includes video signals, audio signals, data signals, and also indicates whether or not the signal will be digital or analog. 3. What type of fiber will be used? As Table 1 notes, the choices are multimode or single-mode. Transmission distance, signal type and application will predetermine the best fiber type. Typically long distance, high speed, or multichannel transmission require single-mode fiber, while short distance, low speed, and single channel transmission will allow the use of less expensive multimode fiber. 4. What optical connectors will be used? As with fiber type, different systems will have different requirements. Connectors may be specified to reduce backreflection, increase ease of installation, meet dense packaging requirements, or interface with connectors in an existing system. 5. What quality is expected at the receive end? This usually refers to video quality, and while it may seem obvious to answer, "the best quality," the helpful answers include: surveillance quality, high quality, broadcast quality, studio quality, etc. The required video quality can impact fiber type and required electronics. 6. What configuration will the system require? This generally refers to the topology of the system, which may be point-to-point, ring, or fanout. In broadcast networks, configurations also include add/drop/repeat topologies. A fiber optic system checklist is available in Adobe Acrobat PDF format by clicking here. This list provides a means to specify these and many other details of a fiber optic system design.
