Fiber-Optic Technologies

Fiber Span Analysis

Span analysis is the calculation and verification of a fiber-optic system's operating characteristics. This encompasses items such as fiber routing, electronics, wavelengths, fiber type, and circuit length. Attenuation and nonlinear considerations are the key parameters for loss-budget analysis. Before implementing or designing a fiber-optic circuit, a span analysis is recommended to make certain the system will work over the proposed link. Both the passive and active components of the circuit have to be included in the loss-budget calculation. Passive loss is made up of fiber loss, connector loss, splice loss, and losses involved with couplers or splitters in the link. Active components are system gain, wavelength, transmitter power, receiver sensitivity, and dynamic range.

Nonlinear effects occur at high bit rates and power levels. These effects must be mitigated using compensators, and a suitable budget allocation must be made during calculations.

The overall span loss, or link budget as it is sometimes called, can be determined by using an optical meter to measure true loss or by computing the loss of system components. The latter method considers the loss associated with span components, such as connectors, splices, patch panels, jumpers, and the optical safety margin. The safety margin sets aside 3 dB to compensate for component aging and repair work in event of fiber cut. Adding all of these factors to make sure their sum total is within the maximum attenuation figure ensures that the system will operate satisfactorily. Allowances must also be made for the type of splice, the age and condition of the fiber, equipment, and the environment (including temperature variations).

Transmitter Launch Power

Power measured in dBm at a particular wavelength generated by the transmitter LED or LD used to launch the signal is known as the transmitter launch power. Generally speaking, the higher the transmitter launch power, the better. However, one must be wary of receiver saturation, which occurs when the received signal has a very high power content and is not within the receiver's dynamic range. If the signal strength is not within the receiver's dynamic range, the receiver cannot decipher the signal and perform an OE conversion. High launch powers can offset attenuation, but they can cause nonlinear effects in the fiber and degrade system performance, especially at high bit rates.

Receiver Sensitivity and Dynamic Range

Receiver sensitivity and dynamic range are the minimum acceptable value of received power needed to achieve an acceptable BER or performance. Receiver sensitivity takes into account power penalties caused by use of a transmitter with worst-case values of extinction ratio, jitter, pulse rise times and fall times, optical return loss, receiver connector degradations, and measurement tolerances. The receiver sensitivity does not include power penalties associated with dispersion or with back reflections from the optical path. These effects are specified separately in the allocation of maximum optical path penalty. Sensitivity usually takes into account worst-case operating and end-of-life (EOL) conditions. Receivers have to cope with optical inputs as high as –5 dBm and as low as –30 dBm. Or stated differently, the receiver needs an optical dynamic range of 25 dB.

Power Budget and Margin Calculations

To ensure that the fiber system has sufficient power for correct operation, you need to calculate the span's power budget, which is the maximum amount of power it can transmit. From a design perspective, worst-case analysis calls for assuming minimum transmitter power and minimum receiver sensitivity. This provides for a margin that compensates for variations of transmitter power and receiver sensitivity levels.

Power budget (PB) = Minimum transmitter power (PTMIN) – Minimum receiver sensitivity (PRMIN)

You can calculate the span losses by adding the various linear and nonlinear losses. Factors that can cause span or link loss include fiber attenuation, splice attenuation, connector attenuation, chromatic dispersion, and other linear and nonlinear losses. Table 3-1 provides typical attenuation characteristics of various kinds of fiber-optic cables. Table 3-2 provides typical insertion losses for various connectors and splices. Table 3-3 provides the margin requirement for nonlinear losses along with their usage criteria. For information about the actual amount of signal loss caused by equipment and other factors, refer to vendor documentation.

Span loss (PS) = (Fiber attenuation * km) + (Splice attenuation * Number of splices) +
(Connector attenuation * Number of connectors) + (In-line device losses) + (Nonlinear losses) + (Safety margin) 

Table 3-1 Typical Fiber-Attenuation Characteristics

Table 3-2 Component Loss Values

Table 3-3 Reference Margin Values

The next calculation involves the power margin (P_M), which represents the amount of power available after subtracting linear and nonlinear span losses (P_S) from the power budget (P_B). A P_M greater than zero indicates that the power budget is sufficient to operate the receiver. The formula for power margin (P_M) is as follows:

Power margin (PM) = Power budget (PB) – Span loss (PS)

To prevent receiver saturation, the input power received by the receiver, after the signal has undergone span loss, must not exceed the maximum receiver sensitivity specification (P_RMAX). This signal level is denoted as (P_IN). The maximum transmitter power (P_TMAX) must be considered as the launch power for this calculation. The span loss (P_S) remains constant.

Input power (PIN) = Maximum transmitter power (PTMAX) – Span loss (PS)

The design equation

Input power (PIN) <= Maximum receiver sensitivity (PRMAX)

must be satisfied to prevent receiver saturation and ensure system viability. If the input power (P_IN) is greater than the maximum receiver sensitivity (P_RMAX), passive attenuation must be considered to reduce signal level and bring it within the dynamic range of the receiver.

Case 1: MMF Span Analysis

Consider the fiber-optic system shown in Figure 3-21 operating at OC-3 (155 Mbps). The minimum optical transmitter launch power is –12.5 dBm, and the maximum optical transmitter launch power is –2 dBm at 1310 nm. The minimum receiver sensitivity is –30 dBm, and the maximum receiver sensitivity is –3 dBm at 1310 nm. The example assumes inclusion of two patch panels in the path, two mechanical splices, with the system operating over 2 km of graded index 50/125-m multimode fiber-optic cable. Refer to Tables 3-1, 3-2, and 3-3 for appropriate attenuation, component, and nonlinear loss values.

Figure 3-21 MMF Span Analysis

The system operates at 155 Mbps or approximately 155 MHz. At such bit rates, there is no need to consider SPM, PMD, or SRS/SBS margin requirements. Because the link is a single-wavelength system, there is no need to include XPM or FWM margins. However, it is safe to consider the potential for a degree of chromatic dispersion, because chromatic dispersion occurs at all bit rates. The span analysis and viability calculations over the link are computed as follows.

Power margin (PM) = Power budget (PB) – Span loss (PS)
PM = 17.5 dB – 11.4 dB
PM = 6.1 dB > 0 dB

In the preceding example, notice that the 11.4-dB total span loss is well within the 17.5-dB power budget or maximum allowable loss over the span.

To prevent receiver saturation, the input power received by the receiver, after the signal has undergone span loss, must not exceed the maximum receiver sensitivity specification (P_RMAX). This signal level is denoted as (P_IN). The maximum transmitter power (P_TMAX) must be considered as the launch power for this calculation. The span loss (P_S) remains constant.

Input power (PIN) = Maximum transmitter power (PTMAX) – Span loss (PS)
PIN = –2 – 11.4
PIN = –13.4 dBm
–13.4 dBm (PIN) <= –3 dBm

This satisfies the receiver sensitivity design equation and ensures viability of the optical system at an OC-3 rate over 2 km without the need for amplification or attenuation.

Case 2: SMF Span Analysis

Consider the fiber-optic system in Figure 3-22 operating at OC-192 (9.953 Gbps). The minimum optical transmitter launch power is –7.5 dBm, and the maximum optical transmitter launch power is 0 dBm at 1550 nm. The minimum receiver sensitivity is –30 dBm, and the maximum receiver sensitivity is –3 dBm at 1550 nm. The example assumes inclusion of two patch panels in the path, four fusion splices, with the system operating over 25 km of step index 8.1/125-m SMF cable. Refer to Tables 3-1, 3-2, and 3-3 for appropriate attenuation, component, and nonlinear loss values.

Figure 3-22 SMF Link-Budget Example

The system is operating at 9.953 Gbps or approximately 10 GHz. At such high bit rates, SPM, PMD, and SRS/SBS margin requirements must be taken into consideration. Also consider the potential for a degree of chromatic dispersion. Because the link is a single-wavelength system, there is no need to include XPM or FWM margins. The link loss and viability calculations over the link are computed as follows.

Power margin (PM) = Power budget (PB) – Span loss (PS)
PM = 22.5 dB – 20.66 dB
PM = 1.84 dB > 0 dB

In the example, notice that the 20.66-dB total span loss is well within the 22.5-dB power budget or maximum allowable loss over the span. To prevent receiver saturation, the input power received by the receiver, after the signal has undergone span loss, must not exceed the maximum receiver sensitivity specification (P_RMAX). This signal level is denoted as (P_IN). The maximum transmitter power (P_TMAX) must be considered as the launch power for this calculation. The span loss (P_S) remains constant.

Input power (PIN) = Maximum transmitter power (PTMAX) – Span loss (PS)
PIN = 0 – 20.66 dBm
PIN = –20.66 dBm
–20.66 dBm (PIN) <= –3 dBm (PRMAX)

This satisfies the receiver sensitivity design equation and ensures viability of the optical system at an OC-192 rate over 50 km without the need for amplification or attenuation. Note, however, that this example has not considered dispersion calculations or dispersion compensation. Dispersion compensation units insert their own loss component into the overall span.