Understanding Optical Power Budget
Optical fiber communications are celebrated for their reliability and high bandwidth capabilities. However, as light signals travel through a medium, they inevitably diminish in power due to scattering and absorption, a phenomenon known as attenuation. This loss of signal strength is measured in decibels per milliwatt (dBm) or decibels per kilometer (dB/km). Various sources of signal loss occur in an optical channel, each potentially affecting the quality and efficiency of the communication link.
Page Contents
- Explaining Optical Power Budget
- Components of Optical Power Budget: Sources of Signal Loss
- Calculating Optical Loss Budget
- Challenges to Calculating Optical Loss Budget
- Takeaways
Explaining Optical Power Budget
An optical power budget (OPB), or the estimated attenuation of an optical line, is a critical figure that accounts for all potential sources of signal loss. It helps determine the amount of signal power that can be lost or absorbed during transmission through an optical channel.
The OPB is typically calculated during the design phase of a proposed fiber optic line and guides the selection of equipment for the future communication channel.
Components of Optical Power Budget: Sources of Signal Loss
The OPB is calculated based on the length of the fiber optic line and is measured in decibels (dB), reflecting the total signal loss in the optical system. It primarily includes the following sources of loss:
- Losses in the optical fiber: These linear losses can arise from various physical processes such as the scattering and absorption of light by the fiber;
- Connection losses: Joining two fibers can lead to additional signal loss due to scattering and reflection;
- Splitter losses: Splitters in optical networks can cause signal loss when dividing the signal into multiple paths;
- Transmitter output losses: Optical transmitters can also incur certain signal losses when generating the optical signal.
Calculating Optical Loss Budget
Attenuation over a specific distance can be measured by emitting a reference light source from one end of the path (for example, with a power of 0 dBm) and receiving such light signal at the other end with a power meter (for example, -15 dBm). The calculation is then made by subtracting one value from the other: 0 - (-15) = 15 dBm.
However, the obtained value of 15 dBm is relative. For example, to attain 15 dBm of attenuation at a wavelength of 1310 nm in lab tests, as much as 41 km of fiber is required. In practical scenarios, such as a communication line laid through marshy areas with bends with a radius of 10 cm and poor installation conditions, this attenuation limit of 15 dBm could be reached in just 10 km of fiber.
Now let's consider the situation from a different perspective, specifically from that of the transceiver. On closer examination of this electronic component, we see that it consists of two main parts: a receiver and a transmitter. Each of these components has its specifications: for the transmitter, it is the emission power, and for the receiver, it is the detection power.
Measurements on an optical line can be performed using an optical time-domain reflectometer, which has a dynamic range not less than the calculated attenuation of the line. To determine if measurements can be conducted with an optical tester, the receiver's sensitivity (the minimum power it can detect) should be subtracted from the transmitter's power. Thus, the OPB of the transceiver is determined by subtracting the detection power from the emission power.
All transmitters and receivers are meticulously selected by communications service providers based on statistical data about existing communication lines. However, there are instances when the statistics do not match reality.
Challenges to Calculating Optical Loss Budget
Let's examine a tangible case. Suppose we have a 40 km communication line. The total attenuation of this line is, say, 25 dBm at a wavelength of 1490 nm. Additionally, we have a pair of 40 km SFP WDM modules, carefully selected by the telecom provider based on statistics that indicated a 22 dBm attenuation margin would be more than sufficient for 40 km of fiber with all its welds and connections.
However, after the modules have been connected, we find out that the network fails to launch. Why does this happen? Because the line's attenuation exceeds the module's OPB by a full 3 dBm: 25 - 22 = 3 dBm. How do we address this problem? One solution is to install transceivers designed for distances of 60 km or 80 km (depending on availability), as the manufacturer has allocated a larger OPB for them.
On top of that, calculations must also account for an operational reserve for any possible additional connections and splices during maintenance works, as well as at least a minimal reserve for the natural aging of optical fiber over time. It is common practice to allocate a power budget margin of 1.5 – 2 dB after the complete connection of the entire communication line from the provider's point to the subscriber's point.
Takeaways
At its core, the OPB represents the sum of all losses that occur within a specific segment or across the entire optical fiber network. Calculating the OPB helps communication engineers determine the maximum distance over which a signal can be transmitted while maintaining an adequate level of quality. The larger the OPB, the greater the potential distance for loss-free signal transmission. Thus, it is crucial to comprehensively calculate the OPB during the design and deployment of optical networks to ensure efficient and reliable communication operations.
