OTDR Measurements in Unbalanced FTTH – PON Networks

Introduction

With the rise of fiber-to-the-home optical networks, which focus on connecting a central point with multiple subscribers via fiber optics, optical splitters or divisors began to be used as a means to “divide” or distribute the optical signal from one input to several outputs to reach a greater number of subscribers. In addition to having multiple outputs, these elements have the ability to distribute power in different ways, which can present different characteristics when implemented in an optical network. These differences give rise to new challenges in OTDR measurement.

Balanced and Unbalanced Networks:

A classification of optical splitters can be:

• Balanced splitters or divisors: These distribute optical power between their outputs in a balanced manner. That is, a splitter with 1 input to 2 outputs (often designated as 1x2) distributes 50% of the input signal to one output and 50% to the other output. Extending this definition, if we have a 1x4 splitter, we get 25% of the input signal at each of the 4 outputs; with 1x8, each output receives 12.5%, and so on.

• Unbalanced splitters or divisors: This type of element can distribute the input power asymmetrically in ratios such as 95/5, 90/10, 85/15, 70/30, 60/40, 55/45, among others. That is, in a splitter with 1 input and 2 outputs with a 90/10 ratio (or simply a 90/10 splitter), the input power is distributed as 10% to one output and the remaining 90% continues to the other output.

According to the characteristics of each type of splitter, two different network types are constructed. Most current networks operate under the paradigm of balanced splitters, giving rise to what is called a centralized network or distributed network. In the first case, the network uses only a single splitter (generally 1x32) and in the second case consists of a cascade of 1, 2 or sometimes even 3 balanced splitters. The splitters are installed in aerial cabinets, connected to each other via fiber optic cable called feeder cable (if it connects the network to the central office), distribution cable (if it connects the feeder to the customer connection point), or drop cable (if it connects the last splitter to the customer terminal). These types of implementation do not have a high cost when there is a large number of subscribers (64 or 128 customers) in close proximity and the capital investment begins before the subscriber is active.

Another topology gaining momentum today is one that uses unbalanced splitters in conjunction with balanced splitters. This consists of a series of splitters with different unbalance ratios (usually 95/5, 90/10, 85/15, 70/30, and 60/40) along the main cable, where small portions of power are diverted to different branches that connect with 1x8 balanced splitters.

Some of the main characteristics of networks with unbalanced splitters are:

• Network growth according to subscriber growth: The network can be easily expanded to new areas simply by using new splitters on the main cable (as long as the optical budget of the deployed PON technology is not exceeded).
• Use of "Plug-and-play" elements for faster and easier implementation: Pre-connectorized connection cables and splitter modules integrated in aerial cabinets using FastConnect / Opti-Tap technology are used.
• Reduction of infrastructure and maintenance costs: Due to the type of implementation, the main cable is longer compared to the feeder in balanced networks, resulting in shorter spans between the unbalanced and balanced splitter. Combined with the fact that if there is no area to connect, there is a reduction in construction and material costs. Additionally, since you will also have a smaller distribution network, it will be easier to locate network defects.

Faced with these types of deployments, without the proper tools to identify this type of unbalanced topology, it is difficult to make correct interpretations, confusing these splitters with high attenuations due to dirty connectors or excessive macro bends, and therefore having failed network certifications or unnecessary evaluations or rework, causing loss of time and money.

OTDR in Unbalanced Networks:

In relation to measuring this type of network and characterizing all parameters of interest, such as insertion loss (IL) and optical return loss (RL) of each network element (splices, connectors, splitters, etc.) or simply searching for faults, the main challenges of these unbalanced topologies are:

• Being able to correctly interpret the end-to-end PON network by performing OTDR testing from the ONT side towards the OLT ("upstream"). The ability to traverse that splitter entails an increase in the effective dynamic range of the OTDR, requiring the use of larger pulse widths, which also gives us large dead zones in the instrument, where the interpretation of some events may be masked.
• The splitters are very close to each other on the main cable (between 100-400 m maximum). The closer the splitters are to each other, the more challenging the scenario becomes.

Taking into account the challenges mentioned and through the association of multiple pulse widths to the different analysis zones of unbalanced splitters, an example is shown obtained from a network where the event view with icons shows the interpretation of a splice that exceeds the allowable attenuation limit, created from the OTDR trace, allowing switching between the two views to analyze the situation in more detail if required. Intuitive Icon View (SLM – Smart Link Mapper) OTDR Trace with Event Table (obtained from the last 1x8 splitter in an unbalanced network).

Conclusions

Due to this new topology incorporating both unbalanced and balanced splitters, there are new challenges in both the implementation and measurement of this type of network. The multi-pulse technique in association with the configuration of the network type you wish to measure simplifies the task to perform simpler and more effective OTDR measurements.