There are five main specifications that are outlined in this standard. The following section outlines each of the specification and their importance for a fully functional optical splitter.
1. Optical Bandpass
For a fiber optic network, there are six nominal optical bandpass ranges.
A PON system has a downstream transmission (data sent from a server to a user) using the 1490nm wavelength while the upstream transmission (data sent from a user to a server) is a 1310nm wavelength. In addition, there needs to be consideration for any requirement for RF video overlay and network testing/maintenance. RF video overlay is generally transmitted through the 1550nm wavelength.
According to the ITU L.41 recommendation, the 1550nm or 1625nm wavelength is used for network for testing and surveillance. With these considerations, the required optical band needs to be determined.
The standard operating wavelength for a PON splitter is the 1260-1650nm which covers most of the optical bands.
The optical bandpass can be tested by connecting the optical splitter to an optical spectrum analyzer with a high powered light source having a central wavelength of the required bandpass. The attenuation across the required bandpass shall meet the splitter requirements.
2. Optical insertion loss
The optical splitter is the component with the largest attenuation in a PON system. The optical insertion loss is the loss of an optical signal resulting from the insertion of a component such as connector or splice in an optical fiber system. In order to conserve the power budget of a PON system, the insertion loss from the splitter needs to be minimized.
Based on the GR-1209 standard, the maximum allowable insertion loss for an optical splitter used in a PON system can be determined by using the calculations outlined in the below table.
1×N Optical Splitter 0.8 + 3.4 log2N
2×N Optical Splitter 1.0 + 3.4 log2N
Note: ‘N’ denotes the number of output ports.
The insertion loss is tested by using a light source and power meter(or) by using an insertion loss meter.The reference power level is obtained and each of the output port of the optical splitter is measured.
3. Optical return loss
The return loss is the loss of power in the light signal returned or reflected by a discontinuity in an optical fiber or transmission line. A high return loss reduces the power reflected back to the transmitting port thus minimizing noise which may result in a system power penalty.
The return loss is tested by using a return loss meter. The input port of the splitter is connected to the return loss meter and all the output ports are connected to a non-reflective index matching gel.
Uniformity is the maximum insertion loss value between one input port and any two output ports or between two input ports and one output port. This requirement ensures that for a PON system, the transmission power at each splitter output port is the same, thus simplifying the network design.
Custom optical splitters with non-uniform coupling ratio can be manufactured for specific network deployment. In such a situation, this criteria is not applicable. The usage of a non-uniform splitter in a PON system increases the complexity in testing, design and maintenance while reducing the network flexibility.
The uniformity of the splitter can be determined by referring to the results from the insertion loss test to ensure that the difference between the highest loss and the lowest loss is within the acceptable uniformity value (≤0.5 dB).
Directivity is the fraction of power transferred from one input port to another input port or from an output port to another output port. For a 2×N optical splitter, when light is injected into one of the input ports, light does not only propagate out of the output ports. Some of the light propagates back through the second input port. Similarly, when light is injected into one of the output ports, some of the light propagates back through the other output ports.
In a bidirectional transmission system such as a PON, directivity is important to reduce the power back to the transmitting port to reduce signal cross talk. In addition, a high directivity value will also cause a higher insertion loss due to the loss in optical power. So it is important to reduce the directivity as much as possible.
Directivity can be measured in a manner similar to the insertion loss test. However, the light source and power meter are connected to each of the input ports of two output ports.