Passive Optical Network Testing faces new challenges

In Passive Optical Network (PON), the difficulty of Installing Optical Fiber Links and passive devices must be re-examined by the current standard testing technology.

One of the other challenges encountered by the testing technology is that the 1490nm wavelength brings special difficulties to the OTDR. When the splitter is installed for testing, OTDR must face the challenges of high loss and difficult signal recovery. This article will discuss the main difficulties of passive optical network installation and some testing processes.

G.986.3 released by the ITU) proposes a broadband access system that improves service capability through Wavelength allocation. We recommend that you specify the 1490nm wavelength for downstream voice and data signals, the 1550nm wavelength for downstream video signals, and the 1310nm for upstream voice and data signals.

Many network devices and device manufacturers have accepted ITU's G.983.3 recommendation. Therefore, today, many optical network operators are installing optical access systems, which are composed of optical line terminal OLT, optical network terminal ONT, WDM coupler and 1 × N filter ). The Optical Fiber Device Used for the optical fiber access system must be able to test all three wavelengths. Optical fiber network operators and testing equipment manufacturers are quite familiar with the 1310nm and 1550nm wavelengths, but the emergence of 1490nm wavelengths poses a new challenge to OTDR.

The. ITU G.983.3 optical access system includes optical line terminals, optical network terminals, and 1 x N filter. These broadband systems have three transmission wavelengths. The new 1490nm wavelength is used for downstream voice and data signals.

Many people in the industry think that tests at nm are sufficient to cover nm. This view is only true for new optical fiber cables, especially G.652C low-water-peak optical fiber cables, which were laid in the late 1990s s or more recently. For the old optical fiber used in the early 1990s s, there was a problem because G.652C had not yet appeared, and the E-band in the water peak area had not aroused people's interest. For this reason, many manufacturers of optical fiber testing equipment now consider developing 1490nm testing functions for their products to test PON. The main optical tests required for PON installation include:

● End-to-end link features.
● Coupling port loss and Reflection characteristics.
● Bidirectional end-to-end optical return loss (ORL) and optical loss.
End-to-end link features

Ideally, PON should be tested after each segment is installed. For example, once the optical cable is installed, end-to-end testing should be completed between the optical fiber end and the Central Bureau co olt. Testing of this section of Indoor equipment is important because it includes many closely linked devices, such as connectors, optical cables, mechanical connectors, and melt connectors. If possible, perform a two-way test because uneven size of the core may lead to different losses in different directions.

In the past, because the blind zone of OTDR was not enough to measure devices close to each other, the characteristics of Indoor equipment were measured using power meters and signal sources. Now, the OTDR provided by the testing equipment manufacturer has 1 m blind zone and 6 m attenuation blind zone, so it is able to locate the bending of each device and optical fiber. When the optical fiber is over-bent, the loss at 1310nm, 1490nm, and 1550nm can be easily detected. When bending is excessive, long wave strengths have higher losses than short wave strengths.

Make sure that there is sufficient optical fiber attenuation at each wavelength-1310nm, 1490nm, and 1550nm. Optical fiber attenuation should be measured using OTDR. The typical Attenuation parameters of the new G.652C optical fiber are 0.33dB/km at 1310nm, 0.21dB/km at 1490nm, and 0.19dB/km at 1550nm. From the CO junction board to the splitter before connection) and from the splitter to the ONT each optical fiber should be measured.
Test After filter is installed

After a filter is installed, end-to-end testing should be performed between each output port of the filter and the OLT. After the current terminal is installed, the test should also be performed between each downstream terminal and the OLT.

OTDR for PON can be successfully tested after the splitter is installed. In fact, the traditional OTDR can identify the loss from 3 dB to 7 dB based on the user's configuration, which is as high as the loss at the end of the optical fiber. By modifying the OTDR analysis method and adjusting the signal recovery capability of the OTDR, The OTDR can detect the loss of up to 20 dB after the filter.

Testing is also recommended from the CO to the shard, and from the Shard to all ONT. This can be achieved through the point-to-point OTDR test. However, only the downlink optical fiber from the splitter to the ONT is different, and point-to-point testing is possible ).

When a passive optical network is installed, OTDR is recommended for testing from the CO to the splitter and from the splitter to all ONT. However, the test is possible only when the downlink optical fiber from the splitter to the ONT is isolated from each other.

When the coupler splitter) is connected to the optical fiber from CO, it is recommended to measure the loss and Reflection Characteristics of the coupler to check whether these parameters are consistent with the manufacturer's. OTDR can be used together with a bare-Fiber adapter and pulse suppression to complete this measurement. Measurement is performed between the coupler output end and the co olt to determine the loss of each filter port at 1310nm, 1490nm, and 1550nm. The measurement can be completed only by using a pulse suppression device. Otherwise, the loss of the filter will fall into the blind zone of the OTDR and cannot be measured. According to ITU-T G.983.1, the coupler port reflection should be-35dB or better.
Bidirectional Loss Measurement

According to ITU-T G.983.4, the total loss budget for Class B PON is 22dB, and class C PON is 27dBB and class C are differentiated based on the quality of lasers and optical devices ). This loss budget is quite tight, especially when using a high port number splitter. Because the input power should be allocated to several output ports, the PON filter has inherent split loss. The split loss depends on the split wave ratio. The split wave at is about 3 dB, and the loss is increased by 3 dB for each added output port. A splitter has at least 15 dB of split loss. This loss exists in both downstream and upstream signals. Coupled with the loss of WDM coupler, connectors, connectors and optical fibers, it is easy to understand that end-to-end optical loss must be measured accurately in two directions during installation.

In addition to optical loss, the measurement of the end-to-end link optical Return Loss ORL is also very important. The adverse effects of ORL include interference with the signal of the light source, increasing the bit error rate of the digital system, reducing the signal-to-noise ratio of the system, fluctuation of the output power of the enhancement laser, and permanent damage to the laser.

Some Optical Loss testers include both loss measurement and ORL measurement. Such testing equipment can perform loss and ORL measurements well on each PON transport link ).

According to ITU G.983.3, the total loss budget of the passive optical network is 22dB or 27dB, which is quite tight, especially when a high port number is used as a filter. The key is to measure bidirectional optical loss and optical return loss during each PON transmission link installation. The new optical loss tester includes both loss measurement and ORL measurement.

Important technologies are being developed in the PON testing field. The OTDR and optical loss tester can meet the needs of installation phase testing. The testing equipment used in the ONT operation and maintenance phase is also under development.