Monitoring Methods in DWDM Systems (Part 2)

There are not so many options for monitoring the status of passive elements, WDM sealing systems, due to the natural limitations associated with the equipment features.

One of the easiest ways to create monitoring of passive WDM multiplexers is to install passive non-uniform dividers on linear ports (in Fig. 1, “4” and “5” are indicated). The uneven divider is often a passive optical splitter of a welded type or a biconical splitter, for more details about the device click here. The inclusion of splitters in the circuit is necessary to organize the removal of test optical power. After the splitter, the test signal can be output via the optical port (indicated by “6” and “7” in Fig. 1) to the measuring equipment or brought to a broadband photodetector.


Fig. 1

Often, only an optical port is organized for connecting measuring equipment, since the installation of photodiodes entails not only the study of the issue of power supply, but also the development of the simplest but control board. And taking into account the fact that a broadband photodetector can only detect a group signal level, there is not much use from this information, which means that the costs are meaningless. For a long time, the role of the connected equipment was either the simplest optical power meters (examples of equipment and detailed information about them by reference), in which case the measurement was evaluative, the presence or absence of “light” (as in the case of built-in photodiodes), or expensive spectrum analyzers (examples of equipment and detailed information about them by reference),

At the beginning of the 2000s, when the sizes of optical compaction devices began to miniaturize, WDM optical power testers began to appear on the measurement equipment market (examples of equipment and detailed information about them by reference and link ). These devices are much simpler to operate, unlike spectrum analyzers, and they have the dimensions and measuring principle of conventional optical power meters, but they allow you to measure each carrier in the selected WDM range. All measurement results are displayed on the LCD display of the device and can be saved. Thus, the measurements of the extracted test signals have become much simpler.

The main disadvantage of passive monitoring is that a very small optical signal is allocated to the test taps, which entails two main problems:
• The final value must be calculated taking into account the percentage division of the coupler;
• Large measuring error associated with the same low allocated power.
It should also be noted that for measurements it is necessary to have good measuring equipment in the arsenal, as well as a large staff of staff who are not only able to use measuring equipment, but also very mobile (since for a frequent provider there is far more than one transmission line built on WDM technology). The main advantage of such monitoring is the simplicity and cost-effectiveness of implementation (measuring equipment and qualified staff are not taken into account).

At the moment there are two solutions for active monitoring of passive WDM components:
• Multiplexer with integrated active monitoring system;
• Tunable multiplexers - ROADM (this type of device is quite complex and has many realizations in hardware, a separate article will be devoted to it).
A multiplexer with a built-in active monitoring system allows simultaneous control of the optical power levels of all signals coming in (from the client side and from the linear side) to the multiplexer.


Fig. 2

The design of a multiplexer with an active monitoring unit repeats a lot of the implementation of simple passive monitoring using a WDM optical signal tester. To divert the test signal, tappers with uneven division are used (“3” is indicated in Fig. 2). Next, the test signal falls on an optical switch of the 2x1 type (indicated by “5” in Fig. 2), with the help of which it is selected which of the two test signals will go to the optical power meter.

The optical power meter consists of an Athermal AWG demultiplexer (in more detail about the device of multiplexers in the first part of the article) and a CCD matrix glued to the output focusing plate. Below is a photograph of a similar unit for CWDM signals; in the case of CWDM, thin-film filters and photodiodes are used. The principle of operation of such an optical power measuring unit is quite simple: the measured group signal is fed to the input optical port, then the signal goes to the focusing green lens (for more details about thin-film filters and the green lens in particular, by reference ), which focuses the signal on the first optical filter, then a system of mirrors using additional skip filters, the group signal is divided into individual wavelengths and received by photo detectors. Information from photodetectors is transmitted to a decisive device, and then to the client in one form or another.


Fig.3

Accordingly, the optoelectronic conversion takes place in the meter and information on the power level of each of the incoming signals at a given time is received on the monitoring board. Further, this information is transmitted to the client program.
In principle, the optical switch can be excluded from the circuit and another meter can be installed instead, but this step increases the cost of the device by 1.5 times.

In connection with the construction architecture features described above (a sufficiently small signal value of ≤5% is allocated to the measurement unit), the control system has a measurement error of ≤ ± 0.8dB. This value of the measurement error was calculated for the demultiplexer (indicated by “2” in Fig. 2) and is maximum, since the incoming signals are very low-power, the average value is -18 ... -8dBm (the absorbed optical power to the measurement unit is -31 ... -21dB). While for the multiplexer (“1” is indicated in Fig. 2), the measurement error will be ≤ ± 0.2dB, since the absorbed optical power is -15 ... -12dB, which is a normal value for FOCL measuring equipment.

The main difference and advantage of the active monitoring system is the use of a client program that allows you to remotely receive operational information to the operator, which simplifies the work with the compaction system and does not require the presence of maintenance personnel in the immediate vicinity of the equipment. It should also be noted that the presence of the multiplexer monitoring function simplifies installation and does not require additional measuring equipment during installation and switching of the system as a whole.

At the moment, the start of sales of multiplexers with an active monitoring system has been announced by several medium-sized manufacturers of telecommunication equipment in Europe (including Russia!) And South Korea. These devices “reached” the Russian market only in the form of brief press releases, but there is no doubt that in the coming year the supply of this kind of equipment in a niche market segment will grow exponentially.