As the structure of optical networks becomes more and more complex, the dynamic reconfiguration of networks becomes more and more frequent, and the management of online optical cable resources and dark fiber becomes more and more important.

   On the other hand, the evolution of optical network intelligence is getting faster and faster, but intelligence is mostly concentrated at the network layer and above. The bottom layer, especially the quality of the optical fiber as the physical basis of the entire optical network, and the visualization and scheduling of optical fiber resources There has not been much progress in real-time and intelligent real-time online management. Real-time monitoring of online fiber link quality and dynamic management of optical fiber resources may become a key to future robust, automated, intelligent, and dispatchable optical networks technology.

At present, the mileage and management complexity of optical fiber resources in optical networks are increasing rapidly, and traditional, inefficient manual maintenance has been difficult to efficiently support the high quality requirements of modern optical networks.

   As a new technology that can meet the above-mentioned demand change direction, online OTDR for transmission network (hereinafter referred to as online OTDR) is increasingly becoming a new essential component unit in WDM system. Unlike traditional OTDR instruments or offline OTDR modules, when real-time monitoring of online fiber links is required, it is necessary to consider that for online fiber links, when optical amplifiers such as EDFA, Raman, Hybrid (EDFA+Raman) are turned on, and When a single or multiple wavelengths carry services, the noise in the system (which may come from the ASE of the optical amplifier, the crosstalk of the signal light and the non-linear products, etc.) will have a huge impact on the online fiber quality monitoring.

Therefore, the online OTDR needs to do the necessary technical research based on the system characteristics of the WDM transmission network to solve the historical problem that the online optical fiber link of the transmission network cannot be monitored in real time.

Several possible implementation methods and application methods of online OTDR in the system are as follows:

Method 1: WDM online OTDR

According to the difference of the used band, this method can be divided into two subdivision methods.

1>L band WDM online OTDR

   In this application mode, the online OTDR borrows the current L band idle band and accesses the existing network by wavelength division multiplexing on the main optical path (e-OTDR in the figure is the online OTDR mentioned above, OA in the figure is the optical amplifier mentioned above, the same below). The application topology is as follows:

It should be noted that the above figure only illustrates the application scenario where the OTDR is accessed from the sending end; the OTDR can also access the system from the receiving end.

The advantages of this application method are:

The dynamic range is large, which has a good effect on the suppression of ASE in the system, and real-time online continuous monitoring of fiber links above 100Km. The reason is that a variety of L-band lasers can be used. The laser output power, wavelength, and spectral width do not have to be too demanding, and there are certain advantages in availability and cost.

The disadvantages are:

a. It is necessary to insert additional WDM in the main optical path to bring extra insertion loss to the main optical path; in addition, it is inconvenient to upgrade the existing old network;

b. In recent years, more and more systems are expanding to C+L-band. The existing L-band OTDR will have wavelength conflicts with the expanded system;

2>OSC band WDM online OTDR

    In this application mode, the online OTDR borrows idle wavelengths in the OSC band (such as 1510±10nm) and accesses the OSC band from the OSC band to the existing network in a wavelength division multiplexing manner. The application principle is as follows:

It should be noted that the above figure only illustrates the application scenario where the OTDR is accessed from the sending end; the OTDR can also access the system from the receiving end.

The advantages of this application method are:

a. No need to modify the existing main optical path and increase the loss of the main optical path. Therefore, it is very easy to smoothly upgrade old stock networks and deploy new systems;

b. The dynamic range is large, which has a good effect on the suppression of ASE and noise in the system, and real-time online continuous monitoring of online fiber links above 100Km;

c. Compatible with C and C+L-band systems.

The disadvantages are:

a. Additional WDM needs to be added to the OSC channel, introducing an additional 0.5dB insertion loss;

b. Due to the use of customized lasers, the cost of 1:1 use is relatively high. However, in practical applications, it can be used with 1*N optical switch according to the system characteristics, which can greatly reduce the cost of single-line monitoring;

Method 2: Time-division multiplexing online OTDR

    In this application mode, the online OTDR multiplexes the OSC operating wavelength (shares the same laser) and connects to the existing network in a time-division multiplexing manner with the OSC module. The application topology is as follows:

It should be noted that the above figure illustrates the application scenario of access from the sending end; however, because the OSC laser is multiplexed, the OTDR needs to be in the same direction as the OSC transmission, which limits the access of the OTDR from the OA receiving end in this mode.

The advantages of this application method are:

a. No need to change the main light path. Therefore, it is very easy for the new network deployment and the smooth upgrade deployment of the old network;

b. The cost of single line monitoring is lower. The reason is that the OTDR and the OSC module share the same laser, sharing part of the hardware cost;

The disadvantages are:

a. It is necessary to insert additional optical devices (such as couplers and similar devices) into the OSC optical path, which increases the insertion loss of the OSC optical path. The fundamental reason is that due to laser multiplexing, additional optical devices need to be inserted in the OSC optical path in order to realize the OTDR function;

b. Intermittent control of the transmission of OSC services needs to be performed at the network management level to coordinate the exclusivity of the OTDR scanning window for specific time slots;

c. Low dynamic range. Since the power of the OSC laser used in large quantities is relatively small (generally around 2mW), the dynamic range when implementing the OTDR function is low;

d. Limited to only access from the sending end, and the laser spectrum width is wider, online performance is more susceptible to the system ASE, online performance is poor.

NOTE: The dynamic range mentioned above follows the definition of IEC61746 RMS dynamic range.

The following table summarizes the application characteristics of the above three topologies:

 

As the world's leading optical device/module manufacturer, Guangxun Technology closely follows the technology development trend. Through continuous and close cooperation with the world's leading telecommunications equipment manufacturers and operators, it has achieved support for multiple OTDR solutions and launched a full range of online OTDR products. This series of products have been shipped on a large scale, serving the real-time monitoring of massive online optical cables worldwide, and providing strong technical support for the automation and intelligent operation and maintenance of online/offline optical cables of WDM systems.