The Last (Active) Person Standing
The Forgotten Art of Active Supervisory and Its Relevance to SDM
By Stuart Barnes
May 26, 2021
Supervisory is a fundamental necessity for long haul amplified submarine links. Its primary application is for fault location in the event of a failure in the system, where it is imperative to both identify the location of the fault and give some indication of its nature. Since optically amplified links were introduced there have been two basic types of supervisory, passive and active.
Passive supervisory is akin to an optical version of sonar. A “ping” is sent along the system and at each repeater a small portion of the signal is carried back to the source via a high loss loopback optical path between the forwards and backwards directions. Through this and some clever analysis it is possible to deduce both the repeater position and an estimation of the attenuation. This is often adequate for fault finding, but the information is limited, especially when compared to active supervisory.
How is active supervisory implemented?
Feedback can be provided from most devices in today’s repeaters. Pumps have multiple feedback features. In addition, anything that senses a physical phenomenon can be inserted into a repeater and in addition to the telemetry from the pumps we can record the following data: pressure, humidity, temperature and angular momentum from internal accelerometers. The key is to find a method for carrying this information back to land where it can be processed and interpolated. Xtera, for example, uses a Sub Carrier Modulation methodology wherein the information is impressed on the traffic signal at a much lower frequency, typically in the MHz range. In doing so sufficient telemetry information is fed back to the shore with minimal impact on the traffic load.
What do we measure?
- Health of the amplifiers
Clearly the primary piece of information is the health of the amplifier itself. Amplifier degradation could impact an entire fibre pair, with the associated financial consequences such as loss of revenue, marine repair, etc. So, in addition to building in redundancy, immensely reliable optical pumps are chosen for submarine systems. Xtera’s 980nm pumps have a FIT rate of ~5 FIT’s and their 14XXnm pumps are even more reliable at circa 1FIT for example.
The pumps are packaged and their drive circuitry enables direct access to a multiplicity of valuable data, temperatures, current, voltage and power. In addition, at the amplifier level active supervisory can determine output power, gain and tilt.
- Additional aspects
In theory any sensor can be put in a repeater housing and used to feed data back to dry land, but the ability to do so can be tempered by space and power considerations. It is also important to avoid anything that can be construed to impact the main function of the repeater.
Xtera have chosen to measure the following additional aspects and we briefly discuss their relevance to the primary goal below.
Xtera inserted accelerometers into the repeater housing to record motion during the early qualification activities (terrestrial testing and sea trials) and fully intended to remove them for the subsequent production units. However, during the sea trials two things became apparent. Firstly, we could view and record how the repeater was positioned on the seabed, and any suspensions that may occur. Secondly, and perhaps of greater value in today’s security conscious world, we could detect any external tampering.
The figure below shows that the repeater could detect movement in the cable a full two hours before it was lifted from the sea floor. Which is especially impressive considering these were still only in-production units.