Underwater optical networks

There is a worldwide network of more than 400 submarine cables. The number of submarine cables will continue to increase to deal with the ever-growing volume of data transmission.
Optical amplifiers were developed in the 1990s, resulting in a phenomenal increase in the transmission distance of submarine cables. Wavelength Division Multiplexing (WDM), which was introduced in the 2000s, made it possible for a single optical fiber to send multiple signals at a time, leading to a substantial increase in the transmission capacity. It is now under consideration to introduce Space Division Multiplexing (SDM), which expands the transmission capacity by packing many cores into an optical fiber. Research and development efforts are in progress to allow cables to accommodate more transmission capacity through the use of multi-core fiber technology that integrates multiple cores into a single fiber.
The service life of a submarine cable is said to be 25 years or so, and there is the need to replace aging submarine cables.

Submarine Cable System

A submarine cable system consists of the following equipment.

Optical cable
Repeater
- Optical amplification function
- Optical distribution function
- Point-of-failure monitoring function
Beach manhole (connection point between submarine cable and land cable)
Cable landing station (power supply equipment, monitoring equipment, circuit-terminating equipment)

The undersea portion from the beach manhole is referred to as the wet plant, and the on-land portion as the dry plant. While a submarine cable typically remains in service for 25 years or so, the equipment of the cable landing station is often upgraded at short intervals due to technological advances.
In recent years, this has resulted in the increasing use of the open cable model in which the wet plant and dry plant are operated by separate providers. The increasing use of the open cable model has made clearer the demarcation point of responsibility at the beach manhole, giving greater importance to pre-operation quality evaluations of submarine cables.

Challenges in Submarine Cable Installation and Maintenance

Challenge 1 Accurate Identification of Point of Failure During Installation and Maintenance
Cables are installed as both the cable landing station and the cable-laying vessel to monitor cable transmission loss and repeater operation. Checking the cable quality from both the land side and ocean side provides long-term quality assurance.
The installed submarine cables may be damaged by fishing operations, ship anchors, and natural disasters. During the maintenance of submarine cables, it is necessary to identify the point of failure, pull up, and repair the damaged cable quickly in order to ensure the rapid recovery of communication. Since pulling up a cable costs considerable time and money, the point of failure in a long-distance cable needs to be identified accurately.

Challenge 2 Signals of Different Wavelengths in Transmission Paths
Technological revolution has made it mainstream to connect submarine cables in a ring topology by means of submarine ROADM (Reconfigurable optical add/drop multiplexer) units and transmit optical signals of multiple wavelengths. Since a different service uses a different wavelength, the test solution is required to be able to change the wavelength of the light source.

Challenge 3 Checking of Signal Level

The gain and loss in repeaters and the loss in optical fibers differ depending on the wavelength. In the WDM system, which transmits optical signals of multiple wavelengths, the signal level change when passing a repeater or cable varies depending on the wavelength of the signal. Since saturation of a specific signal in the optical amplifier leads the signal quality deterioration, it is necessary to check that the variation in the signal level is within a specified range.

Solutions

Solution 1 Point of Failure Identifiable with 10-meter Resolution

Accurate identification of a point of failure in a submarine cable requires high-resolution position detection over a long distance. The high-resolution coherent OTDR MW90010B enables one to find a point of failure in a short measurement time over distances up to 20,000 km.

Solution 2 Changing Wavelengths to Test for Each Transmission Path

Optical signals of multiple wavelengths propagate through the submarine optical network. Since the signal loss and delay time differ depending on the wavelength, tests need to be conducted individually for each wavelength.
The coherent OTDR MW90010B allows the loss to be tested while switching the wavelength in the range of 1527.60 nm to 1567.13 nm.
Furthermore, a tunable optical transceiver can be attached to the Network Master Pro MT1040A. When equipped with such a transceiver, the tester can measure the throughput and delay time while selecting wavelengths.

Solution 3 WDM Signal Analysis Using an Optical Spectrum Analyzer (OSA)

During the submarine cable installation work, an OSA is used to test the signal optical power, optical signal-to-noise ratio (OSNR), gain tilt (flatness of power of each channel), signal optical wavelength, spectrum width, etc.