Next Generation Submarine Network – Innovative Repeater Technology

Submarine cable is highway for data transmission. Submarine networks carry more than 95 percent of the world’s intercontinental electronic communications traffic. Submarine cables are critical infrastructure, which carries more than $10 trillion worth transactions, every day.

Submarine networking has recently seen several technological evolution. Recently, many new subsea announcements with make use of innovative technologies in Dry plant and Wet Plant. One of the latest advancements in wet plant side repeater known as Repeater Pump Farming. 

Submarine fiber has attenuation. Nominal loss of fiber 0.155dB/km at 1550nm (TeraWave® SCUBA Ocean Optical Fiber). In practical scenario, it can vary and may be upto 0.25dB/km or more due various factors.

• Repeaters are used for optical signal amplification. Repeaters are placed every 60km to 70km.
• Power to the repeater is feeded from Power feeding equipment, which is located in Submarine Cable Station.
• In traditional subsea cable, every fiber pair will have their own repeaters. Four fiber pairs will have repeater housing four amplifier chassis. One Amplifier chassis has Dual laser 980nm Pump Units. This is called 2x2 Pump redundancy. These types of redundancy scheme used in the past.
• EDFA - 980-nm technology repeaters–dual pump with high-loss loopback (HLLB) monitoring used in the past.

In traditional Subsea cable, Single pump failure of EDFA (Erbium Doped Fiber Amplifiers) putting entire fiber pair under great risk. If dual, pump fails, resulting in entire repeater failure, which heavily affects live traffic in particular fiber pair of the entire segment.

2x2 Pump redundancy

Scheme is a general design shown in above figure, which is used in past.

Here one amplifier pair i.e two EDFAs (Transmit and receive direction), shares power from two pump lasers. This is high risk. If single pump fails, it is risk. If both pump fails, then entire repeater failure resulting in traffic impact.

 4×2 redundancy

Currently, this scheme is being used. This improves the reliability by having additional 2 pump lasers compared to 2x2 pump redundancy as shown in above figure.

Two EDFAs share the power from four pump lasers, which tolerates three-pump lasers failure at most in each fiber pair. This design doubles the total cost of pump lasers. 

Above-mentioned redundancy schemes are standard. Pump powers dedicated to 1 fiber pair. The pumps cannot be shared to other fiber pair.

Repeater Pump farming

This is the latest advancement in repeater technology. All new Subsea cables will use this type of technology, which provides maximum redundancy and flexibility.

A series of pump lasers and EDFAs are cross connected via two-stage optical fiber couplers. This is highly complex design. 

Pool of pumps cross connected to pool of FPs. 

Repeater Pump Farming Example

Above example shows, 2 fiber pairs with 4 pumps in pump farming scheme. Likewise, we can have different options mentioned below:

• 4 Pumps / 2 Fiber Pairs
• 4 Pumps / 4 Fiber Pairs
• 8 Pumps / 4 Fiber Pairs
• 8 Pumps / 8 Fiber Pairs
• 16 Pumps / 8 Fiber Pairs
• 16 Pumps / 16 Fiber Pairs

 Above options of pump sharing is not fixed. It can be different based on requirements, calculations and design. 

Higher the number of pumps, higher the drop voltage of repeaters. In practical scenario, example below:

3000km Submarine Cable: More fiber pairs / More Pumps

 Higher number of pumps and higher voltage drop of repeaters.Total cable length is small, number of repeaters are less in the cable , and more pumps can be used.

6000km Submarine Cable: More Fiber pairs/ Less Pumps

Total cable length is large, number of repeaters are more in the cable segment, and less pumps can be used

Conclusion

Submarine repeaters are the heart of Submarine cable system. Submarine technology Innovation is continuous process . Do we have new amplifier types, modified EDFA's which can play a role in future submarine systems. To achieve maximum redundancy, flexibility and increase cable capacity Repeater Pump Farming is mandatory.

About the Author

Abdul Ravoof, Senior Engineer - Submarine, Transmission Network & Data Center at Integrated Telecom Company (ITC) in Saudi Arabia