# Toward SDM-Based Submarine Optical Networks: A Review of Their Evolution and Upcoming Trends
## 2 Submarine Optical Networks Basics and Demonstrations Technologies on Submarine Cable Systems  
### The Past Evolution of Submarine Transmission Systems  
The submarine cables' history (pre-optical era) started in Malaya, when by the late 1840s a newly found natural polymer (\"the gutta percha\") provided the basic means to successfully insulate cables for undersea use. A relatively short-distance cable that connected the UK (Dover) to France (Calais) was submerged in the summer of 1850 [1]. Unfortunately, it lasted only for a few messages and was replaced approximately a year later by a more durable successor. Shortly after that, the first cable connecting Europe to the US was deployed and became operational on 16 August 1858 only for telegraphy purposes [5]. Deployment of cables across the Pacific Ocean turned out to be more difficult and therefore transgenic telegraph cables became operational about half a century later, in 1902, connecting the US to Hawaii [1, 5].  
Humanity had to wait approximately fifty more years for the deployment of newer advanced cables to support voice and data communications. TAT-1 (Transatlantic No. 1) was the first transatlantic cable that was used for telephony [1, 5]. The cable that connected Scotland to Clare Ville became operational on 25 September 1956, and initially supported transmission of just 36 telephone channels [1]. The first coaxial cables that could transmit frequency-multiplexed voice signals became operational in the 1960s. Figure 1 depicts some of this history, which led eventually to optical technology (1986, 1988) (optical era) by using optical fiber links for high-speed/capacity submarine communications [1].  
Some significant (as characterized by the IEEE) milestones in the submarine era history are [1] as follows.  
Figure 1: Submarine network elements’ evolution (1956–2022) [1, 2, 3, 4, 5].  
* **1884:** The first submarine cable supporting phone data (from San Francisco to Oakland).
* **1954:** The first submarine (high-voltage direct current) cable connected the island of Gotland to mainland Sweden.
* **1956:** The first deployment of repeaters (in the 1940s) boosted the TAT-1, which was the first telephone cable crossing Atlantic.
* **1964:** The first transpacific submarine coaxial telephone cable linking Japan, Hawaii, and the US mainland.
* **1986:** The first submerged international fiber-optic cable that connected Belgium to the United Kingdom.
* **1988:** The first submerged transoceanic fiber-optic cable, (named TAT-8), that connected the USA to the United Kingdom and France.  
Next, in Figure 2 we illustrate a transatlantic cable capacity comparison beginning with TAT-1 in 1956 and ending with TAT-14 in 2001.  
Figure 3 shows the worldwide submarine cable map, including the majority of submarine cables systems which transport approximately 99% of world's intercontinental traffic and $10 trillion of transactions daily. It comprises 487 cables, stretching over 1.3 million km and 1245 landing stations that are currently in service or under construction.  
Those networks were deployed by consortia of major telecom network operators. However, during the current century, Google has become the most active investor in submarine networks, alongside other major cloud service providers, such as Facebook and Microsoft. From 2016 to 2018, Google invested approximately $47 billion dollars [3] in capital expenditure (capex) to expand and upgrade Google Cloud infrastructure. Nowadays, Google counts 134 points of presence (PoPs) and 14 major subsea cable investments globally [3] interconnecting all six continents, as shown in Figure 4.  
Figure 2: Transatlantic cable capacity (circuits).