Competition in the international telecommunications market is heating up, with yet another private company proposing to lay fiber-optic cable in trenches on the ocean floor.

Last week, Submarine Lightwave Cable Co, proposed a privately owned transatlantic fiber-optic cable to the Federal Communications Commission. The cable would carry voice, video, and data communications between Europe and North America.

Submarine Lightwave is the second company to announce such plans in less than a month. Tel-Optik Ltd., a Washington company, previously announced plans for a $600 million transatlantic cable system.

Fiber-optic cables -- glass strands that transmit voice and data using light pulses -- are a cost-efficient way of sending large-volume communications traffic at high speeds.

The move is sure to give Intelsat's international satellite network and American Telephone & Telegraph Co.'s transatlantic cable system a run for the money -- provided Submarine Lightwave can raise $450 million.

If the cable is built, it will provide 12 times the capacity of a transatlantic fiber-optic cable planned by AT&T and the European telephone systems for 1988, the FCC filing says.

The breakup of the Bell System and blossoming new technologies have created a market ripe for private overseas fiber systems, says Robert E. LaBlanc, president of LaBlanc Associates, a New Jersey consulting firm.

"The reason that everyone's excited is MCI, GTE/Sprint, and others have finally gotten to break into the international market," LaBlancsays. "What they are counting on is the market exploding." But LaBlanc says the financial risks for companies like Submarine Lightwave are still "tremendous," and that the problem of overcapacity may become a reality if all the cables are built.

Submarine's president, Rosario P. Romanelli, says the company plans to be profitable by targeting the Fortune 500 and long-distance telephone companies for its services.

These companies, says Romanelli, are already buying space on land-based fiber cables. "Once a customer has high-quality facilities between San Fransisco and New York, he'll want them to Paris. How else does he go overseas?," asks Romanelli.

"I am convinced if you can get the price of a facility down to a very low number, you will generate new demand . . . for new services," he says. The company plans teleconferencing, closed-circuit television and electronic document transfer services in addition to conventional voice and data traffic, Romanelli says.

But the company has a long way to go before the cable begins operating in 1989. The FCC must approve the cable, the company must obtain financing, choose a headquarters, hire about 100 employes, design and manufacture the system, and secure cable landing rights on both continents.

Romanelli -- an old hand at developing fiber-optic submarine cables, according to the FCC filing -- spent 20 years at Western Union Telegraph Co., serving as president of Western Union International Inc. for 10 years. A few weeks ago, he resigned as president of Teleport Communications, a joint venture between Merril Lynch Telecommunications Inc. and Western Union Communications Systems.

Other company officers include telecommunications lawyer Robert E. Conn and James G. Murray, a telecommunications consultant.