Plans to span the Pacific Ocean floor with a cable that could carry up to 600,000 telephone calls simultaneously were announced by AT&T and a Japanese company yesterday. With eight times the capacity of the most advanced ocean cable now in use, this underwater "information highway" could further stoke already explosive growth of international communications.

Using optical fiber that conveys calls as billions of blips of light, the cable would be switched on in 1996 to meet demand that is expected to continue to surge because of East Asia's emergence as an economic powerhouse. It would require refinement of prototype technology, but American Telephone & Telegraph Co. expressed confidence that could be done on time.

Calling across the Pacific in the past five years has grown at 30 percent to 40 percent a year, said Gregory C. Staple, a Washington communications lawyer. "It's grown sometimes at more than double the rate of the North Atlantic market."

The first transoceanic optical cable went into service in 1988, spanning the Atlantic. Capable of carrying 40,000 calls (the capacity also can be used for data or video), it represented a quantum leap in communications technology. A similar cable began service shortly afterward in the Pacific.

They and others that followed have moved the world away from reliance on satellites, which since the early 1960s have been the technology of choice for transoceanic communications.

By helping lower costs, the new cables are encouraging more spur-of-the moment calls between family and friends across oceans. But the more significant impact, many industry analysts believe, has been in business services such as transmission of high-speed data, computerized designs, facsimile and video conferences. "Three million fax machines in Japan are generating as much international traffic as 49 million telephones," Staple said.

AT&T's partner in the project is Kokusai Denshin Denwa (KDD), Japan's largest international long-distance company. The project must be planned and financed and put to regulators for approval, but both companies said they consider it firm. "We believe we will have sufficient demand in the future," said Takeo Miura, assistant director of KDD's Washington office.

The project depends on perfection of new transmission technology. Current optical fiber cables have "repeaters" spaced along them that transform the signal into electricity, amplify it and resend it as light. The new cable would amplify the light directly, giving greater capacity and flexibility.

"We have engineers working elbow-to-elbow" with counterparts from KDD, said Jack Sipress, who directs work on submarine cables at AT&T Bell Laboratories. "We have done the research and the fundamental development ... and now it's a matter of designing the specific parts and proving that the designs are reliable for the submarine cable environment."

Fiber-optic systems with similar capacity are about to be introduced for land transmission.

In the Pacific, plans call for three more fiber cables to be laid between now and 1993, adding 210,000 more lines even before the newly proposed AT&T cable would come on line. To date, predictions of overcapacity have not been born out. "On all of these cables, the capacity's been oversubscribed almost instantly," said Bill Davidson, a business professor at the University of Southern California.

Even as fiber expands its reach, the international communications satellites operated by the 119-nation Intelsat consortium would continue to grow in capacity, providing routine service, backup for broken cables and service between points of the globe that don't have enough traffic to justify a cable. But their total share of the pie would decline.

Intelsat now has about 51,000 telephone grade circuits between the shores of the Pacific. By 1995, it will have increased that to about 345,000, according to spokesman Tony Trujillo.

Communications Satellite Corp., the U.S. member of Intelsat, welcomed the proposed cable. "The cables are essentially full and the satellites are full," said Bruce Crockett, president of the company's World Systems Division. But he noted that by providing competition, "Clearly, it will keep us on our toes."