The Justice Department said last week that it does not intend to challenge a proposal by National Telecommunications Network of Rockville to construct and operate a high-speed, national fiber-optic network.

"The joint venture has the potential to be significantly procompetitive," wrote Assistant Attorney General Douglas H. Ginsburg. "It represents a possible alternative to existing firms offering telecommunications transmission services through broad regions of the United States and nationwide." NTN had asked the Justice Department to review its proposal.

Fiber optics is a low-cost communications technology that uses tiny glass strands to transmit voice, video or data information using light pulses. Companies such as AT&T, MCI Communications Corp. and US Sprint are installing fiber optics in parts of their networks to offer high-speed, lower cost voice and data transmission.

NTN is a joint venture made up of seven regional companies that own and operate, or plan to build, fiber-optic networks. The seven companies are Consolidated Network Inc., Litel, LDX Net, Southland Fibernet, Microtel Inc., Southern Net Inc. and Williams Telecommunications Co.

Martin F. McDermott III, vice president and general manager of the venture, said the seven companies had decided to link up and build the first all-digital nationwide fiber-optic network because of the growing need for data, as well as voice, communications.

"Data applications are growing at about 25 percent a year. It will be the staple of the country in 15 years," he said.

McDermott said the venture, formed at the beginning of 1985, will be the first nationwide fiber-optic network in the country.

Participants in the venture have raised about $750 million to complete three-fourths of the network, which eventually will have 15,000 miles of fiber-optic cable. The network is to be completed by the end of 1987. Washington is due to be added in August.