The Reagan administration has secretly approved the export to West Germany and France of special lasers for research that could make it simpler and far cheaper to produce highly enriched uranium and super-grade plutonium suitable for nuclear bombs.

The so-called tunable diode lasers, produced by Laser Analytics Inc., were sold to the French and German governments for research in laser isotope separation, a sophisticated process that is also the object of an intensive development program at the Energy Department's Lawrence Livermore and Los Alamos National Laboratories.

The decision to permit export of the lasers for such research, which had to be approved by Energy Secretary James B. Edwards under terms of the Atomic Energy Act, was classified by the Energy Department on grounds that to disclose it might compromise the competitive position of the American manufacturer.

Energy Department officials yesterday, however, confirmed that the exports had been authorized, but said they did not think the lasers would significantly speed either the French or German programs to develop the isotope separation process.

But the approval of the export of technology specifically earmarked for this kind of research would at a minimum appear to be a further departure by the Reagan administration from the longtime U.S. nonproliferation policy of not helping foreign countries develop sensitive nuclear technologies.

The encouragement of foreign laser isotope separation research also is certain to cause concern in Congress, which has strenuously objected to the potential use of this technology even in this country to upgrade plutonium from civilian nuclear fuel reprocessing plants and make it more suitable for use in nuclear weapons.

While the administration spent $47 million in the past year on laser isotope separation research aimed at upgrading plutonium in military programs, Congress is on the verge of enacting legislation that would bar use of commercial nuclear materials for nuclear weapons purposes. A bill containing this prohibition is in conference.

Yet another concern that has been expressed by some members of Congress is that exports of this kind might undercut the apparent American lead in developing a new technology that could significantly cut the cost of enriching uranium to fuel nuclear power plants.

The United States, once the sole supplier of low-enrichment uranium for use as fuel in nuclear power plants around the world, has seen its share of what is currently about an $8 billion annual world market reduced to 35 percent in recent years.

The Energy Department in April signed an eight-year, $300 million contract for "large-scale" engineering development and a demonstration of the laser isotope separation process for enriching uranium at the Lawrence Livermore facility. But those advocating a stronger nonproliferation stance by the administration worry more about the use of this technology to upgrade plutonium.

This concern first surfaced last fall, when the Energy Department disclosed that it would have difficulty producing sufficient plutonium in its military reactors to build the 14,000 new nuclear warheads President Reagan has proposed adding to the U.S. military stockpile.

As a result, the administration said it was investigating the possibility of applying the laser enrichment process to the upgrading of the enormous quantities of plutonium that have been produced by America's 75 operating civilian nuclear power plants. This proposal ran into immediate opposition in Congress, where Sen. Gary Hart (D-Colo.) introduced the amendment that would bar such use.

What concerns Hart and other members of Congress is that any move by the United States or other countries to use the spent fuel from commercial power plants as the raw material for nuclear bombs breaks down a barrier that until now has existed between atoms for peace and atoms for war.

Technically, the plutonium that emerges from a nuclear reactor, once it is separated from the rest of the spent fuel at a reprocessing plant, is suitable in that form for fabrication into a nuclear weapon. But while the Defense Department has tested a nuclear bomb made up of such materials, experts regard a weapon made from this type of plutonium as unreliable.

The reason is that the plutonium that emerges from a nuclear reactor contains both the plutonium-239 isotope, which is ideal for weapons, and the plutonium-240 isotope, which interferes with the fission process.

To make weapons-grade plutonium, the Energy Department extracts the fuel from its military production reactors after it has been in only a short time, and only 3 percent of this super-grade plutonium consists of the isotope 240.

The fuel that comes out of most commercial power stations, having been left in longer, averages at least 12 percent isotope 240. Any plutonium containing more than 6 percent isotope 240 is regarded as less than desirable for weapons use.