A monolithic, environmentally stable Mach-Zehnder device is formed by threading first and second dissimilar optical fibers through the bore of a glass tube. The tube is evacuated and heated to collapse it onto the fibers. The tube is further heated and stretched at two spaced locations to form two couplers that are joined by the two optical fibers. The fibers have different propagation constants in the region between the two couplers, whereby there is a relative delay in the optical signal propagating in one of the fibers with respect to the optical signal propagating in the other fiber.