Tunable laser technology is the basis for a lawsuit in which Photonetics accuses New Focus Inc. of infringing a US patent. Photonetics Inc. of Peabody, Mass., and Photonetics SA of Marly-le-Roi, France, sued New Focus of San Jose, Calif., on July 20. The federal lawsuit alleges that New Focus tunable lasers infringe Photonetics' US Patent No. 5,594,744. Tunable lasers are a core technology in test instruments for wavelength division multiplexed (WDM) communications networks. Because these devices can vary their wavelength, one of them can test several channels of a WDM system. The Photonetics patent, "Singlemode laser source tunable in wavelength with a self-aligned external cavity," is the foundation for Photonetics' Tunics tunable laser, the company's core product line. The patent covers single-mode lasers that change wavelengths via an external cavity with specific characteristics: a resonant cavity with a partly reflective output face, a retroreflecting dispersive device that defines two collimating axes, and an amplifier waveguide that is located inside the cavity. New Focus has its own patent on external-cavity tunable laser technology, US Patent No. 6,108,355, "Continuously tunable external cavity laser." This patent covers a laser comprising a laser amplifier, retroreflector, first and second reflector, and a translator that moves the reflectors to vary the separation between them and thereby tune the laser amplifier.