DFB: Distributed feedback. A resonant-cavity laser using grids etched into the bottom of a semiconductor substrate as a reflective medium.

DBR: Distributed Bragg reflector. A laser in which the etched grating is physically separated from the electronic pumping area of the semiconductor laser. See also SG-DBR.

DWDM: Dense wavelength-division multiplexing. A method of passively combining multiple wavelengths by color. Equipment that combines only a few such wavelengths is known as WDM or CWDM (coarse WDM). The most widely used transponders, however, utilize tens or hundreds of channels in tightly spaced grids, hence the name DWDM.

EAM: Electro-absorption modulators. Chip-level modulation devices often integrated into hybrid transponder devices alongside lasers.

ECDL or External-Cavity: External-cavity diode laser, using a cavity that is separated from the lasing device. Usually employs some form of MEMS to actuate the reflective area for the laser.

FP or Fabry-Perot: The simplest form of fixed-frequency semiconductor laser, based on a resonant-cavity design using the wafer substrate material as the end mirrors.

G-MPLS or Generalized MPLS (see below): An extension of MPLS which allows an IP flow to be associated with a particular wavelength. This would allow a tunable laser to dynamically switch a Voice Over IP call, or a video conference service, to a specific wavelength. Well-defined in Internet Engineering Task Force, but so far used only in lab trials.

ILEC: Incumbent local exchange carrier, or "baby Bell." The primary service provider operating within one local area. With the demise of the majority of CLECs, or Competitive Local Exchange Carriers, the ILEC has become the only local service provider for many businesses and residences.

IP: Internet Protocol, often referred to as TCP/IP for Transport Control Protocol and Internet Protocol. Because of the popularity of the Internet, this packet structure became the most popular way of transporting all information across carriers. Because it was designed for data and not for video and voice, many special protocol tricks have to be employed to carry IP over optical systems, or even for carrying voice and video in electronic networks that utilize IP.

IXC: Interexchange carrier, or a service provider specializing in long-distance transport services across multiple local exchanges.

MEMS: Microelectromechanical systems. Devices operated mechanically on a microscopic level, such as MEMS micromirrors or MEMS waveguides, to guide wavelengths in optical switches and tunable lasers.

Mesh: An any-to-any network in which a node is connected to virtually any other node in a collection of cross-connect links, implemented using OXCs. This can allow very efficient routing, but can be expensive to implement.

MPLS: Multi-protocol label switching. A way to keep all IP packets from the same voice or video session associated with each other in a common "flow," by adding a special label to the IP packet.

OADM: Optical add-drop multiplexer. An optical extension of a common Sonet element called add-drop multiplexer, this is a piece of equipment that allows wavelengths to be added or dropped at particular node points. This would be a particularly useful system to employ a tunable laser.

OXC: Optical cross-connect. This is the true system-level optical switch, employed primarily in long-distance networks, that can shift services from one wavelength to another, thus representing a key target system to utilize a tunable laser. The initial dream of OXC specialists like Sycamore Networks, Tellium and Ciena was to extend the OXC from long-distance to local networks. But this requires the shift in metropolitan networks from rings to meshes, an evolution that has not happened to any great extent. This has soured the fortunes of companies like Sycamore and Tellium, as well as the laser manufacturers that would supply these OEMs.

Sonet: The synchronous optical network, known in Europe as synchronous digital hierarchy, or SDH. This is a 20-year-old standard for metropolitan fiber rings, used by ILECs. Traffic is carried in an electronic packet transported over fiber, using dual counter-rotating rings. The systems are relatively inexpensive to implement, but they were designed in the 1970s for primarily circuit-switched voice traffic, so they make very inefficient use of bandwidth in carrying traffic that is mostly IP-encapsulated data.

SOA: Semiconductor optical amplifier, a semiconductor-based amp often integrated with both fixed-frequency and tunable lasers.