 and are used for sin- gle-mode transmission at 1,550 nm. The VCSEL is now popular for GbE operation at 850 nm. To give a rough view of the relative costs involved with these transmitters at the time of this writing, the Original Equipment Man- ufacturer (OEM) cost in volume of an LED transmitter/receiver (trans- ceiver) component is about $20, an FP laser transceiver is about $100, a DFB laser is about $600, and a gigabit VCSEL is about $70.6 The receivers in these types of relatively inexpensive transceivers are similar. They con- sist of a special diode that converts photons into electrons. Decent receivers are able to detect light levels down to hundreds of nanowatts. Note that the receiver does not care if the transmitter is multimode or single mode; it just knows that photons arrive, regardless of the path they take to get there. Distributed Bragg Reflector Laser (DBR) Distributed Feedback Laser (DFB) Use periodic \-selective Bragg gratings as reflector Etch a "distributed" Bragg grating over active waveguide Tune by injecting current to vary the n of the Bragg grating and Stable, narrow single-mode operation for >2.5 Gb/s of the phase region tuning range ~5 nm Power = 20 mW for signal; >200 mW for pumps Current injection tuning in ~nsec -filter tuning achieved: Variations include: - Thermally: AT of ~ 50°C AN of ~ 5 nm - Sampled grating DBRs Tuning range typically limited to 5~8 nm - Grating assisted codirectional coupler DBRs Multisection DFB have wider tuning but higher losses Mirror Phase Fabrication of multisection DFBs can be complex HR Coating AR Coating X-Selective n-InP Optional Phase Grating Bragg Grating Region Medium MQW Active Region Light into (Waveguide) Fiber p-InP Longer Phase Region Wider Tunability But More Losses Lower Power Thermo-Electric Cooler MQW: Multiple Quantum Wall Vertical Cavity Surface-Emitting Laser (VCSEL) External Cavity Diode Laser (ECDL) Mature technology for 850 nm (GaAs/AIGaAs) Size of cavity set by external mechanical reflectors Tuning achieved via top mirror mounted 