on MEMS actuator X-filtering done external to laser diode. Tuning speed depends Manufacturing advantages over edge-emitting lasers on type of actuation (Motor, MEMS, LC) Easier packaging with vertical emitter and circular beam High power and good spectral purity (such as Low-SMSR) Lower power output than edge-emitting lasers [5-10 mW] Wide tuning range of > 100 nm limited by gain medium, not tunable filter Oth Order Output QW Active Region DBR Mirror p-AIGaAs/GaAs Example of a Littman-Metcalf ECDL Profession InGaAs HR Coating AR Coating Order DBR Mirror n-InAIGaAs/ Optional, Grating InAIAs low-ASE Laser Diode n-InP Substrate Second Diffraction Output Pivot Point -Filter Figure 2-29 Tunable laser technologies. Optics Primer 2.7 Fiber-Optic Receivers Optical receivers are coupled to the fiber strand as a unit. They convert on- off light pulses by photodetection into electrical signals. Those signals are processed by electronic components to provide the output data stream. There are two kinds of photodetector diodes: the Positive Intrinsic Negative (PIN)¹ and the avalanche photodetector diode (APD)¹ type. Typically, the APD type can detect lower optical signal power than the PIN type by about 10 dB (at 10-9 BER) (see Figure 2-30). An ideal receiver should offer good sensitivity (low received power for 10-9 BER), wide dynamic range, relatively low cost to manufacture and operate, reliability, low dependence on temper- ature, and SO on. None of the diodes currently in production have all of the ideal characteristics. For example, the APD has the highest sensitivity and a good dynamic range, but it is difficult to produce. InGaAs photoconductors have poorer sensitivity, but are simpler and have the potential to be inte- grated with low-noise amplifiers. Over time, advances are expected to pro- vide improved receivers with integrated optic and electronic circuits. In summary, Table 2-5 recaps the key parameters associated with the optical transmission links we've just explored. Table 2-6 lays out some link engineering i