cost is not eight times the cost of a PIN diode but, for high volumes, could be less than two times. Telecommunication Networks Electronics Hybrid front end Hybrid Electronics Hybrid front end Hybrid Local oscillator Hybrid Electronics Hybrid front end Hybrid Electronics Hybrid front end Electronics Electronics Electronics Electronics FIGURE 9.29 Block scheme of the quadrature coherent receiver: (a) optical front end; (b) electronic processing. In the optical front end, the gray part can be integrated in an integrated optics circuit: a monolithic Si-SiO2 chip for the hybrids and an InP chip the photodiodes array. Next Generation Transmission Systems Hybrid Hybrid Hybrid Hybrid Thermal phase control FIGURE 9.30 Schematic layout of the hybrids Si-SiO2 chip. (a) Represents the functional scheme of the chip and (b) a simpli- fied layout. At the exit of the balanced receivers, the four photocurrents are modulated at the intermediate frequency Aw and further electronic demodulation allows the four field quadratures to be derived. In order to allow this operation to be performed with high performances, the intermedi- ate frequency Aw has to be stabilized. This can be achieved by a frequency lock loop driven by the wavelength of the incoming signal [31,51]. If the LO frequency is exactly locked to the frequency of the incoming signal the IF demodulation is not needed and the photocurrents are directly in baseband. In this case, the coherent receiver is called homodyne [31]. Under a processing point of view, heterodyne receiver is more complex, but it has been used in radio frequency systems for a long time to alleviate the difficulty of designing wideband components with a very low low-pass frequency (ideally working up to con- tinuous wave [CW]). In optical receivers, heterodyne detection was proposed for the same reason when coherent receivers was first proposed. With the progress of electronics and with the possibility of digital design also for ana- log functions, this difficulty is no more SO important and the simpl