Light-to-light readout system of the CMS electromagnetic calorimeter

For the CMS experiment at the Large Hadron Collider at CERN, an 8OOOO-crysral electromagnetic calorimeter will measure electron and photon energies with high precision over a dynamic range of roughly 16 bits. The readout electronics will be located directly behind the crystals, and must survive a total dose of up to 2x10 Gy along with 5x10**1**3 n/cm**2. A readout chain consisting of a custom wide-range acquisition circuit, commercial ADC and custom optical link for each crystal is presently under construction. An overview of the design is presented, with emphasis on the large-scale fiber communication system. 11 Refs.