COAX products - Modulight

Cable televisionHybrid Fibre Coax (HFC), that incorporates bothfiber optic transmission components and coppercoaxial transmission components is a way ofdelivering video, voice telephony, data, and otherinteractive services over coaxial and fiber opticcables.The HFC network (figure) consists of networkinterface units, fiber nodes, distribution center,and head-end office. The network allows installationof an optical fiber from the cable head-end toserve the distribution center and particular fibernode, in which the optical signal is converted toelectrical signals. The electrical signal is redistributedusing coaxial cable to the subscriber’spremises, where network interface units distributethe signal to different devices. Such network withreturn channel, offers the subscriber a cost-effectiveservice upon demand with high speed.In CATV, for which HFC was originally designed,the AM-modulated data is setting strict requirementsfor the network linearity and noise properties.This requirement on the other hand favoursthe fiber optic transmission path, but at the sametime places stringent requirements on the fiberoptic transmitter i.e. laser module linearity andnoise properties.The relative intensity noise (RIN) is playing a keyrole in Carrier-to-Noise (CNR) performance of thefiber optic link, especially in short transmissiondistances at 1310nm. The RIN of Modulight’spigtailed laser module series designed specificallyfor analogue applications is of the order of150dBc/Hz and 145dBc/Hz, for Distributed Feedbackand Fabry-Perot lasers, respectively.In addition, the analog video signal is also quitesensitive to nonlinearity in the form of intermodulationdistortion. The distortion spectrum containsmany discrete lines deteriorating sensitive analogsignals. Very tight linearity criteria are used indesigning the HFC systems and extra attentionis paid for the optical transmitter distortion, butalso for distortion of the RF cascade. Modulight’spigtailed laser module series designed specificallyfor analogue applications is characterized againstthe stringent intermodulation distortion requirementsof the CATV transmission networks.12