The demand for high-speed communications is growing exponentially. Recent trends in the integration of entire systems-on-chip have spurred the development of Fibre-To-The-Home (FTTH) network for high-speed data and video services. This paper presents a predistortion circuit that integrates all of the functions necessary to implement a high linearity distribution point system for broadband optical fibre communications. The single CMOS chip includes a variable gain amplifier and a predistortion circuit. The circuits have been implemented with 0.35μm CMOS technology and simulation shows that the power consumption is 86mW with a 3.3V supply. The systems and circuits are detailed and their application to analogue optical links discussed.
A CMOS optical fibre transmitter front-end with an analogue predistortion technique is proposed for reducing laser nonlinearity distortion to achieve broadband linearisation of radio frequency (RF) optical fibre communication systems. The technique uses a nonlinearity having the inverse transfer characteristic of the directly modulated vertical cavity surface emitting laser (VCSEL). The analogue predistortion lineariser comprises a linear and a non-linear transconductance differential amplifier. The combined linear and non-linear transconductance differential amplifiers provide the transfer characteristic to counteract the characteristic of the VCSEL laser diode. The post-layout simulation of the linearity of RF optical fibre systems using the predistortion linearisation technique shows 12dBm improvement. The integrated CMOS optical fibre transmitter circuit with the predistortion lineariser is being implemented using the austriamicrosystems (AMS) 0.35μm CMOS technology.
This work is a part of the project entitled Optical Access Key featuring Low-Power Electronic Adaptive Function (OAKLEAF). A partnership has been formed between the Universities of Cambridge and Essex that aims to build on existing research and has two thrusts; verify that VDSL modem compensation techniques can include impairments present in a complete link comprising optoelectronic, radio and copper media components and to produce a demonstrator showing ho VCSEL-based bandpass modulators may offer an effective interface for hybrid xDSL/optoelectronic access networks. In order to analyze and assess the effect of VCSEL non-linearities in a bandpass modulation context a simulation study has been carried out on an 8-channel optical sub-carrier modulated QAM-VDSL system. Also, a new integrated circuit functional block, a combiner and laser driver with predistortion circuit has been designed and simulated to compensate for the non-linearities of the VCSEL.