Free space optical communications (FSO) requires receivers with a wide field of view, large collection area and high bandwidth, as well as good rejection of unwanted ambient illumination. At present most of the optoelectronic components used in these systems are designed for fibre-optic systems and as such are not optimal for this application.
Work at the Universities of Oxford, Cambridge, Huddersfield and Imperial College has produced receivers incorporating detectors and preamplifiers specifically optimised for FSO and these show performance beyond that available commercially available. In this paper we describe the design, fabrication and performance of these integrated components. Further, we describe how this performance might scale with further optimisation, and future directions for optical receiver design.
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.
Most free-space line-of sight systems require tracking in order to keep links aligned, and in order to achieve this a number of disparate optical, optoelectronic and electronic components are required. A key factor in determining the performance of these systems is the ability to integrate these in a scalable compact fashion, and to optimise components for the somewhat distinct requirements of free-space links.
A number of UK universities have been involved in a consortium that has fabricated integrated transceivers that use fully custom components optimised for an indoor free-space link application. The transmitters use arrays of Resonant Cavity LED (RCLED) devices integrated with custom CMOS driver circuitry and the necessary beamshaping optics, so that operating a particular LED in the array transmits light at a particular angle. A similar approach is taken at the receiver; light from a particular angle illuminates one element of a PIN photodiode array. This is integrated with an array of custom CMOS receivers and the necessary optics, creating a compact receiver subsystem. In this paper the components and subsystems are detailed and their application to long-distance links discussed.
This paper describes the development and reports measured performance of integrated CMOS receiver and transmitter circuits for use in an optical wireless link operating at bit rates up to 310 Mb/s. The receiver presented is an angle-diversity design and consists of multiple sectors each driving an individual pre-amplifier channel. The speed limitation for the receiver circuit is determined substantially by the parasitic capacitance introduced by the photodetector. With current PIN devices this capacitance may be comparatively high, of order several picofarads as a relatively large field of view is required for optical wireless applications. The design incorporates an on-chip selector with external controls determined by the signal level. Signals from detectors that receive optical power above a certain threshold level are passed to a combiner circuit. In the transmitter, in order to avoid limiting the optical performance of the emitter, the electrical response of the LED driver is enhanced by current-peaking and charge-extraction circuitry. A novel timing generator is used to achieve fast rise and fall times. Experimental results confirm that the true performance evaluation of high-speed circuits can be severely hindered by parasitics associated with wire bonding and packaging of chips. Flip-chip packaging, advantageous for its small form factor and low capacitance leading to high speed has been investigated. This has led to the development of fully integrated receiver and transmitter systems where the photodetector and photoemitter devices are directly bonded to supporting CMOS substrates which furnish the necessary support electronics.
The widespread use of Optical LANs is dependent on the ability to fabricate low cost transceiver components. These are usually complex, and fabrication involves the integration of optoelectronic and electronic devices, as well as optical components. A consortium of four UK universities are currently involved in a project to demonstrate integrated optical wireless transceiver subsystems that can provide eye-safe line of sight in-building communication at 155Mbit/s and above.
In this paper we discuss the flip-chip integration of two-dimensional arrays of novel microcavity LEDs with custom CMOS integrated circuits in order to produce solid state tracking emitters. Design, fabrication and integration of these structures are detailed. The scaleability and future capability available given further optimisation and development of these systems is also discussed.
The purpose of this work is to develop integrated CMOS designs for optical transceivers at 1.55um wavelength that both meet the current system specification of 155Mb/s and provide a viable upgrade path to higher bit-rates. We present the design and implementation of an integrated multi-channel CMOS transceiver for use in a cellular 155Mb/s Manchester-coded optical wireless link. The receiver is an angle-diversity design and consists of multiple sectors with relatively small field of view; each driving an individual pre-amplifier channel. An on-chip selector selects signals to be passed to the combiner depending on the signal level and external control signals. The outputs of all the selected channels are combined using a current summing junction, implemented using a transconductance-transimpedance approach. In order to achieve a receiver design that will be robust in the face of process variations, an on-chip circuit is provided to maintain the operating point of the amplifier chain. The design has been optimized to achieve -30dBm sensitivity at a BER of 10-<sup>9</sup>. The CMOS transmitter circuit is tailored to match the electro-optic response of the resonant cavity LEDs being used. The transmitter driver incorporates current-peaking and charge-extraction circuitry using a novel timing generator, and has been designed to achieve rise and fall times of better than 0.2ns. Considerable effort is being directed towards the development of integrated designs which do not require significant numbers of discrete components. The prototype designs are being realised in a 0.7μm commodity mixed-signal CMOS process by Alcatel Microelectronics. We report results from the first prototype multi-channel demonstrator system and discuss future research directions.
The widespread use of Optical LANs is dependent on the ability to fabricate low cost transceiver components. These are usually complex, and fabrication involves the integration of optoelectronic and electronic devices, as well as optical components. A number of UK universities are currently involved in a project to demonstrate integrated optical wireless transceiver subsystems that can provide eyesafe line of sight in-building communication at 155Mbit/s and above, using 1550nm eyesafe emitters. The system uses two-dimensional arrays of novel microcavity LED emitters, and arrays of detectors integrated with custom CMOS integrated circuits to implement tracking transceiver components. The project includes design and fabrication of the optoelectronic devices, transimpedance amplifiers and optical systems, as well as flip-chip bonding of the optoelectronic and CMOS integrated circuits to create components scaleable to the large numbers of sources and detectors required. In this paper we report initial results from the first seven channel demonstrator system. Performance of individual components, their limitations and future directions are detailed.
12 This paper presents the design and implementation of a CMOS 310 Mb/s receiver for use in a multi-channel 155 Mb/s Manchester-coded optical wireless link. The receiver consists of a pre-amplifier followed by a post amplifier circuit. The pre-amplifier is a three stage transimpedance amplifier with an NMOS load at the output of each stage to control gain and stability. To allow the sensitivity of the performance to key parameters to be visualized a nomograph technique was developed. The contours of the nomograph show how DC bias, dominant pole frequency and the gain of each stage vary with transistor dimensions. This allows the designer to select transistor sizes for a given bit rate and for stable operation. The design has been optimized to achieve -30 dBm sensitivity at a BER of 10<SUP>-9</SUP>.
12 A 155 Mb/s CMOS LED driver for low-cost optical wireless links is presented. The driver is realized in a 0.7 micrometers commodity mixed-signal CMOS process from Alcatel Microelectronics. The driver employs switching transistors which drive a current mirror to generate modulation current. The circuit also contains a controllable quiescent current source. The transmitter driver incorporates two novel features: adjustable current peaking and adjustable charge extraction. These functions are implemented with the use of original timing generators. Simulations indicate that the design achieves output rise times and fall times of less than 2 ns. Other design parameters were selected to suit the requirements of the InP resonant cavity LED that was developed for this application.
12 Maintaining high bandwidth indoor optical wireless channels under a wide range of operating conditions usually requires relatively complex transceiver components. Integrating optical, optoelectronic and optical components using techniques that are suitable for mass manufacture is an important step in the development of these systems. This paper describes work to develop low cost integrated tracking transmitter and receiver components for use in a cellular indoor optical wireless network. A seven channel demonstrator operating at 155 Mb/s is under construction, using arrays of Resonant Cavity LEDs, PIN detectors, Silicon CMOS driver circuits and associated optics. Development of components, design methodology and initial results are detailed.