Jacques Bellon

Integrated transceivers for optical wireless communications

Line-of-sight free-space optical links can provide extremely high bandwidth communications, but this usually requires that transmitter and receiver are precisely aligned. In order to allow terminals to be mobile, links must be able to track users within their field of view so that the link is maintained. There are various means to do this, but all require complex subsystems with a number of different optical, optoelectronic, and electrical components. A solid-state tracking architecture is introduced and a seven-channel tracking system demonstration described. The system is designed to operate at 155 Mb/s and is, to the best of our knowledge, the first that uses an integrated approach. Arrays of novel resonant cavity LED (RCLED) emitters that operate at 980 nm are used as sources. These are flip-chip bonded to arrays of CMOS driver circuits and integrated with the necessary transmitter optics. The receiver uses a back-illuminated detector array flip-chip bonded to arrays of custom CMOS receivers. All these components are custom and have performance substantially better than nonoptimized commercially available components. In the paper, the design and fabrication of the optics, optoelectronics, and electronics required for this is described. Successful operation of all the subsystems is detailed, together with results from an initial link demonstration.

High-speed integrated transceivers for optical wireless

Optical wireless LANs have the potential to provide bandwidths far in excess of those available with current or planned RF networks. There are several approaches to implementing optical wireless systems, but these usually involve the integration of optical, optoelectronic, and electrical components in order to create transceivers. Such systems are necessarily complex, and the widespread use of optical wireless is likely to be dependent on the ability to fabricate the required transceiver components at low cost. A number of UK universities are currently involved in a project to demonstrate integrated optical wireless subsystems that can provide line-of-sight in-building communications at 155 Mb/s and above. 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. In this article we set out the basic approaches that can be used for in-building optical wireless communication and argue the need for an integrated and scalable approach to the fabrication of transceivers. Our work aimed at implementing these components, including experimental results and potential future directions, is then discussed.

High speed integrated optical wireless transceivers for in-building optical LANs

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.

Transit-time limitations in p-i-n photodiodes

A novel derivation for predicting transit-time effects in p-i-n photodiodes is described in which the diode current is found by simple integration. Experimental results obtained from two wide-area photodiodes compare well with theoretical predictions. It is also shown that previously published theory does not agree with these experimental observations.

Integrated CMOS transmitter driver and diversity receiver for indoor optical wireless links

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.

Flip-chip integrated optical wireless transceivers

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.

High-speed integrated optical wireless system demonstrator

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.

Resonant-cavity LED transceiver arrays for optical wireless communication

155 Mb/s operation of an optical wireless link is achieved by using the spectral characteristics and angular emission spectra of a 7-element tracking array of 980 nm RC-LEDs. Preliminary results show extension to 200 Mb/s/channel.

Solid state tracking integrated optical wireless transceivers for line-of-sight optical links

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.

Advanced receivers for free-space optical communications

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.