Rodney T. Unwin

A comparison of coherent digital PPM with PCM

Digital pulse position modulation (PPM) is a preferred modulation format for the ideal photon counting channel and optical intersateilite links. Here we examine its potential for the coherent optical fibre communications channel. We present a thorough performance and optimisation analysis. Comparisons, at a wavelength of 1.5 m, are made with shot-noise limited coherent PCM (homodyne and heterodyne ASK, FSK and PSK) over a range of fibre band-widths and varying PPM word size. We conclude that for moderate to high fibre bandwidths homodyne digital PPM should achieve an improvement in sensitivity of typically 5 dB over homo-dyne PSK PCM

A monolithic common-collector front-end optical preamplifier

A monolithic transimpedance preamplifier has been developed having a common-collector cascode configuration with shunt feedback, using an advanced bipolar IC process. The measured sensitivity was -35.0 dBm at 140 Mbit/s for an error rate of 10-9and a p-in photodiode responsivity of 0.5 A/W.

The design of tuned front-end GaAs MMIC optical receivers

Recently, much interest has been shown in the design of very low noise tuned front-end optical receivers for use in lightwave systems. This paper looks at the accurate design of such receivers. Simplified design expressions are presented for a number of tuning configurations with theoretical and measured results being presented. The designs were realized as GaAs monolithic microwave/millimeter-wave integrated circuits (MMICs).

10 MB/s optical wireless ethernet: practical results

Optical wireless is a relatively new field, its development encouraged by the ever growing popularity of mobile computing and communications equipment. We present practical results for a 10 MB/s diffused optical wireless Ethernet system which allows a user to freely roam around a room with a laptop computer while maintaining a full Ethernet service. This system has been developed in collaboration with BT labs. Receiver designs and analysis are presented. Also, optical systems have been considered, and practical results presented.

Optical wireless video distribution

In this paper we discuss the design of an optical telepoint receiver. The level of receiver performance required when optical baseband video signals are to be recovered is discussed and a suitable receiver topology is presented. Theoretical and measured results are presented. The effect of ambient light is considered.

Optical soliton transmission with n-ary PPM

In n-ary PPM, a single high energy pulse is used to convey M bits of information. In this paper we propose that advantage be taken of the fiber non-linearity such that the n-ary PPM pulse becomes a temporal soliton. We examine the maximum achievable bit rate of such a system and compare it to that of PCM. We show that under ideal conditions (no fluctuations of the input soliton pulse width) n-ary PPM can be used to achieve a higher bit-rate than PCM. However, in a practical situation fluctuations of the input soliton pulse width are translated to variations in the soliton pulse position. We show that this has a severe effect on the error probability of n-ary PPM and that in this situation PCM offers greater immunity. Finally, we consider the implications of the Gordon-Haus effect on the maximum transmission distance of n-ary PPM and show that it is less than that for PCM due to the narrower pulse arrival time window.

Optical fiber n-ary PPM: approaching fundamental limits in receiver sensitivity

The n-ary pulse position modulation is a means by which excess optical fiber bandwidth can be exchanged for improved receiver sensitivity. Here, we consider both direct detection (PIN- BJT and PIN-FET) and coherent n-ary PPM over a range of fiber bandwidths and PPM word sizes. Our results illustrate that n-ary PPM offers an improvement of between 5 - 11 dB over equivalent PCM systems. Assuming an optical fiber attenuation of 0.2 dB/km this represents an increase in regenerator spacing of between 25 - 55 km. The theoretical results demonstrate that n-ary PPM is a promising modulation format and may have potential for future telecommunication routes.

Coherent detection: n-ary PPM versus PCM

In this paper we present computer results at 1. 5zm comparing coherent n-ary PPM with coherent PCM. We show that for a given fibre bandwidth there is an optimum word size and this occurs when the equivocations attributed to the error sources in n-ary PPM are equal. We conclude that for moderate to high fibre bandwidths homodyne n-ary PPM should achieve an improvement in sensitivity of typically 5 dB over homodyne PSK PCM.© (1991) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Simplified tuned optical receivers for use in lightwave optical subcarrier multiplex systems

In this paper we investigate the design and realization of broadband tee-tuned front-end optical receivers. The receivers will be implemented as a GaAs MMIC using the GEC/Marconi F20 foundry, the active devices are 0.5 micrometers gate-length GaAs MESFETs. Simple design equations are introduced that show the importance of the intrinsic noise parameters P, R and C, which are used to characterize the noise performance of such devices, as well as enabling initial values of tuning elements to be obtained. Finally measured results for a MMIC receiver designed are presented that confirm the expected noise performance as well as the validity of the design process.

Design of tuned optical receivers for use in high-sensitivity light-wave systems

Recently much interest has been shown in the design of very low noise tuned front-end optical receivers for use in Lightwave Sub-carrier Multiplex Systems as well as Coherent Lightwave Systems. This paper looks at the requirements that need to be considered to enable the accurate design of such tuned front-end optical receivers. Simplified design expressions are presented for a number of tuning configurations and theoretical results as well as practical measured results are presented for one of the designs. The design was realized at GaAs MMICs using the GEC/Marconi F20 foundry process.