Burst mode optical receiver and repeater

Details Burst mode optical receiver and repeater Burst mode optical receiver and repeater

1998-03-11

2001-05-08

Negash, Kinfe-Michael (Department: 2633)

Optical: systems and elements

Deflection using a moving element

Using a periodically moving element

C359S199200, C359S199200

Reexamination Certificate

active

06229634

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a receiver for use in an optical network, and in particular to a burst mode digital receiver for use in such a network.
2. Related Art
Currently, in the United Kingdom, the telecommunications network includes a trunk network which is substantially completely constituted by optical fibre, and a local access network which is substantially completely constituted by copper pairs. In future, it would be highly desirable to have a fixed, resilient, transparent telecommunications infrastructure all the way to customer premises, with capacity for all foreseeable service requirements—or at least a point (e.g. the curb closer to such customer premises. One way of achieving this would be to create a fully-managed fibre network for the whole access topography. An attractive option for this is an optical tree access network, such as a passive optical network (PON) which incorporates single mode optical fibre and no bandwidth-limiting active electronics.
In a PON, a single fibre is fed out from a head-end (exchange), and is fanned out via passive optical splitters at cabinets and distribution points (DPs) to feed optical network units (ONUs). The ONUs can be in customers' premises, or in the street serving a number of customers. The use of optical splitters enables sharing of the feeder fibre and the exchange-based optical line termination (OLT) equipment, thereby giving PONs cost advantages. At present, simplex deployment of PONs is the preferred option, that is to say separate upstream and downstream PONs are provided whereby each customer has two fibres. Although simplex working increases the complexity of the infrastructure due to the two fibres per circuit required, it benefits from a low optical insertion loss (due to the absence of duplexing couplers), and a low return loss, since such systems are insensitive to reflections of less than 25 dB with separate transmit and receive paths. However, duplex PONs where one single fibre carries traffic in both directions are also possible. Typically, a PON has a four-way split followed by an eight-way split, so that a single head-end fibre can serve up to 32 customers.
In a known arrangement—TPON (telephony over a passive optical network)—a head-end station broadcasts time division multiplexed (TDM) frames to all the terminations on the network. The transmitted frames include both traffic data and control data. Each termination recognises and responds to appropriately-addressed portions of the data in the broadcast frames, and ignores the remainder of the frames. In the upstream direction, transmission is by time division multiple access (TDMA) where each termination transmits data in a predetermined timeslot, so that the data from the different terminations are assembled into a time division multiple access (TDMA) frame of predetermined format.
Recently, the PON principle has been expanded to form what is known as the SuperPON concept, in which high power optical amplifiers are used to allow very large, high split PONs to be built. For example, the use of optical amplifiers (such as fibre amplifiers) permits at least 3500 customers to be connected to a single head-end station over distances of up to 200 km.
Unfortunately, optical amplifiers can normally only be used on a downstream SuperPON, as the use of amplifiers on an upstream SuperPON would cause noise problems resulting from the superposition of amplified stimulated emissions (ASEs) from the amplifiers. One way of providing amplification in an upstream SuperPON is to replace the last level of split (that is to say the level of split nearest the head-end) by a repeater. This device includes a respective receiver for each incoming branch fibre and a transmitter; the receivers each converting incoming optical signals to electrical signals, amplifying them, and converting the amplified electrical signals to optical signals for onward transmission by the transmitter. Alternatively, each of the fibres leading to the last level of split is provided with a repeater.
Binary data consists of two logic levels, logic level ‘1’ lying above and logic level ‘0’ lying below a pre-set logic threshold. Conventional telecommunications data signals often have a mark-space ratio close to unity. In burst mode, this ratio may be much lower (1:100 or less), as a burst of data may be followed by a long period of silence. Bursts of data from different sources may have different amplitudes, and the bursts may be interleaved, in packet form, to share the same transmission channel.
FIGS. 1
a
and
1
b
illustrate the differences between conventional data signals and burst mode data signals.
A signal which passes through an optical network undergoes attenuation and degradation. Consequently a logic decision threshold must be established at a receiver for the correct identification of each individual bit of data as either a ‘0’ or ‘0’ for subsequent logic processing. It would be desirable, because of its simplicity, to set a constant threshold for all levels of incoming signal. However, if the signal level is varying, a means for automatically altering the threshold may have to be introduced. In the case of a burst mode system, the threshold setting may have to be altered on a burst-by-burst basis, requiring either a very fast acting automatic level detection and threshold setting circuit, or a system with prior knowledge of the level of the incoming bursts which programmes a threshold setting circuit. Clearly, both these approaches lead to some complexity, and often require the use of dc-coupled amplifier stages with consequent problems of drift which may require temperature or other compensation.
SUMMARY OF THE INVENTION
The aim of the invention is to provide a different method of processing incoming data bursts which avoids the need for altering the logic decision threshold, which can operate with bursts which differ considerably in level, which has a capability for wide dynamic range, and which avoids the need for dc-coupled amplifiers.
The present invention provides a burst mode optical digital receiver comprising an opto-electronic detector, up-conversion means for up-converting the frequency of the output of the opto-electronic detector, amplifier means for amplifying the up-converted signal, and down-conversion means for down-converting the frequency of the amplified signal.
Advantageously, the up-conversion means is constituted by a first frequency mixer and an oscillator, the oscillator inputting a carrier of predetermined frequency to the first mixer for mixing with the output of the opto-electronic detector. The down-conversion means may be constituted by a second frequency mixer and an oscillator, the oscillator inputting a carrier of predetermined frequency to the second mixer for mixing with the amplified signal. Preferably, the same oscillator provides the carrier frequency for each of the mixers. For example, if the data rate of the input optical signal is in the region of 300 Mbits
−1
, the oscillator provides a carrier frequency in the region of 500 MHz to each of the mixers.
In a preferred embodiment, the amplifier means is constituted by a first amplifier, a second amplifier and a signal limiter in series, the first and second amplifiers providing good gain, low noise, good linearity and a bandwidth of approximately 10-1000 MHz, and the signal limiter being effective to reduce the range of output pulse amplitudes.
Advantageously, the receiver further comprises a first filter positioned upstream of the first mixer, the first filter being effective to remove the effects of signal degradation caused by the detector and of noise at the output of the detector.
The receiver may further comprise a second filter positioned downstream of the second mixer, the second filter being effective to reduce noise and out-of-band harmonics produced by the signal limiter.
Preferably, a further amplifier and a third filter are positioned between the second mixer and the second filter, the third filter being effective to intro

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