Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Patent
1997-10-27
2000-10-17
Chan, Jason
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
359180, 359181, 359161, H04B 1004, H04B 1000
Patent
active
061340375
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the reduction of crosstalk in an optical network and in particular concerns the reduction of interferometric noise such as incoherent beat noise.
2. Related Art
Interferometric noise may occur when an optical wave sheds a fraction of its power, which later interferes with its parent after having experienced a temporal delay. When the temporal deiay greatly exceeds the source coherence time, the interferometric noise is termed incoherent beat noise. If this condition is not met, the noise is partially coherent or coherent in nature. Several architectures in which such interference takes place have been identified, including delay elements in all-fibre optical bit rate limiters, reflections and Rayleigh backscatter in multi-stage amplified links, and crosstalk in wavelength division multiplexed (WDM) and optical time division multiplexed (OTDM) switching networks.
OTDM switching networks have been demonstrated to support high bandwidth traffic, and are additionally transparent to bit rate, coding format and wavelength. They may be configured to switch continuous high bit rate services such as high definition television (HDTV) or bursty traffic in asynchronous transfer mode (ATM) cells. OTDM switching networks employing amplitude shift keying (ASK) digital transmission/direct detection may be constructed from 2.times.2 switching elements interlinked by optical fibre delay lines (see the specification of PCT/GB92/00400). Inter-channel crosstalk arises in such networks because, in practice, the crosspoints do not possess perfectly isolated outputs. Every TDM channel emerging from the network is corrupted by the unwanted crosstalk waveforms.
In dealing with such unwanted crosstalk, known systems have concentrated on minimising incoherent noise-free crosstalk. The degradation resulting from this crosstalk may be modelled by a "sum of intensities" approach.
SUMMARY OF THE INVENTION
In accordance with the invention, it has been appreciated that interferometric noise, in particular incoherent beat noise crosstalk, can be more significant than incoherent noise-free crosstalk. Incoherent beat noise crosstalk is intensity noise arising from the interference of incoherent waveforms which originate either from very closely-matched wavelength sources such that their beat frequency falls within the receiver bandwidth, or from a single source with a differential delay in the optical network. Incoherent beat noise has been found to induce larger power penalties and bit error rate (BER) floors. The magnitude of the performance degradation induced is dependent upon the crosspoint isolation and the size and configuration of the network. It has been found, however that an isolation of -15 dB, limits the network size to three or four crosspoints, and this is a severe limitation when considering the design of switching networks.
In accordance with the present invention, it has been appreciated that interferometric noise, and in particular incoherent beat noise power arising when crosstalk and data originate from the same source, can be reduced in the optical network by varying the centre frequency of the source.
More particularly, in accordance with the invention there is provided a method of reducing interferometric noise power in an optical network through which data is transmitted as a stream of bits during successive bit periods by an optical source with a given centre frequency, to a receiver for receiving a frequency base band, comprising causing a variation in the centre frequency of the source such as to cause a redistribution of the interferometric noise power from the base band to higher frequencies, thereby reducing noise in the base band.
The method of the invention has particular application to the reduction of incoherent beat noise power.
The source conveniently comprises a laser diode and it has been found that according to the invention the centre frequency can be controlled by controlling the duration of periods when the laser is s
REFERENCES:
patent: 5228043 (1993-07-01), Naito et al.
patent: 5508845 (1996-04-01), Frisken
Burke et al, "128 Line Photonic Switching System Using LiNbO.sub.3 Switch Matrices and Semiconductor Travelling Wave Amplifiers", J. Light Tech. 1992 10(5), pp. 610-615.
Thompson, "Architectures with Improved Signal-To-Noise Ratio in photonic Systems with Fiber-Loop Delay Lines", IEEE Journal of Selected Areas in Communications, 1988, 6(7), pp. 1096-1106.
Gimlett et al, "Effects of Phase-To-Intensity Noise Conversion by Multiple Reflections on Gigabit-per-second DFB Laser Transmission Systems", J. Light. Tech. 1989 7(6), pp. 888-895.
Padmanabhan et al, "Dilated Networks For Photonic Switching", IEEE Trans. Commun. 1987, 35(12), pp. 1357-1365.
Wood et al, "Bi-Directional Fiber-Optical Transmission Using a Multiple-Quantum Wel (MQW) Modulator/Detector", Electronic Letters 22(10), 1986.
Wood et al, "Measurement of the Effect of Optical Beat Interference on the bit Error Rate of a Sub-Carrier-Based Passive Optical Network", presented at OFC 1993, San Jose, California, 1993, paper ThM3.
Yariv et al, "Reduction of Interferometric Phase-to-Intensity conversion Noise in Fiber Links by Large Index Phase Modulation of the Optical Beam", Journal of Lightwave Technology, vol. 10, No. 7, pp. 978-981.
Hunter et al, "Architectures for Optical TDM Switching", OE/Fibers '92, SPIE Boston 1992, Paper 1787-18.
Barnsley Peter Edward
Hunter David Kennedy
Legg Peter Jonathon
Shabeer Mohammed
Tur Moshe
British Telecommunications public limited company
Chan Jason
Sedighian Mohammad
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