Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1996-06-07
2001-06-26
Pascal, Leslie (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200
Reexamination Certificate
active
06252692
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to optical transmission systems, to control systems for optical transmission systems, to dispersion measurement systems, and to elements for receiving or processing signals in optical transmission systems, and to methods of transmitting data along an optical path.
BACKGROUND OF THE INVENTION
The distance between optical terminals of optical fibre transmission systems is limited by the optical power that can be launched into optical fibre by optical transmitters of the optical terminals, the loss and dispersion of optical fibre interconnecting the optical terminals, and the sensitivity of optical receivers of the optical terminals. Where the distance between desired end points of an optical fibre transmission system exceeds the maximum distance between optical terminals, optoelectronic repeaters have been provided. Each optoelectronic repeater comprises an optical receiver for converting the optical signal to an electrical signal, electronics for regenerating the electrical signal, and an optical transmitter for converting the regenerated electrical signal to an optical signal for transmission to the next optoelectronic repeater or to a terminal of the system. There are two main techniques for multiplexing signals in such systems, which operate by wavelength division or time division.
In Wavelength Division Multiplexed (WDM) optical fibre transmission systems which use optoelectronic repeaters, the optical signals are optically demultiplexed at each repeater, so that the signal at each distinct wavelength is coupled to a respective optical receiver for conversion to a respective electrical signal, each respective signal is applied to a respective optical transmitter operating at a distinct wavelength, and the transmitted signals are optically multiplexed for transmission to the next optoelectronic repeater or to a terminal of the system.
As the line rates of optical fibre transmission systems increase into the 2.5 Gbps to 10 Gbps range, higher speed electronics are needed in optoelectronic repeaters, and this increases the cost of optoelectronic repeaters.
Optical amplifiers, for example Erbium Doped Fibre Amplifiers (EDFAs), amplify optical signals directly without converting them to electrical signals. Because EDFAs do not require high speed regeneration electronics, they can be cheaper than optoelectronic repeaters for high speed optical fibre transmission systems.
Moreover, in WDM optical fibre transmission systems, the EDFAs can amplify optical signals at multiple wavelengths without optically demultiplexing them, thereby avoiding the costs of optical multiplexing and demultiplexing, and the costs of multiple optical receivers, multiple regeneration circuits and multiple optical transmitters. Consequently, EDFAs can also be cheaper than optoelectronic repeaters for WDM systems. However, degradation by noise and dispersion effects builds up when optical amplifiers are used. Thus a regenerator may be necessary after several optical amplifier stages, to rebuild the data signal and remove the noise and dispersion degradation.
Disregarding intermodal dispersion which only occurs in multimode fibre (not used in practice for high capacity systems), dispersion, also known as Group Velocity Dispersion, in fibre at least, occurs as a result of two mechanisms:
1 intramodal dispersion—within a single mode different frequencies travel along the fibre at different speeds;
2 material dispersion—the phase velocity of plane waves in glass varies with frequency.
Dispersion is the derivative of the time delay of the optical path with respect to wavelength. The effect of dispersion is measured in picoseconds arrival time spread per nanometer ‘line width’ per kilometer length (ps nm
−1
km
−1
). The magnitude of intramodal and material dispersions both vary with wavelength, and at some frequencies the two effects act in opposite senses. It is generally possible, on a given single mode fibre, to find a wavelength around which there is negligible dispersion, or, conversely, to design a fibre to have minimum dispersion at a desired wavelength. References to dispersion herein will mean the sum total of group velocity dispersion effects.
Dispersion in optical fibre presents serious problems when using light sources whose spectrum is non-ideal for example broad or multispectral-line, or when high data rates are required, e.g. over 2 GB/s. This problem has previously been addressed, at least partially, in four ways. Firstly, by operating at or close to the optical frequency at which the dispersion is a minimum, for example at a wavelength of 1.3 micron in conventional silica fibre. The frequency does not generally correspond with the frequency of minimum transmission loss and attempts to modify the fibre to shift its frequency of minimum dispersion usually result in some loss penalty. This solution has limitations for two reason. Firstly manufacturing variations will always occur. Secondly, a non linearity called four wave mixing seriously degrades WDM signals near the dispersion zero of one piece of fibre. Accordingly, it may be preferable to operate in a given region of dispersion which may not include the dispersion zero.
The second way of overcoming the problem is to use a source with a near ideal narrow linewidth spectrum. The limits for improvement in this respect have been reached since at higher bit rates, the Kerr effect becomes significant. This is where the index of refraction varies with intensity, which causes self phase modulation, or cross phase modulation. The resulting frequency redistribution means that dispersive degradation increases again.
Thirdly, dispersion compensators have been used to equalise the dispersion with an element of equal and opposite dispersion. Such dispersion compensators may take the form of length of fibre, a Mach Zehnder interferometer, an optical resonator, or a Bragg reflector. Some of these compensators can give a variable, controllable amount of compensation.
A fourth technique is to change the modulation at the transmitting end. On example is discussed in EP-A-0643 497. Dispersion produces an FM to AM conversion effect which can facilitate bit detection and thereby extend transmission distance without controlling or compensating dispersion. The dispersion causes shifting of adjacent signal components of different wavelengths, resulting in either energy voids or energy overlaps at the bit transitions. Constructive interference in an overlap causes a positive peak in the optical signal, while a void produces a negative peak. These positive and negative peaks represent an AM signal which may be detected to reproduce the original bit stream.
The document proposes the additional step of adjusting the output power of one or more of the inline amplifiers to further stabilise the dispersion-induced optical signal energy voids and overlaps and thereby further improve the detection thereof. This method requires difficult precision engineering and so is impractical for commercial exploitation.
With the different types of dispersion-shifted fibre, dispersion compensating fibre, and dispersion-compensating filters that could make up a given link, determining the dispersion of a link is no longer the simple operation of multiplying the length in km by the 17 ps
m/km dispersion characteristic of standard single mode fibre. Moreover, when there are optical switches or controllable optical dispersion compensators in the link the dispersion can change as a function of time.
There are several laboratory test instruments available that measure this dispersion, on a static basis. However, they are large, expensive and cannot be used while a signal is present at the same wavelength. Some such instruments require both ends of the fibre be at the same location, and so can only be used to test components of a system before installation. Certainly they are not suitable for incorporation into any element of a practical transmission system.
One attempt to control the effects of dispersion in a high speed transmission syst
Lee Mann Smith McWilliams Sweeney & Ohlson
Nortel Networks Limited
Pascal Leslie
LandOfFree
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