Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
2000-12-06
2003-02-11
Dang, Hung Xuan (Department: 2873)
Optical waveguides
With optical coupler
Input/output coupler
C385S010000
Reexamination Certificate
active
06519390
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to chirped Bragg grating reflectors, in particular to quadratically chirped Bragg grating reflectors, and to adjustable dispersion apparatus and devices (e.g. nodes and repeater units for optical transmission networks) incorporating such reflectors. The apparatus and devices may be for compensating chromatic dispersion in optical transmission systems, in particular, although not exclusively, wavelength division multiplexed digital transmission systems.
BACKGROUND OF THE INVENTION
Chromatic dispersion in optical transmission systems is the variation of group delay as a function of wavelength.
Linear (first order) dispersion, D, is the measure of the rate of change of group delay, &tgr;, with wavelength, &lgr;. (D=d&tgr;/d&lgr;.) Linear dispersion is typically measured in picoseconds per nanometer (ps
m). In the case of a transmission medium, for instance an optical fibre waveguide, whose waveguiding properties are uniform along its length, the linear dispersion exhibited by the medium is proportional to its length and so, for such a medium, it is convenient to define its linear dispersion per unit length, also known as its linear dispersion power. This is typically measured in picoseconds per nanometer per kilometer (ps
m/km).
The value of the linear dispersion of a transmission path is generally itself a function of wavelength, and so there is a quadratic (second order) dispersion term, Q, also known as dispersion slope, which is a measure of the rate of change of linear dispersion with wavelength. (Q=D /d&lgr;=d
2
&tgr;/d&lgr;
2
.) This is typically measured in picoseconds per nanometer squared (ps
m
2
). In some, but not all instances, the effects of quadratic dispersion in NDS and DC fibre (non dispersion shifted fibre, and dispersion compensating fibre) are small enough not to assume significance. There are also higher dispersion terms, whose effects generally assume even less significance.
In a digital transmission system the presence of dispersion leads to pulse broadening, and hence to a curtailment of system reach before some form of pulse regeneration becomes necessary. The problem presented by dispersion increases rapidly with increasing bit rate. This is because, on the one hand, increasing the bit rate produces increased spectral broadening of the pulses, and hence increased dispersion mediated pulse broadening; while on the other hand, increasing the bit rate also produces a reduction in the time interval between consecutive bits. In a WDM (wavelength division multiplexed) digital transmission system, it is not practical to minimize the problems of dispersion by choosing to employ a transmission medium exhibiting near-zero first order dispersive power because low first order dispersive power is associated with aggravated non-linear (e.g. four-wave mixing) distortion.
A known solution to this problem is to employ ‘managed dispersion’ in which near-zero aggregate linear dispersion over a particular transmission path is achieved by the use of alternating sections respectively exhibiting positive linear dispersion and negative linear dispersion, for instance by the use of NDS (non-dispersion-shifted) and DC (dispersion-compensated) optical fibre waveguide. In other words to reduce non-linear distortion resulting from interaction between pulses of different wavelengths travelling in the same direction down a fibre, it is desirable to use a dispersive fibre to reduce the phase matching of different wavelength channels and then at the end of the fibre to compensate for the dispersion introduced by the fibre, i.e. deliberately introduce dispersion having the opposite sign.
Having regard to the manufacturing tolerances in practice encountered in the fabrication of NDS and DC fibre, achieving adequately low aggregate linear dispersion becomes increasingly difficult as the bit rate is increased. Consider for instance a 40 Gbit/s WDM transmission system with a reach of 400 km, and with the shortest and longest wavelength channels separated by 200 nm. The actual amount of linear dispersion in any particular channel that can be tolerated will of course be dependent upon a number of system parameters, but typically may lie in the region of 100 ps
m. A typical NDS fibre exhibits, at a wavelength of 1550 nm, a linear dispersive power of approximately 17 ps/(nm/km), and a quadratic dispersive power of approximately 0.058 ps/(nm
2
·km). Recently DC fibre has been fabricated to a tolerance of ±3% in respect of linear dispersive power, and a tolerance of ±20% in respect of quadratic dispersive power. Therefore, for the 400 km span length, the uncertainty in linear dispersion compensation at the 1550 nm wavelength will amount to approximately 400 ps
m (≈400×17×0.06 ps
m). Given the 200 nm wavelength range, the additional uncertainty at the wavelength extremities produced by the ±20% quadratic tolerance amounts approximately to a further 900 ps
m (400×0.058×200×0.2 ps
m). To this must be added any uncertainty arising from any imprecision in the knowledge of the length and dispersion of the transmission fibre.
The foregoing indicates that, even if the DC fibre were manufactured to tolerances tightened by an order of magnitude, those tolerances would still be large enough to cause difficulty in achieving an accurate enough compensation for the reliable provision of an operating point near the centre of the 100 ps
m window.
There is therefore a useful role for an adjustable amplitude linear dispersion compensation device. Such a device could be one designed for operation on its own to achieve the totality of dispersion compensation. Alternatively, it could be one designed for operation in association with a fixed amplitude dispersion compensation device, such as a length of DC fibre, that provides a level of compensation that is inadequately matched on its own. The adjustable device may be operated with some Corn of feedback control loop to provide active compensation that can respond to dynamic changes of dispersion within the system, and in suitable circumstances to step changes resulting from re-routing occasioned for instance by a partial failure of the system such as a transmission fibre break.
The compensation for linear dispersion already present across an optical signal bandwidth is not the only role for an adjustable dispersion device. There are numerous other roles for apparatus (e.g. a device) which can provide adjustable dispersion (which may not be linear) across an optical signal bandwidth. For example, it may be desirable in certain applications or experimental arrangements to introduce dispersion where none was previously present.
One way of providing dispersion which may be used for dispersion compensation (or other) purposes, utilizes spectrally distributed reflection of light produced by a chirped Bragg grating (described below) extending in the axial direction of an optical waveguide (e.g. reflection from a chirped fibre Bragg grating). Such a method is for instance described in U.S. Pat. No. 4,951,939.
Fibre Bragg gratings (FBGs) are well known and comprise a length of optical fibre (typically monomode fibre) having a refractive index n which is modulated in a periodic fashion along the length. Various techniques are used to produce the modulation &Dgr;n, and the maximum value of &Dgr;n
is typically in the range 10
−6
to 10
−3
. By way of analogy with conventional “line” gratings, the fibre Bragg grating can be thought of as as series of grating elements, each one being a region of modified refractive index, along an optical fibre. In reality, the “edges” of the grating elements are not precisely defined, and in a FBG having constant pitch the refractive index may simply vary in a sinusoidal manner along the fibre.
Other forms of Bragg grating reflectors are also known, such as planar waveguide gratings. In these structures, the refractive index is modulated in some way along an optical path to provide the grating elem
Epworth Richard
Fells Julian A.
Dang Hung Xuan
Dinh Jack
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