Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1998-11-12
2001-11-13
Pascal, Leslie (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200, C359S348000, C359S349000, C385S037000
Reexamination Certificate
active
06317239
ABSTRACT:
FIELD OF INVENTION
The present invention relates to optical repeaters and dispersion equalizers for single- and multi-wavelength operation, with particular application to wavelength division multiplex (WDM) optical telecommunication systems operating at bit rates in the range from 2.5 Gb/s to 10 Gb/s per carrier wavelength.
BACKGROUND OF THE INVENTION
Existing optical fibre networks are based primarily on non-dispersion shifted fiber (NDSF). For wavelength division multiplex (WDM) optical communications systems, chromatic dispersion induced during optical signal propagation through NDSF fibers is a major constraint which limits the length-bandwidth product in an optical communication system operating in the 1550 nm wavelength region.
Dispersion compensation in the 1550 nm wavelength region is an attractive way to overcome this constraint. Various schemes are known to have been suggested or employed for dispersion compensation.
The conventional approach, employing a length of dispersion compensating fiber (DCF) is widely used. Other compensators are known based on fibre gratings, optical interferometers, or cascades of birefringent optical fibers.
To overcome losses in a preceding optical fiber span and in a dispersion compensating element, a dispersion compensating element is used in combination with one or two erbium doped fiber amplifiers, each requiring a pump laser. The latter combination is conventionally used either in discrete or integrated form, with a dispersion compensating fiber. The result is expensive and large sized repeater equipment.
Use of an optical fiber grating as a dispersion equalizer instead of dispersion compensating fibre has the potential for lower cost, but the drawback of known dispersion equalizers based on grating systems is the narrow optical bandwidth and resultant high sensitivity for carrier frequency fluctuations. For example, an optical fibre transmission system using a chirped Bragg reflector and directional coupler for dispersion equalization is described in U.S. Pat. No. 4,953,939 entitled “Optical Fibre Transmission Systems” to Epworth. Epworth uses a piece of fibre in which reflective properties of the grating are graded along the length of the piece of fibre. Different sections of the piece of fibre reflect different optical frequencies and this system provides limited bandwidth. Also, in practice, each optical fiber grating based dispersion equalizer requires an optical amplifier for loss compensation.
The optical bandwidth of a chirped fiber grating is directly proportional to the length of the grating. Given a practical length limitation based upon grating stability and/or manufacturability, the desired optical bandwidth determines the grating chirp parameter, which in turn determines the amount of dispersion compensation from the fiber grating. In known systems, increased dispersion compensation can be achieved only at the expense of optical bandwidth for a given grating length.
SUMMARY OF THE INVENTION
The present invention seeks to provide an optical repeater for single- and multi-wavelength operation with dispersion equalization, which overcomes or avoids the above-mentioned problems.
According to one aspect of the invention there is provided an optical repeater, comprising:
a dispersion equalizer comprising an optical coupler having an input port, an output port and cascaded M optical fiber grating branches coupled in parallel through the optical coupler, and
first and second lengths of an optical fiber amplifier coupled to the input port and output port respectively of the optical coupler,
means for coupling an input optical signal into the first section of the optical fiber amplifier and means for coupling an output optical signal from the second section of the optical fiber amplifier,
each of M optical fiber grating branches comprising a series of N gratings, each grating being designed to selectively reflect one of the series of signal carrier wavelengths &lgr;
a
, &lgr;
b
, &lgr;
c
, . . . &lgr;
N
, to form a cascade of N wavelength selective grating stages of M elements each,
each grating within the grating stage is chirped so that the total optical bandwidth of the equalizer for the stage is the combined optical bandwidths of the gratings within the stage.
Thus, dispersion equalization is provided by the plurality of fiber grating branches that are cascaded so that the bandwidth of the equalizer for a particular stage is enhanced relative to the same amount of dispersion compensation using a single grating approach, e.g. is a multiple of a single grating bandwidth. The insertion loss of the equalizer is efficiently compensated by inserting it between two sections of a fiber amplifier.
Preferably, the fiber amplifier comprises first and second lengths of an erbium doped fiber, and both lengths of fiber are coupled to a single laser source. Because only one pump laser source is required, the cost is substantially reduced. Thus, the optical repeater provides optical loss compensation and chromatic dispersion compensation, while enhancing the bandwidth relative to conventional systems using a grating for dispersion equalization.
Advantageously, the optical repeater is provided for multi wavelength operation, e.g. for a WDM system operating with a plurality of wavelengths. Alternatively, the repeater may provide a single wavelength operation, e.g. when N=1. Conveniently, the gratings have equal bandwidths. Alternatively, they may have unequal bandwidths depending on the requirements for dispersion compensation. Preferably, each grating is an in-fiber Bragg grating with linear chirp. Alternatively, gratings having non-linear chirp are also applicable for dispersion compensation. By way of example, a dispersion equalizer may comprise three fiber grating branches, each branch including one of the first, second and third gratings respectively.
Various modifications to the optical repeater structure described above are possible, including variations in number of branches and number of gratings in each branch. For example, an optical repeater for two or more signal carrier wavelengths may include three fiber grating branches (M=3), each branch having two gratings coupled in series (N=2). Each grating may be chirped for a specific one of the signal carrier wavelengths to reflect one of the two signal carrier wavelengths and transmit the other wavelength. Similarly to the above, an optical repeater having three branches (M=3) with four gratings in each branch (N=4) may also be provided. Another example may include an optical repeater having a three branch (M=3) equalizer for four wavelength (N=4) operation. While the system could be extended for multiple wavelength operation with more than four branches, practical considerations would usually limit the number of fiber grating branches and number of gratings in each branch in the optical repeater to provide the required parameters of the system, e.g. compactness and stability of its operation.
According to another aspect of the invention there is provided a dispersion equalizer, comprising:
an optical coupler having an input port, an output port and cascaded M optical fiber grating branches coupled in parallel through the optical coupler,
each of M optical fiber grating branches comprising a series of N gratings, each gratings being designed to selectively reflect one of the series of signal carrier wavelengths &lgr;
a
, &lgr;
b
, &lgr;
c
, . . . &lgr;
N
, to form a cascade of N wavelength selective grating stages of M elements each,
each grating within the grating stage is chirped so that the total optical bandwidth of the equalizer for the stage is the combined optical bandwidths of the gratings within the stage.
Each optical fiber grating branch of the equalizer includes a single grating (N=1) to form a matched stage of M gratings. Conveniently, the number of branches of the repeater equals three (M=3).
Thus, an optical repeater providing dispersion equalization and loss compensation is provided, based on cascaded fibe
Burbidge Douglas S.
Villeneuve Bernard
de Wilton Angela C.
Donnelly Victoria
Nortel Networks Limited
Pascal Leslie
LandOfFree
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