Dispersion compensating waveguide for optical transmission systems

In order to use one dispersion compensating fiber element for selecting a given value of dispersion, one or more frequency or wavelength dependent optical reflection gratings (G.sub.1, G.sub.2, G.sub.3) is located at such a position along the unit (8) that the double traversal of a section results in a desired value of dispersion at a frequency. A directional coupler (9) diverts the reflected wave to utilization means (10) for its recovery. If a different value of dispersion at the same wavelength, or if some value of dispersion at a different frequency, is required, a reflection grating effective to reflect at the appropriate frequency and at the appropriate position, gives the required values.

BACKGROUND OF THE INVENTION
 The present invention relates to a dispersion compensating waveguide (DCW),
 which has the property of introducing frequency dispersion in transmitted
 optical waves and is generally used for compensating unwanted dispersion
 in a transmission path usually comprising fibre. The invention further
 relates to an optical transmission system incorporating a dispersion
 compensated waveguide.
 Typically the waveguide will comprise dispersion compensating fibre ((DCF).
 In order that a required dispersion be introduced, without a requirement
 for unduly long lengths of dispersion compensating fibre, the fibre may be
 highly doped, e.g. germania doped silica. Whether or not it is highly
 doped, it is usual to cut or otherwise select an appropriate length to
 select a dispersion value, which is somewhat inflexible. If more than one
 wavelength is being used in the transmission of information, there will be
 an inevitable trade-off in throughput resulting from selecting an optimum
 length to suit all the wavelengths used.
 It would be also desirable if losses could be compensated independently of
 the compensation of dispersion, or so as to enable use of dispersion
 compensating fibre to be more flexible.
 SUMMARY OF THE INVENTION
 A particular arrangement to be described below as being helpful in
 understanding the invention proposes the provision of Bragg gratings in
 the dispersion compensating fibre, and makes use of the reflected signals
 from the gratings. The incident signal traverses selectively different
 lengths of the dispersion compensating fibre according to the dispersion
 required by the appropriate positioning of the grating along the
 dispersion compensating fibre.
 If various selected dispersions are required, a prior method is to use
 dispersion compensating fibres of different lengths, or to cut down from a
 starting length. This embodiment uses reflection gratings at intervals
 such that different (double--) lengths of the dispersion compensating
 fibre can be selected for traversal by the signal, and hence different
 dispersions can be selected. The selection would be achieved in practice
 by splicing or writing a grating into the fibre at the required length
 down the fibre.
 In the specification of U.S. Pat. No. 5,404,413 there was proposed an
 optical circulator with three ports. The first and third ports were
 connected to optical fibre systems. The second port was connected to a
 dispersion compensating fibre and return means. A signal passed through
 the dispersion compensating fibre twice, thus permitting the use of
 shorter compensating fibres than previously.
 The following description and drawings disclose a previously proposed
 arrangement and, by means of examples, the invention which is
 characterised in the appended claims, whose terms determine the extent of
 the protection conferred hereby.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 Referring to FIG. 1, a silica optical fibre waveguide 1 typically exhibits
 dispersion which distorts transmitted signals having substantial
 bandwidth. One previously proposed solution which is rather inadequate is
 to operate at 1.311 m or at whatever wavelength around which dispersion is
 a minimum. Unfortunately minimum power loss occurs at very different
 wavelengths from minimum dispersion. Generally, however, a length of
 dispersion compensating fibre 2 is required in series with fibre 1. Thus,
 if the correct length of dispersion compensating fibre 2 is inserted, an
 input pulse of waveform 3 may be broadened to waveform 4 by the
 transmission fibre 1, and then the distortion is compensated by the
 correct length of dispersion compensating fibre 2 to recover the original
 pulse width and shape, as indicated by the approximately square pulse
 3.sup.i resembling substantially the original waveform 3.
 An option to compensate for the loss of power uses the fact that Raman
 scattering increases with increasing germania concentration, so that a
 conventional dispersion compensating fibre, e.g. of highly doped germania
 silica, generates an amplified signal from input signals by molecular
 scattering having a given frequency difference from a pump frequency. FIG.
 2 shows this option in which waveguide 1 is compensated for dispersion by
 the use of dispersion compensating fibre 5. Because the dispersion
 compensating fibre 5 is lossy, it is pumped by means of a diode laser 6 at
 a power just enough to cause stimulated Raman gain at a downshifted
 frequency. Signals at this frequency are amplified, by this Raman effect,
 and the energy of the pumping determines the amount of Raman
 amplification. Thus losses in waveguide 1 can just be compensated by Raman
 amplification in the dispersion compensating fibre 5, for this specific
 downshifted signal frequency corresponding to the Raman frequency shift.
 In one typical sample of dispersion compensating fibre 5, we have
 calculated that 100 mW of pump power is needed to compensate for the 13.6
 dB loss on an 80 kM section of the dispersion compensating fibre 5 (the
 losses in the section of main waveguide 1 being separately accounted for
 or compensated for). This calculation assumes a Raman gain of 10.sup.-12
 cm/w, and is somewhat conservative in assuming a low germania
 concentration in the dispersion compensating fibre material. Assuming a
 higher level of germania dopant concentration, to give a desired high
 dispersion (or need for only a smaller length), the Raman gain coefficient
 would also increase, so that pump power could be decreased for the same
 amplification. The gain bandwidth for Raman amplification is around 10 nm,
 and Raman amplifiers have the useful property that they give quantum
 limited noise performance at any gain.
 Such Raman amplifiers may be pumped at shorter wavelengths by arranging for
 intermediate Raman orders to oscillate in a resonator defined by the
 amplifier A. A second embodiment uses pairs of Bragg gratings in the side
 of the dispersion compensating fibre, each pair defining a respective
 cavity along the dispersion compensating fibre, such as to cause
 oscillations selectively at the respective other orders, and thereby to
 transfer a substantial amount of the power, at one or more of the unwanted
 orders, from the pump signal to the desired order to give amplification to
 the signal. For instance, a diode-pumped Nd:YAG laser transmits 1319 nm
 wavelength pumping power to a dispersion compensating fibre such as that
 schematically shown at 5 in FIG. 2. If the required signal amplification
 is to be at 1.55 .mu.m (i.e. 1550 nm), at which wavelength a standard
 dispersion compensating fibre is highly dispersive, then certain unwanted
 orders at 1380 nm and 1460 nm are generated by oscillations and are
 unrelated to the incident 1.5511 m signal. By the use of spaced grating
 pairs (not shown) reflective at respective 1380 nm and 1460 nm,
 oscillatory cavities are set up which transfer the energy at these
 unwanted wavelengths to energy at a wanted wavelength. This option thus
 comprises a dispersion compensating fibre including oscillatory cavities
 defined by reflective gratings at unrequired oscillatory wavelengths
 unrelated to a required signal wavelength of operation, wherein all these
 wavelengths, required and unrequired, are or tend to be Raman signals
 generated by pumping the dispersion compensating fibre, e.g. by means of a
 standard diode pumped Nd:YAG laser, and the required signal is responsive
 also to an incident signal and is amplified enough to compensate for
 losses in the Ddispersion compensating fibre and in line 1. The wanted
 signal can be recovered from downstream of the reflective grating pair or
 pairs (not shown).
 An embodiment of the invention is described with reference to FIG. 3, in
 which the wanted amplified signal is derived by reflection from an optical
 grating. In FIG. 3, the dispersion compensating fibre 8 is caused to
 reflect the wanted signal from optical transmission line 1 to a circulator
 or other directional coupler 9 and thence to a detector or utilisation
 circuit 10; unwanted signals may be transmitted through a through-path 11
 of the dispersion compensating fibre 8 in FIG. 3. Alternatively all
 signals may be reflected at 11.
 Amplification to compensate for losses if desired may be arranged as in
 FIG. 2 by pumping and by selected Raman molecular transitions. A problem
 with dispersion compensating fibre is that of selecting the appropriate
 length, desirably for economy's sake from a given length, whereby to
 introduce the appropriate amount of compensation for the dispersion caused
 by perhaps 50 kM or some unknown length of transmission waveguide 1. One
 prior proposed way of selecting dispersion compensating fibre lengths is
 by cutting off sections, which tends to be inconvenient and which for WDM
 systems results in a compromise in throughput. Accordingly by this
 embodiment of FIG. 3, reflection points are created at one or more of the
 positions G.sub.1, G.sub.2, G.sub.3 by means of Bragg gratings of
 appropriate element spacing which determine reflection wavelength or
 frequency, and of appropriate grating length which determines bandwidth of
 wavelength energy reflected, If G.sub.1 is operative, the incident signals
 at this frequency will be selectively reflected at this frequency, and
 will traverse the first section, shown leftward of dispersion compensating
 fibre 8, twice, introducing dispersion corresponding to this double
 length, then leftwards to circulator 9 and branched to utilisation circuit
 10. If other dispersions or different wavelengths are to be selected,
 different reflecting gratings G.sub.2 or G.sub.3 are used at appropriate
 positions or appropriate grating element spacings respectively, either of
 these wavelengths being transmitted past grating G.sub.1 with negligible
 reflection. If grating G1 is likely not to be required, it can be erased
 in non-destructive manner by heating or irradiation by UV. New radiation
 gratings G can be added, e.g. they can be spliced in place as required.
 The position of a grating G determines the dispersion, and the spacing
 between grating elements in a grating G determines the wavelength selected
 by reflection.
 Raman amplification by pumping the dispersion compensating fibre material
 will usually be required, as described for FIG. 2, but occasionally may be
 deemed unnecessary if the dispersion compensating fibre 8 or optical
 waveguide line 1 does not introduce excessive power losses.
 Variations of the arrangement of FIG. 3 may be used in further embodiments
 to separate energy at different wavelength in wavelength division
 multiplexers (WDM). The two or more gratings G will be located by splicing
 in place or otherwise, at appropriate distances down the dispersion
 compensating fibre 8, to give the requisite equalisation for energy at
 each of the different wavelengths.
 Referring again to FIG. 3, when a Raman pump is used as in FIG. 2, a
 further reflecting grating (not shown) can be located downstream of the
 other components of grating element spacing selected to reflect energy at
 the pump frequency. The reflected energy will reinforce the incident pump
 energy to result in a higher overall gain.
 Another alternative pumping scheme is to locate a pump source at both ends
 of the dispersion compensating fibre 5 of FIG. 2, when Raman amplification
 is employed, thus again increasing gain when required.