Fibre bragg grating with offset equivalent mirror plane and method for its manufacture

A grating made in the core of an optical fiber presents a nonuniform and asymmetrical profile of the modulation of the refractive index in the direction of the length, which profile is represented by a curve that rises gradually and monotonically from a minimum and substantially null value, with substantially horizontal tangent, in correspondence with an end of the grating to a maximum value, also with substantially horizontal tangent, which is reached in correspondence with the other end of the grating.

FIELD OF THE INVENTION
 The present invention relates to optical fiber components for optical
 telecommunication systems, and more specifically to a fiber Bragg grating
 with offset equivalent mirror plane and to a method of manufacturing such
 gratings.
 BACKGROUND OF THE INVENTION
 The use of fiber Bragg gratings in components for optical telecommunication
 systems such as lasers, amplifiers, filters, add-drop multiplexers,
 wavelength multiplexers/demultiplexers, etc. has been known for some time.
 A review of the use of fiber Bragg gratings as components of optical
 telecommunication systems is found for instance in the papers "Lightwave
 Applications of Fiber Bragg Gratings", by C. R. Giles, Journal of
 Lightwave Technology, Vol. 15, No. 8, August 1997, pp. 1391 et seq., and
 "Fiber Gratings in Lasers and Amplifiers", by J. Archambault and S. G.
 Grubb, ibid., pp. 1379 et seq.
 In particular, in applications in wavelength division multiplexing systems
 it is necessary to have devices capable of separating the various
 channels. For this purpose it is possible to use gratings of which the
 reflection spectrum presents a peak that is, insofar as possible, narrow
 and free of side lobes.
 When fiber Bragg gratings are used to make one or both the reflecting
 elements that delimit a resonant cavity of a component, e.g. a Fabry-Perot
 cavity laser, to be used in such systems, one encounters problems linked
 to the cavity length. This length depends, as is well known, on the
 position of the so-called equivalent mirror plane of the grating. The
 equivalent mirror plane is the plane where a mirror would have to be
 positioned in order that a pulse sent by a source and reflected by the
 mirror returns to the source in the same time the pulse sent into the
 grating would take to return. The distance between the equivalent mirror
 plane and the input end of the grating constitutes the equivalent length
 of the grating. The length of a resonant cavity that makes use of fiber
 Bragg gratings is therefore represented by the distance between the
 equivalent mirror plane of the grating and the other reflecting element of
 the cavity (if only one such element is made by a grating) or between the
 equivalent mirror planes of the two gratings (if both reflecting elements
 are made by gratings). Now, if the linewidth of the laser is to be kept
 limited, the length of the cavity cannot be shorter than a certain minimum
 length, which is determined by manufacturing requirements; on the other
 hand, the longer the cavity, the shorter the distance between the modes
 and hence the harder the separation between the different modes.
 The gratings proposed until now have a modulation of the refractive index
 which, as a function of the length of the grating, presents a symmetrical
 profile with respect to the central point of the grating. In these
 symmetrical gratings the equivalent mirror plane is placed substantially
 at the center of the grating, if the latter is a low-reflecting grating,
 and is located in a more advanced position towards one end if the grating
 is a highly reflecting grating. "Low-reflecting" indicates a value of
 reflectivity such that, when the grating is used as the reflecting element
 of the cavity, the radiation fraction exiting the cavity is sufficient for
 practical uses (typically, a reflectivity of the order of 70% in a laser);
 "highly reflecting" indicates a reflectivity of practically 100% or very
 close to this value. A highly reflecting grating could be used as one of
 the reflecting elements of the cavity, thereby reducing its length,
 provided the other reflecting element presents a sufficiently high
 transmission factor. In the case of a cavity with only one reflecting
 element made by a grating, the latter is positioned in correspondence with
 the output end and the use of a highly reflecting grating under such
 conditions is clearly inconceivable. In the case of a cavity where both
 reflecting elements are made by gratings (in the example, the cavity of an
 all-fiber laser), the use of a highly reflective grating does not solve
 the problem of obtaining a narrow band with a very reduced length of the
 cavity, both because the spectral line of those gratings is in any case
 relatively wide, and because one of the gratings should be a
 low-reflecting grating and hence would present a high equivalent length.
 SUMMARY OF THE INVENTION
 The aforesaid problems are solved by the grating according to the present
 invention, which presents both a narrow reflectivity spectrum, free of
 secondary lobes, and a reduced equivalent length.
 More specifically, a grating is provided that presents a non uniform,
 asymmetrical profile of modulation of the refractive index in the
 direction of the length, which profile is represented by a curve that has
 a minimum and substantially zero value, with substantially horizontal
 tangent, in correspondence with one end of the grating, and rises
 gradually and monotonically until a maximum value, also with a
 substantially horizontal tangent, is reached in correspondence with the
 other end of the grating, where the curve returns to the minimum value
 with substantially vertical slope.
 Preferably such a curve has a trend represented by one of the following
 functions:
EQU y=exp (-x.sup.2) (i.e. a Gaussian function),
EQU y=sin.sup.2 x,
EQU y=tanh x.
 An asymmetrical modulation profile like the one provided according to the
 invention effectively guarantees that the equivalent mirror plane is moved
 forward, in proximity with the maximum of the modulation profile of the
 refractive index, as is readily apparent when applying the description
 provided in L. A. Coldren, S. W. Corzine: "Diode Lasers and Photonic
 Integrated Circuits", Wiley & Sons, 1995. In a practical embodiment of the
 invention, in a grating with length of about 1 cm and reflectivity of the
 order of 70%, with a half-Gaussian modulation profile, the equivalent
 mirror plane was positioned about 2.5 mm from the end closer to the
 modulation maximum; by way of comparison, a conventional grating of the
 same length and similar reflectivity, with symmetrical Gaussian profile of
 the refractive index modulation, would have an equivalent length of the
 order of 5 mm, thus substantially double.
 A grating such as the one described can therefore be advantageously
 employed with a resonant cavity, to form one or both the reflecting
 elements that delimit the cavity. Moreover, tests carried out have
 demonstrated that there are no secondary peaks and that the reflection
 band is narrow.
 To make a grating such as the one described, the conventional techniques
 for writing gratings into optical fibers are used. A review of such
 techniques can be found in the paper "Fiber Bragg Grating Technology
 Fundamentals and Overview", Journal of Lightwave Technology, Vol. 15, No.
 8, August 1997, pp. 1263 et seq. According to the invention, in order to
 obtain the refractive index modulation described above when writing the
 grating by using a phase mask, the diaphragm used to generate the
 intensity distribution of the writing radiation on the phase mask must be
 such as to create an asymmetrical distribution, corresponding to the
 desired profile of the refractive index modulation. Hence the diaphragm
 will be such as to intercept half the beam and to create, with reference
 to the exemplary functions mentioned above, a distribution corresponding
 to the part included between the minimum and the maximum of a Gaussian
 curve or of a curve of the type sin.sup.2 x, tanh x, etc.

SPECIFIC DESCRIPTION
 FIG. 1 schematically shows a conventional device for writing Bragg gratings
 into an optical fiber 1 with the use of a phase mask 2. The phase mask 2
 is illuminated by the UV radiation emitted by a laser 3 through an optical
 system capable of creating, in correspondence with the phase mask 2, an
 image of the source 3 comprising a thin strip whose length corresponds to
 that of the grating to be manufactured. The phase mask, as is well known,
 gives rise to interference, in correspondence with the fiber, to resulting
 in variations of the intensity of the writing radiation which in turn
 cause corresponding periodic variations of the refractive index of the
 core of the fiber in the irradiated area.
 The optical system comprises, in a known manner: a first lens 4 expanding
 the beam emitted by the source; a group of lenses 5, 6, 7 generating a
 collimated beam; a cylindrical lens 8 forming the image of the source on
 the phase mask 2; a diaphragm 9, interposed between the lenses 5, 6, which
 shapes the beam and confers it an intensity distribution corresponding to
 the refractive index modulation profile to be induced in the core of the
 fiber 1 in the irradiated area.
 More specifically, the distribution profile of the intensity of the image
 formed by the cylindrical lens 8 must be asymmetrical and substantially
 zero, with horizontal tangent, in correspondence with one end of the
 image, and must gradually rise until reaching a maximum value, also with
 substantially horizontal tangent, in correspondence with the other end of
 the image, returning then to zero with substantially vertical slope.
 Therefore, the diaphragm 9 shall extend along the path of the beam in such
 a way as to intercept substantially half the beam itself. Intensity
 distribution profiles that meet the demands of the invention are for
 instance those corresponding to half of a Gaussian curve [y=exp(-x.sup.2)]
 or of a curve of the type y=sin.sup.2 x or yet of a curve of the type
 y=tanh x. A half-Gaussian intensity distribution is shown in FIG. 2. This
 distribution can be obtained with a diaphragm 9 like the one shown in FIG.
 3.
 FIG. 4 shows a grating 10 and the modulation of the refractive index
 obtained with an intensity distribution of the writing radiation like the
 one shown in FIG. 2. For the sake of drawing clarity, the pitch of the
 grating has been exaggeratedly lengthened in FIG. 4.
 FIG. 5 shows the reflection spectrum of a grating like the one shown in
 FIG. 4. One can clearly see the single secondary lobe, which however has
 very reduced intensity with respect to the main peak and hence causes no
 problems for wavelength selection.
 It is evident that the description above is provided purely by way of
 example and that variations and modifications are possible without
 departing from the scope of the invention. Thus, for instance, although
 reference has been made to an optical fiber, the invention can relate also
 to gratings obtained in integrated optical guides; also, for the
 manufacture, one can exploit, instead of a phase mask, the interference
 between two beams obtained by splitting the pulses emitted by a source
 between the two branches of an interferometer.