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

Abstract: A grating (10) made in the core of an optical fibre (1) presents a non-uniform 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 (10), until a maximum value, also with substantially horizontal tangent, which is reached in correspondence with the other end of the grating (10).

Description: The present invention relates to optical fibre components for optical telecommunication systems, and more specifically it concerns a fibre Bragg grating with offset equivalent mirror plane and a method of manufacturing such grating. The use of fibre 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 fibre 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 fibre 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 fibre 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 centre 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-fibre 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. 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, i.e. where the asymmetry is along the direction of the fibre axis, which profile is represented by a curve that has a minimum and substantially null value of the modulation of the refractive index, with substantially horizontal tangent, at one end of the grating, and rises steadily and monotonically until a maximum value, also with substantially horizontal tangent, which is reached at 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: y = exp (-x 2 ) (i. e. a Gaussian function), y = sin 2 x, 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, on principle the conventional techniques for writing gratings into optical fibres are used to be carried out as defined in claim 3. 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 2 x, tanh x, etc. For the sake of further clarification, reference is made to the accompanying drawings, wherein: Figure 1 schematically shows a device for the manufacture of an optical fibre grating; Figure 2 shows the intensity distribution of the writing beam required according to the invention; Figure 3 shows the front view of a diaphragm to employ in order to obtain the intensity distribution of the writing beam shown in Figure 2; Figure 4 shows the modulation profile of the refractive index of a grating according to the invention; and Figure 5 shows the reflection spectrum of the grating of Figure 4. Figure 1 schematically shows a conventional device for writing Bragg gratings into an optical fibre 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 by interference, in correspondence with the fibre, to periodic variations of the intensity of the writing radiation which in turn cause corresponding periodic variations of the refractive index of the core of the fibre 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 fibre 1 in the irradiated area. More in detail, the distribution profile of the intensity of the image formed by the cylindrical lens 8 must be asymmetrical and substantially null, 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 2 )] or of a curve of the type y = sin 2 x or yet of a curve of the type y = tanh x. A half-Gaussian intensity distribution is shown in Figure 2. This distribution can be obtained with a diaphragm 9 like the one shown in Figure 3. Figure 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 Figure 2. For the sake of drawing clarity, the pitch of the grating has been exaggeratedly lengthened in Figure 4. Figure 5 shows the reflection spectrum of a grating like the one shown in Figure 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 non limiting example and that variations and modifications are possible without thereby departing from the scope of the invention. Thus, for instance, although reference has been made to an optical fibre, 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.

Claims: A Bragg grating written in a portion of the core of an optical fibre or waveguide (1), characterised in that said grating (10) presents, in the direction of its length, an asymmetrical modulation profile of the refractive index where the asymmetry is along the direction of the fibre axis, which profile is represented by a curve having a substantially null minimum value of the modulation of the refractive index, with substantially horizontal tangent, in correspondence with one end of the grating and rising gradually and monotonically until a maximum value, also with substantially horizontal tangent, which is reached at the other end of the grating. Grating according to claim 1, characterised in that said curve is chosen among the following curves: y = exp (-x 2 ), y = sin 2 x, y = tanh x. Method of manufacturing a Bragg grating (10), with equivalent mirror plane offset towards an end of the grating (10) itself, in an optical fibre or waveguide (1), wherein the fibre or waveguide (1) is exposed, in the portion where the grating (10) is to be formed, to the action of a radiation with such intensity distribution as to give rise to a modulation of the refractive index in the irradiated area, characterised in that said radiation has an intensity distribution shaped in such a way that said modulation profile has an asymmetrical profile where the asymmetry is along the direction of the fibre axis, represented by a curve that has substantially null minimum value of the modulation of the refractive index, with substantially horizontal tangent, in correspondence with one end of the grating (10), and gradually and monotonically rises until a maximum value, also with substantially horizontal tangent, which is reached at the other end of the grating (10). Method according to claim 3, characterised in that said shaping of the intensity distribution is obtained by means of a diaphragm (9) capable of intercepting half the beam and so shaped that the modulation profile, in the portion with gradual variation, is represented by a function chosen among the following functions: y = exp (-x 2 ), y = sin 2 x, y = tanh x.

Inventors: BOSCHIS LAURA, GNAZZO ANGELANTONIO, ROSSOTTO ORIANA, TALLONE LUIGI, BOSCHIS, LAURA, GNAZZO, ANGELANTONIO, ROSSOTTO, ORIANA, TALLONE, LUIGI
Applicants: AGILENT TECHNOLOGIES INC, AGILENT TECHNOLOGIES, INC. (A DELAWARE CORPORATION)
Application year: 1999
Main classification: G02B6
IPC codes: G02B5, G02B6