WO2001050165A1 - Dispositif a fibre optique comprenant un reseau de diffraction - Google Patents

Dispositif a fibre optique comprenant un reseau de diffraction Download PDF

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Publication number
WO2001050165A1
WO2001050165A1 PCT/EP2000/012874 EP0012874W WO0150165A1 WO 2001050165 A1 WO2001050165 A1 WO 2001050165A1 EP 0012874 W EP0012874 W EP 0012874W WO 0150165 A1 WO0150165 A1 WO 0150165A1
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Prior art keywords
fibre
refractive index
optical
core
radius
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PCT/EP2000/012874
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English (en)
Inventor
Paolo Vavassori
Michele Belmonte
Valeria Giuseppina Gusmeroli
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Corning O.T.I. S.P.A.
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Application filed by Corning O.T.I. S.P.A. filed Critical Corning O.T.I. S.P.A.
Priority to AU37261/01A priority Critical patent/AU3726101A/en
Publication of WO2001050165A1 publication Critical patent/WO2001050165A1/fr

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    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/02—Optical fibres with cladding with or without a coating
    • G02B6/02057—Optical fibres with cladding with or without a coating comprising gratings
    • G02B6/02076—Refractive index modulation gratings, e.g. Bragg gratings
    • G02B6/0208—Refractive index modulation gratings, e.g. Bragg gratings characterised by their structure, wavelength response
    • G02B6/021—Refractive index modulation gratings, e.g. Bragg gratings characterised by their structure, wavelength response characterised by the core or cladding or coating, e.g. materials, radial refractive index profiles, cladding shape
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/02—Optical fibres with cladding with or without a coating
    • G02B6/036—Optical fibres with cladding with or without a coating core or cladding comprising multiple layers
    • G02B6/03616—Optical fibres characterised both by the number of different refractive index layers around the central core segment, i.e. around the innermost high index core layer, and their relative refractive index difference
    • G02B6/03638—Optical fibres characterised both by the number of different refractive index layers around the central core segment, i.e. around the innermost high index core layer, and their relative refractive index difference having 3 layers only
    • G02B6/03644—Optical fibres characterised both by the number of different refractive index layers around the central core segment, i.e. around the innermost high index core layer, and their relative refractive index difference having 3 layers only arranged - + -

Definitions

  • Optical-fibre device comprising a diffraction grating
  • the present invention relates to an optical-fibre device comprising a diffraction grating.
  • an optical fibre is a thread-like element suitable to convey light inside it.
  • an optical fibre comprises a central core, typically of doped glass, which confines inside it the light to transmit; a cladding, external to the core, typically of glass as well, having a lower refractive index than that of the core so as to allow confining the light inside the core itself; and one or more external protective and strengthening coatings.
  • optical fibres are also used for producing devices which allow changing the properties of the transmitted light.
  • a wide range of recently developed optical -fibre devices are based on the possibility of making diffraction gratings, in particular Bragg gratings, directly into the core of an optical fibre.
  • a Bragg grating is a typically periodical structure which extends along a predetermined direction, and which comprises longitudinal areas with a high refractive index interposed between longitudinal areas with low refractive index; said structure has the property of back-reflecting the light passing through it at a wavelength (so called Bragg wavelength which is referred to as ⁇ B ) which depends on the reciprocal distance of the above areas and on the optical properties of the means on which the grating itself is made. If the grating is made in the core of a single- mode optical fibre, the Bragg wavelength is given by the following relationship:
  • ⁇ B 2 -n eff LP01 - ⁇ ( 1 )
  • n effjLP01 is the effective refractive index in the fundamental mode LP 01 propagating into the core
  • ⁇ is the grating periodicity
  • Figure 1 shows a typical transmissivity curve of a periodical Bragg grating, in particular a constant- periodicity grating.
  • a main negative peak is present in correspondence with the Bragg wavelength (in the particular case, at about 1551 nm) , because this wavelength is almost completely reflected, and a series of secondary peaks, defining undesired loss peaks, placed at lower wavelengths than the Bragg wavelength.
  • Said loss peaks originated from the fact that in a single-mode optical fibre, after it has been stripped of its protective coating for the purpose of writing the grating, besides the fundamental mode, also a series of cladding modes can propagate.
  • Said cladding modes extend in the entire area taken by the cladding and, for this reason, they have a value of effective refractive index which is lower than that of the fundamental mode (which is prevalently confined into the core) .
  • a power coupling induced by the Bragg grating may occur between the fundamental mode and the cladding modes.
  • the previous relationship (2) can be rewritten as follows:
  • n eff LPnm is the effective refractive index of the generic cladding mode determined by the pair of indexes n and m.
  • the presence of these loss peaks is a disadvantage in the use of Bragg gratings.
  • a Bragg grating is used for filtering a single channel in a multi-channel optical transmission system (that is, multi -wavelength) , for example for the purpose of adding or dropping said channel to/from the transmission system itself
  • the losses due to the coupling of the fundamental mode with cladding modes introduce an undesired attenuation of the signals in the transmission channels adjacent the channel to be filtered.
  • the power coupling between the fundamental mode and the cladding modes substantially providing for a transfer of power from the first one to the second ones, shall be shortly referred to as (power) coupling into cladding modes.
  • Bragg gratings at non-constant periodicity that is, at variable pitch - are known among fibre gratings.
  • An important application of Bragg gratings at non-constant periodicity relates to the compensation of chromatic dispersion.
  • Chromatic dispersion is a phenomenon of broadening of the transmitted optical pulses caused by the different velocity at which the various chromatic components (that is, the different wavelengths) of the transmitted light propagate.
  • the gratings are realised with a periodicity ⁇ variable along the fibre.
  • a chirped grating is a grating with variable pitch; in particular, it is a grating in which the areas with a high refractive index have an increasing or decreasing distance along the grating. Said variation of the grating pitch is defined "chirping factor" .
  • the gratings arranged for compensating the chromatic dispersion usually reflect on a spectrum band having such a width as to include the spectrum region wherein there is a power coupling into cladding modes (with the consequent presence, in said area, of loss peaks)
  • the reflectivity curve of said grating is typically disequalised.
  • the expression "wide band” refers to such a band as to include (at least partly) the above spectrum region seat of coupling phenomena into cladding modes.
  • Figure 2 shows an example of reflectivity curve of a chirped grating.
  • NOI nm ⁇ X ⁇ ⁇ r) -X m (r)-ex V -i - — .
  • NOI 0m J ⁇ (r)- ⁇ 0 ,(r)- r - dr - d ⁇ (4 )
  • the value of the integral (4) must be as small as possible.
  • a first technique for reducing the normalised overlap integral (4) consists in confining the field of the fundamental mode inside the fibre core, where the grating is written. By doing so, the product between fields ⁇ 01 (r)* and ⁇ 0m (r)* outside the area gra i ng tends to zero, and on the basis of the orthonormality relation (5) , the overlap integral tends to be null.
  • V number a fibre parameter
  • the cut-off wavelength corresponds to a value of V equal to 2.405.
  • V the number of fibres with high values of the numerical aperture NA and of the radius of the core R co must be used.
  • Increasing V corresponds to incrementing the ratio between the optical power of the fundamental mode confined into the core and its total optical power, said ratio defining the "confinement factor" of the field. Nevertheless, if the single-mode condition is to be maintained, the V number cannot be increased beyond the value of 2.405.
  • the Applicant has noted that, although the use of a fibre with high NA increments the distance between the Bragg wavelength and the maximum wavelength at which there is a coupling into the cladding modes, thus setting free an useful operating band, the width of said useful band is relatively small and much narrower than that needed for many applications.
  • a second technique for reducing the normalised overlap integral (4) consists in extending the writing area of the grating to the cladding area, at least up to an area wherein the field of the fundamental mode is substantially null. Also in this case, the coupling integral tends to coincide with the orthonormality integral, and thus tends to be null. To make this solution possible, it is necessary to make also the cladding area photo-refractive. For example, in the article by E. Delavaque, S. Boj , J. F. Bayon, H. Poignant, J. Le Mellot, M. Monerie, "Optical fiber design for strong grating photoimprinting with radiation mode suppression", Proc . , PD5, pp.
  • an intermediate layer be inserted between the core and the cladding of a fibre, said layer having a diameter equal to three times as much as the diameter of the core and doped with germanium so as to have the same photo-sensitivity as the core.
  • fluorine be added as co-dopant so as to obtain the same refractive index of the cladding and obtain, thus, a step index fibre.
  • the grating is written into the photosensitive area.
  • the Applicant has noted that, although it is technologically possible to make a photosensitive cladding around a photosensitive core, it is very difficult to obtain, at the same time, a high photosensitivity and a photosensitivity having the same entity in the core and the cladding .
  • a third technique for reducing the normalised overlap integral (4) consists in using a fibre with refractive index profile having a depressed area in correspondence with the most inner area of the cladding, that is, with the area of the cladding adjacent to the core.
  • This type of fibre is usually called "fibre with depressed cladding" .
  • the value of the refractive index is lower than those relating to the core and to the most outer area of the cladding. This characteristic allows reducing the amplitude of the field of the cladding modes in correspondence with the core area (where the grating is written) , thus minimising the value of the integral (4) .
  • the Applicant points out that the fibre used in this article is single-mode, and on the basis of the values of the core radius and of the radius of the depressed area used, the Applicant has estimated that the ratio between the core radius and the radius of the depressed cladding area of said fibre is equal to about 0.86.
  • US Patent 5,852,690 assigned to Minnesota Mining and Manufacturing Company (3M) relates to a fibre with depressed cladding index profile for reducing coupling losses into cladding modes in fibre gratings, wherein the refractive indexes and the geometrical dimensions are selected so as to have a narrow depression area.
  • the ratio between the core radius and the outer radius of the depressed cladding is comprised between 0.5 and 1.
  • the described waveguide improves the characteristics of losses at low wavelengths in case of small but not null asymmetries of the grating, for example as in the typical case of a tilted grating with an angle ranging between 0.25° and 1.5°.
  • said patent in column 3, lines 19-29, specifies that the use of multi-mode fibres for housing the grating - although using the technique of confining as much as possible the core mode so as to reduce the coupling losses - is disadvantageous due to the high losses of optical power in correspondence with the junctions with the single-mode fibres typically used for transmitting signals.
  • further process steps must be adopted in order to have a reduction of said losses, so that the final device is more complex.
  • the European Patent Application EP 0831345 in the name of Sumitomo Electric Industries deals with the problem of coupling into cladding modes, and proposes an optical fibre with depressed cladding refractive index profile and with a grating photo-written into the core.
  • the characteristics of the fibre and of the cladding are preferably such as to meet two conditions that, together, guarantee the single-modality of the fibre.
  • the first condition is that the parameter V must be greater than or equal to 2.4 so that the confinement factor of the fundamental mode is high;
  • the second condition is that the refractive index n 3 of the cladding outer area (herein referred to as second cladding) is greater than the effective refractive index (N 2 ) of the secondary mode but lower than the refractive index (N x ) of the fundamental mode.
  • compression of higher order modes of column 6, they explain the reason for which the presence of higher order modes is regarded as undesired. In practice, in this case, there would be a coupling between the fundamental mode and the higher order modes and the quality of communication would be worse. Moreover, it is specified (col.
  • an optical device comprising an optical fibre with depressed cladding refractive index profile suitable to operate in multi-mode conditions at the wavelengths of interest and in the core of which a diffraction grating having variable periodicity
  • Said device can advantageously be used for dispersion compensation in an optical telecommunication system.
  • the above conditions of multi-modality (which, as already said, are obtained when the operating wavelength is lower than the cut-off wavelength) are met when the effective refractive index ( ⁇ eff ) of at least one higher order mode is greater than the refractive index of the outer cladding area.
  • the Applicant has found that, by using a fibre with a relatively low refractive index step of the core (that is, of the difference between the refractive index of the core and that of the depressed area) , selecting the fibre parameters and, in particular, the core radius so as to make the fibre multi-mode, it is possible to obtain:
  • an optical fibre having a refractive index profile of the depressed cladding type, a cut-off wavelength greater than 1650 and a thickness of the depressed cladding area at least equal to the core radius is especially suitable to house a diffraction grating for making an optical device in which the losses due to power coupling to cladding modes are particularly reduced.
  • the power coupling between the fundamental mode and the higher order guided modes caused by possible asymmetries of the grating can be made lower than the power coupling into the cladding modes; consequently, said effect does not significantly impair the performance of the grating;
  • the present invention relates to an optical-fibre device comprising an optical fibre and a diffraction grating made into said optical fibre, said optical fibre having: - a core, preferably doped with germanium, having a first radius and a first refractive index;
  • said grating being made into said core along a longitudinal axis of said fibre; wherein said grating is of the variable pitch type, and said first and second radiuses and said first, second and third refractive indexes are selected so that the effective refractive index of at least one higher order mode is greater than said third refractive index.
  • the ratio between said second radius and said first radius is greater than 2; more preferably, it is greater than 3.
  • said diffraction grating has an operating spectrum band with width of at least 7 nm.
  • the effective refractive index for mode LP 02 is lower than said third refractive index.
  • Said optical fibre preferably has a cut-off wavelength greater than 1650 nm; more preferably, it is comprised between 1650 nm and 2300 nm; for example, it is comprised between 1700 nm and 1900 nm.
  • the difference between said first refractive index and said third refractive index is preferably comprised between 0.008 and 0.015.
  • the difference between said second refractive index and said third refractive index is preferably comprised between -0.010 and -0.003.
  • Said first radius is preferably comprised between 3.7 and 7 ⁇ m; more preferably, it is comprised between 4.3 and 5.8 ⁇ m.
  • the present invention relates to an optical fibre usable for writing a diffraction grating, having:
  • a core having a first radius and a first refractive index
  • cut-off wavelength is greater than about 1650 nm, and the ratio between said second radius and said first radius is greater than 2.
  • Said cut-off wavelength is preferably comprised between about 1650 nm and 2300 nm; for example, it is comprised between about 1700 nm and 1900 nm.
  • the difference between said first refractive index and said third refractive index is preferably comprised between 0.008 and 0.015.
  • the difference between said second refractive index and said third refractive index is preferably comprised between -0.010 and -0.003.
  • said core is doped with germanium.
  • Said first radius is preferably comprised between 3.7 and 7 ⁇ m, more preferably, between about 4.3 and 5.8 ⁇ m.
  • the ratio between said second radius and said first radius preferably is greater than 3.
  • Figure 1 shows a typical transmissivity curve of a periodical Bragg grating
  • FIG. 2 shows a typical transmissivity curve of a Bragg grating suitable for the compensation of chromatic dispersion
  • FIGS. 3a and 3b respectively show an optical fibre with depressed cladding in which a chirped Bragg grating is obtained and a periodical Bragg grating;
  • Figures 14a- 14c show, on the basis of a numerical simulation, the junction losses between a fibre according to the invention and a single-mode fibre;
  • Figures 16a and 16b respectively show the transmissivity of a device manufactured according to the invention measured with the configuration of figure 15, and simulated with a numerical model;
  • Figures 17a and 17b respectively show the transmissivity of a comparison device operating in a single-mode way, measured with the configuration of figure 15, and simulated with a numerical model;
  • Figure 18 shows a reflectivity curve for the device manufactured according to the invention, obtained from the experimental curve of figure 16a;
  • FIG. 19 shows a reflectivity curve measured for the comparison device operating in a single-mode way, obtained from the experimental curve of figure 17a.
  • numeral 1 refers to an optical fibre comprising a core 2, preferably central with respect to the fibre itself, and with circular section, and a cladding 3 with an annular section, external to the core 2.
  • a Bragg grating 4 Into the core 2 of the optical fibre there is a Bragg grating 4.
  • the optical fibre 1 and the grating 4 define an optical device 5.
  • Grating 4 is a chirped grating comprising a plurality of areas with a high refractive index 4a and of areas with a low refractive index 4b, alternating with one another along the axis of the optical fibre 1.
  • the distance between two consecutive areas with a high refractive index 4a (or between two consecutive areas with a low refractive index 4b) defines the grating pitch ⁇ , which in the present case is progressively increasing shifting from the left rightwards along the fibre axis.
  • Cladding 3 comprises an inner cladding area 3a (also called depressed cladding area and represented with a first grey hue) adjacent to said core 2 and having a second radius R 2 and a second refractive index n 2 lower than the first refractive index , and an outer cladding area 3b (represented with a second grey hue) which is outer with respect to the inner cladding area 3a, and has a third radius R 3 and a third refractive index n 3 greater than the second refractive index n 2 and lower than the first refractive index n x .
  • the inner cladding area 3a is also called depressed cladding area, in relation to the pattern of the refractive index.
  • Figure 3b shows an optical device 5' which differs from device 1 only in that the Bragg grating, herein referred to with 4', is a constant-pitch grating.
  • the characteristic parameters of device 5 havethe following values :
  • the quantity x - n 3 is preferably comprised between 0.008 and 0.015.
  • n 2 - n 3 is preferably comprised between -0.010 and -0.003.
  • the cut-off wavelength is greater than 1650 nm and preferably lower than 2300 nm, for example comprised between 1700 nm and 1900 nm.
  • the first radius R x is preferably comprised between 3.7 and 7 ⁇ m, more preferably it is comprised between 4.3 and 5.8 ⁇ m.
  • the third radius R 3 is preferably comprised between 62 and 63 ⁇ m, for example equal to about 62.5 ⁇ m.
  • the Applicant has found that, using a fibre with a relatively low refractive index step between core and depressed cladding area ( - n 2 , hereafter briefly referred to refractive index step of the core) , selecting the fibre parameters and, in particular, the radius of core 2 so as to make the fibre single-mode, it is possible to obtain:
  • the Applicant has also noted that, to obtain the desired value of the cut-off wavelength of a fibre with depressed cladding refractive index profile, it is possible to intervene, besides on the diameter of the core and on the thickness of the core depressed area, also on the refractive index step of the core through a suitable selection of the type and of the concentration of the dopants.
  • the refractive index step of the core can be controlled by regulating the concentration of Ge into the core itself. Said concentration also determines the photo-refractivity of the core.
  • Figure 4 schematically shows the refractive index profile in fibre 1 in function of radius R, measured starting from the longitudinal axis of the fibre itself.
  • Radiuses R and R 2 of core 2 and of the inner cladding area 3a respectively, and the refractive index steps n 1 -n 2 and n 2 - n 3 between core 2 and inner cladding area 3a, and respectively, between inner cladding area 3a and outer cladding area 3b, are selected so that the guided propagation of at least one further core mode besides the fundamental mode LP 01 is possible, that is, of a higher order core mode.
  • Each further core mode has an effective refractive index n eff with a greater value than that of the refractive index n 3 in the outer cladding area 3b (as exemplified in figure 4) .
  • radiuses R x and R 2 and the refractive index steps n- L -n- 2 and n 2 -n 3 are selected in such a way that the only azimuthal symmetry core mode that can propagate is the mode LP 01 , whereas the propagation of the guided mode LP 02 is not possible.
  • the above parameters are selected so that besides the propagation of the fundamental mode, also the propagation of the higher order mode LP 1:L is possible, and preferably, also that of the higher order modes LP 12 and LP 21 , all of which (at least for fibres with depressed refractive index profile) have an effective refractive index which is greater than that of the mode LP 02 .
  • the effective refractive index of mode LP 1:L and preferably, also the effective refractive index of the higher order modes LP 12 and LP 21 are greater than the third refractive index n 3 .
  • the pattern of the transmissivity characteristic of a Bragg grating written into a depressed cladding fibre as the parameters of the refractive index profile change has been studied using a numerical model.
  • a man skilled in the art that is, a man skilled in the propagation of light in optical fibre, can calculate, once the refractive index profile of the fibre has been defined, the constants of propagation of the core and cladding modes and the distribution of the electromagnetic field for the different modes with azimuthal symmetry LP 0m . Consequently, a skilled man is capable of calculating the overlap integral (4) .
  • the characteristics of the grating that is, the function describing the modulation of the refractive index, it is possible to calculate the pattern of the grating transmissivity in function of the wavelength.
  • the model has been applied to a step index optical fibre, that is, an optical fibre having a predetermined refractive index step between the core and the cladding, comprising a chirped Bragg grating into its core.
  • Said type of fibre can be considered an extreme case of a depressed cladding fibre wherein the depression is null.
  • the grating considered for the model has a Bragg wavelength equal to 1.55 ⁇ m, and a reflectivity at this wavelength equal to 30 dB .
  • the refractive index step between core and cladding has been selected equal to 0.011 (to which corresponds a numerical aperture NA equal to 0.18) and the radius of the core has been made vary so as to obtain three different cut-off wavelength ⁇ c respectively equal to 1200 nm, 1550 nm and 1900 nm, which are respectively lower than, coinciding with, and greater than the Bragg wavelength.
  • the model allowed calculating the pattern of the transmissivity curve of the grating and thus, in particular, the spectrum position and the entity of the loss peaks due to power coupling of mode LP 01 with each of the modes LP 0m with m>l, within a band of about 30 nm starting from the Bragg peak.
  • Figures 5a, 5b and 5c show the pattern of the transmissivity losses (transmission loss) of the grating as the wavelength changed, respectively for ⁇ c equal to 1200 nm, 1550 nm and 1900 nm, obtained with the above numerical model.
  • the peaks in the diagram are to be read as follows. Starting from the Bragg wavelength (that is, 1550 nm) and moving to lower wavelengths (that is, moving leftwards on the axis of wavelengths) , the first peak represents the loss due to the coupling of mode LP 01 with mode LP 02 , the second peak represents the loss due to the coupling of mode LP 01 with mode LP 03 , and so on.
  • the loss are increasing at first, then they reach a maximum level, and decrease until they reach a minimum, and then they are relatively low.
  • loss peaks tend to concentrate in a spectrum region close to the Bragg wavelength. In case of a chirped grating, the spectrum distribution and the entity of loss peaks determine the pattern of the integral loss curve of the grating.
  • Figure 6 shows, always with regard to the above fibre provided with a chirped grating, and operating between 1520 nm and 1550 n , reflectivity curves at the above three cutoff wavelengths .
  • Each curve has been obtained by integrating the transmission losses of the grating by moving from the Bragg wavelength to lower wavelengths.
  • the different curves are substantially flat in the proximity of the Bragg wavelength due to the substantial absence of losses in this spectrum region and, when the loss contributions associated to modes LP 02 , LP 03 , etc., start, they have a decreasing pattern.
  • each reflectivity curve of figure 6 tends to be substantially constant also in this spectrum region, so that the general ⁇ pattern of each curve is assimilable to a "step" profile.
  • an increase in the cut-off wavelength causes a reduction in the height and in the spectrum extension of the step.
  • All of these characteristics contribute to making the pattern of the chirped grating reflectivity "flatter", that is, "more equalised”; said condition is advantageous for the purpose of having an even behaviour of the grating at all the wavelengths it must operate. Said condition especially concern wide-band chirped gratings, that is, chirped gratings wherein the operating band extends beyond the initial area without loss peaks.
  • refractive index step n 2 -n 3 between depressed cladding area 3a and outer cladding area 3b equal to -0.005.
  • Figures 7a, 7b and 7c show the pattern of transmissivity losses of the grating in the case of a fibre with depressed cladding calculated with the above numerical model, respectively for values of the cut-off wavelength ⁇ c equal to 1200 nm, 1550 nm and 1900 nm, and with a value of ratio p between the radius of the depressed cladding R 2 and the radius of core R ⁇ equal to 2.
  • Figures 9a, 9b and 9c show the pattern of transmissivity losses of the grating in the case of a fibre with depressed cladding calculated with the above numerical model, respectively for values of the cut-off wavelength ⁇ c equal to 1200 nm, 1550 nm and 1900 nm, and with a value of ratio p between the radius of the depressed cladding R 2 and the radius of core R equal to 3.
  • Figures 11a, lib and lie show the pattern of transmissivity losses of the grating in the case of a fibre with depressed cladding calculated with the above numerical model, respectively for values of the cut-off wavelength ⁇ c equal to 1200 nm, 1550 nm and 1900 nm, and with a value of ratio p between the radius of the depressed cladding R 2 and the radius of core R ⁇ equal to 4.
  • Figures 8, 10 and 12 show reflectivity curves at the above cut-off wavelengths respectively associated to the patterns of figures 7, 9 and 11 and obtained in a similar way as the curve in figure 6 (that is, by integrating the transmission losses of the grating moving from the Bragg wavelength to lower wavelengths) .
  • the increment of the cut-off wavelength determines a progressive concentration of the highest loss peaks into an increasingly reduced spectrum region.
  • the increment of the cut-off wavelength determines, upon ratio p being equal, a reduction in the height and in the spectrum extension of the step in the reflectivity curves, so that the response curve of the grating is more equalised in the spectrum band of interest.
  • the Applicant has found that, for value of the ratio p greater than 2, the curve is particularly equalised.
  • Figure 13 shows curves relating to the dependence of the reflectivity of the chirped grating on the refractive index step n 1 -n 3 with equal cut-off wavelength (in this case, equal to 1550 nm) , equal ratio p (in this case equal to 3) and equal index step n 3 -n 2 (in this case, equal to -0.005) . From the pattern of the curves of figure 13 , it can be deduced that it is not possible to determine a single selection criterion of the refractive index step n- L *- ⁇ to improve the response of the grating.
  • the grating must substantially reflect only the fundamental mode LP 01 .
  • a first reason for this coupling may be the noise effect generated by the junction between the multi-mode fibre comprising the grating and a single-mode fibre transmitting the radiation, usually present when the above device is used for optical telecommunications.
  • a second reason for the above coupling may be an asymmetry of the grating and, more in particular, a defect in the grating orientation
  • junction losses decrease as the cut-off wavelength increases, and they are especially reduced for cut-off wavelengths greater than those typically used in the third window of optical fibre telecommunications (about 1550 nm) , where the depressed cladding fibre is multi-modal.
  • losses are sensitive to variations of the index step n 1 -n 3 ; in particular, a decrease in the refractive index step n 1 -n 3 causes a decrease of losses.
  • junction losses are less than 0.2 dB, and these values are totally acceptable for the use in telecommunication systems.
  • the shape of the step of the reflectivity curves of figure 13 varies in function of the index step n 1 -n 3 (being the cut-off wavelength and the ratio p fixed) in such a way that it is not possible to identify an optimal value of the index step n- L -n**, in relation to the problem of power coupling into cladding modes, the selection of the index step n 1 -n 3 can be carried out so as to minimise junction losses.
  • a second reason for power coupling into higher order guided modes may be the possible asymmetry of the grating.
  • the Applicant has noted that, using a fibre having a core radius (R x ) comprised between about 4 and 5 ⁇ m, and such a content of Ge in the core as to obtain a index step n 1 -n 3 comprised between about 0.011 and 0.0146, it is possible, by using the process and the parameters of writing specified below, to obtain chirped gratings with such a degree of uniformity of the refractive index in a plane orthogonal to the fibre axis so as to not introduce substantial effects of coupling into core or cladding modes that are not azimuthally symmetric.
  • an optical fibre with depressed cladding refractive index profile has been manufactured, having the following characteristics :
  • n 1 -n 3 between core 2 and outer cladding area 3a equal to 0,011;
  • n 2 -n 3 between depressed cladding area 3a and outer cladding area 3b equal to -0,005;
  • the cut-off wavelengths of higher modes than mode LP 1;L are all lower than 1280 nm; thus, the only guided modes of core are modes LP 01 and LP 1:L .
  • Bragg gratings were written using the technique described in the patent application UK n. 9617688.8 in the name of University of Southampton, briefly described in the following.
  • the fibre is translated with a steady motion.
  • the UV laser beam is sent to the fibre through an acoustic-optical modulator and a phase mask placed in the proximity of the fibre itself.
  • the interference between the beams diffracted by the mask of order 1 and -1 generates a series of fringes which define the periodical modulation of the UV beam.
  • the acoustic-optical modulator the exposure to the UV beam is periodically interrupted and restored when the fibre has been moved by a distance equal to the period ⁇ of the refractive index modulation. In this way, the fringes of the UV beam are overlapped to the previously exposed areas.
  • the grating is made with a relatively high number of subsequent exposures.
  • a suitable technique of apodisation (of the known type, and not described herein) has been used for suppressing the secondary lobes typically present in the grating spectrum response.
  • narrow-band (0.2 nm) gratings with a length equal to 30 mm have been made, with a typical transmissivity greater than 30 dB .
  • the transmissivity spectrum of these gratings has been measured through a spectrum analyser with a resolution of 0.01 nm.
  • the gratings were prevented from having asymmetries .
  • the writing was carried out at a relatively reduced translation speed of the fibre (equal to 0.3 mm/s) , so as to approach the conditions of saturation that bleach the portion of core first impinged by the UV beam, thus guaranteeing that the opposed portion of core would substantially receive the same UV intensity.
  • Figure 15 shows a configuration used for experimental tests on device 5.
  • the characteristics of device 5 are those just described.
  • the ends of fibre 1 were jointed with respective standard SM single-mode fibres 7a, 7b (through fusion and with junction losses equal to about 0.26 dB) .
  • the transmissivity spectrum of device 5 was detected by feeding optical power to one of the single-mode fibres (for example, fibre 7a) and measuring the optical power in output from the other single-mode fibre (fibre 7b) .
  • the multi -mode fibre was curved so as to obtain, both before and after grating 4 (schematically shown) , three fibre rings 10a, 10b with a diameter equal to about 30 mm.
  • the Applicant has checked that these rings allow inducing bending losses on the higher order mode LP 11; whereas there are no significant effects on the fundamental mode.
  • Figure 16a shows the transmissivity spectrum of device 1 measured with the configuration just described.
  • the maximum attenuation in correspondence with the Bragg wavelength is equal to about 38.1 dB .
  • Figure 16b shows the results of the numerical simulation carried out on the same device using the numerical model already used before. It is possible to note the presence, in the experimental graph, of a loss peak at about 1550 nm, not present in the numerical simulation graph. Said peak, probably caused by the presence of residual asymmetries in the grating, has an especially reduced height (about 0.4 dB) ; thus, it is not such as to impair the substantial advantages of using a multi-mode fibre.
  • said further fibre has the same characteristic values as the multi-mode fibre; in particular, it has the same values of the refractive indexes of the different core and cladding areas, as well as the same values of the relative ratios between the radiuses of the core and cladding areas.
  • a Bragg grating was written into said fibre, equal to that made in the multi -mode fibre, for the purpose of obtaining a comparison optical device.
  • Figures 17a and 17b show the corresponding measured and simulated transmissivity spectrums .
  • the attenuation peak of this device is of about 39 dB, similar to that of the device according to the invention. It can be noted that the entity of loss peaks is much greater with respect to the case of the device according to the invention.
  • Figures 18 and 19 show the reflectivity curves relating to the device with multi-mode fibre made according to the invention and to the device with single-mode fibre used for comparison, obtained by calculating the integral losses starting from the experimental spectrums of figures 16a and 17a respectively. The greater equalisation of the reflectivity curve of the device according to the invention is evident.
  • Device 5 can be housed into a container to protect it during use.
  • the Applicant has noted that the insertion of the device into a container wherein the fibre has, in the area taken by the grating, a maximum curvature with a radius of about 50 mm, does not cause substantial variations in the transmissivity spectrum of the grating itself .
  • the device of the present invention can be used in an optical telecommunication system in order to compensate chromatic dispersion.
  • the telecommunication system considered herein is a long-distance WDM (Wavelength Division Multiplexing) telecommunication system, for example, a submarine telecommunication system, wherein the problem of chromatic dispersion is significant.
  • WDM Widelength Division Multiplexing
  • An optical telecommunication system typically comprises a transmitting station, a receiving station and an optical communication line connecting the transmitting and receiving stations.
  • the transmitting station comprises a plurality of optical transmitters each being suitable to transmit an optical signal at a respective wavelength.
  • Each optical transmitter can, for example, comprise a laser source and a wavelength converter suitable to receive the signal generated by the laser and to transmit a signal at a predetermined wavelength.
  • a wavelength multiplexer is connected in input to the transmitters for receiving the plurality of transmitted signals, and it has a single output connected to the communication line for transmitting the wavelength- multiplexed signals on the line.
  • the transmitting station can comprise a power amplifier, connected to the multiplexer output, to impart to the transmitted signals the power needed for the transmission along the line.
  • the receiving station comprises a wavelength demultiplexer connected in input to the line for receiving the transmitted signals, and is provided with a plurality of outputs on which the different wavelengths transmitted are separated.
  • the receiving station also comprises a plurality of optical receivers, each connected to a respective output of the demultiplexer for receiving a signal at a respective wavelength.
  • Each receiver can comprise a wavelength converter suitable to convert the wavelength of the signal into a wavelength which is suitable for the reception of the signal by a photodetector optically connected to the converter itself.
  • the receiving station can comprise a pre-amplifier, arranged upstream of the demultiplexer, for imparting to the transmitted signals the power needed for a correct reception.
  • the communication line comprises several spans of optical fibre (preferably, single-mode optical fibre) and a plurality of line amplifiers, spaced from one another (for example, by about a hundred kilometres) suitable to amplify the signals at a level of power suitable for the transmission on a subsequent optical fibre span.
  • optical fibre preferably, single-mode optical fibre
  • line amplifiers spaced from one another (for example, by about a hundred kilometres) suitable to amplify the signals at a level of power suitable for the transmission on a subsequent optical fibre span.
  • a device according to the invention can be used along the telecommunication line and/or in transmitting and receiving stations.
  • the use of the device according to the invention for compensating chromatic dispersion is especially advantageous when the transmission speeds are of 10 Gbit/s or higher.
  • a dispersion compensator using the device of the invention can comprise a three-port optical circulator, suitable to receive at an input port the transmitted signals and to feed said signals to an output port after they have passed through an intermediate port .
  • Input and output ports can be serially connected to the communication line.
  • a portion of the fibre of device 5 is connected to the intermediate port.
  • Grating 4 reflects the different wavelengths of the signals in correspondence with different areas, thus allowing the reduction of the chromatic dispersion of the signals.

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Optics & Photonics (AREA)
  • Diffracting Gratings Or Hologram Optical Elements (AREA)

Abstract

L'invention concerne un dispositif à fibre optique comprenant une fibre optique (1) et un réseau de diffraction (4) ménagé dans ladite fibre optique (1), qui comporte les éléments suivants : un coeur (2), un premier rayon (R1) et un premier indice de réfraction (n1); une première zone de gaine (3a) entourant le coeur (2) et présentant un deuxième rayon (R2) et un deuxième indice de réfraction (n2) plus faible que le premier indice de réfraction (n1); une deuxième zone de gaine (3b) entourant la première zone de gaine (3a) et présentant un troisième rayon (R3) et un troisième indice de réfraction (n3) plus élevé que le deuxième indice de réfraction (n2) et plus faible que le premier indice de réfraction (n1). Le réseau de diffraction (4) est ménagé dans le coeur (2) le long d'un axe longitudinal de la fibre (1). L'invention est caractérisée en ce que ledit réseau de diffraction (4) est de type à pas variable et en ce que les premier et deuxième rayons (R1R2) ainsi que les premier, deuxième et troisième indices de réfraction (n1, n2, n3) sont sélectionnés de telle sorte que l'indice de réfraction effectif (neff) d'au moins un mode d'ordre supérieur est plus élevé que le troisième indice de réfraction (n3).
PCT/EP2000/012874 1999-12-30 2000-12-14 Dispositif a fibre optique comprenant un reseau de diffraction WO2001050165A1 (fr)

Priority Applications (1)

Application Number Priority Date Filing Date Title
AU37261/01A AU3726101A (en) 1999-12-30 2000-12-14 Optical-fibre device comprising a diffraction grating

Applications Claiming Priority (4)

Application Number Priority Date Filing Date Title
EP99830822.5 1999-12-30
EP99830822 1999-12-30
US17438000P 2000-01-04 2000-01-04
US60/174,380 2000-01-04

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Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN100356212C (zh) * 2003-06-18 2007-12-19 株式会社藤仓 高次模分散补偿光纤和高次模光纤用模转换器
US9933576B2 (en) 2015-12-29 2018-04-03 Stmicroelectronics (Crolles 2) Sas Electro-optic device with an optical grating coupler having a grating period variation and methods of formation thereof

Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0831345A2 (fr) * 1996-09-09 1998-03-25 Sumitomo Electric Industries, Ltd. Réseau à fibre optique
US5852690A (en) * 1997-06-30 1998-12-22 Minnesota Mining And Manufacturing Company Depressed cladding fiber design for reducing coupling to cladding modes in fiber gratings

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0831345A2 (fr) * 1996-09-09 1998-03-25 Sumitomo Electric Industries, Ltd. Réseau à fibre optique
US5852690A (en) * 1997-06-30 1998-12-22 Minnesota Mining And Manufacturing Company Depressed cladding fiber design for reducing coupling to cladding modes in fiber gratings

Non-Patent Citations (2)

* Cited by examiner, † Cited by third party
Title
DONG L ET AL: "OPTICAL FIBERS WITH DEPRESSED CLADDINGS FOR SUPPRESSION OF COUPLINGINTO CLADDING MODES IN FIBER BRAGG GRATINGS", IEEE PHOTONICS TECHNOLOGY LETTERS,US,IEEE INC. NEW YORK, vol. 9, no. 1, 1 January 1997 (1997-01-01), pages 64 - 66, XP000640889, ISSN: 1041-1135 *
HAGGANS C W ET AL: "NARROW-DEPRESSED CLADDING FIBER DESIGN FOR MINIMIZATION OF CLADDINGMODE LOSSES IN AZIMUTHALLY ASYMMETRIC FIBER BRAGG GRATINGS", JOURNAL OF LIGHTWAVE TECHNOLOGY,US,IEEE. NEW YORK, vol. 16, no. 5, 1 May 1998 (1998-05-01), pages 902 - 909, XP000772656, ISSN: 0733-8724 *

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN100356212C (zh) * 2003-06-18 2007-12-19 株式会社藤仓 高次模分散补偿光纤和高次模光纤用模转换器
US9933576B2 (en) 2015-12-29 2018-04-03 Stmicroelectronics (Crolles 2) Sas Electro-optic device with an optical grating coupler having a grating period variation and methods of formation thereof

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