METHOD OF AND APPARATUS FOR THE MEASUREMENT OF MODE FIELD RADIUS IN SINGLE-MODE FIBRES

Abstract: The method of measuring mode field radius in single-­ mode optical fibres comprises the modulation of an optical beam by means of a spatial filter with radial-distribution of transmittivity of Gaussian type. Modulated beam intensity is measured in correspondence with the beam axis for different widths of the Gaussian curve, and the mode field radius is derived from both such intensity values and a parameter dep­ endant on the Gaussian width. The apparatus which imple­ ments the method is also provided.

Description: The present invention relates to methods and apparatus for characterization of optical-fibres for telecommunications and more particularly it concerns a method of and a device for measuring the mode field radius in a single-mode optical fibre. Dimensional parameters are of main importance for single-mode optical fibre characterization. Dimensional parameters include both geometrical/structural features of the fibre (core and cladding radius, maximum numerical aperture, etc.), and properties depending on the propagation (mode-field dimensions and shape). The former properties are obviously of fundamental importance while designing the fibres, while the latter properties (mode parameters) are of main importance in the characterization intended to the use, since they allow joint losses, microbending losses and coupling efficiency to be determined, thus allowing a correct connection dimensioning. Different definitions have been given up to now for such mode parameters and in particular for the mode field radius. A definition frequently applied in the measurements gives mode field radius as the mean square width of the near-field and as inverse of the far-field width. The near-field value gives information on joint losses in case of angular misalignment and on microbending losses, while the far-field value gives information on joint losses due to transverse offset and on guide dispersion. The application of such definitions requires a double series of measurements, namely a near-field and a far-field measurement, for fibre characterization. For this reason, standardizing authorities are oriented towards different definitions of the mode-field radius: among these definitions, the width w b of the gaussian source which affords maximum power coupling into the fibre is of particular interest, since from this definition values are obtained which are a good compromise between near-field and far-field values. Denoting by (r) the field associated with the fibre (r=radial coordinate), and by G(r,w) = exp(-r²/2w²) the source field (w=Gaussian width), value w b is the value which maximizes field super-position integral and is implicitely defined by the relation A measurement method of value w b is described e.g. in the paper entitled "Spot-Size Measurements For Single Mode Fibers - A Comparison of Four Techniques" by W.T. Anderson and D.L. Philen, Journal of Lightwave Technology, Vol. LT-1, N. 1, March 1983, pp. 20 and ff. According to this paper, near-field intensity scanning of a light-beam at the output of the fibre under test is effected and function G(r;w) which better approximates the experimental data is obtained by a computing system. This known method presents two disadvantages, namely: 1) intensity scanning is a rather long operation; 2) complex processings of the experimental data are necessary to obtain w b value, and hence rather sophisticated computing means are required. A method of measurement of the "mode field radius" wherein the beam outgoing from the fibre is modulated by a mask with transmittivity varying in radial direction is described for instance in the European Patent Application No. 145936, published on 26.06.1985, in the name of the same Applicant. The term "mode field radius", however, covers several definitions and the prior art method exploits the definition of mode field radius as the near-field mean-square width (or the inverse of far-field width), and requires modulation by a mask whose transmittivity increases with the square of the transversal beam dimension. Such a method cannot be used for a measurement based on the definition applied here and explained below. In fact, not only is the modulation law different in the two cases and requires masks whose transmittivity variation is of opposite sign (transmittivity increasing towards the edges in the cited application and decreasing in the present invention), but also measurement methods are different, since the known method requires an intensity measurement also on the unmodulated beam and the measurements are of integral type, i. e. the whole beam intensity on the measurement plane and not only the intensity in correspondence with the beam axis, is to be determined. Finally, in the known method the mask effects an intensity modulation, while the present method carries out a field amplitude modulation, as will become clearer hereinafter. The present invention aims at supplying a method and an apparatus, based on the optical processing of the light beam sent into the fibre, which require neither the beam scanning, thus allowing a quicker measurement, nor complex processings of the experimental data, so that less sophisticated and hence less expensive computing means can be used. The method of the invention is characterized in that for measuring the mode field radius w b according to the definition wherein: (r) = radial distribution of the mode field; G(r,w b ) = exp (-r²/2w b ²); r = radial coordinate from the fibre axis in the fibre input or output face; w b = the Gaussian width which maximized S(w b ); said mask has a distribution of transmittivity representable with a Gaussian curve of given width; the intensity of the modulated beam is measured at a location on the axis of the beam at the output of an optical system which performs the optical inverse Fourier transformation; the mask position is varied so as to vary the spatial filtering effect, each new position corresponding to the filtering with a mask having a Gaussian distribution of intensity of different width; the measurement of intensity in correspondence with the modulated beam axis is repeated for different width values; and, for each position of the mask, the product is calculated between the square root of measured intensity and a parameter depending on the mask position, the mode field radius value being the width value which renders that product maximum. The modulation can be indifferently effected on the beam launched into or outgoing from the fibre. In a first embodiment of the invention, the mask position is varied by translating the mask parallel to itself so as to change its distance with respect to the fibre input or output end, and the parameter depending on the mask position is represented by said distance. The mask can be a circularly symmetric gray scale, whose transmittivity is maximum at the centre and minimum at the edges and varies according to a Gaussian law. As an alternative, a mask can be used with perfectly transparent and perfectly opaque zones so that the mean radial transmittivity is a Gaussian function. In a variant embodiment the beam to be modulated is collimated and the mask position is varied by varying the mask inclination with respect to beam propagation direction, and said parameter is the inverse of the square of the gaussian width. Also in this case the mask can consist of either a gray scale such as to maintain the Gaussian distribution, whatever the angle, or perfectly opaque and perfectly transparent zones, such that average transmittivity along a circumference whatever the angle, be a Gaussian function. The theoretical principles whereupon the method provided by the invention is based will be examined hereinafter. The present invention includes also the apparatus for implementing the method to measure the mode field radius as defined above, comprising: a source of an optical beam; a first optical system to transfer the beam to an optical fibre; a second optical system to transfer the beam outgoing from the fibre to a photodetector which supplies electrical signals representing the beam intensity; a spatial filter, coaxial to the beam, which filter has a radially variable transmittivity, radially modulates the beam and is mounted on a movable support allowing the filter position to be varied; and a measuring and computing system which derives value w b of the mode field radius from such electrical signals; which apparatus is characterized in that said filter has a radial distribution of transmittivity which can be represented by a Gaussian function whose width is a function of the filter position, so that the displacement of the filter causes a variation of the width of the Gaussian function, in that said second optical system performs the optical inverse Fourier transformation, and in that said detector is arranged to supply signals representing the beam intensity I o in correspondence with the beam axis, and the measuring and computing system is apt to obtain the value w b of the mode field radius from the product of the square root of the measured values of intensity I o relating to the different positions of the filter and a parameter depending on the mask position, the mode field radius value w b being the width value which renders that product maximum. The invention will become more apparent with reference to the annexed drawings in which: Fig. 1 is a schematic representation of a first embodiment of the invention; Figures 2, 3 are examples of masks usable in the apparatus of Fig. 1; Fig. 4 is a schematic representation of a variant embodiment; Fig. 5 is an example of mask usable in the apparatus of Fig. 4. By way of example, Figures 1 and 4 depict the case in which a beam outgoing from the fibre is modulated, because that arrangement makes the following theoretical disclosure clearer; however, in both embodiments, as an alternative, the beam sent into the fibre can be modulated. As shown in Fig. 1, a light source 1 emits a beam which, through a launching optics 2, is sent into a single-mode fibre 3, whose mode field radius is to be determined. The beam outgoing from fibre 3 is collected by an optical system 4 which forms an image of the fibre output face 3a on the sensitive surface of a detector 5, apt to supply an electrical signal representing the beam intensity in correspondence with the fibre axis. A mask or spatial filter 6 is inserted, coaxially to the beam, between fibre output 3a and optical system 4. The mask modulates the beam field proportionally to exp(-r²/2w²), r being the radial coordinate of the beam with origin on the axis. Mask 6 is mounted on a support, not represented, which is shifted by a suitable controller, schematized by block 7, to allow the distance of mask 6 from fibre end 3a to be varied. Detector 5 is followed by a measuring and computing system 8, which, for each value of distance z between mask 6 and fibre 3, measures the value of light intensity Io in correspondence with the beam axis and calculates product z·√ Io(z) . The value of w b is obtained by computing system 8 from z value, which renders that product maximum. This methodology depends on the characteristics of the system of Fig. 1, which is a kind of "image processor" based on Fourier optics. To evaluate its properties, it is convenient to simulate the filtering operation on plane 6 by considering a suitable "virtual field"  v in replacement of field  at fibre output 3a. Field  v is defined so as to reproduce, by far field propagation, the same distribution generated by the mask on plane 6. Hence the effect of  v in any other system point is equivalent to that produced by  in presence of the mask. More particularly, the image of  v , possibly magnified, is formed on the detector plane in both cases. Thus, neglecting the magnification and diffraction effects of the optical systems, the resulting field on plane 5 can be computed on the basis of the of definition  v , thereby obtaining far-field approximation equation: where r o is the vector of position on plane 6 (with origin on the axis), k=2π/λ is the wave number at the wavelength used for the measurement, z the fibre-mask distance, t the amplitude transmittivity of the mask, ψ and ψ v represent Fourier transforms of  and  v respectively. Antitransforming relation (3), one obtains the expression of  v as convolution integral: where r is the vector radius on plane 3a (with origin on the axis), F⁻¹t is the Fourier antitransform of mask transmittivity t(r), and A o is a constant, whose value depends on the wavelength and on the normalization type chosen for the transform. Since transmittivity t is a Gaussian function, i.e. it is a function t(r)=exp(-r²/2w²) , F⁻¹t(ω) will be proportional to exp(-w²ω²/2), where ω=k/z (r-r') . Substituting in (4), taking into account value of ω and assuming: w b (z) = z/kw (5) virtual field  v in correspondence with the fibre axis becomes: where A is a new constant, proportional to k²/ω² whose value presents no interest. Comparing with (1), one can see that: S ∝ z ·  v (o) ∝ z·√ I o (z) (7) where Io(z) is the field intensity in correspondence with the axis. Since intensity I, as known, is proportional to  v ², the maximum of S may be determined simply by computing maximum of function z·√ Io(z) . Value w b is immediately obtained substituting in (5) value z' which renders such product maximum. To modulate the beam, a circularly symmetric mask can be used, which mask consists of a gray distribution whose transmittivity is maximum at the centre and null at the edges and varies according to the above mentioned law. Such a mask can be obtained with photographic techniques. An example is given in Fig. 2. If field distribution is circularly symmetric, as generally occurs in single-mode fibres, a non-symmetric mask can be used, composed of perfectly opaque and perfectly transparent zones such that transmittivity, along any circumpherence of radius r, is proportional to Some examples of non-symmetric masks are shown in Fig. 3, showing masks comprising 1 to 9 opaque and transparent zones, obtained from suitable repetitions of arcs of curves which, in polar coordinates (r, ϑ), are described by the equation ϑ = C.exp(-r²/2w²) . The masks shown in Fig. 3 can be quartz disks whereupon a null-transmittivity metal layer with desired boundaries has been deposited. In the embodiment shown in Fig. 4, fibre end 3a is placed in the focus of an optical system 40, collimating the beam emitted from the source; the collimated beam is collected by an optical system 41 and concentrated on detector 5. Mask or spatial filter 60, which modulates the beam intensity, is placed between optical systems 40, 41. Mask 60 is mounted on a support, not shown, which can be displaced by controller 70 to rotate the mask about an axis perpendicular to the plane of the drawing, and hence to vary the mask inclination with respect to the beam direction. For each value of angle γ between mask 60 and beam direction, computing system 8 measures the beam intensity in correspondence with the fibre axis and calculates the ratio between the square root of the measured intensity and suitable function of angle γ; ω b value is that maximizing such ratio. In fact, let ξ, η be the coordinates of any point of mask 60 with respect to a reference system in which axis η is the mask rotation axis and axis ξ the perpendicular to such an axis in the plane of mask 60. The mask with variable transmittivity t(ξ,η), for a given value γ of the inclination angle with respect to the beam propagation direction (with x≦45°), acts as a perpendicular mask with transmittivity t'(x,y)=t(x/cos γ,y) , where x and y are the coordinates in a reference system where axis y is still the mask rotation axis and axis x is the perpendicular to such an axis and to the beam propagation direction. To carry out the present invention, the average transmittivity along a circumpherence of radius r of the filter used must be a Gaussian function, whatever the angle γ; in polar coordinates it therefore should be: where µ = 1/cos γ . If the beam has radial symmetry, the inclinable filter acts as an equivalent Gaussian filter whose mean transmittivity t (r), along a circumference of radius r, is given by the second member of (8). Then, the considerations made for the embodiment of Fig. 1 can be applied, by replacing parameter z with focal distance f of the collimation optics placed at the fibre output (Fig. 4) and ω with σ. More particularly, relation (6), in which w b is given by w b (γ) = f kσ (9) is still valid. Consequently, as for Fig. 1, value ω b looked for is the value which maximizes the ratio S ∝ I o (γ)/σ(1/cos γ) . Each function t(ξ,η) satisfying equation (7) can be written as Coefficients α, β determine σ(µ) in the following way Relation (10) is obtained by developing in series of powers the two terms of (8) and then determining the coefficients of the series representing t(ξ,η). Functions (10) can be also negative: this causes no problem since it is sufficient to add a constant to expression (10) so that t≧0 everywhere. A mask with transmittivity given by (10) consists of an alternance of opaque and transparent zones and can be obtained with photographic methods, even though the implementation with the desired accuracy can be complicated. The implementation is simpler if the transmittivity of mask 60 varies in a single dimension, e.g. according to axis ξ. In this case coefficient α of (10) and (11) is zero and the mask presents a trasmittivity: An example of mask with transmittivity given by (12) is shown in Fig. 5. Advantageously, in both embodiments the apparatus is automatized. To obtain that, computing and measurement system 8 can be part of a computer which is connected to control devices 7 or 70 of the mask support 6 or 60 and controls its displacements with the described modalities.

Claims: Method of measurement of the mode field radius in single-mode fibres wherein an optical beam is sent into an optical fibre under test (3) and the intensity of the beam outgoing from the fibre (3) is measured, wherein the light beam is submitted to a spatial filtering with a mask (6; 60) which has a radially variable transmittivity and modulates the beam, characterized in that for measuring the mode field radius w b according to the definition wherein: (r) = radial distribution of the mode field; G(r,w b ) = exp (-r²/2w b ²); r = radial coordinate from the fibre axis in the fibre input or output face; w b = the Gaussian width which maximizes S(w b ); said mask (6, 60) has a distribution of transmittivity representable with a Gaussian curve of given width; the intensity (I o ) of the modulated beam is measured at a location on the axis of the beam at the output of an optical system which performs the optical inverse Fourier transformation; the mask position (6; 60) is varied so as to vary the spatial filtering effect, each new position corresponding to the filtering with a mask having a Gaussian distribution of intensity of different width; the measurement of intensity (I o ) in correspondence with the modulated beam axis is repeated for different width values; and, for each position of the mask (6; 60), the product is calculated between the square root of measured intensity (I o ) and a parameter depending on the mask position, the mode field radius value (w b ) being the width value which renders that product maximum. Method according to claim 1, characterized in that either the beam entering the fibre (3) or the beam outgoing from the fibre (3) is submitted to the spatial filtering. Method according to claim 1 or 2, wherein the position of the mask (6) is varied by translating the mask parallel to itself so as to vary its distance (z) with respect to a fibre end (3a), characterized in that the parameter depending on the mask position is represented by this distance. Method according to claim 1 or 2, characterized in that the beam is collimated prior to modulation and in that the position of the mask (60) is varied by changing the mask inclination with respect to the beam propagation direction, said parameter being the inverse of the square of the Gaussian width. Apparatus for implementing the method according to any one of claims 1 to 4, to measure the mode field radius as defined in claim 1, comprising: a source (1) of an optical beam; a first optical system (2) to transfer the beam to an optical fibre; a second optical system (4) to transfer the beam outgoing from the fibre to a photodetector (5) which supplies electrical signals representing the beam intensity; a spatial filter (6; 60), coaxial to the beam, which filter has a radially variable transmittivity, radially modulates the beam and is mounted on a movable support allowing the filter position to be varied; and a measuring and computing system (8) which derives value (w b ) of the mode field radius from such electrical signals; characterized in that said filter (6, 60) has a radial distribution of transmittivity which can be represented by a Gaussian function whose width is a function of the filter position, so that the displacement of the filter causes a variation of the width of the Gaussian function, in that said second optical system performs the optical inverse Fourier transformation, and in that said detector (5) is arranged to supply signals representing the beam intensity (I o ) in correspondence with the beam axis, and the measuring and computing system (8) is apt to obtain the value (w b ) of the mode field radius from the product of the square root of the measured values of intensity (I o ) relating to the different positions of the filter (6; 60) and a parameter depending on the mask position, the mode field radius value (w b ) being the width value which renders that product maximum. Apparatus according to claim 5, characterized in that said filter (6; 60) is either placed between the source (1) and the input end of the fibre (3) or between the output end of the fibre (3) and the detector (5). Apparatus according to claim 6, wherein the filter support (6) is arranged to translate the filter parallel to itself in the beam propagation direction so as to vary the distance between the filter (6) and the end of the fibre (3), and wherein said filter (6) consists of a circularly symmetric mask, characterized in that the mask has maximum transmittivity at the centre and null transmittivity at the edges. Apparatus according to claim 6, wherein the filter support (6) is arranged to translate the filter parallel to itself in the beam propagation direction so as to vary the distance between the filter (6) and the end of the fibre (3), and wherein said filter (6) consists of a circularly asymmetric mask comprising opaque and transparent zones, characterized in that the mean transmittivity along a circumference of radius r centered on the axis is a Gaussian function of the radius. Apparatus according to claim 6, characterized in that it comprises a further optical system (40) collimating the beam to be modulated, and in that the filter support is connected to control devices (70) operating the support to cause the filter to tilt about an axis perpendicular to the propagation direction of the collimated beam so as to vary the inclination of the filter (60) with respect to propagation direction. Apparatus according to claim 9, characterized in that said filter (60) consists of a plane mask with transmittivity varying at least in one of its dimensions so that for each value of said distance (r) from the beam axis the mean transmittivity on a circumference of radius r with center on the axis is a Gaussian function of the radius.

Inventors: CAPONI, RENATO, COPPA, GIANNI, DI VITA, PIETRO
Applicants: CSELT CENTRO STUDI E LABORATORI TELECOMUNICAZIONI S.P.A.
Application year: 1986
Main classification: G01M11
IPC codes: G01M11, G02B6