METHOD AND DEVICE FOR MEASURING COUPLING LOSSES IN SINGLE MODE OPTICAL FIBRES

Abstract: An optical beam is injected into a single-mode (FO) fibre of a given length and outgoes from an end face of the fibre, is reflected by a reflecting optical system into said fibre; the optical power of the reflected beam guided backward by the fibre is measured. Said end face of the fibre is placed in the optical centre of the reflecting system to obtain the maximum reflected power, and is then deviated from said position detecting the occurring reductions of the reflected optical power. A device and variants to implement the method are also described.

Description: The present invention concerns systems for measuring the characterizing parameters of optical fibres, and more particularly it relates to a method and device for measuring coupling losses in single-mode optical fibres. Coupling losses usually occur at joint points between optical fibre trunks and are chiefly due to transverse and/or longitudinal offsets between the two trunks. A method known in the art of measuring coupling losses is described in the paper entitled: "Direct method of determining equivalent-step-index profiles for monomode fibres", C.A. Millar, Electronics Letters, 25th June 1981, Vol. 17, n° 13. According to this method, two lengths of the same single-mode optical fibre are placed facing each other with the axes aligned, and an optical beam is injected into them. A transverse offset is then introduced between the two fibres which allows the measurement of the optical power transferred from a fibre to the other, and consequently of the amount of the coupling losses. The method allows the measurement of the Mode Field Diameter MFD which is a parameter indicative of microbending and coupling losses in single-mode optical fibres. Mode Field Diameter is given by twice the transverse offset value for which the measured optical power drops to 1/e times the maximum value. According to this method a longitudinal offset between the two fibres can be also introduced; thus also coupling losses between two fibre trunks, due to their longitudinal separation, can be measured. The two fibres are held facing each other by means of a high-precision calibrated mechanical device comprising some micromanipulators for the micrometric regulation of fibre alignment and allowing transverse and/or longitudinal offsets of wanted and measurable amounts to be set. For the maximum optical power transfer the end faces of the two fibres ought to be kept in contact; which, however, would no longer allow the transverse offset regulation, due to the rubbing between the two end faces, which hence are kept at a distance as little as possible from each other; this distance is however, conspicuous (about 5 µm). The two fibres are simply obtained by cutting the same fibre length at a certain point; to obtain the maximum optical power transfer the two end faces ought to be plane, non-corrugated and perpendicular to the fibre axis. However, such a cut is rather difficult to obtain; actually the two end faces are slightly obliquous (with angles even wider than 1° with respect to perpendicularity condition), corrugated and irregular, with convexities, swells and hackles. The presence of hackles requires an increase in the distance between the two end faces to prevent their rubbing. Hence additional optical power losses are introduced due both to the unwanted longitudinal offset between the two trunks, and to the non-perpendicularity and irregularity of the end faces. This effect alters the measurement reducing its accuracy. Besides, by this method, a significant MFD measurement cannot be obtained when the fibre core section has a two-folded symmetry shape (i.e. elliptical shape) instead of circular shape. In fact the two fibres, as a consequence of the cut, usually undergo a reciprocal rotation difficult to be compensated for, which causes in this case a mismatching of the two sections and an uncertainty in the measurement of MFD value due to additional optical power losses which vary with the mutual rotation amount and with the direction and sense of the transverse offset. The foregoing and other disadvantages are overcome by the present invention of a method of measuring coupling losses between single-mode fibres which in stead of providing the optical power transfer from one fibre length to another facing the first, provides the optical beam reflection in the same fibre length emitting it, by means of a reflecting optical system which in an example of embodiment is a spherical mirror, in another example is a system of collimating lenses followed by a plane mirror, and in a further example is a system of magnifying lenses followed by a spherical mirror. In this way the disadvantage of the optical power losses due to the end face imperfections is overcome, since the optical beam is anyway reflected by the reflecting optical system into the same fibre wherefrom it is emitted, even if the end face is not perfectly perpendicular to the fibre axis, hence independently of the emission angle of the optical beam with respect to said axis. The problem of the difficulty of regulating the distance between the end faces of the two fibres, which can arise due to their imperfect planarity, is also solved since now the longitudinal positioning of the end face is no longer hindered by the presence of the second fibre. In the case in which the fibre core section has a two-folded symmetry shape, the problem is resolved of the mutual rotation of the two end faces, since the image of the section reflected by the reflecting optical systems is orientated with the symmetry axes always parallel to those of the fibre core section. In transverse offset conditions, the reflected core image moves without rotating with respect to the emitting core, introducing no measurement errors. The system, of the present invention allows also the measurement of coupling losses due to the longitudinal offset between two fibre trunks of a joint by simply introducing longitudinal offsets between the fibre and the reflecting optical system. The method described in claim 1 is a particular object of the present invention. A device and variants for implementing the method, described in claims 2 to 13, are further objects of the present invention. The method of measuring coupling losses between single-mode optical fibres, provided by the present invention, resides in the measurement of the optical power reflected by a reflecting optical system in the same optical fibre length guiding the direct beam. The core section of the fibre end face opposite to the optical source is placed in the optical centre of the reflecting system to obtain the maximum reflected optical power. A relative controlled offset is then introduced between the fibre end face and its image reflected by the reflecting optical system, either in transversal direction, to measure MFD parameter, or in longitudinal direction, to measure separation-dependent losses. The device provided by the present invention will be now described with reference to an embodiment and to variants thereof given by way of example and not in a limiting sense, and to the annexed drawings in which: - Fig. 1 shows a way of embodiment of the device provided by the invention; - Fig. 2 shows the device of Fig. 1 comprising auxiliary elements for the preliminary focusing operations; - Fig. 3 is an alternative embodiment of the device provided by the invention; - Fig. 4 shows the device of Fig. 3 comprising additional elements for the preliminary focusing operations; - Fig. 5 shows a way of acting on the system of Fig. 3 to obtain MFD measurement; - Fig. 6 shows another alternative embodiment of the device provided by the invention. In Fig. 1 reference FO is single-mode optical fibre of a given length. S denotes a common basically-monochromatic electromagnetic radiation source. S can possibly comprise lenses for collimating the optical beam. A lens array, denoted by L, allows the beam to be focused on fibre FO. The technique used to conve y the optical beam into the fibre is usually known and needs no further explanations. R denotes a conventional optical detector associated with a system measuring the detected optical power. BS1 denotes a common beam splitter inserted along the optical beam path from source S to fibre FO with a determined tilt with respect to the beam. A portion of the optical beam emitted by S crosses BS1 and arrives at FO, while another portion is reflected by BS1 upwards in the direction indicated by the arrow and gets lost. A portion of the optical beam arriving from fibre FO, through L, is reflected by BS1 towards detector R to be measured, while another portion after traversing BS1, goes towards S and gets lost. SPS indicates a spherical mirror which is the first way of embodiment of the reflecting optical system mentioned while describing the method. SPS concavity faces the end face of fibre FO denoted by a : the latter is held, through support SUP, in such a position that the core section of end face a is placed in the curvature centre of spherical mirror SPS. The optical centre of the reflecting system is in this case the curvature centre of SPS. Dotted line SM indicates the presence of a known type mechanical device containing spherical mirror SPS and support SUP with the end portion of fibre FO comprising face a . Mechanical device SM is to allow the micrometric adjustment of the relative positioning of mirror SPS and of end face a , through for instance high-precision calibrated micromanipulators, by which end face a can be positioned in SPS centre, and afterwards a transverse or longitudinal offset of a predetermined measurable value of end face a with respect to centering position can be introduced. The embodiment of device SM is no problem to the skilled in the art. When the fibre core section of end face a is positioned at the exact curvature centre of spherical mirror SPS, the optical beam outgoing from a , which arrives from source S through L, is reflected by SPS towards the same face a , enters fibre FO, is collimated by L and reflected through beam splitter BS1 towards detector R which will measure maximum reflected optical power value. This condition is obtained regardless of the actual emission angle of the optical beam with respect to the fibre axis, since for any emission angle comprised in the mirror aperture, the optical beam will be always reflected on itself owing to the presence of the spherical mirror. Thus the optical power loss found in the known system due to non-orthogonality of face a with respect to the fibre axis is compensated for. Besides, since the second fibre lenght is no longer present, the inconvenience due to the difficulty of regulating the gap between the two end faces is eliminated. Introducing a transverse offset, detector R finds out an optical power reduction, since a more or less considerable portion of optical power is reflected by mirror SPS outside the fibre core section of face a , depending on the value of transverse offset, whereon in turn Mode Field Diameter MFD measurement depends. Under offset conditions the image of the core of face a reflected by the spherical mirror moves without rotating with respect to the real core. In this way if the core is not circular, but it has a two-folded symmetry shape, namely an elliptical shape, a significant MFD measurement is obtained, because additional power losses, due to the mutual rotation of the two end-faces found out in the known MFD measurement system, are eliminated. Spurious Fresnel reflections occurring at both end faces of the fibre and at lens array L have also to be taken into account: such reflections give rise to a consta nt optical power value, which can be detected by the measuring instrument in the absence of coupling which is to be subtracted from the values measured in the presence of coupling. Fig. 2 shows a non-limiting example of modifications to be given to the device of Fig. 1 to facilitate the initial operation of positioning of face a in the curvature centre of mirror SPS. In Fig. 2, MO denotes a usual optical microscope which is inserted into the mechanical device SM in lateral position with respect to the direction of the optical beam which outgoes from the fibre and reaches the mirror. BS2 denotes a beam splitter inserted in the trajectory of the optical beam outgoing from face a of the fibre with a tilt such as to form with microscope MO an image of face a itself. For the initial positioning operation, an optical source is used in the field of the visible e.g. an He-Ne laser denoted by LA which, through a mirror SP, placed between the primary source S and the beam splitter BS1, and lens array L, injects light into fibre FO. Besides mirror SP interrupts the optical beam emitted by the primary source S which can also remain switched on. After the initial operation of position regulation of face a , it will be sufficient to remove mirror SP and beam splitter BS2. The positioning in the centre of SPS of the core section of face a is obtained when by microscope MO there can be seen: - the image of face a focused (condition attained by the adjustment in longitudinal direction of the distance between face a and mirror SPS); - the image of the fibre core reflected by SPS coincident with the direct one (condition attained with relative transversal displacements between face a and mirror SPS). It is worth noting that once the initial fibre positioning phase is finished, microscope MO can be left inserted in SM since it does not interfere with the optical-beam direct-path between the fibre and the spherical mirror, while it is sufficient to remove beam splitter BS2: hence from the structural point of view mechanical device SM can provide the fixed position of microscope MO and the housing place of BS2 which can be inserted and removed at will. What described has been given only by way of non limiting example. Variations and modifications are possible without going out of the scope of the present invention. E.g. Fig. 3 shows an alternative embodiment of the reflecting optical system provided by the method. The optical system of Fig. 3 consists of a collimating lens array denoted by L1, for example consisting of a reflected-light microscope objective, and of a plane mirror SP1. If the optical beam source is in the focus of L1, the beam rays outgoing therefrom are parallel. The optical centre of the lens array corresponds in this example to the focus of lens array L1. By plane mirror SP1, placed perpendicular to said rays outgoing from L1, the optical beam is reflected towards L1 itself, whereby it is converged onto face a of the fibre. Hence system L1, SP1 is equivalent to spherical mirror SPS of Fig. 1. Fig. 4 shows a non limiting example of how the device of Fig. 3 can be modified to facilitate the initial positioning operation of face a in the focus of the lens array L1. The distance between L1 and SP1 (Fig. 4) is not critical and can be adjusted to allow the insertion of beam splitter BS2 along the optical beam path; BS2 is inserted with a tilt, with respect to the optical beam, such as to partly reflect the latter towards a usual telescope OC placed laterally. OC function is the same as that of microscope MO of Fig. 2, i.e. that of allowing the positioning of the core section of face a in the focus of system array L1, according to what alre ady described in relation to Fig. 2, with relative displacements between face a of the fibre and lens array L1. Also for the example of embodiment described in Figures 3 and 4 there are provided, but not shown for the sake of simplicity, generators S, LA, detector R, mirrors SP, BS1, lens array L as for the example of figures 1 and 2. Now mechanical device SM1, of the type of SM of Fig. 2, is to contain fibre FO and its support SUP, lens array L1, mirror SP1, telescope OC and the housing place of beam splitter BS2, which will be inserted only in the initial positioning phase of face a in the focus of L1. To measure the Model Field Diameter (MFD) by the system shown in Figures 3 and 4, either a transverse offset can be introduced between fibre end face a and reflecting optical system L1-SP1, or plane mirror SP1 can be slightly tilted with respect to the perpendicularity condition, as shown in Fig. 5. SP1 hinged at end P; the other end Q is coupled to a micromanipulator, it too inserted in mechanical system SM1 (not shown in the Figure) which regulates end Q movement. Reference f denotes the focal distance of lens L1 at which fibre end face a is placed. To obtain an offset x , mirror SP1 is tilted by an angle α with respect to the perpendicularity condition, so that the reflected optical beam has an angle 2α with respect to the fibre axis. Reference v denotes the longitudinal offset the micromanipulator is to apply to end Q of SP1 to obtain angle α; segment practically corresponds to the length of the support of mirror SP1 from hinge point P to point Q, whereto the micromanipulator is coupled. By geometrical optics formulae it can be derived for small α: x/f = 2α = 2v/ → v:x = :2f A ratio v/x≃10 can be obtained from typical values of ≅10 cm, f≅0.5 cm. Hence an amplification of the movement to be introduced by the micromanipulator is obtained, with respect to the case described in connection with Fig. 3, under the same transverse offset conditions. That makes it easier to regulate the movement and to know with higher accuracy the value of the offset introduced. A partly-reflecting spherical mirror can be used as a reflecting optical system in replacement of the totally-reflecting mirror used in the example of embodiment shown in Figures 1, 2. In this way fibre end face a can be observed through the partly-reflecting surface, since a portion of the optical beam traverses the mirror. The partly-reflecting spherical mirror used is known in the technique as "Luboshez lens" i.e. a positive aplanatic meniscus lens, described e.g. in the book entitled "Modern Geometrical Optics", by M. Herzberger, issued by Interscience Publishers, Inc., New York, 1958 pages 21, 22, 52; the two mirror surfaces consist of a spherical concave surface and a convex one with a smaller curvature radius. For an object placed at the curvature centre of the concave spherical surface a correct virtual image is obtained i.e. with no aberration, but the achromatic one, which, however, in our case does not matter, as monochromatic optical beams are dealt with. The virtual image is magnified n² times the real image, where n is the refractive index of the material the spherical mirror is made of. The concave surface of the spherical mirror is metallized to increase its reflectivity. The spherical mirror positioning in MFD measurement system is analogous to that of the example of embodiment shown in Fig. 1. The fact that a portion of the optical beam emitted by face a is not reflected and traverses the mirror, does not affect MFD measurement, since an optical power ratio is to be measured. By the use of this spherical mirror type the in itial operation of positioning face a in the centre of the mirror is made easier. In fact in this case face a is observed by means of a known lens array placed facing the convex part of the spherical mirror: beam splitter BS2 (Fig. 2) and microscope MO, whose insertion into mechanical device SM is critical owing to the small space available, can thus be eliminated. In this way, the used spherical mirror can have a wider numerical aperture thus obtaining a better definition of the reflected image, and can form the first lens of the lens array permitting the observation of face a of the fibre. A further example of embodiment of the reflecting optical system is shown in Fig. 6. The optical system comprises a magnifying lens array L2, consisting of a direct-light microscope objective, and a spherical mirror SPS1 which can be either of the totally or of the partly-reflecting type, previously described. The distance between L2 and SPS1 is such that if face a of the fibre is placed at the intersecting point of the working plane with the axis of lens array L2, the latter forms a real magnified image I of face a in the centre of the spherical mirror. The magnifying ratio of L2 and the curvature radius of SPS1 must be such that the hypothesis of punctiform image I at the centre of mirror SPS1 may still remain valid. It is worth noting that the presence of lens array L2 allows the use of a spherical mirror SPS1 of reduced numerical aperture. The system of fig. 6 is similar to that shown in Fig. 1, but it makes the offset for MFD measurement easier to control. Holding in fixed position the fibre and lens array L2, spherical mirror SPS1 is transversely displaced; with respect to the transverse offset which ought to be introduced between fibre FO and spherical mirror SPS in the system of Fig. 1, the transverse offset to be given to SPS1 is amplified by a number of times equal to the magnifying ratio of L2 under the same conditions of desired offset between face a and its image reflected by the optical system. Thus the micromanipulator which regulates the transverse offset of mirror SPS1 is easier to control. Said micromanipulator is inserted in the mechanical device (of known type and not shown in the Figure for the sake of simplicity), which comprises support SUP, lens array L2 and mirror SPS1; it is worth noting that the distance on the fibre axis between L2 and SPS1 is fixed and the only possible movement for SPS1 is transverse. As to the initial operation of positioning face a in the working plane of lens L2 (equivalent to that of positioning face a in the centre of mirror SPS in the system of Fig. 2), in the case of totally-reflecting spherical mirror SPS1 a technique is used similar to that shown in Fig. 2 taking care of positioning beam splitter BS2 between lens L2 and the centre of spherical mirror SPS1; in case of a partly-reflecting spherical mirror SPS1, the technique already described in relation to the alternative embodiment of the reflecting optical system consisting of the partly-reflecting spherical mirror only, is used.

Claims: 1. A method of measuring optical coupling losses between single-mode optical fibres, characterized in that an optical beam is injected into one length of optical fibre, said optical beam outgoing from a fibre end face and being reflected, by a reflecting optical system, into said fibre through said face, and re-emerging wherefrom it has been injected, where an optical power of the reflected beam is measured, under a condition taken as a reference, where the core of said end face and its image produced by the optical beam reflected by the reflecting optical system coincide in the optical centre of the reflecting optical system, and under conditions of transversal or longitudinal offset fro m said reference condition. 2. A device for measuring coupling losses between single-mode optical fibres, characterized in that it comprises:- a single-mode optical fibre length (FO);- a primary source (S) of optical signal emitting a primary optical beam penetrating said fibre;- a reflecting optical system (SPS; L1, SP1; L2, SPS1), in the optical centre of which an end face (a) of said fibre is placed wherefrom said primary optical beam outgoes;- a mechanical device (SM) containing said reflecting optical system and a portion of said fibre comprising said end face (a), and also comprising means for mutual micrometric adjustment of the position of the end face with respect to said optical system, so that said primary optical beam, outgoing from the end face (a) and reflected by the reflecting optical system, at least partly reenters the fibre through the end face and outgoes from the opposite end of the fibre as a reflected optical beam;- a first beam splitter (BS1) placed between said primary source (S) and said fibre (FO) with a tilt with respect to the primary optical beam, such that it deviates said reflected optical beam towards detecting and measuring means (R) of the optical power associated with said reflected optical beam, said measurement of coupling losses being carried out by comparing the optical power measured by said detecting and measuring means when the core of said end face (a) and its image, produced by the optical beam reflected by the reflecting optical system, coincide in the optical centre of the reflecting optical system, with the optical power measured by introducing transversal or longitudinal offsets between the end face (a) and its image, taking into account spurious reflections. 3. A device as in claim 2, characterized in that said reflecting optical system consists of a first spherical mirror (SPS) and said optical centre corresponds to the curvature centre of said spherical mirror. 4. A device as in claim 2, characterized in that said reflecting optical system consists of a first magnifying lens array (L2) and a second spherical mirror (SPS1), positioned thus that, when said end-face (a) of the fibre is in the intersection of the working plane with the axis of the first lens array (L2), the latter forms a real magnified image (I) of the end face in the curvature centre of said second spherical mirror (SPS1), said intersection between the working plane and the axis being said optical centre. 5. A device as in claim 2, characterized in that said reflecting optical system consists of a second collimating lens array (L1) and a first plane mirror (SP1) placed perpendicular to the primary optical beam, coming from said end face (a) and collimated by said second lens array (L1), said first plane mirror reflecting said optical beam towards said end face through said second lens array, said optical centre corresponding to the focus of said second lens array (L1). 6. A device as in claim 4, characterized in that said micrometric adjustment means comprised in said mechanical device (SM) produce transverse offsets of said second spherical mirror (SPS1) with respect to the axis of the first lens array (L2), to generate said transverse offsets between the end face (a) and its image. 7. A device as in claim 5, characterized in that said micrometric adjustment means comprised in said mechanical device (SM) produce a tilt of said first plane mirror (SP1) with respect to the perpendicular position, to generate said transverse offsets between the end face (a) and its image. 8. A device as in any of claims 2 to 7, characterized in that for the positioning of said end face (a) of the fibre in the optical centre of said reflecting optical system (SPS; L1, SP1) a secondary optical beam in the field of the visible is used, which replaces the primary beam, and said device further comprises a second beam splitter (BS2) inserted in said mechanical device (SM) in the trajectory of said secondary optical beam with such a tilt that it forms an image of said end face in a first visualizing optical array (MO, OC) this too inserted in said mechanical device (SM), said positioning of the end face (a) of the fibre in the optical centre being carried out acting on said micrometric adjustment means till there can be seen in said first visualizing optical array (MO, OC) the focused image of the end face (a) of the fibre and the image of the core of the fibre reflected by the reflecting optical system coincident with the direct one. 9. A device as in claims 3 and 8, characterized in that said first spherical mirror (SPS) is of the totally-reflecting type and said first visualizing optical array consists of an optical microscope (MO), and said second beam splitter (BS2) is placed between said end face (a) and said first spherical mirror (SPS). 10. A device as in claims 4 and 8, characterized in that said second spherical mirror (SPS1) is of the totally-reflecting type, and in that said first visualizing optical array consists of an optical microscope (MO), and said second beam splitter (BS2) is placed between said first lens array (L2) and the curvature centre of said second spherical mirror (SPS1). 11. A device as in claims 5 and 8, characterized in that said first visualizing optical array consists of a telescope (OC) and said second beam splitter (BS2) is placed between said second lens array (L1) and said first plane mirror (SPS1). 12. A device as in claim 3 or 4 characterized in that for the positioning of said end face (a) of the fibre in the optical centre of said reflecting optical system (SPS; L2, SPS1) a secondary optical beam in the field of the visible is used replacing the primary optical beam, and in that said first or second spherical mirror (SPS; SPS1) is of the partly-reflecting type, and also in that a second visualizing optical array is also provided inserted in said mechanical device (SM) and facing the convex portion of the spherical mirror, said positioning of the end face (a) of the fibre in the optical centre being carried out acting on said micrometric adjustment means till there can be seen in the said second visualizing optical array the focused image of the end face (a) of the fibre coincident with its image refracted by the reflecting optical system. 13. A device as in any one of claims 8 to 12, characterized in that it further comprises:- a secondary source (LA) of an optical signal in the field of the visible emitting said secondary optical beam;- a second plane mirror(SP) placed in the trajectory of said primary optical beam before said first beam splitter (BS1), which reflects said secondary optical beam towards said optical fibre (FO) in replacement of the primary optical beam it interrupts.

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