Method for adjusting the electromagnetic field distribution of an optical fiber, optical fiber and connectors obtained accordingly

Abstract

Method for adjusting an electromagnetic field distribution of an optical fiber (1) comprising a core (2) made of a core material (2a) and a cladding (3) made of at least one cladding material (3a), said method comprising the steps of 5 - providing (110) an optical fiber (1) having an initial first electromagnetic field distribution at a fiber end (4) thereof, - modifying (120) a refractive index of a volume (5) of the optical fiber (1) within an end section of the optical fiber by illuminating said volume (5) with a light source using a multi-photon absorption process in order to obtain a second predefined 0 electromagnetic field distribution at the fiber end, wherein the initial first electromagnetic field distribution has a first initial center core position, and wherein the second predefined electromagnetic field distribution has a second predefined center core position.

Description

[0001] METHOD FOR ADJUSTING THE ELECTROMAGNETIC FIELD DISTRIBUTION OF AN OPTICAL FIBER, OPTICAL FIBER AND CONNECTORS OBTAINED ACCORDINGLY

[0002] FIELD OF INVENTION

[0003] The present invention relates to a method for adjusting the electromagnetic field distribution, in particular the true positioning, of an optical fiber at a facet thereof, to adjusted fibers obtained accordingly and to connectors comprising such adjusted fibers, in particular fiber connectors with a positioning of fibers at a submicron accuracy.

[0004] BACKGROUND

[0005] When coupling two optical elements each comprising an optical waveguide, any misalignment between the optical paths of the waveguides of the optical elements results in transmission losses. In photonics, the alignment of the core of a fiber with a subsequent waveguide in a chip is for instance critical for the transmission efficiency.

[0006] With the development of multiphoton polymerization techniques, new fabrication methods for three dimensional, optical elements have emerged, see for instance EP1295179. Typically, using multiphoton lithography, optical elements are now created in situ in an encapsulating protective monolithic polymeric matrix. Waveguides in polymer materials can for instance be positioned with sub-micron accuracy at the edge of photonic chips.

[0007] Permanently alternating the refractive index of a material using a high-intensity laser is known from the art. The technique is commonly employed for so-called direct writing of optical waveguides. For example, US 2010 / 073676 Al discusses employing the technique for writing secondary waveguides , WO 2016 / 123719 Al discusses it for creating a multi-mode interference (MMI) waveguide and XP001589903 discusses a method for determining the change in refractive index caused by employment of the technique. Additionally, US 2002 / 181910 Al discusses tapering the refractive index for increasing misalignment tolerances and WO 2018 / 022319 Al discusses employing direct writing for connecting two waveguiding arrays with different pitches.

[0008] Yet remains the issue of the true positioning of commercially available optical glass fiber(s) and / or optical glass fiber arrays (also referred in this text as connectors) at the edges of such photonics chips. The positioning of a fiber core within the cladding of the fiber is referred to as the core eccentricity (or concentricity) of the fiber and lies typically in the range of .5 to 1 micron according to manufacturing data. The positioning of a fiber core within the fiber is thus far less accurate than that for the positioning of waveguides in integrated photonics chips. The poor accuracy of the core concentricity is thus a limiting factor for the efficiency. To address the issue of the eccentricity of commercially available products, some mechanical solutions have been proposed. In EP0354173, a method for centering an optical fiber is disclosed using a centering cylinder to actively correct the core alignment of the optical fibers connected with one another. Yet such solutions cause mechanical stress on the fibers reducing their reliability.

[0009] There is thus a need for a method of true positioning of a centre core at an optical fiber end that would not suffer from the drawbacks of the prior art. More generally, there is a need not only for adjusting the eccentricity of optical fibers, but also for adjusting their mode field diameter, the shape of their electromagnetic distribution (round, square, etc) or their type of distribution (Gaussian, non-Gaussian) due to manufacturing inaccuracies and / or to a desired target use (due to a desired characteristics at a subsequent optical element, waveguide). Eccentricity, mode field diameter, shape and type of distribution are all characteristics of the whole electromagnetic field distribution at the end of the fiber. There is thus a need for a method for adjusting the electromagnetic field distribution of an optical fiber in a broad sense without the drawbacks of the prior art mechanical solutions.

[0010] SUMMARY

[0011] The object of the invention is to provide a method for adjusting an electromagnetic field of an optical fiber within an optical assembly, but also optical fibers and connectors obtained accordingly.

[0012] According to a first aspect of the invention, there is provided a method for adjusting an electromagnetic field distribution of an optical fiber comprising a core made of a core material and a cladding made of at least one cladding material. The method comprises the steps of providing an optical fiber having an initial first electromagnetic field distribution at a fiber end thereof, and modifying a refractive index of a volume of the optical fiber within an end section of the optical fiber by illuminating said volume with a light source using a multi-photon absorption process in order to obtain a second predefined electromagnetic field distribution at the fiber end. The initial first electromagnetic field distribution has a first initial center core position, and the second predefined electromagnetic field distribution has a second predefined center core position

[0013] In this way, the electromagnetic field distribution at the optical fiber end can be further tailored to need. In particular, true positioning of a fiber core with respect to a further coupling may be rendered possible by adjusting the electromagnetic field distribution at the fiber end using a multi-photon process to tune the refractive index of material within an end-section of the fiber. Multi-photon absorption may indeed allow modifying the refractive index of any point of a volume within the end section of the optical fiber. The volume with the modified refractive index may in turn modify the initial electromagnetic field distribution to reach a second predefined electromagnetic field distribution. The second predefined electromagnetic field distribution may be regarded as a target electromagnetic field distribution which should be attained at the fiber end after the method is performed. The claimed method is therefore a field method meant to be applied to already manufactured, commercially available, optical (glass) fibers. The optical fibers adjusted according to invention may be optical glass fibers with a core made of glass material having a first refractive index, and a cladding made of at least one glass material having a second refractive index, lower than the first refractive index. As far as the concept still applies, the method may also be applied to optical fibers made of polymer. With this method, inaccuracies in the initial first electromagnetic field due to manufacturing tolerances may be corrected leading in turn to reduced transmission losses at a fiber end when connecting said fiber to a subsequent optical element. In particular, the eccentricity of the fiber may be adjusted. Fiber core eccentricity (also called eccentricity or concentricity in this text) may be defined by the relative displacement of a fiber core center with respect to the geometric center of a cladding outer surface..

[0014] According to a preferred embodiment, the illuminated volume is a volume of the cladding, and modifying a refractive index of a volume of the optical fiber comprises increasing a refractive index in said volume above an initial refractive index of the cladding material, preferably up to an initial refractive index of the core material. In this way, the cladding may be modified at the end section of the fiber to contain a volume with a modified refractive index. Multi-photon absorption may increase the refractive index of an illuminated material. This means a volume of the cladding may get after multi-photon absorption a higher refractive index than its initial refractive index, such that this volume may better guide light than the remaining unmodified cladding. The refractive index of the illuminated volume may be identical, lower or higher than to the refractive index of the core material. This new spatial distribution of refractive indexes in the end section modifies in turn the resultant electromagnetic field distribution at the fiber end. For example, the core may be enlarged or reshaped to a new shape As the light is guided within the modified volume with higher refractive index, the electromagnetic field distribution resulting from the combination of the unmodified part of the initial cladding, the initial core and the illuminated volume may be tailored to meet a target electromagnetic field distribution, with for instance a larger mode field diameter, a new shape or a target eccentricity.

[0015] According to a preferred embodiment, the illuminated volume is a volume of the core, and modifying the refractive index of a volume of the optical fiber comprises increasing a refractive index in said volume above an initial refractive index of the core material. In this way, a new core may for instance be created within the initial core. As the light is better guided within this modified volume with even higher refractive index than the initial core refractive index, the electromagnetic field distribution resulting from the combination of the initial cladding, the unmodified part of the initial core and the illuminated volume acting as a new core may be tailored to meet a target electromagnetic field distribution with for instance a smaller mode field diameter or a target eccentricity. Alternatively, the illuminated volume may comprise both points within the initial core and within the initial cladding. In such an embodiment, the illuminated volume may then have, after multi-photon absorption, one or more refractive indexes. The energy used by the multi-photon absorption process may further be different for different points of the illuminated volume depending on the material at each point and the target refractive index for that respective point.

[0016] According to a preferred embodiment, the illuminated volume is arranged to form with a non-illuminated portion of the fiber a 3D structure creating the second predetermined electromagnetic field distribution. The illuminated volume may have a 3D arrangement such that the combination of the unmodified initial parts of the fiber and the illuminated volume guide the light in a different way than the initial first light distribution. By selecting all the points constituting the illuminated volume, the second target electromagnetic light distribution may be obtained. It is noted that the volume may comprise a plurality of sub-volumes that may not be joining each other physically.

[0017] According to a preferred embodiment, the initial first electromagnetic field distribution has a first mode field diameter and the second predefined electromagnetic field distribution has a second predefined mode field diameter. In this way, a mode field diameter at the fiber end may be adjusted.

[0018] According to a preferred embodiment, the initial first electromagnetic field distribution has a first shape and the second predefined electromagnetic field distribution has a second predefined shape. In this way, a shape of the electromagnetic field distribution may be adjusted. By shape is meant the general shape of a light spot at the end of the fiber, being rather round or square for instance.

[0019] According to a preferred embodiment, the initial first electromagnetic field distribution has a distribution type and the second predefined electromagnetic field distribution has a second predefined distribution type. In this way, the type of electromagnetic field distribution related for instance to a local increase of the intensity of the electromagnetic field distribution, whether Gaussian or non-Gaussian for instance, may be adjusted.

[0020] According to a preferred embodiment, the multiphoton absorption process is a two-photon absorption process. Two-photon absorption (TPA or 2PA), a non-linear absorption process, is the simultaneous absorption of two photons of identical or different frequencies. Absorption of the light leads to permanent changes in the material, such as its refractive index. One of the most distinguishing features of TPA is that the rate of absorption of light by a molecule depends on the square of the light's intensity. This is different from one Photon Absorption, where the rate of absorption is linear with respect to input intensity. As a result of this dependence, if material is illuminated with a high power laser beam, the rate of absorption and thus material alteration decreases very sharply from the center of the beam to its periphery. Because of this, the resolution of a process using TPA is higher than the resolution of a process using normal absorption. Two- photon absorption typically involves a laser device having a controllable laser beam focal point and a controllable laser beam intensity. If a material is illuminated with a laser beam using TPA, the refractive index of the material at the focal point of the laser beam may then be increased. Alternatively, different levels of photon absorption (two or three photons absorption) may be used in the present invention for different points of the illuminated volume to obtain the second predefined electromagnetic field distribution.

[0021] According to a preferred embodiment, the multiphoton absorption process has a three- dimensional spatial resolution. According to a preferred embodiment, the multiphoton absorption process has a sub-micron resolution, preferably a 200nm resolution, more preferably a lOOnm resolution. In this way, the second predefined electromagnetic field distribution may be adjusted with a sub-micron resolution matching thus the resolution typically obtained for waveguides in integrated photonics chips.

[0022] According to a preferred embodiment, providing an optical fiber further comprises observing at least one characteristic of the initial first electromagnetic field distribution. An observation of the initial first electromagnetic field distribution may be performed to determine a starting point for the adjustment method. This observation may enable determining how the characteristic(s) of the initial optical fiber deviate(s) from for instance the ideal characteristic(s) intended by the manufacturer of the optical fiber. A measurement of the initial first electromagnetic field distribution may be performed at the end of the optical fiber while the optical fiber is provided at another end with a reference input light signal.

[0023] According to a preferred embodiment, the volume for obtaining the second predefined electromagnetic field distribution characteristic is determined based on the at least one observed characteristic. In this way, an analysis of the deviation between the at least one observed characteristic of the initial first electromagnetic field distribution and the second predetermined electromagnetic field distribution may conclude on which volume should be illuminated. The analysis may contain a computer-implemented simulation or an iterative analysis.

[0024] According to a preferred embodiment, the second predefined electromagnetic field distribution is based on at least one of the following: a manufacturer ideal electromagnetic field distribution, an electromagnetic field distribution of a target use. In this way, the adjustment method may correct inaccuracies during manufacturing and / or incompatibilities between the optical fiber and an electromagnetic field distribution of a target use. By an electromagnetic field distribution of a target use is meant an electromagnetic field distribution desired at the fiber end when in use connected to a subsequent optical element (whether a photonic chip, another fiber or an end cap). The fiber end forms a connection facet that may be used for example for butt coupling to a subsequent (external) optical element.

[0025] According to a preferred embodiment, the method is for adjusting an electromagnetic field distribution of an optical connector comprising one or more first optical fibers for edge coupling at a connector end, and comprises providing an optical connector with one or more first optical fibers, the optical connector having an initial third electromagnetic field distribution at a connector end thereof, and adjusting an electromagnetic field distribution of at least one of the one or more first optical fibers of the optical connector according to the any of the preceding methods to obtain a fourth predefined electromagnetic field distribution at the connector end. In this way, the electromagnetic field distribution at the optical connector end can be further tailored to need. In particular, true positioning of a fiber within the connector with respect to a further coupling may be rendered possible by adjustment of the electromagnetic field at an end section of that fiber within the connector. In this way, inaccuracies in the initial first electromagnetic field due to manufacturing tolerances of the fiber and / or to positioning inaccuracies within the connector may be corrected. Indeed, the fiber may be produced with a given manufacturing accuracy while the relative positioning of the fiber within the connector may be also subject to manufacturing variations. For instance, the eccentricity of the core within the fiber cladding may be combined with an inaccurate placement of the fiber within V groves in the connector during manufacture of the connector. These two sources of inaccuracies may then combine and affect the electromagnetic field distribution of the whole connector.

[0026] According to a preferred embodiment, providing the optical connector comprises measuring the initial electromagnetic field distributions of the one or more optical fibers of said connector and their relative positions, wherein the fourth predefined electromagnetic field distribution at the connector end is determined according to an average positioning of the measured first initial electromagnetic field distributions of the one or more optical fibers of said connector and their measured relative positions. In this way, the positioning of the second electromagnetic field distributions for the one or more optical fibers may be individually adjusted to better match collectively a fourth predetermined electromagnetic field distribution desired for the connector.

[0027] According to a preferred embodiment, the connector is arranged for edge-coupling the one or more first fibers to one or more second fibers, wherein the fourth predefined electromagnetic field distribution corresponds to a combined electromagnetic field distribution created by the one or more second fibers when arranged in the connector. In this way, an efficient fiber to fiber connector with reduced transmission losses can be arranged.

[0028] According to another aspect of the invention, is provided an adjusted optical fiber comprising a core made of a core material and a cladding made of at least one cladding material. The optical fiber comprises a first section along the optical axis with a cross-sectional first electromagnetic field distribution, and an end section arranged at a fiber end with a second electromagnetic field distribution at said fiber end, wherein a refractive index of at least one point of at least one cross section of the end section differs from a refractive index of a point (or points) with the same polar coordinates in a lateral cross-section (or cross sections) of the first section. The first electromagnetic field distribution has a first initial center core position, and the second predefined electromagnetic field distribution has a second predefined center core position. In this way, a tailored fiber with reduced transmission losses at the adjusted fiber end may be obtained and the eccentricity of the fiber may be adjusted. This means that by cutting an adjusted fiber according to an embodiment of the invention at a distance from the fiber end larger than the depth of a multi-photon absorption process should reveal a new facet at which the electromagnetic field distribution would be different than the electromagnetic field at the fiber end of the adjusted fiber according to an embodiment. However the material composition would remain the same along the fiber axis of the adjusted optical fiber.

[0029] According to a preferred embodiment, a spatial repartition of refractive indexes in a cross section of the first section differs from a spatial repartition of refractive indexes in a cross section of the end-section. This means that cutting an adjusted fiber according to an embodiment of the invention at a distance from the fiber end larger than the depth of a multi-photon absorption process should reveal a new cross section of the fiber wherein areas with different refractive indexes would be arranged spatially differently than for cross sections within the depth of a multiphoton absorption process. In particular the areas with different diffractive indexes at the end facet of the adjusted fiber end may be spatially arranged differently than in a cross section at a distance from the fiber end larger than the depth of a multi-photon absorption process.

[0030] According to a preferred embodiment, the first electromagnetic field distribution has one or more of the following: a first eccentricity of the fiber core with respect to the cladding, a first position of a center of the core with respect to a reference position being said initial center core position, a first mode field diameter and / or a first distribution shape. According to a preferred embodiment, the second electromagnetic field distribution has one or more of the following: a second eccentricity of the fiber core with respect to the cladding, a second position of a center of the core with respect to a reference position being said second predefined center core position, a second mode field diameter and / or a second distribution shape. In this way, the adjusted fiber may be tailored to needs in terms of eccentricity, mode field diameter, shape and or distribution type. Yet this list is not limitative and other characteristics of the second electromagnetic field distribution to be tailored may also be envisaged.

[0031] According to a preferred embodiment, the second electromagnetic field distribution differs from a predefined target electromagnetic field distribution with a sub-micron resolution, preferably a 200nm accuracy, more preferably a lOOnm accuracy. The resolution of the second electromagnetic field distribution may be as good as the resolution of the multi-photon absorption process, i.e. of the laser beam used in the multi-photon absorption process.

[0032] According to a preferred embodiment, the predefined target electromagnetic field distribution is based on at least one of the following a manufacturer ideal electromagnetic field distribution, an electromagnetic field distribution of a target use.

[0033] According to a preferred embodiment, optical connector, comprising one or more optical fibers for edge coupling to a photonic integrated circuit chip, said optical connector comprising at least one adjusted optical fiber according to any of the previous fiber claims.

[0034] According to a preferred embodiment, optical connector according to the previous claim, comprising a support with one or more V grooves for receiving the one or more optical fibers.

[0035] According to a preferred embodiment, optical connector comprising one or more first optical fibers for edge coupling to one or more second optical fibers, said optical connector comprising at least one first optical fiber being an adjusted optical fiber according to any of the previous fiber claims.

[0036] Further embodiments may be defined by the following clauses:

[0037] Clause 1. Method for adjusting an electromagnetic field distribution of an optical fiber comprising a core made of a core material and a cladding made of at least one cladding material, said method comprising the steps of providing an optical fiber having an initial first electromagnetic field distribution at a fiber end thereof, modifying a refractive index of a volume of the optical fiber within an end section of the optical fiber by illuminating said volume with a light source using a multi-photon absorption process in order to obtain a second predefined electromagnetic field distribution at the fiber end.

[0038] Clause 2. Method according to any of the preceding clauses, wherein the illuminated volume is a volume of the cladding, and wherein modifying a refractive index of a volume of the optical fiber comprises increasing a refractive index of the cladding material in said volume above an initial refractive index of the cladding material, preferably up to an initial refractive index of the core material.

[0039] Clause 3. Method according to any of the preceding clauses, wherein the illuminated volume is a volume of the core, and wherein modifying a refractive index of a volume of the optical fiber comprises increasing a refractive index of the core material in said volume above an initial refractive index of the core material.

[0040] Clause 4. Method according to any of the preceding clauses, wherein the illuminated volume is arranged to form with a non-illuminated portion of the fiber a 3D structure creating the second predetermined electromagnetic field distribution. Clause 5. Method according to any of the preceding clauses, wherein the initial first electromagnetic field distribution has a first initial center core position, and wherein the second predefined electromagnetic field distribution has a second predefined center core position.

[0041] Clause 6. Method according to any of the preceding clauses, wherein the initial first electromagnetic field distribution has a first mode field diameter and wherein the second predefined electromagnetic field distribution has a second predefined mode field diameter. Clause 7. Method according to any of the preceding clauses, wherein the initial first electromagnetic field distribution has a first shape and wherein the second predefined electromagnetic field distribution has a second predefined shape.

[0042] Clause 8. Method according to any of the preceding clauses, wherein the initial first electromagnetic field distribution has a distribution type and wherein the second predefined electromagnetic field distribution has a second predefined distribution type.

[0043] Clause 9. Method according to any of the preceding clauses, wherein the multiphoton absorption process is a two-photon absorption process.

[0044] Clause 10. Method according to any of the preceding clauses, wherein the multiphoton absorption process has a three-dimensional spatial resolution.

[0045] Clause 11. Method according to any of the preceding clauses, wherein the multiphoton absorption process has a sub-micron resolution, preferably a 200nm resolution, more preferably a lOOnm resolution.

[0046] Clause 12. Method according to any of the preceding clauses, wherein providing an optical fiber further comprises observing at least one characteristic of the initial first electromagnetic field distribution.

[0047] Clause 13. Method according to the preceding clause, wherein the volume for obtaining the second predefined electromagnetic field distribution characteristic is determined based on the at least one observed characteristic.

[0048] Clause 14. Method according to any of the preceding clauses, wherein the second predefined electromagnetic field distribution is based on at least one of the following a manufacturer ideal electromagnetic field distribution, an electromagnetic field distribution of a target use.

[0049] Clause 15. Method for adjusting an electromagnetic field distribution of an optical connector comprising one or more first optical fibers for edge coupling at a connector end, comprising:

[0050] - providing an optical connector with one or more first optical fibers, the optical connector having an initial third electromagnetic field distribution at a connector end thereof,

[0051] - adjusting an electromagnetic field distribution of at least one of the one or more first optical fibers of the optical connector according to the any of the preceding methods to obtain a fourth predefined electromagnetic field distribution at the connector end. Clause 16. Method according to the preceding clause, wherein providing the optical connector comprises measuring the initial electromagnetic field distributions of the one or more optical fibers of said connector and their relative positions, wherein the fourth predefined electromagnetic field distribution at the connector end is determined according to an average positioning of the measured first initial electromagnetic field distributions of the one or more optical fibers of said connector and their measured relative positions.

[0052] Clause 17. Method according to the clause 16, wherein the connector is arranged for edge-coupling the one or more first fibers to one or more second fibers, wherein the fourth predefined electromagnetic field distribution corresponds to a combined electromagnetic field distribution created by the one or more second fibers when arranged in the connector.

[0053] Clause 18. Adjusted optical fiber comprising a core made of a core material and a cladding made of at least one cladding material, wherein the optical fiber comprises:

[0054] - a first section along the optical axis with a cross-sectional first electromagnetic field distribution, and

[0055] - an end section arranged at a fiber end with a second electromagnetic field distribution at said fiber end,

[0056] - wherein a refractive index of at least one point of at least one cross section of the end section differs from a refractive index of a point with the same polar coordinates in a crosssection of the first section.

[0057] Clause 19. Adjusted optical fiber according to any the preceding fiber clauses, wherein a spatial repartition of refractive indexes in a cross section of the first section differs from a spatial repartition of refractive indexes in a cross section of the end section.

[0058] Clause 20. Adjusted optical fiber according to any of the preceding fiber clauses, wherein the first electromagnetic field distribution has one or more of the following: a first eccentricity of the fiber core with respect to the cladding, a first position of a center of the core with respect to a reference position, a first mode field diameter and / or a first distribution shape.

[0059] Clause 21. Adjusted optical fiber according to any of the preceding fiber clauses, wherein the second electromagnetic field distribution has one or more of the following: a second eccentricity of the fiber core with respect to the cladding, a second position of a center of the core with respect to a reference position, a second mode field diameter and / or a second distribution shape.

[0060] Clause 22. Adjusted optical fiber according to any of the preceding fiber clauses, wherein the second electromagnetic field distribution differs from a predefined target electromagnetic field distribution with a sub-micron resolution, preferably a 200nm accuracy, ore preferably a lOOnm accuracy.

[0061] Clause 23. Adjusted optical fiber according to any of the preceding fiber clauses, wherein the predefined target electromagnetic field distribution is based on at least one of the following a manufacturer ideal electromagnetic field distribution, an electromagnetic field distribution of a target use.

[0062] Clause 24. Optical connector, comprising one or more optical fibers for edge coupling to a photonic integrated circuit chip, said optical connector comprising at least one adjusted optical fiber according to any of the previous fiber clauses.

[0063] Clause 25. Optical connector according to the previous clause, comprising a support with one or more V groves for receiving the one or more optical fibers.

[0064] Clause 26. Optical connector comprising one or more first optical fibers for edge coupling to one or more second optical fibers, said optical connector comprising at least one first optical fiber being an adjusted optical fiber according to any of the previous fiber clauses.

[0065] BRIEF DESCRIPTION OF THE FIGURES

[0066] This and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing currently preferred embodiments of the invention. Like numbers refer to like features throughout the drawings.

[0067] Figure la illustrates a schematic perspective view of an optical fiber according to the art.

[0068] Figure lb illustrates a schematic side view of a fiber end of a fiber of Figure la.

[0069] Figure 1c illustrates a schematic longitudinal cross-section at a fiber end section of a fiber of

[0070] Figure la.

[0071] Figure 2 illustrates a diagram of the transmission losses in dB versus the misalignment of the core of the fiber in pm.

[0072] Figure 3a illustrates an example of a fiber spot having a first given electromagnetic distribution while Figure 3b illustrates an example of a chip spot having a desired electromagnetic distribution. Figure 4 illustrates a block diagram of a method according to an embodiment.

[0073] Figure 5a illustrates a side view of a fiber end of an adjusted fiber according to an embodiment of the invention.

[0074] Figure 5b illustrates a longitudinal cross-section of a fiber end section of an adjusted fiber according to the same embodiment of the invention as Figure 5a.

[0075] Figure 6a illustrates a side view of a fiber end of an adjusted fiber according to another embodiment of the invention.

[0076] Figure 6b illustrates a longitudinal cross-section of a fiber end section of an adjusted fiber according to the same embodiment of the invention as Figure 6a.

[0077] Figure 7a illustrates a side view of a fiber end of an adjusted fiber according to yet another embodiment of the invention. Figure 7b illustrates a longitudinal cross-section of a fiber end section of an adjusted fiber according to the same embodiment of the invention as Figure 7a.

[0078] Figure 8a illustrates a side view of a fiber end of an adjusted fiber according to yet another embodiment of the invention.

[0079] Figure 8b illustrates a longitudinal cross-section of a fiber end section of an adjusted fiber according to the same embodiment of the invention as Figure 8a.

[0080] Figure 9 illustrates a perspective view of a multi-fiber to chip connector according to the art. Figure 10a illustrates a schematic lateral cross section at a connector end of a connector according to Figure 9.

[0081] Figure 10b illustrates a diagram of the optical power intensity measured in mW at the connector end of Figure 6a versus the distance in pm.

[0082] DESCRIPTION OF THE EMBODIMENTS

[0083] Figure la illustrates a schematic perspective view of an optical fiber 1 according to the art. An optical fiber 1 is typically a flexible glass or polymer fiber that can transmit light from one end to the other. Glass optical fibers are typically made by drawing, while polymer fibers can be made either by drawing or by extrusion. An optical fiber 1 has thus typically a tubular shape and comprises a core 2 made of a core material 2a and a cladding 3 made of at least one cladding material 3 a extending along the whole length of the fiber 1. The core 2 and the cladding 3 are ideally concentric. The core material 2a has a first refractive index nl while the cladding material 3a of the cladding 3 has a second refractive index n2, typically a lower than the first refractive index nl of the core material 2a of the core 2. Light is guided within mostly the core 2 which causes the fiber 1 to act as a waveguide. An optical fiber 1 comprises facets at each fiber end and in particular one fiber end 4 may be meant for connection to a subsequent optical element. The subsequent optical element may be for instance another fiber or a waveguide in a photonic chip. The fiber end 4 is typically arranged for butt-coupling with the subsequent optical element. Alternatively the fiber end 4 may be arranged for coupling to a quasi-planer grating coupler. The fiber end 4 is typically a lateral cross section of the fiber 1 perpendicular to a fiber axis F. The fiber axis F may be defined as the axis of symmetry of the whole optical fiber 1. The fiber axis F may be defined as passing through the geometric center C’ of the fiber end 4 with respect to the whole fiber end 4. The fiber 1 with the core 2 and the classing 3 debouches at the fiber end 4. The fiber end 4 is a surface comprising a core area 4a where the core 2 debouches and a cladding area 4b where the cladding 3 debouches.

[0084] Figures lb illustrates a side view of the fiber end 4 of Figure la while Figure 1c illustrates a longitudinal cross-section (along a diametral section axis A) of a fiber of Figure lb. Compared to Figure la, Figures lb and 1c represent additionally an ideal core 2’ having an ideal positioning with a center of the core 2’ at the geometric center C’ of the fiber end 4. With respect to the ideal core 2’, a core 2 may in practice be misaligned due for instance to inaccuracies during the manufacturing (drawing) process of the fiber 1. The core area 4a at the fiber end 4 may have a center C at a distance m from the geometric center C’ of the fiber end 4. The core 2 may have an optical axis O representing the actual optical axis along which the waveguiding function of the fiber is performed. This axis O as can be seen in figure 1c may pass through the centre C and not through the geometric center C’, such that the optical axis O may be misaligned with the fiber axis F. This misalignment of the core 2 with respect to the ideal geometrical alignment is undesirable as it may create transmissions losses when connecting the fiber end 4 to a subsequent optical element.

[0085] Further it is noted that Figures lb and 1c illustrate for ease of representation a relative displacement of the center C and C’ along only one direction (x axis) at the fiber end 4. Yet the relative displacement of the center C and C’ may also be along two directions (x and y axis) at the fiber end 4.

[0086] Figure 2 illustrates a diagram of the transmission losses in dB versus the misalignment of the core of a fiber in pm. The transmission losses depend on a number of factors. This diagram provides an indication of the transmission losses from misalignment. There is thus a need to adjust the positioning of the core, and more generally the electromagnetic field distribution at the end of a fiber 1 , to match a desired eccentricity, more generally a desired electromagnetic field distribution, at the entrance of a subsequent optical element to which a fiber is to be connected.

[0087] Figure 3a illustrates an example of a typical fiber spot of a typical optical fiber of the art, having a first typical electromagnetic field distribution at a fiber end while Figure 3b illustrates an example of a typical chip spot having a typal desired electromagnetic field distribution. As illustrated in these figures, typically a fiber spot at a fiber end 4 may differ from a spot of a chip to which the fiber is to be connected. The electromagnetic distribution of the fiber shown in Figure 3 a may substantially differ from the electromagnetic field distribution shown in Figure 3b in terms of mode field diameter and / or shape among others. The electromagnetic field distribution of Figure 3a shows a rather round distribution of the light intensities, while the electromagnetic field distribution of Figure 3b shows a rather square distribution of the light intensities, typical for square waveguides of photonic chips. There is thus a need to adjust the electromagnetic field distribution (another terminology may be the light distribution) at the end of a fiber to match a desired electromagnetic field distribution at the entrance of the subsequent optical element to which the fiber is to be connected.

[0088] Figure 4 illustrates a block diagram of a method according to an embodiment. The method is for adjusting an electromagnetic field distribution of a commercially available optical fiber 1 as defined in figures la-lc. Such a fiber 1 has an initial first electromagnetic field distribution at a fiber end 4 thereof, which may depend for instance of the manufacturing inaccuracies discussed in relation with Figures 1-3.

[0089] The method of figure 4 will be explained in particular with reference to Figures 5a and 5b representing one example an adjusted fiber obtained by the method of Figure 4. Yet the method may be applied to obtain equally to other embodiments as well. The method may be performed on (commercially) available optical fibers, prior to further connection to a subsequent optical element.

[0090] The method comprises a step 110 of providing an optical fiber 1 having an initial first electromagnetic field distribution at a fiber end 4 thereof, followed by a step 120 of modifying a refractive index of a volume 5 of the optical fiber 1 within an end section 10 of the optical fiber 1 by illuminating said volume 5 with a light source 100 using a multi-photon absorption process in order to obtain a second predefined electromagnetic field distribution at the fiber end 4. The optical fiber 1 as provided in step 110 may be further referred in this text as the initial optical fiber (or in short the initial fiber).

[0091] The end section 10 of the fiber 1 may comprise the fiber end 4 and a portion of the fiber adjacent to the fiber end 4 along a depth p. The depth p may be determined by the depth range of a multi-photon absorption process defining how deep within the fiber the multi-photon absorption process is able to excite the material of the fiber. Typically, a multi-photon process uses a laser device producing a laser beam, the device having a controllable focal spot for focalizing the beam at a focal point and a controllable intensity of the laser beam at the focal point. The length of the end section when measured along the fiber axis F of the optical fiber may then be smaller or equal than the depth range of the multi-photon absorption process. The depth range of a multi-photon absorption process ranges between 0 andl mm, preferably between 0 and 0,5 mm. A multiphoton absorption process may have typically a sub-micron resolution, preferably a 200nm resolution, more preferably a lOOnm resolution.

[0092] The volume 5 may comprise a plurality of points within the fiber 1 that are illuminated with a light source using a multi-photon absorption process. The multiphoton absorption process comprises successively exciting the material present at these points with multiple photons. When absorbing photons, changes in the material occur, in particular the refractive index of the illuminated material may be changed compared to an initial refractive index associated with the initial (ground) state of that material.

[0093] In the embodiment of Figures 5a and 5b, a volume 5 of the initial optical fiber that is illuminated may be a volume of the cladding 3 made of an initial cladding material 3 a having an initial second refractive index n2, lower than the initial first refractive index nl of the core material 2a. After step 120 is performed on the initial optical fiber 1, material 3a is modified into illuminated material 3b with a modified third refractive index n3 different from the initial second refractive n2 of the material 3a. The illuminated volume 5 may then comprise illuminated material 3b. The third refractive index n3 of the illuminated material 3b may be higher than the second refractive index n2 of the initial cladding material 3a. Material 3a and 3b may have the same composition and differ only by their refractive indexes. The third refractive index n3 may be increased up to the first refractive index nl of the core material 2a. However other values of third refractive index n3 may be envisaged.

[0094] It is noted that although in figures 5 through 8, illumination is shown to occur along the optical axis, illumination from all directions, including for instance the sides of the fiber, may also occur. In Figures 5a and 5b, the volume 5 may form with the initial material 2a of the initial core 2 a new substantially tubular core structure within the end section 10 having a refractive index equal to the first refractive index nl. The volume 5 of material 3b may complement the initial core material 2a such that at the fiber end 4 a core area 4a having the first refractive index nl is centered around the geometric center C’ (and thus also with respect to the fiber axis F). This means that at the fiber end 4, after performing the method according to this embodiment, an electromagnetic field distribution centered on the geometric center may be obtained. Such a centered final electromagnetic field distribution may reduce transmission losses for connection to a subsequent optical element aligned with the fiber axis. In other words, the illuminated volume may be arranged to form with the original fiber a 3D structure creating a second predetermined electromagnetic field distribution, preferably thereby improving the match with the desired target. In figure 5b, a distance a may represent the maximum distance between any point of the initial core 2 and the geometric center C’. To recenter the core of the fiber, the volume 5 may then be defined such that at the fiber end 4, an edge region of the volume 5 between the materials 3a and 3b may also be at a distance a from the geometric center C’. In this way, the initial core eccentricity of the initial fiber may be corrected to substantially zero. It is noted that the volume 5 may comprise a sub-volume 5a for tapering the new modified core structure back to the initial core 2 for a smooth transition to a section 11 of the fiber which is not adjusted according to the method.

[0095] Figure 6a illustrates a side view of a fiber end of an adjusted fiber according to another embodiment. Figure 6b illustrates a longitudinal cross-section at a fiber end of an adjusted fiber according to Figure 6a. In this embodiment, the volume 5 is arranged around the initial core 2 to create with the initial core 2 a new tapered core structure between the initial core 2 having an initial diameter d and a core area 4a at the fiber end 4 having a diameter b larger than the initial core diameter d. The volume 5 may comprise illuminated material 3b as in Figure 5 a and 5b obtained by illuminating with a light source 100 the material 3 a of the cladding 3 to obtain a third refractive index n3. The third refractive index n3 may be equal to the refractive index nl of the core material 2a. However, the refractive n3 of the material 3b may also be in between the refractive index nl of the (initial) core material 2a and the refractive index n2 of the (initial) cladding material 3a. In this way, the illuminated volume 5 may thus be arranged to form with the original fiber a 3D structure creating a second predetermined electromagnetic field distribution at the fiber end having a larger mode field diameter than the initial mode field diameter of the initial fiber.

[0096] It is further noted that although a circular core area 4a was represent in Figure 6a, other shapes of core areas may be envisaged to match for instance a desired spot shape at the fiber end 4 having a differ shape than the shape of the initial core 2.

[0097] Figure 7a illustrates a side view of a fiber end of an adjusted fiber according to another embodiment. Figure 7b illustrates a longitudinal cross-section at a fiber end of an adjusted fiber according to Figure 7a. In this embodiment, the volume 5 is arranged inside the initial core 2 to create within the initial core 2 a new core structure having a diameter c larger than the initial core diameter d. The volume 5 may comprise illuminated material 2b obtained by illuminating with a light source 100 the material 2a of the core 2 to obtain a refractive index n4. The refractive index n4 may be higher than the refractive index nl of the core material 2a. In this way, the illuminated volume 5 may thus be arranged to form with the original fiber a 3D structure creating a second predetermined electromagnetic field distribution at the fiber end having a smaller mode field diameter than the initial mode field diameter of the initial fiber. At the fiber end 4, the adjusted fiber may comprise a cladding area 4b having a second refractive index n2, a core area 4a having a first refractive index nl higher than the second refractive index n2 and a “supercore” area 4c having a fourth refractive index n4 higher than the first refractive index nl. The areas 4a and 4c may be concentric. Alternatively, according to another embodiment (not illustrated) the “supercore” area 4c may be decentred with respect to the core area 4a but centered with respect to the geometric centre C’ of the fiber end 4.

[0098] Figure 8a illustrates a side view of a fiber end of an adjusted fiber according to another embodiment. Figure 8b illustrates a longitudinal cross-section at a fiber end of an adjusted fiber according to Figure 8a. In this embodiment, the volume 5 is arranged inside the cladding material 3 to create together with the initial core 2 a new core structure made of distinct volumes. The volume 5 may comprise illuminated material 3b obtained by illuminating with a light source 100 the material 3 a of the cladding 3 to obtain a third refractive index n3. The third refractive index n3 may be higher than the second refractive index n2 of the initial cladding material 3a. In this way, the illuminated volume 5 may thus be arranged to form with the original fiber a 3D structure creating a second predetermined electromagnetic field distribution at the fiber end with a desired type of distribution and / or mode field diameter for instance. At the fiber end 4, the adjusted fiber 1 may comprise a cladding area 4b having a second refractive index n2, a core area 4a having a first refractive index nl higher than the second refractive index n2 and one or more additional core areas 4c having a third refractive index n3 higher than the second refractive index n2.

[0099] Figure 9 illustrates a perspective view of a multi-fiber to chip connector 50 according to the art. A multi-fiber to chip connector 50 may comprise a substrate 50 with one or more V groves 52 for receiving one or more optical fibers 1. A glass lid 54 may then be adhered on top of the fibers 1 to form a connector end 50a for edge coupling to a subsequent optical element (not represented). Alternatively, the connector may be a multi-fiber ferrule, for instance a n MT ferrule in which fiber alignment is dependent on the eccentricity of the fiber and alignment pin holes. The alignment may then be dictated by the alignment pins during mating.

[0100] Figure 10a illustrates a schematic side view at a connector end of a connector according to Figure 9. The method described in Figure 4 for one fiber may be applied to any of the one or more fibers of the connector 50 as illustrated in Figures 9 and 10a. As better shown in Figure 10a, the positioning of the fiber cores 2 of the fibers 1 within the substrate 51 with the V groves 52 may suffer inaccuracies due to the internal inaccuracies of the fiber self and due to inaccuracies of the positioning of the fibers within the groves. A method for adjusting an electromagnetic field distribution of such an optical connector may comprise providing an optical connector with one or more first optical fibers. The optical connector may have an initial third electromagnetic field distribution at a connector end thereof. The method may further comprise adjusting an electromagnetic field distribution of at least one of the one or more first optical fibers of the optical connector according to the method of Figure 4 to obtain a fourth predefined electromagnetic field distribution at the connector end 50a.

[0101] Figure 10b illustrates a diagram of the optical power intensity measured in mW at the connector end 50a of Figure 10a versus the distance in pm with respect to a reference position (see x,y refential). Providing the optical connector may comprise measuring the initial electromagnetic field distributions of the one or more optical fibers of said connector and their relative positions. The fourth predefined electromagnetic field distribution at the connector end 50a may be determined according to an average positioning of the measured first initial electromagnetic field distributions of the one or more optical fibers of said connector and their measured relative positions. Alternatively in an embodiment not represented, where the connector may be arranged for edge-coupling the one or more first fibers to one or more second fibers, the fourth predefined electromagnetic field distribution may correspond to a combined electromagnetic field distribution created by the one or more second fibers when arranged in the connector.

[0102] Whilst the principles of the invention have been set out above in connection with specific embodiments, it is understood that this description is merely made by way of example and not as a limitation of the scope of protection which is determined by the appended claims.

Claims

Claims1. Method for adjusting an electromagnetic field distribution of an optical fiber (1) comprising a core (2) made of a core material (2a) and a cladding (3) made of at least one cladding material (3a), said method comprising the steps of providing (110) an optical fiber (1) having an initial first electromagnetic field distribution at a fiber end (4) thereof, modifying (120) a refractive index of a volume (5) of the optical fiber (1) within an end section of the optical fiber by illuminating said volume (5) with a light source using a multi-photon absorption process in order to obtain a second predefined electromagnetic field distribution at the fiber end, wherein the initial first electromagnetic field distribution has a first initial center core position, and wherein the second predefined electromagnetic field distribution has a second predefined center core position.

2. Method according to any of the preceding claims, wherein the illuminated volume (5) is a volume of the cladding (3), and wherein modifying a refractive index of a volume (5) of the optical fiber (1) comprises increasing a refractive index of the cladding material (3a) in said volume (5) above an initial refractive index of the cladding material (3a), preferably up to an initial refractive index of the core material (2a).

3. Method according to any of the preceding claims, wherein the illuminated volume (5) is a volume of the core (2), and wherein modifying a refractive index of a volume (5) of the optical fiber (1) comprises increasing a refractive index of the core material (2a) in said volume (5) above an initial refractive index of the core material (2a).

4. Method according to any of the preceding claims, wherein the illuminated volume (5) is arranged to form with a non-illuminated portion of the fiber (1) a 3D structure creating the second predetermined electromagnetic field distribution.

5. Method according to any of the preceding claims, wherein the initial first electromagnetic field distribution has a first mode field diameter and wherein the second predefined electromagnetic field distribution has a second predefined mode field diameter.

6. Method according to any of the preceding claims, wherein the initial first electromagnetic field distribution has a first shape and wherein the second predefined electromagnetic field distribution has a second predefined shape.

7. Method according to any of the preceding claims, wherein the initial first electromagnetic field distribution has a distribution type and wherein the second predefined electromagnetic field distribution has a second predefined distribution type.

8. Method according to any of the preceding claims, wherein the multiphoton absorption process is a two-photon absorption process.

9. Method according to any of the preceding claims, wherein the multiphoton absorption process has a three-dimensional spatial resolution.

10. Method according to any of the preceding claims, wherein the multiphoton absorption process has a sub-micron resolution, preferably a 200nm resolution, more preferably a lOOnm resolution.

11. Method according to any of the preceding claims, wherein providing (110) an optical fiber further comprises observing at least one characteristic of the initial first electromagnetic field distribution.

12. Method according to the preceding claim, wherein the volume (5) for obtaining the second predefined electromagnetic field distribution characteristic is determined based on the at least one observed characteristic.

13. Method according to any of the preceding claims, wherein the second predefined electromagnetic field distribution is based on at least one of the following a manufacturer ideal electromagnetic field distribution, an electromagnetic field distribution of a target use.

14. Method for adjusting an electromagnetic field distribution of an optical connector (50) comprising one or more first optical fibers (1) for edge coupling at a connector end (50a), comprising:- providing an optical connector (50) with one or more first optical fibers, the optical connector (50) having an initial third electromagnetic field distribution at a connector end (50a) thereof,- adjusting an electromagnetic field distribution of at least one of the one or more first optical fibers of the optical connector (50) according to the any of the preceding methods to obtain a fourth predefined electromagnetic field distribution at the connector end (50a).

15. Method according to the preceding claim, wherein providing the optical connector (50) comprises measuring the initial electromagnetic field distributions of the one or more optical fibers of said connector (50) and their relative positions, wherein the fourth predefined electromagnetic field distribution at the connector end (50a) is determined according to an average positioning of the measured first initial electromagnetic field distributions of the one or more optical fibers of said connector (50) and their measured relative positions.

16. Method according to the claim 15, wherein the connector (50) is arranged for edge-coupling the one or more first fibers to one or more second fibers, wherein the fourth predefined electromagnetic field distribution corresponds to a combined electromagnetic field distribution created by the one or more second fibers when arranged in the connector.

17. Adjusted optical fiber (1) comprising a core (2) made of a core material (2a) and a cladding (3) made of at least one cladding material (3a), wherein the optical fiber (1) comprises:- a first section along the optical axis with a cross-sectional first electromagnetic field distribution, and- an end section arranged at a fiber end (4) with a second electromagnetic field distribution at said fiber end (4),- wherein a refractive index of at least one point of at least one cross section of the end section differs from a refractive index of a point with the same polar coordinates in a crosssection of the first section- and wherein the first electromagnetic field distribution has a first initial center core position, and wherein the second electromagnetic field distribution has a second predefined center core position.

18. Adjusted optical fiber (1) according to any the preceding fiber claims, wherein a spatial repartition of refractive indexes in a cross section of the first section differs from a spatial repartition of refractive indexes in a cross section of the end section.

19. Adjusted optical fiber (1) according to any of the preceding fiber claims, wherein the first electromagnetic field distribution has one or more of the following: a first eccentricity of the fiber core (2) with respect to the cladding (3), a first position of a center of the core (2) withrespect to a reference position being said initial center core position, a first mode field diameter and / or a first distribution shape.

20. Adjusted optical fiber (1) according to any of the preceding fiber claims, wherein the second electromagnetic field distribution has one or more of the following: a second eccentricity of the fiber core (2) with respect to the cladding (3), a second position of a center of the core (2) with respect to a reference position being said second predefined center core position , a second mode field diameter and / or a second distribution shape.

21. Adjusted optical fiber (1) according to any of the preceding fiber claims, wherein the second electromagnetic field distribution differs from a predefined target electromagnetic field distribution with a sub-micron resolution, preferably a 200nm accuracy, ore preferably a lOOnm accuracy.

22. Adjusted optical fiber (1) according to any of the preceding fiber claims, wherein the predefined target electromagnetic field distribution is based on at least one of the following a manufacturer ideal electromagnetic field distribution, an electromagnetic field distribution of a target use.

23. Optical connector (50), comprising one or more optical fibers for edge coupling to a photonic integrated circuit chip, said optical connector (50) comprising at least one adjusted optical fiber (1) according to any of the previous fiber claims.

24. Optical connector (50) according to the previous claim, comprising a support with one or more V groves (52) for receiving the one or more optical fibers.

25. Optical connector (50) comprising one or more first optical fibers for edge coupling to one or more second optical fibers, said optical connector (50) comprising at least one first optical fiber being an adjusted optical fiber (1) according to any of the previous fiber claims.

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