WO1997042532A1

WO1997042532A1 - Spatial filter for a multicore fibre, and method for making same

Info

Publication number: WO1997042532A1
Application number: PCT/EP1997/002052
Authority: WO
Inventors: Ramiro Conde; Fatemeh Taleblou; Christian Depeursinge
Original assignee: Andromis S.A.
Priority date: 1996-05-03
Filing date: 1997-04-22
Publication date: 1997-11-13
Also published as: FR2748327A1; FR2748327B1

Abstract

An optical device for transmitting incident radiation is disclosed. The device comprises a multicore fibre (40) or multifibre which in turn comprises a set of cores (34, 36, 38) separated by so-called intercore areas (42, 44, 46, 48); and a spatial filter (56) for filtering the propagation modes of a higher order than the fundamental mode in each core, while avoiding the injection of incident radiation into the intercore areas. A method and a device for making said spatial filter, as well as an endoscopic device and an imaging device comprising such an optical device, are also disclosed.

Description

SPACE FILTER FOR MULTI-CORE FIBER AND PROCESS FOR PRODUCING THE SAME

PRODUCTION

DESCRIPTION Technical field and prior art

The invention relates to the field of multicore optical fibers.

A conventional multimode fiber comprises, as illustrated in FIG. 1A, a core 1 and a mantle 3. A multicore fiber is a bundle of fibers, melted and stretched, which therefore forms a continuous whole. The coat of each individual fiber is melted with the coats of neighboring hearts.

Inside a multi-core fiber one can only distinguish individual hearts, the mantle of fibers having become somewhat collective.

FIG. 1B represents a cross-sectional view of a multi-core fiber, the hearts 24 and the coats 26 being grouped inside a first sheath 2b, for example made of silica, and a second sheath 30, called sheath external or "black" coating The external diameter D} of the enseraole can be for example of the order of 200 to 500 μm.

Figure 1C is an enlarged view of the portion 32 of the string of hearts. In this FIG. 1C, it appears that the hearts have cross sections of variable shape, more or less homogeneous. In particular, the diameter d of each heart, that is to say the greatest distance separating two points from the same heart, varies from one heart to another. 1yoiquemer.td can, for example, vary between 3 and 4 μrn for ¹ "the same multicore fiber. Similarly, the average distance from one core to another is not uniform and can for example vary, for the same multicore fiber, from 3 <to 3.5 µm. The concept of multi-core fiber is to be distinguished from that of multi-fiber, which is an assembly or bundle of independent fibers placed together and possibly glued at the end. The invention also applies to multifibers.

Multicore fibers and multifibers are used in imaging, especially in the medical field. Endoscopy, and in particular microendoscopy, allows the practitioner to acquire information, or images, of parts inside the human body, such as the stomach, lungs or heart.

A device for the implementation of such a technique is shown diagrammatically in FIG. 2, or the reference 2 designates a light source which is focused by a lens 4 at the entrance of a light guide 6. The latter is in fact most often connected to a plurality of optical fibers 8, 10 disposed at the periphery of a multi-core fiber 12. An illumination beam 14 can thus be directed onto an area 16 of an organ to be observed, which reflects radiation 18 at the input 20 of a multi-core fiber 12. The latter comprising a coherent beam of individual hearts, these therefore transmit light in an orderly manner between them, and the image obtained at output 22 of the multi-core fiber corresponds to the image formed at the input 20 Means for memorizing, analyzing and / or representing the image can also be provided in combination with this device

This imaging technique is described for example in the articles by A Katzir "Optimal Fibers m Medicine", Scientific American, vol. 260 (5), p. 120-125, 1989 and "Optimal Fiber Techniques (Medicme) ", Encyclopedia of Physical Science and Technology, vol. 9, p. 630-646, 1987.

In practice, a multi-core fiber such as fiber 12 can comprise approximately 700 to 10,000 hearts, for applications in microendoscopy.

During an application to imagery, such as that set out above, the rays 18 reflected by the object or the area 16 to be observed, may be incident on the end of a heart 24 or on an inter-core area 26. In the latter case, the incident light is not transmitted to the outlet end 22. Consequently, only part of the radiation reflected by the object or the area 16 to be observed is transmitted, and the proportion of radiation transmitted depends on the limited number of cores in the multi-core fiber, and on the inter-core distance. The smaller this distance, the higher the number of points in the input image that can be sampled. However, reducing the inter-core distance is not without its problems. Below a certain limit, there is no longer improvement, but on the contrary degradation, of the image obtained at the output of multicore fiber, due to the appearance of a phenomenon of coupling between neighboring hearts. This coupling can be attributed to two physical phenomena.

The first is linked to the existence, for each mode of propagation inside an individual heart, of an evanescent field distributed spatially in the inter-core zones and in the neighboring hearts. This evanescent field is all the more important as the order of the mode is high. It follows that, for an incident intensity I ₀ at the input of a core, the latter transmits a certain intensity I ₀ -ι ₀ , while the neighboring hearts transmit an intensity ι ₀ .

A second phenomenon, of a different nature from the first, is linked to the existence of weakly guided modes. The latter have the property of being guides in a core of the fiber, but with significant losses in the form of radiation directed towards the neighboring hearts.

These phenomena contributing, as explained above, to a decrease in the resolution of the image, it is important to find a way to reduce its importance.

Finally, independently of these physical coupling phenomena, there still remains the problem of the inter-core zone (zone 26 in FIG. 2B). Light incident on this area is not transmitted at the output, but can on the other hand be diffused towards the neighboring cores and thus degrade: the image at the output of the fiber. The same problems arise for multifibers.

Exhibition of the invention

The invention proposes a solution to these problems.

More specifically, the subject of the invention is an optical device for the transmission of incident radiation, comprising:

a multi-core fiber, itself comprising a set of hearts separated by zones called inter-core zones, and

a spatial filter making it possible to filter, in each core, the propagation modes of a higher order than the fundamental mode, and making it possible to avoid the injection of incident radiation in the inter-core zones. 7/42532 PCÏ7EP97 / 02052

"Closing" the inter-core areas with the filter eliminates a source of degradation of the transmitted image. In addition, the fact of reducing the intensity of the propagation modes of a higher order than the fundamental mode makes it possible to reduce the intensity of the evanescent fields, and of the flight modes, and therefore to reduce the sources of coupling between the individual hearts. (The fundamental mode can also be characterized as the mode of order 0, the other modes being of order n:> l, n = l, n = 2, ...).

According to a first particular embodiment, the spatial filter may include a mask, with perforations, deposited on one face of the bundle of multi-core fiber, each perforation being centered on a corresponding individual core, and having an edge closing off the peripheral part of this heart, the inter-core zones being closed by the mask

The mask can for example be a metallic deposit produced on the inlet end of the multi-core fiber.

According to another particular embodiment of the invention, the spatial filter can comprise a photographic emulsion representing the image of a face of one end of the multi-core optical fiber.

The invention therefore also relates to such a spatial filter as such.

The invention also relates to an imaging device, in particular an endoscopic device, comprising a device with multicore fiber and spatial filter, as described above.

The invention also relates to a method for producing a spatial filter at one end of a multi-core fiber comprising a set of hearts, a layer of material being deposited at one end of the multi-core fiber, comprising:

the introduction, by the other end of the multi-core fiber, of a laser beam into at least one individual core of the multi-core fiber, so as to excite the fundamental mode of this core, and

- the perforation of the material layer using the laser beam.

A mask is thus obtained, the perforations of which are self-aligned with the individual hearts and adapted to the shape of each heart.

Due to its production process, this mask is then particularly well suited to allow only a preferential excitation of the fundamental mode inside each core.

In addition, the fact of perforating by directing the laser beam of perforation inside the very heart makes it possible to avoid the formation, during the perforation, of craters in the deposited material. The heat released by this perforation operation is evacuated by the multi-core fiber and by the metal layer, which avoids vaporization of the material deposited at the places where there must be sealing. Before perforation, a centering of the beam, that is to say a positioning making it possible essentially to excite only the fundamental mode of the core, can be carried out by observing the near field transmitted at the fiber outlet, through the layer of material to punch.

The invention also relates to a method for producing a spatial filter, at one end of a multicore fiber, comprising the following steps:

- focusing the image of the face of an end of the multicore fiber on a photographic film, - film development,

- inversion of the image obtained during the development stage, by contact of the film with a photographic emulsion. Again, this method provides an effective filter of higher order modes than the fundamental mode that could propagate in the individual hearts of a multicore fiber.

In addition, the method, whatever it may be, can advantageously include a preliminary step of defining the zones where the laser beam must be successively positioned in order to make the perforations. This makes it possible to automate the process, in particular in the case of perforation of the mask. This definition of the zones can consist in the formation of a global image of the multicore fiber, the digitalization of this image, the search for the barycenter of each core and the calculation of the surface associated with each core in this image. The invention also relates to a device for implementing this process, this device being able in particular to include:

means for emitting a laser beam and means for generating a laser pulse from this laser beam,

means for directing the laser beam onto the entry face of the end of the multi-core fiber on which the layer of material is not deposited,

- Means making it possible to excite essentially, with the laser beam, the fundamental mode of the individual hearts of the multi-core fiber.

The invention also applies to a multifiber. All the objects and processes defined above, in which the multicore fiber is replaced by a multifiber, are therefore part of the invention.

Brief description of the figures In any case, the characteristics and advantages of the invention will appear better in the light of the description which follows. This description relates to the exemplary embodiments, given by way of explanation and without limitation, with reference to annexed drawings in which:

FIGS. 1A, 1B and 1C represent cross-sectional views of an optical fiber and a multi-core fiber,

FIG. 2 is a schematic representation of an endoscopy device,

FIG. 3 represents a first embodiment of the invention,

FIG. 4 represents a device for depositing at the end of a multi-core fiber,

FIG. 5 represents a device for drilling a film deposited on one face of an end of a multi-core fiber,

FIG. 6 represents theoretical and experimental profiles of intensity transmitted at the output of an individual fiber in a multicore fiber,

FIG. 7 represents a core of a multicore fiber, the neighboring hearts and the immediately following hearts, FIGS. 8A and 8B respectively represent the intensity coupled in the fundamental mode, as well as the coupling efficiency in this mode,

FIGS. 9A to 9E illustrate steps for carrying out a process for automating drilling, FIG. 10 represents the formation of a photographic filter,

FIG. 11 represents a device for characterizing a photographic mask according to the invention,

FIG. 12 schematically illustrates a method of interpolating an image obtained with a photographic mask,

FIG. 13 gives modulation and transfer functions with and without a photographic mask,

FIG. 14 represents spatial distributions of intensities, with and without a photographic mask, - FIG. 15 schematically represents a general device for the use of a photographic mask,

- Figure 16 shows the principle of a confocal device with multicore fiber.

Detailed description of embodiments of

The invention

An embodiment of a metal mask on a multi-core fiber will be described. First, the multi-core fiber itself can be purchased commercially (e.g. fiber

Fujikura). Furthermore, a process for manufacturing multicore fibers is described in the article by A.

Katzir, entitled "Optimal Fiber Technics (Medicine)", vol.9, p. 630-646, 1987, and in particular in paragraph IV, C, entitled "Fabrication of Fiber-Optic

Bundles ".

The fiber is then cleaved and polished.

Typically, the individual fibers of a multi-core fiber have a core diameter which varies in the range from 1.8 to 2.1 μm and an inter-core distance which varies in the range from 3 to 3.5 μm.

In order to better understand the different stages of the process used for producing a metal mask, it will be recalled, in connection with FIG. 3, that the principle of masking is twofold:

- the hearts 34, 36, 38 of a multi-core fiber 40 are separated by inter-core zones 42, 44, 46, 48, and one of the objectives is first of all to close these inter-core zones, which remain passive in the process of image transmission; injecting light into these areas could only degrade this transmission,

- It is also a question of closing the peripheral parts 50, 52, 54 of the hearts, in order to minimize the excitation, in the hearts, of the modes other than the fundamental mode.

In FIG. 3, the reference 56 therefore designates a metal mask, making it possible to fulfill these functions when a beam 58 of light is incident on the input face of the multi-core fiber 40.

The formation of the mask 56, at the end of multi-core fiber, can be carried out with any type of material. However, it is preferable to choose materials having a not too high vaporization temperature, in order to be able to make perforations, by laser beam, in the layer of deposited material. It is therefore possible to choose dielectric materials or metals. They are good candidates for the following reasons:

- they allow easy evaporation and precise control of this evaporation (flatness, thickness and surface condition) with good reproducibility, - they have good adhesion properties on dielectric compounds based on silicate, used in particular for the production of cores and sheaths of individual fibers, - they have a precise solid-gas transition point, which conditions the precision of the edges of the perforations,

- they have a high attenuation coefficient, linked to a reflection at the interface, which makes it possible to reduce the layer to be deposited to minute thicknesses A thickness reduced to its minimum limits the effects of differential expansion between the fiber and the layer metallic during heating. Indeed, the stresses are distributed better on a very thin layer which can expand or contract with much more ease. The adhesion of the layer is thereby improved,

- in the form of layers, they have a low thermal conduction. In addition, this property, associated with a high reflection of the laser beams used for the perforations, makes it possible to obtain a layer resistant to the strong illuminations typically used in endoscopy. In comparison, a light absorbing dielectric material presents more risks due to the need to absorb energy in the layer,

- for relatively thin material thicknesses, the absorption / transmission ratio is sufficiently low to allow online control of the perforation.

Among the different possible materials, aluminum is particularly interesting because it is a material which has good adhesion and good layer development properties. Furthermore, the presence of an oxide layer makes the more resistant layer. Among the metals, chromium is also a good candidate for making metal masks. It is possible to provide a combined deposit of chromium, which has excellent adhesion to silica, then to add over the aluminum, which considerably increases the adhesion of the assembly by forming an alloy at 1 ' interface.

Once the material has been chosen, one method that can be used for deposition is to accelerate an electron beam towards a melting metal target. A device used to carry out this deposition step is shown in FIG. 4. A vacuum chamber 60 houses a crucible 62 intended to contain the chosen metallic material, and which can be heated. Means 64 for pumping the interior of the enclosure 60 are also provided. The enclosure also houses the fiber 40 at the end of which it is desired to deposit. The vacuum caused in the vaporization enclosure implies a significant free medium path of the aluminum atoms which will leave the surface of the molten metal located in the crucible 62, and which will gradually deposit on the target, in a non-metallic form. oxidized. In order to obtain a good layer quality, precise control of the growth rate of this layer is carried out. To this end, a vibrating quartz makes it possible to measure very precisely the mass deposited, and therefore the thickness of the layer (nanometric precision). Sufficient distance is maintained between the molten aluminum and the target to avoid angular effects. An example of a device that can be used is the "Edwards Auto-306 Coating Plant" device.

The thickness of the layer to be deposited is preferably as thin as possible. The thickness is preference chosen in order to be able to achieve an attenuation equal to or greater than a factor of 1000, but while allowing sufficient light to pass so as to achieve, before perforation, the centering of the laser beam of perforation relative to the core of the fiber. Thus, centering can be achieved by observing the intensity which is transmitted by the core and which passes through the layer of deposited material.

For thin layers, the thermal conduction in the metal layer is directly proportional to the thickness. To achieve the perforation of a thick layer, the power must be increased, otherwise the layer is liquefied with poor control of the shape, size and edges of the perforation. In this case, it is preferable to reduce the interaction time by limiting the width of the laser pulse.

In the case of aluminum, a layer of thickness between 35 nm and 45 nm, for example 40 nm, constitutes a good compromise for producing a mask making it possible to achieve sufficient spatial filtering, while allowing sufficient light to pass through. , before perforation, to allow centering of the laser beam, and to allow easy evaporation of material. Such a thickness makes it possible to obtain an attenuation factor of approximately 10 ^. Generally, it is possible to produce a mask with a thickness of between 20 and 75 nm.

Once the layer of material has been deposited, the perforations are made in this layer. For this, a device such as that shown schematically in Figure 5 is implemented. The reference 40 always designates the multi-core fiber, with ends 66 and 68, the end 66 being that on which the masking material has been deposited. The beam of a continuous laser 70, for example an ionized argon laser (beam at 514.5 nm) is modulated by an acousto-optical modulator 72 triggered by a pulse generator. Optionally, a beam splitter 73 and a diode 75 make it possible to measure the intensity of the laser beam. The beam is then directed towards a beam expander 74 and is focused by a microscope objective 76 on the entry face 68 of the multi-core fiber. The microscope 76 allows both the observation of the input face of the multicore fiber and the observation of the focused laser beam. A camera 78 transmits the image of the input face and of the focused laser beam to a display device 80. The multi-core fiber is mounted on a table 82 allowing precision adjustments to be made in three directions X, Y, Z by micrometric mechanical control and by submicrometric piezoelectric control. By these adjustment means, the focused laser beam is brought into coincidence with the interior of an individual heart. At the output of the multi-core fiber, and more precisely at the output of the core which receives the focused beam, the laser beam is transmitted through the deposited layer. The image obtained is enlarged by a lens 82, and focused on a camera 84 itself connected to a display device 86.

When the modulator 72 is triggered, a significant part of the intensity of the laser beam is deflected to the first order, and does not reach the beam expander device 74. The intensity transτιιse (a few percent of order 0) is directed towards the entry of the core, crosses it as well as the layer of material deposited at the outlet, and is observed with the second camera 84. The relative position of the transmitted beam can be adjusted with the setting 82, as input, until the best geometric adjustment of the energy transmitted by the heart is obtained. In practice, we try as much as possible to confine the energy injected into the heart in the lowest possible order mode, that is to say in the fundamental mode. For this, we seek to obtain, using the camera 84, an approximately circular, or slightly elliptical, spot at the outlet of the fiber 40. The display means 78, 80 making it possible to control the entry 68 of the fiber 40 in fact make it possible to carry out a coarse adjustment of the laser beam on the input face of the fiber, while the fine adjustment is carried out using means 84, 86. When the modulator 72 is stopped, all of the intensity is transmitted on the input face of the fiber, in order to send a laser pulse of high intensity and, in the case of the example chosen, of a duration of approximately 10 μs The intensity of the beam laser is gradually and successively increased, until a significant increase in the transmitted intensity is observed with the camera 84, which indicates that the perforation of the film or of the layer deposited at the outlet 66 of the fiber 40 is performed. The optimum perforation diameters are obtained for laser powers slightly above the limit power, that is to say the power allowing the perforation limit to be reached.

In general, whatever the material used, for a power substantially equal to the limit power of perforation, the shape and diameter of the perforation are similar to those of the beam introduced into the core. For lower powers, no perforation is not observed, or a perforation is made, but of a diameter much smaller than the diameter of the heart. It is an unstable regime, and the perforation is not performed every time. The more the power is increased, beyond the limit power, and the more the definition of the edge of the perforated area becomes imprecise, but the spatial filter obtained can still be used. Too large an increase in energy produces a parasitic enlargement of the perforations.

In the case of a metallic deposit of aluminum 40 nm thick, perforations were carried out with a laser beam of 40 milliwatts of power and pulses of temporal width 10 μs. The perforation produced is then mainly defined by the geometry of the individual heart. In the perforation zone, the aluminum is evaporated under the action of the laser pulse, since a thickness of 40 nm is not sufficient to remove, by conduction, the heat deposited by the laser pulse. If the aluminum layer is too thick, the material no longer evaporates, but melts. This results in less precise control of the characteristics of the perforation. In general, and whatever the material used, it is therefore advantageous to work with small thicknesses, so as to promote a vaporizing effect under the action of the laser beam.

On the contrary, for powers slightly above 40 milliwatts, the diameter of the hole does not vary significantly with a variation in power. When a perforation has been made in the layer of material deposited at the outlet of multi-core fibers, and more precisely at the outlet of an individual core, a perforation may be performed for a neighboring individual core or another individual core. The process can thus be repeated as many times as there are individual hearts in the multi-core fiber.

The advantages of this method (perforation by injection of the laser beam into the multicore fiber) are manifold. First of all, the multi-core fiber itself can help dissipate the heat or energy deposited by the laser pulse in the layer of material. This avoids parasitic vaporization phenomena. In addition, if the attack is carried out from the front, that is to say if the laser beam comes to perforate the layer while being directed on the face of the latter which is not turned towards the multi-core fiber, craters can form around the perforation, and the latter is much less clear. Finally, the attack on the perforation by the fiber-layer interface of deposited material reduces the importance of inhomogeneities caused by variations in the surface condition of the layer of material. The perforations obtained with this technique are self-centered on the individual hearts and the control of the laser power makes it possible to control their diameter. After completion of all the perforations, a spatial filter is obtained, which essentially only excites the fundamental mode of propagation in the heart (because there is overlap of the mask and the edges of each individual heart), and which allows '' close off the inter-core spaces (because the deposit is made on the entire end of the multi-core fiber, and there can be no perforations in the inter-core zones).

FIG. 6 represents profiles of transmitted intensity, measured at the output of two different hearts of a multi-core fiber having a mask as described above, as a function of the radius measured from the center of the heart. To perform the measurements, a 97/42532 PC17EP97 / 02052

18

magnification device (magnification factor between 40 and 100) focuses the image at the heart output on a CCD camera. The profiles are normalized to a maximum intensity of 1. The slight variations in width from one intensity profile to another come from variations in diameter and ellipticity of the two hearts tested. These variations induce corresponding modifications in the diameter and the shape of the perforations. Curves III and IV represent the theoretical profiles of the fundamental mode and of the order mode 1 respectively. Insofar as curves I and II relate to the total intensity transmitted by each core, the profiles defined by these curves I and II contain the contribution of each of the excited modes. The comparison with the theoretical profiles of the order 0 and order 1 modes shows that the contribution of the order 1 modes, in the intensity actually transmitted, is negligible. This shows that the result sought, namely the reduction of the contribution of the modes of order 1 and more, is obtained.

The average inter-core coupling could also be measured. Due to statistical variations in the diameter of the hearts and in the inter-core distances, the results had to be average to obtain the intensities of inter-core coupling. Different individual hearts were excited using a point source of 3 μm in diameter. For each excited core 91 (see FIG. 7), the intensities leaving the neighboring individual hearts 93-ι, 95-j were measured, on the other side of the multicore fiber with respect to the source, and the intensity distribution. is statistically evaluated It has been noted that 15% of the intensity introduced into a heart 91 are coupled in neighboring hearts 93-1, 93-2,, 93-6, and 6% in the immediately following hearts 95-1, ..., 95-12. The average inter-core coupling is approximately 0.5% for the neighboring hearts and approximately 1.9% for the immediately following hearts. The inter-core coupling is therefore reduced by a factor of 30 for the neighboring hearts and by a factor of 3.2 for the immediately following hearts. The fact that the reduction factor is greater in the first case probably means that it is the reduction of the parasites due to the evanescent waves which is the most effective. This confirms that the order modes greater than or equal to 1 are effectively filtered by the mask obtained according to the present invention. Coupling over longer distances is mainly due to leakage from one heart to the other hearts. The spatial filter obtained is perhaps less effective for the reduction of this type of parasite. However, the degradation of the image quality in the area of high spatial frequencies is mainly due to the contribution of the first type of parasite. Reducing the latter therefore means a significant improvement in the optical performance of multicore fibers.

The diameter φ _p of the perforations, making it possible to obtain the filter effect of the modes of order higher than the fundamental mode, is less than the average diameter φc of the corresponding fiber core, by a value δ substantially between 0 and 1 μm , preferably between 0.2μm and lμm, or, better still, between 0.5μm and 0.8μm

φp = φc-δ. However, it should be noted that optical measurements of these diameters are difficult. An adjustment of the fundamental mode obtained for example theoretically (curve III of FIG. 6), or experimentally (curve I or II of FIG. 6) with a Gaussian function (by definition, close to the real analytical form of this mode) allows a approximate calculation of the intensity I which can be coupled in the fundamental mode, as a function of the radius r of the perforation, with constant illumination over the entire surface. The calculation was carried out at the wavelength 632 nm for a core with a diameter of 2 μm, immersed in a mantle of very large extension, with no neighboring core. The intensity curve was normalized with respect to the maximum, which corresponds to complete illumination of the entire surface of the heart (see Figure 8A). The illumination on a circulated ^ry area with a radius twice the size that the core gives the maximum excitation of the fundamental mode and corresponds to the maximum which can be injected into the heart. If the radius of the perforation is reduced to 1 μm (respectively 0.75 μm, then only 60% (respectively 30%) of this maximum is reached. The reduction in the size of the perforation causes changes in the excitation of the mode. fundamental, and this changes from 60 to 10% when this radius is reduced from 1 to 0.5 μm. Finally, we can note the presence of a highly sensitive zone, between 0.5 μm and approximately 1.3 μm.

FIG. 8B gives, for a core of diameter 2 μm, as a function of the radius r of the perforation, the coupling efficiency δ, that is to say the intensity actually coupled in the fundamental mode, divided by the intensity actually incident on the heart, through the perforation. The units are arbitrary, but a maximum appears clearly for a radius of 0.9 μm. The coupling efficiency is reduced by 30% when the radius goes from 0.9 to 0.5 μm. As the diameter narrows a larger portion of the incident light intensity escapes from the heart in the form of radiative modes; Indeed, a small opening causes very important diffraction effects which are marked by very divergent light rays at the entrance of the heart. The index gradient of the heart cannot compensate for this divergence in order to confine these rays inside the heart and the rays escape. Consequently, ₄ it appears from this FIG. 8B that the zone acceptable for the radius of the perforation is between 0.5 μm and 0.9 μm (for a core with a radius of 1 μm; lμrrKφ _p <1.8 μm ; 0.2μπκδ £ lμm).

As explained above, the mask is pierced heart by heart. A method has been developed which allows this drilling to be carried out automatically. For this, the device described above in connection with Figure 5 is modified; it incorporates, in addition to the elements already described above, means (beam splitter, white light source) making it possible to subject the multicore fiber 40 to uniform white lighting which is introduced through one of the ends of the fiber. By the other end, an image of the fiber core network is obtained, due to this lighting. Means for digitizing the image obtained are provided, as well as means for memorizing the digitized values. The image obtained under uniform white lighting therefore leads to a set of NxN points or pixels. Each pixel corresponds to a digital intensity value, which is a function of the position of the point or of the pixel p (X, Y) in the plane of the image. The process for processing the data thus obtained makes it possible to determine three basic pieces of information: - first of all a digital mask for the image of the fiber is defined, which makes it possible to distinguish the pixels of the multi-core fiber and the pixels located in the region between the multi-core fiber and the outer sheath of the fiber (sheath referenced 30 in Figure 1B); the determination of this mask saves the calculation time and the number of operations;

- the response of each individual heart to the received signal, which makes it possible to search for the barycenter of each individual heart;

- the surface associated with each heart, which makes it possible, in conjunction with the knowledge of the barycenter, to locate each heart;

The first step (definition of a digital mask of the multicore fiber) will now be described, in conjunction with FIGS. 9A and 9B. FIG. 9A represents the reference image obtained in white lighting. The reference 81 designates the circular surface of the network of cores of the multi-core fiber: this surface contains the information useful for image processing. The determination of a digital mask, which delimits this surface, makes it possible to apply the subsequent operations only in this region, and therefore to reduce the number of operations and the calculation time. For this, a scan from left to right of the image is carried out, on each line of the reference image (that is to say of the image obtained with white light illumination) up to the pixel which has an intensity greater than a given threshold. Among the pixels obtained for all the lines, the one with the minimum order column from the left determines the far left column, represented schematically in FIG. 9A by a dashed line 83. In the same way, the extreme right column 85, the row upper extreme 87 and the lower extreme line 89. The applied threshold (the same for all the dimensions of the image) is defined according to the noise level of the image. Next, the pixels located inside the image 81 are coded at "0", while the other pixels are coded at "1". This coding is recorded in a matrix of the same size as the reference image.

The next step is to find the barycenter of each heart. This research is done using the region growth segmentation method. It allows to group the contiguous pixels belonging to the same core. For each pixel of the reference image 81, four connections are defined, according to the diagram shown in FIG. 9C. For each connection, a threshold T is defined, such that, for any coordinate pixel (xι_, y \):

if I _p (x ₁ , y ₁ )> T, then: p (xχ, yι) e C _k

where Ip (xχ, yτ_) designates the intensity of the pixel with coordinates (xi, yτ_) and Ck designates the core number k.

As the neighboring pixels are brought together, a number k is assigned to the corresponding core. It is then possible to calculate the barycenter of the pixels grouped together under the designation "core n ^D k". The method is applied recursively for each pixel of 4 connections. The case of pixels with less than 4 connections finds only for the pixels located at the edge of the image. In the search, there is a test on these pixels and as soon as an edge pixel is detected, the search in this direction stops and continues in another direction.

The threshold T is determined from the cumulative histogram of the intensities of the reference image (taking into account only the pixels located at inside the fiber, using the defined mask), and it is based on the information given by the manufacturer. More precisely, from the data relating to the diameter of the core, to the number of cores of the fiber, the average number of pixels is calculated according to the following relationship:

Npc = ^(N pb ^xA mc> / ^A mb

or A _mc denotes the area of a heart (in μm ^) calculated from the diameter of the heart; A _mr; designates the area of the section of the multicore fiber (in μm ^) defined according to the data of; Np ^ designates the number of pixels in the image of the multicore fiber, or else the area of the section of the multicore fiber in pixels defined from the cumulative histogram; Np _C denotes the number of pixels in the core, or the average area of each individual core, in pixels.

It is possible that the fibers will deform during their hot drawing, and the diameter of the hearts can therefore decrease or increase in one direction. The hearts then no longer have the circular shape. Therefore, the number of pixels obtained has only an average value. By multiplying the number of cores by the surface (pixel) of an individual core, we obtain the "useful" surface of the multi-core fiber. Then, from the cumulative histogram such as that illustrated in FIG. 9D, and which groups, on the abscissa, the cumulative level of gray and on the ordinate the number of pixels, the gray level αe which corresponds to this surface is defined "useful". The segmentation threshold T is defined by the value of this gray level The search is made by moving from right to left (in qris level) on the cumulative histogram until the value of the useful area of hearts be reached. Finally, the area associated with each core is calculated. The response of the heart to the light signal received is defined by the sum of the intensities of the pixels belonging to this heart. To have an exact evaluation of this response, we associate with each heart an area limited by the perpendicular bisectors between the central heart C ^. (see Figure 9E) and each of the neighboring hearts

Thus, the surface of the bundle of hearts is divided into adjacent polygons, and each pixel of the image belongs to one and only one region.

The method described above can be implemented by a suitably programmed microcomputer. The appropriate program instructions are stored on conventional RAM or ROM units, the data obtained, relating to the identification of the center of each core can then control a relative movement mechanism between the end 68 of the fiber (see FIG. 5) and the device which makes it possible to focus the drilling beam at this end 68. These displacement means can be means of displacement of the table 82 on which the end 68 of the multi-core fiber 40 is fixed. A prior calibration makes it possible to locate the coordinates, in the XY plane of the image of the end of the fiber (see FIG. 9A), of a certain position of this table. This position is then chosen as the origin to define any subsequent displacement. After each drilling, the command, to effect the displacement of the end of the fiber in order to carry out the drilling corresponding to a neighboring core, can be sent automatically, from information received by the camera 84 (the intensity transmitted in output 66 of the multicore fiber suddenly increasing after drilling, a threshold condition can be implemented 7/42532 PC17EP97 / 02052

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by comparing the intensity transmitted at fiber output 66 and a reference intensity above which the drilling is considered to have taken place) or can be triggered by the operator (the latter monitoring for example the screen 86 which displays the image of the output 66 of the fiber, which makes it possible to control when the drilling is carried out).

Another embodiment of a device making it possible to perform a filtering of the higher order modes than the fundamental mode of the individual hearts of a multi-core fiber, and which also makes it possible to avoid the propagation of light towards the inter-core zones, will now be described. This device is in fact a photographic mask, or rather a photographic emulsion representing the image of the output of the multi-core fiber.

A device for producing this mask will be described, in connection with FIG. 10. In this figure, the reference 90 designates a multi-core fiber, of the type already described above, in connection with the first embodiment, and for which we want to make the photographic mask. A white light source 92 emits radiation which is focused by a lens 94 at the inlet 96 of the fiber 90. The image of the outlet end 98 of the fiber 90 is focused using focusing means 100, for example a lens, on a photographic film 102, for example a Kodalith ortho type 3 film (Kodak). The magnification of the lens, and therefore the magnification of the image of the output 98 of the fiber on the film 102 is chosen according to the resolution that it is possible to obtain with this film. For example, the resolution of the black and white film mentioned above (Kodak) is in the range of 400 to 600 lp / mm 7/42532 PO7EP97 / 02052

27

(pair of lines per mm), and the magnification factor of the lens 100 chosen is 20. Filters 103 can be interposed on the light path towards the films 102: a red filter allows adjustments to be made without impress the emulsion, while a green filter is introduced to improve the resolution, the adjustment is done by hand by adjusting the position of the photographic film thanks to the micrometric translation stages, and by directly observing the image of the multi-core fiber (for example in red light, the photographic film is not impressed by this color).

The film is then developed, for example with a "fine-line" developer (Kodak), in order to obtain a negative of the mask

Then, the image is reversed by direct contact between the developing film and an emulsion, for example a "kodalith ortho" emulsion deposited on a glass The film is finally developed to obtain the mask

A device for characterizing such a mask will be described, in conjunction with FIG. 11. A beam of white light is emitted by a source 106, and is diffused by a glass 108. The scattered light illuminates the photographic mask 104, of which the image is focused using the lens 100 on the input face 98 of the multi-core fiber 90. The mask 104 is mounted on a table 110 making it possible to move in three directions X, Y, Z (a using mechanical means for micrometric displacements and piezoelectric means for submicrometric displacements) and a rotation around the optical axis. The orientation of the mask 104 and the magnification of the lens 100 are chosen precisely in order to have a complete correspondence between the image of the photographic mask and the network of fibers at the end 98 of the multi-core fiber 90. A test or characterization target 111 is also mounted on a table 112 allowing micrometric displacements in three directions X, Y, Z. The targets are chosen from a set of Ronchi patterns (these patterns are made up of layers of chromium deposited in the form of opaque bars on an optical glass) and the spatial frequency of the patterns is 5, 10, 15,. ... 160 and 170, 180, 190, 200 lp / mm (pairs of lines per mm). Opposite the output 96 of the multicore fiber 90, a lens 114 makes it possible to enlarge and focus, on a CCD camera 116 at 255 gray levels, the image obtained at the output of the fiber. The test patterns arranged opposite the end 98 of the multi-core fiber are precisely adjusted using the table 112, in order to obtain an image, on the camera, for which the lines of the test pattern are arranged vertically. A beam of white light obtained using a source 118 is focused on the exit face 96 of the fiber 90. This beam, after having passed through the fiber, the target positioned opposite the exit 98 of the fiber, the lens 100, and the photographic mask 104, is deflected by a beam splitter 120 in the direction of a microscope 122. The optimal position of the photographic mask 104 and the target, corresponding to the maximum intensity transmitted through the photographic mask and when the fringes of "Moiré" disappear due to the interference between the two networks, is obtained by adjusting the position of these elements using tables 110, 112.

For each test pattern of which an image is formed at output, on the camera 116, an image is recorded in storage means provided for this purpose. Each target is moved laterally and the test is then repeated with the target of higher frequency. The mechanical contact between the target and the input face 98 of the multi-core fiber can cause a slight displacement of the latter, relative to the photographic mask 104. The mask can then be readjusted precisely with the piezoelectric translation means of the table 110, by direct observation.

For each multicore fiber tested, it is possible to carry out a preliminary treatment, identical to that already described above in connection with the production of metal masks, in order to determine the position of the barycenters of each individual core and the surface associated with each heart. For this, in the same way as has already been described above, a beam of white light is introduced into the multi-core fiber, by one of its ends, and the corresponding image is observed by the other end. Said image is digitized, memorized, then, as has already been described above:

- a digital mask is defined making it possible to define the image of the multi-core fiber,

- we are looking for the barycenter of hearts, using the region growth segmentation method, - we calculate the area associated with each heart.

For each image transmitted and stored, obtained with a photographic mask, the intensity transmitted by each individual core can be integrated (the sum of the intensities associated with the pixels contained in the surface C ^ associated with each core is carried out) and the value obtained is associated with the center of the corresponding heart. Then, the intensities are interpolated bilinearly, in order to avoid the presence of the black zones corresponding to the mantle (or inter-core zone) of the fibers. The interpolation procedure can be explained in connection with FIG. 12, which represents a diagram giving the integrated intensity I associated with each barycenter of the heart, as a function of the position (x, y) of the heart in the image. Bilinear interpolation is calculated on each triangular surface determined by the barycenters of three hearts: the intensities of the pixels contained in the triangular area between the centers of hearts a, b and c (area 121 in Figure 12) are replaced by the values of interpolated intensity. With such a treatment, it has been observed that, on an image of a Ronchi test pattern, the black inter-core zones have disappeared and that the image treated has a more continuous appearance.

Finally, the pixel intensity of the interpolated images is averaged along a vertical. The interpolation procedure, combined with the step of calculating the average along a vertical, gives a one-dimensional intensity function. An algorithm, implemented using a conventional computer programmed for this purpose, searches for the extremes of the intensity function, and the corresponding successive contrasts are calculated and averaged. The values obtained for the mean of the contrast are plotted on a graph, as a function of the spatial frequencies of the patterns, and are assimilated to the modulation transfer ionction.

In order to evaluate the efficiency of the mask, the modulation transfer function was measured for a multicore fiber comprising 6000 hearts, on the one hand with and on the other hand without a photographic mask.

FIG. 13 gives the two curves obtained, with mask (curve I) and without mask (curve II), for this modulation transfer function. Values of the function for the masked fiber are greater than those of the function for the unmasked fiber, which clearly shows the filtering effect of the mask. The difference is around 30% for a frequency of 100 lp / mm, and 12% for a frequency of 200 lp / mm. This increase in the modulation transfer function for higher frequencies, with mask, provides an improvement in the quality of the image. FIG. 14 represents a spatial distribution, along a dimension, of the intensities of the image obtained, in the case of an image with mask (curve I) and without mask (curve II). The contrast between individual hearts and the mantle is higher for masked fibers (contrast ~ 0.7) compared to the contrast for unmasked fibers (~ 0.6). As a result, the image quality is enhanced by more contrasting sampling of the fiber cores. The increase in the modulation transfer function, for the masked multi-core fiber, is attributed to a significant reduction in the intensity of inter-core coupling. In turn, this reduction is attributed to a filtering of the guide modes of orders higher than the fundamental mode, and of the modes of leakage in the fiber. Consequently, the photographic mask plays a similar role, with the same technical function, as the metal mask described in the first embodiment: there is spatial filtering, p _α r masking of one by "of the inter-core space between the individual hearts, and on the other hand peripheral areas of the hearts.

Still with regard to the photographic masks, note the possibility of varying the size of the image of the individual hearts by modulating with respect to each other the development parameters that are the exposure time and the development time of the mask. Thus, a good filter can be obtained with a constant exposure time and by working in overdevelopment.

A device for the use of a photographic mask will be described in connection with FIG. 15. In this figure, the references 90, 100 and 104 always represent respectively the multi-core fiber, a lens and a photographic mask obtained for example according to the method which has been described above. An object 124, of which an image is to be obtained, is placed in front of the mask 102, a lens 126 being positioned between these two elements. This lens and the arrangement of the three elements are chosen so that the plane of the photographic mask 104 is coincident with the image plane of the lens. The image obtained on the mask, which then acts as a spatial filter, is then focused by the lens 100, at the input 98 of the multi-core fiber 90. At the output of the latter, one can find the elements 114 and 116 described above in conjunction with FIG. 11 (lens and CCD camera) Any element for processing and / or storing images obtained can be associated with the device. Thus, the images can be stored in a microcomputer 117 for further processing and / or viewing. A possible treatment can be the bilinear interpolation treatment, already described above, this treatment makes it possible to avoid the black zones corresponding to the mantle of the fibers.

Either of the filters described in the present description can be used in combination with a multicore fiber in an endoscopy device such as that described above, in the introduction, in conjunction with Figure 2 and for which further details can be found in the articles by A. Katzir et al. already cited.

More generally, the spatial filters according to the invention can be used in the context of any imaging device implementing a multi-core fiber for which it is necessary to reduce the contributions of the modes of order n> l transmitted by the individual hearts as well as the illumination of the inter-core zones, by the light scattered, re-emitted or reflected by the object studied.

An example of application will be given, which can be implemented with a photographic mask or a metal mask according to the invention. This example concerns an application to confocal microscopy or microendoscopy with multicore fibers, a schematic diagram of which is given in FIG. 16, taken from the article by D Azi? et al., entitled "Imaging performance of the fiber-optic image-bundie confocal microscope", SPIE, Vol. 2083, p 139-146 (1993), or this technique is described.

Confocal techniques use a double spatial filtering system for the injection of light (illumination of the sample) and the collection of the signal emitted by the sample after illumination. This dual system improves the resolution compared to traditional optical microscopy. The use of a multi-core fiber allows the observation of samples outside the microscope area. The size of the hearts and the distance between the hearts should be as small as possible to obtain the best possible resolution. Coupling between hearts plays a less important role than in traditional microendoscopy, because the measurement is done successively and individually on each heart.

Reference 130 designates a multi-core fiber, reference 132 a sample to be studied. A point source 134 emits a beam 136 in the direction of an entry face of the fiber 130, after having passed through a device 138 making it possible to scan the beam and a focusing device 140.

With the aid of the means 138, the beam 136 is successively focused in different individual hearts of the multi-core fiber 130. Between the exit face of the latter and the sample 132 are arranged two objectives 142, 144 which allow, of a On the other hand, to focus the incident light towards the sample and, conversely, to focus the light coming from the area of interest of the sample 132 on the entry face of the multicore fiber. The observation beam is transmitted towards a beam splitter 146 which deflects it towards a point detector 148.

The point detector 148 associated with the detection makes it possible to individually select, heart by heart, the intensity emitted by the illuminated sample. This detector can be as small as desired, the maximum resolution being linked to the size of the heart.

A metal mask according to the present invention can be formed at the input or at the output of the multi-core fiber 130. Likewise, a photographic mask according to the present invention can be placed at several places:

- In the system 142, 144 of lenses close to the sample 132; in this case, the mask allows optimization of the coupling of the light between sample 132 and the multicore fiber: by promoting the excitation of the fundamental mode, we obtain more punctual, and therefore more precise, information of the image. In addition, due to the diffusing effect of the sample, the neighboring hearts are also illuminated and the coupling between the hearts can reduce the resolution. A photographic mask, placed in the lens system close to the sample, helps reduce these effects.

- Installed in the part corresponding to the point source 134, the mask makes it possible to optimize the coupling between the focused beam and the multi-core fiber, by filtering as much as possible the contribution of the modes of order greater than or equal to 1.

- Placed in the part corresponding to the point detector 148, the photographic mask allows the harvest of the signal emitted by the sample, with maximum efficiency. In all cases, (photographic mask or metallic mask), the order modes greater than or equal to 1 see their contribution greatly reduced in the total intensity transmitted by the multicore fiber 130. The various filters according to the invention can be apply to multifibers, which can for example also be used in microendoscopy. In a multifiber, the different fibers, each with its heart and its mantle, can be identified individually; In addition, the densities of the hearts in a multifiber are less important than in a multicore fiber. However, optical isolation problems may arise, as in the case of multicore fibers. Training procedures for masks and the characterization methods described above can be applied directly to multifibers.

Claims

1. Optical device for the transmission of incident radiation, comprising:

- a multi-core fiber (40, 90), or a multi-fiber, itself comprising a set of hearts (34, 36,

38), separated by zones called inter-core zones (42, 44, 46, 48), and

- a spatial filter (56, 104) making it possible to filter, in each core, the propagation modes of a higher order than the fundamental mode, and making it possible to avoid the injection of incident radiation in the inter-core zones

2. Device according to claim 1, the spatial filter comprising a mask (56), provided with perforations, deposited on one face of one end of the multi-core fiber (40), or of the multifiber, each perforation being centered on a heart (34, 36, 38) corresponding and having an edge closing the peripheral part (50, 52, 54) of this heart, the inter-core zones (42, 44, 46, 48) being closed by the mask (56)

3. Device according to claim 2, the mask being a metallic deposit on one face of an end of the multi-core fiber, or of the multifiber.

4. Device according to claim 3, the deposit being aluminum or chromium or chromium and aluminum.

5. Device according to claim 4, the thickness of the deposit being between 20 nm and 75 nm.

6. Device according to claim 5, the thickness of the deposit being between 35 and 45 nm.

7. Device according to claim 6, the thickness of the deposit being substantially equal to 40 nm.

8. An optical device according to claim 1, the spatial filter (102) comprising a photographic emulsion representing the image of a face of one end (98) of the multi-core optical fiber (90) or of the multifiber.

9. Device according to claim 8, the spatial filter (102) being located at a distance from one end (98) of the multi-core fiber or of the multifiber, focusing means (100) being interposed between said filter and said face d 'Entrance.

10. Endoscopic device comprising a device according to one of claims 1 to 9.

11. An imaging device comprising a device according to one of claims 1 to 9.

12. Spatial filter (102) for multicore fiber, comprising a photographic emulsion representing the image of an output end of the multicore fiber or of the multifiber.

13 Proceeds to produce a spatial filter at one end of a multi-core fiber or a multi-fiber, after depositing a layer of material on one face of an end (66) of the multi-core fiber

(40) or multifiber, comprising

- the introduction, by the face of the other end (68) of the multi-core fiber, or of the multi-fiber, of a laser beam into one of the individual hearts of the multi-core fiber or of the multi-fiber, so as to essentially excite the fundamental mode of said individual core, - the perforation of the layer of material using the laser beam.

14 The method of claim 13, the laser beam being center, before perforation, by observation of the near field transmitted by the end (66) of the fiber (40) on which the layer of material is deposited, through this layer of material.

15. Method according to one of claims 13 or 14, the deposited material being a metallic material, and the deposition being carried out by vacuum evaporation.

16. Method according to one of claims 13 or 14, comprising a prior step of defining the zones where the laser beam must be successively positioned in order to make the perforations.

17. Method for producing a spatial filter

(104), for a multi-core fiber, or for a multi-fiber, comprising the following steps:

focusing the image of the face of one end (98) of the multi-core fiber (90), or of the multi-fiber, on a photographic film (102),

- film development,

- inversion of the image obtained during the development stage, by contact of the film with a photographic emulsion.

18. Method according to one of claims 13 a

17, comprising, beforehand, the formation of an image (81) of the bundle of individual hearts, the digitization of this image, the search for the barycenter of each heart and the calculation of the surface associated with each heart (C, C ^ i,... C1-5) in this image.

19. The method of claim 18 comprising, after formation and digitalization of the image of the bundle of individual hearts, the calculation of a digital mask which delimits the area of the image inside which the search for the barycenter and the calculation of the surface of each core must be carried out.

20. The method of claim 18, the search for the barycenter being made by the method of segmentation by growth of regions.

21. The method of claim 18, the surface associated with each heart being obtained by associating with the barycenter of each heart, a surface limited by the perpendicular bisectors between said heart (Cj _j ) and each of the neighboring hearts (Cj ^, C ^ .. ., 0 ^ 5)

22. Device for implementing a method according to one of claims 13 or 14, comprising:

- means (70) for emitting a laser beam and means (72) for generating a laser pulse sequence from this laser beam,

- means (74, 76) for directing the laser beam onto the face of the end (68) of the multi-core fiber (40), or of the multi-fiber, on which the layer of material is not deposited,

means making it possible to excite essentially, with the laser beam, the fundamental mode of the individual hearts of the multi-core fiber or of the multi-fiber.

23. Device according to claim 22 further comprising means (76, 78, 80) for observing the relative position of a spot of the laser beam on the face of the end (68) of the multi-core fiber (40), or of the multifiber, and of this face.

24. Device according to one of claims 22 or 23, further comprising means (82, 84, 86) for observing the near field transmitted by one end (66) of the fiber (40).

25. Device according to one of claims 22 to 24, further comprising means for previously defining the areas or the laser beam must be successively positioned in order to make the perforations.

26. Device according to one of claims 22 to 25, comprising means for forming, before perforation, an image of the bundle of individual hearts, means for digitizing this image, means for calculating the barycenter of the heart, and means for calculate the area associated with each core in this image.