Device and method for controlling group velocity delays of pulses propagating in monomode optical fibers of a fiber bundle

Abstract

According to one aspect, the invention concerns a device for transporting and controlling light pulses for lensless endo-microscopic imaging and comprises: a bundle of N monomode optical fibers (F.sub.i) arranged in a given pattern, each monomode optical fiber being characterized by a relative group delay value (Ax) defined relative to the travel time of a pulse propagating in a reference monomode optical fiber (F.sub.0) of the bundle of fibers (40), an optical device for controlling group velocity (50) comprising a given number M of waveplates (P.sub.j) characterized by a given delay (8t.sub.j); a first spatial light modulator (51) suitable for forming from an incident light beam a number N of elementary light beams (B.sub.i) each of which is intended to enter into one of said optical fibers, each elementary beam being intended to pass into a given waveplate such that the sum of the delay introduced by said waveplate and the relative group delay of the optical fiber intended to receive said elementary light beam is minimal in absolute value; a second spatial light modulator (52) suitable for deviating each of the N elementary light beams such that each elementary light beam penetrates into the corresponding optical fiber perpendicularly to the entrance face of the optical fiber.

Claims

1. A device for transporting and controlling light pulses having at least one first wavelength for lensless endo-microscopic imaging, comprising: a bundle of N monomode optical fibers arranged in a given pattern, configured to receive a light beam at a proximal end and to emit a light beam at a distal end, wherein for each monomode optical fiber a relative group delay value is defined relative to a travel time of a pulse propagating in a reference monomode optical fiber of the bundle of fibers, an optical device for group velocity control, disposed on the proximal side of the optical fibers bundle and comprising: a plurality of M waveplates, each waveplate configured to introduce a given delay; a first spatial light modulator configured to generate, from one or more incident light beams emitted by a light source, a plurality of N elementary light beams comprising optical pulses, wherein, in operation, each elementary beam of the plurality of elementary light beams passes into a waveplate among the waveplates and enters into one of the optical fibers such that the sum of the delay introduced by said waveplate and the relative group delay of the optical fiber is minimal in absolute value; a second spatial light modulator configured to deviate the elementary light beam such that the elementary light beam penetrates into the optical fiber, perpendicularly to the entrance face of the optical fiber; and a phase control device comprising means of programming one and/or the other of the spatial light modulators, wherein the means of programming are configured to apply a phase shift to each of the elementary beams to imprint at the distal end of the bundle of fibers a predetermined phase function and/or to correct the phase variations introduced by each of the fibers of the bundle of fibers.

2. The device for transporting and controlling light pulses according to claim 1, wherein the optical device for controlling the group velocity comprises a first lens and a second lens forming an optical layout with an intermediate focal plane and in which: the waveplates are disposed in the intermediate focal plane of the optical layout; the first spatial light modulator is located in an object focal plane of the first lens; and the second spatial light modulator is located in an image focal plane of the second lens.

3. The device for transporting and controlling light pulses according to claim 1, wherein the optical device for controlling the group velocity comprises a lens and in which: the waveplates are disposed in a plane situated upstream from the first spatial light modulator and are adapted to form, from an incident beam formed of pulses, M light beams, each light beam being formed of pulses characterized by a given group delay; the first spatial light modulator is arranged in the object focal plane of the lens and is intended to receive said M light beams; and the second spatial light modulator is located in an image focal plane of the lens.

4. The device for transporting and controlling light pulses according to claim 3, wherein the first spatial light modulator is formed from M holographic zones, each holographic zone being intended to receive one of said light beams formed of pulses characterized by a given group delay.

5. The device for transporting and controlling light pulses according to claim 1, wherein the bundle of N monomode optical fibers is formed by a multi-core fiber.

6. The device for transporting and controlling light pulses according to claim 1, wherein the N monomode optical fibers are arranged in aperiodic manner.

7. An endo-microscopic imaging system, comprising: a source of light pulses; a device for transporting and controlling the pulses emitted by said source according to claim 1; and a detection path for the light intended to pass through the bundle of monomode optical fibers from its distal end to its proximal end.

8. A method of nonlinear lensless endo-microscopic imaging by means of a bundle of monomode optical fibers arranged in a given pattern, wherein, for each monomode optical fiber a relative group delay value is defined relative to a travel time of a pulse propagating in a reference monomode optical fiber of the bundle of fibers, the method comprising: emitting, using a light source, an incident beam comprising optical pulses having at least one wavelength towards a first spatial light modulator arranged in an object focal plane of a first lens, wherein the first lens forms with a second lens an optical layout with an intermediate focal plane; generating, using the first spatial light modulator, from the incident light beam a plurality N of elementary light beams, wherein each elementary light beam of the plurality of elementary light beams passes into a given waveplate configured to introduce a given delay and arranged in the intermediate focal plane of the optical layout, and enters into one of the optical fibers such that the sum of the delay introduced by said waveplate and the relative group delay of the optical fiber is minimal in absolute value; deviating, using a second spatial light modulator arranged in an image focal plane of the second lens, the elementary light beam such that the elementary light beam penetrates into the optical fiber perpendicularly to the entrance face of the optical fiber; applying a phase shift to each of the elementary beams, using one or the other of the first and second spatial light modulators, to imprint at the distal end of the bundle of fibers a predetermined phase function and/or to correct the phase variations introduced by each of the fibers of the bundle of fibers.

9. A method of nonlinear lensless endo-microscopic imaging by means of a bundle of monomode optical fibers arranged in a given pattern, wherein, for each monomode optical fiber a relative group delay value is defined relative to a travel time of a pulse propagating in a reference monomode optical fiber of the bundle of fibers, the method comprising: emitting, using a light source, an incident beam comprising optical pulses having at least one wavelength and generating, from said incident beam and using a plurality of M waveplates, wherein each waveplate of the plurality of waveplates introduces a given delay, a plurality of M of light beams, each of the M light beams pulses characterized by a given group delay, generating, using a first spatial light modulator arranged in an object focal plane of a first lens and from the M light beams, a plurality N of elementary light beams, wherein each elementary light beams is intended to enter into one of said optical fibers, such that the sum of the delay of the light beam from which is generated the elementary light beam and the relative group delay of the optical fiber is minimal in absolute value; deviating, using a second spatial light modulator arranged in an image focal plane of the first lens, the elementary light beam such that the elementary light beam penetrates into the optical fiber perpendicularly to the entrance face of the optical fiber; applying a phase shift to each of the elementary beams, using one or the other of the first and second spatial light modulators to imprint at the distal end of the bundle of fibers a predetermined phase function and/or to correct the phase variations introduced by each of the fibers of the bundle of fibers.

10. The method of nonlinear lensless endo-microscopic imaging according to claim 8, comprising: emitting at least two incident light beams comprising pulses having different wavelengths, wherein the first spatial light modulator furthermore allows a distributing of the elementary light beams formed of pulses having the same wavelength into a subset of fibers of the bundle of fibers.

11. The method of nonlinear lensless endo-microscopic imaging according to claim 9, comprising: emitting at least two incident light beams comprising pulses having different wavelengths, wherein said first spatial light modulator furthermore of distributes the elementary light beams formed of pulses having the same wavelength into a subset of fibers of the bundle of fibers.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other advantages and characteristics of the invention will appear from the perusal of the description, illustrated by the following figures:

(2) FIGS. 1A and 1B (already described), a schematic diagram of a so-called lensless endoscope based on the use of a bundle of monomode fibers and a diagram illustrating the problems of group delay in the fibers in the case of ultrashort pulses;

(3) FIGS. 2A and 2B, diagrams illustrating an example of a lensless endo-microscopic imaging system according to the present description;

(4) FIGS. 3A to 3D, figures illustrating an example of a multi-core optical fiber and its characterization by the implementing of a method of lensless endo-microscopic imaging according to the present description;

(5) FIG. 4, a diagram illustrating an example of waveplates for the implementing of a method of lensless endo-microscopic imaging according to the present description;

(6) FIGS. 5A and 5B, diagrams illustrating respectively the dispersion of the group delays in the multi-core fiber represented in FIG. 3A, before and after implementing a method of endo-microscopic imaging according to the present description;

(7) FIG. 6, first experimental results comparing the spatial appearance of the focal point at the exit of a multi-core fiber as represented in FIG. 3A with or without application of a method of endo-microscopic imaging according to the present description;

(8) FIG. 7, a diagram illustrating an example of a system of lensless endo-microscopic imaging according to another example of the present description;

(9) FIGS. 8A and 8B, diagrams illustrating examples of a system of lensless endo-microscopic imaging according to other examples of the present description;

(10) FIG. 9, a set of diagrams illustrating a method of distal measurement of the group delays in a bundle of monomode fibers;

(11) FIG. 10, a set of diagrams illustrating a method of proximal measurement of the group delays in a bundle of monomode fibers.

DETAILED DESCRIPTION

(12) In the figures, the identical elements are indicated by the same references.

(13) FIGS. 2A and 2B illustrate schematically a system 200 of lensless endo-microscopic imaging according to the present description as well as the principle of its implementation.

(14) The system 200 generally comprises an emission path, with a light source 10 for the emitting of ultrashort light pulses I.sub.0, typically less than a picosecond, for example between 100 femtoseconds and a picosecond, and a detection path adapted to detect the light intended to pass through the bundle of monomode optical fibers 40 from its distal end to its proximal end. The light detected is, for example, the light coming from the nonlinear process in the specimen after excitation. The detection path comprises a lens 21 and a detector 20 and it is separated from the emission path by a separating plate 22, such as a dichroic plate in the case of nonlinear imaging applications in which the detection wavelength (for example, two-photon fluorescence) is different from the emission wavelength.

(15) The system 200 likewise comprises a device for transporting and controlling the light pulses. According to the present description, the device for transporting and controlling the light pulses comprises an optical device 50 to control the group velocity, or a group delay control (GDC) device, a bundle of N monomode optical fibers F.sub.i, referenced 40, and advantageously an optical system 60 of the telescope type, making it possible to adapt the dimensions of the beam coming from the optical device for control of the group velocity 50 at the entrance face 41 of the bundle of fibers 40. In the example of FIG. 2A, the detection path is represented between the light source 10 and the GDC 50. The detection path could also be found between the GDC 50 and the bundle of fibers 40, for example, between the GDC 50 and the telescope 60.

(16) The N monomode optical fibers F.sub.i of the bundle of fibers 40 are arranged in a given pattern. In the example shown in FIGS. 2A and 2B, the monomode optical fibers F.sub.i are arranged in periodic manner; each fiber F.sub.i forms for example the core of a multi-core fiber or MCF. The bundle of optical fibers 40 comprises an entrance face 41 situated at the proximal side, that is, the side intended to receive an incident luminous flux, and an exit face 42 situated at the distal side, that is, the side intended to emit an outgoing light beam for the illumination of an object of analysis 101.

(17) Each optical fiber F.sub.i of the bundle of fibers is characterized by a relative group delay x.sub.i defined by the difference in the time it takes for an elementary beam B.sub.i formed by a light pulse to move through the fiber F.sub.i and the time it takes for an elementary beam formed from the same light pulse to move through a reference fiber F.sub.0 chosen arbitrarily in the bundle of fibers. The relative group delays x.sub.i thus describe the relative delays of the light pulses propagating in the optical fibers F.sub.i. The characterization of the relative group delays may be done by methods of characterization which are known and which shall be described in further detail below.

(18) According to the present description and as illustrated in general manner in FIG. 2B, the optical device for group velocity control GDC (50) is disposed at the proximal end of the bundle of monomode optical fibers 40 and is designed to reduce, at the distal end of the bundle of optical fibers, the relative discrepancy between the different elementary beams B.sub.i. Thus, the optical device for group velocity control 50 according to the present description is adapted to introducing, in the area of each elementary beam B.sub.i which is intended to enter into a monomode optical fiber F.sub.i of the bundle of fibers 40, a group delay which will compensate at least in part for the group delay x.sub.i characterizing the fiber F.sub.i, such that the relative group delays in the different elementary beams leaving the bundle of fibers 40 are close to zero and at least less than half the duration of the pulses intended to propagate in the bundle of fibers. As shown in FIG. 2B, the group velocity control X.sub.1(i) of the proximal end of the bundle of fibers results in a substantially constant distribution of the group velocities X.sub.2(i) at the distal end.

(19) FIG. 2A illustrates a first sample embodiment of an optical device for group velocity control GDC according to the present description.

(20) The optical device for group velocity control 50 in this example comprises a first lens 53 characterized by a focal distance f.sub.1 and a second lens 54 characterized by a focal distance f.sub.2. The lenses 53 and 54 are defined by any suitable optical system, for example by using lenses and/or mirrors. The first and second lenses 53, 54 are arranged to form an optical layout with an intermediate focal plane (.sub.1) coinciding with the image focal plane of the first lens 53 and the object focal plane of the second lens 54.

(21) The optical device for group velocity control 50 furthermore comprises a given number M of waveplates P.sub.j, advantageously between 2 and 20 plates, spatially distributed in a plane, this plane being, in the example of FIG. 2A, the intermediate focal plane (.sub.1). Each plate is designed to allow the introducing of a given delay t.sub.j.

(22) The velocity control device GDC also comprises a first spatial light modulator 51 adapted to form, from an incident beam formed by pulses I.sub.0 emitted by the light source 10, a number N of elementary light beams B.sub.i intended to enter into each of the N optical fibers F.sub.i of the bundle of fibers 40. In the example of FIG. 2A, the first spatial light modulator 51 is located in an object focal plane of the first lens 53 and is designed to imprint on each elementary beam B.sub.i a deviation such that each elementary beam B.sub.i passes into the appropriate waveplate P.sub.j. The appropriate waveplate P.sub.j is the one which imprints a delay t.sub.j such that the sum of the delay t.sub.j introduced by the waveplate P.sub.j and the relative group delay x.sub.i of the optical fiber F.sub.i which is intended to receive said elementary light beam B.sub.i is close to zero, regardless of the optical fiber F.sub.i or at least less than half the pulse duration. In practice, the number M of waveplates is much less than the number N of monomode optical fibers in the bundle of fibers 40 (for example, a multi-core fiber) and a large number of elementary beams B.sub.i will be imprinted with the same delay. One then seeks to minimize the variance of the histogram of all the values (t.sub.j+x.sub.i) where t.sub.j is the delay applied to the elementary beam B.sub.i which is intended to pass through the fiber F.sub.i characterized by a group delay x.sub.i, such as will be illustrated by way of an example below.

(23) The velocity control device 50 according to the present description likewise comprises a second spatial light modulator 52 adapted to deviating each of the N elementary light beams B.sub.i such that each elementary light beam B.sub.i penetrates into the corresponding optical fiber F.sub.i perpendicularly to the entrance face of the optical fiber. In the example of FIG. 2A, the second spatial light modulator 52 is located in an image focal plane of the second lens 54 and makes it possible to compensate for the deviation introduced into each elementary beam B.sub.i by the first spatial light modulator 51.

(24) In the simplified diagram of FIG. 2A, three elementary beams B.sub.1, B.sub.2, B.sub.3 are thus represented. These beams are formed from an incident beam at the first spatial light modulator 51, the incident beam being formed of pulses I.sub.0 emitted by the light source 10. The beams B.sub.1 and B.sub.2 intended to enter into the optical fibers F.sub.1 and F.sub.2 (not shown) of the bundle of fibers 40, characterized by the group delays x.sub.1 and x.sub.2, are deviated by the first spatial light modulator 51 and focused by the first lens 53 so as to pass through a waveplate P.sub.1 characterized by a delay t.sub.1, whereas the beam B.sub.3 intended to enter into the optical fiber F.sub.3 (not shown) of the bundle of fibers 40, characterized by a group delay x.sub.3, is deviated by the first spatial light modulator 51 and focused by the first lens 53 to move through a waveplate characterized by a delay t.sub.2. The elementary beams B.sub.1, B.sub.2, B.sub.3 are then sent by means of the second lens 54 to the second spatial light modulator 52 which imprints a deviation that compensates for the deviation imprinted by the first spatial light modulator 51 so that the elementary beams each exit with an optical axis perpendicular to the entrance face 41 of the bundle of fibers 40. The beams B.sub.1, B.sub.2, B.sub.3 are formed of light pulses respectively having delays t.sub.1, t.sub.1, t.sub.2 and which, after moving through the monomode optical fibers F.sub.1, F.sub.2, F.sub.3 will have zero or reduced relative differences in the group velocity.

(25) In the example of FIG. 2A, the elementary beams B.sub.i at the exit of the second spatial light modulator 52 are focused in a focal plane .sub.2 and an optical system 60 of telescope type makes it possible to apply a magnification absolutely less than 1 to adapt all of the focusing points formed in the focal plane .sub.2 to the pattern formed by the fibers F.sub.i in the area of the entrance face 41 of the bundle of fibers 40.

(26) According to one variant, the focusing of the elementary beams B.sub.i at the exit of the second spatial light modulator 52 in the focal plane .sub.2 is provided by the spatial light modulator 52 which introduces a parabolic phase into each elementary beam B.sub.i. Alternatively, the velocity control device 50 may comprise, at the output of the second spatial light modulator 52, an optical element (not shown), such as an array of microlenses, which can ensure the focusing of each elementary beam.

(27) The velocity control device 50 as described by means of FIGS. 2A and 2B thus makes possible, in simple fashion, a control of the group velocity for each of the monomode optical fibers F.sub.i of the bundle of fibers 40.

(28) Of course, this velocity control device, or GDC, may well be used to compensate for phase delays which have previously been characterized on the fibers of the fiber bundle and/or to imprint on each elementary beam a phase function which will allow the forming of the sought phase at the distal end of the bundle of fibers 40, for example, a parabolic function for the formation of a focus point.

(29) In the example of FIG. 2A, these functions could be provided by the one and/or the other of the first and second spatial light modulators 51, 52.

(30) In the example of FIG. 2A, the first and/or the second spatial light modulator could be formed by a base modulator with segmented deformable mirrors or membranes, operating by reflection, or by a liquid crystal matrix able to operate by reflection or by transmission.

(31) FIGS. 3 to 6 show initial experimental results obtained with an imaging system as is described in FIG. 2A and allowing a validation of the method according to the present description.

(32) In this example, the light source is a femtosecond laser, emitting pulses of 150 fs at a wavelength of 1.035 m. The device for transporting and controlling of the pulses comprises a bundle of monomode optical fibers formed here by a multi-core fiber.

(33) La multi-core fiber 40 used is illustrated in FIG. 3A. It comprises a group of 169 monomode cores F.sub.i arranged in periodic manner and referenced from a central fiber F.sub.0, as is shown in FIG. 3B. Each monomode core F.sub.i is intended to receive at its proximal end an elementary beam B.sub.i which passes through the core to exit at a distal end, as previously explained. The central core F.sub.0 forms the reference monomode fiber for the determination of the group delay x.sub.i characterizing each monomode core F.sub.i. The multi-core fiber likewise comprises in this example an internal multimode sheath 44 adapted to collecting the light signal from the distal end to the proximal end. In the example shown in FIG. 3A, the distance between cores is 11.8 m, the diameter of a mode in each monomode core is 3.6 m and the corresponding divergence is 0.12 radians; the diameter of the interior multimode sheath 44 is 250 m. The coupling measured between one monomode core F.sub.i and its closest neighbor is less than 25 dB, even with a curvature applied to the multi-core fiber of 12.5 cm radius.

(34) A characterization of the relative group delays of each of the monomode cores of the multi-core fiber 40 is carried out with the help of a known method, such as a method described by means of FIGS. 9 and 10. FIG. 3C thus represents the relative group delays x.sub.i measured for the cores of index i of the multi-core fiber. The group delay is defined as the difference between the time it takes for a light pulse to pass through the fiber F.sub.i and the time it takes for an identical light pulse to pass through the reference fiber F.sub.0. FIG. 3D shows the histogram of all the group delay values.

(35) As described with the aid of FIG. 2A, the velocity control device 50 makes it possible to partition the N elementary beams intended to enter into the N monomode cores of the multi-core fiber 40 into M groups on which M delay values will be imprinted by means of M waveplates P.sub.j.

(36) The M waveplates P.sub.j are formed for example by means of M1 glass plates of identical thickness, the plate of index j comprising j holes, each one able to let through a group of elementary beams; the M1 plates are stacked in order to make up a delay plate comprising M zones able to imprint, on the elementary beams, M delays t.sub.j. The holes can be made, for example, by laser ablation.

(37) Thus, FIG. 4 illustrates the realization of 3 waveplates P.sub.1, P.sub.2, P.sub.3 by means of 2 plates 56, 57 of substantially equal thickness, plate 56 having 2 holes and plate 57 only one hole, the plates being disposed so as to form 3 zones defining the 3 waveplates and which will imprint respectively delays of 0t.sub.g, 1t.sub.g, 2t.sub.g, where t.sub.g is the delay introduced by a pulse passing through a plate.

(38) The waveplates may also be formed by any other known means. For example, there may be M glass bars of equal diameter but different length. Each bar is able to let through a group of elementary beams. The bars are arranged, for example, opposite each other, making it possible to imprint, on the elementary beams, M delays t.sub.j. The length of a bar may be controlled, for example, by polishing. The waveplates may also be formed from a glass plate which is divided into M zones; by a method of micro-fabrication, each of the zones is hollowed out to form M zones of different thickness. The micro-engraving may be a dry engraving (Reactive Ion Etching) or a wet engraving (HF) or it may use a focused ion beam.

(39) As for the spatial light modulators, the waveplates can operate either by transmission or by reflection.

(40) Turning back to the example of FIGS. 3 to 6, each of the N elementary beams B.sub.i will thus pass through one of the 3 waveplates P.sub.1, P.sub.2, P.sub.3, depending on the value of the relative group delay x.sub.i of the fiber F.sub.i which it is intended to pass through. Since M is much smaller than N, a large number of elementary beams B.sub.i will be imprinted with the same delay in the intermediate focal plane .sub.1.

(41) FIGS. 5A and 5B show by histograms all of the values of the relative group delays in a case where there is no group velocity control device (FIG. 5A) and in the case where the group velocity control device is present (FIG. 5B). One observes a clear decrease in the variance from one histogram to the other, and this already with 3 plates introducing 3 distinct delay values.

(42) FIG. 6 shows the spatial appearance of a focal point at the exit from the multi-core fiber with application of the method for control of the group velocity (at left) and without application of that method (at right). In the bottom figures there is shown the image of the focal point, and in the top figures the spatial distribution of the intensity. Here again, these initial experimental results show the gain in intensity achieved thanks to the method according to the present description.

(43) FIG. 7 shows a diagram of one example of a system of lensless endo-microscopic imaging according to another example of the present description.

(44) This example is identical to that of FIG. 2A, but it shows the case of an aperiodic arrangement of the monomode optical fibers in the bundle of fibers 40. One observes that the device and the method for transporting and controlling of pulses according to the present description apply just as well to a bundle of fibers having fibers arranged in aperiodic manner.

(45) FIG. 8A illustrates a diagram of an example of a system of lensless endo-microscopic imaging according to another example of the present description.

(46) In the diagram described in FIG. 8A, one takes advantage of the fact that M<<N to first of all realize a differential delay between M subportions of the collimated incident beam. In FIG. 8A, the collimated incident beam is subdivided into M=2 subportions having a delay t.sub.1 or t.sub.2. This delay may advantageously be realized by a micro-structured plate, as previously described. It is then a matter of assigning, in the N fibers of the bundle of optical fibers, an elementary sub-beam with the chosen delay, here, t.sub.1 or t.sub.2. In this example, the first spatial light modulator 51 advantageously comprises a liquid crystal matrix. For example, one utilizes the additive property of holograms which consists in generating, at M zones of the first spatial light modulator 51, a set of holograms able to diffract the incident beam corresponding to the delay t.sub.i in different directions. These different directions appear as focus points in the plane of the second spatial light modulator 52 and the latter produces a deviation such that each elementary light beam penetrates perpendicularly at the entrance face of the optical fiber. The holograms formed in the area of each of the M zones of the first spatial light modulator are, for example, computer-generated holograms or CGH. Such holograms are described, for example, in Liesener et al., Multi-functional optical tweezers using computer-generated holograms, Opt. Commun., 185, 77 (2000).

(47) FIG. 8B illustrates a diagram of an example of a system of lensless endo-microscopic imaging similar to that of FIG. 8A, but used in an application implementing pulses of two wavelengths, for example, for applications in two-beam nonlinear imaging.

(48) According to this example, each fiber of the fiber bundle 40 is intended to transport an elementary beam at a given wavelength and the relative group delay of this fiber is advantageously characterized at this wavelength. In this example, the first spatial light modulator 51 moreover enables a distribution of the elementary light beams formed from pulses of a given wavelength into an identified subset of the fibers of the fiber bundle 40.

(49) In the example illustrated in FIG. 8B, the beam at the first wavelength .sub.1, indicated by single arrows, passes for example through two waveplates P.sub.1, P.sub.2 characterized by respective delays t.sub.1 and t.sub.2 and the beam at the second wavelength .sub.2, indicated by the double arrows, passes through two waveplates P.sub.3, P.sub.4 characterized by respective delays t.sub.3 and t.sub.4. As in the example of FIG. 8A, the first spatial light modulator 51 makes it possible to form N elementary beams, each elementary beam of given wavelength being characterized by a delay introduced by the plate through which it passes and intended to enter into a previously identified optical fiber of the fiber bundle de. For example, N/2 fibers of the fiber bundle receive elementary beams at the first wavelength .sub.1, whereas the remaining N/2 fibers of the fiber bundle receive elementary beams at the second wavelength .sub.2. In the example illustrated in FIG. 8B, six elementary beams are shown, three of them at the wavelength .sub.1 and three at the wavelength .sub.2. For example, these two groups of fibers are chosen such that the fibers transporting .sub.1 and .sub.2 are interlaced at the proximal face of the bundle of fibers. In the example illustrated in FIG. 8B, the interlacing is illustrated by the fact that, downstream from SLM2, the elementary beams alternate between .sub.1 and .sub.2.

(50) FIGS. 9 and 10 illustrate examples of the method for the characterization of the relative group delays in a bundle of fibers 40 of a device for transporting and controlling of light pulses according to the present description, for example for the characterization of a multi-core fiber. These methods are based on the known techniques of spectral interferometry (see, for example, Lepetit et al., Linear techniques of phase measurement by femtosecond spectral interferometry for applications in spectroscopy, J. Opt. Soc. Am. B, 12(12), 2467 (1995)). FIG. 9 illustrates a method suitable for a distal measurement of group delays, while FIG. 10 illustrates a method suitable for a proximal measurement of group delays, in which it is not necessary to have access to the distal end of the bundle of fibers.

(51) As illustrated in FIG. 9, the method for the characterization of the group delays implements a fiber spectrometer 90 and a spatial light modulator 91. The measurement of the relative group delay x.sub.i of a fiber Fi, defined with regard to the travel time of a pulse propagating in a reference fiber F.sub.0, comprises the following steps. Only the elementary beams B.sub.i and B.sub.0 intended to enter into the optical fibers Fi, F.sub.0 are formed. These pass through the optical fibers F.sub.i and F.sub.0 respectively. Upon emerging from the bundle of fibers 40 at the distal end, B.sub.i and B.sub.0 diverge and overlap spatially. In a plane where the overlapping is near total, an optical fiber 92 collects a portion of each beam. The optical fiber 92 relays the light collected to the spectrum analyzer 90. The spectrum comprises a sinusoidal modulation (curves 94), whose period is equal to (x.sub.i).sup.1; thus, one deduces the sought value, x.sub.i. In practice, in order to eliminate any background signal not coming from B.sub.i or B.sub.0, the spectrum is measured by the principle of phase shift interferometry, where the phase of B.sub.i (with respect to B.sub.0) is scanned by using the spatial light modulator 91, by the technique of phase shift interferometry (see, for example, Bruning et al., Digital Wavefront Measuring Interferometer for Testing Optical Surfaces and Lenses, Appl. Opt. 13(11), 2693 (1976), equations (3-6)).

(52) It is likewise possible to measure x.sub.i without having access to the distal portion of the bundle of fibers 40 as illustrated in FIG. 10. In fact, around 3% of B.sub.0 and B.sub.i is reflected by the distal face of the bundle of fibers, due to the difference in indices of refraction at the interface between the bundle of fibers and air. The reflected beams B.sub.0 and B.sub.i, emerging from the proximal end of the bundle of fibers 40 may be routed to an optical fiber 92 by means of a separating plate 96 (such as a semi-reflecting plate or a polarization splitter cube). The measurement is done as previously described; in this case, one measures (2x.sub.i).sup.1 since the pulses make a round trip in the bundle of fibers.

(53) Although described through a certain number of detailed sample embodiments, the device for transporting and controlling of light pulses for so-called lensless endo-microscopic imaging as well as the systems and methods of lensless endo-microscopic imaging encompass different variants, modifications and improvements which may appear in obvious manner to the person skilled in the art, it being understood that these different variants, modifications and improvements are within the scope of the invention, as defined by the following claims.