Abstract:
An optical head for equipping the distal end of a flexible optical fiber bundle, designed to be urged into contact with an analyzing surface and including optical elements for focusing an excitation signal into a so-called excitation focal point located at a specific depth beneath the analyzing surface and for sampling a signal backscattered by the excitation focal point which is carried back by the fiber bundle. The head includes an optics-holder tube wherein are inserted on one side the distal end portion of the fiber bundle and on the other optical elements, the latter including a plate placed in contact with the end of the fiber bundle whereof the index is close to that of the fiber core and a focusing optical block, an output window being further provided adapted to provide index adaptation so as to eliminate parasitic reflection occurring on the analyzing surface.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a miniaturized optical head provided for equipping the distal end of a flexible optical fibre bundle, said head being intended to be placed in contact with an analyzing surface and adapted for focussing an excitation signal conveyed by said fibre bundle into an excitation focal point which can be situated at different depths relative to the contact surface of the head. The optical head is also adapted for collecting a backscattered signal originating from the subsurface excitation focal point in order for it to be carried by the fibre bundle in particular to detection means and means of analysis and digital processing of the signal. 
   The fields of application concerned are subsurface analysis devices which are confocal in character, the signals conveyed being in particular those of the field of imaging and/or spectroscopy, depending on the source or sources of excitation and detection means used. The confocal character results from the use of the same fibre or fibres for conveying the excitation signal and the backscattered signal. The device can be used for in situ biological analyses, on humans or animals, external, for example in the field of dermatology, or internal and accessible using the instrument channel of an endoscope into which the optical fibre bundle and the optical head can be introduced. It can also be used for cell analyses carried out ex vivo on samples. Moreover also, the optical head can be used for analyzing the interior of a manufactured device. 
   At present, the medical fields of gastroenterology, respirology, gynaecology, urology, otorhinolaryngology, dermatology, ophthalmology, cardiology and neurology are concerned. 
   The magnification of the optical head according to the present invention may or may not be unitary. It is the analysis and signal processing means provided on the side of the proximal end of the optical fibre bundle which allow the restitution of an image or a graph which can be interpreted by a user. 
   The sought objectives for the optical head are in particular the following:
         to have a minimal space requirement in particular in order to be able to be inserted into the instrument channel of an endoscope which in general possesses a diameter comprised between 2 and 4 mm and a given radius of curvature.   to provide a good quality backscattered signal in which aberrations are minimized;   to minimize parasitic reflections at the distal output of the fibre bundle;   to provide a spatial resolution of the excitation focal point of the order of 4 μm, or even less in the case of a non-unitary magnification, allowing the analysis and/or observation of a tissue at a cellular scale;   to be able to be brought into contact with the analyzing surface in order to avoid the problems linked with untimely movements; and   to allow a point focussed in a section plane XY situated at a given depth from the analyzing surface.       

   Miniaturization of the optical head is also advantageous in order to increase the precision of its positioning and also to minimize mechanical inertia in automated uses, for example in extension of a robot arm or telemanipulator. 
   2. Description of the Related Art 
   From the document WO 00/16151, an observation device is known comprising an optical focussing head at the distal end of a flexible channel of optical fibres comprising at the channel output successively three lenses: a ×10 microscope objective, a doublet of 150 mm focal length and a doublet of 50 mm focal length. 
   An optical head is also known, comprising a system of four lenses, the first lens and the fourth lens being two ×10 microscope objectives and the second and third lenses two doublets of 150 mm focal length constituting an afocal system of magnification  1 . 
   These optical systems have the following major drawbacks:
         this type of construction, based on sophisticated microscope objectives (which can contain up to twelve lenses) cannot be miniaturized in order to be introduced into an endoscope instrument channel with a diameter of 2 to 4 mm;   the lateral resolution is of the order of 8 μm, insufficient for analyzing a tissue on the cellular scale;   in the case of confocal imaging, with illumination and scanning of the fibres one by one, a distortion of the image formed is observed (“ballooning” of the lines).       

   BRIEF SUMMARY OF THE INVENTION 
   The aim of the present invention is to overcome these drawbacks and to achieve the objectives mentioned above. 
   This aim is achieved with a miniaturized optical head provided for equipping the distal end of a flexible optical fibre bundle, said optical head being intended to come into contact with an analyzing surface and comprising optical means for focussing an excitation signal coming out of said fibre bundle into a so-called excitation focal point situated at a given depth beneath the analyzing surface and for collecting a signal backscattered by the excitation focal point which is carried back by said fibre bundle, characterized by an optics-holder tube, circular in section, wherein are inserted on one side the distal end portion of the fibre bundle and on the other the optical means, the latter comprising a plate placed in contact with the end of the fibre bundle whereof the index is close to that of the fibre core and a focussing optical block, an output window being moreover intended to come into contact with the analyzing surface and adapted to provide index adaptation in order to eliminate parasitic reflection occurring on the analyzing surface. 
   The optical block comprises a set of lenses which can be standard, the positioning and the curvature of each lens are not allowing a coupling of the signal reflected by the lenses, in particular a coupling of more of 10 −5  relative to the fibre output signal. This makes it possible to avoid interference being caused to the signal originating from the sample observed by this reflected signal. To this end, each lens constituting the optical block possesses an anti-reflection treatment optimal to the working wavelength, and, moreover, it is placed in an extra-focal plane and has a curvature which allows rejection of the signal reflected outside the excitation fibre. The combination of the various lenses allows illumination of the analysis site as needed point by point whilst ensuring a good optical quality necessary for obtaining a high-resolution confocal image. 
   According to a first embodiment, the window is also inserted at the end of the optics-holder tube. 
   According to a second embodiment, allowing analysis at different depths, in particular between 50 and 400 μm, the window is carried by a mobile cap fitted onto the end of the optical head and displaceable using appropriate means, hydraulic, pneumatic, piezo-electric, motorized, electro-optical, etc. the space requirement of which remaining compatible with the miniaturization objective. 
   Other methods of displacement of the depth of the analysis plane can be envisaged, in particular the axial displacement of a mobile optical means provided in the optical block, this mobile optical means being able to be constituted by a refractive optics (standard or with a gradient index) or a diffractive optics. A piezoelectric motor can carry out the displacement of this mobile optical means. A hydraulic actuator can also be used. Another axial scanning mode can also consist of using an optical means specific to the optical block adapted to change the focal distance by the modification of its radius of curvature (or optical power). This optical means can be for example a liquid optical means. 
   For the observation and analysis of biological tissues which are highly diffusing and/or have cellular details requiring a very high spatial resolution, such as the nuclei of healthy cells, an optical head will be preferred with non-unitary magnification, in particular of 0.5 from the distal end of the image guide to the analysis plane. This makes it possible to improve the lateral and axial resolution, and to obtain a larger numerical aperture. 
   Thus, according to the invention, in order to obtain a miniaturization, the microscope objectives conventionally chosen in the focussing heads because of their excellent optical quality are replaced by a combination of mechanical and optical means optimized in order to obtain an optimal coupling of the fibre output signal, i.e. with an optimized transcription of the point spread function (PSF), a wave front quality limited by the diffraction (preferably of the order of λ/30 at the centre of the field to λ/20 at the edge of field) in order to thus obtain a minimization of the aberrations due to the use of more standard lenses for the focussing optical block. 
   For the observation of tissues which are only slightly diffusing and have details greater than 5 μm, in particular for only slightly diffusing biological tissues or manufactured objects such as integrated circuits, a unitary magnification head has the advantage of being simpler to produce and integrate, due to its symmetrical character, and as a result has a lower cost than that of the non-unitary magnification head. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     Other advantages and characteristics of the invention will become clear from the description which follows of a non-limitative embodiment, which description refers to the attached drawing in which: 
       FIG. 1  is an opto-mechanical longitudinal cross-section view of an optical head according to the invention; 
       FIG. 2  is an optical diagram illustrating an embodiment of the focussing optical block with unitary magnification; 
       FIG. 3  is an optical diagram illustrating an embodiment of the focussing optical block with non-unitary magnification; and 
       FIG. 4  is a cross-section view similar to  FIG. 1  illustrating an embodiment of an optical head with adjustable depth of the field. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   According to the embodiment chosen and represented in  FIG. 1 , the optical head comprises a mechanical structure wherein is introduced and fixed on the one side the distal end portion  1  of a bundle  2  of organized flexible optical fibres and on the other side optical means are accommodated, allowing the focussing of a signal issuing from one or more fibres of said fibre bundle. 
   The mechanical structure comprises an optics-holder tube  4  which is circular in section. The bundle  2  is constituted by flexible optical fibres which are organised in the same manner at the input and output of the bundle, and surrounded by a sheath  12 . A tubular metal joining piece  6  open at either end is coupled and adjusted on the end portion  1  of the bundle  2  in such a manner that the end  14  of the bundle  2  is flush with the end of the joining piece  6 . The joining piece  6  allows, prior to assembly in the optics-holder tube  4 , the polishing of the end  14  of the fibre bundle. To this end, the end portion  1  of the fibre bundle  2  comprises a bare portion  9 . Thanks to the most perfect possible surface condition of the end  14 , the parasitic reflections at the input and output of the fibres are minimized and the quality of the signal is enhanced. The joining piece  6  is inserted in an adjusted manner into the optics-holder tube  4 . On the side of the rear end  10  of the optical head, the fibre bundle  2  is fixed using a spot of suitable glue  11  (biocompatible and ensuring tightness) joining the sheath  12  of the fibre bundle  2 , the rear surface  13  of the joining piece  6  and the optics-holder tube  4 , the joining piece  6  being situated slightly retracted in the optics-holder tube. On the side of the bare end  9  of the fibre bundle  2 , the joining piece  6  has an annular collar  16  retracted relative to the outside surface of the joining piece defining one end  17  which is narrow in diameter. An opening  18  is present in the optics-holder tube  4  intended to face the narrow end  17  of the joining piece  6  in order to be able to adjust the position of the joining piece  6  and introduce a second spot of suitable glue  20 . This also allows the gluing to its periphery of an index adaptation plate  21 , with plane and parallel surfaces, said plate being placed in contact with the end  14  of the fibre bundle  2  and the end  17  of the joining piece  6 . The diameter of the plate  21  corresponds to the internal diameter of the optics-holder tube  4 . The characteristics of the plate  21 , its nature and thickness, are chosen in order to obtain a good compromise between the level of backscattering and sufficient resistance for mechanical integration. Its index is chosen to be very close to that of the core of the fibres. The plate  21  thanks to this index and the choice of its thickness makes it possible to minimize and reject from the focal plane, the reflection occurring at the distal end of the image guide by carrying out an index adaptation. In contact with the periphery of the plate  21  there is provided a tubular spacer  22  used to space by a given length a focussing optical block  3  (which is described hereafter in detail), followed in contact with a second tubular spacer  26  used to space an output window  30 . In this front end part  19  of the optical head, the optics-holder tube  4  has an internal recessed annular collar  27 , against which the rear surface of the spacer  26  can be supported. Similarly an annular collar  28  is provided in the internal surface of the spacer  26  against which is positioned the periphery of the rear surface of the output window  30 . The end of the spacer  26  and the window  30  are flush with the end  19  of the optical head. The output window  30  is a plate with parallel and plane surfaces, having here also a thickness sufficient to ensure a good resistance during the mechanical insertion. It is glued at its periphery in contact with the spacer  26 . When it is intended to come into contact with a tissue, the window is chosen in order to be chemically neutral. The window allows at the same time to realise an index adaptation relative to the observation site in the same manner as at the output of the fibre bundle  2 , which produces a minimization of the reflection occurring on the analyzing surface. In the case of the observation of a biological tissue, an anti-reflection treatment in water can moreover be carried out in order to be better adapted to the index of the tissues, and thus to improve the image contrast. The optical system is according to the invention telecentric in the image space. 
   The assembly of the optical head is carried out in the following manner: the joining piece  6  is fitted onto the end portion of the optical fibre bundle having a bare end portion; this assembly is then inserted and adjusted in the optics-holder tube  4  conforming the opening  18  of said tube  4  with the narrow portion  17  of the joining piece  6 ; at the other end of the optics-holder tube  4 , the plate  21  is fitted on so that it comes into contact with the end  14  of the fibre bundle; then the spacer  22 , the optical block  3 , the spacer  26  and finally the window  30  are introduced; spots of glue  11  and  20  are applied in order to fix the joining piece  6  and the plate  21 . 
   The optical block  3  comprises a set of lenses having the function of focussing an excitation beam into an excitation focal point situated in a subsurface analysis plane XY perpendicular to the optical axis. The choice of the position (in an extra-focal plane), the curvature and of an optimal anti-reflection treatment makes it possible to avoid the signal reflected by the lenses causing interference to the signal originating from the sample (the coupling of the reflected signal must not exceed 10–5 relative to the fibre output signal). 
   By way of example,  FIG. 2  shows an optical block  3  with unitary magnification comprising symmetrically on either side of a bi-concave lens  31  of BK7, glass, beyond the plate  21 : a meniscus  32  of SF6 glass, a bi-convex lens  33  of BK7 glass and a planoconvex lens  34  of SF6 glass, and upstream of the output window  30  a planoconvex lens  35  of SF6 glass, a bi-convex lens  36  of BK7 glass and a meniscus  37  of SF6 glass. 
     FIG. 2  shows diagrammatically the optical path of an excitation beam emerging from the optical fibre bundle. A first optical path T 1  of a principal beam centred on the optical axis of the system is represented as a full line and a second optical path T 2  of a beam emerging from an optical fibre or group of fibres not situated on the optical axis as a dotted line. The beam emerging from the window  30  converges in an excitation focal point, for example PT 1  or PT 2 , situated in a subsurface analysis plane XY. The signal backscattered by the excitation focal point then allows the same optical path in the reverse direction. 
   The detailed characteristics (curvature, position etc.) of the different lenses  31  to  37  according to a particular embodiment as well as of the plate  21  and of the output window  30  are given in Table 1 hereafter. 
   
     
       
             
           
             
             
             
             
             
             
             
           
             
             
             
             
             
             
             
           
         
             
               TABLE 1 
             
           
           
             
                 
             
             
               (M = 1) 
             
           
        
         
             
                 
               Surface 
               Type 
               Radius 
               Thickness 
               Class 
               Diameter 
             
             
                 
                 
             
           
        
         
             
                 
               OBJ 
               STANDARD 
               Infinite 
               −100 
                 
               0.7 
             
             
                 
               STO 
               STANDARD 
               Infinite 
               100 
                 
               92.55952 
             
             
               21 
                2 
               STANDARD 
               Infinite 
               0.5 
               BK7 
               0.7 
             
             
                 
                3 
               STANDARD 
               Infinite 
               0.3 
                 
               0.9907993 
             
             
               32 
                4 
               STANDARD 
               −0.8862573 
               0.8 
               BF6 
               1.094269 
             
             
                 
                5 
               STANDARD 
               −1.201577 
               0.2 
                 
               2 
             
             
               33 
                6 
               STANDARD 
               6.25473 
               0.8 
               BK7 
               2.3 
             
             
                 
                7 
               STANDARD 
               −2.246746 
               0.2 
                 
               2.3 
             
             
               34 
                8 
               STANDARD 
               2.819419 
               0.8 
               BF6 
               2.3 
             
             
                 
                9 
               STANDARD 
               Infinite 
               0.4 
                 
               2.3 
             
             
                 
               10 
               STANDARD 
               −2.12778 
               1 
               BK7 
               2.3 
             
             
               31 
               11 
               STANDARD 
               Infinite 
               0 
                 
               2.3 
             
             
                 
               12 
               STANDARD 
               Infinite 
               1 
               BK7 
               2.3 
             
             
                 
               13 
               STANDARD 
               2.12778 
               0.4 
                 
               2.3 
             
             
               35 
               14 
               STANDARD 
               Infinite 
               0.8 
               BF6 
               2.3 
             
             
                 
               15 
               STANDARD 
               −2.819419 
               0.2 
                 
               2.3 
             
             
               36 
               16 
               STANDARD 
               2.246746 
               0.8 
               BK7 
               2.3 
             
             
                 
               17 
               STANDARD 
               −6.25473 
               0.2 
                 
               2.3 
             
             
               37 
               18 
               STANDARD 
               1.201577 
               0.8 
               BF6 
               2 
             
             
                 
               19 
               STANDARD 
               0.8862573 
               0.3 
                 
               1.117908 
             
             
               30 
               20 
               STANDARD 
               Infinite 
               0.5 
               BK7 
               1.029353 
             
             
                 
               21 
               STANDARD 
               Infinite 
               0.08 
               1.330000 
               0.7554534 
             
             
                 
                 
                 
                 
                 
               62.00000 
             
             
                 
               22 
               STANDARD 
               Infinite 
               0 
                 
               0.7049318 
             
             
                 
               IMA 
               STANDARD 
               Infinite 
                 
                 
               0.7049318 
             
             
                 
             
           
        
       
     
   
   The construction according to the invention can be miniaturized while allowing a very good quality signal, as shown by the characteristics hereafter, given by way of example, for an optical head having the characteristics which have just been described with reference to  FIG. 1  and intended to be inserted into the instrument channel of an endoscope, and utilizing at the proximal end of the signal guide confocal imaging means comprising: a light source (for example a pulsed laser), scanning means for injecting the bundle produced fibre by fibre in addressed manner, means for time and spatial filtering of the backscattered signal, detection means, signal-processing means and image-display means, as described in particular in the International Application WO 00/16151. 
   Characteristics of an optical head according to the invention for a coloscope or gastroscope: 
   Dimensions:
         2.5 mm external diameter for the optics-holder tube;   a fibre bundle  2  for example of Sumitomo® trademark constituted by 30,000 fibres with a core diameter of 2.5 μm and of inter-core spacing of 4 μm or of Fujikura® trademark constituted by 30,000 fibres with a core diameter of 1.9 μm and inter-core spacing of 3.3 μm;   an optical block  3  having 1.8 mm in diameter;   a length L (see  FIG. 1 ) between the signal guide fibre output and the external surface of the output window  30  of 8.75 mm, with a front lens varying from 50 to 150 μm;   a total length comprising L and the rigid mechanical joining to the optical fibre bundle of 16.6 mm, compatible with the radius of curvature of the instrument channel of a standard coloscope (Rc=40 mm);   0.5 mm thickness for the index adaptation plate  21  and for the output window  30 , sufficient during the mechanical insertion and allowing a backscattering level of the order of 3.10 −4 .       

   Operating temperature: 37° C. 
   Image quality
         image quality close to the diffraction limit; the WFE, “wave front error”, throughout the field is of the order of λ/30 at the centre of the field to λ/20 at the edge of field; this excellent image quality ensures a good return coupling in the excitation fibre (˜90%);   MTF (modulation transfer function): this corresponds to the relative intensity as a function of the spatial frequency. The cut-off frequency is defined by 1/(2d) where d corresponds to the inter-core distance of the fibres, and is expressed in cycles/mm. Here, with an inter-core distance of 4 μm, the cut-off frequency is 125 cycles/mm. The MTF allows evaluation of the quality of the image by comparing the curve to that of the diffraction limit, and using the criterion according to which the contrast must be 0.5 (value of the relative intensity given by the curve) at the maximum spatial frequency of the device, at a rate of 125 cycles/mm in the present case. The result obtained here is effectively close to the diffraction limit, having a contrast of 0.75 at the spatial frequency of 125 cycles/mm, therefore ensures a very good image quality;   Encircled energy: this allows evaluation of the lateral resolution that can be expected, by evaluating the percentage of energy contained in a diameter. In order to resolve a spot with a diameter of φ, 50% of the minimum energy must be contained in this diameter. In the present case, 50% of the energy originating from the object point is encircled in a diameter of 1.5 μm, whatever its position in the field. 50% of the energy originating from an optical fibre in the signal guide (with a core diameter of 2.5 μm) is therefore encircled in a diameter of 4 μm.   Curvature of field, distortion: The image is curved from 31 μm to 35 μm between the centre and the edge of the field. The residual curvature of field is very low (of the order of 2 μm), as well as the distortion (of the order of 0.8%).       

   Transmission
         on a path: of the order of 0.97%.       

   Thus, the solution proposed according to the invention can be effectively miniaturized and makes it possible to obtain a very good quality image having an expected lateral resolution (namely 4 μm) and to optimize the signal-to-noise ratio by minimizing the parasitic reflection at the image guide output, by optimizing the return coupling level and the transmission of the system. This solution resolves the problem posed and offers the advantages of simplicity of assembly and low cost. 
   It goes without saying that variants of the invention are possible, in particular  FIG. 3  shows a focussing optical block  3  with 0.5 magnification (the same references are used for the elements shared with  FIG. 1 ). Beyond the index adaptation plate  21  there are arranged successively a meniscus  40  of SF6 glass, a planoconvex  41  of BK7 glass, a planoconvex  42  of SF6, a planoconcave  43  of BK7, a planoconcave  44  of BK7, a planoconvex  45  of SF6, a bi-convex lens  46  of BK7, a meniscus  47  of SF6 and a meniscus  48  of SF6. As in  FIG. 2 , there are represented here three optical paths emanating from a different fibre of the bundle: T′ 1 , centred on the optical axis, forming a focussing point PT′ 1  in a subsurface plane P′, and T′ 2  and T′ 3 , non-centred marginal rays forming respectively a focusing point PT′ 2  and PT′ 3  in the plane P′. 
   The detailed characteristics according to a particular embodiment (curvature, position etc.) of the different lenses  40  to  48  as well as the plate  21  and the output window  30  are given in Table 2 hereafter. 
   
     
       
             
           
             
             
             
             
             
             
             
           
             
             
             
             
             
             
             
           
         
             
               TABLE 2 
             
           
           
             
                 
             
             
               (M = 0.5) 
             
           
        
         
             
                 
               Surface 
               Type 
               Radius 
               Thickness 
               Class 
               Diameter 
             
             
                 
                 
             
           
        
         
             
                 
               OBJ 
               STANDARD 
               Infinite 
               −100 
                 
               0.7 
             
             
                 
               STO 
               STANDARD 
               Infinite 
               100 
                 
               92.55952 
             
             
               21 
                2 
               STANDARD 
               Infinite 
               0.3 
               BK7 
               0.7 
             
             
                 
                3 
               STANDARD 
               Infinite 
               0.3 
                 
               0.8744796 
             
             
               40 
                4 
               STANDARD 
               −0.8862573 
               1.3 
               BF6 
               1.044886 
             
             
                 
                5 
               STANDARD 
               −1.201577 
               0.15 
                 
               2 
             
             
               41 
                6 
               STANDARD 
               6.25473 
               0.8 
               BK7 
               2.3 
             
             
                 
                7 
               STANDARD 
               −2.246746 
               0.15 
                 
               2.3 
             
             
               42 
                8 
               STANDARD 
               2.819419 
               0.8 
               BF6 
               2.3 
             
             
                 
                9 
               STANDARD 
               Infinite 
               0.5 
                 
               2.3 
             
             
               43 
               10 
               STANDARD 
               −2.12778 
               0.8 
               BK7 
               2.3 
             
             
                 
               11 
               STANDARD 
               Infinite 
               1.1 
                 
               2.3 
             
             
               44 
               12 
               STANDARD 
               Infinite 
               0.6 
               BK7 
               2.167773 
             
             
                 
               13 
               STANDARD 
               2.12778 
               0.35 
                 
               2.38508 
             
             
               45 
               14 
               STANDARD 
               Infinite 
               0.6 
               BF6 
               2.529293 
             
             
                 
               15 
               STANDARD 
               −2.819419 
               0.1 
                 
               2.774485 
             
             
               46 
               16 
               STANDARD 
               2.246746 
               0.7 
               BK7 
               3.173711 
             
             
                 
               17 
               STANDARD 
               −6.25473 
               0.1 
                 
               3.180204 
             
             
               47 
               18 
               STANDARD 
               1.201577 
               0.7 
               BF6 
               2.856758 
             
             
                 
               19 
               STANDARD 
               0.8862573 
               0.1 
                 
               2.636245 
             
             
               48 
               20 
               STANDARD 
               Infinite 
               0.7 
               BF6 
               1.924121 
             
             
                 
               21 
               STANDARD 
               Infinite 
               0.3 
                 
               1.064745 
             
             
               30 
               22 
               STANDARD 
               Infinite 
               0.3 
               BK7 
               0.85978848 
             
             
                 
               23 
               STANDARD 
               Infinite 
               0.08 
               1.330000 
               0.5069504 
             
             
                 
                 
                 
                 
                 
               62.00000 
             
             
                 
               24 
               STANDARD 
               Infinite 
               0 
                 
               0.3947683 
             
             
                 
               IMA 
               STANDARD 
               Infinite 
                 
                 
               0.3947683 
             
             
                 
             
           
        
       
     
   
   The non-unitary magnification, in this case 0.5 from the distal end of the image guide up to the analysis plane in this example of use, makes it possible to obtain: 
   a better lateral resolution (PSF of 0.75 μm for an extended object with a diameter equal to the core diameter of a fibre (1.9 μm), compared with 1.4 μm for an optical head with unitary magnification). 
   A better axial resolution (of the order of 5 μm compared with 10 μm for the optical head with unitary magnification) 
   A lager illumination numerical aperture (of the order of 0.9 compared with 0.42 for the optical head with unitary magnification), and as a result a more contrasted image. 
     FIG. 4  shows another embodiment of an optical head according to the invention comprising hydraulic-type means for varying the depth of the analysis plane P. The elements similar to those in  FIG. 1  have the same references. The head differs from that in  FIG. 1  in that the window  30  is carried by a cap, having the overall reference  50 , which is fitted onto the optical head. This cap comprises an end portion  51  with a skirt  52  and a front wall  53  in which is fitted an opening  54  with an annular flange  55  adapted for receiving the window  30 , the periphery of the latter being glued onto the flange  55  with a suitable glue. The external diameter of this end portion  51  can be approximately 3 mm, a dimension compatible with the instrument channel of an endoscope. The skirt  52  is fitted onto a so-called intermediate tubular portion  58  of the cap  50 , coupling means being provided between theses two portions comprising on the internal surface of the skirt  52 , a recessed annular flange  56 , and on the external surface of the intermediate portion  58  a collar  59 , a compressible ring seal  60  being arranged between said portions, ensuring the tightness of the coupling. Finally, the cap  50  comprises a rear portion  61 , intended for coupling to an air supply, the diameter of the front end of which  62  is enlarged in order to be coupled onto the rear end of the intermediate portion  58  and the rear diameter  63  is narrow in order to be adapted to the diameter of the optical fibre bundle  2 . The cap  50  has an internal diameter, which is globally greater than the external diameter of the optics-holder tube, in such a manner that a space is arranged between the cap  50  and the optical head which is provided for connection to the air supply. Thus, according to the invention, the adjustment of the position of the subsurface focal plane is carried out not by modifying the position of the lenses inside the optical block  3  but by modifying the position of the window  30  relative to said optical block  3 , thanks to a mobile cap  50  actuated pneumatically, carrying said window. 
   The head which has just been described also differs from that described with reference to  FIG. 1 , in that no spot of glue is provided such as  20  in  FIG. 1  for fixing the end of the joining piece  6 . The fixing is carried out here using the spot of glue  11  behind the head and a ring  65  fixed at the end of the optics-holder tube  4  against a collar.