Abstract:
The invention is directed to a miniaturized optically imaging system with high lateral and axial resolution for endomicroscopic applications. To provide a miniaturized optical head which permits an appreciable increase in photon efficiency with high lateral and axial spatial resolution compared to conventional GRIN optics the plane side of a refractive, plano-convex, homogeneous lens defines a plane entrance surface of the optical system, and first and second GRIN lenses are arranged along the optical axis orthogonal to the entrance surface, wherein the first GRIN lens being arranged downstream of the refractive lens for reducing the divergence of the highly divergent light bundle transmitted from the object through the refractive lens, and the second GRIN lens being provided for adapting the light bundle transmitted by the first GRIN lens to the aperture and object field size of the downstream transmission system.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority of German Application No. 10 2006 046 555.5, filed Sep. 28, 2006, the complete disclosure of which is hereby incorporated by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    a) Field of the Invention 
         [0003]    The invention is directed to a miniaturized optically imaging system with high lateral and axial resolution, particularly for endoscopic applications, which uses at least one GRIN lens system with a radial refractive index profile which generates an optical imaging of near-surface areas of an object in an image plane for an optical transmission system. 
         [0004]    b) Description of the Related Art 
         [0005]    Miniature optics play an important role for microscopy imaging, particularly in endomicroscopy. Endomicroscopic examination instruments are characterized by an optical image recording head which is small in diameter (a few millimeters) and in which the image must be transmitted over very long distances compared to the diameter. 
         [0006]    Miniature optics of this kind image the surface of the object at high resolution in the micrometer range. The image is generated in transmitted light mode, in fluorescence microscopy, reflection microscopy, confocal microscopy or by scanning multiphoton fluorescence excitation. 
         [0007]    In applications of this type, gradient-index (GRIN) lenses are used for the construction of miniature optics based on experience because they have a remarkably good imaging quality with typical small diameters between 0.2 and 2.0 mm and can be favorably integrated in small lens systems because of their generally plane optical surfaces. In GRIN lenses, the optical image-generating action is based on building a refractive index profile in the lens medium and, therefore, the refraction between two media (e.g., between air and glass) at curved surfaces is not necessarily needed for image formation. Therefore, miniature optics are comparatively simple to assemble. Miniaturized optics of this kind are known, for example, for endoscopic applications from EP 1 283 413 B1 and for laser scanning microscopes from DE 100 15 455 A1. 
         [0008]    The solutions mentioned above are disadvantageous with respect to the imaging quality that can be achieved, which is clearly deficient primarily because of a low photon efficiency, a limited lateral and axial resolving capacity (due to the known relatively low numerical aperture of GRIN lenses of about 0.55) and the chromatic aberration of the GRIN lenses. 
       OBJECT AND SUMMARY OF THE INVENTION 
       [0009]    It is the primary object of the invention to provide a novel possibility for realizing a miniaturized optical head, particularly for endomicroscopic applications, which permits an appreciable increase in photon efficiency with higher lateral and axial spatial resolution compared to conventional GRIN optics and avoids the need for complicated multi-lens systems. A further object consists in correcting chromatic aberration in optical imaging which can generally not be corrected particularly in GRIN optics. 
         [0010]    In a miniaturized optically imaging system with high lateral and axial resolution, particularly for endoscopic applications, which uses at least one GRIN lens system with a radial refractive index profile which generates an optical imaging of near-surface areas of an object in an image plane for an optical transmission system with a small diameter compared to the transmission length, the above-stated object is met, according to the invention, in that a refractive plano-convex lens in the form of a homogeneous optical spherical lens element is provided and, together with the GRIN optics, forms an optical system, wherein the plane side of the spherical lens element defines a plane entrance surface of the optical system, and the entrance surface is oriented orthogonal to an optical axis common to both the optical system and the transmission system, in that the spherical lens element is followed by a first GRIN lens for reducing the divergence of the highly divergent light bundle from the spherical lens element, and in that a second GRIN lens for adapting the light bundle transmitted by the first GRIN lens to the aperture and object field of the downstream transmission system is arranged along the optical axis downstream of the first GRIN lens. 
         [0011]    A diffractive lens is advantageously provided in a weakly divergent area of the light bundle of the optical system for correcting the chromatic aberration of the refractive spherical lens element and GRIN optics. The diffractive lens is preferably arranged between the first GRIN lens and the second GRIN lens. 
         [0012]    In an alternative variant, the diffractive lens is advisably arranged between the refractive lens and the object-side first GRIN lens for correcting the chromatic aberration of an optical system. 
         [0013]    The optical system which comprises at least a spherical lens element and a first and second GRIN lens is preferably arranged inside a cylindrical mechanical mount as a miniature optical head, the mechanical mount being hermetically closed by a window as entrance surface in front of the plane surface of the spherical lens element. 
         [0014]    All of the optical components of the optical system are advantageously arranged along the optical axis without gaps. 
         [0015]    For endomicroscopic applications, the miniature optical head is advisably completed by an image-conducting fiber bundle or rigid relay optics arranged downstream of the second GRIN lens on the optical axis. However, an individual optical fiber can also be arranged downstream of the second GRIN lens and is moved with respect to the optical system of the head by means of a microscanner so that it scans an image. 
         [0016]    The transmission system following the second GRIN lens is advantageously fastened in the mechanical mount of the miniature optical head by means of an adapter sleeve. In so doing, a thin excitation fiber for coupling in light is advantageously aligned by means of the adapter sleeve, and either a reception fiber is provided around the thin excitation fiber concentric to the optical axis and aligned by the adapter sleeve or the thin excitation fiber on the optical axis is simultaneously used as a reception fiber. 
         [0017]    In a first construction of the invention, the first GRIN lens and the second GRIN lens advantageously have the same refractive index profile. 
         [0018]    However, it may also be expedient for the first GRIN lens and the second GRIN lens to have different refractive index profiles to adjust magnifications and to correct aberrations. 
         [0019]    In a specific construction of the invention, the second GRIN lens is advantageously replaced by the conventional optics of a microscope body in which the miniature optical head replaces the microscope objective. 
         [0020]    In another construction for which the requirements for chromatic correction are less stringent and in which, consequently, no diffractive lens is required, e.g., in monochromatic operating mode with integral signal reception in case of multiphoton microscopy or when using a fiber-optic scanner as transmission system to the sensor, the first GRIN lens and the second GRIN lens can also advisably be combined to form one GRIN lens. 
         [0021]    Particularly for endoscopic application, it is often advisable to arrange a deflecting element between the first GRIN lens and the second GRIN lens, e.g., for lateral scanning of cavities. The deflecting element is preferably a reflective prism. However, the deflecting element can also advantageously be a beamsplitter, preferably to separate excitation light from radiation emitted by the object. For this purpose, the beamsplitter can advisably have a dichroic coating between two equilateral prisms for spectral separation of the excitation radiation from the object emission. 
         [0022]    In a miniaturized optically imaging system, particularly for endoscopic applications, which uses at least one GRIN lens system with a radial refractive index profile which generates an optical imaging of near-surface areas of an object in an image plane for an optical transmission system with a small diameter compared to the transmission length, the above-stated further object is met in a reduced variant of the invention in that a first GRIN lens is provided for reducing the divergence of the highly divergent light bundle from the object, in that a second GRIN lens is arranged along the optical axis downstream of the first GRIN lens for adapting a light bundle transmitted from the first GRIN lens to the aperture and object field of the downstream transmission system, in that the first GRIN lens defines a plane entrance surface of the optical system comprising the first GRIN lens and second GRIN lens, wherein the entrance surface is oriented orthogonal to an optical axis common to the optical system and to the transmission system, and in that a diffractive lens is provided between the first GRIN lens and the second GRIN lens for correcting the chromatic aberration of the optical system. 
         [0023]    Through the combination of spherical lens element and GRIN lens, the numerical aperture of the miniature optics can be increased appreciably (up to NA=0.85). The spherical lens element and the GRIN lens should be considered as a system because the refractive index profile of the GRIN lens is shaped in such a way that the strong spherical aberration introduced by the spherical lens element is corrected by the GRIN lens. 
         [0024]    Typically, spherical lenses and GRIN lenses have a normal chromatic aberration in which short wavelengths have a shorter focal length than longer wavelengths. A correction of the chromatic aberration of the GRIN lens(es) can be achieved with or without combining with the spherical lens element by means of a diffractive-optical lens element resulting in a total system that is achromatized for two wavelengths. 
         [0025]    Since diffractive lenses have a strong abnormal chromatic aberration in which the focal length f=C/λ is scaled to the reciprocal of the wavelength λ and the proportionality constant C is determined by the geometric structure of the diffractive lens, diffractive lenses have a longer focal length with short wavelengths than with larger wavelengths and are accordingly best suited for compensation of the normal chromatic aberration of GRIN lenses. 
         [0026]    The selection of achromatized wavelengths is suitably adapted depending on application and, for example, can be in the near infrared spectral region in the case of a two-photon microscope for a wavelength in the region of the excitation light and in the region of the half-wavelength of the excitation light for the other wavelength in the region of two-photon fluorescence. In a single-photon laser scanning microscope it can be useful for one wavelength to be the excitation wavelength of the laser and the other wavelength to be in the region of the fluorescence of the object or of the dye that is used. 
         [0027]    The invention makes it possible to realize a miniaturized optical head, particularly for endomicroscopic applications, which has a high lateral and axial spatial resolution and an appreciably increased photon efficiency compared to conventional GRIN optics without using complicated multi-lens systems. The miniature optical head according to the invention has a strongly increased numerical aperture on the object side which causes the substantial increase in photon efficiency and optical resolution. The magnification and image-side numerical aperture of the miniature optical head can be adjusted so as to be favorably adapted to endoscopic image transmission systems. 
         [0028]    Further, the optical system has minimized image errors such as spherical and chromatic aberrations and field curvature, and when using GRIN lenses typical chromatic aberration can be sufficiently compensated in a simple manner by combining with diffractive lenses. 
         [0029]    The invention will be described more fully in the following with reference to embodiment examples. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0030]    In the drawings: 
           [0031]      FIG. 1  shows a general view of the miniature optical system according to the invention; 
           [0032]      FIG. 2  shows a variant of the invention as an endoscopic head with image transmission optics; 
           [0033]      FIG. 3  shows a variant of the invention as an endoscopic head with integral detection; 
           [0034]      FIG. 4  shows a construction of the miniature optical system for particularly simple mounting without gaps; 
           [0035]      FIG. 5  shows a particular construction of the miniature optical system as exclusively achromatic GRIN optics; 
           [0036]      FIG. 6  shows a specific construction of the miniature optical system as input optics for a microscope body; 
           [0037]      FIG. 7  shows a variant of the optical system with a beam-deflecting element for lateral scanning surfaces, particularly in endoscopic applications; and 
           [0038]      FIG. 8  shows a configuration of the miniature optical system with a beamsplitter for separating excitation light and object radiation. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0039]    For imaging an object  1  with high spatial resolution, a miniature optical system  2  according to the invention substantially comprises, as is shown in  FIG. 1 , an entrance surface  21 , which is preferably placed on the object  1  in the form of the plane side of a plano-convex spherical lens element  22 , and GRIN optics  23  which are arranged downstream of the spherical lens  22  along a common optical axis  24  of the optical system  2 . Depending on application-oriented calculation of the total system  2 , its object plane  25 , on which the object points O 1  and O 2  selected in  FIG. 1  lie, can be located at various depths in the volume of the object  1 . 
         [0040]    If it is necessary to compensate for the normal chromatic aberration of the refractive spherical lens element  22  and GRIN optics  23  which leads to a focal length that is shorter with small wavelengths than with larger wavelengths, a diffractive lens  27  is introduced in the beam path of the GRIN optics  23 . Diffractive optical elements which are usually constructed as a diffractive microstructure (e.g., diffraction grating) have a strong abnormal chromatic aberration in which the focal length is scaled to the reciprocal of the wavelength λ (f=C/λ) so that—with corresponding adjustment of the structure constant C of the diffractive lens  27 —an optical system  2  can be adjusted to be achromatized for at least two wavelengths. 
         [0041]    To the extent that achromatization is required, the diffractive lens  27  is advisably positioned in an area with low beam divergence in order to achieve a high color-correcting diffraction efficiency with a small thickness of the diffractive lens  27 . As is shown in  FIG. 1 , an area of this kind is present when the GRIN optics  23  have a first GRIN lens  231  close to the object and a GRIN lens  232  remote of the object between which the diffractive lens  27  can be added. The GRIN lens  232  remote of the object then images the achromatized beam bundle in the image plane  26  in which an intermediate image is generated which is adapted to the aperture and object field of a transmission system  4  to a sensor unit (not shown). 
         [0042]    Alternatively, the diffractive lens  27  can also be arranged in the divergent bundle between the spherical lens element  22  and the object-side GRIN lens  231 . 
         [0043]    As a rule, the miniature optical system  2  images the object points O 1  and O 2  lying within the object  1  with a desired magnification in the image points P 1  and P 2  of an image plane  26  which (at least for endoscopic applications) is only an intermediate image plane for the transmission system  4  arranged downstream. 
         [0044]    For biological or medical applications, the optical system  2  typically has a distance from 20 to 200 μm between the object plane  25  and optical entrance surface  21 . In the specific case of multiphoton microscopy, the distances can be up to and over 1 mm. 
         [0045]    The optical system  2  has the following characteristic quantities:
       diameter, optical thickness and refractive index of the spherical lens  22 ,   diameter, thickness and refractive index profile of the object-side GRIN lens  231  with dispersive (wavelength-dependent) characteristics,   diameter, thickness and refractive index profile of an image-side GRIN lens  232  with dispersive (wavelength-dependent) characteristics,   diameter, thickness and focal lengths at given wavelengths of a diffractive lens  27  for achromatization of the entire GRIN optics  23 , and   gaps  28  between the above-mentioned optical components if advantageous or required for the structural design of the optical system  2 .       
 
         [0051]    Because of the miniaturization of all of the optical components (about 1 mm diameter), the gaps  28  along the optical axis  24  in the system design are optimized as close to zero as possible in order to simplify the alignment of the entire optical system  2 . Insofar as their presence is required or compulsory, the gaps can be filled with a medium or a variety of homogeneous media to influence the optical or mechanical characteristics of the entire system  2 . In the simplest case, they are formed as air gaps or contain another suitable filling gas. In case of optical heads with particularly exacting mechanical requirements (e.g., subject to bending), it may be useful to fill the gaps with optical cement or gel having a suitable refractive index. 
         [0052]    The GRIN lenses  231  and  232  need not have the same refractive index profile. In particular, the GRIN lens  232  remote of the object is essentially intended for adapting the magnification and the numerical aperture to the downstream transmission system  4  which can vary widely. 
         [0053]      FIG. 2  shows an embodiment example of the optical system  2  with a mechanical housing. The entrance surface  21  of the optical system  2  is advantageously realized by an optical window  31  of scratchproof glass or sapphire which is glued or soldered into a mechanical mount  32  for all optical elements of the miniature optical head  3  so as to be hermetically sealed. The spherical lens element  22  is glued on the window  31  internally (with optical cement adapted to the index of refraction) or optically contacted (with polished surfaces by applying with a brief, defined pressure). 
         [0054]    An optical transmission system  4  to a sensor unit (not shown) is provided for endoscopic image transmission from the image plane  26  of the image-side GRIN lens  232 . In this connection,  FIG. 2  shows the end of a flexible image-conducting fiber bundle  41  which is fastened in the mechanical mount  32  of the miniature optical head  3  by means of an adapter sleeve  33 . As alternative constructions of the transmission system  4  indicated by the shaded numbers, rigid relay optics  42  or an individual optical fiber which scans the image field by means of a microscanner (fiber-coupled microscanner  43 ) can also be connected instead of the image-conducting fiber bundle  41  and adjusted to the optical system  2  of the head  3  in the same way by means of the adapter sleeve  33  in the mechanical mount  32 . 
         [0055]      FIG. 3  shows an embodiment example in which the light exiting from a thin excitation fiber  44  is only imaged along the optical axis  24  of the system  2  as a light point. The light coming from the object  1  (back-reflected light or fluorescence light in case of single-photon absorption, two-photon absorption or multiphoton absorption) is captured integrally through a reception fiber  45  and supplied to a detector. Alternatively, it could also be fed back to the excitation fiber  44  and transmitted to the detector (not shown). 
         [0056]    In this case, the image scanning of the object  1  takes place by scanning (controlled movement) of the entire optical head  2 . This arrangement is advantageous in that the correction of the aberrations of the system  2  is more successful because only one image point is active on the optical axis  33  and therefore only spherical and chromatic aberrations are significant. Because of the scanning movement of the entire optical head  2 , all image points have the same imaging quality. In this construction of the invention, the diffractive lens  27  for achromatizing the GRIN optics  23  (in the area of weakly divergent beams between the GRIN lenses  231  and  232 ) can be dispensed with entirely because no chromatic correction is needed, since the fluorescent light is detected integrally. 
         [0057]    A particularly simple system  2  without a diffractive lens  27  results when GRIN lenses  231  and  232  are combined to form a single GRIN lens in that the refractive index profile is formed in such a way that aberrations (primarily spherical aberrations) of the entire system  2  are minimized. The correction of the chromatic aberration which is not carried out in this example can be dispensed with when operating in a narrow wavelength spectrum (monochromatically), when excitation light and detection light are guided in optically separate beam paths (see embodiment example according to  FIG. 8 ), or when the detection (e.g., of fluorescence) is carried out integrally (without optical imaging). The latter is particularly sufficient in multiphoton laser microscopy because the excitation occurs almost exclusively in the laser focus and an image recording is achieved by point-scanning of the object with the laser focus (scanning). 
         [0058]      FIG. 4  shows a construction of the invention in which all of the gaps  28  along the optical axis  24  of the optical system  2  are reduced to zero. This results in a very simple assembly in that all optical components starting with the spherical lens element  22 , followed by the object-side GRIN lens  231 , the diffractive lens  27 , and the image-side GRIN lens  232  are stacked in the mechanical mount  32  (shown only in  FIGS. 2 and 3 ) and align themselves in practice when the concluding adapter sleeve  33  is secured for fastening the transmission system  4 . If an image-conducting fiber bundle  41  is used, as is the case in  FIG. 2 , it is in direct contact with the image-side GRIN lens  232 , since the image plane  26  lies in the exit surface of the GRIN lens  232 . 
       1st Embodiment Example 
       [0059]    In a concrete embodiment form according to  FIG. 5 , the optical system  2  has two identical GRIN lenses  231  and  232  with a nominal numerical aperture of 0.5 and a diameter of 1.0 mm. Achromatization is achieved for 830 nm and 500 nm by the diffractive lens  27  between the two GRIN lenses  231  and  232 . 
         [0060]    The object  1  is located at the entrance surface  21  of the object-side GRIN lens  231  having a thickness of 2.403 mm. 
         [0061]    The refractive index profile of the two GRIN lenses  231  and  232  is reduced to a polynomial formulation at a reference wavelength λ ref =0.67 μm: 
         [0000]        n   ref ( r )= n   0   +n   r2   r   2   +n   r4   r   4   (1) 
         [0000]    where r is the radial coordinate and n 0 , n r2 , n r4  are the refractive index profile parameters. The dependency of the refractive index profile on wavelength λ is described by the following dispersion model: 
         [0000]        n   2 (λ)= n   2 (λ ref )+ K   1 (λ 2 −λ 2   ref )/λ 2   (2) 
         [0000]    where n 2 (λ ref )=n 2   ref  and K 1  is a function of n ref .
 
K 1  is described by the linear equation:
 
         [0000]        K   1 ( n   ref )= K   11   +K   12   n   2   ref   (3) 
         [0062]    Both GRIN lenses  231  and  232  have the same refractive index profile described by the following parameters: 
         [0063]    K 11 =0.11098; 
         [0064]    K 12 =−0.06172; 
         [0065]    n 0 =1.6289; 
         [0066]    n r2 =−0.3482; 
         [0000]    n r4 =0.0620; 
         [0067]    The diffractive lens  27  is assumed to be an ideal thin lens for optimizing the optical design. The focal length depends on the wavelength, where 
         [0000]        f=C/λ,   (4) 
         [0000]    where C=0.004565 mm 2 . Accordingly, the diffractive lens  27  has a focal length of 5.5 mm at 830 nm and a focal length of 9.13 mm at 500 nm. 
         [0068]    The second GRIN lens  232  is adjusted by its thickness of 2.171 mm in such a way that the image plane  26  lies exactly in the exit plane of the image-side GRIN lens for 830 nm and for 500 nm so that the desired achromatization is achieved in this plane. This is especially important for image transmission systems arranged downstream (image-conducting fiber bundle, relay optics, microscope, etc.) which require achromatic imaging. 
       2nd Embodiment Example 
       [0069]    In another specific construction according to the basic optical scheme of  FIG. 4 , the miniature optical head  3  comprises an optical window  31  (material: BK7) with a thickness of 0.170 mm, a spherical lens element  22  (material: LaSFN9, spherical radius: 0.5 mm, thickness: 0.30 mm), two GRIN lenses  231  and  232  with a diameter of 1.0 mm, and a nominal numerical aperture of 0.5. Achromatization for 830 nm and 500 nm is achieved through a diffractive lens  27  between the two GRIN lenses  231  and  232 . 
         [0070]    All optical elements are in direct contact (as is shown in  FIG. 4 ), i.e., there are no gaps  28  along the optical axis  24 . This choice of optical design is advantageous for the alignment of the entire optical system  2  within the mechanical mount  32  with miniaturized head  3 , particularly because of the very small diameters of the optical elements (1 mm). 
         [0071]    In this instance, the object  1  is in water at a working distance of 50 μm from the entrance surface  21  of the object-side spherical lens element  22 . The object-side GRIN lens  231  has a thickness of 1.480 mm. 
         [0072]    As in the first embodiment example above, the refractive index profile of the two GRIN lenses  231  and  232  is described by equations (1), (2) and (3) with the following parameters: 
         [0073]    K 11 =0.11098; 
         [0074]    K 12 =−0.06172; 
         [0075]    n 0 =1.6289; 
         [0076]    n r2 =−0.3532; 
         [0077]    n r4 =0.1032; 
         [0000]    For the calculation of the optical design, the diffractive lens  27  is assumed to be an ideal thin lens. Its focal length depends on the wavelength according to equation (4), where C=0.003901 mm 2  in this example. Accordingly, the diffractive lens  27  has a focal length of 4.7 mm at 830 nm and a focal length of 7.802 mm at 500 nm. The second GRIN lens  232  is adjusted with respect to its thickness of 2.007 mm in such a way that the image plane  26  lies 50 μm behind the exit plane of the GRIN lens  232  in air for 830 nm as well as for 500 nm. The entire optical system  2  is diffraction-limited up to an object-side numerical aperture of 0.85 and, consequently, is provided with a high spatial resolution in axial and lateral dimensions. 
       Third Embodiment Example 
       [0078]    Miniature optical head comprising two GRIN lenses with 1.0 diameter. The object-side GRIN lens has a nominal numerical aperture of 0.5, the image-side GRIN lens has a nominal numerical aperture of 0.2, and an achromatization is achieved for 830 nm and 500 nm by a diffractive lens between the two GRIN lenses. The object is located in the entrance surface of the object-side GRIN lens having a thickness of 2.403 mm. 
         [0079]    As in the first embodiment example above, the refractive index profile of the two GRIN lenses  231  and  232  is defined by the equations (1), (2) and (3) and is calculated for a reference wavelength of λ ref =0.67 μm. Because of the different numerical apertures, they are characterized by different refractive index profiles, and the dependency of the profiles on wavelength also diverges. 
         [0080]    For the object-side lens (NA=0.5), the refractive index profile is described by the following parameters: 
         [0081]    K 11 =0.11098; 
         [0082]    K 12 =−0.06172; 
         [0083]    n 0 =1.6289; 
         [0084]    n r2 =−0.3482; 
         [0085]    n r4 =0.0620; 
         [0086]    For the second GRIN lens  232  (NA=0.2), the refractive index profile is described by the following parameters: 
         [0087]    K 11 =0.05735; 
         [0088]    K 12 =−0.03843; 
         [0089]    n 0 =1.5239; 
         [0090]    n r2 =−0.0522; 
         [0091]    n r4 =0.0015; 
         [0092]    The focal length of the diffractive lens  27  is formulated as in equation (4) above. In this example, C=0.0071961 mm 2 . Accordingly, the diffractive lens  27  has a focal length of 8.67 mm at 830 nm and a focal length of 14.3922 mm at 500 nm. 
         [0093]    The second GRIN lens  232  is adjusted with respect to its thickness of 5.044 mm in such a way that the image plane  26  (in the paraxial area) lies exactly in the exit plane of the GRIN lens  232  for 830 nm and for 500 nm. 
         [0094]    In a reduced embodiment form of the invention in which the numerical aperture of the object-side GRIN lens  231  is sufficient for imaging purposes, an achromatic system  2  can also be constructed—as in FIG.  5 —without a spherical lens element  22 . In this case, the scanning plane of the object  1  is located at a certain working distance from the GRIN lens  23 . As in the preceding examples, the diffractive lens  27  is preferably positioned in the area of low beam divergence between the two GRIN lenses  231  and  232 . The GRIN lens  232  images in the image plane  26  which is an intermediate image plane for a transmission system  4  arranged downstream. 
         [0095]    In another embodiment form of the invention, as is shown schematically in  FIG. 6 , the image-side GRIN lens  232  is replaced by conventional refractive optics (e.g., a multi-lens system). This can be particularly useful when the described miniature optical head  3  is fitted to a microscope body  46  in place of a microscope objective. 
         [0096]    In this case, the optical system  2  is shortened by placing the spherical lens element  22  directly on the window  31  of the miniature optical head  3 , followed after a gap  28  by only an object-side GRIN lens  231  and the diffractive lens  27  for achromatization, and is coupled to the mechanical mount  32  at the microscope body  46  (as transmission system  4 ). The optical head  3  accordingly replaces the microscope objective of any microscope body  46 . In the example according to  FIG. 6 , this body comprises tube optics  461  and an ocular unit  462  through which is generated a real intermediate image that is then available for further image recording (not shown). The function of the second GRIN lens  232  is taken over in practice by the microscope body  46  in this example. 
         [0097]    Particularly for endoscopic applications, it may be advantageous when the optical axis  24  of the object-side entrance surface  21  of the optical system  2  is angled relative to the optical axis  24  of the transmission system  4  because the object  1  to be imaged, e.g., biological tissue, often lies at the side walls of an endoscoped cavity. In this connection, various deflection angles can be advisable. 
         [0098]    An example of a deflection angle of 90° is shown in  FIG. 7 . A beam-deflecting optical element  29  is preferably a microprism  291  which is positioned in the weakly divergent beam path, advisably between the two GRIN lenses  231  and  232 , so that the aberrations introduced by the prism  291  are minimized. As in the preceding example, a diffractive lens  27  can be positioned between the prism  291  and the second GRIN lens  232  for correcting the chromatic aberration of the entire system. The gap  28  serves to minimize the aberration errors. 
         [0099]    Alternatively, it is also possible to replace the microprism  291  by a mirror scanner by which the object  1  imaged by the spherical lens element  22  and first GRIN lens  231  can be scanned line by line. 
         [0100]      FIG. 8  shows a construction in which the beam-deflecting optical element  29  is constructed as a beamsplitter  292 . For applications in fluorescence endomicroscopy or also in multiphoton microscopy and multiphoton endoscopy, it is often useful to separate excitation light and radiation emitted by the object  1  by means of a beamsplitter  292 , since they both generally have a different spectral region. 
         [0101]    The beamsplitter  292  is preferably formed as a cube from two prisms  293  and  294 , and the interface between the two prisms  293  and  294  has a dichroic beam-splitting coating  295 . This can be arranged in such a way that the coating  295  passes the excitation light in transmission and deflects by reflection the radiation emitted by the object  1 . Alternatively, the reverse is also possible. 
         [0102]    In  FIG. 8 , the point A 1  of a light source plane in the excitation beam path is imaged in the object point O 1  by transmission through the beamsplitter  292 . The radiation excited at the object point O 1  is then reflected in the beamsplitter  292  and imaged as an image point P 1  in the image plane  26  of the GRIN lens  233  of the detection beam path. 
         [0103]    In this case, the optical system  2  is formed by an object-side GRIN lens  231 , the beamsplitter  292 , and two image-side GRIN lenses  232  and  233  arranged in the two resulting partial beam paths. The GRIN lenses  232  and  233  can be identically dimensioned, analogous to the second GRIN lens  232  of the preceding examples, or can be adapted differently to specific excitation beam paths and detection beam paths with respect to their optical configuration. 
         [0104]    While the foregoing description and drawings represent the present invention, it will be obvious to those skilled in the art that various changes may be made therein without departing from the true spirit and scope of the present invention. 
       REFERENCE NUMBERS 
       [0000]    
       
           1  object 
           2  (miniature) optical system 
           21  entrance plane 
           22  spherical lens element 
           23  GRIN optics 
           231  (object-side) first GRIN lens 
           232  (image-side) second GRIN lens 
           24  optical axis 
           25  object plane 
           26  image plane 
           27  diffractive lens 
           28  gap 
           29  deflecting element 
           291  microprism (with reflective coating) 
           292  (dichroic) beamsplitter 
           293 ,  294  prisms 
           295  coating 
           3  optical head 
           31  window 
           32  mechanical mount 
           33  adapter sleeve 
           4  transmission system 
           41  image-conducting fiber bundle 
           42  relay optics 
           43  (fiber-coupled) microscanner 
           44  excitation fiber 
           45  reception fiber 
           46  microscope system 
           461  tube optics 
           462  ocular unit 
         A 1  point (in the light source plane) 
         O 1 , O 2  object point 
         P 1 , P 2  image point