Field of the Invention

The invention relates to a composite optical fiber for use in holographic multimodal endoscopy, to endoscopic probes containing said composite fiber and an endoscopic device containing said endoscopic probe.

Background Art

Holographic multimodal endoscopy allows for obtaining of optical images having a high resolution from structures deep inside a sensitive tissue (e.g. brain) of live animal models and humans by means of a minimally invasive optical fiber, wherein the fibers typically have a diameter of 100 pm.

An important imaging modality in microscopy is coherent Raman scattering (CARS) imaging. This contrast mechanism allows for imaging with a chemical contrast of biologically important components - lipids, proteins etc. Apart from the importance of the imaging of lipids, a rapid in situ tumour diagnostics seems to be particularly promising, wherein CARS is one of the most used imaging methods, often together with e.g. two-photon excitation fluorescence (TPEF) and with second harmonic generation (SHG) imaging.

A drawback is that when using optical fibers including multimodal fibers, a signal is generated in the fiber in endoscopes for CARS imaging, said signal being a consequence of four-wave mixing (FWM) and other non-linear processes, particularly when using femtosecond excitation sources. The result of this background is a significant diminishment of contrast on the images. The signal of the background is strong in single-mode fibers, where the entire light is concentrated in a region having a diameter of a few micrometres, and in gradient multimodal fibers (GRIN-type fibers), where the self-imaging property of the fiber leads to creation of narrow foci inside the fiber (the intensity is high in both cases and FWM is thus efficient).

The background spectrally overlaps with the CARS signal, generated in the sample, and thus cannot be filtered when the same fiber (whether the fiber is single-mode or multimodal) is used both for excitation and detection. The background can’t be measured and subtracted for two reasons: a) The background and CARS signal of the sample are both coherent and the detected signal is an interference between them, b) For imaging of tissues, the generated signal is backscattered inside the tissue and the level of the detected signal in the background would thus be affected by the properties of the tissue scattering which would alter the level of the detected signal depending on the position in the sample.

Summary of the invention

Drawbacks of prior art are eliminated by a composite optical fiber for holographic endoscopy, comprising a GRIN fiber having a proximal end for connection with an endoscopic device, it further contains an SI fiber which is fixed to a distal face of the GRIN fiber with its proximal face, the SI fiber having a length of 0.1 to 10 mm, preferably 150 pm to 2 mm, most preferably 200 pm to 1 mm.

Preferably, the length of the GRIN fiber is at least 10 mm, preferably at least 20 mm.

Preferably, the SI fiber is coaxially attached to the GRIN fiber.

It is also advantageous, when the GRIN fiber has an external diameter of 0.05 mm to 2 mm and a core diameter of 0.02 to 1.9 mm, and/or when the SI fiber has an external diameter 0.05 to 2 mm and core diameter of 0.02 to 1.9 mm.

Drawbacks of prior art are also eliminated by an endoscopic probe for holographic endoscopy comprising a connector for connection to an endoscopic device, wherein the probe comprises the above described composite optical fiber, wherein the proximal end of the composite optical fiber is attached to the connector for connection to an endoscopic device.

Drawbacks of prior art are also eliminated by an endoscopic device for holographic endoscopy, which comprises:

- a femtosecond laser radiation source adapted for emission of a Stokes beam and a pump beam,

- a spatial light modulator arranged in the trajectory of the Stokes beam and the pump beam emitted from the femtosecond laser radiation source, - a module for correction of light polarization arranged in the trajectory of the beam brought from the spatial light modulator,

- a first focusing lens arranged in the trajectory of the beam exiting the module for correction of light polarization, and

- the above described composite optical fiber which faces the first focusing lens with its proximal end for entry of the beam focused by the first focusing lens into the composite optical fiber.

Preferably, the endoscopic device comprises a second dichroic mirror arranged in the trajectory of the beam exiting the module for correction of light polarization between the module for correction of light polarization and the first focusing lens, and a third dichroic mirror, wherein the second dichroic mirror is arranged for reflection of back-reflected radiation from a tissue through the composite optical fiber onto the third dichroic mirror.

Preferably, the endoscopic additionally comprises:

- a photomultiplier for detection of a two-photon epi-fluorescent excitation, said photomultiplier being arranged in the trajectory of the beam reflected from the third dichroic mirror, and

- a photomultiplier for detection of coherent Raman scattering, said photomultiplier being arranged in the trajectory of the beam which passes through the third dichroic mirror.

Preferably, the endoscopic device comprises a prism arranged in the trajectory of the beam brought from the module for correction of light polarization between the module for correction of light polarization and the second dichroic mirror.

It is also advantageous, when the endoscopic device comprises a calibration module for calibration of the endoscopic device by focusing of beams brought into the composite optical fiber to a region in front of the distal end of the composite optical fiber.

Brief Description of the Drawings

The invention is further described based on exemplifying embodiments and drawings, wherein Fig. 1 schematically illustrates a conventional gradient multi-modal fiber and generated background when the fiber is used as an endoscopic probe, Fig. 2 schematically illustrates a composite optical fiber according to the invention and generated background, Fig.

3 illustrates a comparison of obtained images with a GRIN fiber only and with the composite optical fiber according to the invention, both of the background itself without samples and of the background with samples (in this case, the samples were 2 pm polystyrene balls), and Fig.

4 illustrates a simplified design of an endoscopic device comprising the composite optical fiber according to the invention.

Description of Exemplifying Embodiments

Gradient multimodal optical fibers are a type of fibers the refractive index of which decreases with increasing distance from the axis of the fiber.

Fig. 1 schematically illustrates a conventional gradient multimodal fiber, i.e. GRIN fiber 1 for use in endoscopy, and background obtained from it, wherein highly intensive foci are clearly visible inside, the foci being formed as images of the excitation point behind a distal end of the GRIN fiber 1, generated as a result of a self-imaging property of the gradient fiber. When imaging a sample using the GRIN fiber 1, a focused point forms in the sample in front of the distal end of the GRIN fiber 1. As a result of the self-imaging property of the GRIN fiber 1, not only this intense focused point, but also its images, forming inside the fiber, will appear in the final image.

Fig. 2 schematically illustrates a composite optical fiber according to the present invention which contains a GRIN fiber 1 which has a proximal end intended for being attached to an endoscopic device and a distal end, to which a SI fiber 2, i.e. a fiber with a step index, is fixed with its proximal face.

The distal end of the composite optical fiber is intended for being inserted to the observed sample.

In the exemplifying embodiment, the GRIN fiber 1 has a length of 30 mm, an external diameter of 125 pm, core diameter of 62.5 pm, a numerical aperture of NA = 0.3 and both faces planar. The core of the GRIN fiber 1 can be e.g. made of germanium-doped silica glass and the sheath of pure, eventually fluorine-doped silica glass.

In general terms, such GRIN fibers 1 can be used for the present invention which have a length of at least 10 mm, better yet at least 15 mm, optimally at least 20 mm. The length of the GRINF fiber 1 is preferably shorter than 100 mm, its external diameter is preferably in the range of 50 pm to 2 mm and the core diameter in the range of 20 pm to 1.9 mm, better yet 40 to 200 pm.

In the above exemplifying embodiment, the SI fiber 2 has a length of 250 pm, an external diameter of 110 pm, core diameter of 100 pm, a numerical aperture of NA = 0.37 and both faces planar. The core can be e.g. made of germanium-doped silica glass and the sheath of fluorine-doped silica glass (e.g. the fiber CeramOptec Optran Ultra WFGE).

In general terms, such SI fibers 2 can be used for the present invention which have a length of 0.1 to 10 mm, better yet 150 pm to 2 mm, an external diameter in the range if 50 pm to 2 mm and a core diameter of 20 pm to 1.9 mm, better yet 40 to 200 pm. Suitable material for making of the SI fiber 2 include also soft glass and eventually plastics.

The fibers 1, 2 are preferably welded to each other.

Thanks to the construction of the composite optical fiber according to the invention, it is prevented that the self-image fields form narrow foci inside the composite optical fiber. Essentially, if a field on an outlet end of the GRIN fiber 1 formed a random speckle pattern, the foci which are nevertheless formed by effect of the self-imaging effect would be broken up to speckles having an intensity that would be orders of magnitude lower.

To achieve this effect, the composite optical fiber according to the invention consists of a relatively long GRIN fiber 1 so that the bandwidth of the probe is satisfactory for focusing of femtosecond pulses, and of a relatively short SI fiber 2, welded on the distal end.

When using an endoscopic probe containing such composite optical fiber, background is thus reduced by one order of magnitude without any increase in the diameter of the endoscopic probe and without a significant decrease of the focusing capability of the endoscopic probe.

An endoscopic probe containing a composite optical fiber according to the invention maintains all other properties of a probe containing a GRIN fiber only: a lateral and axial size of the focused point, a ratio of the intensity of the focused point to the intensity of the background and a range in which the wavelength can be set after an initial calibration. Furthermore, because the attached SI fiber 2 has only a small influence on the scattering of the composite optical fiber, the spectral resolution and the chemical contrast of the so-called “spectrally focused” CARS (required when using femtosecond pulses) is not affected. Because the scattering is the same for all of the points inside the field of view, common means for pulse shaping can be used for maintaining of a short pulse which leads to an efficient excitation of TEPF, SHG and other signals. Because of that, all imaging methods in which femtosecond lasers are used and which are used for optical biopsy, are supported.

For use in endoscopy, this composite optical fiber is preferably glued by the proximal end of the GRIN fiber 1 to a ferrule (not shown) which is adapted to be fixed in a corresponding holder of the endoscopic device.

An exemplifying embodiment of the entire endoscopic device, schematically illustrated in Fig. 5 particularly with regard to its function comprises a femtosecond laser radiation source 3, which represents an excitation source adapted for generating of two synchronized sequences of impulses with different wavelengths, namely a beam with a wavelength of 1040 nm as a Stokes beam and a tuneable beam in the region of 800 nm as a pump beam.

A first block 31 of glass of type SF57 is in this exemplifying embodiment arranged in the trajectory of the Stokes beam, the first block 31 having a thickness of 220 mm, and downstream of it a first corner reflector 33 for setting of a delay of the Stokes beam. A first auxiliary mirror 34 is arranged in the trajectory of the beam reflected from the first corner reflector 33.

A second block 32 of glass of type SF57 is in this exemplifying embodiment arranged in the trajectory of the pump beam, the second block 32 having a thickness of 220 mm, and downstream of it a first dichroic mirror 35 for reflecting the pump beam and a second auxiliary mirror 36 are arranged. Simultaneously, the first dichroic mirror is arranged in the trajectory of the Stokes beam, which passes through this first dichroic mirror 35 after the Stokes beam is reflected from the first auxiliary mirror 34.

The second auxiliary mirror 36 is arranged downstream of the first dichroic mirror 35 in the trajectory of such overlapping beams, and a first converging lens 37 is arranged in their trajectory after they are reflected from the second auxiliary mirror 36 and a first collimator lens 38 is arranged downstream of the first converging lens 37 and a liquid crystals phase spatial light modulator 39 is arranged downstream of the first collimator lens 38.

A second converging lens 38 is arranged in the trajectory of the light reflected from the spatial light modulator 39 and a deflecting mirror 41 is arranged downstream of the second converging lens 38 in order to extract a part of radiation which is further led as a reference beam through the aperture in a first shutter 43 for use in a calibration module 50 as described below.

A directing mirror 21 of a first set of directing mirrors 21 is arranged in the trajectory of the light which has passed through the second converging lens 80, the first set of directing mirrors 21 bringing a first work portion of the light beam which passed through the second converging lens 80, through a third collimator lens 84 into a module 20 for correction of light polarization from one of its sides, wherein it passes through a first half- wave board 85 into a polarization divider 83.

A directing mirror 22 of a second set of directing mirrors 22 is arranged in the trajectory of the light which has passed through the second converging lens 80, the second set of directing mirrors 22 bringing a second work portion of the light beam which has passed through the second converging lens 80, through a fourth collimator lens 81 into the module 20 for correction of light polarization from another of its sides, wherein it passes through a second half- wave board 82 into a polarization divider 83, wherein the first work portion and the second work portion of the beam merge and exit the module 20 for correction of light polarization together.

A seventh auxiliary mirror 86 is arranged in the trajectory of the beam exiting the module 20 for correction of light polarization, i.e. the beam consisting of the beam reflected from the polarization divider 83 and the beam which has passed through the polarization divider 83, and a prism 87, the plane of which is conjugated with the plane of the spatial light modulator 39, is arranged downstream of the seventh auxiliary mirror 86.

A second dichroic mirror 62 is arranged in the trajectory of the beam exiting the prism 87, a first focusing lens 64 is arranged downstream of the second dichroic mirror 62 and the composite optical fiber according to the invention is arranged downstream of the first focusing lens 64, i.e. a GRIN fiber 1 facing with its proximal end the first focusing lens 64 and an SI fiber 2 for insertion into the observed tissue 10 fused to the distal end of the GRIN fiber 1.

A third dichroic mirror 63 is arranged in the trajectory of the light returning through the composite optical fiber through the first focusing lens 64 and subsequently reflected from the second dichroic mirror 62.

A second converging lens 65 is arranged in the trajectory of the partial beam reflected from the third dichroic mirror 63, a first filter 66 is arranged downstream of the second converging lens 65 and a photomultiplier 68 for detection of two-photon epi-fluorescent excitation (epi-TPEF) is arranged downstream of the first filter 66.

A third auxiliary mirror 69 is arranged in the trajectory of the partial beam which has passed through the third dichroic mirror 63, a third converging lens 70 is arranged in the trajectory of the beam reflected from the third auxiliary mirror 69, a second filter 71 is arranged downstream of the third converging lens 70 and a photomultiplier 72 for detection of coherent Raman scattering (epi-CARS) is arranged downstream of the second filter 71.

The figure further shows a calibration module 50 which comprises a second corner reflector 51 which is arranged in the trajectory of the reference beam fed through an aperture 43 and further by reflection from a fourth auxiliary mirror 42, through the second collimator lens 44 and by gradual reflection from a fifth auxiliary mirror 45, a sixth auxiliary mirror 46 and a reflective right-angled prism 47.

A non-polarizing divider 52 is arranged in the calibration module 50 in the trajectory of the reference beam reflected from the second corner reflector 51, wherein a camera 53 is arranged in the trajectory of the beam from the non-polarizing divider 52.

An object lens 54 is also a part of the calibration module 50, said object lens 54 being adjustable to a position, in which a distal end of the SI fiber 2 of the composite optical fiber faces the object lens 54. A fifth dichroic mirror 55 faces the object lens 54 on another side, wherein a second focusing lens 56, a third filter 57 and a multiplier 58 for detection of coherent Raman scattering in the passed light (trans-CARS) are arranged in the trajectory of the beam which has passed through the fifth dichroic mirror 55.

An achromatic doublet 59 is arranged in the trajectory of the beam which has reflected from the fifth dichroic mirror 55 and a non-polarizing divider 52 and a camera 53 are arranged downstream of said achromatic doublet 59. The camera 53 can be based e.g. on CMOS.

The calibration module 50 is removable from the endoscopic device.

The illustrated endoscopic device further comprises a stabilization module 90 which comprises a second shutter 91, a polarizer 92, a fourth dichroic mirror 93 and a pair of photodiodes 94, 95, wherein these component are arranged so that the beam reflected from the second dichroic mirror 62 passes through an aperture of the second shutter 91, through the polarizer 92 and is divided in two portions by the fourth dichroic mirror, wherein each of the two portions impinges on one of the photodiodes 94, 95. When the endoscopic device is in operation, a Stokes beam and a pump beam are generated by the femtosecond laser 3, wherein said beams overlap and are brought on the spatial light modulator 20 with an adjustable delay. Four beams exit the spatial light modulator 20:

- a so-called zeroth diffraction order in the direction of the direct reflection which is neither used nor illustrated in the figure,

- a reference beam used in calibration,

- two work beams.

All of the four beams pass through the second converging lens 80 which allows for their division in the focal plane of the converging lens 80.

The zeroth-order beam and the reference beam are reflected from the deflecting mirror 41. The reference beam passes through an aperture in the first shutter 43. The undesirable zeroth-order beam is filtered by this aperture.

The pair of work beams is brought into the module 20 for correction of light polarization, wherein the polarization of the work beams is turned by half-wave boards 82, 85 and the work beams are merged by the polarization beam divider 83.

After a measurement and a stabilization of a relative phase between a pair of optical trajectories during imaging, a portion of the beam is removed and brought into the stabilization module 90, wherein it passes through the polarizer 92 having axes oriented in the angle of 45° and subsequently is divided by the fourth dichroic mirror 93 into a portion corresponding to the Stokes beam and a portion corresponding to the pump beam, and these portions are scanned by the photodiodes 94, 95.

Because the holograms formed on the module 20 for correction of light polarization are in the form of sums of diffraction gratings, the phase modulation is dependent on wavelength. For tuning of the wavelength of the laser radiation source 3 while maintaining the quality of focusing, a prism arranged in the plane conjugate with the plane of the module 20 for correction of light polarization is used.

During imaging, the CARS signals and the TPEF signals, which have been obtained from the composite optical fiber by back reflection from the examined tissue 10, are reflected from the second dichroic mirror 62 onto the third dichroic mirror 63, where they are divided to a signal lead to a photomultiplier 68 for detection of two-photon epi-fluorescence excitation and a signal lead to a photomultiplier 72 for detection of coherent Raman scattering.

For calibration, the light brought from the composite optical fiber - after passing through the object lens 54 - is reflected from the fifth dichroic mirror 55, and then passes through the achromatic doublet 59 and the non-polarizing divider 52 to the camera 53, wherein the purpose of the calibration is to focus the work portions of the beam in a region in front of the distal end of the composite optical fiber.

After the calibration, the light brought from the composite optical fiber, after passing through the object lens 54 and the fifth dichroic mirror 55, is led through the second focusing lens 56 and the third filter 57 onto the photomultiplier 58.

The above-described composite optical fiber can be used particularly as an endoscopic probe for optical biopsy in sensitive tissue, e.g. brain tissue. By using such a composite optical fiber e.g. multimodal imaging may be carried out, wherein the myelin in the white matter is imaged by coherent Raman scattering and cerebellum fibers with expression of a green fluorescent protein using two-photon fluorescence.

It is clear that a person skilled in the art would readily find further possible alternatives to the embodiments described herein. In particular, the auxiliary mirrors need not to be present and serve merely to direct the beams into the functional components. The scope of the protection is therefore not limited to these exemplifying embodiments but it is rather defined by the appended claims.