US20080304788A1

US20080304788A1 - Broadband Light Source Having a Microstructured Optical Fiber for Endoscopic and Fluorescence Microscopic Examination Devices, in Particular for Special Devices for Optical Biopsy

Info

Publication number: US20080304788A1
Application number: US11/569,848
Authority: US
Inventors: Wolfgang Mannstadt; Bernd Drapp; Wolfram Beier
Original assignee: Schott AG
Current assignee: Schott AG
Priority date: 2004-06-01
Filing date: 2005-06-01
Publication date: 2008-12-11
Also published as: DE102004026931B3; WO2005119355A3; JP2005345474A; DE112005000888A5; US20050265405A1; JP2008501130A; US7680379B2; WO2005119355A2

Abstract

The invention relates to an arrangement for generating a broadband spectrum that can be used in particular as a light source for short coherence interferometry and confocal microscopy as well as endoscopic short coherence interferometry and endoscopic confocal microscopy. The arrangement comprises a laser, in particular a laser diode, for generating a short light pulse of wavelength λ_Pand a microstructured optical fiber (1) of high nonlinearity, that has a null dispersion of the group velocity in the vicinity of the wavelength λ_Pand an anomalous dispersion, as well as means for coupling the light pulse into the microstructured optical fiber.

Description

The invention relates to an arrangement for generating a broadband spectrum that can be used, in particular, as a light source for short coherence interferometry and for endoscopic examination devices and, in particular, for special devices for optical biopsy.
In the case of many medical applications, there is great advantage in the method of endomicroscopy, the use of a combination of endoscope and microscope, since, for example important information relating to tissue structures can be obtained directly from an endomicroscopic intervention. Moreover, the combination of endoscopes and appliances for optical coherence tomography would also, in particular, be very useful in many instances, since it is possible thereby also to acquire histological facts.
A surgeon who is carrying out a classical biopsy in the body interior by means of the endoscope can do so in a more focused and more efficient way when he has a possibility of observing the tissue directly with the aid of a microscope through the endoscope channel.
A desirable further development of endomicroscopy would be the use of optical coherence tomography (OCT) instead of a microscope, but clinical studies in the field of gastroenterology have shown that this quality cannot, however, be achieved to this end with the appliances used so far for coherence tomography, since suitable light sources with a short coherence length and a broadband spectrum have not so far been available, either because of design limitations, because of unstable operation, or else for reasons of cost in operation and in maintenance.
Light sources with a short coherence length and a broadband spectrum are also required, for example, in three-dimensional surface shape recording and, generally, in optical coherence tomography (OCT). OCT constitutes a noninvasive imaging method that visualizes the tomography of surfaces and structures in scattering media. Light in the near IR is used for the examination of, for example, biological tissue on account of its greater penetration depth into the tissue.
The measurement principle is based on an optical interferometer that determines the scattering power and depth position of the structures with a high resolution. The resolution of the OCT is dependent, inter alia, of the light source used. In OCT, the photons scattered in the tissue are filtered out on the basis of their interference capacity. This requires a light beam with the shortest possible coherence length (but >0) and a broadband spectrum in the near IR. The three-dimensional resolution in the beam direction corresponds to the coherence length of the light used, which is also denoted as temporal or longitudinal coherence. The greater the coherence length, the greater is the volume region from which information is backscattered. In modern OCT systems, the resolution is up to 10 micrometers.
Resolutions of approximately 10 micrometers can be reached, for example, using commercially available OCT appliances with superluminescent diodes which emit in the near IR. Although these diodes have a lower light efficiency than comparable laser diodes, their coherence length is short and they therefore allow a good appliance resolution.
However, for many applications, for example in tumor therapy, an improved resolution at cellular level is required but cannot be achieved using commercially available appliances.
Furthermore, it is known to generate a so called supercontinuum by coupling intensive, ultra-short light pulses into a nonlinear optical medium.
In this context, photonic crystal fibres (PCFs) are of increasing interest. These fibres comprise microstructured fibres, for example formed from a fibre core and a microstructured fibre cladding with a periodic structure (photonic band gap fibre), as described by J. C. Knight et al. in Optics Letters, Vol. 21, No. 19, P. 15-47 (October 1996), or a nonperiodic structure, as disclosed by U.S. Pat. No. 5,802,236, which surrounds the core and runs along the fibre length. Suitable structuring and formation of the fibre cladding and dimensioning of the fibre core give rise to index guidance of the radiation in the fibre. The radiation can be concentrated with a high intensity in the core by employing an effective refractive index difference between the fibre core and the fibre cladding (5% to 20%). These fibres typically comprise microstructured silicon oxide fibres.
The third-order nonlinear effects (χ⁽³⁾) which are essential to the generation of a supercontinuum, such as the self-phase modulation, only occur with short light pulses with a high peak intensity. Investigations carried out by Ranka, Windeler, Stenz in Optics Letters, Vol. 25, No. 25 (2000) have shown that sufficiently high field intensities to activate nonlinear processes in order to generate a supercontinuum in microstructured silicon oxide fibres can be achieved using femtosecond laser pulses.
Since the intensity of the light pulse corresponds to the ratio of pulse power to cross-sectional area of the fibre, and since the pulse power is determined by the ratio of pulse energy to pulse duration, to achieve nonlinear effects it is possible, within the context of what is technically feasible, either to shorten the pulse duration and/or to increase the pulse energy, for example by increasing the repetition rate of a laser, and/or to reduce the cross-sectional area of the core of the fibre.
An output spectrum which covers the visible region and the near IR can be achieved with a core diameter of approximately 2 micrometers in microstructured silicon oxide fibres with an anomalous dispersion, as described for example in U.S. Pat. No. 6,097,870, and with a 100 femtosecond laser pulse from a titanium-sapphire laser (typical pulse energy 1 to 12 nJ, pulse power approx. 8 kW). For propagation of the pulse through the fibre, the geometry of the fibre (core, cladding structure) has to be adapted to the wavelength of the laser pulse, in such a manner that the zero dispersion of the group velocity is approximately at the wavelength of the laser pulse.
Therefore, the resolution of measurement arrangements used for short coherence interferometry could be increased by using Ti-sapphire lasers, but such lasers are large, unwieldy, unstable and expensive and are therefore unsuitable for use in a light source for OCT appliances or for other commercial short coherence measuring appliances.
Photonic crystal fibers (PCFs) are of increasing interest in this context. They consist of microstructured fibers, for example from a fiber core and a microstructured fiber cladding with a periodic structure (photonic band gap fiber) as disclosed, for example, by J. C. Knight et al. in Optics Letters, vol. 21, No. 19, pages 15-47 (October 1996), or a nonperiodic structure as disclosed in U.S. Pat. No. 5,802,236, which surrounds the core and runs along the fiber length. Index guidance of the radiation in the fiber results from an appropriate structuring and design of the fiber cladding and dimensioning of the fiber core. The radiation can be concentrated at high intensity in the core by means of an effective difference in refractive index between the fiber core and the fiber cladding (5% to 20%). These fibers typically consist of microstructured silicon oxide fibers.
The nonlinear 3rd order effects (χ⁽³⁾) such as self-phase modulation that are important for generating a supercontinuum appear only in the case of short light pulses of high peak intensity. The investigations by Ranka, Windeler, Stenz in Optics Letters, vol. 25, No. 25 (2000) have shown that femtosecond laser pulses can be used to attain sufficiently high field intensities for activating nonlinear processes in order to generate a supercontinuum in microstructured silicon oxide fibers.
Since the intensity of the light pulse corresponds to the ratio of the pulse power and cross sectional area of the fiber, and the pulse power is determined by the ratio of pulse energy to pulse duration, for the purpose of achieving the nonlinear effects, either the pulse duration can be shortened and/or the pulse energy can be raised within the scope of the technical possibilities, for example by raising the repetition rate of a laser, and/or by reducing the cross sectional area of the core of the fiber.
An output spectrum that covers the visual region and the near IR can be achieved with the aid of a core diameter of approximately 2 micrometers in microstructured silicon oxide fibers with anomalous dispersion such as described, in U.S. Pat. No. 6,097,870, and with the aid of a 100 femtosecond laser pulse of a titanium sapphire laser (typical pulse energy 1 to 12 nJ, pulse power approximately 8 kW). In order to propagate the pulse through the fiber, it is necessary in this case to coordinate the geometry of the fiber (core, cladding structure) with the wavelength of the laser pulse in such a way that the null dispersion of the group velocity lies approximately at the wavelength of the laser pulse.
Thus, the resolution in measuring arrangements used in short coherence interferometry could be raised by using Ti sapphire lasers, but these lasers are large, bulky, unstable and cost intensive and therefore not suitable for use in a light source for OCT appliances or for other commercial short coherence measuring appliances.
US 2005/0024719 describes a confocal microscope of complicated configuration with a number of complex light sources, in particular laser light sources, and a photonic fiber.
WO 03/096900 A2 describes a confocal optical tomography in which the backscattered light from skin layers in vivo is recorded. A point light source generated by an optical fiber is used, being operated with white or infrared light. However, such light sources have only low light intensities, and for this reason image qualities that are worsened either by relatively long recording times or by noise have to be accepted.
In order to increase the angle of the light entering a fiber in which it is thereupon guided, Japanese patent publication JP 2004 344209 A describes the use of a photonic fiber in an endoscope.
A photonic fiber is also mentioned as such in Japanese patent publication JP 2005 025018 A.
Therefore, it is an object of the invention to provide a simple, stable, broadband light source with a short coherence.
Another object of the invention is to provide a simple and stable light source for short coherence interferometry measuring appliances, in particular for OCT and for endoscopic examination devices, in particular for special devices for optical biopsy, which allows a high level of measurement accuracy.
The object is achieved by an arrangement as claimed in claim 1, while advantageous refinements are given in further claims.
It is therefore an object of the invention to make available a simple, stable, broadband light source of low coherence.
A further object of the invention is to make available a simple and stable light source for short coherence interferometry measuring appliances, in particular for OCT and for endoscopic examining devices, in particular for special devices for optical biopsy which enables a higher measuring accuracy.
The object is achieved with the aid of an arrangement as claimed in claim 1, while advantageous refinements are described in further claims.
An important further development of endomicroscopy, the use of optical coherence tomography instead of a microscope, is rendered both economically sensible and technically advantageous by the inventive light source and its use in these medical optical devices.
The use of optical coherence tomography has the greatest advantage in this case that these imaging methods render possible tissue sections similar to those of pathological histology, albeit it without the need for prior tissue extraction.
Clinical studies in the field of gastroenterology have shown that there is a need for this purpose of an optical resolution, in particular a longitudinal resolution, of the images on a cellular scale, that is to say from approximately 1 to 2 μm. However, this quality has not been adequately reached with the aid of the appliances for coherence tomography used so far.
Longitudinal resolution is understood here as a resolution in the direction of the optical axis of an optical appliance, or in the beam direction of the principal direction of propagation of a beam, which is frequently arranged substantially at right angles to the surface of an object to be observed such as, for example, a tissue to be examined.
Thus, in optical coherence tomography (OCT) two-dimensional images of the object observed are produced, in particular in vivo images of tissue to be examined, which correspond to a histological tissue section or are very similar thereto, which section essentially has a thickness comparable to the longitudinal coherence length of the coherence tomography used.
These longitudinal resolution requirements are achieved in a way that is suitable for normal medical operation with the aid of the light source provided by the invention.
Not only is a stable operation of the light source and of the optical appliance very essential here—an important role is also played by costs in the field of operation and maintenance.
Furthermore, better images than previously achievable can be obtained by means of higher light intensities in conjunction with good spatial size of sources or else spatial coherence.
What is understood here as spatial or else as a transverse coherence is the property of a light source that, in particular in an interferometer, influences the lateral extent of an image field within which contrast that is still visible is present inside the image field.
The light source according to the invention, which is essentially punctiform and generates a broadband spectrum comprises a laser for generating a short light pulse of wavelength λ_P, and of a microstructured optical fiber which has a null dispersion of the group velocity in the vicinity of (that is to say with a possible deviation of approximately ±20% from λ_P) the wavelength λ_Pand an anomalous dispersion, as well as of means for coupling the light pulse into the microstructured optical fiber.
With a microstructured optical fiber which preferably has pronounced nonlinear optical properties, i.e. which has a refractive index, dependent on radiant intensity, n (I)=n₀+n₂*I at the wavelength of the incoupled light pulse λ_P, where the nonlinearity factor of the fiber is preferably n₂≧2*10⁻²⁰cm²/W, it is possible to generate a broadband spectrum even using a laser diode which has light pulses in the picosecond range.
This eliminates, for example, complex and expensive sapphire lasers as have hitherto been required to generate femtosecond pulses as the input pulse into microstructured fibers for generating a broadband spectrum.
The optical nonlinearity of the fiber is determined on the one hand by its structure but on the other hand also by the material of the fiber, and therefore the microstructured optical fiber preferably consists of a nonlinear optical material with a high material nonlinearity factor χ⁽³⁾This factor, in conjunction with the fiber geometry, determines the abovementioned nonlinearity factor of the fiber n₂.
Suitable materials for the microstructured optical fiber with a high χ⁽³⁾are preferably materials formed from a multicomponent glass, a multicomponent glass-ceramic, a monocrystalline material, a polycrystalline material, a plastic-matrix composite and/or a liquid-crystal material. The fiber may also be composed of a plurality of these materials.
In further suitable embodiments, the optical material may have isotropic and/or anisotropic properties.
In a further advantageous embodiment, the microstructured optical fiber comprises at least one nonoxidic multicomponent glass, in particular a chalcogenide glass which contains at least As and Sn.
Further suitable materials for the microstructured optical fiber are materials which comprise at least one oxidic multicomponent glass, in particular silicate glasses which have at least one element of the group consisting of alkali metals (Li₂O, Na₂O, K₂O, Rb₂O, Cs₂O) and/or at least one element of the group consisting of alkaline-earth metals (MgO, CaO, SrO, BaO).
It is preferable for the oxidic multicomponent glass also to comprise at least one further representative of the following elements: Al₂O₃, B₂O₃, PbO, ZnO, TiO₂, ThO₂, ZrO₂, La₂O₃, CeO₂and P₂O₅.
Heavy flint glass which has the components SiO₂and PbO and at least one of the components Al₂O₃, B₂O₃, TiO₂, ThO₂, La₂O₃, BaO, Li₂O, Na₂O or K₂O, in particular SF57 with 24.5% by weight of SiO₂, 74.5% by weight of PbO, 0.4% by weight of Na₂O and 0.6% by weight of K₂O, is a particularly suitable material with a high χ⁽³⁾for the microstructured fiber.
In further suitable embodiments, the microstructured optical fiber may comprise at least one oxidic multicomponent glass-ceramic which has crystal phases of strontium niobate, potassium hydrogen phosphate (KTP), BBO, LBO, LiIO₃, LiNbO₃, KnbO₃, AgGaS₃, AgGaSe₂, PPLN and/or BaTiO₃or a plastic-matrix composite formed from an oxidic multicomponent glass and plastics based on PMMA (polymethyl methacrylate), PC (polycarbonate), PA (polyamide) or PE (polyethylene).
Furthermore, in particular the fiber core may comprise at least one mono-crystalline material formed from strontium niobate, potassium hydrogen phosphate (KTP), BBO, LBO, LiIO₃, LiNbO₃, KnbO₃, AgGaS₃, AgGaSe₂, PPLN or BaTiO₃or a polycrystalline material formed from strontium niobate, potassium hydrogen phosphate (KTP), BBO, LBO, LiIO₃, LiNbO₃, KnbO₃, AgGaS₃, AgGaSe₂, PPLN and/or BaTiO₃or a liquid crystal which has a polymeric fraction with a mesogenic group within the main polymer chain or in a side chain which is branched off therefrom.
In addition to the demands relating to the material of the microstructured fiber with a view to achieving nonlinear effects (such as for example self-phase modulation) in the fiber with the lowest possible intensity of the light pulse coupled in, certain demands are also imposed on the geometry of the fiber for generating the broadband spectrum. The wavelength λ_Pof the light pulse coupled into the fiber and the geometry of the fiber determine the propagation of the light pulse through the fiber. To this end, the geometry, in particular the cross section of the fiber, has a design which produces a null dispersion of the group velocity in the vicinity of the wavelength λ_Pof the light pulse and an anomalous dispersion of the light pulse in the microstructured fiber.
The microstructured optical fiber to this end preferably has a fiber core running along the fiber length and a structured fiber cladding arranged around the fiber core, in which case the fiber core in particular comprises a solid body and the fiber cladding comprises hollow structures running parallel to the fiber core.
In particular with a view to production of the microstructured fiber, which will be dealt with in more detail in a subsequent section of the description, it is advantageous if the fiber core comprises a solid rod and the fiber cladding comprises tubes arranged uniformly around the solid rod, preferably so as to form a hexagonal structure.
The minimum possible cross section of the fiber core is required in order to activate the nonlinear effects. For this purpose, in preferred embodiments of the invention the fiber core has a diameter of from 1 μm to 4 μm, and the surrounding tubes of the fiber cladding have a diameter of from 2 μm to 8 μm.
The microstructured optical fiber, in particular comprising the abovementioned materials and/or having the abovementioned small dimensions, is preferably produced using an IR drawing process in accordance with the application in the name of the same Applicant and filed with the USPTO on the same day, entitled “Hot formed Articles and Method and Apparatus for hot-forming” (Applicant's internal reference 03SGLO308USP—application number will be submitted at a later date), and the content of disclosure of this application is incorporated by reference in its entirety. This method makes it possible to draw high-precision microstructured fibers from corresponding preforms made from “difficult” materials, with the semi-homogeneous temperature distribution and heating over the cross section of the fiber (<0.5 K/mm) at a low temperature during the drawing operation allowing effective and accurate manufacture.
To generate the desired nonlinear effects which are required to generate the broadband spectrum, the light pulse coupled in must have a suitably high intensity. Given a high fiber nonlinearity, in particular n₂≧2*10⁻²⁰cm²/W, and a small cross section of the fiber core of the microstructured fiber, in particular with a diameter of from 1 μm to 4 μm, it is preferable for a laser diode to be used to generate the short light pulse to be coupled into the fiber, since the intensity of the light pulse of the laser diode is already sufficient under these conditions.
With laser diodes which have a pulse duration of from 1 picosecond to 10 nanoseconds, preferably from 10 picoseconds to 100 picoseconds, and emit light pulses with a wavelength λ_Pin the range from 500 nm≦λ_P≦1800 nm, preferably a wavelength λ_P=1065 nm, it is possible, by coupling these light pulses into the microstructured fiber, to generate broadband spectra with a wavelength range from 400 nm to 2000 nm, in particular from 700 nm to 1300 nm. Spectra within these wavelength ranges are particularly suitable for optical coherence tomography (OCT), since light with these wavelengths can readily penetrate into tissue which is to be examined.
In one suitable embodiment of the invention, the means for coupling the light pulse into the microstructured optical fiber comprise a free-beam optical system, comprising a positioning unit and an imaging optical system for beam focusing, preferably a microscope objective.
Further suitable means for coupling the light pulse into the microstructured optical fiber comprise a coupling optical waveguide and a plug connection for connecting the coupling optical waveguide to the microstructured fiber.
To couple in the light pulse with the minimum possible losses, the plug connection preferably has a guide which orientates the coupling optical waveguide and the microstructured optical fiber parallel to one another.
A guide of this nature is preferably designed as a ferrule. The ferrule is a small, highly accurate guide tube within the plug connection, which holds the ends of the fibers to be connected such that they are precisely axially aligned with one another and at the same time protects them. This ring may be made from various materials, for example glass, ceramic, plastic or metal.
In a further advantageous embodiment, the means for coupling the light pulse into the microstructured optical fiber comprise a coupling optical waveguide and a splice connection, by means of which the coupling optical waveguide and microstructured fiber are permanently joined.
The light source according to the invention is used as a light source for short coherence interferometry, in particular as a light source for OCT and for confocal microscopy and fluorescence microscopy.
Particular advantages are offered by the light source in the case of use in an endoscope, in particular in an endoscope having a short coherence tomograph device, a short coherence tomograph (OCT), since this combination for the first time enables endoscopic histological examinations and/or optical biopsies in vivo.
It is also advantageous to use such a light source as a broadband radiation source, replacing the lasers, in a fluorescence microscope. The preferred wavelength region of the light source is here from 250 nm to 1200 nm.
It is also very advantageous to use the short coherence light source in an endoscope having a confocal microscope. In this case, a particularly preferred wavelength region of the light source extends from approximately 350 nm to 790 nm and enables cellular states to be examined in vivo.

The invention is explained in more detail on the basis of the following drawings, in which:

FIG. 1 diagrammatically depicts a microstructured optical fiber,

FIG. 2 shows a light source according to the invention,

FIG. 3 shows the diagrammatic structure of an OCT arrangement,

FIG. 4 shows an endoscope having a short coherence tomography device in the case of which use is made of a light source according to the invention, and

FIG. 5 shows an endoscope having a confocal microscope in which a light source according to the invention is used.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In general, the quality of the optical resolution with appliances for coherence tomography is chiefly a function of the so-called coherence length (corresponding to the longitudinal coherence) or the optical bandwidth of the light source used.
A broadband light source based on a microstructured optical fiber such as is described below in detail is advantageously distinguished by an exceptionally short coherence length.
A resolution at the cellular level is achieved by the use of such a light source, and the possibility is thereby provided of supporting the classical biopsy by means of the method of optical biopsy, in particular optical biopsy in vivo.
On the basis of the current legal position in the Federal Republic of Germany, optical biopsy in the clinical field of gastroenterology need scarcely replace classical biopsy in the coming years. However, the surgeon can use the imaging method even before the extraction of the tissue in order to decide at least whether an extraction is necessary at all.
Thus, gastroendoscopic biopsy can also be carried out with fewer tissue sections and thus in a way that is much kinder to the patient.
Moreover, the number of the extracted tissue parts, and thus the tissue parts to be examined histologically, is greatly reduced, and this in turn is attended by a reduction in costs. Thus, it would eventually be possible for the extra costs in the acquisition of equipment to be quickly redeemed through lower laboratory costs and a shortened reconvalescence for the patient.
In addition to the abovedescribed use of the broadband light source, this light source can also be used for endomicroscopy. For endomicroscopy, it is normal to make use of the principle of confocal microscopy for the purpose of the autofluorescence excitation of proteins in the tissue cells.
Microscopes with confocal illumination for the purpose of fluorescence excitation normally make use as light source of one or more lasers of different light wavelengths, and this leads to high operating and maintenance costs.
A broadband light source based on a microstructured optical fiber can here take over the function of a number of lasers for the purpose of fluorescence excitation.
Other broadband light sources such as an incandescent lamp, or a light emitting diode that emits white light are only poorly suited to confocal microscopy because of the excessively low spatial coherence.
Reference is firstly made below to FIG. 2. FIG. 2 illustrates the diagrammatic structure of a light source (8) according to the invention illustrated by way of example. This light source comprises a laser diode (2), means (3) for coupling in a light pulse and a microstructured optical fiber (1).
The laser diode (2) emits light pulses of wavelength λ_P=1065 nm in the direction of the means (3) for coupling in the light pulse.
According to FIG. 2, the means (3) for coupling in the light pulse comprise a diaphragm (4) and a mirror (5) for directing the light pulse onto the microscope objective (6), which focuses the light pulse and couples it into the microstructured optical fiber (1).
The microstructured optical fiber (1), which is diagrammatically depicted in section in FIG. 1, comprises a fiber core (1.1), which is designed as a solid rod, and tubes (1.2), which are arranged uniformly around the fiber core (1.1) and form the fiber cladding. The tubes (1.2) are arranged in a number of layers around the fiber core (1.1), in such a manner that they form a hexagonal structure. Structural arrangements of this type allow the pulse to propagate through the microstructured optical fiber (1).
The fiber material is SF57 comprising 24.5% by weight of SiO₂, 74.5% by weight of PbO, 0.4% by weight of Na₂O and 0.6% by weight of K₂O.
The orders of magnitude of the microstructures are to be matched to the wavelength λ_P=1065 nm of the light pulses of the laser diode (2) and the material nonlinearity factor of the fiber material χ⁽³⁾, in such a manner that the microstructured optical fiber (1) has a null dispersion of the group velocity in the vicinity of the wavelength λ_Pand an anomalous dispersion.
The configuration of the fiber geometry is known per se to the person skilled in the art and has been described, for example, by Barkou, Broeng and Bjarklev in “Dispersion properties of photonic bandgap guiding fibers”, Optical Fiber Communication Conference, paper FG5, 1999 and by R. D. Meade, A. M. Rappe, K. D. Brommer, J. D. Joannopoulos and O. L. Alerhand in “Accurate theoretical analysis of photonic band-gap materials”, Phys. Rev. V 48, 8434-8437 (1993).
The diameter of the fiber core (1.1) is 2.8 μm, the diameter of the tubes (1.2) is 2.9 μm, with the total diameter of the microstructured optical fiber (1) being 125 μm.
The accurate production of this microstructured optical fiber (1) from a heavy flint glass as described above is carried out using an IR drawing process in accordance with US Application 03SGL0308USP.
Once the coupled in light pulse has passed through the microstructured optical fiber (1), it has an output spectrum of from 700 nm to 1300 nm with a substantially uniform intensity. The output spectrum was recorded using a spectrometer (7).
This arrangement provides a stable and broadband light source (8) with a simple structure and a short coherence which can be used, for example, in OCT appliances. This arrangement is selected by way of example; consequently, further embodiments are possible within the scope of the present invention.
FIG. 3 shows the use of the light source (8) according to the invention in an OCT arrangement. The OCT arrangement comprises the light source (8) which emits light with a spectrum in the range from 700 nm to 1300 nm. This light is diverted, via optical waveguides and a 2×2 coupler (9) via a collimator (10), onto the biological specimen (15) to be examined and is then diverted via a lens (11) onto a reference mirror (16). The light which is scattered and reflected from the specimen (15) and the light reflected from the reference mirror (16) are fed in superimposed form, via the 2×2 coupler (9), to the detector unit (12). The detector unit (12) determines the spectrum of the superimposition and uses an electronic processing unit (13) to display it as an image on a monitor (14).
Reference is made below to FIG. 4, which shows an endoscope having a short coherence tomography device or an endoscope having a short coherence tomograph (17), in the case of which use is made of a light source (8) according to the invention and having a photonic fiber 1. In addition, the light source 8 can have a lens 20 downstream of the exit end 1.3 of the photonic fiber 1 with the aid of which the light emerging substantially in a punctiform fashion from the end 1.3 is parallelized.
A further lens 21 for focusing the laser light onto the entry end 1.4 of the fiber 1 can likewise optionally be provided.
The parallel beam produced with the aid of the lens 20 is then split into two component beams 27, 29 in a beam splitter 25, the component beam 27 being directed onto the sample 15, for example tissue to be examined, that is to examined, and is retroreflected. The component beam 29 is retroreflected by a reflector 31.
The two reflecting component beams are combined in the beam splitter 25 to form a light beam 33 that is fed to an optical detector 37 by means of a further lens 35 so that an image of the tissue 15 to be examined is produced on the detector 37 in a fashion superposed on the second component beam of the interferometer.
The detector 37 relays optically acquired image data in electronically converted form to an evaluation unit, which is not illustrated in FIG. 4 and inside which it is possible for these image data to be acquired, recorded and reproduced in a fashion assigned to the respective tissue depth.
The light source 8, the beam splitter 25, as well as the reflector 31 and the retroreflecting or backscattering sample 15 together form a Michelsen interferometer arrangement, the result being interference between the two reflected component beams 27, 29 in the combined beam 33.
Owing substantially to interference between the two component beams 27, 29 of the interferometer, this image of the tissue 15 to be examined exhibits contrast only at those points of the tissue 15 at which the two component beams 27, 29 have an optical path length difference that is smaller than the coherence length or longitudinal coherence.
The depth of the imaged region inside the tissue 15 can be set by displacing the tissue 15 relative to the beam splitter 25 and/or by displacing the mirror 31.
It is possible thereby to compile for various tissue depths two-dimensional pictures resembling a histological tissue section that substantially has the thickness of the longitudinal coherence length.
By being assigned to their depth, it is possible for these two-dimensional pictures to be assembled in the evaluation unit to form three-dimensional tomographic pictures. It can be advantageous here to move only the mirror 31 by means of a linear displacement (not illustrated in FIG. 4) with the aid of a position encoder, and not to vary the distance of the endoscope relative to the tissue 15, something which is rendered possible, for example, by applying the endoscope to the tissue 15.
In a preferred embodiment, the short coherence tomograph 15 can, for example, be entirely accommodated in the head of the endoscope. For example, the laser diode can, however, also be arranged outside the head of the endoscope, and the light for the interferometer arrangement arranged in the head can be supplied through the photonic fiber 1.
Application areas for OCT are in particular in medicine, for example in early diagnosis of cancer, in particular for stomach, bowel, vascular or skin examinations.
In these cases, reflections at the interfaces between materials with different refractive indices (membrane, cell layers, organ boundaries) are measured, preferably in slicewise fashion and in this way a three-dimensional image is reconstructed by computer aided evaluation of the data obtained in slicewise fashion. On account of the high bandwidth of the light source (8) according to the invention, resolutions in the submicrometer range are possible, so that it is possible to produce subcellular structures.
FIG. 5 shows an endoscope having a confocal microscope 38 in the case of which use is made of a light source 8 according to the invention.
A point light source is produced by means of a diaphragm 39 placed in front of the light source 8. The light from the point light source is deflected by a beam splitter 41 and focused onto the sample 15 by means of an objective lens 43. Since the light emerging from the photonic fiber is already substantially punctiform, the diaphragm 39 can also be omitted in a modification of the exemplary embodiment shown in FIG. 5.
The focus of the light beam is scanned laterally along the sample 15 by means of a scanning unit 42 controlled by a computer 45.
The retroreflected light is focused onto a further diaphragm 40, and the light passing through is detected with the aid of an optical detector 44. An image is then calculated in the computer 45 from the location and intensity data.
The confocal arrangement in this case produces a lateral tomogram of the sample to be examined, it being possible to calculate a three-dimensional reconstruction from a number of such images. In particular, it is also possible to carry out a fluorescence measurement with the aid of this arrangement.
In this case, a dichroic mirror that acts, in particular, in a wavelength selected fashion can be used as beam splitter 41.
In the case of such a fluorescence microscope, the light source (8) can have a wavelength region from approximately 250 nm to 1020 nm.
The microscope 38 can, for example, be completely accommodated in the endoscope head, given a sufficiently small design.
Likewise, however, it is also possible for only parts of the microscope to be arranged in the head of the endoscope. For example, the objective lens 43 and the scanning unit 42 can be arranged there, while the light is guided from the beam splitter 41 to the scanning unit through an endoscope tube. It may be stated for the sake of completeness that for the purpose of better understanding various constituents of the embodiments were not illustrated true to scale in the foregoing detailed description of preferred embodiments.

Claims

1. A light source (8) which has a broadband spectrum, comprising

a laser (2) for generating a short light pulse of wavelength λ_P,

a microstructured optical fiber (1) which has a null dispersion of the group velocity in the vicinity of the wavelength λ_Pand an anomalous dispersion, and

means (3) for coupling the light pulse into the microstructured optical fiber (1),

wherein the microstructured optical fiber (1) has a refractive index n which is dependent on radiant intensity and where n(I)=n₀+n₂*I at the wavelength λ_P, where n₂≧2*10⁻²⁰cm²/W.

2. (canceled)

3. The light source (8) as claimed in claim 1, wherein the microstructured optical fiber (1) comprises a nonlinear optical material.

4. The light source (8) as claimed in claim 1, wherein the microstructured optical fiber (1) comprises at least one material formed from at least one of a multicomponent glass, a multicomponent glass-ceramic, a monocrystalline material, a polycrystalline material, a plastic matrix composite and a liquid-crystal material.

5. The light source (8) as claimed in claim 4, wherein the optical material has at least one of isotropic and anisotropic properties.

6. The light source (8) as claimed in claim 4, wherein the microstructured optical fiber (1) comprises at least one nonoxidic multicomponent glass.

7. The light source (8) as claimed in claim 6, wherein the nonoxidic multicomponent glass is a chalcogenide glass that includes at least As and Sn.

8. The light source (8) as claimed in claim 4, wherein the microstructured optical fiber (1) comprises at least one oxidic multicomponent glass.

9. The light source (8) as claimed in claim 8, wherein the oxidic multicomponent glass comprises a silicate glass which has at least one element selected from the group consisting of alkali metals (Li₂O, Na₂O, K₂O, Rb₂O, Cs₂O) or at least one element selected from the group consisting of alkaline-earth metals (MgO, CaO, SrO, BaO), or at least one element selected from each group thereof.

10. The light source (8) as claimed in claim 9, wherein the oxidic multicomponent glass has at least one further element selected from the group consisting of Al₂O₃, B₂O₃, PbO, ZnO, TiO₂, ThO₂, ZrO₂, La₂O₃, CeO₂and P₂O₅.

11. The light source (8) as claimed in claim 10, wherein the oxidic multicomponent glass comprises a heavy flint glass which has the components SiO₂and PbO and at least one of the components Al₂O₃, B₂O₃, TiO₂, ThO₂, La₂O₃, BaO, Li₂O, Na₂O and K₂O.

12. (canceled)

13. The light source (8) as claimed in claim 4, wherein the microstructured optical fiber (1) comprises at least one oxidic multicomponent glass-ceramic.

14. The light source (8) as claimed in claim 13, wherein the multicomponent glass ceramic has at least one crystal phase formed from at least one of the following: strontium niobate, potassium hydrogen phosphate (KTP), BBO, LBO, LiIO₃, LiNbO₃, KnbO₃, AgGaS₃, AgGaSe₂, PPLN and BaTiO₃.

15. The light source (8) as claimed in claim 4, wherein the microstructured optical fiber (1), comprises at least one monocrystalline material formed from one of the following: strontium niobate, potassium hydrogen phosphate (KTP), BBO, LBO, LiIO₃, LiNbO₃, KnbO₃, AgGaS₃, AgGaSe₂, PPLN and BaTiO₃.

16. The light source (8) as claimed in claim 4, wherein the microstructured optical fiber (1) comprises at least one polycrystalline material formed from at least one of the following: strontium niobate, potassium hydrogen phosphate (KTP), BBO, LBO, LiIO₃, LiNbO₃, KnbO₃, AgGaS₃, AgGaSe₂, PPLN and BaTiO₃.

17. The light source (8) as claimed in claim 4, wherein the microstructured optical fiber (1) comprises a plastic-matrix composite formed from an oxidic multicomponent glass, and plastics based on one of the following: PMMA (polymethylmethacrylate), PC (polycarbonate), PA (polyamide) and PE (polyethylene).

18. The light source (8) as claimed in claim 4, wherein the microstructured optical fiber (1) comprises a liquid crystal which has at least one polymeric fraction with a mesogenic group within one of (i) the main polymer chain and (ii) a side chain branching off therefrom.

19. The light source (8) as claimed in claim 1, wherein the microstructured optical fiber (1) has a fiber core (1.1) running along the fiber length, and a structured fiber cladding arranged around the fiber core (1.1).

20. The light source (8) as claimed in claim 19, wherein the fiber core (1.1) comprises a solid body, and the fiber cladding comprises hollow structures running parallel to the fiber core (1.1).

21. The light source (8) as claimed in claim 20, wherein the fiber core (1.1) comprises a solid rod, and the fiber cladding comprises tubes (1.2) arranged uniformly around the solid rod.

22. The light source (8) as claimed in claim 21, wherein the fiber core (1.1) has a diameter of from 1 μm to 4 μm, and the tubes (1.2) have a diameter of from 2 μm to 8 μm.

23. The light source (8) as claimed in claim 19, wherein the microstructured optical fiber (1) can be produced using an IR drawing process in accordance with the US Application 03SGL0308USP.

24. The light source (8) as claimed in claim 1, wherein the laser (2) comprises a laser diode (2) for generating a short light pulse.

25. The light source (8) as claimed in claim 24, wherein the light pulses of the laser diode (2) have a pulse duration of from 1 picosecond to 10 nanoseconds.

26. The light source (8) as claimed in claim 24, wherein the light pulses of the laser diode (2) have a wavelength λ_Pin the region from 500 nm≦λ_P≦1800 nm.

27. The light source (8) as claimed in claim 23, wherein the broadband spectrum comprises a wavelength from 400 nm to 2000 nm.

28. The light source (8) as claimed in claim 1, wherein the means (3) for coupling the light pulse into the microstructured optical fiber (1) comprise a free-beam optical system comprising a positioning unit (4, 5) and an imaging optical system for beam focusing.

29. The light source (8) as claimed in claim 1, wherein the means (3) for coupling the light pulse into the microstructured optical fiber (1) comprise a coupling optical waveguide and a plug connection.

30. The light source (8) as claimed in claim 29, wherein the plug connection has a guide which orientates the coupling optical waveguide and the microstructured optical fiber (1) parallel to one another.

31. The light source (8) as claimed in claim 30, wherein the plug connection has a ferrule.

32. The light source (8) as claimed in claim 1, wherein the means (3) for coupling the light pulse into the microstructured optical fiber (1) comprise a coupling optical waveguide and a splice connection.

33. The light source (8) as claimed in claim 1, defined by its use as a light source (8) for short coherence interferometry.

34. The light source (8) as claimed in claim 33, defined by its use as a light source (8) for OCT.

35. The light source (8) as claimed in claim 1, defined by its use in imaging endoscopy.

36. The light source (8) as claimed in claim 1, defined by its use as a light source (8) for a confocal microscope.

37. The light source (8) as claimed in claim 1, defined by its use as a light source (8) for a fluorescence microscope.

38. The light source (8) as claimed in claim 1, wherein the light source defines a short-coherence measuring appliance.

39. An endoscope defined by a light source (8) which has a broadband spectrum, comprising

a laser (2) for generating a short light pulse of wavelength λ_P,

wherein the microstructured optical fiber (1) has a refractive index n which is dependent on radiant intensity and where n(I)=n₀+n₂*I at the wavelength λ_P, wherein n₂≧2*10⁻²⁰cm²/W.

40. The endoscope as claimed in claim 39 having a short coherence tomography device, wherein the short coherence tomography device is itself defined by the light source (8).

41. The endoscope as claimed in claim 40, in which the short coherence tomography device is arranged in the endoscope and is provided with at least one of an imaging device, a light-guiding fiber and an electronic detector.

42. A confocal microscope defined by a light source (8) which has a broadband spectrum, comprising

a laser (2) for generating a short light pulse of wavelength λ_P,

43. The confocal microscope as claimed in claim 42, wherein said confocal microscope is part of an endoscope.

44. The confocal microscope as claimed in claim 42, defined by a wavelength region of the light source (8) from approximately 350 nm to 790 nm.

45. A fluorescence microscope defined by a light source (8) which has a broadband spectrum, comprising

a laser (2) for generating a short light pulse of wavelength λ_P,

46. The fluorescence microscope as claimed in claim 45, defined by a wavelength region of the light source (8) from approximately 250 nm to 1200 nm.