Abstract:
An optical probe has an optical fiber, a deflecting element, and a protective tube. The optical fiber includes a glass filament having a first diameter for transmitting light between the proximal and distal ends thereof and a resin layer for covering the filament except for the distal end thereof. The deflecting element is made of glass in a circular form having a second diameter larger than the first diameter, and it is connected with the optical fiber and has an end-face having a normal vector whose angle relative to the central axis is larger than the critical angle. The protective tube surrounds a portion of the optical fiber and the entire length of the deflecting element and is adhered to the side of a deflecting optical element, whereas the inside diameter of the part covering the optical fiber is smaller than that of the part covering the deflecting element.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an optical probe for optical measurement using Optical Coherence Tomography (OCT). 
     BACKGROUND ART 
     Optical coherence tomography (OCT) is known as a technique for measuring the intra-lumen tomographic structure of an object having a luminal form, such as a blood vessel. Also, as disclosed in U.S. Pat. No. 6,445,939B (Patent document 1), an optical probe inserted into the lumen of an object for performing such OCT measurement is known. The optical probe has a structure in which the tip (distal end) of a single mode optical fiber is connected to a graded index optical fiber having substantially the same diameter as the single mode optical fiber. By constituting the graded index optical fiber as a lens (GRIN lens) in such manner as the working distance is longer than 1 mm and the spot size is smaller than 100 μm, an object having an inner radius of more than 1 mm can be measured at spatial resolution finer than 100 μm according to the OCT method. 
     Furthermore, as disclosed in U.S. Pat. No. 6,891,984B (Patent document 2) and U.S. Pat. No. 7,813,609B (Patent document 3), by connecting an optical fiber, a GRIN lens, and a deflecting optical element altogether and by covering them with a transparent tube so that the air may be enclosed adjacent to the oblique end face of the deflecting optical element, it is made possible to make an optical probe which can reflect light in a lateral direction at the oblique end face. By rotating such optical probe, the tomographic structure of a luminal object, such as a blood vessel, can be measured by OCT. 
     SUMMARY OF THE INVENTION 
     Technical Problem 
     The object of the present invention is to provide an optical probe capable of stable performance even if it is bent. 
     Solution to Problem 
     An optical probe of the invention has an optical fiber, an optical connector, a deflecting optical element, a protective tube, and a jacketing tube. The optical fiber includes a glass filament in which the circular cross-section perpendicular to the central axis has a first diameter and which can transmit light between the proximal and distal ends thereof, and a resin layer which covers the side of the filament, whereas a portion of predetermined length from the distal end of the filament is not covered with the resin layer. The optical connector is connected with the optical fiber at the proximal end. The deflecting optical element is made of glass having a form in which the circular cross-section perpendicular to the central axis has a second diameter larger than the first diameter, and in which the refractive-index profile is such that the refractive index gradually decreases as it is distanced from the central axis in a cross-section perpendicular to the central axis, whereas a first end thereof is connected with an optical fiber at the distal end, and a second end face thereof has a normal vector in which the angle relative to the central axis is larger than the critical angle of total reflection. The protective tube surrounds a predetermined partial length of the optical fiber and the entire length of the deflecting optical element and is adhered to the side of a deflecting optical element, whereas the inside diameter of the part covering the optical fiber is smaller than the inside diameter of the part covering the deflecting optical element. The jacketing tube extends surrounding the optical fiber and can freely rotate relative to the optical fiber, the optical connector, and the deflecting optical element. 
     In the optical probe of the present invention, the diameter of the deflecting optical element may be 1.02 times to 1.10 times the diameter of the filament of the optical fiber. 
     Advantageous Effects of Invention 
     In the optical probe of the present invention, the tube can be prevented from coming off or being detached when a force is applied to the tube covering the GRIN lens, and the probe can stably be used in a state where it is bent. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic diagram showing an OCT system having an optical probe according to an embodiment of the present invention. 
         FIG. 2  is a sectional view of the distal end of an optical probe according to an embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. In explanation of the drawings, the same mark is given to identical elements, and overlapping explanation will he omitted. The present invention is not limited to the embodiments and is shown by the claims, and it should be noted that all modifications which are equivalent to a claim in terms of meaning or scope are included in the scope of the invention. 
     As disclosed in Patent documents 1 to 3, with respect to optical probes of conventional technologies, the diameter of a GRIN lens and the diameter of an optical fiber are the same with each other. Therefore, it has been a problem that a probe tends to suffer from failures: for example, the tube comes off or the tube is detached from the GRIN lens because of a force applied to the tube covering the GRIN lens at the tip when the optical fiber and the GRIN lens are moved or rotated at a high speed in a state where the optical probe is bent. In particular, such failures easily occur when an optical probe is inserted in a bent blood vessel, because the optical probe is also bent accordingly, whereby the force applied to the tube becomes larger. These failures will degrade the quality of images obtained by OCT because in such cases unnecessary light reflection is caused on the side face of the GRIN lens of the optical probe or reflection efficiency is decreased at the end of the GRIN lens. 
       FIG. 1  is a schematic diagram showing an OCT system  1  having an optical probe  10  according to an embodiment of the present invention. The OCT system  1 , which has an optical probe  10  and a measuring unit  30 , obtains tomographic images of an object  3 . 
     The optical probe  10  comprises: an optical fiber  11  for transmitting light between proximal end  11   a  and distal end  11   b ; an optical connector  12  connected with the optical fiber  11  at the proximal end  11   a ; a deflecting optical element  13  connected with the optical fiber  11  at the distal end  11   b ; a supporting tube  14  surrounding the optical fiber  11  and extending along the optical fiber  11 ; and a jacketing tube  15 . The optical connector  12  is optically connected to a measuring unit  30 . 
     The measuring unit  30  comprises: a light source  31  for generating light; an optical branch  32  for branching the light emitted from the light source  31  and outputting the branched light as illumination light and reference light; an optical detector  33  for detecting the light which has reached from the optical branch  32 ; a terminal  34  for outputting the reference light which has reached from the optical branch  32 ; a mirror  35  for reflecting, toward the terminal  34 , the reference light which has been outputted from the terminal  34 ; an analyzing part  36  for analyzing the spectrum of the light detected by the optical detector  33 ; and an output port  37  for outputting results analyzed by the analyzing part  36 . 
     The fight output from the light source  31  is branched into two by the optical branch  32  at the measuring unit  30  and emitted as illumination light and reference light. The illumination light output from the optical branch  32  enters into the proximal end  11   a  of the optical fiber  11  through the optical connector  12  and is guided by the optical fiber  11  so as to be emitted from the distal end  11   b , and the light is irradiated to an object  3  through the deflecting optical element  13 . The back reflected light which has arisen as a result of the illumination light being irradiated onto the object  3  enters into the distal end  11   b  of the optical fiber  11  through the deflecting optical element  13 , and is guided by the optical fiber  11  so as to be emitted from the proximal end  11   a , and the light is connected to the optical detector  33  through the optical connector  12  and the optical branch  32 . 
     The reference light output from the optical branch  32  is emitted from the terminal  34  and reflected at the mirror  35  and connected to the optical detector  33  through the terminal  34  and the optical branch  32 . The reference light and the light reflected back from the object  3  are combined by the optical branch  32  into interference light, which is detected by the optical detector  33 . The spectrum of interference light is input into the analyzing part  36 , wherein the spectrum of interference light is analyzed and the profile of the back reflection efficiency at each point inside the object  3  is calculated. The tomographic images of the object  3  are calculated on basis of the results of such calculation and outputted from the output port  37  as image signals. 
     Strictly speaking, as to a mechanism in which the illumination light emitted from the distal end  11   b  of the optical fiber  11  returns to the distal end  11   b  of the optical fiber  11  again via the object  3 , there are reflection, refraction, and dispersion. However, such distinction is not essential to the present invention, and therefore, for the sake of concision, they are generically called as back-reflection in this specification. 
     At the measuring unit  30 , the light source  31  generates broadband light which has a spectrum spreading continuously over the wavelength range of 1.6 μm to 1.8 μm. In this wavelength range, a lipid lesion has an absorption peak at a wavelength of 1.70 to 1.75 μm, and in this respect, it differs from a normal blood vessel. Therefore, when an object  3  containing lipid is measured, the spectrum of interference light is affected by absorption of lipid and exhibits significant attenuation at a wavelength of 1.70 to 1.75 μm as compared with a contiguous wavelength band. 
     Furthermore, since the spectrum of interference light has information on the tomographic structure of the object  3 , it is possible to obtain information on the tomographic structure of the object  3  by conducting Fourier analysis of the spectrum by choosing a wavelength zone which is less influenced by absorption of a substance. By analyzing both tomographic structure information and lipid absorption information, it is made possible to calculate a tomographic image in which the lipid distribution is shown. 
     In such calculation, a plurality of lipid distributions may be derived from one spectrum, since a spectrum is influenced by both the absorption of lipid itself and the distribution of lipid. However, as generally known, the scattering intensity of lipid is lower as compared with the scattering intensity of a normal blood vessel, and therefore it is possible to obtain a true lipid distribution by choosing a solution which is most suitable to such known information. 
     The optical fiber  11  constituting the optical probe  10  is made of silica glass and has a length of 1 to 2 m. The optical fiber  11  is a filament having a circular cross-section. The optical fiber  11  has an attenuation of 2 dB or less, preferably 1 dB or less, in the wavelength range of 1.6 μm to 1.8 μm and has a cutoff wavelength of 1.53 μm or less, and operates in a single mode in the above-mentioned wavelength range. An optical fiber based on ITU-T G.652, G.654, or G.657 is suitable. In particular, an optical fiber based on ITU-T G.654 A or C is suitable, because it has an attenuation of 0.22 dB or less at the wavelength of 1.55 μm and has typically a pure silica glass core, and the noise due to nonlinear optical effects, such as a self-phase modulation can be reduced, since its nonlinear optical coefficient is low. 
     As the deflecting optical element  13 , GRIN lens haying a slant end-face is fusion-spliced to the distal end  11   b  of the optical fiber  11 . The deflecting optical element  13  concentrates light emitted from the distal end  11   b  of the optical fiber  11  and deflects the light to a radial direction. The GRIN lens is made of silica glass, has a columnar form, and has an attenuation of 2 dB or less in the wavelength range of wavelength of 1.6 μm to 1.8 μm. The tip of the GRIN lens has a flat reflective surface having the normal vector at an angle beyond a total-reflection critical angle (43 degrees or more in the case of silica glass) relative to the axis. Therefore, at a position of the reflective surface, a cross-section perpendicular to the axis exhibits a partially circular form which lacks a part of the section. 
     If the angle of normal vector agree with 45 degrees, the reflected light reflected at the lens side will return to the optical fiber  11 , resulting in noise. Therefore, it is desirable that the angle of normal vector be different from 45 degrees. On the other hand, if it is too large, the distance of light propagation will be prolonged, resulting in increase in optical loss. Therefore, it is desirable that the angle of normal vector be 46 to 51 degrees. As disclosed in Patent document 2, a hollow contiguous to a reflective surface can be formed by surrounding a lens with plastic tube, such as PET, and thereby total reflection of light can be caused at a reflective surface. 
     The optical fiber  11  is stored in the lumen of a supporting tube  14 . The supporting tube  14  is fixed to an optical connector  12  and at least one portion of the optical fiber  11 . As a result, if the optical connector  12  is rotated, the supporting tube  14  also rotates with it, transmitting the running torque to the optical fiber  11 , and thereby the optical fiber  11 , the deflecting optical element  13 , and the supporting tube  14  will be rotated altogether. Therefore, it is possible to lessen the torque applied to the optical fiber  11  as compared with the torque where only the optical fiber  11  rotates, and thereby the optical fiber  11  can be prevented from fracturing due to torque. 
     It is desirable for the supporting tube  14  to have a thickness of 0.15 mm or more and Young&#39;s modulus of 100 to 300 GPa, which is substantially equivalent to stainless steel. The supporting tube  14  is not necessarily required to be connected in the circumferential direction and may have structure in which about 5 to 20 pieces of wires are twisted together so that its pliability may be adjusted. 
     The optical fiber  11 , the deflecting optical element  13 , and the supporting tube  14  are stored in the lumen of the jacketing tube  15 , and they can be rotated in it. This will prevent an object  3  from being damaged by a rotating part contacting the object  3 . The illumination light is emitted from the deflecting optical element  13 , transmitted through the jacketing tube  15 , and irradiated to the object  3 . The jacketing tube  15 , which is formed with transparent resins, such as PEBA (polyether block amide), Nylon, FEP (fluorinated ethylene-hexafluoopropylene copolymer), PFA (polyfluoroalkoxy), PTFE (polytetrafluoroethylene), and PET (polyethylene terephthalate), has a thickness of 10 to 50 μm and transparency like a transmission loss of 2 dB or less at a wavelength of 1.6 to 1.8 μm. 
       FIG. 2  is a sectional view of the distal end of an optical probe  10  of the present embodiment. A deflecting optical element (GRIN lens)  13  having a slant end-face is fusion spliced with the tip of the optical fiber  11 . The optical fiber  11  is covered with a resin layer  11   c  made of polyimide or acrylate for mechanical protection, whereas the resin layer  11   c  is partially removed by a predetermined length at the tip for the purpose of fusion splicing with the GRIN lens  13 . 
     A part of resin layer  11   c  of the optical fiber  11 , the part where the resin layer  11   c  is removed, and the GRIN lens  13  are covered with a protective tube  16 . The protective tube  16 , which prevents the glass of the GRIN lens  13  and the optical fiber  11  from being damaged and losing strength, forms a hollow  17  which is contiguous to the reflective surface of the tip of the GRIN lens  13 . It is desirable for the protective tube  16  to transmit near-infrared light and to stick to the GRIN lens  13  so as to restrain unnecessary reflection at the interface. Therefore, it is preferable to make the protective tube  16  with PET or PEBA having thermal contraction properties. 
     The optical fiber  11  has an outer diameter of 125 μm. On the other hand, the GRIN lens  13  has an outer diameter of 127 to 135 μm, which is 2 to 10% larger than the outer diameter of the optical fiber  11 . This will increase adhesion between the GRIN lens  13  and the protective tube  16 , Thus, when rotational or back-and-forth movement of the optical fiber  11  is performed in the optical probe  10 , the protective tube  16  will be prevented from detaching from the GRIN lens  13 , and occurrence of unnecessary reflection can he avoided. 
     The refractive power of the GRIN lens  13  varies according to the length of the GRIN lens  13 . Therefore, it is important to manage the length of the GRIN lens  13  with sufficient reproducibility when manufacturing the optical probe  10 . Usually, it is difficult to identify the connection interface between the optical fiber  11  and the GRIN lens  13  because they are both made of silica glass. However, if the optical fiber  11  and the GRIN lens  13  have outer diameters different from each other by 2 to 10% as in this embodiment, it is easy to identify their connection interface, and consequently the length of the GRIN lens  13  can easily be measured, so that the reproducibility of the optical properties of the optical probe  10  can be improved. 
     The GRIN lens  13  and the protective tube  16  are sealed with a transparent resin  18  for mechanical protection. As to the transparent resin  18 , it is desirable to use an ultraviolet-curable epoxy or acrylate resin, since they can be easily molded. However, it is also possible to use a resin which is curable by heat or mixture of two kinds of liquid components.