Patent Publication Number: US-8967885-B2

Title: Stub lens assemblies for use in optical coherence tomography systems

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is related to U.S. patent application Ser. No. 13/403,446, entitled “Methods of making a stub lens element and assemblies using same for optical coherence tomography applications,” and to U.S. patent application Ser. No. 13/403,485, entitled “Probe optical assemblies and probes for optical coherence tomography,” both of which have been filed on the same day as the present application and both of which are incorporated by reference herein. 
     FIELD 
     The present invention relates to optical coherence tomography, and in particular to stub lens assemblies for use in optical coherence tomography systems. 
     BACKGROUND ART 
     Optical coherence tomography (OCT) is used to capture a high-resolution cross-sectional image of scattering biological tissues and is based on fiber-optic interferometry. The core of an OCT system is a Michelson interferometer, wherein a first optical fiber is used as a reference arm and a second optical fiber is used as a sample arm. The sample arm includes the sample to be analyzed as well as a probe that includes optical components. An upstream light source provides the imaging light. A photodetector is arranged in the optical path downstream of the sample and reference arms. 
     Optical interference of light from the sample arm and the reference arm is detected by the photodetector only when the optical path difference between the two arms is within the coherence length of the light from the light source. Depth information from the sample is acquired by axially varying the optical path length of the reference arm and detecting the interference between light from the reference arm and scattered light from the sample arm that originates from within the sample. A three-dimensional image is obtained by transversely scanning in two dimensions the optical path in the sample arm. The axial resolution of the process is determined by the coherence length. 
     To obtain a suitably high-resolution 3D image, the probe typically needs to meet a number of specific requirements, which can include: single-mode operation at a wavelength that can penetrate to a required depth in the sample; a sufficiently small image spot size; a working distance that allows the light beam from the probe to be focused on and within the sample; a depth of focus sufficient to obtain good images from within the sample; a high signal-to-noise ratio (SNR); and a folded optical path that directs the light in the sample arm to the sample. 
     In addition, the probe needs to fit within a catheter, which is then snaked through blood vessels, intestinal tracks, esophageal tubes, and like body cavities and channels. Thus, the probe needs to be as small as possible while still providing robust optical performance. Furthermore, the probe operating parameters (spot size, working distance, etc.) will substantially differ depending on the type of sample to be measured and the type of measurement to be made. 
     SUMMARY 
     An aspect of the disclosure is a stub lens assembly that includes an optical fiber having an end, and an optical fiber ferrule having an end and central bore. The optical fiber ferrule supports the optical fiber within the central bore so that the optical fiber end resides substantially at the optical fiber ferrule end. The stub lens assembly also has a sleeve having a central channel with first and second ends. At least a portion of the optical fiber ferrule is operably supported within the central channel at the first end. The stub lens assembly also has a stub lens element having a stub section with a proximal end that resides adjacent the optical fiber end. The stub section is formed integral with a lens having a distal end that includes a lens surface. At least a portion of the stub section resides within the central channel at the second end so that the optical fiber end and the proximal end of the stub section are in a cooperative optical relationship. 
     Another aspect of the disclosure includes a stub lens assembly that has an optical fiber having an end, and that also has an optical fiber ferrule having an end and that supports the optical fiber end substantially at the optical fiber ferrule end. The stub lens assembly has a sleeve with first and second ends and that operably supports the optical fiber ferrule at the first end. The stub lens assembly also includes a stub lens element having a lens and a stub section integrally formed therewith and having a proximal end, with the stub lens element being operably supported by the sleeve at the second end so that the optical fiber and the stub lens element are in a cooperative optical relationship along a common axis. 
     It is to be understood that both the foregoing general description and the following Detailed Description represent embodiments of the disclosure, and are intended to provide an overview or framework for understanding the nature and character of the disclosure as it is claimed. The accompanying drawings are included to provide a further understanding of the disclosure, and are incorporated into and constitute a part of this specification. The drawings illustrate various embodiments of the disclosure and together with the description serve to explain the principles and operations of the disclosure. 
     Additional features and advantages of the disclosure are set forth in the detailed description that follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the disclosure as described herein, including the detailed description that follows, the claims, and the appended drawings. 
     The claims are incorporated into and constitute part of the Detailed Description set forth below. 
     Any numerical provided herein are inclusive of the limits provided unless otherwise stated. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view of a rod made of an optical material, shown with one of its ends operably arranged relative to a heat source; 
         FIG. 2  is similar to  FIG. 1  and shows a stub lens element formed by heating one end of the rod to form a bulbous end portion that defines a lens having a lens surface; 
         FIG. 3A  is similar to  FIG. 2  and shows the stub lens element in the process of having its lens reduced in size in the lateral dimension by mechanical means; 
         FIG. 3B  is similar to  FIG. 3A  and shows the stub lens element having its lens reduced in size in the lateral dimension via laser processing with a laser beam; 
         FIG. 4A  and  FIG. 4B  illustrate examples of the stub lens element formed by processing the lens to reduce its lateral dimension; 
         FIG. 4C  is similar to  FIG. 4B  and shows an example where the lens surface includes an anti-reflection coating; 
         FIG. 5A  and  FIG. 5B  are similar to  FIG. 4A , with  FIG. 5A  showing a cutting tool being used to cut the stub section to form an angled proximal end, the resulting angled proximal end being shown in  FIG. 5B ; 
         FIG. 6A  is a cross-sectional view of an example stub lens element shown along with a cylindrical sleeve; 
         FIG. 6B  is similar to  FIG. 6A  and shows the stub element operably engaged with the sleeve to form a stub lens sub-assembly; 
         FIG. 6C  is similar to  FIG. 6B  and shows an example sleeve that includes a slot that leads from the sleeve outer surface to the sleeve central channel; 
         FIG. 6D  is similar to  FIG. 6C  and shows an adhesive material disposed in the slot and that serves to secure the stub section of the stub lens element to the sleeve; 
         FIG. 7A  is similar to  FIG. 6B  and shows the stub lens sub-assembly of  FIG. 6A  along with an example optical fiber ferrule; 
         FIG. 7B  is a close-up view of the optical fiber ferrule of  FIG. 7A ; 
         FIG. 7C  shows the optical fiber ferrule with an optical fiber secured therein, engaged with the central channel of the sleeve to form a stub lens assembly, and also shows a photodetector used to measure the mode field diameter of the focused light when performing alignment of the stub lens element relative to the optical fiber; 
         FIG. 8A  is a cross-sectional view that shows the stub lens assembly and a light-deflecting member along with a support member in the form of an outer sleeve, in the process of fabricating a probe optical assembly to be used to form an OCT probe; 
         FIG. 8B  shows the stub lens assembly, light-deflecting member and outer sleeve operably arranged to form the probe optical assembly; 
         FIG. 9A  is a cross-sectional view of an example OCT probe that utilizes the probe optical assembly of  FIG. 8B ; 
         FIG. 9B  is a close-up view of the probe optical assembly contained within the interior of the jacket of probe, showing incident light and scattered light traversing the optical path in opposite directions; 
         FIG. 10  plots the object distance OD (horizontal axis) vs. the working distance WD (left vertical axis, solid-line curve) and the mode field diameter MFD (right vertical axis, dotted-line curve) in connection with designing an example stub lens element, with all dimensions being in microns; 
         FIG. 11  is a plot similar to that of  FIG. 10  that plots the image MFD IM  (microns) versus the working distance WD (microns) for the case of a single-mode optical fiber, but where the fiber MFD F  is changed from 10 um to 7 um. 
         FIG. 12  is similar to  FIG. 7B  and illustrates an example modification of the optical fiber by providing a lens at the optical fiber end either by re-shaping the otherwise flat optical fiber end or by adding a separate lens element; 
         FIG. 13A  through  FIG. 13C  illustrate an example method of forming a fiber pigtail lens assembly using a fusion splicing process; 
         FIG. 13D  is similar to  FIG. 13C  and shows the fiber pigtail assembly operably engaged with the ferrule so that the lens is adjacent one of the ferrule ends; 
         FIG. 13E  is similar to  FIG. 13D  and shows an example embodiment where the lateral dimension of the lens has been reduced in size; 
         FIG. 13F  shows the fiber pigtail assembly of  FIG. 13E  as arranged in a ferrule, with the ferrule and fiber pigtail operably disposed on a transparent support substrate adjacent a light-deflecting member; 
         FIG. 14  is similar to  FIG. 8B  and illustrates an example embodiment of the probe optical assembly that employs a fiber pigtail lens assembly in place of the stub lens assembly; 
         FIG. 15  is a plots the relationship between the length L (mm) of a stub lens element and the lens diameter D 2  (mm) of the stub lens element; 
         FIG. 16A  is a cross-sectional view of another example embodiment of the stub lens assembly that includes the fiber pigtail lens assembly that has a first fused fiber lens element in combination with a second stub lens element; 
         FIG. 16B  is similar to  FIG. 16A , but with the outer sleeve replaced by a support substrate; 
         FIG. 17  is similar to  FIG. 16B  and illustrates another example embodiment of a fused lens assembly wherein the support substrate is made of a transparent material, and the second stub lens has an angled surface that serves as a total-internal-reflection (TIR) mirror; 
         FIG. 18  is similar to  FIG. 17  and illustrates an embodiment that includes a transparent monolithic structure that includes a stub lens element portion; 
         FIG. 19  is a plot of the length L (microns) (horizontal axis) versus the image MFD IM  (microns) (left-hand vertical axis) and working distance WD (microns) (right-hand vertical axis) as defined as the beam-waist location, for an example fused lens element suitable for use in the fiber pigtail lens assembly; 
         FIG. 20  is a side view of an example fused fiber pigtail lens assembly wherein the lens includes an angled surface that defines a TIR mirror that serves to fold the lens axis and direct it through a portion of the lens surface; and 
         FIG. 21  is similar to  FIG. 20  and illustrates another embodiment of the fiber pigtail assembly. 
     
    
    
     Additional features and advantages of the disclosure are set forth in the Detailed Description that follows and will be apparent to those skilled in the art from the description or recognized by practicing the disclosure as described herein, together with the claims and appended drawings. It will be understood that the illustrations are for the purpose of describing particular embodiments and are not intended to limit the disclosure or appended claims thereto. The drawings are not necessarily to scale, and certain features and certain views of the drawings may be shown exaggerated in scale or in schematic in the interest of clarity and conciseness. 
     Cartesian coordinates are shown in certain of the Figures for the sake of reference and are not intended as limiting with respect to direction or orientation. 
     DETAILED DESCRIPTION 
     In the following description, like reference characters designate like or corresponding parts throughout the several views shown in the figures. Unless otherwise specified, a range of values, when recited, includes both the upper and lower limits of the range. As used herein, the indefinite articles “a,” “an,” and the corresponding definite article “the” mean “at least one” or “one or more,” unless otherwise specified. 
     The mode field diameter MFD is a measure of the spot size or beam width of light propagating in a single mode fiber. The mode field diameter MFD is a function of the source wavelength, fiber core radius r and fiber refractive index profile. In an example, the mode field diameter MFD can be measured as the full width at 13.5% of the peak power for a best fit Gaussian beam, while in another example it can be measured by using the Peterman II method, where MFD=2 w, and 
               w   2     =     2   ⁢         ∫   0   ∞     ⁢       E   2     ⁢   r   ⁢           ⁢     ⅆ   r             ∫   0   ∞     ⁢       (       ⅆ   E     /           ⁢     ⅆ   r       )     2         ⁢   r   ⁢     ⅆ   r             
wherein E is the electric field distribution in the optical fiber and r is the radius of the optical fiber core.
 
     With reference to the Figures discussed in greater detail below, the mode field diameter MFD is also referred to herein as a property of an image spot  652  formed at a working distance WD by a probe optical assembly  450  and in this instance is referred to as the image mode field diameter MFD IM , or “image MFD IM ” for short, since the probe optical assembly images an end  324  of an optical fiber  320 , as explained below. The mode field diameter MFD associated with optical fiber  320  is thus called the fiber mode field diameter MFD F , or “fiber MFD F ” for short. An example range for the working distance WD (see  FIG. 9A ) is 5 mm≦WD≦15 mm. An example image MFD IM  is in the range of 15 microns≦MFD IM ≦100 microns. 
     Stub Lens Element 
       FIG. 1  is a side view of a rod  10  made of an optical material. Example optical materials for rod  10  include PYREX® glass, silica, VYCOR® glass or an optical glass. An example rod  10  has a cylindrical shape with any one of a number of possible cross-sectional shapes, such as circular, elliptical, polygonal, etc. The rod  10  has a body  12  that defines a central axis A 1 , a proximal end  14 , a distal end  16 , and a cross-sectional dimension (e.g., a diameter) D 1 . An example diameter D 1  for rod  10  is in the range of 250 microns and 1000 microns for a circular cross-sectional shape. 
       FIG. 1  also shows rod distal end  16  operably disposed relative to a heat source  20 . An example heat source  20  includes at least one heating member  21  that generates heat  22 . An example heating member  21  includes an electrical arc, a laser, a joule-heating element, a flame, a ring burner, etc. The heat  22  is applied to a distal end portion  17  of rod  10  adjacent distal end  16  while the rod is disposed vertically, i.e., with axis A 1  oriented in the direction of gravity. The heat  22  is sufficient to make the distal end portion  17  flow, whereupon surface tension causes the distal end portion to become bulbous, as illustrated in  FIG. 2 . Depending on the cross section of rod  10  and processing conditions, the shape can be spherical, ellipsoid, etc. The now bulbous distal end portion  17  has a diameter D 2 , which in an example is in the range of about 300 microns to 2,500 microns. In an example, a rod  10  made of silica glass and having a diameter D 1  of about 500 microns and having a circular cross-sectional shape allows for diameter D 2  to be about 1.5 mm. 
     The bulbous distal end portion  17  defines a lens  40  having a lens surface  42 . The size of lens  40  and the shape of lens surface  42  can be controlled by controlling the rod-end melting process, e.g., by controlling at least one of: the amount of heat  22  provided by heat source  20 , the feed rate of rod  10  into heat  22 , the rotation of the rod about its central axis A 1 , and the distance over which the rod is lowered into the heat. In particular, lens surface  42  can be made spherical to a high degree of accuracy using this process, though aspherical lens surface shapes can be made as well. In an example, where lens surface  42  is spherical, it can have a radius of curvature R 2  in the range 0.15 mm≦R 2 ≦1.5 mm (see  FIG. 3A  and  FIG. 4A ). 
     In an example, further processing is carried out to change the shape of lens  40 , and in particular to reduce the lateral dimension of the lens.  FIG. 3A  is similar to  FIG. 2 , and illustrates an example embodiment wherein lens  40  is in the process of being reduced in size in the lateral dimension via mechanical grinding by a mechanical grinder  50 .  FIG. 3B  is similar to  FIG. 3A  and illustrates another example of lens  40  being reduced in size in the lateral dimension via laser processing by a laser beam LB. Arrows AR in  FIG. 3A  and  FIG. 3B  show the direction of motion of mechanical grinder  50  and laser beam LB, respectively. 
       FIG. 4A  and  FIG. 4B  illustrate examples of a resultant stub lens element  100  formed by processing lens  40  such that its lateral dimension is reduced. In one example, processed lens  40  has a reduced lateral dimension D 2 ′ in the range about 600 microns to 1 mm, while in another example the reduced lateral dimension D 2 ′ is in the range 700 microns to 800 microns. With reference to  FIG. 4B , in an example, stub lens element  100  has an axial length L from proximate end  14  to lens surface  42  that is in the range 0.5 mm≦L≦5 mm. 
     The stub lens element  100  includes stub section  110  formed by the unaffected portion of rod  10  and the reduced-size lens  40 , which hereinafter is referred to as stub lens  40 . The stub lens  40  has a non-lens outer surface portion  44  adjacent lens surface  42 . The stub section  110  serves as a handle for handling stub lens element  100  and can be cut to have a length suited for its particular application. The stub lens element  100  of  FIG. 4A  has a stub lens  40  with a conic or flared outer surface portion  44 , while the stub lens of  FIG. 4B  has a cylindrical outer surface portion. As shown in  FIG. 4B , stub section  110  has a length L 1 =L−2·(R 2 ). 
     As illustrated in the example stub lens element  100  of  FIG. 4A , the flared shape of lens  40  is employed to accommodate light  650  that diverges as it passes from proximal end  14  to distal end  16 . The stub lens  40  of  FIG. 4A  also shows a relatively short stub section  110  that can be formed by the aforementioned cutting after stub lens element  100  is formed as described above. A transition portion  120  forms the connection between stub section  110  and stub lens  40 . 
     In examples, lens surface  42  of stub lens  40  can be spherical or aspherical. Example aspherical surfaces include bi-conic, parabolic, hyperbolic, etc. The shape of lens surface  42  can be defined by controlling the above-described melt process. In an example, an anti-reflection coating  46  can be applied to lens surface  42 , as illustrated in  FIG. 4C . 
       FIG. 5A  is similar to  FIG. 4A  and shows a cutting tool  150  being used to cut stub section  110  of stub lens element  100  to form an angled proximal end  14  that defines an angle θ relative to axis A 1 , as shown in  FIG. 5B . The angled proximal end  14  can serve to reduce back reflections when stub lens element  100  is used in an OCT probe, as described in greater detail below. An example angle θ is in the range from about 5° to about 12°. 
     OCT Probe Assemblies 
     Aspects of the disclosure are directed to OCT probes and assemblies used in such probes.  FIG. 6A  is a cross-sectional view of an example stub lens element  100  along with a cylindrical sleeve  200 . The cylindrical sleeve  200  has a central axis A 2 , first and second ends  202  and  204 , an outer surface  206 , and a central channel  210  that runs along the central axis and that is open at the first and second ends. The central channel  210  is sized to accommodate stub section  110  of stub lens element  100 . The sleeve  200  can be made of any rigid material, with glass, plastic and metal being some exemplary materials. An exemplary sleeve  200  comprises a section of precision capillary tubing, which can be drawn down to a select size from a much larger tube using a process similar to a redraw process used to make optical fibers. As sleeve  200  is later incorporated into another larger sleeve as is explained below, it is referred to hereinafter as inner sleeve  200 . 
       FIG. 6B  is similar to  FIG. 6A  and shows stub lens element  100  engaged with inner sleeve  200  by inserting stub section  110  into inner sleeve central channel  210  at sleeve second end  204 , thereby forming a stub lens sub-assembly  250 . In an example, an adhesive material  222  can be used to secure stub section  110  in central channel  210 . When stub lens element  100  is engaged with inner sleeve  200 , the stub lens element axis A 1  is substantially co-axial with inner sleeve axis A 2 . The stub lens sub-assembly  250  is thus configured to operably support optical fiber end  324  and stub lens element  100  in a cooperative optical relationship. 
       FIG. 6C  is similar to  FIG. 6B  and shows an example wherein inner sleeve  200  includes a slot  220  formed in inner sleeve outer surface  206  that leads to central channel  210 .  FIG. 6D  is similar to  FIG. 6C  and shows adhesive material  222  disposed in slot  220 . Adhesive material  222  is introduced into slot  220  and contacts stub section  110  of stub lens element  100 . This serves to secure (fix) the stub section to inner sleeve  200 , and providing an alternative to adding the adhesive material to central channel  210  from one of its ends. Once adhesive material  222  hardens, it can be ground, polished or otherwise processed to make its outer surface conform to outer surface  206  of inner sleeve  200 . The slot  220  may be formed in inner sleeve  200  by laser beam LB or by mechanical means, e.g., cutting or grinding. 
     As discussed below, it may be desirable to introduce adhesive material  222  between stub lens element  100  and an optical fiber ferrule, introduced and discussed below. If channel  210  of inner sleeve  200  does not have a means for air to escape, then inserting adhesive material  222  into the channel can be problematic. So slot  220  can serve the additional function of providing a means for air to escape from channel  210  during the fabrication process. 
       FIG. 7A  is similar to  FIG. 6B  and shows stub lens sub-assembly  250  along with an optical fiber ferrule (“ferrule”)  300 , and  FIG. 7B  is a close-up view of ferrule  300 . The ferrule  300  has a central axis A 3 , first and second ends  302  and  304 , an outer surface  306 , and a central bore  310  that runs along the central axis and that is open at the first and second ends. The central bore  310  is sized to fit into channel  210  of inner sleeve  200 . The ferrule  300  can be made of any rigid material, with glass, plastic and metal being some exemplary materials. In an example, ferrule  300  comprises a section of precision capillary tubing. An example diameter of central bore  310  is about 128 microns, and an example outer diameter of ferrule  300  is about 500 microns. 
     In an example, ferrule end  304  is angled at an angle φ relative to central axis A 3 . The central bore  310  of ferrule  300  is sized to accommodate an optical fiber  320 , which in an example is a single-mode optical fiber. The optical fiber  320  includes end  324 , which resides substantially at angled ferrule end  304 . In the example where ferrule end  304  is angled, optical fiber end  324  can also be angled at the same angle φ as the ferrule end. This can be accomplished by inserting optical fiber  320  into ferrule  300  when it has a non-angled end  304 , and then forming the angled ferrule end  304  by a cutting and polishing process that serves also to cut and polish optical fiber end  324 . 
       FIG. 7C  shows ferrule  300 , with optical fiber  320  secured therein, engaged with central channel  210  of inner sleeve  200  at inner sleeve end  202 , so that axes A 1 , A 2  and A 3  are all substantially co-axial. The angled ferrule end  304  and angled end  14  of stub section  110  define a gap  210 G within central channel  210  between the respective angled ends. In an example, gap  210 G can be filled with the aforementioned adhesive material  222  (not shown), e.g., in the form of an index-matching epoxy, to further reduce the back reflections and reduce the sensitivity of the rotational alignment of opposing angled ends. The combination of stub lens sub-assembly  250 , ferrule  300  and optical fiber  320  form a stub lens assembly  350 . The stub lens assembly  350  has an object distance OD, which is defined as the axial distance between optical fiber end  324  and lens surface  42  of lens  40 . In an example, object distance OD is in the range 0.5 mm≦OD≦5 mm, and in a more specific example is 1 mm≦OD≦3 mm. 
     With continuing reference to  FIG. 7C , in an example, during the fabrication of stub lens assembly  350 , one of the fabrication steps includes measuring the image MFD IM . This can be accomplished using, for example, a photodetector PD in the form of a beam-scanning apparatus or a digital camera. A light source LS is optically connected to an end  323  of optical fiber  320 . The photodetector PD is used to measure the size of image spot  652  as formed by light  650  emanating from optical fiber end  324  and being focused by stub lens element  100  at the anticipated working distance WD. The axial position of at least one of ferrule  300  and stub lens element  100  can be adjusted until the object distance OD that minimizes the image MDF IM  is determined. 
     In an example, photodetector PD generates an electrical signal S 1  that is representative of the detected image MFD IM , and this electrical signal is analyzed (e.g., via a computer CU operably connected to photodetector PD) to assess the optimum object distance OD. Once the optimum object distance OD is established, then ferrule  300  and stub lens element  100  are fixed in place within inner sleeve  200  using, e.g., adhesive material  222 , which can be a UV-curable epoxy. In one example, stub lens element  100  is fixed relative to inner sleeve  200  prior to the image MFD IM  measurement, and only the axial position of ferrule  300  is adjusted. In an example fabrication step, gap  210 G can be filled with the aforementioned index-matching material, e.g., UV-curable adhesive material  222 , through slot  220  (see  FIGS. 7A and 7C ). 
     In an example embodiment, stub lens assembly  350  is operably supported by a support member.  FIG. 8A  is a cross-sectional view that shows stub lens assembly  350  and a light-deflecting member  500  having an axis A 5  and arranged relative to a support member  398  in the form of an outer sleeve  400  in the process of forming a probe optical assembly  450 , as shown in  FIG. 8B . The support member  398  is configured to operably support stub lens sub-assembly  350  and light-deflecting member  500  in a cooperative optical relationship that defines folded optical path OP. 
     The light-deflecting member  500  is shown and discussed hereinbelow as a prism by way of illustration. In an alternate example, light-deflecting member  500  comprises a mirror. The outer sleeve  400  has a central axis A 4 , first and second ends  402  and  404 , an outer surface  406 , and an interior  410  that runs along the central axis and that is open at the first and second ends. The interior  410  is configured to accommodate at end  402  stub lens assembly  350  and at end  404  light-deflecting member  500 . In an example, outer sleeve  400  includes a retaining feature  412  disposed within interior  410  at end  402 , with the retaining feature configured to retain inner sleeve  200  of stub lens assembly  350 . The outer sleeve  400  can be made of any rigid material, with glass, plastic and metal being exemplary materials. In an example, outer sleeve  400  comprises a section of precision capillary tube. 
     With continuing reference to  FIG. 8A , light-deflecting member  500  includes a cylindrically curved front surface  502 , a planar angled surface  503  that defines a total-internal-reflection (TIR) mirror  503 M, and a planar bottom surface  504 . The light-deflecting member central axis A 5  is folded by TIR mirror  503 M. The angle of deflection a can be in the range between 90 and 100 degrees. 
     The light-deflecting member  500  is shown along with a retaining feature  512  that serves to retain the light-deflecting member in interior  410  at end  404  of outer sleeve  400  when the light-deflecting member and the outer sleeve are operably engaged. In an example, retaining feature  512  is simply adhesive material  222 . 
       FIG. 8B  shows stub lens assembly  350  and light-deflecting member  500  operably engaged with outer sleeve  400  at respective ends  402  and  404 , thereby forming the aforementioned probe optical assembly  450 . The probe optical assembly  450  includes an optical path OP that begins from optical fiber end  324  and that generally follows the substantially co-axial axes A 1  through A 5 . In an example, optical fiber end  324  defines an object plane OBP and working distance WD defines the distance where light-deflecting-member axis A 5  intersects axis A 4  to an image plane IMP where the smallest image spot  652  is formed. The optical path OP thus comprises the path over which light  650  travels from object plane OBP to image plane IMP. 
     OCT Probe 
       FIG. 9A  is similar to  FIG. 8B  and shows an example of an OCT probe (“probe”)  600  that includes a long (e.g., several meters long) transparent jacket  610  into which probe optical assembly  450  and optical fiber  320  are inserted. An example jacket  610  has a cylindrical body portion  620  that defines an interior  624 . In an example, jacket  610  comprises a long polymer tube having a rounded distal end  616 . The cylindrical body portion  620  has a cylindrically curved outer surface  626 . In an example, jacket  610  has a diameter D 3  in the range 1 mm≦D 3 ≦2 mm. 
     The jacket  610  is configured to contain probe optical assembly  450  in interior  624 .  FIG. 9A  also shows an example where jacket  610  includes a proximal end  614  at which an optical fiber cable  326  that carries optical fiber  320  is operably connected to the jacket. 
       FIG. 9B  is a close-up view of the probe optical assembly  450  contained within interior  624  of jacket  610  of probe  600  and shows light  650  and scattered light traversing optical path OP in opposite directions. The light  650  originates from light source LS, which is optically coupled to end  323  of optical fiber  320 . In an example, light  650  from light source LS has a wavelength of about 1.3 um. The use of a single-mode optical fiber provides the necessary spatial coherence for OCT applications. 
     With reference to  FIG. 9A  and  FIG. 9B , light  650  from light source LS initially travels down optical fiber  320  as guided light. This guided light  650  exits optical fiber end  324  at or near ferrule end  304  and diverges as it begins traveling over optical path OP. This divergent light then passes through gap  210 G and enters proximal end  14  of stub section  110  of stub lens element  100 . The divergent light  650  then travels through stub section  110  to lens  40 , where it exits the lens at lens surface  42  and passes to light-deflecting member  500 . Note that lens surface  42  has positive optical power and so acts to converge light  650 . The now convergent light  650  enters light-deflecting member  500  at curved surface  502 , and is then totally internally reflected at TIR mirror  503 M within the light-deflecting member. This reflection directs convergent light  650  to continue traveling along axis A 5  and to exit light-deflecting member  500  at bottom surface  504 . The light  650  then passes through cylindrical body portion  620  of transparent jacket  610  that resides adjacent light-deflecting member bottom surface  504  and exits probe  600 . Thus, optical path OP passes through transparent jacket  610 . 
     It is noted here that cylindrical curvature of cylindrical body portion  620  of jacket  610  acts as a cylindrical lens surface and so has first optical power in one direction. Accordingly, in an example, cylindrically curved front surface  502  of light-deflecting member  500  is configured to have second optical power that compensates for the first optical power. In an example, this compensation can be provided as negative optical power (i.e., a 1D concave surface) on surface  502  in the same plane of curvature as cylindrical body portion  620  or as positive optical power (i.e., a 1D convex surface) in the plane of curvature orthogonal to the cylindrical body portion. Thus, in one case, the same negative (diverging) optical effect is introduced in both axes, while in another case, the positive (converging) optical effect compensates for the diverging effect of the curved outer surface  626  of jacket  610 . Surface  502  of light-deflecting member  500  can be made curved using standard micro-polishing and micro-finishing techniques. 
     The light  650  that exits probe  600  then travels to a sample  700 , which resides adjacent the probe as shown in  FIG. 9A . The sample  700  has a body (volume)  701  that defines a sample surface  702 . The convergent light  650  is substantially brought to a focus at working distance WD by virtue of lens surface  42  of stub lens element  100  having the aforementioned positive optical power. The focused light  650  forms image spot  652 , which has associated image MFD IM , as illustrated in the close-up inset view of the image spot. 
     A portion  650 S of light  650  incident upon sample  700  is scattered back from sample surface  702  or volume  701  into probe optical assembly  450  through the cylindrical body portion  620  of transparent jacket  610 . This scattered light  650 S then travels back through probe optical assembly  450  over optical path OP but in the reverse direction to that of incident light  650 . The scattered light  650  is then diverted upstream from optical fiber  320  by a fiber coupler FC to travel in another optical fiber section  318  ( FIG. 8B ) to be interfered with reference light (not shown). The interfered light is then detected and processed according to conventional OCT procedures. 
     The stub lens element  100  serves to receive light  650  emitted from optical fiber end  324  and form a high-quality Gaussian beam. In an example, stub lens element  100  and light-deflecting member  500  are configured to meet the requirements for the image mode-field diameter (MFD) and working distance WD for OCT applications. The angled optical fiber end  324  and angled end  14  of stub section  110  serve to reduce back reflections [and thereby?] to improve the SNR. As discussed above, gap  210 G between angled ferrule end  324  and ferrule and angled end  14  of stub section  110  can be filled with an index-matching material to further reduce back reflections as well as to reduce the sensitivity of the rotational alignment of the opposing angled ends that define the gap. 
     Design Considerations 
       FIG. 10  plots the object distance OD (horizontal axis) vs. the working distance WD (left vertical axis, solid-line curve) and the mode field diameter MFD (right vertical axis, dotted-line curve) in connection with designing an example stub lens element  100 . All dimensions are in microns. A Gaussian beam for light  650  was used, along with a radius of curvature of 0.75 mm for lens surface  42 , and silica as the optical material. The wavelength of light  650  was 1.3 microns, at which silica has a refractive index n of about 1.45. 
     Based on the plot of  FIG. 10 , in order to have a working distance WD of about 13.5 mm, the object distance OD needs to be about 2890 microns. The corresponding image MFD IM  is about 60 microns. 
     The plot of  FIG. 10  can also be used to determine the tolerances needed for this design to control working distance WD to within certain limits. As can be seen by a circle C 1  provided on the solid-line curve, object distance OD needs to be controlled to better than about 10 microns in order to control working distance WD to better than 500 microns. Likewise, with reference to a circle C 2  on the dotted-line curve, controlling object distance OD to within 10 microns controls mode field diameter MFD to within about 25 microns. Like plots can be made for the tolerances on the radius of curvature of lens surface  42 . These kinds of tolerance assessments indicate the need for very tight control of working distance WD if good OCT imaging is to be obtained. 
     In OCT applications, the transverse imaging resolution depends on image MFD IM  of image spot  652  formed at working distance WD. A smaller image spot  652  with the same working distance WD is thus desired to achieve higher imaging resolution. 
       FIG. 11  is similar to  FIG. 10  and plots the image MFD IM  (microns) versus the working distance WD (microns) for the case of a single-mode optical fiber  320 , but where the input fiber MFD F  is changed from 10 microns to 7 microns. The solid line and dashed line curves represent two different glass types for stub lens element  100 , namely, PYREX and silica, which has a lower index than PYREX. The dotted line indicates the results with a smaller mode field fiber. The curves plotted in  FIG. 11  indicate that the smaller fiber MFDF leads to a smaller image MFD IM  at the same working distance WD. Similarly, a smaller refractive index n for stub lens element  100  (for example, silica vs. PYREX) leads to a smaller image MFD IM . 
       FIG. 12  is similar to  FIG. 7B  and illustrates an example modification of optical fiber  320  at optical fiber end  324 . In the example, rather than using an optical fiber  320  having a smaller core diameter, a lens  325  is formed on (e.g., via re-shaping via acid etching or melting) or is otherwise added directly to optical fiber end  324 . The lens  325  can have any one of a variety of surface shapes, including spherical and aspherical. Example aspherical surface shapes include parabolic shapes, hyperbolic shapes, biconic shapes, and the like. The shape of lens  325  is limited only by current optical fiber lens-forming techniques. 
     The lens  325  is configured to reduce fiber MFD F , which in turn reduces image MFD IM . Example specifications for image MFD IM , working distance WD, and the M 2  parameter for light  650  are about 80 microns, about 13.5 mm to 15 mm and less than 1.3, respectively. Embodiments of probe  600  fabricated using the components, assemblies and methods as described herein can readily meet these specifications. 
       FIG. 13A  through  FIG. 13C  illustrate an example method of forming a fiber pigtail lens assembly  800  that can serve as a more compact version of the previously described stub lens assembly  350 . With reference first to  FIG. 13A , optical fiber  320  is spliced at optical fiber end  324  to end  14  of rod  10 . The optical fiber  320  has a core  322 , which in an example is comprised of silica or doped silica. In an example, rod  10  is made of silica so that optical fiber core  322  and the rod are substantially index matched. Splicing optical fiber  320  to rod  10  forms a contiguous fiber pigtail structure  348  that is further processed to form fiber pigtail lens assembly  800 . The general process for forming this monolithic fiber pigtail structure  348  is described in U.S. Pat. Nos. 7,228,033 B2, 7,258,495 B1 and 6,904,197 B2, which are incorporated by reference herein. 
     With reference now to  FIG. 13B , rod distal end portion  17  is processed to have a tapered shape and so that rod  10  has a select length to within about +/−20 microns. By controlling the shape of rod distal end portion  17 , the subsequent lens  40  can be made to have a select configuration. 
     With reference now to  FIG. 13C , monolithic fiber pigtail structure  348  of  FIG. 13B  is further processed using for example the thermal methods described above so that rod distal end portion  17  becomes bulbous and forms lens  40  with lens surface  42  having a nominal radius of curvature R 2 . Thus, the resulting fiber pigtail lens assembly  800  includes a stub lens element  100  that includes stub section  110  spliced at end  14  to optical fiber  320 . In fiber pigtail lens assembly  800 , the object distance OD is now essentially the axial distance or length L of the newly formed stub lens element  100 , wherein L is the distance from proximal end  14  of stub section  110  to the apex of lens surface  42  of lens  40 . In an example, length L is in the range 0.5 mm to 5.0 mm. 
       FIG. 13D  is similar to  FIG. 13C  and shows the fiber pigtail assembly  800  operably engaged with ferrule  300  so that lens  40  is adjacent ferrule end  304 . Adhesive material  222  is included within central bore  310  and is used to fix stub section  110  and the spliced-end portion of optical fiber  320  within the ferrule channel. 
       FIG. 13E  is similar to  FIG. 13D  and shows an example embodiment where lens  40  has been reduced in size by polishing, turning, grinding or like manner. This makes fiber pigtail assembly  800  smaller in the lateral dimension, which allows it to be used in different configurations where a wider bulbous lens  40  might prove problematic. For example, with reference to  FIG. 13F , the fiber pigtail assembly  800  and ferrule  300  are shown operably supported by support member  398  in the form of a transparent support substrate  820  having an upper surface  828 . In an example, support substrate  820  supports an example light-deflecting member  500  in the form of a prism atop upper surface  828  adjacent one end of the substrate. The support substrate  820  also supports fiber pigtail assembly  800  and ferrule  300  on surface  828  near the other end of the support substrate. In an example, support substrate  820  has a thickness of about 190 microns. 
     In the example shown, light-deflecting member  500  now has a planar light-deflecting member front surface  502 , along with the aforementioned angled surface  503  that defines a TIR mirror  503 M, and bottom surface  504 , which now resides adjacent the substrate upper surface  828 . The fiber pigtail assembly  800  is disposed on upper surface  828  and in an example is secured thereto, e.g., with adhesive material  222 . The fiber pigtail assembly  800  is arranged so that lens surface  42  of lens  40  confronts planar light-deflecting-member surface  502 . The light-deflecting member TIR mirror  503 M serves to fold axis A 1  so that optical path OP passes through support substrate  820  at the location adjacent light-deflecting member bottom surface  504 . 
     In an example, light-deflecting member  500  can be formed by providing a blank (also called a preform) having a triangular cross-section and that can include a corresponding surface that has either a convex or concave curvature, depending on how the compensation for curved jacket  610  is to be carried out via the subsequently formed light-deflecting member. The blank is then drawn into rods using standard glass drawing techniques, wherein the rods have the same cross-sectional shape as the blank. 
     This light-deflecting member fabrication method requires shaping one blank from which hundreds of meters of light-deflecting-member rods can then be drawn. Centimeter lengths of the light-deflecting-member rods can be mounted on support substrate  820  and then diced into individual light-deflecting members and substrates such as shown in  FIG. 13F   
     In an example, the blank is formed so that light-deflecting-member surface  502  has the appropriate curvature. Moreover, in an example, light-deflecting-member surface  502  can have an amount of tilt relative to light-deflecting member axis A 5  that is capable of reducing back reflections. For example, for most anticipated OCT applications, a tilt of about 2 degrees is sufficient for reducing back reflections to as low as −50 dB to −60 dB. 
       FIG. 14  is similar to  FIG. 8B  and illustrates an example embodiment of probe optical assembly  450  that employs fiber pigtail lens assembly  800  in place of the aforementioned stub lens assembly  350 . The fiber pigtail lens assembly  800  is shown being held in place within outer sleeve  400  by retaining feature  412 . The use of fiber pigtail lens assembly  800  simplifies the design and assembly of probe optical assembly  450  and also eliminates gap  210 G, which was present in stub lens assembly  350  described above. The configuration of fusion-spliced fiber pigtail lens assembly  800  reduces the amount of back reflection to an acceptable level without the need for angled facets. This contributes to pigtail lens assembly  800  having robust performance while also having a relatively low assembly cost.  FIG. 14  also shows an example of light-deflecting member  500  operably engaged at end  404  of outer sleeve  400 . In an example, light-deflecting member  500  can be molded or embossed to sleeve  400  using polymers or UV curable epoxies. 
     The fiber pigtail lens assembly  800  is relatively tolerant to process variations. The underlying reason for this has to do with the fact that the amount of optical material needed to form lens  40  is proportional to the cube of the lens radius R 2 , whereas the amount of material contained in the cylindrical stub section  110  is proportional to the square of the rod radius R 1 , wherein R 1 =(D 1 )/2. For OCT imaging, an example lens radius R 2  for lens  40  of fiber pigtail lens assembly  800  is in the range about 750 microns to about 800 microns. In an example, rod  10  from which lens  40  is formed has a diameter D 1  in the range of about 350 microns to about 500 microns. 
     A variation in the length L of stub lens element  100  is dictated by the shortening of rod  10  during the formation of lens  40 .  FIG. 15  plots the relationship between the stub lens element length L (mm) and the diameter D 2  (mm) of lens  40  of stub lens element  100 . The total variation in length L is ostensibly determined by the accuracy of the mechanism used to feed rod  10  into heat source  20  during the lens formation process (see  FIG. 1 ). Even with a change in length ΔL of 50 microns, the corresponding change in the lens diameter ΔD 2  is only about 2.5 microns. This amount of change in the diameter D 2  of lens  40  does not lead to a significant change in the focusing characteristics of the lens, so the working distance WD and image MFD IM  remain substantially unchanged. 
       FIG. 16A  is a cross-sectional view of another example embodiment of stub lens assembly  350  that includes fiber pigtail lens assembly  800 , wherein its fused stub lens element is denoted as  100 A (with lens  40 A, lens surface  42 A, etc.), as used in combination with a second stub lens element, which is denoted as  100 B (with lens  40 B, lens surface  42 B, etc.). The support member  398  in the form of inner sleeve  200  operably supports ferrule  300  at first end  202  and operably supports stub lens element  100 B at second end  204 . 
     This configuration of stub lens assembly  350  now has two optical surfaces with optical power, namely stub lens surfaces  42 A and  42 B. The stub lens surfaces  42 A and  42 B are arranged in stub lens assembly  350  so that they are confronting. The fiber MFD F  associated with fiber pigtail lens assembly  800  in this example can be made relatively small. This in turn allows for optical fiber  320  to be a conventional optical fiber, such as SMF-28® optical fiber, which is available from Corning, Inc., Corning, N.Y., and which has a core diameter of nominally 10 microns. 
     Another advantage is that the amount of back scattering of light  650  is relatively low by virtue of the pigtail configuration of fiber pigtail lens assembly  800 . Also, the two lenses  40 A and  40 B can be configured so that light  650  is substantially collimated as it travels from lens surface  42 A to lens surface  42 B, as illustrated in  FIG. 16A . This can serve to reduce any beam distortions and also to reduce the diameter requirements for lens  40 B of stub lens element  100 B. 
       FIG. 16B  is similar to  FIG. 16A  and illustrates an example embodiment of stub lens assembly  350  wherein outer sleeve  200  is replaced with support substrate  820 . The support substrate  820  can be made of a rigid material such as glass, plastic, metal and the like. 
       FIG. 17  is similar to  FIG. 16B  and illustrates another example embodiment of stub lens assembly  350  wherein support substrate  820  is made of a transparent material. In an example, support substrate  820  is formed as a unitary molded piece that includes first and second ends  824  and  826 , and a recess  830  formed in upper surface  828 . The ferrule  300  with fiber pigtail lens assembly  800  engaged therewith is disposed on upper surface  828  of support substrate  820  adjacent end  824 . Likewise, stub lens element  100 B is disposed on upper surface  828  of support substrate  820  adjacent end  826 , with the stub lens surface  42 B in opposition to stub lens surface  42 A. Stub lens element  100 B has an angled distal end  14 B that defines a TIR mirror  14 BM. 
     The stub lens elements  100 A and  100 B are aligned (i.e., their respective axes A 1 A and A 1 B are made co-linear) using for example the aforementioned method of monitoring of the image MFD IM  (see  FIG. 7C ). Once so aligned, they are fixed in position. In an example, stub lens element  100 B can be secured to support substrate  820  by an adhesive material  222  introduced into recess  830 , where a portion of lens  40 B resides. Note that the portion of stub section  110 B that includes TIR mirror  14 BM serves essentially the same function as the aforementioned separate light-deflecting member  500 . The fiber pigtail lens assembly  800  can be axially adjusted within ferrule  300  prior to being fixed in place. 
       FIG. 18  is similar to  FIG. 17  and illustrates an embodiment wherein the support member  398  comprises a transparent monolithic structure  850  that includes ends  854  and  856 , and a planar upper surface portion  858  adjacent end  854 . The monolithic structure  850  also includes stub lens element portion  100 B adjacent end  856 , with the stub lens element portion including angled end  14 B and TIR mirror  14 BM. In an example, planar upper surface portion  858  includes at least one alignment feature  860  that facilitates alignment of ferrule  300  and fiber pigtail lens assembly  800  supported thereby with the stub lens element portion  100 B. An example alignment feature  860  is a groove (e.g., a V-groove) that accommodates a corresponding (e.g., complimentary) alignment feature  360  of ferrule  300 . 
     The monolithic structure  850  can be formed, for example, by molding a polymer material, thereby providing for low-cost mass production that can employ reusable molds. The configurations of stub lens assembly  350  of  FIG. 18  has the advantage that the beam dimension of light  650  is substantially larger compared to the single mode fiber mode-field diameter before it is incident on lens surface  42 B. This substantially reduces the light intensity on the lens surfaces so that they can tolerate much higher power levels without degradation, especially when monolithic structure  850  comprises a polymer material. 
       FIG. 19  is a plot of the length L (microns) (horizontal axis) versus the image MFD IM  (microns)(left-hand vertical axis) and working distance WD (microns)(right-hand vertical axis) as defined as the beam-waist location, for an example stub lens element  100  suitable for use in fused fiber pigtail lens assembly  800 . The curve with the circles corresponds to image MFD IM  and the curve with the squares corresponds to working distance WD. The plot of  FIG. 19  is based on lens  40  having a radius R 2 =150 microns and optical fiber  320  having a fiber MFD F  of about 10 microns. The plot shows that fiber pigtail lens assembly  800  can have an image MFD IM  of about 6.5 microns for a length L of 1,200 microns (1.2 mm). 
       FIG. 20  is a side view of an example fiber pigtail lens assembly  800  wherein lens  40  includes an angled surface (facet)  43  that defines a TIR mirror  43 M that serves to fold axis A 1  and direct it through a portion of lens surface  42 . The facet  43  can also include a curvature to compensate for the defocusing effect of curved outer surface  626  of jacket  610 . 
       FIG. 21  is another embodiment of the fiber pigtail assembly  800  assembly similar to that shown in  FIG. 20 , but where optical fiber  320  is not fusion spliced to lens  40 . Rather, optical fiber end  324  is spaced apart from lens  40  and is in optical communication therewith through index-matching material  222 , e.g., UV epoxy. The optical fiber  320  is shown being supported in ferrule  300 , which in turn is supported in inner sleeve  200 . As with lens  40  of  FIG. 20 , lens  40  of  FIG. 21  performs both the beam bending at lens surface  42  and the internal reflection at angled surface (facet)  43 . In the example of  FIG. 21 , lens  40  is formed as a single element fabricated from a hemispherical or biconic ball lens made of a glass or a polymer material. With polymer materials, the fabrication of biconic lens or shaped stub lens is generally easier and can be more readily mass produced, e.g., via a molding process. 
     Although the embodiments herein have been described with reference to particular aspects and features, it is to be understood that these embodiments are merely illustrative of desired principles and applications. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the appended claims.