Abstract:
A fiber interface configuration for a chromatic point sensor optical pen is provided wherein a detector aperture element provides an aperture that is smaller than the light-transmitting core diameter of an optical fiber that is connected to the optical pen. The detector aperture element is fixed relative to the chromatically dispersive optics of the optical pen, the optical fiber abuts the aperture element, and the optical fiber core is aligned to the aperture. The aperture element and the end of the fiber may be inclined relative to the axis of the fiber, to deflect spurious reflections away from the optical signal path. The configuration provides high measuring resolution without using a tapered optical fiber, and provides interchangeability of the optical fiber connected to the optical pen.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates generally to precision measurement instruments, and more particularly to a fiber interface configuration for a point sensor such as may be utilized in an optical pen for chromatic confocal range sensing. 
       BACKGROUND OF THE INVENTION 
       [0002]    Confocal techniques are known for measuring distances. As described in U.S. Pat. No. 5,785,651, it is known to utilize confocal microscopy as an effective means of obtaining high resolution images in both lateral dimension and in depth. In typical confocal microscopy, a monochromatic point source of light is projected onto a surface and a portion of the reflected light is separated and then imaged onto a detector pinhole. The amount of light through the pinhole is measured by a detector. The intensity of the light at the pinhole is diminished for out of focus targets. Therefore, the total light energy reaching the detector is inversely related to the distance a target is from the plane of best focus. This signal can be used to control the position of the microscope until a maximum signal is returned. The position of the microscope can then be recorded and correlated to the height of the object. 
         [0003]    It is also known to use chromatic confocal techniques in optical height sensors. As described in U.S. Patent Application Publication No. US2006/0109483 A1, which is hereby incorporated herein by reference in its entirety, an optical element having axial chromatic aberration, also referred to as axial or longitudinal chromatic dispersion, may be used to focus a broadband light source such that the axial distance to the focus varies with the wavelength. Thus, only one wavelength will be precisely focused on a surface, and the height of the surface determines which wavelength is best focused. Upon reflection from the surface, the light is refocused onto a small detector aperture, such as a pinhole or the end of an optical fiber. In one embodiment, a 50 micron core fiber is utilized as both the source and receiver pinhole (the end of the fiber acting as the pinhole). Upon reflection from a surface and passing back through the optical system to the in/out fiber, only the wavelength that is well-focused on the surface is well-focused on the fiber. All of the other wavelengths are poorly focused on the fiber, and so will not couple much power into the fiber. Therefore, the signal level will be greatest for the wavelength corresponding to the height of the object. A spectrometer at the detector measures the signal level for each wavelength, which effectively indicates the height of the object. 
         [0004]    Certain manufacturers refer to a practical and compact optical assembly that is suitable for chromatic confocal ranging in an industrial setting as a chromatic point sensor (CPS) and/or as an “optical pen”. In such CPS optical pens, the standard design approach has been to use the end of the in/out fiber as the detector aperture. To provide a smaller detector aperture, which generally increases the measurement resolution, the standard technique is to taper the diameter of the fiber and core at its end. For example, certain manufacturers are known to have tapered the end of a 50 micron diameter core fiber to a 20 micron diameter core by a heating and stretching process (e.g. see J. Villatoro et al., “Fabrication and modeling of uniform-waist single mode tapered optical fiber sensors,” Appl. Opt., 42, 2278-2283 (2003)). However, a number of problems or deficiencies have emerged in relation to existing CPS optical pen assemblies that are available for chromatic confocal range sensing. For example, one major problem is that, in order to maintain their specified performance the existing types of CPS optical pen assemblies generally require recalibration and/or factory servicing if the optical fiber needs to be replaced. This is both inconvenient and expensive for users. 
         [0005]    The present invention is directed to providing an apparatus that overcomes the foregoing and other disadvantages. More specifically, an optical fiber interface configuration is provided for a CPS optical pen which increases resolution without requiring a tapered fiber, and provides other advantages. 
       SUMMARY OF THE INVENTION 
       [0006]    This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This summary is not intended to identify key features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. 
         [0007]    A chromatic confocal sensor fiber interface configuration is provided for a compact CPS optical pen. It order to appreciate the value of the design features disclosed herein, it is essential to appreciate that the types of optical pens that are the subject of this invention, are exquisitely sensitive, providing surface height measurement resolution on the order of nanometers. This extraordinary measurement performance is, in turn, exquisitely sensitive to the exact construction of the optical pen. Measuring performance is altered and/or degraded with the smallest variations in the optical pen configuration. In this context, a fiber interface configuration according to this invention provides several advantages over the standard tapered fiber approach that is generally used to improve the measuring resolution of an optical pen. 
         [0008]    The terms light and radiation are generally used interchangeably herein. Accordingly, it will be understood that the term light may include radiation outside the visible spectrum, if such radiation is used in a CPS optical pen. 
         [0009]    In accordance with one aspect of the invention, a detector aperture element provides an aperture that is smaller than the light-transmitting core diameter of an optical fiber that is connected to the optical pen. The detector aperture element is fixed at an aperture operating position relative to the chromatically dispersive optics portion of the optical pen. In various embodiments, in operation, an inner diameter D 2  of the aperture may generally fall within a projection of an outer diameter D 1  of a light-transmitting core of the optical fiber along the direction of the fiber axis. In various embodiments, the optical fiber abuts the aperture element, and the optical fiber core is aligned to the aperture. In one specific example implementation, an optical fiber having a 50 micron core is aligned to, and abuts, a 20 micron aperture. 
         [0010]    By fixing the detector aperture element relative to the chromatically dispersive optics portion of the optical pen, and making the aperture smaller than the optical core diameter, it becomes practical to interchange optical fibers using standard optical fiber connector tolerances, without substantially altering the performance of the optical pen. In contrast, the standard optical pen approach of directly using the end of a tapered fiber as the detector aperture virtually prevents interchangeability, due to variations in the properties and positioning of the tapered ends that are impractical or impossible to control at the required level. Furthermore, relevant properties of the aperture (e.g. its exact size, generation of spurious reflections or scattering, etc.) can be more easily and reliably controlled in the aperture element during manufacturing, in comparison to the corresponding properties of a tapered fiber end. This improved level of control reduces performance variations between different optical pens and simplifies their assembly and manufacturing. 
         [0011]    In accordance with other features of the invention, the CPS optical pen is operable to provide a signal usable to measure a distance to a surface. In one embodiment, the CPS optical pen includes a housing, an optical element portion, and a fiber optic interface sub-assembly. The optical element portion provides a longitudinal chromatic dispersion for radiation emitted from the CPS optical pen, and receives radiation reflected from a surface. The fiber optic interface sub-assembly is fixed relative to the optical element portion and includes a mounting element and an aperture element, and receives an optical fiber. The optical fiber end may be positioned within the fiber optic interface sub-assembly, and the end of the fiber may be inclined at an angle (e.g. 7°) relative to a plane that is normal to a central axis of the fiber, so that reflections off of the fiber end face are directed away from the optical signal path. The end of the fiber is positioned proximate to the aperture element, which includes an aperture with a smaller diameter than that of the optical fiber core (e.g. a 20 micron aperture for a 50 micron fiber core). A surface of the aperture element may be inclined such that it is approximately parallel to the angle at the end of the fiber. 
         [0012]    In accordance with another aspect of the invention, in one embodiment the optical fiber is encased in a ferrule with an angled end that it is approximately parallel to the angle at the end of the fiber. More specifically, in one specific example embodiment, a 50 micron FC/APC fiber is terminated by a ceramic ferrule which has a 7° angled end, and the face of the ferrule and the end of the APC fiber are designed to be flush. 
         [0013]    In accordance with another aspect of the invention, the aperture element is constructed so as to reduce the backscattering of radiation into the fiber core. In one specific example implementation, the aperture element is a pinhole mask formed in 302 stainless steel that has a black copper oxide finish. In another embodiment, the aperture element has a glass substrate which is coated by a dark chrome thin film that includes the aperture. 
         [0014]    In accordance with another aspect of the invention, in one embodiment prior to fixing the fiber optic interface sub-assembly relative to the optical element portion, the aperture is actively aligned. According to one aspect of one exemplary alignment procedure, an optical fiber is positioned in a nominal operating position in the fiber optic interface sub-assembly, which may include positioning the end of the optical fiber against the aperture element in approximate alignment with the aperture. The end of the optical fiber is then illuminated through the aperture using narrowband radiation (e.g. from a HeNe laser), and the power coupled through aperture at the narrowband radiation wavelength (e.g. 633 nm) is measured with a spectrometer (e.g. the spectrometer normally used with the CPS optical pen). The aperture position may then be adjusted until the best spectrometer signal is obtained, and the aperture may then be fixed in position relative to the fiber optic interface sub-assembly. The relationship between the fiber optic interface sub-assembly and the optical portion may then be adjusted under controlled operating conditions using broadband illumination such that the characteristics of the spectrometer signal are approximately optimized. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
           [0016]      FIG. 1  is a diagram of an isometric view of a first exemplary embodiment of a CPS optical pen fiber interface configuration including a fiber interface subassembly; 
           [0017]      FIG. 2  is a diagram of a side view cross-section of a portion of the fiber optic interface subassembly of  FIG. 1  during operation; 
           [0018]      FIG. 3  is a diagram of a side view cross-section of a CPS optical pen including the fiber interface configuration of  FIG. 1 ; 
           [0019]      FIG. 4  is a diagram of an isometric view of a second exemplary embodiment of a CPS optical pen fiber interface configuration including a fiber optic interface subassembly; 
           [0020]      FIG. 5  is a diagram of a side view cross-section of a portion of the fiber optic interface subassembly of  FIG. 4  during operation; and 
           [0021]      FIG. 6  is a diagram of a side view cross-section of a CPS optical pen including the fiber interface configuration of  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0022]      FIG. 1  is a diagram of an isometric view of a first exemplary embodiment of a CPS optical pen fiber interface configuration  100  that includes a fiber interface subassembly  180  comprising a mounting element  182  and an aperture element  190 . Also shown in  FIG. 1 , an optical fiber cable  110  including an optical fiber  112  with a core and cladding is positioned in an FC/APC connector  105 . The connector  105  includes a plug element  106  with a rotational alignment key  107 . The plug element  106  surrounds a ferrule  170  which encases the optical fiber  112 . The optical fiber end  115  of the optical fiber  112  is exposed at the end of the ferrule  170 . In one embodiment, the ferrule  170  may be ceramic. During operation, the fiber interface subassembly  180  receives the ferrule  170  and the optical-fiber end  115 , as the plug element  106  is received by a receiving portion  185  of the mounting element  182 . 
         [0023]    As will be described in more detail below, in one embodiment the mounting element  182  has an angled end surface  184  (e.g. 7° relative to the fiber or ferrule axis). An aperture element  190  is attached to the angled end surface  184 . The aperture element  190  includes an aperture  195  which is aligned with the fiber end  115 . In one embodiment, the aperture  195  is formed in a non-reflective thin film (e.g. dark chrome) that is deposited on a transparent aperture element  190 . The attachment of the aperture element  190  to the angled end surface  184  will be described in more detail below with respect to  FIG. 2 . The end of ferrule  170  and the fiber end  115  are also angled (e.g. 7° relative to a plane that is normal to the fiber or ferrule axis), and the rotational alignment key  107  of the plug  106  and a mating feature (not shown) in the receiving portion  185  are designed to ensure the proper rotational alignment, such that the angled fiber end  115  and end of the ferrule  170  are parallel to the angled surface of the aperture element  190  proximate to angled surface  184  in operation. The angled surfaces help ensure that spurious reflections off of the fiber end  115  or the aperture element  190 , or other elements, is directed away from the optical signal path, that is, away from the core of the optical fiber  112 . In one specific implementation, the optical fiber  112  is an APC fiber and the connector  105  is an FC/APC connector. 
         [0024]    As will be described in more detail below with respect to  FIG. 2 , a core  116  of the optical fiber  112  has a diameter D 1  (e.g. 50 microns), while the aperture  195  has a smaller diameter D 2  (e.g. 20 microns). In this configuration the aperture  195  governs both the lateral and axial resolution of the CPS optical pen, and provides several advantages in comparison to the tapered fiber approach that is conventionally used to increase the resolution in CPS optical pens. As previously outlined, such advantages include more consistent performance between optical pens and optical fiber interchangeability without recalibration. Regarding the tapered fiber approach, tapering is typically difficult to control and the end of the fiber core will often vary by a few microns from the intended size. In addition, in practice, the actual illumination spot size that is produced by the end of a tapered fiber may be larger than one would expect from an “ideal” fiber end. For example, the inventor has observed that for a 20 micron end diameter of a tapered core the resulting spot near the fiber end may have a diameter of up to 35 microns and soft edges, and will vary from fiber to fiber. 
         [0025]      FIG. 2  is a diagram  200  of a side view cross-section of a portion of the fiber interface configuration  100  of  FIG. 1 , during operation. As shown in  FIG. 2 , the optical fiber  112  is encased by the ferrule  170 , which in turn is received in a hole  183  through the mounting element  182 . The hole  183  and the ferrule  170  may be constructed so that the optical fiber  112  is centered to a high degree of precision in the hole  183 . In various embodiments, in operation, the ferrule  170  and the optical fiber  112  are received and constrained in the hole  183  with an axis of the optical fiber  112  approximately coinciding with the axis of the hole  183 . The fiber end  115  and the end of the ferrule  170  may be flush with each other and are beveled to an angle T 1  (e.g. 7°) relative to a plane that is normal to the fiber axis. The aperture element  190  with the opaque thin film  192  is fixed to the angled surface  184  by conventional means (e.g. glue fillets  198 .) The thin film  192  covers the fiber end  115  except for the aperture  195 , which is aligned with the fiber core  116 . The optical fiber  112  has an outer diameter D 1 ′, while the optical fiber core  116  has an outer diameter D 1 , and the aperture  195  has an inner diameter D 2 . In various embodiments the optical fiber end  115  is positioned proximate to the aperture  195 , and aligned with the aperture  195  at least well enough such that the inner diameter D 2  of the aperture falls within a projection of the outer diameter D 1 ′ of the optical fiber along the direction of the fiber axis. In various preferred embodiments, the optical fiber end  115  is aligned with the aperture  195  at least well enough such that the inner diameter D 2  of the aperture  195  falls within a projection of the outer diameter D 1  of the core  116  along the direction of the fiber axis, and is further actively aligned to optimize the measurement signals of the CPS optical pen, as described in greater detail below. 
         [0026]    In one specific example embodiment, for a commercially available optical fiber  112  with an outer diameter D 1 ′ of 125 microns and a core diameter D 1  of 50 microns, the inner diameter D 2  of the aperture  195  is made to be 20 microns. However, in various embodiments a smaller or larger dimension may be chosen for the aperture diameter D 2  (e.g. 35 microns). As will be described in more detail below, smaller diameters tend to provide higher resolution and lower signal (which may increase the signal acquisition time), and larger diameters tend to provide the opposite characteristics. As previously mentioned, the angled surface  184 , the aperture element  190 , and the fiber end  115  are angled at the angle T 1  (e.g. 7°) so that spurious reflections are deflected out of the optical signal path through the fiber core  116  (e.g. into the cladding layer or surrounding environment). In various embodiments, the ferrule  170  is spring-loaded in the FC/APC connector  105  (shown in  FIG. 1 ), and the angled surface  184  is located such that the spring-loading forces the fiber end  115  and/or the ferrule  170  to abut the surface of the aperture element  190  in the region surrounding the aperture  195 . This further reduces or eliminates potential spurious reflections from the optical signal path and ensures that the light received through the aperture  195  enters the core  116  to provide the maximum signal. However, in various embodiments, the fiber end may be recessed by a slight amount proximate to the surface surrounding the aperture  195  (e.g. on the order of the core diameter), and adequate performance may still result. 
         [0027]    In one embodiment, the aperture  195  is actively aligned relative to a nominal operating position of the fiber end  115  and the core  116 . According to one exemplary embodiment of an alignment procedure, an optical fiber  112  is positioned in a nominal operating position in the mounting element  182 , which may include positioning the fiber end  115  against the aperture element  190  in approximate alignment with the aperture  195 . The fiber end  115  is then illuminated through the aperture  195  using narrowband radiation (e.g. from a HeNe laser), and the power coupled into the core  116  through aperture  195  at the narrowband radiation wavelength (e.g. 633 nm) is measured with a spectrometer (e.g. the spectrometer normally used with the CPS optical pen). The aperture position may then be adjusted until the best spectrometer signal is obtained, and the aperture element  190  may then be fixed in position relative to the mounting element  182 . In some embodiments, the alignment procedure may also include simultaneously projecting broadband radiation through the optical fiber  112  (e.g. using the broadband source normally used with the CPS optical pen), such that aperture element  190  backscatters the broadband radiation in the same manner that it would during normal operation. In such a case, the best spectrometer signal is the one that provides approximately the best signal to noise ratio between the narrowband radiation spectral peak and the broadband radiation spectral background. 
         [0028]    In another embodiment, the thin film  192  may include alignment features or patterns (e.g. transparent annuli) that are nominally concentric with the aperture  195 , and that nominally coincide with the diameter of the hole  183 . The features may all be at a diameter substantially larger than the fiber diameter D 1 ′, such that they do not have the possibility of transmitting signal light. The aperture element  190  may then be aligned with the hole  183  under a microscope and bonded in place to the angled surface  184 , such that the alignment features are concentric with the hole  183  within a few microns. This will ensure approximately the same concentricity for the aperture  195  relative to the ferrule  170  and fiber end  115  that are received in the hole  183  during operation. While this visual alignment procedure may be sufficient for some applications, it should be appreciated the active alignment procedure outlined previously is generally preferred in order to provide the best measurement performance in critical applications. 
         [0029]      FIG. 3  is a diagram of a side view cross-section of a first embodiment of a CPS optical pen  300  including the fiber interface configuration  100  of  FIG. 1 . As shown in  FIG. 3 , the CPS optical pen  300  also includes an optical pen assembly housing  320  and an optics portion  350 . In operation, light emitted from the fiber end  115  through the aperture  195  is focused by the optics portion  350 , which includes a lens that provides an axial chromatic dispersion such that the focal point along the optical axis OA is at different distances depending on the wavelength of the light, as is known for chromatic confocal sensor systems. The light is focused on a workpiece surface  390 . Upon reflection from the workpiece surface  390 , the light is refocused by the optics portion  350  onto the aperture  195  as illustrated by the limiting rays LR 1  and LR 2 . A distance X 1  represents the spacing between the rear principal plane of the optics portion  350  and the aperture  195 . Due to the axial chromatic dispersion, only one wavelength will be in focus at the surface  390 , and the distance from the optical pen  300  to the surface  390  determines which wavelength is best focused. The wavelength that is best focused at the surface  390  will also be the wavelength that is best focused at the aperture  195 . Therefore, the light received into the core  116  of the optical fiber cable  110  through aperture  195  will be spatially filtered to have predominantly the best focused wavelength. In various embodiments, the optical fiber cable  110  routes the signal light to a spectrometer (not shown) that is utilized for determining the wavelength corresponding to the highest signal level, thus determining the distance to the workpiece surface  390 . 
         [0030]    The mounting element  182  of the sub-assembly  100  may be attached to the end of the optical pen assembly housing  320  using mounting screws  310 . In various embodiments, the mounting element  182  is mounted such that the axis of the hole  183  is at an angle T 2  relative to the optical axis OA. The angle T 2  is chosen such that the angle (T 1 +T 2 ) is approximately the same as the angle of refraction for light at the surface of the aperture element  190  (e.g. approximately 11° for a fiber end bevel angle of 7° and a glass aperture element), so that light rays parallel to the fiber axis are bent parallel to the optical axis OA as they exit the aperture element  190 . 
         [0031]    In various embodiments, prior to finally tightening the screws  310  and thereby fixing the fiber optic interface sub-assembly  180  relative to the optical element portion  350 , it is actively aligned to the optical portion  350 . The relationship between the fiber optic interface sub-assembly  180  and the optical portion  350  may generally be adjusted under controlled operating conditions using broadband illumination (e.g. as used during normal operation) such that the characteristics of the resulting spectrometer signals are approximately optimized. The relationship may be adjusted using any combination of the axial location and/or tilt of optical portion  350  and the axial and lateral position of the fiber optic interface sub-assembly  180 . In one embodiment, adjustments are made until one or more respective spectral peaks associated with respective measurement distances to a selected surface (e.g. a mirror) exhibit a desirable combination of maximum height and minimum width, so that their peak locations may be estimated with the best accuracy. The aperture  195  is then fixed at the final aperture operating position relative to the optical portion  350 , by fixing the optical portion  350  relative to the housing  320  (if necessary) and fully tightening the screws  310 . 
         [0032]      FIG. 4  is a diagram of an isometric view of a second exemplary embodiment of a CPS optical pen fiber interface configuration  400 . The components of the CPS optical pen fiber interface configuration  400  are similar to those of the CPS optical pen fiber interface configuration  100  of  FIG. 1 , and 4XX series numbers in  FIG. 4  that have the same “XX” suffix as 1XX series numbers in  FIG. 1  may designate similar or identical elements, which may function similarly, except as otherwise described or implied below. The CPS optical pen fiber interface configuration  400  includes a fiber interface subassembly  480  comprising a mounting element  482  and an aperture element  490 . Also shown in  FIG. 4 , is an optical fiber cable  410  including an internal optical fiber  412 . The optical fiber cable  410  may terminate in a suitable commercially available optical fiber connector  414 . A strain relief element  430  may couple a portion of the fiber cable  410  to a rear ferrule  440 . In one embodiment, the rear ferrule  440  may comprise a sleeve having an intermediate portion  450  that has a different diameter then the remainder of the rear ferrule  440 . During assembly, the optical fiber cable  410  may be inserted through the rear ferrule  440  and a bare portion of the optical cable  412  may continue through a front ferrule  470 , with a fiber end  415  exposed at the end of the front ferrule  470 . The front ferrule  470  may be bonded to an intermediate portion  450 . In one embodiment, the ferrule  470  may be ceramic and may include a rotational alignment portion  460 . A hole  483  in the mounting element  482  receives the front ferrule  470  and the optical-fiber end  415 , and internal features in the hole may mate with the rotational alignment portion  460  to insure its proper alignment. A retaining element  420  may slide forward to press the rotational alignment portion  460  forward against an axial stop (e.g. an internal shoulder in the hole  483 ), and the retaining element  420  may be bonded or compression fit into a rear portion of the hole  483  that has a matching diameter. In various embodiments, the ferrule  470  may be ceramic, may be merged with or indistinguishable from the intermediate portion  450  and/or the rear ferrule  440 . 
         [0033]    In various embodiments, the end of the ferrule  470 , the fiber end  415  and an angled surface  484  of the mounting element  482  may all be angled relative to the fiber axis and the axis of the hole  483  as previously outlined for the elements  170 ,  115  and  184 . In the embodiment shown in  FIG. 4 , internal features (not shown) in the hole  483  may mate with the rotational alignment portion  460  to insure their proper rotational alignment, such that the angled fiber end  415  and end of the ferrule  470  are parallel to the angled surface of the aperture element  490  proximate to the angled surface  484  in operation. In alternative embodiments, proper rotational alignment may be achieved by any other convenient means. It will be appreciated that one embodiment, the interior of the hole  483  may be configured similarly to the interior of the holes through the ferrule  470  and the rear ferrule  440 , and the fiber  412  may be received and positioned directly in such a hole in the mounting element  482 . In such an embodiment, the elements  470 ,  460 ,  450  and  440  may be omitted. 
         [0034]    In any case, an aperture element  490  is attached to the angled end surface  484 , and includes an aperture  495  which is aligned with the fiber end  415 . The aperture element  490  may be an opaque material that includes a pinhole aperture. In one specific example embodiment, the aperture element  490  may be formed from 302 stainless steel and have a black copper oxide finish on both sides to reduce spurious reflections of light into the optical signal path. The attachment of the aperture element  490  to the angled end surface  484  will be described in more detail below with respect to  FIG. 5 . 
         [0035]      FIG. 5  is a diagram  500  of a side view cross-section of a portion of the fiber interface configuration  400  of  FIG. 4 , during operation. As shown in  FIG. 5 , the optical fiber  412  is encased by the front ferrule  470  which in turn is received in the hole  483 . In various embodiments the ferrule  470  and the optical fiber  412  are received and constrained in the hole  483  with an axis of the optical fiber  412  approximately coinciding with the axis of the hole  483 . The fiber end  415  and the end of the ferrule  470  may be flush with each other at the angle T 1 . The aperture element  490  may abut and cover the fiber end  415  and may also abut and cover the end of the ferrule  470  as shown. The aperture element  490  may be fixed to the angled surface  484  by conventional means (e.g. glue lines  448 ), with the aperture approximately concentric to the hole  483  and/or the fiber core  416 . In various embodiments, the aperture  495  may be aligned relative to the position of the fiber end  415  and the core  416  using alignment procedures similar or identical to those outlined above with reference to  FIG. 2 . The dimensions D 1  and D 2  may be similar to those discussed above with respect to  FIG. 2 . 
         [0036]    In one specific example embodiment, the fiber end  415 , the end of the ferrule  470  and/or the angled surface  484  may all be polished flush after the components are assembled together. For example, in the initial assembly, the fiber end  415  and/or the ferrule  470  may be inserted so that the ends extend slightly beyond the surface of the mounting element  480 . Thereafter, a polishing operation could be performed to bring the surfaces flush. In such an embodiment, it may be advantageous for the aperture element  490  to abut the flush surfaces and be bonded around its edges with a glue fillet or spot welded to the mounting element  482 . 
         [0037]      FIG. 6  is a diagram of a side view cross-section of a second embodiment of a CPS optical pen  600  including the fiber interface configuration  400  of  FIG. 4 . As shown in  FIG. 6 , the CPS optical pen  600  also includes an optical pen assembly housing  620 , and an optics portion  650 , which may be similar or identical to the optics portion  350  described with reference to  FIG. 3 . In operation, light emitted from the fiber end  415  through the aperture  495  is focused by the optics portion  650 . The light is focused on the workpiece surface  690  with axial chromatic dispersion, reflected, and refocused by the optics portion  650  on to the aperture  495  as illustrated by the limiting rays LR 1 ′ and LR 2 ′. A distance X 1 ′ represents the spacing between the rear principal plane of the optics portion  650  and the aperture  495 . Due to the axial chromatic dispersion, only one wavelength will be in focus at the surface  690  and the same wavelength will focus at the aperture  495 . Therefore, light received into the core  416  of the optical fiber cable  410  through the aperture  495  will be spatially filtered to have predominantly the best focused wavelength. In various embodiments, the optical fiber cable  410  routes the signal light to an optical fiber attached to the connecter  414 , which in turn carries the signal to a spectrometer (not shown) that is utilized for determining the wavelength corresponding to the highest signal level, thus determining the distance to the workpiece surface  690 . 
         [0038]    The mounting element  482  of the sub-assembly  400  may be attached to the end of the CPS optical pen assembly housing  620  using mounting screws  610 . In various embodiments, the mounting element  482  is mounted such the axis of the hole  483  is at an angle T 2 ′ relative to the optical axis OA. The angle T 2 ′ is chosen such that the angle (T 1 +T 2 ′) is approximately the same as the angle of refraction for light at the fiber end  490  (e.g. approximately 11° for a fiber end bevel angle of 7°), so that light rays parallel to the fiber axis are bent parallel to the optical axis OA as they exit the fiber end  490 . 
         [0039]    In various embodiments, prior to finally tightening the screws  610  and thereby fixing the fiber optic interface sub-assembly  480  relative to the optical element portion  650 , it is actively aligned to the optical portion  650 . The fiber optic interface sub-assembly  480  may be actively aligned relative to the optical element portion  650  using alignment procedures similar or identical to those outlined above with reference to  FIG. 3 . The aperture  495  is then fixed at the final aperture operating position relative to the optical portion  650  by fixing the optical portion  650  relative to the housing  620  (if necessary) and fully tightening the screws  610 . 
         [0040]    In practice, CPS optical pens fabricated according to above principles have exhibited superior lateral and axial measurement resolution in comparison to conventional CPS optical pens fabricated using the tapered optical fiber end aperture, despite the fact that the apertures of the aperture elements and the tapered fiber end apertures appear to have the same diameter. While the preferred embodiment of the invention has been illustrated and described, numerous variations in the illustrated and described arrangements of features and sequences of operations will be apparent to one skilled in the art based on this disclosure. Thus, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.