Optical fiber plug connection and adjustment method

Abstract

A fiber plug facilitates optical coupling of a light-guiding fiber to a plug receptacle and includes a plug housing for receiving and locking parts of the fiber plug in position relative to one another. The plug housing has: a fiber inlet and a fiber bearing for the spatially fixed reception of the fiber; optically downstream of the fiber bearing along a beam path, an optical lens for collecting light exiting at an end face of the light-guiding fiber and for collimating the collected light; and a coupling surface with an output of the beam path and with a coupling structure for connection to a receptacle structure that is complementary to the coupling structure. An adjustable optical element is arranged optically downstream of the fiber bearing in the beam path and has a first component of a magnetic coupling consisting of two components and a first component of a kinematic coupling.

Claims

1. A fiber plug for optical coupling of a light-guiding fiber with a plug receptacle, the fiber plug comprising: a plug housing for receiving and locking component parts of the fiber plug in a predetermined position relative to one another; wherein the plug housing includes: a fiber inlet and a fiber bearing configured for spatially fixed reception of the light-guiding fiber; at least one optical lens located optically downstream of the fiber bearing along a beam path of light emitted from an end face of the light-guiding fiber and configured for collecting light exiting from the end face of the light-guiding fiber and for collimating the collected light; a coupling surface with an output of the beam path and with a coupling structure for connection to a receptacle structure, which is complementary to the coupling structure; and at least one adjustable optical element that is arranged in the beam path optically downsteam of the fiber bearing and optically upstream of the coupling surface, wherein the at least one adjustable optical element is configured to reduce an angle of the beam path with respect to an optical axis of the lens and/or to reduce a spatial offset of the beam path with respect to the optical axis of the optical lens, wherein the coupling structure has a first component of a magnetic coupling that includes two components and a first component of a kinematic coupling.

2. The fiber plug according to claim 1, wherein the at least one adjustable optical element includes a prism wedge pair having two mutually adjustable prism wedges arranged in a common mount and wherein the prism wedge pair is tiltable about a plurality of axes.

3. The fiber plug according to claim 1, wherein the at least one adjustable optical element includes a pivot wedge pair that includes two lenses whose side surfaces facing away from one another include planar surfaces and wherein the lenses can be inclined relative to one another and/or displaced laterally.

4. The fiber plug according to claim 1, wherein the optical lens is corrected achromatically to a spectral range or wavelength range intended for use.

5. The fiber plug according to claim 1, wherein the at least one adjustable optical element includes at least one plane-parallel plate.

6. The fiber plug according to claim 5, further comprising a further plane-parallel plate and/or a pair of mutually adjustable prism wedges arranged in the beam path.

7. The fiber plug according to claim 1, wherein the fiber bearing is configured for retaining a light-guiding fiber having a light exit surface directed along the beam path.

8. The fiber plug according to claim 1, wherein the fiber bearing includes a clamping element configured for receiving and retaining a fiber.

9. The fiber plug according to claim 7, wherein the at least one adjustable optical element is configured to be adjusted such that light exiting the fiber is provided at an optical output of the fiber plug parallel and symmetrically with respect to the beam path at the optical output; and wherein the at least one adjustable optical element is locked in a state adjusted in this way.

10. An optical plug connector comprising: a fiber plug that includes: a plug housing for receiving and locking component parts of the fiber plug in a predetermined position relative to one another; wherein the plug housing includes: a fiber inlet and a fiber bearing for the spatially fixed reception of the light-guiding fiber; at least one optical lens located optically downstream of the fiber bearing along a beam path of light emitted from an end face of the light-guiding fiber and configured for collecting light exiting from the end face of the light-guiding fiber and for collimating the collected light; a coupling surface with an output of the beam path and with a coupling structure for connection to a receptacle structure, which is complementary to the coupling structure; and at least one adjustable optical element that is arranged in the beam path optically downsteam of the fiber bearing and optically upstream of the coupling surface, wherein the at least one adjustable optical element is configured to reduce an angle of the beam path with respect to an optical axis of the lens and/or to reduce a spatial offset of the beam path with respect to the optical axis of the optical lens, wherein the coupling structure includes a first component of a magnetic coupling and a first component of a kinematic coupling; and a plug receptacle that includes: a second component of the kinematic coupling, which is complementary to the first component of the kinematic coupling, as a receptacle structure; and a second component of the magnetic coupling that includes two components.

11. The optical plug connector according to claim 10, the adjustable optical element includes a prism wedge pair with two mutually adjustable prism wedges arranged in a common mount and wherein the jointly held prism wedge pair is tiltable about a plurality of axes.

12. The optical plug connector according to claim 10, wherein the adjustable optical element includes a pivot wedge pair, which comprises two lenses whose side surfaces facing away from one another include planar surfaces and wherein the lenses can be inclined relative to one another and/or displaced laterally.

13. The optical plug connector according to claim 10, wherein the at least one optical lens is corrected achromatically to a spectral range or wavelength range intended for use.

14. The optical plug connector according to claim 10, wherein the adjustable optical element includes at least one plane-parallel plate is arranged.

15. The optical plug connector according to claim 14, further comprising a further plane-parallel plate and/or a pair of mutually adjustable prism wedges arranged in the beam path.

16. The optical plug connector according to claim 10, wherein the fiber bearing is configured for retaining a light-guiding fiber having a light exit surface directed along the beam path.

17. The optical plug connector according to claim 10, wherein the fiber bearing includes a clamping element configured for retaining a fiber received in the fiber bearing.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention is explained in more detail below on the basis of exemplary embodiments and figures. In the figures:

(2) FIG. 1a shows a schematic illustration of a resulting angular error;

(3) FIG. 1b shows a schematic illustration of a resulting spatial offset;

(4) FIG. 2 shows a schematic illustration of a first exemplary embodiment of an optical plug connection with a fiber plug and with adjustable optical elements in the infinite beam path;

(5) FIG. 3 shows a schematic illustration of a second exemplary embodiment of an optical plug connection with a fiber plug and with adjustable optical elements in the infinite beam path, and also a schematic illustration of the interface to a microscope;

(6) FIG. 4 shows a schematic illustration of a third exemplary embodiment of an optical plug connection with a fiber plug and with adjustable optical elements in the infinite beam path;

(7) FIG. 5 shows a schematic illustration of a fourth exemplary embodiment of an optical plug connection with a fiber plug and with adjustable optical elements in a divergent section of the beam path;

(8) FIG. 6 shows a schematic illustration of a fifth exemplary embodiment of an optical plug connection with a fiber plug and with adjustable optical elements in a diverging section of the beam path and in the infinite beam path;

(9) FIG. 7a shows a schematic illustration of a zero position of two mutually adjustable prism wedges and the resulting optical effect;

(10) FIG. 7b shows a schematic illustration of two mutually adjusted prism wedges and the resulting optical effect;

(11) FIG. 8a shows a schematic illustration of a pair of pivot wedges in the zero position and the resulting optical effect;

(12) FIG. 8b shows a schematic illustration of the pair of pivot wedges in a pivoted state and the resulting optical effect;

(13) FIG. 9 shows a schematic illustration of an exemplary embodiment of a coupling surface of the plug housing of a fiber plug with first components of a kinematic coupling and with a first component of a magnetic coupling;

(14) FIG. 10 shows a schematic illustration of an exemplary embodiment of a plug receptacle with second components of a kinematic coupling and with a second component of a magnetic coupling;

(15) FIG. 11 shows a schematic illustration of a sixth exemplary embodiment of an optical plug connection with a fiber plug and with adjustable optical elements in the infinite beam path and with a plane-parallel plate in the beam path of the plug receptacle;

(16) FIG. 12 shows a schematic illustration of an arrangement for adjusting a fiber plug; and

(17) FIG. 13 shows a flow chart of a configuration of an adjustment method.

DETAILED DESCRIPTION

(18) The exemplary embodiments shown in FIGS. 2 to 9 are schematic illustrations. The reference signs denote the same technical elements in each case.

(19) FIG. 2 shows an arrangement of a plurality of optical elements in a beam path 6 as an example. In addition, the resulting corrective effect of these optical elements is illustrated.

(20) The fiber 1 held in the ferrule 10 is aligned parallel to but offset from the optical axis 3. The beam path 6 and the optical axis 3 do not coincide in this section, i.e., the beam path 6 extends parallel to the optical axis 3 with an offset. Without correction, a resulting angular error of the collimated beam would occur, as is shown in FIG. 1a. The light exiting at the end face of the fiber 1 diverges and is incident on the optical lens 2, which functions as a collimation optical unit. The light is collimated by the effect of the optical lens 2 and reaches an optical wedge or prism wedge 4 at an angle to the optical axis 3. With regard to its optical properties, in particular with regard to its refractive properties, and also its relative position in the beam path 6, the wedge is designed or arranged in such a way that the rays of the light run parallel to the optical axis 3 after they have passed through the prism wedge 4. As can be seen in particular from the central ray, there may still be a spatial offset. This is corrected by a plane plate 5 that is likewise located in the beam path 6. The plane plate 5 is inclined relative to the optical axis 3 in accordance with the spatial offset that is to be corrected and taking into account its optical properties. As a result, after leaving the plane plate 5, the rays of the light have no angular error and no spatial offset with respect to the plane of the drawing. The beam path 6 and the optical axis 3 extend symmetrically with respect to one another and in the example coincide after the plane plate 5. The beam path 6 is now aligned along the optical axis 3. The light corrected in this way can be made available at an interface 17 (see FIGS. 3 to 6) for further use. In order to achieve the desired corrective effect, the prism wedge 4 and the plane plate 5 can be adjustable, for example they are rotatable or tiltable.

(21) The principle shown in FIG. 2 is implemented in a first exemplary embodiment of an optical plug connection in FIG. 3.

(22) A fiber plug 7 comprises a plug housing 8 with a fiber bearing 11 in which the fiber 1 is inserted. The radiated light is emitted from the fiber plug 7 at an output 9 of the plug housing 8. The fiber held by the ferrule 10 is retained in a fixed location in the fiber bearing 11. After a section of the beam path 6 with diverging rays of the light, the light is incident on the optical lens 2 and is corrected with regard to the angular error and the spatial offset by the effect of the adjustable optical elements downstream in the beam path 6. For the sake of simplicity, the beam path is divided into a section with diverging light beams and a section with collimated light beams.

(23) In the exemplary embodiments shown in FIGS. 3 and 4, the beam manipulation or the correction takes place entirely in the infinite beam, that is to say in the section of the beam path 6 with collimated light rays. In the first exemplary embodiment shown in FIG. 3, a prism wedge pair 12a with mutually adjustable prism wedges is present instead of an individual prism wedge 4 (see FIG. 2). Two independently adjustable plane plates 5 (tiltable about the x-axis and/or about the y-axis) are present in the beam path 6 downstream of the prism wedges. The collimated light is passed at the output 9 to an interface 17, for example of a microscope 17.1. A coupling structure 14 is present in the region of a coupling surface 13, located at the output 9 (see FIGS. 7a, 7b, 8a, and 8b), of the plug housing 8. The former interacts with a receptacle structure 15 of a plug receptacle 16. In the example, the angle is set via the prism wedge pair 12a and the location is set to the respective target values via the pair of plane plates 5.

(24) The plane plates 5 can be dispensed with (FIG. 4) if the pair of prism wedges (prism wedge pair 12b) is arranged in a common mount (symbolized by a common frame), which allows the adjusted wedges to be additionally tilted about two axes (x and y) in order to set a parallel offset of the beam and to be able to correct a spatial offset. The side surfaces of the jointly held prism wedge pair 12b pointing outward in each case, away from the respective other prism wedge, extend in the illustrated starting position of the prism wedge pair 12b orthogonally to the optical axis 3 in an xy plane.

(25) In a fourth exemplary embodiment of the optical plug connection, the adjustable optical elements (adjustment means) are arranged in the divergent section of the beam path 6 (FIG. 5). In contrast to the second and third exemplary embodiments, the plane plates 5 are used to set the angle and the prism wedge pair 12a is used to adjust the spatial offset in relation to the collimated beam. Due to the increased space requirement and the resulting longer focal length of the collimation optical unit 2 and the associated larger beam diameter, the requirements for the interface 17 in the microscope change and the space required for the fiber plug 7 increases. In order to enable the longer focal length of the collimation optical unit 2, a magnifying telescope may be omitted in the subsequent optical system. At the same time, the requirements for the positioning accuracy of the collimation optical unit 2 and for the accuracy of the mechanical interface, that is to say for the kinematic coupling 19, in particular with regard to the angle requirements to be observed, increase. The beam diameter required in the subsequent optical system (microscope) is correspondingly already implemented, entirely or partially, at the output of the fiber plug.

(26) Since the focal length 1:1 is included in the adjustment sensitivity of the optical lens 2 with respect to the fiber ferrule 10, the use of a collimation optical unit 2 with the shortest possible focal length is advantageous for a manageable sensitivity of the adjustment. The corresponding post-enlargement to the necessary beam diameter in the subsequent optical system of the microscope, for example by means of a corresponding telescope, additionally favors the angular sensitivity of the interface 17 per se. With a typical NA of the fiber of <=0.1 in connection with an approximately 6-fold post-enlargement, for example, a good compromise is given with a collimation to a beam diameter in the range of 0.7 mm. For this purpose, the collimation optical unit has, for example, a focal length between 4 mm and 6 mm.

(27) In a fifth exemplary embodiment of the optical plug connection, the optical lens 2 is designed to be adjustable (FIG. 6). A plane plate 5, which is likewise adjustable, is present in the collimated section of the beam path 6, wherein the former is designed in the form of what is known as a “window ball”. The curved lateral surface facing the viewer is symbolized by bow lines. An alternative is provided by two plane plates 5 with tilt axes that are perpendicular to one another (around the x-axis or around the y-axis; not shown).

(28) To adjust the beam position, the lens 2 is positioned using external adjustment aids (see FIG. 9) in all degrees of freedom with respect to the beam coming from the fiber 1 in a manner such that a collimated beam at the output 9 of the fiber plug 7 opposite the coupling surface 13 (see FIGS. 7a, 7b, 8a, and 8b) exits with precise location and angle. The ferrule 10 can be releasably locked in the fiber bearing 11 by means of a clamping element 20. The position setting of the plane plate 5 results in an optimization of the lateral alignment or an optimization of the centering of the beam path 6 with respect to the optical axis 3.

(29) Depending on the wavelength range of the exiting laser radiation, the optical lens 2 acting as a collimation lens is designed to be corrected simply, chromatically or achromatically. The long-term stability of the transmission of short-wave laser radiation at 405 nm must be taken into account in the chromatic and achromatic variants, as required, by means of a correspondingly stable cement layer or a cement-free embodiment.

(30) The operation of two mutually adjustable prism wedges of a prism wedge pair 12a is shown schematically and by way of example in FIGS. 7a and 7b. In a first relative position, the prism wedges are rotated by 180° relative to one another in a zero position. In this zero position, a light ray (arrow) passes along the optical axis 3 or the beam path 6 through the prism wedge pair 12a. In the further relative position according to FIG. 7b, the light ray is deflected downward at a defined angle in the plane of the drawing.

(31) FIG. 8a shows a pivot wedge pair 28 comprising the lenses 28.1 and 28.2. The first lens 28.1 is designed as a plano-concave lens, while the second lens 28.2 is designed as a plano-convex lens. The planar side surfaces of the lenses 28.1 and 28.2 face away from each other so that the planar side surfaces point outward and extend orthogonally to the optical axis 3 or to the beam path 6 in the zero position of the pivot wedge pair 28 shown in FIG. 8a. The optical effect of the pivot wedge pair 28 in the zero position consists in the fact that a light ray that is incident perpendicularly, in particular centrally, on a planar side surface passes through the pivot wedge pair 28 without lateral deflection. In order to substantially reduce the effect of the pivot wedge pair 28 on the collimated beam, the focal lengths of the lenses 28.1 and 28.2 are large and are, for example, 1 m.

(32) The convex and concave side surfaces are matched to one another in terms of their dimensions and radii such that they can be laterally displaced relative to one another. In FIG. 8b, the lens 28.2 provided with a convex side surface is displaced laterally against the first lens 28.1. The incident light ray is deflected from its original direction of propagation.

(33) The lateral displacement of one or both lenses 28.1, 28.2 of the pivot wedge pair 28 takes place, for example, by means of a controlled drive (not shown). The associated control commands can be generated by an evaluation unit 25 and be implemented by an actuating apparatus 26 (see, for example, FIG. 12).

(34) The pivot wedge pair 28 can optionally also be tilted as a whole about the x-axis and/or about the y-axis. The relative position of the lenses 28.1 and 28.2 in this case can be kept constant so that, for example, the optical effect of a plane plate is achieved. Optionally, the pivot wedge pair 28 can be displaced along the z-axis.

(35) An embodiment of the coupling surface 13 of the fiber plug 7 (indicated by a broken solid line) is shown in FIG. 9 in a plan view in the z-direction. The connection of an optical plug connection is achieved by the effect of a magnetic coupling 18 and a kinematic coupling 19.

(36) A ring magnet is present as a first component 18.1 of the magnetic coupling 18 on the coupling surface 13 or is embedded therein partially or flush therewith. The ring magnet rotationally symmetrically surrounds a passage 23 serving as the output 9 of the fiber plug 7. First components 19.1 of the kinematic coupling 19 are provided offset from one another by 120° in each case. In the exemplary embodiment, these are formed by pairs of rods that are parallel to one another and at a distance from one another. In further possible embodiments, the angles between the first components 19.1 and/or their design can be selected differently.

(37) A recess 22 enables a force-free screw connection of the fiber plug 7 as a safeguard against unintentional interruption of the plug connection.

(38) A receptacle structure 15 of the plug receptacle 16 that is compatible with the coupling structure 14 has three spherically protruding second components 19.2 of the kinematic coupling 19 (FIG. 10). These are compatible with the first components 19.1 of the kinematic coupling 19 in terms of their dimensions and positioning. A second component 18.2 of the magnetic coupling 18 is provided symmetrically around a passage 23, the polarity of which with regard to its magnetization is opposite to that of the first component 18.1. The second component 18.2 is compatible with the first component 18.1 in terms of its dimensions and positioning. Fastening holes 29 are additionally shown.

(39) The plug receptacle 16 can have at least one adjustable plane-parallel plate 5 in its beam path 30 (FIG. 11). The possibility of adjusting this plate 5 allows the tolerances to be observed for the beam position of the fiber plug 7 to be more broadly defined than is the case with the previous exemplary embodiments.

(40) The method for adjusting a fiber plug 7 is shown schematically in FIG. 12 on the basis of a fiber plug 7 according to the fifth exemplary embodiment of the optical plug connection (FIG. 6). For the adjustment, the fiber 1 with the ferrule 10 is inserted into the fiber bearing 11. Light is coupled into the fiber 1 and exits at the end face of the fiber 1 located in the fiber plug 7. The fiber plug 7 or its beam path 6 is aligned with an adjustment apparatus 24, and the latter is illuminated with the light exiting the fiber plug 7. The adjustment apparatus 24 can be an autocollimation telescope (AKF) which can be set to two different focal positions and thus combines the functions of both an AKF for angle measurement and an alignment telescope for location determination. For example, the focal length of the AKF can lie in a range of 400 mm in order to achieve a high accuracy of less than or equal to one arc second (≤1″) in the angle measurement.

(41) For the purpose of adjustment, the beam path 6 of the fiber plug 7 is made to coincide with the optical axis of the adjustment apparatus 24. The adjustable optical elements, in this exemplary embodiment the optical lens 2 and the plane plate 5, are subsequently adjusted in a manner such that the light at the optical output 9 is collimated and exits the fiber plug 7 parallel and symmetrically with respect to the beam path 6. The angular and spatial positions are measured with the adjustment apparatus 24. In the example of the AKF, this can be done in two measurement operations. The acquired measurement data relating to the spatial position or angular position can be transmitted to an evaluation unit 25 via a detector 27 connected to the adjustment apparatus 24, for example a CCD camera.

(42) Necessary adjustments of the adjustable optical elements, in this exemplary embodiment the optical lens 2 and the plane plate 5, can be displayed, and manual settings can then be made. Alternatively, the evaluation unit 25 can be configured in such a way that control commands are generated in dependence on the measurement data and transmitted to an actuating apparatus 26 or to a plurality of actuating apparatuses 26. The adjustable optical elements are adjusted according to the control commands. This operation can take place iteratively and be carried out in the sense of a feedback control.

(43) If the acquired measurement data are within permissible tolerances, the adjustment operation is ended. The adjustable optical elements and optionally also the ferrule 10 are locked, optionally non-releasably, by being glued, potted, soldered or welded, for example. The ferrule 10 can also be retained in its installed position by means of the clamping element 20. This embodiment allows the fiber 1 to be reused in the case of a defect in the fiber plug 7.

(44) In a flow chart of the method, the fiber plug 7 and its beam path 6 are aligned relative to an adjustment apparatus 24 (FIG. 13; step 0). The adjustment apparatus 24 can have two beam paths with a reference mark for setting an angle in one beam path and a reference mark for setting a location in the other beam path. In the exemplary configuration of the adjustment method explained below, reference is made to an alternative possibility in the form of an FAKF. The FAKF is refocused accordingly for the purpose of adjusting the angle or the location.

(45) The focusing of the FAKF or the adjustment apparatus 24 is set to infinity. This corresponds to the functional principle of an autocollimation telescope.

(46) In a step 1 of the method, light exiting the fiber 1 divergently in a beam is aligned as symmetrically as possible with respect to the beam path 6 of the fiber plug 7. The beam path 6 of the fiber plug 7 is aligned with the optical axis of the adjustment apparatus 24, with the result that the beam path 6 and the optical axis of the adjustment apparatus 24 coincide.

(47) The optical lens 2 is subsequently positioned in relation to the beam. By displacing the optical lens 2 and/or ferrule 10 relative to one another in the beam direction (z-direction), the beam is collimated by the effect of the optical lens 2. The success of the collimation is checked. Step 1 is repeated if the collimation was unsuccessful.

(48) If the collimation was successful, the angle of the beam is set in a step 2a by laterally displacing the optical lens 2 relative to the ferrule 10 (see also FIG. 1a). The respective changes in the measured angle can be related to a current change in position of the optical lens 2, and the setting can be effected iteratively. Step 2a is repeated as long as a specified target tolerance has not been achieved.

(49) If, on the other hand, the target tolerance has been achieved, the focusing of the FAKF or of the adjustment apparatus 24 is set to finite (alignment position).

(50) Once the setting of the angle has been completed, the optical lens 2 is tilted relative to the ferrule 10 in a step 2b in order to set the desired spatial position of the collimated beam (see also FIG. 1b). This adjustment of the location is comparatively coarse. If, in addition to the optical lens 2, a plane plate 5 or a prism wedge pair 12a, 12b is also located in the beam path 6, tilting it can finely adjust the location. As explained above in relation to angle adjustment, the adjustment of the location can also take place iteratively, that is, the achievement of a target tolerance is checked. Depending on the result of the check, step 2b is repeated, or the method continues with step 2c.

(51) Step 2c is optional and includes a fine adjustment of the location. For this purpose, the plane plate 5, the pivot wedge pair 28 and/or the prism wedge pair 12a, 12b is/are tilted.

(52) In order to evaluate the result of the previous adjustment steps, the achievement of a previously defined target tolerance is checked again. If the target tolerance is not achieved, the adjustment operation is repeated beginning with the setting of the focusing of the FAKF/adjustment apparatus 24 to infinity (autocollimation, between step 0 and step 1).

(53) If the target tolerance of the entire adjustment process is achieved, the method continues with step 3. In step 3, the adjustable optical elements are locked in the state adjusted in this way. The locking can be done either releasably or non-releasably. A fiber plug 7 adjusted in this way can be connected to different plug receptacles 16 once or repeatedly. The correct alignment of the beam of light provided at the output 9 of the fiber plug 7 and, for example, irradiated into a microscope is maintained in this case. Monitoring of the adjustment is advantageously carried out in the now locked state of the fiber plug 7.

REFERENCE SIGNS

(54) 1 Fiber 2 Optical lens 3 Optical axis (of the optical lens) 4 Prism wedge 5 Plane plate 6 Beam path (of the fiber plug 7) 7 Fiber plug 8 Plug housing 9 Output (beam path 6) 10 Ferrule 11 Fiber bearing 12a Prism wedge pair 12b Prism wedge pair in a common mount 13 Coupling surface 14 Coupling structure 15 Receptacle structure 16 Plug receptacle 17 Interface 17.1 Microscope 18 Magnetic coupling 18.1 First component 18.2 Second component 19 Kinematic coupling 19.1 First component 19.2 Second component 20 Clamping element 21 Non-releasable lock 22 Recess (for force-free screw-connection) 23 Passage 24 Adjustment apparatus 25 Evaluation unit 26 Actuating apparatus 27 Detector 28 Pivot wedge pair 28.1 First lens 28.2 Second lens 29 Fastening holes 30 Beam path (of plug receptacle 16)