Self aligning optical fiber terminator

A optical fiber terminator comprising: a cylindrical housing having a through bore that is cylindrical at one end and conical at the other end, a frusto-conical collet mounted inside the conical bore where the collet has a stepped cylindrical through bore with a diameter corresponding to the diameter of an industry standard ST ferrule; and, a means to receive and retain an industry standard ST termination attached to the end of the housing having the conical bore. The housing and collet assembly provide a means for precisely placing the tip of the optical fiber ferrule in both a radial and axial direction to facilitate using standard ST terminations.

FIELD OF THE INVENTION
 This invention generally relates to an optical fiber terminator,
 specifically one that aligns the tip of the fiber both axially and
 concentrically.
 BACKGROUND OF THE INVENTION
 ST style terminators provide a convenient method of terminating optical
 fibers in both digital and analog applications. The qualitative study of
 materials using light usually requires that a sample be illuminated and
 the light either reflected from or transmitted through the sample be
 analyzed. When an optical fiber is used to carry light to a sample, the
 light diverges rapidly as it exits the optical fiber tip and must be
 controlled and made into a collimated beam. When light has passed through
 or has reflected from a sample it must be made to converge, or focus, so
 that the light can be introduced into the small core at the tip of the
 optical fiber for efficient transmission to an analyzing instrument. In
 both cases, a focusing element is placed at a precise distance from the
 tip of the optical fiber. The distance from the optical fiber to the
 focusing element, as well as the concentricity of the optical fiber and
 the focusing element, are critical to ensure a reliable light signal for
 these applications.
 Industry standard ST terminations are not designed to accurately control
 the position of the tip of the optical fiber. The main component of an ST
 terminator is a cylindrical ferrule mounted in a push and twist type
 mechanism, commonly known as a "bayonet" style connection. One end of the
 ferrule has an external shoulder and is constructed so that an optical
 fiber may be inserted. The other end has an opening that is smaller than
 the diameter of the ferrule, which forms an internal shoulder. Upon
 insertion, the tip of the optical fiber extends through the internal
 shoulder and registers axially inside the cylindrical ferrule flush with
 the outside face. Standard ST terminations rely on contact with the
 external shoulder of the connector, distal to the tip, for positioning.
 The ST termination process, during initial manufacture and during field
 repair, involves abrasively polishing the end of the ferrule having the
 internal shoulder and the optical fiber within. This process introduces an
 inconsistency in the final length of finished ferrules, as they extend
 from the external shoulder of the connector, of a significant fraction of
 a millimeter from connector to connector. Attempting to control axial
 position, distal to the optical fiber tip does not account for these
 variations and results in a similar variation in distance between the
 focusing element and the tip of the optical fiber. A means for precisely
 positioning the tip of any ST optical fiber terminator both axially for
 focusing and radially for concentricity would ensure consistency of
 optical focus and efficient light transmission with this common and
 convenient style of optical fiber connector.
 SUMMARY OF THE INVENTION
 It is an object of the invention to improve the axial and concentric
 positioning of an industry standard ST termination. This invention makes
 use of the tip of the finished ST ferrule, the precision diameter of the
 ferrule and the spring loaded design of the ST termination to precisely
 and repeatedly align any ST terminated fiber with an optical element. A
 conical collet, having a concentric counter bore, fits loosely within a
 mating conical housing. The concentric counter bore has a length somewhat
 shorter, and a diameter corresponding to the highly accurate diameter of a
 standard ST ferrule. When an ST ferrule is inserted, the tip of the
 ferrule presses against the bottom of the counter bore, causing the
 outside surface of the conical collet to travel into and wedge against the
 mating surface of the conical bore. The collet travel is precisely
 controlled, causing the ST tip to be precisely positioned axially within
 the housing. As the collet wedges against the conical housing, it in turn
 compresses around the ST ferrule causing the ST tip to be precisely
 positioned concentrically within the housing.

DETAILED DESCRIPTION OF THE INVENTION
 FIG. 1 shows a cross sectional view of a terminator according to the
 invention. A housing 10 is constructed having a through bore that is
 cylindrical at one end and conical at the other. The housing end having
 the cylindrical bore 20 is constructed so that an optical element 120,
 such as a lens, may be mounted to, or in fixed relationship to, the
 housing. The housing end having the conical bore 30 is constructed to
 receive and loosely retain a conical collet 40 within the conical bore 30.
 The housing's conical bore matches the outside surface of the conical
 collet. The conical bore end of the housing is also adapted to receive and
 retain an industry standard ST bayonet style termination.
 FIGS. 2A and 2B show two views of the conical collet 40. The conical collet
 is constructed with a through bore that is cylindrical at one end 50 and
 forms a shoulder 60 at the other end. The diameter of the cylindrical
 portion of the through bore corresponds to the diameter of an industry
 standard ST ferrule. The collet has a plurality of slots 70, equally
 spaced, that are slotted alternately from front to back and back to front,
 of sufficient length and depth to allow the collet to compress and
 decompress uniformly. The collet is slotted and its internal diameter is
 expanded slightly, giving it some spring pressure to enhance the repeated
 insertion and removal of the ST termination ferrule.
 FIG. 3 shows the invention being used with an industry standard ST
 termination 80. An ordinary ST terminated optical fiber connector is
 inserted into the housing end having the conical bore 30. The housing is
 adapted to receive and retain the connector using a bayonet style
 connection. The bayonet style connection is integral to ST connectors. The
 end 90 of the ST ferrule 110 passes into the cylindrical through bore 50
 of the conical collet and registers axially at the conical collet's
 shoulder 60. After contacting the collet's shoulder, the force behind the
 tip of the ferrule causes the collet to travel further into the housing
 and the mating conical bore of the housing 100 uniformly contacts the
 collet in a wedging fashion. The collet reduces slightly in diameter,
 compressing down into the housing bore 100, until the collet's cylindrical
 bore 50 contacts and exerts a positive, inward radial clamping force
 against the length of the ferrule 110 that it contains. When this occurs,
 the collet can travel no further into the housing. The ST connector is
 then fastened in place using the bayonet mechanism that is integral to ST
 style connectors. As stated above, the planar surface of the ferrule tip
 is the initial contact between the ferrule and the collet's shoulder. The
 depth to which the tip pushes the collet into the mating conical housing
 is determined by the diameter of the ferrule, which is closely controlled.
 The final position of the ferrule tip is determined by the trigonometric
 relationship between the ferrule diameter and the half angle of the
 outside conical surface of the collet. In this embodiment a 15 degree half
 angle provides stable concentric positioning and satisfactory axial
 position control of the ferrule tip in light of the consistency in ST
 ferrule diameters.
 In this invention, variations in the position of the ferrule tip are due to
 deviation in the diameter of the ST ferrule, deviation from ideal of the
 conical angles in the housing and the collet and deviation in the
 concentricity of the various bores. The maximum axial range of ferrule tip
 positions due to the ST ferrule diameter will vary according to the
 formula:
 ##EQU1##
 where b is the change in the axial position of the tip of the optical fiber
 as it deviates from the theoretical;
 a is the deviation of the ferrule diameter from the nominal 2.50 mm; and
 A is the half angle of the cone.
 Since ST ferrule diameters are typically within a tolerance range of 0.005
 millimeters for a conical half angle of 15 degrees, the maximum range of
 axial position of the ferrule tip b is:
 ##EQU2##
 The maximum range of ferrule tip positions controlled by the deviation from
 ideal of the conical angles in the housing and on the collet can be
 predicted in a similar fashion. Using a manufacturing tolerance of
 .+-.0.1.degree. for any half angle, the sum of the errors between the
 housing bore and the conical collet angle is a maximum of 0.2.degree.
 producing deviations in both the axial and radial directions. Considering
 the worst case, where the half angle of the collet is favored toward the
 plus side of the tolerance range and the conical half angle of the housing
 bore is favored toward the minus side of the tolerance range, causing the
 collet to contact the housing at the collet's large diameter, the
 deviations will vary according to the formulae:
 ##EQU3##
 x.sub.a =X.sub.h sin(.alpha.+0.2.degree.)-L
 where y.sub.r is the radial displacement of the ferrule tip;
 x.sub.a is the axial displacement of the ferrule tip;
 D is the large diameter of the collet;
 L is the length of the ferrule inserted in the collet;
 X.sub.h is a length of a line from the center of the ferrule tip inserted
 in the collet to a point on the large diameter of the collet where
 ##EQU4##
 and
 .alpha. is the angle between the plane of the large diameter of the collet
 and X.sub.h,
 ##EQU5##
 Using 4.5 mm as the value for L and 7 mm as the value for D yields the
 following values for X.sub.h and .alpha.
 ##EQU6##
 and results in the following values for y.sub.r and x.sub.a :
 ##EQU7##
 x.sub.a =5.7sin(52.13+0.20)-4.5=0.0118 mm
 The range of ferrule tip positions controlled by concentricity deviations
 among the various bores can be determined from the manufacturing
 tolerances. Using a manufacturing tolerance of .+-.0.0127 mm
 concentricity, and assuming the maximum error among the cylindrical and
 conical bores in the housing and the cylindrical bore in the collet,
 yields an error in radial placement of the ferrule tip of 0.0381 mm.
 In accordance with this invention, the total variations in the position of
 the ferrule tip due to deviation in the diameter of the ST ferrule,
 deviation from ideal of the conical angles in the housing and the collet
 and deviation in the concentricity of the various bores becomes 0.0305 mm
 in the axial direction and 00548 mm in the radial direction. This is
 approximately nine times better axial positioning accuracy than can be
 obtained using standard ST hardware and ferrules that can typically vary
 by 0.28 mm in length after polishing, and at least as good radial
 positioning as found in standard ST hardware.