Lensed ferrule with low back reflection

A fiber optic ferrule has an entrance surface that is angled at an angle that other than perpendicular to the optical fiber axis. The optical fibers disposed within the fiber optic ferrule are preferably separated from the entrance surface. These features reduce the amount of reflection of the light back into the optical fiber and increase the performance of the fiber optic ferrule.

BACKGROUND OF THE INVENTION

Existing lens ferrule designs that use single-mode optical fibers create significant return loss, also known as back-reflection. Within a current lensed ferrule, light exits the fiber and if there is a gap between the fiber and ferrule, the light travels through an optically transparent adhesive, followed by several hundreds of microns of the optical polymer used to make the ferrules, and then exits the polymer ferrule material, typically through a lens. The refractive index of the polymer ferrule material differs significantly from the fiber, which causes the primary reflection of light as the light transmits through the index of refraction change of the ferrule from the optically transparent adhesive. The light reflecting back into the optical fiber contributes to a return loss.

In order to reduce the reflection of the light back in to the optical fiber and the ferrule itself, Applicant has determined that if the surface of the ferrule where the light enters after leaving the optical fiber and adhesive is disposed at an angle, the reflection can be reduced to an acceptable amount. The reflection may also be reduced if the front end of the optical fiber is disposed some distance from the entrance surface.

SUMMARY OF THE INVENTION

The present invention is directed to a fiber optic ferrule that includes a main body having a front end, a back end, and a middle portion disposed between the front end and back end, a first opening through the back end of the main body, the first opening configured to receive at least two optical fibers through the back end of the main body, a plurality of optical fiber openings disposed in the middle portion and in communication with and extending from the first opening toward the front end, each of the plurality of optical fiber openings configured to receive an optical fiber inserted through the back end and having a longitudinal axis, a second opening disposed between the middle portion and the front end, the second opening extending through a surface of the main body and being in communication with the plurality of optical fiber openings, and a front portion, the front portion disposed between the second opening and the front end, the front portion having at least one entrance surface that is non-perpendicular to the longitudinal axes of the plurality of optical fiber openings, the at least one entrance surface being a rearward facing surface.

In some embodiments, the at least one entrance surface is in a front hole in the front portion and is in communication with the second opening.

In some other embodiments, the fiber optic ferrule further includes at least one optical fiber stop, the at least one optical fiber stop configured to engage a front end of an optical fiber inserted through at least one of the plurality of optical fiber openings.

In another embodiment, the at least one entrance surface comprises a flat surface having an angle of about 4 degrees.

According to another aspect of the present invention, there is a fiber optic ferrule that includes a main body having a front end, a back end, and a middle portion disposed between the front end and back end, a first opening through the back end of the main body, the first opening configured to receive a plurality of optical fibers through the back end of the main body, a plurality of optical fiber openings disposed in the middle portion and in communication with and extending from the first opening toward the front end, each of the plurality of optical fiber openings configured to receive an optical fiber inserted through the back end and having a longitudinal axis, a second opening disposed between the middle portion and the front end, the second opening extending through a surface of the main body and being in communication with the plurality of optical fiber openings, a front portion, the front portion disposed between the second opening and the front end, the front portion having at least one entrance surface that is non-perpendicular to the longitudinal axes of the plurality of optical fiber openings, the at least one entrance surface being a rearward facing surface, a plurality of optical fibers, each of the plurality of optical fibers disposed in respective one of the plurality of optical fiber openings, and index matching adhesive disposed in the second opening and at ends of the optical fibers.

According to another aspect of the present invention, there is a fiber optic ferrule that includes a main body having a front end, a back end, and a middle portion disposed between the front end and back end, a first opening through the back end of the main body, the first opening configured to receive at least two optical fibers through the back end of the main body, a plurality of optical fiber openings disposed in the middle portion and in communication with and extending from the first opening toward the front end, each of the plurality of optical fiber openings configured to receive an optical fiber inserted through the back end and having a longitudinal axis, a second opening disposed between the middle portion and the front end, the second opening extending through a surface of the main body and being in communication with the plurality of optical fiber openings, and an entrance surface forming a portion of the second opening, the entrance surface being non-perpendicular to the longitudinal axes of the plurality of optical fiber openings

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1-3illustrate one embodiment of a fiber optic ferrule100according to the present invention. The fiber optic ferrule100has a main body102with a front end104and a back end106. Between the front end104and the back end106is a middle portion108.

The fiber optic ferrule100has a first opening110through the back end106of the main body102to receive optical fibers90. The fiber optic ferrule100has a plurality of optical fiber openings112disposed within the middle portion108of the main body102. The plurality of optical fiber openings112extend from the first opening110forward towards the front end104. The plurality of optical fiber openings112are configured to receive the optical fibers90inserted through the back end106. The plurality of optical fiber openings112each have a longitudinal axis A extending along the plurality of optical fiber openings112.

The plurality of optical fiber openings112terminate at a second opening114and are thus in communication therewith. The second opening114is configured to receive an adhesive (preferably epoxy that has a refractive index that is the same as the core of the optical fiber) in order to secure the optical fibers within the fiber optic ferrule100. The second opening114is illustrated as opening through a top surface116of the fiber optic ferrule100. As would be known in the art, the second opening114could be through another surface of the fiber optic ferrule100. Preferably the longitudinal axis A of each of the plurality of optical fiber openings112is parallel to the top surface116and perpendicular to the front face118.

As used herein, the term “front” and “forward” means that direction where the fiber optic connector would mate with another fiber optic connector or device, while the term “rear” or “rearward” is used to mean the direction from which the optical fibers enter into the fiber optic ferrule. So turning toFIG. 2, the front is the direction shown by the arrow and “back” or “rearward” is the opposite direction. Thus, the front of the fiber optic ferrule104is pointed to the right inFIG. 2and the rear or rearward end of the fiber optic ferrule104is pointing to the left. Similarly, the top of the fiber optic ferrule100is that side that has an opening into the fiber optic ferrule on the upper part ofFIG. 2while the bottom is on the bottom ofFIG. 2. Further, it will be appreciated that the optical fibers herein may be single mode or multi-mode and, depending on the type of optical fiber, the point of contact of the optical fibers to the entrance surface of the ferrule will change.

A front portion120is disposed between the second opening114and the front end104. The second opening114has a front wall122that defines the boundary between the second opening114and the front portion120. This front wall122is the entrance surface124of the fiber optic ferrule, where the light from the optical fiber enters the front portion120of the fiber optic ferrule100. Thus, the term “entrance surface” relates to a surface of the front portion120that forms an interface between the second opening114and the front end104where light crosses the boundary between the front portion120and the second opening114. Alternatively, the principles of this boundary act in the same way when the light travels in the opposite direction—through the fiber optic ferrule and into the optical fiber. Typically, the front wall122is perpendicular to the top surface116, the plurality of optical fiber openings112(including longitudinal axis A), and the front end92of the optical fibers90. The front wall122is also typically parallel to the front face118. SeeFIG. 1. In some fiber optic ferrules, the front wall122even acts as an optical fiber stop—essentially eliminating the space between the front end of the optical fiber and the front wall. See, e.g.,FIG. 6. The front portion120may also have a lens126at the front end104. The lens126will collimate the light to allow for optical connection of the fiber optic ferrule100to another ferrule, connector, or device.

In the present invention, the front wall122is disposed at an angle α relative to the front face118and the front end92of the optical fiber as can be seen inFIGS. 2 & 3. As such, the front wall122is also no longer perpendicular to the longitudinal axis A. The angle α is preferably between 1 and 5 degrees. The angle α is very well controlled and is not simply a tolerance error.

In the present invention, the optical fibers90are also separated from the front wall122by a predetermined distance D.FIG. 3illustrates that the optical fiber90is separated from the front wall122. It should be noted that even if the optical fiber90were to engage the front wall122, the core94of the optical fiber would still be separated from the front wall122due to the angle α. Referring back toFIG. 3and then with reference toFIG. 4, the variables used in the calculations to determine the return loss of a fiber optic ferrule are:

λ: wavelength of the light in the optical fiber;

ω0: effective mode radius in single mode fiber at λ, where ω0=4.6 μm at λ=1.31 μm and ω0=5.2 μm at λ=1.55 μm;

z: optical path (twice the distance from fiber to ferrule);

x: lateral offset between beam in fiber and beam reflected by ferrule;

ω2: radius of reflected beam at fiber;

T: coupling efficiency at fiber between fiber mode and reflected mode;

RL: return loss

The refractive indices (ni) of the ferrule material, the fiber core and any adhesive are also variables included in the calculation of the return loss.

Applicant has found that if the angle α is 5°, then the return loss is 45 dB or greater. Although there is not only one return loss specification for all connectors, 45 dB is a common requirement in today's optical networks. SeeFIG. 5showing the effect of the angle and the optical fiber separation on the return loss.

While the angle α improves (i.e., reduces) the back reflection into the fiber, as the light transmits through the angled surface, the beam is skewed away from the angled surface and an aberration is created in the light beam in the fiber optic ferrule100. The top and bottom marginal rays of the light beam then go through two different optical path lengths, which create a wavefront aberration. Such a wavefront aberration can be compensated by making an “irregular” lens126that is not rotationally symmetric about the center of the lens. Another aberration is created due to errors of the fiber tip cleave. If the cleave is not perfectly perpendicular to the fiber core axis, a different aberration is created, which is typically minimized with the index matching material.

FIG. 6illustrates the optical fiber90engaging the front wall122of the fiber optic ferrule100. In this embodiment, the core94of the optical fiber90is about 5 microns from the front wall122or entrance surface of the fiber optic ferrule100because of the angle of the entrance surface. It should be noted that the bottom of the optical fiber90engages the entrance surface124and, with the front end92being perpendicular, the core94is about 5 microns from the entrance surface124(for a fiber of 62.5 micron radius, 62.5×sin 50=5) As noted in the graph inFIG. 5, the optical fiber may be disposed such that the front end92is farther (in a rearward direction) from the entrance surface122than 5 microns.

A second embodiment of a fiber optic ferrule200is illustrated inFIGS. 7 and 8. The fiber optic ferrule200is similar to the fiber optic ferrule100except that is has a different front end. Thus, the fiber optic ferrule200has a main body202with a front end204and a back end206. Between the front end204and the back end206is a middle portion208. The fiber optic ferrule200has a first opening210through the back end206of the main body202to receive optical fibers90. The fiber optic ferrule200has a plurality of optical fiber openings212disposed within the middle portion208of the main body202. The plurality of optical fiber openings212extend from the first opening210forward towards the front end204. The plurality of optical fiber openings212are configured to receive the optical fibers90inserted through the back end206. The plurality of optical fiber openings212each have a longitudinal axis A extending along the plurality of optical fiber openings212.

The plurality of optical fiber openings212terminate at a second opening214and are thus in communication therewith. The second opening214also has a front wall222that mostly defines the boundary between the second opening214and a front portion220—except as noted below. The second opening214is configured to receive an adhesive (preferably epoxy that has a refractive index that is the same as of the core94of the optical fiber) in order to secure the optical fibers within the fiber optic ferrule200. The second opening214is illustrated as opening through a top surface216of the fiber optic ferrule200.

As would be known in the art, the second opening214could be through another surface of the fiber optic ferrule200. Preferably the longitudinal axis A of each of the plurality of optical fiber openings212is parallel to the top surface216and perpendicular to the front face218.

The front portion220has a plurality of front holes230, one for each of the optical fiber openings. SeeFIG. 8. The front holes230are in line with and are a continuation of the optical fiber openings212across the second opening214. Each of the front holes230has an entrance surface234at the end of the front holes230that is also angled at the angle α. The end92of each of the optical fibers90are preferably disposed within the front holes230, but are at least 5 microns from the entrance surface234. The angle α and the distance D are again determined by the equations inFIG. 4as discussed above, depending on the ferrule material, the refractive index of the optical fiber, and the like, the distance D is dependent on these factors, too. The front portion220may also have a lens226at the front end204to correct the aberrations from the angled entrance surface234.

A third embodiment of a fiber optic ferrule300is illustrated inFIG. 9and has the same general construction as the other ferrules. The fiber optic ferrule300has a different front end and second opening. The fiber optic ferrule300has a main body302with a front end304and a middle portion308. The fiber optic ferrule300has a plurality of optical fiber openings312disposed within the middle portion308of the main body302. The plurality of optical fiber openings312extend towards the front end304. The plurality of optical fiber openings312are configured to receive the optical fibers90inserted through the back end306. The plurality of optical fiber openings312each have a longitudinal axis A extending along the plurality of optical fiber openings312.

The plurality of optical fiber openings312terminate at a second opening314and are thus in communication therewith. The second opening314also has a front wall322that mostly defines the boundary between the second opening314and a front portion320—except as noted below. The second opening314is configured to receive an adhesive (preferably epoxy that has a refractive index that is the same as of the core94of the optical fiber) in order to secure the optical fibers within the fiber optic ferrule300. The second opening314is illustrated as opening through a top surface316of the fiber optic ferrule300.

As would be known in the art, the second opening314could be through another surface of the fiber optic ferrule300. Preferably the longitudinal axis A of each of the plurality of optical fiber openings312is parallel to the top surface316and perpendicular to the front face318.

The front wall322functions as the entrance surface in this embodiment and has the angle α. The front portion320(and the second opening314) has a fiber stop330to set the distance D to a predetermined distance. The fiber stop330may be an extension of the front wall322(i.e., made of the same material) and formed at the same time as the rest of the fiber optic ferrule. Alternatively, the fiber stop330can be added after or machined out of the second opening314. The fiber stop330preferably starts at the bottom of the second opening314and extends upward, but could only extend along a portion of the height of the front wall322to where it would engage the bottom of the optical fiber90. The fiber stop330also has an engagement surface332that is parallel to the end92of the optical fibers90(and also the front face318). As illustrated in the figures, the fiber stop330engage the optical fibers90with the engagement surface332. Thus, the fiber stop330engages the bottom of an optical fiber (the cladding). The light would then pass over the fiber stop330on the way to the entrance surface of the fiber optic ferrule100. It would also mean that the angled entrance surface would only have to start above the fiber stops330. The fiber stop330could also be divided into a plurality of fiber stops as well.

It is also a possibility that there could also be a plurality of fiber stops330′ adjacent one another that engage two adjacent optical fibers90. SeeFIG. 10. In this configuration, each of the fiber stops330′ would engage opposing sides of adjacent optical fibers90. In this configuration, the light from the optical fibers would pass between the fiber stops330′ and the entrance surface would have to be angled between the fiber stops.

Another embodiment of a fiber optic ferrule400is illustrated inFIG. 11. The fiber optic ferrule400has the same basic parts as the prior embodiments (e.g, main body, front end, back end, middle portion, plurality of optical fiber openings412, second opening414, front portion, etc.). The fiber optic ferrule400has a plurality of front holes430. The front holes430are in line with and are a continuation of the optical fiber openings412across the second opening414. Each of the front holes430have an entrance surface434at the end of the front holes430that is also angled at the angle α. The front holes430also have a fiber stop432that is a distance D from the entrance surface434. The fiber stop430is a reduced diameter in the front hole430that will engage the cladding of the optical fiber90without blocking the light from the optical fiber core. The angle α and the distance D are again determined by the equations inFIG. 4as discussed above, depending on the ferrule material, the refractive index of the optical fiber. The front holes430can be filled with index matching material to assist in the transmission of the light in the front hole and across the boundary with the entrance surface.

Another embodiment of a fiber optic ferrule500is illustrated inFIG. 12. This fiber optic ferrule500is similar to the fourth embodiment (fiber optic ferrule400), but rather than having a flat surface that is angled in the prior embodiments, the entrance surface has a cone shaped-surface532.

Another embodiment of a fiber optic ferrule600is illustrated inFIG. 13. This fiber optic ferrule600is similar to the fourth and fifth embodiments (fiber optic ferrules400/500), but rather than having a flat or cone-shaped surface of the prior embodiments, the entrance surface has a lensed surface630in the front portion620and accessible from the second opening614. The lensed surface630will collimate the light and will eliminate the need for the irregular lens of the other embodiments.

Another embodiment of a fiber optic ferrule700is illustrated inFIG. 14. The optic ferrule700has the same basic parts as the prior embodiments (e.g, main body, front end, back end, middle portion, plurality of optical fiber openings, second opening714, front portion720, etc.). In this fiber optic ferrule, the optical fiber790is cleaved at an angle that is the same as, or as similar to as possible, the angle α. The optical fiber790is separated from the front portion720and has an index matching adhesive or gel in the second opening714. In this case, the index matching adhesive preferably matches the index of the fiber optic ferrule700rather than the fiber core. Any reflection of the light off the end face of the optical fiber790will not reflect back into the fiber core because of the angled cleave (as indicated by an arrow off the angled cleave of the optical fiber end face). However, it is preferable to have a lens at the front end of the fiber optic ferrule700to correct any aberrations in the light beam as noted above.

Another embodiment of a fiber optic ferrule800is illustrated inFIG. 15. The basics configuration of this fiber optic ferrule are disclosed in U.S. Pat. No. 8,985,865 (the '865 patent), the contents of which are incorporated herein by reference. Fiber optic ferrule800also has an angled entrance surface834relative to the longitudinal axis A of the plurality of optical fiber openings812to reduce the back reflection in this fiber optic ferrule in the same way as the other ferrules noted above. However, in the '865 patent, the entrance surface is still perpendicular to the longitudinal axis of the optical fiber. The lenses826may also be irregular to correct the wavefront aberration caused by the angled surface.