WO2019152620A1

WO2019152620A1 - Fiber optical interface with reduced reflections

Info

Publication number: WO2019152620A1
Application number: PCT/US2019/016001
Authority: WO
Inventors: Eric GRANN; Scott Fluegel
Original assignee: Inneos, Llc
Priority date: 2018-02-05
Filing date: 2019-01-31
Publication date: 2019-08-08

Abstract

Exemplary optical couplers include a molded body comprising a receptacle for an optical fiber, a collimating lens, and an air cavity disposed between the receptacle and the lens; and an optical block having an index of refraction substantially matched to the optical fiber's index of refraction. In some embodiments, the optical block is attached to a receptacle surface proximate to the cavity. In some embodiments, a receptacle wall proximate to the cavity comprises a notch formed by a first portion of the wall recessed into the cavity and one or more surfaces substantially perpendicular to the first portion, and the notch is configured for insertion and retention of the optical block. A first surface of the optical block proximate to the cavity can be coated with an anti-reflective coating or, alternately, the first surface can be uncoated and the optical block configured with a thickness to reduce reflections.

Description

FIBER OPTICAL INTERFACE WITH REDUCED REFLECTIONS

RELATED APPLICATIONS

The present application claims the benefit of priority from U.S. Provisional Patent Application No. 62/626,467 filed on February 5, 2018, the entire disclosure of which is incorporated herein by reference for all purposes.

TECHNICAL FIELD

The present application relates generally to the field of optical communications, and more specifically to devices that couple optical signals (e.g., visible light) from an optical fiber to circuitry for conversion into electric signals.

BACKGROUND

Fiber-optic communication is a method of transmitting information fro one place to another by sending pulses of light through an optical fiber. Modem fiber-optic communication systems generally include an optical transmitter to convert an electrical signal into an optical signal to send into the optical fiber, a cable containing bundles of multiple optical fibers that is routed through underground conduits and buildings, multiple kinds of amplifiers, and an optical receiver to recover the signal as an electrical signal.

Commonly used optical transmitters include semiconductor devices such as light-emitting diodes (LEDs) and laser devices, e.g. vertical cavity surface emitting laser (VCSEL) devices. Such VCSEL de ices can provide a relatively directional output, allowing high coupling efficiency (~50 %) into single-mode and multi-mode fiber. Their narrow spectral width can also facilitate high bit rates since it can reduce the effect of chromatic dispersion.

At the far end, the optical fiber carrying the laser-generated optical signal is coupled to an optical receiver by a device often referred to as an“optical coupler.” A cross-sectional view of an exemplary optical coupler is shown in Figure 1. Optical coupler 100 can be formed, for example, from a molded plastic or similar material, and can include a receptacle 110 for insertion of an optical fiber. An exemplary optical fiber can comprise a ferrule 130 surrounding fiber 120, which can be formed of a glass material. The diameter of ferrule 130 can be dimensioned such that ferrule 130 can be inserted within receptacle 110, thereby holding fiber 120 in place.

The ends of fiber 120 and surrounding ferule 130 can be polished at a 90-degree angle (e.g.,“flat polished”) relative to their centerline. As shown in Figure 1, the end can also abut one end of an air cavity 140 within optical coupler 100, with a collimating lens 150 formed at the other end of cavity 140. When light is transmitted through fiber 120, it passes through cavity 140 and lens 150, exiting as a collimated beam 160. However, not all of the optical energy transmitted through fiber 130 is passed through cavity 140 and lens 150. A typical optical-fiber-to-air interface, such as between fiber 130 and cavity 140, produces a reflection back down the fiber. The amount of reflection can be determined with the Fresnel reflection equation, a simplified version of which is given below:

n-— n₇

R =— - - ni + n₂

For example, with m = 1.0 for air (e.g., cavity 140) and n₂ = 1.4676 for a single-mode optical fiber at 1310 nm wavelength (e.g., fiber 130), the resulting reflection R would be 3.59% of the transmitted energy, which can also be expressed as a return loss of -14.5 dB.

Such reflections can return through the fiber to the optical transmitter and destabilize the laser, increasing the relative intensity noise (RIN) and reducing the optical system performance. In many cases, to avoid such degradations, the return loss must be less than -26 dB. This cannot be achieved with a flat-polished fiber used in the exemplary configuration shown in Figure 1. In some instances, return-loss performance can be improved, to some degree, by“angle polishing” the end of fiber 130 and ferrule 120 to an angle slightly less than 90 degrees (e.g., 82-86 degrees) such that a smaller portion of the light reflecting off the air-glass interface propagates back down the fiber toward the transmitter·

For various reasons, however, it is not always possible to use an angle-polished fiber. For example, angle-polished fibers can require rotational alignment that may not be possible in certain applications. When a flat-polished fiber is required, another solution employed to reduce return loss is to insert a fiber (and ferrule) stub into the coupler receptacle 110, with the stub having an angle-polished end abutting cavity 140 and a flat-polished end that mates with a flat-polished end of fiber 130 and ferrule 120 inserted into receptacle 110. The fiber-stub to fiber connection eliminates the air interface and can significantly reduce reflections. However, achieving and maintaining alignment between fiber 130 and the fiber stub can be difficult. Without proper alignment, only a portion of the light exiting fiber 130 will enter the fiber stub, thereby reducing the efficiency of the optical system.

Accordingly, it can be beneficial to address at least some of these issues and/or problems with an improved optical coupler that reduces return loss to an acceptable level without sacrificing system efficiency. SUMMARY

Accordingly, to address at least some of such issues and/or problems, certain exemplary embodiments according to the present disclosure can reduce reflective return loss back into the optical fiber to a desirable and/or acceptable level while maintaining a desirable and/or acceptable efficiency level of the optical transmission system. As such, exemplary devices according to the present disclosure can vastly out-perform conventional devices and techniques in various known applications, including exemplary applications discussed herein.

Certain exemplary embodiments of the present disclosure include an optical coupler comprising: a molded body comprising: a receptacle for an optical fiber, a collimating lens, and an air cavity disposed between the receptacle and the lens; and an optical block having an index of refraction substantially matched to an index of refraction of the optical fiber, wherein the optical block is attached to a surface of the receptacle proximate to the cavity.

In some exemplary embodiments, the width of the optical block can be less than a diameter of the cavity proximate to the receptacle. In some exemplary embodiments, the optical block can be attached to the surface such that at least a portion of the cavity remains exposed to the receptacle. In other exemplary embodiments, the optical block can be attached to the surface by an adhesive. In some exemplary embodiments, the optical block is rectangular with a thickness of approximately 300 microns.

In some exemplary embodiments, a first surface of the optical block proximate to the cavity can be uncoated, and the optical block can be configured with a thickness such that a return loss due to reflections into the optical fiber from an interface between the first surface and the cavity is less than 25dB. In some exemplary embodiments a first surface of the optical block proximate to the cavity can be coated with an anti-reflective coating.

Other exemplary embodiments of the present disclosure include an optical coupler comprising: a molded body comprising: a receptacle for an optical fiber, a collimating lens, and an air cavity disposed between the receptacle and the lens, wherein a wall of the receptacle proximate to the cavity comprises a notch formed by a first portion of the wall recessed into the cavity and one or more surfaces substantially perpendicular to the first portion; and an optical block having an index of refraction substantially matched to an index of refraction of the optical fiber, wherein the notch is configured for insertion and retention of the optical block.

In some exemplary embodiments, at least a portion of the one or more surfaces can comprise a plurality of flexible protrusions. In some exemplary embodiments, when the optical block is situated in the notch, the flexible protrusions can apply pressure on at least one surface of the optical block, the pressure being sufficient to retain the optical block within the notch. In some exemplary embodiments, the one or more surfaces can comprise two opposing surface having flexible protrusions disposed thereon. In some exemplary embodiments, the one or more surfaces can comprise four surfaces, and the flexible protrusions can be disposed on at least two opposing surfaces of the four surfaces.

In other exemplary embodiments, the optical block can be rectangular with a thickness of approximately 300 microns. In some exemplary embodiments, the width of the optical block can be less than a diameter of the cavity proximate to the receptacle. In other exemplary embodiments, when the optical block is situated within the notch, at least a portion of the cavity can remain exposed to the receptacle. In some exemplary embodiments, a first surface of the optical block proximate to the cavity can be uncoated, and the optical block can be configured with a thickness such that a return loss due to reflections into the optical fiber from an interface between the first surface and the cavity can be less than 25dB. In some exemplary embodiments, a first surface of the optical block proximate to the cavity can be coated with an anti-reflective coating. In some exemplary embodiments, the flexible protrusions can be fin-shaped and extend along at least a portion of the one or more surfaces in a direction perpendicular to the direction of insertion of the optical block into the notch.

Other exemplary embodiments of the present disclosure include an optical receiver comprising one or more embodiments of the optical couplers described hereinabove; a substrate; and a photodiode mounted to the substrate and configured such that the photodiode is optically aligned with the collimating lens.

These and other objects, features and advantages of the exemplary embodiments of the present disclosure will become apparent upon reading the following detailed description of the exemplary embodiments of the present disclosure, when taken in conjunction with the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objects, features and advantages of the present disclosure will become apparent from the following detailed description taken in conjunction with the accompanying Figures showing illustrative embodiments, in which:

Figure 1 is a cross-section side view of an exemplary optical coupler having a return loss that can be insufficient for certain applications; Figure 2 is a cross-sectional side view of an exemplar_)'· optical coupler having an improved return loss, according to one or more exemplary embodiments of the present disclosure;

Figure 3 is a front view of the interior of a fiber receptacle for the optical coupler shown in Figure 2, according to one or more exemplary embodiments of the present disclosure;

Figure 4 is a cross-sectional side view of another exemplary optical coupler having an improved return loss, according to one or more exemplary embodiments of the present disclosure;

Figure 5 is a front view of the interior of a fiber receptacle for the optical coupler shown in Figure 4, according to one or more exemplary embodiments of the present disclosure; and

Figure 6, which includes Figures 6A-B, is a cross-sectional side view of an optical coupler illustrating an exemplary technique for insertion and retention of an optical block within the optical coupler, according to one or more exemplary embodiments of the present disclosure.

While the present disclosure will now be described in detail with reference to the figures, it is done so in connection with the illustrative embodiments and is not limited by the particular embodiments illustrated in the figure(s) or in the appended claims.

DETAILED DESCRIPTION

Embodiments of the present disclosure provide an improved optical coupler comprising a piece (e.g., a block) of optically transparent material situated between the end of the optical fiber and the air cavity within the coupler. The optically transparent material can have an index of refraction that is closely matched to the index of refraction of the optical fiber. Fused silica is an exemplary material having such properties. At 1310 nm wavelength, the group index of fused silica is 1.4616, resulting in a reflectance of 0,00042% or, equivalently, a return loss of -53.7 dB.

Figure 2 is a cross-sectional side view of an exemplary optical coupler 200 having an improved return loss, according to one or more exemplary embodiments of the present disclosure. Similar to exemplary optical coupler 100 shown in Figure 1, optical coupler 200 can be formed of a molded plastic or other suitable material, and can comprise a receptacle 210 for insertion of a fiber 230 surrounded by a ferrule 220. Optical coupler 200 also includes an optical block 270 situated between the end of fiber 230 and cavity 240. In the embodiment shown in Figure 2, optical block 270 can be affixed, attached, and/or mounted to an end wall 290 of receptacle 210. For example, optical block 270 can be attached to end wall 290 by an index-matching adhesive, e.g., an adhesive having an index of refraction approximately midway between the indices of refraction of optical block 270 and fiber 230. Alternately, optical block 270 can be mounted or attached by press fitting. As discussed above, optical block 270 can be formed from a material, such as fused silica, having a group index substantially similar to the group index of fiber 230 at a wavelength (or range of wavelengths) of interest. Reflections from the interface between fiber 230 and an optical block 270 formed of such material can be reduced at least to an acceptable level. In some exemplary embodiments, the surface of optical block 270 adjacent to cavity 240 can be coated with an antireflection coating so as to significantly reduce (e.g., nearly eliminate) reflections from the interface between optical block 270 and cavity 240 back towards fiber 230. The antireflection coating can be chosen according to the wavelength, or range of wavelengths, expected to be carried by fiber 230.

In other exemplary embodiments, the surface of optical block 270 adjacent to cavity 240 can remain uncoated. In such embodiments, the thickness of block 270 can be selected so as to reduce the reflections from the interface between fiber 230 and optical block 270 to at least to an acceptable level. For example, as coherent light exits fiber 230, it disperses such that when it reaches the interface between optical block 270 and cavity 240, the pattern diameter is larger than the diameter of fiber 230. The portion of the light that is reflected by the interface between optical block 270 and cavity 240 further disperses as the reflection propagates through optical block 270, such that the diameter of the reflection pattern is even larger than the fiber diameter when it reaches fiber 230. As such, only a portion of the larger reflection will enter the smaller diameter of fiber 230. The thickness of optical block 270 can be determined based on the dispersion and the desired return loss. For example, to achieve a desired minimum return loss of -25-30 dB, an exemplary thickness for an uncoated optical block 270 is 300 microns.

Figure 3 is a front view of the interior of a fiber receptacle 310 according to one or more exemplary embodiments of the present disclosure. For example, receptacle 310 can correspond to receptacle 210 shown in Figure 2. As shown in Figure 3, an optical block 370 can be placed over a cavity 340. For example, optical block 370 can be attached, affixed, or mounted to an end wall 390 of receptacle 310 adjacent to cavity 340, similar to the manner described above for optical block 270, cavity 240, and end wall 290. The width of optical block 370 can be selected to be less than a diameter of the cavity proximate to the receptacle. In this manner, if optical block 370 is centered with respect to cavity 340, there are air gaps on either side of optical block 370 such that at least a portion of cavity 340 remains exposed to receptacle 310.

Figure 4 is a cross-sectional side view of another exemplary optical coupler 400 having an improved return loss, according to one or more exemplary embodiments of the present disclosure. Similar to exemplary optical coupler 200 shown in Figure 2, optical coupler 400 can be formed of a molded plastic or other suitable material, and can comprise a receptacle 410 for insertion of a fiber 430 surrounded by a ferrule 420. Optical coupler 400 also includes an optical block 470 situated between the end of fiber 430 and one end of cavity 440, with collimating lens 450 situated at an opposite end of cavity 440. In the embodiment shown in Figure 4, the wall of receptacle 410 proximate to cavity 440 can include a first portion 490 recessed (e.g. , as a notch) within cavity 440 and a second portion 480 external to cavity 440. Portions 480 and 490 can be substantially parallel and connected by a surface perpendicular to both.

As shown in Figure 4, portions 480 and 490 can be configured as a“notch” that optical block 470 fits within. This is further illustrated in Figure 5, which is a front view of the interior of a fiber receptacle 510 according to one or more exemplary embodiments of the present disclosure. For example, receptacle 510 can correspond to receptacle 410 shown in Figure 4. As shown in Figure 5, optical block 570 can be situated within a notch in the end wall of receptacle 510 that comprises first portion 590 within cavity 540 (e.g., behind optical block 570 in this view), second portion 580 external to cavity 540, and a side surface 585 connecting the first and second portions of the end wall. In the embodiment shown in Figure 5, side surface 585 is continuous and surrounds optical block 570 completely on all four sides. In other embodiments, such as when the width of optical block 570 is less than the diameter of cavity 540 proximate to receptacle 510, side surface 585 may be discontinuous and/or not surround optical block 570.

In some exemplary embodiments, optical block 570 can be attached, affixed, or mounted within the notch formed by 580, 585, and 590 using an adhesive. In other exemplary embodiments, various pressure-fitting mechanisms can be used to retain optical block 570 after it is inserted within the notch formed by 580, 585, and 590. Figures 6A-6B show cross-sectional side views of the relevant portion of an optical coupler that exemplifies such embodiments. As shown in Figure 6A, collimating lens 650 is situated at one end of cavity 640, while the receptacle end wall at the other end of cavity 640 has a notch similar to the one described above with reference to Figures 4 and 5. The notch comprises a surface 685, which is illustrated here as two surfaces 685a and 685b, but can be continuous or discontinuous in the same manner as discussed above.

Each of surfaces 685a, b can have disposed thereon a plurality of flexible protrusions 695. These flexible protrusions can be formed of the same material as the rest of the optical coupler using, e.g. , the same injection molding process. Flexible protrusions 695 can be a variety of shapes depending on the particular requirements of the application. For example, flexible protrusions 695 can be shaped as fins that extend length-wise along surfaces 685a, b. In the exemplary cross- sectional view of Figure 6, such protrusions can extend length-wise into the drawing page. The length of each fin-shaped protrusion can be less than or equal to the width of the notch in the corresponding dimension. Furthermore, each fin-shaped protrusion can be subdivided length-wise into a plurality of segments. Even so, such arrangements or configurations are merely exemplary.

Figure 6B illustrates the insertion and retention of optical block 670 within the notch comprising surface 685a, b having flexible protrusions 695. When optical block 670 is inserted within the notch in this manner, the insertion force of the block causes flexible protrusions 695 to bend, flex, and/or deform in the direction of the insertion. After the insertion force is removed, the tension in the protrusions caused by the bending, flexion, and/or deformation exerts a force on the sides of optical block 670, causing optical block 670 to be retained within the notch. The amount of retention force provided by flexible protrusions 695 depends on their number (e.g. , how many on each surface 685), constituent material, and amount of bending, flexion, and/or deformation. The latter factor is further related to the lengths of the flexible protrusions versus the difference between the width of optical block 670 and the width of the notch in the receptacle end wall. These parameters can be selected and/or determined based on the size of the optical block 670 and the amount of retention force required. Although Figure 6 shows protrusions on two surfaces 685a, b, other embodiments can have protrusions 695 on only one of 685a, b, such that the retention force of the protrusions 695 can hold optical block 670 against the opposite surface 685a, b.

Although embodiments were described above in the context of an optical coupler for a single optical fiber, the same principles and techniques can be applied to optical couplers for a plurality of optical fibers, e.g., two, eight, or twelve fibers. For example, such embodiments can include a receptacle for each coupled optical fiber, with each receptacle having therein an optical block attached, affixed, and/or mounted according to one of the single-fiber embodiments described above. Such embodiments of multi-fiber optical couplers can be used to improve reflection return loss in devices such as wavelength-division multiplexers, wavelength-division demultiplexers, and optical transceivers. Exemplary devices comprising multi-channel optical couplers that would benefit from reduced return loss are described in application PCT/US20l9/xxxxxx (docket no. 1072-0005) entitled“Multichannel Optical Coupler,” filed concurrently herewith and incorporated herein by reference in its entirety.

The foregoing merely illustrates the principles of the disclosure. Various modifications and alterations to the described embodiments will be apparent to those skilled in the art in view of the teachings herein. It will thus be appreciated that those skilled in the art will be able to devise numerous systems, arrangements, and procedures which, although not explicitly shown or described herein, embody the principles of the disclosure and can be thus within the spirit and scope of the disclosure. Various different exemplary embodiments can be used together with one another, as well as interchangeably therewith, as should be understood by those having ordinary skill in the art. In addition, certain terms used in the present disclosure, including the specification, drawings and claims thereof, can be used synonymously in certain instances, including, but not limited to, e.g. , data and information. It should be understood that, while these words, and/or other words that can be synonymous to one another, can be used synonymously herein, that there can be instances when such words can be intended to not be used synonymously.

Claims

WHAT IS CLAIMED IS:

1. An optical coupler (200) comprising:

a molded body comprising:

a receptacle (210, 310) for an optical fiber (230, 430);

a collimating lens (250); and

an air cavity (240, 340) disposed between the receptacle (210) and the lens (250); and

an optical block (270, 370) having an index of refraction substantially matched to an index of refraction of the optical fiber (230), wherein the optical block is attached to a surface (290, 390) of the receptacle (210) proximate to the air cavity (240, 340).

2. The optical coupler of claim 1, wherein the width of the optical block (270, 370) is less than a diameter of the cavity (240, 340) proximate to the receptacle (210, 310).

3. The optical coupler of claim 2, wherein the optical block (270, 370) is attached to the surface (290, 390) such that at least a portion of the air cavity (240, 340) remains exposed to the receptacle (210, 310).

4. The optical coupler of any of claims 1-3, wherein the optical block (270, 370) is attached to the surface (290, 390) by an adhesive.

5. The optical coupler of any of claims 1-4, wherein the optical block is rectangular with a thickness of approximately 300 microns.

6. The optical coupler of any of claims 1-5, wherein:

a first surface of the optical block (270, 370) proximate to the air cavity (240, 340) is uncoated; and

the optical block is configured with a thickness such that a return loss due to reflections into the optical fiber (230) from an interface between the first surface and the cavity is less than 25dB.

7. The optical coupler of any of claims 1-5, wherein a first surface of the optical block (270, 370) proximate to the air cavity (240, 340) is coated with an anti-reflective coating.

8. An optical coupler (400) comprising:

a molded body comprising:

a receptacle (410, 510) for an optical fiber (430);

a collimating lens (450, 650); and

an air cavity (440, 540, 640) disposed between the receptacle (410, 510) and the lens (450, 650); and

wherein a wall of the receptacle (410, 510) proximate to the air cavity (440, 540, 640) comprises a notch formed by a first portion (490, 590) of the wall recessed into the cavity and one or more surfaces (585, 685) substantially perpendicular to the first portion; and

an optical block (470, 570, 670) having an index of refraction substantially matched to an index of refraction of the optical fiber (430), wherein the notch is configured for insertion and retention of the optical block.

9. The optical coupler of claim 8, wherein at least a portion of the one or more surfaces (585, 685) comprise a plurality of flexible protrusions (695).

10. The optical coupler of claim 9, wherein when the optical block (470, 570, 670) is situated in the notch, the flexible protrusions (695) apply pressure on at least one surface of the optical block, the pressure being sufficient to retain the optical block within the notch.

11. The optical coupler of any of claims 9-10, wherein the one or more surfaces (585, 685) comprise two opposing surfaces (685a, 685b) having flexible protrusions disposed thereon.

12. The optical coupler of any of claims 9-10, wherein:

the one or more surfaces (585, 685) comprise four surfaces; and

the flexible protrusions (695) are disposed on at least two opposing surfaces (685a, 685b) of the four surfaces.

13. The optical coupler of any of claims 9-12, wherein the flexible protrusions (695) are fin- shaped and extend along at least a portion of the one or more surfaces (685) in a direction perpendicular to the direction of insertion of the optical block (470, 570, 670) into the notch.

14. The optical coupler of any of claims 8-13, wherein the optical block is rectangular with a thickness of approximately 300 microns.

15. The optical coupler of claim 14, wherein the width of the optical block is less than a diameter of the air cavity (440, 540, 640) proximate to the receptacle (410, 510).

16. The optical coupler of any of claims 8-15, wherein when the optical block is situated within the notch, at least a portion of the air cavity (440, 640) remains exposed to the receptacle.

17. The optical coupler of any of claims 8-16, wherein:

a first surface of the optical block (470, 570, 670) proximate to the air cavity (440, 540, 640) is uncoated; and

the optical block is configured with a thickness such that a return loss due to reflections into the optical fiber (230) from an interface between the first surface and the air cavity is less than 25dB.

18. The optical coupler of any of claims 8-16, wherein a first surface of the optical block (470, 570, 670) proximate to the air cavity (440, 540, 640) is coated with an anti-reflective coating.

19. An optical receiver comprising:

a substrate;

the optical coupler of any of claims 1-7 mounted to the substrate; and

a photodiode mounted to the substrate and configured such that the photodiode is optically aligned with the collimating lens.

20. An optical receiver comprising:

a substrate;

the optical coupler of any of claims 8-18 mounted to the substrate; and