CN117413211A

CN117413211A - Optical fiber system with multimode fiber optic cable and fiber optic connection to mode-matched single mode fiber optic device

Info

Publication number: CN117413211A
Application number: CN202280039456.1A
Authority: CN
Inventors: 陈皓; 陈欣; 黎康梅; 李明军; 戴维·韦恩·米克; 邬起; 夏晨; 安迪·凤雷·周
Original assignee: Corning Research and Development Corp
Current assignee: Corning Research and Development Corp
Priority date: 2021-04-30
Filing date: 2022-04-20
Publication date: 2024-01-16

Abstract

Disclosed herein is an optical fiber system, comprising: at least one fiber optic cable assembly having a multimode optical fiber for communication of optical data signals at an operating wavelength; and means having a single mode fiber stub having a mode field diameter within 20% of the mode field diameter of the fundamental mode of the multimode fiber at the operating wavelength. The single mode fiber stub is fixed to the ferrule, and a distance of a center of a fiber core of the single mode fiber stub from a center of the ferrule is within 0.5 μm. One end of the single mode fiber stub extends to or beyond the rear end of the ferrule and forms an optical connection with the multimode fiber, wherein the center of the single mode fiber stub is within 2 μm of the center of the multimode fiber.

Description

Optical fiber system with multimode fiber optic cable and fiber optic connection to mode-matched single mode fiber optic device

Priority application

The present application claims priority from U.S. provisional application Ser. No. 63/181,998, U.S. provisional application Ser. No. 63/216,624, and U.S. provisional application Ser. No. 63/305,860, U.S. Ser. No. 2, 2022, filed on 30, 4, and 30, 2021, the contents of which are incorporated herein by reference in their entirety.

Technical Field

The present disclosure relates to fiber optic systems, and more particularly to fiber optic systems having multimode fiber optic cables and fiber optic connections to mode-matched single mode fiber optic devices.

Background

Multimode optical fibers have been widely used for short-range communications, such as data centers, campus networks, and the like. Such optical fibers are used because the light source in the transceiver in the optical device is a multimode light source. In addition, historically, it has been easier to use multimode fibers than single mode fibers. Unfortunately, multimode fibers have a low bandwidth-distance product due to modal dispersion, which makes it difficult and expensive to expand the reach of or increase the data rate of the fiber transmission system. Replacing deployed multimode fiber optic cables is also difficult and expensive in some cases because the cables are buried deep into the building infrastructure.

Furthermore, existing multimode fibers are optimized for operation at a nominal wavelength of 850nm, at which the multimode fibers have high dispersion. For longer reach or higher data rate transmissions, the transmission is typically accomplished through a single mode fiber using a single mode transceiver at about 1310nm or about 1550 nm. In some cases, it is desirable to have an operating wavelength of nominally about 1300nm at which dispersion is lowest.

There is increasing interest in using single-mode transceivers with multimode optical fibers in data centers to improve interoperability, provide smooth upgrade paths, and easier logistics, all of which can bring economic and financial benefits. It would therefore be advantageous to have a way to improve the performance of a multimode optical fiber transmission system without incurring the time, labor and expense of having to replace multimode optical fibers.

Disclosure of Invention

Various devices are disclosed for an optical fiber system, the optical fiber system comprising: at least one fiber optic cable assembly having multimode optical fibers, wherein the system is configured for transmitting optical data signals at an operating wavelength in the range of 1260nm-1360nm or in the range of 1530nm-1565 nm. The device allows single mode transmission over existing multimode optical fibers. The device may be designed with specific single mode fiber stubs that enable the device to perform its desired function with each of the following multimode fiber types: OM1 multimode optical fiber, OM2 multimode optical fiber, OM3 multimode optical fiber, OM4 multimode optical fiber, and OM5 multimode optical fiber.

According to one embodiment, the means for the system mentioned in the preceding paragraph comprises a single mode fiber stub having a mode field diameter at the operating wavelength that is within 20% of the mode field diameter of the fundamental modes of OM1 multimode optical fiber, OM2 multimode optical fiber, OM3 multimode optical fiber, OM4 multimode optical fiber and OM5 multimode optical fiber. The single mode fiber stub also has a fiber cut-off wavelength that is lower than the operating wavelength. The device also includes a ferrule to which the single mode fiber stub is secured. The ferrule includes a front end portion of the single-mode fiber stub providing for optical coupling and a rear end portion opposite the front end portion, a center of the fiber of the single-mode fiber stub being within 0.5 μm from a center of the ferrule at the front end portion of the ferrule.

The device of the preceding paragraph may be a fiber optic connector further comprising a connector body including the ferrule. In such an embodiment, the single mode fiber stub may extend beyond the rear end of the ferrule and have a splice end within the connector body enabling a splice joint to be formed with the multimode fiber within the connector body. In some embodiments wherein the device is a fiber optic connector, the device may further comprise a strain relief positioned over a portion of the connector body, wherein the strain relief has a color for OM1 multimode fiber optic cabling, OM2 multimode fiber optic cabling, OM3 multimode fiber optic cabling, or OM4 multimode fiber optic cabling according to ANSI/TIA-568.3-D-5.2.3 (2016), and wherein the connector body has a color for single mode fiber optic cabling according to ANSI/TIA-568.3-D-5.2.3 (2016). In addition, in some embodiments wherein the device is a fiber optic connector, the mode field diameter of the single mode fiber stub at the spliced end is between 12 μm and 16 μm at an operating wavelength of 1310 nm.

The device of the second paragraph in this summary is alternatively a fiber optic adapter, the fiber optic adapter further comprising an adapter body, the adapter body comprising the ferrule. In such an embodiment, the single mode fiber stub extends to the rear end of the ferrule and the fiber optic adapter defines a male connection interface including the front end of the ferrule and a female connection interface including the rear end of the ferrule.

Also disclosed are embodiments wherein the apparatus of the second paragraph in this summary is an optical transceiver further comprising: a housing; an optoelectronic assembly within the housing configured for single mode transmission at the operating wavelength; and a female connection interface including the front end of the ferrule. In such embodiments, the single mode fiber stub extends from the rear end of the ferrule and is optically coupled to the optoelectronic element within the housing. The female connection interface may be configured to accept duplex connectors (such as LC duplex connectors according to IEC 61754-20:2012) or multi-fiber connectors (e.g., MPO connectors according to IEC 61754-7-3:2019).

The device of any of the preceding paragraphs may include a single mode fiber stub having a length in some embodiments between 0.5cm and 2.0cm or in some embodiments between 0.5cm and 1.5 cm. Additionally, in some embodiments, an apparatus according to any of the preceding paragraphs may include a single mode fiber stub having a fiber cut-off wavelength below 1100 nm. Additionally, in some embodiments, the apparatus of any of the preceding paragraphs may include a single-mode fiber stub having a first end at the front end of the stub and a second end opposite the first end, wherein the mode field diameter of the single-mode fiber stub is greater at the first end than at the second end such that the single-mode fiber stub includes a mode field transition.

Also disclosed is an optical fiber system comprising one or more devices according to the preceding paragraph. According to one embodiment, such an optical fiber system is configured for transmitting optical data signals at an operating wavelength. The fiber optic system includes at least one fiber optic cable assembly including a multimode optical fiber having a mode field diameter of a fundamental mode at the operating wavelength. The fiber optic system further comprises the device of any of the preceding paragraphs, wherein the single mode fiber stub of the device forms an optical connection with the multimode fiber of the at least one fiber optic cable assembly, and wherein a center of the core of the single mode fiber stub is within 2 μm of a center of a core of the multimode fiber at the optical connection.

Associated methods are also disclosed. According to one embodiment, the present disclosure provides a method for upgrading a system comprising at least one link of multimode optical fibers, wherein each of the at least one link comprises an opposite end terminated with a respective multimode optical fiber connector providing the multimode optical fibers of the link for optical coupling, and wherein the multimode optical fibers have a mode field diameter of a fundamental mode at an operating wavelength. The method comprises the following steps: selecting a single mode fiber stub that satisfies the following conditions: having a mode field diameter at the operating wavelength that is within 20% of the mode field diameter of the fundamental mode of the multimode optical fiber, wherein the single mode fiber stub is selected such that conditions can be met when the multimode optical fiber is an OM1 multimode optical fiber, an OM2 multimode optical fiber, an OM3 multimode optical fiber, an OM4 multimode optical fiber, an OM5 multimode optical fiber. The method further comprises the steps of: selecting a ferrule to support the single-mode fiber stub, wherein the ferrule comprises a ferrule bore configured to receive the single-mode fiber stub, and wherein the ferrule is selected such that a ferrule bore eccentricity <0.5 μm, and a diameter of the ferrule bore is within a range of 126±0.5 μm; and setting the ferrule to: replacing a portion of a fiber optic connector intended to replace one of the multimode fiber optic connectors of the link; a portion of an adapter configured to receive one of the multimode fiber connectors of the link; or a portion of an optical transceiver configured to receive one of the multimode fiber optic connectors of the link, wherein the single mode fiber stub is secured to the ferrule.

In an embodiment of the method of using a replacement fiber optic connector, the replacement fiber optic connector further comprises a connector body from which the ferrule extends, the ferrule having a front end portion that provides the single mode fiber stub for optical coupling and a rear end portion opposite the front end portion. The single mode fiber stub extends beyond the rear end of the ferrule. The body of the replacement fiber connector is configured to receive a splice joint between the single mode fiber stub and the multimode fiber.

In an embodiment of the method using an adapter, the ferrule has a front end and a rear end opposite the front end. The single mode fiber stub has a length approximately equal to a distance between the front end and the rear end of the ferrule such that the ferrule provides an opposite end of the single mode fiber stub for optical coupling. The adapter defines a male connection interface including the front end of the ferrule and a female connection interface including the rear end of the ferrule.

In an embodiment of the method of using an optical transceiver, the optical transceiver further comprises a housing and an optoelectronic assembly within the housing, the optoelectronic assembly configured for single mode transmission at the operating wavelength. The ferrule has a front end portion providing the single-mode fiber stub for optical coupling and a rear end portion opposite the front end portion, the single-mode fiber stub extending beyond the rear end portion of the ferrule and being optically coupled to the optoelectronic assembly within the housing, and the housing and the front end portion of the ferrule defining a female connection interface.

Further embodiments of the present disclosure relate to a method comprising: the first multimode fiber optic connector is removed at a first end of the fiber optic cable of the first fiber optic cable assembly. The fiber optic cable of the first fiber optic cable assembly includes a multimode optical fiber. The multimode optical fiber has a mode field diameter of a fundamental mode at an operating wavelength. The method further comprises the steps of: replacing the first multimode fiber optic connector of the first fiber optic cable assembly with a replacement first fiber optic connector at a first end of the fiber optic cable of the first fiber optic cable assembly. The replacement first fiber optic connection includes a first single mode fiber stub that forms a first splice joint with the multimode optical fiber of the first fiber optic cable assembly. The first single mode fiber stub of the first fiber optic cable assembly comprises a mode field diameter within 20% of a mode field diameter of a fundamental mode of the multimode optical fiber of the first fiber optic cable assembly at the operating wavelength. The first single mode fiber stub of the first fiber optic cable assembly includes a fiber cut-off wavelength that is lower than the operating wavelength. The method further comprises the steps of: the second multimode fiber optic connector is removed at a second end of the fiber optic cable of the second fiber optic cable assembly. The fiber optic cable of the second fiber optic cable assembly includes a multimode optical fiber. The multimode optical fiber has a mode field diameter of a fundamental mode at the operating wavelength. The method further comprises the steps of: replacing the second multimode fiber optic connector of the second fiber optic cable assembly with a replacement second fiber optic connector at a second end of the fiber optic cable of the second fiber optic cable assembly. The replacement second fiber optic connector includes a second single mode fiber stub that forms a second splice joint with the multimode optical fibers of the second fiber optic cable assembly. The second single mode fiber stub of the second fiber optic cable assembly comprises a mode field diameter within 20% of a mode field diameter of a fundamental mode of the multimode optical fiber of the second fiber optic cable assembly at the operating wavelength. The second single mode fiber stub of the second fiber optic cable assembly comprises a fiber cut-off wavelength lower than the operating wavelength. The method further comprises the steps of: the replacement first fiber optic connector of the first fiber optic cable assembly is connected with the second fiber optic connector of the second fiber optic cable assembly such that a first single mode fiber stub of the first fiber optic connector of the first fiber optic cable assembly is aligned with and in optical communication with a second single mode fiber stub of the second fiber optic connector of the second fiber optic cable assembly at a fiber optic connection.

Additional embodiments of the present disclosure relate to a method of upgrading a system having links of multimode optical fibers connected in series. Each of the links includes an opposite end terminated with a respective multimode fiber optic connector that provides the multimode optical fibers of the link for optical coupling. The method comprises the following steps: replacing all of the multimode fiber connectors with respective replacement fiber connectors, wherein the replacing does not involve removing all of the multimode fibers such that after the replacing, at least some of the links include the multimode fibers of the respective links that terminate with the respective replacement connectors. The method further comprises the steps of: each of the replacement fiber optic connectors is connected to another one of the replacement fiber optic connectors by a physical contact connection. The connection results in no multimode optical fiber in the system being directly coupled to another multimode optical fiber in the system by a physical contact connection.

Additional features and advantages will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the embodiments as described herein, including the detailed description which follows, the claims, as well as the appended drawings.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and are intended to provide an overview or framework for understanding the nature and character of the claims. The accompanying drawings are included to provide a further understanding, and are incorporated in and constitute a part of this specification. The drawings illustrate one or more embodiments and, together with the description, serve to explain the principles and operations of various embodiments.

Drawings

FIG. 1 is a diagram of a fiber optic system including a multimode fiber optic cable and an optical fiber connection to a multimode fiber optic connector;

FIG. 2 is a diagram of a fiber optic system including a multimode fiber optic cable and a fiber optic connection to a mode-matched single mode fiber optic connector;

FIG. 3 is a side view of a fiber optic cable assembly having multimode optical fibers and fiber optic connectors each including a mode-matching single mode fiber stub;

FIG. 4A is a cross-sectional side view of one embodiment of a portion of the fiber optic connector of FIG. 3;

FIG. 4B is a cross-sectional side view of another embodiment of a portion of the fiber optic connector of FIG. 3;

FIG. 4C is a cross-sectional side view of another embodiment of a portion of the fiber optic connector of FIG. 3;

FIG. 5 is a diagram of one embodiment of the fiber optic system of FIG. 2 showing fiber optic connections to a mode-matched single-mode fiber optic connector;

FIG. 6A is a graph of insertion loss distribution between a multimode fiber connector and a single mode fiber connector;

FIG. 6B is a graph of insertion loss distribution between a multimode fiber and a single mode fiber stub;

FIG. 7A is a graph of the link loss profile of a single multimode fiber having two multimode fibers to a single mode fiber connector;

FIG. 7B is a graph of the link loss profile of a multimode fiber with ends mechanically spliced to a single mode fiber stub;

FIG. 8A is a graph of modeled excitation power using a 14 μm diameter single mode Gaussian emission spot with a different offset from the center of a multimode fiber in each mode group of the multimode fiber, where the multimode fiber comprises an OM1 multimode fiber with a mode field diameter of 13.72 μm at 1310 nm;

FIG. 8B is a graph similar to FIG. 8A, except that the multimode fiber includes an OM2 multimode fiber having a mode field diameter of 14.86 μm at 1310 nm;

FIG. 9A is a graph of a modeled transfer function (frequency response) of a multimode optical fiber using a 14 μm diameter single mode Gaussian emission spot with a different offset from the center of the multimode optical fiber, wherein the multimode optical fiber comprises an OM1 multimode optical fiber having a mode field diameter of 13.72 μm at 1310 nm;

FIG. 9B is a graph similar to FIG. 9A, except that the multimode fiber includes an OM2 multimode fiber having a mode field diameter of 14.86 μm at 1310 nm;

FIG. 10A is a graph of simulated excitation power using a single-mode Gaussian emission spot with different spot sizes and no offset from the center of the multimode fiber in each mode group of the multimode fiber, where the multimode fiber comprises an OM1 multimode fiber with a mode field diameter of 13.72 μm at 1310 nm;

FIG. 10B is a graph similar to FIG. 10A, except that the multimode fiber includes an OM2 multimode fiber having a mode field diameter of 14.86 μm at 1310 nm;

FIG. 11A is a graph of a modeled transfer function (frequency response) of a multimode optical fiber using a single-mode Gaussian emission spot with different spot sizes and no offset from the center of the multimode optical fiber, wherein the multimode optical fiber comprises an OM1 multimode optical fiber having a mode field diameter of 13.72 μm at 1310 nm;

FIG. 11B is a graph similar to FIG. 11A, except that the multimode fiber includes an OM2 multimode fiber having a mode field diameter of 14.86 μm at 1310 nm;

FIG. 12A is a graph of the transfer function (frequency response) measured for a 200m length OM1 multimode under three different emission conditions;

FIG. 12B is a graph of the transfer function (frequency response) measured for a 200m length OM2 multimode at three different emission conditions;

fig. 13A, 13B and 13C are measured optical eye diagrams of a multimode link between 25G LR transceivers/receivers, wherein the diagrams correspond to fiber optic systems comprising: a 1m length of OM1 multimode optical fiber between the transmitter port of the transceiver and the receiver port of the receiver (fig. 13A); a 200m length of OM1 multimode fiber with spliced ends to respective single mode fiber stubs according to the present disclosure (fig. 13B); and a 200m length of OM2 multimode fiber with spliced to both ends of a corresponding single mode fiber stub according to the present disclosure (fig. 13C);

FIG. 14 is a flowchart of steps for using the fiber optic system of FIGS. 2-7B;

FIG. 15 is a flowchart of steps for using the fiber optic system of FIGS. 2-7B;

FIG. 16 is a perspective view of an example of an adapter that may be used in a fiber optic system according to the present disclosure; and is also provided with

Fig. 17 is a cross-sectional view of the adapter of fig. 16.

Fig. 18A is a diagram of an optical fiber system including single-mode transceivers and representative multimode optical fibers, wherein the single-mode transceivers each include a ferrule and a terminating mode-matching single-mode optical fiber stub.

Fig. 18B is a close-up view of one of the single-mode transceivers of fig. 18A, illustrating the manner in which the mode-matching single-mode fiber stub is optically coupled to an optoelectronic element (e.g., a transceiver optical subassembly or a receiver optical subassembly).

Detailed Description

Reference will now be made in detail to the presently preferred embodiments, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.

The embodiments set forth below represent information that enables those skilled in the art to practice the embodiments. Upon reading the following description in light of the accompanying drawing figures, those skilled in the art will understand the concepts of the disclosure and will recognize applications of these concepts not particularly addressed herein. It should be understood that these concepts and applications fall within the scope of the disclosure and the accompanying claims.

Reference numerals and terms

The use of ordinal numbers in combination with elements herein is used only to distinguish between labels (such as "first fiber optic cable" and "second fiber optic cable") that may otherwise be similar or identical, and does not imply a priority, type, importance, or other attribute unless otherwise stated herein.

The term "about" as used herein in connection with a numerical value means any value within a range of ten percent greater or ten percent less than the numerical value.

As used herein, the articles "a" and "an" with respect to an element refer to "one or more" of the element unless otherwise specifically specified. The word "or" as used herein is inclusive unless it is possible depending on the context. For example, recitation of a or B means a, or B, or both a and B.

It will be understood that when an element is referred to as being "connected" or "coupled" to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being "directly connected" or "directly coupled" to another element, there are no intervening elements present.

As used herein, "adjacent" means at … …, near or near … ….

The terms "left", "right", "top", "bottom", "front", "rear", "horizontal", "parallel", "vertical", "lateral", "coplanar" and similar terms are used for convenience in describing the drawings and are not intended to limit the present disclosure. For example, the terms "left" and "right" are used with particular reference to the figures as shown, and embodiments may be in other orientations in use. In addition, as used herein, the terms "horizontal," "parallel," "vertical," "transverse," and the like, include slight variations that may exist in a working example.

As used herein, the terms "optical communication," "in optical communication," and the like mean that two elements are arranged such that an optical signal can be passively or actively emitted therebetween through a medium such as, but not limited to, an optical fiber, connector, free space, index matching structure or gel, reflective surface, or other light directing or emitting means.

In the present disclosure, the abbreviations "SMF" and "MMF" are sometimes used as shorthand for single mode and multimode fibers, respectively. These abbreviations are sometimes used as part of the terms/phrases of the elements associated with such optical fibers. For example, the term "fiber optic connector" is sometimes referred to in this disclosure as "SMF connector" or "MMF connector" depending on the type of optical fiber that the fiber optic connector provides for optical coupling at the end face of the fiber optic connector.

As used herein, fiber core-to-ferrule eccentricity (or simply "core-to-ferrule" eccentricity) refers to the positional relationship between the center of the core of an optical fiber relative to the geometric center of the ferrule to which the optical fiber is secured. Similarly, "ferrule bore eccentricity" or "ferrule bore eccentricity" refers to the positional relationship between the center of the ferrule bore/bore relative to the geometric center of the ferrule.

In this disclosure, for convenience, the discussion regarding the center of an optical fiber assumes that the center of the core of the optical fiber is the same as the center of the optical fiber defined by the cladding (i.e., no core-to-cladding eccentricity). The skilled artisan will appreciate that embodiments are possible in which there is core-to-cladding eccentricity, and that discussion in this disclosure regarding the "center" of an optical fiber actually refers to the center of the core of the optical fiber.

Optical fiber system with multimode connection

Fig. 1 is a diagram of a fiber optic system 100 including a multimode fiber optic cable and an optical fiber connection to a multimode fiber optic connector. Specifically, the fiber optic system 100 includes a single-mode transmitter (Tx) 102A that emits modulated light and a single-mode receiver (Rx) 102B that receives the modulated light. In certain embodiments, the fiber optic system 100 includes single mode fiber optic cable assemblies 104 (1), 104 (2) (generally referred to as SMF cable assemblies 104) at the transmitter and receiver and multimode fiber optic cable assemblies 106 (1) -106 (3) (generally referred to as MMF cable assemblies 106) between the single mode fiber optic cable assemblies. In certain embodiments, the transmitter 102A includes an LR transceiver or an LR4 transceiver. For an LR4 transceiver, the four wavelengths co-propagate within the same fiber to achieve an aggregate data rate of 40 Gb/s. In certain embodiments, the emitter 102A (e.g., a silicon photon-based emitter) emits modulated light having a wavelength in the range of 800nm to 1650nm (e.g., 1300 nm).

The SMF cable assembly 104 (1) includes a single mode fiber 108 and at least one fiber connector 110 (also referred to as "SMF connector 110") that at least partially forms fiber connections 112 (1) -112 (4) (generally referred to as fiber connections 112). The MMF cable assembly 106 includes a multimode optical fiber 114 and a plurality of fiber optic connectors 116 (also referred to as "MMF connectors 116") that at least partially form the optical fiber connections 112. Specifically, the MMF connector 116 fully forms the fiber optic connections 112 (2), 112 (3). In certain embodiments, the multimode optical fiber 114 has a refractive index profile designed to operate optimally around a nominal wavelength of 850nm (e.g., 840nm to 860 nm) (i.e., having an operating wavelength of 850nm where modal dispersion is minimal) or at wavelengths in the range of 800nm to 1600 nm.

Multimode fiber 114 may be widely deployed in local area networks (e.g., campuses, hotels, office buildings, data centers, etc.). Many conventional mounting devices only require the use of OM1 multimode fibers with 2% core delta and/or 62.5 μm core diameter to support a transceiver speed of 1 Gbps. More recent mounting devices may use OM2 multimode, OM3 multimode or OM4 multimode optical fibers with a core increment of 1% and/or a core diameter of 50 μm. The modal bandwidth of the installed fiber presents a difficult obstacle to upgrading the network to higher speeds. In addition, existing multimode fibers 114 in conventional installation equipment are difficult to replace due to cost and interruption. Instead of replacing the multimode optical fiber 114, the signal may instead be transmitted through the fundamental mode of the multimode optical fiber 114. In the event that only one mode propagates in the multimode optical fiber 114, the mode bandwidth problem is reduced (or eliminated).

At 1310nm, the Mode Field Diameter (MFD) of the fundamental mode of the multimode fiber is 13.6 μm for the OM1 multimode fiber and 14.5 μm for the OM2 multimode fiber, the OM3 multimode fiber and the OM4 multimode fiber. In contrast, at 1310nm, the mode field diameter of a standard single mode fiber is about 9.2 μm. In contrast, at 1550nm, the mode field diameter of the fundamental mode of the OM1 multimode optical fiber is 14.85 μm, and the mode field diameters of the fundamental modes of the OM2 multimode optical fiber, OM3 multimode optical fiber and OM4 multimode optical fiber are 15.8 μm. In contrast, at 1550nm, standard single mode fiber has a mode field diameter of about 10.3 μm. In order to efficiently launch light from the single-mode fiber 108 into the fundamental mode of the multimode fiber 114, the mode field diameter of the single-mode fiber 108 needs to be increased to substantially match the fundamental mode of the multimode fiber 114.

Typically, the current solution is directly connected to the existing MMF connector 116 in the field. However, the ferrules used in MMF connectors 116 in the field (i.e., in the installed system) vary greatly in quality (such as materials (stainless steel, composite polymer, zirconia ceramic), geometric tolerances (the geometric tolerances of MMF ferrules are more relaxed than SMF ferrules), etc.). In addition, use of the existing MMF connector 116 can result in significant insertion loss because the MMF connector 116 has a widely distributed fiber core-to-core eccentricity and thus results in dispersion of insertion loss, even when mated with an SMF connector 110 terminating a single mode fiber having an increased mode field diameter. The loss of single-mode optical power will excite higher order modes in the multimode fiber 114, causing further degradation in transmission performance.

Optical fiber system with single mode connection

Fig. 2 is an illustration of an optical fiber system 100' including multimode optical fibers 114 and optical fiber connections 112' of optical fiber connectors 110, 110' associated with single mode optical fibers. Specifically, as noted above with respect to fig. 1, the fiber optic connectors 110 are part of the corresponding SMF cable assembly 104. Fig. 2 presents a fiber optic connector 110' that includes a corresponding single mode fiber stub 108' (referred to herein as an SMF stub 108 '), as will be described in more detail below. Similar to the fiber optic connector 110, the fiber optic connector 110 'is also referred to as an SMF connector 110' in this disclosure.

In certain embodiments, the fiber optic system includes an MMF connector 116 (see fig. 1) that is replaced with an SMF connector 110 'at each fiber optic connection 112'. In other words, all MMF connectors 116 are replaced with SMF connectors 110'. In particular, in certain embodiments, the MMF connector 116 is removed from a receptacle panel, patch panel, or the like, and/or is severed from a cable carrying the multimode optical fiber 114. The multimode optical fiber 114 is prepared by removing a predetermined length of jacket from the associated cable and then removing any coating from the multimode optical fiber 114. The resulting exposed bare multimode optical fiber 114 is cleaned and Separated into predetermined lengths. The SMF connector 110 'is then installed by mechanically splicing or fusion splicing the SMF stub 108' to the bare multimode optical fiber 114. Thus, the cable uses a field-installable SMF connector 110' (e.g., a mechanical splice connector, such as from corning optical communications company (Corning Optical Communications LLC))A connector; fusion splice connector, such as +.>Connector, etc.), wherein the SMF stub 108' functions to provide mode switching, mode conditioning, and/or mode filtering.

Because of the smaller fiber-to-ferrule offset in the SMF connector 110', coupling loss and multipath interference (MPI) are reduced compared to the conventional MMF connector 116. Specifically, the connector center-to-center offset between a pair of SMF connectors 110' is less than or equal to 2.5 microns. As noted above, the loose geometric tolerances of the MMF connector 116 provide a high probability of insertion loss and multipath interference. Additionally, in certain embodiments, the accessible multimode fiber optic cable assembly is replaced with a single mode fiber optic cable assembly. For example, replacing pre-installed MMF connector 116 with SMF connector 110' reduces the loss budget of a single multimode fiber span by 3.7dB. The improvement is even greater for structured cable systems having multi-span multimode optical fibers 114.

With this in mind, and referring back to fig. 2, the fiber optic system 100 'includes at least one fiber optic cable assembly 106'. The fiber optic cable assembly 106' defines at least a portion of an optical path between the transmitter 102A and the receiver 102B. Each of the fiber optic cable assemblies 106' includes a fiber optic cable 200, a first SMF connector 110 (1) ' and a second SMF connector 110 (2) '. The fiber optic cable 200 includes a multimode optical fiber 114 for communicating optical data signals at an operating wavelength (e.g., 1310nm, 1550nm, etc.). Multimode optical fiber 114 includes a mode field diameter of the fundamental mode at the operating wavelength. The first SMF connector 110 (1) 'is located at a first end of the optical fiber cable 200 and the second SMF connector 110 (2)' is located at a second end of the optical fiber cable 200. The first SMF connector 110 (1) ' includes a first SMF stub 108 (1) ' that forms a first splice joint 202 (1) (e.g., a mechanical splice or fusion splice) with the multimode optical fibers 114 of the fiber optic cable assembly 106'. The second SMF connector 110 (2) ' includes a second SMF stub 108 (2) ' that forms a second splice joint 202 (2) (e.g., a mechanical splice or fusion splice) with the multimode optical fibers 114 of the fiber optic cable assembly 106'.

Each of the SMF stubs 108 (1) ', 108 (2) ' includes a mode field diameter at the operating wavelength that is within 20% of the mode field diameter of the fundamental mode of the multimode optical fibers 114 of the first fiber optic cable assembly 106 '. This similarity in mode field diameter in size is considered "mode matching" in this disclosure. In certain embodiments, the SMF stub 108' substantially matches the mode field diameter of the LP01 mode of the OM1 multimode optical fiber, OM2 multimode optical fiber, OM3 multimode optical fiber, OM4 multimode optical fiber, and/or OM5 multimode optical fiber. More specifically, each of the SMF connectors 110 'has an SMF stub 108' with a mode field diameter that closely matches the mode field diameter of the LP01 mode of the multimode optical fiber 114, which may be about 14 μm at 1310nm or about 15 μm at 1550 nm. Each of the pattern matching SMF stubs 108' includes a fiber cut-off wavelength that is lower than the operating wavelength. In some embodiments, the operating wavelength is 1310nm. In certain embodiments, each of the SMF connectors 110 'has an SMF stub 108' with a cutoff length of less than 1200nm and in some embodiments even less than 1100 nm. Each SMF stub 108' may have a mode field diameter that transitions (e.g., by a single stub fiber or by multiple stub fibers) from 9.2 μm at the end of the ferrule of the associated SMF connector 110' to a larger value (e.g., 11.3 μm or 14 μm at 1310 nm) at the opposite end of the SMF stub 108 '. Each of the SMF stubs 108' includes a fiber cut-off wavelength that is lower than the operating wavelength. In some embodiments, the operating wavelength is 1310nm. In one embodiment, the mode field diameter of the end of the first SMF stub 108' (1) of the first fiber optic cable assembly 106' that is proximate to the multimode optical fiber 114 of the first fiber optic cable assembly 106' is between 12 μm and 16 μm at a 1310nm wavelength (e.g., about 14 μm at a 1310nm wavelength). In certain embodiments, the first SMF stub 108 '(1) of the first fiber optic cable assembly 106' has a length of at least 0.5mm, a fiber cut-off wavelength below 1100nm, and/or mode field conversion. In certain embodiments, the first SMF stub 108' (1) of the fiber optic cable assembly 106' comprises a 14 μm mode field diameter at the first end 204 opposite the multimode optical fiber 114' of the fiber optic cable assembly 106' at 1310nm wavelength, a 9.2 μm mode field diameter at the second end 206 of the multimode optical fiber 114 proximate the fiber optic cable assembly 106' at 1310nm wavelength, and/or an adiabatic transition length of at least 0.5 mm.

In certain embodiments, the multimode optical fiber 114 has a core increment of about 1%, a core diameter of about 50 microns, and a mode field diameter of the LP01 mode of 14.5 μm at 1310nm and/or 15.8 μm at 1550 nm. In certain embodiments, the multimode optical fiber 114 is a universal fiber having a multimode core with a mode field diameter of the fundamental mode similar to that of a standard single mode fiber.

In the case where the SMF stub 108' is used as a mode-converting fiber, the purpose is to excite primarily the fundamental mode of the multimode fiber 114. To optimize the fundamental mode emission, the core diameter D of the SMF stub 108 _C And core increment delta ₀ Should be selected such that the mode field diameter of the SMF stub 108' approximates the fundamental mode of the multimode optical fiber 114. Thus, in one example, the core diameter D of the SMF stub 108 _C In the range of 9 mu m to less than or equal to D _C 12 μm or less, and in another example 7 μm or less D _C And is less than or equal to 15 mu m. Additionally, in one example, the core increment (delta ₀ ) In the range of 0.1% to 0.25%, and in another example between 0.07% and 0.5%. In some embodiments, the SMF stub 108' may be a bend insensitive optical fiber. Some of the design examples of the SMF stubs 108' are shown in the table below.

TABLE 1

When the mode field diameters of the LP01 modes of the OM1 multimode optical fiber and the OM2 multimode optical fiber are approximately matched at 1310nm, the mode field diameters of the LP01 modes of these optical fibers at 1550nm are also approximately matched. Thus, the same mode matching of single mode fibers is also applicable to LP01 transmissions using single mode transceivers at about 1550nm for OM1, OM2, etc.

In some other embodiments, the SMF stub 108' may be a gradient index (GRIN) fiber in which the alpha profile has a core delta in the range of 0.1% to 0.6% (e.g., 0.1% to 0.25%) ₀ And the aforementioned core diameter D in the range of 6 to 15 microns _C . In other embodiments, D _C Less than or equal to 50 micrometers. Examples 1 and 2 in table 1 above have graded refractive index profiles.

As shown in fig. 2, the SMF stub 108 (1) ' of the first SMF connector 110 (1) ' of one fiber optic cable assembly 106 (3) is aligned with and in optical communication (e.g., direct optical communication) with the second SMF stub 108 (2) ' of the second fiber optic connector 110 (2) ' of the other fiber optic cable assembly 106 (2) at the fiber optic connection 112 (2) '.

Thus, the optical path includes a plurality of multimode fibers 114 and a plurality of SMF connectors 110, 110' between the transmitter 102A and the receiver 102B. Each of the SMF connectors 110 provides a respective single mode optical fiber 108 for optical coupling, and each of the SMF connectors 110 'provides an SMF stub 108' for optical coupling. In certain embodiments, the multimode optical fibers 114 of the fiber optic cable assembly 106' include at least one of OM1 multimode optical fibers, OM2 multimode optical fibers, OM3 multimode optical fibers, and/or OM4 multimode optical fibers. The SMF stub 108' is configured to propagate optical data signals at the operating wavelengths of a plurality of different types (e.g., OM1, OM2, OM3, and/or OM 4) of multimode fibers, such as, for example, OM1 and OM2 fibers.

Fig. 3 is a side view of a fiber optic cable assembly 106' having multimode optical fibers 114 (see fig. 2) and SMF connectors 110' each including a corresponding SMF stub 108' (see fig. 2).

As noted above, the fiber optic cable assembly 106' includes the fiber optic cable 200 (e.g., multi-fiber backbone cable, multi-mode horizontal distribution cable, etc.), the first SMF connector 110 (1) ' and the second SMF connector 110 (2) '. The fiber optic cable 200 includes multimode optical fibers 114. The first SMF connector 110 (1) 'is located at a first end of the optical fiber cable 200 and the second SMF connector 110 (2)' is located at a second end of the optical fiber cable 200. The first fiber optic connector 110 (1) ' includes a first SMF stub 108 (1) ' that forms a first splice joint 202 (1) (see fig. 2) (e.g., mechanical splice, fusion splice) with the multimode optical fibers 114 of the fiber optic cable assembly 106 '. The second fiber optic connector 110 (2) ' includes a second SMF stub 108 (2) ' that forms a second splice joint 202 (2) (e.g., mechanical splice, fusion splice) with the multimode optical fibers 114 of the fiber optic cable assembly 106 '. Each of the fiber optic connectors 110' defines a single-mode interface 300 for each end of the fiber optic cable assembly 106' (and thus each end of the multimode optical fibers 114 within the fiber optic cable assembly 106 ').

In certain embodiments, the MMF connector 116 is cut from the fiber optic cable 200 and the replacement SMF connector 110' is installed field at both ends of the fiber optic cable 200. In some embodiments, the SMF connector 110' may be connected to other single-mode cable assemblies or directly to a single-mode transceiver to upgrade to higher transmission speeds of 10G and above.

The first SMF connector 110 (1) 'and the second SMF connector 110 (2)' each include a ferrule 301, a body 302 (which may be one or more pieces) from which the ferrule 301 extends, and a strain relief 304 positioned over a portion of the body 302 and a portion of the fiber optic cable 200. The first SMF stub 108' is secured to the ferrule 301. The ferrule 301 includes a front end portion providing a first SMF stub 108' for optical coupling and a rear end portion opposite the front end portion. The first SMF stub 108' extends beyond the rear end of the ferrule such that the splice joint 202 with the multimode optical fiber 114 is located within the body 302. Although the SMF connector 110' is shown in the form of an LC connector (e.g., according to IEC 61754-20:2012), the present disclosure is applicable to other connector types, such as SC (e.g., according to IEC 61754-4:2013) and ST (e.g., according to TIA/EIA 604-2:2004). Both these connector types and other types of mechanical and fusion splice forms are possible.

According to ANSI/TIA-568.3-D-5.2.3 (2016), the strain relief 304 has a color for OM1, OM2, OM3, or OM4 fiber routing and/or the body 302 has a color for single mode fiber routing. With this color definition, the user can clearly distinguish the fiber type of the end face of the optical fiber connector 300 from the fiber type of the optical fiber cable 200.

TABLE 2

Fig. 4A is a schematic cross-sectional side view of one embodiment of a portion of the SMF connector 110' of fig. 3. As similarly noted above, in certain embodiments, the original MMF connector 116 is cut and the multimode optical fibers 114 are stripped and split to form an endface 400 that is connected to the endface 402 of the SMF stub 108' by mechanical splicing (e.g., within splice holder 406 inside the body 302 (fig. 3)). The lateral cladding offset between the fibers is aligned to less than 1 μm in the V-groove of the splice component within splice holder 406. In certain embodiments, the gap between the fiber-optic endfaces 400, 402 may be filled with an index matching material (e.g., an index matching gel). Alternatively, the SMF stub 108' is fusion spliced to the multimode fiber 114, resulting in even less insertion loss (and no index matching gel is required). In some embodiments, splice and SMF stubs 108' are kept straight using splice protectors.

The mode field diameter of the SMF stub 108' at the spliced end closely matches the mode field diameter of the fundamental mode (LP 01) of the multimode fiber 114 at the operating wavelength. At an operating wavelength of 1310nm, the mode field diameter of the LP01 mode of the OM1 multimode fiber is 13.6 μm, while for the OM2 multimode fiber, the OM3 multimode fiber and the OM4 multimode fiber, the mode field diameter of the LP01 mode is 14.5 μm. In certain embodiments, the SMF stub 108' has a mode field diameter of between 12 μm and 16 μm (such as between 13 μm and 15 μm) and specifically about 14.0 μm as a common fiber that can interface with all common types of multimode fibers (i.e., OM1, OM2, OM3, and OM 4). The mode field mismatch is very small and the insertion loss is negligible. The SMF stub 108' has a fiber cut-off wavelength below 1200nm or optionally below 1100nm, so that higher order modes attenuate in very short propagation lengths. In some embodiments, the mode field diameter of the multimode optical fiber 114 may be matched by a number of SMF stubs 108' as bridges to follow the mode field diameter transition. The bridge, which operates in a straight orientation and accommodates as a stub optical fiber, allows for very compact use and tight integration with field-installable connectors and cable assemblies. When the mode field diameter of the fundamental mode (LP 01) of the OM1 multimode optical fiber, OM2 multimode optical fiber, OM3 multimode optical fiber, OM4 multimode optical fiber is approximately matched to the mode field diameter of the SMF stub 108 'at about 1310nm, the mode field diameter of the LP01 mode at about 1550nm is also approximately matched to the mode field diameter of the SMF stub 108'. A typical wavelength range used at about 1550nm is referred to as the C-band, which is between 1530nm and 1565 nm.

If the SMF stub 108' has a uniform core, the mode field diameter at the connector end face 404 (defined by the ferrule 301) may be 14 μm in some embodiments, which is substantially different from that of a standard single mode fiber. To reduce the mismatch of the mode field diameter with the standard single mode fiber, the SMF stub 108' at the connector end face 404 preferably has a mode field diameter that matches the mode field diameter of the standard single mode fiber, i.e., 9.2 μm at 1310 nm. Such mode field transformation and SMF stubs 108' may be fabricated using the low cut-off SMF stubs 108' of standard field-installable connectors, so that the SMF stubs 108' may achieve a mode field diameter of 9.2 μm.

Fig. 4B is a cross-sectional side view of another embodiment of a portion of the SMF connector 110' of fig. 3. In this embodiment, the very short SMF stub 108A '(length of about 5 mm) includes a mode field diameter transition and is fusion spliced to a second SMF stub 108B' having a uniform mode field diameter of 14 μm. The fusion splice may be located inside the ferrule 301 or closely outside the ferrule. The benefit of this embodiment is that the first SMF stub 108A' with the transition of the mode field diameter may be mass produced prior to assembly to the ferrule 301. In some embodiments, the first SMF stub 108A 'may have a uniform mode field diameter of 11.3 μm and the second SMF stub 108B' may have a uniform mode field diameter of 14 μm. The total insertion loss of the connector interface and splice interface is 0.38dB, which is significantly reduced compared to using a single SMF stub 108A' with a 14 μm mode field diameter.

Fig. 4C is a cross-sectional side view of another embodiment of a portion of the fiber optic connector 110' of fig. 3. In this embodiment, an SMF stub 108 "in the fundamental mode is shown for low loss coupling to the multimode optical fiber 114. The SMF stub 108 "transitions adiabatically from one mode field diameter (at the connector end face 404) to another mode field diameter (at the fiber end face 402). In certain embodiments, the mode field diameter increases adiabatically to 14 μm near the fiber-optic endface 402. The adiabatic transition length is at least 0.5mm. The cut-off wavelength at the mode field expansion region is unchanged.

By embedding the SMF stub 108 "inside the body 302 of the SMF connector 110' (and specifically inside the splice holder 406), a very compact form factor is achieved. At the same time, the SMF stub 108 "is protected from bending. In some embodiments, a standard single mode fiber pigtail having an optical fiber end pre-spliced to an SMF stub is spliced from the field to the multimode optical fiber 114. The splice and SMF stubs 108 "are supported by a rigid housing external to the connector.

Fig. 5 is a diagram of an optical fiber system 500 that is used as an exemplary embodiment of optical fiber system 100 (fig. 1) and thus as an exemplary embodiment in which such a system may be upgraded to optical fiber system 100' (fig. 2). Generally, the fiber optic system 500 includes at least one high fiber count backbone cable 502 connecting a horizontal cross-connect 504 with a main cross-connect 506 and at least one horizontal distribution cable 508 connecting receptacles in a subscriber area to the horizontal cross-connect 504. The multimode fiber optic cables 510, 512 in the horizontal cross-connect 504 and the main cross-connect 506 are pre-installed and may be difficult to completely remove and replace with cables composed of single-mode fiber optics. Sometimes, the main cable 502 is cross-connected by an intermediate cross-connect before reaching the horizontal cross-connect 504. The horizontal distribution cable 508 may have a convergence point. Thus, there are many mating connector pairs (connector-to-connector interfaces) in the optical path from transceiver 102A to transceiver 102B. It is noted that each of the connectors in the communication path includes an SMF connector 110', whether by replacing the entire cable or by replacing only the connector.

To upgrade a structured multimode fiber routing system in a local area network, the existing MMF connectors 116 (fig. 1) on both ends of the horizontal distribution cable 508 are replaced with SMF connectors 110 '(fig. 2) having SMF stubs 108'. The accessible short multimode fiber optic cable assemblies in the patch panel and user area may be simply replaced by a single mode fiber optic cable assembly 514. On both ends of the main cable 502, the fibers may be furcated directly, or fusion spliced to the drop panel at the horizontal cross-connect 504 and the main cross-connect 506, or terminated by multi-fiber connectors (e.g., MPO connectors according to IEC 61754-7-3:2019) connected to multi-fiber connectors on the drop boxes. If existing installation equipment uses direct furcation or fusion splicing at the bifurcation points 516, 518, the multimode fibers 114 are well aligned and they support transmission of the fundamental mode with minimal insertion loss and multipath interference. MPO-based connections may be replaced in the field by multiple fusion splices. Alternatively, the MMF connector on the backbone cable 502 may be replaced by a multi-fiber version of the field-installable SMF connector 108', and the junction box with multimode fibers may be simply replaced by a junction box using single-mode fibers.

As similarly noted above, the geometric tolerances of a given connection vary between the SMF stub 108 'and the multimode optical fiber 114 and between the ferrules 301 of the SMF connectors 110, 110' and the ferrules of the MMF connector 116. The ferrule of the MMF connector 116 has a 5-fold loose fit between the ferrule bore eccentricity (the position of the ferrule bore center relative to the ferrule geometric center) and the outer diameter of the multimode fiber 114 that is approximately 6-fold greater. The quality of the ferrule of the MMF connector 116 significantly affects the fiber-to-ferrule eccentricity of the MMF connector 116. The conventional ferrule of the MMF connector 116, which is made of a polymer composite and stainless steel, provides even worse geometric tolerances than those of the ferrule.

	Eccentricity of optical fiber core	Optical fiber OD	Eccentricity of core insertion hole	Core insert aperture
					SMF	<0.5μm	125±0.7μm	<1.0μm	125.5μm～126.5μm
MMF	<1.5μm	125±1.0μm	<6.0μm	127.0μm～132.0μm

TABLE 3 Table 3

The random mating insertion loss distribution of the SMF connector 110 'is quantified using a monte carlo model based on IEC single mode connector insertion loss standards, wherein the SMF stub 108' is connected to the fundamental mode of the multimode fiber 114 terminated with a zirconia ceramic MMF ferrule. In this model, it is assumed that the wavelength is 1310nm and the mode field diameter is 14.0.+ -. 0.6 μm, closely matching the mode field diameters of both the OM1 multimode fiber and the OM2-4 multimode fiber. In addition, it is assumed that the multimode fiber 114 has the same outer diameter tolerance as the SMF stub 108' and that the MMF ferrule outer diameter has the same tolerance as the SMF ferrule to emphasize the effects of ferrule bore eccentricity and diameter tolerance. The probability distribution of multimode fibers and the mmf ferrules scale by the ferrules of the SMF components according to table 3.

Fig. 6A is a graph of insertion loss distribution between MMF connector 116 and SMF connectors 110, 110'. Specifically, the simulated fiber core-to-ferrule eccentricity distribution of the MMF connector 116 is shown. The eccentricity expands to 6 μm mainly due to loose tolerances of the ferrules of the MMF connector 116. Random mating insertion loss of the connector interface between the OM1 multimode fiber in the MMF ferrule and the 14 μm mode field diameter single mode fiber in the SMF ferrule is simulated in fig. 8. The average insertion loss was 0.53dB and 97% maximum insertion loss was 1.82dB. The losses for OM2, OM3 and OM4 fibers were slightly lower with an average IL of 0.50dB and 97% maximum of 1.68dB.

Fig. 6B is a graph of the insertion loss profile between the multimode fiber 114 and the SMF stub 108'. By removing the MMF connector 116 and replacing it with an SMF connector 110' having a mechanical splice located inside the SMF connector 110', the SMF stub 108' and multimode optical fiber 114 are aligned to a much higher precision. Specifically, fig. 6B shows the insertion loss distribution of the mechanical splice. The average insertion loss was 0.09dB and 97% maximum insertion loss was 0.26dB. Fusion splice provides even better performance with an average insertion loss of 0.06dB and 97% maximum insertion loss of 0.14dB. The residual insertion loss can be further reduced by more closely matching the mode field diameter to the type of multimode fiber (e.g., OM1, OM2, OM3, or OM 4) rather than using SMF stub 108' with a 14 μm mode field diameter for all types of multimode fibers.

Fig. 7A is a graph of the link loss profile of a single MMF cable assembly 106 having ends that interface/mate with (i.e., optically and mechanically couple to) the SMF connector 108 such that there are two MMF connectors to the SMF connector interface. Fig. 7B is a graph of the link loss profile of a single multimode optical fiber 114 with a mechanical splice (one at each end) to the SMF stub 108'. In a link consisting of two MMF to SMF interfaces, the link loss includes multipath interference related to core offset between the fibers. The magnitude of multipath interference is proportional to the connection insertion loss. The simulated insertion loss distribution of the total link is summarized in fig. 7A to 7B and table 4 in consideration of multipath interference loss.

	97% maximum link loss (dB)
		Preserving existing MMF connectors	4.20
Mechanical splice connector	0.52
		Fusion splice connector	0.35

TABLE 4 Table 4

As mentioned above, the SMF stub 108 according to the present disclosure includes a mode field diameter within 20% of the mode field diameter of the fundamental (LP 01) mode of the multimode optical fiber at the operating wavelength (e.g., 1310 nm). The SMF stub 108 may be selected for use in an optical fiber system comprising an OM1 multimode optical fiber, an OM2 multimode optical fiber, an OM3 multimode optical fiber, an OM4 multimode optical fiber, or an OM5 multimode optical fiber. In other words, the characteristics of the SMF stub 108 have been found to allow such SMF stub 108 to be used with multiple types of multimode fibers in accordance with the present disclosure, even where the OM1 multimode fiber has a different core diameter (62.5 μm) and core delta (2%) than other types of multimode fibers. This was confirmed by modeling and experimental studies performed using both OM1 multimode fiber and OM2 multimode fiber for tolerance and center-to-center offset of the pattern matching SMF stub 108 according to the present disclosure. As mentioned above, the OM2 multimode fiber has a core diameter of 50 μm and a core delta of 1%. Although OM2 was used for comparison, the results of the study also apply to OM3 multimode optical fibers and OM4 multimode optical fibers each having a core diameter of 50 μm.

Both OM1 multimode fibers and OM2 multimode fibers in modeling have centerline imperfections and non-alpha errors. OM1 multimode fibers have a mode field diameter of 13.72 μm at 1310nm and a full injection mode bandwidth of 710mhz km at 850nm and 506mhz km at 1310nm (as measured by placing a mode modifier (such as Ardent Photonics ModCon modifier) in the light emitting end, for example, depending on the full injection emission conditions). The OM2 multimode fiber has a mode field diameter of 14.86 μm at 1310nm, a full injection mode bandwidth of 1308mhz x km at 850nm and 570mhz x km at 1310 nm.

Fig. 8A and 8B are graphs of modeled excitation power using a 14 μm diameter single mode gaussian emission spot with different offsets from the center of the OM1 multimode fiber and OM2 multimode fiber in each mode group of the OM1 multimode fiber and OM2 multimode fiber, respectively. If the mode field diameter of the SMF stub 108 does not match the mode field diameter of the OM1 multimode fiber or the OM2 multimode fiber at 1310nm, higher order modes may be launched into the multimode fiber due to the mismatch. Similarly, higher order modes may also be excited if there is an offset between the center of the SMF stub 108 and the center of the multimode fiber. As shown in fig. 8A and 8B, since the mode field diameters of both OM1 multimode fiber and OM2 multimode fiber are close to 14 μm, only the fundamental (LP 01) mode is excited in the multimode fiber when the center of the SMF stub 108 is perfectly aligned (0 μm offset) with the multimode fiber. This results in a flat frequency response of the link, as shown in fig. 9A and 9B, which are graphs of modeled transfer functions (frequency responses) for the use of OM1 multimode optical fiber and OM2 multimode optical fiber, respectively, with a single-mode gaussian emission spot of 14 μm diameter with different offsets from the center of the multimode optical fiber. Such a flat transfer function means that the associated link has an extremely high mode bandwidth and thus the link can support high data rates for long distances (i.e., long system reach). Conversely, the more the center of the SMF stub 108 is offset from the center of the multimode fiber, the more higher order modes in the multimode fiber are excited. For example, for the case where the offset is about 2 μm, about 7.5% of the light is emitted into the second order mode in the multimode fiber. For the case where the offset is about 4 μm, this percentage increases to about 23%. Due to this higher order mode excitation in multimode fibers, oscillations occur in the transfer function of the fiber, as shown by the corresponding curves in fig. 9A and 9B. The length of the OM1 multimode fiber and OM2 multimode fiber is assumed to be 1km in modeling. The mode bandwidth of the fiber is defined such that the transmission drops 3dB from 0 frequency. As can be seen from fig. 9A and 9B, the more higher order modes in the OM1 multimode fiber and the OM2 multimode fiber are excited, the deeper the dip in the transfer function, resulting in a drop in bandwidth. In this case, the second SMF stub 108 may be used at the receiving end of the link to filter higher order modes and increase bandwidth, as will be discussed more using experimental results.

Fig. 10A and 10B are graphs of modeled excitation power using single mode gaussian emissions with different spot sizes (diameters) and no offset from the center of the OM1 multimode fiber and OM2 multimode fiber (i.e., SMF stub 108 perfectly aligned with the multimode fiber) in each mode group of the OM1 multimode fiber and OM2 multimode fiber, respectively. As shown, for both OM1 multimode fibers and OM2 multimode fibers, the fundamental mode is excited primarily with an emission spot size ranging from 12 μm to 16 μm, and the power to excite the higher order modes is less than 5%. Thus, the transfer function as shown in fig. 11A and 11B is very flat, indicating a high mode bandwidth. Such results indicate that there is a 2 μm spot size or mode field diameter tolerance for the SMF stub 108.

Experiments were conducted according to modeling to confirm the functionality of using SMF stubs 108 for fundamental mode transmission in multimode fibers. The SMF stub 108 used in the experiment has a mode field diameter of 14.02 μm and is included in an LC-type fiber optic connector (e.g., SMF connector 110). As mentioned above, however, the present disclosure may be applicable to other connector types, such as SC (e.g., according to IEC 61754-4:2013) and ST (e.g., according to TIA/EIA 604-2:2004). Both these connector types and other types of mechanical and fusion splice forms are possible.

Fig. 12A and 12B are graphs of transfer functions (frequency responses) measured under three different emission conditions for a 200m length of OM1 multimode optical fiber and a 200m length of OM2 multimode optical fiber, respectively. One condition is full injection transmission, another condition is that the associated link includes an SMF stub 108 only at the transmitting end, and a third condition is that the associated link includes an SMF stub 108 at both the transmitting end and the receiving end. As shown in fig. 12A and 12B, the transfer function is significantly reduced under full injection emission conditions. The 200m length of OM1 multimode fiber has a full injection mode bandwidth of 4.25GHz corresponding to a scaled bandwidth of 850mhz x km, and the 200m length of OM1 multimode fiber has a full injection mode bandwidth of 6.02GHz corresponding to a scaled bandwidth of 1204mhz x km. Conversely, when the SMF stub 108 is used in the transmitting end (solid line), the transfer function is much flatter, because the fundamental mode is launched mainly into the multimode fiber, resulting in a higher bandwidth. The second SMF stub 108 may be used at the receiving end to filter higher order modes and further increase bandwidth. As shown in this condition in fig. 12A and 12B, when the second SMF stub 108 is included in the receiving end (broken line) of the link, the flatness of the transfer function increases. This in turn increases the mode bandwidth, which is critical in high data rate transmission systems.

Bit Error Rate (BER) was also measured as part of the experiment. The BER measured on condition that the SMF stub 108 is included at each end of the associated link is below 1E-10 for both links of OM1 multimode optical fibers having a length of 200m and links of OM2 multimode optical fibers having a length of 200 m.

Fig. 13A, 13B and 13C are measured optical eye diagrams (e.g., from an oscilloscope) of a multimode link between 25G LR transceivers/receivers. The illustrations correspond to fiber optic systems including: a 1m length of OM1 multimode optical fiber between the transmitter port of the transceiver and the receiver port of the receiver (fig. 13A); a 200m length of OM1 multimode fiber according to the present disclosure having two ends optically coupled to respective single mode fiber stubs (fig. 13B); and a 200m length of OM2 multimode fiber spliced to both ends of a corresponding single mode fiber stub according to the present disclosure (fig. 13C). As can be seen, the eye diagrams for light under different conditions are open and clear.

FIG. 14 is a flowchart of a step 800 for using the fiber optic system of FIGS. 2-7B; step 802 includes removing the first multimode fiber optic connector 116 (1) at a first end of the fiber optic cable 200 of the first fiber optic cable assembly 106 (1). The fiber optic cable 200 of the first fiber optic cable assembly 106 (1) includes multimode optical fibers 114. Multimode optical fiber 114 includes a mode field diameter of a fundamental mode at an operating wavelength.

Step 804 includes replacing the first multimode fiber optic connector 116 (1) of the first fiber optic cable assembly 106 (1) ' with a replacement first fiber optic connector 110 (1) ' at the first end of the fiber optic cable 200 of the first fiber optic cable assembly 106 (1) '. Replacement of the first fiber optic connector 110 (1) ' includes forming a first single mode fiber stub 108 (1) ' of the first splice joint 202 (1) with the multimode optical fiber 114 of the first fiber optic cable assembly 106 (1) '. The first single mode fiber stub 108 (1) ' of the first fiber optic cable assembly 106 (1) ' comprises a mode field diameter within 20% of the mode field diameter of the fundamental mode of the multimode optical fiber 114 of the first fiber optic cable assembly 106' at the operating wavelength. The first single mode fiber stub 108 (1) 'of the first fiber optic cable assembly 106 (1)' comprises a fiber cut-off wavelength that is lower than the operating wavelength.

Step 806 includes removing the second multimode fiber optic connector 116 at the second end of the fiber optic cable 200 of the second fiber optic cable assembly 106 (2). The fiber optic cable 200 of the second fiber optic cable assembly 106 (2) includes multimode optical fibers 114. Multimode optical fiber 114 includes a mode field diameter of the fundamental mode at the operating wavelength.

Step 808 includes replacing the second multimode fiber optic connector 116 (2) of the second fiber optic cable assembly 106 (2) ' with a replacement second fiber optic connector 110 (2) ' at the second end of the fiber optic cable 200 of the second fiber optic cable assembly 106 (2) '. Replacement of the second fiber optic connector 110 (2) 'includes forming a second single mode fiber stub 108 (2)' of the second splice joint 202 (2) with the multimode optical fiber 114 of the second fiber optic cable assembly 106 (2). The second single mode fiber stub 108 (2) ' of the second fiber optic cable assembly 106 (2) ' comprises a mode field diameter within 20% of the mode field diameter of the fundamental mode of the multimode optical fiber 114 of the second fiber optic cable assembly 106 (2) ' at the operating wavelength. The second single mode fiber stub 108 (2) 'of the second fiber optic cable assembly 106 (2)' comprises a fiber cut-off wavelength that is lower than the operating wavelength.

Step 810 includes connecting the replacement first fiber optic connector 110 (1) 'of the first fiber optic cable assembly 106 (1)' with the second fiber optic connector 110 (2) 'of the second fiber optic cable assembly 106 (2)' such that the first single mode fiber stub 108 (1) 'of the first fiber optic connector 110 (1)' of the first fiber optic cable assembly 106 (1) 'is aligned with and in optical communication with the second single mode fiber stub 108 (2)' of the second fiber optic connector 110 (2) 'of the second fiber optic cable assembly 106 (2)' at the fiber optic connection 112.

In certain embodiments, the first single-mode fiber stub 108 (1) 'of the first fiber optic connector 110 (1)' of the first fiber optic cable assembly 106 (1) 'is aligned with and in direct optical communication with the second single-mode fiber stub 108 (2)' of the second fiber optic connector 110 (2) 'of the second fiber optic cable assembly 106 (2)'.

In certain embodiments, the method further comprises replacing the second multimode fiber optic connector 110 (2) ' with a replacement second fiber optic connector 110 (2) ' at the second end of the fiber optic cable 200 of the first fiber optic cable assembly 106 (1) '. The replacement second fiber optic connector 110 (2) 'of the first fiber optic cable assembly 106 (1)' includes a second single mode fiber stub 108 (2) 'that forms a second splice joint 202 (2) with the multimode optical fiber 114 of the first fiber optic cable assembly 106 (1)'. The second single mode fiber stub 108 (2) ' of the first fiber optic cable assembly 106 (1) ' comprises a mode field diameter within 20% deviation of the mode field diameter of the fundamental mode of the multimode optical fiber 114 of the first fiber optic cable assembly 106 (1) ' at the operating wavelength. The second single mode fiber stub 108 (2) ' of the first fiber optic cable assembly 106 (1) ' comprises a fiber cut-off wavelength that is lower than the operating wavelength propagated by the multimode optical fiber 114 of the first fiber optic cable assembly 106 (1) '.

In certain embodiments, the method further comprises replacing the first multimode fiber optic connector 116 (1) with a replacement first fiber optic connector 110 (1) 'at the first end of the fiber optic cable 200 of the second fiber optic cable assembly 106 (2)'. The replacement first fiber optic connector 110 (1) 'of the second fiber optic cable assembly 106 (2)' includes a single mode fiber stub 108 (1) 'that forms a first splice joint 202 (1) with the multimode optical fiber 114 of the second fiber optic cable assembly 106 (2)'. The first single mode fiber stub 108 (1) ' of the second fiber optic cable assembly 106 (2) ' comprises a mode field diameter at the operating wavelength that is within 20% deviation of the mode field diameter of the fundamental mode of the multimode fiber 114 of the second fiber optic cable assembly 106 (2) '. The first single mode fiber stub 108 (1) ' of the second fiber optic cable assembly 106 (2) ' comprises a fiber cut-off wavelength that is lower than the operating wavelength propagated by the multimode optical fiber 114 of the second fiber optic cable assembly 106 (2) '.

In certain embodiments, the method further comprises replacing each multimode fiber optic connector 116 with a replacement fiber optic connector 110' along the optical path between the transmitter 102A and the receiver 102B. Each replacement fiber connector 110' includes a single mode fiber 108. The optical path includes a plurality of multimode optical fibers 114 between each of the replacement fiber optic connectors 110'.

Fig. 15 is a flowchart of steps 900 for upgrading a system 100 having links of serially connected multimode fibers 114. Each of the links includes opposite ends that terminate with respective multimode fiber connectors 116 that provide multimode fibers 114 of the link for optical coupling.

Step 902 includes replacing all of the multimode fiber optic connectors 116 with corresponding replacement fiber optic connectors 110'. The replacement does not involve removing all of the multimode optical fibers 114 such that after the replacement, at least some of the links include multimode optical fibers 114 of the respective links that terminate with the respective replacement connectors 110'.

Step 904 includes connecting each of the replacement fiber connectors 110 'to another one of the replacement fiber connectors 110' via a physical contact connection. The connection results in no multimode optical fiber 114 in the system being directly coupled to another multimode optical fiber 114 in the system by a physical contact connection.

In certain embodiments, at least two of the links are in the form of fiber optic cable assemblies 106 that include fiber optic cables 200 terminated by respective multimode fiber optic connectors 116 prior to replacement. In addition, the respective replacement connectors 110 'of several of the at least two links each include a single mode fiber stub 108' that forms a splice joint 202 with the multimode optical fibers 114 of the fiber optic cable assembly 106. The single mode fiber stub 108' includes a mode field diameter within 20% of the mode field diameter of the fundamental mode of the multimode fiber 114 of the fiber optic cable assembly 106 at the operating wavelength. The single mode fiber stub 108' includes a fiber cut-off wavelength that is lower than the operating wavelength propagated by the multimode fiber 114 of the first fiber optic cable assembly 106.

In some embodiments, at least two links in the form of fiber optic cable assemblies 106 include: a) A first fiber optic cable assembly 106 (1) having respective multimode fiber optic connectors 116 as a first multimode fiber optic connector 116 (1) and a second multimode fiber optic connector 116 (2); and b) a second fiber optic cable assembly 106 (2) having respective multimode fiber optic connectors as a first multimode fiber optic connector 116 (1) and a second multimode fiber optic connector 116 (2). Additionally, the replacement includes removing the first multimode fiber optic connector 116 (1) of the first fiber optic cable assembly 106 (1). The replacement also includes replacing the first multimode fiber optic connector 116 (1) of the first fiber optic cable assembly 106 (1) with a corresponding replacement fiber optic connector 110 'that is a replacement for the first fiber optic connector 110 (1)'. The replacement also includes replacing the first multimode fiber optic connector 116 (1) of the second fiber optic cable assembly 106 (2) with a corresponding replacement fiber optic connector 110 'that is a replacement for the second fiber optic connector 110 (2'). In addition, the connecting includes connecting the replacement first fiber optic connector 110 (1) 'of the first fiber optic cable assembly 106 (1) with the second fiber optic connector 110 (2)' of the second fiber optic cable assembly 106 (2) by a physical contact connection such that the single mode fiber stub 108 'of the replacement first fiber optic connector 110 (1)' is aligned with and in optical communication with the single mode fiber stub 108 'of the replacement second fiber optic connector 110 (2)'.

In certain embodiments, at least three of the links are in the form of fiber optic cable assemblies 106 that include fiber optic cables 200 terminated by respective multimode fiber optic connectors 116 prior to replacement. In addition, the replacing includes replacing at least one cable assembly 106 of the at least three links with a single mode cable assembly 106' including a single mode fiber 108' having opposite ends terminated 110' by respective replacement connectors.

In some embodiments, prior to replacement, at least one of the links is in the form of multimode fiber optic equipment including multimode optical fibers 114 of the link and corresponding multimode fiber optic connectors 116 located in the interior of the multimode fiber optic equipment. In addition, the replacement includes replacing the fiber optic equipment with single-mode fiber optic equipment including single-mode fibers 108 'having opposite ends terminating with respective replacement connectors 110'.

In some embodiments, each of the multimode fibers 114 is associated with the following specifications: the fiber core eccentricity of the multimode fiber is allowed to be >0.5 μm and the outside diameter of the multimode fiber 114 is outside the range of 125±0.7 μm. Each of the multimode fiber optic connectors 116 includes a ferrule 301 having a ferrule interior bore (referred to above as a "ferrule bore"), the ferrule 301 being associated with the following specifications: the eccentricity of the ferrule bore is allowed to be >1.0 μm and the diameter of the ferrule bore is outside the range of 126±0.5 μm. Each of the replacement fiber optic connectors 110' includes a replacement ferrule 301 having a ferrule bore, the replacement ferrules being associated with the following specifications: the eccentricity of the ferrule bore is allowed to be <1.0 μm and the diameter of the ferrule bore is in the range of 126±0.5 μm.

In certain embodiments, the ferrules 301 of each of the replacement fiber connectors 110 'terminate a respective single mode fiber associated with a gauge wherein the fiber core eccentricity is <0.5 μm and the outer diameter of the single mode fiber 108' is in the range of 125±0.7 μm.

It is noted that the principles disclosed above focus on single mode fiber stubs (e.g., SMF stubs 108 ') being included in ferrules assembled into fiber optic connectors (e.g., SMF connectors 110'). For many aspects of the present disclosure, the principles also apply to other form factors of the mode adjustment/mode filtering device. In other words, the SMF connector 108' disclosed above is one exemplary form factor of an apparatus for mode adjustment and/or mode filtering according to the present disclosure. Other form factors for such devices applying the principles of the present disclosure are possible.

To this end, fig. 16 and 17 illustrate an adapter 160, which is another example of a device that may be used for mode adjustment and/or mode filtering according to the present disclosure. The adapter 160 includes a body 162, ferrules 168 (only one ferrule 168 is visible in fig. 17) supported within the body 162, and corresponding SMF stubs 150 secured within the ferrules 168. On a first side of the adapter, the body 162 and the ferrule 168 define a male connection interface 164 (e.g., a duplex connector interface). On a second side of the adapter, the body 162 and the ferrule 168 define a female connection interface 166 (e.g., a duplex receptacle interface). While the illustrated embodiment includes two ferrules 168 and has a duplex (dual channel/dual port) configuration, other embodiments having a single ferrule (e.g., for simplex fiber optic connections) or more than two ferrules (e.g., for multiple duplex fiber optic connections) are possible.

The ferrules 168 each include a first end (front end) associated with the male connection interface 164 and a second end (rear end) associated with the female connection interface 166. The SMF stubs 150 each extend between and terminate at the first and second ends of the associated ferrule 168. Thus, the SMF stubs 150 in this embodiment each have a length approximately equal to the distance between the front and rear ends of the associated ferrule 168. In certain embodiments, the SMF stub 150 has a length of less than 2cm (such as between 0.5cm and 1.5 cm).

When the adapter 160 is used in place of the SMF connector 110', the MMF connector 116 may not need to be removed from the fiber optic system. Alternatively, the MMF connector 116 on the end of the link (MMF cable assembly 106) may be coupled to the female connection interface 166 of the adapter 160. This results in the ferrule of the MMF connector 116 being placed in physical contact with the ferrule 168, with the end of the multimode fiber 114 aligned with the second end of the SMF stub 150 such that they are optically coupled. The modeling described above with respect to fig. 8B-12B applies equally to the adapter 160 used to establish an optical connection between the SMF stub and the multimode optical fiber.

As another example, in some embodiments, the SMF stub 150 may be incorporated directly into the optical transceiver. Such an embodiment enables basic mode transmission over existing MMF wiring infrastructure. For example, fig. 18A is a schematic diagram of a system 1000 including two transceivers 102' connected by one or more MMF cable assemblies 106. Each transceiver 102' includes a housing 1002 and an optical interface 1004. The optical interface 1004 is configured to receive one or more optical connectors (not shown; e.g., the MMF connector 116; see FIG. 1) that include ferrules for alignment and contact with the ferrules 1006 of the optical interface. As schematically illustrated in fig. 18B, the ferrule 1006 terminates an SMF stub 150 that extends into the housing 1002 and is coupled to an optoelectronic component 1008, such as a Transmitter Optical Subassembly (TOSA), a Receiver Optical Subassembly (ROSA), or a bi-directional optical subassembly (BOSA).

Integrating the SMF stub 150 into the transceiver 102' may provide several benefits. First, since the SMF stub 150 enables basic mode transmission through the MMF cabling 106, components such as the SMF connector 110' or the adapter 160 with the SMF stub 150 may not need to be integrated into the system 1000. This avoids increasing the total number of optical connections/bonds in the system 1000, which in turn can help avoid increasing losses (e.g., insertion loss and/or multipath interference) associated with the optical connections/bonds. This is also convenient from an installation point of view, as only the transceiver 102' needs to be installed to upgrade the system originally designed and installed for multimode transmission. Despite these advantages, components such as MF connector 110 'with SMF stub 150 or adapter 160 may be used in conjunction with transceiver 102' if desired. Second, the transceiver 102' may include additional signal processing capabilities for optimizing the performance of the system 1000 for basic mode transmission.

It is noted that the optical interface 1004 may have any suitable design for transceiver applications. In the illustrated embodiment, the optical interface 1004 is configured to accept a dual fiber connector, such as an LC duplex connector (e.g., according to IEC 61754-20:2012) or an MDC, SN, or CS connector (e.g., according to QSFP-DD multisource protocol (MSA)) hardware specification version 6.0 of 2021 and cross-references thereto. Examples of optical transceivers with duplex LC connectors include 25G LR, 100G CWDM4, 100GLR4, 100G ER4, 400G FR4, and 400G LR. In alternative embodiments, the optical interface 1004 may be configured for different connector types, such as multi-fiber push/pull (MPO) connectors (e.g., according to IEC 61754-7) or other multi-fiber connectors. Examples of optical transceivers with MPO connectors include 40GPSM4, 100GPSM4, and 400g DR4. As can be appreciated, the SMF stub 150 may be terminated by different types of ferrules, depending on the type of connector for which the optical interface 1004 is designed. Many other embodiments, modifications and/or variations of the concepts herein disclosed will come to mind to one skilled in the art of optical communications. Therefore, it is to be understood that the description and claims are not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.

Claims

1. An apparatus for use in a fiber optic system including at least one fiber optic cable assembly having multimode optical fibers, wherein the system is configured to transmit optical data signals at an operating wavelength in the range of 1260nm-1360nm or 1530nm-1565nm, the apparatus comprising:

a single mode fiber stub having a mode field diameter at the operating wavelength that is within 20% of the mode field diameter of the fundamental mode of an OM1 multimode fiber, an OM2 multimode fiber, an OM3 multimode fiber, an OM4 multimode fiber, an OM5 multimode fiber, wherein the single mode fiber stub further has a fiber cut-off wavelength that is lower than the operating wavelength; and

a ferrule to which the single-mode fiber stub is fixed, wherein the ferrule includes a front end portion of the single-mode fiber stub provided for optical coupling and a rear end portion opposite the front end portion, and wherein a center of a fiber core of the single-mode fiber stub is within 0.5 μm from a center of the ferrule at the front end portion of the ferrule.

2. The device of claim 1, wherein the device is a fiber optic connector, the fiber optic connector further comprising:

A connector body comprising a ferrule, wherein the single mode fiber stub extends beyond the rear end of the ferrule and has a splice end within the connector body such that a splice joint can be formed with the multimode optical fiber within the connector body.

3. The apparatus of claim 2, further comprising:

a strain relief positioned over a portion of the connector body, wherein the strain relief has a color for OM1 multimode fiber routing, OM2 multimode fiber routing, OM3 multimode fiber routing, or OM4 multimode fiber routing according to ANSI/TIA-568.3-D-5.2.3 (2016), and wherein the connector body has a color for single mode fiber routing according to ANSI/TIA-568.3-D-5.2.3 (2016).

4. The device of claim 2 or 3, wherein a mode field diameter of the single mode fiber stub at the spliced end is between 12 μm and 16 μm at an operating wavelength of 1310 nm.

5. The device of any one of claims 2-4, wherein a mode field diameter of the single mode fiber stub at the spliced end is about 14 μιη at an operating wavelength of 1310 nm.

6. The apparatus of claim 1, wherein the apparatus is a fiber optic adapter, the fiber optic adapter further comprising:

an adapter body comprising the ferrule, wherein the single-mode fiber stub extends to the rear end of the ferrule, and wherein the fiber optic adapter defines a male connection interface comprising the front end of the ferrule and a female connection interface comprising the rear end of the ferrule.

7. The apparatus of claim 1, wherein the apparatus is an optical transceiver, the optical transceiver further comprising:

a housing;

an optoelectronic element within the housing, the optoelectronic element configured for single mode transmission at the operating wavelength; and

a female connection interface comprising the front end of the ferrule, wherein the single-mode fiber stub extends from the rear end of the ferrule and is optically coupled to the optoelectronic assembly within the housing.

8. The apparatus of claim 6 or 7, wherein the female connection interface is configured to accept a duplex connector.

9. The apparatus of claim 8, wherein the female connection interface is configured to accept an LC duplex connector according to IEC 61754-20:2012.

10. The apparatus of claim 6 or 7, wherein the female connection interface is configured to accept a multi-fiber connector.

11. The apparatus of claim 10, wherein the optical interface is configured to accept an MPO connector according to IEC 61754-7-3:2019.

12. The device of any one of claims 1 to 11, wherein the single mode fiber stub has a length between 0.5cm and 2.0 cm.

13. The device of any one of claims 1 to 12, wherein the single mode fiber stub has a fiber cut-off wavelength below 1100 nm.

14. The apparatus of any one of claims 1 to 13, wherein the single-mode fiber stub has a first end at the front end of the ferrule and a second end opposite the first end, and wherein the mode field diameter of the single-mode fiber stub is greater at the first end than at the second end such that the single-mode fiber stub comprises a mode field transformation.

15. An optical fiber system for transmitting an optical data signal at an operating wavelength, the optical fiber system comprising:

at least one fiber optic cable assembly comprising a multimode optical fiber having a mode field diameter of a fundamental mode at the operating wavelength; and

The device of any one of claims 1-14, wherein the single-mode fiber stub of the device forms an optical connection with the multimode fiber of the at least one fiber optic cable assembly, and wherein a center of the core of the single-mode fiber stub is within 2 μιη of a center of a core of the multimode fiber at the optical connection.

16. A method for upgrading a system comprising at least one link of multimode optical fibers, wherein each link of the at least one link comprises an opposite end terminated with a respective multimode optical fiber connector providing the multimode optical fibers of the link for optical coupling, and wherein the multimode optical fibers have a fundamental mode field diameter at an operating wavelength, the method comprising:

selecting a single mode fiber stub that satisfies the following conditions: having a mode field diameter at the operating wavelength that is within 20% of the mode field diameter of the fundamental mode of the multimode optical fiber, wherein the single mode fiber stub is selected such that a condition can be met when the multimode optical fiber is an OM1 multimode optical fiber, an OM2 multimode optical fiber, an OM3 multimode optical fiber, an OM4 multimode optical fiber, an OM5 multimode optical fiber;

Selecting a ferrule to support the single-mode fiber stub, wherein the ferrule comprises a ferrule bore configured to receive the single-mode fiber stub, and wherein the ferrule is selected such that a ferrule bore eccentricity <0.5 μm, and a diameter of the ferrule bore is within a range of 126±0.5 μm; and

setting the ferrule to: replacing a portion of a fiber optic connector intended to replace one of the multimode fiber optic connectors of the link; a portion of an adapter configured to receive one of the multimode fiber connectors of the link; or a portion of an optical transceiver configured to receive one of the multimode fiber optic connectors of the link, wherein the single mode fiber stub is secured to the ferrule.

17. The method of claim 16, wherein the ferrule is provided as part of the replacement fiber optic connector, and wherein:

the replacement fiber optic connector further includes a connector body, the ferrule extending from the connector body,

the ferrule has a front end portion providing the single-mode fiber stub for optical coupling and a rear end portion opposite the front end portion,

The single mode fiber stub extends beyond the rear end of the ferrule, and

the connector body of the replacement fiber connector is configured to receive a splice joint between the single mode fiber stub and the multimode fiber.

18. The method of claim 16, wherein the ferrule is provided as part of the adapter, and wherein:

the ferrule has a front end and a rear end opposite the front end,

the single mode fiber stub has a length approximately equal to a distance between the front end and the rear end of the ferrule such that the ferrule provides an opposite end of the single mode fiber stub for optical coupling, an

The adapter defines a male connection interface including the front end of the ferrule and a female connection interface including the rear end of the ferrule.

19. The method of claim 16, wherein the ferrule is provided as part of the optical transceiver, and wherein:

the optical transceiver further includes a housing and an optoelectronic element within the housing, the optoelectronic element configured for single mode transmission at the operating wavelength,

the single mode fiber stub extends beyond the rear end of the ferrule and is optically coupled to the optoelectronic assembly within the housing, and

the front ends of the housing and the ferrule define a female connection interface.

20. The method of any one of claims 16 to 19, wherein the operating wavelength is in the range 1260nm-1360nm or in the range 1530nm-1565 nm.