WO2018049312A1

WO2018049312A1 - Thin film anti-reflection coating for optical fibers in contact

Info

Publication number: WO2018049312A1
Application number: PCT/US2017/050926
Authority: WO
Inventors: Brian K. Bushnell
Original assignee: Commscope, Inc. Of North Carolina
Priority date: 2016-09-09
Filing date: 2017-09-11
Publication date: 2018-03-15

Abstract

In many situations the end of one optical waveguide, such as an optical fiber, is placed in contact with another waveguide, or fiber, so that light passes from one to the other. Sometimes, such as in multi-fiber connectors, there is a gap between some waveguides and no gap between others. The present invention ensures that the optical transmission through the connector is the same regardless of whether the light passes through a contacted pair of waveguides or a pair of waveguides having a gap therebetween. This is achieved by designing the anti-reflection coatings on the ends of the fibers to produce substantially the same reflectance at the design wavelength, regardless of whether there is a gap between the waveguides or not.

Description

THIN FILM ANTI-REFLECTION COATING

FOR OPTICAL FIBERS IN CONTACT Cross-Reference to Related Application

This application is being filed on September 11, 2017 as a PCT International Patent Application and claims the benefit of U.S. Patent Application Serial No.

62/385,543, filed on September 9, 2016, the disclosure of which is incorporated herein by reference in its entirety. Background of the Invention

The present invention is generally directed to optical transmission networks, and more particularly to coatings on the ends of optical fibers used in optical transmission networks.

Optical communication networks are becoming prevalent in part because service providers want to deliver high bandwidth communication capabilities to customers.

Passive optical networks are a desirable choice for delivering high-speed communication data because they avoid the need for active electronic devices, such as amplifiers and repeaters, between a central office and a subscriber termination. The optical fibers used in such networks preferably introduce as little optical loss to the signal as possible.

Optical communications networks often employ many fibers bundled together in a single cable for ease of handling. The fiber cables have multi-fiber connectors at each end to allow connection to other cables or various components of the network. In some cases, the fiber cable may include as many as 48, 96 or more optical fibers. It is important to the efficient operation of the network that optical losses at the multi-fiber connectors are kept as low as possible and also that losses be uniform across all fibers in the bundle.

Summary of the Invention

One embodiment of the invention is directed to an optical system that has a first optical waveguide having a first end and a second optical waveguide having a second end. There is a first anti -reflective coating on the first end, which has a first reflectance minimum for light at a first wavelength and a second reflectance minimum for light at a second wavelength when the first end of the first optical waveguide is uncoupled. The second end of the second optical waveguide has a second anti -reflective coating that has a third reflectance minimum for light at the first wavelength and a fourth reflectance minimum for light at the second wavelength when the second end of the second optical waveguide is uncoupled. When the first and second reflective coatings are placed in coupling contact to form a contacted anti -reflective coating, a reflectance of the contacted anti -reflective coating at the first wavelength is substantially the same as the reflectance of the first anti -reflective coating at the first wavelength when the first anti -reflective coating is uncoupled and a reflectance of the contacted anti -reflective coating at the second wavelength is substantially the same as the reflectance of the first anti-reflective coating at the second wavelength when the first anti -reflective coating is uncoupled.

Another embodiment of the invention is directed to an optical system that has a first fiber optic connector. A first set of optical fibers terminate in the first fiber optic connector and the ends of the first set of optical fibers are provided with respective first anti -reflection dielectric coatings. A second set of optical fibers terminate in a second fiber optic connector, and ends of the second set of optical fibers are provided with respective second anti -reflection dielectric coatings. When the first and second fiber optic connectors are in a mating relationship, at least some of the second set of optical fibers are transversely aligned to corresponding optical fibers of the first set of optical fibers. When the first and second fiber optic connectors are in a mating relationship, reflection losses for light at a first wavelength propagating from a fiber of the first set of optical fibers into a fiber of the second set of optical fibers are substantially the same regardless of whether an air gap exists between the fiber of the first set of optical fibers and the fiber of the second set of optical fibers or whether the fiber of the first set of optical fibers and the fiber of the second set of optical fibers are in coupling contact. The first wavelength corresponds to a reflection minimum for the first anti -reflection dielectric coating on the fiber of the first set of fibers.

The above summary of the present invention is not intended to describe each illustrated embodiment or every implementation of the present invention. The figures and the detailed description which follow more particularly exemplify these embodiments.

Brief Description of the Drawings

The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which:

FIG. 1 schematically illustrates optical fibers mated in a multi-fiber coupler; FIG. 2A schematically illustrates two optical fibers with multilayer anti -reflective coatings at respective ends, according to an embodiment of the invention;

FIG. 2B schematically illustrates an optical fiber with a multilayer anti -reflective coating at an end, according to an embodiment of the present invention;

FIG. 2C schematically illustrates two optical fibers with multilayer anti-reflective coatings at respective ends in coupling contact, according to an embodiment of the present invention;

FIG. 2D schematically illustrates two optical fibers with multilayer anti -reflective coatings at respective ends not in coupling contact, according to an embodiment of the present invention;

FIG. 3 presents numerical modelling results for the transmission of AR coatings on the ends of respective fibers, where the AR coatings, designed for operation with light at 1350 nm and 1500 nm, are i) in coupling contact, curve 300 and ii) are not in coupling contact, curve 302, according to an embodiment of the present invention; and

FIG. 4 presents numerical modelling results for the transmission of AR coatings on the ends of respective fibers, where the AR coatings, designed for operation with light at 1350 nm, 1500 nm and 1650 nm, are i) in coupling contact, curve 400 and ii) are not in coupling contact, curve 402, according to an embodiment of the present invention.

While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. Detailed Description

Cables that use bundles of optical fibers are commonly used in optical

communications applications. Such cables are commonly terminated with multi-fiber connectors that align the fibers in the bundle with respective fibers in another connector. In order to enhance the coupling of light from a fiber in the first bundle to its respective fiber in the second bundle, it is important that the two fibers be aligned correctly. This requires transverse alignment, i.e. alignment in directions lying in the plane perpendicular to the fiber axis so that there is close overlap between the cores of the two fibers. Correct alignment also requires alignment in the direction parallel to the fiber axis. Ideally, the end faces of the two fibers are in contact: if there is a gap between the fiber ends, a portion of light can escape while traversing the gap as there is no confinement of the light across the gap. If there is no gap, then the light remains confined while passing from one fiber to the other, and losses are reduced.

Axial alignment is relatively easy to achieve in the case of a connector for a single optical fiber. When multiple fibers are included in the connector, on the other hand, axial alignment is harder to achieve, as it requires that fiber ends of each fiber in the connector lie in the same plane. This is illustrated with reference to FIG. 1, which schematically shows two fiber connectors, 100, 102, each carrying respective fibers 104a-104e and 106a- 106e. In many cases the fibers 104, 106 are carried inside respective ferrules, but these are omitted from the drawing for simplicity.

The connectors 100, 102 include respective housings 108, 110, and fixing elements 112, 114 that hold the fibers 104, 106 stationary within the housings 108, 110. Dashed line 116 represents the plane on which the ends of the fibers 104, 106 contact when the connectors are 100, 102 are mated together. Most of the fibers 104, 106 meet in contact at the plane 116. However, if the end of any fiber 104, 106 is not located at the plane 116, then a gap can exist between that fiber 104, 106 and its complement in the other connector. In the illustrated example, the end of fiber 104a does not lie in the plane 116, and a gap 118 exists between it and its complement fiber 106a. Also, the end of fiber 106c does not lie in the plane 116 and a gap 120 exists between it and its complement fiber 104c.

The figure shows connectors carrying five fibers for illustrative purposes only, and it will be appreciated that the connectors may carry different numbers of fibers, for example up to 48, 96 or more. Other fiber connectors can be used, such as 8, 12, 24, 36. The connectors can be traditional connectors such as SC, LC, MPO. The connectors can be other devices that can be used to align the fibers.

In addition to the optical losses being greater upon passing through an inter-fiber gap, where light propagation is undefined, the coatings on the ends of the fibers 104, 106 may also increase the losses. In many cases, the fibers 104, 106 are provided with coatings that reduce reflections at the ends of the fibers 104, 106 when the fiber ends are in contact. It is important, however, that such coatings also have low reflectance for fiber ends that are not in contact. If they do not have low reflectance, then the coatings can increase losses for fiber ends terminate at a gap, rather than in contact with complement fibers, for example fibers 104a, 104c, 106a, 106c. Furthermore, it is important that the reflectance at a fiber end is low, whether a fiber is in contact with its complement fiber or whether its end terminates at an air gap, so as to reduce reflections from the fiber end that may provide feedback to the optical transmitter or transceiver that is sending the optical signal into the fiber system.

FIGs. 2A-2D schematically illustrate various embodiments of coatings on the ends of optical fibers, in coupling contact and with an air gap therebetween. FIG. 2A schematically illustrates a first optical fiber 202 having a core 204 in a cladding 206. The end 208 of the first optical fiber 202 is contained in a first ferrule 210. The end 208 of the first fiber 202 is provided with a first anti -reflective (AR) dielectric coating 212, which may be a multilayer dielectric coating. The core 204 of the first optical fiber 202 has a refractive index that is greater than the refractive index of the cladding 206, which confines a light signal propagating along the fiber 202 to the fiber core 204. In many cases, for example in many optical communications applications, the first fiber 202 is made from silica glass, with the core 204 being doped to increase its refractive index relative to the glass cladding 206. Common wavelengths of operation for optical fiber communication systems include wavelength ranges around 850 nm such as 800-900 nm; around 1310 nm, such as 1270-1350 nm; around 1550 nm, such as 1520-1580 nm; and around 1625 nm, such as 1600-1650 nm, although the present invention may also be used at other desired wavelengths.

In the context of this document, an "anti -reflective coating" or "AR coating" is one that is characterized by a reflectance at the operating wavelength that is less than the reflection of the uncoated material on which the coating is located. Thus, where the fiber 102 is formed of silica glass, reflectance of the uncoated silica fiber end is around 4%. The first AR dielectric coating 112 has a reflectance less than that experienced by light incident on an uncoated end of the silica fiber 102, i.e. less than 4%, more preferably less than 1% and even more preferably less than 0.1%.

FIG. 2A also shows a second optical fiber 222 that has a core 224 in a cladding 226. The end 228 of the second optical fiber 222 is contained in a second ferrule 230. The end 228 of the second fiber 222 is also provided with a second AR dielectric coating 232, which may be a multilayer dielectric coating. The second AR dielectric coating 132 may have the same dielectric structure as the first AR dielectric coating, although this is not a necessary condition.

FIG. 2B schematically illustrates the first optical fiber 202 and the first AR dielectric coating 212 in greater detail. The first AR dielectric coating 212 includes a stack of one or more dielectric layers of alternating low and high refractive index. In the illustrated embodiment, the reflective dielectric coating 212 includes seven layers, four layers 240 of relatively high refractive index (shown in FIG. 2B as white layers) and three layers 242 of relatively low refractive index (shown in FIG. 2B as shaded layers). Where the first AR dielectric coating 212 is designed for low reflection at only one wavelength, λο, the layers 240, 242 may each have an optical thickness (actual thickness times the refractive index) that is around one quarter of the design wavelength, λο/4. The outermost layer of the first reflective dielectric coating 212, i.e. the layer furthest from the first optical fiber 102, is referred to as the first external layer 244.

The different layers in the reflective dielectric coating 112 may be formed of various materials, typically having low absorption at the design wavelength. For example, a "high index" material may be titanium dioxide (Ti0₂) (refractive index (n) ~ 2.49), tantalum pentoxide (Ta₂0₅) (n ~ 2.2) or zinc sulfide (ZnS) (n ~ 2.29 ). "Low index" materials may include, for example, magnesium fluoride (MgF₂) (n ~ 1.37), calcium fluoride (CaF₂) (n ~ 1.43), silicon dioxide (Si0₂) (n ~ 1.45), or aluminum oxide (A1₂0₃) (n ~ 1.76). All refractive index values in this paragraph are given for a wavelength of 1 μτη.

It will be understood that layers of a dielectric coating may have other thicknesses while having the same optical effect in the AR dielectric coating as a single quarter wavelength layer, and the inventions is not intended to place any particular restriction on the thicknesses of the layers. Software packages, for example 'The Essential MacLeod™, marketed by Thin Film Center, Tucson AZ, or 'FilmStar™', marketed by FTG Software, Princeton NJ, are available to determine the reflectance of a coating having different thicknesses, different layer materials, and different numbers of layers. In some

embodiments, the layers of a coating may be made from layers which have an optical thickness that is an odd integer number of quarter wavelengths, i.e. (2ρ+1)λ/4, where "p" is an integer having a minimum value of zero and λ is the wavelength. The value of "p" is zero in many dielectric coatings, since this value allows for the thinnest coatings, which are faster and cheaper to make than coatings where "p" has some value greater than zero. However, a layer having a value of "p" greater than zero has the same optical effect within the stack of dielectric layers as a layer having a value of "p" that is zero. The present invention is intended to cover coatings with different values of "p." Accordingly, where the term "quarter wavelength" is used herein to describe the optical thickness of a dielectric layer, it should be understood that this term also includes an odd integer multiple of quarter wavelengths, i.e. it covers all values of "p," although it is expected that most practical applications will use dielectric layers having a value of "p" that is low, and most usually zero.

FIG. 2C schematically illustrates the first fiber 202 mated to the second fiber 222, with the first core 204 aligned with the second core 224, so that light propagating along one of the fibers 202, 222, couples into the other fiber 222, 202. FIG. 2D shows the same two fibers 202, 222, but with a gap 256 between their ends. Ferrules 210, 230 have been omitted for clarity, although it will be understood that ferrules and/or fiber connectors may be used to hold the two fibers 202, 222 together in alignment. It will further be understood that the fibers 202, 222 may be complementary fibers in a multi-fiber connection.

In the illustrated embodiment, the second fiber 222 is provided with a second AR dielectric coating 232 having a design similar to that of the first AR coating 112, with alternating layers of high index material 250 and low index material 252. The outermost layer of the second reflective dielectric coating is referred to as the second external layer 254.

Coupling losses are reduced when the ends 208, 228 of the fibers 202, 222 are closer together, so it is preferred that the ends of the fibers 202, 222 are in contact in order to reduce coupling losses. Thus, the first external layer 244 of the first AR dielectric coating 212 touches the second external layer 254 of the second AR dielectric coating 232. The condition where the two external layers 244, 254 are contacted together is referred to herein as "coupling contact," and the two coatings 212, 232 in contact with each other may be referred to jointly as a contacted coating.

The two AR dielectric coatings 212, 232 produce anti -reflective effects whether or not the external layers 244, 254 are in coupling contact. In other words, the reflectance of the contacted coating is about the same as the reflectance of either the first AR coating 212 or the second AR coating 232. This is achieved by designing the first and second AR coatings 212, 232 to have anti -reflective properties at the design wavelength, or wavelengths, when the end of the fiber is in air and when the contacted coating has a fiber core on either side.

It will be appreciated that the coatings shown in FIGS. 2A-2D are not drawn to scale relative to the optical fibers.

A dielectric AR coating may be provided with an external layer of a material having a refractive index whose value is intermediate those of the high refractive index material and the low refractive index material, as a scratch-resistant, protective layer. One example of such a material is A1₂0₃, and is useful as a scratch-resistant layer since it is harder than most dielectric materials. The refractive index of A1₂0₃ is around 1.755 at 1 μπι, which generally lies between the refractive indices of the low and high index materials used in the dielectric stack. A layer of A1₂0₃ can be used without adversely affecting the anti -reflection properties of the stack. Other materials may be used as scratch-resistant coatings, such as diamond, aluminum oxynitride (AION), spinel

(MgAl₂0₄) and zirconia (Zr0₂). Scratch-resistant coatings on fibers are discussed in greater detail in U.S. Patent Application titled "Optical Fiber With Scratch-Resistant Ends," filed on May 31, 2016, having serial number 62/343,612, and incorporated herein by reference.

AR dielectric coatings may be fabricated to produce reflection minima at a single wavelength, or at two, three or more design wavelengths. In general, if more reflection minima are required, then the dielectric stack requires more layers to achieve high performance, although this need not always be the case. In multiple wavelength embodiments, the reflection at the design wavelengths is preferably less than 1% and more preferably less than 0.1%, both for coupled and uncoupled operation. The coatings described herein can be designed to operate at many different combinations of design wavelengths, but it is expected that certain more commonly used wavelengths used with optical fibers will be found in many embodiments, including 850 nm, 1310 nm, 1550 nm and 1625 nm. Also, the requirement that the coating demonstrates reflection minima at multiple wavelengths often requires the use of more complex stack designs than the use of quarter wavelength layers. Commercially available programs such as 'The Essential MacLeod™' or 'FilmStar™' referred to above can be used to produce multilayer dielectric stack designs with reflection minima at multiple selected wavelengths. Example 1 : AR Coating at Two Wavelengths

Calculations were performed for a seven-layer dielectric AR coating on a single mode fiber, designed for minimum reflectance at 1350 nm and 1500 nm, for both coupled and uncoupled operation. The dielectric stack design assumed for this coating was as follows, from the fiber surface out to the external layer:

Rel raclh e i ndex 1 hick n ss ( nm )

H 84.22

L 126.48 H 103.38

L 201.57

H 96.54

L 168.16

M 122.45

The material assumed for the high refractive layer (H) was Ti0₂, for the low refractive index layer (L) Si0₂, and the intermediate refractive index material (M) was assumed to the A1₂0₃. The "thickness" column lists the physical thickness of the layer in microns.

The calculated transmittance, T, of light through the coating as a function of wavelength is shown in FIG. 3, where curve 300 shows the connected transmittance, i.e. the transmission going from one fiber to the other fiber, while curve 302 shows the unconnected transmittance, i.e. the transmission from the fiber to air. For both the connected and unconnected situations, the reflectance, R, (R = 1-T) was 0.00% at the design wavelengths of 1350 nm and 1500 nm.

Example 2: AR Coating at Three Wavelengths

Calculations were performed for a seven-layer dielectric AR coating on a single mode fiber, designed for minimum reflectance at 1350 nm, 1500 nm and 1650 nm, for both coupled and uncoupled operation. The dielectric stack design assumed for this coating was as follows, from the fiber surface out to the external layer:

The material assumed for the high refractive layer (H) was Ti0₂, for the low refractive index layer (L) Si0₂, and the intermediate refractive index material (M) was assumed to the AI2O3. The "thickness" column lists the physical thickness of the layer in microns.

The calculated transmittance, T, of light through the coating as a function of wavelength is shown in FIG. 4, where curve 400 shows the connected transmittance, i.e. the transmission going from one fiber to the other fiber, while curve 402 shows the unconnected transmittance, i.e. the transmission from the fiber to air. The reflectance at the design wavelengths when the fiber was connected, curve 400, was R(1350 nm) = 0.00%, R(1500 nm) = 0.27%, and R(1650 nm) = 0.12%. The reflectance at the design wavelengths when the fiber was unconnected, curve 402, was R(1350 nm) = 0.32%, R(1500 nm) = 0.42%, and R(1650 nm) = 0.05%.

It will be appreciated that various other designs may be used for coatings that have reflection minima at two, three or more wavelengths, and that the reflectance at various wavelengths can depend on such factors as layer thickness, number of layers in the dielectric stack and the dielectric materials used. The reflectance for connected and unconnected use is defined to be substantially the same if the difference in the connected and unconnected reflectance is less than 0.5%. In both Examples 1 and 2, the values of the reflectances at the design wavelengths were all below 0.5% for both connected and unconnected operation. Accordingly, the reflectances were approximately the same at the design wavelengths under both connected and unconnected operation.

The graphs shown in FIGS. 3 and 4 are the results of numerical calculations that use certain inputs. In some cases, these inputs assume that the thicknesses of each layer of the dielectric coating, or the refractive index of a material layer, are exactly known values. It will be appreciated that actual implementation of a dielectric coating may result in layer thicknesses being different from exactly desired values or for the refractive index of a deposited material layer to differ from the book value of the refractive index. For example, many thermal evaporation coaters are capable of producing dielectric layers within about 2% of design thickness. The intention is that the invention covers such differences that arise due to manufacturing processes and design changes.

It will be understood that the multilayer dielectric coatings described herein may be deposited on the end of the optical fibers using standard vapor deposition techniques. In situations where the optical fiber ends are held in ferrules prior to deposition of the AR dielectric coatings, the fiber and ferrule may be mounted on a custom flange on the vacuum vessel where the vapor deposition takes place, with the exposed fiber ends within the vacuum vessel. In such situations, it may be preferred that the fiber ferrule to be formed of a material that does not outgas, or only outgasses minimally, when exposed to vacuum, such as an epoxy-less material like glass, ceramic or metal.

Furthermore, the invention is not restricted to being used only to coupling between optical fibers, but may be used in coupling to other elements of an optical communications system that include an optical waveguide. For example, the invention may be used where an optical fiber couples to a waveguide core on an optical chip, the waveguide core having an end at the side of the chip to which the optical fiber couples.

As noted above, the present invention is applicable to fiber optical communication and data transmission systems. Accordingly, the present invention should not be considered limited to the particular examples described above, but rather should be understood to cover all aspects of the invention as fairly set out in the attached claims. For example, the AR dielectric coatings need not be made from the materials listed herein, and may be formed using other suitable dielectric materials. Furthermore, different coating designs may be used in addition to those explained herein.

Various modifications, equivalent processes, as well as numerous structures to which the present invention may be applicable will be readily apparent to those of skill in the art to which the present invention is directed upon review of the present specification. The claims are intended to cover such modifications and devices.

Claims

What is claimed is:

1. An optical system, comprising:

a first optical waveguide having a first end;

a second optical waveguide having a second end;

a first anti -reflective coating on the first end, the first anti -reflective coating having a first reflectance minimum for light at a first wavelength and a second reflectance minimum for light at a second wavelength when the first end of the first optical waveguide is uncoupled;

a second anti -reflective coating on the second end, the second anti- reflective coating having a third reflectance minimum for light at the first wavelength and a fourth reflectance minimum for light at the second wavelength when the second end of the second optical waveguide is uncoupled;

wherein, when the first and second reflective coatings are placed in coupling contact to form a contacted anti-reflective coating, a reflectance of the contacted anti-reflective coating at the first wavelength is substantially the same as the reflectance of the first anti -reflective coating at the first wavelength when the first anti-reflective coating is uncoupled and a reflectance of the contacted anti- reflective coating at the second wavelength is substantially the same as the reflectance of the first anti -reflective coating at the second wavelength when the first anti-reflective coating is uncoupled.

2. An optical system as recited in claim 1, wherein the first and second optical waveguides are single mode optical fibers for light at the first and second wavelengths.

3. An optical system as recited in claim 1, wherein the first and second optical waveguides are multimode optical fibers at the first wavelength.

4. An optical system as recited in claim 1, wherein the reflectance of the first anti-reflective coating is less than 1% at the first wavelength and less than 1% at the second wavelength when the first anti -reflective coating is uncoupled and the reflectance of the second anti-reflective coating is less than 1% at the first wavelength and less than 1% at the second wavelength when the first anti -reflective coating is uncoupled and the reflectance of the coupled anti -reflective coating is less than 1% at the first wavelength and less than 1% at the second wavelength.

5. An optical system as recited in claim 1, wherein the reflectance of the first anti-reflective coating is less than 0.5% at the first wavelength and less than 0.5% at the second wavelength when the first anti -reflective coating is uncoupled and the reflectance of the second anti -reflective coating is less than 0.5% at the first wavelength and less than 0.5% at the second wavelength when the first anti -reflective coating is uncoupled and the reflectance of the coupled anti -reflective coating is less than 0.5% at the first wavelength and less than 0.5% at the second wavelength.

6. An optical system as recited in claim 1, wherein the first anti-reflective coating has a reflectance minimum at a third wavelength and the second anti -reflective coating has a reflectance minimum at the third wavelength and, when the first and second reflective coatings are placed in coupling contact to form the contacted anti -reflective coating, a reflectance of the contacted anti -reflective coating at the third wavelength is approximately the same as the reflectance of the first anti -reflective coating at the first wavelength when the first anti -reflective coating is uncoupled.

7. An optical system as recited in claim 6, wherein the reflectance of the anti- reflection coating is less than 1% at the first wavelength and less than 1% at the second wavelength and less than 1% at the third wavelength when the first anti -reflective coating is uncoupled and the reflectance of the second anti -reflective coating is less than 1% at the first wavelength and less than 1% at the second wavelength and less than 1% at the third wavelength when the first anti -reflective coating is uncoupled and the reflectance of the coupled anti -reflective coating is less than 1% at the first wavelength, less than 1% at the second wavelength and less than 1% at the third wavelength.

8. An optical system as recited in claim 7, wherein the reflectance of the anti- reflection coating is less than 0.5% at the first wavelength and less than 0.5% at the second wavelength and less than 0.5% at the third wavelength when the first anti -reflective coating is uncoupled and the reflectance of the second anti -reflective coating is less than 0.5%) at the first wavelength and less than 0.5% at the second wavelength and less than 0.5%) at the third wavelength when the first anti -reflective coating is uncoupled and the reflectance of the coupled anti -reflective coating is less than 0.5% at the first wavelength, less than 0.5% at the second wavelength and less than 0.5% at the third wavelength.

9. An optical system as recited in claim 1, wherein the first reflective coating comprises a stack of at least two pairs of layers of high and low refractive index material.

10. An optical system as recited in claim 9, wherein the first anti -reflective coating further comprises an outer layer of a scratch resistant material having a refractive index intermediate the refractive index of the low refractive index material and the refractive index of the high refractive index material.

11. An optical system as recited in claim 9, wherein within each pair of layers, the layer of high refractive index is closer to the end of the waveguide than the layer of low refractive index.

12. An optical system as recited in claim 1, wherein the first wavelength is at least 50 nm different from the second wavelength

13. An optical system, comprising:

a first fiber optic connector;

a first set of optical fibers terminating in the first fiber optic connector, ends of the first set of optical fibers being provided with respective first anti -reflection dielectric coatings;

a second fiber optic connector;

a second set of optical fibers terminating in the second fiber optic connector, ends of the second set of optical fibers being provided with respective second anti-reflection dielectric coatings;

wherein, when the first and second fiber optic connectors are in a mating relationship, at least some of the second set of optical fibers are transversely aligned to corresponding optical fibers of the first set of optical fibers; and

wherein, when the first and second fiber optic connectors are in a mating relationship, reflection losses for light at a first wavelength propagating from a fiber of the first set of optical fibers into a fiber of the second set of optical fibers are substantially the same regardless of whether an air gap exists between the fiber of the first set of optical fibers and the fiber of the second set of optical fibers or whether the fiber of the first set of optical fibers and the fiber of the second set of optical fibers are in coupling contact, the first wavelength corresponding to a reflection minimum for the first anti -reflection dielectric coating on the fiber of the first set of fibers.

14. An optical system as recited in claim 13, wherein at least a first subset of the first set of optical fibers are in coupling contact with respective fibers of a second subset of the second set of optical fibers.

15. An optical system as recited in claim 14, wherein the second anti-reflection coating has a reflection minimum at the first wavelength.

16. An optical system as recited in claim 15, wherein the first anti-reflection coating has a second reflection minimum at a second wavelength and the second anti- reflection coating has a second reflection minimum at the second wavelength.

17. An optical system as recited in claim 16, wherein the first wavelength is different form the second wavelength by at least 50 nm.

18. An optical system as recited in claim 15, wherein the first anti-reflection coating has a third reflection minimum at a third wavelength and the second anti-reflection coating has a third reflection minimum at the third wavelength.

19. An optical system as recited in claim 13, wherein reflection losses for light at the first wavelength propagating from a fiber of the first set of optical fibers into a fiber of the second set of optical fibers in coupling contact with the fiber of the first set of optical fibers are less than 0.5% and reflection losses for light at the first wavelength propagating from a fiber of the first set of optical fibers into a fiber of the second set of optical fibers not in coupling contact with the fiber of the first set of optical fibers are less than 0.5%.