Patent Application
20220357516
The present invention is generally directed to optical communications, and more specifically to improved methods for increasing the information transmission capacity for a single optical fiber.
Historically, several steps have been taken to improve the information transmission bandwidth in single mode fiber (SMF) optical communications systems, which are typically used for transmitting information over distances of a kilometer or more. Low transmission loss silica fibers were developed in the late 1970s and early 1980s, permitting the use of silica fibers over greater distances. The advent of erbium-doped fiber amplifiers (EDFAs), providing amplification for signals around 1550 nm, permitted the transmission of signals over even greater distances, while the introduction of wavelength division multiplexing/demultiplexing (WDM) extended the bandwidth of silica fibers by permitting a single mode silica fiber to carry different optical signals at different wavelengths. Optical communication systems have further benefitted from the introduction of advanced techniques such as polarization multiplexing and higher order modulation schemes to increase spectral efficiency (bits/s/Hz). However, current SMF optical transmission systems are now approaching their intrinsic capacity limits, and it is expected that they will be unable to meet future capacity requirements.
One approach being considered for increasing fiber capacity is space division multiplexing (SDM), in which different optical signals are physically (spatially) separated from each other within the same fiber. One particular implementation of SDM is to use a multi-core fiber (MCF), in which a number of different single-mode cores are contained within the same cladding material, laterally separated from each other within the cladding. An important issue for MCF is that crosstalk between cores or modes increases with transmission distance, and/or arises due to bends and fiber imperfections. Extensive digital signal processing is, therefore, needed to perform channel characterization and cope with the crosstalk in a fashion similar to multiple-input multiple-output (MIMO) transmission in radio systems. Furthermore, it is difficult and expensive to manufacture optical fibers having multiple cores within a single cladding. Furthermore, connectivity of the MCF is complicated because the multiple cores require precise rotational alignment of the fiber end about the fiber axis in order for the cores to be aligned to another MCF fiber.
Another proposed implementation of SDM relies on a fiber having a single core with a diameter that is larger than required for single-mode operation and which supports the propagation of a small number of modes. This fiber is referred to as a few-mode fiber (FMF). In a perfectly straight and circularly symmetric fiber, the modal electromagnetic fields do not interact in the sense that the power carried by each mode remains unchanged as the total electromagnetic field propagates in the fiber, thus theoretically each mode can act as an independent transmission channel. However, due to fiber imperfections and/or bends, a mode couples power to other modes, predominantly to those that have similar propagation coefficients. Over long distances, the optical power is likely to be distributed over multiple modes. This can be problematic, however, because a mode couples to a specific linear combination of all FMF modes, and the excitation of another mode couples to a linear combination of all FMF modes that is still orthogonal. With the aid of digital signal processing, the original signals can thus still be recovered. The refractive index profile of a typical FMF has a parabolic shape in the core region, to mitigate differential mode delay, i.e., to assure that the arrival times of all the modes are very similar. This relaxes the requirements on the size of the digital signal processor (DSP) required for signal analysis at the receiver.
Another proposed implementation of SDM relies on optical angular momentum (OAM) multiplexing in a fiber. Difficulties with this approach include the implementation of mode (de)multiplexers having high mode selectivity and avoiding the 1/N insertion loss associated with cascaded beam splitters.
Accordingly, there is a need for improved methods of implementing SDM that can reduce the effects of the problems discussed above. One approach to SDM is to use a fiber with multiple concentric cores. One issue for developing an optical fiber system based around concentric multicore fibers (“concentric MCFs”) is to develop methods of coupling light into and out of such fibers. It would be advantageous to develop efficient methods for coupling light into and out of concentric MCFs, either from/to conventional single core fibers, such as single mode fibers or other concentric MCFs. These coupling methods preferably maintain the advantage of concentric MCFs that, unlike conventional multicore fibers, there is no need to control the axial rotational alignment when connecting fibers. The concentric MCF fiber can, therefore, be implemented in systems using industry-standard connectors, such as SC, LC, MPO and other types of connectors.
This patent application addresses connectivity solutions for implementing a concentric MCF in an optical fiber system.
One embodiment of the invention is directed to an optical device that has a first optical fiber comprising a first central core and at least a second core concentric to the first central core. The first optical fiber has a first end. Light exiting the first central core at the first end propagates along a first axis. Light exiting the second core at the first end propagates along the first axis. A multiplexing/demultiplexing optical coupling unit (mux/demux) is proximate the first end of the first fiber. The mux/demux comprises a first diffractive optical element and a second diffractive optical element arranged so that the light propagating from the first central core is incident on the first diffractive optical element and then incident on the second diffractive optical element and, after passing through the mux/demux, propagates along a second axis. Light from the second core, after passing through the mux/demux, propagates along a third axis that is displaced relative to the second axis.
Another embodiment of the invention is directed to an optical device that has a first fiber having a first end and at least a first core and a second core. The at least a first core and a second core are concentric. A second fiber has a second end and at least a first core. A third fiber has a third end and at least a first core. The device also includes an optical add/drop unit comprising at least one diffractive optical element. The optical add/drop unit is disposed to receive light from the first end of the first fiber and to direct light from one of the first core and the second core of the first fiber into the first core of the second fiber and light from the other of the first core and second core of the first fiber into the first core of the third fiber.
Another embodiment of the invention is an optical device that includes a first fiber having at least an inner concentric core and an outer concentric core. The inner concentric core of the first fiber is closer to an axis of the first fiber than the outer concentric core. The first fiber has a first end. The optical device also includes a second fiber having at least an inner concentric core and an outer concentric core. The inner concentric core is the second fiber is closer to an axis of the second fiber than the outer concentric core of the second fiber. The second fiber has a second end. The optical device also includes an optical coupling unit disposed between the first end of the first fiber and the second end of the second fiber. The optical coupling unit includes at least two diffractive optical elements. Light from the inner concentric core of the first fiber is directed by the optical coupling unit to the outer concentric core of the second fiber, and light from the outer concentric core of the first fiber is directed by the optical coupling unit to the inner concentric core of the second fiber.
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.
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:
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.
An exemplary embodiment of an optical communication system 100 is schematically illustrated in
In this embodiment, the optical communication system 100 is of a space division multiplexing (SDM) design. Optical signals are generated within the transmitter portion 102 and are combined into different modes of a concentric multicore fiber (MCF) 128 in the optical fiber portion 106 and transmitted to the receiver portion 104, where the signals that propagated along different fiber modes are spatially separated and directed to respective detectors. The illustrated embodiment shows an optical communication system 100 that spatially multiplexes four different signals, although it will be appreciated that optical communications systems may spatially multiplex different number of signals, e.g. two, three or more than four.
Transmitter portion 102 has multiple transmitter units 108, 110, 112, 114 producing respective optical signals 116, 118, 120, 122. The optical communication system 100 may operate at any useful wavelength, for example in the range 800-950 nm, or over other wavelength ranges, such as 1250 nm-1350 nm, 1500 nm-1600 nm, or 1600 nm-1650 nm. Each transmitter unit 108, 110, 112, 114 is coupled to the optical fiber system 106 via a space division multiplexed (SDM) multiplexer/demultiplexer (“mux/demux”) 124, that directs the optical signals 116, 118, 120, 122 into respective modes of the concentric MCF 128 of the optical fiber system 106. Embodiments of the concentric MCF 128 and the SDM mux/demux 124 are discussed below. A multiplexer/demultiplexer is an optical device that, for light traveling in one direction, combines signals from two or more fibers into a single fiber and, for light propagating in the opposite direction, splits light from a single fiber into two or more fibers.
The multi-spatial mode optical signal 126 propagates along the optical fiber system 106 to the receiver portion 104, where it is split by a second SDM mux/demux 130 into the optical signals 116, 118, 120, 122 corresponding to the different spatial modes of the concentric MCF 128 that were excited by light from the SDM coupler 124. Thus, according to this embodiment, the transmitter unit 108 produces an optical signal 116, which is transmitted via a first spatial mode of the concentric MCF 128 to the receiver unit 132, the transmitter unit 110 produces an optical signal 118 which is transmitted via a spatial second mode of the concentric MCF 128 to the receiver unit 134, the transmitter unit 112 produces an optical signal 120, which is transmitted via a third spatial mode of the concentric MCF 128 to the receiver unit 136, and the transmitter unit 114 produces an optical signal 122 which is transmitted via a fourth spatial mode of the concentric MCF 128 to the receiver unit 138, with all of the optical signals 116, 118, 120, 122 propagating along the same concentric MCF 128. In this manner, the optical signal 116 may be detected at receiver unit 132 substantially free of optical signals 118, 120 and 122, the optical signal 118 may be detected at receiver unit 134 substantially free of optical signals 116, 120 and 122, the optical signal 120 may be detected at receiver unit 136 substantially free of optical signals 116, 118 and 122, and the optical signal 122 may be detected at receiver unit 138 substantially free of optical signals 116, 118 and 120.
Furthermore, in many optical communications systems there are optical signals propagating in both directions along an optical fiber. This possibility is indicated in
In addition, a signal from a transmitter need not be restricted to only one wavelength. For example, one or more of the transmitter units 108, 110, 112 and 114 may produce respective wavelength division multiplexed signals 116, 118, 120, 122 that propagate along respective modes of the concentric MCF 128. In such a case, the receiver units 132, 134, 136 and 138 may each be equipped with wavelength division demultiplexing units so that the optical signal at one specific wavelength can be detected independently from the optical signals at other wavelengths.
Another embodiment of optical communication system 100′ is schematically illustrated in
For optical signals traveling in the opposite direction, the transceiver 144 may direct signals to the second transmitter unit 110 in the transmitter portion 102, in which case the transmitter units 108, 110, 112, 114 may be transceiver units. Thus, the add/drop filter 140 adds a channel to those propagating from the receiver portion 104 to the transmitter portion 102.
A concentric MCF is an optical fiber that contains two or more concentric volumes of material having a higher refractive index than the immediately surrounding material, designed for different light signals to propagate in a confined manner along each respective concentric volume. For example, the concentric MCF may contain an axial core of relatively high refractive index, surrounded by cylinders of relatively high refractive material, where the volumes of relatively high refractive index material are separated from each other by volumes of relatively low index material. The relatively high and low refractive index material may be, for example, doped or undoped regions of silica glass.
The refractive index profile of one embodiment of a concentric MCF fiber is described with reference to
A concentric MCF can be made using known processes for providing a desired refractive index profile in an optical fiber, such as a silica optical fiber, including chemical vapor deposition techniques such modified chemical vapor deposition (MCVD) or plasma enhanced chemical vapor deposition (PCVD), or processes described in U.S. Pat. No. 6,062,046.
In the particular embodiment of concentric MCF shown in
Other embodiments of concentric MCFs may be employed. For example, there may be more than one concentric cylindrical core surrounding the central core, with different concentric cores positioned at increasing radial distances from the central core. In other embodiments, the values of refractive index in each of the concentric cores need not all be the same. For example, the central core may have a first refractive index and other concentric cores may have refractive indices that are more or less than that of the central core. In other embodiments, the refractive indices of the concentric cores may be highest for the central core and decreasing for concentric cores with increasing radial distance from the central core. In other embodiments still, the refractive index of the cladding material between concentric cores need not be radially uniform. For example, the refractive index of the cladding between the central core and the first cylindrically concentric core may be different from the refractive index between the first cylindrically concentric core and a second cylindrically concentric core. Concentric MCFs are further described in U.S. patent application Ser. No. 15/996,018, filed on Jun. 1, 2018, and the disclosure of which is incorporated herein by reference. The invention is not restricted to the embodiments of concentric MCF specifically described with respect to the figures in U.S. patent application Ser. No. 15/996,018, either to the values of refractive index for the various portions of the fiber, and the concomitant refractive index differences between adjacent fiber regions, nor to the specific radii of the various core and cladding.
Furthermore, it is understood that the change in refractive index between one region of the fiber and another need not be a step index change, but may take place over a non-zero range of radius. Furthermore, in the example discussed above with reference to
A consideration in implementing a concentric MCF fiber in an optical system is the ability to couple different light signals into and out of the concentric MCF. It is desirable that a concentric MCF optic system includes at least the following two functions. The first function is that of a multiplexer/demultiplexer (mux/demux), in which light from two or more single core fibers (“SCFs”) are launched into different cores of a concentric MCF (mux), or the reverse, in which the outputs from different cores of a concentric MCF are directed to the cores of respective SCFs (demux). The second function is that of an add/drop multiplexer, in which light from an SCF is injected into one of the cores of the concentric MCF (add) or light is extracted from one of the cores of the concentric MCF to an SCF (drop) while allowing light in the other cores of the concentric MCF continue propagating. These two functions are standard building blocks for spatially multiplexing and demultiplexing signals from multiple single core fibers, including single mode fibers. Other variations may be useful, however, such as adding or dropping more than one, but not all, of the spatially-multiplexed channels.
In many situations the only difference between a multiplexer and demultiplexer, or between an add filter and a drop filter, is the direction of the light, e.g. from one fiber to many, or from many fibers to one. It will be understood that the propagation of light in many of the optical systems described herein is reversible, i.e. light can travel from a first end of the system to the second end, or from the second end to the first end. For clarity, much of the following description, and the claims, discusses the propagation of light in only one direction. This is not intended to be a limitation on the invention, and it is intended that the description and claims cover optical systems in which light travels in both forwards and reverse directions.
Since one purpose of a concentric MCF fiber is to increase the transmission capacity of fiber, it is often desirable that an optical the optical system that uses a concentric MCF be compatible with wavelength multiplexing and other methods for increasing the transmission capacity of an optical fiber link.
When considering the requirements of a mux/demux, or of an add/drop filter, is it important first to understand how light couples into and out of the concentric MCF.
Some of the optical systems here are useful for coupling light between a concentric MCF and multiple SCFs. One approach for coupling light between cores of a concentric MCF 402 and a number of respective SCFs 404, 406, 408, 410, in other words multiplexing/demultiplexing, is now described with reference to
Diffractive optical elements are fabricated such that transmission through, or reflection from, a diffractive optical element changes the phase of the optical signal at the surface of the DOE. DOEs are typically planar devices and are most often used in transmission. However non-planar DOEs and reflective DOEs may also be used for the same purpose as described herein, with some modification that would be understood by one of ordinary skill. DOEs may also be referred as “digital optical elements” or “holographic optical elements” (HOEs). DOEs may be manufactured by etching a transparent substrate using a series of precise masks, with the resulting surface being an elaborate array of three-dimensional steps that provide the DOE with its optical properties. The fabrication of DOEs takes advantage of the ultra-high precision lithographic techniques that are common in the semiconductor industry. The “digital” nature of many DOEs allows the surface to be patterned in limitless ways, allowing DOEs to accomplish functions that are frequently difficult to achieve with conventional refractive optical elements. Using wafer-scale processing, DOEs can be made in high volumes at low cost. It is also possible to create a “master” DOE using an etching technique, and then to replicate this DOE on polymer substrates using standard microreplication techniques.
It is the flexibility of the patterning of a DOE that allows DOEs to be used in the optical devices for use with concentric MCFs as set forth in this disclosure. Some DOEs are formed with parts having multiple discrete levels or thicknesses, which can be referred to as “digital”, other DOEs are made with levels or thicknesses that vary continuously, in which case the DOE can be referred to as “analog” or “grayscale”. By nature, diffraction depends on wavelength. However, DOEs can be designed to operate efficiently over a wide range of wavelengths. Such achromatic behavior may be advantageous in the present invention, where the wavelength of the light used in the concentric SDM optical system may vary over a range of e.g. 1250 nm-1650 nm.
In this embodiment, the central core 422 of the concentric MCF 402 sustains the propagation of light in a single radial mode. Thus, when the light propagates out of the central core 422, it diffracts in a conical pattern. The intensity distribution of the light from the central core 422 has a spatial distribution that is approximately a Gaussian function of radius. Light emitted from the other cores 424, 426, 428 has the distribution of axially-symmetric “rings” which merge into a cone of light in the far field.
The light from the different concentric cores 422, 424, 426, 428 of the concentric SDM fiber 402 is directed into respective single core fibers (SCFs) 406, 408, 404, 410 via the SDM coupler 400. The single core fibers 404, 406, 408, 410 may be single mode fibers (SMFs). To increase the efficiency of coupling between the concentric SDM fiber and the SCFs, it is preferred that the intensity and phase of light incident on a core match the intensity and phase of light that would be emitted from that core if light were to be passing through the system in the reverse direction. This is true of both single- and multi-core single mode fibers. While a single DOE is able to change the phase of light at the surface of the DOE, resulting in an intensity change in the far field, it is preferred that at least two DOEs 412, 414 are used in the SDM coupler 400 to increase the efficiency of coupling between the concentric SDM fiber 402 and the SCFs. Additional DOEs may be used to further increase coupling efficiency between fibers.
It is useful to consider the optical axes of the various light beams as they propagate from the concentric MCF 402 to their respective SCFs 404-410, with reference to
In another embodiment, schematically illustrated in
In a variation of this embodiment, rather than the SDM coupler 400 having just a single second DOE 414, it may have a number of second DOEs, each one aligned to a respective SCF 404, 406, 408, 410. Furthermore, in certain embodiments, rather than the SDM coupler 400 having a second DOE 414, the SDM coupler 400 may have an array of individual focusing elements, such as lenses, to direct the respective beams from the first DOE 412 to their respective SCFs 404, 406, 408, 410.
Another way of viewing the optical axes described in
The SCF's need not be arranged linearly, and can be arranged in any suitable pattern. For example, in another embodiment, schematically illustrated in
Other arrangements of SCFs are possible. For example, the square arrangement shown in
In another embodiment, the SDM coupler 400 may be used for coupling light from a concentric MCF 402 to another MCF 804 which may be a conventional arrayed-core MCF having an array of non-concentric, single-mode cores 806a-806d, as is schematically illustrated in
An add/drop filter for a concentric MCF may be formed using one, two or more DOEs. In the embodiment schematically illustrated in
In this embodiment, the concentric MCF 902 has 4 concentric cores 924, 926, 928, 930, although it could have more or fewer concentric cores.
In other embodiments, the light from a different concentric core 924, 928, 930 may be directed to the SCF 906 instead of the light from the second concentric core 926. In additional embodiments, the light from more than one core 924, 926, 928, 930 of the concentric MCF 902 may be directed out of the beam directed towards the second concentric MCF. In such a case, the SCF 906 may be replaced by another MCF, for example a third concentric MCF.
It should be appreciated that, although the entrance face to the SCF 906 lies at a distance closer to the second DOE 914 than the entrance face to the second concentric MCF 904, this is not a necessary condition. The two fiber end faces may lie in the same plane, or the entrance face to the SCF 906 may lie at a distance further from the second DOE 914 than the entrance face to the second concentric MCF 904.
Another embodiment of add/drop filter 1000 is now described with reference to
In this embodiment, the concentric MCF 1002 has 4 concentric cores 1024, 1026, 1028, 1030, although it could have more or fewer concentric cores.
In other embodiments, the light from a different concentric core 1024, 1028, 1030 may be directed to the SCF 1006 instead of the light from the second concentric core 1026. In other embodiments, the light from more than one core 924, 926, 928, 930 of the concentric MCF 902 may be directed out of the beam directed towards the second concentric MCF. In such a case, the SCF 906 may be replaced by another MCF, for example a third concentric MCF.
Another embodiment of add/drop filter is now described with reference to
In this embodiment, the concentric MCF 1102 has 4 concentric cores 1124, 1126, 1128, 1130, although it could have more or fewer concentric cores.
In other embodiments, the light from a different concentric core 1124, 1128, 1130 may be directed to the SCF 1106 instead of the light from the second concentric core 1126. In other embodiments, the light from more than one core 1124, 1126, 1128, 1130 of the concentric MCF 1102 may be directed out of the beam directed towards the second concentric MCF. In such a case, the SCF 1106 may be supplemented by another SCF.
Another embodiment of an SDM coupler 1200 for concentric MCFs is schematically illustrated in
In this particular embodiment, the coupler 1220 directs light from cores of the first MCF 1202 to different cores of the second MCF 1204. In other words, rather than simply mapping light from core 1 of the first concentric MCF 1202 to core 1 of the second concentric MCF 1204, it maps from core 1 of the first concentric MCF 1202 to a different core of the second concentric MCF 1204, for example core 4. Thus, in one particular example, the SDM coupler 1200 may direct light between the pairs of cores shown in the following table:
This type of SDM coupler may be useful, for example, to balance out the different speeds of light in the different concentric cores. For example, swapping the light from cores 1, 2, 3, and 4 in the first concentric MCF 1202 respectively to cores 4, 3, 2, and 1 of the second MCF 1204 midway along a path between a starting point and an end point, may reduce skew.
Of course, this is simply one example of mapping light signals between cores of different concentric MCFs, and other mappings may be used, such as from cores 1, 2, 3, 4 respectively to cores 2, 1, 4, 3; or from cores 1, 2, 3, 4, to cores 2, 3, 4, 1, and the like.
In a further embodiment, a concentric MCF path between a start point and an end point may include a number of such couplers that cycle each optical signal through each of the four cores, so that every optical signal is exposed to the same dispersion along the length of the concentric MCF transmission path between the start point and the end point. Thus, in one embodiment, the couplers may cycle the optical signals from cores 1, 2, 3, 4 respectively to cores 2, 3, 4, 1, and then to cores 3, 4, 1, 2, and then to 4, 1, 2, 3 before arriving at the transmission end point.
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. For example, although many of the examples provided herein describe concentric MCFs having four concentric cores, the invention is intended to cover systems that use concentric MCFs having different numbers of concentric cores. In such cases, the number of corresponding SCFs may also change. Likewise, the couplers and filters have been described as using a certain number of DOEs. It may be, however, that the couplers and filters include different numbers of DOEs.
In the various embodiments of devices discussed herein, the DOEs have been shown separated from the optical fibers by an air gap and operating in the far field. This need not be the case, and the DOE may be attached directly to a fiber face. In such a case the alignment between a DOE and its respective fiber, or fibers, may be maintained through the use of an optically transparent adhesive. This approach reduces the number of air/dielectric interfaces and, thus, may reduce reflective losses in the device. Furthermore, the distance between the fiber end face and the patterned surface of the DOE may be controlled by careful selection of the thickness of the DOE substrate. Under this approach, the DOE may operate in the near-field of the concentric MCF.
Furthermore, the embodiments of the present invention described above have used a single concentric MCF as an input. However, this is not intended to be a limit of the invention, and there could be multiple concentric MCFs. For example, if a concentric MCF has M concentric cores, then an SDM coupler can couple to M SCFs. If there are N concentric fibers, then these can couple to N×M SCFs.
In other embodiments, the DOEs may be configured to transfer light between a conventional MCF, having single mode cores arranged in an array, instead of a concentric MCF, and multiple single mode fibers, either in a mux/demux configuration, add/drop configuration, or other configurations.
Finally, the description of the various DOE-based devices for use with concentric MCFs primarily described the optical signals propagating in a single direction, mainly from the concentric MCF on the left of the figure towards the various components on the right of the figure. It will be understood, of course, that optical signals may also be propagated in the opposite directions, and there is no intention in the present description to limit the direction in which optical signal propagate through the claimed optical devices.
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.
This application is being filed on Jun. 19, 2020 as a PCT International Patent Application and claims the benefit of U.S. Patent Application Ser. No. 62/864,774, filed on Jun. 21, 2019, the disclosure of which is incorporated herein by reference in its entirety.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2020/038750 | 6/19/2020 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 62864774 | Jun 2019 | US |
