The invention relates to optical multimode fiber (MMF) communications links over which data in the form of optical signals is transmitted and received over MMFs. More particularly, the invention relates to methods and apparatuses for improving the channel capacity of optical MMF links.
In optical MMF links, optical transceivers are used to transmit and receive optical signals over optical MMFs. An optical transceiver generates amplitude and/or phase and/or polarization modulated optical signals that represent data, which are then transmitted over an optical MMF coupled to the transceiver. Each optical transceiver includes a transmitter side and a receiver side. On the transmitter side of the optical transceiver, a laser light source generates the optical data signals based on a received electrical data signal and an optical coupling system optically couples, or images, the light onto an end facet of an optical fiber. The laser light source typically is made up of one or more laser diodes that generate light of a particular wavelength or wavelength range. The optical coupling system typically includes one or more reflective, refractive and/or diffractive elements. On the receiver side of the optical transceiver, a photodiode detects an optical data signal transmitted over an MMF and converts the optical data signal into an electrical data signal, which is then amplified and processed by electrical circuitry of the receiver side to recover the data.
The majority of the optical fibers that have been installed within buildings are MMFs. MMFs were originally designed for use with light emitting diodes (LEDs) as the light sources. Prior to single mode optical fibers being widely deployed for use in high bit rate, long distance links, a demand for increased channel capacity led to much work being carried out between the late 1970 s and the early 1980 s to improve the performance of optical MMF links. Although much of the development work has ceased, MMFs continue to be used in optical links that operate at lower bit rates and over shorter distances. For example, in building or local area networks, a large installed base of MMFs exist, which represents a significant investment.
In recent years, the demand for high data rate (e.g., 10 Gigabits per second (Gb/s) and higher) local area networks has increased dramatically. Thus, even though an MMF may only be utilised over short distances (e.g., 500 meters (m)), the required data rates cannot be achieved by utilising conventional techniques. In such links, certain link performance characteristics, such as the link transmission distance, for example, are dependent in part on the design of the optical coupling system, the modal bandwidth of the fiber, and the relative intensity noise (RIN) of the laser diode. The modal bandwidth of the fiber and the RIN of the laser diode can be affected by the launch conditions of the laser light into the end of the MMF. The launch conditions are, in turn, dependent upon the properties of the laser diode itself and upon the design and configuration of the optical coupling system. Due to limitations on the manufacturability of optical elements that are typically used in optical coupling systems, the ability to control the launch conditions is limited primarily to designing and configuring the optical coupling system to control the manner in which it optically couples the light from the laser diode onto the entrance facet of the MMF.
Launch techniques such as Center Launch (CL) techniques, Offset Launch (OSL) techniques, or a combination the two, called dual launch (DL) techniques, are known to significantly increase the modal bandwidth of MMF links. For this reason, these launch techniques have been standardized for 10 Gigabit Ethernet links. However, at higher data rates, such as, for example, 40 Gb/s, these launch techniques do not create a sufficient increase in the modal bandwidth of an MMF optical link. Hence, a need exists for a new launch technique that provides MMF optical links with even higher modal bandwidths. One method that is sometimes used to provide an MMF optical link with an increased modal bandwidth is to excite only a small number of fiber mode groups in the MMF. For example, various attempts have been made to excite the lowest-order mode group in MMFs in order to increase the modal bandwidth of the link. However, such attempts generally use CL techniques, which do result in significant increases in modal bandwidth, but only if strict tolerance requirements are met. It has also been proposed to use mode filters in the receivers of the links to increase the modal bandwidth of the links, but mode filters often introduce excessive modal noise into the links.
In order to overcome some of these issues, launch techniques have been proposed that selectively excite one or more higher-order mode groups in an MMF of an optical link in order to increase the bandwidth of the MMF optical link. For example, it is well known that spiral launch techniques can be used to target higher-order mode groups in an MMF, and the use of such techniques have been proposed as part of the 10 GBASE-LRM standards process. Indeed, the use of spiral launch techniques remains a valid approach to increasing the bandwidth of an MMF optical link. Spiral launch techniques target the Laguerre Gaussian (LG) mode groups in the MMF and use a radial phase mask that is matched to a particular LG mode group of the MMF. However, there is reason to believe that spiral launch techniques may not provide significant tolerance to connector offsets. In other words, if the connector that connects the MMF to the receptacle of the optical transceiver is offset in any radial direction relative to the receptacle such that a degree of optical misalignment is introduced into the launch, a radial phase mismatch may exist between the phase of the LG mode group of the MMF that is being targeted and the phase of the light that is being launched into the entrance facet of the MMF. Due to this radial phase mismatch, the target LG mode group may not be sufficiently excited and/or other non-targeted LG mode groups of the MMF may be excited. The consequence of these unintended results may be a failure to sufficiently increase the modal bandwidth of the MMF optical link.
Launch techniques have also been proposed that excite Hermite Gaussian (HMG) mode groups in an MMF. For example, a number of known launch techniques have been proposed for exciting higher-order HMG mode groups, including, for example, techniques that (1) laterally offset the laser beam being launched into the entrance facet of an MMF, (2) angularly offsetting the laser beam being launched into the entrance facet of an MMF, or (3) use masks that match the amplitude and phase of the launched laser beam with the amplitude and phase of a targeted HMG mode group of the MMF. However, with all of the known launch techniques for exciting higher-order HMG mode groups, alignment is critical in that unintended misalignment between the launched light beam and the entrance facet of the MMF can produce undesired results. Consequently, the proposed launch techniques for exciting higher-order HMG mode groups in an MMF generally do not provide for greater connector offset tolerance. In addition, the task of manufacturing masks that match both amplitude and phase can be relatively difficult.
A need exists for a launch technique that is capable of exciting one or more higher-order mode groups of an MMF in order to increase the bandwidth of the MMF optical link. A need also exists for such a launch technique that provides the desired effect of increasing link bandwidth without increasing modal noise in the MMF optical link. A further need exists for such a launch technique that achieves these goals and, at the same time, that obviates the aforementioned problems associated with connector offsets.
The invention is directed to an apparatus and method for providing a line launch of light into an entrance facet of an MMF of an optical communications link to cause at least one targeted HMG mode group of the MMF to be excited. The invention is also directed to an optical link in which the method and apparatus are implemented. The apparatus comprises a laser and an optics system. The laser produces laser light. The optics system receives the laser light. The optics system includes an HMG beam converter that converts the received laser light into a line of HMG spots, which, if launched into an entrance facet of an MMF, causes at least one targeted HMG mode group of the MMF to be excited. Excitation of the targeted HMG mode group causes the effective modal bandwidth of the MMF to be increased.
The method comprises the following: with a laser, producing laser light; in an optics system, receiving the laser light; with an HMG beam converter of the optics system, converting the received laser light into a line of HMG spots. The method further comprises, with an MMF that is optically coupled to the optics system, receiving the line of spots at an entrance facet of the MMF to cause at least one HMG mode group of the MMF to be selectively excited.
The method and apparatus may also be used in conjunction with an apparatus that performs a known center launch technique, in which case at least one higher-order HMG mode group and at least one lower order LG mode group are excited in the link MMF. A mode group demultiplexer (DeMux) is used to separate the higher-order HMG mode group(s) from the lower-order LG mode group(s), which are then coupled to respective optical receivers.
These and other features and advantages of the invention will become apparent from the following description, drawings and claims.
The invention is directed to a method and an apparatus for launching light into an entrance facet of a MMF of an MMF optical link in a way that excites one or more targeted higher-order mode groups in the MMF. The light is launched into the entrance facet of the MMF as a line of phase-modulated spots. Because the light is launched into the entrance facet of the MMF as a line of multiple phase-modulated spots, the launch performed in accordance with the invention will be referred to hereinafter as a “line launch”. The line launch causes one or more targeted higher-order mode groups to be excited in the MMF. The use of the line launch to excite one or more higher-order mode groups in the MMF increases the bandwidth of the link and allows overall link lengths to be increased. In addition, the use of the line launch is reliable and robust despite defects in the MMF and despite connector offsets. Thus, the use of the line launch ensures that a sufficient increase in link bandwidth will be achieved despite the existence of defects in the MMF and even if there is some amount of optical misalignment due to the connector being offset relative to the corresponding receptacle.
In accordance with the illustrative, or exemplary, embodiments, the line launch targets one or more higher-order HMG mode groups. HMG mode groups are relatively insensitive to launch offsets that are orthogonal to their axes, which are generally coaxial with the optical axis of the MMF in which the HMG mode groups propagate. For this reason, using the line launch technique to selectively excite one or more higher-order HMG mode groups of an MMF increases the bandwidth of an MMF optical link, while, at the same time, providing at least some tolerance to connector offsets.
The operation of the line launch is best understood using a mode theory for an MMF. The basic mode calculation will therefore now be described and used to develop a solution for the line launch. In solving the scalar wave equation for the guided spatial modes in MMFs, a set of mathematical expressions are developed for the launch transverse electric field that enables a distinct excitation of a particular mode group to occur. The properties of light propagation along an MMF are determined by its core diameter as well as its refractive index profile. For the initial calculations, it is assumed that the refractive index profile of a typical graded-index MMF can be generalized as a power-law function:
where n1 is the axial refractive index at the core centre, r is the radial distance from the core centre, a is the core radius, α is the alpha profile parameter, and Δ=(n12−n22)/2n12 is the relative refractive index difference between core and cladding for a cladding refractive index n2. An infinite alpha profile parameter α≈∞ gives a step refractive index profile; whereas α≈2 is typically used for graded-index MMFs. For the analysis of graded-index MMFs, the Helmholtz wave equation in polar coordinate is used:
where ∇2 is the Laplacian operator.
The propagation of signal at carrier frequency ω0 along the z-axis in an MMF is the superposition of multiple time-harmonic, electric-field plane waves of mode (μ, ν), described by the following phasor notation expression
E(r, θ, z, t)ΣêμνE(t)ψμ(r)exp(iνθ)exp[i(ω0t−βμνz)] Eq. (3)
where êμν, is the unit vector in the direction of the electric field, E(t) is the signal envelope, ψμ(r) is the unitless normalized transverse profile in a radial order μ, and βμν is propagation constant. The constituent plane waves that define E(r, θ, z, t) are uniquely defined by βμν and a corresponding orthonormal basis set ψμ(r)exp(iνθ) which has an oscillatory behavior within the fiber core in an azimuthal order ν. By expending the Laplacian operator with the Fourier decomposited optical field in polar coordinate, the Helmholtz wave equation can be simplified to a scalar wave equation when the time-varying envelope Eμ(t) is small compared with the carrier frequency ω0 [20],
For signal modulation at a rate smaller than 100 Gb/s, the signal envelope Eμ(t) varies at least four orders of magnitude slower than the carrier frequency ω0 in the near infrared regime. Hence, the slowly varying approximation holds in arriving at Eq. (4). Modern graded-index MMFs used for data communication applications are approximately square law media in which the optimum refractive index profile for minimal mode dispersion is α≈2. However the actual value typically ranges between 1.8 and 2.2 depending on the material and the optimized operating wavelength. When the relative refractive index difference is small Δ<<1, the mode field distribution can be modeled, to at least a first approximation, as the field distribution of the modes of a square law medium (α=2) which can be solved analytically. For a square law medium, the scale wave equation can be written as [21]
Eq. (5) is in the form of Whittaker's equation. The only solution to this equation for which ψμ(0) is finite can be expressed as a confluent hypergeometric function [21,22] which is bounded as r→∞ when
Hence, the exact solutions to the wave equation can be written in a form of the associated Laguerre function Lμ(ν)(x) as
where the associated Laguerre polynomials Lμ(ν)(x) are defined as
For α≈2 and Δ<<1, linearly polarized modes may be used, where the transverse fields are represented to a good approximation by Laguerre-Gaussian functions. The associated Laguerre polynomial function in Eq. (8) in polar coordinate may be expressed in terms of Hermite polynomial in Cartesian coordinate using x=r cos θ and y=r sin θ [23]
from which the Hermite polynomials Hm(x) to be used in our discussion are defined as
where m is the order of the Hermite polynomial and x is the displacement. The expression └.┘ in Eq. (10) rounds the value to the nearest integers less than or equal to itself.
The modal excitation from a given launch field to a multimode fiber is determined by calculating the power coupling into each fiber mode through the overlap integral of the electric fields of the incident beam Ein and the respective fiber mode Eμν,
in polar coordinate. The symbol * in (11) denotes the complex conjugate. Equation (9) shows that the associated Laguerre polynomial (as the basic transverse mode set for graded-index fiber) can be expressed in the terms of Hermite polynomial. For this reason, to achieve selective mode group excitation, Hermite-Gaussian beams are used as a basic set for generating the launch field. The Hermite-Gaussian transverse electric field in polar coordinates is defined as:
where w0=√{square root over (2/κ)}=√{square root over (a/(k0n1)}√{square root over (Δ)}) is the waist of the fundamental mode in an ideal graded-index multimode fiber. A higher order Hermite-Gaussian mode (m>0) is considered in one direction; whereas a fundamental Gaussian mode (n=0) is assumed for the other. The resultant two-dimensional electric field distribution is, therefore, written as
For a given azimuthal order ν and radial order μ, a linearly polarized mode degenerates into four possible forms accounting two polarizations Ex, Ey and two orientations cos υθ, sin υθ. It is assumed that the coupling efficiency is independent of the state-of-polarization and that the MMF is radially symmetric. Hence, by taking the real part of Eq. (7) without loss of generality, the electric field of the guided transverse modes can be written as:
By expressing the Hermite polynomials in terms of Laguerre polynomials, a Hermite-Gaussian beam in Eq. (12) may be expressed in terms of a set of Laguerre-Gaussian beam with the same mode group order,
for even m and n,
for odd m and even n,
for even m and odd n,
for odd m and n. In all cases, the mode group order of the guided transverse modes remains the same as m+n+1. Consequently, by setting the incident beam Ein to a Hermite-Gaussian beam Emn, only those guided modes with the same mode group order as m+n+1 will have non-zero values in the overlap integral in Eq. (11) given the orthogonal property of the guided transverse modes. In other words, the excited mode group in a MMF is determined solely by the order of the incident Hermite-Gaussian beam. As different modes in a mode group propagate at a similar speed, this method of selectively exciting a target mode group effectively enables single-mode operation in a MMF, thereby providing an improved effective modal bandwidth that is near the ultimate transmission limit of the link.
The laser diode 111 receives an electrical input signal, IN, which is typically an electrical data signal, and produces an optical output signal, OUT, which is typically an optical data signal. The optical signal OUT is coupled into the entrance facet of the single mode optical fiber stub 112. The optical signal OUT is then output from an exit facet of the stub 112 and coupled into the optical coupling system 140. The optical coupling system 140 includes a light collimator 140a, such as a collimating lens, and an HMG beam converter 140b, which may take on any one of a variety of forms, as will be described below in more detail. The light collimator 140a collimates the optical signal output from the exit facet of the stub 112. The HMG beam converter 140b receives the collimated light beam and converts the collimated light beam into a line of collimated spots such as that depicted in
The aforementioned line of spots may be produced in a variety of ways. For example, the HMG line of spots may be generated by either a far-field approach using lenses and a holographic technique, or by near-field beam shaping using a single-to-multi-mode fiber coupling scenario of the type depicted in and described above with reference to
With reference again to
The manner in which the HMG beam converter 140b may be fabricated in accordance with an illustrative or exemplary embodiment is described with reference to
In addition, a study of the tolerance to the differential phase difference and consequently the differential etch depth was also carried out for an OM1 fibre. In this study, an ideal line launch without launch offset was considered, and an extra phase term exp(iΔε) is added to Eq. (13). The calculated mode group power extinction degradation was plotted (not shown) for OM1 fibres. The degradation is defined as the difference in the mode group power extinction ratio from the ideal case of Δε=0. The results indicated that a phase error Δε of ±0.3π can be supported for a mode group power extinction ratio degradation smaller than 3 dB. This ±0.3π differential phase difference tolerance corresponds to an acceptable etch depth range of 450±150 nanometers (nm) in fused silica.
Typical laser sources for optical communications have a single transverse mode and are single-mode fiber-coupled, which yields an ideal Gaussian shape in the output beam profile. The generation of the HMG beam for the line launch implementation, therefore, becomes a problem of converting the input Gaussian beam into a HMG beam of a specific order in one direction whilst retaining the Gaussian profile in the other direction. A simple solution has been developed in accordance with embodiments of the invention to realize the proposed line launch by converting a Gaussian beam into the desired HMG beam profile so that the beam converter 140b, functioning as an intermediate connection between the optical Tx 110 and the fiber plant, MMF 101, can be used to achieve the line launch implementation of the type shown in
A experiment was conducted to demonstrate the principles and concepts of the overall line launch using the setup shown in
Thus, it has been shown that the line launch technique of the invention takes advantage of one of the observations of the invention; in particular, that the phase profile of the launch beam is the most important parameter for high bandwidth operation. Consequently, an exact HMG intensity distribution is not required in the line launch implementation. Therefore, in accordance with the invention, a simple near-field approach has been developed to realize a practical line launch using a binary phase mask and a binary intensity mask, which are implemented in a single binary phase and intensity mask. The combined phase and intensity mask, which comprises the HMG beam converter 140B shown in
The frequency responses for the case of the line launch are smooth over a wide frequency range. Measured results indicate a strong evidence for a single mode group excitation in which the excited mode group is determined by the order of the line launch. In addition, the measured frequency response under line launch remains smooth over the frequency range of interest even though the measured 3 dBo bandwidth drops as the launch offset increases. This unique characteristic can only be realised by the phase-modulated line launch of the invention and is not observed for centre launches or offset launches. In addition, the smooth frequency response associated with the line launch obviates the need to perform equalization in the optical Rx, or at the very least simplifies the complexity of electronic dispersion compensation circuits used at high bit rate operation. For example, the absence of deep nulls in the frequency response enables the use of simple zero forcing equalisers. Furthermore, the improvement in the effective modal bandwidth provided by the line launch technique enables the length of the optical MMF link to be substantially increased (e.g., three times as long) as that which may be achievable using centre and offset launch techniques.
The connector 200 has a body 210 having a front face plate 210a and a back face plate (not shown), which are identical in construction. The connector 200 is similar in construction to a conventional fiber connector/physical contact (FC/PC) square adapter except that the connector 200 has multiple keyways, as described above, whereas the conventional FC/PC adapter has a single keyway and is only connectable to an end of an optical fiber cable in a single position. In the interest of brevity and for ease of illustration, only the front face plate 210a is illustrated in the drawings and described herein, as the back face plate (not shown) is substantially identical in construction to the front face plate 210a.
The front face plate 210a and the back face plate (not shown) are connected together via fastening devices 211a-211d that mate with threaded holes 212a-212d, respectively, formed in the face plates. The front face plate 210a has inner connector portions 213a, 213b and 213c formed thereon and an outer connector portion 214 formed thereon, which come into contact with respective connector portions (not shown) disposed on the end of the optical fiber cable (not shown) that couples to the connector 200. In accordance with this embodiment, the components 210a, 211a-211d, 212a-212d, and 214 are identical to the corresponding components of a conventional FC/PC square adapter. The term “square” is used in the industry to denote that the face plates are generally square in shape, resulting in the shape of the adapter being generally square in shape. The connector 200, however, also includes a multi-way keyway component 220 that provides multiple keyways 220a-220d as compared to the corresponding keyway component of the conventional FC/PC square adapter, which provides a single keyway, as will now be described with reference to
It should be noted that although a four-way connector 200 has been described herein, the principles and concepts of the invention apply to any multi-way connector configuration. For example, instead of having four cylindrical portions 220a and four flat portions 220b, the keyway component may have eight cylindrical portions 220a and eight flat portions 220b, in which case each cylindrical portion 220a would be separated from its nearest neighbouring cylindrical portions 220a by 45°. In the latter case, a cable could be connected to the connector 200 in any one of eight radial positions. As another example, instead of having four cylindrical portions 220a and four flat portions 220b, the keyway component may have two cylindrical portions 220a and two flat portions 220b, in which case each cylindrical portion 220a would be separated from its nearest neighbouring cylindrical portions 220a by 180°. In the latter case, a cable could be connected to the connector 200 in either one of two radial positions.
The arrangement of the optical link 300 enables higher bandwidths and longer link lengths to be achieved due to the reduced modal dispersion caused by exciting only targeted mode groups. For example, each optical channel may have a bandwidth of 10 Gb/s, thereby resulting in an overall link bandwidth of 20 Gb/s. The length of the link MMF 306 can be at least about 220 meters (m) for OM1 MMF fiber, for example.
Most of the optical energy associated with the lower order mode groups remains generally coaxial with the optical axis of the MMF 306 than that associated with the higher order mode groups as the optical energy propagates through free space inside of the mode group DeMux 310. Therefore, the optical energy associated with the lower order mode groups simply propagates from the connector 325 to the connector 326. From the connector 326, the optical energy associated with the lower order mode groups is coupled into the fiber 311 and propagates along the fiber 311 to the optical Rx 314 where it is received. The optical energy associated with the higher order mode groups propagates in directions that are at greater angles to the optical axis of the MMF 306. Optical elements 330a and 330b that are partially or wholly reflective are positioned to receive the optical signals associated with the higher order mode groups and to direct the optical signals toward the connector 327. The connector 327 couples the optical signals into the fiber 312, which carries the signals to the optical Rx 315.
It should be noted that the configuration of the mode group Demux 310 shown in
The invention has been described with reference to preferred or illustrative embodiments for the purposes of demonstrating the principles and concepts of the invention. Those skilled in the art, however, will understand that the invention is not limited to these demonstrative embodiments. A variety of modifications can be made to the embodiments described above without deviating from the scope of the invention.
This application is a continuation-in-part of, and claims the benefit of the filing date of, provisional application Ser. No. 61/286,980, which was filed in the United State Patent and Trademark Office on Dec. 16, 2009, entitled “METHOD AND APPARATUS FOR PROVIDING A LINE OF SPOTS LAUNCH OF LIGHT INTO AN END OF A MULTIMODE OPTICAL FIBER”, which is hereby incorporated in its entirety.
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/IB10/00839 | 3/19/2010 | WO | 00 | 6/15/2012 |
Number | Date | Country | |
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61286980 | Dec 2009 | US |