Abstract:
A 2-dimensional grating generates multi-mode light from two or more single-mode inputs. One or more waveguides couple the light from the single-mode inputs to the 2-dimensional grating. The 2-dimensional grading couples and transmits the generated multi-mode light to an optical fiber configured to support multiple modes.

Description:
[0001]    REFERENCE TO RELATED APPLICATIONS 
         [0002]    This application claims the benefit of U.S. Provisional Patent Application No. 62/327,981 filed Apr. 26, 2016, and also claims the benefit of U.S. Provisional Patent Application No. 62/327,977 filed Apr. 26, 2016, and also claims the benefit of Australian Provisional Application No. 2015901995 filed May 29, 2015, and also claims the benefit of Australian Provisional Application No. 2015901395 filed Apr. 19, 2015, all of which are herein incorporated by reference. 
       TECHNICAL FIELD 
       [0003]    The subject application teaches embodiments that relate generally to mode division multiplexing over a fiber optic channel, and specifically to a circuit for emitting and detecting multiple modes simultaneously on multi-mode fiber. 
       SUMMARY 
       [0004]    In an example embodiment, a method includes receiving light from two or more single-mode fiber inputs, coupling the received light to a 2-dimensional grating using one or more waveguides, and generating multi-mode light by the 2-dimensional grating. The multi-mode light is coupled and transmitted to an optical fiber that is configured to support multi-mode light. 
         [0005]    In an example embodiment, a system includes first and second single-mode fiber inputs, and one or more waveguides to couple light from the single-mode fiber inputs to a 2-dimensional grating. The 2-dimensional grating is configured to generate multi-mode light from the received single-mode light and couple the generated multi-mode light to an optical fiber configured to support multiple modes. 
       BACKGROUND 
       [0006]    Rapid growth in various forms of communications through all available data transmission networks using current single-mode fibre over the next decade is expected to exceed the data transport capacity of optical fibre links which currently utilize time, frequency and optical wavelength domain multiplexing techniques. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a diagram of a comparison of OFDM, WDM &amp; TDM waves. 
           [0008]      FIG. 2  is a diagram of guided modes in a step-index optical fibre versus the normalized light frequency according to an embodiment of the disclosure. 
           [0009]      FIG. 3  is a diagram of the electrical field E Z  of the LP 01  mode according to an embodiment of the disclosure. 
           [0010]      FIG. 4  is a diagram of electrical field E Z  of the LP 11A  mode, and where the field of the LP 11B  mode is rotated by π/2 according to an embodiment of the disclosure. 
           [0011]      FIG. 5  is a diagram of mode multiplexing utilizing a bulk optics device according to an embodiment of the disclosure. 
           [0012]      FIG. 6  is a diagram of a polarization splitter utilizing a light-splitting grating coupler according to an embodiment of the disclosure. 
           [0013]      FIG. 7  is a diagram of a multi-channel integrated optical circuit device which multiplexing light in to several modes according to an embodiment of the disclosure. 
           [0014]      FIG. 7   bis  is a diagram of a system for generating a bipolar LP 11  mode field from a unipolar LP 01  mode field according to an embodiment of the disclosure. 
           [0015]      FIG. 8  is a diagram of an internal schematic of a first multi-channel integrated optical circuit for mode multiplexing according to an embodiment of the disclosure. 
           [0016]      FIG. 9  is a diagram of a second multi-channel integrated optical circuit for mode multiplexing according to an embodiment of the disclosure. 
           [0017]      FIG. 10  is a diagram of a third multi-channel integrated optical circuit for mode multiplexing according to an embodiment of the disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    The systems and methods disclosed herein are described in detail by way of examples and with reference to the figures. It will be appreciated that modifications to disclosed and described examples, arrangements, configurations, components, elements, apparatuses, devices methods, systems, etc. can suitably be made and may be desired for a specific application. In this disclosure, any identification of specific techniques, arrangements, etc. are either related to a specific example presented or are merely a general description of such a technique, arrangement, etc. Identifications of specific details or examples are not intended to be, and should not be, construed as mandatory or limiting unless specifically designated as such. 
         [0019]    Single-mode fibre is the most used optical fibre type in today&#39;s data transmission and communications systems. Single-mode fibre is used beyond its so-called cut-off wavelength, where it guides only one mode, the LP 01  mode. This single mode may be used in two orthogonal (i.e., at a 90° angle) polarization states, thereby providing two independent channels. This process is referred to as polarization multiplexing (PolMUX). (See  FIG. 1 ). 
         [0020]    The systems and methods disclosed herein extend this capacity using a new methodology for multiplexing. Such a methodology is created by the existence of multiple modes that are guided in an optical fibre when it is used below its cut-off wavelength. The disclosed system and method offers a stable and compact solution for exciting and detecting individual light mode fields in multi-mode fibre field environment. 
         [0021]    Solutions for exciting and detecting individual mode fields in a fibre can use bulk optics (See  FIG. 5 ), which are complex to fabricate and generally unstable. 
         [0022]    Time-division multiplexing (TDM), or optical time-division multiplexing (OTDM), are methods of transmitting and receiving independent signals over a common signal path by means of synchronized switches at each end of the transmission line so that each signal appears on the line only a fraction of time later in an alternating pattern. (See  FIG. 1 ) 
         [0023]    Wavelength division multiplexing (WDM) is a technique where optical signals with different wavelengths are combined, transmitted together, and separated again. No time synchronization among these signals is needed. It is primarily used to transmit data separately in several channels with slightly different wavelengths. It may also be used to submit a signal in parts in many channels simultaneously, with due synchronization among these signal parts. 
         [0024]    Systems and methods of the present disclosure enable one to excite multiple light modes (i.e., light beams) in the fibre at the transmitting end, and to separate them at the receiving end. 
         [0025]    Pursuant to Maxwell&#39;s field equations, fibre typically guides light in various mode field patterns (See  FIG. 2 ). For a weakly guiding fibre, the mode solutions which follow from solving the relevant Maxwell&#39;s field equation, where n 1 /n 2 ≈1, may be grouped in linearly polarized (LP) modes. 
         [0026]    The fundamental mode (HE 11 ) equals the LP 01  mode, and has no cut-off. Stated another way, light is guided for every normalized frequency V, and thus for every wavelength λ. When increasing V beyond 2.405, which is equivalent with lowering the wavelength below the cut-off wavelength corresponding with V=2.405, the fibre will guide multiple modes; when increasing V, these additional modes will subsequently be the LP 11  mode, LP 21  mode, LP 02  mode, LP 31  mode, L 12  mode, etc. 
         [0027]    In an embodiment, a miniaturized integrated circuit significantly increases the speed of data transmission in fiber optic systems. It is particularly applicable to overcoming the limitations of fiber optic digital communication systems which utilize a single transmission mode optical fiber. Historical systems rely on time (or frequency) division multiplexing, wavelength division multiplexing, or a combination of both, to increase data transmission capacity. (See  FIG. 2 ) 
         [0028]    The present disclosure is applicable to high data rate transmission, such as the transmission of conventional or high definition video, but is equally applicable to transmission of any form of digital information, including but not limited to, real time sharing of medical &amp; scientific research; defense response systems; corporate data sharing; security monitoring; real time sharing of educational libraries; personal medical information &amp; condition tracking &amp; integration; home &amp; commercial robotic control systems to assist the elderly or infirmed stay in their homes; 3-D (i.e., holographic) telecommunications; field to fridge produce/consumables tracking &amp; automated reordering; machine-to-machine communications systems; home &amp; office ambience video walls (i.e., video walls that real-time broadcast a live scene from another place in the world); enhanced internet search results; and the spectrum of commercial and non-commercial internet based enterprises that exist currently and will be created in the future. 
         [0029]    Through mode division multiplexing (MDM), different spatial modes in a multi-mode fiber can serve as discrete communication channels. This possibility has not been exploited in practice yet. Current systems typically rely on single mode fiber. 
         [0030]    Mode division multiplexing requires technology for combining several data streams into different modes of a multi-mode fiber (i.e., multiplexing), and for receiving a signal from a multi-mode fiber and separating out individual mode signals (i.e., de-multiplexing). 
         [0031]    The MDM function using a bulk optics approach (See  FIG. 5 ) is costly and has difficulty providing the required performance in a stable way. Therefore, such MDM technology has not to date been commercially deployed. 
         [0032]    In an embodiment of the disclosure, an optical integrated circuit (IC) provides MDM multiplexers and de-multiplexers. These optical ICs are reciprocal devices, so the same device can be made to function as both multiplexer and de-multiplexer, depending on how it is configured. 
         [0033]    Optical ICs according to the present approach guide several on-chip optical signals in distinct single mode waveguides. These waveguides feed the signals to two-dimensional (2-D) grating couplers which are placed so that light is radiated from the waveguides and emitted in two distinct polarization states from the surface of the optical IC. 
         [0034]    When V&gt;2.405, the LP 11  mode (comprised of TE 01 , HE 21  and TM 01  modes) light can likewise be guided in the fibre. When the wavelength of light is further lowered, V likewise increases, resulting in more and more modes being guided. The integrated mode multiplexer circuit according to the disclosure (See  FIG. 7 ) has multiple single-mode input fibres, of which the LP 01  fields are transformed by the circuit respectively into LP 01 , LP 11A  and LP 11B  modes. These modes are subsequently multiplexed and launched into the output multimode fibre. The LP 11  mode has a bipolar electrical field, as shown in  FIG. 4 . The transformation of the input unipolar LP 01  mode into a bipolar LP 11  mode is according the disclosure done by the converter circuit shown in  FIG. 7 . bis.    
         [0035]    The launching into the multi-mode fiber can be accomplished by vertically emitting 2D grating couplers. The 2-D grating couplers are arranged on the IC surface and coupled to the waveguides so that each waveguide signal creates a pattern of emitted radiation (i.e., light) from the grating couplers that is an approximate match to the multi-mode fibre, in amplitude, phase and/or polarization. Each waveguide is thus associated with a distinct mode of the fiber, thereby providing the multiplexing function. 
         [0036]    This same structure can provide de-multiplexing, with input radiation (i.e., light beam) from the multi-mode fiber being received by the 2-D grating couplers and eventually divided among the on-chip single mode waveguides such that each waveguide signal is associated with a corresponding mode signal on the multi-mode fiber. 
         [0037]    Although not an exhaustive list, ‘Alternative Embodiments’ describes examples of this approach, where the multi-mode fiber has 3 spatial modes (LP 01 , LP 11A , L 11B ) with 2 polarization states each in order to provide a total of 6 independent channels in the current embodiment. The two inputs of the 2D grating coupler each launch a polarization state. 
         [0038]    In an embodiment, a multi-mode fiber with several independent channels (i.e., cores) can be used. 
         [0039]    After the LP 01  has been created, the first modes to be guided are the LP 11  modes, which have a distinctly different mode field pattern from the LP 01  mode. The LP 11  mode comes in two orthogonal (i.e., rotated at 90°) field patterns, named LP 11a  and LP 11b , and each of these modes can again support two orthogonal (i.e., rotated at 90°) polarization states, which thus creates four independent channels. 
         [0040]    When the applied wavelength of the LP 11  mode is below the cut-off wavelength, but above the cut-off wavelength of the next-higher mode (i.e., LP 21 ), one can create six (6) independent channels on the fibre (2×LP 01 , and 4×LP 11 ). 
         [0041]    Each mode can constitute a transmission path in the fibre which is independent from the other paths; this “mode division multiplexing” can multiply the transport capacity of the fibre. In addition to mode division multiplexing (MDM), one can use multiple wavelengths within each mode (WDM), and within each wavelength, one can use time division multiplexing (TDM). 
         [0042]    Thus MDM creates a transmission rate compounding effect that can be seen as opening an extra dimension in fibre optic communication, and promises to be an attractive route for increasing fibre capacity in response to the booming capacity needs of a rapidly expanding Internet. 
         [0043]    The excitation and detection of 6 mode field channels can be accomplished using a device constructed with bulk optics (i.e. with lenses and phase plates). Such devices are mechanically elaborate, and require special measures to assure stability. Moreover, the launching with phase plates exploits only the phase domain for excitation of the modes, and not the amplitude domain. 
         [0044]    The present system and method provides a low cost miniaturized integrated optical circuit for exciting six (6) mode field patterns. It connects 6 individual input fibres, thereby providing individual channel data signals to the common output fibre (i.e., Few-Mode Fibre). The disclosure carries 6 mode field channels (See  FIG. 7 ). 
         [0045]    The multi-channel miniaturized integrated optical circuit is reciprocal, so it can also be used as a mode de-multiplexer. It can be reversed to separate the 6 mode field channels arriving via the common fibre into 6 individual output fibres. (See  FIG. 8 ). 
         [0046]    The miniaturized integrated circuit is based on planar integrated optical waveguide technology, and can be fabricated using Iridium-Phosphide integrated optical circuit processes, or silicon photonic integrated optical circuit processes. 
         [0047]    At the left side of the circuit, the input signals to be multiplexed are fed by standard single-mode fibres (SMF) and coupled into the waveguides on the chip by means of grating couplers. Optimum coupling takes place when the SMF launches the grating coupler at a specific angle, according to grating coupler principles, known to those familiar with the art. 
         [0048]    Five (5) two-dimensional (2-D) grating couplers, situated on the right of the circuit, launch into the Few-Mode Fibre (FMF), positioned vertically above these couplers. Each coupler can handle two orthogonal polarizations, each launched via an input port of the coupler. 
         [0049]    Such 2D grating couplers are feasible because they are able to launch or receive light vertically from a fibre as has been previously reported. In order to match the field distribution of the LP 01  mode in the FMF (See  FIG. 3 ), the signals launched into the two orthogonal polarization states of this mode are fed to the central 2D grating coupler. 
         [0050]    To launch the LP 11a  mode in the FMF, two 2D grating couplers located at diagonally opposite positions are used, from where the emerging optical fields need to differ π in phase in order to optimally match the LP 11A  mode field distribution (See  FIGS. 3 &amp; 4 ). 
         [0051]    The two signals needed for driving the two 2D grating couplers are derived from a single input by means of a 50%/50% field splitter (i.e., 1×2 multimode interference coupler), from which two waveguides feed them to the respective 2D grating coupler. 
         [0052]    The required π phase difference is obtained by designing the waveguides so that their length is approximately equal, which mathematically means that one of the waveguides has additional curves, plus a phase tuning element which can tune the phase to assure the π phase difference; See  FIG. 7   .bis.    
         [0053]    In one example, such phase tuning can be implemented by a heater element which can effect a slight local change in the refractive index of the waveguide. 
         [0054]    Similarly as described above for the LP 01  mode, the two polarizations of the LP 11A  mode are launched by deploying both the x-polarization and y-polarization operation modes of the 2D grating couplers. 
         [0055]    Similarly as for the LP 11A  mode, the two polarizations of the LP 11B  mode are launched via the two 2D grating couplers located at the diagonally opposite positions. The design considerations for the waveguides and the phase tuning elements are identical to those for the LP 11A  mode. 
         [0056]    The disclosed miniaturized integrated optical circuit is compact and stable, hence resolving the mechanical issues and reducing cost. Moreover, it uses both the phase and the amplitude domain for selectively launching the modes, and yields higher performance in terms of launching efficiency, with better selectivity, better stability and less crosstalk between modes, when compared to the bulk optics approach (See  FIG. 5 ). 
         [0057]    In another embodiment of the disclosure, four 2D grating couplers are used as shown in  FIG. 8 . It deploys push-pull driving of opposite 2D grating couplers to excite an LP 11  mode. It should be noted that the differential mode field pattern of an LP 11  mode can be demonstrated (See  FIG. 3 ), and common mode driving of the same 2D grating couplers which thereby excite the LP 01  mode relying on the rotational symmetric mode field pattern of the LP 01  mode (See  FIG. 2 ). 
         [0058]    The push-pull impulse signals are obtained by using a π phase shifter in combination with the relevant 1×2 Multi-Mode Interference (MMI) power splitter/combiner (See  FIG. 7 . bis  and  FIG. 8 ). 
         [0059]    It is also shown that the driving signals of the 2D grating couplers are formed out of the individual input signals launched into the photonic integrated circuit (See  FIG. 8 ). The E-fields are illustrated with x1 and x2 for common-mode driving of the two LP 01  modes, and the E-fields indicated with y1 through y4 for push-pull driving the four LP 11  modes. 
         [0060]    The advantages of this embodiment are that it allows longer tapered waveguide regions to feed the 2D grating couplers, and that the 2D grating couplers themselves can be slightly larger which improves the coupling into the LP 11  and LP 01  modes (See  FIG. 7 ). 
         [0061]    The disadvantage of this embodiment is that in order to assure the correct phase relations at the inputs of the 2D grating couplers, there is created a less efficient coupling into the LP 01  mode, and more complex on-chip waveguide structures because it requires phase tuning sections in each outlet of the 1×2 MMI couplers at the left side (See  FIG. 8 ). 
         [0062]    In another embodiment of the disclosure three 2D grating couplers are used as shown in  FIG. 9 . It has a centrally positioned 2D grating coupler for driving the LP 01  mode, and two outer positioned 2D grating couplers to drive the LP 11A  and LP 11B  modes, respectively. The LP 11  modes are, however, not driven in push-pull mode, rather they are driven in an asymmetric (i.e., one sided) mode. 
         [0063]    The advantages of this embodiment are a very simple integrated optical circuit structure, with only three 2D grating couplers; no need for phase-tuning sections; and, the allowance for long tapered waveguide regions to feed the 2D grating couplers. 
         [0064]    The disadvantages are a less efficient coupling into the LP 11  modes, and potential significant crosstalk into the LP 01  mode when driving the LP 11  mode. However, crosstalk in this embodiment is not present when driving in push-pull mode (See  FIG. 7  &amp;  FIG. 8 ). Elimination of possible crosstalk requires extra signal processing in the electrical domain, and will to a small degree decrease the performance of the system. 
         [0065]    While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the spirit and scope of the inventions.