Patent Description:
Optical wireless communication (OWC) is receiving interest not only from academia, but increasingly also from industry. It offers a powerful solution for creating high-capacity wireless connectivity while not compromising the already overcrowded radio spectrum. , it is a powerful alternative for fronthaul connections from the baseband units (BBUs) to the remote radio head (RRH) antenna stations in the upcoming <NUM> wireless networks which should offer 1000x more capacity at 100x less power consumption than current <NUM> networks, with <<NUM> latency. The unbeatable bandwidth of the optical beams used in OWC provides the unique opportunity to meet these challenges, while in addition offering improved security (as tapping a beam is nearly impossible, and these OWC links do not penetrate walls) and reliable connections (as OWC links are not bothered by electromagnetic interference (EMI) from radio sources, nor generate EMI).

This prior art document described an indoor optical wireless communication system based on 2D optical beam steering at l. <NUM> using arrayed waveguide gratings.

To address the needs in the art, an optical beam steering device is provided that includes only one input optical fiber carrying multiple input optical signals, where each input optical signal includes a unique wavelength, an arrayed waveguide grating router (AWGR) having multiple output fibers, where the input optical fiber is connected to the AWGR, where distal ends of the multiple output fibers are arranged in a two-dimensional fiber array, where the multiple input optical signals are routed by the AWGR according to each unique wavelength to a unique AWGR output fiber in the two-dimensional fiber array, and a lens, where the distal ends of the output fibers are disposed proximal to a focal plane of the lens, where for each unique position of each output fiber distal end with respect to a the lens, each input optical signal is steered at a unique angle as an output beam emitted from the lens, where changing the wavelength of the input optical signal changes the output signal angles for two-dimensional beam steering, wherein a profile of said output beam comprises a diverging beam, wherein said output fiber ends are positioned somewhat closer to the principal plane of said lens than said focal plane of said lens.

In an embodiment of the invention, the two-dimensional fiber array is configured for scanning with discrete steps in both two-dimensional angular directions when the input optical signal wavelength is varied.

The current invention provides two-dimensional steering of optical beams by use of an arrayed waveguide grating router (AWGR) device, which is an integrated device with fiber pigtails, where the outgoing fibers are arranged in a well-designed 2D matrix followed by a lens. 2D steering is enabled by tuning the wavelength of the optical signal fed into the arrayed waveguide grating. Multiple beams can be emitted simultaneously and each be independently steered by tuning the wavelength of each signal.

According to the prior art document, a 2D beam steering device is shown in <FIG>. The backbone fiber is feeding the multiple optical signals carrying the high-speed data into the AWGR, where each signal has a different specific wavelength. As indicated, the AWGR has many output fibers, and according to the wavelength filtering characteristics of the AWGR, the wavelength of each input signal determines to which specific output fiber this signal will go. The output fibers are arranged in a two-dimensional matrix structure (indicated as 2D fiber array). This array is put in the focal plane of a lens. Thus, depending on the position of a fiber with respect to the lens, the optical signal out of that fiber will be steered as a collimated beam emitted by the lens under a certain angle. By changing the wavelength of the optical signal, that signal will emerge out of another fiber, and thus be emitted by the lens under another angle. The 2D fiber matrix structure will thus yield the two-dimensional beam steering.

The 2D beam steering device of <FIG> provides scanning with discrete steps in both the 2D angular directions when the wavelength is varied. The module with two cross-aligned gratings provides continuous scanning in one angular direction, and discrete scanning in the orthogonal angular direction. The emitted optical beams provide full coverage of the target area which can be reached by both the module according to the current invention and by the module with two cross-aligned gratings according to a previous invention (<CIT>).

The area covered by the beam steering can be derived from the spacing Δy of the fibers in the 2D fiber array and the focal length f of the lens. Each beam will create a spot with a diameter Dspot in the image plane. The maximum allowable spot size is dictated by the aperture of the optical wireless communications (OWC) receiver, and the minimum amount of received power needed for the data rate to be reliably transferred. As shown in <FIG>, given that the image area of size L×L needs to be fully covered by the beam spots (so the various spots should be directly adjacent to each other, touching each other without any room in between) and given a certain number M<NUM> of output ports of the AWGR, the spot size Dspot, the spacing of the fibers Δy and the lens focal length f can be calculated as <MAT> where tan α is the half emission angle with which the light is radiated by an output fiber of the AWGR. For single-mode fiber (SMF) in the λ=<NUM> wavelength range, typically tan α = λ/(π·w<NUM>) ≈ <NUM>. 1where w<NUM> is the SMF's beam waist.

For typical commercially available AWGRs, this commonly leads to a quite bulky module, e.g., with an AWGR having <NUM> output ports M = <NUM>, in order to cover an area of only <NUM> × <NUM><NUM> (so L = <NUM>), a large condenser lens is needed with f = <NUM> and diameter Dlens = <NUM>, and a fiber spacing Δy = <NUM> which yields a 2D fiber array total size of <NUM> × <NUM><NUM>.

To circumvent this bulkiness, the current invention employs defocusing for the module. The layout of the defocused module is actually the same as in <FIG>, but now the fiber array is not put in the focal plane of the lens, but somewhat closer, at a distance v = (<NUM>-p) · f from the lens' principal plane, where the defocusing parameter p satisfies <NUM> ≤ p < <NUM> ; see <FIG>. Note that p = <NUM> yields a module without defocusing, i.e. the module according to <FIG>. As a result of the defocusing, the beam after the lens is not collimated anymore, but is slightly diverging. Thus the diameter of the beam spot in the image area Dspot gets larger; so alternatively, given a certain area to be covered, the lens focal length f and the lens diameter Dlens can be reduced. This beam spot diameter Dspot is independent on the output fiber position, and can be derived to be <MAT>.

As b > f, increasing the defocusing p therefore implies a larger spot diameter Dspot ·.

Applying a fractional defocusing parameter p (with <NUM> < p < <NUM>; p = <NUM> means no defocusing, so the case shown in <FIG>), a considerable reduction of the module's size is achieved. It can be derived that the lens focal length f, fiber spacing Δy and lens diameter Dlens needed are <MAT> where b is the distance between the lens and the image plane (i.e. the area to be covered by the beam steering). <FIG> shows the paraxial ray tracing results in case of no defocusing (p = <NUM>, so with collimated beams) and defocusing (p = <NUM>, so with diverging beams). Note that the system parameters have been set according to equation (<NUM>) above, such that the spots in the coverage area stay touching each other without intermediate space, in order to keep the full coverage of the image area.

<FIG> shows how the size of the 2D beam steering module can be reduced (in terms of reduction of the lens focal length f, its diameter Dlens, and the fiber spacing Δy), by increasing the defocusing parameter p. The results have been calculated for a spot diameter Dspot = <NUM> (with which reliable transmission of more than 10Gbit/s per beam has been achieved), a distance b = <NUM> (a typical indoor OWC working distance when the beam steering module is mounted on the ceiling of a room in a building), a coverage area <NUM> × <NUM><NUM> (so L = <NUM>), and an AWGR with <NUM> output ports (so M = <NUM>), similar as before. Note that compared to the case with collimated beams (p = <NUM>) above where f = <NUM>, Dlens = <NUM>, and Δy = <NUM>, with a defocusing of p = <NUM> giving f = <NUM>, Δy = <NUM>, Dlens = <NUM>. This illustrates the significant size reduction of the 2D beam steering module, which can be achieved with the proposed defocusing method.

The current invention can be extended to an AWGR with a larger port count. AWGR-s having <NUM> output ports are available for the C-band (wavelength range λ = <NUM> to <NUM>), and also for the L-band (λ = <NUM> to <NUM>). By putting such AWGR-s in parallel, an equivalent AWGR with <NUM> output ports is obtained for operation over the C + L bands (λ = <NUM> to <NUM>). Deploying such a high-port-count AWGR yields M = <NUM>, and assuming the same acceptable beam size Dspot = <NUM>, the coverage area is increased to <NUM> × <NUM><NUM> (as L = M · Dspot = <NUM>). As shown in <FIG> with e.g. a defocusing of p = <NUM> the required lens focal length is f = <NUM>, fiber spacing Δy = <NUM>, and lens diameter Dlens = <NUM>. The f-number of the lens is then f / Dlens = <NUM>, which is a realistic number for e.g. well-established <NUM> camera lenses.

In system experiments for the current invention, 20Gbit/s OWC data transfer was successfully achieved with binary on-off modulated light using a spot size Dspot=<NUM>. The measurements indicated that the spot size could be increased further to <NUM>, even <NUM> without loosing significantly on performance. <FIG> shows the system's design parameters for a spot size Dspot = <NUM>. With a defocusing p = <NUM> the required lens focal length is f = <NUM>, fiber spacing Δy = <NUM>, and lens diameter Dlens = <NUM>. The f-number of the lens is then f / Dlens = <NUM>, which is a demanding but still realistic and available number for well-established <NUM> camera lenses. The coverage area can thus be further increased to <NUM> × <NUM><NUM>.

Further increasing the allowable spot size Dspot will also increase the coverage area further, but in addition makes the required f-number ever higher. This makes the lens harder to realize, and hence (significantly) more expensive. , for Dspot = <NUM>, a p = <NUM> is needed for f = <NUM>, Δy = <NUM>, Dlens = <NUM>, implying an f-number f / Dlens = <NUM>, which leads to a quite complex and expensive lens design.

The current invention provides ease of assembly and improved stability as the wavelength-dependent functions are done by an integrated optical circuit. The 2D beam steering by the alternative module is slightly different from the steering by the two cross-aligned diffraction gratings module as described in <CIT>. In the alternative one, the steered beam follows discrete steps in both the two angular directions when the wavelength is varied, whereas in the latter one the steered beam is continuous in one angular direction, but takes discrete steps in the orthogonal angular direction. With a well-designed beam, full coverage of the target area can be achieved with either module.

Another advantage of the defocusing approach is a better fill factor of the covered area. When adopting the lens focal length f and fiber spacing Δy calculated for the image plane distance b<NUM>, the spots will exactly touch each other in the image plane (see <FIG>, where the e-<NUM> intensity contours of the Gaussian spot footprints are shown). When the image plane is moved to a larger distance from the lens (so b > b<NUM>), the spacing between the spot centers Δyc increases (Δyc = Δy · (b/f - <NUM>)). Without defocusing (i.e. p = <NUM>), the spot diameter Dspot stays constant and becomes smaller than the spot spacing Δyc, hence the fill factor decreases when b increases (see <FIG>). For b < b<NUM>, overlap between the spots occurs (see <FIG>). <FIG> shows how the spot diameter, the spacing between the spots, and the fill factor (i.e. the summation of the footprint areas of all spots divided by the total coverage area) evolve when the distance b of the image plane to the lens is increased, both for the collimated-beams case (p = <NUM>) and the defocused case (p = <NUM>), for the design where Dspot = <NUM> at room height b<NUM> = <NUM>, and M = <NUM> ports. Clearly the defocusing has reduced the dependency of the fill factor on the distance b to the lens.

According to this invention disclosure, a compact and stable 2D optical beam steering module can be achieved by the proposed concept based on a high fiber-port count optically integrated AWGR followed by a lens including proper defocusing of the 2D fiber matrix.

Claim 1:
An optical beam steering device, comprising:
a) only one optical fiber carrying multiple input optical signals, wherein each said input optical signal comprises a unique wavelength;
b) an arrayed waveguide grating router, AWGR, having multiple output fibers, wherein said input optical fiber is connected to said AWGR, wherein distal ends of said multiple output fibers are arranged in a two-dimensional fiber array, wherein said multiple input optical signals are routed by said AWGR according to each said unique wavelength to a unique said AWGR output fiber in said two-dimensional fiber array; and
c) a lens, wherein said distal ends of said output fibers are disposed proximal to a focal plane of said lens, wherein for each unique position of each said output fiber distal end with respect to a said lens, each said input optical signal is steered at a unique angle as an output beam emitted from said lens, wherein changing said wavelength of said input optical signal changes said output beam angles for two-dimensional beam steering;
characterized in that:
said output fiber ends are positioned somewhat closer to the principal plane of said lens than said focal plane of said lens such that a profile of said output beam comprises a diverging beam.