{"id":860,"date":"2020-03-24T22:39:23","date_gmt":"2020-03-25T05:39:23","guid":{"rendered":"https:\/\/sierra.ece.ucdavis.edu\/?p=860"},"modified":"2020-03-25T15:31:25","modified_gmt":"2020-03-25T22:31:25","slug":"3d-arbitrary-photonics","status":"publish","type":"post","link":"https:\/\/sierra.ece.ucdavis.edu\/index.php\/2020\/03\/24\/3d-arbitrary-photonics\/","title":{"rendered":"3D Arbitrary Photonics"},"content":{"rendered":"\n

 <\/h1>\n\n\n\n

Technology Overview<\/strong><\/p>\n\n\n\n

There are two main methods of realizing 3D PICs.  The first method utilizing ultrafast laser inscription (ULI) offers freeform shaping of waveguides in arbitrary contours and formations.  The second method utilizing multi-layer stacking and coupling of planar PICs exploit relatively mature 2D PIC fabrication processes applied to each layer sequentially.  Both fabrication methods for 3D PICs have advantages and disadvantages such that certain applications may favor one method over the other.  On the other hand, a joining of 2D PICs with 3D PICs can help realize integrated microsystems of new functionalities such as non-mechanical beam steering, space-division multiplexing (SDM), programmable arbitrary beam shaping, and photonic signal processing. We discuss examples of 3D PICs and 2D\/3D integrated PICs in two applications, SDM via orbital-angular-momentum (OAM) multiplexing\/demultiplexing and optical beam steering using optical phased arrays.<\/p>\n\n\n\n

Perhaps the most distinctive 3D PICs with no electronic counterpart is realized by 3D waveguide formation by ultrafast laser inscription (ULI).  Direct laser writing of waveguides in dielectric material is an extremely powerful fabrication technique [34]. It utilizes the multi-photon nonlinear absorption of sub-bandgap photons to create permanent structural changes in a material with dimensions comparable to the writing laser\u00e2\u20ac\u2122s wavelength (e.g., ~1 \u00c2\u00b5m3<\/sup>). The types of structural changes include refractive index and an increased susceptibility to chemical etching [35]. The induced modifications from a femto-second train of optical pulses are strongly localized in three dimensions to the high intensity region at the focus of a lens driven by a nonlinear absorption mechanism. This unique characteristic is what provides direct laser writing its unique advantage over other waveguide fabrication techniques; the capability to freely form truly three-dimensional structures [36]. ULI has been widely demonstrated in many types of materials including amorphous glasses and crystals with measured propagation losses \u00e2\u2030\u00a40.3 dB\/cm [37, 38] in fused silica.<\/p>\n\n\n\n

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Figure 1. Schematic illustration of 3D photonic inscribing where the waveguide is formed by increasing the local index of the material. The table shows how optical pulse energy, pulse repetition rate, and waveguide scan speed are adjusted to optimize the quality of the inscribed waveguide when the pulsewidth is kept constant. Depending on the combination of laser repetition rate and mechanical scan rate, the material modification regions formed by each pulse will overlap by varying amounts.<\/figcaption><\/figure><\/div>\n\n\n\n

Current Research Activitie<\/strong>s<\/p>\n\n\n\n

3D multi-ring waveguides for OAM applications<\/strong><\/p>\n\n\n\n

Multi-ring 3D waveguide design and fabrication for the OAM application are important first step towards scalable spatial division multiplexing (SDM).  Our group has developed a 3D waveguide design tool utilizing the routing algorithm that maintains the same photonic path lengths, the minimum radius of curvature, and the minimum inter-waveguide distance. Figure 6 (a) shows the computer-aided-design, and (b) a photograph of the fabricated multi-ring 3D waveguides at the facet.  In such cases, the multi-ring OAM PICs can possibly realize generation of arbitrary spatial waveforms such as Laguerre-Gaussian modes by adding the radial control of the optical field to the azimuthal control achieved in individual OAM PICs of each ring.<\/p>\n\n\n\n

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Figure 2. (a) Dual ring 3D waveguide design utilizing the routing algorithm that maintains the same photonic path lengths, the minimum radius of curvature, and the minimum inter-waveguide distance, (b) a photograph of the fabricated multi-ring 3D waveguides (Corning Eagle 2000) seen at the facet.<\/figcaption><\/figure><\/div>\n\n\n\n

<\/p>\n\n\n\n

3D waveguides for optical phased arrays and beam steering<\/strong><\/p>\n\n\n\n

In addition to the spatial waveform shaping using multi-ring 3-D PICs fabricated by ULI methods, the same ULI can form 3-D PICs of arbitrary array shapes, including a rectangular array.  Figure 7 shows the details of a 3-D PIC that transforms a linear 1 \u00c3\u2014 256 input waveguide array to a 16 \u00c3\u2014 16 rectangular array of output waveguides using the ULI technique. Figure 7 (a) show a computer aided design with detail of the output array, (b) a photograph of the output facet and (c) photograph of a portion of the input facet. As is the case with most dielectric waveguides, the index difference between the waveguide core and cladding determines the minimum bending radius due to radiation losses from the bent waveguide. Since the index difference for these ULI waveguides is ~0.5%, we chose a minimum bending radius of 20 mm, which showed negligible bending loss. Similarly to the multi-ring 3-D PICs, the rectangular PICs also utilized the 3D computer aided design for precise optical path length matching of all 256 waveguides.  Such 3D PICs are useful for non-mechanical beam steering [51] as well as SDM MIMO optical communications and beam forming (e.g. Hermite-Gaussian modes).  Figure 8 (a) shows an example of optical beam steering realized by hetergneous integration of such a 3D waveguide array and a 2D PIC consisting of optical splitter waveguides and waveguide array phase shifters. Figure 8 (b-i) illustrates optical beam steering results experimentally obrained from phase tuning the array waveguides in the 2D PIC by \u00cf\u20ac, 0.75\u00cf\u20ac, 0.5\u00cf\u20ac, 0.25\u00cf\u20ac, 0, -0.25\u00cf\u20ac, -0.5\u00cf\u20ac, and -0.75\u00cf\u20ac  between the waveguides [51].  Here, phase error correction [11] is conducted by phase tuning the array waveguides.<\/p>\n\n\n\n

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Figure 7. (a) 16 \u00c3\u2014 16 3D waveguide design utilizing a routing algorithm that maintains the same photonic path lengths, the minimum radius of curvature, and the minimum inter-waveguide distance, (b) a photograph of the fabricated 16 \u00c3\u2014 16 3D waveguides seen at the output facet. (c) Magnified micrograph of a portion of the 256 input waveguides.<\/figcaption><\/figure><\/div>\n\n\n\n
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Figure 8. (a) Optical phase array beam steering module realized by hetergneous integration of a 3D waveguide array similar to Figure 7 (b) and a 2D PIC consisting of optical splitter waveguides and waveguide array phase shifters. (b-i) optical beam steering results obrained from phase tuning the array waveguides in the 2D PIC by \u00cf\u20ac, 0.75\u00cf\u20ac, 0.5\u00cf\u20ac, 0.25\u00cf\u20ac, 0, -0.25\u00cf\u20ac, -0.5\u00cf\u20ac, and -0.75\u00cf\u20ac between the waveguides [51]. The optical waveguide propagation loss value of the 3D waveguide was approximately 0.2 dB\/cm.<\/figcaption><\/figure><\/div>\n\n\n\n

<\/p>\n\n\n\n

3D Photonic Packaging<\/strong><\/p>\n\n\n\n

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Figure 5. 3D ULI capabilities at UC Davis producing 16 core fiber interface shown in (a1-a2) and 256 core fiber interface shown in (b1-b2). On the opposite end of the 3D ULI block can utilize adiabatic coupling between negative-tapered silicon nitride waveguides (150 or 200 nm thickness) and the ULI waveguide as shown in (c1, c2, and c3) where nearly perfect mode transformation from (c2) the entrance from SiN waveguide to (c3) the exit to 3D waveguide can be possible to achieve 0.04 dB loss. (d) the silicon photonic 3D fan-out chip including the 3D ULI MCF interfaces. Shown is an 8-core MCF with 4 strands producing 32 silicon nitride waveguide interface to silicon photonic dies.<\/figcaption><\/figure><\/div>\n\n\n\n

The most scalable solutions to\nhigh-density MCF interface for silicon photonics is to utilize 3D ultrafast\nlaser inscription capabilities. UCDavis has developed low-loss (0.1~0.2 dB\/cm)\nULI capabilities for arbitrarily shaped 3D waveguides capable of low loss (<\n0.3 dB) MCF coupling using 3D CAD design tools with compatible design-rule\nchecks. Figure 5\n(a) and (b) show 16 core 3D ULI waveguides and 256 3D ULI waveguides designed\nto interface specialty MCFs to silicon photonic waveguides on the other end. In\nparticular, UCDavis demonstrated ULI directly on an oxide dielectric layer\ndeposited on silicon wafers. Hence, as Figure 5\n(d) illustrates we propose to develop a silicon photonic 3D fan-out chip\ninterfacing an array of MCFs with an array of silicon nitride waveguides with\nthe 3D ULI waveguides (shown example shows 4 strands of 8-core MCFs interfacing\nwith a silicon photonic transceiver die which will also have a matching 32 silicon\nnitride waveguide interface.<\/p>\n\n\n\n

UC Davis has recently demonstrated an optical interposer\nwith vertical evanescent waveguide coupling to a silicon photonic die with 0.38\ndB loss (limited by the airgap caused by the non-planar metal layer) providing\n\u00c2\u00b1 2 \u00c2\u00b5m alignment tolerance. Elimination of the airgap at the planarized\ninterface expects to reduce the optical loss to below 0.1 dB. Wafer-scale\nfabrication of 3D ULI is possible utilizing UC Davis\u00e2\u20ac\u2122 fabrication method,\nespecially because the optical beam can be split to multiple spots (100 spots\nis possible at the current laser power level and the required power for the\nULI) for simultaneous writing of multiple spots. From the CAD design to actual\ncompletion takes only several hours, and the proposed method is viable for\nrapid prototyping. Foundry transition to AIM Photonics have been discussed\nmultiple times over the past two years with keen interest from the AIM\nPhotonics community including industry members. <\/p>\n\n\n\n

References<\/strong><\/p>\n\n\n\n

[1]        S. Cheung, Y. Kawakita, K. Shang, and S.\nJ. Ben Yoo, “Highly efficient chip-scale III-V\/silicon hybrid optical\namplifiers,” Optics Express, <\/em>vol.\n23, pp. 22431-22443, 2015\/08\/24 2015.<\/a><\/p>\n\n\n\n

[2]        S. Cheung, S.\nTiehui, K. Okamoto, and S. J. B. Yoo, “Ultra-Compact Silicon Photonic 512\nx 512 25 GHz Arrayed Waveguide Grating Router,” Selected Topics in Quantum Electronics, IEEE Journal of, <\/em>vol. 20,\npp. 310-316, 2014.<\/a><\/p>\n\n\n\n

[3]        R. S. Weis and\nT. K. Gaylord, “Lithium niobate: Summary of physical properties and\ncrystal structure,” Applied Physics\nA, <\/em>vol. 37, pp. 191-203, 1985.<\/a><\/p>\n\n\n\n

[4]        E. L. Wooten,\nK. M. Kissa, A. Yi-Yan, E. J. Murphy, D. A. Lafaw, P. F. Hallemeier, D. Maack,\nD. V. Attanasio, D. J. Fritz, G. J. McBrien, and D. E. Bossi, “A review of\nlithium niobate modulators for fiber-optic communications systems,” IEEE Journal of Selected Topics in Quantum\nElectronics, <\/em>vol. 6, pp. 69-82, 2000.<\/a><\/p>\n\n\n\n

[5]        D. Janner, D.\nTulli, M. Garc\u00c4\u00b1\u00cc\u0081a-Granda, M. Belmonte, and V. Pruneri, “Micro-structured\nintegrated electro-optic LiNbO3 modulators,” Laser & Photonics Review, <\/em>vol. 3, pp. 301-313, 2009.<\/a><\/p>\n\n\n\n

[6]        G. Poberaj, H.\nHu, W. Sohler, and P. G\u00c3\u00bcnter, “Lithium niobate on insulator (LNOI) for\nmicro-photonic devices,” Laser &\nPhotonics Reviews, <\/em>vol. 6, pp. 488-503, 2012.<\/a><\/p>\n\n\n\n

[7]        A. Rao, A.\nPatil, P. Rabiei, A. Honardoost, R. DeSalvo, A. Paolella, and S. Fathpour,\n“High-performance and linear thin-film lithium niobate Mach\u00e2\u20ac\u201cZehnder\nmodulators on silicon up to 50GHz,” Optics\nLetters, <\/em>vol. 41, pp. 5700-5700, 2016.<\/a><\/p>\n\n\n\n

[8]        M. Prost, G.\nLiu, and S. J. B. Yoo, “A Compact Thin-Film Lithium Niobate Platform with\nArrayed Waveguide Gratings and MMIs,” in 2018 Optical Fiber Communications Conference and Exposition (OFC)<\/em>,\n2018, pp. 1-3.<\/a><\/p>\n","protected":false},"excerpt":{"rendered":"

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