Research ArticleAPPLIED PHYSICS

Ultra-compact broadband polarization diversity orbital angular momentum generator with 3.6 × 3.6 μm2 footprint

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Science Advances  31 May 2019:
Vol. 5, no. 5, eaau9593
DOI: 10.1126/sciadv.aau9593
  • Fig. 1 Concept and principle of chip-scale broadband polarization diversity OAM generator on a silicon platform.

    (A) Zoom-in 2D subwavelength surface structure (superposed holographic fork gratings) region. (B and C) Illustration of holographic method producing fork gratings. The coupled interference between the vertically incident x-pol. (B) or y-pol. (C) OAM mode and the x-pol. (B) or y-pol. (C) TE0 in-plane guided mode forms a fork grating on top of the silicon waveguide with the fork opening direction along x (B) or y (C). (D to F) Superposed holographic fork gratings G(x, y) (D) formed by the superposition of two fork gratings of G1(x, y) with the fork opening direction along x (E) and G2(x, y) with the fork opening direction along y (F). (D) to (F) correspond to (A) to (C), respectively. (G to J) Superposed holographic fork gratings for generating broadband polarization diversity x-pol. OAM+1 (G), x-pol. OAM−1 (H), y-pol. OAM+1 (I), and y-pol. OAM−1 (J) under different incident conditions of −y-input x-pol. (G), y-input x-pol. (H), −x-input y-pol. (I), and x-input y-pol. (J) TE0 in-plane waveguide mode. x-Pol., x-polarization; y-Pol., y-polarization.

  • Fig. 2 Simulation results for characterizing the polarization diversity OAM generator.

    (A) Purity (y-pol. OAM+1) versus mesh size used in the 3D-FDTD simulations (length l: 3.6 μm; depth h: 60 nm). (B) Purity of polarization diversity OAM modes versus length l of the grating region (depth h: 60 nm). (C) Purity of polarization diversity OAM modes versus depth h (length l: 3.6 μm). (D) Purity of polarization diversity OAM modes versus wavelength (length l: 3.6 μm; depth h: 60 nm). (E) Scattering efficiency (y-pol. OAM+1) without and with a reflector versus depth h (length l: 3.6 μm). (F) Scattering efficiency (y-pol. OAM+1) without and with a reflector versus thickness of the SiO2 substrate (length l: 3.6 μm; depth h: 120 nm). (G) Transmission/reflection efficiency versus depth h (length l: 3.6 μm). (H) Central wavelength versus depth h (length l: 3.6 μm). (I) Scattering efficiency versus wavelength at two depths of 100 and 120 nm (length l: 3.6 μm).

  • Fig. 3 Simulation results for the generation of polarization diversity OAM modes and the evolution process of in-plane guided mode to out-plane OAM mode.

    (A to D) Intensity profile, phase distribution, and interferogram of the generated polarization diversity OAM modes without a reflector in the C-band (1530 to 1565 nm). (E to H) Electric field distributions (amplitude of electric field components) along the waveguide (from waveguide region to grating region) and at different height of the grating region (z = 0.1, 0.2, 0.3, 0.4, 0.5 μm, far field) without a reflector. (A and E) x-pol. OAM+1. (B and F) x-pol. OAM−1. (C and G) y-pol. OAM+1. (D and H) y-pol. OAM−1. (I to L) Intensity profile, phase distribution, and interferogram of the generated polarization diversity OAM modes with a reflector at 1550 nm. (I) x-pol. OAM+1. (J) x-pol. OAM−1. (K) y-pol. OAM+1. (L) y-pol. OAM−1. (A to L) Length l: 3.6 μm. (A to D) Depth h: 60 nm. (E to H) Depth h: 100 nm. (I to L) Depth h: 120 nm.

  • Fig. 4 Fabricated devices, experimental setup, and measured results for the generation of broadband polarization diversity OAM modes.

    (A) Measured optical microscope image of the layout of superposed holographic fork gratings connected by four adiabatic tapers. (B) Measured SEM image of the superposed holographic fork grating region. (C) Measured optical microscope image of the device without incidence of the in-plane guided mode. (D) Measured near-field view (bright spot from the center grating region) for generating y-pol. OAM−1 with incidence of the in-plane guided mode. (E to H) Measured optical microscope images of the fabricated devices with different lengths of the grating region of 2.4 μm (E), 3.6 μm (F), 5 μm (G), and 10 μm (H). (I) Experimental setup. PC, polarization controller; VOA, variable optical attenuator; Col., collimator; Pol., polarizer; HWP, half-wave plate; BS, beam splitter. (J) Measured far-field intensity profiles and interferograms for the broadband generation of polarization diversity OAM modes (x-pol. OAM+1, x-pol. OAM−1, y-pol. OAM+1, y-pol. OAM−1) in the C-band. (K) Measured far-field intensity profiles of x-pol. OAM+1 (1550 nm) and y-pol. OAM+1 (1500, 1550, 1600, 1630 nm) after a rotating polarizer (90°, 45°, 0°).

  • Fig. 5 Experimental measurements of the phase reconstruction, phase purity, and scattering efficiency for the generation of polarization diversity OAM modes.

    (A) Measured far-field intensity profiles, collinear interferograms, tilt interferograms, and reconstructed phase structures of the generated y-pol. OAM+1 at 1500, 1550, 1600, and 1630 nm. (B) Measured phase purity of the generated y-pol. OAM+1 versus wavelength (1500 to 1630 nm). (C) Measured phase purity of the generated y-pol. OAM+1 versus length l of the grating region with two depths of 100 and 120 nm. (D) Measured scattering efficiency of the generated y-pol. OAM+1 versus wavelength with two depths of 100 and 120 nm.

  • Fig. 6 Experimental measurements of the cross-talk matrix and accumulated cross-talk for the generation of polarization diversity OAM modes (x-pol. OAM+1, x-pol. OAM−1, y-pol. OAM+1, y-pol. OAM−1).

    (A) Experimental configuration. Pol., polarization; HWP, half-wave plate; SLM, spatial light modulator; Col., Collimator. (B) Measured 4 × 4 intensity distribution matrix. (C) Measured histograms of the 4 × 4 cross-talk matrix. (D) Measured 4 × 4 cross-talk matrix. (E) Measured accumulated cross-talk summing up all the noise (all the unwanted channels are on at the same time).

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/5/eaau9593/DC1

    Section S1. Evolution process of in-plane guided mode to out-plane OAM mode

    Section S2. Device fabrication process

    Section S3. Simulated intensity distribution matrix

    Fig. S1. Simulated 3D-FDTD results of electric field distributions (real part of electric field components) along the waveguide (from waveguide region to grating region) and at different height of the fork grating region (z = 0.1, 0.2, 0.3, 0.4, 0.5 μm, far field).

    Fig. S2. Illustration of the device fabrication process of the silicon OAM generator (spin coating, electron-beam lithography, inductively coupled plasma, photoresist removal).

    Fig. S3. Simulated 4 × 4 intensity distribution matrix for the generation of polarization diversity OAM modes (x-pol. OAM+1, x-pol. OAM−1, y-pol. OAM+1, y-pol. OAM−1).

  • Supplementary Materials

    This PDF file includes:

    • Section S1. Evolution process of in-plane guided mode to out-plane OAM mode
    • Section S2. Device fabrication process
    • Section S3. Simulated intensity distribution matrix
    • Fig. S1. Simulated 3D-FDTD results of electric field distributions (real part of electric field components) along the waveguide (from waveguide region to grating region) and at different height of the fork grating region (z = 0.1, 0.2, 0.3, 0.4, 0.5 μm, far field).
    • Fig. S2. Illustration of the device fabrication process of the silicon OAM generator (spin coating, electron-beam lithography, inductively coupled plasma, photoresist removal).
    • Fig. S3. Simulated 4 × 4 intensity distribution matrix for the generation of polarization diversity OAM modes (x-pol. OAM+1, x-pol. OAM−1, y-pol. OAM+1, y-pol. OAM−1).

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