Polarization-independent Modulator Research Articles

Polarization-independent electro-absorption optical modulator based on trapezoid polymer-graphene waveguide

A polarization-independent electro-absorption modulator based on the trapezoid polymer-graphene waveguide (PGW) was proposed. The modulator was constructed on a trapezoid polymer waveguide, and the insulting dielectric spacer sandwiched in the two graphene layers was placed on the surface of the trapezoid polymer waveguide core. The simulation results show that by applying different gate voltages on the graphene layers, effective mode index of the TE and TM modes in the PGW can realize the similar changes in the C-band, which provides the possibility for realizing the polarization-independent modulation. Here, through simulation and optimization, the presented modulator has a compact size and the extinction ratio (ER) of 37 dB can be achieved by reasonably setting working points of “OFF” and “ON” states with 800 μm long active graphene length. The ER variation between the two operating modes is about 0.46 dB. The corresponding power consumption of the modulator is about 23.6 pJ/bit.

Parallel directional coupler based dual-polarization electro-absorption modulator using epsilon near-zero material

Electro-optical modulation, where a radio frequency signal can be encoded in an optical field, is crucial to decide the overall performance of an integrated photonics system. Due to the growing internet penetration rate worldwide, polarization-division-multiplexing (PDM) technique has emerged to increase the link capacity, where polarization-independent modulators are desirable to reduce system complexity. In this study, we propose a novel parallel directional coupler based dual-polarization electro-absorption modulator based on epsilon-near-zero (ENZ) material. The proposed design is capable of independent and synchronized modulation of two fundamental modes viz. transverse magnetic (TM) and transverse electric (TE) mode of a standard silicon (Si) rib waveguide. Indium-tin-oxide (ITO)–Si based two parallel hybrid plasmonic waveguides (HPW1 and HPW2) are placed such that fundamental TM (TE) mode of the input bus waveguide can be coupled to HPW1 (HPW2). The ENZ-state of ITO, acquired upon two independent electrical gating, enables large modulation depth by utilizing enhancement of electric field at the absorptive carrier accumulation layer. With a 27 μm active length, the extinction ratio (ER) of the proposed design is 10.11 dB (9.66 dB) for TM (TE) modulation at 1550 nm wavelength. This results in a 0.45 dB ER-discrepancy and indicates the polarization-insensitive nature of the modulator. The insertion losses and modulation bandwidths of our design are less than 1 dB and more than 100 GHz, respectively, for both polarizations over the entire C-band of wavelength. The proposed design can find potential applications in the PDM-enabled integrated photonics systems and high speed optical interconnections at data center networks.

Polarization-Independent Optoelectronic Modulator Based on Graphene Ridge Structure.

In this paper, we propose a polarization-independent optoelectronic modulator based on the electrical absorption effect of graphene. Firstly, we use the simulation software COMSOL Multiphysics to design the structure, and find via changing the applied voltage on both ends of the graphene that the equivalent refractive index of graphene can be changed, thus changing the light absorption capacity of the modulator. The waveguides in the transverse magnetic (TM) and transverse electric (TE) modes have almost the same extinction coefficient by making a double-layer graphene ridge structure in the center of the silicon-based waveguide, which can achieve approaching modulation depth in the TM and TE modes. At 1550 nm wavelength, the two-dimensional cross-section of the structure is analyzed by the FEM method using COMSOL Multiphysics to obtain the effective refractive index of the structure. The simulation results show that when the distance between the double-layer graphene isolation layer is d = 20 nm, the TE and TM modes can achieve extinction ratios up to 110 dB over the wide communication band by selecting appropriate “ON” and “OFF” switching points. The bandwidth is 173.78 GHz and the insertion loss is only 0.0338 dB.

3-micron wave band mid-infrared polarization-independent graphene modulator

3-micron wave band mid-infrared polarization-independent graphene modulator

All-Dielectric Graphene-induced T-Slot Waveguide Electro-Optic Modulator With Polarization-Independent Operation

Developing efficient techniques to realize polarization-independent modulators nowadays is still important for on-chip photonic circuits. Here, we propose and theoretically demonstrate a graphene-based T-slot waveguide modulator, which simultaneously supports the TE and TM polarized modes, offering a greater possibility to achieve the polarization-independent modulation. The modulation performance is comprehensively studied in terms of attenuation loss, insertion loss, modulation depth, normalized mode area, bandwidth and power consumption. Optimizations of key characteristic parameters illustrate that the modulator not only offers a mode confinement beyond the diffraction limit (<; λ <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /12 and <; λ <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /8 for TM and TE modes, respectively), but also exhibits an outstanding performance with a modulation depth (MD) of >0.35 dB/μm, a insertion loss (IL) of <; 0.1 dB/μm and a MD discrepancy of <; 0.012 dB/μm for both polarized modes covering the whole S, C and L bands. Particularly, the modulator possesses a compact footprint of 11.6 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , a 3-dB modulation bandwidth of 50 GHz and a power consumption per bit of 2.07 pJ/bit based on theoretical predictions. The polarization-independent operation is also quite robust against the possible fabrication perturbations. Our work may inspire new potential for developing the integrated modulators and opto-electronic interconnects in the advanced polarization-diversity circuits.

Tunable graphene–dielectric metasurfaces for terahertz all-optical modulation

In this paper, two graphene-based all-optical terahertz (THz) modulators are proposed. The operations of the structures are based on high quality factor (Q-factor) resonances in graphene–dielectric metasurfaces. The first modulator is a polarization-dependent metasurface where a sharp resonance is originated from the effect of electromagnetically induced transparency. The structure is analyzed using the finite element method. The modulation is performed by photoexcitation of carriers in graphene by infrared optical pumping. Interestingly, owing to the stimulated emission of THz photons, a high transmission change of more than 120% and a high modulation depth of 93% at the frequency of 5 THz are obtained. The second structure is a polarization-independent modulator that operates based on a high Q-factor Fano resonance in asymmetric dielectric disks. It is presented that a high modulation depth of 98% is achieved. The required optical intensity for both structures is calculated to be as low as 7.35 W/cm2. As the proposed graphene-based metasurfaces are optically tunable, they are promising for the realization of fast and efficient spatial THz modulation.

Polarization-insensitive wavefront shaping using the Pancharatnam-Berry phase.

We discuss a method to achieve a polarization-independent modulation of the electromagnetic wavefront based upon the Pancharatnam-Berry phase. When the length of the twisted anisotropic material is equal to the birefringence length (i.e., full-wave plate length), a phase delay proportional to the squared transverse derivative of the twisting angle appears. Physically, the phase delay is associated with the Kapitza effect applied to the Pancharatnam-Berry phase. Our theoretical results are confirmed by finite-difference time-domain (FDTD)-based numerical simulations.

Double-layer graphene optical modulators based on Fano resonance in all-dielectric metasurfaces

In this paper, two double-layer graphene optical modulators based on all-dielectric metasurfaces are proposed. The double-layer graphene modulators remove the requirement of doped silicon back gates which would cause excessive loss and limit the modulation speed. The first structure is based on the electromagnetically induced transparency phenomenon, and the second one is a polarization-independent modulator that is based on the Fano resonance. The structures are simulated and analyzed using the finite element method. According to the simulation results, large modulation depth of about 95% at the wavelength of 1.55 μm can be obtained. The required voltages of 4.95 V are obtained for both of the modulators. By assuming the device of 50 × 50 unit cells, the 3 dB bandwidth of the first (second) structure is calculated as 630 MHz (482 MHz). It is estimated that by improving the quality of graphene, the 3 dB bandwidth of more than 2 GHz could be attained. The achieved modulation performances are much better than the previously reported free-space modulators with the same device area. The proposed high-performance optical modulators are promising for free-space optics technology especially free-space optical communication networks.

Waveguide Engineering of Graphene Optoelectronics—Modulators and Polarizers

The concept of incorporating graphene into nanophotonic waveguides has pullulated into massive broadband optoelectronic applications with compact footprint. We theoretically demonstrate that by solely altering the dimension design of graphene-laminated silicon waveguides, the phase, amplitude, and polarization of the fundamental propagating modes can all be effectively tailored under different bias voltages. Different device functionalities, including optical amplitude/phase modulators and polarizers, are ascribed into the devising of the effective mode index. A comprehensive analysis and unified design scenarios upon waveguide geometries are summarized, with fabrication robustness and moderate process complexity. Moreover, design examples are manifested. We report a TM-mode-based phase modulator, achieving a π phase shift within an active length of 49.2 μm with dual graphene layers. A feasible polarization-independent amplitude modulator is also demonstrated, where the discrepancy of the imaginary parts of the effective mode index between the two fundamental modes is kept at an order of 10−5 over a broad wavelength range from 1.35 to 1.65 μm.

Polarization-Independent Modulator by Partly Tilted Graphene-Induced Electro-Absorption Effect

A polarization-independent graphene-based electro-absorption optical modulator concept is presented. The device is based on a slanted silicon stripe upon which two graphene flakes reside; on this surface, another silicon waveguide is capped, forming a rectangular silicon-graphene waveguide (SGW). Simulation results show that effective mode index of both TE and TM modes in the SGW undergo almost the same variations under different biases across a broad wavelength range. For a 30- $\mu \text{m}$ -long silicon-graphene waveguide, throughout the S, C, and L communication bands, extinction ratio (ER) higher than 20 dB can be obtained by proper choosing of switching points for “ON” and “OFF” states. The ER discrepancy between TE and TM modes is smaller than 2.3 dB, and the polarization dependency is lower than 0.1 dB for “ON” state, which fulfills the requirement of polarization-independent modulation. Calculations show that the 3-dB modulation bandwidth higher than 100 GHz of our modulator is possible.

Lithium Niobate Polarization-Independent Modulator Using Integrated Polarization Splitters and Mode Converters

Compact lithium niobate polarization-independent electrooptic modulators using integration of polarization splitters and mode converters are presented. Experimental results show polarization-dependent loss as low as 0.04 dB can be achieved for devices fabricated on Z -cut substrates with an applied voltage of 26 V. The electrode lengths required for electrooptic modulation are 10 and 6 mm for TE and TM modes, respectively, and the total device length is only 1.6 cm. Moreover, only one diffusion step is required that greatly simplifies the fabrication process.

Mechanical Modulation of a Hybrid Graphene–Microfiber Structure

Strain manipulation of a hybrid graphene-integrated microfiber structure is investigated experimentally and theoretically. Because of the ultralong light–graphene interaction length (tens of millimeters), as much as 30% polarization-independent modulation with a moderate strain of 5% is achieved. This platform may provide new guidelines for designing flexible graphene-integrated devices. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Polarization-independent terahertz wave modulator based on graphene-silicon hybrid structure**Project supported by the National Natural Science Foundation of China (Grant No. 61565004), the Natural Science Foundation of Guangxi Zhuang Autonomous Region, China (Grant Nos. 2013GXNSFDA019002 and 2014GXNSFGA118003), the Guangxi Scientific Research and Technology Development Program, China (Grant No. 1598017-1), the Guilin Scientific Research and Technology Development Program, China (Grant Nos. 20140127-1

In this study, we propose and demonstrate a broadband polarization-independent terahertz modulator based on graphene/silicon hybrid structure through a combination of continuous wave optical illumination and electrical gating. Under a pump power of 400 mW and the voltages ranging from –1.8 V to 1.4 V, modulation depths in a range of –23%–62% are achieved in a frequency range from 0.25 THz to 0.65 THz. The modulator is also found to have a transition from unidirectional modulation to bidirectional modulation with the increase of pump power. Combining the Raman spectra and Schottky current–voltage characteristics of the device, it is found that the large amplitude modulation is ascribed to the electric-field controlled carrier concentration in silicon with assistance of the graphene electrode and Schottky junction.

Polarization-independent submillisecond phase modulation utilizing polymer/short-pitch cholesteric liquid crystal composite.

A broadband, polarization-independent phase modulation spanning the visible range is demonstrated using a polymer/cholesteric liquid crystal composite with optical pitch in the ultraviolet. Polarization insensitivity is achieved as a result of two effects: (1) optical anisotropy of the rod-like molecules is canceled out by the short helical pitch, and (2) stabilization of the Grandjean texture by the polymer network suppresses depolarization. Polarization-independent modulation of the refractive index by approximately 0.045, corresponding to a phase modulation of π at 500 nm, is achieved with submillisecond response times. Our material system opens new avenues for polarization-independent, tunable optical devices, such as narrow bandpass filters, gratings, and adaptive lenses.

Relative Intensity Noise Suppression of Spectrum-Sliced Channels Using Polarization-Independent Optical Modulators

Performances of spectrum-sliced channels are strongly affected by their relative intensity noise (RIN). We use polarization-independent optical modulators (PIOMs) for spectrum-sliced channels to suppress their RIN. The PIOM driven by a high sinusoidal voltage signal evenly redistributes optical frequency components in the spectral domain and reduces the RIN. It can be used at a broadband light source (BLS) output to produce spectrum-sliced channels having lower RIN values. Also, it can be used for each spectrum-sliced channel within each optical network unit (ONU). In our experiment, where 12.5-GHz-spaced spectrum-sliced channels are used in 1-GbE speed, the use of PIOM at the BLS output reduces the bit error rate (BER) of the spectrum-sliced channel by more than an order of magnitude. The use of PIOM within the ONU reduces the BER by approximately 3 orders of magnitude.

Polarization-Independent Acousto-Optical Modulation of Bessel Light Beams

The acousto-optical diffraction of Bessel light beams (BLB) propagating near the optical axis of uniaxial gyrotropic crystals is examined. It is shown that polarization-independent modulation of Bessel beams is possible in paratellurite crystals; that is, the Bragg diffraction efficiency is independent of the polarization state of the incident beam. The physical reason for this kind of modulation is found to be the simultaneous occurrence of two anisotropic diffraction processes, such that the conditions for Bragg synchrony are satisfied for orthogonally polarized elliptical Bessel beams. The coupled wave equations and an overlap integral method are used to study the dependence of the diffraction efficiency on the length of the acousto-optical interaction, the ultrasonic power, and the polarization state of the incident Bessel beam.

Polarization-independent electro-optic modulator based on PMNT electrically-controlled birefringence effect and Sagnac interferometer

Abstract A novel polarization-independent electro-optic modulator is proposed and investigated by using the electrically-controlled birefringence effect of transparent lead magnesium niobate titanate (PMNT) electro-optic ceramic and a Sagnac interferometer structure. The PMNT electro-optic ceramic is used as a phase retarder for two counter-propagating waves in which their polarization directions are adjusted to parallel and vertical to the electrical-field vector respectively. Then the output light intensity from the Sagnac interferometer will be modulated completely with the driven electrical signal and it is independent with the polarization direction of incident light. The extinction ratio is more than 21 dB. This modulated device will be very appropriate as a Q-switch or optical attenuator in the high power laser system.

High efficiency reflective liquid crystal polarization gratings

We experimentally demonstrate a reflective-mode liquid crystal polarization grating with high reflectance, small grating period, and subms switching times. This switchable optical element can diffract ∼100% into a single order, have highly polarization-sensitive first orders, and have a polarization-insensitive zero order. Here we introduce an absorbing layer that overcomes the reflection of the (ultraviolet) holographic beams, which otherwise prevents high quality fabrication. At a grating period of 2.1 μm, we report 98% diffraction efficiency, 90% reflectance, ∼600:1 contrast-ratio, and ∼3000:1 polarization contrast. These elements can therefore be configured as polarization-independent modulators or switchable polarizing beam splitters, for use in telecommunications, displays, spatial-light modulators, and polarimetry.

P-209: Evaluation of Projection Schemes for the Liquid Crystal Polarization Grating Operating on Unpolarized Light

We report on our first experimental comparison of polarizationindependent projection schemes using the liquid crystal polarization grating (LCPG) modulator as the active element. This recently-demonstrated LC grating has unique diffraction properties, and we evaluate its performance using the bright- and dark-field Schlieren configurations and consider its use with a TIR-prism assembly. Each scheme shows promise for highbrightness and good-contrast projection for different applications. 1. Introduction crystal polarization gratings (LCPGs) show great promise as projection display modulators of unpolarized light, exhibiting high contrast and experimentally-demonstrated 100% diffraction efficiency [1]. This type of modulator is particularly suited for mobile, battery-powered “pocket” projectors that demand high contrast and brightness with minimal power consumption. As with the other types of projection display systems using diffractive modulators, the contrast and brightness of our system depends on the projection scheme employed. Since the principle of operation in the LCPG is distinct from previous LC gratings, we anticipate comparably improved projected image properties (contrast and brightness). As only very limited experimental and theoretical investigations into LCPG switches have been carried out (and none utilizing a realistic projection scheme), here we report on the first examination of such. This information should strongly inform the design of any realistic projection system based on the LCPG modulator. The key feature of our polarization-independent modulator is the diffractive LCPG, whose operation and structure is outlined in Fig. 1. This grating is unique from conventional gratings in that only three orders (m=±1,0) are present, and its diffraction efficiency for unpolarized light follows [1]:

P-167: FDTD and Elastic Continuum Analysis of the Liquid Crystal Polarization Grating

liquid crystal polarization grating (LCPG) has advantages of polarization independence of the zero diffraction order, nearly 100% diffraction efficiency in the first orders, and higher resolution capability over previously reported binary LC gratings. Here we analyze the LCPG, an electrically controlled, polarization-independent light modulator using the finite- difference time-domain (FDTD) method and the elastic continuum theory. The optical performance is studied and critical electrical parameters for a LCPG cell are presented. 1. Introduction of liquid crystal (LC) light modulators operating on unpolarized light have been proposed in an effort to overcome the substantial losses in conventional LC displays that require polarized light. Most prominently, Bos and coworkers suggested several potentially efficient and polarization-independent diffraction gratings using liquid crystals (1-4). These approaches, however, face crucial limitations for real applications due to their binary nature: fabrication difficulty and the easy appearance of defects. They therefore tend to have inherently large periods that result in many diffraction orders with small diffraction angles. A newly demonstrated liquid crystal polarization grating (LCPG) (5) is a switchable diffractive optical element with a continuously varying, periodic, anisotropic index profile. As shown in Fig. 1(a), this can be embodied as a nematic director that follows (6): n(x) = ˆ cos(x / ) + ˆ y sin(x /) + ˆ (0) (1)