Mode Conversion Efficiency Research Articles

Ultracompact and broadband asymmetric directional-coupler-based mode division (de)multiplexer

This paper presents an ultracompact, low-loss, and broadband two-mode division (de)multiplexer. The device structure is based on a two-waveguide asymmetric directional coupler. By maximizing the effective index difference of the even and odd supermodes through proper design of the geometrical parameters of the waveguides, the coupling length without taking the S-bend into account is obtained as 1.4 μm. This is the shortest coupling length, to the best of our knowledge, for transverse-electric modes in mode (de)multiplexers reported until now. In addition to reducing the footprint, this structure improves the mode conversion efficiency while maintaining low-loss and low-crosstalk behavior. The three-dimensional finite-difference-time-domain simulation results show that at the wavelength of 1.55 μm, the mode conversion efficiency, crosstalk, and insertion loss are 99.86%, −32.7 dB, and −0.49 dB, respectively. Simulation results also demonstrate that the proposed mode (de)multiplexer exhibits low crosstalk of less than −11 dB over a broad bandwidth of 100 nm around the central wavelength of 1.55 μm.

Optimal configuration of the waveguide acousto-optic TE-TM polarization mode convertor on X-cut lithium niobate substrate

An excitation, distribution, and interaction of surface acoustic waves (SAW) with the guided light in integrated optical structures on X-cut lithium niobate substrates were studied. The resonance excitation frequencies, the spatial SAW distribution and the TE-TM optical mode conversion efficiency were investigated. An optimal configuration and most suited material of interdigital transducer (IDT) for integrated acousto-optic devices were found.

13 μm fiber grating in a thin-core fiber for LP01–LP11 mode converters and sensing ability

We demonstrate an all-fiber structure that can realize LP01-LP11 mode conversion and twist measurement. It is a thin-core fiber (TCF) grating at a wavelength of 1310nm cascaded to a short segment of a TCF of a different core size. It is found that the different core size of the TCF between the fiber and the grating has an impact on the excitation of a higher-order mode and mode conversion efficiency. The fiber structure exhibits a good linear response to twisting, strain, and temperature. Depending on the associated mode, the mode intensity and the wavelength for exciting the peaks of the grating have different sensitivities to twisting angle, applied strain, and temperature. These properties can be exploited for simultaneous measurement.

Millimeter wave □ TE 10 to ○ TE 01 mode transducer for electron cyclotron resonance heating applications

In this manuscript, a new design approach is proposed for □TE10 (rectangular) to ○TE01 (circular) mode transducer for electron cyclotron resonance heating (ECRH) applications. The design consists of two parts: first part has a rectangular waveguide, which transforms □TE10 to □TE20 mode, and in the second part, conversion of □TE20 into ○TE01 mode takes place. The electric field pattern, mode conversion efficiency or transmission efficiency and the operating principle of the individual parts are discussed, optimized, and analyzed. The novelty of this integrated design lies in the fact that it has merits of sidewall and inline coupling. Simulation study and optimization of the individual sections of the proposed mode converter were carried out using computer simulation technology (CST) microwave studio software (MWS). The simulation result shows that the design gives more than 97% of mode conversion efficiency with −1 dB bandwidth of 14.65% at 42 GHz. Copper material is used for fabrication of the mode transducers and are assembled with precise doweling. The simulated and measured high frequency (HF)-field pattern of the proposed mode transducer is also highlighted in the manuscript.

Efficient TE-Polarized Mode-Order Converter Based on High-Index-Contrast Polygonal Slot in a Silicon-on-Insulator Waveguide

We proposed a new type of transverse electric (TE) polarized mode-order converter based on a deeply-etched polygonal slot on a silicon-on-insulator waveguide. Along the transverse direction of the waveguide, two irregular boundary surfaces of the slot can introduce high-contrast index modulation on guided modes, leading to multimode interference in the slot. Therefore, when the slot is optimized, we can achieve efficient mode conversions based on the multimode interference. As examples, mode converters from the fundamental TE mode (TE0) to the first-order TE mode (TE1) and to the second-order TE mode (TE2) have both been demonstrated with a short device length ( μ m), a high mode conversion efficiency (>97.6%), and a low modal crosstalk (< −20.0 dB) over a broad wavelength range from 1500 to 1600 nm (∼100 nm). In addition, based on different polygonal slots, other types of mode conversions such as from TE1 to TE3 and from TE2 to TE1 have be realized. Fabrication tolerance of the proposed structure is analyzed. Owing to the high efficiency and compact size, the proposed structure could be applied to on-chip mode division multiplexing systems for high-density integration.

Orbital angular momentum generation in two-mode fiber, based on the modal interference principle.

In this Letter, we demonstrate, to the best of our knowledge, a novel method to generate an orbital angular momentum (OAM) based on the principle of the modal interference in a two-mode fiber. At the interference dips, the left- or right-handed circular polarized HE11 modes can be ideally converted into the ±1-order OAM beam. To verify this concept, we employed the femtosecond laser micro-processing technology to write micro-waveguides in the two-mode fiber and hence realized the in-line modal interferometer. To enhance the mode conversion efficiency at the dips, we optimized the waveguide parameters both theoretically and experimentally. The interference spectrum and spiral/fork patterns confirm the OAM beam generation with an efficiency as high as 99%.

All-Optical Mode Conversion and Temperature Sensing via Transient Grating in Step-Index Fiber

We demonstrate the formation of Kerr-induced transient long period grating in a standard single-mode fiber (SMF28) using sub-nanosecond laser pulses, which can be used to realize an efficient all-optical mode conversion and temperature sensing. The fiber supports $LP_{01}$ and $LP_{11}$ spatial modes at 1064-nm wavelength. The beating between two guided modes induces refractive index grating exploiting the Kerr effect. The probe beam co-propagating with the pump at the same wavelength but with different polarization exhibits efficient mode conversion efficiency from $LP_{01}$ to $LP_{11}$ mode. Temperature sensitivity is measured owing to the change in mode conversion efficiency due to the shift in resonant peak wavelength with the increase in temperature. The change in temperature is manifested by the change in spectral power of probe beam at the output. The experimental result demonstrates an average temperature sensitivity of 0.15 Watt/°C. The transient nature of the grating with high sensitivity makes our approach unique from the existing results and can open a route for potential applications.

Mode-conversion of the extraordinary wave at the upper hybrid resonance in the presence of small-amplitude density fluctuations

In spherical tokamaks, the electron plasma frequency is greater than the electron cyclotron (EC) frequency. Electromagnetic waves in the EC range of frequencies are unsuitable for directly heating such plasmas due to their reduced accessibility. However, mode-conversion of the extraordinary wave to the electron Bernstein wave (X–B mode-conversion) at the upper hybrid resonance makes it possible to efficiently couple externally-launched electromagnetic wave energy into an overdense plasma core. Traditional mode-conversion models describe an X-mode wave propagating in a potential containing two cutoffs that bracket a single wave resonance. Often, however, the mode-conversion region is in the edge, where turbulent fluctuations and blobs can generate abrupt cutoffs and scattering of the incident X-mode wave. We present a new framework for studying the X–B mode-conversion which makes the inclusion of these fluctuations analytically tractable. In the new approach, the high-field cutoff is modeled as an infinite barrier, which manifests as a boundary condition applied to a wave equation involving only one cutoff adjacent to the resonance on the low-field side. The new model reproduces the main features of the previous approach, yet is more suitable for analyzing experimental observations and extrapolating to higher dimensions. We then develop an analytical estimate for the effect of small-amplitude, quasi-monochromatic density fluctuations on the X–B mode-conversion efficiency using perturbation theory. We find that Bragg backscattering of the launched X-mode wave reduces the mode-conversion efficiency significantly when the fluctuation wavenumber is resonant with the wavenumber of the incident X-mode wave. These analytical results are corroborated by numerically integrating the mode-conversion equations.

Efficient Grating Couplers for Space Division Multiplexing Applications

Grating couplers are developed as one of the building blocks in silicon photonics to couple light between on-chip devices and optical fibers. For emerging high-capacity datacom and telecom systems enabled by space division multiplexing, it is essential to couple light together with mode conversion functions. In this work, we show the feasibility of combining conventional grating couplers and mode converters, both with a high efficiency. The proposed devices are very compact, built on standard silicon-on-insulator wafers, with no extra fabrication efforts required. It is shown that the LP01, LP11a, LP11b, and LP21b modes in a few-mode optical fiber can be successfully excited directly from the proposed grating couplers, which have the fundamental mode as an input from an integrated waveguide. The overall coupling efficiency, including mode conversion efficiency, can be as high as 50%. We also analyze the performance of the grating couplers in terms of bandwidth, efficiency, and tolerance to device fabrication errors.

Compact Temperature Sensor With Highly Germania-Doped Fiber-Based Michelson Interferometer

A compact temperature sensor with highly germania-doped fiber (Ge-fiber)-based Michelson interferometer (MI) is proposed and experimentally demonstrated. It is constructed by splicing a 1-mm-long, 75 mol.% GeO2-doped fiber to a single-mode fiber and fabricating a taper at the splicing point. The fiber taper improves mode conversion efficiency and thus increases the extinction ratio of the interference fringe. The free spectral range is only 18.9 nm owing to the much larger differential refractive index of the Ge-fiber than normal fibers. By monitoring wavelength shift of the reflection spectrum, temperature measurement within a wide range of 30°C–400°C is successfully achieved with sensitivity up to 100 pm/°C, which is much higher than normal fiber-based MI sensors owing to the higher thermo-optic coefficient of the Ge-fiber. The compact size and high sensitivity make the proposed sensor a highly potential candidate in point measurement of temperature.

Mode conversion via wavefront shaping.

In recent years, wavefront shaping has been utilized to control and correct distorted light for enhancing a bright spot, generation of a Bessel beam or darkening a complete area at the output of a scattering system. All these outcomes can be thought of as enhancing a particular mode of the output field. In this letter, we study the relation between the attainable enhancement factor, corresponding to the efficiency of mode conversion, and the field distribution of the target mode. Working in the limit of a thin diffuser enables not only a comparison between experimental and simulated results, but also allows for derivation of an analytic formula. These results shed light on the ability to use a scattering medium as a mode converter and on the relationship between the desired shape and the efficiency.

Ultra-compact and fabrication-tolerant mode multiplexer and demultiplexer based on angled silicon waveguides

An ultra-compact and fabrication-tolerant mode multiplexer (MUX) and demultiplexer (DeMUX) is proposed based on the angled silicon waveguides. The proposed mode (De)MUX is optimized by using the full-vectorial finite element method (FV-FEM) and three-dimensional full-vectorial finite difference time domain method (3D-FV-FDTD). The simulated results show that the optimal mode (De)MUX is with an ultra-compact length of 7.0μm, a high mode conversion efficiency of 98.5%, an ultra-low excess loss of 0.05 dB and a low mode crosstalk of −18.2 dB at the operating wavelength of 1550 nm. The fabrication tolerances are also studied for the proposed mode (De)MUX.

Investigation on adiabatic mode evolution in a few-mode optical waveguide

The mode-conversion characteristics of a few-mode optical waveguide based on adiabatic mode evolution are investigated. The mode-conversion efficiency and cross talks of the configuration are found dependent on the position and angle between the assistant waveguide and the main waveguide, and also the size of the assistant waveguide. By setting the bottoms of the two waveguides at the same plane, the symmetry of the configuration can be broken. The configuration is able to convert each mode to a higher order or lower order mode by launching light from different ports. In particular, mode conversion between the two degenerated LP11 modes can be achieved with wide bandwidth.

LP01-LP11a mode converters based on long-period fiber gratings in a two-mode polarization-maintaining photonic crystal fiber

A long-period fiber grating (LPFG) mode converter based on a two-mode polarization-maintaining photonic crystal fiber (PM-PCF) is proposed and demonstrated. The mode converters realize conversions between the LP01 modes and LP11a modes with parallel polarization directions. Different from typical conventional mode converters, the PM-PCF-LPFG mode converters most notably can separate out two linearly polarized LP11a modes at different wavelengths. The highest mode-conversion efficiency is more than 99%. In addition, the bandwidth of the mode converter is adjustable by changing the grating number of the LPFG.

Fiber-based mode converter for generating optical vortex beams

In this work, an all-fiber-based mode converter for generating orbital angular momentum (OAM) beams is proposed and numerically investigated. Its structure is constructed by cascading a mode selective coupler (MSC) and an inner elliptical cladding fiber (IECF). OAM modes refer to a combination of two orthogonal LPlm modes with a phase difference of ±π/2. By adjusting the parameters and controlling the splicing angle of MSC and IECF appropriately, higher-order OAM modes with topological charges of l = ±1, ±2, ±3 can be obtained with the injection of the fundamental mode LP01, resulting in a mode-conversion efficiency of almost 100%. This achievement may pave the way towards the realization of a compact, all-fiber, and high-efficiency device for increasing the transmission capacity and spectral efficiency in optical communication systems with OAM mode multiplexing.

Multi-channel mode converter based on a modal interferometer in a two-mode fiber.

In this Letter, we propose a multi-channel mode converter with the concept of a modal interferometer in a two-mode fiber (TMF). Two lateral stress points in a TMF function as in-line fiber mode couplers to construct the modal interferometer, and both transmission spectra and near-field patterns confirm that the LP01 mode is successfully converted into an LP11 mode at the multiple channels. The measured mode conversion efficiency almost completely follows the theoretical tendency. Finally, the mode conversion is realized at 20 channels in the C+L wavelength band with conversion efficiency up to 99.5% and insertion loss as low as 0.6dB. Furthermore, the channel spacing can be freely tailored by adjusting the distance between two stress points.

Hybrid plasmonic mode converter: theoretical formulation and design with a graphical approach

The theoretical formulation based on rigorous transmission-line networks is developed for a general mode conversion problem and lays the groundwork for a simpler yet more efficient graphical design approach for hybrid plasmonic mode converters (HPMCs). The concurrence of co- and cross-polarization conversion to and among higher-order photonic and HP modes followed by subsequent power redistributions and losses over the course of the HPMC can lead to performance degradation and largely determines the silicon core thickness. Using gradient ascent of the TM polarization fraction incorporated with modal index contours sets critical perturbation parameters for required transverse structural asymmetry. Polarization reversal estimates are shown to be practically applicable for about 60% of the total device length. The mode conversion efficiency (MCE), insertion loss (IL), and the polarization conversion efficiency of the proposed HPMC (<7×0.4 μm2) at λ0=1550 nm are 90.04%, 0.4691 dB, and 99.96%, respectively. The 85%-bandwidth of the MCE is 135 nm, while the IL stays below 0.5 dB over a 68-nm spectral range.

Laguerre-Gaussian modal q-plates.

We propose space-variant uniaxial flat optical elements designed to generate pure Laguerre-Gaussian modes with arbitrary azimuthal and radial indices l and p from an incident Gaussian beam. This is done via the combined use of the dynamic and the geometric phases. Optimal design protocol for the mode conversion efficiency is derived, and the corresponding characteristics are given for -6≤l≤6 and 0≤p≤5. The obtained "modal q-plates" may find many applications whenever the radial degree of freedom of a light field is at play.

Embedded-Silicon-Strip-to-Hybrid-Plasmonic Waveguide Polarization Mode Converter

A novel polarization mode converter bridging a conventional embedded silicon strip waveguide and a metal-capped hybrid plasmonic waveguide is proposed. Innovative design principles based on gradient ascent of the TM polarization fraction in conjunction with the modal index contour for practical initial designs are developed and numerically verified. The mode conversion efficiency (MCE), insertion loss (IL), and the polarization conversion efficiency (PCE) at $\lambda _{0}=1550$ nm are 87.58%, 0.5899 dB, and 99.84%, respectively. The respective 1-dB optical bandwidths for the MCE and IL are about 133 and 140 nm, while the PCE is > 98% over a 102-nm spectral range. The low-index silica spacer is found to impose the most stringent fabrication requirement.

Using reservoir-engineering to convert a coherent signal in optomechanics with small optomechanical cooperativity

Optomechanical dark mode plays a central role in effective mechanically-mediated conversion of two different cavity fields. In this paper, we present a more efficient method to utilize the dark mode to transfer a coherent signal. When an auxiliary cavity mode is exploited, two approaches are proposed to effectively eliminate the optomechanical bright mode, and only the optomechanical dark mode is left to facilitate state transfer. Even with small cooperativity and different losses for the two target modes, the internal cavity mode-conversion efficiency can also reach unity.