Mode Splitter Research Articles

Radially polarized cylindrical vector beam generation in all-fibre narrow linewidth single-longitudinal-mode laser

A new method to generate the radially polarized cylindrical vector beams (CVBs) is proposed and experimentally demonstrated. The method is based on a narrow linewidth single-longitudinal-mode erbium-doped fiber ring laser incorporating an all-fiber fused mode selective coupler (MSC). The MSC, made of a single-mode fiber (SMF) and a homemade few-mode fiber (FMF), is inserted into the cavity of the laser to act as both a mode convertor and a mode splitter. It is able to directly convert the fundamental mode HE11 in the SMF to the radially polarized TM01 mode in the FMF and accordingly output the converted radially polarized CVB. The FMF used in the MSC is specially designed to break the near-degeneracy of the vector modes in the LP11 group, adding to the ease and effectiveness of mode conversion from HE11 mode to TM01 mode in the MSC. The measured linewidth of the laser is approximately 7.8 kHz with a 20 dB linewidth and the mode purity of the radially polarized CVB is about 94%.

Design of compact mode splitters using identical coupled waveguides with slots

In the past, multimode transmission through fibers has not been considered a preferred means of signal propagation due to the intermodal dispersion, which limits the data rate. However, recently, there has been an interest in using different modes as individual channels to increase the overall data rate. However, the challenging task in implementing such a system is that each individual mode has to be excited separately without mixing, and these should remain separated even at the bents and splices, and finally, they need to be collected separately. In this paper, design and optimization of a compact optical mode splitter by introducing a small slot in a silicon nanowire waveguide is demonstrated by employing a full-vectorial finite element method. The authors report here that by creating a slot within a waveguide, the desired coupling length ratio of 1:2 between the fundamental and the second modes can be obtained. The waveguide junctions have also been analyzed by using a rigorous least squares boundary residual method to study power transfer efficiency and effect of fabrication tolerances.

Broadband on-chip polarization mode splitters in lithium niobate integrated adiabatic couplers.

We report, to the best of our knowledge, the first broadband polarization mode splitter (PMS) based on the adiabatic light passage mechanism in the lithium niobate (LiNbO3) waveguide platform. A broad bandwidth of ~140 nm spanning telecom S, C, and L bands at polarization-extinction ratios (PER) of >20 dB and >18 dB for the TE and TM polarization modes, respectively, is found in a five-waveguide adiabatic coupler scheme whose structure is optimized by an adiabaticity engineering process in titanium-diffused LiNbO3 waveguides. When the five-waveguide PMS is integrated with a three-waveguide "shortcut to adiabaticity" structure, we realize a broadband, high splitting-ratio (ηc) mode splitter for spatial separation of TE- (H-) polarized pump (700-850 nm for ηc>99%), TM- (V-) polarized signal (1510-1630 nm for ηc>97%), and TE- (H-) polarized idler (1480-1650 nm for ηc>97%) modes. Such a unique integrated-optical device is of potential for facilitating the on-chip implementation of a pump-filtered, broadband tunable entangled quantum-state generator.

Dissipative soliton resonance Ytterbium-doped fiber laser with cylindrical vector beam generation

We experimentally demonstrate a dissipative soliton resonance (DSR) Ytterbium-doped fiber laser with high-purity cylindrical vector beam (CVB) generation, using a mode-selective coupler (MSC) as transverse mode converter and splitter. The all-normal-dispersion mode-locked Ytterbium-doped fiber laser operating at DSR regime without spectral filter can deliver CVB pulses with a pulse duration of ∼733 ps and a pulse energy as high as ∼0.439 nJ. The radially and azimuthally polarized beams can be switched by adjusting the polarization controller, with a high mode purity measured to be >94.5%. This compact DSR fiber laser could find potential applications in the field of material processing, nonlinear optics and so on.

Temperature-Independent Micro-Refractometer Based on Cascaded In-Fiber Air Cavities With Strain-Error Correction

We proposed a new fabrication technology of in-fiber air-cavity by splicing single-mode fiber and hollow core fiber (HCF) filled with the glycerol solution in this paper. A fiber-optic interferometer composed of two cascaded ellipsoidal in-fiber air cavities was fabricated by the above-mentioned method. The two air cavities are the mode splitter and the mode combiner of the transmission-type multimode interferometer (MMI), respectively. The collapsed HCF with the length of $973~\mu \text{m}$ between them can be used as the refractive index (RI) sensing area of the MMI, and the temperature sensitivity of the MMI can be ignored compared with the RI sensitivity. Owing to the RI insensitivity and temperature insensitivity, the air cavity with the minimum wall thickness of $6.2~\mu \text{m}$ is used as the reflection-type Fabry–Perot interferometer to compensate the strain of the MMI. Research shows that the sensor can eliminate the effects of temperature and strain in the measurement of RI.

Passively Mode-Locked Ytterbium-Doped Fiber Laser With Cylindrical Vector Beam Generation Based on Mode Selective Coupler

We propose and demonstrate, for the first time to the best of our knowledge, a passively mode-locked ytterbium-doped fiber laser generating cylindrical vector beams (CVBs) using a mode selective coupler (MSC) as the transverse mode converter and splitter, and a semiconductor saturable absorber mirror for mode-locking. According to the coupling mode theory and the phase matching principle, the MSC was fabricated by weakly fused technology, having a low loss of about 0.5 dB and achieving LP11 mode with a high purity of >96%. The CVB fiber laser operates at a center wavelength of 1042.3 nm, with a 3 dB spectral width of 1.5 nm. The repetition rate of the mode-locked laser pulses is 18.58 MHz. The radially and azimuthally polarized vector beams can be switched by adjusting the polarization controllers in the fiber ring cavity, with a high mode purity measured to be >95%. The mode-locked CVB ytterbium-doped fiber laser has potential applications in optical tweezers, optical imaging, etc.

A 1- $\mu$ m Cylindrical Vector Beam Fiber Ring Laser Based on a Mode Selective Coupler

We propose and demonstrate a continuous-wave all-fiber ring laser generating cylindrical vector beams (CVBs) using a mode selective coupler (MSC) as transverse mode converter and mode splitter. The MSC is fabricated by a novel method free of pre-tapering, achieving LP11 mode with a high purity of >96% near the wavelength of 1064 nm. The CVB fiber laser operates at a center wavelength of 1053.9 nm, with a 3-dB linewidth of less than 0.04 nm and a signal-to-background ratio of >60 dB. The laser slope efficiency is >9%. The radially and azimuthally polarized beams can be switched by adjusting the polarization controllers in the fiber ring cavity, with a high mode purity measured to be >96%.

High efficiency mode-locked, cylindrical vector beam fiber laser based on a mode selective coupler.

We propose and demonstrate an all-fiber passively mode-locked laser with a figure-8 cavity, which generates pulsed cylindrical vector beam output based on a mode selective coupler (MSC). The MSC made of a two mode fiber and a standard single mode fiber is used as both the intracavity transverse mode converter and mode splitter with a low insertion loss of about 0.65 dB. The slope efficiency of the fiber laser is > 3%. Through adjusting the polarization state in the laser cavity, both radially and azimuthally polarized beams have been obtained with high mode purity which are measured to be > 94%. The laser operates at 1556.3 nm with a spectral bandwidth of 3.2 nm. The mode-locked pulses have duration of 17 ns and a repetition rate of 0.66 MHz.

Mode Splitter Without Changing the Mode Order in SOI Waveguide

We proposed a mode splitter based on directional coupler (DC), which is capable of splitting the TE0 & TE1 modes without changing the mode order. It is found that high coupling efficiency (>;1 dB), low mode crosstalk (<;16 dB), wide bandwidth (~100 nm) and large fabrication tolerance (±10 nm) can be obtained by a 24.4 μm coupling length DC structure, which is composed of a strip and a slot silicon waveguide. This design represents a first step towards mode splitter without changing the mode order, which can find important potential applications, such as optical isolation and mode division multiplexing (MDM), in large scale photonic integrated circuits (PICs). © 1989;2012 IEEE.

Accurate Analysis of the Mode (de)multiplexer Using Asymmetric Directional Coupler

Design optimizations of asymmetric directional couplers for the fundamental quasi-TE (TM) mode with the higher order quasi-TE (TM) modes (de)multiplexer including the ${\bf H}_y^{21} $ ( ${\bf H}_x^{21} $ ), ${\bf H}_y^{31} ({\bf H}_x^{31})$ , ${\bf H}_y^{41} ({\bf H}_x^{41})$ , and ${\bf H}_y^{51} ({\bf H}_x^{51})$ modes are studied by a full-vectorial H-field finite-element method. Phase matching of nonidentical nanowires (NWs) are more critical and accuracy of the numerical method for the design is tested here. The effect of possible fabrication tolerance is also discussed thoroughly. These results show that a narrow separation of 100 nm or similar value can be employed for the quasi-TE mode splitters, but for the quasi-TM modes, a relatively wider separation of ∼ 500 nm may be used. The fabrication of the single mode NW should be strictly accurate within ±5 nm of the design values.

Waveguide Mode Splitter Based on Multi-mode Dielectric-Loaded Surface Plasmon Polariton Waveguide**Supported by the National Basic Research Program of China under Grant Nos 2011CBA00200 and 2011CB921200, the Strategic Priority Research Program (B) of the Chinese Academy of Sciences under Grant No XDB01030200, the National Natural Science Foundation of China under Grant No 11374289, the Fundamental Research Funds for the Central Universities

In photonic integrated circuits, information is usually encoded in the optical path. In this work, based on the multi-mode dielectric-loaded surface plasmon polariton waveguide, we numerically design a directional coupler, which can divide the different waveguide eigenmodes into different optical paths. The results show a possibility to encode information onto different waveguide modes. We also experimentally demonstrate that the splitting ratio of this directional coupler structure can be tuned without changing its size.

Tunable omnidirectional absorber and mode splitter based on semiconductor photonic crystal

In this paper, the properties of one-dimensional (1D) photonic crystals (PCs) composed of the semiconductor (GaAs) and dielectric layers are theoretically investigated by the transfer matrix method (TMM). The absorption of semiconductor layers is investigated theoretically. Due to the magneto-optical Voigt effect, the dielectric constant of the semiconductor is modified differently in different modes and frequency ranges. If the frequency range of the incident wave is larger than the plasma frequency, TE and TM modes of the incident wave will be absorbed in a wide incident angle. TM wave will be absorbed but TE wave will be reflected while the frequency range is less than the plasma frequency. The absorption of semiconductor can also be tuned by varying the external magnetic field. The proposed PCs have a reconfigurable application to design a tunable omnidirectional absorber and mode splitter at same time.

Self-imaging of orbital angular momentum (OAM) modes in rectangular multimode interference waveguides.

We study the propagation of orbital angular momentum (OAM) modes in rectangular multimode waveguides. Due to the multimode interference effect, an OAM mode input forms self-images at certain propagation distances. As OAM modes can be decomposed as the superposition of a pair of quarter-wave phase-shifted even and odd modes, their symmetry properties lead to two different self-imaging categories - forming the OAM-maintaining and the field-splitting self-images. We analyze these phenomena using multimode interference theory, and establish the rules governing the OAM-maintaining self-imaging, which allows the multi-mode interference waveguides to be used as OAM mode splitters and couplers.

Large bandwidth mode order converter by differential waveguides.

In this article, we propose a large bandwidth mode-order converter design by dielectric waveguides with equal lengths but different cross-sectional areas. The efficient conversion between even and odd modes is verified by inducing required phase difference between the equal length waveguides of different widths. Y-junctions are composed of both tapered mode splitter and combiner to connect mono-mode waveguide to multi-mode waveguide. The converted mode profiles at the output port show that the device operates successfully at designed wavelengths with wide bandwidth. This study provides a novel technique to implement compact mode order converters and direction selective/sensitive photonic structures.

Miniature Fiber-Optic Strain Sensor Based on a Hybrid Interferometric Structure

We present a compact fiber-optic strain sensor with a hybrid interferometric structure, including a micro-cavity Fabry-Pérot interferometer (MC-FPI) and a Mach-Zehnder interferometer (MZI). The sensor is formed by splicing two conventional single-mode fibers (SMFs), one as the lead-in fiber and the other as the lead-out fiber, to both ends of a photonic crystal fiber (PCF). Through controlling the splicing parameters, the lead-in SMF/PCF splice forms a micro-cavity and causes the collapse of the microholes in the PCF, which acts as a mode splitter/combiner. For the lead-out SMF/PCF splice, no micro-cavity is formed and the splicing point only functions as a mode splitter and combiner. The PCF together with the two splicing points form the MZI. Sensor prototypes with different MC-FPI lengths are developed. Combining reflection and transmission spectra together, the sensor achieves a high strain sensitivity, which can find application in real strain measurement.

GPS Antenna Used in Tracking and Identifying Cargo

In order to track and identify remote cargo with electronic label, the electronic label may carry GPS module. To GPS modules antenna, not only the factors such as impedance bandwidth and axial ratio bandwidth are considered, but also the stability of the phase center is more concerned about, due to it determines the precision and accuracy of location. At the same time it requires that axial ratio is less than 3 db in wide angle range of elevation theta. Realizes circular polarized GPS antenna through the orthogonal 4-fed mode and Wilkinson power splitters, in addition, calibrates and optimizes the average phase center of antenna through the optimization algorithm. The simulation results show that GPS antenna works in the L-band 1.575 GHz,impedance bandwidth in which return loss less than-10 db achieves 91.4 MHz,and coverages: 1.5358 ~ 1.6272 GHz; 3db axis ratio bandwidth achieves 305 MHz in the biggest gain direction of XOZ section;In the center frequency point 1.575 GHz, elevation theta which corresponds 3db axis ratio coverages: -119.2°to 111.5°; The stability of the phase center is less than 3 mm.

Multi-spectral bands optical coherence tomography assisted by ultrawide band photonic crystal fiber splitter

We report fabrication and performance of the 2×2 photonic crystal fiber (PCF) splitter that enables multi-spectral bands operation of an optical coherence tomography (OCT) system. The PCF splitter was made by coupling PCFs to a planar lightwave circuit (PLC) splitter chip, which worked as a single mode splitter from visible to near infrared wavelengths. The core size of the PLC splitter chip was about 3μm×3μm and the core-cladding index difference was about 0.25%, which gave the cutoff wavelength at 630nm. The implemented PCF-PLC splitter showed a small excess loss of 1.2∼3.3dB and a low polarization-dependent loss of 0.15∼0.19dB over a wide wavelength range from 640nm to 1000nm. With the splitter, OCT images of a human finger and a nail have been successfully obtained at several spectral bands; 680nm, 840nm, and 930nm center wavelengths.

Characteristics of bend sensor based on two-notch Mach–Zehnder fiber interferometer

A compact all-fiber modal Mach–Zehnder interferometer is presented based on two notches at different locations along normal single-mode fibers by CO2 laser irradiation. Two notches are mode splitter and mode combiner, respectively. This configuration which does not need any troublesome cleaving or aligning process is demonstrated as a bending sensor. The results suggest that the transmission loss varies and the interference fringe shifts with the bending increasing. The fluctuation of temperature dose not affects the shape of the interferometer fringe and transmission. Therefore, the proposed device offers salient advantages of simple fabrication technique and low cost. In addition, a high stability over time was expected since the irradiated notches do not degrade over time or with temperature. Due to its asymmetry, it also possesses higher sensitivity to orientation, which would be a promising unit for sensor applications.

Photonic Crystal Fiber-Based Interferometric Biosensor for Streptavidin and Biotin Detection

We report the fabrication of a highly sensitive refractometer based on photonic crystal fiber (PCF) interferometry. Using fusion arc splicing technique, a short length of PCF (∼3 mm) was spliced to conventional single-mode fibers on both sides. The voids of the PCF were completely collapsed in the splice region, which acted as mode splitter and combiner on both sides. Microbubbles were formed in the splice region and acted as diverging lens for exciting cladding modes efficiently. The transmission spectrum exhibited detectable “dips” for refractive index (RI) sensing. The bulk liquid sensitivity can reach up to 320 nm/RIU (RI unit) based on the linear curve fit for RI ranging from 1.33 to 1.34. Biosensing experiment was carried out using streptavidin and biotin as respective analyte and receptor. To the best of authors’ knowledge, this is the first time of reporting bioaffinity test on the external surface of PCF using modal interferometry.

Mode Conversion/Splitting by Optical Analogy of Multistate Stimulated Raman Adiabatic Passage in Multimode Waveguides

We propose and describe mode converter/splitter based on optical analogy of multistate stimulated Raman adiabatic passage in multimode waveguides. Computer-generated planar holograms are used to implement the coupling coefficients that mimic the optical pulses used in the transfer among quantum states of atoms and molecules. The mode coupling properties in multimode waveguides are analyzed using the coupled-mode theory and shown to resemble the multistate stimulated Raman adiabatic passage process. Key features of multistate systems are illustrated with theoretical calculations and numerical examples. Mode converter and splitter are designed based on the theoretical analysis and verified using beam propagation simulations.