HE11 Mode Research Articles

Generating broadband cylindrical vector modes based on polarization-dependent acoustically induced fiber gratings using the dispersion turning point

Cylindrical vector beams (CVBs) with special polarization distribution have been extensively investigated due to the unique ways of interacting with matter. Although several configurations have been developed to generate CVBs, such as Q-plates and subwavelength gratings, the bandwidth of a single CVB is inherently narrow due to the phase geometry, which would limit its application for femtosecond lasers. Here, a broadband CVB mode converter based on an acoustically induced fiber grating (AIFG) and a tuning method of dispersion turning point (DTP) is demonstrated both theoretically and experimentally with the 3-dB bandwidth of 125 nm, which is more than 10 times that of conventional AIFGs. Not only can the DTP wavelength be tuned from the original 1500 nm to 1650 nm by thinning the fiber, but also the stable generation of a single broadband HE21odd/even mode can be controllably implemented by adjusting the polarization state of the incident light, owing to the larger beat length difference between HE21 and other CV modes. Additionally, the femtosecond CVBs and orbital angular momentum (OAM) modes are successfully generated and amplified by combining the broadband AIFG with a figure-9 mode-locked fiber laser. Meanwhile, it is verified by simulation that the choice of broadband CV mode and the tunability of DTP wavelength can be realized by designing ring-core fibers with different structures, which can furthermore improve the flexibility of generating high purity CVBs. This study provides a highly controllable technique for the generation of broadband CVBs and OAMs paving the way for high-capacity CVBs communication.

Multiphysics simulations of a cylindrical waveguide optical switch using phase change materials on silicon

This work presents the design and multiphysics simulation of a cylindrical waveguide-based optical switch using germanium-antimony-tellurium (GST) as an active phase change material. The innovative cylindrical architecture is theoretically analyzed and evaluated at 1550 nm wavelength for telecommunication applications. The dispersion relation is derived analytically for the first time to model the optical switch, while finite element method (FEM) and finite difference time domain (FDTD) techniques are utilized to simulate the optical modes, light propagation, and phase change dynamics. The fundamental TE01 and HE11 modes are studied in detail, enabling switching between low-loss amorphous and high-loss crystalline GST phases. Increasing the GST thickness is found to increase absorption loss in the crystalline state but also slows down phase transition kinetics, reducing switching speeds. A 10 nm GST layer results in competitive performance metrics of 0.79 dB insertion loss, 13.47 dB extinction ratio, 30 nJ average power consumption, and 3.5 Mb/s bit rate. The combined optical, thermal, and electrical simulation provides comprehensive insights towards developing integrated non-volatile photonic switches and modulators utilizing phase change materials.

Bi-directional high-order orbital angular momentum mode generation using gratings in high numerical aperture fiber

We propose a novel design and theoretically analyse it for bi-directional orbital angular momentum (OAM) mode generation in high numerical aperture (NA) optical fibers using the combination of a long period grating (LPG), a short-period (Bragg) grating and a three-port optical circulator. The proposed design consists only of standard fiber gratings rather than specially designed fibers, such as ring-shaped or other types of fibers, making our model relatively simple. Full-vector true-mode analysis has been used to study the modal characteristics of the supported modes in the high-NA fiber. High-NA optical fibers can effectively relieve the degeneracy of the linearly polarized modes. A circularly polarized (CP) light is injected into a single mode fiber, the output end of which is axially spliced into a high-NA fiber. An appropriately designed raised-cosine apodized tilted LPG (TLPG) is used for broad bandwidth coupling of the fundamental HE11 mode to the co-propagating HE21 mode. The employed TLPG converts the CP fundamental core mode (HE 11x ± iHE 11y ) to ±1 order OAM modes (HE 21even ± iHE 21odd ). Both the CP mode and the generated OAM pass through the circulator in the direction of port 1 to port 2. A second fiber is connected at the output of port 2 and a tilted fiber Bragg grating (TFBG) is used to couple the forward-propagating HE11 core mode with the counter-propagating HE21, EH11 and HE31 core modes. The TFBG reflected higher order (±1 and ±2) OAM modes are transmitted through port 3. We observe that the TFBG has a mode-conversion efficiency of more than 95 % for each mode conversion. The generated vortex beam topological charges have been verified using the coaxial interference method.

Low-threshold single ternary GaAsSb nanowire lasers emitting at silicon transparent wavelengths

Conventional binary III–V nanowire (NW) lasers face substantial challenges in tuning their lasing emission to silicon transparent wavelengths and require complex quantum heterostructure designs for realizing on-chip integrated nanolasers. Here, an alternative and straightforward approach is reported by developing ternary III–V NW-lasers in the form of surface-passivated GaAsSb NW-lasers grown on silicon. High-quality GaAsSb NW-cavities with high Sb-content (>20%) and extended lengths (>5 μm) are shown to exhibit striking radiative efficiency enhancements (∼200-fold) when passivated by closely lattice-matched InAlGaAs shell layers. Utilizing this core–shell approach, optically pumped lasing is then demonstrated from single GaAsSb NW-lasers with lasing threshold as low as 3.2 μJ/cm2 at temperatures up to 250 K and emission wavelengths of ∼1.1–1.2 μm. Analysis of the optical mode spectra and mode-dependent threshold gain further shows that lasing is induced by the fundamental HE11 modes, and likely even lower thresholds may be achieved by establishing the TE01 mode at increased NW-cavity diameters.

Optimization and Dispersion Tailoring of Chalcogenide M-Type Fibers Using a Modified Genetic Algorithm

M-type optical fibers in which a core is surrounded by a thin ring layer with a higher refractive index have attracted increasing attention in recent years. One of their advantageous features is the ability to operate a non-fundamental LP02 mode possessing unusual dispersion properties, namely, a zero-dispersion wavelength (ZDW) shifted to the short wavelength region relative to the material ZDW. The LP02 mode can be selectively excited since it is predominantly localized near the core, while the fundamental LP01 and other higher modes are localized near the ring (for proper fiber parameters). In this paper, we present a comprehensive theoretical analysis of effective dispersion tailoring for the HE12 mode of highly nonlinear chalcogenide glass fibers (for which the LP mode approximation fails due to large refractive index contrasts). We demonstrate fiber designs for which ZDWs can be shifted to the spectral region < 2 μm, which is of great interest for the development of mid-IR supercontinuum sources and frequency-tunable pulse sources with standard near-IR pumping. We obtained the characteristic equation and solved it numerically to find mode fields and dispersion characteristics. We show the possibility of achieving dispersion characteristics of the HE12 mode with one, two, three, and four ZDWs in the wavelength range of 1.5–5.5 μm. We used a modified genetic algorithm (MGA) to design fibers with desired dispersion parameters. In particular, by applying an MGA, we optimized four fiber parameters and constructed a fiber for which HE12 mode dispersion is anomalous in the 1.735–5.155 μm range.

Differential derivative method for highly accurate calculation of fiber refractive index curves

The refractive index of an optical fiber is an important parameter, which is closely related to the characteristics of dispersion, birefringence, and attenuation. It is vital to accurately calculate the refractive index in designing special optical fibers. However, the refractive index curves for different transmission modes of an optical fiber may be crossed or closed at some specific wavelengths, where the calculator may easily be tricked. A differential derivation method (DDM) is demonstrated to accurately calculate the refractive index curves by enhancing the mode tracking ability. The effectiveness is verified in the calculations of the refractive index curves for the HE11, HE12, TE01, and EH11 modes of a custom-designed fiber.

Nanofibre-based trap for Rb2 molecule

We describe a theoretical proposal of a nanofibre-based trap for a Rb2 molecule prepared in the metastable state . The trapping potential results from the combination of a travelling and a standing-wave fields, both carried by the fundamental guided mode HE11 of the fibre. We show that, with an experimentally realistic choice of laser frequencies and powers, one can implement a ≈200μK-deep well at ≈140 nm from the fibre surface accomodating for ≈500 translational molecular states.

Evaluation of transmission efficiency of the ECH/CD transmission lines in integrated commissioning phase on JT-60SA

An electron cyclotron heating and current drive (ECH/CD) system with two transmission lines (TLs), TL#1 (waveguide inner diameter ⌀=31.75 mm) and TL#2 (⌀=60.3 mm), has been installed and operated on JT-60SA for the integrated commissioning phase. In order to evaluate the transmission efficiency of the TLs, the theoretical loss of each TL component is calculated assuming pure HE11 mode. In addition, the millimeter wave power at the beginning and end of two TLs was measured experimentally by calorimetric dummy loads (DLs) at high-power of ∼ 400 kW for ∼ 0.3 s. The output power of the gyrotron is estimated by using the measured power at the beginning of each TL with the theoretical loss between the DL and the gyrotron. The injection power to the tokamak from a launcher is also estimated by using the measured power at the end of each TL with the theoretical loss between the DL and the launcher. Using these values, the transmission efficiencies, defined as the ratio of the gyrotron output power to the tokamak injection power, are evaluated for both TLs, which are ∼70% for TL#1 at 110 GHz, and 80 % (82 GHz), 82 % (110 GHz), and 84 % (138 GHz) for TL#2. The total power injected into the tokamak by the combination of TL#1 and TL#2 is expected to be 1.5 MW. It is found that the measured transmission efficiency of TL#2 shows a good agreement with the theory, while the measured transmission efficiency of TL#1 was slightly lower than the theoretical value. To investigate this result, power measurements are also performed at additional locations on the TL#1 with additional DLs.

3D-Printed Dielectric Rods for Radar Range Enhancement

A design strategy to alter the radiation characteristics of modular radar printed circuit boards without the need for expensive retooling and remanufacturing is presented in this paper. To this end, a compact radar module including microstrip array antennas integrated with a dielectric rod lens is considered for a demonstration of an X-band radar antenna gain improvement leading to radar detection range enhancement. Using travelling wave theory, the proposed lens is designed to target the excitation of HE11 mode to achieve gain improvement without disturbing reflection coefficients. Using a low-cost rapid-manufacturing 3D-printing technology, two pairs of the 3D-printed dielectric rods integrated with a dielectric housing are designed and fabricated uniformly for a commercially available off-the-shelf radar module. The radar integration with the dielectric rod lens leads to a low-cost and easy-to-fabricate long-range radar system. Compared with the radar without the rods, the design system achieved a measured 6.6 dB gain improvement of the transmitter and receiver antennas which causes doubling the detection range for both elevation and azimuth directions at 10.525 GHz.

Design of an all hollow fiber‐coupled middle infrared laser heterodyne radiometer (LHR)

AbstractAn integrated optics method, using hollow fibers which easily ensures optimal beam alignment of the local oscillator (LO) and incoherent solar radiation and promotes high photomixing efficiency, is proposed for a compact, rugged, and high‐efficiency all hollow fiber‐coupled photomixing assembly in the mid‐infrared (IR). The method is adopted for the design of an all‐hollow fiber‐coupled mid‐IR laser heterodyne radiometer (LHR) using an 8 µm external‐cavity quantum cascade laser as LO. Coupling efficiency, attenuation coefficients, total‐loss transmission, and output beam divergence of the HE11 mode in hollow fibers for the all hollow fiber‐coupled mid‐IR LHR are discussed in detail and are shown to highly depend on the launch conditions. Meanwhile, the selection of input and output coupling lenses that are associated with the hollow fibers is also studied for achieving the theoretical loss of the HE1m mode in hollow fibers and high heterodyne performance. It shows that using a hollow fiber for photomixing supersedes conventional spatially separated optics.

Study on the temperature characteristic of tapered twin-core fiber working at dispersion turning point

We present a study on the spectral characteristics and ultrahigh sensitivity near the dispersion turning point (DTP) of tapered twin-core fiber (TCF). The tapered TCF with the DTP can be fabricated by the one-step taper method by the flame taper machine. Numerical simulations reveal that DTP exists when the group effective refractive index difference between the HE11 mode and the HE21 mode equals zero, where ultrahigh sensitivities can be achieved. The position of the DTP is dependent on the diameter of the taper waist and the refractive index of the external environment. We experimentally demonstrate the temperature characteristic of tapered TCF with DTP in the air, and its maximum sensitivity is − 994 pm/ºC. Finally, by coating polydimethylsiloxane (PDMS) material with the high refractive index and the high thermal-optical coefficient on the tapered TCF, not only the DTP can be achieved in large diameter, but also ultra-high temperature sensitivity can be obtained. The measured temperature sensitivity of the tapered TCF coated with PDMS is as high as 36.71 nm/ºC.

Ultra-broadband mid-infrared TE01 vector mode selector based on ring-core few-mode photonic crystal fiber

A mid-infrared TE01 vector mode selector based on chalcogenide ring-core few-mode fiber is proposed and investigated numerically by the finite element method (FEM). By embedding metal composite nanowire in the PCF core to generate surface plasmon resonance, TE01 vector mode could propagate due to low loss transmission, whereas the fundamental mode, TM01 and HE21 modes could hardly be maintained due to high loss. The effects of structural parameters on mode transmission characteristics and mode loss in photonic crystal fibers in the mid-infrared wavelength range are investigated. Under the optimal structural parameters, the loss ratios of each mode to TE01 mode are more than 50 in the waveband range from 2 to 3 μm, which means the TE01 single-mode transmission can be maintained in ultra-wide mid-infrared band. The designed fiber can be connected with the mid-infrared laser as a vector mode selection device to efficiently output pure TE01 mode cylindrical vector beam, which makes preferable application in mid-infrared optical communication, optical capture, super-resolution imaging, biochemical sensing and other fields.

Power scaling of normal-dispersion continuum generation using higher-order modes in microstructured optical fibers.

The use of a higher-order HE12-like mode to produce weak normal dispersion over a substantial wavelength range in a microstructured optical fiber is investigated numerically. It is shown that the effective area, and thereby the pulse energy, can in this way be scaled by an order of magnitude compared to using the fundamental mode in a single-mode fiber. Multimode nonlinear simulations indicate that nonlinear mode coupling will not disturb single-mode operation in the HE12 mode at least up to the threshold where polarization modulation instability sets in.

Development of high-power, long-pulse, multi-frequency ECH/CD system for JT-60SA

A gyrotron and a matching optics unit (MOU) for the multi-frequency electron cyclotron heating and current drive (ECH/CD) system in JT-60SA have been developed successfully. The gyrotron demonstrated stable operation at high powers of 1.5 MW for 5 s and 1.9 MW for 1 s at 110 GHz. To obtain high HE11 mode purity (>90%) at the outlet of the waveguide in the ECH/CD launcher, a MOU for operating at three frequencies of 82 GHz, 110 GHz, and 138 GHz that includes three pairs of water-cooled phase correcting mirrors has been developed, which allows the mirrors to be changed without opening the evacuated MOU. The mode purity at the inlet of a dummy load in the transmission line was evaluated at high-power and an HE11 mode purity of >90% was obtained at three frequencies.

Wideband Corrugated Feedhorns, for Radar, Communications, Radiometry and Quasi-Optics

A wide variety of desirable antenna beam patterns can be synthesized by optimal excitation and phasing of the HE11 and HE12 modes in scalar corrugated feedhorns. However, the bandwidth of such two-mode horns is often limited by modal dispersion. In this paper we introduce a class of low dispersion, two-mode feedhorns that can operate, in some cases, over operating bandwidths of 40-50%. We provide example designs that include horns with high coupling efficiency to: 1) a pure HE11 mode for single-mode excitation of corrugated pipe transmission lines; 2) a LG00 and LG02 combination for radiometry, with narrow beams; 3) a pure Laguerre Gaussian LG00 mode for quasi-optical instrumentation with constant phase centers; 4) a constant gain antenna for uniform illumination with frequency; 5) Airy patterns or “top hat” patterns for radar or communications applications, designed to maximize aperture efficiencies when used with larger reflect or lens antennas. More generally, we show methods to generate and phase multiple HE1n modes, to synthesize symmetric output beams at any desired frequency or gain.

Multimode -symmetry thresholds and third-order exceptional points in coupled dielectric waveguides with loss and gain

A pair of coupled dielectric waveguides with balanced loss and gain is the basic system for observing the phenomena of parity-time () symmetry and exceptional points (EPs). We use the multimode analytical approach to study the peculiarities of interaction between the modes excited in such an archetypal structure. With this approach, the conditions for symmetry breaking are obtained in the special case of the hybrid HE and EH modes resulting in the ordinary second-order EPs (or -symmetry thresholds). The coupling of modes with the different azimuthal indices (the TM01 and HE21 modes in our study) generates the uncommon types of EPs. First, there are the so-called multimode -symmetry thresholds due to interaction of hybrid and transverse modes. Second, the third-order EPs appear as a result of three-modes degeneracy of real eigenvalues. It is noteworthy that an increase in the EPs order is achieved without both breaking the symmetry and introducing additional elements into the waveguide structure. The results obtained with the multimode analytical approach are in perfect agreement with the independent full-wave simulations.

Characterization of intra-mode forward Brillouin scattering induced by vector high-order optical mode in nanofibers

We present the characteristics of intra-mode forward stimulated Brillouin scattering (FSBS) induced by HE11 and high-order optical mode (HOM) in nanofibers, respectively. Multiple transverse acoustic modes (TAMs) would be excited in the FSBS process. However, only a few specific low-order TAMs participate in the interaction of different optical mode pairs and then contribute to Brillouin gain. Meanwhile, the changes of dominating Brillouin peaks with the nanofiber radius are shown in detail, which further clearly confirms the difference in the gain characteristics of different intra-mode Brillouin gain spectrums. Especially, several obvious maximum and minimum gain values exist at the nanofiber radius of around 0.8 μm. The HOM induced intra-mode FSBS is of great importance for understanding of the phonon–photon interaction in multi-mode waveguides.

Purcell Effect and Beaming of Emission in Hybrid AlGaAs Nanowires with GaAs Quantum Dots.

Control of directionality of emissions is an important task for the realization of novel nanophotonic devices based on nanowires. Most of the existing approaches providing high directionality of the light emitted from nanowires are based on the utilization of the tapered shape of nanowires, serving as nanoantenna coupling with the light waveguided in nanowire and the directional output beam. Here we report the beaming of the emitted light with wavelength near 800 nm by naturally formed core-shell AlGaAs NW with multiply GaAs quantum dots (QDs) diameter 30 nm and height 10 nm, while the diameter of NW 130 nm, what does not support efficient emission into waveguided modes, including the mode HE11. Experimental measurements show that intensity of emission for directions in the vicinity of the axis of NW is about two orders of magnitude higher than for perpendicular directions. The developed theoretical approach allowed us to calculate the probability of spontaneous emission for various directions and into waveguided modes and showed that highly directional radiation can be provided by the intrinsic emission properties of cylindrical NW. Our results suggest that for the small diameter of NW, directional emissions are associated with an TM0 leaky mode (when electric field oriented in axial direction) and therefore manifests in an existence of axial electric dipole transitions in quantum dots.

Ultrasensitive interferometers based on zigzag-shaped tapered optical microfibers operating at the dispersion turning point.

This work proposes and demonstrates a novel interferometric sensor based on a zigzag-shaped tapered optical microfiber (Z-OMF) working at the dispersion turning point (DTP). The Z-OMF can be fabricated in a controllable manner through a modified fiber tapering method. Our study shows that the bending taper can transfer a portion of the fundamental HE11 mode to higher-order modes, and when the bending angle of the Z-OMF reaches 1.61°, high contrast interference fringes can be formed between the HE11 and the HE21 modes. More importantly, we find that by optimizing the diameter of the OMF, the group effective refractive index (RI) difference between HE11 and HE21 mode equals zero, and the refractive index sensing performance can be dramatically improved. To validate our proposed sensing mechanism, we experimentally demonstrate an ultrahigh sensitivity of 1.46×105 ± 0.09×105 nm/RIU. The proposed Z-OMF interferometer has the advantage of high sensitivity and low cost and shows excellent potential in chemical and biological detection.

Forward Brillouin scattering between vector high-order optical modes in a few-mode nanofiber

Gain and acoustic characteristics of inter-mode forward stimulated Brillouin scattering (FSBS) induced by discrete vector high-order optical mode (HOM) pairs in a few-mode nanofiber (FMNF) are presented in this paper. Three completely different acoustic modes of TA02, R22, and R21 preferentially interact with the TE01–TM01, TE01–HE21, and TM01–HE21 mode pairs, respectively, and dominate the highest Brillouin peak at different frequencies. Moreover, the influence of FMNF radius on Brillouin gain spectrum (BGS) characteristics is also investigated. The R21-caused Brillouin peak in the BGS of TM01–HE21 is much higher than that of TE01–TM01 (TE01–HE21). Especially, the R21 acoustic mode that interacts with the TM01–HE21 mode pair can provide the maximum Brillouin gain, which is also the highest peak at frequency ranges greater than 1 GHz, compared with all inter-mode and even intra-mode BGSs. And a definite functional relationship is found between the incident wavelength, the optimal FMNF radius, and the maximum of R21-caused Brillouin gain. Vector HOMs induced inter-mode FSBS may provide a promising approach for applications, such as sensing and nonlinear dynamics of the Brillouin interaction.