Fundamental HE11 Mode Research Articles

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.

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.

Study on dual-mode large-mode-area multi-core fiber with air-hole

In this paper, a multi-core fiber with 22 air holes and 15 conventional cores is proposed. The fiber can realize dual-mode operation of fundamental mode HE11 and second-order mode HE21 under certain conditions. The relationship between performance of the fiber and the structural parameters that include core pitch, refractive index difference between core and cladding, and fiber core radius is investigated systematically. By adjusting the above parameters, the effective area of fundamental mode can be up to 1423.972 µm2 and keep a low bending loss at 1550 nm. Due to its flexibility of the structure, the proposed fiber is significant for theoretical research and practical manufacturing.

Quantum interface between light and a one-dimensional atomic system

We investigate optimal conditions for the quantum interface between a signal photon pulse and one-dimensional chain consisting of a varied number of atoms. The tested object is physically designed as an atomic array of tripod-type atoms confined with a nanoscale dielectric waveguide and experiencing its fundamental HE11 mode. The efficiency of interaction is mainly limited by achieved overlap and coupling of the waveguide evanescent field with the trapped atoms. We verify those physical conditions when the coupling within the main scattering channels would be sufficient for further providing various elementary interface protocols such as light storage, light-matter entanglement, preparation of a few photon states on demand, etc.

Terahertz rectangular corrugated transmission line with Gaussian mode filtering for DNP‐NMR

Gyrotrons have been developed as terahertz wave sources in dynamic nuclear polarisation (DNP) system to increase the sensitivity of nuclear magnetic resonance (NMR) in the high-magnetic field regime. However, the transmission of the Gaussian beam from the gyrotron output window to the NMR system proves to be a critical challenge. In this study, a terahertz transmission system, composed of a rectangular corrugated waveguide and a back-to-back rectangular corrugated horn for Gaussian mode filtering, is proposed. The attenuation in the optimised 0.33 THz corrugated waveguide remains lower than 0.01 dB/m in a broad frequency range from 280 GHz to 450 GHz. Besides, the coupling coefficient from the electric field in the 0.33 THz mode-filtering horn to the fundamental HE11 mode in the rectangular corrugated waveguide is obtained nearly 99.5%. This terahertz transmission system is potential to be applied in DNP-NMR spectrometer based on continuous wave gyrotrons.

Investigation of electromagnetic mode transition and filtering of an asymptotically single-mode hollow THz Bragg fibre

The mechanism for electromagnetic (EM) mode transition and filtering in an asymptotically single-HE11-mode hollow THz Bragg fibre is investigated. We designed, fabricated, and measured Bragg fibres with an asymptotically single-mode pattern, achieving measured signal propagation loss of better than 3 dB m−1 at 0.265 THz and with an operating frequency range from 0.246 to 0.276 THz. Mode transition and filtering effects are both verified by 3D full-wave simulations using measured material properties, with geometrical parameters extracted from the fabricated Bragg fibre prototypes. By optimizing the coupling efficiency between the free-space Gaussian beam and the guided Bessel function mode, the optimum distance of mode transition from the Gaussian-beam excitation into the guided mode is calculated to be ~13.7 free-space wavelengths in our fibre, to ensure fast EM-field convergence to the desired asymptotically single-mode mode pattern in the Bragg fibre. After this mode transition region, the electric field amplitude ratio between the desired HE11 mode and the main competing HE12 mode is approximately 7 times, with the HE12 mode attenuation being more than 10 dB m−1 larger than the fundamental HE11 mode; the results indicate that our fibre is one of the best candidates for low-loss asymptotically single-mode terahertz signal interconnections.

Unique loss characteristics in TE01 modes of conventional photonic bandgap fibers.

We report modeling that demonstrates the reduction of transmission loss and broadening of the bandwidth of a conventional hollow-core photonic bandgap fiber (PBF). Numerical investigation reveals that transmission loss of the high-order TE01 mode is lower than that of the fundamental HE11 mode in fibers with thick cladding walls. By comparing dispersion curves of PBFs with different core-wall thicknesses, we show that the TE01 mode has weaker coupling strength to a surface mode than the HE11 mode. This result opens the way for a wider transmission band and lower transmission loss in PBFs that are subject to the detrimental effects of surface modes.

Chirality of Light in Hybrid Modes of Vacuum-clad Ultrathin Optical Fibers

We investigate chirality of light in the quasicircularly polarized fundamental HE11 mode and thequasicircularly polarized higher-order hybrid modes of vacuum-clad ultrathin optical fibers. Weshow that, for a given fiber with the parameters in the range of experimental interest, the higherorder modes have smaller optical chirality per unit energy than the fundamental mode. The sign ofthe chirality per unit energy of a HE or EH mode is the same as or opposite to, respectively, thesign of the phase circulation direction. Outside the fiber, the fields in the quasicircularly polarizedHE11 mode and the quasicircularly polarized higher-order HE21 and HE31 modes are superchiral.

Comparative study of guided modes in hollow core Bragg fibers with binary and ternary photonic bandgap claddings

We comparatively investigate the modal properties of guided modes in hollow core Bragg fibers (HC-BFs) with the binary and ternary photonic bandgap claddings by using a full-vector finite element method. The influences of structural changes from the binary periodic cladding (BPC) to the ternary periodic cladding (TPC) on the dispersion relations, confinement losses, and modal field distributions of the lowest five guided modes are thoroughly discussed under different core diameters. The contrasts in bandwidths for the guided modes between both types of HC-BFs strongly depend on their distinct frequency cutoff characteristics. The bandwidth contrast for a given guided mode in large-core HC-BFs is different from that in small-core HC-BFs. The confinement loss of the fundamental HE11 mode in the TPC-based HC-BF is much lower than that in the BPC-based HC-BF, which is essentially attributed to the stronger modal field confinement by the TPC. The differences in minimum confinement losses of the HE11 mode between both types of HC-BFs decrease with decreased core diameters. In contrast, the changes of cladding structures have very limited influence on the confinement loss of the TE01 mode in both the large-core and small-core HC-BFs. Moreover, we explore the reasons behind the evolutions of the minimum loss contrasts for the HE11 and TE01 modes between both types of HC-BFs with the core diameter, by quasi-quantificationally analyzing the influences of reflection characteristics of the transverse electric (TE) and transverse magnetic (TM) waves on the confinement loss.

Corrugated Waveguide Mode Content Analysis Using Irradiance Moments.

We present a novel, relatively simple method for determining the mode content of the linearly polarized modes of a corrugated waveguide using the moments of the intensity pattern of the field radiated from the end of the waveguide. This irradiance moment method is based on calculating the low-order irradiance moments, using measured intensity profiles only, of the radiated field from the waveguide aperture. Unlike the phase retrieval method, this method does not use or determine the phase distribution at the waveguide aperture. The new method was benchmarked numerically by comparison with sample mode mixtures. The results predict less than ±0.7% error bar in the retrieval of the mode content. The method was also tested using high-resolution experimental data from beams radiated from 63.5 mm and 19 mm corrugated waveguides at 170 and 250 GHz, respectively. The results showed a very good agreement of the mode content retrieved using the irradiance moment method versus the phase retrieval technique. The irradiance moment method is most suitable for cases where the modal power is primarily in the fundamental HE11 mode, with <8% of the power in high-order modes.

Simple Correctors for Elimination of High-Order Modes in Corrugated Waveguide Transmission Lines.

When using overmoded corrugated waveguide transmission lines for high power applications, it is necessary to control the mode content of the system. Ideally, overmoded corrugated transmission lines operate in the fundamental HE11 mode and provide low losses for long distances. Unwanted higher order modes (HOMs), particularly LP11 and HE12, are often excited in the experimental systems due to practical misalignments in the transmission line system. This paper discusses how the unwanted modes propagate along with the fundamental mode in the transmission line system by formulating an equation that relates the center of power offset and angle of propagation of a beam (for the HE11 and LP11 modes) or the waist size and phase front radius of curvature of a beam (for the HE11 and HE12 modes). By introducing two miter bend correctors into the transmission system-miter bends that have slightly angled or ellipsoidal mirrors-the HOMs can be precisely manipulated in the system. This technique can be used to eliminate small quantities of unwanted modes, thereby creating a nearly pure fundamental mode beam with minimal losses. Examples of these applications are calculated and show the theoretical conversion of up to 10% HOM content into the fundamental HE11 mode with minimal losses.

Cold atom guidance in a capillary using blue-detuned, hollow optical modes

We demonstrate guiding of cold 85Rb atoms through a 100-micron-diameter hollow core dielectric waveguide using cylindrical hollow modes. We have transported atoms using blue-detuned light in the 1st order, azimuthally-polarized TE01 hollow mode, and the 2nd order hollow modes (HE31, EH11, and HE12), and compared these results with guidance in the red-detuned, fundamental HE11 mode. The blue-detuned hollow modes confine atoms to low intensity along the capillary axis, far from the walls. We determine scattering rates in the guides by directly measuring the effect of recoil on the atoms. We observe higher atom numbers guided using red-detuned light in the HE11 mode, but a 10-fold reduction in scattering rate using the 2nd order modes, which have an r4 radial intensity profile to lowest order. We show that the red-detuned guides can be used to load atoms into the blue-detuned modes when both high atom number and low perturbation are desired.

Modal Properties of Hybrid Plasmonic Waveguides for Nanolaser Applications

We investigate the modal properties of nanowire-metal hybrid plasmonic waveguides by use of the coupled-mode theory and finite-element method. We show that the coupling between the nanowire fundamental HE11 mode and surface plasmon mode results in three different supermodes. We numerically calculate the dispersion relations, normalized mode areas, and confinement factors of these different modes for plasmonic nanolaser applications and show that the lowest threshold mode is determined by the waveguide geometry.

Design and development of a dual‐mode corrugated horn for an offset reflector antenna

AbstractThis article presents the design and radiation characteristics of a dual‐mode corrugated matched feed horn. This type of feed eliminates the unwanted high cross‐polarization of an offset parabolic reflector antenna and improves the overall performance of the antenna sub‐system. In a cylindrical corrugated structure, higher order HE21 mode is added with the fundamental HE11 mode to configure a dual‐mode corrugated horn. For the proposed horn, the return‐loss characteristics and the far‐field radiation patterns were measured and the results are compared with the simulated results. The horn has been used as a primary feed device to illuminate a linearly polarized offset parabolic reflector antenna and the improvement in the cross‐polarization has been estimated. © 2009 Wiley Periodicals, Inc. Microwave Opt Technol Lett 52: 113–116, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24852

Cross-Polarization Cancellation in an Offset Parabolic Reflector Antenna Using a Corrugated Matched Feed

This letter presents the design of a dual-mode corrugated matched feed to cancel the unwanted high cross polarization and beam squinting in an offset parabolic reflector antenna. The matched feed concept has been implemented in a cylindrical corrugated structure by adding a higher order HE21 mode to the fundamental HE11 mode. The designed feed was then used to illuminate a linearly, as well as a circularly, polarized offset parabolic reflector antenna, and the secondary radiation patterns were computed. Significant improvement in the cross polarization was observed when the results were compared to that of a conventional corrugated horn illuminated offset reflector antenna.

An antenna-feed system with an RF drive source for a Cherenkov maser—an antenna-amplifier

The microwave transmission system of an X-band antenna-amplifier (a relativistic traveling-wave tube intended for operation in the fundamental nonaxisymmetric HE11 mode of a dielectric rod) has been investigated. A specially designed two-frequency tunable resonance microwave pulse compressor is used as an RF drive source. The microwave transmission system is incorporated into a hollow high-voltage electrode of a linear induction accelerator. It is shown that this transmission system can be reasonably matched in a frequency band of about 700 MHz, and microwave pulses with a duration of ∼4 ns and a peak power of at least up to 1.6 MW can be transmitted via it without substantial losses and envelope distortions. It is also shown that the actual configuration of the device virtually does not affect the angular distribution of the far-field radiation pattern of the rod antenna, which is an extension of the slow-wave structure of the amplifier.

采用基本空间充填模式的掺铒多孔光纤分析

Erbium-doped holey fiber with hexagonal lattice was modeled by using effective index method. In order to calculate the equivalent step index of the periodic structure of the cladding holey optical fiber, all-vectorial fundamental space filling mode approach was utilized. By using EH11 mode, we have numerically solved the rate equations of a three-level pumping scheme for a fiber laser. The obtained results have shown a good agreement with the other experimental results, recently. The results have predicted amplifiers with gain efficiencies as high as 10 dB/mW. OCIS codes: 060.2310, 060.2410, 060.2320, 140.3500, 140.4480. doi: 10.3788/COL20080609.0644. In erbium doped fiber amplifiers (EDFAs), to obtain high gain efficiency one needs to use fibers with high numerical aperture (NA) to achieve the maximum population inversion of the erbium ions. It is useful to confine the erbium ions to the central part of the core to enforce the pumping light propagating with the fundamental HE11 mode [1,2] . Germanium-doping, which is used in the core of high NA fiber, reduces the emission bandwidth of the Er 3+ ions and limits its concentrations. This results in narrower gain bandwidths and increases the device

Application of effective index method to higher order modes of photonic crystal fibers

AbstractThe application of effective index method (EIM) to calculate the modal indices of higher order modes (HOMs) of photonic crystal fibers (PCFs) is investigated. The effective index of the fiber cladding is obtained by a plane wave expansion method because analytical solution is found to induce large errors for PCFs with large air holes. The modal indices of the TE01 and TM01 modes of a PCF are solved using EIM as a demonstration of the applicability of this method. It is shown that there exists a different optimized core for these two modes when compared with the case for the fundamental HE11 mode. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 567–570, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.22196

A novel two-mode fiber-optic interferometer based on HE11–TE01 modal interference utilizing a liquid-crystal-clad fiber modal filter

A novel scheme of two-mode fiber-optic interferometer based on interference between HE11 and TE01 modes of a circular symmetric fiber is reported. This interferometer exploits a modal filter made of a piece of liquid-crystal-clad fiber, to select one of the second-order modes TE01 and filter the other TM01 and HE21 modes. Thus, two spatial modes, fundamental mode HE11 and second-order mode TE01, serve as the two arms of the interferometer to exhibit their phase modulation produced within a sensing region, on the output intensity pattern. A polarizer (analyzer) is employed at the output end of the fiber to achieve a two-lobe radiation pattern. Theoretical and experimental studies on the operating characteristics of such a two-mode fiber-optic interferometer are presented. The variation of the two-lobe pattern has a predictable dependence on the phase retardation between the two modes and can easily obtain an optimized visibility by rotating the polarizer (analyzer). The application of this interferometer for temperature sensing between 20 and 100°C is demonstrated. The measurement results show a phase shift sensitivity of 0.9rad/m°C, comparable to that of the existing two-mode fiber interferometer using an elliptical-core fiber.

Use of liquid-crystal-clad fiber as a modal filter for a two-mode fiber-optic interferometer

A novel two-mode fiber-optic interferometer employing a liquid-crystal-clad fiber as a modal filter is proposed. The liquid-crystal-clad fiber can guide one of the second-order modes, TE01, and leak out the other TM01 and HE21 modes. Thus two spatial modes, fundamental mode HE11 and second-order mode TE01 serve as the two arms of the interferometer to detect an external perturbation. The experimental results obtained when this interferometer was used to sense temperatures ranging from 20 to 100 degrees C are demonstrated.