Mode amplification fiber laser based on photonic band gap fiber

We theoretically propose the structure of all-solid photonic band gap fiber to support LP₀₁ mode at 980 nm and four modes of LP₀₁, LP₁₁, LP₀₂ and LP₂₁ at famplification of characteristics of the photonic band gab and apply photonic fiber as gain medium to fiber lasers, finally realizing separate amplification of LP₁₁ and LP₂₁ high-order modes with single-mode stable pump light of LP₀₁ mode at 980 nm. Besides, we realize LP₁₁ mode and LP₂₁ mode laser output of dual-wavelength.     

C-band fundamental/first-order mode converter based on multimode interference coupler on InP substrate

The design, fabrication and characterization of a fundamental/first-order mode converter based on multimode interference coupler on InP substrate were reported. Detailed optimization of the device parameters were investigated using 3D beam propagation method. In the experiments, the fabricated mode converter realized mode conversion from the fundamental mode to the first-order mode in the wavelength range of 1530-1565 nm with excess loss less than 3 dB. Moreover, LP₀₁ and LP₁₁ fiber modes were successfully excited from a few-mode fiber by using the device. This InP-based mode converter can be a possible candidate for integrated transceivers for future mode-division multiplexing system.     

LP01 to LP11 mode convertor based on side-polished small-core single-mode fiber

An all-fiber LP01-LP11 mode convertor based on side-polished small-core single-mode fibers (SMFs) is numerically demonstrated. The linearly polarized incident beam in one arm experiences π shift through a fiber half waveplate, and the side-polished parts merge into an equivalent twin-core fiber (TCF) which spatially shapes the incident LP01 modes to the LP11 mode supported by the step-index few-mode fiber (FMF). Optimum conditions for the highest conversion efficiency are investigated using the beam propagation method (BPM) with an approximate efficiency as high as 96.7%. The proposed scheme can operate within a wide wavelength range from 1.3 μm to1.7 μm with overall conversion efficiency greater than 95%. The effective mode area and coupling loss are also characterized in detail by finite element method (FEM).     

Spectral control of optical gain in a rare earth-doped optical fiber using novel triple layered structures

Novel techniques to control the effective emission cross section of a rare earth-doped optical fiber with a functional cladding structure are reviewed for applications in active fiber devices. We discuss evanescent wave filtering (EWF) technique based on the overlap in the radiative transition cross sections between the emitting ion in the core and the absorbing ion in the inner cladding. Experimental applications of EWF concept are reviewed for a composite gain medium with the erbium ions in the core and samarium ions in the inner cladding for self-gain flattening in C-band. W-type three layered fiber structure is also analyzed in terms of modal guidance and location of the LP₀₁ mode cut-off. Its application in neodymium and thulium-doped silica fiber laser is discussed to facilitate the radiative transition near 940 and 1600 nm region, respectively. The design concepts, fabrication process, and device performances are discussed for each application.     

Interféromètres modaux à symétrie circulate dans les fibres optiques

The spectral response of a few-mode fiber in the case of LP₀₁/LP₀₂ excitation is studied showing the similarities and differences between the transmission of such a bimodal interferometer and that of a classical interferometer. The applications envisaged are, on the one hand, the characterization of fibers (in particular by measurement of an equalization wavelength), and on the other hand, the development of all-fiber interferometric sensors without the need of a separate reference arm.     

All-fiber mode converter based on superimposed long period fiber gratings

In this paper, a novel broadband all-fiber mode converter is proposed and experimentally demonstrated. Through writing a pair of superimposed long period fiber gratings (SLPFGs) in tow-mode fiber (TMF) with a CO₂ laser, the mode converter can realize the conversion from LP₀₁ to LP₁₁ owing to the phase matching condition. Numerical and experimental results show that the bandwidth of this mode converter is 3 times broader than that of a single grating converter. The converter has low loss, high coupling efficiency, small size and is easy to fabricate, so it can be widely used in mode-division multiplexing.     

A novel four-air-hole multicore dual-mode large-mode-area fiber: Proposal and design

A novel four-air-hole multicore dual-mode large-mode-area optical fiber is proposed in this paper. The characteristics of the mode field distribution, the operating wavelength, the effective area A_eff and the bending loss of fundamental modes are calculated. The influence of the structural parameters on the operating wavelength, the effective index n_eff and the effective area of fundamental modes is also discussed. This fiber makes the second-order modes TE₀₁ and TM₀₁ cut-off while increasing the effective area of fundamental modes, which can realize strictly dual-mode operation. The maximum effective area of fundamental modes is approximately 4025 μm². The single-mode operation can be also realized by adjusting the structural parameters. This fiber is simple to fabricate, and flexible to design. It can be used for large-mode-area high-power optical fiber laser and amplifier.     

Proposal for sum-frequency generation based on modal phase-matching in photonic crystal fibers

In this paper, we report on a feasible phase matching between three low-order modes guided in an appropriately designed photonic crystal fiber (PCF). The phase matching condition for sum-frequency generation (SFG) can be realized over a large wavelength range by modifying the air holes size and the lattice pitch. This can be achieved in the proposed photonic crystal fiber because the sum-frequency wave propagating in a LP₀₂ mode leaks more into the low refractive index cladding than does the fundamental wave. By “feeling the cladding”, the LP₀₂ mode compensates for the increased refractive index of the silica at the sum-frequency wavelength. The numerical results show that the SFG in the proposed nonlinear PCF can be tunable, and the pump and sum-frequency waves are well-confined in the fiber core region.     

Measurement of RF Transmission Mode in ITER Relevant EC H&;CD Transmission Line

The preliminary results of mode measurement in the ITER relevant 40 m long transmission test line composed from 63.5 mm diameter corrugated waveguides and miter bends are presented. The field patterns were measured by taking temperature profiles on a paper screen placed in front of the waveguide end using an infrared imaging camera. The complex electric field at the waveguide end was retrieved from the measured temperature profiles. As a result, the transmission power includes 87% of HE₁₁ mode and 6% of LP₁₁ odd (HE₂₁+TE₀₁) mode and small ratios of other modes. The mode content had small dependence on length of the transmission line. This result indicated that the higher order mode content generated at the input of the transmission line is conserved and propagated through the transmission line. This suggests that the initial RF coupling to the waveguide is critical since it affects the launcher efficiency.     

Effects of circuit loss and space charge in gyro-travelling-wave tubes

A modified dispersion relation of a gyro-TWT using a cylindrical waveguide excited in aT E _mn -mode is derived incorporating the effect of the transverse space charge forces. The relation is subsequently studied considering the circuit losses and interpreted for the conventional TWT-type gain equation. The second harmonic operation using theT E ₀₂ cylindrical waveguide mode resonated with the second beam-harmonic mode is found to be more sensitive to the Pierce's loss and space-charge parameters for a gyro-TWT than the fundamental operation using theT E ₀₁ cylindrical waveguide mode resonated with the first beam-harmonic mode. It is established that these parameters substantially influence the device gain and must be considered for the prediction of its accurate value. Furthermore, these parameters when properly controlled can also improve the device bandwidth.     

High power millimeter-wave TE03 to TM11 mode converters

Transmission of power from 42 ± 0.2 GHz gyrotron (TE₀₃ mode) to tokamak or dummy load requires a set of transmission line components. It includes a set of mode converters that converts circularly unpolarised TE₀₃ mode to polarised HE₁₁ mode. The mode conversion sequence is methodised in two steps; first from TE₀₃ to TE₀₁ mode and then from TE₀₁ to TM₁₁ mode. The proposed mode converters performance depends on the parameters such as waveguide radius, beat wavelength, number of ripples, its perturbation amplitude (η) and bending angle. These parameters are numerically optimised and verified in CST microwave studio. TE₀₃ to TE₀₁ mode conversion is realised by the generation of intermediate mode TE₀₂. The mode conversion length for TE₀₃ to TE₀₂ mode converter is6λ₀, (where λ_b is the beat wavelength of corresponding mode conversion) which converts 99.15% of an incident circular TE₀₃ mode power into TE₀₂ mode power. Next, mode converter converts TE₀₂ mode into TE₀₁ mode with 99.06% efficiency along the optimal length of6λ_b. For TE₀₁ to TM₁₁ mode conversion, two conversion methodologies have been proposed: first using axis arc bend (34.94°) technique to convert TE₀₁ into its analogous degenerate mode with 99.01% conversion efficiency and second using parabolic curve with 90° bend, which gives a conversion efficiency of 97.5%.     

Loss characteristics of helical-core fiber

A special optical fiber is investigated,which has a helical core in the cylindrical cladding.The beam propagation method(BPM) is used for analyzing the impacts of the geometric and physical parameters on the properties of mode losses of the helical-core fiber.The propagation loss is 0.32 dB/m for the fundamental mode and the propagation loss is 20.95 dB/m for the LP11 mode in the wavelength range of 1050-1065 nm when the core diameter is 19 μm,the pitch of the core’s helix is 2.66 mm,and the offset of the helix core from the center of the fiber axis is 31 μm.The core diameter of the single-mode helical-core fiber well exceeds that of the conventional large-mode-area fiber.The helical-core fiber can provide the effective large-mode-area single-mode operation without coiling fiber or selecting excitation mode.     

Non-polarization and high-coupling-efficiency coupler using multilevel grating structure

A multilevel grating coupler based on silicon-on-insulator (SOI) material structure is proposed to realize the coupling between waveguide and waveguide or waveguide and fiber. This coupler is compatible with the current fabrication facilities for complementary metal oxide semiconductor (CMOS) technology with vertical coupling. This structure can realize coupling when the beams with transverse electric (TE) polarization and transverse magnetic (TM) polarization are incident at the same time. The influences of the grating coupler parameters including wavelength, the thickness of waveguide layer, the thickness of SiO₂ layer and the number of steps on the TE mode and TM mode coupling efficiencies are discussed. Theory researches and simulation results indicate that the wavelength range is from 1533 nm to 1580 nm when the TE mode and TM mode coupling efficiencies are both more than 40% as the grating period is 0.99 μm. The coupling efficiencies of the incident TE and TM modes are 49.9% and 49.5% at the wavelength of 1565 nm, respectively, and the difference between them is only 0.4%.     

Single Mode Laser Oscillation in an Nd‐Doped Large Core Double Clad Fiber Cavity with Concatenated Adiabatic Tapers

We created a new design for an Nd‐doped clad‐pumped silica fiber laser to enhance the pump absorption and lasing efficiency for a butt‐coupled, end‐pumped scheme. Two concatenated adiabatic tapers formed within the laser cavity simultaneously removed higher order modes and were spliced to conventional single mode fibers. We theoretically analyzed mode propagation along the composite cavity and experimentally achieved continuous wave oscillation in the LP₀₁ mode at 1.06 µm and a laser output power of over 820 mW with a slope efficiency of 27%.     

Tunable mode converter using electromagnet-induced long-period grating in two-mode fiber

We present an in-fiber mode converter that changes the propagation ratio of two core modes. A dynamic long-period grating is constructed in a two-mode fiber by the combination of an electromagnet with a nonmagnetic spring coiled around a stiff iron rod. The fundamental mode is converted to the LP₁₁ mode according to the voltage applied on the electromagnet. The mode-conversion wavelength can be tuned by stretching the coil spring. Compared to the coupling result with leaky cladding modes, the mode-conversion bandwidth expands from 5 nm to 90 nm and the drive voltage decreases almost by half.     

Mode selective directional couplers for overmoded waveguide systems

Two types of directional couplers for transverse electric (TE) modes are described: short and multihole couplers, respectively. They selectively pick one mode out of a mode mixture in an overmoded circular waveguide system. Unwanted modes are either statistically kept at low level or are suppressed by destructive interference in the coupling waveguide. Mode selectivity and directivity in multihole couplers oscillate up and down with an increasing number of holes, finally reaching a minimum of approximately 20 dB, unless there are competing modes with rational fractions of the beat wavelength. A multihole coupler for the TE₀₂ mode (28 GHz, 63.4 mm waveguide diameter, 41 holes) and a length of 1.6 m shows a calculated directivity of 68 dB and suppresses the unwanted modes TE₀₁ with 34 dB (24 dB), TE₂₂ with 37 dB (45 dB), and further modes TE_?m (?<5, m<6) with 17 dB to 34 dB in forward direction (figures in parentheses are for unwanted modes propagating in backward direction). A short directional coupler for the TE₀₁ mode (28 GHz, 63.4 mm waveguide diameter) with 16 holes and a length of 230 mm shows a directivity of 55 to 100 dB between 27.9 and 28.1 GHz, suppressing the TE₀₂ mode with 35 to 80 dB, the TE₀₃ mode with 30 to 65 dB, and the TE₂₂ mode with 30 to 70 dB.     

Gyrotrons: relativistic case,E and H modes,output converter design

Self-consistent non-linear equations of a relativistic gyrotron with the operational mode E ₀₁ as well as the E ₀₁ and H ₀₁ modes interacting are presented. On their basis, we have optimized the efficiency of a number of gyrotron variants—with uniform and non-uniform magnetic field and regular or irregular waveguide systems. An analysis of the peculiarities of the interaction in such variants has been performed. It has been shown that the total electron efficiency of relativistic gyrotrons with E₀₁ working mode can reach 35-45%. The interaction of the E₀₁ and H₀₁ modes (at the E ₀₁ working mode) is of an interference nature and under optimal conditions the efficiency may increase by 10%. An algorithm has been suggested to optimize the profile of the gyrotron waveguide output converter based on the optimization parameters separation technique. Results are presented for the case of the H₀₁ working mode.     

Cord identification technique for ultra-low bending loss fibers using higher order modes of visible light

We propose a cord identification technique for ultra-low bending loss fiber using higher order modes of visible light. With this kind of fiber, bending losses are greatly reduced and it is difficult to obtain sufficient leaked light with a conventional macro-bending technique. The bending loss of higher order modes is several orders larger than that of fundamental modes. Higher order modes can exist at shorter wavelengths and their guiding loss is small when the fiber is not tightly bent. As a result, higher order modes are suitable for cord identification purposes with ultra-low bending loss fiber. We determined that the LP₂₁ and LP₀₂ modes at 650 nm (red) and the LP₃₁ mode at 532 nm (green) are the most effective for cord identification purposes. We employed an offset launch technique to excite higher order modes, and achieved a sensitivity improvement of more than 14 dB. By using our method, a cord can be identified by red or green light even with the naked eye.     

Transmission Properties of Closed Trapezoidal-Groove Guide for Millimeter Waves

The transmission characteristics of the mode TE₁₁ for a new type of closed trapezoidal-groove guide with arbitrary inclined angle are analysed in detail with the mode-matching method in this paper. The calculated results are in good agreement with the experimental values in the published reference. The dispersion of the closed trapezoidal-groove guide is very low. The relationships of the cut-off wavelength and the attenuation constant with the structural sizes of the closed trapezoidal-groove waveguide are also obtained. The given results have very important value in study, design, manufacture and application for closed trapezoidal-groove waveguide.     

Symmetric waves in a circular metal waveguide containing an anisotropically conducting coaxial cylindrical surface

Symmetric waves propagating in a hollow metallic waveguide that contains a coaxial cylindrical surface which conducts perfectly along helical lines with a small twist angle are investigated. It is found that, at certain ratios between the structure’s geometric parameters, there exist two modes with close propagation constants. It is shown that the superposition of these modes results in a field that, during propagation along the waveguide, periodically coincides either with the field of the E ₀₁ mode of the inner cylindrical channel (in this case, the field is zero in the coaxial area) or with the field of the H ₀₁ mode of the outer channel. A circular-waveguide E ₀₁ ? H ₀₁ mode converter has been developed and experimentally investigated. It is shown that the conversion loss does not exceed 0.8 dB.