LP11 Mode Research Articles

Design of a Nested Hollow-Core Anti-Resonant Fiber Sensor for Simultaneous Measurement of Temperature and Strain

A highly sensitive sensor, which can detect the temperature and strain simultaneously, is proposed using a hollow-core anti-resonant fiber with composite nested tubes. The sensing fiber contains two kinds of nested tubes, and two different sensing mechanisms, the resonance coupling effect and the intermodal interference, are realized in the same section of a hollow-core anti-resonant fiber fully filled with ethanol. Five conjoined nested anti-resonant tubes are introduced to suppress the confinement loss of the higher-order mode LP02. One hybrid conjoined nested tube, which consists of a half-circular anti-resonant tube and a half-circular resonant tube, is introduced to induce a resonant coupling between the LP02 mode in the core and the dielectric mode in the nested resonant tubes. Numerical investigations demonstrate the shifts of the feature wavelengths of the resonance coupling effect, and the intermodal interference shows different velocities with temperature and strain, while a simultaneous measurement of temperature and strain can be realized with high sensitivities (3.36 nm/°C and −0.003 nm/με to temperature and strain, respectively). Since the sensor can be fabricated by full infiltration with liquid into the large-size core and cladding tubes of hollow-core anti-resonant fibers, and special post-processing, such as selective infiltration or coating, is notneeded. The proposed sensors based on hollow-core anti-resonant fibers with functional liquid infiltration provide a more efficient and versatile platform for the temperature and strain sensing.

Efficient mode coupling/(de)multiplexing between a few-mode fiber and a silicon photonic chip

Mode-division multiplexing (MDM) has attracted much attention due to its ability to further increase the transmission capacity of optical interconnects. While further developments of MDM optical interconnects are hindered by the coupling of few-mode fibers (FMFs) and silicon photonic chips, a high-efficiency, broadband, and scalable multimode FMF-chip interface is still eagerly desired. To address this challenge, a novel scheme for efficient multimode coupling is proposed by introducing a silica planar lightwave circuit as an intermediate. The core idea is to couple and demultiplex higher-order modes by leveraging the superiorities of silica optical waveguides for manipulating LP modes, facilitated through tailoring the mode conversion related to different mode symmetric properties. The demultiplexed modes are consequently butt-coupled to the silicon photonic chip in single-mode manner, thus being available for fulfilling further data transmitting/receiving/routing directly. As a proof of concept, a six-channel FMF-chip coupler working with the LP01-x/y, LP11a-x/y, and LP11b-x/y modes is designed with low coupling losses of 0.77–1.39 dB and low intermode crosstalk of <−27.2 dB in a broad bandwidth (>150 nm). Minimum coupling losses of 1.36–2.48 dB are experimentally demonstrated. It is the first demonstration for the integrated multimode FMF-chip coupler enabling the simultaneous coupling of six mode-channels, to the best of our knowledge. We believe that this work has the great potential for developing energy-efficient and low-cost chip-to-chip MDM interconnections in the future.

Unlocking mode programming with multi-plane light conversion using computer-generated hologram optimisation

Abstract Programmable optical devices provide performance enhancement and flexibility to spatial multiplexing systems enabling transmission of tributaries in high-order eigenmodes of spatially-diverse transmission media, like multimode fibre (MMF). Wavefront shaping with spatial light modulators (SLMs) facilitates scalability of the transmission media by allowing for channel diagonalization and quasi-single-input single-output operation. Programmable mode multiplexing configurations like multi-plane light conversion (MPLC) utilise the SLM and offer the potential to simultaneously launch an arbitrary subset of spatial tributaries in any N -mode MMF. Such programmable optical processor would enable the throughput of space-division multiplexing (SDM) systems to be progressively increased by addressing a growing number of tributaries over one MMF and in this way meet a growing traffic demand – similarly to the wavelength-division multiplexing evolution path. Conventionally, MPLC phasemasks are calculated using the wavefront matching algorithm (WMA). However, this method does not exploit the full potential of programmable mode multiplexers. We show, that computer-generated hologram algorithms like direct search enable significant improvement compared to the traditional WMA-approach. Such gains are enabled by tailored cost functions with dynamic constraints concerning insertion loss as wellas mode extinction ratio. We show that average mode extinction ratio can be greatly improved by as much as ≈ 15 dB at the expense of a small insertion loss35
deterioration of < 3 dB. One particular feature of programmable mode multiplexers is the adaptability to optimised transmission functions. Besides conventional LP modes transmission, we employ our approach on Schmidt modes, which are spatial eigenchannels with minimum crosstalk derived from a measured transmission matrix

Enhancing the performance of LP mode multiplexing devices via incident angle optimization of the array beam

Enhancing the performance of LP mode multiplexing devices via incident angle optimization of the array beam

Multi-wavelength Erbium-doped Fiber Laser with High-purity LP11 Mode Output Based on Mechanically Induced Long-period Fiber Gratings and Double Sagnac Fiber Filter

Multi-wavelength Erbium-doped Fiber Laser with High-purity LP11 Mode Output Based on Mechanically Induced Long-period Fiber Gratings and Double Sagnac Fiber Filter

A quad-polarization reconfigurable transmitarray antenna with beam-scanning and reconfigurable polarization capabilities

Abstract In this paper, a novel quad-polarized electronically reconfigurable transmitarray antenna(QRTA) is proposed, which can actively switch among four linear-polarization modes and steer beam-scanning in these polarization modes. The QRTA element follows a receiving-transmitting structure. Its receiver or transmitter is composed of four rotationally symmetric patches and a coupling patch. Four rotationally symmetric patches generate x- and y-polarization modes, while coupling patches on the top and bottom layers of the QRTA element are designed to synthesize x-, 45◦-, y-, and 135◦-polarization modes. The QRTA element utilizes a reverse symmetry current method to achieve a 180◦ phase difference in the radiated electromagnetic waves. By manipulating the state of the p-i-n diodes, the proposed QRTA element can achieve four LP functions and a 1-bit phase shift. Based on the proposed novel QRTA element, a 10 × 10 elements QRTA prototype is fabricated and measured. The measured results demonstrate that the proposed QRTA can electronically switch between four LP modes and achieve beam-scanning in those LP modes. The proposed QRTA shows several advantages, including low cost, multi-polarization capability, high gain, and beam-scanning, which means that it can enhance the polarization capabilities and intelligence of wireless communication systems.

Multi-plane light conversion (MPLC) LP mode multiplexer based on grayscale maskless lithography.

Multi-plane light conversion (MPLC) is a technique that uses multiple phase plates to modulate a light beam step-by-step. This technique has attracted widespread attention in the field of mode-division multiplexing (MDM) communications due to its high flexibility. MPLC device requires precisely controlled fabrication accuracy in experiments, but conventional multi-etching processes will accumulate alignment errors. Here, the fabrication of the MPLC device using maskless grayscale lithography was proposed, which requires only a single-exposure process. Through single-exposure lithography, the continuous phase of the digital mask ranging from 0 to 2π on MPLC is discretized into 128 steps. The digital masks of the MPLC with more steps of phase can reduce the insertion loss and mode crosstalk of LP modes. By using the fabricated MPLC, we experimentally demonstrate the MDM of LP01, LP11a, LP11b, and LP21 modes with mode crosstalk less than -22 dB, and the insertion loss less than 4 dB. In high-speed optical communications, each LP mode carries a 10 Gbit/s on-off keying (OOK) signals, and the experimentally measured bit error rates (BER) curves power penalty is less than -7 dB. The experiment demonstrated that maskless grayscale lithography can efficiently and accurately fabricate MPLC mode multiplexers.

Ultra-broadband multimode fiber-to-chip edge coupler based on periodically segmented waveguides.

Mode-division multiplexing (MDM) technology demonstrates a bright outlook for enhancing the capacity of chip-scale or fiber-based optical communication. Nevertheless, the fiber-to-chip MDM optical interconnects are hindered by the considerable mode mismatch and inter-modal cross talk between the few-mode fiber (FMF) and on-chip few-mode waveguide (FMW). In this Letter, a new, to the best of our knowledge, multimode coupling solution based on periodically segmented waveguides for the MDM system is proposed, which achieves efficient conversion between LP01, LP11a, and LP11b modes in FMF and E11, E21, and E12 modes in FMW with low refractive index difference. The simulation results show that the coupling loss is less than 0.41, 0.27, and 0.90 dB for the three modes, over the wavelength range of 1100-1800 nm. The fabricated device based on a polymer platform shows low fiber-to-chip coupling losses of less than 1.8, 1.7, and 3.0 dB, respectively, over a 130 nm wavelength. The presented scheme provides a competitive solution for realizing the ultra-efficient integration of prospective fiber-chip optical interconnections and communications.

Generation of High-Quality Cylindrical Vector Beams from All-Few-Mode Fiber Laser

Transverse mode control of laser intracavity oscillation is crucial for generating high-purity cylindrical vector beams (CVBs). We utilized the mode conversion and mode selection properties of two-mode long-period fiber gratings (TM-LPFGs) and two-mode fiber Bragg gratings (TM-FBGs) to achieve intracavity hybrid-mode oscillations of LP01 and LP11 from an all-few-mode fiber laser. A mode-locked pulse output with a repetition rate of 12.46 MHz and a signal-to-noise ratio of 53 dB was achieved with a semiconductor saturable absorber mirror (SESAM) for mode-locking, at a wavelength of 1550.32 nm. The 30 dB spectrum bandwidth of the mode-locked pulse was 0.13 nm. Furthermore, a high-purity CVB containing radially polarized and azimuthally polarized LP11 modes was generated. The purity of the obtained CVB was greater than 99%. The high-purity CVB pulses have great potential for applications in optical tweezers, high-speed mode-division multiplexing communication, and more.

Mode conversion in graded-index few-mode fiber via hollow cylindrical long-period fiber gratings.

We demonstrate the realization of mode conversion using hollow cylindrical long-period fiber gratings (LPFGs) inscribed in graded-index few-mode fibers (FMFs) by a femtosecond laser. By precisely shaping the refractive index modulation into a hollow cylindrical structure, we enable efficient coupling from the fundamental mode to high-order modes (LP11, LP02, LP21, LP12, LP31, LP03, and LP22 modes). The method achieves high-efficiency and low-loss mode conversion across various mode groups, marking a first for graded-index FMFs with different grating periods. The influence of the hollow cylindrical LPFG radius on mode conversion efficiency and spectral characteristics is thoroughly investigated. Additionally, incorporating a linear polarizer allowed for distinguishing between LP modes within a mode group, enhancing the tunability of the mode converter. These mode conversion devices have significant potential for applications in mode gain equalization and mode scrambling for mode division multiplexing (MDM) systems.

Forward design method for the design of panda polarization-maintaining few-mode optical fiber based on artificial neural network.

Panda polarization-maintaining few-mode optical fiber (PPMFMOF) has important research significance in the short distance optical transmission field owing to its advantages of weak nonlinear effects, which is benefit to reduce the use of digital signal processing equipment. Designing a high-performance PPMFMOF quickly and efficiently is expected and yet challenging. In this article, we demonstrated a forward design method for the design of PPMFMOF based on artificial neural network (ANN) to solve the problems of inefficient and time-consuming PPMFMOF design in traditional design method. By studying the influence of different ANN models on the fiber performance, the approximate range of the optimal value was obtained in advance, then the minimum effective refractive index difference (Δneff,min) between adjacent LP modes was used as the optimization object, finally design of PPMFMOF supporting 10 LP modes in C + L band was successfully realized. This method provided low time-consuming, high-efficiency and high-accuracy for the fast design of PPMFMOF and the maximum mean absolute percentage error (MAPE) of the ANN model to predict the effective refractive index (neff) of 10 LP modes is only 3.2211 × 10-7. We believe that the proposed method could also be quickly and accurately applied to other functional optical fiber designs.

Modal gain characteristics of few-mode erbium-doped fiber amplifiers with pump mode beating

Using the linear polarization (LP) mode decomposition method, we theoretically analyze the beat effect between the same polarization components of the vector pump modes and propose a pump beat model of few-mode erbium-doped fiber amplifiers (FM-EDFAs). The relationship of the overlap factor with the signal gain in the beat model is verified by the amplification of LP01 and LP11 modes for the first time, that is, the differential modal gain (DMG) is dependent on the integral of the overlap factor difference over the EDF length. We also simulate the effect of pump mode beating on signal amplification: the modal gain varies periodically with the pump mode phase, and the beat length can affect the fluctuation period and amplitude of the DMG. The DMG can further be reduced by controlling the initial phases of the vector modes and properly designing the beat length of the FM-EDF. The similar process can expand to more signal modes, such as the simultaneous amplification of LP01x, LP11ex, LP11oy, LP21ex and LP21oy modes. The LP mode decomposition method is also applied to the beat effect between signal modes or the amplification of orbital angular momentum (OAM) modes. One can expect to implement an experiment on the FM-EDFA amplification with pump mode beating by optimally designing the FM-EDF structure, using a properly narrow linewidth pump laser and precisely controlling the pump mode coherence.

Measurements of microwave transmission mode in HL-3 tokamak relevant ECH corrugated waveguides transmission line

The transmission modes of high-power millimeter wave in corrugated waveguides transmission line (TL) are crucial, especially for the long-pulse operation for long-distance TL. The microwave transmission mode in the TL on HL-3 tokamak relevant the electron cyclotron heating system are studied for the purpose according to the phase retrieval method based on intensity distribution measured by an infrared camera at several different locations away from the outlet of TL. A specific high power test platform with about 40-meter TL is established and the mode contents in the middle and the end of the TL are analyzed. In the middle of the TL, the proportion of main transmission mode LP01 mode is about 97 % and other high-order modes contents are <3 %. In the end of the TL, the main mode purity is about 94 %, and the high-order mode, LP11 even mode is excited about 4.6 %. These mode analysis results indicate that the high-power and long-pulse millimeter wave corrugated waveguides TL are designed and machined well, and the alignment method for long distance transmission line installation is highly useful for high-purity wave mode transmission.

1.3 μm higher order LP11 mode pulses in praseodymium-doped fluoride fiber laser

Abstract This work demonstrated an LP11 mode pulse laser at O-band using praseodymium-doped fluoride fiber as a gain medium. The Q-switched laser was initially generated using antinomy-telluride/polyvinyl-alcohol (Sb2Te3/PVA) thin film, and the LP11 modes were analyzed using a beam profiler. The pulse laser was achieved at a pumped power of 273–303 mW. With the increase in pump power, the pulse repetition rate increased to a maximum of 60.2 kHz, and the pulse width decreased to a minimum of 2.86 μs. The operating wavelength of the Q-switched output was 1303.2 nm. The corresponding pulse energy and peak power also increased with the increase in pump powers to the maximum of 12.12 nJ and 4.24 mW. The higher-order modes were obtained after the fiber was offset between the single-mode fiber and the two-mode fiber (TMF). The TMF output was then connected to the multimode fiber, a collimator, and the scanning-slit optical beam profiler to observe the higher-order modes. A uniform two-lobe structure of higher-order LP11 modes was observed.

Fiber Bragg grating sensing multiplexing method using photonic lantern spatial mode diversity

This paper introduces a method for fiber Bragg grating (FBG) multiplexing based on spatial mode diversity. By introducing the dimension of spatial modes in few-mode fibers (FMF), the FBGs are allocated into multiple optical paths carried by different spatial modes. The method involves multiplexing and detecting the reflected spectra of FBGs in various spatial modes, effectively enhancing the system's sensing capacity. The paper establishes a mode-diversity multiplexing sensing system using mode-selective photonic lanterns and FMF circulator. As a proof of concept, the system is experimentally validated for multipoint temperature sensing using four spatial modes LP01, LP11, LP21 and LP02, and its performance is compared with that of traditional multiplexing methods. The experimental results indicate that the proposed mode division multiplexing (MDM) and hybrid MDM/wavelength division multiplexing (WDM) schemes effectively enhance the sensing capacity. Furthermore, the sensitivity to temperature response for the LP01, LP11, LP21 and LP02 modes in the spatial mode diversity multiplexed sensing surpasses 0.0096 nm/°C, maintaining linearity above 97%, and the maximum error contained within 5%. The proposed method offers a novel approach to expand the capacity of sensor networks.

Design of low loss THz dual guided photonic crystal fiber with supporting of 68 OAM modes and 8 LP modes

Design of low loss THz dual guided photonic crystal fiber with supporting of 68 OAM modes and 8 LP modes

Error Analysis and Experimental Research of Temperature/Strain Sensing Based on Few-Mode Fiber Bragg Grating

ABSTRACT To solve the temperature/strain cross-sensitivity problem, simultaneous measurement of temperature and strain can be achieved by using the two spectra of LP01 mode and LP11 mode in few-mode fiber Bragg grating (FM-FBG). However, in the same fiber the temperature/strain sensitivity difference is small, so the sensing accuracy is limited greatly. In this paper, we calculate the relation between sensing accuracy and sensitivity-difference and obtain that the enlarged sensitivity difference could improve the sensing accuracy. FBGs are engraved on two kinds fibers single-mode (SM) and few-mode (FM) with different geometrical dimensions; the sensing coefficients of LP01 mode in SM-FBG and LP11 mode/LP01 mode in FM-FBG are calibrated firstly and then are used to sense the temperature/strain. The experimental results show that parallelly using LP01 mode in SM-FBG and LP11 mode in FM-FBG achieves simultaneous measurement of temperature and strain, and error is reduced greatly compared with the sensing results using LP11 mode and LP01 mode in FM-FBG or using LP01 mode in SM-FBG and LP01 mode in FM-FBG, where the wavelength-temperature/strain coefficient difference is maximum. The temperature measurement error is 1.64°C, and the strain measurement error is 20.04 με, which is consistent with the theoretical analysis. The sensing results provide technical reference for FBG multi-parameter sensing measurement and the practical engineering application.

Low crosstalk heterogeneous sixteen-core four-mode fiber with concave-convex double-type refractive index profiles.

Multi-core few-mode fiber based on space division multiplexing technology is widely regarded as the primary solution to the optical communication capacity issue. However, the quality of the communication signal of the multi-core few-mode fiber depends on the degree of energy coupling between the cores. Thus, we propose a heterogeneous sixteen-core four-mode fiber that achieves low inter-core crosstalk, which first employs a combination of concave and convex double-type refractive index profiles. The advantage of this double-type refractive index profile is that the fiber can more conveniently regulate the phase differences of the same modes in adjacent cores. At the wavelength of 1550 nm and the optimal fiber parameters, all cores of the proposed fiber support four LP modes, and the inter-core crosstalk of all LP modes is less than -72 dB/km. This research demonstrates the efficacy of the proposed fiber in enhancing signal quality and presents innovative ideas and solutions for the future design of various low crosstalk multi-core few-mode fiber.

Numerical modeling of an ASE suppression and power scalable approach in 1908 nm LMA-TDFO.

High-power Tm-doped fiber lasers operating at short wavelengths (< 1940 nm) are widely used in biomedical engineering, remote sensing, and pumping Ho-doped solid-state lasers. However, it is challenging to increase their output power. Amplified spontaneous emission (ASE) is a major bottleneck in scaling the power of Tm-doped fiber lasers operating at short wavelengths. This paper presents what we believe to be a novel approach for suppressing ASE and increasing output power in large-mode-area Tm-doped fiber oscillators (LMA-TDFOs) operating at short wavelengths. By designing a core with some particular doping profiles in LMA fibers, most ASE can be concentrated in the LP11 mode and suppressed by bending the fiber to a suitable radius. At the same time, the signal laser can be wholly coupled to the LP01 mode in the oscillator. A model describing the transverse mode competition and ASE is developed. A novel nested-ring doping scheme for LMA fibers is designed, and the output characteristics of a 1908 nm oscillator based on this scheme are numerically investigated. Theoretical analysis confirmed that this sophisticated doping design can effectively achieve the intended ASE suppression effect. Further simulations show that this method is expected to increase the output power of the 1908 nm LMA-TDFO to the kW level and has also demonstrated good practicability across a broader signal spectrum range (1848-1940 nm).

Radial diversification of pump and signal intensities in EDFA comprising LP01 mode dispersion compensation fibers in third order nonlinear condition

Radial diversification of pump and signal intensities in EDFA comprising LP01 mode dispersion compensation fibers in third order nonlinear condition