Two-mode Fiber Research Articles

Study on Stimulated Raman Scattering Threshold Over Two-Mode Fiber

Study on Stimulated Raman Scattering Threshold Over Two-Mode Fiber

Versatile MDM-OFDM system with selectable single-mode half-subcarrier or two-mode full-subcarrier multiplexing

We propose a novel all-optical hybrid MDM-OFDM system that enables variable transmission capacity by seamlessly switching between two-mode full subcarriers and single-mode sparse subcarriers multiplexing through the use of power loading. The operation depends on the mode crosstalk present between the arrayed waveguide grating and the two-mode fiber. Through rigorous analysis, our results indicate that for the two-mode full subcarrier multiplexing, a power loading of 0.78 dB for the TE10 mode guarantees uniform performance across all subcarriers. Furthermore, by effectively suppressing the TE10 mode using a power loading of 12 dB, our system achieves similar performances and functionality to single-mode sparse subcarrier multiplexing, wherein only the TE00 mode is utilized. Our system offers variable transmission rates for data center connections, using all-optical signal processing, without varying the modulation scheme and the symbol rate.

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.

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.

Modal group refractive index measurement of few-mode fibers based on time-domain cross-correlation

We propose a measurement method based on the time-domain cross-correlation technique, combined with the cut-back method, enabling the measurement of group refractive indices (n g ) in few-mode fibers (FMF). A Mach–Zehnder interferometric system, equipped with high-precision and extensive range delay devices, is established. The system records off-axis holograms of spatial reference light at various delays interfering with the emitted light from the fiber under test. The interference energy is extracted from these holograms, and a time-domain mode energy curve is developed utilizing the principle of cross correlation. Optimal holograms at each of the curve peaks are used to reconstruct the modal field distribution, effectively separating and accurately identifying each mode within the FMF. By integrating the cut-back method, the n g corresponding to each mode is calculated based on the changes in group delay before and after fiber cutting. The n g of modes in the two-mode fibers was measured and the differential group delay calculated from the measurement agrees with the manufacturer’s specifications. The measured n g of a standard single-mode fiber aligns with the manufacturer’s specifications. Furthermore, the n g of the higher-order modes in four-mode fibers were measured by exciting them at different angles and validating the wave optics theory that the n g of the fiber modes is independent of the excitation angle. This method can simultaneously measure the n g of several modes in a fiber, providing support for the development and application of FMFs.

Performance Enhancement in a Few-Mode Rayleigh-Brillouin Optical Time Domain Analysis System Using Pulse Coding and LMD Algorithm

Rayleigh Brillouin optical time domain analysis (BOTDA) uses the backscattered Rayleigh light generated in the fiber as the probe light, which has a lower detection light intensity compared to the BOTDA technique. As a result, its temperature-sensing technology suffers from a low signal-to-noise ratio (SNR) and severe sensing unreliability due to the influence of the low probe signal and high noise level. The pulse coding and LMD denoising method are applied to enhance the performance of the Brillouin frequency shift detection and temperature measurement. In this study, the mechanism of Rayleigh BOTDA based on a few-mode fiber (FMF) is investigated, the principles of the Golay code and local mean decomposition (LMD) algorithm are analyzed, and the experimental setup of the Rayleigh BOTDA system using an FMF is constructed to analyze the performance of the sensing system. Compared with a single pulse of 50 ns, the 32-bit Golay coding with a pulse width of 10 ns improves the spatial resolution to 1 m. Further enhanced by the LMD algorithm, the SNR and temperature measurement accuracy are increased by 5.5 dB and 1.05 °C, respectively. Finally, a spatial resolution of 1.12 m and a temperature measurement accuracy of 2.85 °C are achieved using a two-mode fiber with a length of 1 km.

Mode-selective power splitters for mode-division multiplexing optical networks

We propose an all-fiber mode-selective power splitter (MSPS) for non-circular-symmetric LP lm (l = 1, 2, …) modes, which is suitable for multicasting and optical performance monitoring in mode-division multiplexing optical fiber networks. The MSPSs are asymmetric two-core few-mode directional couplers composed of a few-mode fiber and a two-mode fiber. We theoretically studied the three conditions required by the MSPSs. By carefully choosing the core-to-core distance and coupling length, the MSPS can achieve arbitrary splitting ratio regardless of the modal field orientation of the input non-circular-symmetric LP mode. By using an asymmetric structure, the MSPS can ensure the power splitting only happens on the target non-circular-symmetric LP mode when the phase matching condition is satisfied. In addition, we designed and numerically simulated LP31 MSPSs with four kinds of splitting ratios, among which the one with 90/10 splitting ratio was fabricated based on tapering and polishing method. The fabricated LP31 MSPS is characterized and the results show that its splitting ratio is much more stable than regular LP31 mode-selective coupler.

Thermally stable vector torsion sensing based on cascaded long-period fiber gratings in two-mode fiber

We demonstrated a torsion sensor based on an in-fiber Mach-Zehnder interferometer (MZI), which consists of two identical cascaded long-period fiber gratings (CLPFGs) inscribed in two-mode fiber (TMF) by high-frequency CO2 laser. The transmission and mode coupling performances, as well as the spectral properties have been proved theoretically and experimentally. The proposed TMF-CLPFGs could not only be used as an efficient multi-wavelength LP01-LP11 mode converter, but also achieve the vector torsion measurements. As the twist induced, the proposed TMF-CLPFGs present vector torsion ability through the intensity of interference dips varying. The torsion sensitivity of TMF-CLPFGs with spacing length 1 cm can reach −1.07 dB/(rad/m) in counterclockwise direction and 0.9 dB/(rad/m) in clockwise direction. Moreover, the temperature sensitivity of TMF-CLPFGs was measured to be 0.055 dB/°C, which could be negligible to torsion measurement. Therefore, the proposed TMF-CLPFGs could be potential in few-mode fiber-based communication systems and sensing fields.

All-Fiber Low-Modal-Crosstalk Demultiplexers for DSP-Free IM/DD LP-Mode MDM Transmission

Weakly coupled mode-division multiplexing (MDM) techniques supporting intensity modulation and direct detection (IM/DD) transmission are promising methods of enhancing the capacity of short-reach scenarios in which low-modal-crosstalk-mode demultiplexers for degenerate linear polarized (LP) modes are highly desired. In this paper, we review two degenerate-mode reception schemes. Firstly, a low-modal-crosstalk orthogonal combined reception method for degenerate modes is proposed based on all-fiber mode-selective couplers, in which signals in both degenerate modes are demultiplexed into the LP01 mode of single-mode fibers and then are multiplexed into the mutually orthogonal LP01 and LP11 modes of a two-mode fiber (TMF) for simultaneous detection. Secondly, a novel degenerate-mode-selective coupler consisting of an input few-mode fiber and an output TMF is proposed, which could demultiplex degenerate LP modes without any digital signal processing (DSP). Both demultiplexers are achieved based on the taper and polish process. The fabricated devices are characterized and compared. The results show that the proposed schemes can pave the way to the practical implementation of DSP-free IM/DD LP-mode MDM transmission systems.

Coherence-tailorable vector fiber source

Apart from the polarization and spatial mode, the degree of coherence (DoC), as a new degree of freedom (DoF), has actuated the control of structured optical fields beyond into a higher dimension. However, the control of DoC mainly focuses on the spatial coherence, and so far, there are no reports on the modulation of temporal coherence of the structured light. Here, a temporal coherence-tailorable vector fiber source is proposed and experimentally demonstrated. With a two-mode long-period fiber grating and polarization control, switchable transverse-mode outputs including azimuthally polarized mode TE01 and radially polarized mode TM01 are performed in an all-fiber compact regime. Benefitting from a tunable superfluorescent fiber source as the illumination of spatial mode control, the temporal coherence and the operating wavelength of vector beams can be simultaneously modulated. This work may not only advance a considerable protocol for creating and manipulating high-dimensional vectorial optical field, but also open up a promising avenue for potential applications in optical communication, high-resolution imaging, and particle trapping.

All-optical tunable multi-channel mode conversion based on a modal interferometer using two-mode fiber

This paper presents all-optical tunable multi-channel mode conversion based on a Norland Optical Adhesive (NOA)-coated modal interferometer constructed by two-mode fiber. The modal interferometer consists of a tapered two-mode fiber (TTMF) sandwiched between two pieces of single-mode fibers. The TTMF provides a uniform modal interferometer with LP01 and LP11 modes as well as a strong evanescent field on its surface. The NOA is illuminated by a 405 nm laser which changes the refractive index of the NOA, thus the output spectrum of the modal interferometer can be controlled. Due to the strong light-matter interaction between NOA and the evanescent field around the TTMF, a large wavelength shift of the output spectrum can be observed under external pump light excitation. The experimental results show the transmission spectrum exhibits a blue shift with increasing pump power. A wavelength variation rate of 289 pm/mW is obtained under external pump light excitation. The interference spectrum has multiple dips where the input LP01 mode is converted to the LP11 mode. Thus, the modal interferometer acts as a tunable multi-channel mode converter. The proposed device might have potential applications in all-optical controllable devices and mode division multiplexing systems, etc.

Tracing the origins of mode coupling for beam quality optimization in high-power fiber laser delivery systems

Abstract Suppressing mode degradation is the key issue for high-power laser delivery; however, diagnosing mode degradation in its entirety, ranging from the contents and origins to locations, has always been a major obstacle. Here, a versatile approach for tracing the origins of mode coupling is demonstrated through addressing the differential intermodal dispersions of fiber modes. Full recognition for modal contents and the origins of mode degradation are experimentally completed in a two-mode fiber laser delivery system, which assists a significant improvement of beam quality M2 from 1.35 to 1.15 at the highest power of over 300 W. This method yields a quantitative characterization for manipulating the individual mode of dual-mode coupling origins or their combinations. This work points toward a promising strategy for the online tracing of mode coupling in cascade fiber links, thus enabling further pursuit of seeking extreme beam quality in high-power fiber laser systems.

Standard Single-Mode Fiber with High Modal Bandwidth as Two-Mode Fiber around 1060 nm for High Data Rate Transmission

A step-index standard single-mode fiber as a two-mode fiber at 1060 nm can have a high modal bandwidth. In the current work, we conducted a detailed study and found that the LP11 mode of such a fiber is bending-sensitive and that the light excited to LP11 mode can be stripped out due to bending. The transmission experiments were conducted using offset launch with both LP01 and LP11 modes excited and center launch with only LP01 mode excited to show transmission performance in different conditions. We demonstrated the feasibility of 25 Gb/s NRZ transmission over 1 km of the fiber when both LP01 and LP11 modes were excited. We further explored the feasibility of a trench-assisted bending-insensitive step-index standard single-mode fiber with good bending properties for both LP01 and LP11 modes for two-mode transmission at 1060 nm. We found a fiber that has high modal bandwidth at 1060 nm and can sustain bending down to at least a 20 mm diameter. The high-bandwidth two-mode fiber can be potentially useful for future 1060 nm-based VCSEL transmission.

The intrinsic mode activation and evolution in fiber splicing based on 3D refractive index profile characterization

Fiber fusion splicing tremendously affects the modal stability, insertion loss and transmission quality of the optical fiber communication systems. Especially, in the few-mode fibers, a tiny dissatisfactory fiber splicing operation may lead to a big modal perturbation or unexpected mode coupling. In this paper, we propose a 3D refractive index profile visualization method to demonstrate the mode activation and evolution in fiber fusion splicing. A refractive index profile measuring instrument is established and applied to detect the 3D refractive index profile deformation caused by the fiber splicing. Then, based on the measured values of 3D refractive index profile, the mode activation is analyzed for fiber splicing between a single-mode fiber and a two-mode fiber with different core-offset distance. Results show that the mode begins to tremble with a core-offset of 4 μm and the high-order modes are effectively activated at the core-offset of 7 μm. The basic theoretical model and experiment results provide a profound support in almost all kinds of fiber splicing operation, mode interference construction and mode coupling modulation. Additionally, the method can be expanded to the research on fiber cleaving and bending.

Generation of cylindrical vector beam pulses by using multi-wavelength EDFL with two-mode fiber filter

A multi-wavelength erbium-doped fiber laser (MW-EDFL) operating in the L-band with cylindrical vector beam outputs is proposed and experimentally investigated. The multi-wavelength lasing is realized by exploiting a two-mode fiber filter (TMFF) as the comb filter and a single-mode fiber spool with 2-km in length to suppress the mode competition. The multi-wavelength combs with channel spacing of 0.36 nm, 0.58 nm, and 1.12 nm are achieved by regulating the interference length of the TMFF, and up to 18 lasing channels are obtained with a wavelength spacing of 0.36 nm. Good long-term stability is manifested within a 100-minute duration, as the maximum peak power fluctuation of 1.96 dB and the maximum wavelength shift of 0.03 nm are measured. By adjusting the polarization controller associated with the two-mode long-period grating as a high-efficiency mode converter, radially and azimuthally polarized cylindrical vector beams of high purity are thus generated.

Tunable spectrally-flat multi-wavelength cylindrical vector beam fiber laser by using a re-circulating frequency shifter loop with two external injections

We experimentally demonstrate a tunable spectrally -flat multi-wavelength cylindrical vector beam (CVB) fiber laser based on a two-mode long-period fiber grating (TM-LPFG) and a re-circulating frequency shifter loop with two external injections. The TM-LPFG is utilized as a mode converter in the re-circulating frequency shifter loop to excite the LP11 mode in the two-mode fiber. By adjusting the wavelength interval of the two injected seed signals, the power of the injected seed signals, and the pump power can produce a flat intensity envelope between the two injected wavelengths. Multi-wavelength CVBs of 12, 18, or 30 channels with a peak-to-peak power variation lower than 2 dB between the two injected wavelengths are obtained with a wavelength spacing of approximately 0.08 nm. The wavelength spacing can be adjusted by changing the frequency of the radio frequency signal. Multi-wavelength CVBs of 14 channels with a peak-to-peak power variation lower than 2 dB between the two injected wavelengths are obtained with a wavelength spacing of approximately 0.2 nm. The purity of the generated multi-wavelength CVBs is higher than 98%. Due to the absence of interference-type filter elements, the multi-wavelength CVB fiber laser exhibits excellent stability. The maximum wavelength deviation of each channel is less than 6.5 pm, and the maximum intensity fluctuation is less than 0.7 dB over a period of one hour.

Two-mode fiber Mach-Zehnder interferometric temperature sensor in the 50 °C – 650 °C range

In this work a two-mode fiber Mach Zehnder interferometer (TMF-MZI) temperature sensor in the 50 – 650° C range is successfully demonstrated. The sensor design incorporated a tapered fiber with dimensions of 20 mm/20 mm/20 mm for the down-taper, waist-length, and up-taper, respectively, while the waist-diameter was set to 30 μm. Around this taper dimensions, multiple geometric configurations were explored in this research, and it was found that devices with a spectral period around 20 nm held the most favorable spectral characteristics, avoiding the appearance of shorter spectral ripples at high temperatures. The sensor exhibited consistent and repeatable behavior, maintaining its spectral characteristics throughout multiple heating and cooling cycles. The sensor interference spectrum exhibits a consistent wavelength shift through 1350–1700 nm, such shift fits a linear function with an R-square value of ∼0.985 and a sensitivity of 70 pm/°C. The simple fabrication, low cost, and stability of the optical response make this sensor relevant for high-temperature measurements.

All-Fiber Electric Field Sensor With Tapered Two-Mode Fiber Structure on DR1/PMMA Polymer

All-Fiber Electric Field Sensor With Tapered Two-Mode Fiber Structure on DR1/PMMA Polymer

High-precision magnetic field sensor via optoelectronic oscillator having taper-based in-line modal interferometer

A magnetic field sensor with high precision based on a dual-loop optoelectronic oscillator (OEO) in series with a compact in-line modal interferometer (MI) is designed and experimentally demonstrated. According to the sensing principles of the OEO-based sensor, the mapping relationship between the wavelength sensitivity of the designed MI and the microwave frequency sensitivity of the OEO is established, revealing that the frequency sensitivity of the OEO enlarges with the increment in the wavelength sensitivity. Given to this, we design a kind of high-sensitivity fiber magnetic field probe comprised of a tapered two-mode fiber (TMF) coated with magnetic fluid (MF), which is sandwiched between two segments of single-mode fiber (SMF) with unilateral core-offset. Compared to the bilateral core-offset, the unilateral core-offset reduces the mode mismatching loss further. At the same time, it can make the cladding mode have high intensity and interference spectrum generate large contrast. The above treatment ensures high sensing performance of the proposed OEO-based sensor. In the experiment, the in-line MI with a core-offset distance of 5 µm and a waist diameter of ∼9.2 µm is prepared. Firstly, we use Fourier phase unwrapping method to characterize the magnetic field in order to overcome the extraction problem of inaccurate dip wavelength due to the raw interference spectra with irregular burrs, showing maximum wavelength sensitivity of 691.5 pm/mT (655.4 pm/mT) for the light wave vector parallel (perpendicular) to the magnetic field. In order to enhance the measurement accuracy further, by means of the designed dual-loop OEO with high signal-to-noise ratio (SNR), we directly put the designed in-line MI into the above OEO system, obtaining the frequency sensitivity of − 1.508 kHz/mT (−1.429 kHz/mT) in the range of 6.69–8.35 mT (1.96–3.17 mT) for the light wave vector parallel (perpendicular) to the magnetic field, corresponding to the measurement error of within ± 0.089 mT ( ± 0.01 mT). The designed device can have potential in the fields of high-accuracy magnetic field measurement in a confined space.

Controlled excitation and regulation of LP11a and LP11b modes based on long period gratings in a slightly elliptical core two-mode fiber

Controlled excitation and regulation of LP11a and LP11b modes based on long period gratings in a slightly elliptical core two-mode fiber