Few-mode Fiber Research Articles

Extended S2 diagnosis of mode degradation in fiber components through group delay stretching.

The separate diagnosis of mode degradation in few-mode fiber components (FMFCs) is a challenging task due to the reciprocal mode cross talk. In this work, we propose the group delay stretching, combined with the extended spatially and spectrally (S2) resolved imaging technique, to decouple and analyze the mode coupling within the body of the FMFC with short-length pigtails. Through stretching the mode delay by a delay fiber, the degraded modes related to different origins are effectively separated, and the extended S2 technique quantifies the individual modal weight for each component. Experiments on different types of FMFCs have verified the validity of our method. This method paves the way for optimizing and manipulating the fiber components in the dimensionality of modes.

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.

Generation of noise-like pulses in a linear-cavity Tm fiber mode-locked laser based on FMF saturable absorber

As a real saturable absorber (SA) based on the nonlinear multimode interference (NL-MMI) effect, the devices based on the few-mode fiber possess numerous exciting characteristics due to their all-fiber structure, straightforward manufacturing process. In this paper, by employing a SA with the SMF-FMF-SMF structure in a linear cavity, we demonstrate a stable mode-locked Tm-doped fiber laser. At the pump power of 1 W, a single noise-like pulse (NLP) with a pedestal pulse duration of 28 ps and coherent peak width of 0.77 ps is generated at a central wavelength of 1940 nm with a 3 dB bandwidth of 10.51 nm. The stable noise-like pulse can be sustained from the lasing pump threshold up to the maximum pump power of 2.2 W without experiencing pulse splitting. The experiment results in a peak average output power of 116 mW, along with a corresponding maximum pulse energy of 24.73 nJ at a repetition frequency of 4.69 MHz. The high stability of our fiber laser is confirmed by the signal-to-Noise Ratio (SNR) of 63 dB, and its enduring stability is additionally validated over an 8-hour period. The experimental findings suggest that the SMF-FMF-SMF structure has significant potential in the simple and robust all-fiber mode-locked lasers operating in the noise-like pulse regime.

Inter-Mode Crosstalk Estimation between Cores for LPmn Modes in Weakly Coupled Few-Mode Multicore Fiber with Perturbations.

A novel inter-mode crosstalk (IMXT) model of LPmn mode for weakly coupled few-mode multicore fiber is proposed based on the coupled mode theory (CMT) with bending and twisting perturbations. A universal expression of the mode coupling coefficient (MCC) between LPmn modes is derived. By employing this MCC, the universal semi-analytical model (USAM) of inter-core crosstalk (ICXT) can be applied to calculate the IMXT. Simulation results show that our model is generally consistent with previous theories when stochastic perturbations are absent. Moreover, our model can work effectively when stochastic perturbations are present, where former theories are not able to work properly. It has been theoretically found that the MCC has an intimate relationship with core pitch. Our model, based on the CMT, can provide physical characteristics in detail, which has not been reported clearly by former theories. In addition, our model is applicable to phase-matching and non-phase-matching regions of both real homogeneous and heterogeneous few-mode multicore fibers (FM-MCFs) with a wider range of applications.

In-fiber waveguide-based mode-locker for generating diverse ultrafast pulses

The pursuit of an optimal mode-locker in nonlinear photonics is crucial for advancing high-performance laser technologies. Addressing the market demands and technical complexities in creating highly integrated and robust saturable absorbers, we present a femtosecond laser-inscribed in-fiber straight waveguide, seamlessly integrating a few-mode fiber segment into a single-mode fiber. This design reduces costs, simplifies fabrication, and enhances system integration and stability. As a saturable absorber, it enables nonlinear polarization rotation and multimode interference, which are essential for ultrafast pulse generation. It exhibits slight polarization-dependent loss and significant modulation depth, effectively inducing mode-locking. Based on the waveguide, 1.36 ps soliton pulses with a 2.9 nm bandwidth at a central wavelength of 1573 nm are initially achieved in an anomalous dispersion regime. Transitioning to the normal dispersion regime, at 190 mW pump power, the system produces Q-switched mode-locked pulses at 1574 nm. Increasing the pump power to 295 mW results in 712 fs noise-like pulses at 1572 nm, featuring an 8 nm bandwidth and 4.57 MHz repetition rate. This advancement in fiber lasers, facilitated by hybrid nonlinear effects in the waveguide, represents a significant milestone, promising for nonlinear optics and photonics due to its simplicity, cost-effectiveness, compactness, robustness, and high damage threshold.

The mode detangler.

Astrophotonics aims to transfer photonic technology to the development of compact astronomical instruments. However, light coupling from a multimode fiber, typically adopted in modern observatories, to a single-mode photonic device still poses a challenge. Though a photonic lantern can enable this transition in a low-loss way, it requires that the number of single-mode fibers (SMFs) at the output is the same as the number of guided modes in the multimode fiber, resulting in a cumbersome fan-out of many single-mode devices to be connected. Herein, we invent an active device in a waveguide form called "the mode detangler" (MD). We show that it can adaptively transform a complex light field from a multimode fiber to a single-mode-like spot. In this way, only one single-mode device is required at the end. The path leading to the idea and the theory behind the mode detangling effect is explained, followed by numerical simulations and experimental demonstrations using a few-mode fiber as proof of concept. We believe this device has the potential to address the multimode-to-single-mode conversion challenge in astrophotonics but also sheds light on (de)multiplexing applications regarding spatial mode technology in optical communications.

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.

Experimental study of intramodal XPM reduction by intramodal dispersion in weakly coupled FMF.

The understanding of nonlinear propagation effects in low-crosstalk few-mode fiber is crucial for a weakly coupled mode-division multiplexed system. In this Letter, we report the first, to the best of our knowledge, experimental verification of the advantage of intramodal dispersion on mitigating intramodal cross-phase modulation in a weakly coupled few-mode fiber transmission. The experimental system is established over a 70-km multiple-ring-core few-mode fiber accommodating 6 linearly polarized modes, based on which the influences of intramodal cross-phase modulation on transmission performances of each linearly polarized mode are evaluated. Experimental results show that the intramodal cross-phase modulation of degenerate linearly polarized modes with much larger intramodal dispersion values are significantly weaker than those of non-degenerate linearly polarized modes, in which the maximum suppression of intramodal cross-phase modulation noise is up to 9.7 dB. We believe that this work would be beneficial to practical applications of weakly coupled mode-division multiplexing technologies.

Fiber Specklegram Sensor Based on Reflective Mode Rotation in Few-Mode Fiber

Fiber Specklegram Sensor Based on Reflective Mode Rotation in Few-Mode Fiber

112-GBaud mode-division-multiplexing IM-DD transmission beyond net 6.4 Tb/s over weakly coupled FMF for optical interconnections.

Weakly coupled mode-division-multiplexing (MDM) systems based on intensity modulation and direct detection (IM-DD) are a good candidate for further improving the capacity of short-reach optical interconnections. However, restrained by the modal crosstalk of the transmission link and the reception of degenerate mode groups (DMGs) utilizing bandwidth-limited multimode photodetectors (PDs), high-speed MDM IM-DD has encountered a capacity bottleneck. In this Letter, we investigate a high-speed weakly coupled MDM IM-DD transmission system utilizing a degenerate mode diversity receiver scheme adopting high-bandwidth single-mode PDs over a multiple-ring-core (MRC) few-mode fiber (FMF) and a low-crosstalk mode multiplexer/demultiplexer (MUX/DMUX). An MDM IM-DD transmission with four DMGs and eight wavelengths is experimentally demonstrated with 112-GBaud four-level pulse-amplitude modulation (PAM4) and probabilistically shaped PAM8 per lane over 200-m weakly coupled MRC-FMF. To the best of our knowledge, this is the first experimental demonstration of the MDM IM-DD transmission system with up to 112-GBaud baud rate and beyond 6.4-Tb/s net rate. Meanwhile, the experimental results show that the proposed MDM IM-DD transmission link has a superior performance only adopting a low-complexity feedforward equalizer, making it a promising candidate for high-speed optical interconnections.

Ultrahigh-channel-count OAM mode conversion utilizing a hybrid few-mode fiber configuration.

We propose and demonstrate a hybrid few-mode fiber configuration (HFMFC) that enables ultrahigh-channel-count orbital angular momentum (OAM) mode conversion. The HFMFC consists of periodically twisted graded-index few-mode fiber segments and a step-index few-mode fiber segment. Our proposed HFMFC-based multichannel OAM mode converter (OAM-MC) offers an exceptionally high channel count in a wide bandwidth, with customizable channel spacing down to 50 GHz (∼0.4 nm), achieved through optimization of the structural parameters of the HFMFC. By employing this methodology, we have successfully demonstrated 10, 32, 117, and 233 channel OAM mode conversions covering the entire C + L band, representing the highest performance among all reported fiber-based multichannel OAM-MCs to date, for the first time to the best of our knowledge. The suggested ultrahigh-channel-count OAM-MC exhibits promising potential for applications in various fields such as OAM fiber communication, OAM holography, OAM information processing, and OAM metrology.

Few-mode optical fiber-based flexible pressure feedback tentacle doped with fluorescence temperature pointer

Abstract In this paper, a flexible fiber pressure feedback whisker is proposed, which consists of a water droplet shaped polydimethylsiloxane (PDMS) elastomer with an embedded balloon shaped few mode fiber. The mechanical sensing performance of the device was analyzed and optimized using a combination of finite element method and beam propagation method (BPM). The built-in cladding corroded few-mode fiber increases pressure sensitivity by more than four times. The collection efficiency of fluorescence signal is improved by cladding corrosion. The PDMS elastomer was doped with upconversion nanoparticles NaYF4:Yb, Er@NaYF4 in order to achieve temperature measurement by fluorescence intensity ratio technology. The combination of fluorescence signal and interference spectrum can not only achieve real-time and accurate pressure detection at different temperatures, but also incorporate fluorescent materials into flexible bionic skin for temperature self-compensation, which has potential application value for the development of bionic fiber micro-nano sensing and control devices.

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.

Optical cooling of a Yb-doped alumino-phosphosilicate fiber in air by -250 mK.

Recent progress in the fabrication of Yb-doped silicate fibers with low concentration quenching and low background absorption loss has led to the demonstration of anti-Stokes-fluorescence cooling in several aluminosilicate compositions. This breakthrough is critical to combat deleterious thermal effects due to the quantum defect in fiber lasers and amplifiers. Since cooling efficiencies remain low (1-2.7%), it is paramount to engineer compositions that improve this metric. We report a silica fiber with a core glass heavily doped with aluminum and phosphorus that sets, to our knowledge, a few new records. This few-mode fiber (16-µm core) was cooled in air by -0.25 K from room temperature with ∼0.5 W of 1040-nm power. The measured cooling efficiency is 3.3% at low pump power and 2.8% at the power that produced maximum cooling. The critical quenching concentration inferred from the measured dependence of cooling on pump power and careful calibration of the pump absorption and saturation is 79 wt.%. The inferred background absorption loss is 15 dB/km. Together with the fiber's average Yb concentration of 4.2 wt.%, these metrics rank among the best reported in a silica glass.

Low-modal-crosstalk doped-fiber amplifiers in few-mode-fiber-based systems

Independent light propagation through one or multiple modes is commonly considered as a basic demand for mode manipulation in few-mode fiber (FMF)- or multimode fiber (MMF)-based optical systems such as transmission links, optical fiber lasers, or distributed optical fiber sensors. However, the insertion of doped-fiber amplifiers always kills the entire effort by inducing significant modal crosstalk. In this paper, we propose the design of doped-fiber amplifiers in FMF-based systems adopting identical multiple-ring-core (MRC) index profiles for both passive and doped fibers to achieve low modal crosstalk. We develop the direct-glass-transition (DGT) modified chemical vapor deposition (MCVD) processing for precise fabrication of few-mode erbium-doped fibers (FM-EDFs) with MRC profiles of both refractive index and erbium-ion doping distribution. Then, a few-mode erbium-doped-fiber amplifier (FM-EDFA) with a maximum gain of 26.08 dB and differential modal gain (DMG) of 2.3 dB is realized based on fabricated FM-EDF matched with a transmission FMF supporting four linearly polarized (LP) modes. With the insertion of the FM-EDFA, 60 + 60 km simultaneous LP01/LP11/LP21/LP02 transmission without inter-modal multiple-input multiple-output digital signal processing (MIMO-DSP) is successfully demonstrated. The proposed design of low-modal-crosstalk doped-fiber amplifiers provides, to our knowledge, new insights into mode manipulation methods in various applications.

Large measurement range curvature sensor based on all-fiber Mach-Zehnder interferometer using triple-ring-core few-mode fiber.

This study experimentally demonstrates a large measurement range curvature sensor based on a Mach-Zehnder interferometer (MZI) in a triple-ring-core fiber (TRCF). The sensor is fabricated by fusion splicing a segment of TRCF between two pieces of single-mode fiber (SMF), forming the SMF-TRCF-SMF sandwich structure. Since the TRCF can support the propagation of a few guided modes, the fundamental mode interferes with high-order modes in the sensing part to produce a periodic interference spectrum. Bending causes the effective refractive index of the mode to change, which shifts the interference spectrum. The bending measurement is achieved by monitoring the wavelength variation of interference dips. The proposed sensors can realize curvature measurements up to 100 m-1, with a bending sensitivity of -225.9 pm/m-1. It is believed that the proposed MZI sensor may have potential applications in robot hand gesture recognition and control.

Novel mirror-flipped mode permutation technique for long-haul 6-mode transmission

Mode-division multiplexing based on few-mode fibers is expected to overcome capacity limits of current single-mode fiber communications. Mode permutation as an approach for extending transmission reach has attracted great attention. In this paper, we propose a mirror-flipped mode permutation (MFMP) scheme based on newly developed fibers with symmetric differential mode delay (DMD). We demonstrate a 28-GBaud transmission over a 1542-km 6-mode fiber link, which suppresses modal-dispersion impact by 36% and mitigates mode-dependent-loss effect with 3.1-dB Q-factor improvement, compared with the traditional cyclic mode permutation at 1233 km. Then, we experimentally verify that the proposed MFMP works well even using a fiber with a not exactly symmetric group delay distribution. Benefiting from a low DMD in this fiber, we demonstrate a 6-mode PM-QPSK transmission at 25 GBaud with 16-λ channels, extending the transmission distance to 3000 km.

A novel heterogeneous sixteen-core four-mode fiber with two types of refractive index profiles

To this day, the inter-core crosstalk and intra-core crosstalk are potential factors limiting optical fiber transmission over long distances and serious degradation of signal quality in communication systems of multi-core few-mode fibers. To this end, this work proposes a heterogeneous sixteen-core four-mode fiber with two types of refractive index profiles is proposed in this paper, which consists of a graded-index core and a step-index core. After optimization, the four modes can transmit independently and steadily at 1550 nm, and the effective refractive index difference between adjacent modes is more than 1 × 10−3. Under the optimal fiber parameters, the inter-core crosstalk results can be obtained from the research results and are less than −69 dB/km, which reflects the significant effect of the proposed fiber in reducing the inter-core crosstalk. The proposed fiber is the first to use the combination of step-index core and graded-index core so that the inter-core crosstalk of heterogeneous fibers is successfully suppressed at a low level, while providing new ideas and solutions for future research on novel low-crosstalk multi-core few-mode fibers that can be applied to terrestrial and undersea communication systems.

High-gain far-detuned nonlinear frequency conversion in a few-mode graded-index optical fiber

We present theoretical and experimental evidence of high-gain far-detuned nonlinear frequency conversion, extending towards both the visible and the mid-infrared, in a few-mode graded-index silica fiber pumped at 1.064 μm\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\upmu \\hbox {m}$$\\end{document}, and more specifically achieving gains of hundreds of dB per meter below 0.65 μm\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\upmu \\hbox {m}$$\\end{document} and beyond 3.5 μm\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\upmu \\hbox {m}$$\\end{document}. Interestingly, our findings highlight the potential of graded-index fibers for enabling high-gain wavelength conversion into the strong-loss spectral region of silica. Such advancements require an accurate interpretation of intramodal and intermodal four-wave mixing processes.

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.