Higher Order Mode Fibers Research Articles

Cross-axis Bragg gratings in few-mode fibers inscribed with a femtosecond laser point-by-point technique

Point-by-point (PbP) cross-axis Bragg gratings in a step-index two-mode fiber are fabricated by tightly focused femtosecond laser pulses. The grating is formed by inscribing a line of periodic damage points crossing the fiber axis at a certain tilting angle. We experimentally show that the cross-axis structure can introduce multiple mainband resonances associated with the higher-order modes, with wavelength spacing proportional to the tilting angle. We also develop a numerical model for the PbP-inscribed gratings based on pulse-wave approximation and the standard coupled-mode theory, the resulting simulation exhibits a good agreement with the experimental findings. This work suggests the higher-order fiber modes, synergizing with the PbP technique, has the potential to add a new dimension to designing gratings with desired spectral spacing; such an inverse problem of PbP gratings can be assisted by the numerical modal we developed.

High-order mode erbium-doped fiber laser based on cavity LPFG converters.

We experimentally investigate high-order mode (HOM) generation at a wavelength of 1.5µm in an all-fiber erbium-doped laser based on a long-period fiber grating (LPFG). The CW laser emission is achieved when the pump power is above the threshold of 10mW. An LPFG with a 15dB bandwidth of 147.76nm from 1502.76nm to 1650.52nm is used as a mode converter inside the cavity. The generation of the broadband L P 11 mode is ultimately obtained. By using a few-mode output coupler, we can obtain the intracavity conversion of the linear polarization mode. Single-, dual-, triple-, and quadruple-wavelength operations can be achieved by changing the polarization state of the polarization controllers in the cavity. The tunable range of the output wavelengths is up to ∼23.20n m. The output power and slope efficiency of the HOMs are presented and discussed. We believe our work might benefit the investigation of HOM fiber lasers, and might be further applied to the intracavity conversion of the linear polarization mode or orbital angular momentum beams.

Self-starting high-order mode oscillation fiber laser.

In this paper, we proposed and demonstrated two kinds of all few-mode fiber lasers with self-starting high-order mode (HOM) oscillation. The fundamental mode can be completely suppressed by using a bandpass filter with a few-mode fiber pigtail. In the continuous-wave (CW) regime, the fiber laser directly oscillates in HOM with a signal-to-noise ratio as high as 70dB, and the slope efficiency is up to 46%. The self-starting HOM mode-locked pulse can be easily achieved by employing a saturable absorber. The HOM oscillation pulsed fiber laser stably operates at 1063.72nm with 3dB of 0.05nm, which can deliver cylindrical vector beams with a high mode purity of over 98%. To our knowledge, this is the first demonstration for self-starting HOM direct oscillation in stable CW and pulsed operation states without additional adjustment. This compact and stable HOM fiber laser with a simple structure can have important applications in materials processing, optical trapping, and spatiotemporal nonlinear optics. Moreover, this work may offer a promising approach to realizing high-power fiber laser with arbitrary HOMs stable output.

High-order mode fiber laser based on few-mode fiber gratings

We propose and experimentally demonstrate a high-order mode fiber laser based on few-mode fiber gratings. The fiber laser has the structure of three sub-ring-cavities. LP01 mode, LP11 mode and LP21 mode can be obtained by different few-mode long-period fiber gratings. The reflection of LP01 mode, LP11 mode and LP21 mode can be realized by different few-mode fiber Bragg gratings. Therefore, the simultaneous lasing of LP01 mode, LP11 mode and LP21 mode at the same wavelength can be achieved. A few-mode fiber Bragg grating functions as a discrete filter. By adjusting a polarization controller, switchable output among single-, dual- and triple-wavelength lasing can be realized, and three wavelengths correspond to LP01 mode, the mixing of LP01 mode and LP11 mode, and LP11 mode. Therefore, the simultaneous lasing of different modes at the same wavelength, and the simultaneous lasing of different modes at different wavelengths can be achieved in this fiber laser, which can be applied to optical communication systems based on mode-division multiplexing-wavelength-division multiplexing.

Effect of Thermal Blooming on the Higher-Order Mode Fiber Laser Array Propagation Through the Atmosphere

The influence of thermal blooming on the propagation properties of higher-order mode (HOM) fiber laser array is studied by using the algorithm for simulating the laser beam propagation in the atmosphere. Based on the multiphase screen method and finite-difference method, the four-dimensional (4D) computer code of time-dependent propagation is designed to simulate the propagation of HOM fiber laser array through the atmosphere. In this study, the laser energy focusability of the LP11 mode beam array is investigated in detail for different beamlet arrangements, transverse wind speed, and the content of LP01 mode under the conditions of thermal blooming. In free space, the focal shape of the LP11 mode beam array depends on the arrangement of the second circle of the initial beam array, whereas the influence of the central beamlets is weak. The number of side lobes can be tailored by changing the arrangement of the beamlets. In contrast, under the conditions of thermal blooming, the central beamlet has a significant effect on focal beam shape. It is demonstrated that the laser energy focusability can be improved by rotating the central beamlet or increasing the transverse wind speed. As the content of the LP01 mode increases, the energy is gradually concentrated from the side lobes to the center lobe. Furthermore, the effects of initial beam array arrangements on the energy focus and focal shape are investigated. The optimal arrangement for obtaining high energy focusability is discussed in detail. These results could provide useful references for applications of the HOM beam array.

Simultaneous nonlinear wavelength and mode conversion for high-brightness blue sources

We investigate frequency doubling of focused Bessel-like higher order fiber modes in a one-dimensionally quasi-phase matched structured KTP crystal. A single higher-order fiber mode, L P 0 , 7 , was generated at 971 nm using a fiber optical parametric amplifier pumped by sub-nanosecond pulses at 1064 nm. Frequency doubling to 485.5 nm was achieved with a maximum conversion efficiency of 48%, which produced a highly structured blue beam without any brightness improvement with respect to the fundamental beam. By optimizing the phase mismatch, we also experimentally demonstrate clean on-axis conversion—resulting from cascaded χ ( 2 ) : χ ( 2 ) processes, which is in good agreement with our numerical simulations, that give rise to brightness enhancement.

Higher-order mode supercontinuum generation in dispersion-engineered liquid-core fibers

Supercontinuum generation enabled a series of key technologies such as frequency comb sources, ultrashort pulse sources in the ultraviolet or the mid-infrared, as well as broadband light sources for spectroscopic methods in biophotonics. Recent advances utilizing higher-order modes have shown the potential to boost both bandwidth and modal output distribution of supercontinuum sources. However, the strive towards a breakthrough technology is hampered by the limited control over the intra- and intermodal nonlinear processes in the highly multi-modal silica fibers commonly used. Here, we investigate the ultrafast nonlinear dynamics of soliton-based supercontinuum generation and the associated mode coupling within the first three lowest-order modes of accurately dispersion-engineered liquid-core fibers. By measuring the energy-spectral evolutions and the spatial distributions of the various generated spectral features polarization-resolved, soliton fission and dispersive wave formation are identified as the origins of the nonlinear broadening. Measured results are confirmed by nonlinear simulations taking advantage of the accurate modeling capabilities of the ideal step-index geometry of our liquid-core platform. While operating in the telecommunications domain, our study allows further advances in nonlinear switching in emerging higher-order mode fiber networks as well as novel insights into the sophisticated nonlinear dynamics and broadband light generation in pre-selected polarization states.

Fast simulation and design of the fiber probe with a fiber-based pupil filter for optical coherence tomography using the eigenmode expansion approach.

Fiber probes for optical coherence tomography (OCT) recently employ a short section of step-index multimode fiber (SIMMF) to generate output beams with extended depth of focus (DOF). As the focusing region of the output beam is generally close to the probe end, it is not feasible to adopt the methods for bulk-optics with spatial pupil filters to the fiber probes with fiber-based filters. On the other hand, the applicable method of the beam propagation method (BPM) to the fiber probes is computationally inefficient to perform parameter scan and exhaustive search optimization. In this paper, we propose the method which analyzes the non-Gaussian beams from the fiber probes with fiber-based filters using the eigenmode expansion (EME) method. Furthermore, we confirm the power of this method in designing fiber-based filters with increased DOF gain and uniformly focusing by introducing more and higher-order fiber modes. These results using the EME method are in good agreement with that by the BPM, while the latter takes 1-2 orders more computation time. With higher-order fiber modes involved, a novel probe design with increased DOF gain and suppressed sidelobe is proposed. Our findings reveal that the fiber probes based on SIMMFs are able to achieve about four times DOF gain at maximum with uniformly focusing under acceptable modal dispersion. The EME method enables fast and accurate simulation of fiber probes based on SIMMFs, which is important in the design of high-performance fiber-based micro-imaging systems for biomedical applications.

Designing tunable narrowband parametric source in Chalcogenide-based dynamic fiber geometry

We numerically investigate the generation of spectrally isolated narrowband tunable parametric sources by continuous-wave pumping a specially designed gelatin-coated chalcogenide-silica dynamic fiber in the normal dispersion regime. The relative humidity (RH) dependent phase matching dominated by fourth order dispersion has been exploited for the first time, to the best of our knowledge, to produce far-detuned new wavelengths. A tunable source in the range of 0.96–2.5 μm with a tuning rate of 1.3 THz/%RH has been designed by milliwatt pumping a 30 cm long fiber at 1.395 μm. Additionally, the sideband tailorability of about 50 THz close to the visible band was achieved by selectively exciting higher-order fiber modes (HE21) in the anomalous dispersion regime at 0.91 μm. We emphasize that the parametric sources from the proposed host would be extremely useful in short-wave infrared spectroscopy and biomedical applications.

Recent progress of dynamic mode manipulation via acousto-optic interactions in few-mode fiber lasers: mechanism, device and applications

Abstract The burgeoning advances of spatial mode conversion in few-mode fibers emerge as the investigative hotspot in novel structured light manipulation, in that, high-order modes possess a novel fundamental signature of various intensity profiles and unique polarization distributions, especially orbital angular momentum modes carrying with phase singularity and spiral wave front. Thus, control of spatial mode generation becomes a crucial technique especially in fiber optics, which has been exploited to high capacity space division multiplexing. The acousto-optic interactions in few-mode fibers provide a potential solution to tackle the bottleneck of traditional spatial mode conversion devices. Acousto-optic mode conversion controlled by microwave signals brings tremendous new opportunities in spatial mode generation with fast mode tuning and dynamic switching capabilities. Besides, dynamic mode switching induced by acousto-optic effects contributes an energy modulation inside a laser cavity through nonlinear effects of multi-mode interaction, competition, which endows the fiber laser with new functions and leads to the exploration of new physical mechanism. In this review, we present the recent advances of controlling mode switch and generation employing acousto-optic interactions in few-mode fibers, which includes acousto-optic mechanisms, optical field manipulating devices and novel applications of spatial mode control especially in high-order mode fiber lasers.

Formation and Evolution of Soliton in Two-Mode Fiber Laser

High-order mode fiber lasers are gathering the rising attentions from fundamental researches to practical applications due to their distinctive spatial and temporal properties. Here, we demonstrate a carbon nanotube mode-locked two-mode fiber laser and investigate the formation and evolution process of the soliton based on the time-stretch dispersive Fourier transform method. Stable single pulse and bound-state pulses are observed at pump powers of 32.0 and 41.3 mW, respectively. Attributing to the coexistence of fundamental mode and first group high-order modes, the optical spectrum exhibits slight intensity modulations and the autocorrelation trace shows additional small side lobes. Similar to that of single-mode fiber lasers, the buildup process of soliton includes the relaxation oscillation, beating behavior, and stable mode-locking state. However, owing to the interplay between two transverse modes in the two-mode fiber laser, the duration and fluctuation intensity of the relaxation oscillation are much larger than that of single-mode fiber lasers.

Nonlinear four-wave mixing with enhanced diversity and selectivity via spin and orbital angular momentum conservation

Light that can carry orbital angular momentum (OAM) has found a variety of applications in super-resolution microscopy, optical communications, and laser machining, bringing up the need for pure OAM light generation at on-demand power levels and wavelengths. Parametric four-wave mixing is a promising platform for such source generation, and while investigations of higher-order fiber modes have revealed enhanced phase-matching possibilities, the role of the angular momentum of light in this process has not yet been substantially considered. Here, with a specially designed ring-core fiber in which over 16 OAM modes can be stably guided, we demonstrate the first experiments, to our knowledge, investigating nonlinear four wave mixing between OAM modes in an optical fiber. The large modal space as well as spin and OAM conservation rules enable a high diversity of phase matching conditions while also providing high selectivity. We report parametric wavelength translations of over 438 nm and the ability to obtain kilowatt peak-power level ∼ nanosecond pulses of pure OAM beams at user defined colors.

Reconversion of higher-order-mode (HOM) output from cladding-pumped hybrid Yb:HOM fiber amplifier.

We demonstrate operation of a cladding-pumped hybrid ytterbium-doped HOM fiber amplifier and reconversion of the HOM output to Gaussian-like beam by using an axicon based reconversion system. The amplifier was constructed by concatenating single-mode and HOM ytterbium-doped double clad fibers, and was excited by a common multimode pump source. A continuous wave (cw) input signal of 97mW was amplified to 100W at the amplifier output, which yielded a gain of more than 30dB. The LP0,10 output of the HOM amplifier could be converted to a Gaussian-like beam with 67% conversion efficiency. We present, both analytically and numerically, the effects of scaling the beam size on axicon's apex angle, and how shape imperfections affect the mode converter's performance.

Generation of optical vortices using asymmetrically spliced fibers

We experimentally demonstrate a simple approach to generate optical vortex (OV) beams using optical fibers. The main structure is realized by asymmetrically splicing a standard simple-mode fiber with a two-mode fiber (TMF). The asymmetrical fusion joint can partly convert input fundamental mode to higher-order fiber modes. The unconverted fundamental mode component can be eventually filtered out through adjusting the states of two polarization controllers (PCs) applied on the two fibers and one polarizer located behind the output end of the TMF. By adjusting the two PCs, one can also selectively generate LP modes or OV modes with ℓ = ±1. The relationship between offsets and conversion efficiency is measured and analyzed, which accords well with the simulation. Mode purity and working bandwidth are also measured in the experiment.

All-fiber high-order mode laser using a metal-clad transverse mode filter.

We propose and demonstrate an all-fiber laser with LP11 mode output. A transverse mode filter is designed and fabricated to suppress the fundamental mode and enable the fiber laser to oscillate in the second-order (LP11) transverse mode. The mechanism is to introduce relatively low ohmic loss for the TE01 mode and much higher ohmic losses for other modes through the loss of evanescent waves in the metal clad. The fiber laser operates at the center wavelength of 1053.9 nm with a narrow 3 dB linewidth of 0.019 nm. Four states of cylindrical vector mode with high modal purity are obtained through adjusting the intra-cavity polarization controller. This approach has great potentiality and scalability of realizing single high-order mode fiber laser, from which a wide range of applications could benefit.

Linear-cavity cylindrical vector lasers based on all-fiber mode converters

We propose three mode converters based on coupling of the fundamental mode and the first group higher-order modes in fiber tapers, angled fiber facets, and transversely dislocated fibers, respectively. The insertion loss and coupling efficiency of the fiber tapers are 0.541 dB and 10.6% respectively, which are much better than those of the angled fiber facets (2.4 dB and 2.9%) or the transversely dislocated fibers (1.2 dB and 5.6%). Cylindrical vector beams are delivered from an erbium-doped fiber laser after separately incorporating the mode converters into the cavity. The output spectra of three lasers are located at 1548.7 nm with the same 3-dB bandwidth of 0.03 nm, and spatial distributions are switchable between radially and azimuthally polarized states by adjusting the incident polarization state of the fundamental mode. The linear-cavity all-fiber lasers are capable of generating flexible and cost-effective cylindrical vector beams, which are quite attractive for fiber-based sensing and communications.

Ultrahigh-sensitive multimode interference-based fiber optic liquid-level sensor realized using illuminating zero-order Bessel–Gauss beam

A proposal toward the enhancement in the sensitivity of a multimode interference-based fiber optic liquid-level sensor is explored analytically using a zero-order Bessel–Gauss (BG) beam as the input source. The sensor head consists of a suitable length of no-core fiber (NCF) sandwiched between two specialty high-order mode fibers. The coupling efficiency of various order modes inside the sensor structure is assessed using guided-mode propagation analysis and the performance of the proposed sensor has been benchmarked against the conventional sensor using a Gaussian beam. Furthermore, the study has been corroborated using a finite-difference beam propagation method in Lumerical’s Mode Solutions software to investigate the propagation of the zero-order BG beam inside the sensor structure. Based on the simulation outcomes, the proposed scheme yields a maximum absolute sensitivity of up to 3.551 dB / mm and a sensing resolution of 2.816 × 10 − 3 mm through the choice of an appropriate length of NCF at an operating wavelength of 1.55 μm. Owing to this superior sensing performance, the reported sensing technology expedites an avenue to devise a high-performance fiber optic-level sensor that finds profound implication in different physical, biological, and chemical sensing purposes.

Polarization-maintaining, large-effective-area, higher-order-mode fiber.

Higher-order-mode (HOM) fibers guiding light in large-effective-area (Aeff) Bessel-like modes have recently generated great interest for high-power laser applications. A polarization-maintaining (PM) version of HOM fibers can afford the added possibility of coherent beam combination, improved material processing, and polarization multiplexing of high-power fiber lasers. We report a PM-HOM fiber for guiding Bessel-like modes with Aeff ranging from 1200-2800 μm2. The fiber modes exhibit a birefringence value that compares well with that of a conventional single-mode PM fiber (2×10-4), and exhibit a polarization extinction ratio ranging from 13-23dB over meter-long fiber lengths, practical for amplifier systems. This fiber presents a unique platform for next-generation high-power fiber systems, as well as for the fundamental studies on deterministically polarized Bessel-like modes in fibers.

Wavelength-agile high-power sources via four-wave mixing in higher-order fiber modes.

Frequency doubling of conventional fiber lasers in the near-infrared remains the most promising method for generating integrated high-peak-power lasers in the visible, while maintaining the benefits of a fiber geometry; but since the shortest wavelength power-scalable fiber laser sources are currently restricted to either the 10XX nm or 15XX nm wavelength ranges, accessing colors other than green or red remains a challenge with this schematic. Four-wave mixing using higher-order fiber modes allows for control of dispersion while maintaining large effective areas, thus enabling a power-scalable method to extend the bandwidth of near-infrared fiber lasers, and in turn, the bandwidth of potential high-power sources in the visible. Here, two parametric sources using the LP0,7 and LP0,6 modes of two step-index multi-mode fibers are presented. The output wavelengths for the sources are 880, 974, 1173, and 1347 nm with peak powers of 10.0, 16.2, 14.7, and 6.4 kW respectively, and ~300-ps pulse durations. The efficiencies of the sources are analyzed, along with a discussion of wavelength tuning and further power scaling, representing an advance in increasing the bandwidth of near-infrared lasers as a step towards high-peak-power sources at wavelengths across the visible spectrum.

Generation of a vacuum ultraviolet to visible Raman frequency comb in H2-filled kagomé photonic crystal fiber.

We report on the generation of a purely vibrational Raman comb, extending from the vacuum ultraviolet (184 nm) to the visible (478 nm), in hydrogen-filled kagomé-style photonic crystal fiber pumped at 266 nm. Stimulated Raman scattering and molecular modulation processes are enhanced by higher Raman gain in the ultraviolet. Owing to the pressure-tunable normal dispersion landscape of the "fiber + gas" system in the ultraviolet, higher-order anti-Stokes bands are generated preferentially in higher-order fiber modes. The results pave the way toward tunable fiber-based sources of deep and vacuum ultraviolet light for applications in, e.g., spectroscopy and biomedicine.