Step-index Few-mode Fiber Research Articles

Experimental Multi-Bit Header Classification at 28.5 Gb/s With Step-Index Few-Mode Fibers

Experimental Multi-Bit Header Classification at 28.5 Gb/s With Step-Index Few-Mode Fibers

Transverse magnetic supermodes in plasmonic optical fibers excited by radially polarized light.

The overlap integrals method, with a fully vectorial formulation, is used to model the selective excitation of the TM01 mode in a few-mode optical fiber with a radially polarized donut beam, and its coupling to guided modes having a plasmonic character (supermodes). The analyses were performed on a waveguide formed as a step-index few-mode optical fiber coated with a thin gold film, at an operating wavelength of 1310 nm. The waveguide was found to support modes having optical fiber, circular metallic waveguide, and surface plasmon characteristics, depending on geometrical and material parameters. Three purely bound transverse magnetic (radially polarized) supermodes were identified: Two symmetric, labeled sTM01 and sTM02 modes, and one asymmetric, labeled ab mode, where symmetry pertains to the transverse electric field distribution over the gold film. The effective mode indices of the supermodes were studied as a function of the thickness of the gold film and its proximity to the fiber core. Considerations for the selective excitation of the sTM01 mode are discussed along with its possible applications. The transmittance of the supermodes is found to be robust even at sharp waveguide transitions. The results predict that effective excitation of TM supermodes with strong plasmonic character, without significant coupling losses, can be achieved by exciting the fiber with a radially polarized donut beam.

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.

Microwave fractional Hilbert transformer implemented in a dispersion-tailored few-mode fiber

We experimentally demonstrate, for the first time to our knowledge, a microwave fractional Hilbert transformer in a few-mode fiber using a transversal filtering approach. The filter taps are provided by a tunable true-time delay line that is realized by exploiting the spatial dimension of a dispersion-engineered double-clad step-index few-mode fiber. Both the fractional order and operational bandwidth of the fractional Hilbert transformer can be continuously tuned by adjusting the tap coefficients and varying the operational optical wavelength, respectively. The magnitude and phase response for different fractional orders, ranging from 0.17 to 1.00 that correspond to phase shifts of 15° to 90°, are measured. Operational bandwidths of 7.4 to 10.6 GHz are demonstrated for a classical Hilbert transformer. Real-time temporal fractional Hilbert transform of a Gaussian-like pulse is also performed. Our results are in good agreement with theory, validating the viability of our approach for implementation of microwave fractional Hilbert transformers.

Ultra-low-loss 5-LP mode selective coupler based on fused biconical taper technique.

Trapped in the stringent adiabatic transmission condition of high-order modes, low-loss fused biconical taper mode selective coupler (FBT-MSC) has long been challenging to achieve. We identify the adiabatic predicament of high-order modes to stem from the rapid variation of the eigenmode field diameter, which is caused by the large core-cladding diameter difference of few-mode fiber (FMF). We demonstrate that introducing a positive-index inner cladding in FMF is an effective approach to address this predicament. The optimized FMF can be used as dedicated fiber for FBT-MSC fabrication, and exhibits good compatibility with the original fibers, which is critical for the wide adoption of MSC. As an example, we add inner cladding in a step-index FMF to achieve excellent adiabatic high-order mode characteristics. The optimized fiber is used to manufacture ultra-low-loss 5-LP MSC. The insertion losses of the fabricated LP01, LP11, LP21, LP02 and LP12 MSCs are 0.13 dB at 1541 nm, 0.02 dB at 1553 nm, 0.08 dB at 1538 nm, 0.20 dB at 1523 nm, and 0.15 dB at 1539 nm, respectively, with smoothly varying insertion loss across the wavelength domain. Additional loss is less than 0.20 dB from 1465.00 nm to 1639.31 nm, and the 90% conversion bandwidth exceeds 68.03 nm, 166.68 nm, 174.31 nm, 132.83 nm, and 84.17 nm, respectively. MSCs are manufactured using commercial equipment and a standardized process that takes just 15 minutes, making them a potential candidate for low-cost batch manufacturing in a space division multiplexing system.

Design of few-mode erbium-doped fiber with a low differential modal gain and weak coupling based on layered doping.

A step index few-mode erbium-doped fiber (FM-EDF) for mode gain equalization is designed and proposed in this paper, which uses the layered-doping method to reduce the differential mode gain (DMG). The optimum structure of FM-EDF is obtained by adjusting the doping radius and doping concentration. When this structure is applied to a few-mode erbium-doped fiber amplifier (FM-EDFA), the DMG in the range of 1550-1565nm is ∼0.28d B, and the DMG of the whole C-band is usually less than 0.5dB. At the same time, the gain of each mode in 1530-1555nm is ∼20d B, while the gain decreases gradually in the 1555-1565nm due to the absorption characteristics of erbium ions. In addition, the minimum refractive index difference (Δ n eff) between modes is 1.29∗10-3 due to the selection of the refractive index and radius of the fiber core, which will greatly reduce the coupling between modes in practical application. Tolerances in the fiber manufacturing process are also considered for reliable FM-EDFA performance. When the doping concentration or the doping radius changes based on the precise value, the DMG will increase to a certain extent. In general, the DMG can maintain a small value, which is beneficial to applications in optical communication systems.

Optical fiber polarization-entangled photon pair source using intermodal spontaneous four-wave mixing in the visible spectral band

The theoretical and experimental investigation results of the generation of the polarization-entangled photon pairs operating at visible wavelengths are reported. The generation of polarization-entangled photon pairs was based on intermodal spontaneous four-wave mixing (IM-SFWM) using standard step-index few-mode fiber. It was shown theoretically that several combinations of IM-SFWM processes could occur depending on spatial modes of a pump beam. A polarization-entangled photon pair source based on Sagnac loop incorporating a segment of few-mode fiber were then experimentally created. A two-photon interference fringe visibility of our implemented photon pair source were 91.7% and 88.8% in H/V bases and D/A bases, respectively. A quantum state tomography was also conducted to reconstruct the density matrix of the generated state with a fidelity to a maximum entangled state of 93.1%.

Active learning aided four-mode fiber design with equalized zero dispersion for short-reach MDM optical communications

In this work, we demonstrate an active learning method for the optimized design of a few-mode fiber (FMF) with equalized zero dispersion between four modes, which can be used for short-reach mode-division-multiplexed (MDM) optical communication without multi-input-multi-output (MIMO) processing and chromatic dispersion compensation (CDC). To obtain the desired FMF, a multi-parameter design of a complex fiber structure is needed, which is usually very difficult, inaccurate, and time-consuming. The proposed active learning can utilize fewer data than the neural network to achieve improved prediction performance by selecting more valuable data. By balancing zero dispersion, equalized dispersion, and manufacturing feasibility, structure parameters of the four-ring step-index FMF supporting four modes are predicted by the active-learning-based inverse design. The standard deviation of four-mode dispersion of the designed fiber is 0.016. The total dataset is significantly reduced to 400 by using active learning and equalized zero dispersion is obtained. The equalized zero dispersion performance is characterized by using an optical parametric amplification (OPA) modal which is highly sensitive to dispersion. The broad OPA gains with high pump power and low amplification cross talk indicate that the designed FMF has low dispersion near to zero, low nonlinearity, and weak coupling for all four modes, which is highly suitable for high-speed MIMO-less and CDC-less MDM optical communications.

Tunable mode-locked fiber laser based on nonlinear multimode interference with double offset-splicing step-index few-mode fiber.

A tunable mode-locked all-fiber Yb-doped laser with a double offset-splicing step-index few-mode fiber (DOS-SIFMF) is demonstrated, to the best of our knowledge, for the first time. The structure of DOS-SIFMF, which constructs a micro Mach-Zehnder interferometer as a consequence of introducing offset splicing, has characteristics of both a saturable absorber and filter and is more accessible to obtain mode-locking operation in an all-normal dispersive region. The results of simulation show that interference with fewer modes is more reliable to acquire mode-locking operation of the fiber laser. The central wavelength, spectrum, and pulse widths are 1032 nm, 6.15 nm, and 28.8 ps, respectively. The output pulse in time and spectrum domains can be tuned in the range of 168.7 ps and 10.7 nm, respectively. This structure has effects of both mode-locking and filtering, showing potential application in communication and sensing. Furthermore, the influence on mode number to interference is generally discussed in the end.

Reconfigurable Few-Mode Fiber-Based Microwave Photonic Filter

We experimentally demonstrate, for the first-time to our knowledge, tunable true-time delay line operation over a few-mode fiber link, in which the time delay can be continuously and widely tuned, from 46.3 to 105.6 ps, by simply sweeping the operating optical wavelength over the 35-nm range of the C-band. This has become possible thanks to the particular modal dispersion properties of the developed double-clad step-index few-mode fiber, as it features relatively evenly-spaced incremental chromatic dispersion values among 5 spatial modes. To date, it is the first time a dispersion-diversity FMF with this property has been experimentally reported. We assess the performance of this true-time delay line when applied to microwave signal filtering in both space and wavelength diversities, where a variety of reconfigurable 5, 7 and 10-tap microwave filters with free spectral ranges ranging from 7.7 to 27.1 GHz are experimentally demonstrated.

Nonlinear spatial reshaping of pulsed beam in a step-index few-mode optical fiber.

In this paper we demonstrate the spatial and spectral dynamics of pulse propagation in a step-index few-mode optical fiber, through an experimental and numerical analysis. The Kerr induced spatial self-cleaning is demonstrated by coupling a sub-nanosecond pulsed laser at 532nm into the fiber supporting above 10 modes. A bell-shaped and approximately single mode beam can be obtained for peak powers above 6kW and it remained relatively unchanged up to 25kW. But at significantly higher input peak powers, the spatial contents of spectral sidebands change dramatically, because of intermodal four wave mixing effect.

Spatiotemporal characterization of nonlinear intermodal interference between selectively excited modes of a few-mode fiber

Nonlinear propagation of signals in single-mode fibers is well understood, and is typically observed by measuring the temporal profile or optical spectrum of an emerging signal. In multimode fibers, the nonlinearity has both a spatial and a temporal element, and a complete investigation of the interactions between propagating modes requires resolving the output in both space and time. We report here spatiotemporal measurements of a time-dependent mode interference effect, arising from the Kerr nonlinearity, of two selectively excited LP 0 m modes of a step-index few-mode fiber. We describe a method to selectively excite two propagating modes through the use of a phase mask directly patterned on the entrance face of the fiber. The output is resolved by raster-scanning a near-field tapered single-mode optical fiber probe that is connected to a high-speed detector. The results show that in the presence of nonlinearity, the output exhibits a spatiotemporal character that cannot be adequately characterized by a camera image or pulse shape alone.

Dispersion-tailored few-mode fiber design for tunable microwave photonic signal processing.

We present a novel double-clad step-index few-mode fiber that operates as a five-sampled tunable true-time delay line. The unique feature of this design lies in its particular modal chromatic dispersion behavior, which varies in constant incremental steps among adjacent groups of modes. This property, which to the best of our knowledge has not been reported in any other few-mode fiber to date, is the key to tunable operation of radiofrequency signal processing functionalities implemented in few-mode fibers. The performance of the designed true-time delay line is theoretically evaluated for two different microwave photonics applications, namely tunable signal filtering and optical beamforming networks for phased array antennas. In the 35-nm optical wavelength tuning range of the C-band, the free spectral range of the microwave filter and the beam-pointing angle in the phased array antenna can be continuously tuned from 12.4 up to 57 GHz and 12.6° up to 90°, respectively.

Machine learning aided inverse design for few-mode fiber weak-coupling optimization.

Few-mode fiber (FMF) supporting many modes with weak-coupling is highly desired in mode division multiplexing (MDM) systems. The multi-parameter design of FMF becomes comparably difficult, inaccurate and time-consuming when it comes for complex fiber structures and many high order modes. In this work, we demonstrate a machine learning method using neural network to inversely design the desired FMF based on multiple-ring structure. By using the minimum index difference between adjacent modes as the weak-coupling optimization aim, we realize the inverse design of 4-ring step-index FMFs for supporting 4, 6 and 10 -mode operation, and 6-ring step-index FMF for supporting 20-mode operation. This method provides high-accuracy, high-efficiency and low-complexity for fast and reusable design of optical fibers, including particularly weak-coupling FMF in this work. It can be widely extended to a lot of fibers and has great potential for instantaneous applications in the optical fiber industry.

Inter-mode stimulated Brillouin scattering and simultaneous temperature and strain sensing in M-shaped few-mode fiber

Stimulated Brillouin scattering (SBS) in a few-mode fiber (FMF) is of significance for the distributed temperature and strain sensing. An FMF with M-shaped refractive index distribution (M-FMF) is proposed in order to improve the performance of simultaneous temperature and strain sensing based on SBS. Propagation of four optical modes is supported by the M-FMF, so that the Brillouin gain spectrum (BGS) can be obtained by both intra-mode and inter-mode SBS. The BGSs produced by the interactions of LP<sub>01</sub>-LP<sub>01</sub> mode pair, LP<sub>01</sub>-LP<sub>11</sub> mode pair, and LP<sub>11</sub>-LP<sub>11</sub> mode pair are analyzed, respectively. Meanwhile, the temperature and strain sensing performance based on the BGS of LP<sub>01</sub>-LP<sub>11</sub> mode pair are studied in detail. Considering a common step-index FMF, only one obvious scattering peak is usually present in the BGS obtained from the interaction between different optical mode pairs, therefore, it is inconvenient to achieve multi-parameter sensing measurement. In this paper, the BGS of LP<sub>01</sub>-LP<sub>11</sub> mode pair has two scattering peaks, which are contributed by the acousto-optic coupling between the acoustic modes L<sub>1<i>n </i></sub>(<i>n</i> = 1, 2) and the optical modes LP<sub>01</sub> and LP<sub>11</sub>. The two Brillouin scattering peaks have large gain values of 0.1004 m<sup>–1</sup>·W<sup>–1</sup> and 0.0463 m<sup>–1</sup>·W<sup>–1</sup>, respectively. More importantly, the gain difference between two Brillouin scattering peaks is small, and the frequency interval is 75 MHz, which can be applied to simultaneous temperature and strain sensing. The influences of the refractive index and the fiber core radius on the BGS of LP<sub>01</sub>-LP<sub>11</sub> mode pair are studied. By selecting the optimal structure parameters, we discuss the effect of temperature and strain on the BGS of LP<sub>01</sub>-LP<sub>11</sub> mode pair. The errors for simultaneous temperature and strain measurement are reduced to 0.23 ℃ and 5.67 με. Compared with other reported results, our obtained temperature and strain sensitivity are high and sensing errors are low in the considered M-FMF. In other words, based on the BGS of LP<sub>01</sub>-LP<sub>11</sub> mode pair, the performance of temperature and strain sensing are improved in the M-FMF. This work is of great significance for studying intra-mode and inter-mode SBS in an FMF. Moreover, the results also provide a guideline for further improving the performance of simultaneous temperature and strain sensing.

All-Fiber Cylindrical Vector Beams Multiplexing Through a Mode-Selective Coupler

In order to improve channel capacity of a communication system, a novel concept of all-fiber cylindrical vector beams (CVB) multiplexing through a mode-selective coupler (MSC) is proposed. According to the proposed concept, a step-index few-mode fiber (FMF) is also designed and fabricated for supporting the first-order CVBs (TE01 and TM01) propagation. In addition, for generating and multiplexing the mutually orthogonal TE01 and TM01, a novel mode-selective coupler with two inputs and one output is also proposed and manufactured by the pre-tapering technology, which can be considered to be the superposition of two independent MSCs with one input and one output. The corresponding theory analysis and simulation with respect to the MSC are performed to prove the validity of the proposed concept in principle. To verify the feasibility of the proposed concept in practice, a novel experimental setup is also proposed and established. The measured result agrees well with the simulation on the whole. Bit error rate (BER) and constellation graph (CG) are employed to evaluate the system performances of the proposed concept. The measured results demonstrate that the BER performance of TM01 (TE01) decreases with the increase of FMF length and data rate, respectively. In addition, compared to the case of only TM01 (TE01) propagation, the measured system performances seriously decline when TM01 and TE01 are multiplexed. The received power distributions demonstrate that the crosstalk among first-order vector modes increases with the increase of FMF length when TM01 (TE01) is excited and propagated. Some advanced technologies [with respect to fibers, fabrication, (de)coding, etc.] are also discussed, which can be employed to improve the system performance in the future.

Efficient Mode Multiplexer for Few-Mode Fibers Using Integrated Silicon-on-Insulator Waveguide Grating Coupler

We propose a novel and highly efficient multimode waveguide grating coupler which can simultaneously launch four channels including two polarizations of the linear polarized (LP) modes, LP 01 and LP 11 of a step-index few-mode fiber (FMF). The waveguide grating coupler on the silicon-on-insulator (SOI) platform is based on a subwavelength structure which was optimized using the genetic optimization approach with 2-dimensional finite-difference time-domain (2-D FDTD) simulations combined with the second - order effective medium theory (EMT). Simulations predicted the coupling efficiencies to be -4.3 dB for LP 01 mode and -5.0 dB for the LP 11 mode. The design was fabricated in a multi-project wafer (MPW) run for silicon photonics. Coupling efficiency of -4.9 dB and -6.1 dB was experimentally demonstrated for LP 01 mode and LP 11 mode, respectively. The proposed mode multiplexer is entirely passive and suitable for future applications with FMFs in the space-division-multiplexing (SDM) networks.

Design and analysis of a combined-ring core weakly coupled few-mode fiber with six linearly polarized modes and an ultra-large effective index difference.

The theory and properties of weakly coupled few-mode fiber (FMF) have been investigated by means of mode power distribution. According to that, a novel weakly coupled FMF with a combined-ring core structure has been proposed. To the best of our knowledge, it is the first time a weakly coupled FMF has realized the goal of supporting six modes and ultra-large minimum effective index difference (min Δneff) of 1.83*10-3 between all neighboring modes. The combined-ring core structure evidently relieves the pressure of modifying neff via inserting a single low-index area and a single high-index ring in step-index FMFs. Due to this structure, a weakly coupled FMF design can be realized by the smaller low-index central area (central area radius d=0.82 μm) and lightly doped high-index ring (the refractive index difference between the ring and the core Δn2=0.11%) compared with former designs. The effective area (Aeff) and differential mode delay characteristics of the designed FMF have been optimized. Furthermore, we calculated the bending loss and fiber properties at the C and L bands to prove that our proposed FMF satisfies the requirement of large-capacity mode-division multiplexing optical networking.

Threshold for stimulated Brillouin scattering in few-mode fibers.

In this paper, the stimulated Brillouin scattering (SBS) threshold of step-index few-mode fibers (FMFs) is theoretically analyzed. The scattering characteristics of FMFs are analyzed using the mathematical model of the Brillouin scattering spectrum. The gain spectrum of Brillouin scattering and the influence of fiber parameters on the SBS threshold of FMFs are discussed, and the SBS theoretical threshold is obtained according to the parameters of FMFs. A new experimental system for measurement of the SBS threshold of FMFs based on two photonic lanterns is built that can eliminate the Fresnel reflection. The measured SBS thresholds of the LP01 and LP11 modes over 9.8km two-mode fiber are 10.93dBm and 11.35dBm, respectively, and the measured SBS thresholds of LP01, LP11, LP21, and LP02 over 11km four-mode fiber are 10.18, 10.38, 11.71, and 12.23dBm, respectively. The experimental results agree well with the theoretical analysis, and the deviation is less than 0.7dB. The theoretical analysis has guiding significance for setting the incident power in FMF communication and sensing. The experimental system can accurately measure the SBS thresholds of FMFs.

Propagation characteristics of orbital angular momentum modes at 810 nm in step-index few-mode fibers

In free-space or in optical fibers, orbital angular momentum (OAM) multiplexing for information transmission has been greatly developed. The light sources used were well coherent communication bands, and the fibers used were customized. Here, we use an 810 nm femtosecond laser to generate optical vortices carrying OAM and then feed them into two kinds of commercial step-index few-mode fibers to explore the transmission characteristics of OAM modes. We also propose a method without multiple-input multiple-output digital signal processing to identify the input OAMs. It is of great guiding significance for high-dimensional quantum information experiments via the OAMs as a degree of freedom, using the light generated by the spontaneous parametric down-conversion as the source and the commercial fibers for information transmission.