Mode resolved bend loss in few-mode optical fibers

Seeing the strong suppression of higher order modes in single trench fiber using the S2 technique

The single trench fiber (STF) is a promising fiber design for mode area scaling and higher order mode (HOM) suppression. In this Letter, we experimentally demonstrate the strong HOM-suppression in a homemade STF using the spatially and spectrally resolved imaging (S²) technique. This STF has a 20-µm core and its performance is compared to a conventional step-index fiber with almost the same parameter. Results show that the bending loss of the HOM in STF is 8-times larger than conventional fiber at a bend radius of 7 cm. In addition, when severe coupling mismatch is introduced at the input end of the fiber, the STF can keep the fundamental-mode output while the conventional fiber cannot. To the best of our knowledge, this is the first time to experimentally analyze the HOM content in an STF and compare its performance with that of a conventional fiber. Our results indicate the great potential of the STF for filtering the HOM in fiber laser applications.

Contribution to the Improvement of the Correlation Filter Method for Modal Analysis with a Spatial Light Modulator

Modal decomposition of light is essential to study its propagation properties in waveguides and photonic devices. Modal analysis can be carried out by implementing a computer-generated hologram acting as a match filter in a spatial light modulator. In this work, a series of aspects to be taken into account in order to get the most out of this method are presented, aiming to provide useful operational procedures. First of all, a method for filter size adjustment based on the standard fiber LP-mode symmetry is presented. The influence of the mode normalization in the complex amplitude encoding-inherent noise is then investigated. Finally, a robust method to measure the phase difference between modes is proposed. These procedures are tested by wavefront reconstruction in a conventional few-mode fiber.

M2 factor estimation in few-mode fibers based on a shallow neural network

A high-accuracy, high-speed, and low-cost M² factor estimation method for few-mode fibers based on a shallow neural network is presented in this work. Benefiting from the dimensionality reduction technique, which transforms the two-dimension near-field image into a one-dimension vector, a neural network with only two hidden layers can estimate the M² factor directly. In the simulation, the mean estimation error is smaller than 3% even when the mode number increases to 10. The estimation time of 10000 simulation test samples is around 0.16s, which indicates a high potential for real-time applications. The experiment results of 50 samples from the 3-mode fiber have a mean estimation error of 0.86%. The strategies involved in this method can be easily extended to other applications related to laser characterization.

Macrobending loss in wrapped fiber optic for load detections

Macrobending-based fiber optic sensors are constructed to measure load. Two sensor models are investigated to find a better linearity response; i.e., narrow helix-shaped and wide helix-shaped sensors. The sensing mechanism of those sensors is due to a macrobending loss of the traveling light inside the optical fibers. This work employs a distributed feedback laser diode and a Ge-based photodetector as the light source and the light loss detector, respectively. The results indicated that the linear response to a static-load exposure apparently can be improved by spacing the coiled fiber on the silicon rubber at a specific diameter. Several investigations reveal the optimum diameter and fiber spacing to design an accurate fiber optic based sensor for load measurement. A sensitivity of 398V/kg/cm² and 100V/kg/cm² are obtained on a diameter of 25 mm and 38 mm cylindrical rubber with fiber spacing of 15 mm. Furthermore, an assessment under a dynamic object can exhibit uniform responses presenting minor deviations.

Enhanced refractometer for aqueous solutions based on perfluorinated polymer optical fibres

The use of the new CYTOP (Cyclized Transparent Optical Polymer) fibres for the inscription of optical structures and the detection of different parameters has started to gain importance in the past decade. This work presents the design, simulation and manufacture of a CYTOP-based surrounding refractive index sensor for aqueous solutions, given its high sensitivity in the range 1.315 - 1.333 (at 1550 nm wavelength). The structure is based on a bent and polished fibre (in order to increase its sensitivity), the polished area being the surface on which a diffraction grating is inscribed with a femtosecond laser. The interaction of the field propagated by the fibre with the grating causes diffraction of certain orders towards the outside, depending, among other things, on the refractive index of the fluid. In addition to a maximum sensitivity of -208.8 nm/RIU and a remarkable insensitivity to temperature, it offers a spectral fingerprint of each sensed fluid.

Modal analysis of structured light with spatial light modulators: a practical tutorial

A quantitative analysis of optical fields is essential, particularly when the light is structured in some desired manner, or when there is perhaps an undesired structure that must be corrected for. A ubiquitous procedure in the optical community is that of optical mode projections-a modal analysis of light-for the unveiling of amplitude and phase information of a light field. When correctly performed, all the salient features of the field can be deduced with high fidelity, including its orbital angular momentum, vectorial properties, wavefront, and Poynting vector. Here, we present a practical tutorial on how to perform an efficient and effective optical modal decomposition, with emphasis on holographic approaches using spatial light modulators, highlighting the care required at each step of the process.

Influence of pedestal diameter on mode instabilities in Yb/Ce/Al-doped fibers

In this paper we present numerical and experimental results revealing that the mode instability threshold of highly Yb-doped, Ce/Al co-doped pedestal fibers is affected by the size of the index-increased pedestal structure surrounding the core. An alternative preparation technology for the realization of large mode area fibers with very large Al-doped silica pedestals is introduced. Three different pedestal fiber design iterations characterized by low photodarkening were manufactured and tested in counter-pumped amplifier setups. Up to 1.9 kW continuous-wave output power of near-diffraction-limited beam quality (M² = 1.26) was achieved with an 18/200/420 µm fiber of very low NA = 0.042, limited only by the occurrence of mode instabilities.

Focal field analysis of highly multi-mode fiber beams based on modal decomposition

In this work, a numerical modal decomposition approach is applied to model the optical field of laser light after propagating through a highly multi-mode fiber. The algorithm for the decomposition is based on the reconstruction of measured intensity profiles along the laser beam caustic with consideration of intermodal degrees of coherence derived from spectral analysis. To enhance the accuracy of the model, different approaches and strategies are applied and discussed. The presented decomposition into a set of linearly polarized modes enables both the wave-optical simulation of radiation transport by highly multi-mode fibers and, additionally, the analysis of free-space propagation with arbitrarily modified complex amplitude distributions.

Trench-assisted multimode fiber used in Brillouin optical time domain sensors

The trench-assisted multimode fiber TA-OM4 is used as a sensing fiber to achieve a higher signal-to-noise (SNR) in Brillouin optical time domain sensors, due to its high stimulated Brillouin threshold and high modulation instability threshold. The Brillouin gain spectrum (BGS), stimulated Brillouin scattering (SBS) and modulation instability (MI) thresholds of TA-OM4 at 1550 nm are characterized and demonstrated theoretically and experimentally. The SNR improvements of TA-OM4 over G655 and G657 at the end of 15.5 km-long fibers, which are respectively 1.1 dB and 2.3 dB are verified experimentally. We achieve a temperature uncertainty of 0.3°C in 15.5 km TA-OM4 with 5 m spatial resolution by use of a Brillouin optical time domain reflectometry (BOTDR) sensor. The good bend resistance and high SBS and MI thresholds of TA-OM4 with better SNR improvements over SMFs works at the extreme bending conditions.

Learning to decompose the modes in few-mode fibers with deep convolutional neural network

We introduce a deep-learning technique to perform complete mode decomposition for few-mode optical fibers for the first time. Our goal is to learn a fast and accurate mapping from near-field beam patterns to the complete mode coefficients, including both modal amplitudes and phases. We train the convolutional neural network with simulated beam patterns and evaluate the network on both the simulated beam data and the real beam data. In simulated beam data testing, the correlation between the reconstructed and the ideal beam patterns can achieve 0.9993 and 0.995 for 3-mode case and 5-mode case, respectively. While in the real 3-mode beam data testing, the average correlation is 0.9912 and the mode decomposition can be potentially performed at 33 Hz frequency on a graphic processing unit, indicating real-time processing ability. The quantitative evaluations demonstrate the superiority of our deep learning-based approach.

Theoretical and experimental studies on direct-liquid-cooling fiber-coiling mode control technology in high-power fiber lasers

We present a direct-liquid-cooling fiber-coiling mode control technology to flexibly and effectively control the output beam quality of high-power fiber lasers. For theoretical guidelines, we propose a theoretical model of the fiber-coiling mode control method based on the bend loss estimated using the beam propagation method. The model is used to analyze the effects of fiber-coiling methods and fiber laser parameters on fiber-coiling mode control. Based on a theoretical analysis, we constructed two master oscillator power amplifiers with 20/400 and 25/400 fibers, respectively, to carry out experiments. The results demonstrate the accuracy of the theoretical model and the effectiveness of the direct-liquid-cooling fiber-coiling method in controlling the output beam quality.

Orientation-dependent fiber-optic displacement sensor using a fiber Bragg grating inscribed in a side-hole fiber

We propose and experimentally demonstrate an orientation-dependent fiber-optic bending sensor. The sensing probe consists of a fiber Bragg grating inscribed in both the fiber core and the surrounding cladding of a section of a side-hole fiber. We utilized a side-illumination technique using a femtosecond laser to achieve the grating structure formation. The transmission intensities of both resonances are highly sensitive bending of the fiber, and the bending response shows orientation dependence. The surrounding temperature fluctuation causes a wavelength shift, but not an intensity variation. Therefore, the proposed sensor can be employed for simultaneous measurement of bending and temperature.

Highly sensitive fiber-optic accelerometer by grating inscription in specific core dip fiber

A highly sensitive fiber-optic accelerometer based on detecting the power output of resonances from the core dip is demonstrated. The sensing probe comprises a compact structure, hereby a short section of specific core (with a significant core dip) fiber stub containing a straight fiber Bragg grating is spliced to another single-mode fiber via a core self-alignment process. The femtosecond laser side-illumination technique was utilized to ensure that the grating inscription region is precisely positioned and compact in size. Two well-defined core resonances were achieved in reflection: one originates from the core dip and the other originates from fiber core. The key point is that only one of these two reflective resonances exhibits a high sensitivity to fiber bend (and vibration), whereas the other is immune to it. For low frequency (<10 Hz) and weak vibration excitation (<0.3 m/s²) measurement, the proposed sensor shows a much higher resolution (1.7 × 10^-3 m/s²) by simply monitoring the total power output of the high-order core mode reflection. Moreover, the sensor simultaneously provides an inherent power reference to eliminate unwanted power fluctuations from the light source and transmission lines, thus providing a means of evaluating weak seismic wave at low frequency.

Multimode fiber modal decomposition based on hybrid genetic global optimization algorithm

Numerical modal decomposition (MD) is an effective approach to reveal modal characteristics in high power fiber lasers. The main challenge is to find a suitable multi-dimensional optimization algorithm to reveal exact superposition of eigenmodes, especially for multimode fiber. A novel hybrid genetic global optimization algorithm, named GA-SPGD, which combines the advantages of genetic algorithm (GA) and stochastic parallel gradient descent (SPGD) algorithm, is firstly proposed to reduce local minima possibilities caused by sensitivity to initial values. Firstly, GA is applied to search the rough global optimization position based on near- and far-field intensity distribution with high accuracy. Upon those initial values, SPGD algorithm is afterwards used to find the exact optimization values based on near-field intensity distribution with fast convergence speed. Numerical simulations validate the feasibility and reliability.

Tunable higher-order orbital angular momentum using polarization-maintaining fiber

For the first time, to the best of our knowledge, light with orbital angular momentum (OAM) of ±2ℏ per photon is produced using commercially available polarization-maintaining fiber with modal purity of 96%. Twist measurements demonstrate that the average orbital angular momentum can be continuously tuned between ±2ℏ. The authors consider beams of non-integer OAM, created using the presented method, as superpositions of integer OAM states.

Single mode 4.3 kW output power from a diode-pumped Yb-doped fiber amplifier

We investigate the average power scaling of two diode-pumped Yb-doped fiber amplifiers emitting a diffraction-limited beam. The first fiber under investigation with a core diameter of 30 µm was able to amplify a 10 W narrow linewidth seed laser up to 2.8 kW average output power before the onset of transverse mode instabilities (TMI). A further power scaling was achieved using a second fiber with a smaller core size (23µm), which allowed for a narrow linewidth output power of 3.5 kW limited by stimulated Brillouin scattering (SBS). We mitigated SBS using a spectral broadening mechanism, which allowed us to further increase the output power to 4.3 kW only limited by the available pump power. Up to this power level, a high slope efficiency of 90% with diffraction-limited beam quality and without any sign of TMI or stimulated Raman scattering for a spectral dynamic range of higher than -80 dB was obtained.

On the measurement of fundamental mode bend loss in large-mode-area optical fibers

The measurement of fundamental mode bend loss is thoroughly studied in large-mode-area few-mode optical fibers. The influencing factors, including spectral properties of the light source, modal power content and cladding light, are experimentally investigated. Monte Carlo simulations are performed to help in understanding and illustrating the distribution of the variations. Practical guidelines and an example setup are provided for precise and accurate measurements.

Twist-induced guidance in coreless photonic crystal fiber: A helical channel for light

A century ago, Einstein proposed that gravitational forces were the result of the curvature of space-time and predicted that light rays would deflect when passing a massive celestial object. We report that twisting the periodically structured "space" within a coreless photonic crystal fiber creates a helical channel where guided modes can form despite the absence of any discernible core structure. Using a Hamiltonian optics analysis, we show that the light rays follow closed spiral or oscillatory paths within the helical channel, in close analogy with the geodesics of motion in a two-dimensional gravitational field. The mode diameter shrinks, and its refractive index rises, as the twist rate increases. The birefringence, orbital angular momentum, and dispersion of these unusual modes are explored.

Bending losses of trench-assisted few-mode optical fibers

A semianalytical method based on the perturbation theory is developed to calculate the bending losses of individual modes of few-mode fibers (FMFs); it is applicable for optical fibers with arbitrary circularly symmetric index profile, especially for trench-assisted fibers. The bending performance of trench-assisted step-index FMFs and parabolic-index FMFs are investigated with three key parameters (i.e., the refractive index difference of trench-cladding, the width of the trench, and the distance of the core-trench). Then, a performance index is defined to estimate the bending performance for FMFs. It is shown that changing the distance of the trench-core, for each order of mode, there is a minimum bending loss, which can be used for fiber optimization. This optimization position (core-trench distance) decreases as the mode order increases. It is found that the bending performance of parabolic-index FMFs is better than that of step-index FMFs with fixed core radius and cutoff wavelength. The conclusions are helpful for understanding the mechanism of bending loss for FMFs, and make contributions to designing and manufacturing of few-mode bend-insensitive fibers.

Mode resolved bend-loss analysis in few-mode fibers using spatially and spectrally resolved imaging

The increasing use of few-mode fibers for high-speed optical communication systems in space division multiplexing has created a need for mode resolved characterization of few-mode fibers. In this Letter, we present a new method to characterize the bend loss of the individual modes in a few-mode fiber. This procedure uses a simple setup for spatially and spectrally resolved imaging and allows the measurement of the bend loss of each and every guided mode at once. It does not require the use of mode converters in contrast to other methods. Results for graded-index two- and four-mode fibers are presented, together with comparisons against direct bend-loss measurements for the four-mode and standard single-mode fibers.

Real-time mode decomposition for few-mode fiber based on numerical method

Today a specific attention has been paid to look into the modal characteristics of the high-power laser beam. And the instantaneous monitoring and analyzing on modal content via the mode decomposition technique will provide a novel route. We implement the first-ever experimental investigation on the real-time mode decomposition technique for few-mode laser beam based on stochastic parallel gradient descent algorithm. It will reduce the cost and the complexity of the mode decomposition system. We have succeeded to decompose the mode spectra as well as calculating the beam quality factor at about 9 Hz monitoring rate, while the high agreement between the measured and reconstructed intensity profiles in each frame indicating the high accuracy and stability during the process. By employing a fiber-squeezing-based polarization controller, the modal content under test can be time-varying automatically.

Three-dimensional modeling of CPA to the multimillijoule level in tapered Yb-doped fibers for coherent combining systems

We developed a three-dimensional numerical model of Large-Mode-Area chirped pulse fiber amplifiers which includes nonlinear beam propagation in nonuniform multimode waveguides as well as gain spectrum dynamics in quasi-three-level active ions. We used our model in tapered Yb-doped fiber amplifiers and showed that single-mode propagation is maintained along the taper even in the presence of strong Kerr nonlinearity and saturated gain, allowing extraction of up to 3 mJ of output energy in 1 ns pulse. Energy scaling and its limitation as well as the influence of fiber taper bending and core irregularities on the amplifier performance were studied. We also investigated numerically the capabilities for compression and coherent combining of up to 36 perturbed amplifying channels and showed more than 70% combining efficiency, even with up to 11% of high-order modes in individual channels.

Measurement of effective refractive index differences in multimode optical fibers based on modal decomposition

We demonstrate the nondestructive measurement of the effective refractive index difference of two arbitrary modes within a multimode optical fiber by utilizing a tunable fiber grating. We use a mechanical grating of variable period to couple the respective modes and measure the mode content at the fiber output based on the correlation filter technique. From the dependence of the coupling efficiency on the grating period, the effective index difference of the modes can be extracted with high accuracy.

Polarization-resolved near-field mapping of plasmonic aperture emission by a dual-SNOM system

We study the polarization characteristics of light emission and collection in the near field by the tips of a Dual-SNOM (two scanning near-field optical microscopes) setup. We find that cantilevered fiber probes can serve as emitters of polarized light, or as polarization-sensitive detectors. The polarization characteristics depend on the fiber type used for tip fabrication. In Dual-SNOM measurements, we demonstrate mapping of different field components of the plasmonic dipole pattern emitted by an aperture probe.

Few-mode fiber with inverse-parabolic graded-index profile for transmission of OAM-carrying modes

A novel type of few-mode fiber, characterized by an inverse-parabolic graded-index profile, is proposed for the robust transmission of cylindrical vector modes as well as modes carrying quantized orbital angular momentum (OAM). Large effective index separations between vector modes (>2.1 × 10(-4)) are numerically calculated and experimentally confirmed in this fiber over the whole C-band, enabling transmission of OAM(+/-1,1) modes for distances up to 1.1 km. Simple design rules are provided for the optimization of the fiber parameters.

Measurement of higher-order mode propagation losses in effectively single mode fibers

We present a direct and nondestructive measurement of the propagation loss of higher-order modes (HOMs) in effectively single-mode fibers. Lossy HOMs are excited by applying local stress at various points alongside a straight single mode fiber. The change of the HOM power as a function of the propagation distance is recorded at the fiber end by performing a modal decomposition with a correlation filter. The results for the HOM propagation loss are compared to simulations yielding very good agreement.

Comparative analysis of numerical methods for the mode analysis of laser beams

We present a comparative study of four numerical methods to detect the mode content of a laser beam from, at most, two intensity images. The techniques are compared regarding temporal effort, stability, and accuracy, using the example of three multimode optical fibers that differ in the number of supported modes.

Modal characterization of fiber-to-fiber coupling processes

We present a detailed experimental investigation of a fiber-to-fiber coupling process by characterizing the mode content at the output of the system. In our experiment a single-mode fiber is transversally scanned with respect to a multimode fiber, revealing position-dependent higher-order mode excitation. The outlined measurement system can be used for automated optimization of fundamental mode content and beam quality. Additionally, our approach characterizes the modal transmission properties of the multimode waveguide in its present state and is hence of high relevance for the conception of transport fibers and fiber laser systems.