GRIN Fiber Research Articles

Generation of second harmonic at wide conversion band in GRIN multimode fibers

Optical poling of multimode graded-index fibers (GRIN) has emerged as a promising technique for creating periodic inscriptions of the second-order nonlinear optical susceptibility χ(2), enabling the generation of a second harmonic in silica fibers. In this work, we investigate the generation of multiple spectral peaks using a continuous broadband source in the infrared domain, generated in the same fiber by a femtosecond laser with a central wavelength different from the one used for the poling process. Building upon theoretical foundations, this work contributes at explaining how second and third-order nonlinear processes participate to the broad generation of the second harmonic in GRIN fibers.

Harmonic motion modes in parabolic GRIN fibers

We report electromagnetic field modes solving the inhomogeneous Maxwell equations for parabolic gradient index fibers in the low refractive index contrast approximation. The first family comprises accelerating fields characterized by an intensity distribution center tracing a circular trajectory transverse to the fiber optical axis. These fields maintain an invariant shape for both their intensity and phase distributions while rotating around their center. The second family comprises breathing fields characterized by an intensity distribution center aligned with the fiber optical axis. These fields exhibit intensity distribution scaling along propagation, while their phase swirls and rotates around the optical axis without changing their intensity distribution shape and topological charge.

Spatial Beam Cleaning in Multimode GRIN Fibers: Polarization Effects

Spatial Beam Cleaning in Multimode GRIN Fibers: Polarization Effects

High-temperature wave thermalization spoils beam self-cleaning in nonlinear multimode GRIN fibers.

In our experiments, we reveal a so-far unnoticed power limitation of beam self-cleaning in graded-index nonlinear multimode optical fibers. As the optical pulse power is progressively increased, we observed that the initial Kerr-induced improvement of the spatial beam quality is eventually lost. Based on a holographic mode decomposition of the output field, we show that beam spoiling is associated with high-temperature wave thermalization, which depletes the fundamental mode in favor of a highly multimode power distribution.

Focusing optimization in multimodal graded index fiber coupling by wavefront shaping

Graded index fibers are important technological elements that have improved the performance of systems in many scientific fields due to their particular refractive index profile. However, the necessity to control focus at their distal end makes the design and fabrication of additional lenses at the tip of the fiber an essential step to their applications, such as bio imaging and integrated systems. An alternative and more general approach that does not require additional fabrication steps is highly desirable. We hypothesize that controlling the wavefront at the entrance of the GRIN fiber is such a method. In this work, the convergence of the beam at the distal end of graded index fibers was controlled by phase shaping the input wavefronts. By optimizing phase masks, focus was generated and tuned according to their desired dimensions, eliminating any further modifications to the fiber. To explore the applications enabled by focusing, coupling experiments were performed fiber-to-fiber and fiber-to-chip. It was found that by using optimized wavefronts to tune the convergence in a fiber lensless system, the output power of the experiments was significantly enhanced to more than 4.5dB when compared to the non-optimal configuration.

Mechanism of kisspeptin neuron synchronization for pulsatile hormone secretion in male mice

The mechanism by which arcuate nucleus kisspeptin (ARNKISS) neurons co-expressing glutamate, neurokinin B, and dynorphin intermittently synchronize their activity to generate pulsatile hormone secretion remains unknown. An acute brain slice preparation maintaining synchronized ARNKISS neuron burst firing was used alongside invivo GCaMP GRIN lens microendoscope and fiber photometry imaging coupled with intra-ARN microinfusion. Studies in intact and gonadectomized male mice revealed that ARNKISS neuron synchronizations result from near-random emergent network activity within the population and that this was critically dependent on local glutamate-AMPA signaling. Whereas neurokinin B operated to potentiate glutamate-generated synchronizations, dynorphin-kappa opioid tone within the network served as a gate for synchronization initiation. These observations force a departure from the existing "KNDy hypothesis" for ARNKISS neuron synchronization. A "glutamate two-transition" mechanism is proposed to underlie synchronizations in this key hypothalamic central pattern generator driving mammalian fertility.

Influence of Ultra-Small GRIN Fiber Probe on Fiber F-P Interference Signal

Influence of Ultra-Small GRIN Fiber Probe on Fiber F-P Interference Signal

High Power All-Fiber Supercontinuum System Based on Graded-Index Multimode Fibers

An all-fiber supercontinuum source based on graded-index multimode fibers is reported. The supercontinuum source is based on a homemade mode-locked oscillator and a three-stage picosecond amplifier, which obtained the supercontinuum by a graded-index multimode fiber. The laser output with a spectral range of 480–2440 nm, an average power of 25 W, and a repetition frequency of 8.27 MHz is obtained. To the best of our knowledge, this is the highest average power for generating a supercontinuum with an all-fiber structure based on the graded-index fiber. The effects of GRIN fiber length and different pump peak powers on the supercontinuum generation are also verified. The results showed that the graded-index multimode fiber can effectively obtain a supercontinuum with high power.

Comparison of multimode GRIN-fiber Raman lasers with FBG and random DFB cavity

Abstract Raman lasing in multimode GRIN fibers is accompanied by sufficient improvement of the output beam quality in comparison with that for pump radiation, which offers opportunities to wavelength-agile fiber lasers of new type. Here we compare power scaling and brightness enhancement capabilities of Raman laser based on multimode GRIN-fiber of 62.5/125 um core/cladding diameters pumped by ∼700 W multimode source with beam quality M2∼10, performed in two different cavity configurations: 1) linear cavity based on two fiber Bragg gratings and 2) half-open cavity with one FBG and random distributed feedback via Rayleigh backscattering along the GRIN fiber.

Mode-resolved analysis of pump and Stokes beams in LD-pumped GRIN fiber Raman lasers.

All-fiber Raman lasers have demonstrated their potential for efficient conversion of highly multimode pump beams into high-quality Stokes beams. However, the modal content of these beams has not yet been investigated. In this work, based on a mode decomposition technique, we are able to reveal the details of intermodal interactions in the different operation regimes of continuous wave multimode graded-index fiber Raman lasers. We observed that, above the laser threshold, the residual pump beam is strongly depleted in its transverse modes with principal quantum number below 10. However, the generated Stokes signal beam mainly consists of the fundamental mode, but higher-order modes are also present, albeit with exponentially decreasing population.

MEMS fiber optic acoustic sensor and performance testing method based on ultra-small GRIN fiber lens

MEMS fiber optic acoustic sensor and performance testing method based on ultra-small GRIN fiber lens

Origin of spontaneous wave mixing processes in multimode GRIN fibers

We show that geometric parametric instability (GPI) in graded-index multimode fibers is strongly influenced by higher-order dispersion. By measuring the output spectrum for different core radii, we distinguish peaks generated by GPI from other coexisting parametric processes using phase-matching arguments and numerical simulations. We highlight for the first time a non-degenerate GPI process involving two pumps at different wavelengths.

Kilowatt level Raman amplifier based on 100 µm core diameter multimode GRIN fiber with M2 = 1.6.

In this Letter, we demonstrate a high-power Raman fiber amplifier with excellent beam quality based on graded-index fiber. The Yb-doped fiber laser (YDFL) and bandwidth-tunable amplified spontaneous emission (ASE) source are employed as the pump source to compare the laser performance separately. When the ASE with a bandwidth of 8 nm is employed, a maximum power of 943 W at 1130 nm is achieved, which is twice that pumped by YDFL. The beam quality factor M2 at maximum output power is 1.6, with a brightness enhancement (BE) factor of 27. To the best of our knowledge, this is the best beam quality and BE factor based on pure Raman gain with output power of over 100 W.

F-P interferometer vibration measurement experimental system based on ultra-small grin fiber probe

F-P interferometer vibration measurement experimental system based on ultra-small grin fiber probe

Research on SS-OCT Vibration Measurement Method Based on Ultra-Small GRIN Fiber Probe

Research on SS-OCT Vibration Measurement Method Based on Ultra-Small GRIN Fiber Probe

Managing Self-Phase Modulation in Pseudo-Linear Multimodal and Monomodal Systems

We propose a new semi-analytical model, describing the bandwidth evolution of pulses propagating in dispersion managed (DM) transmission systems using multimodal graded-index fibers (GRIN) with parabolic index. The model also applies to monomodal fiber DM systems, representing the limit case where beam self-imaging vanishes. The model is successfully compared with the direct integration of the (1+1)D nonlinear Schr\"odinger equation for parabolic GRIN fibers, and to experimental results performed by using the transmission of femtosecond pulses over a 5 m span of GRIN fiber. At the high pulse powers that are possible in multimodal fibers, the pulse bandwidth variations produced by the interplay of cumulated dispersion and self-phase modulation can become the most detrimental effect, if not properly managed. The analytical model, numerical and experimental results all point to the existence of an optimal amount of chromatic dispersion, that must be provided to the input pulse, for obtaining a periodic evolution of its bandwidth. Results are promising for the generation of spatio-temporal DM solitons in parabolic GRIN fibers, where the stable, periodic time-bandwidth behaviour that was already observed in monomodal systems is added to the characteristic spatial beam self-imaging.

Research on the application of the ultra-small GRIN fiber probe

Research on the application of the ultra-small GRIN fiber probe

Beam cleaning effects in multimode GRIN-fiber Raman lasers and amplifiers

Raman conversion in multimode GRIN fibers is accompanied by sufficient improvement of the output beam quality in comparison with that for pump radiation, which offers opportunities to create amplifiers and lasers of new type based on directly 9xx-nm diode pumped passive GRIN fibers, which may operate at almost any wavelength in transmission window (0.95-2 micron) of silica fibers. Here we review experimental results and physical mechanisms leading to the beam quality improvement (beam cleaning) in multimode Raman lasers and amplifiers, including effects of Raman gain, fiber Bragg gratings and Rayleigh backscattering feedback in GRIN fibers.

Label-free CARS microscopy through a multimode fiber endoscope.

Multimode fibres have recently been employed as high-resolution ultra-thin endoscopes, capable of imaging biological structures deep inside tissue in vivo. Here, we extend this technique to label-free non-linear microscopy with chemical contrast using coherent anti-Stokes Raman scattering (CARS) through a multimode fibre endoscope, which opens up new avenues for instant and in-situ diagnosis of potentially malignant tissue. We use a commercial 125 µm diameter, 0.29 NA GRIN fibre, and wavefront shaping on an SLM is used to create foci that are scanned behind the fibre facet across the sample. The chemical selectivity is demonstrated by imaging 2 µm polystyrene and 2.5 µm PMMA beads with per pixel integration time as low as 1 ms for epi-detection.

Further study of coupling efficiency of ultra-small gradient-index fiber probe

In this paper, the relationship between the coupling efficiency and transmission distance of different fiber optic probes is compared for further studying the focusing performance of ultra-small GRIN fiber probes. Taking an example of five typical ultra-small GRIN fiber probes such as single mode fiber (SMF), "SMF + 0.31mm 50/125 GRIN", "SMF + 0.36mm NCF + 0.12mm 50/125 GRIN", "SMF + 0.48mm 100/140 GRIN", "SMF + 0.48mm NCF + 0.14mm 50/125 GRIN", the relationships between the theoretical coupling efficiency and transmission distance are compared with experimental data. The obtained results show that an increase in the working distance of a probe causes increase of the transmission distance to maintain a higher coupling efficiency, and leads to gain of the waist diameter. Therefore, the comprehensive effects of the working distance, waist diameter and coupling efficiency must be taken into consideration in the process of optimizing the design parameters of ultra-small GRIN fiber probes.