All-fiber spatial rotation manipulation for radially asymmetric modes

High-power polarization-maintaining LP11-mode fiber laser based on long-period fiber grating for precise welding

An LP11-mode output all-fiber laser was presented, utilizing long-period fiber gratings (LPFGs) and polarization-maintaining optical fiber (PMF). The LPFG was designed and fabricated, achieving a 90.56% efficiency in LP01 to LP11 mode conversion. Furthermore, the transmission stability of LP11-mode in the PMF was also explored, with the spatial mode overlap ratio exceeding 0.95. Ultimately, the high-power polarization-maintaining (PM) fiber laser, capable of the LP11 mode output, was constructed, with the output power of 600 W and the beam quality M2 of 2.84. During the process of welding a thick Al-plate, the LP11 fiber laser exhibits a notable 1.88 times greater depth of fusion compared to the commercial single-mode fiber laser, when operating at the laser welding head speed of 100 mm/s. For applications demanding non-circular symmetric high-order modes, this research holds substantial potential for widespread adoption within the field of industrial processing.

Experimental investigation of point-by-point off-axis Bragg gratings inscribed by a femtosecond laser in few-mode fibers

Off-axis Bragg gratings with varied horizontal and vertical distances off the center in a step-index two-mode fiber were fabricated by 800 nm infrared-femtosecond laser pulses through a point-by-point technique. In this article, we experimentally investigate these gratings via measuring the transmitted power and the reflected intensity profiles under different input polarization, with multiple characteristics reported for the first time to the best of our knowledge. To highlight, we find that the birefringence induced to the LP₀₁ reaches its maximum magnitude at an intermediate offset, followed by the fast and slow axes switching at a further slightly increased offset. We also show that the peak reflectivity of the LP₁₁ exhibits strong polarization dependence, with the much stronger peak reflectivity constantly corresponding to the polarization perpendicular to the damage-point-to-center line, whereas the peak reflectivity of the LP₀₁ has almost no polarization dependence. Moreover, we report that the reflected mode patterns of the cross-coupling of the LP₀₁ and LP₁₁ are linked to the direction of linear polarization, through which one can selectively excite an arbitrarily oriented LP₁₁ by merely altering the polarization.

Selective transverse mode operation of a fiber laser based on few-mode FBG for rotation sensing

A fiber laser with selective transverse mode operation based on few-mode FBG was designed, and rotation sensing via hybrid mode operation was successfully demonstrated. The mode selection was achieved by a few-mode FBG inscribed in homemade elliptical multilayer-core fiber (EMCF). The particular designed EMCF only supports LP₀₁ and LP₁₁ ^even mode groups, and the resonance of a few-mode FBG could be adjusted through mode excitation. Therefore, selective transverse mode operation was realized by switching the resonance wavelengths corresponding to the self-coupling mode or cross-coupling mode. Besides, rotation sensing was achieved in the hybrid mode operation due to the asymmetric multilayer-core design. A sensitivity of 0.074 mW/° was preliminarily demonstrated. The measured angle of the rotation sensing system is within ± 2° in the temperature range of 10-90 °C, showing that this system was inherently insensitive to temperature, eliminating the requirement for temperature compensation.

Realization of linear-mapping between polarization Poincaré sphere and orbital Poincaré sphere based on stress birefringence in the few-mode fiber

Based on the spatial profiles and polarization states evolution process of the first-order modes resulted from stress-induced birefringence in the few-mode fiber (FMF), we analyze the mapping relationship between the input polarization states represented on polarization PS and the output spatial profiles represented on the orbital PS of the FMF with respect to the magnitude and orientation of birefringence. When the input mode lobe orientation and the phase differences between the four eigenmodes of FMF induced by the stress birefringence satisfy a given condition, the mapping relationship between the input polarization PS and the output orbital PS is linear. Thus, the arbitrary points on the orbit PS can be generated at the output of stressed FMF by controlling the polarization state of the input modes. Then we experimentally verify that, an electrical single-mode polarization controller, a mode converter for converting fundamental mode to higher-order mode, a polarization controller mounting a coil of two-mode fiber and a polarizer can be employed to generate arbitrary first-order spatial modes on the orbital PS by controlling the input single-mode polarization states. The positions on the orbital PS of the generated first-order modes, which are obtained by calculating the three normalized Stokes parameters of output modes, agree well with the simulation ones. The correlation coefficients between the theoretical mode profiles and the experimental ones are higher than 80%. Since the spatial profile evolutions depend on the variations of the input polarization states, a potential advantage of this method is high-speed switching among desired first-order modes by using the commercial devices switching the state of polarization.

Two-mode fiber based directional torsion sensor with intensity modulation and 0° turning point

In this paper, we report a novel in-fiber Mach-Zehnder interferometer based directional torsion sensor, in which a section of two-mode fiber is sandwiched between two single mode fibers by over core-offset splicing technique. The variety of fringe visibility demonstrates the strong dependence on offset and fiber length. For the first time the near zero visibility at 0° rotating state is obtained by fine offset-modulation. The experimental results show that, with 0° turning point, the counter-clockwise to the clockwise direction can be recognized by the reversal from peak to dip of fringes. Moreover, a competitive sensitivity of 21.485 dB/(rad/cm) is gained with high linearity and low-temperature crosstalk in the range from -40 rad/m to 40 rad/m. Without any pre-twisting, our fiber torsion sensor is small size, ease of fabrication, cost efficiency and very potential in the applications of industrial and artificial intelligence.

Generation of LP11/LP21 modes with tunable mode lobe orientation controlled by polarization states

We propose and experimentally demonstrate a novel scheme to generate LP₁₁/LP₂₁ modes with tunable mode lobe orientation (MLO). Wherein, the MLOs have an excellent linear relationship with the linearly-polarized states of input fundamental modes. The proposed scheme is composed of a polarization controller (PC), a mode converter, a mode and polarization controller (PMC) which is twined with the few mode fiber (FMF) and a polarizer. Experimental results show that the deviations of MLOs between generated LP₁₁/LP₂₁ modes and simulated ones are less than 3.5 and 8 degrees over C band. Since polarization control up to nanosecond scale is available with GaAs or lithium based electro-optic modulator, the proposed scheme could enable nanosecond time scale MLO control, which would be immensely useful for optical trapping, fiber sensors and optical communications.

All-fiber polarization-division multiplexing to mode-division multiplexing conversion for hybrid optical networks

This paper proposes and experimentally demonstrates an all-fiber conversion method that transfers the polarization-division multiplexing (PDM) signals to the mode-division multiplexing (MDM) signals. The conversion scheme is based on a mode converter and a polarization-mode controller. The input X-polarized/Y-polarized fundamental modes are converted to the first-order linear-polarized LP11A/LP11B modes with crosstalk of -10  dB/-18  dB and insertion losses of 3.04 dB/3.1 dB at 1550 nm, respectively. Using the proposed converter, 11.2 GBaud/s polarization-division-multiplexed quadrature-phase-shift keying (PDM-QPSK) and 16 quadrature-amplitude-modulation (PDM-16QAM) signals are successfully converted to 11.2 GBaud/s MDM-QPSK and MDM-16QAM signals.