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Generation of High-order Laguerre-Gaussian modes in Coated and Uncoated Graded-Index and Step-Index Multimode Fibers

Wasyhun Asefa Gemechu, Umberto Minoni, Michela Borghetti, Fabrizio Frezza, Daniele Modotto, Stefan Wabnitz, Fabio Mangini

Abstract

We report an efficient experimental method for generating high-order Laguerre-Gauss (HOLG) modes by simply coupling a Gaussian beam into the cladding of a multimode fiber (MMF). In particular, the order of the HOLG mode remains invariant with respect to input power, propagation distance, and pulse duration. Furthermore, spectral and power measurements confirm that the beam-shaping mechanism is predominantly linear, whereas Kerr nonlinearity primarily affects the longitudinal phase-matching condition and conversion efficiency, without altering the generated mode order. Altogether, these findings establish our approach as a highly robust and scalable platform for generating tailored optical beams.

Generation of High-order Laguerre-Gaussian modes in Coated and Uncoated Graded-Index and Step-Index Multimode Fibers

Abstract

We report an efficient experimental method for generating high-order Laguerre-Gauss (HOLG) modes by simply coupling a Gaussian beam into the cladding of a multimode fiber (MMF). In particular, the order of the HOLG mode remains invariant with respect to input power, propagation distance, and pulse duration. Furthermore, spectral and power measurements confirm that the beam-shaping mechanism is predominantly linear, whereas Kerr nonlinearity primarily affects the longitudinal phase-matching condition and conversion efficiency, without altering the generated mode order. Altogether, these findings establish our approach as a highly robust and scalable platform for generating tailored optical beams.
Paper Structure (5 sections, 10 figures)

This paper contains 5 sections, 10 figures.

Table of Contents

  1. Introduction
  2. Experimental Setup
  3. Experimental results: GRIN MMFs
  4. Experimental results: SI MMFs
  5. Conclusion

Figures (10)

  • Figure 1: Experimental setup for generating HOLG mode in MMFs.The components include: VDF (Variable neutral density filter), HWP (Half-wave plate), $\text{L}_{1-3}$ (Lenses), SI/GRIN MMF (Step-index/Graded-index multimode fiber), BPF (Band-pass filter), FM (Flipping Mirror), PM (Power Meter), and OSA (Optical Spectrum Analyzer). The inset details the injection conditions and the $\pm\{x,y\}$ offsets between the fiber axis and the input laser beam.
  • Figure 2: The experimentally generated HOLG modes arise from selected symmetrical offset values in both the x- and y-directions within the GRIN MMF. These HOLG modes are formed in the core, with the edge of the intensity profile corresponding to the core-cladding interface.
  • Figure 3: Experimentally measured relationship between input and output power (black dots) as a function of fiber length for the GRIN MMF at different lengths: (a) $\text{L} = 0.5$ m, (b) $\text{L} = 2.0$ m, and (c) $\text{L} = 4.0$ m. The linear fit is indicated by the red line.
  • Figure 4: Stability of generated HOLG beams. Transverse near-field intensity distributions measured as a function of input power and propagation length ($0.4 \leq \text{L} \leq 2.0$ m).
  • Figure 5: Near-field intensity distribution in an uncoated GRIN fiber. The bare cladding-air interface prevents the absorption of cladding modes, making both the generated HOLG beam (core) and the residual pump light (cladding) distinctly visible. The blue ring immediately preceding the intense green ring marks the boundary between the core and the cladding.
  • ...and 5 more figures