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Fabrication and study of femtosecond laser micromachined few-mode elliptical core waveguides

Prajal Chettri, Shailesh Srivastava

Abstract

Femtosecond laser micromachining (FLM) fabricated waveguides inherently form elliptical cores due to differences in focal spot size and the Rayleigh range of the microscope objective. Consequently, it is essential to study their propagation characteristics, which differ from those of conventional circular-core waveguides. In this work, we present the results of a parametric optimization of these waveguides to identify fabrication parameters that lead to minimal loss. A propagation loss characterization study revealed that, for a laser wavelength of 1030 nm, a pulse width of $\sim$300 fs, a pulse energy of 600 nJ, a scan speed of 2 mm/s, and a repetition rate of 100 kHz, a transparent and micro-bubble-free waveguide with a propagation loss of $\sim$0.4 dB/cm was formed. The modal analysis further demonstrated that the V-number depends on the core aspect ratio. The waveguide modes were compared with computationally generated modes, revealing a correlation that aligns well with existing literature.

Fabrication and study of femtosecond laser micromachined few-mode elliptical core waveguides

Abstract

Femtosecond laser micromachining (FLM) fabricated waveguides inherently form elliptical cores due to differences in focal spot size and the Rayleigh range of the microscope objective. Consequently, it is essential to study their propagation characteristics, which differ from those of conventional circular-core waveguides. In this work, we present the results of a parametric optimization of these waveguides to identify fabrication parameters that lead to minimal loss. A propagation loss characterization study revealed that, for a laser wavelength of 1030 nm, a pulse width of 300 fs, a pulse energy of 600 nJ, a scan speed of 2 mm/s, and a repetition rate of 100 kHz, a transparent and micro-bubble-free waveguide with a propagation loss of 0.4 dB/cm was formed. The modal analysis further demonstrated that the V-number depends on the core aspect ratio. The waveguide modes were compared with computationally generated modes, revealing a correlation that aligns well with existing literature.
Paper Structure (10 sections, 4 equations, 10 figures)

This paper contains 10 sections, 4 equations, 10 figures.

Table of Contents

  1. Introduction
  2. Methods
  3. Fabrication of Waveguides
  4. Characterization of the Waveguides
  5. Results and discussion
  6. Cross-sectional ellipticity dependence on laser fabrication parameters
  7. Waveguide width and modification dependence on laser fabrication parameters
  8. Propagation loss characterization
  9. Modal analysis of elliptical core waveguides
  10. Conclusion

Figures (10)

  • Figure 1: Experimental schematic for waveguide characterization
  • Figure 2: Optical microscope image of the waveguides depicting the dark and bright zones within the cross-section.
  • Figure 3: Cross-sectional optical microscope images of optical waveguides fabricated with different pulse energies and scanning speeds.
  • Figure 4: Eccentricities as a function of pulse energies for different scanning speeds.
  • Figure 5: Longitudinal optical microscope images of optical waveguides fabricated with different pulse energies and scanning speeds
  • ...and 5 more figures