Fig. 1. Schematic fabrication setup for BGWs: the waveguide is first photoinscribed by using femtosecond Gaussian pulses in a slit shaping method, then the picosecond Bessel pulses are generated by using an insertable 4f system with an axicon element to induce grating voids inside the waveguide along the x-axis direction.

Fig. 2. (a) Resonant spectral transmission of first-order BGWs inscribed with different Bessel pulse energies. (b) Resonant spectral transmission of first order BGWs inscribed with pulse energy of 1.6 μJ upon injection with two orthogonal linear polarizations. The insets show the energy flux profiles for the parallel-polarized (left) and perpendicular-polarized (right) modes.

Fig. 3. (a) Longitudinal section of the grating void (2.8 μJ) after material removal by FIB. (b) Cross sections of the post-polished grating voids induced by picosecond Bessel beam with different pulse energies. (c) Dependence of the voids’ diameters and surrounding condensed region on pulse energies.

Fig. 4. (a) Measured resonant spectra of BGWs with grating segment length of 8 mm. The picosecond Bessel pulses used for inducing grating voids range from 1.5 to 5.0 μJ. (b) Measured and modelled effective refractive index change of BGWs fabricated with different Bessel pulse energies. The inset is the enlarged version. (c) Measured and modelled coupling strength of BGWs fabricated with different Bessel pulse energies. (d) Measured and modelled bandwidth of resonance peak of BGWs fabricated with different Bessel pulse energies.

Fig. 5. Bessel voids were inserted into the waveguide with different offsets relative to the center of the waveguide’s cross section. The pulse energy of the picosecond Bessel beam for inducing grating void was fixed at 2.2 μJ. (a) Diagram of the offset BGWs. (b) PCM top images of the spatially displaced BGWs. (c) Resonance spectra of third-order BGWs with different offset values. (d) Dependence of coupling strength on the offset value.

Fig. 6. BGW with multiple resonance peaks inscribed with Bessel laser pulses. Case in which eight third- and fourth-order gratings with different resonance peaks were inserted into the same waveguide in a parallel-serial-combined method. Both the lengths of Part 1 and Part 2 are 9 mm.

Fig. 7. (a) Schematic experimental setup for thermal characterization of the two-resonance-peaks BGW sensors. (b) Schematic diagram and PCM images of the two-resonance-peaks BGW. (c) Responses of resonance peaks to temperature. (d) Linear relationship between the shift of the resonance peak and temperature.