Fig. 1. Experimental set-up. (a) A 1064 nm laser beam is expanded 5× so as to homogeneously fill the spatial light modulator (SLM). The SLM is placed in the front Fourier plane of lens L1 (focal length 30 cm). It shapes the light beam in a volume around the back Fourier plane of lens L1. A second lens L2 (focal length 5 cm) is used to compress and collimate the modulated beam. Part of the beam is reflected to a CCD camera (CCD 1) for online monitoring of the generated intensity profiles. A 10×0.25 NA microscope objective is used to couple the beam into the sample fiber. The end-face of the fiber is imaged on to a second camera (CCD 2) using a 40×0.65 NA objective. HWP: half wave plate; P: polarizer; NDF: neutral density filter; BE: 5× beam expander; BS: beam splitter; L1 and L2: lenses; PC: computer. (b) A blazed phase grating is used to redirect the shaped beam into the +1 diffracted order, the other orders being filtered out using a Fourier-plane aperture A.

Fig. 2. (a–c) Normalized target intensity distributions for LG₀₁ mode (a) radial HG₀₁ mode (b) radial and HG₀₂ mode (c); (d–f) corresponding holograms generated by the SLM (black is 0, white is 2π); (g–i) resulting normalized intensity distribution measured by CCD1.

Fig. 3. Micrographs and loss spectra of the fibers used in our experiments: (a) SEM of the cross-section of the HC-bandgap fiber with pitch Λ=3.2±0.1 µm and core diameter d_core= 12.0±0.2 µm; (b) loss spectrum of the fiber shown in (a); (c) optical micrograph of the near-field emerging from a 3 cm long piece of HC-kagome PCF with pitch Λ=11.9±0.2 µm and d_core=23.4±0.2 µm, illuminated from below with a halogen lamp. The thickness of the glass struts in the cladding varied between 350 and 450 nm; (d) loss spectrum of the fiber in (c); the shaded areas indicate measured bands of loss.

Fig. 4. Normalized near-field intensity profiles measured at the end-face of a HC-kagome fiber: (a–d) 55 cm long and (e–h) after cutback to 30 cm. The excited core resonances are: (a) fundamental LP₀₁; (b) LP₁₁; (c) LP₂₁; (d) LP₃₁. The polarization state at input to the fiber is shown on the left. The yellow arrows and circles indicate the dominant polarization state at the end-face. In (b) the transmitted signal did not change significantly as the polarizer was rotated, indicating that the light was in an elliptical polarization state (indicated by the yellow circles), and in (a) linear polarization was preserved, though it rotated slowly along the fiber, being aligned horizontally in (a) and vertically in (e).

Fig. 5. Normalized mode intensity profiles measured at the end-face of a 19 m long HC-bandgap fiber: (a) fundamental LP₀₁ mode; (b) LG₀₁ mode; (c,d) the corresponding normalized intensity profiles after cut-back to 3 m of fiber.