Fig. 1. Cross-sectional image of a photonic bandgap fiber. The fiber shown exhibits the transmission pictured in Fig. 4. and was used to obtain spectral data shown in Fig. 5a.

Fig. 2. Measured OPO idler tuning range in 1.2 µm cavity length steps. Every other spectrum is shown using a dashed line for clarity.

Fig. 3. Experimental configuration. OPO idler pulses (3.0–3.4 µm) leave the cavity through mirror M1 and are then collimated with a CaF₂ lens before entering a Michelson interferometer. After the interferometer the pulses are coupled into the fiber and detected with a PbSe photodiode or steered directly onto the detector, omitting the fiber (dashed beam path). Mirrors M1–M6 have high reflectivity from 900–1100 nm and high transmission at other wavelengths. Mirror M1 is coated on a CaF₂ substrate. PZT, piezoelectric translator; BS, 50:50 mid-IR beamsplitter; Ge, uncoated germanium filter; ZnSe, zinc selenide lens; PBF, photonic bandgap fiber.

Fig. 4. Spectral transmissions of six different fibers having core diameters and cladding pitches similar to the example pictured in Fig. 1.

Fig. 5. Experimental results. (a) Idler spectra recorded after the fiber. Thick line, fiber filled with pure nitrogen; thin line, fiber filled with 5:95 methane:nitrogen mixture. (b) Experimental (thick line) and calculated (thin line) methane transmission profiles.