Both the fundamental wavelength of the neodymium 4F3/2 → 4I9/2 transition (890 to 950 nm) and its second harmonic have applications in spectroscopy, and the harmonic also is useful in displays and medical sensing. Quasi-phase matching makes it easy enough to generate the second harmonic efficiently; the problem is forcing the 4F3/2 → 4I9/2 transition to lase in the first place. Normally, neodymium’s 4F3/2 → 4I11/2 transition (1060 to 1150 nm) usurps all the population inversion of the upper 4F3/2 level and extinguishes the weaker line.Figure 1. The cladding of the all-solid photonic bandgap fiber comprised high-index rods in a triangular lattice, so the effective refractive index of the cladding was greater than that of the core. Images ©OSA.Recently, researchers at the University of Bath in the UK used a length of photonic bandgap fiber as a spectral filter to force a neodymium-doped fiber laser to lase on the weaker 4F3/2 → 4I9/2 transition. Photonic bandgap fiber guides light not by total internal reflection, but by creating a photonic bandgap in the cladding that is analogous to the electronic bandgap in a semiconductor.Figure 2. The fiber exhibited high loss at the spectral region of the 4F3/2 → 4I11/2 transition (1060 to 1150 nm) and low loss for the F3/2 → 4I11/2 transition (890 to 950 nm).To facilitate splicing the photonic bandgap fiber to conventional fiber, the researchers fabricated it with a standard 125-μm outer diameter. Unlike a conventional holey fiber, the all-solid photonic bandgap fiber had a microstructure composed of high-index germanium rods of 3.1-μm diameter arranged in a triangular lattice with a 6.5-μm pitch (Figure 1). They measured the spectral transmission of a 10-cm length of the fiber to confirm its high loss (>20 dB) in the 1060- to 1150-nm region and low loss — ~1.8 dB, due almost entirely to splice losses — in the 890- to 950-nm region (Figure 2).Figure 3. Above, spliced into the resonator as a spectral filter (a), the photonic bandgap fiber forced the laser to oscillate on the weaker F3/2 → 4I9/2 transition (b).They spliced the 10-cm length into a fiber laser resonator, using a short length of standard single-mode fiber between the 2.5-m-long Nd-doped fiber and the photonic bandgap fiber to minimize mode mismatch between the two (Figure 3a). A metallic mirror defined one end of the laser resonator, and the Fresnel reflection from the facet of the Nd-doped fiber defined the other. The investigators pumped the fiber laser with continuous-wave 808-nm radiation from a Ti:sapphire laser and, after achieving threshold at 70 mW, measured the laser’s slope efficiency at 32 percent. The laser output was clearly due to the weaker 4F3/2 → 4I9/2 transition, which was 55 dB above the stronger 4F3/2 → 4I11/2 transition (Figure 3b).Figure 4. Below, without the fiber in the resonator, the laser lased on the stronger 4F3/2 → 4I11/2 transition (a). With an intracavity photonic bandgap fiber whose spectral transmission is represented by the broken line, the laser lased on two lines (b). A different photonic bandgap fiber forced one of the lines below threshold so that lasing occurred on only one line (c).To demonstrate the versatility of using an intracavity photonic bandgap fiber to control a fiber laser’s spectral output, the researchers fabricated several lasers with various intracavity photonic bandgap fiber in them. One made without the photonic bandgap fiber lased on the stronger 4F3/2 → 4I11/2 transition, as expected (Figure 4a). By substituting various photonic bandgap fiber into the laser, the researchers obtained simultaneous lasing on two 4F3/2 → 4I9/2 transitions or on a single one (Figures 4b and -c).Optics Letters, May 15, 2006, pp. 1388-1390.