title: Comment on the reported fiber attenuations in the visible regime in "Fabrication of glass photonic crystal fibers with a die-cast process".
abstract: We comment on the recent paper by Zhou et al. [Appl. Opt.45, 4433 (2006)APOPAI0003-693510.1364/AO.45.004433], in which transmission losses of 0.2-0.3 dB/m were claimed across the wavelength range 420-900 nm in a high-index (n(d)=1.80518 at 587.6 nm) SF6 glass-based photonic crystal fiber fabricated by novel die-cast technique. If confirmed, these losses are at least 1 order of magnitude lower than previous reported losses of SF6 photonic crystal fibers from other fabrication approaches. Here we present a statistic survey on the relationship between the refractive index and the bulk material attenuation, based on a large number of commercial Schott optical glasses with the n(d) ranging between 1.40 and 2.05. It shows that the loss of a high-index (n(d)=1.80) glass optical fiber should be at the levels of 10-50 dB/m at 420 nm and 1-10 dB/m at 500 nm, respectively. Moreover, the material attenuation of such a high-index glass fiber should intrinsically show a large decay, from 10-50 dB/m at 420 nm to the level of 1 dB/m at 700 nm, which arises from the tail on the UV absorption edge of the high-index glass extending to the visible region. Therefore, we conclude that: (1) the low loss of 0.2-0.3 dB/m reported in the cited paper is abnormally one or two magnitudes lower than the material attenuation that a high-index (n(d)=1.80) glass optical fiber should have in the range between 420 and 500 nm and that (2) the flat loss curve between 420 and 700 nm in the cited paper deviates greatly from the intrinsic behavior of a high-index (n(d)=1.80) glass fiber.
authors.affiliation: Optoelectronics Research Centre, University of Southampton, Highfield, Southampton, UK, SO17 1BJ. if@orc.soton.ac.uk
authors.forename: Xian
authors.initials: X
authors.lastname: Feng
authors.forename: Wei H
authors.initials: WH
authors.lastname: Loh
authors.forename: David J
authors.initials: DJ
authors.lastname: Richardson