Photoscattering effect in supercontinuum-generating photonic crystal fiber

Abstract

A photosensitivity different from that responsible for fiber grating inscription is found in a supercontinuum-generating photonic crystal fiber transmitting intense 818 nm femtosecond pulses. This photosensitivity progressively generates a waveguide at the entrance of the fiber to scatter light of specific wavelengths and is termed as the photoscattering effect. This effect is linked to the ~800 nm photosensitivity in the microlithography of bulk silica glass. While the effect somewhat limits fiber-optic supercontinuum applications, it can be beneficial to produce new photonic devices.In a previous study,1 we reported a relatively surprising photosensitivity for supercontinuum (SC) generating heavily Ge-doped germanosilicate fibers. That is, the prolonged exposure of 800 nm femtosecond pulses produces a multi-millimeter long waveguide at the fiber entrance to dramatically decrease the fiber transmission property over a broad spectral range. This phenomenon resembles the photodarkening effect widely observed in rare-earth doped silica fibers if the Ge dopant plays the role of the rare-earth ions to produce certain absorptive photoproducts.2 Alternatively, it resembles the type I photosensitivity well documented in germanosilicate fiber grating inscription in terms that the waveguide is thermally erasable.3 The observed reduction of fiber transmission may be due to the formation of a structure analogous to a long period grating (LPG), which can attenuate fiber transmission by coupling light from core modes into cladding modes. The nature of the photosensitivity remains controversial mainly because the role of the Ge dopant is unclear. In this work, we report the presence of this photosensitivity in a pure silica photonic crystal fiber (PCF) so that no special photosensitive impurity is essential for such photosensitivity. We study the mechanism underlying the near-infrared photosensitivity, which has not been reported in PCFs, and point out its potential applications.The PCF (LMA-8, Crystal Fibre A/S) under study is an endlessly single-mode pure silica fiber having a cross section of hexagonally arranged 1.7 μm diameter air holes with a pitch size of 4.9 μm. The absence of the central hole results in a core diameter of 8.5 μm, a mode field diameter of 6.0 μm, and a numerical aperture (NA) of 0.10 (at 818 nm). The zero dispersion wavelength of the fiber is estimated to be 1.1 μm. 4 The pump laser is a 250 kHz regenerative amplifier (Reg9000, Coherent) pulsed at 818 nm with ~25 nm full width at half maximum bandwidth. The 1 mm diameter laser beam is coupled into the fiber by a NA of 0.40, 5.0 mm diameter aspheric lens. The pulse energy (pump power) is varied by a neutraldensity filter between 0.04 and 1 μJ (10-250 mW). The pulse width is controlled by a compressor to yield transform-limited 35 fs or positively chirped 210 fs pulses immediately a) htu@uiuc.edu. b) boppart@uiuc.edu. NIH Public Access Author ManuscriptAppl Phys Lett. Author manuscript; available in PMC 2011 February 22. NIH-PA Author Manuscrip...

Emergence of self-organized long-period fiber gratings in supercontinuum-generating optical fibers

“…The phenomenon similar to the LPFG formation has been observed from a pure silica photonic crystal fiber (PCF) [10] and another highly nonlinear PCF pumped at a largely anomalous dispersion region. This raises concern about the long-term stability of all fiber-based SC applications, regardless of the nonlinear optics of the SC generation.…”

Band-gap nonlinear optical generation: The structure of internal optical field and the structural light focusing

“…This structural feature causes numerous physical effects. 1-7,9 Therefore, PCs and quasi-PCs could be used in various applications, including waveguiding [13][14][15][16] and PC fibers, [17][18][19][20][21] for optical supercontinuum generation, [22][23][24][25][26] nonlinear light conversion, high-harmonic generation, and active media pumping. [27][28][29][30][31][32][33][34][35][36] Recently, the surface electromagnetic modes of the PC and surface Bloch waves [37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53]67,68 have attracted much interest in the scientific community.…”