Abstract

Recently, second-order nonlinearities (SONs) of the order of 1 pm/V have been observed in balk glasses using a variety of different poling techniques.^1–3 We report here the first successful poling of optical fiber using both thermal and electron implantation. techniques. Compared to previous work in fibers,^4,5 a 200-fold improvement in SON is obtained. First, the following preforms were thermally poled: MCVD preform A (starting tube: electrically fused natural quartz (GE-100, low OH ~ 1 ppm); P + F-doped silica cladding and Ge-doped core); MCVD preform B (starting tube: flame fused natural quartz (Herasil-1, OH ~ 150 ppm); P + F-doped silica cladding and Ge + Na-doped core); MCVD preform C (same starting tube as B, no cladding and Ge- doped core); MCVD preform D (same starting tube as B and C, no cladding and Ge + Na-doped core); VAD preform E with a Ge-doped core. Slices ~1 mm thick were poled at 2.5 kV and 280°C for 15 min. A pump beam from a Nd:YAG mode-locked and Q-switched laser (λ = 1064 run) crossed the sample at the Brewster angle. The second harmonic (SH) signal was monitored while translating the sample in a direction perpendicular to the plane of incidence. No SH was detected anywhere in preforms A and E. A strong SH signal was observed in the starting tubes in B, C and D, and no SH signal was observed in the P + F doped cladding while scanning along preform B with a p-polarized pump (Fig. 1). We observed ~15% stronger SH signal in the Ge- doped core compared to the starting tube (Fig. 2). The SH signal in the core was also found to follow the Ge concentration. The measured absorption spectrum in the core region of preforms C and D indicated an OH concentration of ~80 ppm.

© 1994 Optical Society of America