We found that marked increases in refractive index of chemically amplified photoresists induced by highly repetitive femtosecond laser irradiation without post-exposure baking treatment. For laser writing speed less than 30 μm/s, the refractive index change of the nonlinear absorption region was as large as 8 × 10(-3). Moreover, cross-linking reactions of the resists were induced. The refractive index changes can generate optical confinement and subsequent channel propagations of femtosecond laser pulses. The coupling efficiency was estimated as high as 87% using a low numerical aperture objective lens. The peak intensities of the guiding modes exceeded the polymerization threshold of the resist.