Femtosecond, subablation threshold photomodification of poly(methyl methacrylate) (PMMA) at $387\mathrm{nm}$ is explored to enable fabrication of optical components. Volatile fragment analysis (thermal desorption gas chromatography–mass spectrometry) and molecular weight distribution monitoring (size exclusion chromatography) suggest photochemical modification, involving direct cleavage of the polymer backbone and propagation via chain unzipping under formation of monomers, similar to the pyrolytic degradation of PMMA. Waveguides were produced in undoped, clinical-grade PMMA, showing an increased refractive index in the laser focal region $(\mathrm{\Delta }{n}_{\max }=4\times {10}^{-3})$ .