Abstract
Permanent change of the refractive index of photo-thermo-refractive glass can be achieved by exposure to ultraviolet light followed by thermal development. The dependence of the change on the dosage is nonlinear because the photochemical process of photosensitivity involves saturation. With a holographic recording of Bragg gratings inside glass, a sinusoidally modulated dosage is imprinted into distorted modulation of the refractive index. We investigate this distorted modulation and derive analytical expressions for the amplitudes of its spatial harmonics.
Highlights
Photosensitive materials such as photo-thermo-refractive (PTR) glass [JL1] have become the material of choice over the past twenty years for the manufacturing of resonant diffractive optical elements such as volume Bragg gratings (VBGs) [JL2,JL2a]
The fabrication process of VBGs begins with the initial ultraviolet (UV) exposure of a glass blank with periodically modulated intensity produced by a holographic recording
The typical amplitude of refractive index modulation (RIM) required for high reflectivity of a narrow-band VBG is a few hundred ppm, and it is developed in the regime of the linear photosensitivity of glass
Summary
Photosensitive materials such as photo-thermo-refractive (PTR) glass [JL1] have become the material of choice over the past twenty years for the manufacturing of resonant diffractive optical elements such as volume Bragg gratings (VBGs) [JL2,JL2a]. Among the various types of VBGs, chirped Bragg gratings (CBG) with linearly varying modulation periods are used to stretch and compress short laser pulses with a relatively wide spectral range [JL6] In this case, the RIM amplitude should be a maximum since the monochromatic components of the pulse spectrum are reflected over a short length of matched resonant conditions inside the CBG. An example is the reduction of the visibility of the dosage interference fringes over a long exposure time due to instabilities in the setup for holographic recording Another is the increase of material absorption in overexposed glass specimens that leads to the degradation of the quality of the fabricated Bragg gratings. Our objective is to determine the optimum value of the dosage that provides the maximum amplitude of effective Bragg modulation of the refractive index
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