High spatial resolution measurement of volume holographic gratings

The conventional approach for measuring volume holographic gratings typically requires measuring the transmitted and diffracted beams simultaneously while varying the angle of incidence. To obtain the spectral response a tunable laser is used with a fixed angle of incidence. In the former case, the motion of the diffracted beam from reflection gratings requires the detector to move with it, or otherwise the range of angles must be limited. Furthermore it is often difficult to separate the diffraction from the specular surface reflections, increasing the error of the measurement. In the latter case, a high cost tunable laser is required. We describe methods for the measurement of volume holographic gratings with high spatial resolution. A fixed wavelength laser in conjunction with a high-resolution digital camera is used to measure the angle selectivity of the transmitted beam only. The measured data are fit to a model of the grating diffraction combined with the cyclical interference from the surface reflections in order to increase the accuracy when measuring uncoated gratings. The system is capable of simultaneously measuring diffraction efficiency, loss, surface reflectivity, Bragg angle, and grating tilt in one plane, with a resolution of better than 250 micrometers over the area of a 45 mm by 35 mm wafer. Through a transformation utilizing the de-phasing term of the coupled wave analysis of thick hologram gratings, the wavelength selectivity is also obtained.