Design of an optical spectrometer using a holographic grating having a spatially variable period

We present a new concept of optical spectrometer based on a plane holographic grating having a spatially variable period (chirped grating) used as the component for spectral selection. The chirped grating is made through a holographic technique. We have developed a mathematical model to predict the properties of the grating, as a function of the writing beam parameters. The experimental results confirmed the predictions from the model. The spatially variable period provides focusing properties to the grating which eliminate the need for the usual curved mirrors in the spectrometer. Once calibrated, this spectrometer has a resolution comparable to the one of a high-resolution commercial device, considering their different dimensions. This comparison is achieved with the help of a performance criterion we have introduced. The new concept would make it possible to reduce the optical dimensions of usual spectrometers approximately by half, for a given resolution. The aberrations produced by the chirped grating, the spectral range that can be analyzed and the positioning control of the detected image will be discussed.