acousto-optic spectrum analyzer
acousto-optic spectrum analyzer noises
Acoustooptic spectrum analyzers were first proposed and implemented in 1960s. Modern devices based on acousto-optic interaction can compete with the digital devices based on FFT when analysis bands of more than 300 MHz are point of interest. Spectrum analysis in optics is a natural procedure implemented by means of cylindrical or spherical lenses. To perform spectrum analysis radiosignal should be represented in spatial dimension, what can be done by the Bragg cell. Spectrum registration is executed by linear or array photodetector (PD), placed in lens’s focal plane. The wave carrying data for this type of devices is the laser monochromatic optic wave.
Main characteristics and parameters of acousto-optic spectrum analyzer are derived from instrumental function (IF) and its representation on output of discrete PD. In most cases IF of spatial integration spectrum analyzer can be described with the function of [sin(x)/x]2. We should take into the consideration that frequency resolution will be decreased if main lobe of IF leis on three pixels of PD. Keeping this in mind one can calculate signal current on output of PD if photoretector current sensitivity, laser radiation power, modulation index for Bragg cell and optical losses are given. Dynamic range of spectrum analyzer is limited by the noises of PD (shot noises) and noises of the device on PD output (thermal noises) at the bottom, and at the top – by natural acousto-optic interaction nonlinearity. With the acceptable level of nonlinearity is given available dynamic range can be counted for different diffraction regimes. It is shown that dynamic range in Bragg regime is 4.77 dB more than one in Rahman-Nat regime.