Spectral Demodulation System Based on Two Transmission Volume Bragg Gratings

In this paper, a fiber grating demodulation system based on two transmission volume Bragg gratings (VBGs) was proposed. In order to resolve the problem that the spectral resolution of the present fiber grating demodulation system is not high enough, the two transmission VBGs were applied to improve the spectral resolution and reduce the volume of the spectrometer. The diffraction characteristics of the transmission VBGs were analyzed, and the optical path of the two transmission VBGs demodulation system was designed based on the diffraction characteristics. The grating constant, lens parameters, and aberration correction of the system were analyzed and calculated. The calculation showed that the theoretical wavelength range of the demodulation system was from 1525 nm to 1565 nm and the theoretical optical resolution was 60 pm when the grating constant was 0.9168, the angle between two transmission VBGs was 89 degrees, the focal length of the collimator was 60 mm, and the focal length of the imaging lens was 131.5 mm. The aberration of the system was well corrected by using a lens as the collimator and a reflector as the imaging lens. The system principle prototype was assembled and calibrated, and its performances were experimentally investigated. The results showed that the spectrometer worked stably, with a wavelength range from 1525 nm to 1565 nm, an optical wavelength resolution of 65.3 pm, and a high demodulation speed of 10 kHz.