Abstract:We have demonstrated the use of microspectroscopy for measuring the Cr3+ and Cr4+ fluorescence spectra in double-clad Cr:YAG crystal fiber. The emission spectra of Cr3+ and Cr4+ are detected from core and inner cladding. The Cr3+ spectrum in the inner cladding shows a broad-band emission from 650 to 950 nm, while the emission of Cr4+ occurs in the range of 1.15–1.55 μm with a peak around 1.22 μm. The characteristic of Cr ion at high-field sites shows a narrow-band emission (2E→4A2 for Cr3+;1E→3A2 for Cr4+), whereas that at low-field sites shows a broad-band emission (4T2→4A2 for Cr3+; 3T2→3A2 for Cr4+). The emission intensity ratio of high-field sites to low-field sites in the inner cladding with different compositions has been investigated. It varies from 20% to 29% for Cr3+ and from 7.1% to 11.3% for Cr4+ when the concentration of SiO2 increases from 26.9 to 43.0 wt %.