Influence of liquid nitrogen cooling on the spectral performance of BAC-P in bismuth-doped phosphosilicate fibers under liquid nitrogen temperature

The first results of the liquid nitrogen cooling effect on the spectral properties of a phosphor-related bismuth active center (BAC-P) have been presented. It is found that the small-signal absorption bands of BACs are mildly affected at liquid nitrogen temperature (LNT, 77 K), while the unsaturable absorption level is lowered moderately. Meanwhile, noticeable shape changes of luminescence spectra and enhanced luminescence intensity (∼ twice) of BAC bands were revealed at LNT upon excitation at 830 or 980 nm, which is contributed by the notable growth of BAC-P. In addition, the influence of cooling on the on-off gain performance of BACs is also explored. Furthermore, the laser-induced effect on the luminescence of BAC-P upon cooling at LNT is also investigated, revealing a noticeable increase (∼10%) instead of photobleaching. These experimental results bring new insights into the temperature-related spectral properties of BACs and provide an effective way for tuning the luminescence scheme for Bi-doped fibers by cryogenics for the desired spectral range 1.26–1.36 µm.