Ultralow-threshold laser in a Nd3+ doped silica microsphere

A neodymium-doped silica microsphere (NDSM) laser is described in this work, which was a pure silica sphere coated with Nd3+ doped sol-gel film and showed a rather simple production procedure, good characteristics, low cost and flexibility. To create the NDSM lasers, the neodymium-doped sol-gel film was applied to the surface of a silica microsphere, an optical tapered fiber (OTF) which was used to couple an 808 nm continuous-wave diode laser into the inner surface of the NDSM, served as a pump light source. Multi-mode and single-mode laser oscillations around 1100 nm were observed due to the 4F3/2-4I11/2 transitions of Nd3+ ions. Highly confined whispering gallery modes of pump laser made an ultralow threshold microlaser, we could observe the laser oscillations when the pump power was as low as 15.7 µW. The multi-mode laser oscillation spectrum depends on the microsphere morphology characteristics. The free spectral range (FSR) was in good agreement with the calculated value which was based on the Mie scattering theory.