High power Al-free DFB laser diode for atomic clocks : narrow linewidth and demonstration of saturation spectra of the Cesium D2 line

摘要

There is a growing demand for precise gyroscopes and atomic clocks for positioning, flight navigation systems and aerospatial applications. One of the prerequisites for atomic optical pumps is a laser diode with high power (a few 10mW), narrow linewidth (<2MHz), and beam qualities (M~2<1.5). Another important factor for aerospatial applications is a very high reliability performance of the laser devices. With an aim to address these issues, we have laid down the technological foundation and further developed ridge waveguide distributed feedback (DFB) laser diodes with an emission wavelength of 852nm corresponding to the D2 cesium transition in atomic clocks. The epitaxy is based on an Al free active region with a GaInP large optical cavity and a single compressive strained GaInAsP quantum well. Fabricated DFB uncoated lasers have shown wavelength emission at 852.12nm with an output optical power of 40mW, a SMSR >30dB at the D_2 line, at 37°C. Low self-heterodyne linewidths of 0.8MHz and 1.2MHz were measured respectively at 20mW 12°C and 40mW 37°C. With this uncoated diode, we have realized saturation spectra of cesium atoms to determine the resolution and the stability of the laser diode working on Cs. The saturation spectrum of the D_2 line of ~(133)Cs was recorded with a resolution close to the natural line width. Preliminary studies of reliability were the measurement of catastrophical optical mirror damage (COMD) for different anti-reflection (AR) coatings. We obtained a COMD density of 19MW/cm~2.