Results of very long time ageing tests of DFB ridge laser diodes emitting at 852 nm and 894 nm for Cesium atomic clocks
Time and Frequency applications need high accuracy and high stability clocks. Optically pumped compact industrial Cesium atomic clocks are a promising approach that could satisfy these demands. However, the stability of these clocks relies, among others, on the performances of the laser diodes that are used. This issue has led the III-V Lab to commit to the European Euripides-LAMA project that aims to provide competitive compact optical Cesium clocks for ground applications. This work will provide key experience for further space technology qualification. III-V Lab was in charge of the design, fabrication and reliability of Distributed- Feedback diodes (DFB) at 894 nm (D1 line of Cesium) and 852 nm (D2 line). The use of D1 line for pumping will provide simplified clock architecture compared to the D2 line pumping thanks to simpler atomic transitions and a larger spectral separation between lines in the 894 nm case. Also, D1 line pumping overcomes the issue of unpumped “idle states” that occur with D2 line. The modules should provide narrow linewidth (<1 MHz), very good reliability in time and, crucially, be less sensitive to optical feedback. We show here results from Al-free active region with InGaAsP quantum well Ridge DFB lasers. We obtain the D1 Cs line (894.4nm) at 67°C and 165mA (optical power of 40mW) with a high side mode suppression ratio. The D2 Cs line (852.1nm) is obtained at room temperature at 40mW. By a small increase of temperature it is also possible to get Cs lines at 20mW. In order to address the long term reliability of these lasers, long duration ageing tests (more than one year targeted) have been carried out for both wavelengths at 20mW and 25°C The laser diodes have aged during 10 000 hours and 9336 hours for DFB addressing Cesium D2 and D1 lines, respectively. These ageing tests show very low increase of the operating current, almost linear with time. Lasers designed for D1 and D2 Cesium lines show an average increase of operating current of 0.15% and 0.37% per 1000hours, respectively.