Time-frequency applications are in need of high accuracy and high stability clocks. Compact industrial Cesium atomic clocks optically pumped is a promising area that could satisfy these demands. However, the stability of these clocks relies, among others, on the performances of laser diodes that are used for atomic pumping. 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 earth applications. This work will provide key experience for further space technology qualification. We are in charge of the design, fabrication and reliability of Distributed-Feedback diodes (DFB) at 894nm (D1 line of Cesium) and 852nm (D2 line). The use of D1 line for pumping will provide simplified clock architecture compared to D2 line pumping thanks to simpler atomic transitions and larger spectral separation between lines in the 894nm case. Also, D1 line pumping overcomes the issue of unpumped “dark states” that occur with D2 line. The modules should provide narrow linewidth (<1MHz), very good reliability in time and, crucially, be insensitive to optical feedback. The development of the 894nm wavelength is grounded on our previous results for 852nm DFB. Thus, we show our first results from Al-free active region with InGaAsP quantum well broad-area lasers (100μm width, with lengths ranging from 2mm to 4mm), for further DFB operation at 894nm. We obtained low internal losses below 2cm-1, the external differential efficiency is 0.49W/A with uncoated facets and a low threshold current density of 190A/cm², for 2mm lasers at 20°C.
Laser diodes emitting at different wavelengths can address various applications. 852nm or 894nm
single frequency low linewidth laser diodes are needed for Cs pumping for realization of atomic
clocks. 780nm high power low linewidth laser diodes and amplifiers are needed for Rb pumping for
realization of cooled atoms based inertial sensors. High power lasers at 793nm and 975nm with
wavelength stabilization are required to pump Tm and Yb doped fibres respectively. We have
developed the building blocks and have realize the different kinds of laser diodes needed for various
pumping applications. One of these key building blocks are the Al free active region laser structures,
which allow epitaxial regrowth on a Bragg grating necessary to get single frequency or wavelength
Ultra precise and stable gravimeters and gyrometers are highly demanded for various applications like fundamental
physics, geophysics, navigation systems. Interferometry of Rubidium cold atoms requires high power, narrow linewidth,
low frequency noise and highly reliable optical sources emitting at 780 nm.
In this context, we developed basic bricks for realization of a distributed feedback (DFB) laser.
On one hand, 100 μm broad area devices achieve at 20°C a continuous wave (CW) output power of more than 4 W per
facet. On the other hand, we demonstrated excellent performances on Fabry-Perot ridge-waveguide lasers with a
threshold current of 35 mA, emitting up to 120 mW per facet, in single lateral mode at 780 nm. We already achieved an
output power of 20 mW with a small spectral linewidth of less than 1 MHz on a DFB laser.
We present here the results on a new and systematic investigation of the low frequency noise of such laser structures, in
order to better understand and improve their performances.
By using an appropriate current source and very low noise voltage amplifier (10-19 V2/Hz at 10 Hz), we can measure the
intrinsic Terminal Electrical Noise (TEN), due to the fluctuations of the laser voltage. The measurements have been
performed at low frequency (1Hz < f <100 kHz) and different laser currents (around the threshold current, above and at
high laser current). On broad band area lasers, we obtained very low 1/f level noise (10-13 V2/Hz at 1 Hz) due to optical
gain fluctuations. The white noise(shot and thermal noise) level is about 10-18 V2/Hz. The corner frequency between 1/f
and white noise is about 3 kHz, which is a good result for this kind of structures. Electrical noise measurements will be
interpreted by using lasers noise theory.