Since a few years, atomic pumping technology has spread a lot, in particular for applications like very sensitive and stable atomic clocks, gyrometer, gradiometer, and gravimeter. Development of a compact optical source with a high power and a very narrow linewidth has become very important. At the moment, using an extended cavity laser (ECL) is a common solution in laboratory. For integrated system in spaces applications which need an excellent mechanical stability, the distributed feedback laser diode could be a good alternative. In this paper, we will present the performances of two kinds of lasers: Fabry-Perot laser diode (FP) and distributed feedback laser diode (DFB), centered at 780nm, for the pumping of the D2 line of Rubidium atoms (Rb). The Fabry-Perot laser diode presents the advantage of a high power emission but with a multimode optical spectrum. It can be integrated in an extended cavity which permits to filter the frequency and therefore to obtain a very narrow linewidth (less than 100 kHz). The DFB laser has the advantage to show a single frequency emission thanks to the integration of the filter in the device waveguide.
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
Development of techniques such as atom optical pumping, for atomic clocks and matter-wave
interferometers (gyrometer, accelerometer, gravimeter and gradiometer), requires laser diodes with high power
and excellent spectral (narrow linewidth) and spatial qualities together with high reliability.
We have developed high power Fabry-Perot and distributed Feedback lasers diodes (DFB) emitting at
780nm corresponding to the D<sub>2</sub> line of Rubidium.
We have realized a Fabry-Perot laser diode, without aluminium in the active region, with a cavity length
and a waveguide width of respectively 1mm and 4μm and an AR/HR coating on the facets. We obtain a low
threshold current around 40mA and a high slope efficiency of 1W/A at 20°C. We obtain a good beam quality M²
of 2 at 200mW. These lasers diodes are very interesting to be inserted in an external cavity.
For DFB lasers, we used a second order diffraction grating in a GaInP/GaAsP/GaInP waveguide. We
calculated the coupling coefficient KL of 1.5 for a length of 2mm, for a width of 4μm. These lasers show a low
threshold current (around 65mA) with a slope efficiency around 0.37W/A. We have obtained at 25°C, 145mA an
optical power of 25mW at the D<sub>2</sub> line of Rubidium with a side mode suppression ratio around 44dB. By the selfheterodyne
method, we measured a low linewidth of our DFB laser at 780nm around 1.25MHz (lorentzian fit).
The development of techniques such as atom optical pumping, for atomics clocks or precise gyroscopes, requires
laser diodes with high power and excellent spectral (narrow linewidth) and spatial qualities together with high
We have realized a six months ageing test on Al-free DFB lasers emitting at 852nm for Cs pumping. Ten DFB
lasers were aged at 40°C and 20mW. The extrapolated lifetimes at 40°C, based on 20mW operating current, of
our DFB lasers are higher than 500000 hours which confirms the excellent potential of this Al-free technology
for long life spatial mission. Furthermore, the evolution of the operating current (initially around 70mA), after
six months, is less than 5% (corresponding to 3mA).
We obtain a very good stability of optical spectra: an average variation of the Side Mode Suppression Ratio
(SMSR) of less than 2dB and a variation of the wavelength of less than 0.12 nm.
We also measured the linewidth of our DFB lasers with the delayed self-heterodyne method after the six months
ageing: we obtain a very narrow linewidth at 25°C (measurement temperature) around 215kHz (lorentzian fit,
white noise) or 330kHz (gaussian fit, 1/f noise).
Precise gyroscopes and atomic clocks are in high demand for positioning and flight navigation systems or measurement
of fundamental constants. The development of techniques such as atom optical pumping (Cs or Rb) requires laser diodes
with high power and excellent spectral (narrow linewidth) and beam qualities. For spatial applications a high reliability is
required (mission lifetime is around 15 years).
We have realized different studies of reliability on our Al-free DFB lasers: Catastrophically Optical Mirror Damage
(COMD) evaluation, lifetest, optical and spectral measurements before and after ageing. We obtained high COMD
densities (respectively 13MW/cm<sup>2</sup> in continuous wave CW and 19MW/cm<sup>2</sup> in pulsed mode. Furthermore, we have
realized ageing test on these DFB laser diodes emitting at 852.12nm (D2 line of Cs). We used five different ageing
conditions (power and temperature) to determine ageing properties.
The extrapolated lifetimes of our DFB laser (for operating current variation equal to 100%) are higher than 140000 hours
(about 15 years) for an ageing at T= 25°C and P= 40mW. This confirms the excellent potential of this Al-free technology
for long life spatial mission.
The Side Mode Suppression Ration (SMSR) of the aged D2 line DFB lasers remains very high with a measured change
of -1.4dB ± 8dB. There are no significant drifts of the DFB laser wavelength after aging (average ~0.03 nm).
We also measured the linewidth of our aged DFB lasers by the self-heterodyne technique and obtained narrow beating
linewidths of around 900kHz.