Atomic clocks will be used in the future European positioning system Galileo. Among them, the optically pumped clocks provide a better alternative with comparable accuracy for a more compact system. For these systems, diode lasers emitting at 852nm are strategic components. The laser in a conventional bench for atomic clocks presents disadvantages for spatial applications. A better approach would be to realise a system based on a distributed-feedback laser (DFB). We have developed the technological foundations of such lasers operating at 852nm. These include an Al free active region, a single spatial mode ridge waveguide and a DFB structure.
The device is a separate confinement heterostructure with a GaInP large optical cavity and a single compressive strained GaInAsP quantum well. The broad area laser diodes are characterised by low internal losses (<3cm -1 ), a high internal efficiency (94%) and a low transparency current density (100A/cm2). For an AR-HR coated ridge Fabry Perot laser, we obtain a power of 230mW with M2=1.3.
An optical power of 150mW was obtained at 854nm wavelength, 20°C for AR-HR coated devices. We obtain a single spatial mode emission with M2=1.21 and a SMSR over 30dB, both at 150mW.
DFB Lasers at 852.12nm, corresponding to the D2 caesium transition, were then realised with a power of 40mW, 37°C for uncoated devices. The SMSR is over 30dB and the M2=1.33 at 40mW. Furthermore, the preliminary results of the linewidth obtained with a Fabry Perot interferometer give a value of less than 2MHz.
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
stabilized lasers.
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
reliability.
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/cm2 in continuous wave CW and 19MW/cm2 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.
KEYWORDS: Semiconductor lasers, Cesium, Atomic clocks, High power lasers, Indium gallium phosphide, Laser development, Quantum wells, Chemical species, Mirrors, Near field optics
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 (M2<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 D2 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 D2
line of 133Cs 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/cm2.
KEYWORDS: Semiconductor lasers, Cesium, High power lasers, Indium gallium phosphide, Laser development, Quantum wells, Atomic clocks, Near field optics, Laser applications, Chemical species
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 (≈100mW),
narrow linewidth (<2MHz), and spatial qualities (M2<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) lasers 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 D2 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 D2 line of
133Cs was recorded with a resolution close to the natural line width.
Single frequency and single spatial mode diode lasers emitting at 852nm are strategic components for systems such as
atomic clocks (positioning systems for navigation, in space atomic clock like Galileo or Pharao (cold atom),
measurement of fundamental constants), or interferometry applications. We have developed the technological
foundations of lasers at 852nm to address these different applications. These include an Al free active region, a single
spatial mode ridge waveguide and a DFB (distributed feedback) structure.
The device is a separate confinement heterostructure with a GaInP large optical cavity and a single compressive strained
GaInAsP quantum well. For an AR-HR coated ridge Fabry Perot laser, we obtain a power of 230mW with M2=1.3.
An optical power of 150mW was obtained at 854nm, 20°C for AR-HR coated devices. We obtain a single spatial mode
emission and a SMSR over 50dB, both at 150mW.
DFB Lasers at 852.12nm, corresponding to the D2 caesium transition, were then realised with a power of 40mW per
facet, 37°C for uncoated devices. At 40mW, we determine a M2 value of 1.3. We measure a SMSR value around 50dB
between 10°C and 80°C.
On this last laser run, we obtain very homogeneous spectral linewidth values for five different lasers, measured with a
Fabry Perot interferometer. We obtain at 20°C a low average linewidth value of 1.40MHz and 1.10MHz at respectively
40mW and 20mW, together with a low standard deviation of 0.1MHz. At 852.12nm (37°C, 40mW), a low linewidth
value of 1MHz was measured, for one laser preliminary tested.
Atomic clocks will be used in the future European positioning system Galileo. Among them, the optically pumped clocks provide a better alternative with comparable accuracy for a more compact system. For these systems, diode lasers emitting at 852nm are strategic components. The laser in a conventional bench for atomic clocks presents disadvantages for spatial applications. A better approach would be to realise a system based on a distributed-feedback laser (DFB). We have developed the technological foundations of such lasers operating at 852nm. These include an Al-free active region, a single spatial mode waveguide and a DFB structure. The device is a separate confinement heterostructure with a GaInP large optical cavity and a single compressive-strained GaInAsP quantum well. The broad-area laser diodes are characterised by low internal losses (<3 cm-1), a high internal efficiency (94%) and a low transparency current density (100A/cm2). For an AR/HR coated 2mm long around 4μm wide ridge diode, we obtain a low threshold current (40mA) and a high slope efficiency (0.90W/A). With the Fabry-Perot laser structure we obtain 852nm wavelength at 145mW (I=200mA, 15°C). We measure an optical power of 230mW (I=280mA) in a single spatial mode with the beam quality parameter M2=1.3. With the DFB laser structure, we have obtained single frequency (side-mode-suppression ratio : SMSR over 30dB) and single mode lasers (M2<1.5) with a high optical power. An optical power of 150mW was obtained at 854nm wavelength and 20°C for AR-HR coated 2mm long, ~ 4μm wide devices. At this power, both near and far fields in the slow axis are gaussian-shaped with respective full widths at 1/e2 of 8μm and 9.2° respectively, corresponding to a single spatial mode emission with a beam quality parameter M2=1.29. The SMSR is over 30dB. Furthermore, the preliminary results of the linewidth obtained with a Fabry-Perot interferometer give a value of less than 2MHz.
Atomic clocks will be used in the future European positioning system Galileo. Among them, the optically pumped clocks provide a more accurate alternative. For these systems, diode lasers emitting at 852nm are strategic components. The laser in a conventional bench for atomic clocks presents disadvantages for spatial applications. A better approach would be to realise a system based on a distributed-feedback laser. Thus we have developed laser structures emitting at λ=852nm, using an aluminium free active region. The device is a separate confinement heterostructure with a GaInP large optical cavity and a single compressive-strained GaInAsP quantum well. The broad-area laser diodes are characterised by low internal losses (<3 cm-1), a high internal efficiency (94%) and a low transparency current density (100A/cm2). For an AR/HR coated 2mm long around 4μm wide ridge diode, we obtain a low threshold current (40mA) and a high slope efficiency (0.90W/A). We obtain 852nm wavelength at 145mW (I=200mA, 15°C). We measure an optical power of 230mW (I=280mA) in a single spatial mode with the beam quality parameter M2=1.3. From our first attempt for a DFB laser, we obtained a threshold at 20°C of 45mA and a slope efficiency about 0.45W/A with an uncoated 2mm long around 4μm wide device. At 40mW (I=140mA,), both near and far fields in the slow axis are gaussian-shaped with respective full widths at 1/e2 of 7μm and 10.4°, corresponding to a single spatial mode emission with the beam quality parameter M2=1.2. At this power, the laser wavelength is 853.8nm with a side-modesuppression ratio over 30dB.
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