V. Ligeret

High-power Al-free active region (λ = 852nm) DFB laser diodes for atomic clocks and interferometry applications

We have developed single frequency and single spatial mode laser structures with high optical power, using an aluminium free active region, which are to our knowledge the first demonstration for Cs pumping at 852 nm.

A new design of ECLD for compact atomic clocks

Cold atom clocks would take great benefits of microgravity environment. In Relation with the clock developments, we work on miniature laser-cooling optical benches. In this article we describe a new design of compact external cavity laser diode, which exhibits up to 80 mW at the output of the ECLD for a diode current of 100 mA and a line width of about 150 kHz.

High-power, high-reliability, and narrow linewidth, Al-free DFB laser diode for Cs pumping (852nm)

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).

Al-free active region (λ = 852nm) DFB laser diodes for atomic clocks and interferometry applications

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.

High-Power Al-free active region (λ = 852nm) laser diodes for atomic clocks and interferometry applications

We have developed Fabry-Perot lasers at λ=852nm, using an aluminium free active region with the aim to develop a single-frequency and single spatial mode device for atomic clocks and interferometry applications. The device is a separate confinement heterostructure with a GaInP large optical cavity and a 8nm compressive-strained GaInAsP quantum well. The broad-area (100μm wide) laser diodes are characterised by low internal losses (<3 cm-1), a high internal efficiency (94%) and a low transparency current density (100A/cm2) which illustrates the quality of the laser structure. For an AR/HR coated 2mm long broad area laser diodes we measure a low threshold current density (245A/cm2) and a high slope efficiency (0.9 W/A). We obtain an optical power of more than 5.5W (I= 8.5A), under CW operation at 15°C, with a maximum wall-plug efficiency of 0.45. The lasing emission is achieved up to at least 115°C An optical power of more than 1.4W is obtained at 100°C (I=3.6A). A power of 1.2W (I=1.7A, 15°C) is achieved at 852nm. For an AR/HR coated 2mm long 4μm wide ridge waveguide laser diode, we obtain a low threshold current (46mA) and a high slope efficiency (0.9W/A). We obtain 852nm wavelength at 145mW (I=200mA, 15°C). We measure an optical power of 180mW (I=240mA) in a single spatial mode with the beam quality parameter M2=1.5. At 180mW both near and far field are gaussian-shaped with respective full widths at 1/e2 of 6μm and 12°.

High-power high-reliability narrow-linewidth Al-free DFB laser diode for Cs pumping (852 nm)

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.

Measurement of Irradiation Impact on Carrier Lifetime in a Quantum Well Laser Diode

We report on a novel method to measure radiation induced non radiative carrier lifetime modifications in laser diodes. This method is based on the conjugation of theoretical gain calculation, experimental gain measurements, and threshold variation measurements. We show that lifetime variations measured that way are in good agreement with those measured using time resolved photoluminescence (TRPL), and that the methodology can be applied to annealed samples, whose carrier lifetime cannot be otherwise measured.

Narrow spectral linewidth of al-free active region DFB laser diodes operating at 852nm

We have developed single frequency and single spatial mode laser structures with stable narrow linewidth (<1MHz) and high optical power (40mW), using an aluminium free active region for Cs pumping at 852nm.

Narrow linewidth destributed feedback laser diodes at 852nm

We have developed single frequency and single spatial mode laser structures with very narrow linewidth (~850 kHz @ 15 mW measured with an ECL). We demonstrate also a very good interaction between the DFB laser and Cs atoms.

A compact optical bench for laser cooling

We present the architecture of a A4 size optical bench developed for the compact cold atom clock HORACE. This proceeding focuses on two original sub systems as compact extended cavity laser diode with optical isolator places inside the laser cavity, and very compact saturated absorption systems.