We developed and tested a miniature optical bench to implement a cesium CPT clock based on a Vertical External Cavity Surface Emitting Laser (VECSEL) that provides two orthogonally polarized frequencies separated by a difference of 9.2 GHz. This work presents characterizations of the miniature optical power stabilization set-up as well as preliminary investigations of the VECSEL properties.

The requirements for coherent combination of high power GaAs-based single-pass tapered amplifiers are studied. Changes to the epitaxial layer structure are shown to bring higher beam quality and hence improved combining efficiency for one fixed device geometry. Specifically, structures with large vertical near field and low wave-guiding from the active region show 10% higher beam quality and coherent combining efficiency than reference devices. As a result, coherent combining efficiency is shown to be limited by beam quality, being directly proportional to the power content in the central lobe across a wide range of devices with different construction. In contrast, changes to the in-plane structure did not improve beam quality or combining efficiency. Although poor beam quality does correlate with increased optical intensities near the input aperture, locating monolithically-integrated absorption regions in these areas did not lead to any performance improvement. However, large area devices with subsequently improved cooling do achieve higher output powers. Phase noise can limit coherent combining, but this is shown to be small and independent of device design. Overall, tapered amplifiers are well suited for high power coherent combining applications.

We report on a coherent beam combination of three high-brightness tapered amplifiers, which are seeded by a single-frequency laser at λ = 976 nm in a simple architecture with efficiently cooled emitters. The maximal combined power of 12.9 W is achieved at a combining efficiency of > 65%, which is limited by the amplifiers' intrinsic beam quality. The coherent combination cleans up the spatial profile, as the central lobe's power content increases by up to 86%. This high-brightness infrared beam is converted into the visible by second harmonic generation. This results in a high non-linear conversion efficiency of 4.5%/W and a maximum power over 2 W at 488 nm, which is limited by thermal effects in the periodically poled lithium niobate (PPLN).

Coherent beam combining (CBC) of high brightness tapered amplifiers (TPAs) is a promising approach for the development of high power semiconductor laser sources with excellent beam quality and offers a pathway to future direct diode high brightness system. Operation of the amplifiers in quasi continuous wave (QCW) mode allows to overcome the thermal limitations in the amplifiers and to test their full power scaling potential. Additionally, high brightness QCW sources with high spectral purity are in demand as NIR pump sources for nonlinear frequency conversion to address medical applications of modulated high power visible laser sources. In this paper we discuss dynamic effects in pulsed high brightness tapered amplifiers and demonstrate their coherent superposition for brightness scaling. We use state-of the art TPAs with the so called ELOD2 epitaxial design, which allows us to maintain close-to-diffraction limited beam quality at high powers.

High brightness diode laser beam combining techniques are in demand for efficient high power nonlinear conversion. Coherent beam combining (CBC) is the only method that has the potential for brightness scaling by maintaining one single narrow spectral linewidth. CBC in a master oscillator power amplifier (MOPA) configuration using a small number of efficiently cooled tapered amplifiers is a promising approach for efficient brightness scaling in a simple architecture. We present the application of such a source based on CBC of three tapered amplifiers seeded by a DFB laser at λ = 976 nm for second harmonic generation (SHG). A maximum power of 2.1 W at 488 nm was generated by SHG in a MgO:PPLN bulk crystal limited by thermal effects. A clear benefit of the beam clean-up resulting from the CBC setup was documented leading to an improved nonlinear efficiency. As part of our ongoing studies into further brightness scaling in CBC architectures, we present an experimental analysis of the phase dynamics of tapered amplifiers in quasi continuous operation (QCW) at high currents. Furthermore, we are investigating different amplifier designs for improved beam quality at high powers and therefore improved combining efficiency.

Coherent population trapping (CPT) has been demonstrated as an interesting technique for miniature atomic frequency references [1,2] and quantum information. It is based on the coupling of the two hyperfine ground states of an alkali atom – namely cesium (133Cs) for atomic clocks – through excitation to a common atomic level by two phase-coherent laser fields nearly resonant with the atomic transitions. The frequency difference between the two laser fields is tuned at the atomic frequency splitting in the microwave range, equal to 9.192 GHz for 133Cs atoms. Outputs powers in the mW range and narrow-linewidth emission (<500 kHz) are required for the two laser beams.

Improved diode laser beam combining techniques are in strong demand for applications in material processing. Coherent beam combining (CBC) is the only combining approach that has the potential to maintain or even improve all laser properties, and thus has high potential for future systems. As part of our ongoing studies into CBC of diode lasers, we present recent progress in the coherent superposition of high-power single-pass tapered laser amplifiers. The amplifiers are seeded by a DFB laser at λ = 976 nm, where the seed is injected into a laterally single-mode ridge-waveguide input section. The phase pistons on each beam are actively controlled by varying the current in the ridge section of each amplifier, using a sequential hill-climbing algorithm, resulting in a combined beam with power fluctuations of below 1%. The currents into the tapered sections of the amplifiers are separately controlled, and remain constant. In contrast to our previous studies, we favour a limited number of individual high-power amplifiers, in order to preserve a high extracted power per emitter in a simple, low-loss coupling arrangement. Specifically, a multi-arm interferometer architecture with only three devices is used, constructed using 6 mm-long tapered amplifiers, mounted junction up on C-mounts, to allow separate contact to single mode and amplifier sections. A maximum coherently combined power of 12.9 W is demonstrated in a nearly diffraction-limited beam, corresponding to a 65% combining efficiency, with power mainly limited by the intrinsic beam quality of the amplifiers. Further increased combined power is currently sought.

Coherent beam combining (CBC) aims at increasing the spatial brightness of lasers. It consists in maintaining a constant phase relationship between different emitters, in order to combine them constructively in one single beam. We have investigated the CBC of an array of five individually-addressable high-power tapered laser diodes at λ = 976 nm, in two architectures: the first one utilizes the self-organization of the lasers in an interferometric extended-cavity, which ensures their mutual coherence; the second one relies on the injection of the emitters by a single-frequency laser diode. In both cases, the coherent combining of the phase-locked beams is ensured on the front side of the array by a transmission diffractive grating with 98% efficiency. The passive phase-locking of the laser bar is obtained up to 5 A (per emitter). An optimization algorithm is implemented to find the proper currents in the five ridge sections that ensured the maximum combined power on the front side. Under these conditions we achieve a maximum combined power of 7.5 W. In the active MOPA configuration, we can increase the currents in the tapered sections up to 6 A and get a combined power of 11.5 W, corresponding to a combining efficiency of 76%. It is limited by the beam quality of the tapered emitters and by fast phase fluctuations between emitters. Still, these results confirm the potential of CBC approaches with tapered lasers to provide a high-power and high-brightness beam, and compare with the current state-of-the-art with laser diodes.

We describe a new coherent beam combining architecture based on passive phase locking of emitters in an extended cavity on the rear facet and their coherent combination on the front facet. This rear-side technique provides strong optical feedback for phase locking while maintaining a high electrical-to-optical efficiency. Two high-brightness high-power tapered laser diodes are coherently combined using a Michelson-based cavity. The combining efficiency is above 82% and results in an output power of 6.7 W in a nearly diffraction-limited beam with an M4σ2≤1.2. A semi-active automatic adjustment of the current enhances the long-term stability of the combination, while the short-term stability is passively ensured by the extended cavity. This new laser configuration exhibits the simplicity of passive self-organizing architectures while providing a power conversion efficiency of 27% that is comparable to master oscillator power amplifier architectures.

We demonstrate a rear-side phase-locking architecture with two high-brightness diode lasers. This technique is based on the passive phase-locking of emitters in an external cavity on their rear facet, and their coherent combination on the front facet. Two high-brightness high-power tapered laser diodes are coherently combined using a Michelson-based cavity. The combining efficiency is above 80% and results in an output power of 6.5 W in a nearly diffraction-limited beam.

The availability of small atomic clocks should greatly improve the mobility and robustness of, for example, satellite positioning system and high bitrate communication networks. In this context, coherent population trapping (CPT) is an interesting technique for the development of compact atomic references. It is based on the coupling of the two hyperfine ground states of an alkali atom, through excitation to a common atomic level, by two phase-coherent laser fields. We propose an innovating laser source for the production of such bifrequency field based on the dual-frequency and dual-polarization operation of an optically-pumped vertical external-cavity semiconductor laser (OP-VECSEL). Laser noise contributions to clock performance have been evaluated and results in an estimated clock frequency stability of 2.7 x 10-12 over a second limited by laser intensity noise. Stability level below 3 x 10-13 over one second is reasonably reachable with small changes in clock working conditions or laser set-up.

An external cavity using a binary phase grating has been developed to achieve coherent combining of five quantum-cascade lasers emitting at 4.65 μm. The grating phase profile is designed to combine five beams of equal intensities into a single beam with a good efficiency (~75%). The performances of this cavity concerning output power, stability, combining efficiency and beam quality are detailed. We report a CW combining efficiency of 66% corresponding to an output power of ~0.5 W with a good beam quality (M(2)<1.6).

We describe both coherent combining and wavelength stabilization of 10 tapered lasers in an external Talbot cavity with a volume Bragg grating. 1.7W at 976 nm in a narrow bandwidth, high coherent beam are obtained.

High-brightness single laser diodes based on the widespread taper design have demonstrated output powers of a few Watts with a single transverse mode operation [1]. The use of arrays of such lasers result in a further increase of the laser power, but with the drawback of a loss in the spatial brightness. To overcome this limitation numerous external-cavity configurations have been proposed which induce a coherence between the individual emitters of the array and result in a brightness improvement [2]. In this work we describe two external cavities intended to improve the spatial brightness of a bar of N = 6 index-guided tapered laser diodes emitting around 975 nm. The lateral structure of the emitters consists of a short ridge single-mode section, a 2.3 mm-long narrow-angle tapered ridge and a common amplified free-space 0.2 mm-long section. The array pitch is p = 30 µm, and the near-field 1/e2 full-width (1/e2-FW) of each emitter is 30 µm too, so the filling factor of this array is 100% on the front facet and the emission section is w = 180 µm wide. No adjacent coupling between emitters is evidenced in the free running laser emission of the array alone, and its 1/e2-FW divergence is ∼80 mrad in the slow axis. Our external cavity designs aim at controlling the slow-axis beam divergence of the whole array by inducing an angular-filtered feedback into the lasers [3,4]. The configuration forces the array to operate in the out-of-phase mode, which has two main lobes in its far-field profile at ± λ/2p = ± 16 mrad. We take benefit of the angular selectivity of volume Bragg gratings to favour an asymmetrical feedback on one of these peaks. The far-field of the extended-cavity array is thus expected to exhibit one diffraction-limited peak in the symmetric direction (Figure 1). Two different setups have been investigated experimentally:

We report on the design and characterization of a single-frequency diode-pumped vertical external-cavity surface-emitting laser emitting at 852 nm for Caesium atomic clock experiments. Up to 120 mW under 1.1 W pumping is achieved.

We report on a diode-pumped vertical external-cavity surface-emitting laser emitting around 852 nm for Cesium atomic clocks experiments. We have designed a 7-quantum-well semiconductor structure optimized for low laser threshold. An output power of 330 mW was achieved for 1.1 W of incident pump power. Furthermore, a compact setup was built for low-power single-frequency emission. We obtained an output power of 17 mW in a single longitudinal mode, exhibiting both broad (9 nm) and continuous (14 GHz) tunability around the Cesium D2 line. The laser frequency has been stabilized on an atomic transition with residual frequency fluctuations ∼300 kHz. Through a beatnote experiment the −3 dB laser linewidth has been measured to <500 kHz over 10 ms.

We present the diode pumping of a Nd:ASL (Sr1-xLax-yNdyMgxAl12-xO19) crystal for second harmonic generation at 453 nm. We have developed a high-brightness pump source based on a tapered amplifier in an extended cavity with a volume Bragg grating for wavelength stabilization. A pump brightness of 110 MW cm(-2) sr(-1) is obtained with a linewidth lower than 80 pm at 798 nm. This laser source is used to pump a Nd:ASL crystal to obtain 300 mW at 906 nm and 53 mW at 453 nm by intracavity doubling with a LBO crystal.

We describe the coherent combining and the wavelength stabilization of 10 tapered lasers in an external Talbot cavity. The use of a volume Bragg grating as feedback element to narrow the spectrum is demonstrated.

We describe the coherent combining and the wavelength stabilization of 10 tapered lasers in an external Talbot cavity. The use of a volume Bragg grating as feedback element to narrow the spectrum is demonstrated.

This work reports on an optically-pumped vertical external-cavity surface- emitting laser (VECSEL) emitting around 852 nm for Cesium atomic clocks experiments. We describe in the following our first results on the design and the characterization of a VECSEL’s semiconductor structure suitable for these applications. We optimized the parameters of the structure in order to have a low threshold and a high gain structure emitting around 852 nm. With a compact setup, we obtained a 5-mW single frequency emission exhibiting broad and fine tunability around the Cesium D 2 line.