Fabiola Camargo

Coherent Dual-Frequency Emission of a Vertical External-Cavity Semiconductor Laser at the Cesium

We describe the dual-frequency and dual-polarization emission of a diode-pumped vertical external-cavity semiconductor laser at 852 nm dedicated to the coherent population trapping of cesium atoms. The output power reaches ~20 mW on each frequency, with a frequency difference in the gigahertz range.

{\rm D}_{2}

Cs atomic clocks based on coherent population trapping require two phase-locked laser lines with output power in the 10-mW range and a frequency difference of about 9 GHz to provide the microwave interrogation. Stabilization of one laser frequency to the reference laser with a phase-locked loop and high-frequency modulation of a diode laser are the two most frequent solutions. Alternatively, dual-frequency operation of a single laser source might provide the simplest architecture. It is based on the simultaneous emission of two orthogonally-polarized laser beams sharing the same laser cavity, but with a slight anisotropy resulting in the frequency difference. The major advantage of this configuration lies in the fact that the frequency fluctuations of the two beams are strongly correlated. Such dual frequency oscillation has already been observed with rare-earth doped material lasers [1]; recently it has also been demonstrated with a 1-µm vertical-external cavity semiconductor laser (VECSEL), with the benefit of low phase and intensity noise thanks to the class-A regime dynamics of VECSEL [2]. In this work, we describe the first dual-frequency operation of an optically-pumped VECSEL emitting around the Cs D2 line at 852 nm.

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We present in this work the study of a short vertical external cavity semiconductor laser in single longitudinal operation at 852 nm without intracavity elements. Two different configurations were studied, a plane-plane configuration, stabilized by the thermal lens and a plane-concave configuration. The influence of the output coupler transmission and the thermal lens has been studied. In the plane concave configuration we have demonstrated more than 100mW in stable single frequency operation using a very compact cavity emitting around 852 nm.

Dual-frequency operation of a vertical external cavity semiconductor laser for coherent population trapping cesium atomic clocks

We evaluate a dual-frequency and dual-polarization optically-pumped semiconductor laser emitting at 852 nm as a new laser source for compact atomic clocks based on the coherent population trapping (CPT) technique. The frequency difference between the laser modes is tunable to 9.2 GHz corresponding to the ground state hyperfine-split of Cs. Impact of the laser noise has been investigated. Laser relative intensity noise is limited by the pump-𝑅𝐼𝑁 transfer to a level of - 110 dB/Hz. Laser frequency noise shows excess mechanical and technical noise resulting in a laser linewidth of 1 MHz at 1 s in lock operation. The noise performance and spectral properties of the laser are already adequate to realize CPT experiments and should result in Allan standard-deviation of the clock below 1 × 10-12 at 1 second.

Evaluation of the single-frequency operation of a short vertical external-cavity semiconductor laser at 852 nm

Coherent population trapping (CPT) of metal-alkali atoms is an interesting technique for the development of compact atomic frequency references; it relies on the excitation of the atoms by two phase-coherent laser fields. We describe the design and operation of an innovating dual-frequency laser source dedicated to Cs CPT atomic clocks, based on the direct dual-frequency and dual-polarization operation of an optically-pumped semiconductor laser at 852 nm. The phase noise of beatnote generated by the laser source is at maximum of -90 dBrad2/Hz with active stabilization, and the relative intensity noise (RIN) has been measured at -115 dB/Hz. It would potentially results in a clock frequency stability of 1.6.10-12 at 1 second, limited by the laser RIN. With proper adjustments in the laser and clock set-up, we target a stability of 3.10-13 at 1 second.

Evaluation of the noise properties of a dual-frequency VECSEL for compact Cs atomic clocks

Coherent population trapping (CPT) is an interesting technique for the development of compact atomic frequency references. We describe an innovating laser source for the production of the two cross-polarized coherent laser fields which are necessary in CPT-based atomic clocks. It relies on the dual-frequency and dual-polarization operation of an optically-pumped vertical external-cavity semiconductor laser. This particular laser emission is induced by intracavity birefringent components which produce a controllable phase anisotropy within the laser cavity and force emission on two cross-polarized longitudinal modes. The laser emission is tuned at the Cs D2 line (λ = 852.14 nm), and the frequency difference Δν between the two laser modes is tunable in the microwave range. The laser line wavelength is stabilized onto an atomic hyperfine transition, and concurrently the frequency difference is locked to an ultra-low noise RF oscillator at 9.2 GHz. The high spectral purity of the optically-carried microwave signal resulting from the beatnote of the two cross-polarized laser lines is assessed through its narrow spectral linewidth (<30 Hz) as well as its low phase noise (≤ -100 dBrad2/Hz). The performance of this laser source is already adequate for the interrogation of atoms in a CPT atomic clock, and should result in an estimated relative stability of 3.10-13τ-1/2 – one order of magnitude better than commercial atomic clocks.

High-purity microwave signal from a dual-frequency semiconductor laser for CPT atomic clocks

We describe the dual-frequency operation of an optically-pumped vertical external cavity semiconductor laser (VECSEL) stabilized onto an Cs atomic transition. It is based on the simultaneous emission of two cross-polarized adjacent longitudinal modes inside the same laser cavity, which provides a strong correlation between the two laser lines. The frequency difference, in the GHz range, is fixed by the intracavity phase anisotropy, and precisely tuned with an electro-optic modulator (EOM). For this work, we additionally take benefit of the class-A dynamical behaviour of VECSEL which results in a shot-noise limited relative-intensity-noise on a wide spectral range. The GaAs/AlGaAs active structure is pumped with a 1W-fiber coupled laser diode at 670 nm. The laser cavity has been carefully designed for improved thermal and mechanical stability, and compactness. It consists in a 15-mm concave output coupler, a glass Fabry-Perot etalon, a YVO4 birefringent plate and a MgO:SLT EOM. The output power at each frequency reaches 20 mW. The frequency difference is phase-locked to a microwave reference source through the EOM voltage with a MHz bandwidth, resulting in a high-purity optically-carried microwave signal. Simultaneously, one laser line is locked on a Cs atomic hyperfine transition at 852 nm through a low-bandwidth servo-loop on the cavity length. The performance of our laser source is thus fully compatible with the excitation of Cs atoms in coherent population trapping atomic clocks.

Generation of high-purity microwave signals from a dual-frequency OP-VECSEL

Stable single-frequency laser with narrow linewidth emission (&#60; 500kHz), fine tunability over a few GHz and output power in the 0.1W range is required to cold atom interferometry experiments. Until now different approaches have been studied in order to fulfil all these characteristics. Single frequency high power lasers have been demonstrated with distributed feedback diode lasers and tapered extended cavity diode lasers. However, these two solutions suffer from beam quality degradation at high power. A possible alternative solution to obtain a compact and simple single frequency source with narrow linewidth and good beam quality is an optically-pumped semiconductor vertical external-cavity surface-emitting laser (OPS-VECSEL), which has already demonstrated multi watt output power in the fundamental transverse mode [1] as well as high power single frequency operation [2]. Nevertheless the majority of these works using VECSEL are focused around 1µm emission. In this work we evaluate short external-cavities VECSEL for single-frequency emission at 852nm dedicated to Cs atom spectroscopy.