Abstract—We evaluate the influence of 2 types of laser sources with different spectral profiles on the performance of vapor cell atomic clocks based on lin‖lin coherent population trapping (CPT) resonances. We show that a short-term stabil- ity of 1 – 2 · 10 -11 τ -1/2 may be reached in a compact system using a modulated vertical cavity surface-emitting laser. Here the stability is limited by the detection noise level and can be improved up to a factor of 4 by increasing the lock-in detec- tion frequency to several tens of kilohertz, which is not pos- sible in standard double resonance atomic clocks. We compare these results with CPT prepared under the same experimental conditions, using 2 phase-locked extended cavity diode lasers, with which we predict a challenging short-term stability of 1 – 3 · 10 -13 τ -1/2 , comparable to the state-of-the-art laser-pumped Rb-clocks. C P T, V C F S C to the microwave-optical double resonance scheme, coherent population trapping (CPT) does not require a microwave cavity, which may be an advantage in compact or miniature atomic clocks [1], [2]. CPT usually suffers from smaller signal contrast, because of the losses induced by the presence of trapping states not contribut- ing to the clock signal. The short-term stability of an atomic clock can be eval- uated by the Allan deviation [3]: s t y k Q SN = , 12 × × - / / (1) where S is the signal (here the CPT resonance) amplitude measured in microamperes; N is the detection noise ex- pressed in terms of noise density (μA Hz -1/2 ); k is a con- stant that depends on the type of modulation used and is of the order of 0.2; and Q is the resonance quality factor, i.e., the ratio between the resonance frequency (v) and the signal (here CPT resonance) line width (Γ). We have investigated the case of 87 Rb D 1 when a longi- tudinal magnetic field is applied and CPT resonances are excited by a linearly polarized, unidirectional, and mul- tifrequency light field (the so-called lin‖lin CPT) [4]–[6], [8]. The main advantage of this interaction scheme with respect to the CPT state prepared with circularly polar- ized light field is that there is no trapped state in which the atomic population can be accumulated through the optical pumping. The CPT resonances were prepared by using either a modulated vertical cavity surface-emitting laser (VCSEL) or 2 phase-locked (PL) extended cavity diode lasers (ECDL). We predict the achievable clock fre- quency stability, basing our considerations on measure- ments of the signal-to-noise ratio in our setup. The cur- rent-modulated VCSEL has a multifrequency spectrum with broad line width (in our case, 100 MHz, measured by a heterodyne beat-note), but it is very compact, ro- bust, and has low power consumption. Therefore, VCSELs are suitable for applications in commercial devices. The VCSEL was modulated with the frequency v ≈ 3.4 GHz (i.e., half of ground state hyperfine separation in 87 Rb). The phase-modulation index was estimated to be about 1.8, corresponding to the maximum power in the 1st-order side-bands 1 which are used for CPT preparation. On the other hand, the PL ECDL have, in good approximation, a pure bichromatic light field. The frequency difference between the 2 spectral components (containing 99.5% of the laser power) is Δv ≈ 6.8 GHz. The laser line width of each spectral component is about 40 kHz reached by a fast stabilization servo loop. The root mean square phase noise level of the PL ECDL system is ϕ rms ≤ 50 mrad (measured in the band of 1 Hz to 1 MHz) [9]. However, the PL ECDL system is bulkier than a modulated VCSEL and has been used only in the laboratory so far. During the experiments, we ensured that we had almost the same resonant power of 50 μW in those frequency components used for CPT resonance excitation. As in the best case, a fraction of 68% of the spectral VCSEL power is contained in both 1st-order sidebands used for CPT resonance exci- Influence of Laser Sources with Different Spectral Properties on the Performance of Vapor Cell Atomic Clocks Based on lin‖lin CPT Evelina Breschi, George Kazakov, Roland Lammegger, Boris Matisov, Laurentius Windholz, and Gaetano Mileti E. Breschi and G. Mileti are with the Laboratoire Temps-Fréquence, University of Neuchâtel, Neuchâtel, Switzerland. G. Kazakov and B. Matisov are with the St. Petersburg State Poly- technic University, St. Petersburg, Russia. R. Lammegger and L. Windholz are with the Institute of Experimen- tal Physics, Graz, Austria. 1 The maximum power that can be stored up in the 1st-order side- bands is the 68% of the total laser power. Published in IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control 56, issue 5, 926-930, 2009 which should be used for any reference to this work 1