We measure the coupling constant between the two perpendicularly polarized eigenstates of a two-frequency Vertical External Cavity Surface Emitting Laser (VECSEL). This measurement is performed for different values of the transverse spatial separation between the two perpendicularly polarized modes. The consequences of these measurements on the two-frequency operation of such class-A semiconductor lasers are discussed.

The role of coherent population oscillations is evidenced in the noise spectrum of an ultra-low noise lasers. This effect is isolated in the intensity noise spectrum of an optimized single-frequency vertical external cavity surface emitting laser. The coherent population oscillations induced by the lasing mode manifest themselves through their associated dispersion that leads to slow light effects probed by the spontaneous emission present in the non-lasing side modes.PACS numbers: 42.55. Ah, 42.60.Lh, 42.50.Lc Since the early works of Sommerfeld [1] and Brillouin [2, 3] on light propagation through resonant atomic systems, slow and fast light (SFL) have been the subject of considerable research efforts. To control the group velocity of light, various approaches have been proposed and demonstrated, such as, e. g., electromagnetically induced transparency [4,5], coherent population oscillations (CPO) [6,7], and stimulated Brillouin scattering [8]. All these approaches are based on the well known Kramers-Krönig relations stating that a narrow resonance in a given absorption profile gives rise to very strong index dispersion in the medium. Consequently, a pulse of light can propagate through a material slower or faster than the velocity of light in vacuum without violating Einstein's causality [9]. In this framework, the major part of the studies reported in the literature is devoted to single-pass propagation in the considered dispersive medium: the pulse shape or the amplitude modulation of the light is fixed at the entrance of the SFL system. The point is then to investigate how these characteristics evolve during propagation through the medium.Systems, such as lasers, in which the light is self organized, have not attracted so much attention in this context. Yet, CPO, an ubiquitous mechanism inducing SFL, is present in any active medium provided that a strong optical beam saturates this medium. Thus, CPO must be present in any single frequency laser since the oscillating beam acts as a strong pump which, by definition, saturates the active medium. This effect could be observed using an external probe whose angular frequency is detuned with respect to the oscillating mode, by less than the inverse of the population inversion lifetime 1/τ c . Besides, it has been shown in semiconductor optical amplifiers (SOAs) that CPO induced SFL leads to a significant modification of the spectral noise characteristics at the output of the SOA [10,11]. Consequently, this effect should be also visible in the laser excess noise, using the * Electronic address: karim.el-amili@u-psud.fr spontaneous emission present in the non-lasing side longitudinal modes of a single-frequency laser as probe of the CPO effect. To reach this situation, the free spectral range (FSR) of the laser must not be larger than 1/τ c . This is seldom fulfilled in most common lasers. For instance, in ion-doped solid-state lasers, τ c is in the range of 1 µs -10 ms [12]. Thus, the FSR of the laser should be smaller than 1 MHz, forbidding single-frequency op...