Observation of noise reduction in spatially coupled microchip lasers

Abstract

We report on the observation of noise-correlated light beams emitted from a pair of spatially coupled microchip lasers. We study in particular, both experimentally and theoretically, the noise reduction induced in the intensity difference fluctuations, by examining the spectral density ratio between the difference and the sum of the two laser intensities in different frequency domains. The noise in the intensity difference is strongly reduced in the spectral domain of the relaxation oscillations of the coupled lasers. The agreement between experiment and the theoretical model findings is very good. d between the two lasers, ~ii! their relative frequency detun- ing D5d 22d 1 ~expressed in Hz!, with d 1 and d 2 the indi- vidual laser cavity detunings, and ~iii! the normalized pump- ing rates of the two lasers M 1 and M 2. Fine adjustments of one of these two latter parameters allow us to vary the rela- tive frequency detuning D. Typically we observe a 20 MHz shift for a pumping rate variation equal to 0.01 @4#. The other relevant parameters will be defined throughout the text. The dependence of the noise reduction on these parameters is studied and a detailed comparison of the experimental results with the extended theoretical model @5# is given. In order to observe the synchronization of the noise- driven fluctuations in the intensities of the two lasers and the consequent noise reduction effect, we first analyzed the time evolution of the intensity of each laser for different values of the distance d in the region of phase locking @3-5#, keeping the system close to resonance, because far from resonance the two microlasers are weakly coupled and the noise corre- lation effects are greatly reduced, as will be shown later. All experiments were performed in nearly identical conditions ~same pump and laser beam waist, and same region of the crystal!, except for the pumping rates M 1 and M 2, which were adjusted in order to achieve the phase locking. The values of the pumping rates remain very close and their variations are moderate ( M 153.060.6 and M 253.360.3). Table I shows the values corresponding to the series of ex- periments performed for D'0.