  We analyze, both theoretically and experimentally, the phase noise of the
radio frequency (RF) beatnote generated by optical mixing of two orthogonally
polarized modes in an optically pumped dual-frequency Vertical External Cavity
Surface Emitting Laser (VECSEL). The characteristics of the RF phase noise
within the frequency range of 10 kHz - 50 MHz are investigated for three
different nonlinear coupling strengths between the two lasing modes. In the
theoretical model, we consider two different physical mechanisms responsible
for the RF phase noise. In the low frequency domain (typically below 500 kHz),
the dominant contribution to the RF phase noise is shown to come from the
thermal fluctuations of the semicondutor active medium induced by pump
intensity fluctuations. However, in the higher frequency domain (typically
above 500 kHz), the main source of RF phase noise is shown to be the pump
intensity fluctuations which are transfered to the intensity noises of the two
lasing modes and then to the phase noise via the large Henry factor of the
semiconductor gain medium. For this latter mechanism, the nonlinear coupling
strength between the two lasing modes is shown to play an important role in the
value of the RF phase noise. All experimental results are shown to be in good
agreement with theory.
