Excess intensity noise in a low-noise single-frequency class-A VECSEL is experimentally investigated over the frequency range 10 kHz–18 GHz. An analytical model is derived, based on multimode Langevin equations, to describe the observed laser excess noise over the whole bandwidth. From 50 MHz to 18 GHz, class-A operation leads to a shot noise limited relative intensity noise (RIN), namely -155 dB/Hz for 1-mA detected photocurrent, except at harmonics of the cavity free spectral range (FSR). At these frequencies, the excess noise is shown to be due to the amplified spontaneous emission contained in the nonlasing side modes. The measured levels of excess noise correspond to side mode suppression ratios (SMSRs) ranging from 70 to 90 dB, in agreement with the model. At low frequencies, 10 kHz–50 MHz, the observed excess noise spectrum has the expected Lorentzian shape. Its bandwidth increases with the pumping rate to an upper limit given by the cavity photon lifetime. Below this cutoff frequency, we show that the pump RIN is the dominant source of noise, while it is filtered by the laser dynamics above. Finally, our model permits to design a semiconductor class-A laser with an intensity noise limited to the shot noise level over the whole 10 kHz–18 GHz bandwidth.
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