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Static and dynamic properties of a GaAlAs laser diode with an external cavity (1.5-50 cm) have been studied experimentally, it is found that the optical feedback induced intensity noise is strikingly suppressed by 30 dB when a single frequency oscillation is realized with good phase matching. The threshold of modulation depth, over which intensity noise increases abruptly, has been measured as a function of the modulation frequency in a range of 10-400 MHz. It has the maximum at about 40-70 MHz and increases with the decrease of the external cavity length. On the basis of the compound cavity model the phase and gain conditions for the single frequency oscillation are discussed and the threshold modulation depth is calculated, which is shown to be in good agreement with the experimental results.