Intensity Noise Reduction Of Semiconductor Lasers Using External Optical Feedback

A clear choice in design of an optically fed phased array antenna is T/R level data mixing architecture [l]. The advantage of this architecture, however, lies in the use of optically distributed frequency reference and data links. The performance of directly modulated fiberoptic links at microwave frequencies is very poor in terms of gain and dynamic range. We have been able to devise methods to extend the effective bandwidth of directly modulated fiberoptic links to high microwave frequencies, resulting in synchronization of millimeter wave oscillators [2]. However, the measured dynamic range of the receivers in the T/R level data mixing is limited by the data signal fiber optic link performance [1]. Furthermore, the dynamic range of the directly modulated fiber optic links is dictated by the linearity and relative intensity noise (RIN) of semiconductor laser diode [3]. A method based on the coherent feedback to the laser diode, which achieved an efficient frequency response over a narrow band corresponding to the frequency reference signals was demonstrated [ 4]. In this paper we demonstrate the potential of this technique to significantly reduce the RIN of laser at data signal frequencies.