Accurate knowledge of the laser linewidth is essential for many applications in coherent optical communication, optical metrology and high-resolution spectroscopy. The self-heterodyne/homo-dyne measurement technique is an established method for measuring the linewidth of semiconductor lasers [1, 2]. The basic idea of the technique is to convert the optical phase or frequency fluctuations of the laser into variations of light intensity in a fiber-optic interferometer. In the interferometer, the optical signal is compared with a delayed replica of itself and detected with a fast photodiode. In the heterodyne case, an acousto-optic modulator is used to shift the spectrum in order to reject the delected DC signal in the photodiode and to allow use of a standard RF spectrum analyzer to measure the spectrum. The optical losses in the acousto-optic modulator arc, however, substantial and reduce the overall sensitivity of the setup, typically by several dB.
The delayed self-homodyne detection technique gives a convenient method to study the linewidth of laser sources. We have performed linewidth measurements on a 1.55-µm noncommercial multi-quantum-well DFB laser using the modified homodyne technique with delays short and by comparing with the coherence time of the laser [1]. The setup includes a phase modulator to average out the critical dependence on the phase matching of the interfering fields when using a short delay. Furthermore, in the detection stage the signal is shifted up to a carrier frequency by an RF-oscillator and a mixer to allow the use of a standard spectrum analyzer. A fit of a theoretical model to the measurement data gives a value for the linewidth.
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