High-resolution laser chirp measurement using quadrature sampling technique

A high-resolution laser chirp diagnosing technique is proposed and experimentally demonstrated by quadrature sampling of a phase-modulated Mach-Zehnder (M/Z) interferometer. A light signal from an injection current-modulated DFB source passes through an M/Z interferometer whose output is sampled with quadrature time delay. Digital arctangent demodulation is used to extract the laser chirp information from the sampled data. To remove the amplitude modulation (AM) effect, the intensities were divided by the modulation signal. Using a 30 cm path unbalanced M/Z interferometer, an 84 kHz readout resolution in the 19 GHz range is demonstrated with frequency increase/decrease discerning capability.