MINIATURE ND-YAG LASERS - NOISE AND MODULATION CHARACTERISTICS

The frequency and intensity noise spectra, as well as the frequency modulation (FM) response of 1320-nm laser-diode-pumped miniature Nd:YAG ring lasers have been measured. The frequency noise spectrum has a resonance peak at the relaxation oscillation frequency of the laser (between 123 and 150 kHz) and is flat beyond 200 kHz with the spectral density of 613 rad2 ∙ Hz, much smaller than that of semiconductor lasers; the corresponding laser linewidth is less than 49 Hz. The relative intensity noise is -140 dB/Hz at the valley and has a resonance peak at the relaxation oscillation frequency of the laser. The FM response is flat from dc to 110 kHz and is in the 0.65−3 MHz/V range; the modulation frequency is limited by the relaxation oscillation frequency of the laser. The small frequency noise makes this laser an excellent source for phase-shift keying coherent lightwave transmission systems. The FM modulation bandwidth is much larger than the linewidth; therefore, phase-locked loops can be constructed using Nd:YAG lasers. Both phase-locked loops and 100-Mbit/s homodyne receivers employing miniature Nd: YAG lasers have been demonstrated experimentally. © 1990 IEEE