Laser frequency stabilization by means of optical self-heterodyne beat-frequency control

Abstract

Summary form only given. We present a new method for laser frequency stabilization based on control of a fiber interferometer-derived heterodyne beat signal that is proportional in specific limits to the laser frequency derivative. The output of an external-cavity diode laser equipped with an intracavity electro-optic crystal (EOC) is split and coupled into a Mach-Zehnder interferometer with one short and one long (fiber delayed) arm. Light passing through the short leg is acousto-optically frequency shifted by /spl Delta//spl nu//sub AOM/ relative to the initial laser frequency /spl nu/(t). The difference in optical path length between the two arms creates a time delay /spl tau//sub D/ between the two fields. Thus, the resulting heterodyne signal as detected by the avalanche photodiode has a frequency /spl nu//sub h/(t) given by /spl nu//sub h/(t)=/spl nu/(t)-/spl nu/(t-/spl tau//sub D/)+/spl Delta//spl nu//sub AOM/. The output of the photodiode is subsequently coupled into a phase-locked loop (PLL) operating as a FM demodulator. The PLL's output signal V(t) is proportional to the difference quotient of the laser's frequency during the time delay /spl tau//sub D/. AC coupling blocks the constant term and subsequent integration electronics yield a voltage that in specific limits is proportional to the overall laser frequency excursion during the integration time. After appropriate amplification, this voltage is applied to the EOC and provides negative feedback to the laser, stabilizing its frequency.