Precise optical experiments, like the measurement of variations in the Earth’s gravity field, employ lasers with highly stabilized emission frequency, or, in other words, extremely narrow linewidth. To develop the lasers with high spectral purity, a number frequency-stabilization techniques have been applied. The techniques are mostly based on the embedded feedback loop (optical, electronic, or their combination) that attempts to compensate for frequency fluctuations around a stable frequency reference. A simple Fabry-Perot (FP) interferometer, consisting of two high reflectivity mirrors separated by a spacer, usually serves as a convenient tool for such frequency reference. When laser and FP interferometer are properly aligned to be mode matched, the laser becomes locked to the external FP cavity. Then radio-frequency modulation techniques are utilized to derive an electronic error signal that represents the deviations of the laser frequency from a given FP cavity reference fringe. An error signal is used for electronic feedback (primarily through the laser current in the case of diode lasers) or optical feedback (cavity mirror on piezoelectric translator) to control the laser frequency and minimize its deviations.
The idea of a distributed feedback diode laser with Hertz linewidth, reported by Zhao and co-authors in this Optics Letters publication, is based on two achievements in the design of frequency stable lasers. First, they used the technique proposed by Salomon, Hils and Hall, where sub-Hertz relative frequency stabilization was realized by the locking of two lasers to the same FP interferometer. This technique provides common mode rejection against cavity fluctuations. Then, the active control of length of an external optical path by the Pound-Drever-Hall technique was used to compensate for environmental influence on the frequency instability. The application of these two approaches allowed the authors to reduce the linewidth of the DFB laser diodes used in the experiment to the Hertz limit, and to efficiently suppress laser phase noise, keeping the low white phase noise plateau level above the Fourier frequency value of ~17 kHz.
The employment of the developed laser in commercial tools may enhance the quality of measurements in applications such as metrology, gravity wave astronomy and quantum optics.