Coherent optical fiber system with Zeeman lasers

Abstract

The high-quality stabilized laser source is the most Important device in a coherent optical fiber system.1 Here we try to use a Zeeman laser for this purpose. For convenience, our experiments are done in the 0.633-μm waveband. It has been reported2,3 that the Zeeman He-Ne laser has obtained a frequency stability of <1 MHz (10−9) for a 1-s average time and a long-term frequency reproducibility of ±1 MHz and that the equipment for frequency stabilization is not complex. But the reported stabilized laser comes up short in its application to a coherent optical system. As stated in Ref. 1 the frequency of this laser is locked at the lowest point A of the Zeeman beat frequency curve as shown in Fig. 1. This results in an optical frequency modulation of ~8 MHz peak-peak, and frequency tuning is impossible because the frequency of the laser is fixed. The frequency of our Zeeman laser is locked at point B of the Zeeman beat frequency curve, as shown in Fig. 1, by comparing the Zeeman beat frequency with the frequency of a quartz oscillator. By careful and correct design this stabilized Zeeman laser gives even better frequency stability (~10−11) and nearly the same frequency reproducibility (~10−9) as that reported in Ref. 1. There is no optical frequency modulation, and the optical frequency can be tuned by the change of the magnetic field strength. The frequency tuning range is ~400 MHz.