Ultra-narrow linewidth optical filter based on Faraday effect at isotope 87Rb 420 nm transitions

An ultra-narrow linewidth optical filter with isotope 87Rb vapor at 420 nm, within the best waveband 400-500 nm for deep sea communication is achieved for the first time. The Faraday effect, circular dichroism, and nonlinear saturation techniques are utilized to narrow the bandwidth from previous 2.5 GHz to about 15 MHz level on the energy transition 5S1/2→6P3/2. By changing the temperature and magnetic field, the maximum transmission is obtained when the temperature and the magnetic field of the 87Rb cell are at 100 °C and 12 G. We discuss the varying influences of temperature, magnetic field, and pump power on the transmission of the atomic filter. The maximum single peak transmission at 5S1/2, F=2→6P3/2, F′=3 transition is 2.1% with a bandwidth of 17.8 MHz, and 1.9% at the 5S1/2, F=2→6P3/2, F′=2,3 (cross-over) transition with that of 14.2 MHz. The calculated equivalent noise bandwidth of this system is 32.5 MHz. Compared with the conventional Faraday anomalous dispersion optical filter, the bandwidth of our system is narrowed at least two orders of magnitude and is closer to the natural linewidth. This ultra-narrow linewidth filter has the potential to be applied to submarine communication or the pump laser in a four-level Rb-based active optical clock.