New atomic filter with bandwidth close to the atomic natural linewidth
87Rb atoms are irradiated with the circularly polarized coupling light, and in the case of the two-photon resonance, the polarization plane of the linearly polarized signal light will be rotated over an angle when passing through the atomic vapor. An atomic filter with bandwidth of 10 MHz and transmission of 33.2% has been realized based on the quantum interference induced Faraday effect. The center frequency of the filter can be tuned by altering the coupling frequency.
Unltranarrow bandwidth optical filters can be used to efficiently suppress the background light noise while extracting the weak signal. The atomic filtering method has been proved to be one of the most effective ways to realize unltranarrow bandwidth optical filtering, and is able to significantly enhance the sensitivity of the signal detection by the use of absorption, emission and internal energy exchange of atoms. Atomic filters have been applied in the areas of LIDAR, atmospheric remote sensing, laser and quantum communications.
In order to explore new applications for atomic-ensemble-based quantum memories and narrow band photon sources, researchers are working on ultranarrow bandwidth atomic filters with high transmission and large frequency tunability, meanwhile the passband matching the atomic resonant frequency.
The researchers realized an atomic optical filter at 87Rb D2 line with a bandwidth of 25 MHz and a transmission of 18% based on Faraday anomalous dispersion effect and velocity-selective pumping. The filter is composed of two permanent magnets producing the axial magnetic field, two polarization-orthogonal Glan-Tayor prisms with an extinction ratio 105:1, and a frequency stabilized 780 nm diode laser.
The research group, led by Prof. Mingsheng Zhan, from Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, achieved an ultranarrow bandwidth atomic filter working at the D1 line (795nm) of 87Rb atoms based on the quantum interference induced Faraday effect. It is reported in Chinese Optics Letters, Volume 12,Issue 12.
In their research, circular birefringence of the atomic medium was induced by a bunch of circularly polarized coupling light , which led to the polarization rotation of the linearly polarized signal light while passing through the atomic vapor. The frequency of the coupling light was locked to a specific atomic transition, and in the case that the frequency difference of the signal light and the coupling light equals to the separation of 87Rb hyperfine ground state, the signal light with frequency in the narrow EIT window could transmit through the two crossed Glan-Thompson prisms, while the light with frequency outside the EIT window would be rejected. Based on such quantum inference induced Faraday effect, the researchers achieved filter bandwidth of 10 MHz, which is close to the natural linewidth of Rb atoms, and the transmission of 33.2%. The center frequency tuning of the filter was realized by altering the coupling frequency. The dependence of the transmission on frequency detuning and intensity of the coupling light was also studied.
The following work will be focused on further enhancement of the transmission of the atomic filter by selecting different energy levels. Meanwhile, the researchers are trying to suppress the bandwidth through filling the vapor cell with buffer gas to increase the light-atom interaction time.
Atomic filter with ultranarrow bandwith may find applications in the areas of long-distance free space quantum key distributions,quantum internet, laser Doppler velocimetry and laser remote sensing with high sensitivity, allowing them to be operated under conditions of intense sunlight and tiny signal. It may also have potential applications in atomic-based quantum memories and narrow band single photon sources.