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Applied Physics Letters

A laser pumped ultranarrow bandwidth optical filter

Zhilin Hu, Xizhi Zeng

Published Oct 12, 1998• 4 citations
Publisher

Abstract

An atomic optical filter with ultranarrow bandwidth has been developed. This filter has two operating channels generated from two hyperfine energy levels in the Cs ground state with a 9.2 GHz frequency separation. By employing a cw Ti:sapphire laser to alternatively pump the polarized Cs atoms to a selected hyperfine energy level, each of the two channels can be, respectively, used as emitting and receiving terminals. Compared to a normal Faraday anomalous dispersion optical filter, there is an increase in transmittance and a decrease in total equivalent bandwidth in this filter.

DOI
10.1063/1.122380

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An atomic optical filter with ultranarrow bandwidth has been developed. This filter has two operating channels generated from two hyperfine energy levels in the Cs ground state with a 9.2 GHz frequency separation. By employing a cw Ti:sapphire laser to alternatively pump the polarized Cs atoms to a selected hyperfine energy level, each of the two channels can be, respectively, used as emitting and receiving terminals. Compared to a normal Faraday anomalous dispersion optical filter, there is an increase in transmittance and a decrease in total equivalent bandwidth in this filter.
We propose and implement two ultranarrow-bandwidth optical filters with different principles based on laser-cooled87Rb atoms. Using the cold atomic cloud trapped in magneto-optical trap (MOT) to replace the vapor cell of traditional Faraday anomalous dispersion optical filter (FADOF), the effect of Doppler broadening on the transmitted bandwidth of FADOF will be effectively suppressed to the order of natural linewidth. Thus, an ultranarrow-bandwidth FADOF with a bandwidth of 2.7(2) MHz and a peak transmission of 3.2% is realized. It is the narrowest bandwidth FADOF known at present. Moreover, a transmission-enhancing method based on optical pumping is proposed. The two circularly polarized components of a linearly polarized probe laser have a large refractive index difference (induced dichroism), when it goes through the unidirectional population of atoms. Thus, forming a large rotation angle with high transmission. The two schemes can be extended to almost all kinds of atomic optical filters and may find applications in active optical frequency standard and self-stabilizing laser.
To achieve quantum-noise-limited performance, background-limited laser receivers require narrow-band optical filters. We measured and modeled the ultranarrow-band transmission spectrum of a Cs Faraday filter at 852 nm. The transmission spectrum consisted of passbands on either side of the 6 (2)S((1/2))-6 (2)P(3/2) hyperfine doublet lines, making a total of four. The passbands may be simple peaks or highly modulated, depending on the operating parameters. We observed peaked passbands of near-unity transmission with a 0.6-GHz bandwidth and modulated bands with features as sharp as 100 MHz. Excellent agreement with our calculations at 852 nm allows us to predict confidently a 0.7-GHz transmission band for Cs at 455 nm.
Atomic optical filters such as Faraday anomalous dispersion optical filters (FADOFs) or similar technologies can achieve very narrow optical bandwidth close to the scale of atomic linewidth, which can be greatly reduced in cold atoms. However, limited by the number of cold atoms and the size of the cold atomic cloud, the number of atoms interacting with the laser is reduced, and the transmission remains as low as 2%. In this work, we introduce the optical pumping into the cold atomic optical filter to solve this problem. Circular polarized optical pumping can produce polarization of the atomic ensemble and induce dichromatic as well as the Faraday rotation. We demonstrate a cold-atom optical filter which operates on the 87Rb 52S1/2 (F=2) to 52P3/2 (F′=2) transition at 780 nm. The filter achieves an ultranarrow bandwidth of 6.6(4) MHz, and its peak transmission is 15.6%, which is nearly 14 times higher than that of the cold-atom optical filter realized by Faraday magneto-optic effect. This scheme can be extended to almost all kinds of atomic optical filters and may find applications in self-stabilizing laser and active optical clock.
We demonstrate a nonlinear optical filter with ultranarrow bandwidth in cesium vapor. The optical filter operates on the 6S(1/2)→7P(3/2) transition at 455 nm. The single peak transmission at the 6S(1/2), F=4→7P(31/2), F'=5 transition is 9.7% with a bandwidth of 6.2 MHz, whereas at the 6S(1/2), F=3→7P(3/2), F'=2, 3 (cross-over) transition is 6.1% with a bandwidth of 3.9 MHz. The bandwidth approaching the natural linewidth is improved at least two orders of magnitude compared with conventional Faraday anomalous dispersion optical filters. This technique can also be applied to other alkali atoms.
Using laser cooling technology, we demonstrate an ultranarrow-bandwidth Faraday anomalous dispersion optical filter (FADOF) which operates on the52S1/25^{2}\text{S}_{1/2}52S1/2​(F=2F = 2F=2) to62P3/26^{2}\text{P}_{3/2}62P3/2​(F′=3F' = 3F′=3) transition at 420 nm based on cold 87Rb atoms. The filter achieves a single 2.7(2) MHz passband at a peak transmission of 3.2%. It is the narrowest bandwidth FADOF known at present, as the influence of Doppler broadening on the bandwidth narrowing of FADOF is effectively overcome by cold atoms. This ultranarrow-bandwidth FADOF has a figure of merit (FOM) value of 11.85 GHz−1, which is seven times larger than the largest FOM achieved by thermal vapor atomic optical filter published to date. Furthermore, the scheme can be extended to almost all kinds of atomic optical filters and may find applications in active Faraday optical frequency standard and self-stabilizing laser.