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Joint Conference of the European Frequency and Time Forum and IEEE International Frequency Control Symposium (EFTF/IFCS)

Ultranarrow Bandwidth Optical Filter Based on Laser Cooled 87Rb Atoms

Xiaolei Guan, Wei Zhuang, Tiantian Shi, Jianxiang Miao, Jia Zhang, Jingbiao Chen, Bin Luo

Published May 15, 2023• 0 citations
Publisher

Abstract

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.

DOI
10.1109/eftf/ifcs57587.2023.10272172

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To obtain the weak signal light detection from the high background noise,a study on ultra-narrow bandwidth Faraday anomalous dispersion optical filter(FADOF) at the 87Rb D1 line,5S1/2F=2→5P1/2 F′=1transmission(795 nm) is demonstrated experimentally.A sample cell filled with Rubidium atoms in which the proportion of 87Rb is greater than that of the nature rubidium is placed in a homogeneous magnetic field between two crossed Glan-Thompson polarizers.The incident probe light goes through the sample cell,and interacts with the atoms.Because of the Faraday rotation,the function of the filter can be realized.When the experiment condition changed,the result of transmission spectrum changed accordingly.With the temperature rising from 340 K to 360 K,the variation of the transmission spectrum is recorded particularly.And the reason for the variation is analyzed carefully.At a proper temperature and magnetic field,a transmission spectrum narrowed to ~220 MHz is obtained,and the peak transmission is about 48%.The performance of the87Rb D1 line is better than that of the 85Rb.

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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.
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.
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.
We present a theoretical and experimental study of the ultra-narrow bandwidth Faraday anomalous dispersion optical filter (FADOF) operating at the rubidium D1 line (795 nm). This atomic line gives better performance than other lines for the main FADOF figures of merit, e.g. simultaneously 71 % transmission, 445MHz bandwidth and 1.2GHz equivalent noise bandwidth. c © 2013 Optical Society of America OCIS codes: 020.1335, 010.3640 Ultra-narrow bandwidth optical filters are key el-ements in laser remote sensing (LIDAR), observa-tional astronomy, free-space communications and quan-tum optics. Relative to conventional interference fil-ters, FADOFs offer high background-rejection, mechani-cal robustness, imaging capability and high transmission. FADOFs have been developed for several alkali atom res-onances – Cs D2 [1] and 6S1/2 → 7P3/2 [2] lines, Rb D2 line [3, 4], K (three lines) [5], Na D lines [6], and for Ca [7]. We demonstrate a FADOF on the D1 line of Rb (wave-length 795 nm). This line, efficiently detected with Si detectors, accessible with a variety of laser technolo-gies, and with large hyperfine splittings, is a favorite for coherent and quantum optics with warm atomic va-pors. Applications include electromagnetically-induced transparency [8], stopped light [9], optical magnetom-etry [10, 11], laser oscillators [12], polarization squeez-ing [13, 14], quantum memory [15] and high-coherence heralded single photons [16,17]. Here we show that the Rb D1 line provides supe-rior FADOF performance. We demonstrate a FADOF surpassing other atoms and other Rb transitions for key figures of merit, including peak transmission Tmax, transmission bandwidth and equivalent noise bandwidth ENBW = T−1max T (ν)dν, where T (ν) is the filter trans-mission versus frequency ν [4].
We demonstrate a Faraday anomalous dispersion optical filter (FADOF) operating on 5S(1/2)-6P(3/2) transition at 420 nm with a 5 cm long cell of 96.5% enriched 87Rb and measure the transmission as a function of the magnetic field and temperature. The isotope 87Rb FADOF achieves a peak transmission of 98% with a bandwidth of 2.5 GHz when the temperature of the isotope 87Rb cell is 280°C and the magnetic field is 500 G. The corresponding equivalent noise bandwidth is 5.9 GHz. The isotope 87Rb FADOF is applied to submarine communications, is used as the laser frequency stabilization for some atomic lines, and is also used as the pump laser in four-level Rb active optical clock.
To obtain the weak signal light detection from the high background noise,a study on ultra-narrow bandwidth Faraday anomalous dispersion optical filter(FADOF) at the 87Rb D1 line,5S1/2F=2→5P1/2 F′=1transmission(795 nm) is demonstrated experimentally.A sample cell filled with Rubidium atoms in which the proportion of 87Rb is greater than that of the nature rubidium is placed in a homogeneous magnetic field between two crossed Glan-Thompson polarizers.The incident probe light goes through the sample cell,and interacts with the atoms.Because of the Faraday rotation,the function of the filter can be realized.When the experiment condition changed,the result of transmission spectrum changed accordingly.With the temperature rising from 340 K to 360 K,the variation of the transmission spectrum is recorded particularly.And the reason for the variation is analyzed carefully.At a proper temperature and magnetic field,a transmission spectrum narrowed to ~220 MHz is obtained,and the peak transmission is about 48%.The performance of the87Rb D1 line is better than that of the 85Rb.