Using laser cooling technology, we demonstrate an ultranarrow-bandwidth Faraday anomalous dispersion optical filter (FADOF) which operates on the$5^{2}\text{S}_{1/2}$($F = 2$) to$6^{2}\text{P}_{3/2}$($F' = 3$) 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.
Using laser cooling technology, we demonstrate an ultranarrow-bandwidth Faraday anomalous dispersion optical filter (FADOF) which operates on the52S1/2(F=2) to62P3/2(F′=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 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.
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.
We demonstrate a narrowband optical filter which operates on the 52S1/2 to 52P3/2 transition at 780 nm in rubidium vapor based on optical-pumping-induced dichroism combined with Faraday anomalous dispersion effects. Its peak transmission is 18.4(2)% at the 52S1/2, F = 2 to 52P3/2, F ' = 1, 3 crossover transition, whereas at the 52S1/2, F = 2 to 52P3/2, F ' = 2, 3 crossover transition the transmission is 18.6(2)%. Both transitions have a bandwidth (full-width at half-maximum of peak transmission) as narrow as 24.5(8) MHz, which is remarkably improved compared with the narrowest bandwidth as we know. This technique can also be applied to other alkali atoms.
In the development of atomic physics, laser sources with Frequencies corresponding to atomic transition and high stability are essential. The Faraday laser is a special diode laser using the Faraday anomalous dispersion optical filter (FADOF) to realize frequency selection, so the output laser frequency is automatically limited to the atomic Doppler broadening. However, the frequency of a Faraday laser corresponds to the range around only one atomic hyperfine transition. Here, we realize a switchable Faraday laser with two isotopes laser frequencies corresponding to 85Rb 52S1/2 (F=3)→52P3/2 and 87Rb 52S1/2 (F=2)→52P3/2 transitions based on a single isotope 87Rb-FADOF. The laser has good robustness against the fluctuation of diode current and temperature, with wavelength fluctuating within 0.8 pm from 16 to 30 °C of diode temperature, and has a free-running linewidth of 18 kHz. We also lock the laser frequency to the two cycling transitions of 85Rb 52S1/2 (F=3)→52P3/2 (F′=4) and 87Rb 52S1/2 (F=2)→52P3/2 (F′=3) by the modulation transfer spectroscopy technique. The Allan deviation of the residual error signal is 3×10−14/τ, and the frequency stability of the beat detection reaches 2.8×10−12 at 1 s integration time. This 780 nm switchable Faraday laser expands the application scenarios of Faraday lasers, which can be used in laser cooling atoms, optical frequency standards, and other quantum precision measurement fields.