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Abstract
We demonstrate an isotope 87Rb Faraday anomalous dispersion optical filter (FADOF) with a single transmission peak resonant with the 5S1/2, F = 2 → 5P3/2, F’ = 1, 2, 3 transitions at 780 nm with an enriched 87Rb isotope at low temperature. The isotope 87Rb FADOF achieves a single peak transmission of 74.8% with a bandwidth of 0.96 GHz. Compared with most of FADOFs operated at frequencies far from absorption, the isotope 87Rb FADOF that we have achieved can provide a transmission band exactly covering atomic transitions for many practical applications.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
As FADOFs have many excellent properties, such as narrow bandwidth [1], high transmission, and high noise rejection [2], they play a key role in many applications such as free space optical communication, lidar remote sensing systems and so on [3,4]. FADOFs are also used in the field of laser frequency stabilization, which makes the laser immune to the fluctuation of current and temperature [5,6]. FADOFs composed of all kinds of elements such as sodium [3], potassium [7–9], rubidium [10–15], cesium [16–18], calcium [19], magnesium [20], and strontium [21], have been studied for many years.
FADOFs researched before had many transmission peaks, which could lead to instability of the laser frequency. A single transmission peak is achieved by using two linked FADOF systems in a cascading configuration [22,23]. But there are still some small transmission peaks. A single transmission peak is also realized by using a Zeeman absorption cell, but their experimental setup is very complicated [24]. In order to simplify the system, our group achieves a FADOF with a single transmission peak using a buffer gas filled rubidium cell [11]. A single transmission peak is achieved, but this FADOF operates at frequencies far from absorption. To overcome this shortcoming, our group achieves a Cs FADOF with a single transmission peak resonant with the atomic transition at 455 nm [17], which is used as the pump laser in four-level active optical clock [25–27].
So far, most of scientific Rb experiments have focused on 87Rb while natural Rb atoms contain 72.8% 85Rb. When 87Rb is used in the field of laser frequency stabilization, 85Rb will bring many uncertain factors. Therefore, we conduct a study using 96.5% enriched isotope 87Rb to eliminate the negative impact of 85Rb particularly for all optical frequency stabilization 87Rb laser at 780 nm.
In this work, we demonstrate the isotope 87Rb FADOF with a single transmission peak resonant with the 5S1/2, F = 2 → 5P3/2, F’ = 1, 2, 3 transitions at 780 nm with an isotope 87Rb cell of 96.5% enriched 87Rb and 3.5% 85Rb at low temperature. We measure the transmission as a function of magnetic field and temperature. The isotope 87Rb FADOF achieves a single peak transmission of 74.8% with a bandwidth of 0.96 GHz. Compared with most of FADOFs operated at frequencies far from absorption, the isotope 87Rb FADOF realized here can provide a transmission band exactly covering atomic transitions and can be used in an all-optical locking of a semiconductor laser to the atomic resonance line [5,6].
2. Experimental setup
The relevant energy levels of 87Rb and the experimental setup are shown in Fig. 1 and Fig. 2. In Fig. 2, ECDL represents a 780 nm external cavity diode laser which can be tuned to cover all of the 87Rb 52S1/2 →52P3/2 transitions. The laser is divided into two beams by BS1. One is used for saturated absorption spectrum and the other one as a probe laser of the isotope 87Rb FADOF. G1 and G2 are a pair of crossed polarizers whose extinction ratio is 1 × 10−5. The saturated absorption spectrum of a natural Rb cell is collected by PD2 and the transmission spectra of the isotope 87Rb FADOF is collected by PD1. The 96.5% enriched isotope 87Rb cell and the natural Rb cell are 5 cm in length. The temperature of the isotope 87Rb cell is controlled by a heating wire with precision of 1°C. The magnetic field is generated by a piece of permanent magnets. Different magnetic fields are obtained by adjusting the distance of magnets. We make use of magnetic shield in order to reduce the influence of the external earth’s magnetic field.
Fig. 2 Experimental schematics. ECDL, 780 nm external cavity diode laser; ISO, optical isolator; BS, beam splitter; G, Glan-Taylor prism; R, high reflection mirror for 780 nm; PD, photodiode.
3. Results and discussion
Since we apply 96.5% enriched isotope 87Rb cell with only 3.5% 85Rb, the corresponding transmission spectra of 85Rb is dramatically reduced in the measured transmission spectra of isotope 87Rb FADOF shown in Fig. 3. The upper line is the saturated absorption spectrum of natural Rb(with 27.2% 87Rb and 72.8% 85Rb) which shows four absorption valleys. Two peaks on both sides are corresponding to 87Rb F = 2 and F = 1 transitions, and two peaks in the middle corresponding to 85Rb transitions. The four peaks of saturated absorption are used as frequency reference. The bottom line in Fig. 3 is the transmission spectrum of the isotope 87Rb FADOF whose two single transmission peaks are assigned to transitions of 5S1/2 corresponding to 87Rb F = 2 and F = 1. The magnetic field is 250 G and the temperature of the isotope 87Rb cell is 75°C. Transmission of single peak corresponding to F = 2 is 74.8% with a bandwidth of 0.96 GHz. The isotope 87Rb FADOF we have achieved works at frequencies resonant with atomic transitions rather than far from absorption. As a FADOF is used for laser frequency stabilization, a single peak is better than multi-peaks. The experimental setup with low temperature and low magnetic field is more convenient than that of high temperature and large magnetic field. So, the results have important applications for laser frequency stabilization [5,6].
Fig. 3 Transmission spectra of the isotope 87Rb FADOF with a single transmission peak at 250 G and 75°C. The upper line is the saturated absorption spectrum of natural Rb and the bottom line is the transmission spectrum of the isotope 87Rb FADOF.
For laser frequency stabilization, FADOFs operating at low magnetic field and low temperature are preferred. At a particular low magnetic field, a single peak transmission transforms into multi-peaks transmission if the temperature of the isotope 87Rb cell is gradually increased. Figure 4 shows the transforming behavior corresponding to 87Rb F = 1, 2 transitions. At 60°C and 65°C, there are only two single transmission peaks corresponding to F = 1, 2 transitions respectively which are resonant with atomic transitions. The transmission corresponding to F = 2 transition at 60°C and 65°C is 37.5% and 33.8%, respectively, which is used for laser frequency stabilization. However, the transmission corresponding to F = 1 transition at 60°C and 65°C is only 12.5% and 15.7%, respectively, although they are single peak, the transmission rate is not too high, which is not suitable for laser frequency stabilization. At 70°C, two single transmission peaks corresponding to F = 1, 2 transitions simultaneously transform into two double transmission peaks. The single peak transmission transforms into multi-peaks transmission corresponding to F = 2 transition at 75°C. At 75°C, transmission peaks corresponding to F = 1 transitions split in three peaks. The transmission corresponding to F = 2 transition decreases with the increase of the isotope 87Rb cell temperature. However, contrary to F = 2 transition, the transmission corresponding to F = 1 transition increases with the cell temperature increasing. The transmission corresponding to F = 1 transition at 86°C is 37.8%, which can be used for laser frequency stabilization at the line. For F = 1, 2 transmission peak, the absorption of light resonant with atomic transition increases with temperature increasing. Therefore, the FADOF exhibits the properties of two or more transmission peaks far from the atomic transition.
Fig. 4 Transmission spectra of the isotope 87Rb FADOF corresponding to 87Rb F = 2, 1 transitions at different cell temperature. The magnetic field is 70 G. The upper line is the saturated absorption spectrum of natural Rb. The five bottom lines are the transmission spectra of the isotope 87Rb FADOF at 60°C, 65°C, 70°C, 75°C and 86°C, respectively.
The transmission spectrum of the isotope 87Rb FADOF is measured to check its property at low cell temperature. Figure 5 indicates that when the magnetic field is 70 G, we find that the transmission increases with the increase of temperature at first. At 60°C, the transmission achieves a maximum of 37.5%. Then, the transmission decreases with temperature increasing. The laser frequency stabilization doesn't need so very high transmission as communication, therefore, such transmission is enough.
Fig. 5 Transmission of the isotope 87Rb FADOF corresponding to 87Rb F = 2 transition as a function of cell temperature. The magnetic field of the isotope 87Rb cell is 70 G.
We measure the transmission spectra corresponding to 87Rb F = 1, 2 transitions as shown in Fig. 6 to check the transforming behavior via the change of magnetic field. At a particular temperature, we find that a single peak turns into multi-peaks if the magnetic field of the isotope 87Rb cell is gradually increased.
Fig. 6 Transmission spectra of the isotope 87Rb FADOF corresponding to 87Rb F = 2, 1 transitions as a function of magnetic field at 75°C. The upper line is the saturated absorption spectrum of natural Rb. The five bottom lines are the transmission spectra of the 87Rb FADOF at 80 G, 150 G, 250 G, 400 G, 470G, respectively.
At 250 G, the transmission of peak corresponding to F = 2 transition arrives at a maximum of 74.8%. Its bandwidth is 0.96 GHz. When it is used for laser frequency stabilization, the signal has a long tail which is difficult to process in a phase sensitive detection system for peak locking. Hence it becomes evident that side locking may be the only option if the system is operated at that condition. When the magnetic field changes from 80 to 250 G, the peak transmission corresponding to F = 2 transition increases gradually, arriving at its maximum at 250 G. From 250 G to larger magnetic field, the transmission peak starts to decrease. At 400 G, the peak transmission starts to split apart into double peaks with a transmission of 58.1%. The bandwidth of the transmission peak becomes larger and larger with the increase of magnetic field [17]. From Fig. 6, the peak transmission corresponding to F = 1 transition as a function of magnetic field is similar to that of F = 2 transition.
Comparing the Fig. 4 and Fig. 6, there are multiple F = 2 transmission peaks at 75°C and 70 G in Fig. 4, whereas only two F = 2 transmission peaks at 75°C and 80 G in Fig. 6, which indicates that the magnetic field affects quantities of the peaks. Because of low magnetic field, Faraday Effect is not obvious, therefore F = 2 transmission peaks show multi – peaks at 70 G. As are shown in Fig. 4 and Fig. 6, the transmission peak corresponding to F = 1 transition is more immune to the effect of temperature than magnetic field before splitting. The temperature only affects absorption of atoms, while the magnetic field affects the Zeeman level movement, which results in the splitting of the transmission peak.
Figure 7 indicates the peak transmission corresponding to 87Rb F = 2 transition as a function of magnetic field. The peak transmission increases with magnetic field increasing at first at 75°C. At 250 G, the transmission achieves a peak transmission of 74.8%. Then, the transmission decreases with the increase of magnetic field rapidly. When the magnetic field began to increase, Faraday Effect becomes more obvious. However, the stronger magnetic field expands the bandwidth of the transmission peak. So, the transmission decreases with stronger magnetic field.
Fig. 7 Transmission of the isotope 87Rb FADOF corresponding to 87Rb F = 2 transition as a function of magnetic field. The temperature of the isotope 87Rb cell is 75°C.
4. Conclusion
In conclusion, we experimentally demonstrate an isotope 87Rb FADOF with a single peak transmission of 74.8% with a bandwidth of 0.96 GHz at modest temperature around 75°C for laser stabilization application. The isotope 87Rb FADOF operates at frequencies resonant with the 5S1/2, F = 2 → 5P3/2, F’ = 1, 2, 3 transition at 780 nm. Compared with most of tradition FADOFs operated at frequencies far from absorption, the isotope 87Rb FADOF that we have achieved can provide a transmission band exactly covering atomic transitions for many practical applications and be used for laser frequency stabilization to some atomic lines. We have realized an external cavity laser system which is immune to current and temperature fluctuations with this kind of isotope 87Rb FADOF [6]. As a FADOF with multi-peaks transition, the locked laser frequency is far from the atomic transitions and is apt to drift to neighbor transition peak. The isotope 87Rb FADOF we have realized can solve this challenge perfectly, thus the frequency stabilized lasers with this isotope 87Rb FADOF can provide light resonant with atomic transitions used for atom-photon interaction experiments.
Funding
National Natural Science Foundation of China (NSFC) (11847108); Education Department Fund of Jiangxi Province (6000074-255); Shangrao Technology Fund (5000109-255).
Acknowledgments
The authors thank Professor Jingbiao Chen for his beneficial discussion and direction of this research.
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