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We demonstrate a Cs FADOF (Faraday anomalous dispersion optical filter) with a single transmission peak resonant with the 6S1/2, F = 4 → 7P3/2, F′ = 3, 4, 5 transition at 455 nm. The filter achieves a single peak transmission of 86%. With the technique of saturated absorption spectra, we obtain the bandwidth of the single peak, which is 1.5 GHz. While most of other FADOFs operate at frequencies far from absorption, the filter we have realized can provide light exactly resonant with atomic transitions with a high transmission. We also find that, at a particular temperature, we can achieve a single transmission peak rather than many peaks far from absorption by changing the strength of magnetic field. This technique can also be applied to other alkali atoms.
© 2012 Optical Society of America
1. Introduction
At present, ultra-narrow bandwidth optical filters play a key role in a broad range of applications such as free space optical communication [1], lidar remote sensing systems [2–4], transmitting and receiving terminals in satellite laser linking acquisition systems [5], and the generation of narrowband quantum light [6, 7]. Additionally, FADOFs can also be used to realize an external cavity diode laser, which is immune to current and temperature fluctuations [8]. This method for laser stabilization is similar to that based on narrow interference filters [9]. Since FADOFs have the advantages of high transmission with narrow bandwidth, fast response, and high noise rejection, they have been extensively studied both theoretically and experimentally for several elements, including Cs [10–14], Rb [15–20], K [21–25], Na [25, 26], and Ca [27]. However, most of FADOFs discussed above work at frequencies far from absorption and actually, we need light resonant with atomic transitions for atom-photon interaction experiments. Therefore, the conventional FADOFs can not satisfy our special demands.
In 1995, an induced-dichroism-excited atomic line filter was realized [28]. In the induced-dichroism-excited atomic line filter, pumping laser is used to induce circular dichroism instead of magnetic field. Based on the mechanism of induced-dichroism-excited atomic line filter, optical filters with ultra-narrow bandwidth are realized [29–31, 14]. This kind of FADOF can provide light resonant with atomic transitions. However, the transmission at a 10% level and the need of pumping laser with high intensity limit this kind of FADOF to be used widely. In order to achieve a FADOF with a single transmission peak, we also used a buffer-gas-filled rubidium cell instead of pure rubidium cell [19]. Although we did achieve a single transmission peak with this method, the FADOF operated at frequencies far from absorption.
Another method to obtain light resonant with atomic transitions is the line-center operation. The conventional FADOFs discussed above operate in a relatively weak magnetic field and in the line-wing operation. At a particular temperature and magnetic field, FADOFs will work in the line-center operation. The K-FADOFs [23, 24] and Na-FADOF [26] working in line-center operation have been realized, respectively.
In this article, we demonstrate a Cs FADOF with a single transmission peak resonant with the 6S1/2, F = 4 → 7P3/2, F′ = 3, 4, 5 transition at 455 nm. We study the transmission as a function of magnetic field and temperature in detail. With the technique of saturated absorption spectra, we obtain the bandwidth of the single peak. Compared with most of other FADOFs that operate at frequencies far from absorption, the filter we have realized can provide light exactly resonant with atomic transitions with a high transmission.
2. Experimental schematics
Figure 1 shows relevant energy levels of Cs [32]. Figure 2 shows the experimental setup. ECDL represents 455 nm external cavity diode laser and M is a high reflection mirror for 455 nm. The laser beam is divided into two parts by a beam splitter (BS). One is used for saturated absorption spectra whereas the other one as probe laser of the FADOF. P1 and P2 are Glan-Taylor prisms with an extinction ratio 105 : 1. The reference saturated spectroscopy signal is collected by a photodiode (PD2) and the transmitted signal of FADOF is collected by PD1. We can obtain the bandwidth of the transmitted signal with the saturated spectroscopy signal collected by PD2. The Cs cell is 5 cm long. The temperature of the Cs cell is controlled by a heating wire with precision of 1°C. The magnetic field is produced by a piece of permanent magnets. The fluctuation and homogeneity of the magnetic field can be neglected. We can obtain different magnetic fields by changing the distance of magnets. Magnetic shields are used to reduce the external earth’s magnetic field. The transmission is measured as the ratio of the maximum transmitted laser power when the two polarizers are perpendicular and the maximum transmitted laser power when the two polarizers are in parallel.
Fig. 2 Experimental schematics. ECDL, 455 nm external cavity diode laser; BS, beam splitter; P, Glan-Taylor prism; M, high reflection mirror for 455 nm; PD, photodiode.
3. Experimental results
3.1. FADOF with multi-peaks
Figure 3 shows the transmitted signal of the FADOF with multi-peaks. The magnetic field is 100 G and the temperature of Cs cell is 250°C. The inset is the magnified reference saturated absorption signal. The six peaks of the inset from left to right represent transitions of F = 4 → F′ = 5, F = 4 → F′ = 4, 5, F = 4 → F′ = 3, 5, F = 4 → F′ = 4, F = 4 → F′ = 3, 4, F = 4 → F′ = 3, respectively. The upper line is the saturated absorption signal collected by PD2 while the bottom line is the transmitted signal of the FADOF. The FADOF works at frequencies far from absorption.
Fig. 3 Transmission of the FADOF with multi-peaks at 100 G and 250°C. The upper line is saturated absorption signal and the bottom line is the transmitted signal of the FADOF. The inset is the magnified reference saturated absorption signal.
3.2. Multi-peaks transforming into a single peak
At a particular temperature, we find that multi-peaks transform into a single peak gradually if we enlarge the magnetic field. Figure 4 shows the transforming behavior corresponding to F = 4 → F′ = 3, 4, 5 transitions. The temperature of Cs cell is 190°C. The upper line is the reference saturated absorption signal. The inset is the magnified saturated absorption signal. The three bottom lines are the transmitted signals of the FADOF at 200 G (blue), 700 G (green), and 900 G (red), respectively. At 200 G, there are two transmitted peaks. At 700 G the two peaks are closer to each other. When the magnetic field is 900 G, there is only a single peak which is resonant with atomic transitions.
Fig. 4 Multi-peaks transforming into a single peak corresponding to F = 4 → F′ = 3, 4, 5 transitions. The temperature of Cs cell is 190°C. The upper line is the reference saturated absorption signal. The inset is the magnified saturated absorption signal. The three bottom lines are the transmitted signals of the FADOF at 200 G (blue), 700 G (green), and 900 G (red), respectively.
We also study the transforming behavior corresponding to F = 3 → F′ = 2, 3, 4 transitions as shown in Fig. 5 to check whether the transforming behavior is applied to other transitions. The temperature of Cs cell is 190°C. The upper line is the reference saturated absorption signal. The three bottom lines are the transmitted signals of the FADOF at 200 G (blue), 600 G (green), and 700 G (red), respectively. The inset is the magnified reference saturated absorption signal. The six peaks of the inset from left to right represent transitions of F = 3 → F′ = 4, F = 3 → F′ = 3, 4, F = 3 → F′ = 2, 4, F = 3 → F′ = 3, F = 3 → F′ = 2, 3, F = 3 → F′ = 2, respectively. From Fig. 5 we know that the transforming behavior corresponding to F = 3 → F′ = 2, 3, 4 transitions is similar to that corresponding to F = 4 → F′ = 3, 4, 5 transitions.
Fig. 5 Multi-peaks transforming into a single peak corresponding to F = 3 → F′ = 2, 3, 4 transitions. The temperature of Cs cell is 190°C. The upper line is the reference saturated absorption signal. The inset is the magnified reference saturated absorption signal. The three bottom lines are the transmitted signals of the FADOF at 200 G (blue), 600 G (green), and 700 G (red), respectively.
Figure 6 shows transmission peaks corresponding to F = 3 → F′ = 2, 3, 4 transitions and F = 4 → F′ = 3, 4, 5 transitions in a full spectrum region. The temperature of Cs cell is 190°C. The magnetic field is 800 G. The upper line is the reference saturated absorption signal and the bottom line is the transmitted signal of the FADOF. Compared with other Cs FADOF [12], the FADOF we have achieved works at frequencies resonant with atomic transitions rather than far from absorption.
Fig. 6 Transmission peaks corresponding to F = 3 → F′ = 2, 3, 4 transitions and F = 4 → F′ = 3, 4, 5 transitions in a full spectrum region. The temperature of Cs cell is 190°C. The magnetic field is 800 G. The upper line is the reference saturated absorption signal and the bottom line is the transmitted signal of the FADOF.
At high temperature and low magnetic field, the light resonant with atomic transition is absorbed almost totally. As a result, the FADOF displays the characteristic of two transmission peaks or even more with frequencies far from the atomic transition. With the magnetic field increasing, the Zeeman levels move far from the atomic transition. The absorption effect relevant to the light resonant with the atomic transition becomes much weaker. Therefore, the light resonant with the atomic transition passes through the orthogonal polarizers under Faraday effect. Theoretical calculations are discussed in detail previously [10, 24].
3.3. Transmission of the single peak as a function of magnetic field and temperature
Since we have obtain a single transmission peak resonant with atomic transitions, we begin to investigate dependence of the single peak transmission on both the magnetic field and the temperature in order to make the single peak transmission as large as possible.
Figure 7 shows transmitted signals at different magnetic fields. The temperature of Cs cell is 130°C. The upper line is the reference saturated absorption signal. The inset is the magnified reference saturated absorption signal. The three bottom lines are the transmitted signals of the FADOF at 300 G (blue), 700 G (green), and 1000 G (red), respectively. The bandwidth of the transmitted peak becomes larger and larger with the magnetic field increasing.
Fig. 7 Transmitted signals at different magnetic fields. The temperature of Cs cell is 130°C. The upper line is reference saturated absorption signal. The inset is the magnified reference saturated absorption signal. The three bottom lines are the transmitted signals of the FADOF at 300 G (blue), 700 G (green), and 1000 G (red), respectively.
Figure 8 shows transmission of the single peak as a function of magnetic field. At first, the transmission increases rapidly with the magnetic field increasing. At 700 G, the FADOF achieves a peak transmission 81%. Then, the transmission decreases with magnetic field increasing rapidly. Faraday effect becomes obvious when the magnetic field increases initially. However, the large magnetic field enlarges the bandwidth of the transmission peak. Therefore, the transmission will decrease.
Fig. 8 Transmission of the single peak as a function of magnetic field. The temperature of Cs cell is 130°C.
Figure 9 shows transmitted signals at different temperature. The magnetic field is 900 G. The upper line is the reference saturated absorption signal. The inset is the magnified reference saturated absorption signal. The three bottom lines are the transmitted signals of the FADOF at 110°C (blue), 190°C (green), and 210°C (red), respectively. The bandwidth of the transmitted peak becomes larger and larger with temperature increasing. At 190°C, the FADOF achieves a peak transmission of 86% with a bandwidth of 1.5 GHz. The bandwidth is obtained with the reference saturated absorption spectra.
Fig. 9 Transmitted signals at different temperature. The magnetic field is 900 G. The upper line is the reference saturated absorption signal. The inset is the magnified reference saturated absorption signal. The three bottom lines are the transmitted signals of the FADOF at 110°C (blue), 190°C (green), and 210°C (red), respectively.
Figure 10 shows the transmission of the single peak as a function of temperature. We find that the transmission increases with the temperature increasing at first. At 190°C, the FADOF achieves a peak transmission. Then, the transmission decreases with temperature increasing rapidly. The reason is that the high temperature makes the absorption effect obvious.
4. Conclusion
In conclusion, we demonstrate a Cs FADOF with a single transmission peak resonant with the 6S1/2, F = 4 → 7P3/2, F′ = 3, 4, 5 transition at 455 nm. The filter achieves a single peak transmission of 86% with a bandwidth of 1.5 GHz. We obtain the bandwidth of the single peak with the technique of saturated absorption spectra. Compared with most of other FADOFs that operate at frequencies far from absorption, the filter we have realized can provide light exactly resonant with atomic transitions. We also find that, at a particular temperature, we can achieve a single transmission peak rather than many peaks by applying proper strength of the magnetic field in the system. Based on this work, we expect to realize an external cavity laser system which is immune to current and temperature fluctuations with this FADOF [8]. If we use FADOFs with many peaks, the external cavity laser will not only work at a single frequency. In addition, the laser frequency is far from the atomic transitions. The filter we have realized can solve this problem perfectly. Additionally, this filter can provide light resonant with atomic transitions for other atom-photon interaction experiments.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Grant Nos. 10874009 and 11074011).
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