1. Introduction

Atomic optical filters can realize ultra-narrow bandwidth optical filtering by using transitions of atoms [1, 2]. There are several types like the atomic resonance filter (ARF) [3], the Faraday anomalous dispersion optical filter (FADOF) [4, 5], the induced-dichroism excited atomic line (IDEAL) filter [6,7] and the Voigt magneto-optic atomic line filter [8]. These filters have found their way into free space laser communications [9], lidar detections [10–12 ], laser stabilizations [13], active optical clocks [14], ghost imaging [15] and quantum memory systems [16] for their narrow band, high transmittance, large field of view and excellent noise rejection abilities.

In a wireless optical communication system, Doppler effect shifts the signal frequency a lot when the terminal is moving in a high speed. The transmittance peak of the atomic filter needs to follow the signal frequency, which can be realized by adjusting the magnetic field. Due to the requirement of the magnetic field along axial direction as in Fig. 1(a), FADOF can use a solenoid scheme to generate a tunable magnetic field. But that consumes an extremely high power [17]. To overcome this problem, the Voigt scheme, whose magnetic field is perpendicular to the propagation direction of the light as shown in Fig. 1(b), could be used. The Voigt-based filter can make use of electromagnet cores in a field spool to generate a tunable magnetic field without blocking the signal light, while the solenoid in FADOF cannot use any cores along the light path. Electromagnet cores make the Voigt setup easier to reach the same magnetic field in a Faraday setup, because it needs much lower current in the coil. So compared to FADOF, magnetic field in Voigt type is easier to realize [18]. A tunable electromagnet with power consumption much lower than a solenoid makes the Voigt type better for transmittance peaks tuning. On the other hand, Faraday type filters have been realized by various transitions from atomic ground states or excited states, of which excited state FADOFs (ES-FADOFs) can provide much more wavelength choices [12,19–22 ]. However, so far, the Voigt type filters have only ground state realizations [8,18].

 

Fig. 1 (a) Diagram of the Faraday type. (b) Diagram of the Voigt type. (c) The Voigt filter scheme. The pump and signal lights propagate along the x axis and the magnetic field the z axis. The polarization of the signal laser has a 45-degree angle with the y axis.

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In this Letter, we propose and realize a magnetically tunable rubidium excited state Voigt atomic optical filter at 1.5 μm. A tunable electromagnet is used to generate the magnetic field. The influences of the pump frequency, the magnetic field and the pump polarization on the pump effect are experimentally investigated. Transmittance spectra in different magnetic fields are also measured to get the tunability of this filter.

2. Experimental schematics

As shown in Fig. 1(c), the excited state Voigt atomic optical filter contains an atomic cell along the x axis, so as the signal light and pump light. The magnetic field is along the z axis, perpendicular to the propagation direction of the lights. Vertical linear (y direction), horizontal linear (z direction), left circular, right circular, +45 degree and −45 degree polarizations (VLP, HLP, LCP, RCP, +45P and −45P for short) laser pumping are examined respectively. This excited state Voigt atomic optical filter works at the 5²P_3/2 → 4²D_5/2 transition of rubidium, whose wavelength is 1529 nm, with 780 nm pumping from 5²S_1/2 to 5²P_3/2, as shown in Fig. 2(a). The experimental setup for the excited state Voigt filter scheme is shown in Fig. 2(b). The pump laser is produced by a diode laser at 780 nm and its polarization is adjusted by HWP1 and QWP1. Then it is reflected to the signal light path by M2. The 1529 nm signal is produced by an external cavity diode laser and divided into two parts by HWP2 and PBS. The reflected light enters the Fabry-Perot cavity to measure the bandwidth of the transmittance. The transmitted light is adjusted by HWP3 and enters the Voigt filter. The polarization direction of P1 is parallel to that of the input. The rubidium cell is 40 mm long, with a diameter of 20 mm. The magnetic field inhomogeneity along the x axis is less than 10% in the 40 mm cell length. The linearly polarized dichroism produced by the transition between 5²P_3/2 and 4²D_5/2 rotates the signal light in a narrow band and it can pass through P2.

 

Fig. 2 (a) The energy levels of rubidium D2 line [23]. The hyperfine structures of 5²P_3/2 can not be detected without Doppler-free measurement and are drawn as a band. (b) Schematic experimental setup. The 1529 nm laser and the 780 nm laser are the signal and pump laser, respectively. P1 and P2 are orthogonal polarizers, with antireflection coating from 1050 to 1620 nm. M1 and M2 are mirrors. The rubidium vapor is in a 40 mm long quartz cell. HWP1 is half wave plate on 780 nm, while HWP2 and HWP3 are on 1550 nm. QWP1 is quarter wave plate on 780 nm. PBS is polarized beam splitter on 1550 nm. PD1, PD2 and PD3 are photodiodes. The magnetic field is generated by an electromagnet. P_A and P_B are probe points in experiments.

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The pump efficiency is one of the key factors in excited state atomic filters and it is quite important to find the most efficient pump condition. According to Zeeman effect, Δm_J=±1, Δm_J=0 transitions split in different ways. The VLP only resonates with transitions of Δm_J=±1 and the HLP only resonates with transitions of Δm_J=0. Based on our previous research result that a mixed line of Δm_J=±1 transitions from A2 and A3 shows up in a certain range of the magnetic field, at which an efficient pumping can be obtained [22]. So the VLP may achieve the best pump efficiency. To verify this, we have tested different polarizations and measured the pump absorptions and the signal transmittances.

3. Experimental results

The 780 nm laser with frequency tuning passes through the rubidium vapor at 90 °C, and the absorption spectra in different magnetic fields and with different pumping polarizations are recorded, as shown in Fig. 3. The absorption lines of pump lasers with different polarizations are the same without the magnetic field, all of which are 4 main lines, A1–A4. As the magnetic field increases, the transitions split. Each of the VLP line splits into 2 parts and a mixed line [22,24] with stronger absorption appears as the magnetic field is over 400 gauss, as shown in Fig. 3(a). The HLP absorption lines split less than the VLP and is shown in Fig. 3(b). The LCP, RCP, +45P and −45P have all transitions of Δm_J=±1, Δm_J=0 and their splits are shown in Fig. 3(c–f). The mixed line with VLP pumping obtains the maximum absorption ratio among all the pump polarizations.

 

Fig. 3 Measured absorption ratio spectra of rubidium D2 line under a temperature of 90 °C in various magnetic fields from 0 to 1600 gauss. (a–f) are results corresponding to vertical linear, horizontal linear, left circular, right circular, +45 degree linear and −45 degree linear polarizations.

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Based on the pump efficiency results, the transmittance spectra of the excited state Voigt filter with the six kinds of pump polarizations are measured. The filter works at 150 °C and the magnetic field of 872 gauss. The transmittance of the Voigt filter avoiding the system loss from optical attenuations is shown in Fig. 4(a). Peak transmittances by the VLP are significantly higher than other polarizations. Especially, its peak transmittances are more than four times higher than those with the HLP pumping. Take the result by 30 mW pumping as an example, the peak transmittance by the VLP reaches 57.6%, while that by the HLP is only 13.5% under the same conditions, as shown in Fig. 4(b). Transmittances by LCP, HCP, +45P and −45P pump lasers are higher than those by the HLP and lower than those by the VLP, and spectra of these four kinds are almost the same, which indicates that the phase difference between the vertical component and the horizontal component of the pump laser does not affect the transmittance. This result is in accordance with the previous absorption results of the pump laser and the VLP is indeed the most efficient pump polarization for the excited state Voigt optical filter. Thus, to obtain the best transmittance, the VLP pumping is used in the following experiments.

 

Fig. 4 (a) Peak transmittances and (b) transmittance spectra of the Voigt filter by different pump polarizations.

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To examine the tunability by the magnetic field, the influence of the magnetic field to the transmittance spectrum is then experimentally analyzed. The atomic vapor is kept at 150 °C and the pump laser is fixed at 30 mW. The density plot of the transmittance under magnetic fields varying from 0 to 1600 gauss is shown in Fig. 5(a). As the magnetic field increases, the transmittance of the Voigt filter appears, increases and then reduces and splits. To be more clear, the transmittance peaks are sorted out and drawn in Fig. 5(b). Below 1000 gauss, the transmittance spectra have two main peaks (Peak_A and Peak_B) whose frequencies shift as the magnetic field increases. Above 1000 gauss, another two side peaks (Peak_C and Peak_D) appear. The side transmittance peaks shift over 1.6 GHz as the magnetic field change from 125 to 1500 gauss. The tunability of transmittance peaks could be used to follow the signal frequency shifting [25]. Transmittance peaks are drawn in Fig. 5(c). The maximum transmittance is obtained at 925 gauss, above which the transmittance decreases and the peaks split. Figure 5(d) shows the transmittance spectra with 872 gauss and 1369 gauss. At 872 gauss, the transmittance spectrum is a double-peak structure, and its each single peak is about 600 MHz width. At 1369 gauss, the transmittance spectrum has changed into a four-peak structure, and its total transmission bandwidth is about 4 GHz. Generally speaking, transitions of 5²P_3/2 → 4²D_5/2 are divided into 3 groups in high magnetic fields, Δm_J=−1, Δm_J=0 and Δm_J=1. The transmittance peaks all appear between transitions of Δm_J=±1 and transitions of Δm_J=0. When the magnetic field is high enough, like 1396 G, that transitions of Δm_J=±1 are too far from transitions of Δm_J=0, the dispersion in the middle point reduces and the transmittance reduces too, thus two peaks change into four. The multi-peak transmittance structure may be used to match modulated laser signal.

 

Fig. 5 (a) The density plot of the Voigt filter transmittance for various magnetic fields. (b) Frequency shifts of transmittance peaks for various magnetic fields. (c) Peak transmittances for various magnetic fields. (d) Transmittance spectra of the excited state Voigt filter under 872 gauss and 1369 gauss.

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4. Conclusion

In conclusion, we demonstrate an excited state Voigt atomic optical filter working at optical communication wavelength of 1.5 μm with 57.6% peak transmittance. The pump efficiency for the Voigt filter is experimentally analyzed. Pump laser with linear polarization vertical to the magnetic field and resonates with the mixed transition line can obtain the most efficient pumping for this filter. The pump polarization choice can be conveniently extended to excited state Voigt atomic filters based on other elements like sodium, potassium and cesium. The influence of the magnetic field to the transmittance is also studied. The magnetic field can be tuned from 0 to 1600 gauss by a tunable electromagnet, and transmittance spectra can be observed from 125 to 1500 gauss. In that range, a transmittance peak tunability of 1.6 GHz is reached via the magnetic field adjusting. Compared with FADOF, the Voigt filter realizes the tunability with much lower power consumption and improves the practicability of tunable optical filters.