An excited state Faraday anomalous dispersion optical filter (ES-FADOF) working at the optical communication wavelength (1.5 μm) is realized. Unlike the usual ES-FADOF schemes using an external frequency stabilization, an internal frequency stabilization scheme is proposed and the working atoms inside the filter are adopted as the reference. A particular cross line of multiple transitions is used for the frequency stabilization for the pump laser and thus, a higher pump efficiency is achieved. For example, compared with previous ES-FADOF schemes, this method can increase the transmittance from 10% to 60% at 100 °C. Moreover, in this scheme, the external frequency stabilization is not necessary and the volume of the atomic filter can be reduced. This simplifies the whole structure and a compact ES-FADOF can thus be realized.
© 2014 Optical Society of America
Faraday anomalous dispersion optical filter (FADOF), well-known for high transmittance, narrow bandwidth, large field of view and excellent out-of-band rejection [1,2], has drawn a lot of attentions since it was introduced in 1982 . It uses resonant Faraday anomalous dispersion effect in atomic vapor to rotate the polarization of light within a narrow band encompassing the atomic transition frequencies, and filters out the out-of-band light by a pair of orthogonal linear polarizers. FADOFs working at atom ground state transition lines have been realized at some wavelengths in various elements, such as rubidium , cesium [2,4], sodium , potassium  and calcium . These filters cover several wavelengths in a range from 423 nm to 894 nm. They have particular advantages that they have highly stable transmission peaks and they are insensitive to the incident angle, which are very important for wireless optical systems. So they are introduced into many applications, such as free space laser communication [8–10], laser system , lidar [12–14], etc.
However, the working wavelength of FADOF is restricted by the atomic transition frequencies. The ground state transitions cannot meet all the upcoming needs for more and more wavelengths from different applications. A lot of those wavelengths only exist in the transitions between excited states. Hence, the excited state FADOF (ES-FADOF) was proposed and realized in some wavelengths, such as green [14–16], red  and near-infrared [18, 19] wavelengthes, especially wavelengths beyond 1 μm in fiber communications. ES-FADOF at this waveband is significant for it can combine wireless optical communication researches with the mature fiber optical communication devices.
ES-FADOF requires the atoms to be pumped to the excited state. Electrodeless discharge lamp  and laser pumping  are two methods which are reported to transfer population to the excited state. However, the electrodeless discharge rubidium vapor lamp based filter  has low efficiency and this limits the performance of the device. The laser pumping based filter cannot work continuously when using a pulsed pump laser , while a continuous-wave pump laser usually requires a frequency stabilization system with an atomic transition reference. The traditional frequency stabilization needs an external frequency reference and that increases the volume and complexity. Moreover, the atoms for Faraday rotation experience Zeeman splitting and hence their transition frequencies are different from those of the external atoms for stabilization. Atoms with shifted levels cannot be pumped efficiently when the pump laser is locked to the external reference and this limits the performance of the filter.
In this article, we propose an ES-FADOF scheme working at 1529 nm using rubidium. An internal frequency stabilization scheme is used to overcome the restrictions mentioned above. The frequency used for stabilization is chosen based on one particular absorption line of the working atomic ensemble constituting the filter. Experiment results show that this method increases the pump efficiency. One example is that it increases the transmittance from 10% to 60% at 100 °C. It has a higher transmittance at lower working temperature than 21.9% (without taking account of the system loss from optical attenuations) at 220 °C, the transmittance of electrodeless discharge rubidium vapor lamp based filter .
2. Experimental schematics
A schematic diagram of the rubidium energy levels used for this ES-FADOF is depicted in Fig. 1. A 780 nm tunable laser pumps the atoms from the ground state to 52P3/2 for Faraday rotation at 1529 nm by the transition between 52P3/2 and 42D5/2. The hyperfine structure band of 52P3/2 is one order of magnitude smaller than that of the ground state, and it is also narrower than the Doppler width. Thus, the hyperfine structures of state 52P3/2 are not detailed here.
The experimental setup for the internal stabilization method is shown in Fig. 2(a), together with the usual external frequency stabilization system for comparison. The filter is made up of a rubidium vapor cell placed in a static magnetic field of 550 gauss between a pair of orthogonal linear polarizers. An external cavity diode laser working at 1529 nm generates the probe signal. By scanning the frequency of the probe laser and recording the transmitted power of the probe through the filter, the transmittance spectrum can be obtained. Two dichroic mirrors are placed on both sides of the cell to reflect 780 nm laser and allow 1529 nm laser to pass. Traditionally, the pump laser is usually locked to an external frequency stabilization system, which could be an optical cavity or an atomic reference cell, as in the dashed box in Fig. 2(a). In our experiment, a frequency stabilization system based on a rubidium absorption spectrum of D2 line with Doppler broadening and a first harmonic frequency stabilization method is used for comparison. The strongest absorption line, 85Rb, 52S1/2 (F = 3) → 52P3/2, is chosen as the external frequency reference (EFR). This method isolates the frequency stabilization system from the filter and leads to a mismatch between the frequency of the pump laser and the resonant frequency of the working atoms in the magnetic field. In 550 gauss magnetic field of the ES-FADOF, the absorption peak moves away from all the original peaks as shown in Fig. 2(b). The new peak is slightly increased for the reason that the absorption lines of the left and right circularly polarized lights split and shift to opposite sides of the original transition due to Zeeman effect, thus the split transitions of adjacent levels could mix together and generate a multiple transition line for linearly polarized light .
Our new scheme uses this line and we call it the internal mixed line reference (IMR). This line contains the left and right circularly polarized resonances, which are caused, separately, by two adjacent atomic transitions. So the difference of the two parts of the pump laser can indicate the frequency position between the two transitions. The output of the difference circuit is set to zero when the laser frequency matches the resonant frequency of atoms best. Then the positive and negative difference signals can indicate the directions of the frequency detuning. In the experiment, absorptions of the left and right circularly polarized lights are separately detected and electronically subtracted to generate a difference signal as shown in Fig. 2(c). The frequency of the probe laser can be tuned by changing the external-cavity length through a piezoelectric transducer (PZT). By processing the difference signal though the servo circuit and inputting them to the PZT controller, a closed loop feedback can be formed. Thus, the pump laser frequency can be locked to the strongest resonance of atoms constituting the filter, the IMR labeled in Fig. 2(b). This frequency stabilization method is similar to the dichroic atomic-vapor laser lock (DAVLL) , and our method refers to a mixed atomic line in much higher magnetic strength. This multiple transition line reflects the resonant frequency shifts of the working atoms, which cannot be achieved via the external method.
3. Experimental results
The ES-FADOF with pump frequency locked to this mixed line has significant advantages compared with the external method as shown in Fig. 3. At a relatively low vapor cell temperature of 100 °C, the peak transmittance of the IMR solution reaches 60% with 30 mW pump power. This is about 5 times higher than that of the EFR. When the cell temperature is increased to 110 °C, the performances of both the methods improves. However, in this case, the transmittance of the IMR is still higher than that of the EFR for the same pump power. When the vapor cell temperature is further increased to 120 °C, the transmittance of the IMR gets lower than that of the EFR for some pump powers. It is a typical FADOF phenomenon of the operation modes conversion , which indicates that the optimal working condition for line-center operation has been reached by the IMR at 12 mW pump power, and the polarization of probe over-rotates beyond 90 deg as the pump power further increases. Then the line-center performance decreases, but the performance of the wings keeps increasing, as shown in Fig. 4(ac). By the EFR method, the transmittance also experiences this process, as shown in Fig. 4(d–f). The difference is that the optimal operating condition for line-center operation of the EFR is reached at a pump power 18 mW as shown in Fig. 4(e). The conversion of line-center and wings operation occurs at about 30 mW as shown in Fig. 4(f). The pump powers are all higher than those of the IMR. The comparison between Fig. 4(a–c) and Fig. 4(d–f) indicates that the IMR is still more efficient than the EFR at 120 °C, for the new scheme needs lower pump powers to reach the same performances, though the transmittance of the IMR is lower than that of the EFR at 18 mW pump.
When the working temperature reaches 130 °C, the difference between the IMR and EFR is not distinct. The pump lasers in both methods are almost completely absorbed by the rubidium cell in the filter. But the absorption lengths of the pump lasers are different between the IMR and EFR. By a rough measurement, at 130 °C and 9 mW pump power, the EFR shows a 45 mm spontaneous emission fluorescence line almost all along the cell. While using the IMR, the fluorescence disappears in a distance of about 25 mm. It means that, to get the same transmittance under same temperature and pump power, the IMR requires much shorter cell length than the EFR, which is important for applications with volume limitations.
In conclusion, we demonstrate an ES-FADOF working at the optical communication wavelength of 1.5 μm. An internal pump method is adopted which uses the ES-FADOF transition line itself. Experiment for comparing with the external ES-FADOF shows that this method can increase the transmittance from 10% to 60% at 100 °C. It also allows the filter working on lower pump power or shorter cell size when the working temperature increases. All of the improvements benefit from the higher pump efficiency of the new solution. Moreover, the whole system can be more compact without the external frequency reference and the volume can be reduced. This method can be conveniently extended to all kinds of ES-FADOFs.
This work is supported by the National Science Fund for Distinguished Young Scholars of China (Grant No. 61225003), National Natural Science Foundation of China (Grant No. 61101081), and the National Hi-Tech Research and Development (863) Program.
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