What is a FADOF?

Generally speaking, a Faraday anomalous dispersion optical filter (FADOF) consists of an atomic vapor cell placed between two crossed polarizers. An homogenous, constant magnetic field of the strength B is applied parallel to the optical axis of the gas cell. Two effects govern the transmission of such a filter: the absorption of the atomic vapor and the polarization rotation of the incident light. The polarization rotation originates from the Zeeman splitting of the atomic transitions, which causes a difference in index of refraction for right and left circular polarized light. The latter effect is similar to the regular Faraday effect, but is much enhanced in the vicinity of absorption lines(1). The degree of rotation is strongly wavelength dependent and thus the crossed polarizers in combination with the gas cell act as sensitive transmission filters(2).

FADOF systems have been considered in the literature mostly as daylight suppression filters for use in communication(3). Since off resonant light is blocked by crossed polarizers FADOFs typically show passbands of a few GHz only and a high out-of-band suppression(3). Other advantages of FADOF systems include a large field of view and their relative insensitivity to vibrations. A possible disadvantage is the restriction to the vicinity of atomic transitions. However, FADOF systems of excited states have been implemented(4,5). These works are of particular interests as some of the alkali have excited state transitions within the range of frequency doubled Nd:YLF or Nd:YAG lasers.

In remote sensing applications such as our Opens internal link in current windowBrillouin project often small frequency shifts must be detected. An established method for this task is the use of edge filters5. The basic idea is to provide steep transmission edges of the filter in the regions of interest. Small shifts in the frequency of the signal will thus yield a large change in transmission. After calibration of the system, the determination of the frequency shift is therefore transfered to an intensity measurement.

Currently we are evaluating the potential of FADOF systems as frequency analyzing tools for our Opens internal link in current windowBrillouin project. By setting the temperature of the gas cell and appropriately tuning the magnetic field FADOF systems are capable of producing steep transition edges at the desired frequency separation. The first step was to set up a FADOF experiment, work out the theory and compare the experiment with the theoretical prediction. The agreement between theory and experiment is excellent. Therefore, we are now working on implementing an excited state FADOF (ESFADOF) system in order to evaluate the FADOF's potential as an edge filter in the 532-nm wavelength range.

Experimental Setup of a ground state FADOF based on Rb.
Comparison of the experimental data to the theoretical fit of the data.


  1. D. Macaluso and O. Corbino, C.R. Acad. Sci. 127, 548 (1898).
  2. Y. Ohman, Stockholm Obs. Ann. 19, 3 (1956).
  3. D. Dick and T. Shay, Opt. Lett. 16, 867 (1991).
  4. T. Shay, in Proceedings of the IEEE Lasers and Electro-Optical Society's Annual Meeting (1993), pp. 359-360.
  5. R. Billmers, S. Gayen, M. Squicciarini, V. Contarino, W. Scharpf, and D. Alloca, Opt. Lett. 20, 106 (1995).
  6. C. Korb, B. Gentry, and C. Weng, Appl. Opt. 31, 4202 (1992).


Prof. Dr. Thomas Walther

Laser und Quantenoptik
Institut für Angewandte Physik
Fachbereich 05 - Physik
Technische Universität Darmstadt
Schlossgartenstr. 7
D-64289 Darmstadt

+49 6151 16-20831 (Sekretariat)

+49 6151 16-20834 (Fax)




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