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arXiv:0904.4142 · Optics Letters

Faraday dichroic beam splitter for Raman light using an isotopically pure alkali-metal-vapor cell

R. P. Abel, Ulrich Krohn, Paul Siddons, Ifan G. Hughes, Charles S. Adams

Published Oct 3, 2009• 45 citations
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Abstract

We present an application of the Faraday effect to produce a narrowband dichroic beam splitter using an alkali metal vapor. Two Raman beams detuned in frequency by the ground-state hyperfine splitting in (87)Rb are produced using an electro-optic modulator and then separated using the Faraday effect in an isotopically pure (85)Rb thermal vapor. An experimental transmission spectra for the beam splitter is presented along with a theoretical calculation. The performance of the beam splitter is then demonstrated and characterized using a Fabry-Perot etalon. For a temperature of 70 degrees C and a longitudinal magnetic field of 80 G, a suppression of one frequency of 18 dB is achieved, limited by the quality of the polarizers.

DOI
10.1364/ol.34.003071

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We present an application of the Faraday effect to produce a narrowband dichroic beam splitter using an alkali metal vapor. Two Raman beams detuned in frequency by the ground-state hyperfine splitting in (87)Rb are produced using an electro-optic modulator and then separated using the Faraday effect in an isotopically pure (85)Rb thermal vapor. An experimental transmission spectra for the beam splitter is presented along with a theoretical calculation. The performance of the beam splitter is then demonstrated and characterized using a Fabry-Perot etalon. For a temperature of 70 degrees C and a longitudinal magnetic field of 80 G, a suppression of one frequency of 18 dB is achieved, limited by the quality of the polarizers.
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Results of a numerical study of the Faraday effect arising upon propagation of the light beams with the frequencies ω L1 (resonant to the nS 1/2-nP 1/2, 3/2 transitions) and ω L2 (resonant to the nP 1/2, 3/2-(n+2)S 1/2 transitions) through alkali-metal vapors are presented. Characteristics of the magneto-optical rotation spectra at each of the frequencies are strongly affected by the second intense radiation field resonant to the adjacent transition. When the atoms interact with two strong light waves, resonant to adjacent transitions, and with a magnetic field, the shape of the Faraday rotation spectra depends on the energy shifts of the atomic states that arise due to the dynamic Stark effect and the Zeeman effect (the Paschen-Back or an intermediate-type effect), as well as due to the difference of populations of these states caused by the interaction of the atoms with the fields. The results obtained show that in the frequency selection method, based on the resonance Faraday effect, the frequency of the generated narrow-band beam can be tuned by the intensity of the strong wave, resonant to the transition between the excited states.
This thesis presents both a computational and experimental investigation into light propagation in hot alkali-metal vapours, with a particular focus on utilizing the Faraday effect for practical applications. A model to calculate various spectra for a weak-probe laser beam in an atomic medium with an applied axial magnetic field is presented. A computer program (ElecSus) was developed which implements this model efficiently. Using ElecSus we design optical devices such as Faraday filters and laser frequency stabilizing references. The design of Faraday filters utilizing compact vapour cells is shown, along with excellent agreement with experiment. The importance of including the effect of self broadening in the model is shown for these short path length vapour cells. Also, a Faraday filter is presented that can potentially be used for quantum optics experiments on the caesium D1_11​ line (894~nm). The filter displays the highest ratio of transmission to equivalent noise bandwidth to date for a linear Faraday filter, demonstrating the power of computerized optimization for this application. Furthermore, a Faraday filter is presented for use as an intra-cavity element in an external-cavity diode laser. A proof-of-principle experiment is demonstrated which shows that using a short external cavity with the Faraday filter eliminates mode-hops. Experimentally and theoretically the Faraday effect is investigated in large magnetic fields where alkali-metal atoms enter the hyperfine Paschen-Back regime. This hyperfine Paschen-Back Faraday effect is shown to allow a direct measure of the refractive indices for left and right circular polarized light. Furthermore, fitting the weak-probe spectra using ElecSus is found to give measures of the magnetic field with a fractional precision of the order of 10−410^{-4}10−4. In addition we study slow-light pulse propagation in a high density rubidium vapour, showing that our theoretical model for the electric susceptibility is valid for short pulses as well as continuous-wave light. This shows that the model is accurate for predicting weak-probe pulse propagation.
We propose a far off-resonance laser frequency stabilization method by using multipass cells in Rb Faraday rotation spectroscopy. Based on the detuning equation, if multipass cells with several meters optical path length are used in the conventional Faraday spectroscopy, the detuning of the lock point can be extended much further from the alkali metal resonance. A plate beam splitter was used to generate two different Faraday signals at the same time. The transmitted optical path length was L=50 mm and the reflected optical path length was 2L=100 mm. When the optical path length doubled, the detuning of the lock points moved further away from the atomic resonance. The temperature dependence of the detuning of the lock point was also analyzed. A temperature-insensitive lock point was found near resonance when the cell temperature was between 110°C and 130°C. We achieved an rms fluctuation of 0.9 MHz/23 h at a detuning of 0.5 GHz. A frequency drift of 16 MHz/h at a detuning of -5.6 GHz and 4 MHz/h at a detuning of -5.2 GHz were also obtained for the transmitted and reflected light Faraday signal.
We investigate experimentally and theoretically the Faraday effect in an atomic medium in the hyperfine Paschen–Back regime, where the Zeeman interaction is larger than the hyperfine splitting. We use a small permanent magnet and a micro-fabricated vapour cell, giving magnetic fields of the order of a tesla. We show that for low absorption and small rotation angles, the refractive index is well approximated by the Faraday rotation signal, giving a simple way to measure the atomic refractive index. Fitting to the atomic spectra, we achieve magnetic field sensitivity at the 10 −4 level. Finally we note that the Faraday signal shows zero crossings which can be used as temperature insensitive error signals for laser frequency stabilization at large detuning. The theoretical sensitivity for 87 Rb is found to be ∼40 kHz °C −1 .