Magnetometer based on the opto-electronic oscillator
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Magnetometer based on the opto-electronic oscillator Andrey B. Matsko, Dmitry Strekalov, and Lute Maleki Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, California 91109-8099
ABSTRACT We discuss properties of an all-optical self-oscillating magnetometer based on an opto-electronic oscillator stabilized with an atomic vapor cell. Proof of the principle DC magnetic field measurements characterized with 2 × 10−7 G sensitivity and 1 − 1000 mG dynamic range in one of the schemes are demonstrated. INTRODUCTION An Opto-Electronic Oscillator (OEO) is a photonic device that produces microwaves characterized with high spectral purity.1 We propose to create a miniature active magnetometer (Fig. 1) using an atomic vapor cell stabilized OEO. The magnetometer is intended to generate a stable narrowband microwave signal with frequency proportional to the external magnetic field. In a generic OEO a laser beam sent consecutively to an amplitude modulator and a fast photodiode produces the electrical current that includes a component at the frequency, generally microwave, of the modulation. The electrical output of the photodetector is amplified, filtered, and fed back to the microwave input port of the modulator. The system is self-oscillating if the laser power and microwave amplification are of sufficient magnitude.
Figure 1. Two possible schematics of OEO magnetometers. (a) Continuous wave (cw) laser light is detected with a photodiode after passing an amplitude modulator (EOM) and atomic vapor cell. The microwave signal is amplified and fed back into the modulator. The solid and dashed lines stand for light and microwaves (mw) respectively. (b) The cw laser and modulator are replaced with VCSEL.
An OEO becomes a magnetometer if an atomic vapor cell with proper chemical content is inserted on the path of the modulated light. The cell transmits sidebands of modulated light only when the modulation frequency coincides with a half of the eigenfrequency of the
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magnetic-field-sensitive hyperfine transition of the atoms. In other words, the atomic cell operates as a pass-band microwave filter. The OEO signal locks to the frequency of the filter. We here discuss two schemes of OEO magnetometers. The schemes are different in their optical parts. One scheme containing an electro-optical modulator and a CW monochromatic laser (Fig. 1a) was partially studied very recently,2 while the other, based on a directly modulated vertical cavity surface emitting laser (VCSEL) (Fig. 1b) is a novel one. Our self-oscillating scheme is conceptually different from the external oscillator-based magnetometer scheme.3 We calculate the phase noise, stability, and estimate performance of our OEO magnetometers. Finally, we include the results of the proof of the principle measurements of the magnetic field with the devices. The devices are self-oscillating and, obviously, differ from the passive electromagnetically induced transparency (EIT) magnetometers. They are also differen
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