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IEEE photonics journal

Magnetic Field Measurement Using Peak-Locked Zeeman Coherent Population Trapping Resonance in Rubidium Vapor

Renu Tripathi, G. S. Pati

Published Jun 13, 2019• 8 citations
Publisher

Abstract

A sensor for magnetic field measurement is developed using a small (centimeter size) rubidium cell. A high contrast Zeeman coherent population trapping (CPT) resonance is produced by the magnetic dither in the sensor solenoid, and a peak-locked servo is implemented for magnetic field measurement. Sensitivity close to 110 pT/ √Hz has been achieved in axial field measurement. The CPT-based magnetic sensor is capable of measuring sub-nanotesla level magnetic field variations at high (microtesla) field strength. We discussed possible sources of noise and broadening which are affecting the performance of the sensor, and suggest changes that will further improve the performance of the sensor.

DOI
10.1109/jphot.2019.2922831

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A sensor for magnetic field measurement is developed using a small (centimeter size) rubidium cell. A high contrast Zeeman coherent population trapping (CPT) resonance is produced by the magnetic dither in the sensor solenoid, and a peak-locked servo is implemented for magnetic field measurement. Sensitivity close to 110 pT/ √Hz has been achieved in axial field measurement. The CPT-based magnetic sensor is capable of measuring sub-nanotesla level magnetic field variations at high (microtesla) field strength. We discussed possible sources of noise and broadening which are affecting the performance of the sensor, and suggest changes that will further improve the performance of the sensor.
We have developed a field-deployable vector magnetometer using a small rubidium vapor cell, which is capable of measuring the magnitude and direction of the earth's magnetic field with high accuracy and sensitivity. The magnetometer measures Larmor precession of rubidium atoms using the phenomenon, called synchronous coherent population trapping (SCPT). This gives the advantage of producing a high contrast and narrow linewidth resonance needed for measuring the magnetic field with high accuracy and sensitivity. The magnetometer exhibits sub-nT/√Hz sensitivities in measuring the field components along the three directions.
We demonstrate a real-time, single-shot vector magnetic measurement technique using synchronous coherent population trapping (SCPT) in an apparatus consisting of a small rubidium vapor cell. Vector modality of our magnetometer is enabled by designing a feedback system based on performing a peak-lock on a particularly strong 2ΩL magnetic resonance produced in the SCPT spectrum and compensating the external magnetic field via a three-axis field coil. With its current design, this magnetometer exhibits high sensitivities of approximately 155, 129, and 57pT/Hz in measuring the magnetic field vector components along the (x,y,z) axes. Sensitivities closer to the shot-noise limit can be achieved in the future by reducing the technical noise of our equipment and by employing a differential detection and polarization rotation measurement scheme in our system.
We have set up a coherent population trapping (CPT)-based magnetometer prototype with the D1 line of ⁸⁷Rb atoms. The dichromatic light field is derived from a fiber electro-optic modulator (FEOM) connected to an external cavity laser diode. A CPT resonance signal with a 516 Hz linewidth is observed. By feeding back the derivative of the resonance curve to the FEOM with a proportional integral controller, of which the voltage output is directly converted to the measured magnetic field intensity, the resonance peak is locked to the environmental magnetic field. The measurement data we have achieved are well matched with the data measured by a commercial fluxgate magnetometer within 2 nT, and the sensitivity is better than 8 pT/√Hz in a parallel B field.
Vector magnetic field measurement is required in several applications including magnetic surveys, navigation and anomalies detection. We have designed and developed a vector magnetometer prototype that can be deployed in the field to survey earth’s magnetic field. Vector operation is enabled in this prototype by using the phenomenon of synchronous coherent population trapping (SCPT), and combining it with a feedback compensation mechanism. The prototype utilizes a small rubidium cell which is placed inside a three-axis field coil. Magnetic field along each axis is measured by performing a peak-lock on one of the strongest resonances produced by SCPT and compensating the external magnetic field via servo control of the three-axis field coil. Under this locked condition, the feedback current applied to each coil in the three-axis system gives a real-time, single-shot measurement of the magnetic field components. Extensive testing of the prototype is being done by performing magnetic field measurements in the magnetically noisy environment inside the laboratory and in-comparison with that of a fluxgate sensor.
An automated magnetometer suitable for long lasting measurement under stable and controllable experimental conditions has been implemented. The device is based on coherent population trapping (CPT) produced by a multifrequency excitation. CPT resonance is observed when a frequency comb, generated by diode laser current modulation, excites Cs atoms confined in a π/4×(2.5)2×1 cm3, 2 TorrN2 buffered cell. A fully optical sensor is connected through an optical fiber to the laser head allowing for truly remote sensing and minimization of the field perturbation. A detailed analysis of the CPT resonance parameters as a function of the optical detuning has been made in order to get high sensitivity measurements. The magnetic field monitoring performances and the best sensitivity obtained in a balanced differential configuration of the sensor are presented.
An atomic magnetometer based on coherent population trapping (CPT) resonances in microfabricated vapor cells is demonstrated. Fabricated by the micro-electro-mechanical-system (MEMS) technology, the cells are filled with Rb and Ne at a controlled pressure. An experimental apparatus is built for characterizing properties of microfabricated vapor cells via the CPT effects. The typical CPT linewidth is measured to be about 3 kHz (1.46 kHz with approximately zero laser intensity) for the rubidium D1 line at about 90 °C. The effects of pressure, temperature and laser intensity on CPT linewidth are studied experimentally. A closed-loop atomic magnetometer is finally finished with a sensitivity of 210.5 pT/Hz1/2 at 1 Hz bandwidth. This work paves the way for developing an integrated chip-scale atomic magnetometer in the future.