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arXiv:1805.07391 · Laser Technology for Defense and Security

Investigations of optical pumping for magnetometry using an auto-locking laser system

Alexander Pouliot, Hermina C. Beica, Adam C. Carew, B. Barrett, Andrew Vorozcovs, Gehrig Carlse, Anantharaman Kumarakrishnan

Published May 4, 2018• 7 citations

Abstract

We have developed a versatile pulsed laser system for high precision magnetometry. The operating wavelength of the system can be configured to optically pump alkali vapors such as rubidium and cesium. The laser system consists of an auto-locked, interference filter stabilized, external cavity diode laser (ECDL), a tapered waveguide amplifier, and a pulsing module. The auto-locking controller can be used by an untrained operator to stabilize the laser frequency with respect to a library of atomic, molecular, and solid-state spectral markers. The ECDL output can be amplified from 20 mW to 2 W in continuous wave (CW) mode. The pulsing module, which includes an acousto-optic modulator (AOM), can generate pulses with durations of 20 ns and repetition rates of several MHz. Accordingly, the laser system is well suited for applications such as gravimetry, magnetometry, and differential absorption lidar. In this work, we focus on magnetometric applications and demonstrate that the laser source is suitable for optically pumping rubidium vapor. We also describe numerical simulations of optical pumping relevant to the rubidium D1 and D2 transitions at 795 nm and 780 nm respectively. These studies are relevant to the design and construction of a new generation of portable, rubidium, spin exchange relaxation-free (SERF) magnetometers, capable of sensitivities of 1 fT Hz-1/2 1.

DOI: 10.1117/12.2304598

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We have developed a versatile pulsed laser system for high precision magnetometry. The operating wavelength of the system can be configured to optically pump alkali vapors such as rubidium and cesium. The laser system consists of an auto-locked, interference filter stabilized, external cavity diode laser (ECDL), a tapered waveguide amplifier, and a pulsing module. The auto-locking controller can be used by an untrained operator to stabilize the laser frequency with respect to a library of atomic, molecular, and solid-state spectral markers. The ECDL output can be amplified from 20 mW to 2 W in continuous wave (CW) mode. The pulsing module, which includes an acousto-optic modulator (AOM), can generate pulses with durations of 20 ns and repetition rates of several MHz. Accordingly, the laser system is well suited for applications such as gravimetry, magnetometry, and differential absorption lidar. In this work, we focus on magnetometric applications and demonstrate that the laser source is suitable for optically pumping rubidium vapor. We also describe numerical simulations of optical pumping relevant to the rubidium D1 and D2 transitions at 795 nm and 780 nm respectively. These studies are relevant to the design and construction of a new generation of portable, rubidium, spin exchange relaxation-free (SERF) magnetometers, capable of sensitivities of 1 fT Hz-1/2 1.

We describe a compact waveguide amplifier system that is suitable for optically pumping rubidium magnetometers. The system consists of an auto-locking vacuum-sealed external cavity diode laser, a semiconductor tapered amplifier and a pulsing unit based on an acousto-optic modulator. The diode laser utilises optical feedback from an interference filter to narrow the linewidth of an inexpensive laser diode to ~500 kHz. This output is scannable over an 8 GHz range (at 780 nm) and can be locked without human intervention to any spectral marker in an expandable library of reference spectra, using the autolocking controller. The tapered amplifier amplifies the output from 50 mW up to 2 W with negligible distortions in the spectral quality. The system can operate at visible and near infrared wavelengths with MHz repetition rates. We demonstrate optical pumping of rubidium vapour with this system for magnetometric applications. The magnetometer detects the differential absorption of two orthogonally polarized components of a linearly polarized probe laser following optical pumping by a circularly polarized pump laser. The differential absorption signal is studied for a range of pulse lengths, pulse amplitudes and DC magnetic fields. Our results suggest that this laser system is suitable for optically pumping spin-exchange free magnetometers.

The atomic magnetometer exhibits high sensitivity in weak magnetic field detection, with diverse applications in dark matter detection, biomagnetic sensing, wireless communication, etc. However, its performance, especially in environmental robustness and sensitivity, is largely limited by laser characteristics. Here, we realized two potassium Faraday lasers operating at potassium D1 (770 nm) and D2 (767 nm) lines, serving as pump and probe lasers for a potassium atomic magnetometer. Utilizing the Faraday anomalous dispersive effect for frequency selection, the lasers achieve automatic wavelength locking within the atomic Doppler broadening, enabling robust, self-stabilizing operation without tedious adjustments. Furthermore, both lasers exhibit strong immunity to environmental disturbances, such as vibration and temperature variations. By implementing the modulation transfer spectroscopy (MTS) technique, the frequency stability of the pump laser and probe laser reaches approximately 5.1 × 10 − 13 / τ and 6.2 × 10 − 13 / τ , respectively. Notably, the optimized frequency stability is accompanied by better power stability, showing threefold and fivefold improvements over their free-running counterparts, which reaches the 10 − 5 level at 1 s. The enhancements in laser performance directly improved the magnetometer’s performance, yielding a magnetic noise floor of approximately 100 fT / Hz at 10 Hz under a bias field of 1000 nT. Therefore, with hands-off operation, environmental robustness and competitive sensitivity, this achievement holds promising prospects for both laboratory and field-deployable applications.

Spin-exchange relaxation free alkali-metal magnetometers typically operate in the regime of high optical density, presenting challenges for simple and efficient optical pumping and detection. We describe a high-sensitivity Rb magnetometer using a single elliptically polarized off-resonant laser beam. Circular component of the light creates relatively uniform spin polarization while the linear component is used to measure optical rotation generated by the atoms. Modulation of the atomic spin direction with an oscillating magnetic field shifts the detected signal to high frequencies. Using a fiber-coupled distributed feedback laser, we achieve magnetic field sensitivity of

7\text{ }\text{fT}/\sqrt{\text{Hz}}

with a miniature $5\ifmmode\times\else\texttimes\fi{}5\ifmmode\times\else\texttimes\fi{}5\text{ }\text{mm}\text{ }\text{Rb}$ vapor cell.

We describe our approach to atomic magnetometry based on the push-pull optical pumping technique. Cesium vapor is pumped and probed by a resonant laser beam whose circular polarization is modulated synchronously with the spin evolution dynamics induced by a static magnetic field. The magnetometer is operated in a phase-locked loop, and it has an intrinsic sensitivity below 20fT/Hz, using a room temperature paraffin-coated cell. We use the magnetometer to monitor magnetic field fluctuations with a sensitivity of 300fT/Hz.

We consider the use of a Cr3+:BeAl2O4 laser in free-running operating as a source of emission for optical pumping rubidium alkali metal vapors. The use of dispersive elements in the composition of the laser cavity makes it possible to smoothly tune lasing wavelength and to realize generation at wavelengths corresponding to the D1 and D2 lines of the 85Rb and 87Rb isotopes. Optical pumping of rubidium isotopes by laser emission with wavelengths of 795 and 780 nm, respectively, is experimentally implemented, and their fluorescence is demonstrated. The question of using a wavelength-tunable laser in the method of spin-exchange optical pumping of noble gases is discussed.

We use a

^{87}\text{Rb}

atomic vapor, suitable for an optically-pumped magnetometer (OPM) in Earth-field conditions, to study the noise properties of three strategies for generating pulsed optical pumping. We compare a frequency-modulated (FM) laser, amplitude modulation (AM) via an acousto-optic modulator (AOM), and amplitude modulation via a semiconductor optical amplifier (SOA). Pumping the ensemble to operate as a Bell-Bloom OPM, and with an equal degree of spin polarization, the three methods give nearly identical sensitivity, showing that the SOA, despite being an active device, can introduce negligible additional noise. Pumping the ensemble to operate as a free-induction-decay OPM, we observe longer unpumped coherence times with the SOA-AM method than with the FM method. Finally, using the higher power available from the SOA, we demonstrate an environment-limited sensitivity of

80\text{fT}/\sqrt{\text{Hz}}

at

600\text{Hz}

and 200fT

200\text{fT}/\sqrt{\text{Hz}}

at

4\text{kHz}

, one to two orders of magnitude beyond what was achievable with the other pumping methods.