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arXiv:1805.07387 · High-Power Diode Laser Technology

Auto-locking waveguide amplifier system for lidar and magnetometric applications

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

Published Feb 19, 2018• 11 citations
arXivHTMLPDF

Abstract

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.

DOI
10.1117/12.2286952

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Review of Scientific Instruments•May 1, 2009•7 citations
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.
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.
A robust, compact, highly accurate rubidium optical frequency standard module was developed to overcome the delicate performance of conventional frequency stabilized lasers. A frequency doubled 1560 nm distributed feedback diode laser locked to a rubidium D(2) saturated absorption line without using an optical amplifier was demonstrated, and dithering-free optical output was obtained. In addition, the sensitivity of the developed optical frequency standard to magnetic fields was investigated. We confirmed that the influence of the magnetic fields on the optical frequency standard can be almost negligible when using appropriate magnetic-shield films. As a result, the magnetic-field-insensitive optical frequency standard, which can be embedded in optical systems, exhibiting uncertainty less than at least 100 kHz, was successfully realized for the first time to the best of our knowledge.
A scalar magnetic field sensor based on a millimeter-size Rb87 vapor cell is described. The magnetometer uses nearly copropagating pump and probe laser beams, amplitude modulation of the pump beam, and detection through monitoring the polarization rotation of the detuned probe beam. The circularly polarized pump laser resonantly drives a spin precession in the alkali atoms at the Larmor frequency. A modulation signal on the probe laser polarization is detected with a lock-in amplifier. Since the Larmor precession is driven all-optically, potential cross talk between sensors is minimized. And since the pump light is turned off during most of the precession cycle, large offsets of the resonance, typically present in a single-beam Bell–Bloom scheme, are avoided. At the same time, relatively high sensitivities can be reached even in millimeter-size vapor cells: The magnetometer achieves a sensitivity of 1 pT/Hz1/2 in a sensitive volume of 16 mm3, limited by environmental noise. When a gradiometer configuration is used to cancel the environmental noise, the magnetometer sensitivity reaches 300 fT/Hz1/2. We systematically study the dependence of the magnetometer performance on the optical duty cycles of the pump light and find that better performance is achieved with shorter duty cycles, with the highest values measured at 1.25% duty cycle.
We describe an inexpensive lock-in amplifier that can be built using discrete off-the-shelf RF components and home-built analog circuits. This lock-in has been used in a feedback loop to lock the frequency of a laser to an atomic transition. The frequency stability was tested by trapping rubidium atoms. The feedback loop involves obtaining a modulated saturated absorption signal from a vapor cell using an acousto-optic modulator to modulate the laser frequency. The absorption signal is sent to the lock-in to generate an error signal, proportional to the first derivative of the absorption, which is fed back to the laser to complete the feedback loop. We also demonstrate a simplified optical setup for viewing the saturated absorption spectrum. In this case, the signal consists of narrow saturated absorption spectra riding on top of a Doppler-broadened pedestal. We show that it is possible to greatly reduce the effect of this background and lock the laser to the atomic resonance by modifying the home-built lock-in to generate an error signal proportional to the third derivative of the absorption. The results of this work can be adapted for advanced undergraduate laboratory work.PACS Nos.: 01.50Pa, 07.50.–e, 42.62Fi, 42.60Fc
We use a 87Rb^{87}\text{Rb}87Rb 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 80fT/Hz80\text{fT}/\sqrt{\text{Hz}}80fT/Hz​ at 600Hz600\text{Hz}600Hz and 200fT200fT/Hz200\text{fT}/\sqrt{\text{Hz}}200fT/Hz​ at 4kHz4\text{kHz}4kHz, one to two orders of magnitude beyond what was achievable with the other pumping methods.