A multi-pass system can be used as a technique for effectively reducing the photon shot noise contribution in certain measurements. This is because increasing the optical path length increases the light-matter interaction in a medium, which reduces the effective contribution of photon shot noise and enhances the signal to noise ratio (SNR). The work presented here would describe a ray tracing Python algorithm that uses rudimentary ABCD matrix formalism and matrix eigen decomposition to calculate the position of the beam as it travels between mirror pairs. The code accepts arbitrary combinations of mirrors with varying focal lengths, distance apart, and rotation. Evolution of Gaussian beam parameters can also be calculated as the beam propagates through the multi-pass system. Beyond that, errors in both tilt and translations of the mirrors can be introduced to calculate the tolerance for misalignment in any multi-pass optical design. This serves as a great alignment tool when experimentally aligning the mirrors of a multi-pass system. Beyond applications in magnetometry, multi-pass systems can also be used for gas detection through spectroscopy and laser amplification for short-pulsed lasers.
A multi-pass system can be used as a technique for effectively reducing the photon shot noise contribution in certain measurements. This is because increasing the optical path length increases the light-matter interaction in a medium, which reduces the effective contribution of photon shot noise and enhances the signal to noise ratio (SNR). The work presented here would describe a ray tracing Python algorithm that uses rudimentary ABCD matrix formalism and matrix eigen decomposition to calculate the position of the beam as it travels between mirror pairs. The code accepts arbitrary combinations of mirrors with varying focal lengths, distance apart, and rotation. Evolution of Gaussian beam parameters can also be calculated as the beam propagates through the multi-pass system. Beyond that, errors in both tilt and translations of the mirrors can be introduced to calculate the tolerance for misalignment in any multi-pass optical design. This serves as a great alignment tool when experimentally aligning the mirrors of a multi-pass system. Beyond applications in magnetometry, multi-pass systems can also be used for gas detection through spectroscopy and laser amplification for short-pulsed lasers.
• Recirculating multipass cell enables improved beam coverage and active volume. • Analytical model predicts beam distribution and astigmatism effects. • General framework developed for spin noise in multipass alkali cells. • Beam geometry shown to influence spin diffusion noise. • Recirculating design demonstrates reduced spin noise under optimized conditions. Multipass cells enable long optical path lengths in compact volumes and are central to quantum technologies such as atomic magnetometers and optical quantum memories. In optical magnetometry, multipass geometries enhance sensitivity by increasing optical depth, reducing photon shot noise, and enabling quantum non-demolition detection. However, in conventional cylindrical multipass cells, Lissajous beam trajectories lead to repeated revisiting and incomplete mirror coverage, limiting effective volume utilization. Here we present a recirculating multipass alkali cell that overcomes these limitations by increasing the active-to-cell volume ratio and minimizing beam spot overlap. We develop an analytical ABCD-matrix model to predict beam trajectories, spot sizes, and astigmatism, validated by Zemax simulations. We further introduce a general analytical framework for spin correlation noise that incorporates astigmatism and spatial intensity distributions. By deriving the spin-noise time-correlation function and spectrum, we show how beam intensity profiles influence spin diffusion noise. Our results demonstrate improved beam coverage, reduced spot overlap, and enhanced spin correlation, particularly for concave mirrors with long focal lengths, while showing that avoiding tightly-focused regions significantly suppresses spin diffusion noise. These findings establish recirculating multipass cells as a practical, high-performance platform for precision atomic sensing and other multipass-cavity-based quantum devices.
Atomic magnetometry has spectacular magnetic field sensitivity at room temperature. Here, we theoretically and experimentally investigate the benefits of a multi-pass cell in magnetometers using nonlinear magneto-optical rotation interrogation. Our theoretical analysis shows that there is an improvement in the signal-to-noise ratio (SNR) and consequently on the magnetic field sensitivity by carefully choosing the number of passes through the medium. In our specific case, we experimentally demonstrate a 160% enhancement in the magnetometer sensitivity by using a triple-pass cell, and it is consistent with our analysis on the SNR. This work provides a pathway to evaluate the benefits of multi-pass cells in high-performance atomic magnetometers.
Abstract We propose a signal- and sensitivity-enhanced spin-exchange-relaxation-free atomic magnetometer utilizing a multi-pass cell with a highly spatially homogeneous optical pumping scheme. The relationship between the magnetometer’s performance and the number of probe beam passes is measured. The optical rotation angle exhibits a linear correlation with the number of passes, achieving 89 mrad nT−1 for a quadruple-pass cell. However, the sensitivity does not increase linearly when further increasing the optical length due to the noise limit. This work provides a method to study the enhancement performance of the magnetic response and sensitivity in atomic magnetometers by using multi-pass cells.
Multi-passed probe light can enhance the signal of atomic magnetometers by increasing the effective optical path length between the alkali metal atomic ensemble and the probe light, thereby strengthening the sensitivity. In recent years, multi-passed probe light has become an important method for improving the response signal and sensitivity of atomic magnetometers. To achieve biaxial magnetic field measurement while maintaining high sensitivity, a biaxial measurement spin-exchange relaxation-free (SERF) atomic magnetometer with a multi-passed probe light is proposed in this paper. The theoretical model and experimental advantages of using multi-passed probe light in longitudinal magnetic field modulation (Z-mode) SERF atomic magnetometers are analyzed. In this paper, a new model of output response of the Z-mode SERF atomic magnetometer based on multi-passed probe light is established, a compact reflective optical path is designed to increase the optical path between the probe light and the polarized alkali metal atomic ensemble, enhance the signal response, and achieve the suppression of low-frequency 1/f noise through longitudinal magnetic field modulation. Experimental results confirmed that the magnetic field sensitivity in the x- and y-axis of the multi-passed probe light configuration improved by 29% and 18%, respectively, compared to the single-pass configuration.
SERF atomic co-magnetometer offers high sensitivity and navigation-grade potential. However, the performance of traditional single-pass detection methods is limited due to insufficient Faraday rotation signals. To address this issue, this paper proposes a vertical-reflection multi-pass detection method. By introducing hollow roof mirrors and half-wave plates, the optical interaction path is significantly extended while ensuring strict orthogonality between the pump and probe beams. This compact square shaped optical path design effectively amplifies the Faraday rotation effect, suppresses cross-axis coupling, and significantly improves the signal-to-noise ratio and measurement sensitivity. Through theoretical modeling analysis, this approach enhances the rotation signal while extending the optical path, thereby improving the system response. Experimental results demonstrate that the multi-pass structure improves the signal-to-noise ratio by several times compared to single-pass detection, and the design offers excellent integration potential, providing a feasible technical path for high-performance SERF quantum sensors for navigation and high-precision physical research
In this Letter, we propose a single-beam nonlinear magneto-optical rotation (NMOR) magnetometer with a multipass 4 He gas-discharged cell. In contrast to the single-pass cell, the multipass cell allowed laser beams to pass through the metastable-state atomic ensemble 22 times, which directly increases the optical path length and significantly enhances magneto-optical rotation in the 4 He gas sample. Based on nonlinear Faraday rotation, the 4 He magnetometer with the multipass cell demonstrates a noise floor of 9 fT/Hz 1/2 , which approaches the photon-shot noise floor limit of 6.4 fT/Hz 1/2 . In addition, the wider linewidth in metastable-state atoms realizes an NMOR 4 He magnetometer with a 3 dB bandwidth of 4.3 kHz, in contrast to the ultranarrow linewidth in the antirelaxation-coated cells or spin-exchange relaxation-free regime alkali-metal cells with buffer gas. Since the 4 He cell functions without heating or cryogenic cooling, the femtotesla sensitivity and kilohertz-bandwidth 4 He magnetometer exhibits potential in biomagnetic applications such as magnetocardiography and magnetoencephalography.