Application of VCSEL in Bio-Sensing Atomic Magnetometers: Comparison
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There is a rapid development of chip-scale atomic devices due to their great potential in the field of biomedical imaging, namely chip-scale atomic magnetometers that enable high resolution magnetocardiography (MCG) and magnetoencephalography (MEG). For atomic devices of this kind, vertical cavity surface emitting lasers (VCSELs) have become the most crucial components as integrated pumping sources, which are attracting growing interest.

  • VCSEL
  • chip-scale atomic magnetometers
  • magnetoencephalography

1. Introduction

Nowadays, magnetocardiography (MCG) is attracting growing interest due to outstanding performance on screening and diagnosis of heart disease. Compared with the traditional electrocardiogram (ECG), multi-channel magnetocardiographic mapping (MMCG) is a faster and contactless method for 3D imaging and localization of cardiac electrophysiologic phenomena with higher spatial and temporal resolution [1]. In addition to MCG, magnetoencephalography (MEG) emerges as an important approach for the characterization of functionality and effective connectivity of the brain and has already been applied for the research of cognitive science [2,3][2][3]. In conventional practice, the superconducting quantum interference device (SQUID) is considered as a feasible approach of neuromagnetic fields, namely brain and heart [4]. However, SQUID systems is expensive and cumbersome as it requires liquid Helium circulation for temperature conditioning, which has made it challenging to be applied to common research and clinical sites [5]. On the other hand, the emerging atomic magnetometer has the same level of sensitivity with much lower cost and volume, which has long been considered as a feasible substitute of SQUID. Driven by the urgent demand, the atomic magnetometer (AM) has experienced dramatical development in the past two decades accompanied by the development of nano fabrication technologies. Researchers nowadays are able to utilize AM as an alternative (of SQUID) for clinical applications. One of the major trends for AM is the volume reduction that facilitates the integration of multiple sensors in a small area in pursuit of high imaging resolution—especially chip-scale atomic magnetometers that enable integration of sensor arrays to obtain three-dimensional magnetic field signals with flexible signal localization and fast screening [6,7,8,9][6][7][8][9]. However, due to the requirement of chip integration, conventional bulk lasers are not suitable for emerging chip-scale atomic magnetometers. On the other hand, VCSEL have advantages including low power consumption, feasible digital modulation, circular beam profiles, and compact volume, which are promising characteristics for the realization of chip-scale atomic magnetometers.

2. The Fundamentals of VCSEL and Atomic Magnetometer

2.1. VCSEL Fundamentals

Nowadays, the VCSEL is widely acknowledged as a commercialized light source for emerging applications, namely integrated light detection and ranging (LIDAR) and Virtual Reality/Augmented Reality (VR/AR). While the wavelength needed for atomic devices is normally fixed by transition band gap of alkali atoms, namely 795 nm(Rb), 895 nm(Cs). However, VCSEL for these wavelengths have not been fully commercialized, especially high power VCSEL over tens of mW (at atomic transition wavelengths). Many efforts have been made to develop VCSELs and the integration into atomic devices in recent years. In this chapter, the history and fundamental structure of VCSEL are briefly reviewed. VCSEL, since it first came out in 1977, has experienced dramatic development in recent decades. In 1977, Kenichi Iga first proposed the concept of a vertical cavity surface emitting laser and successfully made the world’s first VCSEL in 1979 [10], which can only work at ultra-low temperatures. Later, with the advancement of nanofabrication technologies, researchers made improvements to the structure of VCSELs. The first is the growth of distributed Bragg reflectors using molecular beam epitaxy (MBE) or metal organic vapor chemical deposition (MOCVD) [11,12,13,14][11][12][13][14]. The second is the use of a quantum well structure in the active region, which effectively reduces the threshold current of the VCSEL and makes it possible for the VCSEL to operate at room temperature [15,16,17,18,19,20][15][16][17][18][19][20]. The third one is the proposed oxidation aperture technique, which can be limited to both optical and electric fields [21,22,23][21][22][23]. Meanwhile, even better performance can be obtained by slight modification to the VCSEL structure, such as surface relief [24,25,26][24][25][26] and high-index-contrast subwavelength grating [27,28,29][27][28][29]. The VCSEL is composed of an optical cavity with a two distributed Bragg reflector (DBR) and a gain region. The top and bottom mirrors are periodic structures consisting of high and low refractive index materials of quarter wavelength, and their reflectivity should generally exceed 99%. By changing the number of periods of the high and low refractive index materials of the top and bottom mirrors, it is possible to achieve either top-emission or bottom-emission of the VCSEL. The intermediate active region is usually composed of one or several quantum well structures that are only a few tens of nanometers thick. VCSELs with different emission wavelengths can be obtained by designing active region materials with different components. For 795 nm (Rb) and 894.6 nm (Cs), the active materials are usually composed of InxAlGa1−xAsor AlxGa1−xAs. To precisely control the thickness of each layer of the structure, DBR is generally grown by MOCVD or MBE. The top and bottom of the VCSEL need to be coated with a ring-shaped metal layer to allow the current to be injected through the ohmic contact. Substrates usually use a p-on-n doping sequence to reduce absorption loss of n-type mirrors. To increase the current limitation and light field limitation of VCSELs, oxide apertures are formed by selectively lateral oxidizing a 10 nm–30 nm thick semiconductor layer with high aluminum content. The material of the oxide layer is typically AlAs. 

2.2. Atomic Magnetometer

With the increase of sensitivity to the order of femtotesla, chip-scale atomic magnetometers have been used for cardiac and cerebral magnetic measurements. Generally, the magnetometer relies on the Zeeman splitting at the atomic level, not the hyperfine level splitting [30]. Chip-scale atomic magnetometers suitable for miniaturization are based on optical pumping. When σ+− polarized light in resonance with the D1 line pass through the alkali metal vapor, the alkali metal atoms in the ground state are pumped to the excited state and made to become spin-polarized along the direction of the pumped light. Consequently, the atoms are pumped into the optically non-absorbing dark state using σ+− polarized laser beam, and then the oscillation frequency of the magnetic field and the Zeeman splitting resonance is set to drive the atoms from the dark state into the absorbing state, so that the magnetic resonance jump can be detected by changes in transmitted light through cell [31]. Atomic magnetometers can be divided into many categories. The first is, the chip-scale atomic magnetometer is based on the CPT resonance principle, which probes the ground-state hyperfine splitting between two magnetically sensitive Zeeman states to measure the magnetic flux density experienced by an atom ensemble. The second one is a chip-scale atomic magnetometer based on the spin–exchange relaxation-free (SERF) principle. Atoms with high alkali metal atomic densities and very low magnetic fields have Larmor frequencies much smaller than the relaxation rate of the atoms. When a σ+− polarized pump beam of a specific frequency passes through alkali vapor cell, the spin direction of the atomic ensemble is redirected. If there is a weak magnetic field perpendicular to the direction of polarization, the polarization of the atoms will be deflected. There are two main ways to detect this deflection signal. One is based on the Faraday effect, where a linearly polarized light is applied perpendicular to the direction of atomic polarization, and the polarization plane is deflected when the line polarized light passes through the alkali vapor cell [32]. Another way is to frequency modulate or amplitude modulate the pump light. The process of atomic polarization reorientation leads to an increase of light absorption in the alkali vapor cell, so that the transmission spectrum of light as a function of the magnetic field will have a zero-field resonance. The measured absorption curve is transformed into a dispersion curve by a lock-in amplifier, at which point there is a maximum slope suitable for detection at the zero fields. The above description of the atomic device principle shows that the basic requirement for a VCSEL is a single frequency output, which means that the VCSEL must be single mode and single polarized. Maintaining single mode and single polarization suppresses laser phase noise and intensity noise, which can narrow the laser linewidth and increase instrument accuracy. For VCSEL applications in atomic devices, the line width of the laser should be less than 100 MHz [33]. Next, I will introduce the development of VCSEL mode control and polarization control, respectively.

3. Mode Control of VCSEL for the Chip-Scale Atomic Magnetometer

In the chip-scale atomic magnetometer, the transverse higher-order modes affect the polarization rate of alkali metal atoms and reduce the accuracy of chip-scale atomic devices. Therefore, it is necessary to ensure the single transverse mode output of the laser. Generally, the VCSEL can be approximated as a cylindrical waveguide structure, which provides transverse optical confinement. The cavity length of the VCSEL is only one or a few wavelengths, and it is generally a single longitudinal mode output. However, its transverse dimension is larger than its effective cavity length, and higher-order modes often appear in the transverse direction. There are many ways to achieve single-mode VCSELs, such as using external mirrors for mode control, which results in a VCSEL with high single-mode power.

3.1. Transverse Optical Guiding

There are many ways to realize lateral optical guiding, and the simplest way is to reduce the oxide aperture diameter. In 1997, Grabherr et al. realized an 850 nm VCSEL with a single-mode output power of 2.8 mW at a 3 μm oxide aperture [35][34]. In the same year, Jung et al. realized an 840 nm VCSEL with a single-mode power output of 4.8 mW at a 3.5 μm oxide aperture [36][35]. However, the smaller oxide aperture increases the thermal resistance of the VCSEL, leading to an increase in the self-heating effect of the VCSEL. The high temperature causes changes in VCSEL output power and threshold current, shortening the lifetime of the VCSEL. To solve the problem of serious heat generation in small oxide aperture VCSELs, the extended cavity technique is proposed. The diffraction loss increases with increasing the length of a resonant cavity. The higher-order mode has a larger lateral range with respect to the fundamental modes, and their losses are greater. By using the extended cavity technique, a single mode VCSEL has been achieved. In 2000, Unold et al. demonstrated a single mode VCSEL with long monolithic cavity, whose output power reached 5 mW at 7 μm oxide aperture [37][36]. Later, Unold et al. further realized a single mode output power of 5.4 mW at 980 nm emission, which had an effective cavity length of 9.2 μm [24]. The extended cavity VCSEL increases the cavity length, which may lead to an increase in the longitudinal modes. Moreover, increasing the cavity length will attenuate the fundamental mode while attenuating the higher-order modes, reducing the output power of the VCSEL. and an ion implantation aperture of 6 μm [38][37]. In 2004, an 850 nm VCSEL fabricated by Fang-I Lai et al. achieved an output power of 3.8 mW with an oxide aperture of 8 μm [39][38]. The Zn-diffusion approach is similar to the ion implantation approach in that it achieves single mode output by increasing the optical loss in higher modes, but the deep Zn-diffusion layer increases the threshold current and reduces the output power. In 2001, C.C. Chen et al. solved the problem by a shallow Zn-diffusion technique [40][39]. Later, J.W. Shi et al. used the Zn-diffusion technique to enhance the single-mode VCSEL to obtain a bandwidth of 8 GHz and a maximum output power of 3 mW [41][40]. In 2013, J.W. Shi et al. integrated a single-mode VCSEL with Zn-diffusion technique into a 6 × 6 two-dimensional array with a dispersion angle of 4° and an output power of 104 mW [42][41]. The top DBR deposited metal layer to form a metal aperture is a further improvement of the above structure, and the metal aperture is slightly larger than the oxidation aperture, the lateral range of the higher order die is larger and more easily blocked by the metal layer, the base die is mainly in the central region, and the blocking effect is weak. In 2006, Otoma et al. fabricated an 850 nm single-mode VCSEL with an output power of 4.7 mW and an SMSR of 27 dB using a structure limited by metal aperture and oxide-confined [43][42].

3.2. Mode-Selective Losses or Gain

The single-mode output can be achieved by keeping the loss in the center region constant through surface relief. The reflectivity of the DBR structure is very sensitive to changes in the number of layers of high and low reflectivity as well as changes in layer thickness; therefore, the loss in the high-order mode region can be increased by surface relief while the loss in the fundamental mode part of the central region is kept constant to achieve single-mode output. In 1999, H.J. Unold et al. first proposed a surface relief technique for mode control and achieved a single-mode output VCSEL with a wavelength of 850 nm at μm In 2001, H.J. Unold et al. proposed a surface relief self-alignment method, which requires only one additional photoresist step, and achieves single-mode output at 16 μm oxide aperture, which has an output power of 3.4 mW at room temperature [24]. In 2009, Pierluigi Debernardi et al. established a hot-cavity model for surface relief VCSELs and standard VCSELs and found that the thermal lensing effect of surface relief structure is more obvious [26]. Surface relief VCSELs are single-mode outputs at the same drive current, while standard VCSELs are multimode outputs. Compared to other methods, the surface relief technique causes little damage to electrical or thermal characteristics. However, the surface etching technique requires very high precision etching accuracy, while it is not easy to batch production. An inverted surface relief technique can overcome the drawback. In 2016, Benjamin Kesler et al. proposed the method of depositing patterned dielectric layers on VCSELs, which has the advantage that it can be applied to any VCSEL without etching or epitaxial growth process, and the output power is 3.5 mW at 850 nm wavelength [45][43]. In 2017, for the application of VCSELs in CPT atomic clocks, Lei Xiang et al. performed surface relief etching in the center of the top reflector to implement a single-mode VCSEL [46][44]. The device uses gain cavity detuning to solve the emission wavelength shift problem. The single-mode output power reaches 0.45 mW at 80 °C, while the side mode rejection ratio exceeds 30 dB. In atomic devices, large output power is usually one of the requirements for a laser. In 2018, Zuhaib Khan combined surface etching and Zn diffusion layer techniques for high-speed VCSELs to increase the output power while reducing the number of modes to achieve quasi-single-mode output [47][45]. Another effective method for VCSELs to achieve single-mode output is to form a two-dimensional photonic crystal structure by etching the top DBR. A single-mode cylindrical waveguide is formed in the central region by varying the depth, shape, and spacing of the etched holes. In 2002, Dae-Sung Song et al. fabricated an 850 nm photonic crystal VCSEL with an SMSR of 45 dB, modeled after the structure of a photonic crystal fiber [48][46]. However, photonic crystal structure increases optical losses and thermal resistance, resulting in a large threshold current and low energy efficiency. Danner and Yang made improvements for photonic crystal power enhancement, achieving an output power of 3.1 mW at 850 nm and 5.7 mW at 990 nm, respectively [49][47]. In 2013, Meng Peun Tan et al. combined ion-implanted structures and photonic crystals to reduce the series resistance and current density of VCSELs by separating electrical and optical apertures, with an output power of more than 2.5 mW at 850 nm [50][48]. VCSELs generate high order modes at higher currents due to the complex doping of DBR, oxide hole limiting structure, and other factors. The previously adopted method is to integrate photonic crystals into the DBR, and this structure still suffers from multi-mode problems at high currents. Directly replacing the DBR with a two-dimensional photonic crystal can obtain a single fundamental mode output at higher currents. Photonic crystal surface emission lasers (PCSELs) are used to achieve large range surface laser emission through multi-directional Bragg diffraction of photonic crystals, while the beam direction and polarization characteristics of the laser can be easily controlled. In 2014, Kazuyoshi’s work led to a breakthrough in photonic crystal SEL power enhancement by using the MOCVD method instead of wafer bonding to construct a triangular hole to form a photonic crystal structure with a single-mode output power of 0.5 W [51][49]. In 2017, Ming-Yang Hsu et al. achieved the first 1.3 μm quantum dot photonic crystal surface emission laser(QDPCSEL) by using indium tin oxide (ITO) deposition instead of the sacrificial layer etching technique and epitaxial regrowth technique or wafer bonding technique, simplifying the process flow while obtaining a better current uniformity and finally achieving an output power of 2 mW [52][50]. In 2019, Huan-Yu Lu et al. designed QDPCSEL with an output power of 13.3 mW at a high temperature of 90 °C and a wavelength of 1.3 μm by introducing an additional lateral feedback mechanism [53][51].
The high-contrast subwavelength grating (HCG) structure provides a new implementation idea for the single-mode output of VCSELs. The HCG structure is a periodic structure consisting of a high refractive index grating strip and a low refractive index medium with a grating period between the high and low refractive index wavelengths. The output light mode and reflectivity can be easily controlled by changing the grating period, thickness, duty cycle, and other parameters. In 2007, Michael C.Y. HUANG et al. replaced the HCG structure with a partial VCSEL top DBR, achieving an output power of about 1 mW at room temperature with an SMSR of up to 45 dB [55][52]. HCG has a lower thermal resistance compared to DBR and can achieve a higher reflectivity in a smaller volume. Therefore, it is well suited for single-mode output at large oxide apertures. In 2008, Michael C.Y. HUANG et al. compared the output power of VCSEL with different grating areas to investigate the single-mode characteristics of HCG-VCSEL with an output power of 2.3 mW at 10 μm oxide aperture and its SMSR over 40 dB [56][53]. Because of the characteristics of HCG structure such as high emissivity and easy tuning, long wavelength single mode VCSELs come to be used for optical communication. In 2010, Werner Hofmann et al. using amorphous silicon to fabricate HCG, achieved the first electrically pumped VCSEL at 1310 nm using an HCG structure and obtained single-mode emission with an output power of more than 0.4 mW at an oxide aperture of 11 μm [27]. In 2013, Y. Rao et al. realized an InPd-based 1550 nm VCSEL with 2.4 mW single-mode output at 15 °C continuous wave operation [29]. In 2018, KunLi et al. constructed a novel beam shaping element using HCG to realize a double-sided VCSEL with different mode distributions, where one side has a single-mode output and the other side has a different far-field emission mode. 

4. Application of VCSEL in a Chip-Scale Atomic Magnetometer

As mentioned above, chip-scale atomic magnetometers have gradually improved in sensitivity and have reached subfemtotesla magnitude [88[54][55],89], enabling sensitivity similar to that of SQUIDs, and without the need for ultra-low temperature environments. Generally, the light sources of choice are diode lasers, where distributed feedback (DFB) lasers are widely used because of their extremely narrow linewidth and high frequency stability. However, the volume of DFB lasers is too large for a chip-scale magnetometer. To solve this problem, the light source of the atomic magnetometer was replaced by an optical fiber instead of a spatial light. Nevertheless, because of the coupling efficiency, optical fiber leads to a loss of optical power and reduces the portability of the device. The method of using VCSELs as light sources not only makes it easy to integrate miniaturized prototypes and even achieve battery-powered portability, but also results in less loss of optical power. The following will introduce the latest progress of three different types of chip-scale atomic magnetometers.

4.1. CPT Atomic Magnetometer

CPT atomic magnetometers are generally used to measure cardiac magnetism because of its low sensitivity. The structure of CPT atomic magnetometer is similar to that of CPT atomic clock, and the technology of miniaturized CPT atomic clock is relatively mature, so miniaturized CPT atomic magnetometer is developing rapidly. In 2004, Peter D. D. Schwindt et al. fabricated a chip-scale atomic magnetometer based on the CPT resonance principle, in which the power of the VCSEL was attenuated to 5 μW and the beam diameter was 170 μm after collimation. The sensitivity of the device is 50 pT/Hz1/2 and the volume is 12 mm3 [90][56]. To improve sensitivity of the miniaturization of the CPT atomic magnetometer, Michael Rosenbluh et al. suppressed the noise in the CPT resonance detection process by differential detection with a reduction of two orders of magnitude in the low-frequency noise power spectral density. This provides a new approach to the sensitivity enhancement of magnetometer [91][57]. In 2006, Lammegger presented a coupled dark state magnetometer (CSDM), which was based on two-photon spectroscopy of free alkali atoms. The vertical cavity of VCSELs leads to the characteristic of low parasitic capacity and therefore can be modulated by microwave signals with good efficiency [92][58]. In 2010, Pollinger et al. designed a laser carrier frequency stabilization control loop for CSDM which had been proven in long-term magnetic field measurements up to 11 h [93][59]. Due to its high accuracy and stability, CSDM was used to detect a space magnetic field [94,95,96][60][61][62]. In 2010, Guobin Liu et al. improved the microwave modulation scheme of the CPT atomic clock by proposing an energy-level modulation scheme that extends the detection resolution of the magnetic field [97][63]. However, this scheme can reduce the CPT signal amplitude due to the uneven magnetic field of the AC coil. Generally, the circular polarization multichromatic laser was used to generate CPT resonance that causes the problems of leaky trap-state atoms and the unwanted light background. In order to improve the atom utilization, many new CPT configurations have been proposed, such as lin//lin [98][64], push–pull [99][65], σ+−σ− [100][66], etc. In 2012, Tan et al. proposed a chip-level lin⊥lin quasi-bichromatic laser beam scheme in which two VCSELs are set up as a master–slave scheme. The master laser is driven by DC, and the slave laser is driven by a 6.834 GHz microwave [101][67]. Frequency injection locking is achieved by feeding output of the master circuit back to the slave circuit. In 2014, Shangqing Liang et al. proposed a new differential detection method for CPT magnetometers, which is able to improve both sensitivity and absolute accuracy [102][68]. In 2016, Haje Korth et al. fabricated a miniaturized scalar magnetometer for space exploration applications [9]. The VCSEL in the magnetometer is temperature and current controlled by a custom mixed-signal integrated circuit. The wavelength is modified by slowly scanning the temperature of VCSEL to find the temperature point when the 87Rb D1 line is reached, and this temperature point is used as the set value for feedback control to achieve closed-loop control of the frequency. In 2022, V. Andryushkov et al. used a compensated modulation coil to convert a CPT atomic clock into a vector magnetometer [103][69]. The method allows for configuring the atomic magnetometer without changing the CPT atomic clock components. Despite the rapid miniaturization of CPT atomic magnetometers, they are gradually being replaced by other types of optical pumping magnetometers in the medical field due to their inherent low sensitivity.

4.2. M-x Atomic Magnetometer and Bell–Bloom Atomic Magnetometer

M-x atomic magnetometers use Helmholtz coils to coherently drive the precession of the atomic spin about the static magnetic field. The alkali atomic spin precession modulates the transmitted light intensity, which is collected by a photodetector. Unlike the M-x magnetometer, the Bell–Bloom magnetometer modulates the pump light, which drives the coherent precession of the alkali atoms about the external magnetic field to be measured. In 2006, S. Groeger designed a highly sensitive laser-pumped M-x magnetometer with higher magnetometric sensitivity for laser pumping compared to discharge lamp pumping. The light source used in the device is a frequency-stabilized laser introduced through a 10 m long fiber. Although the magnetometer was not miniaturized and integrated in this experiment, it was demonstrated that the sensitivity of the M-x magnetometer with the laser as the light source can reach 29 fT [31]. In 2007, Peter D. D. Schwindt et al. constructed the first miniature optically pumped atomic magnetometer, which reduces power consumption, simplifies operation, and achieves higher sensitivity than previous magnetometers based on the CPT resonance principle [104][70]. The laser used in the experiment is a 795 nm VCSEL, and the heating of the gas chamber and laser is performed by a double-layer coil magnetic field counteracting technique, which can greatly reduce the influence of the magnetic field brought by the heating current. The overall power consumption of the device is 194 mW, and the overall volume is 25 mm3. In 2010, in order to overcome the crosstalk problem in arraying the mid-drive coils of the M-x magnetometer, Ricardo Jimenez-Martinez et al. abandoned the scheme of using RF coils in favor of direct modulation of the VCSEL [105][71]. By adjusting the modulation amplitude and duty cycle in the Bell–Bloom magnetometer, a sensitivity similar to that of the M-x magnetometer was obtained. In 2017, M. Ranjbaran used square-wave RF magnetic fields with different duty cycles instead of sinusoidal fields to enhance the harmonic component [106][72]. It was found that the steepest slope of the dispersive signal was obtained for the detection of nuclear resonance at the 5th harmonic of the square wave field with a duty cycle of 10%, resulting in a 4.5-fold increase in magnetic field sensitivity relative to the first harmonic. Although this experiment uses a DFB laser, the results are also valid for chip-scale atomic magnetometers with VCSEL as the light source. In the Bell–Bloom magnetometer, modulating laser power reduces its linearity, and to address this issue, Levy et al. improved the linearity of the Bell–Bloom magnetometer by offsetting the large signal generated at the photodetector due to light not absorbed by the magnetometer [107][73]. Enabling the Bell–Bloom magnetometer to operate in magnetic interference-rich environments, it is suitable for measuring human biomagnetic fields in non-magnetically shielded environments. In 2018, Hunter et al. presented the application of a free-induction-decay (FID) technique using the amplitude modulation (AM) and frequency modulation (FM) implementations with a microfabricated Cs vapor cell. VCSEL was the solitary laser source for atomic magnetometer due to compact structure and large modulation bandwidth. However, due to the limitation of optical power, the noise floor of the root spectral density was measured to be 3 pT/Hz1/2 and 16 pT/Hz1/2 for the AM and FM configurations, respectively [108][74]. In the same year, Deans et al. reported on a single-channel rubidium radio-frequency (RF) atomic magnetometer operating in unshielded environments and near temperature with a measured sensitivity of 130 fT/Hz1/2 [109][75]. The volume of sensor was only 57 mm3, which increases the spatial resolution of measurements. To demonstrate the feasibility of practical use of electromagnetic induction imaging with atomic magnetometers, Marmugi et al. reported a 50 fT/Hz1/2 87Rb RF atomic magnetometer operating in an unshielded environment and near room temperature. Phase stability better than 0.03∘ across the imaging area over several hours was obtained by biasing the RF atomic magnetometer to near-resonant operation [110][76]. In 2021, Deans et al. realized a compact RF atomic magnetometer with the sensor head including the required laser sources, the RF source as well as the magnetic field coils. VCSELs were tuned by varying the current and temperatures but do not have independent control of frequency and intensity. The advantage of this technique is that it does not require any background subtraction, which further validates its applicability to bio-medicine [111][77].

4.3. Dual-Beam SERF Atomic Magnetometer

The dual-beam SERF atomic magnetometer has a high sensitivity and is currently used in a wide range of applications in the field of brain magnetism. In 2003, I. K. Kominis et al. realized the first small volume multi-channel subfemtotesla atomic magnetometer based on SERF. Its sensitivity reached 0.54 fT/Hz1/2, which exceeded that of superconducting quantum interference devices, and the measurement volume was only 0.3 cm3 [88][54]. The experiment uses a high power laser with a pump optical power of 1W and a detection optical power of 100 mW, and a single VCSEL cannot achieve such a high output power. In 2007, Vishal Shah et al. realized a single-beam SERF chip-scale atomic magnetometer with a sensitivity below 70 fT/Hz1/2 [112][78]. The laser power of the sensor is only 0.35 mW for single-beam measurement and 0.12 mW for dual-beam measurement of the detection light, which can use VCSEL as the light source. Suppression of magnetic field noise can effectively improve the sensitivity of SERF magnetometers. An effective way is to use magnetically shielded materials with less magnetic noise, such as magnetic shielding using Mn-Zn ferrites. Another way is to minimize the magnetic field introduced by the current of the electronic components. In 2009, Jan Preusser et al. performed optical pumping and heating by moving electronic components such as VCSELs and heating membranes to a remote location and using optical fiber introduction, a method that effectively avoids the effects of magnetic noise generated by electric currents [113][79]. In 2013, R. Mhaskar et al. improved the laser heating of the gas chamber by using a filter attached to the chamber wall to absorb the laser light for heating, reducing the reflection and refraction from the chamber wall and reducing the power required for laser heating [114][80]. With the development of integrated optoelectronics, great breakthroughs have been made in nano-chip atomic magnetometers. In 2020, Yoel Sebbag et al. designed a chip-scale nanophotonic atomic magnetic sensor. The magnetic field detection is performed by measuring the change in ellipticity due to magnetically correlated circular dichroism by means of a photon spin sorter (PSS) [115][81]. In 2021, Yoel Sebbag et al. implemented a new high-precision atomic magnetometer with a magnetic sensitivity of 700 pT/Hz1/2 by the computer-aided reverse design of a photon spin sorter [116][82].

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