In the compact atomic gyroscopes, the vertical-cavity surface-emitting laser (VCSEL) is usually utilized as the laser source. However, the power of VCSEL is not sufficient to polarize the nuclear spin efficiently. To solve this problem, we put forward a multi-laser module design for optical pumping in compact atomic gyroscopes. In this module, several lasers are integrated in one module and utilized together for pumping. We fabricated and tested a laser module with two VCSELs for demonstration. First of all, the beam collimation system is simulated, and it is concluded that the two lasers can be integrated on the same micro-heater and collimated by the same lens. This is proved experimentally by beam profile measurement. Furthermore, the polarization distribution is simulated and investigated, and the results indicate that the polarization and its homogeneity can be increased significantly when pumping with two lasers. Finally, the nuclear polarization is measured experimentally, and it is demonstrated that the nuclear polarization when pumping with two 1 mW lasers is increased from 3.71% to 5.52%, and the proportion of the increase is more than 48% compared to that when pumping with a single 1 mW laser. These results prove the feasibility of the design, and this multi-laser pumped regime provides new inspirations for the development of compact atomic gyroscopes.
Atomics magnetometers achieve remarkable accuracy, applying to production and scientific research. However, their size and bulk components make it difficult to achieve higher accuracy. We investigate the influence of the skew angle of the pump beam on the optical pumping rate in an atomic magnetometer. An analysis based on the Bloch equation is proposed to decrease evaluate optical errors in the process of production and assembly. When the incident angle is non-zero, the pumping rate has a projection in the direction of a static magnetic field. By establishing the pumping rate equation, the pumping rate of each position in the vapor cell in the direction of static magnetic field at different pump light skew angles is calculated in our study. The sensitivity was measured experimentally to demonstrate the simulation results. The results indicate that the optical pumping rate decreases as the amplitude of skew angle and propagation distance increasing which can be evaluated by one-dimensional distribution while the decay rate increases with the rise of the angle. The simulation values of the rubidium pumping rate, obtained with an incident angle of 0.5° , in the center of the vapor cell are reduced by 46%. The sensitivity decreases with the increasing skew angle similar to the attenuation trend of the optical pumping rate but not the same. Our work provides a reference for evaluating the optical error of atomic magnetometer which is useful for miniaturization design.
Atomic sensing devices usually contain fiber coupling systems. A two-mirror fiber coupling system is usually used in our research to couple spatial light into polarization maintaining fiber. In order to improve the axes alignment accuracy and optical extinction ratio of this fiber coupling system for atomic sensing devices, we propose an improved method based on the influence of mirrors on beam polarization. The polarization maintaining fiber can maintain the polarization of linear polarized light only when the polarization direction coincides with the fiber. However, according to theoretical analysis by Jones Matrix and experimental results, we demonstrate that mirrors have non-negligible influence on beam polarization, which causes difficulty in axes alignment. Both dielectrical mirrors and metallic mirrors have influence on the azimuth and ellipticity of polarized light, and the influence of dielectrical mirrors is more remarkable than that of metallic mirrors. Thus we propose to add a half wave plate or a quarter wave plate in the system to compensate for the influence of mirrors, and the extinction ratio of fiber output light is consequently increased. According to the experimental results, our new approach can increase the extinction ratio by about 30dB.
In recent years, atomic gyroscope has become an important direction of inertial navigation. Nuclear magnetic resonance gyroscope has a stronger advantage in the miniaturization of the size. In atomic gyroscope, the lasers are indispensable devices which has an important effect on the improvement of the gyroscope performance. The frequency stability of the VCSEL lasers requires high precision control of temperature. However, the heating current of the laser will definitely bring in the magnetic field, and the sensitive device, alkali vapor cell, is very sensitive to the magnetic field, so that the metal pattern of the heating chip should be designed ingeniously to eliminate the magnetic field introduced by the heating current. In this paper, a heating chip was fabricated by MEMS process, i.e. depositing platinum on semiconductor substrates. Platinum has long been considered as a good resistance material used for measuring temperature The VCSEL laser chip is fixed in the center of the heating chip. The thermometer resistor measures the temperature of the heating chip, which can be considered as the same temperature of the VCSEL laser chip, by turning the temperature signal into voltage signal. The FPGA chip is used as a micro controller, and combined with PID control algorithm constitute a closed loop control circuit. The voltage applied to the heating resistor wire is modified to achieve the temperature control of the VCSEL laser. In this way, the laser frequency can be controlled stably and easily. Ultimately, the temperature stability can be achieved better than 100mK.
The nuclear magnetic resonance gyroscope is based on spin-exchange optical pumping of noble gases to detect and measure the angular velocity of the carrier, but it would be challenging to measure the precession signal of noble gas nuclei directly. To solve the problem, the primary detection method utilizes alkali atoms, the precession of nuclear magnetization modulates the alkali atoms at the Larmor frequency of nuclei, relatively speaking, and it is easier to detect the precession signal of alkali atoms. The precession frequency of alkali atoms is detected by the rotation angle of linearly polarized probe light; and differential detection method is commonly used in NMRG in order to detect the linearly polarized light rotation angle. Thus, the detection accuracy of differential detection system will affect the sensitivity of the NMRG. For the purpose of further improvement of the sensitivity level of the NMRG, this paper focuses on the aspects of signal detection, and aims to do an error analysis as well as an experimental research of the linearly light rotation angle detection. Through the theoretical analysis and the experimental illustration, we found that the extinction ratio σ2 and DC bias are the factors that will produce detective noise in the differential detection method.
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