Atomic magnetometer which uses atoms as sensitive elements have ultra-high precision and has wide applications in scientific researches. The photoelastic modulation method based on photoelastic modulator (PEM) is used in the atomic magnetometer to detect the small optical rotation angle of a linearly polarized light. However, the modulation amplitude of the PEM will drift due to the environmental factors, which reduces the precision and long-term stability of the atomic magnetometer. Consequently, stabilizing the PEM’s modulation amplitude is essential to precision measurement. In this paper, a modulation amplitude stabilization method for PEM based on Field Programmable Gate Array (FPGA) is proposed. The designed control system contains an optical setup and an electrical part. The optical setup is used to measure the PEM’s modulation amplitude. The FPGA chip, with the PID control algorithm implemented in it, is used as the electrical part’s micro controller. The closed loop control method based on the photoelastic modulation detection system can directly measure the PEM’s modulation amplitude in real time, without increasing the additional optical devices. In addition, the operating speed of the modulation amplitude stabilization control system can be greatly improved because of the FPGA’s parallel computing feature, and the PID control algorithm ensures flexibility to meet different needs of the PEM’s modulation amplitude set values. The Modelsim simulation results show the correctness of the PID control algorithm, and the long-term stability of the PEM’s modulation amplitude reaches 0.35% in a 3-hour continuous measurement.
The spin-exchange relaxation-free (SERF) magnetometer as an ultra-precision magnetometer has been researched during recent times. The sensitivity of the signal measurement limits the accuracy of the magnetometer. The optical modulation method is used to detect the tiny optical rotation angle of the linear polarized (LP) light, and the modulator is improving from Faraday magneto-optic modulator to photoelastic modulator (PEM). However, the current commercial PEMs have several defects in the adoption of the magnetometer. First, considerable heat will reduce the PEM’s modulation precision; In addition, the big appearance will hamper the assembly of the magnetometer; Moreover, the products are unreliable in the small amplitude modulation. In order to overcome these drawbacks, a sort of PEM is designed by theoretical calculation and finite element simulation in the paper. The target PEM with 50kHz intrinsic frequency and 795nm transmission is composed of one hexahedron piezoelectric transducer (PZT) glued with one optical glass each other. About the PZT, the alpha quartz is determined by considering the vibration and temperature properties of the material, then a proper cut angel and size is calculated to satisfy the design target. Subsequently, the fused silica is used for its well optical property. In the final, a simulation is conducted to verify the feasibility and validity of the design.
Atomic magnetometer which uses alkali atoms as the sensors can realize ultrahigh sensitivity magnetic field measurement and has extensive applications scientific researches. Semiconductor lasers are used as the pump and probe laser in atomic magnetometer. Due to structural characteristics of semiconductor laser, beam divergence angles in vertical and horizontal direction have large deviation and laser beam diverges extremely fast. However, poor laser beam affects the implementation of atomic magnetometer sensitivity adversely. Only the circular laser beam with Gaussian distribution guarantees the homogeneous polarization of alkali atom vapor and high efficiency of atomic magnetometer. Consequently, a beam collimation system must be designed. In this paper, a collimation method using a thin lens and a pair of anamorphic prisms is proposed to guarantee the laser spot size approximately constant. The thin lens is used to decrease fast-axis divergence angle and ensure transmitted light is parallel. The anamorphic prisms pairs expand the laser beam in slow-axis and make the beam spot nearly round. Initially, the effect of thin lenses and anamorphic prisms on the relationship of input and output beam profiles is theoretically analyzed based on principle of geometrical optics. Then the software Zemax is used to simulate the collimation system. Finally, a beam collimation system is designed and tested. The experiment result shows that the laser beam size is approximately 2×2cm<sup>2</sup> and the beam approximate a Gaussian profile, which can meet the requirement of the atomic magnetometer.