Nuclear magnetic resonance gyroscopes (NMRGs) are a kind of rotation-speed sensor that senses the angular velocity by measuring a frequency shift in the Larmor precession of the nuclear spin in a constant magnetic field[
Chinese Optics Letters, Volume. 15, Issue 8, 082302(2017)
Noise suppression for the detection laser of a nuclear magnetic resonance gyroscope based on a liquid crystal variable retarder
In this Letter, the liquid crystal variable phase retarder is applied for the accurate modulation of the laser power in a detection system and the construction of a system that suppresses the influence of laser noise on the gyro’s bias instability. A closed-loop control method for a laser noise suppression system is proposed. We obtain a power stability index of 0.038% in a 3-h continuous test, and the nuclear magnetic resonance gyroscope bias instability reaches
Nuclear magnetic resonance gyroscopes (NMRGs) are a kind of rotation-speed sensor that senses the angular velocity by measuring a frequency shift in the Larmor precession of the nuclear spin in a constant magnetic field[
A liquid crystal variable retarder (LCVR) can be used to modulate the laser intensity[
The NMRG is a kind of atomic gyroscope designed on the principle of nuclear magnetic resonance. The principle for the angular rate
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The pumping laser and detecting laser shall be used in the operation of the magnetic resonance gyroscope. Firstly, the electron inside the alkali metal shall be driven into a polarized state with a pumping laser. Then, the status information of the atom shall be obtained through the detecting laser, and the angular rate information of the carrier can be obtained through signal processing.
To measure the angular rate of the carrier with an NMRG, it is first necessary to obtain the information of the detecting laser passing the alkali-metal air chamber through the photoelectric detector. From the information of the detecting laser, the
The demodulated signal is, therefore,
Then, the angular rate
Figure 1.Diagram of second demodulation process of
Therefore, the noise of the detecting laser is certain to result in the instability of the
The LCVR is a wave plate with continuously adjustable phase retardation produced by the electro-optic birefringent effect of the nematic liquid crystal. The molecule of the nematic liquid crystal has a birefringent effect, and its refractive index difference is mainly related to the voltage (denoted as
The phase retardation of the LCVR can be expressed as
The change relation between
Figure 2.LCVR phase retardation and transmittance. (a) Change relationship between LCVR phase retardation and voltage and (b) change relationship between unified transmittance and voltage.
The schematic diagram of a liquid-crystal variable laser power attenuator based on the LCVR is shown in Fig.
Figure 3.Schematic diagram of the liquid crystal variable attenuator.
The polarization direction of incident light shall be adjusted into the horizontal polarization light; when the polarizer will not work, it can be omitted. The incident light power shall be denoted as
The laser power stabilization system based on the LCVR uses the above liquid-crystal variable power attenuator as the actuator, which stabilizes the laser power through negative feedback control by using the photodiode-sampling laser power signal. When
Figure
Figure 4.Schematic diagram of the laser intensity stabilization system.
The optical axis of the polarizer is perpendicular to the optical axis of the polarizer. The angle between the optical axis of the polarizer and the fast axis of the liquid crystal variable phase retarder is 45°. The laser generates a linear polarized light with a wavelength of 795 nm and a power of about 1 mW, which is separated by a beam splitter (BS1) and received by a photo detector (PD3). The PD3 receiving laser is mainly used to monitor the original intensity information. Another beam of light enters a variable laser power attenuator formed by a polarizer, a liquid crystal variable phase retarder, and a polarizer. By changing the driving voltage applied to the LCVR, the polarization state of the laser can be changed so as to change the output power of the laser. The laser emitted from the detector is divided into two beams by BS2. The detector PD1 is used for the sampling of the optical power, and the detector PD2 is used to test the stability of the output laser power. The optical power sampled from PD1 is converted to a voltage signal in the control unit. The control unit compares the actual optical power with a set value to produce the appropriate control voltage. At the same time, a symmetrical square wave signal with a frequency of 2 kHz, a duty cycle of 50%, and an average value of 0 is generated in the circuit. The amplitude of the square wave is modulated by the control voltage, which is loaded on the liquid-crystal controllable phase retarder, and the optical power of the outgoing laser is changed to realize the closed-loop stability of the power.
The results of continuous 3-h testing on the system are shown in Fig.
Figure 5.(Color online) Results of laser inspection system stabilization experiment.
In order to compare the effect of the laser power on the detection of the NMRG, a stability test of the NMRG detection signal was carried out on the NMRG prototype. It is known that the angular rate
The results of the
Figure 6.Results of the laser without intensity stabilization.
Figure 7.Results of the laser with intensity stabilization.
Figure
Figure 8.(Color online) Detection signal spectrum analysis.
It can be seen from Fig.
Figure 9.(Color online) Allan standard deviation curve.
The Letter analyzes the relationship between the power stability of a detecting laser of an NMRG and the noise of the detecting laser signal, which introduces the working principle of the laser-power stability system based on the LCVR, and proposes the closed-loop control method for the laser noise suppression system by design of the circuit control system. By using such a system, the precision test for the NMRG is carried out, and a power stability index of 0.038% is achieved in 3-h continuous testing. The SNR of the detection signal of the NMRG is improved effectively, and the bias instability achieves an index of
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Binquan Zhou, Guanqun Lei, Linlin Chen, Wenfeng Wu, Zhuo Wang, Xiaofeng Meng, Jiancheng Fang, "Noise suppression for the detection laser of a nuclear magnetic resonance gyroscope based on a liquid crystal variable retarder," Chin. Opt. Lett. 15, 082302 (2017)
Category: Optical devices
Received: Jan. 6, 2017
Accepted: May. 19, 2017
Published Online: Jul. 20, 2018
The Author Email: Zhuo Wang (zhuowang@buaa.edu.cn)