Bias Characteristics Of Four-mode Differential Laser Gyroscope With Nonplanar Cavity And Its Electronic System Design

The nonplanar four-mode differential laser gyro (FMDLG) has important potential applications in the field of industry and national defense due to its advantages such as excellent scale factor precision, absence of mechanical noise and small data delay. In order to improve the performance of nonplanar FMDLG, physical mechanisms and characterics of its bias are investigated both theorctially and experimentally. Moreover, the methods to reduce zero drift in aspect of electronic system design are discussed.The influence of nonreciprocal effects in ring lasers on the difference frequency of nonplanar FMDLG is analyzed. It is shown that the total bias can be resolved into seven sub-biases, which are called main bias, differential polarization loss bias, differential direction loss bias, magnetic circular dichroism loss bias, scattering bias, Verdet constant dispersion bias and Langmuir flow bias respectively. In addition, the characteristics of each sub-bias are discussed.A convient experimental system is designed, including electronic circuit modules such as digital signal processing, analog to digital converter, digital to analog converter, temperature mesasurement, piezoelectric transducer driver, magnetic field generator, current stabilized high voltage power supply, radio frequency amplifier and amplitude detector. With the experimental system, an readout system based on electronic signal processing is realized. Furthermore, the resolution is improved to better than 0.001(″) with resolution enhancement technique. Through configuring the circuit modules accompanied with corresponding codes, the experimental system can realize functions such as mode scanning, countering and path length control.Based on theoretical analysis, main characteristics of the bias are investigated through variation of longtitudal mode order, detuning frequency (operating point), operating style, magnetic field, discharge current and temperature. Minor difference of optical circuit among different orders of longitudal mode can lead to bias difference on the order of 0.01 Hz. Therefore, it is desirable that the FMDLG maintains the same longitudal mode throughout its mission. Bias sensitivity to operating point variation will be reduced greatly when a longitudinal magnetic field is applied to the gain media with specific amount. Bias sensitivity to magnetic field variation can be eliminated when the FMDLG is operated at the optimal operating point. Magnetic circular dichroism loss bias varies greatly with temperature, which is the major cuase of bias temperature sensitivity in the experimental FMDLG..Disadvantages of the path length control system using the difference between the left and right circularly polarized intensities are disscussed. Experimental results indicate that operating point affect bias, turn-on drift and temperature sensitivity greatly. When the difference between the left and right circularly polarized intensities is used to control path length, operating point will be related to parameters of beam combining and path length control circuit due to asymmetry in the process of opto-electronic convertion and amplification. A path length control system with maximizing clockwise (or anticlockwise) beat wave amplitude is designed, which obtains a frequency stability of 1.9×10-10. Moreover, it is immune to variations of gain and offset in the electronic circuit. In order to eliminate randomness in acquiring initial longitudinal mode at power up and mode jump at large range of operating temperature, smart modes acquisition program is added into the path length control system which makes the FMDLG maintain the same mode all the while.Shortcomings of path length control systems only using beam intensities are analyzed theoretically. Because of the asymmetry of gain curve composed of two isotropes, even with ideal opto-electronic conversion process, it is nearly impossible for the operating points determined by these path length control systems to be consistent with the optimal one. As a result, zero drift caused by variations of parameters such as gain curve, cavity loss and magnetic field can not be cancelled out. In order to solve this problem, a generalized path length control system is designed to make sure the FMDLG working with dispersion equalized and at the optimal operating point. Comparative exeripment showed that the magnetic sensitivity of one FMDLG is 62.63(°)/(h·mT) when controlled with equal intensity, while the magnetic sensitivity is only 0.88(°)/(h·mT) when controlled at the optimal operating point.With temperature cycling experiments, thermal compensation model is established by using stepwise regression method. Zero drift caused by both environmental temperature variation and self heating can be compenstated effectively. Test results of one experimental FMDLG give the following data: at (-40⁶0)°C, bias stability within 75 min after turn-on is 0.005(°)/h; bias repeatability is 0.005(°)/h with 2 hour off time; max bias error (MBE) and root mean square (RMS) are 0.03(°)/h and 0.009(°)/h respectively in static temperature compensation; MBE and RMS are 0.115(°)/h and 0.015(°)/h respectively with temperature varying at 1oC/min; MBE and RMS are 0.153(°)/h and 0.028(°)/h respectively with temperature varying randomly at (1⁵)oC/min; random walk is 3.9×10^-4(°)/h^-1/2.The nonplanar FMDLG has obtained inertial navigation capability in benign environment with slow temperature variation. However, more work should be done so as to realize inertial navigation capability in severe environment such as violent temperature variation, intense magnetic field disturbance, vibration and shock.