Laser frequency splitting technology, widely used
Laser frequency splitting technology is widely used in quality, weight, pressure, length, angle measurement and other fields. There are many produce laser frequency splitting method, such as tools Chengdu Institute of dual-frequency laser interferometer used in Zeeman effect, Zygo laser interferometer company used Bragg effect of laser gyro (angle interferometer) used in the Sagnac effect, etc. these methods can lead to frequency splitting. In measuring instruments, the use of frequency splitting the biggest advantage is that the system can work in exchange of a state of great gain and a good anti-interference ability. However, the frequency of the above methods are limited to split: Zeeman effect, because of the impact of traction mode it difficult to raise the frequency difference; Bragg effect, in order to ensure multi-beam interference, it is difficult to reduce the frequency difference, the former affected the performance of the system equipment, the latter being circuit devices speed limit.
Since 1985, Chinese and foreign scientists have been proposed based on the principle of birefringence frequency splitting technology, after two decades of continuous research, has been gradually moving towards a practical use is meter design a technical matter of concern.
Laser cavity can be considered similar to Pb interferometer, when the standing wave frequencies to meet the conditions to obtain maximum interference, that is, the output cavity, the frequency splitting principle birefringence technique is based on laser standing wave conditions, lasers physical cavity length and output a constant frequency relationship, a certain degree of physical cavity length output certain frequency, if the laser cavity length geometric certain circumstances, the principle of the use of birefringence have two physical cavity length, so the output of two frequency laser . Scientists have proposed a variety of produce in the laser cavity birefringence methods, such as in the resonant cavity insert two 1 / 4 wave plate, KDP crystal and so on, as a result of fast axis and slow axis refractive index difference between the refractive index, the production of different physical cavity length, resulting in different frequency output.
Tsinghua University, precision testing technology and equipment, State Key Laboratory of birefringence frequency splitting technology for a large number of studies, a quartz crystal in the cavity resettlement or electro-optical crystal, the use of natural birefringence effect, electro-optical birefringence effect or stress birefringence Effect of laser frequency splitting method can produce more than 40MHz frequency of poor e-ray and o-ray. However, due to the reasons for the competition model, in the frequency deviation of less than 40MHz, the system will have a "frequency locking", went back to the single-frequency output state, in order to avoid this kind of "latch-up" phenomenon in the laser lab, additional horizontal magnetic field, will be Larger media divided into two categories, so as to avoid the e-ray and o-ray pattern of competition, the two stable oscillation frequency, this method can generate from 0Hz to hundreds of MHz frequency difference.
The emergence of birefringent lasers, not only for dual-frequency heterodyne laser interferometer provides a frequency difference can adjust the laser light source, at the same time, induced many of the new measurement methods, such as the frequency splitting technology and mode competition effects of combining able to a longitudinal mode interval of bandwidth in accordance with the e-ray and o the order of the oscillation is divided into four parts, you can prove that when the laser cavity changes λ / 2, the output frequency shift of a longitudinal mode interval, followed by the above-mentioned four oscillation a point of order. Take advantage of this phenomenon, If we take the laser cavity length as a sensor, when the cavity length changes in λ / 8 when the transition to a new oscillation state, use of the oscillation state of the emergence of the order can be used as sensors are usually identified to the basis This sensor is based on the length of the laser wavelength of the benchmark, it can be said that the length of the most high-precision sensors. At present, the scope of such a sensor has reached 25mm; Similarly, the principle and the phenomenon of laser optical feedback combination is expected to appear practical optical interferometer feedback.
Based on the principles of birefringence frequency splitting is a new technology, with in-depth study, this physical phenomenon is bound to continue to permeate the field of metrology, a new measurement principle and method, and promote the development of measurement technology.
