Objective With the rapid development of the aerospace and microelectronics industries, the demand for ultra precision measurement is also increasing. He-Ne lasers are widely used in mechanical and ultra precision measurement fields due to their excellent coherence and other characteristics. Among them, the thermally stabilized He-Ne laser is suitable as a wavelength scale laser for laser interferometry due to its high frequency stability, good beam quality, and low cost. However, traditional thermally stabilized lasers have poor frequency stability and reproducibility, which cannot further meet the requirements of high-precision laser interferometry for frequency stability and accuracy. This article reports a frequency biased locking system for thermally stable He-Ne laser based on a combination of an acousto-optic modulator and an optical phase-locked loop. This system combines the high-frequency response characteristics of an acousto-optic modulator with the high sensitivity characteristics of an optical phase-locked loop, enabling fast and accurate frequency locking of a thermally stable He-Ne laser.
Methods This article reports an optical phase-locked loop bias locking system based on an acousto-optic modulator. An iodine stabilized frequency laser is chosen as master laser, and a thermally stabilized He-Ne laser as the slave laser. The beam of the slave laser is modulated by an acousto-optic modulator and locked onto the master laser. The reference signal for frequency offset locking is a 30 MHz signal generated by a signal generator. Data are collected using a frequency counter. The locking result is shown (Fig.9).
Results and Discussions In the experiment, a highly stable He-Ne laser based on intracavity saturation absorption stabilization was used as the wavelength reference source for thermal stabilization laser locking. Through beat frequency measurement with iodine stabilized laser wavelength reference, the results show that the 1 s wavelength stability of the iodine stabilized laser is 1.3\times 10^-11 , reaching 4.1\times 10^-13 in 1 000 s, reproducibility better than 1.0\times 10^-11 . The frequency jitter of the laser beat frequency after the system is locked is shown (Fig.10). As a comparison, the figure shows the drift of the beat frequency under free operation. In the experiment, a frequency counter was used to count the beat frequency signal for 30 min in the open-loop state of the optical phase-locked loop. Then, the reference frequency was set to 30 MHz to lock the thermal stabilized frequency laser to the iodine stabilized frequency laser, and the beat frequency after the loop locking was continued to be counted for 180 min. The beat frequency was locked at a bias frequency of 30 MHz, with a fluctuation range below 0.2 Hz. We have achieved high stability frequency locking of thermally stabilized lasers compared to iodine stabilized lasers. The relative Allen variance of the frequency offset of the optical phase-locked loop is shown (Fig.11). Among them, the relative Allen variance of the integration time of 1 s and 1000 s is 3.3\times 10^-9 and 1.4\times 10^-12 respectively.
Conclusions This article introduces a high stability laser frequency stabilization method based on the combination of an acousto-optic modulator and an optical phase-locked loop. An experiment was conducted using a self-developed optical phase-locked loop system to lock the bias of a thermally stable all cavity He-Ne laser to an iodine stable frequency laser. The signal-to-noise ratio of the beat frequency signal was increased to over 40 dB (Fig.9) through a beat frequency signal detection unit based on an acousto-optic modulator. A digital frequency discriminator and PI control circuit were used to feedback control the acousto-optic modulator, achieving closed-loop control of the optical phase-locked loop. The frequency stability of the thermally stable He-Ne laser is significantly improved, enabling it to meet the requirements for laser frequency stability and accuracy in the fields such as ultra precision interferometry and ultra sensitive spectral detection.
DownLoad: