To assess the long-term stability of the laser wavelength, a 14-h beat frequency experiment is conducted using an optical frequency comb as the reference laser. The experimental results are presented in Fig. 11. The findings indicate that over the 14-h measurement period, the average vacuum frequency value obtained is 473612353616 kHz, with a deviation of 12 kHz from the internationally recommended value by the International Committee for Weights and Measures. Without altering the analog PID parameters, the laser overall PID gain is adjusted through a digital potentiometer. The wavelength stability (Allan standard deviation) of the laser, as determined by the beat frequency with an optical frequency comb, before and after PID gain adjustment, is illustrated in Fig. 12. Maintaining the same PID parameters and appropriately adjusting the overall PID gain after locking contribute to an improvement in frequency stability. After gain adjustment, the frequency stability is measured as 7.3×10^-11 for 0.1 s, 1.4×10^-11 for 1 s, 3.0×10^-12 for 10 s, 8.5×10^-13 for 100 s, 3.1×10^-13 for 10³ s, and 2.0×10^-13 for 10⁴ s. The experimental results demonstrate that the laser exhibits a high level of frequency stability.

Based on the principle of iodine molecule saturation absorption for frequency stabilization, the 633-nm He-Ne laser holds considerable application value in geometric metrology, precision interferometric measurements, atomic spectroscopy, and gravity measurements. Recently, digital metrology has emerged as the future direction of metrology. Although 633-nm iodine-stabilized He-Ne lasers based on analog control systems offer high frequency stability and cost-effectiveness, their further development is constrained by limitations in intelligence and digitization levels. The 633-nm iodine-stabilized He-Ne laser, which is based on a digital control system, not only satisfies the demands of digitization but is also more suitable for miniaturization. However, due to the precision limitations of the employed analog-digital (AD) and digital-analog (DA) converters, the frequency stabilization performance of the 633-nm iodine-stabilized He-Ne laser based on digital circuits is often less ideal, with frequency stability typically lower than that of iodine-stabilized He-Ne lasers based on purely analog control systems. To address the aforementioned issues, in this study, a 633-nm iodine-stabilized laser is proposed based on a modulo-mixed control approach. It combines the advantages of high levels of digitization and stability.

The structure of the laser system is depicted in Fig. 1. The laser comprises two main components: the laser head and control system. The laser head incorporates a self-developed high-power iodine-stabilized He-Ne laser head, comprising a laser tube, iodine cell, high-reflectivity mirror, piezoelectric ceramic, photodetector, and thermoelectric cooling element. The control system consists of two parts: analog circuitry and digital circuitry. The analog circuitry section consists of three parts: a sinusoidal signal generator, an optical power signal demodulator, and a proportional-integral-differential (PID) controller. The sinusoidal signal generator employs a Wien bridge sinusoidal signal generation circuit. By selecting low-temperature drift precision components and finely tuning the component parameters, high-quality output can be realized. The optical power signal demodulator is realized via a mixer. The demodulated third harmonic signal used for locking realizes a satisfactory level, with a signal-to-noise ratio of 8∶1. The PID controller allows for gain adjustment through a digital potentiometer, which is managed by an mirco-controller unit (MCU), expanding the adaptability of laser frequency locking to different scenarios. The digital circuitry section, employing two absorption peak recognition algorithms, achieves automatic locking of absorption peaks. It enables the uploading of laser operational data to a computer, enhancing the digitization level of the laser system. Combining the aforementioned sections, the final design of the laser system is successfully implemented.

To assess the automatic peak-locking functionality of the laser, the newly developed laser is compared with the iodine-stabilized He-Ne laser, a national length standard, with both locked onto the d-peak and g-peak. The frequency difference between the two peaks is set at 39.422 MHz. In the experiment, the newly developed laser is intentionally unlocked, and the locking process is monitored via beat frequency. The experimental results are depicted in Fig. 10. The results demonstrate that the system can successfully relock, with a relocking duration of 30 s. After locking, the laser remains positioned in a stable manner at the target absorption peak.

The 633-nm iodine-stabilized He-Ne laser proposed in this study, based on the modulo-mixed control method, exhibits crucial digital features such as automatic peak recognition. It satisfies the urgent demand in the metrology industry for digitally-enabled length measurement standards. Simultaneously, leveraging analog circuitry technology, the laser achieves high-precision locking of absorption peaks. Experimental results demonstrate a frequency stability of 1.4×10^-11 for 1 s and 3.1×10^-13 for 10³ s, satisfying the metrology industry requirements for high stability in 633-nm iodine-stabilized He-Ne lasers. The laser system presented in this study provides essential technical support for the application of the 633-nm iodine-stabilized He-Ne laser in precision measurements and digital metrology.