Objective In modern frontier physics application fields, such as ultra-high resolution spectroscopy, precision measurement, laser cooling and the trapping of atoms, and optical frequency standard, very high laser performance (excellent frequency stability, narrow output linewidth, etc.) is required. However, the monochromaticity and the linewidth of the free-running laser cannot meet the application requirements, so it is necessary to stabilize the frequency and power. A common method with good maturity and a high signal-to-noise ratio is the laser frequency stabilization technology based on saturated absorption spectrum and wavelength modulation frequency locking. Therefore, it can stably lock the laser frequency for a long time. However, the laser is a type of electro-optic converter that is very sensitive to the environment. The temperature and vibration significantly influence its frequency stability, which requires good environmental adaptability by the frequency stabilization scheme that is used to improve the laser performance. As a result, this paper focuses on the environmental adaptability of semiconductor laser lock-in systems. The frequency locking scheme is based on the saturated absorption spectrum modulation and demodulation technology. The findings may be useful in guiding the stability and the practical optical path design of semiconductor laser frequency locking.
Methods In the laboratory environment, we use three typical frequency locking schemes: doppler-free locking (DFL), cell-reflection locking (CRL), and mirror-reflection locking (MRL) optical path to lock the frequency of the semiconductor laser. The three schemes use a lock-in regulator (LIR) for optical amplification. Then, through the modulation and demodulation technology to lock the laser on the saturation absorption peak. Meanwhile, the acousto-optic frequency-shifting optical path generates the laser frequency required for the experiment. Finally, we tested the environmental adaptability of the three schemes. The research method is to change some environmental parameters such as temperature, vibration frequency, and vibration amplitude to test the stability of the laser frequency and describe the environment tolerance of each scheme. A heater is used to heat the shell of the semiconductor laser in the temperature experiment, and the error signal and the temperature of the locking point are monitored. The vibration experiment is conducted on the optical platform. The influences of vibration amplitude (the driving power of vibration source) and vibration frequency on the three frequency locking schemes are studied. Because the optical platform can effectively block the high-frequency signal, only the low-frequency signal of 0--300 Hz is studied in the experiment. The duration of each vibration is set to 30 s.
Results and Discussions The temperature experiment shows that the laser unlocking temperature of the DFL scheme is 33.2 ℃, locking time is 214 s, and frequency drift is approximately 2% (Fig.4). In CRL, the unlocking temperature is 27.1 ℃, locking time is 80 s, and frequency drift is approximately 6% (Fig.5). In MRL, the unlocking temperature is 28.7 ℃, locking time is 89 s, and frequency drift is approximately 10% (Fig.6). This is because although the increase of the laser temperature leads to the fluctuation of the laser frequency and power, in DFL, because of the existence of the reference light and difference between the signals collected by two photodiodes, these interference signals can be greatly eliminated. Finally, the interference signal into the proportional integral derivative(PID) is weak. As a result, the feedback circuit can lock the laser frequency more precisely at the peak of the saturation absorption spectrum. However, as the temperature rises and reaches the critical mark, the temperature-induced interference exceeds the regulation ability of the feedback circuit. The negative feedback is destroyed, resulting in laser unlocking. Because there is no reference light for the CRL and MRL, the interference signal caused due to temperature change will enter the PID directly, making its temperature tolerance less than DFL. In the vibration amplitude experiment, the vibration frequency is set to 100 Hz. The fluctuation of the DFL optical path is relatively stable at approximately 1%, when the driving power of the vibration source increases from -4 dBm to 4 dBm; CRL fluctuates the least (approximately 0.3%), and MRL changes a lot (approximately 10%) (Fig. 7). In the vibration frequency experiment, the driving power of the vibration source is set at 0 dBm. The fluctuation of the DFL optical path is relatively stable at approximately 0.7%, when the vibration frequency increases from 0 Hz to 300 Hz; CRL has the smallest fluctuation (approximately 0.3%). MRL varies from 0.3% to 0.9% (Fig. 8). The results show that when the vibration frequency and amplitude change, the influence on DFL and CRL optical path is small and the influence on MRL is obvious. For DFL, the influence of noise caused due to vibration is greatly weakened due to the existence of reference light. For CRL, the pumping light comes from the weak reflection of the atomic gas cell, which is very weak. The fluctuation of the error signal is small due to the small saturated absorption signal and noise level, but the stability is weaker than that of DFL. The saturation absorption signal is relatively strong for MRL, and the probing light is reflected from the mirror. The small change in mirror position caused due to vibration impacts the spectral signal, increasing the noise and decreasing the stability.
Conclusions Three typical laser frequency locking schemes are studied using the experimental idea of controlling variables based on the principle of the saturated absorption spectrum and wavelength modulation frequency locking. The results show that DFL has the best temperature tolerance and antivibration interference, followed by CRL and MRL, indicating that DFL is more suitable for harsh environments. However, CRL and MRL also have certain application values. For example, because the probing and pumping lights in DFL need to coincide as much as possible, the distance between the two mirrors in the optical path must be as far as possible, and the actual optical path occupies a larger space. Therefore, DFL may not be applicable when the optical path size is limited. However, CRL and MRL have small optical path sizes and are easy to integrate and miniaturize for future laser units.
