Ultrastable lasers have important applications in precision measurement, precision spectroscopy, and quantum information. However, it is difficult to meet the requirements of the aforementioned applications using lasers in the free-running state because of frequency jitter and drift caused by external environmental factors. Various active techniques for stabilizing the laser frequency have been proposed and implemented. Among them, the ultrastable cavity Pound-Drever-Hall (PDH) frequency stabilization technology has a high locking accuracy and it is mature with wide application. Single-frequency fiber lasers have undergone rapid development in recent years. Based on the fiber laser's characteristics of narrow linewidth and low noise, ultrastable lasers with superior performance are developed. However, according to current reports, the most widely used ultra-stable laser sources are DFB fiber lasers. Because the resonance cavity and the fiber grating cavity mirror of DFB fiber lasers are integrated into the same structure, the length of the laser resonance cavity and the fiber grating cavity mirror can be simultaneously controlled when used for frequency stabilization of an ultrastable cavity PDH, which is very beneficial for obtaining a long-term locked ultrastable laser. Compared with DFB fiber lasers, DBR fiber lasers have more practical value for achieving ultra-stable cavity PDH frequency stabilization because DBR fiber lasers do not require rare earth-doped fibers to exhibit photosensitivity, are easier to manufacture, and have more advantages in terms of the laser band, wavelength flexibility, and other aspects. However, when DBR fiber lasers are used for ultrastable cavity PDH frequency stabilization, owing to the independent active temperature control of the fiber resonant cavity, coordinating the frequency of the FBG center with the frequency of the ultrastable cavity mode locked by the PZT during the PDH frequency-locking process is difficult. This makes long-term locking of the frequency of the DBR fiber laser difficult due to degradation of the frequency locking owing to laser mode hopping, making it difficult to meet the requirements of special applications, such as quantum entanglement experiments. Therefore, further study is required to achieve long-term locking of DBR fiber lasers based on ultra-stable cavity PDH frequency stabilization.
A home-made 2 μm band DBR single-frequency fiber laser was used as the laser source. In order to quickly tune the frequency of the laser, a PZT that can be stretched axially along the fiber was pasted on the side of the laser resonator. The laser resonator was strictly insulated and equipped with an active temperature control device to reduce the influence of the external environment on the frequency stability of the laser so that it can meet the requirements of ultrastable cavity PDH frequency stabilization. A 1.0 μm band ultrastable cavity with an FSR of 1.5 GHz and a fineness of 15000 was used as the frequency reference, and the 1950 nm laser was locked to a transmission peak of the ultrastable cavity by using the ultrastable cavity PDH frequency stabilization scheme after using PPLN crystal frequency doubling. We experimentally confirmed that it is difficult to achieve long-term locking of ultrastable cavity PDH frequency stabilization based on a DBR single-frequency fiber laser. Therefore, herein, we propose and demonstrate a real-time temperature control scheme for DBR fiber resonators using a PZT feedback control signal to generate temperature control signals based on the frequency reference of the ultrastable cavity. This method first generates a temperature control signal by calculating and processing the PZT feedback voltage using a single-chip microcomputer and then realizes real-time temperature control based on an ultrastable cavity frequency reference for the DBR fiber resonator and its cavity mirror FBG through the temperature control signal, thereby resolving the issue of long-term locking of the DBR fiber laser.
The quality of the laser output characteristics is a determinant of whether the laser can be locked; therefore, we first tested the laser output characteristics. Herein, the laser temperature is set to change from 15 ℃ to 35 ℃, the laser wavelength is changed by 1.06 nm [Fig. 2(a)], and when the temperature is stable, the laser can ensure a single-longitudinal-mode operation. The results of single-longitudinal-mode operation measured using the F-P scanning interferometer are shown in Fig. 2(b). By applying triangular wave modulation signals of different frequencies and voltages to the PZT, the measured frequency tuning range of the laser is found to be 1.6 GHz@52 V and the response bandwidth is approximately 8 kHz [Fig. 2(c)]. To characterize the quality of the laser-locking results, the frequency noise is measured before and after laser locking. The measurement results [Fig. 4(a)] show that the laser frequency noise is decreased by to 3‒4 orders of magnitude compared with that before locking, reaching a minimum of 0.08 Hz2/Hz@18 kHz. Through indirect beat frequency measurements, the laser linewidth after locking is determined to reach 255 Hz [Fig. 4 (b)], the frequency jitter of the laser reaches approximately 7 kHz within 2 h, and frequency instability reaches 3.76×10-13@1000 s (Fig. 5). Implementing a real-time temperature control scheme based on the frequency reference of the ultrastable cavity for the laser prevents the PZT voltage of the laser from reaching the voltage value at which the laser generates mode hopping [Fig. 7(a)]; thus, the laser will not lose its lock because of mode hopping. By monitoring the light intensity at the transmission port of the ultrastable cavity for 240 h using a photodetector followed by a digital multimeter, it is found that the transmitted light intensity remains relatively stable [Fig. 7(b)], which indicates that the developed laser achieves long-term locking.
We report a custom-built 2 μm band DBR fiber laser that can be used as an ultrastable laser source, in which frequency locking was achieved based on ultrastable cavity PDH frequency stabilization. The adiabatic constant-temperature packaging of the laser and built-in PZT with a frequency-tuning function meet the requirements of ultrastable cavity PDH frequency stabilization experiments. After frequency doubling using a PPLN crystal, the 1950 nm fiber laser successfully achieves frequency locking by using a ultrastable cavity with an FSR of 1.5 GHz, a fineness of 15000, and operation in 1 μm band as a frequency reference. DBR fiber lasers are difficult to lock in for a long time when implementing ultrastable cavity PDH frequency stabilization. We propose and demonstrate a scheme for using the PZT feedback control signal to trigger the generation of temperature control signals. Real-time temperature control is implemented based on an ultrastable cavity frequency reference for DBR fiber resonators to achieve long-term frequency locking of such DBR fiber lasers based on ultrastable cavity PDH frequency stabilization. The real-time temperature control scheme based on the frequency reference of the ultrastable cavity for DBR fiber resonators proposed herein provides an important reference point for realizing long-term ultrastable cavity PDH frequency stabilization of DBR fiber lasers.
