Fig. 1. Schematic diagram of our 2 μm single-longitudinal-mode fiber laser and its PDH frequency stabilization structure based on ultra-stable optical cavity. The yellow area represents the DBR fiber resonator located inside a brass block, the light green area represents the polytetrafluoroethylene insulated packaging box, the light yellow area represents the PDH feedback loop, and the gray area represents the temperature feedback circuit

Fig. 2. Measured output characteristics of our single-longitudinal-mode fiber laser. (a) Temperature tuning characteristics of the output laser wavelength; (b) single-longitudinal-mode oscillating characterized by F-P scanning interferometer; (c) response characteristics of PZT frequency tuning mechanism

Fig. 3. Error signals recorded by an oscilloscope. (a) Error signal before frequency locking; (b) error signal after frequency locking

Fig. 4. Frequency noise power spectra and beat spectrum. (a) Measured frequency noise power spectra of the laser before and after its frequency is locked; (b) collected beat spectral data between two frequency-stabilized 1342 nm lasers using an FFT and the Lorentz fitting curve obtained from these data

Fig. 5. Variation of beat frequency signal with time and its Allan variance. (a) Beat frequency signal between two 1342 nm frequency-stabilized lasers as a function of time measured by a frequency counter locked to rubidium atomic clock with a gating time of 100 ms. In the measurement, the output laser from one 1342 nm laser is transmitted to our lab through a 20 km telecommunications optical cable. (b) Allan variance calculated from the frequency jitter data over time recorded in Fig. 5(a)

Fig. 6. PZT control voltage signal and single-longitudinal-mode oscillating behavior of the laser at the moment when the laser frequency just loses the locked state. (a) Recorded PZT control voltage signal when the laser frequency is locked; (b) single-longitudinal-mode oscillating behavior of the laser captured by an F-P scanning interferometer at the moment when the laser frequency just loses the locked state

Fig. 7. Variation of voltage with time. (a) PZT feedback control voltage signal recorded within 160 h with a digital multimeter; (b) variation of voltage over time within 240 h collected at the transmission end of the optical cavity with a digital multimeter following a photodetector