Objective Low-noise narrow-linewidth single-longitudinal-mode (SLM) lasers have several important applications in many fields, such as gravitational wave detection, atomic and molecular physics, time and frequency transfer, and precision measurement. Currently, SLM fiber lasers are divided into three main types: distributed feedback, distributed Bragg reflection (DBR), and ring cavity laser. Among them, the ring cavity laser, which employs a long cavity design, facilitates increase in cavity gain to improve output power using relatively high-gain fibers. A ring cavity laser can also be used in other applications such as frequency tuning and feedback control by simply inserting optical components into a cavity. Moreover, the Q-value of a fiber-laser resonator can be increased by employing a long ring cavity. This long cavity helps in obtaining a narrow free-running laser linewidth and reduces the relaxation oscillation frequency of the laser. This is useful in suppressing a relaxation oscillation peak through photoelectric feedback control. However, a long ring cavity is easily affected because of external-environment noise disturbances, which leads to SLM instability, e.g., mode hopping. Thus far, the achieved continuous operation time of a free-running compound ring cavity fiber laser without mode hopping is only several hours. Therefore, although in principal, a fiber-laser source exhibits excellent performance, it cannot meet practical application requirements. In this study, a low-noise narrow-linewidth-ring erbium-doped fiber laser operating in a SLM and maintaining its polarization is reported.
Methods A polarization-maintaining(PM) fiber's strong ability to resist environmental disturbances allows the parametric optimization of a compound ring cavity through repeatable experiments in a laboratory. During experimental optimization, Fabry-Perot(F-P) type very narrow resonant transmission peaks of a secondary cavity are used to filter potentially oscillating dominant longitudinal modes from a large number of nearby main-cavity modes. Then, notch bands of the band-notch type secondary cavity are used to suppress excess dominant main-cavity modes, which are filtered using the F-P type secondary cavity. Length of each secondary cavity can be designed and optimized according to the length of the main cavity so that it meets the Vernier-effect requirements to broaden the effective longitudinal mode spacing of a compound ring cavity. During laser optimization, a vibrational acoustic wave and a thermal-isolation design are employed to protect the laser from environmental vibrations and thermal disturbances. Length of each cavity in a compound ring cavity fiber laser can be fine-tuned to strictly meet the Vernier-effect requirements by changing the control temperature of a laser. Thus, mode hopping is efficiently suppressed.
Results and Discussions After optimizing the cavity length and employing the vibrational acoustic wave and thermal-isolation design, the laser is capable of performing the SLM operation at room temperature [Fig. 2(a)]. By fine-tuning the length of each cavity through temperature compensation, the laser is capable of achieving continuous SLM operation without mode hopping for more than 14 h. To the best of our knowledge, this is the longest SLM free-running time ever achieved using the proposed laser type [Fig. 2(b]. By adopting a laser cavity structure that maintains the laser polarization and exhibits a high-Q design, the laser output signal-to-noise ratio measured is up to 80dB [Fig. 3(a)], and linewidth is approximately 400Hz [Fig. 3(b)]. The long-term SLM operating characteristics of the compound ring cavity fiber laser designed by our experiment allowed us to measure its broadband noise characteristics. The relative intensity noise-power (Fig. 4) and frequency noise-power spectra (Fig. 5) of the ring cavity fiber-laser type were measured in the mHz--MHz frequency band for the first time. The noise measurement results demonstrate that the proposed laser exhibits excellent noise characteristics, similar to those of the DBR fiber laser. Moreover, its intensity noise and frequency noise are lower than those of the DBR fiber laser in the 1mHz--10Hz frequency band. Additionally, its relaxation oscillation frequency is lower than that of the DBR fiber laser.
Conclusions A low-noise compound ring cavity fiber laser with stable SLM operation is proposed in this study. Using PM fibers with strong resistance to external disturbances, compound ring cavity parameters were accurately optimized to achieve a broad effective longitudinal mode spacing. After fabricating vibration isolation and thermal insulation packaging, temperature control was adopted to further fine-tune the FSRs of a compound ring cavity. Then, the Vernier-effect requirements were strictly followed to suppress mode hopping. Finally, the proposed laser was capable of achieving long-term SLM stable operation for more than 14 h, which is the longest SLM stable operation time ever achieved by a compound ring cavity fiber laser to the best of our knowledge. Owing to the laser high-Q characteristics, its output signal-to-noise ratio is up to 80dB, and 3-dB linewidth is approximately 400Hz. The long-term SLM-operation characteristics of the compound ring cavity fiber laser allowed us to measure the noise behavior of the laser type. The noise measurement results demonstrate that the compound ring cavity fiber laser exhibits low intensity and frequency noise in the mHz--MHz frequency band. Compared with a typical DBR fiber laser, the proposed laser exhibits lower relaxation oscillation frequency, relative intensity noise, and frequency noise in the 1mHz--10Hz frequency band.
