Objective Ultrastable low-noise ultranarrow linewidth laser light source has a wide range of applications in precision measurement, optical atomic clock, time-frequency transmission, and low-noise microwave generation. Ultrastable cavity Pound-Drever-Hall(PDH) frequency stabilization technology is one of the most important solutions for obtaining such ultrastable lasers. Based on this, the linewidth of the distributed feedback(DFB) single-frequency fiber laser has reached the order of millihertz. In addition to the performances of the ultrastable cavity and servo system, the available linewidth and frequency instability of the ultrastable fiber laser depend on the performances of the linewidth, frequency drift and noise level of the free-running fiber laser, and the laser frequency-tuning mechanism. Although many research institutions in China have developed single-longitudinal-mode fiber lasers with frequency-tuning mechanisms and the laser frequency has been stabilized within a hundred kilohertz for a long time by the saturated-absorption frequency stabilization technology based on fine transition spectral lines of gas molecules, home-made single-frequency fiber laser has not been applied to ultrastable cavity PDH frequency stabilization technology to obtain subhertz-linewidth ultrastable fiber laser source. Therefore, it is very necessary to investigate the PDH frequency stabilization with such home-made single-frequency fiber lasers.
Methods By optimizing the structure parameters of the laser cavity, adopting adiabatic packaging and precision temperature control, and integrating the piezoelectric transducer (PZT) in the cavity that can quickly and widely tune the laser frequency, a free-running DBR fiber laser that can be used to obtain an ultrastable laser via ultrastable-optical-cavity PDH frequency stabilization was developed. The laser power was boosted by a low-noise single-mode polarization-maintaining fiber amplifier. The ultrastable optical cavity used for frequency stabilization was made of ultralow expansion (ULE) glass with a cavity length of 10 cm. The corresponding free spectral range(FSR) was 1.5 GHz and the fineness was 360000. The fiber laser was modulated by an electro-optical modulator (EOM) and then coupled into the optical cavity. The laser with modulated sidebands reflected by the optical cavity was detected with a photodetector and then mixed with the drive signal of the EOM through a mixer to obtain the error signal for laser frequency stabilization. The error signal was processed by the servo system, and the low-frequency component was fed back to control the voltage of the PZT to compensate the low-frequency fluctuations of the laser frequency. The high-frequency component was fed back to the acousto-optic modulator (AOM) drive controller to achieve the high-frequency fluctuation compensation of the laser frequency. To evaluate the effect of PDH frequency stabilization of our fiber laser, two DBR fiber lasers were stabilized to the two adjacent cavity modes of the optical cavity at the same time. The performance parameters of the frequency stabilization fiber laser were then measured by the beat frequency of the two frequency-stabilized lasers.
Results and Discussions The developed DBR fiber laser exhibits stable single-longitudinal-mode oscillation characteristics [Fig. 2(a)]. The relationship between the amount of change in the output laser frequency and the modulation frequency of the tuning voltage when the PZT is applied with different voltage values is given [Fig. 2(b)]. The tuning bandwidth of the laser frequency adjusted by PZT is about 8--10 kHz and the maximum tuning range exceeds 3.2 GHz. The signal-to-noise ratio of the output laser is about 60 dB [Fig. 3(a)] and the 3-dB linewidth of the laser is about 1.25 kHz [Fig. 3(b)]. The error signal is recorded by the oscilloscope when a triangular-wave sweep voltage of 7 V at 20 Hz is applied to PZT [Fig. 5(a)]. The error signal then changes to a straight line after the laser frequency is locked to the reference cavity [Fig. 5(b)]. There is a drift in the frequency of the beat frequency signal. The range of drift within 1 h is less than ±20 Hz [Fig. 6(a)] and the frequency drift of each laser after frequency stabilization is less than ±10 Hz. The frequency instability of the frequency-stabilized fiber laser corresponding to 1 s and 100 s is 6×10 -16 and 8×10 -15, respectively [Fig. 6(b)]. Fig. 7(a) displays the measured frequency noise power spectrum of the frequency-stabilized fiber laser in the range of 1 mHz--100 kHz. The frequency noise is reduced by more than eight orders in the range of 1 mHz--10 Hz. The frequency noise is reduced to about 8×10 -3 Hz 2/Hz especially from 1 to 10 Hz. Using the measured beat frequency data, the laser linewidth after Lorentz fitting is 280 mHz [Fig. 7(b)].
Conclusions We have demonstrated the results of ultrastable cavity PDH frequency stabilization based on a home-made single-frequency DBR fiber laser at 1550 nm. By optimizing the structure parameters of the laser cavity, adopting adiabatic packaging and precision temperature control, and integrating the PZT in the cavity that can quickly and widely tune the laser frequency, a free-running DBR fiber laser that can be used to obtain an ultrastable laser via ultrastable-optical-cavity PDH frequency stabilization is developed. Using an ultrastable optical cavity with a length of 10 cm and a fineness of 360000, the frequency drift of the fiber laser after PDH frequency stabilization is less than ±10 Hz and the frequency instability at 1 s and 100 s is 6×10 -16 and 8×10 -15, respectively. The frequency noise is reduced to 8×10 -3 Hz 2/Hz at 1--10 Hz and the linewidth is narrowed down to 280 mHz. It is shown that the main performances of our lasers can be used to construct subhertz-linewidth ultrastable laser light sources, which can be used in fields such as gravitational wave detection, precision measurement, and time-frequency transmission.
