The National Time Service Center of China is developing a compact, highly stable, 698 nm external-cavity diode laser (ECDL) for dedicated use in a space strontium optical clock. This article presents the optical design, structural design, and preliminary performance of this ECDL. The ECDL uses a narrow-bandwidth interference filter for spectral selection and a cat’s-eye reflector for light feedback. To ensure long-term stable laser operation suitable for space applications, the connections among all the components are rigid and the design avoids any spring-loaded adjustment. The frequency of the first lateral rocking eigenmode is 2316 Hz. The ECDL operates near 698.45 nm, and it has a current-controlled tuning range over 40 GHz and a PZT-controlled tuning range of 3 GHz. The linewidth measured by the heterodyne beating between the ECDL and an ultra-stable laser with 1 Hz linewidth is about 180 kHz. At present, the ECDL has been applied to the principle prototype of the space ultra-stable laser system.

Thermal noise in optical cavities sets a fundamental limit to the frequency instability of ultra-stable lasers. Numata et al. derived three equations based on strain energy and the fluctuation–dissipation theorem to estimate the thermal noise contributions of the spacer, substrates, and coating. These equations work well for cylindrical cavities. Extending from that, an expression for the thermal noise for a cubic spacer based on the fluctuation–dissipation theorem is derived, and the thermal noise in cubic optical cavities is investigated in detail by theoretical analysis and finite element simulation. The result shows that the thermal noise of the analytic estimate fits well with that of finite element analysis. Meanwhile, the influence of the compressive force F_p on the thermal noise in cubic optical cavities is analyzed for the first time. For a 50 mm long ultra-low expansion cubic cavity with fused silica substrates and GaAs/AlGaAs crystalline coating, the displacement noise contributed from every F_p of 100 N is about three times more than that of the substrate and coating.

Optical reference cavities are a key component of ultra-stable lasers. Based on the theory of elastic deformation, the displacement difference between the top and bottom mirrors of an optical vertical cavity supported on the midplane was analyzed in the present research. The displacements and tilt angles of the two mirrors of the optical reference cavity were deduced based on Poisson’s ratio. In order to verify the validity of the theoretical analysis, the vibration sensitivity induced by the elastic deformation of vertical optical reference cavities was simulated by finite element analysis. In order to compensate for the displacement difference between the top and bottom mirrors under an external force, the zero vibration sensitivity of the optical cavities was achieved by changing the thicknesses of Ultra-Low Expansion glass rings and the mirrors. The results and methods presented in the present paper may facilitate the design of vertical reference cavities, especially transportable optical reference cavities.

Two diode lasers at 698 nm are separately locked to two independent optical reference cavities with a finesse of about 128,000 by the Pound–Drever–Hall method. The more accurate coefficient between voltage and frequency of the error signal is measured, with which quantitative evaluation of the effect of many noises on the frequency stability can be made much more conveniently. A temperature-insensitive method is taken to reduce the effect of residual amplitude modulation on laser frequency stability. With an active fiber noise cancellation, the optical heterodyne beat between two independent lasers shows that the linewidth of one diode laser reaches 1 Hz. The fractional Allan deviation removed linear frequency shift less than $30mHz/s$ is below $2.6×10−15$ with 1- to 100-s average time.

In many scientific fields, the continuous operation of ultra-stable lasers is crucial for applications. To speed up the frequency stabilization process in case of the occurence of unexpected interruptions, a prompt frequency stabilization approach based on an improved mean shift algorithm is proposed and verified with a homemade laser system. We developed a double-loop feedback controller to steer the laser frequency with fast and slow channels, respectively. In this study, an improved mean shift algorithm is utilized to intelligently search for the transmission signal, which involves adaptively updating the sliding window radius and incorporating a Gaussian kernel function to update the shift vector. The number of lock points on the left and right sides of the central point determines the scanning direction to search for the transmission signal quickly. The laser is intentionally interrupted 306 times within 10,000 s to evaluate the relocking performance. The median auto-locking time of the laser is improved from 16 s to 4 s. By beating with another ultra-stable laser system, the laser frequency instability is measured to be less than $2.1\times {10}^{-14}$ and the linewidth is 5 Hz. This work improves the adaptation and relocking ability of the ultra-stable laser in a complex environment.