An automatic frequency stabilized laser with hertz-level linewidth
Introduction
Nowadays, ultra-stable laser system, benefiting from low-frequency noise and short-term stability, is crucial to many research areas such as the test of fundamental physics constants [1], [2], the detection of gravitational wave [3], [4] or dark matter [5], [6], low phase-noise microwave generation [7], [8], high precision phase-coherent frequency transfer, and atomic frequency standard [9], [10]. Recently some unmanned applications require long-term robust frequency locking especially for space-borne scientific missions such as eLISA, TianQin, GRACE-FO, GOCE, NGGM, etc. [11], [12], [13], [14], [15], [16].
Various automatic approaches for laser frequency relocking are proposed and developed to retrieve the lock without any human intervention and enhance system robustness in a harsh environment. Novel automatic relock systems based on analog circuits have been developed for locking the laser to the optical resonant cavity [17], [18]. However, a versatile control algorithm is difficult to be included in analog circuits, which makes it not qualified for various conditions. Later various digital automatic relock systems have been developed for diverse applications [19], [20]. A digital laser frequency auto-locking system based on FPGA is developed for inter-satellite laser ranging and a control bandwidth of 100 kHz is achieved which is limited by the speed of FPGA [21], [22], [23]. Furthermore, the commercial FPGA Laser Frequency Offset Unit from Stable Laser Systems Inc. offers auto-locking and re-locking too. However, the bandwidth of the PID controller should be wide enough to cover the whole laser’s frequency noise in the free-running mode. To achieve the ultra-stable laser system with serval hertz linewidth, the bandwidth of the controller needs to be at the level of MHz in general, which is difficult to be achieved only by digital circuits due to its long response time.
Benefited from the high speed and low noise, the analog circuit is attractive to apply in servo systems. The complex algorithms can be flexibly used by digital circuits [24]. However, the latency of algorithms in digital systems limits the feedback bandwidth of digital circuits. The analog-digital hybrid circuit is attractive to realize automatic frequency stabilization. A system consists of an analog loop filter for the fast feedback loop and a digital circuit for the slow control loop has been proposed [25]. The 1.5 MHz bandwidth is obtained in the fast feedback loop, and a digital circuit for the slow control loop can achieve a dynamic range to achieve relock. However, the parameters of the analog fast feedback loop can not be adjusted flexibly. In this paper, to take advantage of each, the analog circuit is used for PID control which requires wide bandwidth, while the digital circuit is used for automatic relocking and PID parameter adjustment. A wide enough control bandwidth and built-in flexibly automatic control algorithms are employed to improve its applicability. This paper is to develop an automatic laser frequency stabilization system for field and space applications.
Section snippets
System design
The layout of the overall system is presented in Fig. 1. In the optical part, a robust interference-filter ECDL emitted at 698 nm with 180 kHz linewidth is employed as a single-frequency source, which is designed and presented in our previous work [26]. The laser beam is coupled to an optical isolator (ISO) to avoid light feedback. The output beam passes through a half-wave plate() for adjusting the laser polarization to match the main axial of the electro-optic modulator (EOM), then sends
Result and discussion
To evaluate the performance of the homemade locking system, the transfer functions of the Toptica FALC110 module and the homemade relocking module are measured both in open loop (P gain only) by the Frequency Response Analyzer. As shown in Fig. 5 (a), the −3 dB bandwidth is about 0.5 MHz and 2 MHz, respectively. The phase loss reaches −180 is about 8 MHz and 15 MHz, respectively. The electrical bandwidth of the homemade module is slightly larger than the Toptica module. To evaluate noise
Conclusion
A novel automatic unmanned intervention laser frequency stabilization system is developed based on an analog-digital hybrid servo controller. The measured PID bandwidth is 2.1 MHz mainly thanks to the analog circuits. With the help of digital circuits, the average auto-relocking time is about 4.86 s. To demonstrate the robustness of the laser control system, one week of continuous laser frequency lock without human intervention is demonstrated and no momentary unlock occurs. Beat with another
CRediT authorship contribution statement
Xinqian Guo: Investigation, Methodology, Data curation, Writing – original draft. Linbo Zhang: Investigation, Methodology, Data curation. Jun Liu: Software, Validation. Long Chen: Validation, Investigation. Le Fan: Software, Data curation. Guanjun Xu: Supervision. Tao Liu: Conceptualization, Resources, Writing – review & editing. Ruifang Dong: Writing – review & editing. Shougang Zhang: Supervision, Resources.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the National Key Research and Development Program of China (2016YF-F0200200); National Natural Science Foundation of China (NSFC) (11803041, 61127901, 91636101); Strategic Priority Research Program of the Chinese Academy of Sciences (CAS) (XDB21000000); Open Research Fund of State Key Laboratory of Transient Optics and Photonics (SKLST201909).
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