Experimental Study on Narrowing 632. 8 nm External Cavity Diode Laser Linewidth Based on Self Made Ultra-Stable F-P Cavity

Narrow linewidth lasers are widely used in the field of optoelectronic detection due to their advantages such as good monochromaticity, high stability, and long coherence length, including coherent communication, precision measurement, optical frequency standards, absorption spectrum measurement, and research on the interaction between light and matter. At present, the linewidth of stable He-Ne lasers can reach the order of MHz. The linewidth of distributed feedback (DFB) fiber lasers can reach the order of kHz. The linewidth of DFB semiconductor lasers can also reach the order of MHz. However, grating feedback semiconductor lasers can achieve Hundreds of kHz line width output. In order to further narrow the line width of various lasers, it is necessary to lock the laser to a certain frequency reference through feedback control technology. In this paper, a self-designed ultra-stable cavity is used as the frequency reference, and the effective narrowing of the linewidth of 632. 8 nm external cavity semiconductor laser (ECDL) is achieved. The narrow linewidth laser generation system includes the design of an ultra-stable cavity, the design of the optical path, the frequency control of ECDLand the integration of the system. The super-stable cavity adopts a two-mirror Fabry-Perot cavity (F-P cavity) structure, and the cavity is glass-ceramic with an expansion coefficient of about 10(-)(6) K-1. The cavity mirror is a pair of flat and concave mirrors with a reflectivity of 99. 988 5% +/- 0. 003 5%. In order to reduce the influence of the external environment on the cavity length of the FP cavity, the temperature of the cavity needs to be designed. This system uses four peltiers with a total power of 96 W and a water-cooled heat dissipation design. In order to reduce the influence of sound and air flow on the cavity mode frequency, the F-P cavity is placed in a vacuum chamber with a vacuum of 10(-)(5 )torr. For effective vibration isolation, the cavity is isolated from the vacuum chamber with a silicone rubber material. The ECDL used by this system is the DL pro series laser from German Toptica Company, which has two frequency control terminals of piezoelectric (PZT) and current modulation, and the response bandwidth is 1 kHz and 100 MHz respectively. The frequency control of the laser uses Pound-Drever-Hall ( PDH) frequency locking technology. The modulation frequency of 18 MHz is loaded on the current modulation terminals of the laser. The error signal is obtained by demodulating the reflected signal of the FP cavity. Through two-way feedback control, a locked bandwidth of nearly 1 MHz is achieved. Through continuous optimization of the system, we finally narrowed the laser line width of about 300 kHz in the free-running state to the order of 10 kHz, and the system operated stably, and the frequency drift locked for 12 hours was about 30 MHz. The 632. 8 nm narrow linewidth laser source developed in this paper can not only be applied to the field of absorption spectrum measurement, but also play an important role in the field of optical surface precision measurement.