Objective The near-infrared 1240 nm lasers have been widely used in many scientific research fields such as optical time domain reflectometer (OTDR) and water remote sensing, owing to their intrinsic merits including narrow linewidth, perfect beam quality, higher stability, and lower noise. However, it is impossible to directly obtain the 1240 nm laser by the existing laser gain media because there are not suitable gain media covering the 1240 nm spectrum. In recent years, some groups have successfully obtained the stable single-frequency (SF) 1240 nm lasers by means of the stimulated Raman scattering (SRS) based on Raman gain media. Nonetheless, due to small Raman scattering cross-section of Raman gain media, the threshold power of the attained Raman laser is so high that more incident pump power is necessary to scale up the output power of 1240 nm laser. In this paper, we present a cavity resonance-enhanced watt-level SF 1240 nm Raman laser. After the cavity is stably locked to the incident pump laser, the pump and Raman lasers resonate simultaneously in the designed resonator. In this case, the pump threshold of the Raman laser is effectively decreased, and the stable SF Raman laser is obtained at the same time, which provides an effective and feasible way to obtain stable high power SF Raman laser.
Methods In this study, firstly, a high-quality single-crystal diamond grown by chemical vapor deposition (CVD) is chosen as the Raman gain crystal, which has good optical properties of low-nitrogen, low-birefringence, and so on. And an all-solid-state CW SF 1064 nm infrared laser with good performance is served as the pump source to avoid mode competition in the process of SRS. On this basis, according to the transmission matrix theory of the optical resonant cavity, a symmetrical bow-tie double-resonance cavity for both pump and Raman laser is reasonably designed. Furtherly, based on the SRS process rate equation and the principle of cavity resonance-enhancement technology, the transmissivity of the input coupling mirror for the pump laser and the transmissivity of the output coupling mirror for the Raman laser are optimized as 3.5% and 0.5%, respectively. Then, the H?nsch-Couillaud (H-C) locking system is used to accurately lock the resonating frequency of the cavity to the frequency of the pump laser. In addition, a retro-reflecting device consisting of a plane mirror (M5) coated with high-reflection film at the wavelength of the Raman laser is used to reflect the backward wave leaking from the output coupling mirror to ensure the unidirectional operation of the Raman laser. Finally, an SF 1240 nm Raman laser with stable unidirectional operation is attained.
Results and Discussions In order to accurately lock the Raman cavity, the cavity length is scanned by changing the voltage loaded on the piezoelectric transducer (PZT), and the transmission peak of the cavity is detected by the photodetector. When the incident pump power is lower than the threshold power, the transmission peak curve is a standard Gaussian curve. However, when the incident pump power exceeds the threshold, the transmission peak curve has a broad detuning range, and the detuning becomes severe with the increasing incident pump power (Fig. 4). In the experiment, the cavity detuning is well compensated by reducing the temperature of the diamond crystal and then the cavity length of the Raman laser can be stably locked at the resonant frequency of the pump laser at high incident pump power. After the cavity is stably locked, the maximal output power of the stable SF 1240 nm Raman laser reaches up to 1.48 W with the incident pump power of 9.17 W (Fig. 5). The threshold pump power is as low as 2.73 W, which indicates that the double-resonance cavity can effectively decrease the threshold of the Raman laser.
Conclusions A watt-level SF 1240 nm Raman laser with low pump threshold is demonstrated in this paper, which is implemented by using an SF 1064 nm laser and a diamond crystal as the pump source and Raman gain medium, respectively. In order to decrease the threshold of the Raman laser, a double-resonance cavity is designed and adopted with assistance of cavity resonance-enhancement technology. After the parameters of the optical resonator are optimized and the resonating frequency of the Raman resonator is locked to the frequency of the incident pump laser by the H-C locking system in the experiment, the pump and Raman lasers resonate simultaneously in the designed resonator. The attained pump threshold is as low as 2.73 W. On this basis, the output power of SF 1240 nm laser reaches up to 1.48 W when the pump power is 9.17 W, and the corresponding slope efficiency is 24.9%. The measured long-term power stability in 30 min and the beam quality M2 are better than 1.10% (RMS) and 1.2, respectively. The achieved Raman laser with double-resonance cavity can provide a feasible way to decrease the threshold pump power of Raman laser, and the obtained stable SF 1240 nm laser source can be used in atmospheric monitoring and biomedicine field.
