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摘要
Objective: Compact, high-power, low-cost yellow lasers at ~589 nm have potential in dermatological applications. There is no solid-state gain medium that can directly lase at 589 nm, so nonlinear frequency conversion of near-infrared laser is an indispensable technology. The yellow solid laser is usually generated by sum mixing from two Nd:YAG lasers at 1064 nm and 1319 nm, which the multiple-cavity systems are too complex to use. Yb-doped silica fiber has gain at 1178 nm, but lasing at this wavelength is difficult. So, the Raman fiber laser and amplifier are known for their unique advantage of flexibility in wavelength. The common frequency doubling method employs an external enhancement cavity to achieve high-efficiency and high-power frequency doubling. Nevertheless, it adds complexity to the laser system. In this article, we report a compact, low-cost, high-power, narrow-linewidth yellow laser by single-pass frequency doubling of a cascaded Raman fiber laser in a periodically poled MgO-doped near-stoichiometric LiTaO3 crystal (PPSLT). Up to 10.19-W 589-nm laser is obtained with a conversion efficiency of 18.12%, which is limited by the fundamental laser linewidth. To the best of our knowledge, this laser system has the simplest structure, is the easiest to operate, and is the most suitable for commercial use. Methods: The experimental configuration of the yellow fiber-based laser is shown in Fig. 1, including three functionally different parts-a 1070 nm fiber laser used as a Raman pump source, a cascaded Raman fiber laser, and a single-pass frequency doubling device. The 1070-nm source is a conventional fiber Bragg grating (FBG)-based fiber oscillator. The gain fiber is 10/125-μm polarization-maintaining (PM) Yb-doped fiber (YDF). Using one 1070/1120-nm wavelength division multiplexing (WDM), the generated 1070 nm laser is coupled into the cascaded Raman oscillator, which comprised PM980 gain fiber and two pairs of FBGs with wavelengths of 1120 nm and 1178 nm, respectively. The 1120 nm FBG had a high reflection, which could improve the conversion efficiency from 1120 to 1178 nm. This structure reduced the length of the gain fiber and improved the conversion efficiency. The collimated 1178 nm fiber laser output is optically isolated and focused to incident on a periodically poled MgO-doped near-stoichiometric LiTaO3 crystal (PPSLT). The diameter of the 1178 nm output laser is ~1.3 mm. The length of PPSLT is 20 mm, and the end faces are coated to low reflectivity (R<0.2%) at both 1178 and 589 nm. The PPSLT crystal is installed in a homemade oven operating at 51 ℃ with a control accuracy of ±0.01 ℃. The input polarization of the isolator is matched by rotating the collimator, and the output polarization of the isolator is designed for vertical polarization, which is parallel to the poling direction of the crystal. A dichroic mirror is employed to separate the incident fundamental light and frequency-doubled light, which is highly transmissive at 589 nm (T>95%) and highly reflective at 1178 nm (R>99.5%). Results and Discussions: The CW 1070 nm output power and conversion efficiency are considered as functions of diode pump power. When the diode pump power reaches 209 W, the 1070 nm output power is scaled to 122 W, corresponding to ~58% optical-optical conversion efficiency [Fig. 2 (a)]. The 1070 nm fiber laser is injected into cascaded Raman oscillation cavities. The maximum output power of 1178 nm laser is scaled to 56.23 W [Fig. 2 (b)]. The central wavelength of the 1178 nm laser is almost the same at different output powers. The 3 dB linewidth of the 1178 nm laser increases with power from 0.04 to 0.38 nm [Fig. 3 (b)], which means the proportion of fundamental light that can be effectively converted is decreasing. The second-harmonic power and second-harmonic generation (SHG) efficiency are considered as functions of the fundamental power at the optimum phase-matching temperature (Fig. 4). When the fundamental power reaches 56.23 W, the maximum SHG output power is 10.19 W, corresponding to a conversion efficiency of 18.12%. The stability of 589 nm laser power measured during 1 h is shown in Fig. 5. Since the fundamental laser is generated with a single-mode fiber and the frequency doubling is achieved with a PPSLT crystal, near-diffraction-limited beam quality is expected (Fig. 6). Conclusions: A 589 nm yellow laser is developed by single-pass frequency doubling of a linearly-polarized narrow-linewidth 1178 nm cascaded Raman fiber laser in a PPSLT. A high-power cladding-pumped Yb-doped 1070 nm fiber laser is built as a Raman pump source. The cascaded Raman process is implemented by 1120- and 1178-nm Raman oscillators. WDM is used in the setup to couple the input pump light and filter the reflected Raman light. Up to 10.19 W continuous-wave 589-nm laser is obtained with a conversion efficiency of 18.12%. The wavelength flexible cascaded Raman fiber laser combined with the single-pass frequency doubling device has advantages of small volume, low cost, good robust performance, and easy operation, which are suitable for use in the medical field.
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