33 W continuous-wave single-frequency green laser by frequency doubling of a single-mode YDFA
Continuous-wave (CW) green lasers have many scientific and industrial applications, including laser display[1,2], pumping of Ti-sapphire and dye lasers[3], semiconductor mask inspection[4], the generation of CW UV radiation by second-harmonic generation (SHG)[5], etc. Due to the lack of solid state gain medium that can directly lase at the green spectral range, the standard method for generating a green laser is nonlinear frequency conversion. Periodically poled lithium niobate/tantalite (
Yb-doped fiber (YDF) lasers have advanced greatly in recent years, and are replacing bulk solid state lasers in many fields of application[12,13]. The fiber laser geometry is beneficial for thermal management, thus, it supports higher power operation[14,15]. So, the frequency doubling of a YDF laser is a superior route to produce high power green sources. Normal intracavity frequency doubling is not efficient for fiber lasers due to the limited intracavity power enhancement. A novel method of SHG in an enhancement resonator within a fiber laser cavity has been proposed and generated up to a 19 W output[16]. However, the laser stability and power scalability remain to be demonstrated. Instead, SHG in an external enhancement cavity is possibly a more scalable technique for generating a fiber-based visible laser, because it separates the fundamental laser and frequency doubling stage. With a 1064 nm single-frequency fiber master oscillator power amplifier (MOPA) and a resonant cavity for enhanced SHG in LBO, a more than 20 W CW single-frequency 532 nm laser was reported[17]. Similar results have been reported at other wavelengths[18]. The challenge for further power scaling is on increasing the power of the single-frequency YDF MOPA, which is usually limited by stimulated Brillouin scattering (SBS), and thermal management of the resonant doubling cavity.
In this Letter, we report a high power CW single-frequency 532 nm laser by frequency doubling of a YDF MOPA. The CW linearly polarized single-frequency YDF amplifier (YDFA) produces a 60 W laser at 1064 nm with an optical conversion efficiency of 58%. A bow-tie external resonant cavity incorporating an LBO nonlinear optical crystal is used for generating the second harmonic of the fiber laser. The LBO crystal is type-I noncritically phase matched, which avoids a walk-off effect between the fundamental and harmonic waves[19,20]. A 33.2 W green laser at 532 nm is generated with a 45 W incident fundamental laser. The corresponding conversion efficiency is 74%.
The experimental configuration of the green fiber laser is shown in Fig.
The collimated 1064 nm fiber amplifier output is optically isolated, and then coupled into a homemade bow-tie ring resonant cavity with a pair of steering mirrors and a mode-matching lens. The resonant cavity consists of two plane mirrors (M1, M2) and two concave mirrors (M3, M4). The total geometrical cavity length is 157 mm. A 30-mm-long, noncritically phase-matched, antireflection-coated LBO crystal is installed in an oven and placed between two concave mirrors. Both mirrors have a curvature radius of 50 mm. In the experiment, the optimum crystal temperature is found to be 154.6°C, which is higher than the theoretical value of 149°C. This is because one side face of the crystal is exposed to ambient air of 20°C. The beam waist radius inside the LBO crystal is
In order to achieve efficient SHG, it also needs to ensure optimum overlap between the fundamental light beam and the eigenmode of the resonant cavity. The waist radius at the mid-point of the resonator arm defined by two plane mirrors is calculated to be
Limited by the nonlinear effect of SBS, the power scaling has been difficult for a single-frequency fiber amplifier. So far, there are several methods for increasing the SBS threshold, for example, by using a large mode area (LMA) fiber[21], decreasing the gain fiber length[22], broadening the effective SBS gain spectrum[23], and so on. In the experiment, the SBS is avoided by using a Yb-doped gain fiber and a delivery fiber as short as possible. Only a 1.5 m gain fiber is used. The linearly polarized single-frequency 1064 nm laser output as a function of diode pump power is shown in Fig.
Fig. 2. 1064 nm output power and backward light power of the main amplifier as a function of the pump power.
The spectra of the collimated 1064 nm laser at different output powers are measured with an optical spectra analyzer (AQ6370), as shown in Fig.
Fig. 3. Output spectra of the 1064 nm fundamental laser at different output powers: (a) wide spectra, (b) fine spectra.
Fig. 4. Spectra of the backward light from the 1064 nm main amplifier at different output powers: (a) wide spectra, (b) fine spectra.
The amplified single-frequency 1064 nm laser is frequency doubled in a resonant cavity described in the setup section. Mode-match lenses with various focal lengths are compared for efficient coupling. A lens with a focal length of 300 mm is finally chosen for high power experiments. The optimum position of the lens is fine-tuned experimentally. Figure
Fig. 5. Dependence of the second-harmonic output power and the conversion efficiency on the pump power. A numerical fitting is also given.
The linewidth of the 1064 and 532 nm lasers are measured with a Fabry–Perot interferometer (FPI) of 4 GHz free spectral range for the respective wavelength region. As shown in Fig.
In conclusion, we present a simple structure, high power 532 nm fiber laser system based on a 1064 nm YDFA and a highly efficient cavity-enhanced second-harmonic converter. Stable and reliable locking of the enhancement cavity is achieved with a piezoelectric ceramic transducer (PZT) actuator, which is controlled in a PDH scheme. Frequency doubling is achieved in a noncritically phase-matched LBO crystal. More than 30 W CW single-frequency 532 nm radiation with great beam quality is generated with an external conversion efficiency of 74%. The fiber-based single-frequency green laser is an efficient and reliable alternative pump source for a Ti-saphire laser, and may find applications in laser projects, laser shows, material processing, etc.
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Shuzhen Cui, Lei Zhang, Huawei Jiang, Yan Feng. 33 W continuous-wave single-frequency green laser by frequency doubling of a single-mode YDFA[J]. Chinese Optics Letters, 2017, 15(4): 041402.