Objective Thanks to the promising performances of narrow spectral linewidth, low noise and high coherence, single-frequency fiber lasers (SFFLs) have attracted considerable interests for a variety of applications including gravitational wave detection, LIDAR and nonlinear frequency conversion. In the 1.0 µm spectral band, single-frequency lasing from rare earth doped fibers mainly operates in the 1000-1120 nm wavelength region. However, for applications such as metrology and atomic spectroscopy that employ SFFLs in the visible band, extending the operation wavelength range to meet the application needs is urgently required. In particular, frequency doubling of 1178 nm SFFL to produce yellow light is crucially demanded in laser-guide-star detection. However, the large gain of ytterbium-doped fiber (YDF) between 1030 and 1100 nm would induce very strong amplified spontaneous emission (ASE) and lead to parasitic lasing, limiting the available lasing at 1178 nm.
Methods This paper presents the realization of a high-performance 1178 nm SFFL based on the DFB (distributed feedback) structure. By inscribing a periodic structure on a 5 cm long YDF and introducing a π-phase shift point in the fiber Bragg grating, an extremely narrow spectral transmission window was formed, ultimately resulting in single-frequency laser output. The structural setup of the laser is depicted in Fig.1, where pumping light was coupled into the phase-shifted grating through a wavelength division multiplexer (WDM). The output laser was then tested through the backward output end of the WDM.
Results and Discussions The output power of the laser varies with the pumping power was recorded and shown in Fig.2, where it can be seen that the maximum output power of the laser is 13.0 mW and the slope efficiency is 6.92%. The spectrum at maximum output power is illustrated in Fig.3, showing an output wavelength of 1 178.01 nm with an overall signal-to-noise ratio up to 63 dB. To verify the single-frequency characteristics of the laser operation, its longitudinal mode was tested and shown in Fig.4. Only two main peaks were observed within one scanning voltage cycle, indicating the stable single longitudinal mode operation. Subsequently, the polarization extinction ratio (PER) was measured to be more than 19 dB across different power levels, demonstrating a consistent stability in polarization state of the laser. The relative intensity noise (RIN) was further examined and shown in Fig.5(a), in which the intensity noise levels remained around −120 dB/Hz within low frequencies ranging from 1 to 100 kHz. Furthermore, the phase/frequency noise of the laser was tested as depicted in Fig. 5(b). Within a 0.1 ms integration time, the linewidth of the 1178 nm single-frequency laser was calculated to be 27.61 kHz.
Conclusions In this work, a 5.0 cm long Yb-doped fiber was employed to implement a DFB structure to realize 1178.014 nm single-frequency laser output. This fiber laser has a maximum output power of 13 mW with a signal-to-noise ratio of 63 dB and a polarization extinction ratio of 19.7 dB. As far as we know, this is the first demonstration of a 1178 nm Yb-doped single-frequency fiber laser.
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