We report a low noise continuous-wave (CW) single-frequency 1.5-µm laser source obtained by a singly resonant optical parametric oscillator (SRO) based on periodically poled lithium niobate (PPLN). The SRO was pumped by a CW single-frequency Nd:YVO 4 laser at 1.06 µm. The 1.02 W of CW single-frequency signal laser at 1.5 µm was obtained at pump power of 6 W. At the output power of around 0.75 W, the power stability was better than ±1.5% and no mode-hopping was observed in 30 min and frequency stability was better than 8.5 MHz in 1 min. The signal wavelength could be tuned from 1.57 to 1.59 µm by varying the PPLN temperature. The 1.5-µm laser exhibits low noise characteristics, the intensity noise of the laser reaches the shot noise limit (SNL) at an analysis frequency of 4 MHz and the phase noise is less than 1 dB above the SNL at analysis frequencies above 10 MHz.

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The molecular structures, UV-Vis absorption spectra, and energy level structures of the dyes D-SS and D-ST were simulated using density functional theory, time-dependent density functional theory (TDDFT), and natural bond orbital analysis, which provided the physical mechanisms of dye-sensitized solar cells (DSSCs) containing D-ST and D-SS. The UV-Vis absorption spectrum of D-SS showed a significant red shift compared with that of D-ST and the molar absorption coefficient of D-SS was higher than that of D-ST. D-SS molecules should have a higher solar radiation photon-harvesting ability than D-ST molecules, but the energy level of the highest occupied molecular orbital (HOMO) of D-SS was higher than the redox energy level of the electrolyte (I-/I-3). As a result, an optically excited D-SS molecule cannot be successfully recovered by accepting an electron from the electrolyte after being oxidized by injecting an electron towards the TiO2 electrode. This limits the photon harvesting ability of D-SS molecules, and thereby significantly decreases the photovoltaic properties and energy conversion efficiency of DSSCs containing D-SS. This allows the photovoltaic properties of DSSCs containing D-SS to be understood, especially why its photovoltaic energy conversion efficiency is lower than that of DSSCs containing D-ST. The position of the HOMO energy level of dye-sensitized molecules is very important for the operation of DSSCs, and that of the organic sensitizer molecules used in DSSCs must be lower than the redox energy

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