This paper primarily discusses the structural performance analysis of a single/double/quadruple tapered optical fiber (TOF) structure based on single-mode fiber (SMF) and multi-mode fiber (MMF). Furthermore, the TOF’s performance, including its diameter distribution, transmitted intensity, and reproducibility, is also evaluated. According to the experimental results, it can be concluded that the quadruple TOF structure based on SMF has a higher density of evanescent waves (EWs) on the surface of the tapered area, which is essential for the fabrication of high-sensitivity optical fiber sensors. The structure proposed in this article is feasible, and it can be used for optical fiber sensing while offering significant practical and promising applications as well.

Stimulated polariton scattering (SPS) and stimulated Raman scattering (SRS) in R b T i O P O 4 (RTP) crystal are combined in an intracavity-pumped Stokes parametric oscillator (SPO) to extend the tunable Stokes laser spectral range. The pumping laser wavelength is 1064 nm from a diode-end-pumped acousto-optically Q-switched Nd:YAG laser. By the SPS process in the SPO, the SPS-Stokes wave can be discontinuously tuned in the range of 1075.7–1076.0 nm, 1077.7–1080.4 nm, 1081.8–1082.2 nm, and 1084.8–1087.8 nm, respectively. By the following SRS process in the same RTP crystal, the laser wavelength is further shifted in the range of 1107.7–1108.1 nm, 1109.0–1112.7 nm, 1114.3–1115.1 nm, and 1117.8–1121.1 nm, respectively. A maximal average output power of 970 mW is achieved for the SRS-Stokes wave at the peak wavelength of 1118.8 nm. It is obtained when the diode power is 7.9 W, and the pulse repetition frequency (PRF) is 10 kHz.

This paper reports an intracavity terahertz parametric source based on the stimulated polariton scattering in RbTiOPO4 crystal with a high repetition rate and a high average power. The side pumping for the gain medium by laser diodes is adopted to get a higher fundamental power and a larger fundamental laser beam size than those in the diode-end-pumping in order to improve the THz output power. A non-collinear convex-plane fundamental cavity is adopted so that the thermal effect can be offset in some degree and the fundamental beam size is further increased. The obtained maximum average THz output power is 367 μW at 3.88 THz when the diode pump power is 105 W and the pulse repetition frequency (PRF) is 7 kHz. The terahertz power of 367 μW is the highest ever obtained in SPS sources.

In this paper, a KTiOPO₄ (KTP) crystal is used as the nonlinear medium in an intracavity-pumped terahertz parametric oscillator (TPO) based on stimulated polariton scattering (SPS). Almost all the reported intracavity SPS sources adopt the diode-end pumped configuration, and the THz output power is limited by the serious thermal effect and small fundamental beam size. For improving the THz output power, we propose diode-side pumping for the laser medium to get a higher fundamental power and a larger fundamental beam size. A convex–plane fundamental laser resonator is used to further offset the thermal effect and increase the fundamental beam size. The THz frequency of the intracavity-pumped KTP terahertz parametric oscillator can be discontinuously tuned from 3.19 THz to 5.94 THz with three gaps. The fundamental beam diameter in the KTP crystal is about 1.3 mm. The maximum average THz power is 166 μW at 5.74 THz under a pulse repetition frequency (PRF) of 6 kHz and a diode pump power of 98 W. By means of the diode-side pumped configuration, the maximum THz output power is more than two-fold higher compared to the diode-end pumped configuration reported using the KTP crystal.

In this paper, the rate equations describing the operation of intracavity-pumped Q-switched terahertz parametric oscillators based on stimulated polariton scattering are given for the first time. The rate equations are obtained under the plane-wave approximation, the oscillating fundamental and Stokes waves are supposed to be round uniform beam spots. Considering the fact that the terahertz wave nearly traverses the pump and Stokes beams and using the coupled wave equations, the terahertz wave intensity is expressed as the function of the fundamental and Stokes intensities. Thus, the rate equations describing the evolution processes of the fundamental and Stokes waves are obtained in the first step. The THz wave properties are then obtained. Several curves based on the rate equations are generated to illustrate the effects of the nonlinear coefficient, the THz wave absorption coefficient, and pulse repetition rate on the THz laser characteristics. Taking the intracavity-pumped Mg:LiNbO₃ TPO as an example, the THz frequency tuning characteristic and the dependences of the fundamental, Stokes, and THz wave powers on the incident diode pump power are calculated. The theoretical results are in agreement with the experimental results on the whole.

For the first time, a diode-pumped actively Q-switched Nd:YVO₄/RbTiOPO₄ (RTP) intracavity Raman laser at 1.49 µm was demonstrated to the best of our knowledge. Experimentally, a dual-end diffusion-bonded YVO₄–Nd:YVO₄–YVO₄ crystal was employed as the laser medium to generate 1.34 µm laser radiation, and an RTP crystal as the Raman medium to enable the frequency conversion, by which radiation at 1.49 µm was achieved successfully. With an incident pump power of 10.4 W, an average output power of 502 mW was obtained at a pulse repetition rate of 15 kHz, corresponding to a conversion efficiency of 4.8%.