<p>We report efficient frequency downconversion of a continuous-wave low-power Raman laser using intracavity difference frequency mixing in a periodically poled lithium niobate (PPLN)-adopted optical parametric oscillator (OPO). The Raman laser was obtained using a 1060-nm fiber laser and 137-m passive fiber. Its central wavelength was fixed at 1111 nm with the power ranging from 320 mW to 5.984 W. The 1060- and 1111-nm pump beams were incident into the OPO at the same time. The high-power 1060-nm pump beam built parametric oscillation first, and the difference frequency generation (DFG) occurred between the low-power Raman laser and intracavity signal laser. The PPLN temperature was properly controlled at 45°C to ensure both the OPO and DFG processes synchronously satisfy phase matching conditions. Benefiting from intracavity high signal power, the Raman laser was successfully converted to the 3560-nm midinfrared radiation under every investigated pump power level. The maximum 3560-nm idler power reached 1.026 W, indicating a 17.4% pump-to-idler slope efficiency and about 15% optical-to-optical conversion efficiency. The comparative experiments also verified that the phase matching conditions were satisfied maximally at 45°C.</p>
In this paper, a dual-wavelength(DW) mid-infrared optical parametric oscillator(OPO) pumped by a DW fiber laser based on the stimulated Raman scattering(SRS) effect, is demonstrated. When the pump power satisfied the threshold condition, obvious SRS effect was observed and a DW fiber laser with center wavelengths at 1060 nm and 1113 nm was obtained. The DW fiber laser was injected into a single-period crystal, and the whole process went through four stages. In the first three stages, 1060 nm pump laser achieved parametric oscillation and generated 1602 nm signal laser and 3138 nm idler laser and the conversion efficiency was seriously affected by SRS effect. In the fourth stage, two independent parametric processes were realized, which generated two mid-infrared output at 3131nm and 3580nm with powers of 2.46W and 40mW, respectively. The efficiency characteristics in the four stages were also discussed separately.
The thermal stress damage of optical elements always restrict the development of high power laser system. We studied the thermal damage mechanism of the optical elements with contaminants induced by high power continuous wave (CW) lasers. An experiment was carried out by a self-build optical element testing platform and a model based on the temperature field theory and thermodynamic theory was set up. We recorded the thermal stress damage process based on a 10 kW/cm<sup>2</sup> level mid-infrared continuous wave laser. Then we calculated the thermal damage process of optical elements. The calculated results are in agreement with our experimental record. The results showed the success of modeling calculation in the thermal damage mechanism caused by contaminants.
In this paper, a four-wavelength continuous-wave mid-infrared optical parametric oscillator was demonstrated for the first time. The pump source was a home-built linearly polarized Yb-doped fiber laser and the maximum output power was 72.5 W. The pump source had three central wavelengths locating at 1060 nm, 1065 nm and 1080 nm. Four idler emissions with different wavelengths were generated which were 3132 nm, 3171 nm, 3310 nm and 3349 nm under the maximum pump power. The maximum idler output reached 8.7 W, indicating a 15% pump-to-idler slope efficiency. The signal wave generated in the experiment had two wavelengths which were 1595 nm and 1603 nm under the maximum pump power. It was analyzed that four nonlinear progresses occurred in the experiment, two of them being optical parametric oscillation and the rest two being intracavity difference frequency generation.
We use the fiber-pumped MgO:PPLN crystal to realize the MIR CW-OPO operation, and observe the nonuniform temperature distribution on the central axis of the crystal. Then we use the heat transfer model in COMSOL software to simulate the temperature distribution in the crystal and find the near linear temperature gradient on the central axis of the crystal. Input the axial temperature distribution to our SRO model based on MATLAB and the simulation results show that the linear axial temperature gradient distribution will not only cause the center wavelength shift of the signal light, but also reduce the parametric gain of the signal light, the uniform temperature along the crystal axis will get the maximum gain. This feature limits the prospect of the single OPO in high-power narrow linewidth laser.
The spread of the pump model, established based on MATLAB, simulates the distribution of the pump in End-Pumped single crystal fiber. Simulation results show that the pump in the rod single crystal fiber will converge again. By changing the crystal absorption coefficient, it can be found that smaller the absorption coefficient is, more uniform the pump distribution is; when it is greater, the pump will concentrate to the pump end more seriously. Establish End- Pumped Experimental platform in the experiment, the crystal is 1 mm in diameter and length of 30 mm, Nd<sup>3+</sup> doping concentration is 1%. Change the position of the pump light's focus in the crystal, we can see different distribution of the pump light by different focus location in the crystal and find that the pump light has the most homogeneous distribution when the focus is on the crystal axis and has 1mm distance to the pump end face. At this time, the second convergence of the pump is clearly visible. By changing the pump wavelength, crystal absorption coefficient changes. It is found that under the same pump power, absorption coefficient is greater, the pump will concentrate to the pump end more seriously. And the temperature of crystal pump end rises, which is identical with the simulation results. The results indicate that for the single crystal fiber, the higher absorption coefficient is not better, low absorption coefficient leads to the uniform distribution of the pump, there will be a better absorption in a relatively long length of single crystal fiber. And due to the lower end face temperature, end pump power upper limit will also increase.
The near and far field intensity distribution as well as the beam quality of the combination between the hollow beam generated by double axicons and the Gaussian beam were simulated in this paper. The simulation results revealed that several parameters like the interval between two axicons and the phase difference between the two beams would influence the intensity distribution of the combined beam, especially the phase difference between the hollow beam and Gaussian beam which could transforms the far-field intensity distribution into quasi-hollow distribution or peak shaped distribution and was of great potentiality in the industry application.
Limited by the thermal effects and the laser-induced damage characteristics of the non-linear crystals, mid-infrared (MIR) output power of single optical parametric oscillator (OPO) is hard to get further promoted with excellent beam quality. An alternative solution is the multiple-beams combination technology, which exactly provided an effective approach for decreasing the thermal effects and the damage risk of the OPO system under high power operation. In this letter, the experimental study on the spectral beam combination of three idler MIR lasers was carried out for the first time. An optical parametric system with MIR output power of 30 W at 3130nm, 3352nm, and 3670nm was finally obtained. Experimental results indicated that the beam quality M<sup>2</sup> factors of the combined laser were measured to be ~1.76 and ~2.42 in the horizontal and vertical directions, respectively, which confirmed the feasibility of the schematic design.
The hollow beam has a variety of special physical properties and can be applied to the optical catheter, optical trap, generation of the light trap and many other important fields. In this paper the light-field conversion of the Gaussian beam passing through double axicons and generating the hollow beam is theoretical derived and simulated using the light-field propagation method. The influence of several parameters on the near and far field intensity distribution of the hollow beam is discussed. We find that the hollow beam with different light-field can be generated by controlling these parameters and this has a great potential in terms of micro manipulation, optical trap and other fields.
The experimental results of a temporally stable, continuous wave, midinfrared (MIR), singly resonant optical parametric oscillator pumped by an all-fiberized master oscillator power-amplifier structured random fiber laser (RFL) is presented. The maximal idler output power of 4.35 W was achieved at 3271 nm with good beam quality, and the corresponding pump-to-idler slope efficiency was up to 17.1%. The idler output power exhibited a peak-to-peak fluctuation better than 3.2% RMS at the maximum output power over 20 min. Meanwhile, other characteristics of the generated idler MIR laser had been discussed in details, which offered effective guidance on the research of the frequency downconversion process in the case of temporally incoherent light and broadened the range of RFL applications.
A polarization-converting system is designed by using axicons and wave plate transforming naturally polarized laser to linearly polarized laser at real time to resolve difficulties of generating high-power linearly polarized laser. The energy conversion efficiency reaches 96.9% with an enhancement of extinction ratio from 29.7% to 98%. The system also keeps excellent far field divergence. In the one-way SHG experiment the double frequency efficiency reached 4.32% using the generated linearly polarized laser, much higher than that of the naturally polarized laser but lower than that of the linearly polarized laser from PBS. And the phenomenon of the SHG experiment satisfies the principle of phase matching. The experiment proves that this polarization-converting system will not affect laser structure which controls easily and needs no feedback and controlling system with stable and reliable properties at the same time. It can absolutely be applied to the polarization-conversion of high power laser and enhance the SHG efficiency and the energy efficiency.
3~5μm mid-infrared laser has many important applications, such as gas detection, spectral analysis, remote sensing, medical treatment, and also in the military laser radar, infrared countermine, and so on. Optical parametric oscillator (OPO) is an efficient way to generate laser in this wavelength range, which has attracted the eyes of many people. In this paper, the recent development of mid-infrared OPO is overviewed. Meanwhile, detailed introduction on our recent work is given. Maximum idler output power of 34.2W at center wavelength of 3.35μm was obtained, to our knowledge, which is the new power record of the international public reporting for the continue-wave (CW) mid-infrared OPO. It is worth mentioning that the pump source, the quasi single-frequency (SF) narrow line width fiber laser, was also developed by our groups. According to the current status of research, some solutions is proposed in order to achieve higher power, narrower line width, and compact volume mid-infrared OPO in a wide tunable range.
3~5μm mid-infrared laser has many important applications, such as gas detection, spectral analysis, remote sensing,
medical treatment, and also in the military laser radar, infrared countermine, and so on. Optical parametric oscillator
(OPO) is an efficient way to generate laser in this wavelength range, which has attracted the eyes of many people. In
this paper, the recent development of mid-infrared OPO is overviewed. Meanwhile, detailed introduction on our
recent work is given. Maximum idler output power of 34.2W at center wavelength of 3.35μm was obtained, to our
knowledge, which is the new power record of the international public reporting for the continue-wave (CW)
mid-infrared OPO. It is worth mentioning that the pump source, the quasi single-frequency (SF) narrow line width
fiber laser, was also developed by our groups. According to the current status of research, some solutions is proposed
in order to achieve higher power, narrower line width, and compact volume mid-infrared OPO in a wide tunable