We report a more than 150 W spectrally-clean continuous wave Raman fiber laser at 1120 nm with an optical efficiency of 85%. A approximately 30 m standard single mode silica fiber is used as Raman gain fiber to avoid second Stokes emission. A spectrally asymmetric resonator (in the sense of mirror reflection bandwidth) with usual fiber Bragg gratings is designed to minimize the laser power lost into the unwanted direction, even when the effective reflectivity of the rear fiber Bragg grating becomes as low as 81.5%.
Context. Sodium laser guide stars (LGS) are about to enter a new range of laser powers. Previous theoretical and numerical methods are inadequate for accurate computations of the return flux, hence for the design of the next-generation LGS systems. Aims. We numerically optimize the cw (continuous wave) laser format, in particular, the light polarization and spectrum. Methods. Using Bloch equations, we simulate the mesospheric sodium atoms, including Doppler broadening, saturation, collisional relaxation, Larmor precession, and recoil, taking all 24 sodium hyperfine states into account and 100-300 velocity groups. Results.LGS return flux is limited by "three evils": Larmor precession due to the geomagnetic field, atomic recoil due to radiation pressure, and transition saturation. We study their impact and show that the return flux can be boosted by repumping (simultaneous excitation of the sodium D 2 a and D 2 b lines with 10−20% of the laser power in the latter). Conclusions. We strongly recommend the use of circularly polarized lasers and repumping. As a rule of thumb, the bandwidth of laser radiation in MHz (at each line) should approximately equal the launched laser power in Watts divided by six, assuming a diffraction-limited spot size.
We report on a 25 W continuous wave narrow linewidth (< 2.3 MHz) 589 nm laser by efficient (> 95%) coherent beam combination of two narrow linewidth (< 1.5 MHz) Raman fiber amplifiers with a Mach-Zehnder interferometer scheme and frequency doubling in an external resonant cavity with an efficiency of 86%. The results demonstrate the narrow linewidth Raman fiber amplifier technology as a promising solution for developing laser for sodium laser guide star adaptive optics.
We demonstrate the cascaded coherent collinear combination of a seed-split triplet of 1178nm high-power narrow-band (sub-1.5MHz) SBS-suppressed CW Raman fibre amplifiers via nested free-space constructive quasi-Mach-Zehnder interferometry, after analysing the combination of the first two amplifiers in detail. Near-unity combination and cascaded-combination efficiencies are obtained at all power levels up to a maximum P(1178) > 60W. Frequency doubling of this cascaded-combined output in an external resonant cavity yields P(589) > 50W with peak conversion efficiency eta(589) ~85%. We observe no significant differences between the SHG of a single, combined pair or triplet of amplifiers. Although the system represents a successful power scalability demonstrator for fibre-based Na-D(2a)-tuned mesospheric laser-guide-star systems, we emphasise its inherent wavelength versatility and consider its spectroscopic and near-diffraction-limited qualities equally well suited to other applications.
Aims. To aid the design of laser guide star (LGS) assisted adaptive optics (AO) systems, we present an analysis of the statistics of the mesospheric sodium layer based on long-term observations (35 years). Methods. We analyze measurements of the Na-layer characteristics covering a long period , acquired at latitude 23 • south, in São José dos Compos, São Paulo, Brazil. We note that Paranal (Chile) is located at latitude 24 • south, approximately the same latitude as São Paulo.Results. This study allowed us to assess the availability of LGS-assisted AO systems depending on the sodium layer properties. We also present an analysis of the LGSs spot elongation over the year, as well as the nocturnal and the seasonal variation in the mesospheric sodium layer parameters. Conclusions. The average values of the sodium layer parameters are 92.09 km for the centroid height, 11.37 km for the layer thickness, and 5 × 10 13 m −2 for the column abundance. Assuming a laser of sufficient power to produce an adequate photon return flux for an AO system with a column abundance of 4 × 10 13 m −2 , a telescope could observe at low geographic latitudes with the sodium LGS more than 250 days per year. Increasing this power by 20%, we could observe throughout the entire year.
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