A 2 kw single-mode fiber laser with a 20-m long delivery fiber and high back reflection resistance has been demonstrated. An Yb-doped fiber with large core size and differential modal gain is used to realize high SRS suppression and single-mode operation simultaneously. The 20 m-long delivery fiber gives flexibility to the design of processing systems. An output power of 2 kW is achieved at a pump power of 2.86 kW. The slope efficiency is 70%. The power of the Stokes light is less than -50 dB below the laser power at the output power of 2 kW even with a 20-m delivery fiber. Nearly diffraction-limited beam quality is also confirmed (M<sup>2</sup> = 1.2). An output power of 3 kW is believed to be achieved by increasing pumping power. The back reflection resistance properties of the fabricated singlemode fiber laser is evaluated numerically by the SRS gain calculated from measured laser output spectra and fiber characteristics. The acceptable power of the back reflection light into the fiber core is estimated to be 500 W which is high enough for processing of highly reflective materials. The output power fluctuation caused by SRS and back reflection in materials processing will be well suppressed. Our high power single-mode fiber lasers can provide high quality and stable processing of highly reflective materials.
Latest computational and experimental studies on high-speed monolithic silicon-based Mach-Zehnder optical modulators
are studied in the light of photonic integrated circuits for digital coherent communication at a bit rate as fast as 128 Gb/s
per wavelength channel. Lateral PN-junction rib-waveguide phase shifters are elaborated with experimental
characteristics of DC phase shifter response in comparison with computational characteristics. High-speed response in
refractive-index dynamics including electron and hole transport in the PN junction is simulated to study speed limit of
the phase shifters. The performance in quadrature phase-shift keying signal generation is characterized in experimental
and computational constellation diagrams. Silicon waveguides for polarization-division multiplexing are designed in
common design rules with the rib-waveguide phase shifters. Long-haul transmission in polarization-multiplexed
quadrature phase-shift keying in 1000-km single-mode fiber link is confirmed with a monolithic silicon Mach-Zehnder
modulator assembled with modulator drivers in a ceramic-based metal package.