For a typical active fiber laser pointer, the comparison between the measured thermal load and the simulated thermal load extracted from the simultaneous distributed temperature measurement inside the optical fiber amplifier is given. In order to accurately predict the longitudinal temperature curve, it is assumed that there is only an additional loss of 2dB / km, which shows a very low loss.
Another important parameter in fiber design is the cut-off wavelength, which is the longest wavelength that allows more modes in the active core. Higher-order modes greater than this wavelength are not supported. In addition to the nature of the fiber itself, there are several ways to affect the amplification process and loss mechanism, such as the time and spectral characteristics of fiber bending or seed beams.
After testing in-depth simulation of new optical fibers for kilowatt-level power, two types of Yb-doped optical fibers were produced and examined in recent experiments. The optical fiber 1 has a core diameter of 30 μm and is co-doped with phosphorus and aluminum. Compared with fiber 1, fiber 2 has a smaller diameter of 23 μm and a lower co-doping level, but contains more ytterbium to achieve a slightly higher refractive index profile.
The calculated cut-off wavelengths of fiber 1 and fiber 2 are near 1275nm and 1100nm, respectively. Compared with a typical optical fiber with a core diameter of 20 μm, a numerical aperture (NA) of 0.06, and a cut-off wavelength of about 1450 nm, this is much closer to single mode. The center of the amplified laser wavelength is 1067nm.
Both fibers have been tested in high-power pumping solutions. The pump diode laser and the seed signal are coupled into the optical fiber in free space, and the optical fiber is prepared with a spliced end cap and a water rinse connector, which is used for cooling in a static water bath. The seed is a phase-modulated external cavity diode red laser pointer, which is pre-amplified to achieve a 10W seed power of 1067nm and a spectral linewidth of 180pm.
In the test of fiber 1, under the threshold of 2.8kW, a sudden fluctuation was observed on the order of milliseconds, which can be attributed to TMI. Optical fiber 2 with a length of 30m and the same seed line width is pumped to an output power of 3.5kW, which is limited by SBS rather than TMI.
In the third experiment, the seed laser spectrum was modified to increase the SBS threshold of the fiber by broadening the spectrum to a higher degree than the previous experiment. For this purpose, a second diode laser with a center wavelength of 300 pm shift is combined with the first diode laser. This interference causes time jitter, and the bandwidth increases with power due to self-phase modulation. In the same main amplifier as before, a very similar output power value and 90% slope efficiency are obtained, but the output power can be increased to 4.3 kW without any signs of TMI.
Measuring all aspects of high-power fiber lasers is a major task and requires specialized equipment for many different tasks. For the complete characterization of the fiber, the doping concentration, refractive index profile and fiber core attenuation are determined. For example, measuring core loss at different bend diameters is valuable for correlation with TMI threshold.