Study on Scanning Velocity of Germanium Core Fibers with Different Outer Diameters Annealed by CO2 Laser
As the germanium(Ge)core is mostly in an amorphous or polycrystalline state after fabrication,laser annealing is an effective way to improve the properties of semiconductor core fiber.During the laser annealing process,the axial scanning velocity of the laser along the fiber is an important parameter that affects the properties of the annealed fiber.Therefore,it is of great significance to investigate the modification mechanism of laser annealing on the Ge core to improve the properties of annealed fibers.In this study,three sets of Ge core fibers with different outer diameters(OD)and the same inner diameter(ID)were annealed by CO2 laser at different scanning velocities.The laser annealing experiments were carried out on Ge core optical fibers with an ID of 41～43 μm and the ODs of 188 μm,251 μm,and 270 μm,respectively.The Ge core fibers were annealed by the SK-3D30 CO2 laser.The laser spot is 1 mm in diameter,the output power is 0～30 W,and the laser wavelength is 10.6 μm.The scanning region along the fiber axis is 1 mm×50 mm,which can completely cover the Ge core fiber,and the laser scanning in this region was reciprocated along the fiber axis during the annealing time.After laser annealing,samples were analyzed by using the spectrometer.Raman experiments were carried out on the cross-section of the Ge core fiber to collect the Raman peak frequency information.The obtained data was processed into mapping by MATLAB software.The optical transmission loss of Ge core fiber was measured by the cutback method.The system consists of a laser,photodetector,and optical power meter.The samples were cut off at 5 mm each time and measured 3 times per fiber.All measurements were made at room temperature.Three sets of experiments were carried out,the laser frequency is 50 kHz,the laser power is 20%(6 W),and the laser scanning time is 20 s.1)The Ge core fiber with an OD of about 188 μm was annealed by laser,and the scanning velocities were set at 8 mm·s-1,10 mm·s-1,12 mm·s-1,and 14 mm·s-1.The Raman frequency distribution and average value at 10 mm·s-1 laser scanning velocity closest to Ge bulk crystal and optical transmission loss values was 3.435 dB·cm-1.2)The Ge core fiber with an OD of about 251 μm was annealed by laser,the scanning velocities were set at 10 mm·s-1,12 mm·s-1,14 mm·s-1,16 mm·s-1,and 20 mm·s-1.The Raman frequency distribution and average value at 14 mm·s-1 laser scanning velocity closest to Ge bulk crystal and optical transmission loss values was 2.147 dB·cm-1.3)The Ge core fiber with an OD of about 270 μm was annealed by laser,and the scanning velocities were set at 12 mm·s-1,14 mm·s-1,16 mm·s-1,and 18 mm·s-1.The Raman frequency distribution and average value at 16 mm·s-1 laser scanning velocity closest to Ge bulk crystal and optical transmission loss values was 3.578 dB·cm-1.The experimental results show that under the same OD conditions,the laser annealing effect becomes better first and then worse with the increase in laser scanning velocity,and the scanning velocity for obtaining the optimal annealing effect increases with the increase of the OD of the fiber.Temperature variation at fixed points on the surface of the Ge core on the laser-irradiated side during the annealing process was simulated by COMSOL Multiphysics.The simulation results indicated that under the same OD conditions,the faster scanning velocity leads to the formation of denser temperature pulses,so that the Ge core is in the relatively high-temperature region most of the time,and the strength of the modification effect of this temperature field structure on the Ge core is more enhanced.The experimental results and the simulation of temperature variation indicate that the laser scanning velocity is an important factor affecting the annealing effect of Ge core fiber.The annealing intensity of the laser-annealed Ge core fiber can be enhanced as the laser scanning velocity is increased.
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