Lidar has been widely used in wind ranging, automatic drive and sensing mapping. The reflected light signal is obtained through first emitting a Gaussian beam from the laser source and then reflecting after reaching the surface of the object. After the computer analysis, the information of the object such as orientation, attitude and distance can be obtained. However, as for a lidar system, its laser source is an important unit influencing the performance of the whole system. A fiber laser has become the best choice of the light source for a lidar system, because of its good beam quality, high pulse energy and high repetition rate. At the same time, an erbium-ytterbium co-doped fiber has attracted the attention of many researches due to its advantages such as "eye-safe" and low atmospheric transmission loss. Therefore, as the most important gain medium for the laser lidar, polarization-maintaining erbium-ytterbium co-doped fiber has important research significance. In this paper, a 10 μm/128 μm polarization-maintaining erbium-ytterbium co-doped fiber is successfully fabricated by the modified chemical vapor deposition (MCVD) technique combined with the solution doping technology (SDT). The structural parameters and optical properties of this polarization-maintaining erbium-ytterbium co-doped fiber are measured. And its laser performance is also studied.
MCVD combined with SDT is used to fabricate the erbium-ytterbium co-doped fiber. The content (mole fraction) of P2O5 in the core is increased by more than 10% with reverse phosphorus doping and gas phase compensation. In order to avoid the defect of the core bursting during drilling, the fiber prefabricated rod is first annealed due to the high stress of its core. Through the Sagnac interferometer and the optical spectrum analyzer (OSA), the birefringence value is measured. The measurement structure is shown in Fig. 3. And the measurement structure of polarization extinction ratio is also shown in Fig. 5. In order to analyze the laser performance, the structure of an erbium-ytterbium co-doped fiber laser is shown in Fig. 6. The seed source has a power of 20 mW and a central wavelength of 1551 nm. An isolator (ISO) connected to the seed is used to protect the seed source. The isolator is followed by a (2+ 1)×1 forward pump combiner (PC), and one of its pump fiber is used to monitor the backward power and observe the backward spectrum. The 940 nm light generated by the laser diode (LD) is coupled to the active fiber through the pump fiber of the (2+ 1)×1 backward PC, and the pump power is 16.5 W. The coiling diameter of the active fiber is 10 cm. The cladding pump stripper (CPS) is implemented by coating a high refractive index adhesive to filter cladding light from the fiber. Finally, an isolator is fused at the end to prevent reflection.
The dimension of the fiber is shown in the inset of Fig. 2(b). The diameters of core and cladding are measured to be 10.19 μm and 128.69 μm, respectively. The diameter of the boron rod is measured to be 32.59 μm. Figure 2(b) shows the refractive index profile of the prefabricated rod. A numerical aperture of 0.24 is finally achieved. The absorption coefficient measured by the truncation method is 2.42 dB/m at 940 nm. The interference image at 1500-1600 nm is observed in the OSA (Fig. 4). The beat length at 1550 nm is calculated to be 9 mm with a birefringence coefficient of 1.29×10-4. At the same time, a polarization extinction ratio of 24 dB at 1310 nm is measured through the erbium-ytterbium co-doped fiber with a length of 4 m. As for the laser performance, due to the inherent loss, the final seed power coupling into the active fiber is 17.5 mW. Figure 7(a) shows the slope efficiency under different fiber lengths and pump powers. It can be seen from this figure that the optimal length is 7.5 m. When the pump power is 16.5 W, the output power and the slope efficiency reach the maximum, which are 5.8 W and 36%, respectively. The polarization extinction ratio is measured to be 21 dB. In addition, as shown in Fig. 7(b), the optical-to-optical efficiency tends to be saturated with the increase of pump power at different lengths. After reaching the saturation state, the optical-to-optical efficiency is more than 33% without a downward trend, which indicates that the laser power can be further increased at this time. The spectrum at 7.5 m is shown in Fig. 8. It can be observed from this output spectrum that the amplified spontaneous emission (ASE) power increases gradually with the increase of pump power, but the signal-to-noise ratio remains above 50 dB. From the backward spectrum, one can observe that the remaining pump light intensity is stable, which may be caused by the fact that some spiral pump light in the polarization-maintaining fiber is not absorbed by the fiber and the CPS is not added. Meanwhile, there is no parasitic oscillation at 1 μm. It shows that the polarization-maintaining erbium-ytterbium co-doped fiber prepared in this paper has good laser performances.
In this paper, a polarization-maintaining erbium-ytterbium co-doped fiber for lidar is successfully fabricated by MCVD combined with SDT. The performance of this polarization-maintaining fiber is measured. A birefringence coefficient of 1.29×10-4 and a polarization extinction ratio of 24 dB@4 m at 1310 nm are achieved. In addition, a polarization-maintaining all-fiber erbium-ytterbium co-doped fiber laser system is built, and the slope efficiency reaches 36%. Above all, the highest efficiency of the polarization-maintaining erbium-ytterbium co-doped fiber is achieved, which provides the possibility for exact localization of a military lidar.
