In order to filter out the residual pump light, the high-order mode laser, the core leakage laser, and the reflected laser in the power delivery fiber (PDF) cladding of a high power fiber laser, it is urgent to improve the beam quality and keep the operational stability of a high power fiber laser. Cladding light stripper (CLS) as the critical component for cladding light filtering, is capable of efficiently stripping cladding light and ensuring the stability and beam quality of a fiber laser. In the practical laser applications, another significant effect of CLS is to resist returning light. Due to the reflection from the workpiece surface, the light returning back to the fiber damages the resonant tank and the fiber components, disturbs the stability of the resonant tank, burns the fiber components including the fiber Bragg grating (FBG) and the combiner, and breaks down the red light chip. CLS is able to effectively eliminate the returning light transmitting in the fiber cladding, so that the damage can be suppressed enormously. At present, the main research direction of CLS is to achieve a higher power attenuation coefficient, a lower temperature rise coefficient, and a tolerance of high power through different methods.

In practical application of low numerical aperture(NA)cladding light, it is difficult to withstand high power and high attenuation coefficient, and the problem of high power CLS temperature rise is still difficult to solve, which is mainly due to the long filtering distance required for low NA light and the concentrated leakage of high NA light. Most researches about CLS devices use the PDF materials at abroad.

Here the self-developed 20 μm/400 μm power delivery optical fiber is used as the preparation material of a CLS device, the KHF₂ solution, glass frosted paste, and hydrofluoric acid are used as the etchants, and the segmented etching method and the composite application of the three etchants are adopted to prepare the gradient structured CLS. In order to test the CLS performance during stripping of high NA cladding light, a test device (Fig. 1) is built. The filtering performance and heating of CLS for 976 nm pump LD input are measured. This device uses three 140 W LDs as the input laser source. In order to test the filtering performance of CLS under different etchant processes, the samples (Table 1) are connected to the test device for a test. The input power of the pump laser is 180 W.

According to the corrosion characteristics of each corrosive agent, the glass frosted paste and KHF₂ are suitable for making rough structures on the PDF surface, and the HF is suitable for preparing a section of optical fiber with a special diameter. After filtering the cladding light, the power proportion of low NA light in the fiber increases relatively. Reasonable gradient transition and increase in roughness can improve the filtering performance of CLS. Therefore, the lengths of four sections of the gradually corroded CLS are designed as 70 mm, 25 mm, 25 mm, and 25 mm, respectively. According to the test result of a filtered pump laser, the KHF₂ is used as the corrosive agent in the first section, and the glass frosted paste and HF are used as the corrosive agents in the last section. The process parameters of the designed CLS samples are shown in Table 2. The NAs of the high-order mode laser leaked from the actual core and the laser returned by the workpiece are much smaller than that of the pump light. In order to obtain high-power cladding light with the same brightness as that of the low NA core laser, the self-developed ytterbium doped fiber with a size of 19 μm/400 μm is adopted as the gain medium. When the ytterbium doped fiber is fused with an energy transfer fiber at the high reflective grid end, the core laser in the energy transfer fiber leaks to the cladding layer of the ytterbium doped fiber through the fiber core mismatch. Furthermore, the bending diameter of the fiber at the end of the high reflective fiber grating is less than 80 mm, which promotes the leakage of the high-order mode laser in the fiber core, so as to obtain the hundreds of watts level low NA cladding laser.

The samples in Table 1 are used to strip the 180 W high NA pump laser with the test device shown in Fig. 1, and the obtained results are shown in Fig. 3. The power attenuation coefficient of the sample prepared by HF corrosion is the smallest, which is within 1 dB. The power attenuation coefficient of the sample prepared by KHF₂ corrosion increases gradually with the increase of corrosion time, and its value approaches 2 dB and then remains balanced. The power attenuation coefficient of the sample prepared by glass frosted paste corrosion is the largest, which is up to 16 dB, and its filtering performance for semiconductor laser is the best. The CLS light leakage diagrams of different samples are shown in Fig. 4. Judging from the light leakage shape, the scattering power density in the cladding layer is reduced. If the corrosion length increases, the scattering power density decreases. Therefore, KHF₂ is suitable for the first stage.

The prepared CLS by the composite corrosion process (Table 2) is used to strip the 343 W pump laser power with the device shown in Fig. 1, and the output power is only 2.22 W with no obvious fever. The test results are shown in Fig. 9(a), and the corresponding power attenuation coefficient is 21.9 dB. As shown in Table 2, the composite corrosion process is more suitable for a high power laser stripper. Connecting the CLS (Table 2) to the test device (Fig. 2) can reflect the actual application effect of a CLS. The power difference before and after CLS access is calculated and recorded as the stripping power, so as to evaluate the optical stripping ability of the CLS cladding. The test results are shown in Fig. 9(b). Both the laser output power and the CLS stripping power increase linearly with the increase of input power, indicating that CLS can work stably and strip a low NA 208 W cladding laser when the laser input power is 1173 W. The CLS device is suspended without heat sink scattering. The direction of the bold arrow in Fig. 11 indicates the laser transmission direction, and the thermal imaging diagram shows that the maximum temperature of CLS is only 41.3 ℃, and the temperature field is evenly distributed.

Based on the self-development PDF, the anhydrous cold CLS is prepared for a high power fiber laser. The gradient structure of CLS is prepared by optimizing the corrosion process and the surface structure of the optical fiber. When the pump light input power is 343 W, the power attenuation coefficient is 21.9 dB. The 1173 W input laser can be used to strip the 208 W low NA cladding light, and the working temperature of the stripper is stable below 45 ℃. The CLS performance can meet the application requirements of high beam quality and stability high-intensity fiber lasers.