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Abstract
We fabricated a Yb-doped yttrium aluminium garnet (Yb:YAG) crystal-derived silica fiber (YCDSF) using an assembly consisting of a YAG crystal rod and silica tube on a CO2 laser-heated drawing tower. The fiber has a Yb concentration of 5.66 wt%, and absorption coefficient of 32 dB/cm at 980 nm. The figure of merit of the unsaturated absorption and gain per unit length of the YCDSF are 93% and 4.4 dB/cm, respectively. Based on the results of the numerical simulation, an all-fiber distributed Bragg reflector (DBR) laser using only a 1.5-cm-long YCDSF is experimentally demonstrated to have a maximum output power of 360 mW with a pump threshold power of 21 mW. The fiber laser also achieved an optical signal-to-noise ratio of 80 dB, a beam quality factor of 1.022 in two orthogonal directions and a slope efficiency of up to 50.5%. These results indicate that the all-fiber DBR laser has potential applications in high-quality seed sources and coherent optical communications.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Distributed Bragg reflector (DBR) fiber lasers operating at ∼1.0 µm have attracted considerable attention because of their all-fiber design, high performance (with low noise and good beam quality), and wide applications (e.g. laser radar, seed source, and high-resolution spectrum) [1–4]. Yb ions have a broad absorption and emission cross-section, long upper-level lifetime and high efficiency; hence, they play an important role in the 1.0-µm fiber lasers and amplifiers [5,6]. For a compact DBR all-fiber laser at 1.0 µm, the ideal gain fibers have high doping concentration and high gain coefficient [1]. However, it is difficult to prepare high concentration Yb-doped silica fibers using traditional vapour deposition methods, because of the severe concentration quenching effect [7]. To overcome this problem, the melt-in-tube method is used extensively because of its low material composition limit and wide applicability [8,9]. Recently, gain fibers with a multi-component glass core, such as tellurite [10], phosphate [11], and germanate [12] glasses, have been successfully fabricated using the melt-in-tube method, and they have achieved high gain coefficients. For example, the gain coefficient of Yb-doped phosphate glass fibers can reach 5.7 dB/cm. However, the poor thermal stability and weak mechanical strength of these fibers restrict their development [13]. Moreover, fibers based on these glasses are incompatible with commercial silica-based fibers because of the difficulty in fusing fiber. In recent years, yttrium aluminium garnet (YAG) crystal/ceramic-derived fibers with an yttrium aluminosilicate glass core have exhibited numerous advantages, such as a high rare-earth doping concentration, thermal conductivity, and stimulated Brillouin scattering threshold [14–16]. In addition, the high Y and Al content of the fiber core can increase the refractive index and improve the thermal expansion coefficient, resulting in fibers that are compatible with silica-based commercial fibers [9,17,18].
Several studies have been conducted on the performance of lasers based on Yb:YAG derived silica fibers. In a study by Zhang et al. [19], a laser based on 10-cm doped fiber showed a slope efficiency of 23%. In another study, Xie et al. [20] developed an all-fiber laser with a slope efficiency of up to 45.4% by 20-cm-long, which is the highest efficiency value achieved till date (YCDSF-1). When the active fiber inserted into a system is short, the total length of the fiber cavity is only several centimeters (this type of cavity is called “short-cavity”). The advantage of a short cavity structure is that it can effectively control the longitudinal mode interval of the laser. Liu et al. [21] developed a short-cavity single-longitudinal-mode fiber laser with a slope efficiency of approximately 18.5% and an optical signal-to-noise ratio (OSNR) of 80 dB by using a short fiber (YCDSF-2) of 1.4 cm. Short-cavity DBR fiber lasers have high sensitivity and stability deeming them suitable for use in DBR fiber laser sensors [22]. A short fiber with a high gain coefficient is of great significant. However, the slope efficiency of a short-cavity DBR fiber laser based on YCDSF remains low compared with the above non-short-cavity lasers. In other words, the performance of an all-fiber short-cavity DBR laser based on Yb:YAG derived silica fiber requires further optimization, especially with regard to YCDSF properties.
In this study, a YCDSF is fabricated using the melt-in-tube method, in which a Yb:YAG crystal rod is used as the preform core. The structural characteristics of the YCDSFs are characterized based on their Raman spectrum and refractive index distributions. At the same time, the obtained fibers are systematically characterized, including their chemical composition, absorption, fluorescence, and amplification performance. Moreover, short-cavity all-fiber DBR lasers with different YCDSF samples are built experimentally as well as via simulations, and their laser properties are investigated.
2. Fiber fabrication and properties
The YCDSF samples were fabricated using the melt-in-tube method on a CO2 laser-heated drawing tower. The schematic diagram of the fiber drawing process is presented in Fig. 1(a). The preform of the YCDSF consisted of a commercially available Yb:YAG crystal rod, with 10.0 at% Yb (Lanjing Optoelectronic Technology, Shanghai, China) and a high-purity silica substrate tube. The Yb:YAG crystal rod was 4.0-cm-long with a diameter of 0.5-mm. The inner and outer diameters of the silica tube were 1.0 mm and 6.0 mm, respectively. After the crystal rod was inserted into a high-purity silica substrate tube, a vacuum environment (2.0 Pa) was created and maintained inside the silica tube during the fiber drawing process. The low-pressure inside the tube promotes the formation of a bubble-free interface between the core and cladding of the fiber [23]. The CO2-laser was focused on the preform by using an optical system. The temperature of the hot zone induced by the CO2 laser was controlled by adjusting the laser power output. The fiber drawing temperature was approximately 2000-2100 °C. Uniform YCDSFs were obtained by simultaneously controlling the fiber drawing and feeding speeds, forming a closed-loop. Compared with the case of a graphite-heated drawing tower [19–22], the CO2 laser-heated drawing tower has a small high-temperature zone. Moreover, the CO2 laser at 10.6 µm can be strongly absorbed by the silica tube; Hence, the Yb:YAG crystal core was melted by conductive heating and the tube was softened [24]. This could help maintain the geometric structural features of the preform on the drawn fibers and suppress the elemental migration between the core and cladding layers, as reported in Ref. [25,26]. The fabricated YCDSFs had a uniform core diameter of 15.0 µm and a cladding layer of 130.0 µm, as illustrated in cross-sectional view and side-view in Fig. 1(b) and Fig. 1(c), respectively. This indicates that the YCDSF sample possessed a clear boundary between the core and the cladding and a complete waveguide structure. Figure 1(d) shows the transmission of red light through the fiber. No obvious scattering points were observed, which indicates that there wre no large bubbles or defects in this fiber.
Fig. 1. (a) Schematic diagram of YCDSF fabrication using a CO2 laser-heated drawing tower; optical images of the fabricated YCDSFs, (b) cross-sectional view, (c) side view, and (d) image of YCDSF transmission with red light.
The Raman spectra of the YCDSF and YAG crystal were measured by using a Raman spectrometer (LabRam HR800, Horiba Jobin Yvon, France) equipped with a laser operating at a wavelength of 532 nm, and the results are shown in Fig. 2(a). The spectrum of the fiber cladding layer shows typical broad Raman peaks at 487 cm-1 and 793 cm-1, indicating the nature of the fused silica glass network [27]. Broadband Raman peaks were observed at 451 cm-1, 782 cm-1, and 970 cm-1 in the fiber core region, and a series of sharp Raman peaks were observed at 262 cm-1, 370 cm-1, and 790 cm-1 in the YAG crystal. This implies that the materials in the fiber core were essentially amorphous [7,8,28]. Tiny vibration peaks of the silica glass network (e.g., 487 cm-1 and 795 cm-1) appeared in fiber core layer, suggesting that silica in the cladding layers may diffused into the fiber core during the drawing process.
Fig. 2. (a) Raman spectra of the fiber core and cladding layers, and YAG crystal rod; (b) excitation and emission spectra of the YAG crystal rod; (c) excitation and emission spectra of YCDSF; (d) fluorescence decay curves of the Yb:YAG crystal rod and YCDSF.
The excitation and emission spectra of the Yb:YAG crystal and the fiber as well as the fluorescence decay time curves of Yb3+ ions were measured using a fluorescence spectrophotometer (Edinburgh FLS-980, England). A side-pumping scheme was used, in which light was pumped into one fiber axis at an angle of ${90^\circ }$, as the spectrum was collected. In the case of the Yb:YAG crystal, a sharp emission peak was observed at 1029 nm, and the excitation spectrum monitored at 1029 nm showed a series of narrow band peaks, as shown in Fig. 2(b). In contrast, the excitation and emission peaks associated with the YCDSF were broad with shifting wavelengths, as shown in Fig. 2(c). The optimal excitation and emission peaks of the Yb:YAG crystal, were located at 940 nm and 1029 nm, respectively, while those of the YCDSF were located at 978 nm and 1018 nm, respectively. All the excitation and emission peaks of the crystal and fiber materials can be attributed to the electronic transitions between 2F5/2 and 2F7/2 of the Yb3+ ion. Nevertheless, different spectral profiles exist due to the local structural change around the Yb ions [29]. These results can be observed in the Raman spectra of the crystal and fiber materials in Fig. 2(a). The Raman peaks of the Yb:YAG crystal were relatively sharp (purple curves in Fig. 2(a)), as a results of an ordered lattice structural arrangement. The Yb3+ ions are replaced with Y3+ ions sites in the YAG crystal. The microenvironment around the Yb3+ ions were the same and ordered. Therefore, the Yb3+ ions in YAG crystal materials formed an emission center. For YCDSF materials, the emission spectra excited at 918 nm and 940 nm presented broadband peaks at 976 nm and 1018 nm, respectively. Combining these results with the Raman spectrum of the fiber core (blue curves in Fig. 2(a)), establishes that there exists a non-crystalline and disordered coordination environment in the YCDSF core materials. The Yb3+ ions in the disordered environment formed a different emission center. The interaction between the emission centers of Yb3+ ions resulted in the spectral broadening and wavelength shifting of the optimal excitation and emission peaks.
Figure 2(d) depicts the fluorescence lifetime measurement of Yb3+ ions on both the YAG crystal and its derived fiber under 980 nm excitation. The YCDSF sample has a much shorter lifetime (498 µs), than the Yb:YAG crystal (1025 µs). The shorter lifetime of the YCDSF is attributed to the large difference between the lattice environments around the Yb ions in the crystal and the YCDSF [30]. On the one hand, the non-radiative transition probability is proportional to the phonon energy of the matrix material [31]. The maximum phonon energy (ћω) of the fiber core material is significantly higher than that of the YAG crystal, which results in a sharp decrease in the fluorescence lifetime. On the other hand, the solubility of Yb ions in YAG crystal-derived fiber is lower than that in the YAG crystal [32]. The concentration quenching effect of Yb may occur in the YCDSF. It is important for Yb-doped fiber materials to suppress the concentration quenching process.
To further understand the Si element diffusion in YCDSFs, the elemental distribution and refractive index distribution of the YCDSF cross-section were measured using an electron probe micro-analyser (EPMA-1720, SHIMADZU, Japan) system and a refractive index profiler (S14, Photon Kinetics, Inc., U.S.), respectively. The cladding and core of the YCDSF presented an obvious mutation in the elemental distribution. The fiber core contained Al, Y, and Yb elements from the YAG crystal materials, and Si from the silica cladding layer, as shown in Fig. 3(a). The exact amount of dissolved SiO2 in the core was 49.58 wt%, while the amounts of Yb, Y, and Al are 5.66, 14.80, and 8.59 wt%, respectively. The diffusion and migration of Si are obvious, which is also consistent with the results of the Raman spectra of YCDSF. The two-dimensional mappings of the elements of the YCDSF cross-section yielded the same results, as shown in Fig. 3(b). There exists a significant delamination at the boundary between the cladding and core of the YCDSF. In particular, the distribution of Yb in the fiber core is relatively uniform. This result is consistent with the absence of a fast decay component in the fluorescence lifetime decay curve (Fig. 2(d)).
Fig. 3. (a) Elemental distribution curve along the diameter of the fiber; (b) elemental distribution two-dimensional mapping in the YCDSF cross-section; (c) the refractive index distribution of the fiber sample.
The refractive index difference (Δn) between the fiber core and cladding was approximately 0.058, as shown in Fig. 3(c). The calculated refractive index of the fiber core layer was 1.514. and its numerical aperture was 0.42. The high refractive index in the core region mainly results from the role of high-concentration doped materials.
The loss spectrum of the YCDSF was measured using a broadband light source (BBS, YSL Photonics, SC-5) and an optical spectrum analyser (OSA, AQ6370, YOKOGAWA, Japan), as shown in Fig. 4(a). Two typical absorption peaks of Yb3+ ions exist at 918 and 980 nm, respectively. The absorption coefficient of the YCDSF reached 32 dB/cm at 980 nm. The strong absorption coefficient of the YCDSF can provide increased pump energy. The high absorption coefficient of the YCDSF is attributed to the high doping concentration of Yb3+ ions. According to the loss spectrum, the estimated background losses were approximately 0.011 dB/cm at 1200 nm. The background losses mainly come from the impurities of the precursor materials, the elements diffusion between YAG crystal and silica tube, and the change in the YAG crystal structure [14,20]. It is necessary to further optimize the background losses in terms of the purity of the precursor materials and drawing process [19].
The pump absorption coefficient (${\alpha _p}$) was determined from the nature of the fiber materials, which can describe the process via which pumped photons are absorbed [33,34]. Therefore, the ${\alpha _p}$ of YCDSFs under 980 nm pumping was analysed, as shown in Fig. 4(b). With the increase of the 980 nm pump power, ${\alpha _p}$ showed a trend of an initial increase, followed by a decrease, and finally stabilization. The stable value of ${\alpha _p}$ is also called the unsaturated absorption coefficient (${\alpha _{us}}$), 1.46 dB/cm. This part of the pump light does not work during the process of Yb3+ ion stimulated transition. In contrast, the absorption coefficient of the working pump light is the saturated absorption coefficient (${\alpha _s}$), 19.14 dB/cm. To assess the effective pump absorption in the YCDSF, the figure of merit of the unsaturated absorption (Ma) is defined as Ma=${\alpha _s}$/ (${\alpha _s} + {\alpha _{us}}$). The value of Ma is approximately 93%. A high value of Ma means that a large part of the pump photons will participate in the excitation of the active ions, promoting the desirable luminescence process at the corresponding bands. In other words, the larger the Ma value, the higher the laser efficiency. Therefore, it is very important for the YCDSF to be used in high-conversion-efficiency fiber lasers and amplifiers.
3. Amplification and laser performance
An amplification gain experimental system was built, as shown in Fig. 5. A homemade fiber laser with a 1030 nm signal light and 980 nm pump light were coupled to a 5.4-cm-long YCDSF using a single mode 980/1030 nm wavelength division multiplexer (WDM). A fiber isolator (ISO) was used to ensure unidirectional light transmission.
Three signal light powers, -25, -5 and 2 dBm, were selected, and the gain coefficients were measured as a function of the pump power, as shown in Fig. 6. With the increasing of in the pump light power at 980 nm, the gain coefficient is increased gradually. The gain coefficients of the signal light power at -25, -5, and 2 dBm were 4.4, 3.8, and 3.3 dB/cm, respectively. As the signal light power is increased, the corresponding gain of the YCDSF is decreased. The maximum gain coefficient of the YCDSF reached 4.4 dB/cm, which is beneficial for a fiber laser with a short-cavity single-frequency fiber laser configuration [14]. Compared with that in Refs. [20,21], the gain coefficient of YCDSF is higher in this experiment, mainly because of the large Ma value and high doping concentration.
For all-fiber linear DBR laser systems, the pumping method directly affects the laser performance [35]. To obtain a high slope efficiency and large output power, a backward pumped all-fiber linear DBR laser system was established using WDMs and a pair of fiber Bragg gratings (FBGs), as shown in Fig. 7. A 980 nm laser diode (LD) was used as the pump light source. The WDM was used to input the pump light and output the laser at 1030 nm. The two ports of the YCDSF were fusion spliced to a high-reflectivity FBG (HR-FBG) and a low-reflectivity FBG (LR-FBG) using a CO2 laser splice machine (LZM-100, AFL Fujikura, Japan). To optimize the laser system in terms of the length of the YCDSF and the reflectivity of the LR-FBG, numerical modelling was performed based on the theoretical model in Ref. [36]. The rate equations and propagation equations can be expressed as:
Fig. 7. Schematic diagram of the all-fiber DBR laser system. The insect pictures are (1) the transmission spectra of the HR-FBG and LR-FBG, (2) the image of the YCDSFs, and (3) the image of the fusion point.
The boundary conditions for the powers of the signal and pump can be given by
where ${N_2}$ is the population concentration in the 2F5/2 level; N is the total concentration of Yb3+ ions; ${\lambda _p}$ and ${\lambda _s}$ are the wavelengths of the pump and signal, respectively; ${P_p}$ and ${P_s}$ are the powers of the pump and signal, respectively; “+” and “–” represent the forward and backward directions, respectively; A and L are the area and length of the YCDSF, respectively; ${\delta _{ap}}$ (${\delta _{as}}$) and$\; {\delta _{ep}}$ (${\delta _{es}}$) are the absorption and emission cross sections of the pump (signal) light, respectively;$\; {\mathrm{\Gamma }_p}$ and ${\mathrm{\Gamma }_s}$ are the confinement factors of the pump and signal light, respectively;$\; {\alpha _p}$ and ${\alpha _s}$ are the losses of the pump and signal light, respectively; $\tau $ is the lifetime of the 2F5/2 level; R1 and R2 are the reflectivity of the HR-FBG and LR-FBG, respectively; h is Planck’s constant and c is the speed of light. The values of the physical quantities in the equation are listed in Table 1.
Table 1. Values of the physical quantities in the equation
When the pump power is 500 mW, the simulation results of the output power as a function of the reflectivity of R2 are calculated and shown in Fig. 8(a). For a fixed the YCDSF length, the output power first increases and then decreases with increase in R2. As the fiber length increases, the R2 corresponding to the maximum output power of the laser decreases gradually. The rectangular area marked in Fig. 8(a) is the ideal area for the selection of L and R2, which provides a powerful reference for the construction of the laser system. When the pump power is 500 mW and R2 is 70%, the relationship between the fiber length and output power is as shown in Fig. 8(b). As the YCDSF length is increased, the output power of the laser also first increases and then decreases. When the fiber length is ∼1.5 cm, the output power is at its maximum value.
Fig. 8. Simulation result: (a) the output power as a function of R2 for different fiber lengths; (b) the laser output power as a function of the YCDSF length, when the pump power is 500 mW and R2 is 70%.
In the experiment, the reflectivity of the LR-FBG (R2) was 70.0%, with a center wavelength of 1029.6 nm, and a 3 dB bandwidth of 0.09 nm. The reflectivity of the HR-FBG was above 99.5% with a 3 dB bandwidth of 0.35 nm. The spectrum characteristics are shown in Fig. 7(1). These FBGs were written in a large-mode-field double-clad fiber (LM-DCF) (Nufern, Inc., USA). The core and cladding diameters were 11.0 µm and 130.0 µm, respectively, which match the YCDSF geometry as closely as possible. The image of the fusion point is shown in Fig. 7(3), and the total splice loss between the FBGs and YCDSF was approximately 0.5 dB. Here, three YCDSF samples and different locations in the same fiber, were chosen in the DBR fiber laser. The fiber lengths were 0.7, 1.5, and 5.4 cm, respectively, as shown in Fig. 7(2).
In the numerical simulation, the slope efficiencies of fiber lasers with YCDSF samples having lengths of 0.7, 1.5, and 5.4-cm, were 28.7%, 59.1%, and 31.7%, respectively. In the experiments, their slope efficiencies were 24.1%, 50.5%, and 22.4%, respectively. And their corresponding threshold powers were 24, 21, and 84 mW, respectively, as shown in Fig. 9. Our experimental results and simulation results for the output power of the laser as a function of pump power with different fiber lengths were basically consistent. The slope efficiencies obtained in the experiments were lower than the theoretically calculated values, which is caused by the theoretical calculation ignoring the splice loss between YCDSF and FBG pigtail fiber, FBG’s actual reflection efficiency, bandwidth of the FBGs, the influence of ambient temperature and so on. We found that when the YCDSF is relatively short, the gain is small and limited, resulting in the relatively larger threshold power and lower slope efficiency of the fiber laser, such as the fiber laser with a 0.7-cm-long YCDSF, shown by the blue curve in Fig. 9. However, when the YCDSF is relatively long, the transmission loss of YCDSF is high, resulting in a large threshold power and low slope efficiency for the fiber laser, such as the fiber laser with 5.4-cm-long YCDSF, shown by the purple curve in Fig. 9. When the fiber length matches its gain and loss, the slope efficiency and threshold power of the fiber laser are optimized, such as the fiber laser with a 1.5-cm-long YCDSF, shown by the black curve in Fig. 9. Its maximum output power can reach 360 mW, which is three times that of the short-cavity all-fiber DBR laser with other Yb:YAG-derived silica fibers; its slope efficiency is also doubled [20,22]. These results are presented in Table 2, which shows the property parameters of different Yb-doped fibers.
Fig. 9. Output power of the linear laser as a function of pump power with backward pump of different fiber length. The inset shows a magnified view of the graph for a pump power range of 0 to 300 mW.
Table 2. Property parameters of Yb-doped silica fiber, YAG crystal fiber, and YAG-derived silica fibers
To further analyze the output spectrum property of the fiber laser with a 1.5-cm-long YCDSF, the optical spectrum of laser output, under the maximum output power, was recorded in the range of 900-1200 nm, as shown in Fig. 10(a). The laser output spectrum consists of a strong narrow-bandwidth laser peak and a weak spontaneous emission spectrum. Its OSNR is approximately 80 dB, and the central wavelength is 1029.6 nm. The inset shows an enlarged view of the laser spectrum ranging from 1027 nm to 1032 nm. It is obvious that the laser output spectrum is highly symmetrical and has a narrow bandwidth, which were inherited from the LR-FBG and YCDSF, respectively. The fluctuation of the output power (FOP) is only 0.35% of the average power within 80 min, as shown in Fig. 10(b). The laser output power is relatively stable. The small fluctuations may be attributed to the fluctuations in pump power and ambient temperature. The beam quality was characterized using a the BeamSquared system (SP920, Spiricon), as shown in Fig. 10(c). The quality factor was 1.022 in both the vertical and horizontal directions. Its waist width and Rayleigh length along the x-axis and y-axis are 471.71 µm and 165.97 mm, 469.75 µm and 164.63 mm, respectively. The two-dimensional beam profile is almost the same as the standard Gaussian distribution. These excellent spectrum properties make this fiber laser a good candidate for use as a seed light in high-power lasers.
Fig. 10. (a) Output spectrum of the laser with a 1.5-cm-long YCDSF under maximum output power, and the inset shows an enlarged view; (b) the laser stability record in 80 min; (c) beam quality of the fiber laser and its two-dimensional beam profile.
Compared with the commercial Yb-doped silica fiber (Fibercore, DF1100) [37,38], the Yb:YAG-derived silica fibers showed a more flexible rare earth ion doping concentration, high slope efficiency, and high output power. However, their threshold power was slightly higher. Compared with Yb-doped silica fiber or Yb:YAG-derived silica fibers, the YAG crystal fiber [39] has obvious advantages in terms of material properties. At a low Yb ion doping concentration, its slope efficiency is up to 68.70% and the maximum output power is 50.0 W [40]. However, this fiber core is 100 µm, and forms a non-all-fiber laser system. Furthermore, it is difficult to couple it with silica optical fiber, and to apply it to all-fiber laser systems at this stage. The YCDSF with a large core diameter (YCDSF-3), can be used in a non-all-fiber laser system. The output power of the laser is 670 mW, and its slope efficiency exceeds 76.0%. However, its OSNR is 70 dB, and the threshold power is up to 140 mW. Thus, the beam quality is also poor [41]. Therefore, for a short-cavity all-fiber DBR laser, the Yb:YAG-derived silica fiber is an ideal candidate because of its high gain coefficient, high doping concentration, and good compatibility. The comparison with other fibers is shown in Table 2 in details. With regard to the fabricated fiber, its gain per unit length is up to 4.4 dB/cm, and the slope efficiency exceeded 50.0%. The maximum output power is up to 360 mW, and it remained unsaturated. Therefore, the performance of fiber laser can be further improved, especially the performance of YCDSF.
4. Conclusion
The YCDSFs were fabricated by via the CO2 laser-heated drawing technique. The Yb ion doping concentration was up to 5.66 wt%. The absorption coefficient, unsaturated absorption coefficient, and merit of unsaturated absorption of the YCDSF at 980 nm were 19.14 dB/cm, 1.46 dB/cm, and 93%, respectively. The gain per unit length of YCDSF is up to 4.4 dB/cm. In addition, we built an all-fiber DBR laser with a 1.5-cm-long YCDSF through simulation and experiment. Its maximum output power was up to 360 mW, and the slope efficiency exceeded 50.0%, making it superior to short-cavity all-fiber DBR lasers with other Yb:YAG-derived silica fibers. The output laser spectrum had a high OSNR of 80 dB, and a small FOP within 80 min of only 0.35%. The short-cavity all-fiber DBR laser based on the high gain coefficient YCDSF is compact, efficient, and stable, and has the potential to be used in applications such as high-quality seed source and high-precision coherent communications.
Funding
National Natural Science Foundation of China (61520106014, 61635006, 61935002, 61975113); Pre-Research Fund Project (6140414030203).
Acknowledgments
We are grateful to Prof. Chengbo Mou at the Shanghai University for invaluable analyses and discussions on the beam quality of the fiber laser.
Disclosures
The authors declare no conflicts of interest.
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