Single-frequency fiber lasers (SFFLs) have attracted considerable attention because of their remarkable narrow linewidth, low noise, high stability, and compact all-fiber configuration. These merits have inevitably driven the increasing applications of SFFLs in the generation of optical frequency combs and in nonlinear frequency conversion, gravitational wave detection, and coherent beam combining [1–4]. The lasing performance of SFFLs, such as slope efficiency, maximum output power, linewidth, and stability, is strongly influenced by the cavity configuration and the properties of the gain fiber. In the cavity design, various approaches have been utilized to accomplish single-longitudinal-mode (SLM) operation, such as distributed Bragg reflectors (DBRs) [5], a distributed feedback (DFB) configuration [6], a long linear cavity [7], and a traveling-wave ring cavity [8]. In the last two designs, the cavity length on the order of meters has to be applied, together with a narrow filter, in order to maintain the SLM operation, whereas the DBR and DFB configurations have cavity lengths of only several centimeters. Such ultrashort cavity configurations allow them to obtain a longitudinal mode interval of up to several gigahertz (GHz) and achieve a stable SLM operation more easily. In general, the DFB cavity is formed by writing a pair of fiber Bragg gratings (FBGs) on the gain fiber, which necessarily imposes a high requirement on the grating writing technology. On the contrary, building the DBR cavity is relatively easy, and the resultant fiber laser has a more robust, compact structure and a high slope efficiency [5]. As far as the gain fiber is concerned, recent decades have witnessed great efforts in developing novel gain fibers to realize high-efficiency SFFLs, such as highly rare-earth-doped phosphate fibers, tellurite-based fibers, and crystal-derived silica fibers [9–14]. Among these gain fibers, yttrium aluminum garnet (YAG) crystal-derived silica fibers are of great interest. Because of their excellent physical and chemical properties, the derived fibers are compatible with commercialized silica-based fibers and have the capability of doping a high level of rare-earth ions [15–18]. On the other hand, optical fiber fabrication technology has progressed rapidly in the recent two decades, providing an opportunity for the development of high-quality gain fibers. For instance, the melt-in-tube method has contributed greatly to the development of YAG crystal-derived silica optical fibers [19,20], which can be used as promising gain fiber materials in single-frequency DBR fiber lasers.

In 2012, the Yb-doped YAG crystal-derived silica fiber (YCDSF) was investigated as a heavily doped gain medium for fiber-amplifier applications [15], and the obtained fibers showed a higher Yb doping level with a greater uniformity over conventional Yb-doped aluminosilicates. Moreover, it is also expected that the YCDSF-based fiber lasers could have a better performance in output power over the silica-based fiber lasers due to the increased threshold of the stimulated Brillouin scattering [21]. Since then, a number of studies on high-power fiber lasers and SFFLs have been carried out based on the developed YCDSFs [13,22–25]. To date, the maximum output power of the SFFLs can be up to 110 mW, and the associated slope efficiency is 18.5% [13]. The improvement of the SFFL performance is deemed to go along with the development of high-quality gain fibers. Previously, we fabricated a high-gain YCDSF based on the melt-in-tube method on a CO2 laser-heated drawing tower and built a DBR fiber laser based on a 1.5 cm long YCDSF [26]. The technique has a much shorter hot zone compared to the traditional graphite-heated fiber drawing method [13,26,27], i.e., about 13 mm versus 40 mm. As a consequence, it may suppress silica diffusion from the cladding region into the YAG crystal core, sustaining a high Yb doping level in the resultant YCDSF. The fiber laser showed an optimum performance with a maximum output power of 360 mW and a slope efficiency of 50.5%. However, it did not operate at a single frequency.

In this work, single transverse and longitudinal mode operation at 1030 nm has been accomplished through innovatively incorporating an ultrashort, highly efficient YCDSF into an all-fiber DBR cavity with the aid of theoretical calculations. A significant breakthrough has been made in the maximum output powers and the slope efficiency through the built DBR laser cavity containing a 0.7 cm long YCDSF. The absorption and emission cross sections of the YCDSF are calculated and analyzed as well as the gain cross section. The lasing characteristics of the SFFL have been systematically investigated in terms of its longitudinal mode operation, slope efficiency, output spectrum, power stability, beam quality, and noise. The established approach could provide insight into the study of a range of high-efficiency SFFLs doped with other rare-earth ions at other operating wavelengths.

In the measurement system, the laser output can be selectively connected to either an optical spectrum analyzer (OSA, AQ6370, Yokogawa, Japan), power meter (PM, PM100D, Thorlabs), electrical spectrum analyzer (ESA, Agilent), or BeamSquared system (SP920, Spiricon) depending upon the requirement. The OSA was used to characterize the spectral properties of the output lasing light, and the output power of the laser was measured by the PM. With a variable optical attenuator (VOA), the laser output was converted to the electrical signals by a photodetector (PD, 818-BB-51F, Newport), and subsequently these signals were analyzed by the ESA in terms of the longitudinal mode and noise characteristics of the fiber laser. The beam quality of the laser output was also evaluated by the BeamSquared system.

To ensure the SLM operation, twice the longitudinal mode interval should be more than the reflection bandwidth (Δυ) of the laser. It is known that the reflection bandwidth of the single-frequency DBR fiber laser is mainly determined by the LR-FBG instead of the HR-FBG. In the present work, the reflection bandwidth of the LR-FBG (0.05 nm) is equivalent to 14.15 GHz [the solid blue line in Fig. 4(b)], half of which is 7.075 GHz as designated by the blue dotted line in Fig. 4(b). This determines a critical fiber length of 0.7 cm, less than which the DBR cavity will be the SLM operation as shown in the left inset in Fig. 4(b). Otherwise, the laser would operate at the multiple longitudinal mode (MLM) state [the right inset in Fig. 4(b)], i.e., with further extending the fiber length or increasing the Δυ value.

At the maximum output power, the output spectrum of the SFFL was scrutinized using an OSA with the resolution of 0.02 nm. The examined spectrum ranges from 900 to 1200 nm as illustrated in Fig. 6(b), where the inset is part of magnified region of 1028 to 1031 nm. In this figure, a strong narrow-bandwidth lasing peak centered at 1029.6 nm can be seen, associated with an optical signal-to-noise ratio (OSNR) of about 79 dB and an ultranarrow bandwidth of 0.05 nm at 3 dB. Besides, there is a weak broad peak at 1025 nm, a typical amplified stimulated emission (ASE) of Yb3+ ions [23,26]. The highly symmetrical profile of the lasing peak and its associated ultranarrow bandwidth imply the good confinement accomplished within the laser cavity.

The performance of the SFFL in this work is compared with other phosphate- and silica-based all-fiber SFFLs, as shown in Table 1. First, the phosphate-based fibers [34,35] show a much higher Yb doping level than silica-based fibers, leading to a larger fiber gain coefficient and a higher slope efficiency in the constructed SFFLs. However, they suffer from relatively poor mechanical strengths over the YCDSFs. Besides, the SFFLs based on phosphates generally show a larger lasing threshold, for example, over 40 mW in Refs. [34,35], which may be accounted for from the mismatch occurring in the splicing Table 1.

Summary of the All-Fiber DBR SFFLs Based on Different Gain Fibers

According to the delayed self-heterodyne method, the linewidth of the SFFL was studied, in which a 10 km long single fiber was used to provide a delay of 0.05 ms and a linewidth resolution of 15 kHz. During the measurement, a 200 MHz frequency shift was introduced using the AOM so as to avoid the interference of low-frequency signals, and the applied ESA had a sweep time of 0.69 s with a 30 Hz bandwidth resolution. Figure 8(b) shows these typical heterodyne signals with respect to the pump power. As can be seen from the figure, the 20 dB linewidth of the heterodyne signals gradually increases from 0.49 MHz at 242 mW to 1.7 MHz at 722 mW and 3.43 MHz at 965 mW. The linewidth of the laser can be estimated by dividing the 20 dB linewidth of the heterodyne signal by 20. A clear relationship between the linewidth and the pump power is also presented in Fig. 8(c). When the launched pump power rises from 170 to 965 mW, the measured linewidth increases from 20 to 171 kHz. A rapid increase in linewidth after 600 mW is also observed, which might be due to the significant heat accumulation in the YCDSFs. This is because that part of the pump light might not contribute to the stimulated transition process of Yb3+ ions, but rather than would be released in the form of heat [39]. The generated heat could modulate the refractive index of the YCDSF, causing linewidth broadening [40]. In general, the obtained linewidth of the laser in this work agrees with those of reported silica-based SFFLs [13,36], but it is broader than those of other phosphate-based SFFLs [10,34,35]. Further improvement can be realized by self-injection locking technology with passive optical feedback loops [41].

In summary, we have demonstrated an all-fiber DBR SFFL at 1030 nm based on an ultrashort YCDSF highly doped with 5.66% (mass fraction) Yb. The absorption, emission, and gain cross sections of the fabricated YCDSF are calculated. A stable single transverse and longitudinal mode operation has been accomplished with the DBR cavity based on a 0.7 cm long YCDSF. The SFFL built has a slope efficiency of 35% with an FOP of 0.85% within the examined duration over 10 h, showing the maximum output power of up to 258 mW. The OSNR of the laser is 79 dB with a beam quality of 1.016 in both the x and y axial directions. As the pump power increases, the measured RINs gradually decreases from −135 to −138.7dB/Hz at 10 MHz, while the linewidth gradually increases from 20 to 171 kHz. The study suggests that the high-performance single-frequency DBR fiber lasers constructed from high-gain YCDSF could be applied as a seed source for a high-power fiber laser and detection source for the optical sensing.

Acknowledgment. We are grateful to Profs. Hairun Guo, Liang Zhang, and Chengbo Mou at Shanghai University, China, for their help in data analysis and insightful discussion on the performance of the DBR SFFL.