Narrow linewidth, low noise, and good beam quality make single-frequency fiber lasers (SFFLs) desirable as light sources in fiber sensing, LIDAR, gravitational-wave detection, atomic physics, coherent communications, and quantum physics^[1–5]. Unpolarized outputs of many SFFLs limit their use in certain situations, including the generation of second harmonics, optical parametric oscillations, and beam synthesis^[6–11]. Comparatively speaking, linearly polarized single-frequency fiber lasers (LPSFFLs) with the linear cavity possess the features of compactness and low cavity loss, making them more suitable and popular. However, efficient linear cavity lasers greatly depend on high-gain active fibers. For applications in the 1.5 µm band, silica fibers with high Er3+ concentrations are prone to concentration quenching, which limits laser performance^[12]. So far, SFFLs have mostly adopted high-concentration-doped soft glass fibers, e.g.,  phosphate-based fibers^[13,14], tellurite-based fibers^[15,16], and bismuthate-based fibers^[17]. In recent years, crystal-derived fibers have been particularly attractive, due to the high doping concentration of rare-earth ions, strong mechanical strength, and excellent chemical stability. To date, YAG crystal-derived silica fibers (YDSFs) doped with rare-earth ions, e.g.,  Yb3+, Cr3+, and Tm3+, have been developed and used for linear cavity fiber lasers^[18-20].

In 2009, the first Er:YAG crystal-derived silica fiber (EYDSF) was prepared using the melt-in-tube method, and its material and optical properties were studied^[21]. Since then, EYDSFs have widely been used and investigated. For example, Dragic et al. investigated the stimulated Brillouin scattering (SBS) effect in EYDSFs^[22]. They found that both Y2O3 and Al2O3 increased the phonon rate of the material, and Er ions were beneficial for the bandwidth extension of the Brillouin gain spectrum resulting in a high Brillouin scattering threshold. Because of these features, they can be used as promising gain fiber in lasers. In 2021, the first EYDSF-based fiber laser was constructed, achieving pulsed lasing output with a slope efficiency of 15.1% and a power of 24.2 mW at 1550 nm^[23]. Later on, Er/Yb co-doped crystal-derived fibers were fabricated by UV-curable nanocomposites to achieve a single-frequency fiber laser of ∼2mW output at 1552 nm^[24]. Recently, EYDSFs were used in a ring-cavity system, achieving continuous-wave lasing output at 1560 nm with a slope efficiency of 5.3% and a power of 33 mW^[25]. Moreover, the laser performance, e.g.,  the slope efficiency, the output power, and the polarization state, can further be optimized through the enhanced properties of EYDSFs. Especially, LPSFFLs not only effectively suppress the competition between polarization modes in the cavity but also are less susceptible to environmental disturbances. Hence, the exploration of EYDSF-based LPSFFLs is of great significance.

In this work, EYDSFs were fabricated using the melt-in-tube method with an Er:YAG crystal rod. A polarization-maintaining Bragg grating-based distributed feedback (DBR) laser was constructed with a 1480 nm pump. The laser performance was systematically evaluated in terms of output power, linewidth, polarization extinction ratio, noise, and so on. Lastly, the laser achieved a significant breakthrough in the maximum output power and slope efficiency.

Using a CO2-laser-heating drawing tower, the EYDSFs were fabricated with the melt-in-tube method at ∼2100°C. The detailed preparation process has been reported in the previous work^[25]. Then, the refractive index distribution of the EYDSF was measured by a fiber refractive index analyzer (S14, Photon Kinetics Inc., U.S.). The core and fiber diameters are 12.4 and 125.6 µm, respectively, as shown in Fig. 1(a). The refractive index difference between the core and cladding and the numerical aperture (NA) are 0.0470 and 0.37 at 633 nm, respectively. The splicing loss of the single-mode fiber (SMF) and single-mode polarization-maintaining fiber (SM-PMF) with EYDSF was tested using the cut-back method at 1610 nm, where the absorption band of Er3+ is negligible. The splicing losses are measured to be ∼0.11dB (SMF–EYDSF) and 0.10 dB (SM-PMF–EYDSF). The core diameters of the SMF and the SM-PMF are 8.0 and 9.0 µm, respectively. Mismatched mode fields by the differences in the core diameter and the NA can cause losses at splicing, leading to a negative impact on the output power of the laser. So the preparation process needs to be further optimized to decrease the loss.

The absorption spectrum of the EYDSF was measured with the cut-back method using an optical spectrum analyzer (OSA, AQ6315, Yokogawa, Japan), as exhibited in Fig. 1(b). The absorption peaks of the EYDSFs were at 647, 798, 980, 1480, and 1531 nm, corresponding to the transitions from the ground state of I415/2 to the excited states of F49/2, I49/2, I411/2, and I413/2, respectively. The excitation-emission spectrum and fluorescence decay curves were measured using a fluorescence spectrophotometer (FLS-980, Edinburgh, UK). The excitation peaks of the EYDSF were at 980 and 1480 nm, and the emission peak was at 1531 nm, as shown in Fig. 1(c). The fluorescence decay curves are shown in Fig. 1(d). The fluorescence lifetime of the 1480 nm excitation (6.3 ms) is shorter than that of the 980 nm excitation (7.4 ms). This is because electrons need an additional relaxation process from the I411/2 to the I413/2 energy level under 980 nm excitation, while they directly transfer from the I413/2 to the I415/2 energy level upon 1480 nm excitation. It is noted that the shorter fluorescence lifetime under 1480 nm excitation is not favorable for cavity energy storage and the suppression of self-pulse effects getting continuous-wave lasing output.

Amplification characteristics at 1560 nm were investigated with 980 and 1480 nm pumps. The amplification system is based on a forward-pumping structure with an EYDSF of 2-cm-long and a signal intensity of −20dBm. The variation of the gain coefficient with different pump powers is plotted in Fig. 2(a). The net gain coefficient of the 980 nm pump gradually increases with the increase of the pump power and eventually reaches saturation. That of the 1480 nm pump is limited by the pump power and cannot reach saturation. Nevertheless, the maximum gain coefficient (2.11 dB/cm) with the 1480 nm pump is still larger than that with the 980 nm pump (1.75 dB/cm). The unsaturated absorption coefficient (αus) at 1480 nm is as low as 0.38 dB/cm, which is lower than 0.49 dB/cm at 980 nm, as depicted in Fig. 2(b) (of which α_s is the saturated absorption coefficient). The low αus could improve the pump utilization efficiency.

The 980 and 1480 nm laser diodes were used as the pump source in a DBR laser system with a 2-cm-long EYDSF. The system is shown in the inset of Fig. 2(c). Under the same condition (the same components except for pump wavelength), the output power with the 980 nm pump is only 33 mW, and its optical-to-optical conversion efficiency is 18.0%. The output power with the 1480 nm pump is as high as 54 mW, and its optical-to-optical conversion efficiency is up to 27.0%, as shown in Fig. 2(c).

The properties of the EYDSF are compared with different EYDSFs, as shown in Table 1. The gain coefficient of the EYDSF decreases as the Er3+ doping concentration is increased from 2.96% (mass fraction) (EYDSF-1) to 4.06% (mass fraction) (EYDSF-2). This is due to the fact that higher doping concentrations exacerbate concentration quenching, resulting in performance degradation. So it is very important to choose the proper doping concentration. In this work, the EYDSF-4 with the Er3+ doping concentration of 3.40% (mass fraction), achieved the highest gain coefficient of 2.11 dB/cm, as shown in Table 1. It is beneficial to obtain a higher output power. Their core diameters listed in the table are closed. In addition, the EYDSF-4 has the lowest NA, which reduces its splicing loss with the SMF, as low as 0.11 dB, as displayed in Table 1.

A linearly polarized DBR cavity system was constructed to study the laser properties of the EYDSF, as shown in Fig. 3(a). The 1480 nm pump was selected because of its advantages in gain coefficient, unsaturated absorption coefficient, and fluorescence lifetime as demonstrated above. A single-mode 1480/1560 nm polarization-maintaining wavelength-division multiplexer (PM-WDM) was used to launch the pump light at 1480 nm and output the lasing at 1560 nm. The DBR cavity consists of a polarization-maintaining low reflectivity fiber Bragg grating (FBG) (PM-LR-FBG), a 2-cm-long EYDSF, and a high reflectivity FBG (HR-FBG). The reflectivity corresponding to the slow-axis of the PM-LR-FBG and the reflectivity of the HR-FBG were 60% and 99.9%, respectively. The center wavelengths of the PM-LR-FBG’s fast and slow axes were 1559.6 and 1560.0 nm, respectively, as depicted in the transmission spectra of Fig. 3(b). The central wavelength of the HR-FBG was 1560.0 nm, coinciding with the slow axis of the PM-LR-FBG so that the slow-axis polarized light can oscillate to form a resonant cavity. A large amount of residual pump was detected at the outer end of the cavity of the HR-FBG. Therefore, a fiber mirror with reflected wavelengths of 1470–1565 nm was spliced at the tail end of the HR-FBG, which can reflect the residual pump power into the cavity and increase the output power through the enhancement of the absorbed power by EYDSF.

To achieve the single longitudinal mode (SLM) operation of the laser, twice the longitudinal mode spacing (υFSR) should be larger than the reflection bandwidth of the laser. The υFSR is calculated by the formula c/2nl, where c is the speed of light in a vacuum, n is the refractive index of the fiber core, and l is the cavity length of the laser cavity. The total cavity length is the sum of the EYDSF and the effective length of the grating pairs. The effective lengths of the PM-LR-FBG and the HR-FBG are approximately 0.35 and 0.10 cm, respectively. The 3 dB bandwidth of the PM-LR-FBG and the HR-FBG are approximately 0.07 and 0.40 nm, respectively. The reflection bandwidth (Δυ) of the PM-LR-FBG is less than 8 GHz, only half of which (4 GHz) was designated by the dotted line in Fig. 3(c). When the length of the EYDSF is 2 cm, the corresponding longitudinal mode spacing of the laser cavity is 4.05 GHz, which is larger than half of the reflectivity bandwidth of the PM-LR-FBG. Thus, a single longitudinal mode output can be achieved.

The longitudinal mode performance of the laser was evaluated by a delayed self-heterodyne system based on the Mach–Zehnder interference. In the system, an 80-MHz acousto-optic modulator was utilized for frequency shifting. The measurement range of the spectrum analyzer is from 0 to 10 GHz, with a resolution of 10 kHz. The radio frequency beat signal spectra of the laser at different pump powers were measured using an electrical spectrum analyzer (ESA, Agilent, U.S.). Only one obvious beating peak at 80 MHz could be seen under different pump powers from Fig. 4(a). The measurement range is wider than twice the longitudinal mode spacing, indicating that the laser could run stably in a single longitudinal mode state. The linewidth of the output lasing at the maximum pump power was measured with the same system using the delayed self-heterodyne method. In this system, a 50 km single-mode fiber was used as a delay line, which provided a delay of 0.243 ms and a bandwidth resolution of 3–4 kHz. The resolution of the spectrometer was selected to be 1 kHz. A typical Lorentz-like signal was observed in the ESA. The 20 dB bandwidth of the heterodyne signals is 48 kHz, demonstrating the laser linewidth of 2.4 kHz, as shown in Fig. 4(b). This indicates that the LP-SFFL has good linewidth characteristics.

The variation of the output power for the DBR laser with different pump powers was further analyzed. For lasers without and with the fiber mirror, the pump thresholds are 159 and 105 mW, respectively. The output power increases linearly with the pump power increasing after exceeding the excitation threshold as shown in Fig. 5(a). The slope efficiency is only 12.7% without a fiber mirror, while the slope efficiency with a fiber mirror is up to 22.4%. Under the same maximum pump power, the output powers are 54 and 103 mW, respectively. The fiber mirror significantly improves the slope efficiency and the output power. Since no saturation of the output power was observed, this implies that a higher output power can be obtained if the pump power further increases. A strong narrow-bandwidth lasing peak centered at 1560 nm was observed using an optical spectrum analyzer with a resolution of 0.02 nm, as shown in Fig. 5(b). It corresponds to an optical signal-to-noise ratio (OSNR) of ∼71.0dB. The inset in Fig. 5 shows a portion of the amplified region of 1559–1561 nm, which shows a highly symmetric lasing peak.

In order to evaluate the polarization characteristics of the laser, a power meter (PM100D, Thorlabs, U.S.) was used to measure the output power of the laser in different polarization directions by rotating the polarizer. A good fit can be obtained using Malus’s equation as shown by the red line in Fig. 6(a). This indicates that the lasing output has good linear polarization characteristics. The polarization extinction ratios (PER) were calculated at different pump powers. The PER of the LP DBR laser is almost constant, about 25 dB, for the different pump powers, as shown in Fig. 6(b), which means that the PER is independent of the pump power.

Subsequently, the frequency spectrum at the maximum output power was measured using an InGaAs photodetector (PD- DET08CFC/M, Thorlabs, U.S.) and an electrical spectrum analyzer in the range of 10 Hz–10 MHz. Relative intensity noises (RIN) of the laser were calculated. There is a relaxation oscillation peak at 316 kHz, and the noise stabilizes at −139dB/Hz in the high-frequency band (>1MHz), as illustrated in Fig. 7(a). A continuous wave is observed on the oscilloscope, as displayed in the inset of Fig. 7(a). The suppression of the self-pulsing is related to the high gain coefficient and short fluorescence lifetime with a 1480 nm pump.

The power stability was tested continuously at an output power of ∼103mW. A power meter is used to monitor power fluctuations over a two-hour period sampling at 1-s intervals, as shown in Fig. 7(b). The inset of Fig. 7(b) shows an enlarged view of the power fluctuations over a half-hour period. The fluctuation of the output power (FOP) is <0.7% of the average output power. The power fluctuations are caused by the temperature fluctuation, small vibrations in the environment, and random fluctuations of the pump power.

The linear cavity SFFLs based on different Er-doped fibers were compared, as shown in Table 2. As seen from Table 2, Er-doped silica fibers have a lower gain coefficient than soft glass fibers and crystal-derived fibers. Compared with other crystal-derived fibers, the gain coefficient of the EYDSF in this work is up to 2.11 dB/cm, which, to our knowledge, is the highest gain coefficient reported so far. In addition, the output power and slope efficiency are also the highest of all the EYDSF-based SFFLs as a result of the higher doping concentration of the Er3+ ions, lower fiber splicing losses, proper pump wavelength (1480 nm), and the laser design. Better linear polarization characteristics are achieved compared with other lasers listed in Table 2. The extinction ratio of the output lasing is as high as 25.0 dB attributed to a high-quality component and a gain fiber in the laser system. Moreover, the setting of fusion parameters and the fusion angle of the PMF with other fibers are also crucial.

In summary, we prepared an Er3+-doped crystal-derived fiber with a high-concentration using the melt-in-tube method. The gain coefficient of the fiber was 2.11 dB/cm with a 1480 nm pump. We achieved linearly polarized single-frequency lasing output in a PM-FBG-based DBR laser cavity at 1560 nm with a linewidth of 2.4 kHz. The length of the EYDSF was only 2 cm. Using a 1480 nm pump, the optical-to-optical conversion efficiency and the output power were raised compared with a 980 nm pump, and the continuous-wave lasing output was achieved. A fiber mirror was introduced at the end of the laser cavity to reflect the residual pump, which made the output power increase from 54 to 103 mW. This work highlights the significant enhancement of the lasing performance, especially the slope efficiency of 22.4% and the maximum output power of 103 mW, which are the highest efficiency and output power achieved in EYDSF-based SFFL, respectively. The PER of the laser is 25 dB, and the RIN is <−139dB/Hz. It exhibits long-period stability with an FOP of less than 0.7% over 2 h. It is believed that all-fiber linearly polarized single-frequency fiber lasers constructed in this work is a good seed source for coherent optical communications, LIDAR, laser combining system, and other applications.