1. Introduction

High-power diffraction-limited Yb-doped fiber laser operating near 980 nm is highly desirable for high-power laser system due to their potential applications as the pump source for ultrafast solid-state lasers and fiber lasers [1,2]. Moreover, it can also produce blue radiation and deep-ultraviolet radiation via nonlinear frequency conversion, which plays an important role in the field of biomedicine and underwater exploration [3–5]. However, developing a high-power high-brightness three-level fiber laser near 980 nm is a challenging endeavor, because of the well-known obstacle, i.e., the unavoidable amplified spontaneous emission (ASE) around 1030 nm.

In order to facilitate power up-scaling, cladding-pumped scheme is indispensable, but it is imperative to note that sufficiently large core-to-cladding area ratio (CCAR) is critical to mitigate the ASE around 1030 nm [6]. In general, there are two straightforward methods to enlarge the CCAR, i.e., increase the core diameter, or reduce the cladding diameter. However, some issues will also be brought with these methods. The large core will be detrimental to the beam quality, while the small inner-cladding will restrict the available pump power, and correspondingly restrict the output power.

In spite of that, some impressive progresses have been made by using micro-structured fibers [7–13]. In 2008, based on an ultra-large core PCF (80-µm core diameter), the first hundred-watt-level near-diffraction-limited fiber laser near 980 nm was demonstrated with output power being 94 W and M² factor being 1.2 [7]. The micro-structure of the PCF was designed to suppress high-order modes, and thus the near-diffraction-limited beam quality was realized. Later, a 151-W 978-nm monolithic near-diffraction-limited fiber laser was demonstrated with the utilization of Yb-doped all-solid photonics bandgap fiber (AS-PBF) [11]. By designing the bandgap loss of the AS-PBF to effectively suppress the ASE around 1030 nm, the ASE around 1030 nm can be sufficiently suppressed even with a low CCAR. Therefore, with a small core diameter AS-PBF (about 21-µm core diameter), the near-diffraction-limited beam quality at 151-W output power was obtained. From above studies, it can be known that these micro-structures play very important roles in generating the high-power diffraction-limited lasing near 980 nm. However, these micro-structures also bring one issue, i.e., the difficulty in the fabrications of these fibers, which limits the promotion of pertinent studies and results.

Comparatively, step-index Yb-doped fibers (YDFs) were attempted more widely for generating the lasing near 980 nm, because they can be fabricated and obtained more conveniently. Correspondingly, their solutions can be more cost-effective and promotable than the solutions of micro-structured fibers. However, without the help of micro-structure, it is more challenging to achieve high-power and near-diffraction-limited operation simultaneously by using step-index YDFs. Nowadays, by utilizing the ultra-large core YDFs [14–18], the output power of fiber laser near 980 nm has been rapidly up-scaled, from hundred-watt-level to kilo-watt-level [14–16]. However, the ultra-large core also resulted in the poor-quality beam, e.g., the M² factor is larger than 16 in the kilo-watt-level fiber laser near 980 nm [16].

On the other sides, a number of step-index YDFs were designed for the single-mode operation, e.g., such as jacketed-air-clad YDF [19], tapered step-index YDF [20], saddle-shaped YDF [21], W-profile YDF [22], clad-etched YDF [23], low core numerical aperture (NA) YDF [24] and so on, but only tens-of-watt output power was achieved. Then, some studies were made in order to balance the output power and beam quality [25–27]. In 2022, based on a step-index YDF with the core/cladding diameter of 20/125 µm, a hundred-watt few-mode fiber laser near 980 nm was demonstrated, and 109-W lasing power with about M² factor being 1.9 was obtained [27]. However, another obstacle was revealed in Ref. [27], i.e., the in-band ASE. Different from the ASE around 1030 nm generated by the four-level transmission of Yb-ion, the in-band ASE is generated by the three-level transmission of Yb-ion, because its wavelength is still located within the band around 980 nm (generally ranging from 974 nm to 982 nm). As a result, the output properties of in-band ASE are very different from the ASE around 1030 nm. References [15,27] have revealed that the strong in-band ASE can be present even with the ASE around 1030 nm well suppressed. Therefore, the in-band ASE should be studied in detail.

In fact, the negative effect of in-band ASE on the performance of fiber lasers near 980 nm has been revealed in some former literatures [10,15,27–33]. In Ref. [10], it has been warned that the in-band ASE should be paid attention in high-power fiber laser near 980 nm. In 2021, Ref. [15] demonstrated that the in-band ASE can be the key limitation to the power scalability of Yb-doped fiber amplifier near 980 nm. Later, by improving the suppression of in-band ASE with enlarged seed power, the kilo-Watt level fiber laser was demonstrated. However, the in-band ASE observed in Ref. [27] is obviously stronger than those literatures. It is found that the even with more-than 24-W seed power, the in-band ASE cannot still be well suppressed. As the result, only 17-dB suppression of in-band ASE was achieved by using the seed power as large as 34 W. Moreover, such strong in-band ASE can still do harm to the beam quality because of the high-order modes of in-band ASE in the few-mode active fiber. Therefore, besides the ASE around 1030 nm, the in-band ASE also enhances the difficulty of generating the high-power diffraction-limited lasing near 980 nm with the step-index YDF.

In this paper, we demonstrate the first hundred-watt-level step-index YDF fiber laser near 980 nm with near-diffraction-limited beam quality (M² factor smaller than 1.3), to the best of our knowledge. It is found, unexpectedly to some extent, that the suppression of in-band ASE can be greatly improved by adjusting the seed wavelength to around 979 nm. Then, only with 11.7-W 979-nm single-mode seed light, 101.5-W output power was achieved, and there is almost no in-band ASE observed in the output spectra. The M² factor is about 1.285 at the maximum output power.

2. Experiment setup

The experimental setup of the monolithic master oscillator power amplifier (MOPA) fiber laser is illustrated in Fig. 1, which is composed of a lower-power seed oscillator and a high-power amplifier. Different from former works where the seed/signal wavelength is mainly within the range from 976 nm to 978 nm, the seed wavelength is adopted as 979 nm here with the consideration of in-band ASE suppression. In fact, former studies have shown that the seed wavelength smaller than 978 nm cannot well suppress the in-band ASE. Despite that, those studies revealed that the in-band ASE with longer wavelength can be enhanced faster with the increment of pump power [10,15,33], which implied that the wavelength longer than 978 nm can be more dominant in the gain competition. In addition, some studies also revealed that with the seed wavelength longer than 980 nm, the in-band ASE can also be present within the band around 977 nm [28,33]. Therefore, based on those studies, we decided to make a try with the seed wavelength of around 979 nm which is located in the range from 978 nm to 980 nm.

 

Fig. 1. Experimental setup of the monolithic fiber laser. Inset: Cross section images of the active fiber (a: 10/130 YDF; b: 20/125 YDF).

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Besides the wavelength of seed light, the single-mode operation is still indispensable to achieve the diffraction-limited beam quality. Thus, instead of the few-mode seed oscillator in Ref. [27], the single-mode 979-nm seed oscillator is fabricated firstly (see Fig. 1). The single-mode operation is ensured by the active fiber which is a step-index YDF with the core of 10-µm diameter and 0.072 numerical aperture (NA). Then, the normalized frequency is about 2.31 at 979 nm, and thus the YDF is single-mode fiber. The inner-cladding is octagonal (see the inset (a) of Fig. 1), and its diameter (i.e., the distance of flat-to-flat or the diameter of inscribed circle of the octagonal inner-cladding) of the YDF is about 130 µm, and the cladding absorption is about 0.73 dB/m at 915 nm.

The resonant cavity of the fiber oscillator is established by a pair of fiber Bragg gratings (FBGs) with center wavelength of about 979.2 nm. The high-reflectivity FBG (HR FBG) provides reflectivity of 99.5% and 3-dB bandwidth of 1.1 nm, and the low-reflectivity FBG (LR FBG) provides reflectivity of 9.4% and 3-dB bandwidth of 0.5 nm. A counter-pumped configuration is utilized in the seed oscillator, and the pump sources are two 915-nm laser diodes (LDs) pigtailed with a passive fiber owning the core/cladding diameter of 105/125 µm and the core NA of 0.15. The residual pump light and the signal light leaking from fiber core are eliminated by the high-power cladding light stripper (CLS) which is fabricated by chemical etching method [34,35]. To filter out the 1030-nm ASE generated from the fiber oscillator, a 975-nm filter with bandwidth of 30 nm is added after the LR FBG.

Then, a high-power fiber amplifier is fabricated with a step-index YDF. The step-index YDF owns 20 µm core diameter and 125 µm inner-cladding diameter. The NAs of the core and cladding are 0.08 and 0.46, respectively. The inner-cladding is octagonal (see the inset (b) of Fig. 1), and its peak cladding absorption is about 5.47dB/m at 915nm. The bidirectional pumping configuration is utilized in the amplifier to ensure enough pump power, and the pump light generated from four 915-nm LDs (owning the same parameters with the LDs used in the seed oscillator) is coupled into active fiber through two (2 + 1) × 1 combiners, which provides the maximum 545-W pump power. Two CLSs are employed to get rid of the unwanted cladding light. The seed light is injected into the fiber amplifier via a mode field adaptor (MFA). The output fiber is angle-cleaved to suppress the undesired optical feedback. All the components of the MOPA fiber laser are water-cooled.

3. Experimental results and discussions

3.1 Experimental study of the seed oscillator

Firstly, the properties of the seed light were studies experimentally, and the pertinent results are given in Fig. 2, which were measured before filter. Figure 2(a) gives the variation of seed power before filter with pump power, and it can be found that the seed power exhibits a linear increase with the pump power, and the maximum seed power of 13.24 W was obtained when the pump power reached to 397 W. The slope efficiency was about 3.55%. Such a low efficiency is mainly caused by the ultra-low CCAR of 0.0059 (i.e., 10²/130²). According to the previous study [6], the small CCAR will make it difficult to suppress the 1030-nm ASE, and then the length of the active fiber used in the seed oscillator has to be shortened to 0.61 m to suppress the ASE. However, such a short active fiber will lead to the insufficient absorption of the pump power, and thus resulting in relatively low slope efficiency.

 

Fig. 2. (a) The variation of seed power before filter with pump power. (b) The variation of output spectra before filter with seed power, inset: the local output spectra near 980 nm. (c) The beam quality and the beam profile after filter.

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Figure 2(b) shows the variation of output spectra before filter with seed power. It can be found that the 1030-nm ASE is well suppressed, and the peak-to-peak suppression of 1030-nm ASE reaches about 43 dB at the maximum output power. Besides, there is no residual pump light observed in the output spectra, indicating that the residual pump light is fully eliminated by the CLS. The inset in Fig. 2(b) shows the local spectra near 980 nm. It can be found that the central wavelength and 3-dB bandwidth of the seed light are about 979.2 nm and 0.17 nm, respectively.

In addition, in order to verify the single-mode operation of the fiber oscillator, the beam quality of the seed oscillator after the filter was also measured based on the 4-sigma method, and the beam quality and the beam profile at the seed power of 13.24 W are given in Fig. 2(c). It can be found that the measured beam quality factor (M² factor) was about 1.08, indicating that the beam quality of seed oscillator after the filter should be single-mode.

Then, the output properties of seed light after MFA were measured in order to reveal the effect of MFA on the seed light, and the pertinent results are given in Fig. 3. Figure 3(a) gives the variation of seed power after MFA with pump power. It can be found that the maximum output power and the slope efficiency are lowered to 11.72 W and 3.2%, respectively, which is mainly caused by the insertion loss of components (e.g., filter, MFA). Figure 3(b) shows the output spectra at various seed power. It can be found that there is no 1030-nm ASE observed in the output spectra, indicating that the 1030-nm ASE has been filtered out by the filter. The inset in Fig. 3(b) shows the local spectra near 980nm, which can be found that the spectral profile of seed light around 980 nm keeps well after the MFA.

 

Fig. 3. (a) The variation of seed power with pump power. (b) The variation of output spectra with seed power, inset: the local output spectra near 980 nm. (c) The beam quality and the beam profile at 11.72-W seed power.

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Meanwhile, the measured beam quality of the seed oscillator after the MFA is given in Fig. 3(c). It can be found that the measured beam quality factor (M² factor) was about 1.1, indicating that the MFA can well maintain the beam quality of single-mode seed light.

3.2 Experimental study on the MOPA fiber laser near 980 nm

With the seed light, the experimental study on the MOPA fiber laser was carried out. The length of the active fiber in the amplifier was optimized to 0.51 m in order to suppress the 1030-nm ASE. The pertinent results are given in Fig. 4. Figure 4(a) gives the output power versus the pump power. It can be found that the output power increases linearly with the pump power, and the maximum output power of 101.5 W was achieved at a pump power of 545 W with the slope efficiency of 17.38%. Figure 4(b) gives the output spectra at various output powers. It can be found that the 1030-nm ASE is well suppressed, and the peak-to-peak suppression reached 38 dB at the maximum output power, which only takes up 0.6% of total output power (estimated by spectral integration).

 

Fig. 4. (a) The variation of output power with pump power. (b) The variation of output spectra with output power (inset: the local output spectra near 980 nm at various output powers). (c) The variation of spectral bandwidth of signal light with pump power. (d) The beam quality and the beam profile at 101.5-W output power.

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Besides the 1030-nm ASE, the in-band ASE should also be paid attention to, because it can also affect the beam quality of the fiber laser. Then, the inset of Fig. 4(c) gives the local spectra near 980nm. It can be found that there is no in-band ASE observed in the output spectra, which means that the in-band ASE can be fully suppressed with the 979.2-nm seed light. This observation is unexpected to some extent, because the experimental study in Ref. [27] shows that even with the 34-W seed power, only 17-dB suppression of in-band ASE can be achieved with the 978-nm seed light. However, in this experiment, by adjusting the seed wavelength to 979.2 nm, the in-band ASE can be suppressed so well that no obvious in-band ASE is present in the spectrum with only 11.72-W seed power. The specific origin of such huge improvement is still not so clear. One possible reason is the rapid variations of absorption and emission cross sections of Yb-ion within the band near 980 nm, which makes the 979.2-nm wavelength more dominant in the gain competition with the in-band ASE than other wavelengths. It should be noted that the gain of signal light is simultaneously determined by the emission cross section and absorption cross section [6]. The larger emission cross section will be helpful for the gain, while the larger absorption cross section will do harm to the gain. Then, compared with the shorter signal wavelength such as 976 nm [15] or 978 nm [27], although the emission cross section at 979 nm is smaller, the smaller absorption cross section can make the net gain larger, because the small absorption cross section means that the lower Yb-ion population excitation is required to provide the gain for the signal light amplification, and thus the pump threshold is lower, which is beneficial to the suppression of the in-band ASE. Thus, the better suppression of the in-band ASE can be realized. Such explanation can also be verified by the experiment observations that the in-band ASE with the longer wavelength is stronger with the 976-nm [15] or 978-nm [27] signal light, which implied that the gain at 976 or 978 nm should be lower than the gain at longer wavelength. However, when the signal wavelength is longer than 979 nm (e.g., 981 nm [28] or 982 nm [33]), although the absorption cross section at 981 nm or 982 nm is smaller, the smaller emission cross section will make the net gain lower, and thus the suppression of in-band ASE at shorter wavelength will become more difficult. The pertinent explanation can also be verified by the experiment observations that the obvious in-band ASE was present at shorter wavelength of the spectra [28,33]. Therefore, the optimal signal wavelength is around 979 nm, which is the result of the balance between the effects of absorption and emission cross sections. As a result, the in-band ASE can be suppressed much better by adjusting the signal wavelength to around 979 nm. This experiment also demonstrates that the performance of Yb-doped fiber lasers near 980 nm should be sensitive to the option of seed or signal wavelength.

Moreover, Fig. 4(c) gives the variation of spectral bandwidth of signal light with pump power. It can be found that the central wavelength and 3-dB bandwidth of the signal light are 979.2 nm and 0.16 nm, respectively, which are very close to the seed light (see the inset of Fig. 1(b)). In spite of that, it can be seen that the spectrum is obviously broadened only at the bottom (larger than 20 dB from the maximum) with the increment of pump power, otherwise, it is not obviously broadened. Besides, it can also be found that the spectral broadening at the bottom is asymmetrical which is different from the symmetrical spectral broadening caused by the four-wave mixing (FWM) [36] or self phase modulation (SPM) [37]. Considering that the laser power is only about 100 W, it is difficult to induce significant nonlinear effects. Therefore, the broadening of spectral bottom may be induced by the in-band ASE.

Then, the beam quality was also measured, and the beam quality and the beam profile at the maximum output power are shown in Fig. 4(d). The measured M² factor was 1.285, which means that the MOPA fiber laser achieves near-diffraction-limited operation. Compared with the beam quality of seed light (about M² factor being 1.1), the output beam quality of the amplifier exhibits degradation to some extent, which is mainly because the active fiber employed in the amplifier is few-mode fiber and can support several high-order modes. Then, considering the few-mode operation of in-band ASE, the sufficient suppression of in-band ASE with the 979.2-nm seed light should play an important role for achieving such good beam quality.

4. Conclusion

In conclusion, we demonstrated the first hundred-watt-level step-index YDF laser near 980nm with M² factor smaller than 1.3. This demonstration can be achieved with the help of single-mode 979.2-nm seed light. It is found that when the seed wavelength is adjusted to around 979 nm, the in-band ASE can be well suppressed with only 11.72W seed power (obviously lower than 34 W used in Ref. [27]). Then, the 101.5-W output power is achieved with the M² factor smaller than 1.3. The power up-scaling is mainly limited by the low efficiency, which is induced by the low CCAR of the active fiber. By enlarging the core diameter (i.e., enlarging the CCAR), the better suppression of the 1030-nm ASE can be achieved, and thus the efficiency can be improved. In this work, it is also revealed that the option of seed wavelength should be paid more attention for improving the performance of YDF fiber near 980 nm. Besides, as far as its application as a pump source, although the 979 nm as the pump wavelength will suffer from the smaller absorption of Yb-ion than the 976-nm wavelength, it is beneficial to reduce the quantum defect, and thus mitigate thermal effects (e. g., thermal lens (ThL), transverse modal instability (TMI)) which is the important factor limiting the power up-scaling of the fiber laser [38–40]. Besides, Ref. [41] has been revealed that TMI threshold can be elevated by using the pump wavelength longer than 976 nm. This work does not only provide an effective method to suppress in-band ASE, but also provides a cost-effective solution to fabricate the hundred-Watt-level near-diffraction-limited fiber lasers near 980 nm. The pertinent results can also provide guidance for studying other sorts of three-level lasers and amplifiers.