In this paper, we demonstrated a monolithic fiber-Bragg-grating-based (FBG-based) master oscillator power amplification configuration fiber laser with a narrow linewidth at high-power level. Several approaches were implemented to reduce the seed laser linewidth and the magnification of spectrum broadening in order to achieve a narrow output linewidth. The narrow seed laser linewidth was obtained by restricting the reflection bandwidth of the FBG. To reduce the magnification of spectrum broadening, a backward pumping scheme was employed in the amplifier stage after its capacity to suppress laser spectrum broadening was preliminarily investigated experimentally. Further, by intentionally shortening the length of the active fiber in the amplifier and sharing the backward pumping power with the oscillator, the spectrum broadening was further inhibited without sacrificing optical efficiency. A maximum output power of 2.19 kW was achieved with a 3 dB spectrum bandwidth of only 86.5 pm. The beam quality at the maximum power was measured to be M2~1.46. No sign of transverse mode instability was shown during the experiments.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Over the past decades, there have been significant advances in the development of high-power continuous-wave fiber lasers. As a result of breakthroughs in techniques involving double-clad fibers, pump sources, and beam combining, the power level of fiber lasers has exceeded tens of kilowatts and is still far from the theoretical limit [1–3]. Among these techniques, spectral beam combining (SBC) has attracted worldwide attention and kept refreshing the power record of fiber lasers [3,4]. In SBC, multiple fiber lasers with different wavelengths are combined into a single output to achieve high power. Obviously, to further increase laser power, the power output of each individual fiber laser should be increased. Moreover, to increase the number of combined laser wavelengths, the spectrum of individual fiber lasers should be narrower. Therefore, it is very meaningful to pursuit high laser power output with narrow linewidth from individual fiber laser.
Nevertheless, during the process of increasing laser power, the occurrence of nonlinear effects within fiber such as self-phase modulation (SPM), cross-phase modulation (XPM), and four-wave mixing (FWM) is inevitable. These effects lead to spectrum broadening in the laser output [5–7]. In order to suppress spectrum broadening, numerous studies have been performed on nonlinearity suppression and spectrum control. One of the effective spectrum control methods is using a phase modulation single frequency seed laser [8–10]. The single frequency output, which is usually generated by a semiconductor distributed feedback laser (DFB) laser or an ultra-short-cavity laser, is then spectrally broadened via a phase modulator. After pre-amplification, the phase-modulated signal laser enters the main amplifier stage to achieve a high power output. Experimental data and theoretical analyses have found out that during the power amplification process, there is no evident broadening in the output laser spectrum [8–11]. This method can achieve output laser power in kilowatts range with extremely narrow linewidths. However, there are limits to the phase modulation seed laser method. For example, such fiber laser usually experiences severe stimulated Brillouin scattering (SBS) because of the non-broadening spectrum characteristic . To improve SBS threshold, higher speed phase modulation or a more complicated configuration is required, which increases cost and puts strict demand on the modulator. In addition, the power scaling of such lasers is also limited by transverse mode instability (TMI) .
Apart from phase modulation seed lasers, traditional fiber Bragg grating (FBG) based oscillators may also be utilized. In contrast, it is much simpler to achieve a high output laser power with an FBG-based seed laser in a master-oscillator-power-amplification (MOPA) configuration. Because the SBS threshold is positively correlated with the laser linewidth, the introduction of an appropriate amount of spectrum broadening control allows the SBS threshold to increase synchronously with the output power of the laser . However, it is difficult to prevent the laser spectrum from being over-expanded once the laser reaches high power. Much effort has been directed toward reducing the bandwidth of the spectrum of high power FBG-based lasers. In 2015, Hao et.al presented a forward-pumped MOPA fiber laser with a 3 dB spectrum bandwidth of 80 pm at a power level of 823 W . In the same year, Xu et.al demonstrated a two-stage MOPA fiber laser with an output power of 2 kW. The 3 dB bandwidth of the laser spectrum was expanded from 0.1 nm (25 GHz) to 0.3 nm (75 GHz) . In 2016, Huang et al. developed a 2.9 kW laser based on a forward-pumped MOPA fiber laser seeded by a narrow linewidth oscillator. The final output 3 dB bandwidth of the laser spectrum at the maximum power was 0.31 nm . Irrespective of these impressive results, it is still difficult to achieve a laser linewidth of less than 0.1 nm at a power level above 2 kW.
In this report, we present a monolithic backward-pumped MOPA-configuration fiber laser. In order to achieve a narrow output laser linewidth, various measures were implemented to control both the seed laser linewidth and the magnification of spectrum broadening. A narrow seed laser linewidth was obtained by restricting the reflection bandwidth of the FBG. With respect to the magnification of spectrum broadening, backward pumping was employed in the amplifier after its suppression effect on spectrum broadening was evaluated via preliminary experiments. Then, a pumping sharing setup was built between the oscillator and the amplifier stage by intentionally shortening the length of the active fiber to suppress the spectrum broadening without sacrificing optical efficiency. In addition, the central wavelength shifting property of the non-wavelength-stabilized laser diode was also studied to prevent the over-expansion of the seed laser linewidth. The output laser power reached its peak at 2.19 kW while the 3 dB spectrum bandwidth was broadened to only 86.5 pm. To the best of our knowledge, this is the best result from an FBG-based MOPA fiber laser at this power level. In addition, the M2 factor of the output laser beam was measured to be 1.46. The evolution of the beam quality suggests that transverse mode instability did not occur in the laser output.
2. Experiment setup
Figure 1 illustrates the schematic diagram of the fiber laser. The MOPA configuration includes an oscillator stage and an amplifier stage. In the oscillator stage, the linear resonator cavity consists of a pair of fiber Bragg gratings (FBGs) with reflectivity spectra centering at 1070 nm. The reflectivity of high reflection (HR) grating and output coupling (OC) grating at 1070 nm is 99% and 10%, respectively. The active fiber used in the oscillator was a 10 m long double-clad Yb-doped fiber (YDF) with a core and inner clad diameter of 20/400 μm. The absorption coefficient of the active fiber was 1.1 dB/m @975 nm. A 975 nm non-wavelength-stabilized (NWS) laser diode (LD) injected 140 W of pumping power into the active fiber through a (1 + 1) × 1 fiber coupler. The coupler’s pump fiber had a core and inner clad diameter of 200/220 μm and the numerical aperture (NA) of the core was 0.22. The signal fiber of the fiber coupler had a core and inner clad diameter of 20/400 μm and a core/inner-clad NA of 0.06/0.46. The input end of the signal fiber was cleaved at 8 degrees to prevent the formation of parasitic laser.
The amplifier stage utilized a 25/400 μm YDF that was 14.5 meters long. The core/inner-clad NA of the YDF was 0.06/0.46, and the absorption coefficient was 1.7 dB/m @975 nm. The amplifier was backward pumped. Five 975 nm NWS LDs served as the pumping sources and their pumping power was coupled into the active fiber via a (6 + 1) × 1 fiber coupler. Each LD can provide pumping power of approximately 550 W. The core and inner clad diameter of the coupler’s signal fiber was 30/400 μm. The other parameters of the fiber coupler are the same as those of the coupler in the oscillator stage. A coated end cap was installed at the output end to reduce reflected laser. It should be noted that there was no isolator between the oscillator and the amplifier. So if there was SBS happening in the fiber laser, the generated Stokes wave will propagate backward, enter the oscillator and be reflected by the HR-FBG. The reflected Stokes wave will then be amplified by both YDF and finally be measured in the output laser spectrum. In this way, the existence of SBS could be monitored.
3. Spectrum broadening control
In general, in a MOPA configuration fiber laser, the output laser linewidth from the amplifier can be considered as the outcome of the broadening of the input seed laser’s linewidth in the amplifier. Therefore, to obtain a narrow output laser linewidth, appropriate measures should be taken to reduce both narrow input laser linewidth and the magnification of spectrum broadening in the amplifier.
The reflection bandwidth of the FBGs is the key factor in controlling the linewidth of the seed laser. However, it is technically difficult to achieve a very narrow reflection bandwidth on an HR-FBG, while it is possible to inscribe narrow reflection bandwidth grating on an OC-FBG. Therefore, we customized the FBGs with the different reflection bandwidth demands. The 3 dB bandwidth of the HR-FBG and OC-FBG were 1 nm and 30 pm, respectively. As a result, a narrow 3 dB linewidth of 36.6 pm was achieved from the oscillator.
Even with this narrow linewidth seed laser, it is still necessary to prevent over-broadening of the laser linewidth during power scaling process in the amplifier stage. As mentioned above, spectrum broadening in the amplifier is induced due to nonlinearity. Therefore, to reduce the magnification of spectrum broadening, two measures were taken to suppress the nonlinearity in the amplifier stage.
Backward pumping scheme was one of the approaches used for nonlinearity suppression. It has been determined in previous investigations that compared to forward pumping, backward pumping has a higher threshold for some nonlinear effects such as stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS) [15–18]. This advantage is attributed to the different laser power distribution along the active fiber between the two pumping schemes, as illustrated in Fig. 2. The backward pumping results in a lower average power along the active fiber, which is suggestive of a less intense interaction between the signal laser and the fiber [5,18]. Thus, nonlinear effects are often suppressed in backward pumping schemes.
In order to investigate the impact of different pumping schemes on laser spectrum broadening, a comparative experiment was set up, as illustrated in Fig. 3. The seed laser, which generated 15 W power, was connected to an amplifier via a mode-field adapter (MFA). The active fiber was a 15 m long 25/400 μm YDF with a core/inner-clad NA of 0.06/0.46 and an absorption coefficient of 1.7 dB/m @975 nm. The amplifier was bi-directionally pumped with an NWS LD from each direction. Each LD can provide a pump laser power of 600 W. Both fiber couplers share the same parameters with the coupler in the oscillator in Fig. 1. A cladding light stripper (CLS) was installed just before the output end cap to remove residual pumping laser.
With the seed laser fixed at 15 W, each LD worked separately at its maximum power and a 500 W output laser power was achieved. The laser spectrum was monitored throughout this process. Figure 4 presents the laser spectrum broadening trend in both pumping schemes. It is evident that the forward pump laser was more adversely influenced by spectrum broadening than the backward pumped laser. At 500 W, the 3 dB bandwidth of the forward pumped laser spectrum was 1.8 times that of the backward pumped laser. With respect to the 10 dB bandwidth, the ratio is 2.1 times. Therefore, it is safe to conclude that similar to other nonlinear effects, spectrum broadening can be effectively suppressed using a backward pumping scheme.
Apart from backward pumping, there is another approach to control spectrum broadening. In the amplifier stage, we intentionally shortened the length of the active fiber. As such, the YDF in the amplifier was not able to absorb all the pumping power in the backward direction. Therefore, the residual pump power entered the oscillator stage. This indicates that the total pumping power in the backward direction was shared between the oscillator and the amplifier, as illustrated in Fig. 5. On one hand, a shorter active fiber length can effectively suppress nonlinear effects and the correspondingly laser spectrum broadening. On the other hand, the pumping power shared between the oscillator and the amplifier prevented the optical efficiency from decreasing too much because of the short active fiber length.
Apart from the two aforementioned approaches, there was another factor that could potentially influence the experimental results that needed to be considered. As mentioned before, the pump source employed in the fiber laser was NWS LD, which is known for its central wavelength shifting feature. It was demonstrated in our previous research that the central wavelength of the NWS LD spectrum would shift from 968 nm at low current to 975 nm at the maximum current . Since the absorption cross-section of Yb3+ ion varies with wavelength, this shift would also result in a difference in the absorption coefficient of YDF, as shown in Fig. 6(a). As a result, the pump power entering the oscillator and the signal seed laser originating from the oscillator varies depending on the different LD working currents, as shown in Fig. 6(b). If the NWS LDs operates at 6 A or 7 A, the high seed laser power would result in a broad seed laser spectrum and consequently a broad output laser spectrum. To achieve a narrow output laser spectrum, the LDs in the backward pumping direction should all finally operate at or near the maximum current where the seed laser power is low.
4. Experimental result
With a seed laser power of 80 W, the fiber laser finally reached the maximum output power of 2190 W while a total pump laser power of 2694 W was injected into the gain fiber, corresponding to an amplifier extraction efficiency of 78.3%. The optical efficiency was low until the laser power reached approximately 900 W, which was due to the central wavelength shifting feature of the NWS LD mentioned above. The low-efficiency region in Fig. 7 corresponds to the time when the NWS LD in the backward direction operated at low currents. The central wavelength of the pumping laser shifted away from 975 nm, resulting in a low absorption coefficient of the YDF for the pumping laser and a corresponding low optical efficiency.
The laser spectrum evolution with power scaling was recorded using a Yokogawa AQ6370D Optical Spectrum Analyzer, and the results are depicted in Fig. 8. The curve suggests that a linear relationship exists between the 3 dB bandwidth and the laser power, which agrees with existing theory and data. The variation of the central wavelength is most likely due to the changing laser power and fiber temperature that alters the period of both FBGs. When the output laser power reached 2190 W, the 3 dB and 10 dB bandwidth was broadened to 86.5 pm and 288 pm, respectively, as shown in Fig. 9. In the output laser spectrum at the maximum power, there was no evident peak at the Brillouin shift of 10~20 GHz (38~76 pm) where the SBS Stokes wave typically located . This suggested no SBS occurred in the fiber laser.
Nevertheless, the data points at 904 W and 1763 W of the bandwidth curve in Fig. 8 seems to deviate from the linear relationship. This phenomenon is also most likely due to the central wavelength shifting feature of the NWS LD. In the experiment, the five NWS LDs were divided into three groups and launched in order. The upper part of Fig. 8 illustrates the LD launching order. Firstly, two LDs were launched from 1 A to 10 A. Then another two LDs followed the same routine. Finally, the last LD was launched at 9 A. Thus, those two points correspond to the situations where the two launched LDs operate at the maximum current. Hence, the central wavelength of the pumping laser was shifted to 975 nm where the peak of the Yb3+ ion absorption cross-section is located . Therefore, more pumping power was absorbed in the amplifier stage than in the oscillator stage. According to the simulation results in Fig. 6(b), the seed laser power decreased and led to a spectrum narrowing. This effect can also explain the shift of the central wavelength to a shorter wavelength at these two points in Fig. 8.
To investigate transverse mode instability (TMI) in the fiber laser, the beam quality factor M2 was monitored at different power levels, as shown in Fig. 10(a). The trend did not show any sign of drastic degeneration of the beam quality which is the typical indication of the occurrence of TMI [21,22]. This implies that the laser power was still less than the TMI threshold, indicating the potential for a further power improvement. At the maximum laser power of 2190 W, the M2 factor was measured to be 1.46, as shown in Fig. 10(b).
In this report, an FBG-based MOPA configuration narrow spectrum high power fiber laser was constructed. In order to achieve a narrow output laser linewidth, several measures were taken to control both the input seed laser linewidth and the magnification of the spectrum broadening. To reduce the input linewidth, the reflection bandwidth of the FBGs was constrained to obtain a narrow seed laser linewidth. With regard to the magnification of spectrum broadening, preliminary experiments were conducted to study the impact of the pumping schemes on spectrum broadening. As a result, the backward pumping scheme was verified to have an outstanding advantage for laser spectrum broadening suppression and this scheme was employed in the amplifier stage. In addition, the active fiber length was intentionally reduced to cause insufficient pump absorption. In this way, the pump power was shared between the oscillator and amplifier, so that the nonlinearity induced spectrum broadening can be suppressed without sacrificing optical efficiency. Further, the central wavelength shifting feature of the NWS LD was investigated to prevent over-expansion of the input seed laser linewidth. Finally, a 2.19 kW output power was achieved from the fiber laser. As a result of the spectrum broadening suppression methods that were utilized, the 3 dB bandwidth of the spectrum at the maximum power was broadened to only 86.5 pm. To the best of our knowledge, this is the best result from an FBG-based MOPA configuration fiber laser at this power level. The beam quality remained at approximately M2~1.46 throughout the process of increasing the power, suggesting there was no TMI occurrence in the laser.
National Natural Science Foundation of China (61675114, 61875103); Tsinghua University Initiative Scientific Research Program (20151080709).
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