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Abstract
In this work, experimental investigations were presented on the variation of spectral broadening and optical efficiency in a kW-level narrow linewidth MOPA fiber laser under different pumping configurations. The results revealed that the backward-pumping scheme could reduce the spectral broadening and enhance the optical efficiency compared with the forward-pumping scheme. For a detailed study, the bidirectional pumping setup was employed. It could be seen that by removing the pump diodes from the forward direction and adding them to the backward direction one by one, the spectral bandwidth and the optical efficiency gradually decreased and increased, respectively. By coiling the gain fiber on a plate with a bending diameter of 9-13 cm, a multi-peak and unstable spectrum was observed. When this fiber was coiled on a cylinder with a fixed diameter of 10.5 cm, the spectral instability was removed and a single-peak spectrum was observed. For forward pumping only, the spectral bandwidth, the optical efficiency, and the maximum output power were measured to be 0.121 nm, 78.92%, and 822 W, respectively, while for backward pumping only, these values were measured to be 0.084 nm, 82.65%, and 860 W.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
High power fiber lasers with narrow linewidth and excellent beam quality have attracted great interest in various applications such as nonlinear frequency conversion [1,2], gravitational wave detection [3,4], and spectral beam combining (SBC) [5–7]. Among these applications, the SBC has drawn more attention for power scaling. Clearly, for the best efficiency of the SBC, the output power of each fiber laser with a specific wavelength must be increased and its spectral bandwidth must be reduced as much as possible. The power scaling of such narrow linewidth fiber lasers is challenging due to nonlinear effects such as four-wave mixing (FWM), self-phase modulation (SPM), and cross-phase modulation (XPM) [8]. The broadening of the laser spectrum due to these phenomena is unavoidable and must be controlled by some approaches [8,9].
Power amplification of different types of low power seed source such as phase-modulated single-frequency seed laser [10–12] and fiber Bragg grating (FBG)-based narrow linewidth oscillator [13,14] is a main approach in designing narrow linewidth high power fiber lasers. However, employing complex electronic equipment and multi-stage amplification of low power seed makes the phase-modulated method costly. A common approach is to use a FBG-based seed laser in a master oscillator power amplifier (MOPA) configuration. This configuration is able to maintain the spectral purity of the seed [8].
In narrow linewidth high power fiber lasers, stimulated Brillouin scattering (SBS) has become one of the main barriers. The laser spectral bandwidth, the Brillouin gain bandwidth, and the Brillouin gain coefficient affect the SBS threshold [15,16]. Mode size scaling (by lowering the power density in the fiber core) and fiber shortening are two effective approaches to overcome the SBS effect [17–19]. To mitigate the SBS effect, other techniques such as thermal gradients [20,21], phase modulation of the single-frequency seed [22,23], and fiber stress [24] are also used.
Many studies have been published on FBG-based high power narrow linewidth MOPA fiber lasers using different pumping schemes. In 2015, a two-stage forward-pumped fiber laser with a $3$-dB spectral bandwidth of $0.3$ nm and power of $2$ kW was presented by Xu et al. [16]. In the next year a forward-pumped fiber laser with a $3$-dB spectral bandwidth of $0.31$ nm and power of $2.9$ kW was developed by Huang et al. [25]. In 2021, a backward-pumped fiber laser with a $3$-dB spectral bandwidth of $0.103$ nm and power of $3.01$ kW power was proposed by Yan et al. [26]. Another backward-pumped fiber laser with a $3$-dB spectral bandwidth of $0.086$ nm and power of $2.19$ kW power was introduced by Xiao et al. [8] in 2019. In 2020, a bidirectional-pumped fiber laser with a $3$-dB spectral bandwidth of $0.2$ nm and $3$ kW power was developed by Wang et al. [27]. In the same year a bidirectional-pumped fiber laser with a $3$-dB spectral bandwidth of $0.24$ nm and $2.4$ kW power was introduced by wang et al. [28]. According to previous studies, the backward-pumping method is one of the approaches used for nonlinearity suppression. In addition, compared with the forward-pumping method, it has a higher optical efficiency and a higher threshold for SBS and stimulated Raman scattering (SRS) [8,29–31].
In the current study, laser spectrum was stabilized by a proper coiling of the gain fiber. The spectral bandwidth and the optical efficiency of the narrow linewidth FBG-based MOPA configuration fiber laser were compared in different pumping schemes. In addition, the pump power was assumed to be the same in all experiments. At first, by coiling the amplifier gain fiber in the planar spiral shape, a multi-peak and unstable spectrum was observed at the output power of greater than $100$ W. Hence, the fiber was coiled in a cylindrical spiral shape to extract properly the higher order modes (HOMs) as the origin of spectral instability. Then, the forward and backward pumping schemes were compared in separate setups. In order to have the same conditions for comparison, bidirectional pumping was used and all pumping arrangements were tested. For forward pumping only, the spectral bandwidth and maximum output power were measured to be $0.121$ nm and $822$ W, respectively, whereas, for backward pumping only, these values were measured to be $0.084$ nm and $860$ W, respectively. The amplifiers operated quite stably without any sign of transverse mode instability (TMI) and SBS phenomena.
2. Experimental setup
2.1 Forward pumping
Figure 1 illustrates the schematic diagram of the proposed forward-pumped narrow linewidth MOPA configuration fiber laser. The main amplifier was seeded by a linear cavity oscillator based on a pair of homemade FBGs centered at the wavelength of $1079$ nm. The FBGs were inscribed on an $8/130$ $\mu m$ (core/inner-clad diameter) photosensitive fiber using a phase-mask scanned by the fourth harmonic of a $Nd$-$YVO_{4}$ laser beam [32]. Using homemade FBGs provides the possibility of fabricating output couplers (OCs) with different $3$-dB reflection bandwidths of $0.05$, $0.1$, $0.15$ and $0.2$ nm. These FBGs were tested to obtain the narrowest spectral linewidth of the seed at the desired output power without observing the SBS signal in the amplifier output spectrum.The $3$-dB bandwidth of the seed output spectrum was measured to be $0.020$, $0.027$, and $0.031$ nm, respectively. But the SBS signal was observed at the amplifier output spectrum at the power of 180, 420, and 650 W, respectively. The difference between the main signal and the SBS signal was measured to be 13, 21, and 25 dB, respectively. On the other hand, In the case of OC with $3$-dB reflection bandwidth of $0.2$ nm, no SBS signal was viewed at the amplifier output spectrum up to $30$ dB. Therefore the $3$-dB reflection bandwidth of the high reflector (HR) and OC was selected as $2$ nm and $0.2$ nm and their reflectivity was $99$% and $10$%, respectively. The central wavelengths of the HR and OC were measured to be $1078.74$ nm and $1078.55$ nm, respectively. A $25$ W $975$ nm non-wavelength stabilized (NWS) laser diode (LD) with a central wavelength tolerance of $\pm$ 3 nm and a (2+1)${\times }$1 combiner were used for pumping. However, as known the pump power of the NWS diode is absorbed over a relatively large length of the gain fiber so the seed spectrum widens to some extent and prevents to occurrence of SBS effect on the amplifier in the process of power scaling.The pump port of the combiner had a core/inner-clad diameter of $105/125$ $\mu m$ and a core numerical aperture (NA) of $0.22$. The signal input and output fibers of the combiner were $6.5/125$ $\mu m$ and the core/inner-clad NAs of the signal input and output fibers were $0.12/0.46$. The oscillator consisted of FBGs and a $4$-m-long $10/125$ $\mu m$ ytterbium-doped fiber (YDF) with the absorption coefficient of $5.7$ dB/m at $976$ nm. The gain fiber was coiled in a planar spiral shape and placed on an aluminum plate. The minimum bending diameter (BD) was $\sim 6$ cm and the maximum bending diameter was $\sim 9$ cm. The measured seed laser spectrum was slightly noisy. Therefore, it was not suitable for noiseless amplification. As known, the spectral purity of the amplifier is firmly dependent on the spectral purity of the seed. So it was essential to increase the signal-to-noise ratio in the seed output spectrum. As illustrated in Fig. 2(a), the seed spectrum was slightly noisy, and the signal-to-noise ratio, $3$-dB reflection bandwidth and output power were measured to be about $22$ dB, $0.064$ nm and $15$ W respectively. The selection of FBGs and gain fiber in the seed was based on existing fibers and some considerations. The single-mode $8/130$ $\mu m$ photosensitive fiber with a core NA of $0.095$ was used to inscribe FBGs, and the short length ($4$ m) $10/125$ $\mu m$ YDF fiber with a core NA of $0.12$ and high absorption coefficient was selected as a gain fiber to suppress SBS. However, the core NA of this fiber was $0.12$, so the fiber was not completely single-mode, and the $LP_{11}$ mode also can be guided and amplified in it. At the splice point of HR to the gain fiber, the mismatched fusion led to the formation of $LP_{11}$ mode in the gain fiber. At the splice point of the gain fiber to the OC, also a mismatched fusion led to the loss of $LP_{11}$ mode, but a part of $LP_{11}$ mode is back-reflected to the gain fiber. This loss decreased the finesse of the laser resonator and made the spectrum noisy. The noises were decreased by tight coiling of the gain fiber on the cylindrical spiral shape with a bending diameter of $6.5$ cm, and the signal-to-noise ratio was increased up to $28.5$ dB. The $3$-dB spectrum bandwidth was decreased to $0.035$ nm and output power was increased to $17$ W, as demonstrated in Fig. 2(b). Therefore, it is better to coil the gain fiber in a cylindrical spiral shape with an appropriate diameter for the small core size fiber to suppress $LP_{11}$ mode. A homemade CLS was employed to leak out the residual signal and the pump light in the cladding.
Fig. 2. The output seed spectrum after coiling the gain fiber in (a) the planar spiral shape, and (b) the cylindrical spiral shape.
The amplifier stage used six wavelength-stabilized (WS) LDs, a (6+1)${\times }$1 tapered fused bundle (TFB) combiner, a $6.5$-m-long $25/400$ $\mu m$ YDF with an absorption coefficient of $1.8$ dB/m at $976$ nm, and a CLS. Each LD provided the output power of $170$ W at $976$ nm with a central wavelength tolerance of $\pm$1 nm, spectral bandwidth of $0.7$ nm, pigtail fiber of $200/230$ $\mu m$, and core NA of $0.22$. The pump fiber of the combiner had pump a core/inner-clad diameter of $200/220$ $\mu m$ and a core NA of $0.22$. The signal input and output fibers of the combiner were $6/125$ $\mu m$ and $20/400$ $\mu m$ respectively. The core/inner-clad NA of the signal input and output fibers were $0.12/0.46$ and $0.06/0.46$, respectively. The output fiber of the combiner was spliced to the $25/400$ $\mu m$ $0.065/0.46$ NA YDF and the signal input fiber was connected to the seed. The gain fiber was coiled in a planar spiral shape and placed on a heat sink. Laser signal was injected from the coiled YDF with a diameter of $9$ cm, and the largest diameter of the coiled YDF was $13$ cm. Although the seed output had a stable, narrow linewidth, and noiseless spectrum, a multi-peak and unstable spectrum was observed at the amplifier output for a laser power of greater than $100$ W as shown in Fig. 3(a). It was speculated that this spectral instability was due to the amplification of very low power spectral sidebands by HOMs. Hence, the gain fiber was coiled in a cylindrical spiral shape with a fixed bending diameter of $10.5$ cm. By this coiling, a stable single-peak spectrum was observed at the laser output (Fig. 3(b)). Therefore, this type of coiling was chosen for all setups.
Fig. 3. The output laser spectrum at the power of $100$ W after coiling the gain fiber in (a) the planar spiral shape, and (b) the cylindrical spiral shape.
A $6.5$-m $25/400$ $\mu m$ YDF was selected based on the cut-back method to suppress the SBS signal. Using a gain fiber with this short length causes a rather high amount of pump power to remain unabsorbed in the cladding. Therefore, a homemade high power CLS was employed to attenuate the cladding light more than $20$ dB [33,34]. To monitor the onset of SBS, the backward signal power was measured continuously. A probable sudden increase of the backward power shows the initiation of SBS. If SBS occurs in the amplifier, the generated signal will propagate toward the seed and will be reflected by the HR. Hence, the SBS peak will be amplified by the seed and amplifier gain media and will be seen in the laser output spectrum. Therefore, the spectrum was measured from the end of the amplifier. Figure 4 shows the output spectra of the seed and amplifier at different power levels. As can be seen, by increasing the output power, the spectra become wider and there is no SBS or other signals. The amplifier reached the maximum output power of $830$ W under the pump power of $1020$ W. The optical efficiency of the amplifier was $79.71\%$ and the $3$-dB linewidth of the amplifier at maximum power was measured to be $0.082$ nm. Figure 5 (a) illustrates the spectral broadening of the amplifier output at the $3$ dB and $10$ dB in different power levels. As shown, the slope of spectral broadening is greater at $10$ dB than at $3$ dB. It seems this is due to nonlinear effects and the amplification of spectral sidebands. Figure 5(b) depicts the output power (PM1) and the backward power (PM2) versus the pump power. No sudden increase of the backward power is observed in this figure. Accordingly, there is no onset of SBS even at the highest output power.
Fig. 4. The output spectra of the seed and amplifier in the forward-pumping scheme at different power levels. The inset shows the measured laser spectrum in broad range.
Fig. 5. (a) The spectral broadening of the amplifier output in the forward-pumping scheme at $3$ dB and $10$ dB in different power levels; (b) The output and backward powers versus the pump power levels in the forward-pumping scheme.
Tight coiling of the gain fiber around the aluminum cylinder increased the loss of HOMs and TMI threshold [11]. Figure 4(b) shows that the output power increases linearly (has no roll-over) versus the pump power levels, and the optical efficiency does not drop at the high power levels. Furthermore, the CLS temperature (which its unusual increase due to leak of HOMs, mainly $LP_{11}$ is a typical sign of the TMI) did not increase at high power levels. Also, the beam quality factor ($M^{2}$) was monitored at different power levels to investigate TMI in this pumping configuration. To measure $M^{2}$, a $3$-m-long QBH with a delivery fiber of $30/400$ $\mu$m was spliced to the output end of the amplifier. This fiber was used with a core diameter of $30$ $\mu$m increase the cross-section area of the core and hence to reduce the power density. This prevented nonlinear effects such as SBS. However, using this fiber decreased the beam quality to some extent. As shown in Fig. 6(a), the trend of the $M^{2}$ factor did not depict any sign of severe deterioration of the beam quality, which is the usual indication of TMI happening [8]. These evidences show no TMI in the forward pumping setup. At the maximum laser power of $830$ W, $M_{x}^{2}$ and $M_{y}^{2}$ were measured to be 2.11 and 2.05, respectively, as shown in Fig. 6(b).
Fig. 6. The beam quality factors ($M_{x}^{2}$ and $M_{y}^{2}$) of the output laser at (a) different power levels and (b) $830 W$.
2.2 Backward pumping
In the next step, the forward combiner was removed from the setup of Fig. 1 and the backward combiner was spliced to the end of the gain fiber. The pump fiber of the backward combiner had a core/inner-clad diameter of $200/220$ $\mu m$ and a core NA of $0.22$. The signal input and output fibers of the combiner were $30/400$ $\mu m$ and $30/250$ $\mu m$, respectively. The core/inner-clad NA of the signal input and output fibers was $0.065/0.46$. The same LDs which were used in the forward-pumping scheme were also employed for the backward-pumping scheme. The seed laser was connected to the amplifier via a mode field adapter (MFA) whose input and output fibers were $6/125$ $\mu m$ and $20/400$ $\mu m$, respectively. Figure 7 shows the schematic diagram of the backward-pumped narrow linewidth fiber amplifier.
The seed output power is still $17$ W and the amplifier output was measured to be $860$ W. The $3$-dB spectral bandwidth and the optical efficiency of the amplifier at the maximum output power were measured to be $0.080$ nm and $82.65\%$, respectively. According to the results, the optical efficiency was higher in the backward-pumping scheme than in the forward-pumping scheme. The laser power distribution along the gain fiber differs for the forward and backward pumping schemes. For the forward-pumping scheme, the laser power density grows to nearly maximum at the primary parts of the gain fiber. For the backward pumping scheme, the laser power density reaches the maximum at the output end of the amplifier. So the effective length which all the photons experienced in the forward pumping scheme is longer than that in the backward pumping scheme. As a result, the photons experienced more loss or lower efficiency in the forward pumping scheme [8]. The effective length of the gain fiber is greater in forward pumping than in backward pumping. This makes spectral broadening larger in forward pumping than in backward pumping [25]. However, due to $4$ m length of the signal fiber port in the backward combiner, the effective length in both pumping methods was almost equal. Therefore, the spectral bandwidths of these schemes were approximately equal. Figure 8 depicts the output spectra of the seed and amplifier at different power levels. Figure 9(a) illustrates the spectral broadening of the amplifier output at the $3$ dB and $10$ dB in different power levels. Figure 9(b) shows the output power (PM1) and backward power (PM2) versus the pump power levels. No sudden increase of backward power is seen. This shows that SBS does not occur in the amplifier even at the maximum output power.
Fig. 8. The output spectra of the seed and amplifier in the backward-pumping scheme at different power levels. The inset shows the measured laser spectrum in broad range.
Fig. 9. (a) The spectral broadening of the amplifier output in the backward-pumping scheme at $3$ dB and $10$ dB in different power levels; (b) The output and the backward power in the backward-pumping scheme versus the pump power levels.
Like the previous section, the gain fiber was still coiled around the aluminum cylinder with a fixed diameter of 10.5 cm to extract HOMs and enhance the TMI threshold. Figure 9(b) shows that the output power increases linearly versus the pump power, and there is no drop in optical efficiency. Furthermore, the CLS temperature was almost constant at high power levels. The $M^{2}$ parameter, illustrated in Fig. 10(a), shows no serious degeneration of beam quality. These proofs demonstrate no TMI in the backward pumping setup. At the maximum laser power of $860$ W, $M_{x}^{2}$ and $M_{y}^{2}$ were measured to be $2.14$ and $2.07$, respectively, as shown in Fig. 10(b).
Fig. 10. The beam quality factors ($M_{x}^{2}$ and $M_{y}^{2}$) of the output laser at (a) different power levels and (b) $860 W$.
2.3 Bidirectional pumping
To have the same conditions, the bidirectional pumping was employed. The combiners which were used in the previous setups were also utilized in the bidirectional-pumping setup. Figure 11 shows the schematic diagram of the bidirectional-pumped narrow linewidth fiber amplifier. At first, all LDs were used for pumping in the forward direction and no LD was used in the backward direction (shown as state $(6, 0)$). Then, the LDs were removed from the forward combiner one by one and added to the backward combiner. These configurations corresponded to states $(5, 1)$, $(4, 2)$, $(3, 3)$, $(2, 4)$, $(1, 5)$, and $(0, 6)$. The first and second numbers in the parentheses indicate the number of diodes used for pumping in the forward and backward directions, respectively. Figure 12 shows the output spectra of the seed and amplifier at the maximum output power in states $(6, 0)$, $(3, 3)$, and $(0, 6)$. The maximum achieved output power for forward pumping only and backward pumping only was $822$ W and $860$ W, respectively.
Fig. 12. The output spectra of the seed and amplifier in the bidirectional-pumping scheme in states $(6,0)$, $(3,3)$, and $(0,6)$ at the maximum output power. The inset shows the measured laser spectrum in broad range.
Fig. 13(a) and Fig. 13(b) show the spectral broadening of the amplifier output at the $3$ dB and $10$ dB, respectively, in the states $(6, 0)$, $(3, 3)$, and $(0, 6)$ at different power levels. Obviously, by removing the pump LDs from the forward direction and adding them to the backward direction, the spectral bandwidth gradually decreased. Figure 14 shows the $3$-dB spectral bandwidth and the optical efficiency of the amplifier in different states at the maximum output power. The optical efficiency was increased by using more LDs in the backward direction. At the maximum output power in backward pumping only, the $3$-dB spectral bandwidth which was measured to be $0.084$ nm was $\sim 1.5$ times narrower than that of forward pumping only which was measured to be $0.121$ nm. These results may be due to different laser power distributions along the gain fiber which imposes more effective length in the forward-pumping scheme than the backward-pumping scheme. In other words, there is less average power along the gain fiber in the backward pumping configuration and hence less intense interaction between the laser signal and the gain fiber [8].
Fig. 13. The spectral broadening of the amplifier output at (a) $3$ dB, and (b) $10$ dB in different states at different power levels.
Similar to the previous sections, as depicted in Fig. 15(a), again, the trend of the $M^{2}$ factor in state $(0, 6)$ did not show any sign of drastic deterioration of the beam quality. This indicates that the laser power was still lower than the TMI threshold. At the maximum laser power of $860$ W, $M_{x}^{2}$ and $M_{y}^{2}$ were measured to be $2.13$ and $2.08$, respectively, as shown in Fig. 15(b).
Fig. 15. The beam quality factors ($M_{x}^{2}$ and $M_{y}^{2}$) of the output laser at (a) different power levels and (b) $860 W$.
3. Conclusion
In conclusion, an FBG-based MOPA configuration narrow linewidth fiber amplifier with different pumping schemes was constructed and its spectral bandwidths were compared. With an output power of $17$ W, the seed source was fabricated using homemade FBGs which had different reflection bandwidths. The desired narrow linewidth seed without a SBS peak at amplifier output spectrum was selected. It was found that the origin of the observed multi-peak and unstable spectrum in the amplifier output was due to the HOMs. To prevent this instability, the gain fiber was coiled on an aluminum cylinder with a fixed bending diameter of $10.5$ cm. Therefore, a stable single-peak spectrum was observed at the laser output. Because spectral broadening is affected by pumping schemes, different pumping arrangements from the forward and backward directions were examined. The results showed that by removing the LDs from the forward direction and adding them to the backward direction, the spectral bandwidth and the optical efficiency gradually decreased and increased, respectively, for a certain value of pump power. In backward pumping only, the $3$-dB linewidth which was measured to be $0.084$ nm was $\sim 1.5$ times narrower than that of forward pumping only which was measured to be $0.121$ nm. The maximum output powers for forward pumping only and backward pumping only were measured to be $822$ W and $860$ W, respectively. There was no power roll-over, showing no sign of the onset of TMI. The backward power did not have a sudden increase, showing no sign of SBS even at the maximum output power.
Disclosures
The authors declare no conflicts of interest.
Data availability
The data supporting this study’s findings are available from the corresponding author on request.
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