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Abstract
In this manuscript, a narrow linewidth fiber amplifier based on confined-doped fiber is established, and the power scaling and beam quality maintaining capabilities of this amplifier are investigated. Benefitted from the large mode area of the confined-doped fiber and precisely controlling the Yb-doped region in the fiber core, the stimulated Brillouin scattering (SBS) and transverse mode instability (TMI) effects are effectively balanced. As a result, a 1007 W signal laser with just 1.28 GHz linewidth is obtained by combining the advantages of confined-doped fiber, near-rectangular spectral injection, and 915 nm pump manner. As far as we know, this result is the first beyond kilowatt-level demonstration of all-fiber lasers with GHz-level linewidth, which could provide a well reference for simultaneously controlling spectral linewidth, suppressing the SBS and TMI effects in high-power, narrow-linewidth fiber lasers.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
The salient properties of high-power narrow-linewidth fiber lasers on linewidth, beam quality and stability makes it a desired light source for numerous applications, such as coherent/spectral beam combining [1–5], coherent lidar [6,7] and nonlinear frequency conversion [8]. However, the power scaling and beam quality maintaining of the narrow-linewidth fiber lasers are primarily limited by the nonlinear effects and the transverse mode instability (TMI) effect [9–12]. In cooperation with the optimizations in system design, the development of high brightness pump source, and the fiber fabrication technique, narrow-linewidth fiber lasers have flourished in recent years. Till now, narrow linewidth fiber lasers with linewidths of tens of GHz level have made significant progress in brightness enhancement both in linear-polarized and non-polarized states [13–21].
However, for near-single-mode fiber lasers with linewidths of ≤10 GHz, especially for the ones with ∼GHz level linewidth, power scaling is still challenging. The limitation is mainly attributed to the inner contradictions of the system design for providing comprehensive suppression to the SBS and TMI effects in the conventional active fiber assisted amplifiers [22–24]. Currently, the implementation and optimization of the high-power narrow-linewidth fiber laser systems involves from the generation of the injected seed to the amplification systems. Firstly, for the generation of injected seed, single frequency seed in collaboration with the phase modulation technique is a prevailing route. And a variety of modulated signals have also been developed, such as the white noise source (WNS) [24], pseudo random bit sequence (PRBS) [25–28], sinusoidal signal (Sine) [24,29], multi-phase coded signal (MPCS) [30], dual modulation with sinusoid and noise (DMSN) [21]. As for the construction of the amplifiers, series of effective methods for suppressing the SBS and/or TMI effects are proposed, like employing the laser gain competition technology [29–31], applying a temperature gradient [32], using active fibers with special core/cladding diameter [33] or properly coiling the active fiber [24]. Based on the optimization above-mentioned, plenty results have been made which are shown in Table 1. In 2008, a kilowatt-level fiber amplifier with full width at half maximum (FWHM) linewidth of 8 GHz was achieved by using optimized fiber with core/cladding meter of 25/440 µm [33]. Based on the pseudo-random bit sequence (PRBS) phase modulation format, a 1 kW output laser with linewidth of 3.5 GHz is reported in 2016 [25]. Further incorporating thermal gradient or laser gain competition technology, the linewidth could be compressed to 2-3 GHz while maintaining the output power at kilowatt-level [31,32]. In 2021, W. Lai et. al. utilizing multi-phase coded signal (MPCS) modulation realized a 737 W fiber amplifier with a linewidth of 4.6 GHz and further importing the laser gain competition technology, the output power was improved to 1023 W [30]. More recently, our group has reported a 694 W all-fiber laser with linewidth of 0.79 GHz by combining the advantages of Sine modulation and tapered active fiber [24].
Table 1. Recent progress of all-fiber high-power narrow linewidth fiber lasers
Benefited from the external assisted strategies, the linewidth of kilowatt fiber lasers has been compressed to ∼2 GHz. Nevertheless, it is necessary to avoid using assisted strategies in engineering application to guarantee the robustness and reliability of the whole system. In addition, high-power lasers with linewidth of ∼1 GHz are demanded to ensure the temporal coherence and combining efficiency of the CBC system in long distance propagation [35]. In case of further narrowing the spectral linewidth in the no external strategy assisted kilowatt fiber laser systems, the simultaneous suppression of the SBS and TMI effects will be more difficult. Till now, different special designed active fibers have been tentatively employed for the comprehensive suppression on the SBS and TMI effects, such as tapered Yb-doped fiber, Chirally-Coupled-Core fiber, photonic crystal fiber, all-solid photonic bandgap fiber, and confined-doped fiber [36–40]. As one of the special designed fibers, confined-doped fiber has well potential for the comprehensive suppression of SBS and TMI effects, due to its large mode area and mode selection character [41–43]. Besides, the confined-doped fiber also has some unique advantages comparing with other special designed fibers, like convenient for constructing an all-fiber structure and compatible with multiple pumping schemes et. al. [34,44]. And based on the confined-doped fiber, a 1099 W TMI-limited all-fiber laser with 10 GHz linewidth has been achieved in 2021 [34]. Further optimization on the fabrication parameters of confined-doped fibers, like absorption coefficient, core/cladding diameter, doping ratio and refractive index distribution, is promising to realize a kilowatt all-fiber laser with narrower spectral linewidth.
In this paper, we demonstrate the power scaling of a fiber laser with 1.28 GHz linewidth by comprehensively suppressing the SBS and TMI effects. Benefited from its large mode area and mode selection feature, the confined-doped fiber is employed to mitigate the SBS effect while maintaining good beam quality. Besides, a near-rectangular inject spectrum is applied to further suppress the SBS effect while pumping wavelength of 915 nm is selected to whittle the heat load of the active fiber and increase the gain saturation effect for further suppressing the TMI effect. Overall, an all-fiber laser with 1007 W output power and near-diffraction-limited beam quality is obtained.
2. Fiber design
As for the active fibers applied in the fiber laser systems with GHz level linewidth, the characters like relatively high absorption coefficient, large mode area and single-mode operation are simultaneously required. With comprehensive consideration of the characters mentioned above, the confined-doped active fiber used in our experiment is fabricated accordingly and the detailed information of this fiber is shared in Fig. 1.
Fig. 1. (a) Cross-section of the confine-doped fiber; (b) Measured refractive index profile along the diameter.
The cross-section of this confined-doped fiber is demonstrated in Fig. 1(a). As shown in the picture, the core diameter of the confined-doped fiber is fabricated as large as 42 µm while the inner-cladding diameter is fabricated into an octagonal shape with a diameter (side-to-side) of 250 µm. Figure 1(b) illustrates the refractive index profile along the diameter which could be divided into three parts. From the outermost layer to the center, the refractive index increases in a gradient, representing to the inner-cladding, undoped core and Yb-doped core, respectively. The diameter of the Yb-doped region is controlled at a diameter of 30 µm, corresponding to a doping ratio (defined as the diameter of the doped zone to the diameter of the entire core) of 71.5%, which is benefit to provide a preferential gain for the fundamental mode (resulting in the suppression of the TMI effect). Moreover, the absorption coefficient of this active fiber is measured to be 3.25 dB/m while pumped by the 915 nm LDs and the numerical aperture (NA) is measured to be 0.084. According to the refractive index profile shown in Fig. 1(b), the effective mode area is calculated to be 519.5 µm2.
3. Experimental setup
The experimental layout of the high-power narrow-linewidth fiber amplifier is shown schematically in Fig. 2, in which a classical master oscillator power amplifier (MOPA) configuration with forward pumping scheme is applied. The single-frequency source (SFS) is a linearly polarized fiber laser with a central wavelength of 1064.0 nm [45]. The output power and the 3 dB linewidth of this seed are measured to be ∼53 mW and ∼20 kHz, respectively. A LiNbO3 electro-optical modulator (EOM) with a half-wave voltage of 2.0 V and a bandwidth of 150 MHz is used after the seed to provide external modulation. The EOM is driven by a sine signal with a driven frequency and voltage of 50 MHz and 10 V. Then the modulated seed is injected into the commercial cascaded pre-amplifiers to further amplified to ∼10 W. After the Pre-amplifiers, a high-power circulator is inserted to export the backward power from the main amplifier for power and backward spectrum measurements, in which a power meter with the maximum operating power of 3 W and an optical spectrum analyzer (OSA) with 0.02 nm resolution are applied. A high-power band-pass filter (BPF) with a bandwidth of 1064 ± 1 nm is employed before the main amplifier to remove the sideband noise and suppress the amplified spontaneous emission (ASE) in the main amplifier.
The main amplifier is constructed in forward pumping scheme and six 915 nm laser diodes (LDs) are combined to pump the confined-doped active fiber (above-mentioned) via a (6 + 1) × 1 pump and signal combiner. In this combiner, the core/inner cladding diameter of the signal input port is 10/130 µm while that of the output port is 42/250 µm. The pump delivery fibers of the LDs and the combiner are all 105/125 µm passive fibers with numerical aperture (NA) of 0.22. A piece of 3.5-meter-long confined-doped YDF is applied in the main amplifier to provide active gain. Especially, in the process of constructing the main amplifier, the fused points are carefully processed to avoid the excitation of high order modes while the active fiber is properly coiled on a water-cooled plate with a diameter of 15 cm to balance the energy efficiency as well as the compactness of the system. Finally, the amplified laser is output through a quartz block holder (QBH). The collimated output laser is then transmitted to a dichroic mirror to remove the residual pump. After the dichroic mirror, 0.1% of the signal laser is stripped out for the measurements like spectrum, linewidth, temporal trace as well as the beam quality while the major signal power is transferred to a power meter for power recording.
4. Experimental results
In the experiment, we firstly measured the linewidth, spectrum, and beam quality of the seed laser at the full power of the pre-amplifiers. As shared in Fig. 3 (a), it is the scanning spectrum of the output laser, which is measured by a Fabry-Perot interferometer (FPI) with a free spectral range (FSR) of 10 GHz. Benefitting from the external modulation, a near-rectangular spectrum is obtained with FWHM linewidth of 1.27 GHz. Figure 3 (b) is the spectrum of the signal laser measured at the full power of the pre-amplifiers which is recorded by another OSA placed at the end of the system. In the spectrum, no ASE component is found at the spectral signal-to-noise ratio (SNR) of ∼60 dB. Meanwhile, the beam quality is measured with M2 value of 1.27 and 1.30 for the x and y direction (the D4σ method is applied). The beam profile recorded at the focus point is shown inserted in Fig. 3, which illustrates that the seed is operating at near-diffraction-limited beam quality.
Fig. 3. (a) scanning spectrum of the input signal.; (b) the spectrum measured by the OSA at the full power of pre-amplifiers (inset: the beam profile and corresponding beam quality M2).
Then, we investigate the performance of this confined-doped fiber assisted narrow-linewidth amplifier through the power amplification process. Figure 4(a) illustrates the output power as well as the backward power versus the pump power. This figure demonstrates that the output power arises linearly with the pump power and the slope efficiency is measured to be 74.9% (the optic-to-optic efficiency at the maximum output power is 74.6%). For especial, when the output power exceeds 897 W (pumped by 1164 W), the backward power begins to increase nonlinearly, indicating that the Stokes light induced by the SBS effect has taken the dominance in backward propagating light. When the output power is scaled to 1007 W at 1335 W pump power, the backward power reaches 422 mW, accounting for 0.04% of the output power. Figure 4(b) are the spectra of the backward propagating light, in which two peaks could be observed, corresponding to the Rayleigh peak and Stokes peak. The four spectra shown in the picture are recorded at the output power of 424 W, 616 W, 796 W and 1007 W. With the increase of output power, the SBS induced Stokes peak grows much quicker than the Rayleigh peak. And at the maximum output power, the Stokes peak is 3.4 dB higher than the Rayleigh peak.
Fig. 4. (a) Output power and backward power versus the pump power; (b) Spectra of the backward propagating laser.
The spectral properties of this narrow-linewidth fiber laser system are also recorded at the maximum output power, which have been shared in Fig. 5. The spectrum of the output signal is shown in Fig. 5(a). As shown in this picture, the intensity of the output signal is ∼50 dB higher than the residual pump laser and ∼59 dB higher than the ASE component. Meanwhile, the scanning spectrum of the output laser measured by the FPI is demonstrated in Fig. 5(b). At the maximum output power, the scanning spectrum of the output laser still remains in a near-rectangular shape, and the FWHM linewidth of the output laser is measured to be 1.28 GHz. The linewidth measured at the maximum output power is nearly the same as that of the seed laser (as shared in Fig. 3(a)).
Fig. 5. (a) The spectrum of the output laser at the maximum output power. (b) the scanning spectrum for the seed laser with near-rectangular spectrum.
Subsequently, the time-frequency characteristics of the output laser are measured in the experiment, as shown in Fig. 6. The temporal traces of the seed laser and the maximum output laser are recorded by a photodetector of 150 MHz bandwidth and an oscilloscope of 100 MHz bandwidth. The normalized intensity of the temporal traces is demonstrated in Fig. 6(a), in which we can see that slight fluctuation appears at the maximum output power and the normalized standard deviation of the temporal trace increases from 1.0% to 3.4% comparing with the seed laser. In the corresponding Fourier spectra, shared in Fig. 6(b), no characteristic envelope of TMI effect below 10 kHz is observed, indicating that thermal induced dynamic mode coupling doesn’t happen at 1007 W. But, some instable peaks are found at the maximum output power, which may be induced by the power extraction of the Stokes light [46].
In addition, the beam quality of the output laser is recorded through the amplification process. Figure 7 demonstrates the dependence of the equivalent M2 factor on the output laser. Specifically, the equivalent M2 factor is defined as:
As shown in Fig. 7, slight degradation in beam quality is observed when the output power exceeds 800 W, and the equivalent M2 factor increases from ∼1.35 to ∼1.50 correspondingly. As we mentioned above, no thermal induced dynamic mode coupling happens through the amplification, therefore, we speculate that the degradation of beam quality is induced by the spatial hole burning effect [47]. Moreover, the near-field intensity profiles shared inserted in the picture all distribute in a near-Gaussian shape, proving that the amplifier works with a near-diffraction limited beam quality through the amplification process.
5. Conclusion
In this paper, the power scaling of an all-fiber amplifier was investigated which was constructed by a confined-doped fiber with the core/cladding diameter of 42.0/250.0 µm (fabricated at a doping ratio of 71.5%). Benefitted from the large more area and modes selection features of the confined-doped fiber in cooperation with a near rectangular spectrum and 915 pump manner, a 1007 W all-fiber laser with linewidth of 1.28 GHz was obtained and the beam quality (equivalent M2 value) was measured to be 1.47, indicating a near-diffraction-limited beam quality. This result verified the advantage of confined-doped fibers in the comprehensive suppression of the SBS and TMI effects in the kilowatt all-fiber laser systems with GHz level linewidth.
Funding
National Natural Science Foundation of China (62005313, 62035015, 62075242).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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