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Abstract
In this paper, we report a high power single frequency 1030 nm fiber laser with near-diffraction-limited beam quality based on a polarization-maintaining tapered Yb-doped fiber (T-YDF). The T-YDF has advantages of effectively suppressing stimulated Brillouin scattering (SBS) while maintaining good beam quality. As a result, a record output power of 379 W single frequency, linearly polarized, nearly single-mode fiber amplifier operating at 1030 nm is demonstrated. The polarization extinction ratio is as high as 16.3 dB, and the M2 is measured to be 1.12. Further, the dependence of the thermal-induced mode instability (TMI) threshold on the polarization state of an input signal laser is investigated for the first time. By changing the polarization state of the injected seed laser, the output power can increase to 550 W while the beam quality can be maintained well (M2=1.47). The slope efficiency of the whole amplifier is about 80%. No sign of SBS appears even at the highest output power and the further brightness scaling of both situations is limited by the TMI effect. To the best of our knowledge, this result is the highest output power of all-fiberized single frequency fiber amplifiers.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
High power single frequency fiber amplifiers develops rapidly in recent years due to its wide application in gravitational wave detection (GVD) [1], coherent LIDAR [2], nonlinear frequency conversion (NFC) [3], coherent beam combining (CBC) [4] and so on. Up to now, single frequency fiber laser with a central wavelength of near 1 µm has been amplified to several-hundred-watt level based on master oscillator power amplifier (MOPA) configuration [5]. However, kW-level single frequency fiber amplifiers with good beam quality are strongly demanded in the next generation GVD [1]. Therefore, pursuing higher output power has always been the research hotpot in single frequency fiber amplifier. The primary limiting factor in the power scaling process of single frequency fiber laser is usually considered to be stimulated Brillouin scattering (SBS) [5]. It can be suppressed effectively by a number of approaches, such as employing large-mode-area (LMA) active fiber [6,7], applying strain or temperature gradient [8–10], using acoustically tailoring technology [11–13], introducing laser gain competition [14], shortening fiber length [15], counter-pumping manner [16,17] and so on. As the output power increases based on these SBS suppressing methods, another limitation factor called thermal-induced mode instability (TMI) is observed to damage the beam quality and prohibit the further brightness scaling [18]. In order to mitigate the TMI effect, several methods were also proposed, such as reducing the fiber core to pump cladding ratio [19,20], suppressing the photo-darkening of fiber [21,22], increasing the relative mode loss of high order modes [23,24], optimizing the pumping manner [25,26], optimizing pumping wavelength or signal wavelength [27–29] and so on. Besides, further investigations showed that TMI suppressing is more challenging in narrow linewidth fiber amplifiers than that in conventional broadband ones [30,31]. Nowadays, to achieve single frequency fiber laser with higher output power, the suppressing of SBS and TMI effects should be considered simultaneously. With this consideration, new-type of active fiber design and fulfillment is one of the most promising techniques for balancing the two detrimental effects in single frequency fiber amplifiers [13].
Up to now, significant progresses of high power single frequency fiber amplifier have been achieved (as shown in Table 1). Typically, as high as 811 W output power, which is on behalf of the highest output power of single frequency fiber amplifiers, has been realized based on bulk optics configuration by applying temperature gradient (about 70°C) on a gain tailored active fiber [13]. In fact, single frequency fiber amplifier with all-fiber structure is preferable in many applications due to its high compactness and reliability [5]. As for all-fiber single frequency fiber amplifier, 332 W linear-polarized output power has been reported where a high-dopant-concentration polarization- maintained (PM) LMA active fiber with a core diameter of 30 µm was adopted [7]. Further, the output power was improved to 414 W by imposing strain gradient on a high-concentration PM LMA active fiber [9]. Notably that low noise, single frequency fiber amplifiers have been presented with output powers of 200 W and 365 W quite recently [32,33]. As shown in Table 1, additional SBS suppressing methods such as thermal or strain gradients were applied in single frequency fiber amplifier with output power higher than 400 W level. However, it may bring inconvenience and sacrifice the stability of fiber laser system in practical applications.
Table 1. Typical progress on single frequency fiber laser (Non: nonlinearly polarized state, NA: not available, ATF: Acoustically tailored fiber, T-YDF: tapered Yb-doped fiber, LMA: large mode area).
Recently, to reduce the impact of nonlinear effect and maintain good beam quality of fiber laser, some special fiber types have been developed. In addition to the gain tailored photonic crystal fiber mentioned above, the photonic bandgap fiber and chirally coupled-core fiber have also been demonstrated for high power single mode operation [36,37]. Besides, as a potential approach for effectively managing SBS effect and maintaining good beam quality, the LMA tapered Yb-doped fiber (T-YDF) has also been proven to achieve high power single frequency output [34,38–41]. The SBS frequency shift would vary with the gradually-changed diameter of tapered active fiber, which results in a broadening SBS gain spectrum and can suppress SBS effectively [42]. What’s more, the tapered structure has a near identical core to inner cladding ratio along the longitudinal profile. Therefore, the T-YDF could promise excellent beam quality in LMA fiber amplifier with increasing core area along the active fiber for SBS management [43,44]. Besides, all the single frequency fiber amplifiers mentioned above operate in the wavelength range of 1060∼1080 nm. The other operating range in the Ytterbium band has also been investigated widely in high power fiber amplifier [45–48]. In fact, 1030 nm signal laser has larger emission cross-section so that the gain saturation can be stronger, so the TMI threshold tends to be increased [49]. Moreover, the quantum defect in 1030 nm fiber amplifier is smaller than that in fiber amplifier with wavelength between 1060 nm and 1080 nm, which further helps suppress TMI effect. However, the large absorption cross-section at 1030 nm could lead to amplified spontaneous emission (ASE) easily due to strong reabsorption [50,51]. It would bring challenges to improve output power of 1030 nm single frequency fiber amplifier. So far, there are no reports on 1030 nm single frequency fiber amplifier above 100-watt level.
In this manuscript, we demonstrate an all-fiber single-frequency fiber amplifier emitting at 1030 nm based on a high-dopant-concentration T-YDF. Due to the excellent SBS suppressing ability of T-YDF, without any other special SBS suppressing methods, the nonlinear increase of backward light was not observed on experiment and the slope efficiency of main amplifier is about 80%. The TMI effect does not appear until the output power of this fiber amplifier reaches to 379 W at linearly polarized state and 550 W at non-polarization-maintained state. When it operates at linearly polarized state, the polarization extinction ratio (PER) is about 16.3 dB and the M2 can be maintained to be 1.12 at 379 W. Moreover, the M2 is measured to be 1.47 when the output power increases to 550 W at non-polarization-maintained state. As far as we know, this is the highest demonstrated power for all-fiber single frequency MOPAs with near-diffraction-limited beam quality.
2. Experiment setup
Our experiment setup is shown in Fig. 1. The whole configuration employs three stages polarization-maintained Yb-doped fiber (PM-YDF) amplifiers. The linearly polarized single frequency seed is a fiber laser with linewidth of 20 kHz [52], and its central wavelength and output power are 1030 nm and 40 mW, respectively. Firstly, the output power of seed laser is amplified to be 1.5 W after passing through a commercial pre-amplifier (P-A). A polarization-maintained band pass filter (BPF) is used to mitigate the amplified simultaneous emission (ASE) stemming from the P-A. Then the laser is launched into the second pre-amplifier utilizing a piece of 1.5 m gain fiber with core/cladding diameter of 12/125 µm. A laser diode (LD) with 976 nm central wavelength is used to pump the active double-cladding fiber via a (2 + 1)×1 pump combiner. The output power after passing through the second stage is measured to be about 8.4 W. In front of the third stage, a polarization-maintained circulator is employed to monitor the backward power of the main amplifier and protect the forestage amplification chain.
Fig. 1. Experiment setup. (P-A: pre-amplifier, BPF: band pass filter, LD: laser diode, PM: polarization maintained, YDF: Yb-doped fiber).
As for the third stage, six 180 W level, 976 nm LDs are combined to pump a polarization-maintained tapered Yb-doped fiber (T-YDF) via a (6 + 1)×1 PM pump combiner. In this pump combiner, the core/inner cladding diameter of the signal input port is 15/130 µm while that of the output port is 25/250 µm. The T-YDF has core/inner cladding diameters of 36.1/249.3 µm at the input port and 57.8/397.3 µm at the output port and the whole length is 1.27 m. The tapered region is as long as 0.74 m that locates at the middle part of fiber with approximately linear longitudinal profile. Different from the typical T-YDF with gradually-changing diameter through the whole length [34,38–41], the input port and the output port of this T-YDF have a smooth region with length of about 0.26 m. The core radius along with the fiber length is shown in Fig. 2. It has a typical cylindrical fiber cross-section shape with double-cladding. The cladding absorption efficient of this T-YDF is about 12.1 dB/m near 976 nm and the core numerical aperture (NA) is 0.064. A homemade endcap is spliced behind the output port of Tapered-YDF and the output beam is collimated by a free-space collimator, behind which a dichroic mirror is used to filter the residual pump light and ASE light.
3. Experiment results and discussion
3.1 379 W linearly polarized single frequency near-diffraction-limited fiber amplifier
Firstly, this section shows the experiment results when the whole fiber amplifier system operates at a linearly polarized state. As shown in Fig. 3, the output power of the main amplifier has a linear fitting to 80% slope efficiency against pump power. We can find that there is no power roll-off in the amplifying process. The output power could reach to 427 W when the pump power increases to 535 W.
In the experiment, the TMI effect is analyzed by measuring the temporal intensity of output laser and its corresponding Fourier transform spectrum. When the TMI effect happens, the laser power would present fluctuations in temporal domain and the kHz-level characteristic frequency would appear in its corresponding Fourier transform spectrum [53]. The temporal intensity is measured by using a high-speed detector whose response frequency is about 150 MHz. A pinhole is placed before the detector to eliminate the false light and control the size of laser beam. As a result, it is found that the temporal intensity has been kept stable and no special characteristic frequency has been observed until the output power increases to 379 W. For comparison, Figs. 4(a) and 4(b) give the time trace during a period of 5 ms and the corresponding Fourier transform spectrum at a typical output power of 353 W, where the standard deviation of temporal intensity is calculated to be 0.015. However, when the output power just reaches 379 W, a sudden intensity fluctuation and the corresponding spectral peaks within 0∼5 kHz are observed [as shown in Figs. 4(c) and 4(d), respectively]. The standard deviation of temporal intensity increases to 0.319. The overall results reveal that the TMI effect occurs at the output power of 379 W.
Fig. 4. (a) The normalized temporal intensities at 353 W, (b) the corresponding Fourier transform spectra at 353 W, (c) the normalized temporal intensities at 379 W, (d) the corresponding Fourier transform spectra at 379 W.
Moreover, we measure the beam quality of laser in the power scaling process by using a Laser Quality Monitor [as shown in Fig. 5(a)]. We can see that the output laser maintains near-diffraction-limited beam quality before reaching TMI threshold. At the power of 379 W, where the TMI effect occurs, the M2 factor is measured to be 1.12 and the beam profile indicates that it still operates at a near single-mode state. When the output power increases to 405 W, some extent of distortion is observed in its beam profile and the M2 factor is measured to be 1.55. In the experiment, the beam quality of output laser can be maintained well even though its temporal intensity appears to be just instable in this fiber amplifier. It is also shown that the TMI threshold diagnosed by temporal and Fourier transform spectrum method is a little lower than that of the beam quality degradation metric. As the output power further increases to 427 W, a serious distortion happens on the beam profile of output laser. The M2 factor could increase to be 2.52 at this time, which means the laser beam quality deteriorates seriously. Meanwhile, the polarization extinction ratio (PER) of output laser is also measured by using a polarization beam splitter and λ/2 wavelength plate [(as shown in Fig. 5(b)]. According to Ref. [20], the polarized degree of output laser could decrease when the TMI occurs. However, different from the results shown in Ref. [20], the polarization extinction ratio in our experiment could still maintain when the TMI occurs. When the output power increases from 379 W to 427 W, the PER of laser varies between 16.3 dB and 16.7 dB. Specifically, it is measured to be 16.3 dB at the power of 379 W. One of the possible reason is that the polarization states of the TMI-excited high-order modes is identical to that of the fundamental mode in the present fiber amplifier.
Fig. 5. (a) The beam quality (M2) versus output power (insets: the beam profiles at 379 W, 405 W and 427 W), (b) the polarization extinction ratio (PER) versus output power.
3.2 Discussion of the dependence of the TMI threshold on the injection polarization state
According to the previous literature [54], the TMI threshold in a non-polarization maintaining fiber amplifier could be higher than that in polarization maintaining fiber amplifier. It indicates that the TMI threshold could be closely related to the polarization state of injected fiber laser. To explore the influence of the polarization state of injected signal laser on the TMI threshold of this T-YDF amplifier, we spliced the fiber between the circulator and combiner with different off-axis angle offsets. Then the polarization state of the input laser could change from linearly polarized to other states. Meanwhile, the TMI thresholds at different splicing angle errors were determined by using temporal and Fourier transform spectrum method. To promise the reliability of experimental results, the output power increasing process was measured twice at each splicing angle offset and the results presented excellent repeatability on experiment. The experiment results are shown in Fig. 6. The TMI thresholds present symmetry with the different off-axis splicing angle offsets. When the fiber before main amplifier was spliced with off-axis angle offset of π/6 or 5π/6, the TMI thresholds, which were 379 W and 395 W respectively, were not improved significantly compared with that at linearly polarized state. However, the TMI threshold could increase to 550 W at splicing angle offset of π/3 or 2π/3. At the splicing angle offset of π/2, the TMI threshold was measured to be 427 W. We infer that the TMI threshold characteristics shown in Fig. 6 are the comprehensively influence of polarization state of the injected signal laser, the initial excited state of linear-polarized modes and the polarization evolution process along the gain fiber. Overall, the TMI threshold of a linearly polarized laser in such T-YDF amplifier is lower than that with different off-axis splicing angle offsets, which reveals that brightness scaling of linear-polarized single frequency fiber laser is indeed more challenge than that in non-polarized case.
3.3 550 W single frequency and a near-diffraction-limited fiber amplifier
As introduced above, as high as 550 W single frequency near-diffraction-limited fiber laser can be achieved by splicing the fiber between the circulator and combiner with angle offset of π/3 or 2π/3. In this section, the detailed results at this situation would be presented. Firstly, the output power and backward power are shown in Fig. 7. It is noted that the output power increases linearly with the pump power and the slope efficiency still maintains to be 80%. The output power can be boosted to 550 W at the pump power of 676 W. No sign of nonlinear increase of backward power is observed in the whole process, which means that it does not reach to the SBS threshold in the main amplifier.
To investigate the TMI effect roundly, we measure the normalized temporal intensity of output laser and the corresponding Fourier transform spectrum in the power amplifying process. Similar to the linearly polarized situation, the temporal intensity has been kept stable and its Fourier transform spectrum has always been smooth until the output power increases to 550 W. As an example, Fig. 8(a) and Fig. 8(b) present the measuring results at the power of 536 W. The standard deviation of temporal intensity in Fig. 8(a) is calculated to be 0.040. In Fig. 8(b), the several discrete spectral lines at low frequency stage in Fourier transform spectrum mainly come from the noise of the experiment environment and the fiber amplifier system. As the output power increases to 550 W, sudden instability and fluctuation can be observed obviously in the temporal traces and its standard deviation increases to 0.489 [as shown in Fig. 8(c)]. The corresponding spectral peaks within 0∼5 kHz can be seen clearly in its Fourier transform spectrum as well [as shown in Fig. 8(d)]. Therefore, it can be concluded that the TMI threshold is about 550 W at this situation. Moreover, the PER at the power of 550 W is found to fluctuate from 4.7 dB to 7.7 dB, which indicates that the polarization state of laser appears to be instable at this situation.
Fig. 8. (a) The normalized temporal intensities at 536 W, (b) the corresponding Fourier transform spectra at 536 W, (c) the normalized temporal intensities at 550 W, (d) the corresponding Fourier transform spectra at 550 W.
In addition to measuring the time and its Fourier transform spectrum of output laser in the amplifying process, we also monitor the beam quality of laser by using the Laser Quality Monitor (LQM) through 2nd moment criterion. In the experiment, when the laser power is scaled to be 458 W, the output beam is still maintained to be near single mode and the M2 factor is measured to be ∼1.11. When the output power further scales from 458 W to 550 W, despite that TMI effect is not observed, the beam quality is still degraded slowly along with power scaling. This beam quality degradation could be attributed to the statistic mode coupling effect at high power operation. At the power of 550 W, where the onset of TMI happens, the M2 is examined to be 1.47 (as shown in Fig. 9).
Finally, Fig. 10 shows the optical spectrum at this maximum output power. It indicates that little ASE and residual pump light appear in the output laser beam and the signal-to-noise ratio is about 50 dB. The inset picture is the scanning spectrum of the laser measured by a Fabry-Perot interferometer (FPI) with a free spectral range of 1.5 GHz, which indicates that the output laser has a resolution-limited linewidth of 45 MHz.
Fig. 10. The optical spectrum of a laser beam at the power of 550 W (inset: the scanning spectrum by the FPI).
4. Conclusion
In summary, we have established a 1030 nm all-fiber single frequency fiber amplifier based on a tapered Yb-doped fiber. It can achieve a 379 W linearly polarized near-diffraction-limited output laser with the PER of 16.3 dB and the M2 of 1.12. Further, by splicing the fiber before main amplifier with different off-axis angle offsets, we investigate and discuss the dependence of TMI threshold on the polarization state of input signal laser. Specifically, at the off-axis angle offset of π/3 or 2π/3, the output power can be boosted to 550 W and the M2 is maintained to be 1.47 at the same time. The slope efficiency of the main amplifier is as high as 80%. We believe that this work could give a well reference for further brightness scaling of all-fiberized single frequency fiber amplifiers with near-diffraction-limited beam quality.
Funding
Science and Technology Innovative Research Team in Higher Educational Institutions of Hunan Province (2019JJ10005); Construct Program of the Key Discipline in Hunan Province (2019RS3017); National Natural Science Foundation of China (61705264); Research Plan in Key Areas of Guangdong province, China (2018B090904001).
Disclosures
The authors declare no conflicts of interest.
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