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Abstract
In this paper, we study the emerging 1535 nm Er: Yb codoped fiber MOPA with high power and high brightness. To characterize the interstage influence of this ASE-sensitive system, we conduct an interstage numerical model based on steady energy transfer model, where the seed and amplifier converge together. We analyze the amplifier setup, the seed pumping scheme, and feedback from inner reflection based on the model. Afterwards, we experimentally demonstrate a 1535 nm all fiber large mode area Er: Yb codoped fiber MOPA with the output power of 174.5 W, the brightness of 13.97 W/μm2sr, and ASE suppression ratio of 45 dB. To the best of our knowledge, this is the highest power and brightness of 1535 nm fiber lasers to date.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
There is great interest in high-power Erbium doped fiber (EDF) lasers operating at 1.53-1.62 $\mu m$ in recent years, due to their atmospheric transparency at eye safe wavelengths [1,2]. The power scaling of Er-only fibers [3–6]suffers a lot from the low absorption cross section, the intensive heat of quantum defect, and the concentration quenching [7]. 656 W is currently the record power of this kind of fiber lasers, however, the inevitable enlargement of core diameter worsened the beam quality factor ($M^2$) to 10.5 [3]. Er-Yb codoped fibers (EYDF), enhancing the absorption via the energy transfer from Yb ions to Er ions, and avoiding the quenching effect at high doping concentration, is critical for the power scaling of fiber lasers operating at this wavelength with near-diffraction-limit beam quality. However, the bottlenecking of the energy transfer between the Yb ions and Er ions (Yb-bottlenecking) limits the power scaling of EYDF lasers and amplifiers, accompanied by the observation of parasitic lasing at 1060 nm and even fiber damage. In general, the two main problems for high-power 1.5 $\mu m$ lasers and amplifiers are the Yb-bottlenecking and the thermal problems under high quantum defect.
For the Yb-bottlenecking problems, researchers have proposed the method of auxiliary seed [8,9] and Yb-band co-pumping [10–14] to overcome the Yb-parasitic lasing. However, due to device limitations and other factors, these methods have limited progress in high-power field yet. For the thermal problems, researchers have studied the method of off-peak pumping, in which the pump sources with lower absorption coefficients are utilized in the EYDF, avoiding the peak around 975 nm. This method avoids violent heat accumulation near the pumping end in the EYDF. In 2020, the power scaling of 1560 nm EYDF reached 302 W under backward pumping by 915 nm laser diodes (LDs) with a near-diffraction-limit beam quality ($M^2$<1.1), where the low-absorption pumping (avoiding the 975 nm peak) increased the threshold of Yb-parasitic lasing and the direct immersion water cooling released the influence of the overheat [15]. In 2021, our group demonstrated a 220 W high-power EYDF master oscillator power-amplifier (MOPA) operating at 1600 nm off-peak pumped by 1018 nm fiber lasers [16,17]. The absorption cross section of 1018 nm is much lower than that of 915 nm, thus, the thermal problems are less severe. However, the Yb- bottlenecking is still the limiting factor for the power scaling. For further power scaling of EYDF lasers and amplifiers, some better approaches should be proposed to solve these problems.
The power scaling of high power and high brightness 1535 nm lasers is one of the interesting branches of Er-band fiber lasers. 1535 nm is an appropriate wavelength for in-band pumping [18–20] of fiber lasers operating at 1580 nm to 1610 nm, which may give a solution to the Yb-bottlenecking and thermal problems.
The highest output power of Er doped fiber laser pumped by $\sim$1535 nm LDs was reported at 88 W [19], which is far behind other pumping schemes [3,15,18,21], due to the low brightness of the pumping source. Calculated in the terms of $W/\mu m^2sr$, the brightness of the $\sim$1535 nm LD in [19] was only 0.012 $W/\mu m^2sr$. In 2014, 264 W output power at 1585 nm in in-band pumping EYDF laser was demonstrated [18], using 37 single mode EYDF lasers of 11 W at 1535 nm. The brightness of the 1535 nm fiber laser was 0.92 $W/\mu m^2sr$. Therefore, the development of high-power high-brightness 1535nm fiber laser is critical for EDF (EYDF) in-band pumping. For the research of the 1535 nm fiber laser, to the best of our knowledge, the highest power of free-space 1535 nm fiber laser was reported at 108 W to date [22], the free space grating was essential for establishment of 1535 nm feedback in the EYDF cavity. The brightness of the report was 4.37 $W/\mu m^2sr$, however, the free-space structure with high maintenance cost is not suitable for in-band pumping. The highest power of the all-fiber 1535 nm fiber laser was reported at 60 W [5] , while the brightness was only 0.086 $W/\mu m^2sr$ due to the utilization of large-core Er doped fiber. Large-mode-area (LMA) EYDF cladding pumped MOPA is successful in power scaling of laser at 1550-1565 nm [15,21]. However, for the power scaling of 1535 nm, this scheme encounters the challenge of ASE at 1540-1565 nm, because the 1535 nm laser has strong re-absorption effect in the amplifier, which has not been studied systematically in previous reports.
In this report, we demonstrate a high-power 1535 nm EYDF fiber laser based on MOPA scheme. We utilize heavily doped concentration LMA EYDF as the gain fiber of the amplifier. The suppression of the feedback from the inner reflection, the optimization of the seed and amplifier, and the backward-pumping configuration contribute to the power breakthrough and the system stability. The power of our report reaches 174.5 W with the brightness of 13.97 $W/\mu m^2sr$ and the ASE suppression ratio of over 45 dB. To the best of our knowledge, this is the highest power and highest brightness for 1535 nm fiber laser. Also, we conduct an interstage model based on steady energy transfer equation for the design of the lasering system.
2. Method
The 1535 nm MOPA is configured as Fig. 1. The limiting problems of the 1535 nm EYDF laser are the Yb-bottlencking, the thermal problem, and the 1560 nm ASE. In this report, we take a series of methods to conquer these problems. Firstly, we utilize 915 nm LD off-peak backward-pumping scheme in the amplifier to improve the threshold of the Yb-bottlenecking and reduce the influence of thermal effects, which have been studied in [16,23]. Secondly, we conduct an optimization of the LMA amplifier to suppress the 1560 nm ASE. Also, the feedback from inner reflection and the interstage ASE distribution are characterized by the steady energy transfer equations.
Fig. 1. Schematic diagram of the 1535 nm MOPA (CLS: cladding light striper; PC: pump combiner; HR: high-reflection fiber gratting; OC: low-reflection fiber gratting; SM: single-mode; LMA: large-mode-area)
The rate equations of our interstage model are similar to [16,23]. The differences between our model and other models are the boundary conditions and the convergence methods, in other words, we take the amplifier and the seed as a combing system in our model. The combing calculation of the seed and the amplifier in the 1535 nm MOPA is essential, since the change of the initial condition of one stage may have significant influence on another stage, especially for the prediction of the ASE. Specifically, on one hand, the output ASE is accumulated from the seed to the amplifier, and the seed setup may have significant influence on the final output. If we only take the isolated amplifier into consideration as traditional model [23], the important interaction between the seed and the amplifier will be ignored; on the other hand, the Yb-ASE generated in the amplifier may propagate backward to the seed and have important influence on the establishment of the laser. The boundary conditions are shown in the following equation:
Table 1. Fiber parameters
For the designing of the 1535 nm MOPA, we study the spectral distribution at different conditions as shown in Fig. 2. We investigate the evolution of output spectrum with output power. As shown in Fig. 2(a), the intensity of the 1560 nm ASE shows little change as the power increases, and we attribute it to that there is a fixed ratio between the gain of the Er-ASE and the Er-signal along the system. Similarly, we investigate the Yb-ASE evolution in Fig. 2(b). The Yb-ASE is enhanced significantly as the power increases, compared to the Er-ASE, which is due to the accumulation of upper energy level concentration of the Yb ions. We can conclude from the comparison of the Er-ASE and the Yb-ASE, that the Yb-parasitic lasing limitation can be solved by the control of the pump power, and the suppress of the Er-ASE need optimization of the system.
Fig. 2. Simulated spectral spectrum. a,b: Er and Yb band spectrum at different power; c,d: spectrum at different inner-reflection conditions; e:optimization of the amplifier; f: analysis of the pumping wavelength of the seed.
To increase the Er-ASE suppression ratio and the Yb-bottlenecking threshold, we investigate the feedback from inner reflection. If the LDs were spliced to the laser oscillator directly, the internal reflection in the LD would bring around 4$\%$ feedback to the seed cavity. For the 1560nm and Yb-band with very high gain at high power, the 4$\%$ inner-reflection could be very dangerous. Therefore, we insert a pump combiner for transition between the LD and the seed cavity to make the angle-cleaved end, which has been verified in the similar 1018 nm fiber oscillator [24]. The simulated results of the 1535 nm amplifier were shown in Fig. 2(c) and (d), where the angle-cleaved scheme and the direct splicing scheme have the same pumping power of 360W. The results indicate the angle-cleaved scheme increases the 1560nm ASE suppression ratio by 20 dB, and avoids the generation of Yb-parasitic lasing.
In Fig. 2(e), we study the influence of the amplifier length, and the result indicates that the amplifier length has a significant influence on the suppression ratio of the 1560 nm ASE. To attain an over 40 dB suppression ratio of ASE, the fiber length of the LMA fiber needs be shortened to 4m. However, the amplifier with too short EYDF will face the problem of inadequate absorption. Therefore, the most optimized fiber length is estimated around 4 m for this kind of fiber.
The seed laser cavity could also have influence on the output spectrum of the amplifier, and for the first time, we study the pumping wavelength of the seed laser. As we all know, due to the Yb-parasitic lasing and the generated heat, the amplifier tends to utilize LDs with wavelength of 915 nm (or 940 nm) as the pumping source instead of 975 nm, which is at the absorption peak. However, for the seed laser, 975 nm pumping scheme is possible since the seed power doesn’t reach the threshold level of the problem above. As shown in Fig. 2(f), we study the spectral performance of the MOPA with seed pumped by the common 915 nm LD and 975 nm LD. The fiber lengths of the seed lasers are set based on the 20 dB absorption of the pumping light, and all the other conditions, including the amplifier and inner reflection, are set the same. The results in Fig. 2(f) illustrate that the MOPA with 975 nm pumping seed has significant advantage over the 915 nm pumping. This effect can be attributed to the shorter seed the former MOPA used, that weaken the interstage accumulation of the 1560 nm ASE.
We can conclude from the numerical investigation: to attain high power 1535 nm with high 1560 nm ASE suppression ratio and high Yb-parasitic lasing threshold, the design points should include inserting pumping combiner for transition with angle-cleaved end, optimization of the amplifier length, and pumping the short seed cavity by 975 nm LD.
3. Pre-experiments and results
We conduct a series of pre-experiments under low pump power before the high power experiments to give evidence to the numerical optimization, which are shown in Fig. 3. Firstly, We conduct end-cleaving control experiments to verify the influence of the inner-feedback on the output spectrum, as shown in Fig. 3(a). The seed end and the output end are perpendicularly cleaved, 8-degree-angle cleaved, and angle-cleaved with inserted CLSs. The pumping power of the angle-cleaved amplifier are controlled as 88 W, while the pumping power of the perpendicularly cleaved system is 14.8 W since higher power would be dangerous for the parasitic lasing. The results indicate the perpendicularly cleaved ends have very low threshold of parasitic lasing at 1567 nm. The CLSs near the seed end and the output end are beneficial for the suppression of 1550 ASE, which makes sense because in the angle-cleaved system, some light is reflected backward to the cladding, thus the CLSs could avoid this feedback in the EYDF.
Fig. 3. The pre-test experiments results: (a): The end-cleaving control experiments, PC for perpendicularly cleaved ends, while AC for angle-cleaved ends, e.g. ’PC-AC’ means the seed end is perpendicularly cleaved and the output end is angle-cleaved, both ends are not inserted with a CLS; (b): The control experiments of pumping wavelength of the seed, that seed 1 for 1.8 m EYDF in the seed pumped by 975 nm LD (around 20 dB total absorption for 975 nm), the seed 2 for 1.8 m EYDF pumped by 915 nm LD, and the seed 3 for 8 m EYDF pumped by 915 nm LD (around 20 dB absorption for 915 nm); (c): The control experiments of the seed ends with the output end is angle-cleaved with a CLS; (d): The control experiments of the amplifier length.
We study the spectral behavior of different pumping wavelength of the seed, whose results are shown in Fig. 3(b). All the seeds are pumped by around 14 W LD (either 915 nm or 975 nm) , and the amplifiers are pumped by 915 nm LD with controlled power of 88 W. The tested seed are 1.8 m EYDF in the cavity pumped by 975 nm LD (around 20 dB total absorption for 975 nm), 1.8 m EYDF pumped by 915 nm LD, and 8 m EYDF pumped by 915 nm LD (around 20 dB absorption for 915 nm). The spectrum figures are shifted to have the same peak level at 1535 nm for better presentation of ASE comparison. The results in Fig. 3(b) indicate the 975 nm pumping seed shows significant progress over 915 nm pumping seed on the performance of the amplifier output spectrum, just as our prediction in the numerical investigation. It is noted that even for 915 nm seed 2 with the same length as 975 nm seed 1, the ASE behavior is still weakly worse than seed 1. We attribute it to the inadequate absorption which causes the power decrease of the seed, and lower its competitiveness with the 1550 nm ASE in the amplifier.
Afterward, we study the seed-end management influence on the output spectral behavior from the same amplifier with CLS-inserted angle-cleaved end. In this experiment, the amplifier is with 4 m length under 88 W 915 nm pump. The seed end is handled by perpendicular cleaving, angular cleaving, and angular cleaving with a CLS, respectively. The results are shown in Fig. 3(c), indicating that the perpendicular/angle-cleaved ends have significant influence on the output spectral behavior, verifying the cross-stage accumulation of ASE discussed in the method part of this article. It is noted that the spectrum with/without a CLS near the seed end show little difference, and we think the CLS between the two stages weakens the cladding-ASE accumulation starting from the angle-cleaved feedback of the seed end.
The experiments of Fig. 3(a)-(c) are all conducted with a 4-m backward-pumping amplifier. In addition, We test the spectral behavior of other lengths, whose results are shown in Fig. 3(d). The experiments utilize forward pumping scheme to calculate the pump absorption of the amplifier. A dichroic beam splitter is located to split the 1535 nm signal and 915 nm unabsorbed pump. The optical efficiency versus launched pump is measured as 33.0 $\%$ for 2 m (output power of 118 W) and 37.5 $\%$ for 3.2 m (output power of 102.5 W), while the pump absorption is 5.9 dB for 2 m, and 8.6 dB for 3.2 m. The low pump absorption causes the decrease of optical efficiency in the short amplifier. The 4 m amplifier balance the high optical efficiency of over 40$\%$ and good spectral performance. As for the 5-m amplifier experiment, the ASE suppression ratio is decreased to less than 40 dB, facing parasitic lasing risk. Therefore, we think a 4-m LMA-EYDF with a CLS-inserted angle-cleaved output end is suitable as the amplifier and a 1.8-m SM-EYDF with CLS-inserted angle -cleaved seed end for the seed, just as Fig. 1 shows.
4. High-power experiments
The high-power experiment is configured as these points as Fig. 1. The LMA gain fiber we use in the amplifier is Coherence LMA-EYDF-25p/300-HE, and the SM gain fiber in the seed cavity is Coherence SM-EYDF-10/125-xp. All the passive fibers are chosen to match the gain fiber. We utilize a pair of home-made fiber Bragg grating with center wavelength at 1534.7 nm to provide the feedback in the seed cavity. The reflectivity is 0.99 for the HR with the bandwidth of around 0.4 nm and 0.1 for the OC with the bandwidth of around 0.1 nm. The 975 nm pumping power of the seed is 14.7 W, and the output power of the seed is 5.8 W. The amplifier with the EYDF length of 4 m is pumped by a 915 nm LD with the maximum output power of 374.8 W.
We measure the output power and spectrum using a thermal power meter and an optical spectrum analyzer (YOKOGAVA-AQD6370D). The highest output power reaches 174.5W as shown in Fig. 4, without observation of Yb parasitic lasing, shown in the spectra of Fig. 5. The 5.8 W seed is attenuated to 0.42 W during passing through the amplifier without any pump. The high attenuation is mostly due to the high absorption of Er ions at 1535 nm. We calculate the optical efficiency deducting the passing through power of 0.42 W in Fig. 4. The optical efficiency reaches 46.5$\%$ at the output power of 174.5 W, and shows no sign of saturation, indicating further power scaling potential. The output spectrum has good purity even at the highest output power with the 3dB spectral width of around 80 pm (the measuring resolution is 20 pm) and the ASE suppression ratio of over 45 dB. To measure the far-field divergence angle, a metal circle aperture is located at a certain distance from the output end to guarantee 86.5$\%$ of the total power pass through it. The half far-field divergence angle is measured as 0.058 rad at the output power of 129.2 W. We calculate the brightness by assuming the core area as the laser emitting surface and the $\pi NA^2$ as the solid angle of the emitting laser. The laser brightness of our 1535 nm MOPA is estimated as 13.97 $W/\mu m^2sr$, higher than 0.92 $W/\mu m^2sr$ in [18], 4.37 $W/\mu m^2sr$ in [22], and 0.086$W/\mu m^2sr$ in [5], calculated in the same way. Further power scaling is limited by the 915 nm pumping source. The power and spectrum results indicate the 1535 nm fiber laser is suitable for in-band pumping.
Fig. 5. Measurements of the spectra. (a): Er-band spectra; inset: detailed spectrum at output power of 174.5 W; (b):Yb-band spectra
5. Summary
In summary, we demonstrate a high-power and high-brightness 1535 nm EYDF fiber laser based on MOPA scheme. We utilize high-doping-concentration LMA EYDF as the gain fiber of the amplifier. We propose the interstage model for the analysis of the interstage ASE accumulation. In this paper, we analyze the suppression of the feedback from the inner reflection, the pumping wavelength of seed laser, and the optimization of the gain fiber. Based on the theoretical optimization, the power of experimental reports reaches 174.5 W with the brightness of 13.97 $W/\mu m^2sr$ and the signal-noise ratio of over 45 dB. To the best of our knowledge, this is the highest power and highest brightness for 1535 nm fiber laser. We believe our 1535 MOPA is suitable for the next-generation in-band pumping of Er-band fiber laser with much higher power that the lasers reported to date.
Funding
National Natural Science Foundation of China (61875103, 62122040, 62075113, 11604177).
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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