We present an efficient 976 nm laser generation from an ytterbium (Yb)-doped step-index multicore fiber (MCF) with six cores placed in a ring shape. Each of the six cores has a large-mode-area (LMA) and a low numerical aperture (NA), which makes the MCF equipped with the features of a large core-to-cladding area ratio and differential bending loss for wavelength and mode selection. Hence, the Yb-doped MCF benefits 976 nm laser generation by simultaneously suppressing unwanted 1030 nm emission and higher-order modes (HOMs). A 976 nm laser is obtained in a short piece (88 cm) of the Yb MCF, with a good slope efficiency of 46% with respect to launched pump power and the maximum output power of 25 W (pump power limited). A mode area of 1432 µm2 at the 976 nm is expected for the fundamental in-phase mode.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
High brightness laser sources operating around 976 nm have attracted much attention for great values of essential applications such as a promising pumping source for Erbium (Er)-doped and Yb-doped fiber lasers and amplifiers. Moreover, with frequency-doubling, the 976 nm laser can be efficiently converted to a 488-490 nm blue laser which is important for a broad applications in biomedical and biological fields, as a replacement for the bulky and inefficient argon ion laser for blue laser generation [1,2]. A 976 nm fiber laser up to a few tens of watts can be achieved, relying on a cladding-pumped three-level Yb-doped fiber lasing system . The absorption and emission cross sections are nearly identical around 976 nm in a Yb-doped fiber (YDF), which implies that at least 50% Yb-ion population inversion is required to overcome reabsorption and create optical gain at 976 nm. Moreover, its competing four-level transition emission at 1030 nm becomes dominant at low population inversions. Therefore, the main challenge of achieving an efficient cladding pumped Yb-doped fiber laser operating at 976 nm wavelength is related to suppression of the undesired emission around 1030 nm.
In the past decades, several special fiber designs for efficient fiber lasers operating at 976 nm have been demonstrated. Building spectral filtering and spectral gain discrimination in a fiber are found efficient for the 3-level 976 nm lasing generation. A spectral filtering effect can be built in a photonic bandgap fiber to suppress the unwanted 1 µm emission in a YDF [4–6]. Recently, a output power of 151 W with 63% slope efficiency was realized in an all-solid photonic bandgap fiber . It is also noteworthy that core composition plays a part to build a spectral gain discrimination for an advantage of 976 nm lasing .
In addition to the spectral filtering and spectral gain discrimination, there is a simple yet popular approach which utilizes a large core-to-cladding area ratio (CCAR) in favor of efficient three-level fiber lasers . The large CCAR makes pump intensity increase relatively and enables a high inversion (> = 50%) along the fiber which is required for the three-level laser. Such large CCAR has been realized in all-solid single core Yb-doped fibers for efficient 976 nm lasers and amplifiers [9–11] including a fiber laser at 977 nm up to 5.5 W output power with a 25% efficiency achieved in a YDF with a 0.1225 CCAR (28 µm core and 80 µm cladding) . A tapered single core solid YDF with a 0.060 CCAR (12 µm core and 49 µm cladding at fiber center) achieved 10.6 W output power but with a lower efficiency of 18.4% . A higher efficiency of 31% for 13 W output power was presented in a 0.097 CCAR YDF amplifier (14 µm core and 45 µm cladding) . Much larger CCAR is able to be built in a rod-type photonic crystal fiber (PCF) or a jacketed air-clad PCF [3,12–14]. A 94 W output power was reported with 63% slope efficiency based on a rod-type YDF PCF with CCAR of 0.16 (80 µm core and 200 µm cladding) . Although the large CCAR design in an all-solid step-index single core fiber has been successful, its mode area scalability is likely to be limited practically, since there is a difficult requirement on ultra-low core NA to meet for achieving good beam quality.
In this work, we report an alternative fiber design, a multicore Yb-doped LMA fiber, for efficient cladding-pumped 976 nm fiber laser. The MCF has been extensively investigated over the past decades, since it is a promising approach to mode area scaling which is essential for suppressing unwanted nonlinear effects [15–20]. It was recently reported that a LMA MCF could reach above 100 W mainly in the fundamental in-phase mode at 1 µm [19,20]. In this paper, we investigate the features of LMA MCF for 976 nm lasing. The LMA MCF has a large doping area which leads to a large CCAR, suggesting potential of its application for the three-level lasing system. Furthermore, the LMA MCF is found to have a sensitive wavelength selective and mode selective bending technique enabled by the low NA of its individual cores. These features are found essential to suppress 1030 nm emission and HOMs, thus generating 976 nm lasing efficiently with a number of HOM suppressed. The MCF based 976 nm laser generated up to 25 W output power and a 46% lasing slope efficiency (limited by a pump power).
2. Fiber design and theoretical analysis
The fiber was fabricated by a conventional stack and draw method. First, we etched the cladding layer of a single core Yb-doped fiber preform fabricated by the combination of modified chemical vapor deposition process with chelate delivery system (MCVD-CDS), then cut the single core preform into six pieces. The six Yb-doped rods were placed in a hexagonal ring configuration and inserted into a substrate silica tube together with silica rods. Subsequently it was drawn into an all-solid MCF with a low-index coating in the draw tower. Theoretically, MCF with a hexagonal-ring structure offers the best performance in aspects of power handling and good beam quality in contrast to MCFs with closed-packed hexagonal structures or squared arrays . The fiber fabrication details were reported in . The cross-section microscope image and the three-dimensional (3D) index profile of the MCF are shown in Fig. 1(a) and Fig. 1(b) respectively. It is found that the cores are well placed in a ring with each core having diameter of ∼19 µm and being separated with a 2.3 µm edge-to-edge gap. The one-dimensional (1D) index profile of the MCF, measured across two opposite cores, is shown in Fig. 1(c). It shows that index difference between the core and the cladding is 0.00155 which corresponds to a core NA of ∼0.067 (very similar to the available commercial LMA fibers). The small signal cladding absorption is 3.1 dB/m at 915 nm. The cladding diameter of the MCF is ∼235 µm with 0.45 NA, provided by a low index coating material. Consequently, CCAR of the MCF is 0.04 which is equivalent to a single core fiber (SCF) with a large core to cladding diameter ratio of 1:5 (e.g. 47 µm core diameter if the same cladding diameter (235 µm) is assumed). The V-number of the individual cores is 4.1 at 976 nm, which supports four LP modes of LP01, LP11, LP21 and LP02. Since the six cores are close enough, the modes supported by individual core can form supermodes by evanescent field coupling between adjacent cores.
We use the commercial software COMSOL to calculate the near field supermodes distribution of the MCF. Since each individual core can support four LP modes, four groups of supermodes are formed as LP01 supermodes, LP11 supermodes, LP21 supermodes and LP02 supermodes. We select the supermodes of the four groups which formed by individual cores with the same phase distribution and present them in Fig. 2. The first row shows the mode distribution of the supermodes. The corresponding phase distributions of the four supermodes are shown in the second row of Fig. 2. Furthermore, the LP01 supermodes distribution at near field are shown in Fig. 3. It reveals that the individual cores support LP01 mode, and the phase distribution of the supermodes differs from each other. On the basis of the near field mode distribution, the corresponding far field mode distribution after propagating a distance can be obtained by numerically solving the Fresnel diffraction integral using a standard fast Fourier transform method . It is worth noting that the supermode shown in Fig. 3(a) is called in-phase mode which is of great interest because its far field profile has a dominant central lobe which is bright and nearly diffraction limited . The LP01 in-phase mode can be preferentially selected in a Talbot cavity . In this paper, we resort on a bending technique for mode selection as well as wavelength selection at the same time.
To investigate an effectiveness of the preferential selection via fiber bending, a SCF with a same doping area (23.27 µm core radius), cladding diameter (hence, the same CCAR) and a core NA is employed for comparison. We resort on a finite element method using COMSOL to calculate bending losses at 976 nm and 1030 nm for the MCF (in-phase LP01) and the SCF (LP01). Figure 4(a) presents the calculated bending loss results of the two fibers. It is clearly observed that both of the two fibers show a similar tendency of bending loss increasing with tighter bending, inflicting much larger loss at the 1030 nm than 976 nm. It is noteworthy that bending losses in MCF are much larger than SCF in both wavelengths. The difference is emphasized in Fig. 4(b). The two loss curves at 1030 and 976 nm from a same fiber are subtracted to clearly examine bending loss difference (loss at 976 nm - loss at 1030 nm) for the two fibers. It apparently reveals that the MCF has a stronger ability to suppress the unwanted 1030 nm wavelength for a fundamental mode. We note that the 976 nm LP01 in-phase mode experience non-negligible bending loss per meter at tight bending. Therefore, we only apply the bending to a short section of a MCF (15 cm) in experiment, which introduces severe bending loss at 1030 nm, but insignificant 976 nm bending loss. The detailed experiments are provided in the following section.
In addition to the 1030 nm suppression which is essential for the 976 nm lasing efficiency, a fundamental mode operation is also desirable for better beam quality. We investigate a mode selection based on a bending technique for the considered MCF. Each LP mode supported in the individual cores can interact and form several supermodes with different phase distributions. For example, the LP01 supermode 1 (SP1) is the supermode which is formed by six LP01 modes from the individual cores with the same phase. Hence, the LP01 SP1 is the LP01 in-phase mode as shown in Fig. 3(a). As another example, the LP01 SP6 is the supermode which is formed by the six LP01 modes but with the phase difference of π between two adjacent cores. Hence, the LP01 SP6 is an out-of-phase mode. We calculate the 976 nm bending loss of the LP01 SPs and LP11 SPs in the MCF using COMSOL. Calculation results of the six LP01 SPs are shown in Fig. 5(a). All the SPs present a similar growth tendency of bending loss when bending turns tighter. However, bending losses of the four supermodes (LP01 SP3 - LP01 SP6) increase much faster, indicating possibility of selective suppression.
Figure 5(b) shows bending loss calculation of LP11 SPs, compared to the LP01 SP1 and LP01 SP2. Similar to the LP01 SPs in Fig. 5(a), the bending losses of the LP11 SP3 - SP5 are greater than the LP11 SP1 and SP2. At 2.0 cm bending radius, bending loss of LP11 SP3, LP11 SP4 and LP11 SP5 are at least 1200 dB larger than the LP11 SP1 and LP11 SP2. To have a clear comparison, the LP11 SP1, LP11 SP2 and LP01 SP1, LP01 SP2 are selected for the Fig. 5(c) in a reduced y-axis scale for a larger version. It is clearly observed that the bending losses of LP11 SP1 and LP11 SP2 are nearly 200 dB larger than the LP01 SP1 and LP01 SP2 at 2.0 cm bending radius. To sum up, according to the calculation results shown in Fig. 5, the bending loss discrimination of supermodes are evident when bending radius gets smaller. The LP01 SP1 and LP01 SP2 have the smallest bending loss while the other supermodes have much larger bending losses, which makes them significantly suppressed at tight bending (bending radius < = 2.5 cm). It reveals that mode selection in MCF can be achieved by bending technique which is very beneficial for achieving a good beam quality in this 976 nm fiber laser. It is worth noting here that there are seven more supermodes of LP11 which are LP11 SP6 to LP11 SP12. Their bending losses are not presented in Fig. 5(b), since the LP11 SP6 to LP11 SP12 are suffering super large bending loss and several individual cores in these supermodes hardly confine the energy in their core areas from 4 cm bending radius. It is noteworthy that the bending loss calculation results shown here are in the core-cross bent direction. Bending losses with gap-cross bent direction are calculated as well, the results are very similar to the core-cross bent direction presented here.
3. Experiment results
We provide experimental investigation on 976 nm lasing with a Yb-doped MCF. Firstly, a piece of Yb-doped MCF with 52.5 cm length is employed. Before establishing the laser cavity, the wavelength selective bending technique of the MCF is confirmed by generating an amplified spontaneous emission (ASE) with different bending radii. The MCF is cladding pumped by a 915 nm laser diode with a delivery fiber (0.22 NA, 105/125 µm) without feedback. The forward ASE spectrum is collected and measured by an optical spectrum analyzer (OSA). The spectra are measured with different bending radii and shown in Fig. 6(a). By applying one full round bending in a small section of the whole fiber, the emission at 1030 nm is significantly suppressed, whereas the emission peak at 976 nm is nearly unaffected. As the bending radius reducing to 2.25 cm, around 6 dB suppression at 1030 nm is achieved, and the 976 nm emission peak becomes evidently higher than 1030 nm. Therefore, this 52.5 cm length MCF enables forming a laser cavity without an extra filter for 1030 nm suppression. It is worth noting here that the smallest bending radius in calculation is 2.0 cm, but the smallest bending radius applicable in the experiment is 2.25 cm to prevent fiber breaking. To test laser performance, a resonant cavity is built as depicted in Fig. 6(b). The MCF is bent to one full round with a 2.25 cm radius, hence more than 80% of the fiber is kept straight for being mounted on translational stages. Both of the two fiber ends are flat cleaved and a 90:10 high reflection (HR) mirror directly butts to the pump output end of the MCF. It reflects nearly 90% of the 976 nm lasing signal power and the residual 915 nm pump power back into the MCF. The Dichroic Mirror (DM) 1 and DM2 are employed for coupling a pump light into the MCF, and outcoupling the generated 976 nm signal for characterization. The lenses employed in the pump coupling end have an effective focal length of 8 mm, NA of 0.5 and a 0.23 dB insertion loss. The spatial distribution of the output beam through the 90:10 HR mirror is profiled at the far field with a beam profiler located at 50 mm behind the HR mirror.
The 976 nm lasing spectrum measured at the maximum lasing power after DM1 is shown in Fig. 7(a). The wavelength of the lasing signal is 976 nm and there no lasing signal is observed around 1 µm. The 976 nm lasing slope efficiency is 25% with respect to the launched power as presented in Fig. 7(b). The maximum 976 nm lasing power is 15 W with this 52.5 cm fiber length. We measured the beam profile at the far field after the HR mirror. Similar to , an adjustable aperture is placed before the beam profiler for filtering out the pedestal formed by the supermodes, the power carried by the pedestal is measured to be around 30% of the total power. The measured M2 after the aperture is shown in Fig. 7(c). The mean M2 is 1.91 and a Gaussian-like mode distribution is found as shown in the inset. The beam quality can be further improved by further reducing the V-number of each individual cores and the S2 measurement is necessary to clearly reveal the mode content .
In order to achieve more efficient 976 nm lasing, a laser cavity with a longer piece of MCF (88 cm), hence making stronger pump absorption, is investigated. Firstly, the ASE measurement is done with the 88 cm long MCF. Similar to Fig. 6(a), the emission at 1030 nm is largely suppressed with one full round tighter bending as seen in Fig. 8(a). When the bending radius reduces to 2.25 cm, 5.5 dB suppression at 1030 nm is obtained. However, it can be observed that the 1030 nm emission peak is still higher than the emission at 976 nm even at the 2.25 cm bending radius. Therefore, a short bandpass filter is inserted to further suppress the 1030 nm in the laser cavity as shown in Fig. 8(b). A small section of the MCF in the cavity is coiled to one full round of 2.25 cm bending radius. The pump launching end of the MFC is flat cleaved, while the other fiber end is angle-cleaved to prevent reflection of 1030 nm. A broadband high reflection mirror follows the filter, and it reflects nearly 100% of the generated 976 nm lasing power and the residual pump power back into the MCF. The spectrum of 976 nm lasing at the maximum lasing power is measured. Similar to Fig. 7(a), the lasing wavelength is 976 nm and there no 1 µm lasing is observed. The signal power linearly increases with the launched pump power, marking the slope efficiency to 46% as shown in Fig. 9(a). The maximum 976 nm lasing power is 25 W which is only limited by the available pump power. The Fig. 9(b) shows the measured 976 nm laser beam image after DM1. Since the beam profile of 976 nm laser is imaged by the lens, the collected beam image from DM1 is the near field intensity distribution. Thus, the beam image shown in Fig. 9(b) is very similar to the calculated mode distribution shown in Fig. 3(a). It is worth noting here that the 88 cm length was the optimal fiber length for generating 976 nm laser in our experiments. Increasing the fiber length strengthened the 1030 nm emission which was not able to be suppressed even with the 1030 nm filter.
In conclusion, we demonstrate an efficient 976 nm laser based on a Yb-doped LMA MCF fiber. To the best of our knowledge, this is the first proposal and experiment demonstration of using a piece of MCF to realize 976 nm lasing. In contrast to the commercial LMA YDFs, this MCF has a large CCAR of 0.04 which is helpful for 976 nm lasing. Furthermore, it is found that the wavelength selective and mode selective bending technique of the MCF are beneficial for the efficient 976 nm laser with a number of HOM suppressed. A filter-free laser cavity with a 52.5 cm long MCF generates a slope efficiency of 25% at 976 nm. The cavity can be further simplified by replacing the mirror with a fiber grating or a reflection coating. To achieve higher lasing efficiency, a longer MCF (88 cm) is employed in the laser cavity with an additional 1030 nm filter. This laser setup enables the 976 nm lasing signal power reaching to 25 W (limited by available pump power) with a slope efficiency of 46%.
The authors declare no conflicts of interest.
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.
1. A. Bouchier, G. Lucas-Leclin, P. Georges, and J. M. Maillard, “Frequency doubling of an efficient continuous wave single-mode Yb-doped fiber laser at 978 nm in a periodically-poled MgO:LiNbO3 waveguide,” Opt. Express 13(18), 6974 (2005). [CrossRef]
2. T. D. Raymond, W. J. Alford, M. H. Crawford, and A. A. Allerman, “Intracavity frequency doubling of a diode-pumped external-cavity surface-emitting semiconductor laser,” Opt. Lett. 24(16), 1127 (1999). [CrossRef]
3. J. Boullet, Y. Zaouter, R. Desmarchelier, M. Cazaux, J. Saby, R. Bello-doua, and E. Cormier, “High power ytterbium-doped rod-type three- level photonic crystal fiber laser,” Opt. Express 16(22), 17891–17902 (2008). [CrossRef]
4. V. Pureur, L. Bigot, G. Bouwmans, Y. Quiquempois, M. Douay, and Y. Jaouen, “Ytterbium-doped solid core photonic bandgap fiber for laser operation around 980 nm,” Appl. Phys. Lett. 92(6), 061113–4 (2008). [CrossRef]
5. T. Matniyaz, W. Li, M. Kalichevsky-Dong, T. W. Hawkins, J. Parsons, G. Gu, and L. Dong, “Highly efficient cladding-pumped single-mode three-level Yb all-solid photonic bandgap fiber lasers,” Opt. Lett. 44(4), 807 (2019). [CrossRef]
6. W. Li, T. Matniyaz, S. Gafsi, M. T. Kalichevsky-Dong, T. W. Hawkins, J. Parsons, G. Gu, and L. Dong, “151W monolithic diffraction-limited Yb-doped photonic bandgap fiber laser at ∼978 nm,” Opt. Express 27(18), 24972 (2019). [CrossRef]
7. H. Li, S. Chen, R. Sidharthan, J. Ma, N. Xia, C. J. Chang, J. Kim, and S. Yoo, “Investigation of Core Compositions for Efficient 976 nm Lasing From Step Index Large-mode-area Fiber,” IEEE Photonics Technol. Lett. 32(23), 1457–1460 (2020). [CrossRef]
8. J. Nilsson, J. D. Minelly, R. Paschotta, A. C. Tropper, and D. C. Hanna, “Ring-doped cladding-pumped single-mode three-level fiber laser,” Opt. Lett. 23(5), 355–357 (1998). [CrossRef]
9. S. S. Aleshkina, M. E. Likhachev, D. S. Lipatov, O. I. Medvedkov, K. K. Bobkov, M. M. Bubnov, and A. N. Guryanov, “5.5 W monolitic single-mode fiber laser and amplifier operating near 976 nm,” Fiber Lasers XIII Technol. Syst. Appl. 9728(October 2017), 97281C (2016).
10. S. S. Aleshkina, A. E. Levchenko, O. I. Medvedkov, K. K. Bobkov, M. M. Bubnov, D. S. Lipatov, A. N. Guryanov, and M. E. Likhachev, “Photodarkening-free Yb-doped saddle-shaped fiber for high power single-mode 976-nm laser,” IEEE Photonics Technol. Lett. 30(1), 127–130 (2018). [CrossRef]
11. L. Kotov, V. Temyanko, S. Aleshkina, M. Bubnov, D. Lipatov, and M. Likhachev, “Efficient single-mode 976 nm amplifier based on a 45 micron outer diameter Yb-doped fiber,” Opt. Lett. 45(15), 4292 (2020). [CrossRef]
12. F. Roeser, C. Jauregui, J. Limpert, and A. Tünnermann, “94 W 980 nm high brightness Yb-doped fiber laser,” Opt. Express 16(22), 17310 (2008). [CrossRef]
13. R. Selvas, J. K. Sahu, L. B. Fu, J. N. Jang, J. Nilsson, and A. B. Grudinin, “High-power, low-noise, Yb-doped, cladding-pumped, three-level fiber sources at 980 nm,” Opt. Lett. 28(13), 1093–1095 (2003). [CrossRef]
14. D. B. S. Soh, C. Codemard, S. Wang, J. Nilsson, J. K. Sahu, F. Laurell, V. Philippov, Y. Jeong, C. Alegria, and S. Baek, “A 980-nm Yb-doped fiber MOPA source and its frequency doubling,” IEEE Photonics Technol. Lett. 16(4), 1032–1034 (2004). [CrossRef]
15. Y. Huo, P. K. Cheo, and G. G. King, “Fundamental mode operation of a 19-core phase-locked Yb-doped fiber amplifier,” Opt. Express 12(25), 6230 (2004). [CrossRef]
16. P. K. Cheo, A. Liu, and G. G. King, “A high-brightness laser beam from a phase-locked multicore Yb-doped fiber laser array,” IEEE Photonics Technol. Lett. 13(5), 439–441 (2001). [CrossRef]
17. K. Saitoh, “Multicore fiber technology,” Opt. Fiber Commun. Conf. OFC 201534(1), 55–66 (2015).
18. A. V. Andrianov, N. A. Kalinin, E. A. Anashkina, O. N. Egorova, D. S. Lipatov, A. V. Kim, S. L. Semjonov, and A. G. Litvak, “Selective Excitation and Amplification of Peak-Power-Scalable Out-of-Phase Supermode in Yb-Doped Multicore Fiber,” J. Lightwave Technol. 38(8), 2464–2470 (2020). [CrossRef]
19. J. Ji, S. Raghuraman, X. Huang, J. Zang, D. Ho, Y. Zhou, Y. Benudiz, U. Ben Ami, A. A. Ishaaya, and S. Yoo, “115 W fiber laser with an all solid-structure and a large-mode-area multicore fiber,” Opt. Lett. 43(14), 3369 (2018). [CrossRef]
20. R. Sidharthan, J. Ji, N. Xia, Y. Zhou, J. Zang, X. Huang, W. J. Lai, Y. Benudiz, U. Ben Ami, A. A. Ishaaya, and S. Yoo, “Mode Selection in Large-Mode-Area Step-Index Multicore Fiber Laser and Amplifier,” IEEE Photonics Technol. Lett. 32(12), 722–725 (2020). [CrossRef]
21. P. Zhou, X. L. Wang, Y. X. Ma, H. T. Ma, X. J. Xu, and Z. J. Liu, “Beam quality and power scalability of various multicore fiber lasers,” Chinese Phys. Lett. 26(8), 1–3 (2009). [CrossRef]
22. Y. Li and T. Erdogan, “Cladding-mode assisted fiber-to-fiber and fiber-to-free-space coupling,” Opt. Commun. 183(5-6), 377–388 (2000). [CrossRef]
23. J. W. Nicholson, A. D. Yablon, S. Ramachandran, and S. Ghalmi, “Spatially and spectrally resolved imaging of modal content in large-mode-area fibers,” Opt. Express 16(10), 7233–7243 (2008). [CrossRef]