Open Access
Add to BibSonomy
Abstract
In this work, we present a monolithic single-frequency, single-mode and polarization maintaining Yb-doped fiber (YDF) amplifier delivering up to 6.9 W at 972 nm with a high efficiency of 53.6%. Core pumping at 915 nm and elevated temperature of 300 °C were applied to suppress the unwanted 977 nm and 1030 nm ASE in YDF, so as to improve the 972 nm laser efficiency. In addition, the amplifier was further used to generate a single-frequency 486 nm blue laser with 590 mW of output power by single-pass frequency doubling.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
The 97x nm short wavelength laser of ytterbium doped fiber (YDF) has several promising applications [1–3], such as being a high power, high brightness pumping source for Yb and Er, and generating coherent blue light and deep-ultraviolet light. Specially, the single-frequency laser operating at 972 nm can be applied to produce a single-frequency 486 nm blue laser through frequency doubling, which coincides with one of the Fraunhofer lines (i.e., dark lines in solar spectrum) in wavelength and therefore is preferred by deep-sea underwater applications [4]. In addition, the 486 nm laser is required in the spectroscopy research of hydrogen and positronium, e.g. isotope shift [5], 1S–2S transition frequency [6] and hyperfine structure [7].
Compared with those YDF lasers operating around 976 nm, it is more challenging to obtain a 972 nm YDF laser. In addition to the common challenge of an amplified spontaneous emission (ASE) around 1030 nm (a low-threshold emission band of Yb), the ASE around 977 nm is another one for the 972 nm laser, because the emission band of Yb at ∼977 nm has a small FWHM of 8 nm, and the emission cross section (ECS) at 972 nm is reduced to one third of that at 977 nm. However, learning from the achievements of 976 nm YDF amplifiers [8–10], a master oscillator power amplifier (MOPA) with cladding pumping at 915 nm can be used to boost the laser power at 972 nm. To deal with the ASE problems, a YDF with large core-to-cladding diameter ratio is generally required, although it is detrimental to beam quality, to some extent, due to the multimode problem. Besides, a short fiber length is better since the gain of ASE is proportional to the fiber length. In 2016, Z. Burkley et al. reported a 6.3 W single-frequency, polarization maintaining (PM) YDF amplifier (a free-space structure) running at 972.5 nm with a low slope efficiency of 13% [11], in which a YDF with a length of 10 cm and a core/cladding diameter of 20/128 was adopted to suppress the unwanted ASE. Later, the output power was scaled to 10.8 W at 120 W pump power [2]. The nearly 100 W unabsorbed pump power, undoubtedly, will be a trouble to a compact, robust all-fiber structure amplifier.
For a high efficiency 972 nm YDF amplifier, it needs to absorb pump power as much as possible (by improving pump rate rather than doping concentration). Wherein tailoring fibers to own large core-to-cladding diameter ratio is one efficient way. The reported 80/200 rod-type YDF (63% at 979 nm) [1], 105/250 multimode YDF (65.3% at 978 nm) [12], and multicore YDF (46% at 976 nm) [13], are good examples. For a single-mode operation requirement, however, decreasing the diameter of cladding while maintaining a single-mode core is a better way, such as the 14/45 YDF in [9], where a 13 W single-mode 976 nm laser with a good slope efficiency of 31% was demonstrated. But it requires complex processes to build a fiber laser.
Core pumping is another strategy, but smarter one, to improve the efficiency of 972 nm YDF laser, which can offer >100 dB/m pump absorption in single-mode YDF (e.g. 5/125 YDF), two orders of magnitude higher than cladding pumping. The fiber length thus can be shorten to tens of centimeters, which will enable the 97x nm fiber laser to get rid of 1030 nm ASE. To date, the available pump power of a single-mode 915 nm laser diode does not exceed 300 mW. Fortunately, however, 900 nm Nd-doped fiber (NDF) laser, a powerful alternative, can overcome the limitation of pump power [14–16], although it also faces several challenges in power scale-up. This core pumping scheme was previously demonstrated by M. Laroche et al. in a pulsed 978 nm YDF amplifier [17]. In addition, elevated temperature imposed on YDF will cause changes in the absorption cross section (ACS) and ECS of Yb ions [18], which has been applied to obtain a short wavelength YDF laser at 960 nm [19].
In this work, we report on an efficient single-frequency all-fiber 972 nm MOPA, based on a single-mode PM YDF, with core pumping at 915 nm and elevated temperature, and its single-pass frequency doubling to 486 nm blue laser. The 915 nm pumping source is a previously constructed NDF laser with single-mode output, and can provide a maximum output power greater than 10 W. The elevated temperature applied on YDF allows the amplification of 972 nm in a normal length fiber without being seriously affected by the 977 nm ASE. As a result, 6.9 W of output power with a slope efficiency of 53.6% is generated at 972 nm, and ∼600 mW blue laser is obtained at 486 nm using a periodically poled MgO:LiNbO3.
2. Experiment setup
The schematic diagram of the 972 nm single-frequency MOPA is shown in Fig. 1 and can be decribed as follows. The 972 nm seed is a commercial distributed feedback (DFB) diode laser (LD-PD INC), with an output power of 50 mW and a linewidth of < 2 MHz. In order to achieve a high output power, a pre-amplifier was designed. It consists of a 30-cm-long PM YDF (Coractive DCF-YB-6/128S-PM), a PM980 fiber based 915/976 wavelength division multiplexing (WDM), and a 915 nm NDF pumping laser. Two isolates are spliced at the input and output ports of the amplifier to protect the seed and the pre-amplifier from backward-propagation light, respectively. To filter the 977 nm ASE generated in the pre-amplifier which has a high gain in main amplifier, a 972 nm band-pass filter with bandwidth of 2 nm is used behind the output isolator of the pre-amplifier. The main amplifier is the same as the pre-amplifier, and the output end-face is angle-cleaved at 8° to avoid parasitic oscillation around 980 nm. For an elevated temperature, a tubular furnace with 30-cm-long heating zone was applied to heat the two YDFs. A highest temperature of 400 °C was reached in the experiment, so the coating layers of both YDFs, with ignition temperature less than 200 °C, were removed before being put into the tubular furnace.
Fig. 1. Experimental setup of the core-pumping single-frequency YDF amplifier with elevated temperature. ISO: isolator; WDM: wavelength division multiplexing.
In the experiment, the output power was measured with a power meter (Thorlabs S442C) and the optical spectra were recorded by an optical spectrum analyzer (OSA, Yokogawa AQ6370D). Besides, an integrating sphere (Labsphere, 3P-GPS-020-SL) was used in the spectral measurement to eliminate the spatial inhomogeneity of the output laser.
3. Results and discussion
3.1 Influence of temperature on laser performance
A separate online heating experiment was conducted on the pre-amplifier to discuss the influence of temperature (T) on 972 nm output power. Figure 2(a) depicts the monitoring result in terms of output power versus time (measured behind the 972 nm filter), with temperature changing from 20 °C to 300 °C, in which the 915 nm pump power and seed power were fixed at 950 mW and 50 mW, respectively. Initially, the tubular furnace is in a thermal insulation state, and at this moment the 972 nm output power remains at 127 mW. When the heating time goes to 3 min, a heating program starts, which can heat to 300 °C in 30 min. Then there is another keeping warm process.
Fig. 2. (a) Variation of output power with time (or temperature) in online heating experiment; (b) the influence of elevated temperature on laser spectrum; the emission spectrum (c) and absorption spectrum (d) of YDF at different temperature.
As shown in Fig. 2(a), the 972 nm output power increases linearly with time in the heating process, and it gradually increases to 325 mW. As the temperature of tubular furnace changes uniformly with heating time, the output power is linear with temperature in this range with a slope of 0.7 mW/°C, although there is a saturation tendency when temperature closes to 300 °C. After the tubular furnace enters 300 °C for thermal insulation, there is still a small increase in output power. It may be caused by the heat balance between the tubular furnace and the YDF. Finally, the output power stabilizes at 335 mW. In another single-mode amplification experiment with a seed at 972.4 nm, 1.90 W, 1.72 W and 1.54 W output power were obtained at 300, 350 and 400 °C, respectively, for a 2.9 W of pump power. It shows that higher temperature does not mean higher output power, because high temperature may lead to thermal quenching of Yb ions [18]. Thanks to the positive effect of elevated temperature, 2.6 times of output power at 300 °C was obtained compared with that at room temperature.
Figure 2(b) shows the laser spectrum of a single-mode YDF amplifier (with a seed at 972.4 nm) in the range of 150 °C – 300 °C. Obviously, a higher temperature corresponds to a lower 977 nm ASE; on the contrary, the intensity of laser peak at 972.4 nm increases slightly. So the signal-to-noise ratio (SNR) of 972.4 nm is significantly improved, from 10.2 dB at 150 °C to 19.4 dB at 300 °C (nearly two times). The ECS change of Yb ions at elevated temperature is the main reason [18].
To illustrate this point intuitively, the emission spectrum of the 6/128 YDF at T = 20/100/200/300/400 °C was tested, as shown in Fig. 2(c). In the experiment, a short fiber length of 1.5 cm, and a small pump power of 9 mW at 915 nm (core pumping), were adopted. Besides, a filter-type 915/980 WDM was used to extract signal. For T = 20 °C, the intensity at 977 nm (emission peak) is about four times that at 1030 nm, which is close to the ESC ratio of Yb ions, indicating the suitability of the experiment parameters (fiber length and pump power). By comparison, it can be found that the intensity at 977 nm and 1030 nm decrease significantly with the increase of temperature, respectively; however, on both sides of the 977 nm peak (i.e., 970 nm and 990 nm bands), the change is not obvious. The cut-off edge at 965 nm is caused by the 915/980 WDM. The inset in Fig. 2(c) shows a local spectrum near 970 nm. For the wavelength at 972 nm, its intensity does not change within 200 °C, but it decreases at 300 °C and 400 °C. Therefore, the result in Fig. 2(a) can be explained by the reduction of 977 nm emission and rate competition (between 972 nm and 977 nm). In addition, the increase in emission below 970 nm can also be used to develop new YDF lasers with wavelengths < 970 nm.
For a full understanding of the result in Fig. 2(a), the absorption spectrum of the 6/128 YDF, with a length of 3 cm, was measured by using a broadband light source (Thorlabs SLD920P), as shown in Fig. 2(d). The absorption coefficients at 976 nm and 915 nm, two main pump wavelengths for YDF laser, decrease with increasing temperature, respectively; however, that at 960 nm and 990 nm bands increase gradually. Interestingly, there are some special wavelengths, such as 943 nm, 972 nm and 983 nm, which basically do not change with temperature. Two useful data can be obtained from the tested spectra in Fig. 2(d), namely –0.6 dB/m/°C at 976 nm and –0.12 dB/m/°C at 915 nm. In the experiment in Fig. 2(b), we observed the unabsorbed power at 915 nm is in line with the variation of temperature. Therefore, longer fiber lengths should be used at high temperature. Using a pump laser close to 943 nm instead of the 915 nm pump laser can avoid this the problem.
3.2 Single-frequency 972 nm YDF amplifier
Based on the above heating experiment result, an elevated temperature of 300 °C was selected in our 972 nm single-frequency MOPA. The output power of the pre-amplifier with respect to 915 nm pump power is shown Fig. 3(a), which is measured behind the 972 nm filter. An output power of 0.85 W is achieved at a pump power of 2.4 W, corresponding to a slope efficiency of 35.8%. The insertion loss of isolator and band-pass filter decreases the conversion efficiency from 915 nm to 972 nm. For the safety of the MOPA, the output power of pre-amplifier was set at a small power of 0.32 W to avoid possible stimulated Brillouin scattering (SBS), because the passive fiber in MOPA is long and has a small core diameter.
Fig. 3. Output power of (a) pre-amplifier and (b) main amplifier with respect to pump power at 915 nm with an elevated temperature of 300 °C.
In the main amplifier, first, the optical spectrum measured by OSA indicated that the output power contains 15% of unabsorbed pump power at 915 nm. So a long-pass filter (an optical element, not depicted in Fig. 1) was placed in front of the output port to obtain a clear output at 972 nm. As shown in Fig. 3(b), the 972 nm output power increases with increasing pump power, and a maximum output power of 6.9 W is obtained at 12.9 W of pump power with a slope efficiency of 53.6% (optical-to-optical efficiency of 51%). If we subtract the loss of pump power in WDM, the slope efficiency can be calibrated to 57.7%. In other laser experiment around 976 nm, we observed photodarkening effect in the YDF, which also led to the reduction of laser efficiency. By increasing pump power and using a larger core fiber, such as 10/125 YDF, the 972 nm output power can be increased to more than 10 W in the future. In addition, for a further exploration, for example, to simplify experimental conditions, we have tried the method of shortening fiber length of YDF in the main amplifier optical path. At this time, the YDF does not need heat. Finally, a slope efficiency of ∼30% (the output power is 1.9 W) was achieved using a 9-cm-long YDF, but the spectral SNR degraded fast (26 dB at 1.9 W). Maybe there is a trade-off between fiber length and elevated temperature in some practical applications.
Compared with the 915 nm cladding pumping scheme [2], the proposed scheme with both core pumping and elevated temperature is not only easy to obtain single-mode output, but improves the laser efficiency of YDF at 972 nm by several times. As demonstrated in the MOPA, a single-mode 972 nm laser with a laser efficiency of > 50% was generated.
Figure 4(a) displays the laser spectrum of the MOPA at different output power (T = 300 °C), tested by OSA with integrating sphere and a multimode fiber. In addition to the main laser peak at 972 nm, a small but clear ASE signal around 977 nm is observed, and due to the accumulation of upper-level population its intensity increases with the increase of output power or pump power. The small peak at 972.5 nm in Fig. 4(a) is a false signal, which is generated by the cut-off sideband of the 972 nm band-pass filter. It should be noted that the problem of 1030 nm ASE in cladding-pumped 97x nm fiber lasers does not exist here. At the maximum output power of 6.9 W, the SNR is 31 dB. By spectral integration, the energy ratio at 972 nm is calculated to be 99.3%; thus, the energy of 977 nm ASE can be neglected. A single-mode PM980 fiber is used to obtain a fine laser spectrum, as shown in Fig. 4(b). Benefiting from the modification effect of elevated temperature on ECS of Yb ions, the 972 nm YDF amplifier is almost not affected by the unwanted 977 nm ASE, so the amplification factor of the main amplifier reaches 20 times. Without this limitation, a long YDF can be used to fully absorb pump power, thus creating the opportunity to improve laser efficiency at 972 nm.
Fig. 4. Laser spectra of the main amplifier tested by OSA with: (a) integrating sphere and a multimode fiber; (b) a single-mode fiber.
3.3 Frequency doubling to 486 nm blue laser
Further, a simple and compact single-pass second-harmonic generation (SHG) scheme was applied to generate 486 nm single-frequency laser based on the above 972 nm single-frequency MOPA. The experiment setup is illustrated in Fig. 5(a). A periodically poled MgO-doped lithium niobite (MgO:PPLN) crystal with a length of 20 mm is used here, whose end-faces are deposited antireflective coatings at both 972 nm and 486 nm. The crystal is wrapped with indium foil and placed in an oven with a stable temperature of ∼40 °C. In order to prevent backward-propagation light and conveniently control the polarization direction of incident 972 nm laser (by rotating), an isolator and a collimator, both made of PM1060L fiber, are spliced behind the output fiber in turn. The output spot size of the collimator and the focal length of the space lens are 1.3 mm and 40 mm, respectively.
Fig. 5. (a) Schematic diagram of the single-pass generation of 486 nm blue laser (DM: dichroic mirror); (b) output power at 486 nm as a function of the fundamental power at 972 nm; (c) laser spectrum at 590 mW. The inset shows the beam profile of 486 nm blue laser.
The dependence of the 486 nm power with the fundamental power at 972 nm is shown in Fig. 5(b). Influenced by the insertion loss of the isolator and mode area mismatch between PM980 and PM1060L, the total power at 972 nm decreases to 5.4 W. Therefore, a maximum output power of 590 mW is obtained at 486 nm for a 5.4 W fundamental power, corresponding to a conversion efficiency of 11%. Figure 5(b) depicts the laser spectrum of the 486 nm blue laser at maximum power measured by Yokogawa AQ6373B, and the inset shows the beam profile of 486 nm laser which has a Gaussian shape. Current, the output power at 486 nm is mainly limited by the 972 nm laser power, so it is possible to obtain several watts output in the future. As far as we known, 590 mW is the highest output power generated by the single-pass frequency doubling of 972 nm YDF amplifier.
4. Conclusion
In summary, we have demonstrated a monolithic single-frequency MOPA based on a PM single-mode YDF operating in a continuous wave regime at 972 nm. It delivers up to 6.9 W with SNR > 30 dB, diffraction-limited beam quality, and a record high efficiency of 53.6%. Core pumping at 915 nm and elevated temperature strategies were applied to improve 972 nm laser efficiency by suppressing the undesired 977 nm and 1030 nm ASE in YDF. A high temperature of 300 °C was used in the MOPA. To understand the mechanism of high temperature imposing positive effect on the lasing at 972 nm, the emission and absorption spectra of YDF were explored. The reduction of 977 nm emission at elevated temperature is the main reason, which leads to the significant decrease of ECS difference between 972 nm and 977 nm. By using a periodically poled MgO:LiNbO3 crystal and the single-pass SHG scheme, a single-frequency 486 nm blue laser with 590 mW of output power and a conversion efficiency of 11% was demonstrated. To the best of our knowledge, it represents the highest power achieved in such a configuration.
Funding
National Key Research and Development Program of China (2020YFC2200300).
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.
References
1. F. Röser, C. Jauregui, J. Limpert, and A. Tünnermann, “94 W 980 nm high brightness Yb-doped fiber laser,” Opt. Express 16(22), 17310–17318 (2008). [CrossRef]
2. Z. Burkley, A. D. Brandt, C. Rasor, S. F. Cooper, and D. C. Yost, “Highly coherent, watt-level deep-UV radiation via a frequency-quadrupled Yb-fiber laser system,” Appl. Opt. 58(7), 1657–1661 (2019). [CrossRef]
3. W. Li, T. Matniyaz, S. Gafsi, M. T. Kalichevsky-Dong, T. W. Hawkins, J. Parsons, G. Gu, and L. Dong, “151 W monolithic diffraction-limited Yb-doped photonic bandgap fiber laser at ∼978 nm,” Opt. Express 27(18), 24972–24977 (2019). [CrossRef]
4. K. Li, Y. He, J. Ma, Z. Jiang, C. Hou, W. Chen, X. Zhu, P. Chen, J. Tang, S. Wu, F. Liu, Y. Luo, Y. Zhang, and Y. Chen, “A dual-wavelength ocean Lidar for vertical profiling of oceanic backscatter and attenuation,” Remote Sensing 12(17), 2844 (2020). [CrossRef]
5. D. Leibfried, M. Weitz, T. W. Hänsch, and F. Schmidt-Kaler, “Precision measurement of the isotope shift of the 1S-2S transition of atomic hydrogen and deuterium,” Phys. Rev. Lett. 70(15), 2261–2264 (1993). [CrossRef]
6. A. Matveev, J. Alnis, B. Bernhardt, A. Beyer, R. Holzwarth, A. Maistrou, R. Pohl, K. Predehl, T. Udem, T. Wilken, N. Kolachevsky, M. Abgrall, D. Rovera, C. Salomon, P. Laurent, T. W. Hänsch, and C. G. Parthey, “Improved measurement of the hydrogen 1S–2S transition frequency,” Phys. Rev. Lett. 107(20), 203001 (2011). [CrossRef]
7. M. Heiss, G. Wichmann, A. Rubbia, and P. Crivelli, “The positronium hyperfine structure: Progress towards a direct measurement of the 23S1 → 21S0 transition in vacuum,” J. Phys.: Conf. Ser. 1138, 012007 (2018). [CrossRef]
8. N. Valero, C. Feral, J. Lhermite, S. Petit, R. Royon, Y. Bardin, M. Goeppner, C. Dixneuf, G. Guiraud, A. Proulx, Y. Taillon, and E. Cormier, “39 W narrow spectral linewidth monolithic ytterbium-doped fiber MOPA system operating at 976 nm,” Opt. Lett. 45(6), 1495–1498 (2020). [CrossRef]
9. 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), 4292–4295 (2020). [CrossRef]
10. J. Wu, X. Zhu, H. Wei, K. Wiersma, M. Li, J. Zong, A. Chavez-Pirson, V. Temyanko, L. J. LaComb, R. A. Norwood, and N. Peyghambarian, “Power scalable 10 W 976 nm single-frequency linearly polarized laser source,” Opt. Lett. 43(4), 951–954 (2018). [CrossRef]
11. Z. Burkley, C. Rasor, S. F. Cooper, A. D. Brandt, and D. C. Yost, “Yb fiber amplifier at 972.5 nm with frequency quadrupling to 243.1 nm,” Appl. Phys. B 123(1), 5 (2017). [CrossRef]
12. M. Chen, J. Cao, A. Liu, Z. Huang, Z. Pan, Z. Chen, and J. Chen, “Demonstration of kilowatt monolithic Yb-doped fiber laser operation near 980 nm,” Opt. Lett. 46(21), 5340–5343 (2021). [CrossRef]
13. H. Li, J. Zang, S. Raghuraman, S. Chen, C. Goel, N. Xia, A. Ishaaya, and S. Yoo, “Large-mode-area multicore Yb-doped fiber for an efficient high power 976 nm laser,” Opt. Express 29(14), 21992–22000 (2021). [CrossRef]
14. K. Le Corre, H. Gilles, S. Girard, A. Barnini, L. Kervella, P. Guitton, B. Cadier, T. Robin, G. Santarelli, and M. Laroche, “83 W efficient Nd-doped LMA fiber laser at 910 nm,” Proc. SPIE PC 11981, 11 (2022). [CrossRef]
15. M. Laroche, B. Cadier, H. Gilles, S. Girard, L. Lablonde, and T. Robin, “20 W continuous-wave cladding-pumped Nd-doped fiber laser at 910 nm,” Opt. Lett. 38(16), 3065–3067 (2013). [CrossRef]
16. P. H. Pax, V. V. Khitrov, D. R. Drachenberg, G. S. Allen, B. Ward, M. Dubinskii, M. J. Messerly, and J. W. Dawson, “Scalable waveguide design for three-level operation in Neodymium doped fiber laser,” Opt. Express 24(25), 28633–28647 (2016). [CrossRef]
17. M. Laroche, C. Bartolacci, B. Cadier, H. Gilles, S. Girard, L. Lablonde, and T. Robin, “Generation of 520 mW pulsed blue light by frequency doubling of an all-fiberized 978 nm Yb-doped fiber laser source,” Opt. Lett. 36(19), 3909–3911 (2011). [CrossRef]
18. S. W. Moore, T. Barnett, T. A. Reichardt, and R. L. Farrow, “Optical properties of Yb3+-doped fibers and fiber lasers at high temperature,” Opt. Commun. 284(24), 5774–5780 (2011). [CrossRef]
19. J. Y. Yi, Y. W. Fan, and S. L. Huang, “Study of short-wavelength Yb: fiber laser,” IEEE Photonics J. 4(6), 2278–2284 (2012). [CrossRef]
