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Abstract
We report a Yb-doped 25/400 phosphosilicate binary fiber with a pedestal structure by conventional modified chemical vapor deposition (MCVD) technology and solution doping process. Through Ge-doped raised fiber cladding, the fiber provides a low 0.054 core NA. The core dopant concentration of Yb2O3 and P2O5 is estimated to be 0.48 mol% and 7.4 mol%, respectively. It is found that the Yb-doped phosphosilicate binary fiber shows very low photodarkening loss of 3.7 dB/m at 633 nm, and emission spectrum also shows obvious blue shift. Tested in an all-fiber master oscillator power amplifier (MOPA) system, more than 3.2 kW laser at 1046 nm is achieved with a suitable 976 nm pump power injected, the slope efficiency is about 85.8%, and the beam factor of M2 is 1.79.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
High power fiber lasers and amplifiers have a great development in the latest decade, thanks to the high conversion efficiency, small volume, good beam quality, convenient thermal management, and good stability [1–5]. Yb-doped fiber lasers are now widely used in industrial processing, military, medical care, and scientific research [4–8]. With the development of large mode area fibers and pump technologies, IPG Photonics (USA) achieved output power 10 kW [9] under single fiber and single-mode operation, and in 2012, the single mode fiber laser reached 20 kW [10], which has remain to be a record for many years. However, the nonlinear effects [2,3], photodarkening effect [3,11], transverse mode instability [2,12] and high-power laser damage [3,12] limit the further improvement of the output power of a single fiber.
Laser synthesis technology of high-power fiber lasers is considered to be an effective method to scale laser output power and still maintain excellent beam quality [13,14]. Multi-fiber spectral synthesis needs lasers at different wavelengths [13]. The output wavelength of conventional commercial Yb-doped aluminosilicate fiber is typically 1064 nm. There are also some reports on the research for short wavelength emission of Yb-doped fiber, and phosphosilicate fiber with Yb-doped is favorable for the shorter wavelength operation [15,16]. On the other hand, the phosphate glasses have very high solubilities of rare-earth ions [17,18] and no clustering effect, which is beneficial to the inhibition of photodarkening [7,11,19]. Even through Yb-doped multi-component phosphosilicate fibers such as aluminophosphosilicate are widely reported [5,7], there are few reports on phosphosilicate binary fiber, especially for kW power output at short wavelengths.
Doping phosphorus and rare earth materials into the silica glass will increase the numerical aperture of the fiber core and reduce the beam quality of the fiber laser [20,21]. Pedestal structure design is reported by some articles, by improving the refractive index around the core, it can reduce the numerical aperture of the core and achieve high beam quality control [8,22,23]. For the purpose of achieving the output of high power laser at short wavelength with high beam quality, phosphosilicate binary system and pedestal design technologies are combined together.
In this study, Yb-doped 25/400 phosphosilicate fiber with a pedestal structure is prepared by MCVD technology in combination with conventional solution doping process. The fiber core achieves a low NA design through pedestal structure. Blue shift of emission peak is caused by doping a large amount of P2O5 without Al2O3 co-doping. Yb-doped fiber reported in this paper shows that more than 3.2 kW laser output at 1046 nm with a suitable 975 nm pump power is injected, the slope efficiency of the all-fiber amplifier is about 85.8% and the beam factor of M2 is 1.79. This work may provide new insights into the smart design and preparation of Yb-doped double cladding fibers for high power short wavelength lasers.
2. Experimental details
2.1 Preform and fiber fabrication
The Yb-doped phosphosilicate core preform with a pedestal structure is prepared by MCVD and conventional solution doping process. Firstly, germanosilicate glass with a certain thickness and refractive index distribution is deposited in the inner wall of a deposition tube. Secondly, porous phosphorous-doped silica is deposited on the inner wall of the deposition tube and then immersed in a Yb-containing alcoholic solution, followed by drying and sintering steps. Thirdly, the deposited tube is condensed into a solid glass rod at higher temperatures. After jacketed with a suitable silica tube, the perform is ground into an octagonal shape to achieve the design core/cladding ratio (25/400).Then the preform is drawn into a 400 µm (external diameter, flat to flat) fiber with an octagonal shaped cladding at the temperatures of 2050-2250°C and coated with a low index acrylate which provides a pump NA of 0.46.
2.2 Laser performance testing
Laser performances of Yb-doped 25/400 phosphosilicate binary fiber with a pedestal structure are tested by an all-fiber amplifier configuration of bi-directional pump, which is reported in our previous work [4,24] and has a small part of difference, as showed in Fig. 1. The seed laser is a single-mode CW all-fiber laser with output wavelength at 1046 nm controlled by fiber Bragg gratings, and the output power is about 500 W with good beam quality (M2=1.2). The all-fiber amplifier is pumped by twelve pump lasers (output power of ∼400 W each) with central wavelength of 976 nm. The pump delivery fibers of semiconductor lasers are 200/220 µm with NA 0.22 and are fused with the signal fiber of the (6 + 1) x1 fiber coupler directly. 8-meter long Yb-doped phosphosilicate binary fiber is fused between two 25/400 µm passive fiber from input and output of the (6 + 1) x1 coupler. A home-made cladding mode stripper (CMS) is fused between the (6 + 1) x1 fiber coupler and the quartz block holder (QBH) and removes leaked signal light and unabsorbed pump light. The fiber laser beam is collimated and output by a QBH, and it is split by beam splitters and attenuated to mW-lever for spectrum and beam quality analysis. The splice spots of the fiber components take advantage of low fuse-loss as 0.15 dB and are coated with a low index acrylate which provides a pump NA of 0.46 in the all-fiber amplifier system. The pump lasers and the all-fiber amplifier system are cooled by the water chiller in the experiments.
Fig. 1. Schematic diagram of MOPA laser set-up. (LDs: laser diodes; CMS: cladding mode stripper; QBH: quartz block holder; PM: power meter.)
3. Experimental results and discussion
3.1 Fiber characteristics
Figure 2(a) shows the cross-section of a Yb-doped phosphosilicate fiber. The fiber shows a conventional octagonal cladding with a core of 24.8 µm and a cladding of 400.3 µm (flat to flat) in diameter. Unlike commercial octagonal 25/400 µm fiber, the prepared fiber includes a pedestal ring of 50.1 µm in diameter, surrounding the Yb-doped core. For further study and analysis, the measured refractive index profile of the Yb-doped phosphosilicate fiber is given in Fig. 2(b) and is measured by a refractive near field method. It can been seen that the refractive index difference of the Ge doped pedestal structure is ∼0.0077, relative to the silica glass cladding. Refractive index difference between the core and pedestal structure is ∼0.0010, and the NA of the Yb-doped core relative to the pedestal is calculated to be ∼0.054. It is important to note that there exists a central dip at the core center which results from dopant re-evaporation during the core rod collapse process [5,25] and is considered by some to be favorable for the laser output control of few-moded fibers [26,27].
The element distribution of 25/400 Yb-doped phosphosilicate fiber with pedestal structure is showed in Fig. 3, which is tested by an electron probe micro-analyzer (EPMA). Peak concentrations of Yb2O3 and P2O5 in the core are found to be ∼0.48 mol% and 7.4 mol%, respectively. The central dip of P2O5 in the core concentration profiles of Fig. 3(a), which comes from the evaporation of P2O5 in the collapse process [5,25], and core concentration of doped P2O5 reduces to 5.3 mol%. Yb2O3 also decreases in the fiber core center and with a value is 0.17 mol% following the evaporation of P2O5. A remarkable phenomenon is that there is a small amount of P2O5 in the core layer diffusion out the core from the diameter of 25 to 30 µm. This elemental distribution result also conforms with the refractive index distribution as shown in Fig. 2(b). The pedestal structure of 25/400 Yb-doped phosphosilicate fiber is also tested by EPMA, the radial concentration profiles is as showed in Fig. 3(b), and doped concentration of GeO2 is ∼5.9 mol%, with a diameter of 51 µm, which supports the cross section as shown in Fig. 2(b). According to the increment of molar composition of GeO2 (+13×10−4), P2O5 (+9×10−4), and Yb2O3 (+67×10−4) [16,28], the Δn of fiber core and pedestal structure are calculated as 0.0099 and 0.0077 by simple additivity rule, and the calculated Δn supports result of refractive index distribution test as shown in Fig. 2(b).
Fig. 3. Radial concentration profiles of 25/400 Yb-doped phosphosilicate fiber: (a) core: P2O5 and Yb2O3; and (b) pedestal structure: GeO2.
With the cut-back method, the measured cladding pump absorption spectra of Yb-doped phosphosilicate binary fiber is shown in Fig. 4(a). Pump absorption coefficient of Yb-doped phosphosilicate fiber at 915 nm and 976 nm are 0.61 dB/m and 2.20 dB/m, respectively. Figure 4(b) shows the spectra of absorption and emission cross-section of the Yb-doped phosphosilicate fiber, and emission cross-section is calculated by McCumber formulabased on the measured absorption specrum. It is interesting to note that there is a small emission peak around 1006 nm, which indicates that the phosphosilicate binary system exhibits blue shift as compared to alumosilicate fibers of the 1030 nm emission peak [15,29], and shows more beneficial for the short wavelength fiber lasers. Photodarkening effect of the Yb-doped phosphosilicate fiber is examined with a core-pumping configuration, using 976 nm laser as the pump source and 633 nm laser as the detection light source, keeping 50% population inversion during the whole fiber test. The measured temporal excess loss at 633 nm is depicted in Fig. 4(c), which indicates a very low photodarkening loss of ∼3.7 dB/m, and photodarkening test wavelength at 633 nm is 71 times larger than the signal wavelength loss [30], it reflects that the phosphosilicate binary system has a good inhibition effect of photodarkening.
Fig. 4. (a) Absorption spectrum, (b) Absorption and emission cross-section and (c) Photodarkening -induced temporal excess loss at 633 nm under 976 nm pumping of Yb-doped phosphosilicate fiber.
3.2 Laser performance
Fiber laser performance is characterized in a MOPA system as shown in Fig. 1, the output wavelength 1046 nm of seed laser is controlled by fiber Bragg gratings. 8-meter-long Yb-doped 25/400 phosphosilicate binary fiber presents a maximum laser output power of 3.2 kW at 1046 nm as shown in Fig. 5. The linearly-fitted slope efficiency reaches 85.8%. It reflects the lower background loss and higher optical-optical conversion efficiency of the Yb-doped 25/400 phosphosilicate binary fiber.
The black line of Fig. 6(a) shows a typical spectrum curve of 500 W seed light with a 4.7 nm FWHM, and the central wavelength of output laser of seed light is ∼1046 nm. The red line of Fig. 6(a) shows the laser spectrum of Yb-doped 25/400 phosphosilicate binary fiber at 3.2 kW. As it can be seen, the central wavelength of output laser is ∼1046 nm with FWHM of ∼11.3 nm. No other peak is found in the figure, this phenomenon states that the Yb-doped 25/400 phosphosilicate binary fiber shows good nonlinear suppression. Commercial Yb-doped 25/400 aluminosilicate fiber (NA 0.062, Cladding pump absorption 2.20 dB/m@976 nm) is also tested and compared in the same MOPA system, as shown in Fig. 6(b), redundant spectral peaks of ∼1017 nm and ∼1075 nm begin to appear at the output power 1.7 kW (black line of Fig. 6(b)), and become more severe at 2.2 kW (red line of Fig. 6(b)). The additional peaks are caused by four-wave mixing and self-phase modulation. From the results of the emission cross section as shown in Fig. 4(b), Yb-doped phosphosilicate binary system exhibits blue shift of the emission peak as compared to the alumosilicate fibers, thus shows high performance for the short wavelength laser operation.
Fig. 6. Spectrums of the all-fiber laser amplifier at: (a) 500 W seed light and 3.2 kW output laser of Yb-doped 25/400 phosphosilicate binary fiber, (b) 1.7 kW and 2.2 kW output laser of commercial Yb-doped 25/400 aluminosilicate fiber.
The output laser beam quality factor M2 is measured using the Ophir’s Beam Squared system in the experiment. Figure 7 presents the spatial beam quality of the constructed Yb-doped phosphosilicate binary fiber amplifier in this study, giving M2 = 1.79 and 1.77 for X and Y directions respectively, it shows excellent beam quality of the signal light at high power levels. The good beam quality is partly due to the small loss of fiber fusion, and the more important reason is low core NA achieved by pedestal design, which makes it easier to control the beam quality through modal filtering by bending. It shows that pedestal design can effectively control the beam quality. The beam quality of 1046 nm laser from commercial Yb-doped 25/400 aluminosilicate fiber is also analyzed by an Ophir’s BeamSquared system, which is 1.90 under the situation that the output power is 2.2 kW.
4. Conclusion
In conclusion, we adopted a conventional MCVD and solution doping process to fabricate Yb-doped 25/400 phosphosilicate binary fiber, and tested its 1046 nm laser performance. The low core NA is obtained by means of raised Ge-doped cladding as well as Yb:P:Si core. Doping a large amount of P2O5 with 7.4 mol% in the core inhibits the photodarkening attenuation effectively, and causes the emission peak of fiber laser blue shift to 1006 nm. With an all-fiber amplifier configuration, 3.2 kW laser output power is achieved with a slope efficiency of 85.8% at 1046 nm, and the beam factor of M2 is 1.79. The results demonstrate that Yb-doped phosphosilicate binary fiber with a pedestal structure can be a choice for shorter wavelength high power fiber laser applications.
Funding
National Key R&D Program of China (2016YFB0402200, 2017YFB1104400).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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