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A polycrystalline 1.5% Ho: YAG fiber with a diameter of 31 µm was prepared. Surface roughness from grain boundary grooving was reduced by polishing, which decreased the fiber scattering coefficient from 76 m−1 to 35 m−1. Lasing tests were done on this fiber with a SF57 Schott glass cladding. Lasing was confirmed by spectrum narrowing with threshold pump power lower than 500 mW and a slope efficiency of 7%. To our knowledge, this is the first lasing demonstration from a small diameter polycrystalline ceramic fiber.
© 2017 Optical Society of America
1. Introduction
Fiber lasers can be smaller, more reliable, and more powerful than slab lasers. Fiber lasers currently use silica cores and achieve high power in multimode use, but are power limited in single-mode operation necessary for long distance beam propagation [1]. Low silica thermal conductivity causes high thermal gradients at high power, which causes mechanical failure and thermal lensing that degrades beam quality. Stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS) also limit the power of silica based fiber lasers. YAG has much higher thermal conductivity than silica and a lower thermos-optic coefficient [2–5]. This enables high power laser generation without wavefront/modal distortion or mechanical failure. YAG also has a higher laser damage threshold than silica [6,7], and has a peak SBS gain coefficient at least 100 × lower than silica [8–10]. The relative performance of single-mode YAG and silica fibers can be estimated [11,12]. The maximum CW power is about 0.2 kW for a 30 µm diameter silica fiber. Predicted maximum power levels in YAG are 1.1 kW for a fiber with the same diameter, but may be as high as 20 - 50 kW if YAG SBS gain coefficients are 10−12 m/W [8–11].
Melt-grown single-crystal YAG slabs are used in high-power lasers [13]. Single-crystal YAG fiber lasers with diameter >75 µm have also been demonstrated [14–18]. These diameters do not allow single-mode operation, but recently single-crystal fibers with diameters as small as 20 µm have been grown by laser heated pedestal growth [9,10,19,20]. Surface roughening by faceting, cost, and reproducibility are concerns for single-crystal YAG fibers [19]. Despite the issues, doped single crystal YAG fiber with diameters as low as 25 µm and loss coefficients of 0.1 m−1 have been produced [21], typically with growth rates of ~1 mm/min [19], and are recently commercially available, albeit at high cost [20,21]. Lasing has been demonstrated from 20 µm diameter Cr4+: YAG single crystal fiber, but with a cladding that did not support single-mode operation [22].
Polycrystalline (ceramic) YAG slab laser hosts are more attractive than single-crystals for a variety of reasons [23–25]. Laser power >100 kW has been demonstrated in ceramic YAG slabs [25]. Ceramic YAG has higher fracture toughness than single-crystals, allows incorporation of higher concentrations of many dopants, supports higher power densities, and is adaptable to economical processing for many shapes, including fine-diameter fibers.
Despite anticipated advantages, polycrystalline YAG fiber lasers have not been demonstrated, although there have been attempts [26,27]. We report lasing of polycrystalline YAG fiber that has been under development for several years [28–32]. It was difficult to identify features that dominated scattering in these fibers. Surface roughness from grain boundary grooving, index of refraction gradients from compositional inhomogeneity, dopant segregation to grain boundaries, and pore and second-phase size distributions are features that may degrade optical quality. We report the effect of surface roughness reduction by polishing on light propagation in polycrystalline YAG fiber. Lasing in a 31µm diameter surface-polished polycrystalline Ho: YAG fiber with SF57 Schott glass cladding is demonstrated.
2. Experiments
2.1 Fiber processing and characterization
Fiber processing is described in detail elsewhere [32]. Briefly, green fibers were prepared with undoped YAG powder (Nanocerox, Ann Arbor, MI), MethocelTM (E4M grade, Dow, Midland, MI) as a binder, glycerol (Sigma-Aldrich, St. Louis, MO) as a plasticizer, Ho2O3 powder (Sigma-Aldrich, St. Louis, MO) and deionized water. The YAG powder was heat-treated at 700°C in air to remove organics. Heat-treated powder was ball-milled using high purity alumina balls in deionized water. After ball-milling the slurry was centrifuged (Super T21, Sorvall, UK) to classify the powder. Classified YAG powder, filtered MethocelTM solution, glycerol, and Ho2O3 powder (1.5%) were mixed using a planetary vacuum mixer (ARV-310, Thinky, Japan). The mixture was extruded at 20-35 MPa pressure with a high pressure syringe pump (100DM, Teledyne, Lincoln, NE) through a 50µm diameter nozzle.
The extruded fibers were dried at room temperature, burned out at 600°C in air to remove the organics, and densified between 1700 and 1800°C using a 4 × 10−4 Pa vacuum (Centorr, Nashua, NH). The densified fibers were heat-treated at 1400-1500°C in air. The fiber surfaces were mechanically polished to remove grain boundary grooves, using a smooth cloth and 1 and 0.25 µm diamond slurries. Circular cross-section was maintained during polishing. The surface-polished fibers was coated with glass powder (SF57, Schott, Elmsford, NY) and heat-treated at 700-800°C to form a cladding. Fiber ends were polished to improve optical characterization. Final length of the 31-µm diameter fiber was 62 mm. Microstructural features were characterized with a scanning electron microscope (Quanta, FEI, Hillsboro, OR) using 15 kV acceleration voltage. Fiber sectioning was done with a dual beam-focused ion beam (Lyra, Tescan, Czech Republic).
Fiber scattering was characterized using dark-field confocal microscopy, which has been used to study nanoparticle and plasmonic materials [33], as well as features in waveguides [34]. Dark field imaging was done with dual six-axis motorized Thorlabs NanoMax stages with a side-coupled fiber and waveguide measurement station. A single-mode silica fiber was placed on one six-axis stage, and a YAG fiber on the other. This provides six degrees of alignment adjustment for the input, enabling precise alignment. Near-infrared light at 1060 nm was coupled through the passive single-mode fiber into the YAG fiber to facilitate loss measurements. Images were collected using a near-infrared-enhanced CCD camera coupled to a 10 × microscope objective.
The scattering coefficient of the surface-polished fiber was measured using brightness decay in the image taken during 1980 nm laser injection. For unpolished fibers, a cutback method was used with 1480 nm laser injection. This involved loss measurements of fibers of different lengths that were sectioned with a razor blade. Lasing was attempted on surface-polished fiber when scattering losses were found to be low enough.
2.2 Lasing setup
A resonator was formed around the fiber by collimating lenses and flat mirrors at fiber ends to produce a laser cavity. Two set-up variants are shown in Fig. 1. The setup in Fig. 1(a) used two 2.5 cm focal length lenses (L1 and L2), a dichroic mirror M1 (highly reflective (HR)) at 2.1 μm and anti-reflective (AR) at 1.9 μm), and a variable output coupler at 2.1 μm (M2). A 20W thulium fiber laser operating at 1908 nm was the pump source, and was focused to a spot size of approximately 30 μm. The YAG fiber facets were uncoated, resulting in a per-facet Fresnel reflection of approximately 8%, which was used as the cavity mirrors for the Fresnel + Fresnel configuration in Fig. 1(b). In both setups the fiber was mounted on a water- cooled copper mount capable of 5-axis adjustment.
Fig. 1 Configurations of the Ho:YAG fiber laser setup for (a) HR + Fresnel and HR + various R and (b) Fresnel + Fresnel.
3. Results and discussion
3.1. Surface roughness
Surface roughness forms by grain boundary grooving at high temperatures. Significant scattering from rough YAG fiber surfaces was found using confocal microscopy, which is consistent with reports for other fibers [35,36]. A set of images from lengths of unpolished fiber in Fig. 2(a) and polished fiber in Fig. 2(b) illuminated with 1060 nm light is shown in Fig. 2. Particularly bright spots in the images indicate high scattering regions, or “hotspots”, in the fiber. Light travels further and the illumination is smoother in the surface-polished fiber.
Fig. 2 Optical microscope images of the polycrystalline YAG fibers illuminated with 1060 nm (a) before and (b) after polishing.
More highly polished YAG fibers with 25 µm diameter are shown in Fig. 3. Quantitative measurements of surface roughness were made from SEM micrographs of fiber surface cross-sections. These measurements are shown in Fig. 3. The RMS surface roughness decreased from 70 to 30 nm after extensive fiber polishing.
Fig. 3 SEM micrographs of the surfaces of the polycrystalline YAG fibers with different degrees of surface polishing: (a) shows the surface of the as-sintered fiber and (c) is further polished than (b). Surface roughness values of (a) and (c) are given in root mean square (RMS).
3.2. Lasing of surface-polished 1.5% Ho-doped polycrystalline YAG fiber
Figure 4 shows an SEM micrograph of a cross-section of the 31-µm diameter 1.5% Ho-doped polycrystalline YAG fiber used for lasing. The average grain size is 1.5 µm. The image wastaken before surface polishing. Scattering losses of the surface-polished and unpolished fibers were 35 m−1 and 76 m−1, respectively, which are higher than those reported for 20 µm diameter Cr4+: YAG single-crystal fiber that supported lasing [22].
Fig. 4 SEM micrograph of the cross section of the polycrystalline Ho: YAG fiber before surface polishing.
Using an InGaAs detector, the cavity mirrors were adjusted until lasing was observed from the fiber. For initial testing, the pump was chopped at 60 Hz with a 50% duty cycle.
The Fresnel + Fresnel configuration achieved a maximum total output power of 34 mW. This is cumulative power that accounts for the output from both facets. The performance and slope efficiency of each setup can be seen in Fig. 5 and is summarized in Table 1.
Table 1. Summary of laser performance as a function of mirror reflectivities.
The spectral characteristics before and after the threshold were measured and the results are shown in Fig. 6(a). Once the laser threshold was reached, the optical spectrum narrowed as anticipated. While the threshold obtained in this work was larger than the 50 mW threshold reported for 20 µm diameter single-crystal Cr:YAG fiber [22], it is expected that the smaller diameter of the single-crystal fiber should have a lower pump threshold.
Fig. 6 (a) Output spectrum of the fluorescence (blue) and the laser output (red), and (b) Output power as a function of input power for the HR + Fresnel configuration during power scaling efforts.
After initial laser demonstrations, we measured the maximum output power for this fiber laser. The HR + Fresnel configuration was used because it had the best slope efficiency of the two setups in Fig. 1. The 50% duty cycle input power (green) in Fig. 6(b) was measured with a chopped pump (quasi-continuous wave). The data points plotted in red were taken during continuous wave (CW) pumping. A maximum output power of 701 mW was obtained at 10.5 W of incident pump power. At maximum output power the laser had a slope efficiency of 7%, which was higher than the 4% slope at lower output powers (Table 1), but slightly lower than the 7.3% to 11.2% reported for 20-μm diameter single-crystal Cr:YAG [22]. Slope efficiencies from the single crystal fibers with larger diameter are as high as 67.5% from 320 µm diameter of 0.5% Ho-doped YAG fiber [37] and 58% from 100 µm diameter of 1% Yb-doped YAG fiber [38], which implies that there is room for improvements on lasing efficiency of the polycrystalline YAG fibers.
In conventional step-index cladded fibers, the number of modes (N) can be predicted from the fiber factor (V) using fiber core radius (r), light wavelength (λ), and the index of refraction of the core (ncore) and cladding (nclad):
The refractive index of YAG increases by Δn = 0.0028 per 1% of holmium dopant. Using values for 1.5% Ho: YAG fibers [39], ncore = 1.7994, nclad = 1.7896, r = 15.5 µm, and λ = 2.09 µm, the fiber supports ~30 modes; it is not a single mode fiber. The number of modes can be lowered by reducing fiber diameter and increasing nclad by using undoped YAG as cladding.
In future work, fibers with smaller diameter, lower doping concentrations of 0.5% for reduced upconversion loss, and lower surface roughness by further polishing will be prepared for higher efficiency. Methods to apply cladding with higher index of refraction and thermal conductivity are also in development.
4. Conclusion
Using confocal microscopy, fiber surface roughness caused by grain boundary grooving was found to result in significant light scattering in 25 µm diameter polycrystalline YAG fiber. The surface roughness of the fiber was reduced by surface polishing. A 1.5% Ho:YAG polycrystalline fiber with 31 µm diameter and 1.5 µm grain size was prepared, and the surface was polished and glass-clad. Lasing was demonstrated for this fiber at 2091 nm with a threshold lower than 500 mW and 7% slope efficiency.
Funding
United States Air Force (USAF) FA8650-10-D-5226 and FA8650-11-D-5400
Acknowledgments
The work of N. G. Usechak was supported by AFOSR LRIR 16RYCOR332. The views and opinions expressed in this paper are those of the authors and do not reflect the official policy or position of the United States Air Force, Department of Defense, or the U.S. Government.
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