Yttrium aluminium garnet (YAG) crystal fibers with a core diameter of 40 μm were cladded by high index glasses using the co-drawing laser-heated pedestal growth method. Due to the extremely large cooling rates in the fabrication processes, unexpected and phenomenally large index drops of 0.018 and at least 0.02 were found from the as-grown capillary and the YAG crystal fiber cladding compared with bulk N-SF57’s, respectively. The high-index glass cladding is effective in reducing the number of guided modes, and the intensity profiles of the crystal fiber show there are only four guided modes at 532 nm.
©2013 Optical Society of America
Yttrium aluminum garnet (YAG) has been widely used as a laser host because of its superior optical, thermal, mechanical properties, as well as its plurality in hosting active ions such as Ce3+, Yb3+, Nd3+, Cr4+, Er3+, Tm3+, and Ho3+ ions with fluorescence spectra ranging from 0.5 to 3 μm. The gain medium in a fiber form has the advantages over bulk gain medium including the better thermal management of the fiber structure having a larger surface-to-volume ratio and also large pump absorption. The laser-heated pedestal growth (LHPG) has been shown an effective process for single-crystal fiber growth . Adding a cladding layer to the crystal fiber is helpful in improving the propagation loss and output beam quality. Growth methods of YAG-based crystal fibers with a cladding structure reported include the surface regrowth for dopant out-diffusion method , the extrusion method using high-index glasses as the cladding materials , and the co-drawing LHPG (CD-LHPG) method capable of growing both single- and double-cladding crystal fiber structures [4,5]. The recent advancement on the CD-LHPG technique has results in various applications in broadband light source, laser, and optical amplifier [6–9]. The quality of the crystal fiber fabricated by the LHPG method is confirmed by the record-low pump threshold and record-high slope efficiency of the Cr4+:YAG crystal fiber laser at room temperature . Compared with silica fiber lasers, the crystalline core offers high emission cross section for transition metal ions because of the unique local matrix. Also the thermal conductivity and stimulated Brillouin scattering threshold of the YAG crystal are higher than those of the fused silica. However, so far, the cladded crystal fibers are highly multimode.
The Yb:YAG single-crystal un-clad fibers fabricated by the micro-pulling down (μPD) method have been used as gain medium or as the last stage power amplifier for the 1030-nm high power laser applications recently [10,11]. The pump beams were guided by the crystal fibers of 1-mm diameter but the intra-cavity laser or the signal beam was free propagating inside the fiber in both cases. Nd:GdVO4 single crystal fiber with a core diameter of 3 mm was cladded by pure GdVO4 using the double-die edge-defined film-fed growth (EFG) method . High power Er:YAG lasers at eye-safe 1617- and 1645-nm for remote sensing and ranging applications have attracted intensive research activities in recent years. The laser efficiency degrades when the doping concentration of erbium ions exceeds 1 at.%, due to the energy transfer up-conversion . The Er:YAG crystal fiber provides a solution for achieving sufficient pump absorption, in the case of using low erbium ion concentration Er:YAG crystal. The 0.5% Er:YAG crystal fiber grown by μPD method has a large diameter of 800 μm but no cladding . The Er:YAG double-clad crystalline channel waveguide was fabricated with a core of 500 μm × 500 μm using the adhesive-free bonding method and used in cladding-pump laser at 1645 nm with an improved beam quality . The LHPG method has the advantage of growing crystal fibers with smaller core diameters, compared with other fiber or waveguide fabrication methods.
Here we report the fabrication of YAG crystal fibers with core diameters of 40 μm and cladded by SF57HHT and N-SF57 high-index glasses using the CD-LHPG method for reducing the number of guided modes. A phenomenally large index drop of the glass cladding of the crystal fiber was found due to the extremely large cooling rate in the fabrication process. The measured output intensity profiles at several wavelengths show that the glass-clad crystal fiber is guiding at 532, 633 and 780 nm wavelengths where it should be un-guiding because the index of bulk N-SF57 optical glass is larger than the YAG’s. The output intensity profile at 532 nm shows the glass cladding is effective in reducing the number of guided modes and consists of mainly four modes. The finding of the index drop during CD-LHPG crystal fiber fabrication process is critical in selecting proper glass as the cladding material when further pursuing single-mode crystal fibers operating at the 1-μm Yb:YAG, 1.3- to 1.5-μm Cr4+:YAG, and 1.6-μm Er:YAG laser wavelengths.
2. Theoretical grounds
The normalized frequency, , and numerical aperture, NA, of a glass-clad YAG crystal fiber are defined as:16].
The available glass capillaries for the CD-LHPG process were made of fused silica or borosilicate glass [4–9]. The index of YAG crystal is quite high so the index difference between the YAG crystal core and the glass cladding is large. As a result, the glass-clad crystal fibers are all multi-mode. The N-SF57 (Schott) glass having a comparable high index is chosed as the cladding material. The index dispersion curves of YAG  and N-SF57  are calculated using the Sellmeier equation and shown in Fig. 1. The index of YAG is higher than the index of bulk N-SF57 glass for wavelengths longer than 0.872 μm so the N-SF57-clad YAG crystal fiber should be guiding. The NA of N-SF57-clad YAG crystal fiber was expected to be from 0 to 0.17 for wavelength from 0.872 to 1.7 μm, as shown in Fig. 1. However, a large index drop of the as-drawn N-SF57 capillary was observed which is attributed to the extremely large cooling rate in the drawing process. The fit index curve of N-SF57 capillary to the index measurement data plotted as dots is also shown in Fig. 1. As will be depicted later, an even larger index drop of the N-SF57 cladding of the YAG crystal fiber was found after the CD-LHPG process. The N-SF57-clad YAG crystal fiber is guiding at wavelengths shorter than the original cross-over wavelength at 0.872 μm. SF57HHT glass (Schott) has a refractive index similar to N-SF57’s. The refractive index difference between N-SF57 and SF57HHT is less than 10−4 in the wavelength range from 472 to 812 nm.
The index of an optical glass changes in a heat treatment process. The change of refractive index is related to the ratio of annealing (cooling) rates in the processes in the logarithmic scale :Fig. 2. The index change is compared with an original annealing rate of 7 °C/hr. The thermal treatment induced index change is pretty small, usually in the order of 10−4 to 10−5 .
3. Experimental results and discussion
3.1 Growth process of glass-clad YAG crystal fibers
Pure YAG crystal rods with a 0.5 mm × 0.5 mm rectangular cross-section were used as the source materials for the growth of crystal fibers. Single-crystalline 40-μm YAG fibers were grown at a growth speed of 3 mm/min through a three-step size-reduction process using the LHPG method. Two glasses including SF57HHT and N-SF57 to be used as the cladding materials were made into capillaries for the first time, to facilitate the CD-LHPG process. A 40-μm YAG single-crystal fiber was inserted into the glass capillary. Then the crystal-fiber-filled glass capillary was sealed at one end using the CO2 laser of the LHPG system, and vacuumed through the other end to remove the unwanted air in the gap between the inner wall of the glass capillary and the single crystal fiber. The glass-clad YAG crystal fiber was grown using the CD-LHPG process at a growth speed of 3 mm/min. The thermal parameters of the materials are shown in Table 1. The softening temperatures of the SF57HHT and N-SF57 glasses are 716 and 519 °C , and both are much lower than the 1970 °C melting temperature of YAG crystal . The CO2 laser power during the co-drawing process was controlled so only the glass capillary got softened and attached to the YAG single-crystal fiber, while the crystal fiber remained unmelt, as shown in Fig. 3. During the CD-LHPG process, a very large cooling rate is expected. The reasons are two-fold. The heating zone is small since it is located approximately at the beam waist of the focused CO2 laser. Also the volume of the heated and being grown section of crystal fiber is small since the crystal fiber is so thin and consequently the heat capacitance of the heated section is small. Therefore the temperature gradient is large outside the heating zone and a large index drop of the cladding glass was induced, as described in Eq. (2). Then the index cross-over wavelength of the crystal fiber is likely to move from the original 872 nm toward a shorter wavelength.
3.2 SF57HHT-clad YAG crystal fiber
Due to the material availability, SF57HHT capillary was tried as a preliminary test for cladding the YAG crystal fiber. The SF57HHT glass was made into capillaries with fine machining and torch drawing. The ~1-mm outer diameters are not quite uniform, and the inner hole diameters are also large, i.e. 300 to 500 µm. The length of the SF57HHT-clad YAG crystal fiber is 5.4 cm. The measured propagation loss is 0.1 dB/cm using the cut-back method. The output intensity profiles of the crystal fibers were measured for verification of the numbers of guided modes. As shown in Fig. 4(a), the SF57HHT-clad sample was guiding even at 532 nm wavelength. The index drop of the SF57HHT cladding was at least 0.02. As shown in Figs. 4(b) and 4(c), the patterns of the as-grown sample at 633 and 1064 nm were non-circular, possibly due to the stress induced index non-uniformity of the non-concentric inner and outer diameters. An annealing process with a 24-hour soaking time at the transformation temperature Tg of SF57HHT, and a subsequently controlled slow cooling rate of 7 °C/hr from Tg to 120 °C below it was used for stress releasing and index fine-tuning. The result after annealing showed the SF57HHT-clad YAG crystal fiber sample becomes un un-guiding at 532 and 633 nm. The index drop of the SF57HHT cladding is reduced to no larger than 0.01. As shown in Fig. 4(d), the output intensity profile at 1064 nm of the annealed fiber sample becomes circular and has less speckles, which indicates that the stress was effectively released and the index of glass cladding was increased.
3.3 N-SF57-clad YAG crystal fiber
To improve the cladding uniformity, N-SF57 capillaries were prepared from a drawing tower with inner and outer diameters of 130 and 330 μm, respectively. The length of the N-SF57-clad YAG crystal fiber was 4.8 cm. The endface photo of N-SF57 capillary is shown in Fig. 3(b). The refractive index of the as-grown N-SF57 capillary was measured using the V-block refractometer method at four wavelengths including 441, 639, 947, and 1550 nm . The index dispersion curve of the glass capillary was fit using the Sellmeier equation. Figure 5 shows the wavelength-dependent index drop of the as-grown N-SF57 capillary compared with its bulk index. At 639 and 947 nm, the measured index drops were as large as 0.017, which are at least two or three orders of magnitude larger than the typical data . The index drop decreased only slightly towards longer wavelength when the wavlength was larger than 1 μm, but increased significantly toward shorter wavelength when the wavelength was shorter than 0.6 μm. Large refractive index drops of 0.005 (core) and 0.01 (cladding) were ever reported after a glass fiber drawing process using two different optical glasses as the core and the cladding materials, but were assumed to be constant over the spectrum . The phenomenally large and wavelength dependent index drop of the thermally treated glass is reported for the first time, to our knowledge. The simplification of wavelength-independent index drop is more appropriate for the long (infrared) wavelength range, but is less accurate in the short (visible) wavelength range.
The endface photo of N-SF57-clad YAG crystal fiber grown by the CD-LHPG process is shown in Fig. 3(c). The core of the crystal fiber is not so clear because the indices of the core and cladding are very close to each other. The 23.7-μm eccentricity of the core is due to the large spacing between the 40-μm diameter of YAG crystal fiber and the 130-μm inner diameter of the N-SF57 capillary. Measurement of the surface reflection of a polished crytal fiber endface was attempted for deriving the index profile. However, the surface index of the glass cladding changes as much as 0.03~0.05 after the polishing process . The surface reflection measurement is unable to discern the minute index difference between the crystal core and glass cladding. The output intensity profile of the crystal provides a more convincing information of the guiding condition. Figures 6(a) to 6(c) are the measured intensity profiles at 780, 633, and 532 nm, respectively. It is clear that the number of guides modes decreases at shorter wavelengths, as the difference between the indices of YAG crystal core and N-SF57 glass cladding becomes smaller. Like the SF57HHT-clad sample, the N-SF57-clad sample is also guiding at the short wavelength at 532 nm. And the intensity profile shows the number of guided modes is limited and indeed significantly reduced. Simulation was used to fit the measured profile at 532 nm. The simulated intensity distribution as shown in Fig. 6(d) is a linear combination of the intensities of the lowest four fiber modes, i.e., LP01, LP11, LP21, and LP02 modes. Existence of other higher order modes are possible but with relatively small intensities . The calculated beam quality parameter, M2, based on the measured profile of Fig. 6(c) are about 2.9 . If considering only four guided modes, the normalized frequncy should be less than 5.14. The corresponding NA and index difference between fiber core and cladding are smaller than 0.022 and 1.3 × 10−4, respectively. The index drop of the N-SF57 glass cladding of the crystal fiber sample with respect to its original bulk value is at least 0.02, similar to the value of SF57HHT-clad sample, but larger than the 0.018 index drop of the as-grown N-SF57 capillary as shown in Fig. 5. This implies the cooling rate in the crystal fiber cladding growth process is even larger than that of the glass capillary pulling process. The annealing process was applied to the N-SF57-clad YAG crystal fiber for tuning up the index of the glass cladding, and consequently moving the single-mode or few-mode wavelength toward the longer wavelength. However, the N-SF57 glass cladding is tentative to crack after the annealing process.
In conclusion, we have successfully fabricated a YAG crystal fiber with an N-SF57 glass cladding. The glass-clad crystal fiber was guiding at wavelengths where it was supposed to be un-guiding. A phenomenally large index drop of the N-SF57 glass cladding after the CD-LHPG growth process was unexpectedly found so the guiding region was extended to the visible wavelength range. The effectiveness of reducing the number of guided modes is confirmed by the measured intensity profiles. At 532 nm, the output profile shows that there are four dominant guided modes. To tune the few-mode region toward the longer wavelength, the large index drop of the glass cladding should be taken into account when choosing the suitable glass as the fiber cladding material. The high-index-glass-clad crystal fiber has the potential for designing the large-core gain fibers for high power laser applications with a good beam quality.
This work is partially supported by the National Science Council, Taiwan.
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