1. Introduction

The laser-heated pedestal growth (LHPG) technique is a known crystal growth technique used to make high-quality crystal rods and fibers [1, 2]. In our group, this technique is also being extendedly explored to make single crystal core optical fiber (SCCOF) waveguides, i.e. single crystal core with glass cladding in contrast to conventional optical fiber with glass (amorphous) core and glass cladding [3]. The single crystal core optical fiber can greatly extend the functionality of optical fiber devices because single crystal core possesses commonly better conversion efficiency of lasing and nonlinear optical properties than glass core [4]. In this study, YAG single crystal core with silica glass cladding fibers were fabricated by sapphire tube-assisted CO₂ LHPG technique, namely adopting a sapphire tube as an intermediate heat reservoir to fuse the silica glass and YAG crystal rod together so as to mitigate the power fluctuation of CO₂ laser, other than direct heating on silica/YAG assembly by CO₂ laser in the conventional LHPG technique [5].

In the course of this SCCOF development, one of great concerns of adapting this sapphire tube-assisted LHPG technique is about the final quality of cladded crystal core optical fiber. For instance, the extent of residual stress distribution, induced by the differential thermal expansion coefficients between the crystal core (e.g. ~7 × 10⁻⁶ K⁻¹ in YAG) and cladding (e.g. ~5 × 10⁻⁷ K⁻¹ in silica) during the modified (sapphire tube-assisted) LHPG cladding process, is a crucial concern because its existence affects not only the mechanical integrality of the crystal core optical fiber but also all kinds of optical property [6, 7]. Therefore, seeking for a rapid characterization tool with high spatial resolution down to submicron level is very important for the advancing development of such SCCOF made by the sapphire tube-assisted CO₂ LHPG technique.

It is known that the Nd³⁺ emission of ⁴F_3/2 to ⁴I_9/2 transition is quite sensitive to surrounding stress as Hua and Vohra pointed out in the literature [8]. Therefore, it might be possible to estimate the residual stress reminded in the cladded SCCOF by measuring the Nd³⁺ emission of ⁴F_3/2 to ⁴I_9/2 transition in Nd³⁺:YAG after and before the modified LHPG cladding process. However, in contrast to traditional emission measurements (~µm spatial resolution), a submicron spatial resolution of emission is desired owing to the few microns size of SCCOF in this study. Thus a scanning near field optical microscope (SNOM, NT-MDT/NTEGRA Spectra), which consists of a 532 nm laser beam as excitation source for ⁴I_9/2→²K1_3/2/⁴G_7/2 + ⁴G_9/2 absorptions in Nd³⁺ [9,10] and an IRCCD (ANDOR, DV401-BV) as detector, was employed for providing ~0.2µm spatial resolution for all emission shift measurements in this study [11].

In summary, we extended the sapphire tube-assisted CO₂ LHPG technique to make Nd³⁺:YAG SCCOFs and revealed their residual stress distributions by doping Nd³⁺ in YAG as distributive stress detection probes and hereafter measuring their emission spectra at various positions from the core toward the outer cladding by SNOM. Scanning electron micrographs and elemental line profiles were also acquired with field-emission SEM-EDX (JEOL-6330) to visually characterize the interdiffusion regions of SCCOF. Note that the Nd³⁺ emission of ⁴F_3/2 to ⁴I_9/2 transition can also be aroused up by structural change other than stress alone. Hence, in order to monitor any potential YAG structural variation in the course of fabrication process, their Raman spectra were acquired with a micro-Raman spectroscope (HORIBA, Model HR800) as the first-step screening tool. For the further structural identification, the crystal structures in core, inner and outer cladding regions were characterized with transmission electron microscope (Tecnai, F20 G2 MAT S-TWI).

2. Fabrication

The LHPG process is characterized as float-zone method. The un-melted crystal rod itself supports the molten zone on the top [12], i.e. the molten zone can be held in its place by surface tension without a crucible so that inevitable contamination from the crucible can be totally eliminated. The initial dimensions of [111]-cut 1 wt% Nd³⁺:YAG single crystal rod (CASIX) are 0.5 mm in diameter and 30 mm in length. In order to achieve a few-mode SCCOF, two consecutive reductions of diameter of the single crystal rod from ~250 µm to 70 µm were necessary and performed by repeating the conventional (without the sapphire tube) CO₂ LHPG process twice. Based upon the mass conservation law, the diameter reducing ratio of fiber-to-crystal rod is inversely equal to the square root of fiber-to-crystal rod translation velocity ratio and could be formulated as:

 

Fig. 1 A photograph of Nd³⁺:YAG single crystal rod regrowth with conventional CO₂ LHPG technique.

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To make Nd³⁺:YAG SCCOF waveguides, the sapphire tube-assisted CO₂ LHPG technique was employed. Its schematic layout of setup is shown in Fig. 2.The sapphire tube alleviates the thermal fluctuation from the CO₂ laser power instability because it has higher thermal specific heat [13]. Hence, less temperature fluctuation on fabrication process is obtained with the modified CO₂ LHPG technique. The length of the sapphire tube is 1.6 mm and its outer and inner diameters are 1200 ± 3 μm and 480 ± 2μm, respectively. The Nd³⁺:YAG crystal rod with 70 ± 2 μm in diameter, obtained from the last two consecutive regrowth, was then carefully inserted into a silica capillary tube which has 76 μm inner and 320 μm outer diameter. This assembly was then fused together at a speed of 5 mm/min while the radiation heat emitting from the sapphire tube can soften and then collapse the silica capillary onto the inside Nd³⁺:YAG crystal rod. The radiation heat had caused an inevitable interdiffusion between the center Nd³⁺:YAG and outer capillary tube silica during the modified LHPG cladding process and subsequently resulted in the formation of an inner cladding layer, an outer cladding, and reduced core size of Nd³⁺:YAG from 70 µm to 12 µm; i.e. an inner cladding formed by interdiffusion between the outer layer of the Nd³⁺:YAG and the inner region of the capillary tube silica and an outer cladding which is indeed the outer un-interdiffusion region of the capillary tube silica.

 

Fig. 2 The schematic layout of the sapphire tube-assisted LHPG setup and CO₂ laser beam path.

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3. Measurement and results

As described above, residual stresses can be built up in the crystal core optical fiber and consequently can cause the emergence of anisotropic behavior in the crystal core and furthermore the cracking in the optical fiber if the residual stress exceeds the local bonding strength [14]. A scanning electron micrograph of end face of the crystal optical fiber clearly reveal good quality of SCCOF obtained with proper growth processing control, as shown in Fig. 3(a).The bright, gray, and dark regions represent the core, inner and outer claddings, respectively. The HRTEM image of Nd³⁺:YAG core and inner clad region and selected area electron diffraction (SAED) pattern of the crystal core along <111> crystallographic zone axis, showing [2 −2 0] plane are shown in Figs. 3(b) and 3(c), respectively. The SAED pattern reveals the d₂₂₀ = 4.235 Å of the cladded Nd³⁺:YAG, which is similar d-space as that of pristine pure YAG crystal d₂₂₀ = 4.243 Å (JCPDS #88-2048, 2001) [15]. The clean HRTEM image and closely matching lattice parameters indicate the maintenance of YAG single crystal nature in the core without structural modification caused by the fabrication process with modified CO₂ LHPG technique. In comparison to conventional optical fibers which have always amorphous core, the new type of crystal core optical fiber is very attractive to optical fiber community since many excellent optical properties are only achievable from single crystal core, not amorphous core [16].

 

Fig. 3 (a) SEM, (b) HRTEM image, and (c) SAED pattern along <1 1 1> zone of the core of Nd³⁺:YAG single crystal optical fiber.

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To monitor structural changes of the Nd³⁺:YAG crystal during the modified LHPG cladding process, Raman peaks of the Nd³⁺:YAG crystal rod and the core of silica cladded Nd³⁺:YAG SCCOF were obtained from HORIBA micro-Raman spectroscope that is equipped with a He-Ne laser that provides a spectral resolution below 0.6 cm⁻¹ at 633 nm. The incident light at 633 nm was focused to ~1 μm spot on the end surface of the crystal core fiber at positions 1, 2, 3, and 4 with a 100x objective lens, as depicted in Fig. 4(a).The Raman spectra of the initial Nd³⁺:YAG crystal rod and the crystal core optical fiber at positions 1, 2, 3, and 4, and their corresponding normal mode assignments are plotted in Fig. 4(b). All the Raman peaks of the silica cladded Nd³⁺:YAG crystal fiber are mimic to those of the initial Nd³⁺:YAG crystal rod. The results demonstrate that the core is still a Nd³⁺:YAG crystal without structural modification from the modified LHPG cladding process. This conclusion is important to guarantee the valid of residual stress measurement obtained by detecting the emission shift of ⁴F_3/2 to ⁴I_9/2 transition because the emission shift can also be brought up by structural change as well.

 

Fig. 4 (a) The SEM image of the core of Nd³⁺:YAG SCCOF. The numbers 1, 2, 3, and 4 indicate the positions of Raman and emission acquisition, (b) The Micro-Raman spectra, and (c) The emission spectra of ⁴F_3/2→⁴I_9/2 transitions of initial Nd³⁺:YAG crystal rod and the core of Nd³⁺:YAG SCCOF at positions 1, 2, 3, and 4.

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As shown from previous literature, the most stress sensitive emission of Nd³⁺:YAG is around 950 nm (10.5 × 1000 cm⁻¹), associated with the ⁴F_3/2→⁴I_9/2 transitions [8]. Furthermore, based upon Kobyakov’s report, the value of compressive stress coefficient of the ⁴F_3/2→⁴I_9/2 transition is about 0.75 cm⁻¹/K_bar [17]. Therefore, we can estimate the residual stress built-up in the core by using this relationship, given the magnitudes of the emission shift. Figure 4(c) shows the emission spectra of ⁴F_3/2→⁴I_9/2 transition at various positions inside the core of the SCCOF and initial Nd³⁺:YAG (CASIX), acquired by SNOM. In summary, the residual stress R_stress in Nd³⁺:YAG core is then estimated by [18]:

 

Fig. 5 The emission peak shifts of ⁴F_3/2→⁴I_9/2 transition with respect to that of the initial Nd³⁺:YAG crystal rod and their estimated refractive indices at 1.06 µm on positions 1, 2, 3, and 4 in the core region.

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The Nd³⁺:YAG single crystal core optical fiber made by the sapphire tube-assisted CO₂ LHPG technique indeed have residual compressive stresses built up and subsequently emerges the birefringence (~10⁻³) in the core region. However, the quality of Nd³⁺:YAG SCCOF is excellent without any mechanical damage and the single crystal nature is still maintained after the cladding process. Therefore, the sapphire tube-assisted CO₂ LHPG technique is useful technique for making single crystal core optical fiber waveguides.

4. Conclusion

A Nd³⁺:YAG single crystal core optical fiber waveguide was successfully fabricated by the sapphire tube-assisted CO₂ LHPG technique without apparent damage. The single crystal core and waveguide characteristics, enabled by the silica cladding process, are beneficial to enable and extend many functionalities in optical fiber devices. The reasons are: (1) the single crystal core can provide better lasing and optical nonlinear conversion efficiencies than glass core and (2) the waveguide nature can greatly enhance the interaction length without the shortcoming of divergence in free space. Furthermore, a stress mapping technique with submicron spatial resolution is feasible with Nd³⁺ as distributive stress detection probe and SNOM as enabling submicron–spatial-resolution detection tool, as described in this study.