1. Introduction

Benefiting from the long length, small cross section, light weight, and relatively low cost, glass based optical fibers have considerable advantages as a basic matrix for realizing flexible and economically viable photonic devices. Low-loss silica glass optical fiber waveguides have been widely used in traditional long-haul fiber optical telecommunications [1]. Glass composites are composed of glass and other non-glass materials, for example, polymer, semiconductor, metal, or non-metallic dielectric crystal, and glass composite based optical fibres will allow us to realise compact devices with a wide range of desired functionalities encompassing the areas of optics, electro-optics, magneto-optics, etc. Recently, glass composite based optical fiber waveguides, using the combination of highly dissimilar materials, such as glass/polymer, glass/semiconductor, glass/metal, glass/dielectric crystal, etc, have been put forward as promising approaches to create compact novel photonic devices with multiple functionalities [2,3]. Amongst the variety of non-glass materials, ferroelectric crystals are a particularly interesting class of materials capable of having multiple functionalities in optical waveguides. With built-in electrical dipoles in the crystal structure, the spontaneous polarization in ferroelectric crystals causes structural anisotropy which results in useful nonlinear optical properties, such as the electro-optic effect, harmonic generation and photorefraction [4]. For example, the electro-optic effect, which has a fast response time of picoseconds or less, is used in high-speed devices such as Mach-Zehnder interferometeric modulators and switches.

Essentially the above nonlinear optical properties are based on the second order nonlinearity χ⁽²⁾. The second harmonic conversion efficiency η_2ω/ω of the fundamental wave to the harmonic wave is a good measure for evaluating the χ⁽²⁾ based nonlinear process. In the simple case of plane waves, non-depletable pump, and perfect phase matching, the second harmonic conversion efficiency η_2ω/ω is proportional to IL²(χ⁽²⁾)², where I is the fundamental wave intensity, L is the interaction length of the fundamental wave and the harmonic wave, and χ⁽²⁾ is the second order nonlinear coefficient, respectively [5]. To achieve a high conversion efficiency for second harmonic generation (SHG), a long length of a nonlinear material with a high second order nonlinear coefficient χ⁽²⁾ is required. A fiber-based χ⁽²⁾ nonlinear medium is clearly advantageous as glass optical fibers can be readily drawn to very long lengths. In previous work, thermal or electrical poling methods were applied to glass - mainly silica for the low fiber attenuation - and a second order nonlinearity χ⁽²⁾ be induced in the originally isotropic glass [6]. A relatively high SHG efficiency (15.2%) and high SHG output powers (236mW) were obtained from a 32cm-long periodically poled silica fiber (PPSF) [7]. However, the second order nonlinear coefficient d₃₃ of the poled silica is very low, 0.054pm/V at 1.55µm [7], e.g. almost two orders of magnitude lower than that of the commonly used nonlinear crystal β-BaB₂O₄ (BBO) which has a second-order nonlinear coefficient |d₂₂(1.064μm)| = 2.2pm/V [8]. Furthermore, electrical poling of the silica fiber requires inserting metal wires with 30µm diameter into holes around the fiber core. In order to create a quasi-phase-matching (QPM) structure in the PPSF to compensate the mismatch in the phase velocities of the copropagating fundamental wave and harmonic wave, fiber Bragg grating techniques had to be employed. Both these techniques are time-consuming and inefficient, and limit the useable fiber length in practice.

Instead of a poling approach, glass ceramic technology can create micron to submicron size nonlinear crystalline features inside the glass matrix [4], combining the advantages of glass and nonlinear optical crystals for realizing multiple functionalities in the form of optical fibers. Using the traditional heating approach of glass ceramic technology, nonlinear optical crystals such as BBO, LiNbO₃ (LN), LaBGeO₅ (LBGO) etc have been grown inside the glass matrices of B₂O₃-Al₂O₃-BaO, SiO₂-Al₂O₃-Li₂O-Nb₂O₅ and GeO₂-B₂O₃-La₂O₃, respectively, after reheating the glass bulk at high temperature for several hours [9–11]. A drawback of this traditional heating approach, however, is that the size, orientation, and spatial distribution of the nonlinear crystalline features in the glass matrix cannot be well controlled; it is thus not an ideal method for obtaining the desired glass composite device. Since the nucleation and the subsequent growth of the crystallites are facilitated by energy applied to the glass matrix, other forms of energy should be able to accomplish the same result as heating, e.g. optical and electrical fields. The technique of laser induced crystal growth in bulk glass was developed from 2000 [12]. With laser induced crystallization, the laser beam is focused on a small local area of the glass. From the non-radiative relaxation, the laser photons are converted into phonons, heating up the local area. In this manner, space selective crystalline architectures can be built inside the glass with the use of a femtosecond pulsed laser or continuous wave (CW) laser [12, 13]. The main advantages of the laser induced crystallization are: (1) the laser beam can be localized in an area of the transparent glass and (2) the spot size of laser beam can be extremely small (down to the Rayleigh limit) so that wavelength-scale crystalline features can be created.

Despite the above advantages of the laser induced crystallization method, so far, no success has been reported in creating a nonlinear crystalline architecture in the format of the optical glass fiber due to two contradictory requirements: the thermal stability of the glass host should be sufficiently good for stable fiber drawing; however, the glass should be not too thermally stable in order to facilitate the nucleation and crystal growth inside the glass fiber easily.

One of the ferroelectric crystals, the stillwellite-type (La,Ln)BGeO₅, in which Ln are the lanthanide elements, the group of 15 metallic chemical elements with atomic numbers 57 through 71 from lanthanum through lutetium – has attracted considerable attention since continuous wave green laser emission due to self-frequency-doubling was observed in Nd³⁺ doped LaBGeO₅ single crystals [14–16]. The second-order nonlinear coefficients (d₁₁, d₂₂, d₃₁, and d₃₃) of undoped LaBGeO₅ (LBGO) crystals were reported to be between 0.23 and 0.46pm/V at 1.064μm [8], about one order of magnitude higher than that of poled SiO₂ fiber [7]. The LBGO crystalline phase has been realized in the glass bulk based on the composition of 50GeO₂-25B₂O₃-25(La,Ln)₂O₃ (mol.%) using both reheating and laser inducing methods [11,17]. The 50GeO₂-25B₂O₃-25(La,Ln)₂O₃ glass (LBGO glass) composition is uniquely stoichiometric to the targeted LBGO crystal, important for obtaining single crystalline architecture without the involvement of heterogeneous phases. In addition, the molar percentage of the glass former oxides, GeO₂ and B₂O₃, is high in the glass so quick glass crystallization can be suppressed during the fiber drawing.

In this work, we choose the glass composition of 50GeO₂-25B₂O₃-17.5La₂O₃-7.5Yb₂O₃ (mol.%) (Yb:LBGO) to grow LBGO crystals using the laser induction approach. The replacement of the large amount of Yb₂O₃ to La₂O₃ is to further increase the glass thermal stability. In comparison with other commonly used rare earth dopants, such as Sm³⁺, etc, in the LBGO glass [17], there is only one absorption band peak at 975nm due to the Yb^{3+ 2}F_7/2 → ²F_5/2 transition, indicating low material attenuation for most of the usable wavelength range from the visible to the near-infrared. Using Yb:LBGO glass, ribbon fibers were fabricated. With a CW UV 244nm laser, laser induced crystalline waveguides were created on the surface of the glass ribbon fibers. The crystalline nature was confirmed via micro-Raman spectroscopy. No obvious grain boundaries were observed along the crystalline waveguides. The waveguide properties of the crystalline channel have been measured. To the best of our knowledge, this is the first report of using the laser-inducing approach to fabricate crystalline waveguides in the glass optical fiber format.

2. Fabrication

The LBGO glass was prepared by the conventional melting-quenching method. Stoichiometric amounts of Yb₂O₃, La₂O₃ (99.99%), B₂O₃ (99.99%) and GeO₂ (99.99%) were weighed in the composition of 50GeO₂-25B₂O₃-17.5La₂O₃-7.5Yb₂O₃ (mol.%,) to provide a 70-gram batch, and melted in a platinum crucible at 1450°C for 90 minutes. The melt was then cast into a stainless steel mold, which was preheated at 600°C, to form a rectangular slab preform with dimensions 5x15x75mm. The glass slab preform was annealed at 650°C, around the glass transition temperature T_g, for 2hours and then drawn into ribbon shaped fiber with a width of 450µm and a thickness of 150µm. The yield of the fiber draw was greater than 50 meters.

Figure 1(a) shows the absorption spectrum of the polished Yb:LBGO glass with a thickness of 6.6mm. It is seen that the glass is highly transparent in the range between 500 and 2200nm except for the Yb³⁺ absorption band at 975nm.

 

Fig. 1 (a) Absorption spectrum of LBGO glass doped with 7.5mol.% of Yb₂O₃. (b) Schematic of fabricating laser induced crystalline waveguide on glass ribbon fiber.

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A CW, single polarization 244nm frequency doubled argon laser was employed to create crystalline waveguides on the surface of the ribbon fiber. As schematically shown in Fig. 1(b), the fibers with a length of 15-20cm were fixed on a grooved metal plate and mounted on a programmable, motor-controlled high-resolution XYZ translation stage. The collimated UV laser beam was focused onto the surface of the ribbon through a focal lens. The focused spot size was controlled to be 10µm in diameter. The XYZ stage moved along the length of the ribbon fiber according to a pre-set program. Two key parameters in the experiment were the laser power focused onto the fiber and the scanning speed of the stage along the fiber length (Y direction in Fig. 1(b)).

3. Characterizations of material properties

Figure 2 shows the scanning electron microscope (SEM) photograph of the cross-sectional view of Yb:LBGO ribbon fiber after the exposure from the 244nm laser, for a laser power of 38mW and a scanning speed of 60mm/min. It is seen that a channel area with a width of 10µm is densified after the laser exposure, indicating a positive index change after laser irradiation. It is also seen that there is a curved crack with a radius of 10µm around the laser induced channel (Fig. 2(b)). The crack arose from the mechanical cleaving and is believed to be reflecting the boundary between the thermally stressed area after the rapid laser irradiation and the unstressed area. It can also be deduced that the penetration depth of the UV 244nm laser in the heavily Yb doped LBGO glass is on the order of µm due to the high absorption coeffcient of the Yb:LBGO glass at 244nm (see Fig. 1(a)).

 

Fig. 2 (a) SEM photograph of cross-sectional view of laser irradiated channel on Yb:LBGO ribbon fiber with laser power of 38mW and scanning speed of 60mm/min. (b) Zoom-in view of rectangular area in (a).

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In order to investigate the chemical structural change of the laser irradiated area, micro-Raman spectra of various LBGO samples were measured using a Renishaw Raman Microscope with a depolarized 632.8nm HeNe laser source. From the CCD camera on the Raman microscope, the 632.8nm laser spot size focused on the sample surface was estimated to be between 4 and 5µm for all the measurements below.

Figure 3(a) and (b) compare the micro-Raman spectra of the original Yb-doped LBGO glass ribbon fiber sample without UV laser irradiation and the laser induced channel on the surface of the glass ribbon. Note that the original slab preform and the area outside the laser induced channel on the glass ribbon show the same Raman spectra as the ribbon fiber without the UV laser irradiation, the change on the glass ribbon occurred only after it was irradiated by the laser. It can be seen that with laser treatment under the power of 20mW and scan speed of 60mm/min, multiple sharp Raman lines show up from the Raman shift of 700-1500 cm⁻¹ indicating the formation of crystal. To further understand the crystal formation, an original Yb:LBGO ribbon fiber was reheated at 838°C for 5hours. The Raman spectrum of the fiber after heating treatment is shown in Fig. 3(c). Figure 3(d) shows the Raman spectrum of an undoped LBGO crystal (from Ref [19].). Essentially, the Yb:LBGO fiber after heat treatment shows similar Raman spectral lines as that of standard undoped LBGO crystal, except that the latter has a clearer 870 cm⁻¹ peak arising from the symmetric GeO₄ stretching vibration [18–20], showing that the LBGO crystal grown from the glass matrix by the traditional heating approach possesses less symmetry in the crystal structure than that of a standard crystal. In comparison with the standard LBGO crystal, the laser induced crystalline waveguide in this work shows more sharp Raman lines at (i) 700-800cm⁻¹, (ii) 900-1000cm⁻¹, and (iii) 1200-1400cm⁻¹, which correspond to the GeO₄ stretching vibration, anti-symmetric stretching vibrations of BO₄ tetrahedra, and BO₃ unit vibrations respectively [18–20]. The standard LBGO crystal has the crystal structure of stillwellite and crystallizes at room temperature in the trigonal polar space group P3₁ (C₃²) [21]. It consists of spiral BGeO₅ anion chains interconnected by La³⁺. The chain is built of a BO₄ corner-connected tetrahedron and three neighbouring BO₄ tetrahedra which form one link of the spiral. The outer oxygen atoms of each of two neighbouring BO₄ tetrahedra are interconnected with GeO₄ tetrahedra, which are outside the BO₃ chain. Therefore, it can be deduced that the laser induced Yb:LBGO crystal has a significant amount of antisymmetric stretching vibrations of BO₃, BO₄ and GeO₄ units, indicating structure defects in the crystal. This is probably because the laser induced crystallization here is a very rapid process and the structure relaxation for completing the crystallization cannot be accomplished. In other words, the laser-induced crystalline features are of a highly deformed LBGO crystal structure, or more specifically of deformed BGeO₅ chains.

 

Fig. 3 Comparison of micro-Raman spectra of (a) Yb:LBGO glass ribbon before UV laser irradiation, (b) laser induced channel on the ribbon fiber, and (c) glass ribbon through heating treatment at 838°C for 5hours. (d) Raman spectrum of undoped LBGO crystal [18].

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Figure 4 shows the SEM photographs of the laser induced channels under different laser irradiation conditions. With a laser power of 53mW and a scan speed of 20mm/min, significant grain boundaries and surface cracks can be seen in the irradiated crystalline channel. By reducing the laser power and increasing the scan speed, the grain boundaries within the crystalline channel almost vanish from (a) to (c). In the close-up view of Fig. 4(c), it is seen that there are some features with dimension of ~200nm on the surface of the channel. These may be residual grain boundaries or just surface submicron cracks in the crystalline channel.

 

Fig. 4 SEM photographs of channel waveguides under different laser irradiation conditions. In (a), (b) and (c), the laser power was 53, 38, and 20 mW, and the scanning speed is 20, 60, and 60 mm/min, respectively. Each SEM picture on the right side is zoom-in view of the left one.

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Initial attempts at using electron backscattered diffraction (EBSD) carried out with a field-emission gun scanning electron microscope (FEG-SEM) (model: JEOL JSM-6500F) with resolution down to 1.5nm (for ideal conducting materials) to investigate the nature and orientation of the crystalline channel did not yield the expected EBSD diffraction (Kikuchi) patterns from the top surface of the crystalline channel (shown in Fig. 4(c)). As EBSD occurs at a shallow depth just under the surface of the sample, careful surface preparation is typically needed to remove any remnant deformation layers and surface irregularities (e.g., concentrated hydrochloric acid to etch the crystal surface [17].) However, no such surface preparation was undertaken prior to this preliminary EBSD measurement, because it was difficult to fashion the local area of the micron-size crystalline channel without damaging it. Work is continuing to develop the appropriate surface treatment for analyzing this waveguide structure.

Based on the material characterization to date, we infer that the laser-created waveguide possesses good crystalline characteristics, but further material characterization remains using high resolution electron microscopy, and achieving an acceptable sample surface quality with which to map out the nature and size of the crystalline grain features.

4. Characterization of the optical performance of the crystalline waveguide

Using the optimum irradiation conditions, i.e., laser power of 20mW (corresponding to an intensity of 25kW/cm²) and scan speed of 60mm/min, three crystalline channel waveguides were created on the top surface of the Yb:LBGO ribbon fiber (as shown in Fig. 5(a-c) ). The widths of the channels were measured to be 6.5µm, from the SEM picture in Fig. 5(b).

 

Fig. 5 SEM photographs of (a) cross sectional view and (b) top surface view of ribbon fiber after laser irradiation (Conditions: 20mW and 60mm/min). (c) Optical photograph of top view of the channel waveguides on ribbon fiber. (d) Near field image of guidance at 1060nm after 4.7cm long crystalline channel 1

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Figure 5(d) shows the near field image of the guidance at 1060nm from channel 1 which had a length of 4.7cm (seen in Fig. 5(b) and 5(c)). More than four lower-order optical modes are observed, indicating that the enhancement of the refractive index of the laser induced crystalline core above the glass is > 0.01, corresponding to a numerical aperture of 0.19 or higher. The refractive index n of the LBGO glass at 1.06µm is 1.80 [11]), while the average index (i.e., n_av = (n_o²n_e)^1/3 where n_o and n_e are the refractive indices of the ordinary and extraordinary waves respectively) of LBGO crystal [8,11] at 1060nm is calculated to be 1.813, supporting our estimates.

From scattered light measurements [22], the loss of the crystalline waveguide is estimated to be 3dB/cm, which includes the strong absorption tail of Yb³⁺ at 1060nm arising from the heavy Yb doping level (~19wt.%).

To test the nonlinear behavior of the waveguide, a single transverse mode, linearly polarized, Yb-doped silica fiber based master oscillator power amplifier (MOPA) with a pulse duration of 10ns and repetition rate of 50kHz was used as the pump source. The collimated pump beam was focused onto channel 1 (waveguide length of 4.7cm as shown in Fig. 5(b) and 5(c)). A half-wave plate was put in front of the focal lens. By carefully rotating the half-wave plate, the TE polarization state of the pump was aligned with the TE polarization of the waveguide. The output of the crystalline waveguide was butt-coupled into a conventional silica fiber (SMF28), and then recorded by an optical spectrum analyser (OSA). Owing to the lack of periodic structure built in the channel waveguide needed to satisfy phase matching, no efficient second harmonic generation was observed, apart from weak green light seen at the beginning of the crystalline waveguide. Instead, four-wave-mixing (FWM) took place inside the channel waveguide (see Fig. 6(a) ). The launched pump average power at 1060 nm is estimated to be 200mW (corresponding to the peak power of 400W). Four peaks (at ~1050nm, ~1055nm, ~1065nm and ~1070nm respectively) symmetrically exist on both sides of the 1060nm pump, corresponding to cascaded first and second order FWM processes. The exact location of these peaks, i.e., the 5nm shift between them, is expected to be determined by a phase matching condition involving the waveguide structure as well as the crystal orientation. However, since the latter is currently unknown, a detailed analysis is still ongoing. In addition, the two peaks at the red side of the pump show a stronger intensity than the other two because of the strong Yb absorption fading out from 1000 to 1100nm (see Fig. 6(a)). A fully vectorial finite element method was used to calculate the dispersion profile of the waveguide, Fig. 6(b). The crystal material dispersion [8] was averaged over its orientation, an index step with respect to the glass substrate of 0.013 was assumed, and rectangular waveguide dimensions of 6.5µm by 2.2µm were chosen to match the observed mode profiles (see Fig. 5(d)). The dispersion values for the TE (vertically polarized) and TM polarizations (horizontally polarized) at 1060nm are calculated to be −169 and −173ps/nm/km respectively. Using the formula derived by Boling, Glass, and Owyoung (BGO formula) [23], the average Kerr nonlinear refractive index n₂ of the LBGO crystal is calculated to be 10 x 10⁻²⁰ m²/W, which is almost 5 times higher than that of the fused silica. Note that the BGO formula provides a good approximation for low-index glasses and crystals as well as high-index optical materials [24]. It shows that the LBGO crystalline waveguide also has the potential to be a promising Kerr nonlinearity χ⁽³⁾ based device.

 

Fig. 6 (a) Bulk Yb:LBGO absorption spectrum and spectral trace (with 1nm resolution) of four wave mixing observed after 4.7cm long crystalline channel 1 using the 1060nm pulsed laser source; (b) Calculated dispersion curves of TE-TM polarizations of LBGO crystal/glass waveguide (Inset: mode fields of fundamental TE/TM modes.)

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5. Conclusions

We report the first fabrication of a laser-induced crystalline waveguide in the traditional glass optical fiber format. The glass and crystal are based on the composition (La,Yb)BGeO₅. Laser induced waveguides were fabricated on the surface of the ribbon glass fiber using tens of milliwatts of continuous wave UV laser radiation and a fast scanning speed of >5cm/min. The crystalline nature of the created material architecture was observed using micro-Raman spectroscopy and scanning electron microscopy. From the optical characterization results, this crystalline waveguide in fiber format is potentially promising for realizing nonlinear optical devices. However, the waveguide loss will need to be reduced to a level between 0.1 and 0.5dB/cm before this novel optical waveguide can be practical. The concept of laser-induced crystalline waveguide in optical glass fiber format opens up the prospect of fabricating functional photonic circuits in a traditional glass optical fiber format in an efficient and cost-effective way.