- Email: [email protected]
Laser-induced nonlinear crystalline waveguide on glass fiber format and diode-pumped second harmonic generation
Optical Fiber Technology 41 (2018) 118–124
Contents lists available at ScienceDirect
Optical Fiber Technology journal homepage: www.elsevier.com/locate/yofte
Regular Articles
Laser-induced nonlinear crystalline waveguide on glass fiber format and diode-pumped second harmonic generation Jindan Shia,b, Xian Fengc,
T
⁎
a
Jiangsu Key Laboratory of Advanced Laser Materials and Devices, School of Physics and Electronic Engineering, Jiangsu Normal University, Xuzhou 221116, China Jiangsu Collaborative Innovation Centre of Advanced Laser Technology and Emerging Industry, Jiangsu Normal University, Xuzhou 221116, China c Laser Institute of Engineering, Beijing University of Technology, 100 Pingleyuan, Chaoyang District, Beijing 100124, China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Second harmonic generation Laser induced crystallization Fiber design and fabrication Laser direct writing
We report a diode pumped self-frequency-doubled nonlinear crystalline waveguide on glass fiber. A ribbon fiber has been drawn on the glass composition of 50GeO2-25B2O3-25(La,Yb)2O3. Surface channel waveguides have been written on the surface of the ribbon fiber, using space-selective laser heating method with the assistance of a 244 nm CW UV laser. The Raman spectrum of the written area indicates that the waveguide is composed of structure-deformed nonlinear (La,Yb)BGeO5 crystal. The laser-induced surface wavy cracks have also been observed and the forming mechanism of the wavy cracks has been discussed. Efficient second harmonic generation has been observed from the laser-induced crystalline waveguide, using a 976 nm diode pump. 13 μW of 488 nm output has been observed from a 17 mm long waveguide with 26.0 mW of launched diode pump power, corresponding to a normalized conversion efficiency of 4.4%W−1.
1. Introduction Since the first observation of second harmonic generation (SHG) from a quartz crystal pumped by a ruby laser in 1961 [1], χ(2) based second-order nonlinear optical effects have been of great interest, especially for making the efficient, powerful and compact solid state ultraviolet (UV) and visible laser sources. This is currently mainly driven by many industrial applications, for example, micromachining using photolithograph in the semiconductor industry using UV lasers and laser display using the primary colours of red, green and blue (RGB) lasers. In the simple case of plane waves, non-depletable pump, and perfect phase matching, the second harmonic conversion efficiency η2ω/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 [2]. To achieve a high conversion efficiency for second harmonic generation (SHG), a long length of a nonlinear material with a high secondorder nonlinear coefficient χ(2) is required. Also small spatial mode size helps to enhance the fundamental wave intensity I. A fiber-based χ(2) nonlinear medium is thus an ideal medium format. Glass optical fibers have been successfully employed in the long-haul and short-haul optical communications, since (i) glass is a material readily drawn to very long lengths and (ii) the loss of the conventional glass optical fiber is low.
⁎
Unfortunately, glass is an isotropic material and thus the χ(2) of an untreated glass is nil. In previous works, thermal or electrical poling methods [3] were applied to fiber to induce a second-order nonlinearity χ(2) into the originally isotropic glass fiber. A relatively high SHG efficiency (15.2%) as well as high SHG output power (236 mW) was obtained from a 32 cm-long periodically poled silica fiber (PPSF) [4]. However, the second-order nonlinear coefficient d33 of the poled silica is very low, only 0.054 pm/V at 1.55 µm [4], almost two orders of magnitude lower than that of the commonly used nonlinear crystal βBaB2O4 (BBO) with a second-order nonlinear coefficient |d22(1.064 μm)| of 2.2 pm/V [5]. Furthermore, electrical poling of the silica fiber requires inserting metal wires with 30 µm diameter into holes around the fiber core [4]. In order to create a quasi-phasematching (QPM) structure in the PPSF to compensate the mismatch in the phase velocities of the copropagating fundamental wave and harmonic wave, fiber Bragg grating technique had to be employed as well. Both these techniques are time-consuming and inefficient and thus largely limit the useable fiber length in practice. Glass, known as a supercooled liquid, is thermally metastable. In other words, the glass has higher energy than the crystal, which is a true thermodynamic stable phase. When a type of physical energy, for example the heat, is applied on the glass, and let the glass temperature above its transition temperature Tg, which is the boundary between solid and liquid, the glass will have opportunity to be crystallized. In
Corresponding author. E-mail addresses: [email protected] (J. Shi), [email protected] (X. Feng).
https://doi.org/10.1016/j.yofte.2018.01.015 Received 1 December 2017; Received in revised form 14 January 2018; Accepted 16 January 2018 1068-5200/ © 2018 Elsevier Inc. All rights reserved.
Optical Fiber Technology 41 (2018) 118–124
J. Shi, X. Feng
the crystallization process, the external energy drives the atoms to overcome the energy barrier and consequently transfer through the solid/liquid interface. The atoms from the liquid thus get rearranged in a certain periodic way to form crystal. The traditional glass ceramic technology is such a way to control nucleation and crystallization of glass [6]. In the traditional glass ceramic technology, the whole glass workpiece is reheated in a high temperature furnace. More generally, one can find other alternative approaches to apply certain types of physical energy onto the whole piece of glass or the selected local area on the glass. For example, high-energy laser beam can be focused on the glass and the laser energy is converted to the heat at the selected micron-size domain. The local solid glass becomes into molten state and then the desired crystalline phase can be formed along the laser scanning direction. This method is called as space-selective laser heating [7]. In this work, we report a diode-pumped self-frequency-doubled nonlinear crystalline waveguide on glass fiber format. A ribbon fiber has been drawn on the glass composition of 50GeO2-25B2O325(La,Yb)2O3. Surface channel waveguides have been written on the surface of the ribbon fiber, using space-selective laser heating method. The crystalline nature of the crystalline (La,Yb)BGeO5 waveguide has been analyzed by the micro-Raman spectroscopy. The laser-induced surface wavy cracks have also been observed and the forming mechanism of the wavy cracks has been discussed. Efficient second harmonic generation has been observed from the laser-induced crystalline waveguide, using a CW diode pump.
Fig. 1. Schematic diagram of fabricating crystalline waveguide on ribbon fiber using space-selective laser heating.
2. Experiment
the ribbon fiber, confirmed by the observation of micro-Raman spectroscopy.
2.1. Glass preparation and fiber drawing
2.3. Crystallinity characterizations using Raman spectroscopy
Stoichiometric amounts of Yb2O3 (Alfa Aesar, 99.99%), La2O3 (Alfa Aesar, 99.99%), B2O3 (Alfa Aesar, 99.99%) and GeO2 (Alfa Aesar, 99.99%) were weighed in the composition of 50GeO2-25B2O3-24La2O31Yb2O3 (mol.%) (Yb1:LBGO glass) to provide a 70-gram batch, and melted in a platinum crucible at 1450 °C for 90 min. The melt was then cast into a stainless steel mould, which was preheated at 600 °C, to form a rectangular slab preform with dimensions of 5 × 15 × 75 mm. The glass slab preform was annealed at 650 °C, around the glass transition temperature Tg, for 2 h and then drawn into ribbon shaped fiber with a width of 450 µm and a thickness of 150 µm. The yield of the fiber draw was greater than 50 m. The transmission spectrum of a polished Yb1:LBGO glass with a thickness of 5.11 mm was measured by a CARY UV-VIS-NIR spectrometer. The Differential Thermal Analysis (DTA) curve of Yb1:LBGO glass powder is measured by PerkinElmer DTA7 with a ramp rate of 10 K/ min.
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 532 nm laser source. With the assistance of a CCD camera on the Raman microscope, the spot size of the 532 nm laser focused on the sample surface was estimated to be between 4 and 5 µm for all the measurements below. In addition, Yb1:LBGO fibers are reheated for crystallization in a small home-made furnace at 780 °C for a certain period from 0.5 min to 5 h. The Raman spectra of these samples are measured. 2.4. Second harmonic generation A fiber-pigtailed 976 nm diode laser was used as the pump source for SHG, as seen in Fig. 2. A half wave plate was placed in front of the ribbon fiber to control the polarization of the input light. The collimated pump beam was focused into the crystalline waveguide on a 17 mm long ribbon fiber. A CCD camera was used to monitor the nearfield image from the waveguide output, ensuring that the pump light
2.2. Laser-induced crystalline waveguide on glass fiber format A CW, single polarization 244-nm frequency-doubled argon-ion laser was employed to create crystalline waveguides on the surface of the ribbon fiber. As schematically shown in Fig. 1, the fibers with a length of 15–20 cm 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 10 µm in diameter. The XYZ stage moved along the length of the ribbon fiber according to a pre-set program. Laser power focused onto the fiber and the scanning speed of the stage along the fiber length (Y direction in Fig. 1) were the two key parameters in the above process. The power of the exposed UV laser was 20-70mW and the stage scanning rate was 10–120 mm/min. As described in Ref. [8], the absorbed UV laser energy was transformed to thermal energy and the stoichiometric nonlinear crystal (La,Yb)BGeO5 (Yb1:LBGO) was created on the surface of
Fig. 2. Schematic diagram of laser diode pumped SHG from crystallized LBGO fiber.
119
Optical Fiber Technology 41 (2018) 118–124
J. Shi, X. Feng
Fig. 3. Transmission spectrum of Yb1:LBGO glass with a thickness of 5.11 mm.
Fig. 4. DTA curve of Yb1:LBGO glass powder with ramp rate of 10 K/min.
was focused into the waveguide. The output spectrum was then collected by a multimoded silica fiber with a core diameter of 100 µm by butt-coupling and recorded by an Ocean-Optics HR4000 miniature spectrometer ranging from 200 to 1100 nm. A 3 mm-thick band-pass filter (Thorlabs FGS900, 315–710 nm BPF) was then added directly after the ribbon fiber to block the laser radiations beyond 900 nm. A power meter with high sensitivity was put after the filter to measure the output power of the generated SHG signal. The launching efficiency was estimated to be 5%. 3. Results and discussion 3.1. Laser-induced crystalline waveguides under various conditions Fig. 3 shows the measured transmission spectrum of the polished Yb1:LBGO glass with a thickness of 5.11 mm. It is seen that the glass is highly transparent in the range between 400 and 2200 nm except for the Yb3+ absorption band (2F7/2 → 2F5/2 transition) peaking at 975 nm. The bandgap of the glass is located at ∼330 nm. The penetration depth of the 244 nm laser on the glass is estimated with the order of tens of microns, according to the transmission curve. Fig. 4 plots the measured DTA curve of Yb1:LBGO glass powder. It is seen that the glass transition temperature Tg is 675 ( ± 2) °C and the onset of crystallization Tx is 790 ( ± 2) °C. Note that the glass transition temperature Tg is the transition between the solid glass state and the liquid state. Thermodynamically, the solid-state glass should be given with some certain energy to cross the energy barrier towards the liquid state. On the DTA curve shown in Fig. 4, the temperature of Tg is determined by the intersection of the two straight lines when the endothermal process happens. The onset of the crystallization temperature Tx, which instead is an exothermal process, is determined by the same way. Fig. 5 shows the optical photographs of the laser-induced channels under different laser irradiation conditions. Table 1 summarizes the laser writing conditions and the outcome of the channel waveguides. With a laser power of 70 mW and a scan speed of 20 mm/min, significant grain boundaries and periodic surface wavy cracks can be observed in the irradiated crystalline channel (see Fig. 5(a)). The width of the channel waveguide is 12.7 µm, which is close to the focused spot size of 10 ± 1 µm. Out of the channel, one can see that there is an area with overheated trace. This area is formed due to the thermal diffusion from the central molten channel to the side solid glass area. The thickness of the thermal diffusion layer here is 17.3 µm. By reducing the laser power down to 38 mW and increasing the scan
Fig. 5. Optical photographs of top view of laser-induced waveguides on ribbon fiber. The writing conditions of (a)-(e) are seen in Table 1. Note that the UV laser scans from the left to the right.
120
Optical Fiber Technology 41 (2018) 118–124
J. Shi, X. Feng
Table 1 Laser writing conditions and waveguide parameters. Sample number
a b c d e
Laser power mW
Scanning speed mm/min
70 38 38 20 20
20 120 10 120 60
Waveguide width (µm)
12.7 9.7 11.4 6.2 8.2
Table 2 Assignation of Raman bands of LBGO samples [12]. Thermal diffusion thickness (µm) 17.3 7.6 10.5 / /
Raman shift
Assignation
300–400 cm−1 550 cm−1
Bending vibrations of GeO4 tetrahedra Combination of the bending vibrations of T–O–T (T = Ge or B) bonds GeO4 stretching vibrations Symmetric BO4 and GeO4 stretching vibrations Antisymmetric stretching vibrations of BO4 tetrahedra
700–800 cm−1 800–900 cm−1 900–1000 cm−1
interconnected by La3+. The BGeO5 chain is built of one [BO4] cornerconnected tetrahedron and three neighbouring [BO4] tetrahedra which form one link of the spiral. The outer oxygen atoms (O2) of each of two neighbouring [BO4] tetrahedra are interconnected with [GeO4] tetrahedra, which are outside the [BO3] chain [12]. It is seen from Fig. 6 that the main Raman band of the LBGO glass is located at 800 cm−1, which is arising from the bone structure (i.e., spiral [BGeO5] chains) of LBGO glass [11,12]. After laser irradiation, sharp peaks at 720 cm−1, 750 cm−1, 890 cm−1 and 910 cm−1 appear above the 800 cm−1 band, indicating crystallization occurring. These peaks are due to the enhanced [GeO4] stretching vibrations, and symmetric [BO4] and [GeO4] stretching vibrations (see Table 2). It is therefore believed that with laser irradiation, a significant amount of antisymmetric stretching vibrations of [BO3], [BO4] and [GeO4] units form on the spiral [BGeO5] chains, because the laser-induced crystallization is a very fast procedure and the structure relaxation for completing the crystallization cannot be accomplished [8]. Fig. 7 illustrates the evolution of micro-Raman spectra of Yb1:LBGO glass fiber reheated at 780 °C for 0.5 min–5 h, compared with the untreated Yb1:LBGO glass fiber and the standard LBGO crystal [11]. The Raman intensity of each trace has been normalized at the 800 cm−1 peak. The reheating temperature is set 10 °C below the crystallization onset Tx of the Yb1:LBGO glass, for clearly investigating the evolution of the crystallization under high temperature. It is seen that obvious crystallization occurs after the sample has been reheated between 2 and 5 min. With the reheating time increase from 5 min to 5 h, essentially the relative intensity of 850 cm−1 band enhances, indicating the relaxation of symmetric [BO4] and [GeO4] stretching vibrations. But compared with the standard LBGO crystal, even the sample reheated for 5 h still does not show the minor peaks at 350 cm−1, 550 cm−1, 900 cm−1 (marked with stars in Fig. 7), which are arising from the bending vibrations of [GeO4] tetrahedra, the combined bending vibrations of Ge–O–Ge and B–O–B bonds, and antisymmetric stretching vibrations of [BO4] tetrahedral, respectively. It means that the actual relaxation of crystal structure takes time. It is therefore not surprising that laser-induced crystallization tends to form defects or deformed structures in the crystal, because the time of the laser-induced
speed to 120 mm/min, the width of the waveguide channel narrows down to 9.7 µm and a damped periodical wavy crack is seen inside the channel area, extending along the laser scanning direction (see Fig. 5(b)). The thickness of the thermal diffusion layer is 7.6 µm. When the laser power is 38 mW and the scan speed is 10 mm/min, the width of the waveguide channel is measured to be 11.4 µm. A straight wavy crack is seen nearly in the middle of the channel (see Fig. 5(c)). The thickness of the thermal diffusion layer is 10.5 µm. By further lowering the laser power down to 20 mW, proper channel waveguides without obvious thermal diffusion layer are formed (see Fig. 5(d) & (e)). With a fast scanning speed of 120 mm/min (see Fig. 5(d)), the waveguide shows relatively dim. With a slower scanning speed of 60 mm/min (see Fig. 5(e)), the center of the channel waveguide is bright with good contrast, indicating that it is the optimum condition for making laser-induced crystalline waveguides with decent quality.
3.2. Crystalline nature of laser-induced waveguide on fiber Fig. 6 illustrates the micro-Raman spectra of the original Yb-doped LBGO glass ribbon fiber sample without UV laser irradiation and the laser induced waveguide on the surface of the glass ribbon. The Raman spectrum of the standard LBGO crystal [11] is plotted on Fig. 6 as well. Table 2 lists the assignation of the Raman bands of LBGO glass and crystal provided by Ref. [12]. Note that the original preform and the area outside the laser induced waveguide on the glass ribbon show the same Raman spectra as the ribbon fiber without the UV laser irradiation, indicating that no crystallization occurs during the fiber drawing. It is known that standard LBGO crystal has the crystal structure of stillwellite and crystallizes at room temperature in the trigonal polar space group P31 (C32) [12]. It consists of spiral BGeO5 chains
Fig. 7. Evolution of micro-Raman spectra of Yb1:LBGO glass fiber reheated at 780 °C for 0.5 min–5 h, in comparison with untreated glass fiber and standard LBGO crystal [11].
Fig. 6. Comparison of micro-Raman spectra of Yb1:LBGO glass ribbon before and after UV laser irradiation, and undoped LBGO crystal [11].
121
Optical Fiber Technology 41 (2018) 118–124
J. Shi, X. Feng
other than LaBGeO5 between 700 and 900 cm−1 were observed. 3.3. Analysis of wavy cracks and thermal model of laser-induced crystallization As illustrated in Fig. 8(a), during the scanning of the 244-nm laser on the surface of the glass ribbon fiber, the solid glass absorbs the exposed laser power and converts it into heat. The local area is then molten. After the laser spot moves forward, the molten area is cooled down. At the same time, because only a slot area is molten from the laser exposure, huge temperature contrast exists between the molten area and the surrounding glass. The formation of such wavy (or oscillatory) cracks is due to the thermal stress arising from the rapid cooling rate of the molten area during solidification and the thermal mismatch of the thermal expansion coefficients of the crystalline phase and the surrounding glass [14–17]. To obtain a crack-free waveguide with desired crystalline phase and crystallinity from the glass matrix, the exposed laser power and the scanning speed are the two crucial parameters, because they determine the initial temperature and the cooling rate of the melt. If the laser power is too high, the starting temperature of the melt will be too high and the molten area will be largely expanded out of the laser spot due to the thermal diffusion. The cooling rate in the local area will then be highly uneven because of the large temperature gradient. Undesired crystalline phases and/or poor crystallinity might be formed. So might the cracks. When the cooling rate is too fast, the molten state might be quenched to glass state again and also the crack might occur. But if the cooling rate is too slow, the heat converted from the laser power might not be proportionally accumulated, due to the thermal diffusion rate of the glass and the cooling effect from the air moving above the glass. When the laser power and the scanning speed are 70 mW and 20 mm/min respectively, a wavy crack with a periodicity of 180.1 µm is formed (see Fig. 8(b)). Since the laser spot focusing on the surface of the glass fiber is a transient thermal source (see Fig. 8(a)), such a periodic crack forms only when the crack propagation can keep pace with the laser scanning. Therefore the time to form each period of the wavy crack is calculated to be 0.54 s. When the laser power is reduced down to 38 mW, a periodically damped crack with a periodicity of 54.8 µm is formed with the laser scanning speed of 120 mm/min (see Fig. 8(c)). The decrease of the laser power reduces the temperature in the exposed area and the increase of the scanning speed enhances the moving speed of the front of the molten area and the solid glass. Thus the energy of the wavy crack damps quickly because the crack propagation along the laser scanning direction cannot follow the movement of the laser-induced hot spot. Thermal diffusion layers are seen on both sides of the laser induced channels in Fig. 8(b) and (c). Based on the definition of thermal conductivity, W/A = λ * (Th − Tl)/t, in which W is the power input into the area, A is the contact area between the hot area and cold area, λ is the thermal conductivity of glass, Th and Tl are the temperatures on both ends in the thermal diffusion layer, t is the thickness of the thermal diffusion layer (see Fig. 8(d)). Since the laser scans with a speed of v, the area A per unit time is then calculated to be H·v, where H is the depth of the molten area under the laser spot. Given that the LBGO glass has a thermal conductivity λ of 1.5 W/(m·K) [18] at high temperature and a depth H of 5 µm [8], the temperature difference ΔT(= (Th − Tl)) is calculated to be 500 °C and 300 °C respectively in the cases of Fig. 8(b) and Fig. 8(c). Since the glass transition temperature Tg is the turning point between the viscous liquid and the solid glass, when the temperature is near the Tg, the viscosity of the liquid is extremely high (∼1013 P) and it is difficult to form the thermal diffusion layer. Therefore, it is reasonable to assume that the Tl here is only slightly higher than the glass transition temperature Tg. Consequently, the actual temperature in the cases of Fig. 8(b) and (c) can be estimated to be in the range between 1000 and 1200 °C. And under the optimum laser
Fig. 8. Analysis of periodic wavy cracks formed on waveguide (Damped sine wave fitted as the guidance).
crystallization procedure is much shorter than that in traditional crystal growth technology. This is consistent with the observation in Ref. [13], in which after laser writing, Raman lines due to unknown crystal phases 122
Optical Fiber Technology 41 (2018) 118–124
J. Shi, X. Feng
Fig. 9. Temperature effect on the rates of homogeneous nucleation and crystal growth (After Tammann [9,10]).
condition, i.e., when laser power is 20 mW and the scanning speed is 60 mm/min, the temperature of the local area is estimated to be in the range of 800–900 °C, at which the molten area is with relatively high viscosity. Fig. 9 illustrates a classical diagram of Tammann [9,10], showing the effect of temperature on the rates of homogenous nucleation and crystal growth. It is seen that under the temperature in close proximity to the melting temperature Tm or the glass transition Tg, it is not suitable for controllable nucleation and crystal growth, because of the lack of thermodynamic driving forces in the former case or the too high viscosity in the latter case. For obtaining the glass ceramics with desired controlled crystalline phase size, the optimum combination of the rate of nucleation and the rate of crystal growth is necessary. Fig. 10 gives a comparison of laser induced crystallization and traditional Bridgman method growing crystal. It can be clearly seen that
Fig. 11. (a) Measured spectrum of SHG from 17 mm-long crystalline waveguide. (b) Near field image (blue/green 488 nm SHG) of output end of waveguide. Note that the launched 976 nm pump power was 26.0 mW.
these two methods are very similar in terms of the solidification procedure of the crystal/melt interface. The major difference between these two methods is that the laser induced crystallization is free of seed and the containing crucible. The former is a disadvantage, because without a seed with the controlled crystal direction, the laser induced crystallization will show random crystallographic direction on the final crystalline phase. But the crucible-free laser-induced crystal growth approach provides the opportunity to efficiently fabricate functional photonic circuits in a traditional glass optical fiber format. 3.4. Second harmonic generation from crystallized LBGO waveguide To further investigate the crystallization of the UV laser induced waveguide, SHG experiment has been carried out. The schematic setup is shown in Fig. 2, using a CW 976 nm fiber-pigtailed laser diode as the pump. A 17 mm-long Sample e was used in the experiment. The fabrication conditions of Sample e can be found in Table 1. As seen in Fig. 11(a), a strong and narrow peak at 488 nm is observed, while the 976 nm pump was depleted. The amplified spontaneous emission (ASE) of Yb3+ ions is also observed from 1000 to 1100 nm on the emission spectrum without the bandpass filter at the output of fiber. The nearfield image of the output end of the crystalline LBGO fiber was recorded by a CCD camera as shown in Fig. 11(b), ensuring that the output signal was only from the crystalline fiber. Clear blue/green spot can be seen on the top surface of the output end of the ribbon fiber. The cross section of the guided crystalline waveguide is estimated to be 11( ± 1) µm × 3( ± 1) µm, according to its near field image of in Fig. 11(b). It is seen that the waveguide supports a few modes at 488 nm. Fig. 12 shows the spectrum at 976 nm from pump source and the one at 488 nm from the LBGO fiber. The fundamental pump wave
Fig. 10. Analogous comparison of laser induced crystallization and traditional Bridgman method growing crystal.
123
Optical Fiber Technology 41 (2018) 118–124
J. Shi, X. Feng
pump and the 488 nm SHG wave. Fig. 13 plots the refractive indices (ne and no) of pure LBGO crystal extracted from Ref. [19]. In the plot, for both refractive index of no and ne, 1% uncertainty has been taken into account due to the differential of the compositions. As the meshed zone illustrated in Fig. 13, it is possible to satisfy the phase matching requirement between the fundamental pump wave at 976 nm and the second harmonic wave at 488 nm. 4. Conclusions In summary, we report a laser-induced nonlinear crystal waveguide in the glass fiber format. The glass and crystal are based on the stoichiometric composition of (La,Yb)BGeO5. Laser induced waveguides have been fabricated on the surface of the ribbon glass fiber using tens of milliwatts of CW UV laser radiation and a fast scanning speed of > 5 cm/min. The crystalline nature of the created waveguide has been observed using micro-Raman spectroscopy. The formation of wavy cracks during the laser induced crystallization process has been observed and analyzed. Using a CW 976 nm diode as the pump, second harmonic generation has been observed from the laser-induced crystalline waveguide. 13 μW of 488 nm output has been observed from a 17 mm long waveguide with 26.0 mW of launched diode pump power, corresponding to a normalized conversion efficiency of 4.4%W−1. 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.
Fig. 12. Measured spectra of fundamental wave at 976 nm (top) and SHG wave at 488 nm (bottom) from 17 mm-long crystalline fiber. Note that the launched 976 nm pump power was 26.0 mW.
Acknowledgment This work is supported by the National Natural Science Foundation of China (NSFC, Nos. 61307055, 61527822, 61235010 and 61307054). References [1] P.A. Franken, A.E. Hill, C.W. Peters, G. Weinreich, Generation of optical harmonics, Phys. Rev. Lett. 7 (1961) 118–119. [2] B.E.A. Saleh, M.C. Teich, Fundamentals of Photonics, John Wiley & Sons, Inc., 1991. [3] R.A. Myers, N. Mukherjee, S.R.J. Brueck, Large second-order nonlinearity in poled fused silica, Opt. Lett. 16 (22) (1991) 1732–1734. [4] A. Canagasabey, C. Corbari, A.V. Gladyshev, F. Liegeois, S. Guillemet, Y. Hernandez, M.V. Yashkov, A. Kosolapov, E.M. Dianov, M. Ibsen, P.G. Kazansky, High-average-power second-harmonic generation from periodically poled silica fibers, Opt. Lett. 34 (16) (2009) 2483–2485. [5] D.N. Nikogosyan, Nonlinear Optical Crystals: A Complete Survey, Springer, 2005. [6] G.H. Beall, L.R. Pinckney, Nanophase Glass-Ceramics, J. Am. Ceram. Soc. 82 (1) (1999) 5–16. [7] K. Miura, J. Qiu, T. Mitsuya, K. Hirao, Space-selected growth of frequency-conversion crystals in glasses with ultrashort infrared laser pulses, Opt. Lett. 25 (2000) 408–410. [8] X. Feng, J. Shi, C. Huang, P. Horak, P.S. Teh, S. Alam, M. Ibsen, W.H. Loh, Laser-induced crystalline optical waveguide in glass fiber format, Opt. Express 20 (26) (2012) B85–B93. [9] G. Tammann, The States of Aggregation, Van Nostrand Reinhold, New York, 1925. [10] P.W. McMillan, Glass-Ceramics, Academic Press, London, U.K., 1979. [11] C. Coussa, C. Martinet, B. Champagnon, L. Grosvalet, D. Vouagner, V. Sigaev, In situ Raman spectroscopy of pressure-induced changes in LaBGeO5 glass: hysteresis and plastic deformation, J. Phys.: Condens. Matter 19 (2007) 266220. [12] M.B. Smirnov, A.V. Menschikova, I. Kratochvilova-Hruba, Z. Zikmund, Lattice dynamics and phase transition in LaBGeO5, Phys. Status Solidi (b) 241 (5) (2004) 1017–1025. [13] A. Stone, M. Sakakura, Y. Shimotsuma, G. Stone, P. Gupta, K. Miura, K. Hirao, V. Dierolf, H. Jain, Formation of ferroelectric single-crystal architectures in LaBGeO5 glass by femtosecond vs. continuous-wave lasers, J. Non-Crystal. Solids 356 (52-54) (2010) 3059–3065. [14] A. Yuse, M. Sano, Transition between crack patterns in quenched glass plates, Nature 362 (1993) 329–331. [15] Y. Hayakawa, Numerical study of oscillatory crack propagation through a two-dimensional crystal, Phys. Rev. E 49 (3) (1994) R1804–1807. [16] H.-A. Bahr, A. Gerbatsch, U. Bahr, H.-J. Weiss, Oscillatory instability in thermal cracking: a first-order phase-transition phenomenon, Phys. Rev. E 52 (1) (1995) 240–243. [17] M. Adda-Bedia, Y. Pomeau, Crack instabilities of a heated glass strip, Phys. Rev. E 52 (4) (1995) 4105–4113. [18] R.C. Zeller, R.O. Pohl, Thermal conductivity and specific heat of noncrystalline solids, Phys. Rev. B 4 (1971) 2029–2041. [19] Y. Takahashi, Y. Benino, T. Fujiwara, T. Komatsu, Second harmonic generation in transparent surface crystallized glasses with stillwellite-type LaBGeO5, J. Appl. Phys. 89 (10) (2001) 5282–5287.
Fig. 13. Refractive indices (ne and no) of LBGO crystal in the wavelength region of 450–1150 nm (after Ref. [19]).
shows a peak wavelength of 976.6 nm and a full-width-half-maximum (FWHM) width of 1.5 nm, while the second harmonic wave is with a peak wavelength of 487.8 nm and a full-width-half-maximum (FWHM) width of 1.0 nm. Clearly, the bandwidth and central wavelength of the second-harmonic wave (488 nm) match well with its fundamental wave (976 nm) within the resolution uncertainty of the spectrometer. All the above results indicate that the generated 488 nm was the second harmonic wave from the 976 nm fundamental wave. A power meter with high sensitivity was put after the bandpass filter to measure the output power of the generated SHG signal. The 488 nm output has been measured to be 13 μW with a launched pump power of 26.0 mW. This is corresponding to a normalized conversion efficiency of 4.4%W−1, translating to a 60-fold improvement over the previous reported best value that from PPSF [4]. Lack of the precise information of the crystalline orientation and the corresponding refractive index of the formed Yb-doped LBGO crystal on the waveguide, it is difficult to precisely calculate the phase matching condition in the obtained crystalline waveguide between the 976 nm
124
Contents lists available at ScienceDirect
Optical Fiber Technology journal homepage: www.elsevier.com/locate/yofte
Regular Articles
Laser-induced nonlinear crystalline waveguide on glass fiber format and diode-pumped second harmonic generation Jindan Shia,b, Xian Fengc,
T
⁎
a
Jiangsu Key Laboratory of Advanced Laser Materials and Devices, School of Physics and Electronic Engineering, Jiangsu Normal University, Xuzhou 221116, China Jiangsu Collaborative Innovation Centre of Advanced Laser Technology and Emerging Industry, Jiangsu Normal University, Xuzhou 221116, China c Laser Institute of Engineering, Beijing University of Technology, 100 Pingleyuan, Chaoyang District, Beijing 100124, China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Second harmonic generation Laser induced crystallization Fiber design and fabrication Laser direct writing
We report a diode pumped self-frequency-doubled nonlinear crystalline waveguide on glass fiber. A ribbon fiber has been drawn on the glass composition of 50GeO2-25B2O3-25(La,Yb)2O3. Surface channel waveguides have been written on the surface of the ribbon fiber, using space-selective laser heating method with the assistance of a 244 nm CW UV laser. The Raman spectrum of the written area indicates that the waveguide is composed of structure-deformed nonlinear (La,Yb)BGeO5 crystal. The laser-induced surface wavy cracks have also been observed and the forming mechanism of the wavy cracks has been discussed. Efficient second harmonic generation has been observed from the laser-induced crystalline waveguide, using a 976 nm diode pump. 13 μW of 488 nm output has been observed from a 17 mm long waveguide with 26.0 mW of launched diode pump power, corresponding to a normalized conversion efficiency of 4.4%W−1.
1. Introduction Since the first observation of second harmonic generation (SHG) from a quartz crystal pumped by a ruby laser in 1961 [1], χ(2) based second-order nonlinear optical effects have been of great interest, especially for making the efficient, powerful and compact solid state ultraviolet (UV) and visible laser sources. This is currently mainly driven by many industrial applications, for example, micromachining using photolithograph in the semiconductor industry using UV lasers and laser display using the primary colours of red, green and blue (RGB) lasers. In the simple case of plane waves, non-depletable pump, and perfect phase matching, the second harmonic conversion efficiency η2ω/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 [2]. To achieve a high conversion efficiency for second harmonic generation (SHG), a long length of a nonlinear material with a high secondorder nonlinear coefficient χ(2) is required. Also small spatial mode size helps to enhance the fundamental wave intensity I. A fiber-based χ(2) nonlinear medium is thus an ideal medium format. Glass optical fibers have been successfully employed in the long-haul and short-haul optical communications, since (i) glass is a material readily drawn to very long lengths and (ii) the loss of the conventional glass optical fiber is low.
⁎
Unfortunately, glass is an isotropic material and thus the χ(2) of an untreated glass is nil. In previous works, thermal or electrical poling methods [3] were applied to fiber to induce a second-order nonlinearity χ(2) into the originally isotropic glass fiber. A relatively high SHG efficiency (15.2%) as well as high SHG output power (236 mW) was obtained from a 32 cm-long periodically poled silica fiber (PPSF) [4]. However, the second-order nonlinear coefficient d33 of the poled silica is very low, only 0.054 pm/V at 1.55 µm [4], almost two orders of magnitude lower than that of the commonly used nonlinear crystal βBaB2O4 (BBO) with a second-order nonlinear coefficient |d22(1.064 μm)| of 2.2 pm/V [5]. Furthermore, electrical poling of the silica fiber requires inserting metal wires with 30 µm diameter into holes around the fiber core [4]. In order to create a quasi-phasematching (QPM) structure in the PPSF to compensate the mismatch in the phase velocities of the copropagating fundamental wave and harmonic wave, fiber Bragg grating technique had to be employed as well. Both these techniques are time-consuming and inefficient and thus largely limit the useable fiber length in practice. Glass, known as a supercooled liquid, is thermally metastable. In other words, the glass has higher energy than the crystal, which is a true thermodynamic stable phase. When a type of physical energy, for example the heat, is applied on the glass, and let the glass temperature above its transition temperature Tg, which is the boundary between solid and liquid, the glass will have opportunity to be crystallized. In
Corresponding author. E-mail addresses: [email protected] (J. Shi), [email protected] (X. Feng).
https://doi.org/10.1016/j.yofte.2018.01.015 Received 1 December 2017; Received in revised form 14 January 2018; Accepted 16 January 2018 1068-5200/ © 2018 Elsevier Inc. All rights reserved.
Optical Fiber Technology 41 (2018) 118–124
J. Shi, X. Feng
the crystallization process, the external energy drives the atoms to overcome the energy barrier and consequently transfer through the solid/liquid interface. The atoms from the liquid thus get rearranged in a certain periodic way to form crystal. The traditional glass ceramic technology is such a way to control nucleation and crystallization of glass [6]. In the traditional glass ceramic technology, the whole glass workpiece is reheated in a high temperature furnace. More generally, one can find other alternative approaches to apply certain types of physical energy onto the whole piece of glass or the selected local area on the glass. For example, high-energy laser beam can be focused on the glass and the laser energy is converted to the heat at the selected micron-size domain. The local solid glass becomes into molten state and then the desired crystalline phase can be formed along the laser scanning direction. This method is called as space-selective laser heating [7]. In this work, we report a diode-pumped self-frequency-doubled nonlinear crystalline waveguide on glass fiber format. A ribbon fiber has been drawn on the glass composition of 50GeO2-25B2O325(La,Yb)2O3. Surface channel waveguides have been written on the surface of the ribbon fiber, using space-selective laser heating method. The crystalline nature of the crystalline (La,Yb)BGeO5 waveguide has been analyzed by the micro-Raman spectroscopy. The laser-induced surface wavy cracks have also been observed and the forming mechanism of the wavy cracks has been discussed. Efficient second harmonic generation has been observed from the laser-induced crystalline waveguide, using a CW diode pump.
Fig. 1. Schematic diagram of fabricating crystalline waveguide on ribbon fiber using space-selective laser heating.
2. Experiment
the ribbon fiber, confirmed by the observation of micro-Raman spectroscopy.
2.1. Glass preparation and fiber drawing
2.3. Crystallinity characterizations using Raman spectroscopy
Stoichiometric amounts of Yb2O3 (Alfa Aesar, 99.99%), La2O3 (Alfa Aesar, 99.99%), B2O3 (Alfa Aesar, 99.99%) and GeO2 (Alfa Aesar, 99.99%) were weighed in the composition of 50GeO2-25B2O3-24La2O31Yb2O3 (mol.%) (Yb1:LBGO glass) to provide a 70-gram batch, and melted in a platinum crucible at 1450 °C for 90 min. The melt was then cast into a stainless steel mould, which was preheated at 600 °C, to form a rectangular slab preform with dimensions of 5 × 15 × 75 mm. The glass slab preform was annealed at 650 °C, around the glass transition temperature Tg, for 2 h and then drawn into ribbon shaped fiber with a width of 450 µm and a thickness of 150 µm. The yield of the fiber draw was greater than 50 m. The transmission spectrum of a polished Yb1:LBGO glass with a thickness of 5.11 mm was measured by a CARY UV-VIS-NIR spectrometer. The Differential Thermal Analysis (DTA) curve of Yb1:LBGO glass powder is measured by PerkinElmer DTA7 with a ramp rate of 10 K/ min.
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 532 nm laser source. With the assistance of a CCD camera on the Raman microscope, the spot size of the 532 nm laser focused on the sample surface was estimated to be between 4 and 5 µm for all the measurements below. In addition, Yb1:LBGO fibers are reheated for crystallization in a small home-made furnace at 780 °C for a certain period from 0.5 min to 5 h. The Raman spectra of these samples are measured. 2.4. Second harmonic generation A fiber-pigtailed 976 nm diode laser was used as the pump source for SHG, as seen in Fig. 2. A half wave plate was placed in front of the ribbon fiber to control the polarization of the input light. The collimated pump beam was focused into the crystalline waveguide on a 17 mm long ribbon fiber. A CCD camera was used to monitor the nearfield image from the waveguide output, ensuring that the pump light
2.2. Laser-induced crystalline waveguide on glass fiber format A CW, single polarization 244-nm frequency-doubled argon-ion laser was employed to create crystalline waveguides on the surface of the ribbon fiber. As schematically shown in Fig. 1, the fibers with a length of 15–20 cm 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 10 µm in diameter. The XYZ stage moved along the length of the ribbon fiber according to a pre-set program. Laser power focused onto the fiber and the scanning speed of the stage along the fiber length (Y direction in Fig. 1) were the two key parameters in the above process. The power of the exposed UV laser was 20-70mW and the stage scanning rate was 10–120 mm/min. As described in Ref. [8], the absorbed UV laser energy was transformed to thermal energy and the stoichiometric nonlinear crystal (La,Yb)BGeO5 (Yb1:LBGO) was created on the surface of
Fig. 2. Schematic diagram of laser diode pumped SHG from crystallized LBGO fiber.
119
Optical Fiber Technology 41 (2018) 118–124
J. Shi, X. Feng
Fig. 3. Transmission spectrum of Yb1:LBGO glass with a thickness of 5.11 mm.
Fig. 4. DTA curve of Yb1:LBGO glass powder with ramp rate of 10 K/min.
was focused into the waveguide. The output spectrum was then collected by a multimoded silica fiber with a core diameter of 100 µm by butt-coupling and recorded by an Ocean-Optics HR4000 miniature spectrometer ranging from 200 to 1100 nm. A 3 mm-thick band-pass filter (Thorlabs FGS900, 315–710 nm BPF) was then added directly after the ribbon fiber to block the laser radiations beyond 900 nm. A power meter with high sensitivity was put after the filter to measure the output power of the generated SHG signal. The launching efficiency was estimated to be 5%. 3. Results and discussion 3.1. Laser-induced crystalline waveguides under various conditions Fig. 3 shows the measured transmission spectrum of the polished Yb1:LBGO glass with a thickness of 5.11 mm. It is seen that the glass is highly transparent in the range between 400 and 2200 nm except for the Yb3+ absorption band (2F7/2 → 2F5/2 transition) peaking at 975 nm. The bandgap of the glass is located at ∼330 nm. The penetration depth of the 244 nm laser on the glass is estimated with the order of tens of microns, according to the transmission curve. Fig. 4 plots the measured DTA curve of Yb1:LBGO glass powder. It is seen that the glass transition temperature Tg is 675 ( ± 2) °C and the onset of crystallization Tx is 790 ( ± 2) °C. Note that the glass transition temperature Tg is the transition between the solid glass state and the liquid state. Thermodynamically, the solid-state glass should be given with some certain energy to cross the energy barrier towards the liquid state. On the DTA curve shown in Fig. 4, the temperature of Tg is determined by the intersection of the two straight lines when the endothermal process happens. The onset of the crystallization temperature Tx, which instead is an exothermal process, is determined by the same way. Fig. 5 shows the optical photographs of the laser-induced channels under different laser irradiation conditions. Table 1 summarizes the laser writing conditions and the outcome of the channel waveguides. With a laser power of 70 mW and a scan speed of 20 mm/min, significant grain boundaries and periodic surface wavy cracks can be observed in the irradiated crystalline channel (see Fig. 5(a)). The width of the channel waveguide is 12.7 µm, which is close to the focused spot size of 10 ± 1 µm. Out of the channel, one can see that there is an area with overheated trace. This area is formed due to the thermal diffusion from the central molten channel to the side solid glass area. The thickness of the thermal diffusion layer here is 17.3 µm. By reducing the laser power down to 38 mW and increasing the scan
Fig. 5. Optical photographs of top view of laser-induced waveguides on ribbon fiber. The writing conditions of (a)-(e) are seen in Table 1. Note that the UV laser scans from the left to the right.
120
Optical Fiber Technology 41 (2018) 118–124
J. Shi, X. Feng
Table 1 Laser writing conditions and waveguide parameters. Sample number
a b c d e
Laser power mW
Scanning speed mm/min
70 38 38 20 20
20 120 10 120 60
Waveguide width (µm)
12.7 9.7 11.4 6.2 8.2
Table 2 Assignation of Raman bands of LBGO samples [12]. Thermal diffusion thickness (µm) 17.3 7.6 10.5 / /
Raman shift
Assignation
300–400 cm−1 550 cm−1
Bending vibrations of GeO4 tetrahedra Combination of the bending vibrations of T–O–T (T = Ge or B) bonds GeO4 stretching vibrations Symmetric BO4 and GeO4 stretching vibrations Antisymmetric stretching vibrations of BO4 tetrahedra
700–800 cm−1 800–900 cm−1 900–1000 cm−1
interconnected by La3+. The BGeO5 chain is built of one [BO4] cornerconnected tetrahedron and three neighbouring [BO4] tetrahedra which form one link of the spiral. The outer oxygen atoms (O2) of each of two neighbouring [BO4] tetrahedra are interconnected with [GeO4] tetrahedra, which are outside the [BO3] chain [12]. It is seen from Fig. 6 that the main Raman band of the LBGO glass is located at 800 cm−1, which is arising from the bone structure (i.e., spiral [BGeO5] chains) of LBGO glass [11,12]. After laser irradiation, sharp peaks at 720 cm−1, 750 cm−1, 890 cm−1 and 910 cm−1 appear above the 800 cm−1 band, indicating crystallization occurring. These peaks are due to the enhanced [GeO4] stretching vibrations, and symmetric [BO4] and [GeO4] stretching vibrations (see Table 2). It is therefore believed that with laser irradiation, a significant amount of antisymmetric stretching vibrations of [BO3], [BO4] and [GeO4] units form on the spiral [BGeO5] chains, because the laser-induced crystallization is a very fast procedure and the structure relaxation for completing the crystallization cannot be accomplished [8]. Fig. 7 illustrates the evolution of micro-Raman spectra of Yb1:LBGO glass fiber reheated at 780 °C for 0.5 min–5 h, compared with the untreated Yb1:LBGO glass fiber and the standard LBGO crystal [11]. The Raman intensity of each trace has been normalized at the 800 cm−1 peak. The reheating temperature is set 10 °C below the crystallization onset Tx of the Yb1:LBGO glass, for clearly investigating the evolution of the crystallization under high temperature. It is seen that obvious crystallization occurs after the sample has been reheated between 2 and 5 min. With the reheating time increase from 5 min to 5 h, essentially the relative intensity of 850 cm−1 band enhances, indicating the relaxation of symmetric [BO4] and [GeO4] stretching vibrations. But compared with the standard LBGO crystal, even the sample reheated for 5 h still does not show the minor peaks at 350 cm−1, 550 cm−1, 900 cm−1 (marked with stars in Fig. 7), which are arising from the bending vibrations of [GeO4] tetrahedra, the combined bending vibrations of Ge–O–Ge and B–O–B bonds, and antisymmetric stretching vibrations of [BO4] tetrahedral, respectively. It means that the actual relaxation of crystal structure takes time. It is therefore not surprising that laser-induced crystallization tends to form defects or deformed structures in the crystal, because the time of the laser-induced
speed to 120 mm/min, the width of the waveguide channel narrows down to 9.7 µm and a damped periodical wavy crack is seen inside the channel area, extending along the laser scanning direction (see Fig. 5(b)). The thickness of the thermal diffusion layer is 7.6 µm. When the laser power is 38 mW and the scan speed is 10 mm/min, the width of the waveguide channel is measured to be 11.4 µm. A straight wavy crack is seen nearly in the middle of the channel (see Fig. 5(c)). The thickness of the thermal diffusion layer is 10.5 µm. By further lowering the laser power down to 20 mW, proper channel waveguides without obvious thermal diffusion layer are formed (see Fig. 5(d) & (e)). With a fast scanning speed of 120 mm/min (see Fig. 5(d)), the waveguide shows relatively dim. With a slower scanning speed of 60 mm/min (see Fig. 5(e)), the center of the channel waveguide is bright with good contrast, indicating that it is the optimum condition for making laser-induced crystalline waveguides with decent quality.
3.2. Crystalline nature of laser-induced waveguide on fiber Fig. 6 illustrates the micro-Raman spectra of the original Yb-doped LBGO glass ribbon fiber sample without UV laser irradiation and the laser induced waveguide on the surface of the glass ribbon. The Raman spectrum of the standard LBGO crystal [11] is plotted on Fig. 6 as well. Table 2 lists the assignation of the Raman bands of LBGO glass and crystal provided by Ref. [12]. Note that the original preform and the area outside the laser induced waveguide on the glass ribbon show the same Raman spectra as the ribbon fiber without the UV laser irradiation, indicating that no crystallization occurs during the fiber drawing. It is known that standard LBGO crystal has the crystal structure of stillwellite and crystallizes at room temperature in the trigonal polar space group P31 (C32) [12]. It consists of spiral BGeO5 chains
Fig. 7. Evolution of micro-Raman spectra of Yb1:LBGO glass fiber reheated at 780 °C for 0.5 min–5 h, in comparison with untreated glass fiber and standard LBGO crystal [11].
Fig. 6. Comparison of micro-Raman spectra of Yb1:LBGO glass ribbon before and after UV laser irradiation, and undoped LBGO crystal [11].
121
Optical Fiber Technology 41 (2018) 118–124
J. Shi, X. Feng
other than LaBGeO5 between 700 and 900 cm−1 were observed. 3.3. Analysis of wavy cracks and thermal model of laser-induced crystallization As illustrated in Fig. 8(a), during the scanning of the 244-nm laser on the surface of the glass ribbon fiber, the solid glass absorbs the exposed laser power and converts it into heat. The local area is then molten. After the laser spot moves forward, the molten area is cooled down. At the same time, because only a slot area is molten from the laser exposure, huge temperature contrast exists between the molten area and the surrounding glass. The formation of such wavy (or oscillatory) cracks is due to the thermal stress arising from the rapid cooling rate of the molten area during solidification and the thermal mismatch of the thermal expansion coefficients of the crystalline phase and the surrounding glass [14–17]. To obtain a crack-free waveguide with desired crystalline phase and crystallinity from the glass matrix, the exposed laser power and the scanning speed are the two crucial parameters, because they determine the initial temperature and the cooling rate of the melt. If the laser power is too high, the starting temperature of the melt will be too high and the molten area will be largely expanded out of the laser spot due to the thermal diffusion. The cooling rate in the local area will then be highly uneven because of the large temperature gradient. Undesired crystalline phases and/or poor crystallinity might be formed. So might the cracks. When the cooling rate is too fast, the molten state might be quenched to glass state again and also the crack might occur. But if the cooling rate is too slow, the heat converted from the laser power might not be proportionally accumulated, due to the thermal diffusion rate of the glass and the cooling effect from the air moving above the glass. When the laser power and the scanning speed are 70 mW and 20 mm/min respectively, a wavy crack with a periodicity of 180.1 µm is formed (see Fig. 8(b)). Since the laser spot focusing on the surface of the glass fiber is a transient thermal source (see Fig. 8(a)), such a periodic crack forms only when the crack propagation can keep pace with the laser scanning. Therefore the time to form each period of the wavy crack is calculated to be 0.54 s. When the laser power is reduced down to 38 mW, a periodically damped crack with a periodicity of 54.8 µm is formed with the laser scanning speed of 120 mm/min (see Fig. 8(c)). The decrease of the laser power reduces the temperature in the exposed area and the increase of the scanning speed enhances the moving speed of the front of the molten area and the solid glass. Thus the energy of the wavy crack damps quickly because the crack propagation along the laser scanning direction cannot follow the movement of the laser-induced hot spot. Thermal diffusion layers are seen on both sides of the laser induced channels in Fig. 8(b) and (c). Based on the definition of thermal conductivity, W/A = λ * (Th − Tl)/t, in which W is the power input into the area, A is the contact area between the hot area and cold area, λ is the thermal conductivity of glass, Th and Tl are the temperatures on both ends in the thermal diffusion layer, t is the thickness of the thermal diffusion layer (see Fig. 8(d)). Since the laser scans with a speed of v, the area A per unit time is then calculated to be H·v, where H is the depth of the molten area under the laser spot. Given that the LBGO glass has a thermal conductivity λ of 1.5 W/(m·K) [18] at high temperature and a depth H of 5 µm [8], the temperature difference ΔT(= (Th − Tl)) is calculated to be 500 °C and 300 °C respectively in the cases of Fig. 8(b) and Fig. 8(c). Since the glass transition temperature Tg is the turning point between the viscous liquid and the solid glass, when the temperature is near the Tg, the viscosity of the liquid is extremely high (∼1013 P) and it is difficult to form the thermal diffusion layer. Therefore, it is reasonable to assume that the Tl here is only slightly higher than the glass transition temperature Tg. Consequently, the actual temperature in the cases of Fig. 8(b) and (c) can be estimated to be in the range between 1000 and 1200 °C. And under the optimum laser
Fig. 8. Analysis of periodic wavy cracks formed on waveguide (Damped sine wave fitted as the guidance).
crystallization procedure is much shorter than that in traditional crystal growth technology. This is consistent with the observation in Ref. [13], in which after laser writing, Raman lines due to unknown crystal phases 122
Optical Fiber Technology 41 (2018) 118–124
J. Shi, X. Feng
Fig. 9. Temperature effect on the rates of homogeneous nucleation and crystal growth (After Tammann [9,10]).
condition, i.e., when laser power is 20 mW and the scanning speed is 60 mm/min, the temperature of the local area is estimated to be in the range of 800–900 °C, at which the molten area is with relatively high viscosity. Fig. 9 illustrates a classical diagram of Tammann [9,10], showing the effect of temperature on the rates of homogenous nucleation and crystal growth. It is seen that under the temperature in close proximity to the melting temperature Tm or the glass transition Tg, it is not suitable for controllable nucleation and crystal growth, because of the lack of thermodynamic driving forces in the former case or the too high viscosity in the latter case. For obtaining the glass ceramics with desired controlled crystalline phase size, the optimum combination of the rate of nucleation and the rate of crystal growth is necessary. Fig. 10 gives a comparison of laser induced crystallization and traditional Bridgman method growing crystal. It can be clearly seen that
Fig. 11. (a) Measured spectrum of SHG from 17 mm-long crystalline waveguide. (b) Near field image (blue/green 488 nm SHG) of output end of waveguide. Note that the launched 976 nm pump power was 26.0 mW.
these two methods are very similar in terms of the solidification procedure of the crystal/melt interface. The major difference between these two methods is that the laser induced crystallization is free of seed and the containing crucible. The former is a disadvantage, because without a seed with the controlled crystal direction, the laser induced crystallization will show random crystallographic direction on the final crystalline phase. But the crucible-free laser-induced crystal growth approach provides the opportunity to efficiently fabricate functional photonic circuits in a traditional glass optical fiber format. 3.4. Second harmonic generation from crystallized LBGO waveguide To further investigate the crystallization of the UV laser induced waveguide, SHG experiment has been carried out. The schematic setup is shown in Fig. 2, using a CW 976 nm fiber-pigtailed laser diode as the pump. A 17 mm-long Sample e was used in the experiment. The fabrication conditions of Sample e can be found in Table 1. As seen in Fig. 11(a), a strong and narrow peak at 488 nm is observed, while the 976 nm pump was depleted. The amplified spontaneous emission (ASE) of Yb3+ ions is also observed from 1000 to 1100 nm on the emission spectrum without the bandpass filter at the output of fiber. The nearfield image of the output end of the crystalline LBGO fiber was recorded by a CCD camera as shown in Fig. 11(b), ensuring that the output signal was only from the crystalline fiber. Clear blue/green spot can be seen on the top surface of the output end of the ribbon fiber. The cross section of the guided crystalline waveguide is estimated to be 11( ± 1) µm × 3( ± 1) µm, according to its near field image of in Fig. 11(b). It is seen that the waveguide supports a few modes at 488 nm. Fig. 12 shows the spectrum at 976 nm from pump source and the one at 488 nm from the LBGO fiber. The fundamental pump wave
Fig. 10. Analogous comparison of laser induced crystallization and traditional Bridgman method growing crystal.
123
Optical Fiber Technology 41 (2018) 118–124
J. Shi, X. Feng
pump and the 488 nm SHG wave. Fig. 13 plots the refractive indices (ne and no) of pure LBGO crystal extracted from Ref. [19]. In the plot, for both refractive index of no and ne, 1% uncertainty has been taken into account due to the differential of the compositions. As the meshed zone illustrated in Fig. 13, it is possible to satisfy the phase matching requirement between the fundamental pump wave at 976 nm and the second harmonic wave at 488 nm. 4. Conclusions In summary, we report a laser-induced nonlinear crystal waveguide in the glass fiber format. The glass and crystal are based on the stoichiometric composition of (La,Yb)BGeO5. Laser induced waveguides have been fabricated on the surface of the ribbon glass fiber using tens of milliwatts of CW UV laser radiation and a fast scanning speed of > 5 cm/min. The crystalline nature of the created waveguide has been observed using micro-Raman spectroscopy. The formation of wavy cracks during the laser induced crystallization process has been observed and analyzed. Using a CW 976 nm diode as the pump, second harmonic generation has been observed from the laser-induced crystalline waveguide. 13 μW of 488 nm output has been observed from a 17 mm long waveguide with 26.0 mW of launched diode pump power, corresponding to a normalized conversion efficiency of 4.4%W−1. 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.
Fig. 12. Measured spectra of fundamental wave at 976 nm (top) and SHG wave at 488 nm (bottom) from 17 mm-long crystalline fiber. Note that the launched 976 nm pump power was 26.0 mW.
Acknowledgment This work is supported by the National Natural Science Foundation of China (NSFC, Nos. 61307055, 61527822, 61235010 and 61307054). References [1] P.A. Franken, A.E. Hill, C.W. Peters, G. Weinreich, Generation of optical harmonics, Phys. Rev. Lett. 7 (1961) 118–119. [2] B.E.A. Saleh, M.C. Teich, Fundamentals of Photonics, John Wiley & Sons, Inc., 1991. [3] R.A. Myers, N. Mukherjee, S.R.J. Brueck, Large second-order nonlinearity in poled fused silica, Opt. Lett. 16 (22) (1991) 1732–1734. [4] A. Canagasabey, C. Corbari, A.V. Gladyshev, F. Liegeois, S. Guillemet, Y. Hernandez, M.V. Yashkov, A. Kosolapov, E.M. Dianov, M. Ibsen, P.G. Kazansky, High-average-power second-harmonic generation from periodically poled silica fibers, Opt. Lett. 34 (16) (2009) 2483–2485. [5] D.N. Nikogosyan, Nonlinear Optical Crystals: A Complete Survey, Springer, 2005. [6] G.H. Beall, L.R. Pinckney, Nanophase Glass-Ceramics, J. Am. Ceram. Soc. 82 (1) (1999) 5–16. [7] K. Miura, J. Qiu, T. Mitsuya, K. Hirao, Space-selected growth of frequency-conversion crystals in glasses with ultrashort infrared laser pulses, Opt. Lett. 25 (2000) 408–410. [8] X. Feng, J. Shi, C. Huang, P. Horak, P.S. Teh, S. Alam, M. Ibsen, W.H. Loh, Laser-induced crystalline optical waveguide in glass fiber format, Opt. Express 20 (26) (2012) B85–B93. [9] G. Tammann, The States of Aggregation, Van Nostrand Reinhold, New York, 1925. [10] P.W. McMillan, Glass-Ceramics, Academic Press, London, U.K., 1979. [11] C. Coussa, C. Martinet, B. Champagnon, L. Grosvalet, D. Vouagner, V. Sigaev, In situ Raman spectroscopy of pressure-induced changes in LaBGeO5 glass: hysteresis and plastic deformation, J. Phys.: Condens. Matter 19 (2007) 266220. [12] M.B. Smirnov, A.V. Menschikova, I. Kratochvilova-Hruba, Z. Zikmund, Lattice dynamics and phase transition in LaBGeO5, Phys. Status Solidi (b) 241 (5) (2004) 1017–1025. [13] A. Stone, M. Sakakura, Y. Shimotsuma, G. Stone, P. Gupta, K. Miura, K. Hirao, V. Dierolf, H. Jain, Formation of ferroelectric single-crystal architectures in LaBGeO5 glass by femtosecond vs. continuous-wave lasers, J. Non-Crystal. Solids 356 (52-54) (2010) 3059–3065. [14] A. Yuse, M. Sano, Transition between crack patterns in quenched glass plates, Nature 362 (1993) 329–331. [15] Y. Hayakawa, Numerical study of oscillatory crack propagation through a two-dimensional crystal, Phys. Rev. E 49 (3) (1994) R1804–1807. [16] H.-A. Bahr, A. Gerbatsch, U. Bahr, H.-J. Weiss, Oscillatory instability in thermal cracking: a first-order phase-transition phenomenon, Phys. Rev. E 52 (1) (1995) 240–243. [17] M. Adda-Bedia, Y. Pomeau, Crack instabilities of a heated glass strip, Phys. Rev. E 52 (4) (1995) 4105–4113. [18] R.C. Zeller, R.O. Pohl, Thermal conductivity and specific heat of noncrystalline solids, Phys. Rev. B 4 (1971) 2029–2041. [19] Y. Takahashi, Y. Benino, T. Fujiwara, T. Komatsu, Second harmonic generation in transparent surface crystallized glasses with stillwellite-type LaBGeO5, J. Appl. Phys. 89 (10) (2001) 5282–5287.
Fig. 13. Refractive indices (ne and no) of LBGO crystal in the wavelength region of 450–1150 nm (after Ref. [19]).
shows a peak wavelength of 976.6 nm and a full-width-half-maximum (FWHM) width of 1.5 nm, while the second harmonic wave is with a peak wavelength of 487.8 nm and a full-width-half-maximum (FWHM) width of 1.0 nm. Clearly, the bandwidth and central wavelength of the second-harmonic wave (488 nm) match well with its fundamental wave (976 nm) within the resolution uncertainty of the spectrometer. All the above results indicate that the generated 488 nm was the second harmonic wave from the 976 nm fundamental wave. A power meter with high sensitivity was put after the bandpass filter to measure the output power of the generated SHG signal. The 488 nm output has been measured to be 13 μW with a launched pump power of 26.0 mW. This is corresponding to a normalized conversion efficiency of 4.4%W−1, translating to a 60-fold improvement over the previous reported best value that from PPSF [4]. Lack of the precise information of the crystalline orientation and the corresponding refractive index of the formed Yb-doped LBGO crystal on the waveguide, it is difficult to precisely calculate the phase matching condition in the obtained crystalline waveguide between the 976 nm
124