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Abstract
In this work, the depressed cladding waveguide in PMN-PT crystal is fabricated by using a femtosecond laser with a central wavelength of 800 nm for the first time. The result of the confocal micro-Raman shows the change of Raman signal in each region. When coupling a laser at 980 nm into the depressed cladding waveguide in the PMN-PT sample, the output characteristics with temperatures under TM and TE polarizations are also explored, and the result shows that the insertion loss under TM polarization is only 1.33 dB at 50 °C.
© 2021 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
In recent decades, the perovskite-like oxides with the stoichiometry ABO3 have drawn considerable attention because of their significant properties and potential applications. As one kind of these crystalline materials, Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) crystal or ceramics owns excellent voltage stability, high electro-optic effect and converse-piezoelectric effect, and becomes a hot research topic [1–3]. Up to now, the PMN-PT-based devices have been successfully developed, including optical limiter, polarization controller, sensor and transducer, etc [4–6]. These promising and widespread applications demonstrate the PMN-PT crystal or ceramic can be a suitable candidate for integrated photonics circuits.
Optical waveguide structure can confine light propagating in a small volume with dimensions of several micrometers, and lead to a much high intensity with respect to the bulk materials [7–9]. As the basic component in the integrated photonic system, different types of optical waveguides have been designed and fabricated, such as planar waveguides, channel waveguides and ridge waveguides to meet the requirement of various scenarios. Femtosecond laser directly writing (FLDW) is a mature, powerful and unique technique to implement guiding configurations in optical materials [10–13]. Compared with the traditional techniques, i.e. ion/proton exchange, ion implantation, and proton beam writing, FLDW performs advantages of maskless 3D processing ability, wide adaptability of materials and negligible thermal-diffusion effect. Normally, the processing of one channel waveguide in a 10-mm length crystal will cost less than 10 minutes. Since the pioneering work of Davis et al. in 1996, FLDW has already been utilized to manufacture guiding devices in glasses, dielectric crystals, ceramics, organic materials, and semiconductor materials [14–19]. Furthermore, due to the different interaction mechanism between the femtosecond laser and the materials, the single-line waveguides (the induced tracks of Δn >0), the stress-induced dual-line waveguides (the induced two-paralleled tracks of Δn <0), and the depressed cladding waveguides (surrounded by the numbers of tracks of Δn <0) have been produced [20–23]. The depressed cladding waveguides possess flexible diameters and shapes, and are popular owing to a further compatible device with the gratings, the fibers, etc. Meanwhile, the circular cladding waveguides always support the guidance both along the TE polarization and the TM polarization in most materials [24–27]. To the best of our knowledge, ridge and planar waveguides have been achieved in PMN-PT films, and the channel waveguides have not been produced inside this fantastic material.
In this work, we firstly report on the depressed cladding waveguide in PMN-PT crystal fabricated by FLDW. The micro-Raman spectra of the depressed cladding structure have been investigated to elementarily reveal the laser modification mechanism. The all-angle guiding performance at 980 nm has been studied. Our work manifests that the guiding structures inside the PMN-PT crystal can be achieved easily, and the significant features of it will pave a solid way for further applications of PMN-PT crystals in integrated photonic circuits with reservation of the material characteristics.
2. Experimental details
The PMN-PT ((purchased from Hefei Kejing materials technology co. LTD) is cut to a size of 10 (x) 5 (y) 0.5 (z) mm3 along the crystallographic directions ([100], [110] and [111], respectively,) with three faces optically polished. The moistureproof film has been coated to protect the crystal. The waveguide is fabricated in the PMN-PT crystal by using an amplified Ti: sapphire femtosecond laser system (Astrella, Coherent Inc., USA). The system can deliver 1-kHz linear polarized pulses with the width changing from 35 fs to 150 fs, and the energy of 6 mJ at a central wavelength of 800 nm. A watt pilot motorized attenuator was utilized to adjust the incident energy precisely. The crystal was placed on a 6-axes motorized stage to achieve the designed configuration during the fabricating process. Finally, based on our testing results, the micromachining pulse width was set to 65 fs, and the pulse energy is 80 nJ after being focused by a microscope objective lens (50×, N.A. 0.55). The fabricating process is illustrated in Fig. 1.
Fig. 1. Schematic diagram of femtosecond laser direct writing PMNPT depressed cladding waveguide. The insets are the top view and the cross-section view of the waveguide.
As shown in the insets of Fig. 1, the waveguide with a diameter of 30 μm is located at 50 μm beneath the surface of the crystal. The depressed cladding waveguide is surrounded by 24 tracks directly written by femtosecond laser with the refractive index reduced about 0.001 according to our previous experience. Certainly, we can add the pulse energy and increase the scanning speed to reduce the inscription time to some extent. According to the previous report, cladding waveguides with circular cross-section have better performance on the propagation losses and propagation at different polarizations, and so on. Also, another waveguide has been fabricated with a diameter of 50 μm. However, the insertion loss is quite larger due to the laser leaking out through the tracks.
3. Results and discussions
Firstly, we used a confocal micro-Raman spectroscopy system (Nanobase XperRam 200, South Korea) to characterize the waveguide cross-section face directly modified by femtosecond laser. As shown in Fig. 2, the Raman spectra of three regions of the cross-section are measured. The peaks of Raman shift at 270, 443, 583, and 812 cm-1 correspond to four phonon modes, E + B1, A1(LO), E1(TO) and A1(LO), respectively. The A1(LO) mode is located at 812 cm−1 and can be assigned to stretching vibration of Nb–O–Mg. Mode E1(TO) at 583 cm-1 is assigned to bending vibrations of oxygen octahedra. Mode A1(LO) located at 443 cm-1 originates from stretching vibrations of Mg–O–Mg. The spectral peak located at 270 cm-1 is a mixed-mode E + B1, which represents stretching vibrations of oxygen octahedra around B-cations. When comparing the Raman signal of three different regions in the figure, it can be seen that the intensity of the filament region is much lower than that of the waveguide region and bulk region. This indicates that the lattice is damaged obviously at the filament region caused by the FLDW. The width of the Raman spectrum of filament at E + B1 mode is wider than that of bulk, which also confirms the lattice damage. The blue shift of filament relative to bulk at E + B1 mode can also be obviously compared. This suggests that there is not only damage to the lattice during the action of the femtosecond laser but also extrusion and expansion of the lattice, which can lead to anisotropic characteristics of the guiding area.
Fig. 2. The Raman spectra of PMN-PT depressed cladding waveguide measured by the confocal-Raman microscope at room temperature (26°C). Blue, yellow and red lines are the spectrum collected from the guiding region, the filament and the bulk, respectively, as pointed by the arrows in the inset figure.
In order to investigate the guiding features of the depressed cladding waveguide processed by FLDW in PMN-PT crystal, we used a CW 980-nm circularly polarized laser in the end-face coupling system. According to the inspection report, the transmittance of the PMN-PT crystal at 980 nm is about 62%. As shown in Fig. 3(a), the cladding waveguide supports a single-mode laser propagation approximately at room temperature (26 °C). As we all know, the characteristics of the PMN-PT crystal will be changed a lot during increasing the temperature. Thus, to further explore the waveguide properties at different temperatures, we utilized a TEC (Linkam THMS600) with an accuracy of 0.01 °C to control the sample temperature.
Fig. 3. Near-field modal profiles of the waveguide at 26,30,35,40 and 50 °C (from (a) to (e)) without polarization. Near-field modal profiles of the waveguide at 26,30,35,40 and 50 °C (from (f) to (j)) at TM polarization. The scale bar in the figure is 30 μm.
At the meantime, we also use a Gran Taylor prism and a halfwave plate to change the polarizations of the laser beam. Figures 3(a) to 3(e) show the near-field modal profiles generated by the waveguide at different temperatures. It can be seen that the mode of PMN-PT cladding waveguide keeps quasi-single-mode with the increase of temperature from the room temperature (26 °C) to 50 °C. When the input power is 1.6mW, it is measured that the output power increases with the increase of temperature, which is 371 μW at room temperature (26 °C) and 674 μW at 50 °C.
Meanwhile, the near-field modal profiles of the PMN-PT cladding waveguide have been observed at TE and TM polarization states respectively. Figures 3(f) to 3(j) show that the near-field modal profiles at TM polarizations can keep a quasi-single-mode propagation with the increase of temperature from 26 °C to 50 °C. The output power at TM and TE polarizations are measured, and it is found that the output power at TM polarization is higher than the TE polarization. In particular, the output power at TM polarization varies significantly with temperature increase from room temperature to 50 °C. The value of output power at TM polarization is 181.3 μW at 26 °C and 606 μW at 50 °C. However, over the same range of temperature variations, the output power at TE polarization appears relatively stable and the output power at TE polarization is 176 μW at 26 °C, 243 μW at 50 °C.
In order to form a more intuitive understanding of the guiding characteristics of the PMN-PT depressed cladding waveguide under different temperatures, the insertion loss at TM and TE polarizations were calculated. As Fig. 4(a) shows, the insertion loss at TM polarization is close to the TE polarization, but it is smaller than that at TE polarization. As the temperature rises, the insertion loss of both TM and TE polarization tends to decrease, but the loss at TM polarization decreases m3ore significantly than that at TE polarization. The insertion loss at TM polarization reduces from 6.58 dB at 26 °C to 1.33 dB at 50 °C. Through simulation and calculation, the propagation loss of PMN-PT crystal at TM polarization can be obtained as 5.97 dB. Although the insertion loss at TE polarization decreases as the temperature increases, it only decreases by 1.4 dB, which is not as obvious as that at TM polarization. This indicates that the depressed cladding waveguide in PMN-PT crystal is more suitable for TM polarization guiding, which will offer an experimental reference for further applications of waveguides in PMN-PT crystal.
Fig. 4. (a) Insertion loss variation diagram of TM and TE polarization under temperature change from 26°C to 50°C, and the error bar indicates the influence of the coupling loss. (b) The output power of PMN-PT cladding waveguide for all-angle polarizations at 26°C and 50°C.
The unequal insertion loss in the case of TM polarization and TE polarization can be ascribed to the following two reasons. On the one hand, there exists general asymmetry in the depressed cladding waveguide prepared by FLDW due to its relatively large waveguide scale. The asymmetry of the guiding configuration will be reflected in the presence of some tiny bends in the waveguide. These tiny bends in the waveguide collectively affect the characteristics of the waveguide at different polarizations. On the other hand, the polarization characteristics of the PMN-PT crystal depressed cladding waveguides are also related to the different guiding features along different crystallographic directions. In terms of the polarization characteristics of the waveguide, the guiding performance along the [111] direction (TM polarization direction) of the PMN-PT crystal depressed cladding waveguide is significantly better than that along the [110] direction (TE polarization direction).
Figure 4(b) records the all-angle polarization of PMN-PT depressed cladding waveguide at different temperatures. It can be clearly observed in the figure that the PMN-PT depressed cladding waveguide has an obvious polarization dependence. In addition, the polarization dependence keeps the same during the investigation of temperature changing. Due to the properties of this material, the insertion loss of this crystal will be decreased with the increasement of the temperature. The output power along the TM polarization direction (0 degree) changes obviously with the increase of temperature, but the change at the TE polarization direction (90 degrees) is extremely small. It also suggests that PMN-PT depressed cladding waveguide has an access to the significant optical guidance properties at TM polarization.
4. Conclusion
In summary, we have successfully fabricated depressed cladding waveguide in PMN-PT crystal by femtosecond laser, and successfully explored its guiding performance at 980 nm at different temperature range. The insertion loss will decrease with the increasing the temperature. Furthermore, it is found that the guiding performance at TM polarization is much better than that at TE polarization when the temperature is higher than the room temperature. According to our calculation, the loss at TM polarization will be decrease from 6.6 dB to 1.3 dB. Although the structure still supports laser propagation at TE polarization, the insertion loss is still as high as 5.5 dB during the measurement. Our results demonstrate the fabrication of cladding waveguide in PMN-PT crystal, and extend the applications of guiding devices based on this material. Its unique temperature-sensitive guiding performance will make contributions to the development of PMN-PT based photonic devices.
Funding
National Natural Science Foundation of China (1170420111874226); Fundamental Research Funds for the Central Universities.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (NSFC) (Grants No. 11704201 and 11874226); the Fundamental Research Funds for the Central Universities.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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