We have investigated that femtosecond laser induced optical waveguides in AF32 and in Borofloat-33 glasses. Type-I positive and type-II negative refractive index change mechanisms were established in AF32 and in Borofloat-33 glass waveguides, respectively. Lowest propagation loss of 1.1 ± 0.31 dB/cm could be attained in AF32 glass, which has a typically higher index change than in Borofloat-33 glass. Resultant losses are directly correlated with densification and non-bridging oxygen hole centers, which can be authenticated by Raman and photoluminescence studies.
© 2017 Optical Society of America
The ability of three dimensional (3D)-writing micro and nano-structures inside transparent dielectric materials using femtosecond (fs) laser can help manipulate the properties of guiding light through 3D shaping of the refractive index contrast at micro or nano-scale level [1–4]. Furthermore, the integration of optical waveguides inside materials has several advantages over bulk materials and plays a vital role in the scalable photonic devices with outstanding precision [1–25]. The initial modification aspect rather took place on the silica glass structure followed by low laser pulse energy (below an intermediate level) and low-repetition rate result in a nearly isotropic index change typically in the order of + 3-7 × 10−3 magnitude in the type-I waveguides [3,6,8]. On the contrary, the optical waveguides inside materials (at the intermediate level of pulse energy) exhibit quite dissimilar properties accompanied by the realization of local anisotropic nature. Change in the refractive index reaches to be about −10−2 significantly in the type-II waveguides, where there are nanogratings formation in specific compositions (i.e., fused silica, alkali-free or low alkali aluminoborosilicate glass, ULE glass and germano-silicate glass) [2–5].
Firstly, the fabrication of 3D-waveguides with the unique technique of ultrafast laser focusing inside silica glass were pioneered by Hirao’s group in 1996 . Since then, the technology has been enabled to virtually make a variety of 3D-devices inside materials with and without rare earth doping, leading to enlarge applications in an increasing number of fields in the daily life [1–25]. One of the most promising and exciting application of fs laser direct writing and related components (splitters, couplers and more generally photonics integrated circuits) is to fabricate low-loss waveguides in a wide variety of transparent materials [10–24]. There are few reported significant examples of the low-loss waveguides written in the glassy or crystalline optical materials that has been reported in the literature [10–24], as follows. The lowest propagation loss is of about 0.15-1.47 dB/cm achieved with chalcogenide glasses [10,11], 0.3-1.0 dB/cm for silica glasses [12–14], 0.4-0.85 dB/cm for phosphate glasses , 0.2-0.5 dB/cm for borosilicate BK7 glasses [17,26], 0.2 dB/cm for bismuth borate glasses , 0.3-3 dB/cm for PMMA polymers [19,20] and 0.5-3.5 dB/cm for crystals [21–24]. Despite silicate, chalcogenide, phosphate, borate and polymer based optical components and photonic devices have already implemented, the precision and quality are still a challenging issue [10–17]. However, the main characteristics of silica based multicomponent glass materials are high surface quality, excellent thermal and chemical stability as well as ultralow transmission losses in the visible and near-infrared range. These superior properties of silica materials substantially overcome the shortcoming of polymer, chalcogenide and phosphate based materials. Concurrently, silica based multicomponent glass materials will be enhanced 3D integrated photonic device efficiency and associated functionality prudently to diversify the application fields.
For instance, Eaton et al. have demonstrated low loss-waveguides (0.2 dB/cm at 1550 nm) in the heat accumulation regime using fs laser of 1045 nm with a 1 MHz repetition rate in AF45 and in Eagle 2000 alkali free or low alkali alumino-borosilicate glasses . Recent results from Thiel et al. have shown an inscription of waveguide bundles using Bragg grating structures inside fused silica (1.0 dB/cm at 1550 nm) . Moreover, the Bragg grating structures are also effectively inscribed in AF32 thin echo and Borofloat-33 glasses . It has been pointed out that the high content of Al2O3 and alkaline-earth metal oxides (AF32 glass) can achieve the anisotropic refractive index change due to the formation of nanograting consisting of sheet like cavities with a sub-wavelength periodicity, which increase the refractive index by the same order of magnitude as observed in silica glass . These advances enable us to develop low-loss waveguides and explore underlying structure inside AF32 glass. For comparison, we also took Schott Borofloat-33 commercially available glass. The present work mainly deals with the fabrication and controlling optical properties of AF32 and Borofloat-33 glass waveguides as a function of pulse energy. The lowest propagation loss of AF32 glass attained was of 1.1 ± 0.31 dB/cm at 632.8 nm. We performed micro-Raman spectroscopy mainly to explore the laser induced structural modifications. To this end, the laser-induced point defects was also tested by the help of photoluminescence studies.
2. Experimental details
We use the standard 3D-direct fs laser writing technique inside Schott AF32 and Schott Borofloat-33 glasses. These commercial glasses were dissected into rectangular shape with a typical dimension of 10 (x-axis) × 10 (y-axis) × 1 (z-axis) mm3 and then polished to reach an optical quality. The modification of the glass plates was accomplished using a commercial fs laser operating at an emitting wavelength of 1031 nm (Origami-10 XP, Onefive) with a pulse duration of 420 fs and tunability of repetition rate accessible between 1 kHz and 1000 kHz. Further, 800 µm width of the slit was introduced in front of an objective lens, in this manner, to regulate the focal spot shape at the focus. An aspheric lens (40 × microscopic objective with a 0.6 NA, numerical aperture) was used to focalize the laser beam into 200 µm below surface of the glass plates. Subsequently, the laser pulse energy was reckoned after microscopic objective focusing the laser beam straightly into the glass plates. Ultimately, the laser inscription was carried out across the glass plates and tested with different pulse energies, while keeping the line spacing of 100 µm to avoid any overlapping stress effects. Type-I and type-II waveguides were promptly made by using a constant writing speed of 5.6 mm/s and a repetition rate of 50 kHz. After laser processing, the end-facet of the glass waveguides was polished to optical quality, before, optical measurements.
First, the fs laser induced permanent modifications were ensured using an optical microscope in transmission mode. Then, the end-face coupling technique and quantitative phase microscopy (QPM) were used to discriminate not only type of mode formation, but also study the complicated structures inside modified glasses. QPM measurement was done carefully in the presence of white light through a 20 × focusing objective into the glass plates. Finally, a quantitative phase mapping of the structures was produced by the QPM software (Iatia vision sciences) to estimate an average index change across the laser track. Micro-Raman spectroscopy (Renishaw in Via Raman spectrometer) was useful to analyze the Raman signal at the cross-section of the glass waveguides with an excitation wavelength of 532 nm and an objective with a magnification of 20 × NA of 0.4. Photoluminescence (PL) spectra were recorded at the cross-section of the glass waveguides using a spectrophotometer (Nanofinder FLEX2, Tokyo Instruments, Inc). The set up was integrated with an optical microscope, where green laser (532 nm) was used to focus on the end-facet of the glass plates through a 50 × focusing objective with a 0.25 NA. All spectroscopic measurements were done post-mortem and carried out at room temperature (RT).
3. Results and discussion
3.1 Optical properties of waveguides
Figure 1 exemplifies an optical microscope (Axio Imager, Carl Zeiss) images of the waveguide cross-section for different pulse energies. It is worth noting that there is a significant increment in diameter and length of the laser track, when increasing pulse energy from 0.25 up to 2.61 µJ. These laser-induced single and long “striations” are usually referred to type-I or type-II waveguide formation depending on the sign of the refractive index change [3,27]. Further, this type can be tentatively confirmed by the QPM measurement, which can be correlated with 3D-image analysis of the cross-section. As it can be seen in Fig. 1, both diameter and length of the laser tracks are growing with pulse energy followed by small pits on the waveguide cross-section has been identified beyond this pulse energy of 0.75 µJ in AF32 glass. It should be noted that Borofloat-33 glass waveguides, whose diameter and length follows the same trend observed for AF32 glass. However, there are black lines in the center part of the waveguide cross-section for Borofloat-33 glass, which is a characteristic feature of negative index changes and thus type-II waveguide modifications. In contrast, surrounding “white color” section is likely related to the stress induced by the permanent volume changes within the irradiated area and this can be exploited to guide the light. By comparing the results obtained in both glasses, one can recognize that the diameter of AF32 glass waveguides is nearly two-times higher than that seen in diameter related to Borofloat-33 glass waveguides despite the same laser parameters were used.
The experimental setup to probe the waveguides by means of end-face coupling with a He-Ne laser source (632.8 nm) is described elsewhere [13,22–24]. Figure 1 shows the experimental mode profile cartography of AF32 and Borofloat-33 glass waveguides for TE polarization. It assesses the intensity distribution of guiding mode profile images and reveal almost circular as well as symmetric features, whatever pulse energy may be used in the investigated energy range. From Fig. 1, it should be noted that the diameter of mode is marginally increasing with respect to pulse energy in AF32 glass. This represents higher coupling losses due to the substantial mismatch between an incoming light and numerical aperture of respective waveguides . On the other side, it is clear that rising in the pulse energy in Borofloat-33 glass follows significant reduction in the waveguide mode diameter. This can be understood as a fingerprint of declining in the transmission losses of Borofloat-33 glass. Other remarkable features are that the mode diameter is higher for AF32 glass than for Borofloat-33 glass waveguides. Additionally, there is no documentation of the mode formation below the threshold pulse energy of 0.25 µJ for AF32 and 0.43 µJ for Borofloat-33 glasses, respectively.
Using the end-face coupling technique, the insertion losses (ILs), the propagation losses (PLs) and the coupling losses (CLs) of AF32 and Borofloat-33 glass waveguides are calculated and discussed, as follows. The ILs are computed using the following formula , where Pout is output beam power coming out of the waveguide end facet and Pin is power of the input beam passing into the waveguide [13,15,22]. The CLs are roughly estimated by using the formula dB , taking into consideration the overlap of the incident light beam (w1) and the waveguide mode (w2) profile. Finally, the PLs are computed from (ILs-CLs)/L (dB/cm), here L is the length of the waveguides . In the present case, at first, the experiment is carried out carefully from low pulse energy to high pulse energy and vice versa. Similarly, it is repeated five times, in such a way, we can get reliable losses because the coupling between an incoming laser beam and the objective waveguide is slightly varied for each measurement. In addition, we took into consideration the Fresnel losses of AF32 and Borofloat-33 glass waveguides at end-facets. The calculated PLs as a function of pulse energy are shown in Fig. 2 with error bars. It is worth mentioning that the obvious reduction in PLs, when increasing pulse energy until 1.13 µJ and thereafter we observe slightly higher losses in AF32 glass. Similarly, an identical trend, but more pronounced was observed in Borofloat-33 glass. There is likely a significant contribution of not only Rayleigh scattering, but also of the non-bridging oxygen hole centers (NBOHCs), which is confirmed by the photoluminescence and other possible point defect centers (i.e., AlOHC, AlE’ and SiE’) [9,28].
It is necessary to evaluate index change across the waveguide in order to precisely define the type-I or type-II modifications. The positive or negative index changes rely on the thermal characteristics of materials and laser writing parameters. The mean value of the refractive index across the laser written waveguides can be inspected by QPM measurement, where the laser polarization (E) is normal to the writing direction (v) [3,27]. However, this is not a direct pathway to evaluate refractive index change across the written laser track, but it is a mean value. Typical QPM images of type-I (AF32) and type-II (Borofloat-33) glass waveguides are shown in Figs. 3(a) and 3(b), respectively. The indication of white color line corresponds to positive phase change (i.e., increase in index change), while the black color area demonstrates a negative phase change in the modified materials. Corresponding the typical integrated profile picture of AF32 (type-I) (Fig. 3(a)) seems to be quite complementary to Borofloat-33 profile (type-II) (Fig. 3(b)) image across the waveguide. In the case of Borofloat-33 profile image, the positive part is principally attributed to stress generated around type-II modifications, as shown in Fig. 3(b).
An extracted phase change (Δφ, rad) across the waveguide is plotted versus pulse energies along with error bars, as presented in Fig. 4. It is noteworthy that there is ascending order in the amplitude of positive phase change area (i.e., there is an increment in the refractive index) with increasing pulse energy in AF32 glass waveguides. The AF32 glass waveguides exhibit higher index positive contrast likely due to the higher densification, defects and related structural changes of the glass network. This will be discussed below, when we are analyzing Raman and PL studies. On the other hand, the refractive index changes in Borofloat-33 glass waveguides are characterized by a central black region with a negative phase shift corresponding to the glass expansion, as shown in Fig. 4 [3,27]. It is important to underline that the amplitude of the negative part (decreasing) are following opposite trend with pulse energy.
The simple and feasible way to correlate our loss measurement with composition of the glasses was adopted in order to realize type-I and type-II waveguide formation. The chemical composition of AF32 glass is typically in the range of (55% - 65%) SiO2 – (15% - 20%) Al2O3 – (5% - 10%) B2O3 – (10% −15%) alkaline earth metal oxides (CaO, MgO) . Whereas, composition of Borofloat-33 glass is about 81% SiO2 – 13% B2O3 – 4% Na2O / K2O – 2% Al2O3 . The glass transition temperatures (Tg), and densities are 525 °C, 2.23 g/cm3 for Borofloat-33 glass and 717 °C, 2.43 g/cm3 for AF32 glass, respectively. Hence, the higher content of Al2O3 led to the higher rigidity, higher glass transition temperature and higher densification in comparison to the low content of Al2O3 and high content of SiO2 in Borofloat-33 glass . When the repetition rate is typically below 100 kHz, the time between two successive laser pulses is higher than the thermal diffusion time (3-4 µs) around the laser focal spot . At below laser threshold pulse energy, the maximum temperature remains below the glass melting temperature . In the course of thermal diffusion time, the glass is, thus, unable to have structural rearrangements themselves completely taken place. So, the energy relaxation leads to the formation of color centers (e.g. non-bridging oxygens hole centers and SiE’) likely accompanied by a slight defect assisted densification that are at the root of permanent modifications in AF32 glass .
The mechanism is quite different in Borofloat-33 glass, because it has lower glass transition temperature (Tg) and higher ability to absorb the pump wavelength. When increasing the laser pulse energy above threshold, more and more energy is deposited within the focal volume, leading to the local melting and resolidification of the glass. This results in an increase of the glass fictive temperature give rise in a glass expansion and thus negative index changes. This permanent negative volume change is accompanied by a positive elastic strain in and out the laser, resulting in the surrounding positive index changes.
Therefore, we suggest that such alumino-borosilicate glasses consisting of high Al2O3 content and alkaline earth metal oxides (rather than alkali elements) might be helpful to have good waveguides with minimal propagation losses. Note that, similar glass composition (AF32) allows to induce nanogratings and related form of birefringence as well provided that adequate irradiation conditions are used (mainly a high pulse number >104), as reported by Fedotov et al. in Ref. 4.
3.2 Structural and photoluminescence studies
The renowned spectroscopic techniques (µ-Raman and PL) can be applied to investigate femtosecond laser induced structural modifications, including densification or expansion and point defect formation in AF32 and Borofloat-33 glass waveguides.
Raman spectra were recorded on cross-section of the glass waveguides before and after femtosecond laser and typical normalized Raman spectra are shown in Fig. 5 in the range of 150-1700 cm−1. The main Raman band located at about 453 cm−1 is ascribed to the bending vibration mode of the Si-O-Si bond (T-O-T bending mode of the TO4 network structure) in six-membered rings . Two relatively sharp Raman bands recognized at ~488 cm−1 and ~599 cm−1, known as D1 and D2 defect bands, commonly referred to symmetrical stretching mode of bridging oxygens in four- and three- membered silicate ring structures [29–32]. The presence of small hump clearly visible at about 760 cm−1 belongs to the six membered rings with a BO4 tetrahedra . The band at 800 cm−1 is the signature of Si-O-Si bending . Besides, a feeble band at about 806 cm−1 is arisen due to the three-membered rings of BO3 triangles (“boroxyl rings”) in borate network [29,32]. There are four Raman bands assessed at about 910 cm−1, 980 cm−1, 1050 cm−1 and 1130 cm−1 in the high wavenumber region, which has been assigned to the Qn units (n = 1, 2, 3 and 4, bridging oxygens) in the silica group [30,31]. Further, a broadband spanning in the range of 1300-1500 cm−1 is a characteristic feature of the B-O- stretching bonds associated with a large number of borate groups [32–34].
The remarkable increment in height of the D1 and D2 defect bands after femtosecond laser irradiation are observed in contrast to non-irradiated Borofloat-33 glass, see in Fig. 5 inset. This is an indication of an increment in the three- and four-membered ring structures in the silica network following femtosecond laser ionization. The increment is consistent obviously with higher densification, higher fictive temperature and compaction of the glass network [29–31]. Further, it is of notified that the main band related to the Si-O-Si units is shifted towards higher wavenumber side in the irradiated zone in comparison with the non-irradiated one. Thus, we can say that the blue shift is in the irradiated zone as a consequence of rigidity, decrease in an average bond angle and compaction of the glass network [29,31]. It is observed that the broadband spanning in the range of 1300-1500 cm−1, high wavenumber region, is found to be enhanced and at the same time, Si-O-Si (800 cm−1) bending band is quenched. This behavior evinces transparently an increment in the non-bridging oxygens with BO3 triangular units and loosing of diborate groups [33,34].
In comparison to the Raman spectrum of Borofloat-33 glass (non-irradiated), the main band is shifted by about 30 cm−1 towards higher wavenumber side in AF32 glass. This might be due to the presence of low-content of silica, chemical changes and structural changes in silicate and borate networks . One can underline that the presence of high content of Al2O3 and alkaline earth metal oxides results in the intensity of Raman bands in the range of 900-1200 cm−1 drastically amplified. While, other bands especially at about 760 cm−1 completely vanished and 800 cm−1 is quenched as well, as shown in Fig. 5. It is suggested that these structural modification could be related to the Al2O3 preferentially bonding with silicon that led to an enhancement not only in the number of SiO4 tetrahedral units with non-bridging oxygens, but also the Si-O-Al units including diborate groups. Upon femtosecond laser irradiation, taking close look on spectrum, there is a slight densification occurred in AF32 glass as compared to non-irradiated one. The rest of the bands are found to be remaining the same level under femtosecond laser irradiation.
PL spectra of AF32 and Borofloat-33 glass waveguides are recorded in the 540-865 nm spectral region, while exciting at 532 nm laser, before and after femtosecond laser irradiations. PL spectra of Borofloat-33 glass waveguides are shown in Fig. 6(a). PL spectraconsist of a strong and broad asymmetric red emission band identified at about 680 nm over the range of 560-865 nm. This is attributed to the signature of non-bridging oxygen hole centers (NBOHCs), which are mainly originating from the modified glasses [1,35]. In addition, the weak band identified at about 546 nm might be ascertained to silanol groups. In fact, the intensity of the NBOHCs PL modified region is at least fivefold higher than that of PL in unmodified Borofloat-33 glass [1,35]. When comparing the peak positions in modified and unmodified glass, we can perceive a red-shift in the modified glass, as shown in Fig. 6(a). Such a shift could be related to changes in the bond-angle distribution and bond length between silicon (Si) and oxygen (O) inside the network. Further, it was pointed out that an increment in the NBOHCs luminescence and full-width at half maximum (FWHM) with pulse energy, because the perturbed structural changes and enhancement of strained bonds of Borofloat-33 glass waveguides. Similar behavior of PL was notified in AF32 type-I glass waveguides with respect to pulse energy (not shown here). Normalized PL spectra of AF32 and Borofloat-33 glasses for typical pulse energy of 2.61 µJ are shown in Fig. 6(b). It can be clearly seen from Fig. 6(b), a red-shift of NBOHCs in AF32 glass has been notified as compared to Borofloat-33 glass. Moreover, there are slight changes in the FWHM. It is numerically estimated that an integrated area of the NBOHCs band in AF32 is found to be higher than the integrated area of the NBOHC band in Borofloat-33 glass. Furthermore, the point defects probably increase in the densification of multicomponent glasses due to reduction of valence in the SiO2 ring structure under femtosecond laser irradiation as evidenced by the Raman spectroscopy. From a comparison of the Raman and PL spectra of two studied glasses, it is concluded that higher content of Al2O3 and alkaline earth metal oxide (AF32) glasses provide higher densification, NBOHC defect centers and Si-O-Al networks that could likely be offered lowest propagation losses of the written optical waveguides.
In summary, type-I and type-II modifications in Schott AF32 and in Schott Borofloat-33 glasses, respectively, have been confirmed by QPM and optical microscope images. Higher refractive index and higher densified waveguides can provide lowest propagation losses typically 1.1 ± 0.31 dB/cm at 632.8 nm in AF32 glass in contrast to Borofloat-33 glass waveguides. Higher NBOHCs defect concentration and red-shift have also been observed in AF32 glass using PL measurement in comparison to Borofloat-33 glass. The results are emphasized here that AF32 glass should be potentially improved in the production of highly efficient light guiding structures with relatively low propagation losses. Therefore, isotropic nature of Schott AF32 (alkali free alumino-borosilicate) glass waveguides are promising candidates and expected to apply for not only passive waveguides, optical communication applications, non-linear devices, but also integrated photonic devices.
National Natural Science Foundation of China (No.11574181, 61631166001); the Fundamental Research Funds of Shandong University (2014JC047),; Open Research Fund of State Key Laboratory Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences (201514). It is also funded by Opto-Flag Project a “Programme Innovation and entrepreneuriat Prématuration” of IDEX Paris Saclay.
References and links
1. J. Martínez, A. Ródenas, T. Fernandez, J. R. Vázquez de Aldana, R. R. Thomson, M. Aguiló, A. K. Kar, J. Solis, and F. Díaz, “3D laser-written silica glass step-index high-contrast waveguides for the 3.5 μm mid-infrared range,” Opt. Lett. 40(24), 5818–5821 (2015). [CrossRef] [PubMed]
2. M. Lancry, B. Poumellec, J. Canning, K. Cook, J.-C. Poulin, and F. Brisset, “Ultrafast nanoporous silica formation driven by femtosecond laser irradiation,” Laser Photonics Rev. 7(6), 953–962 (2013). [CrossRef]
3. M. Lancry, B. Poumellec, A. Chahid-Erraji, M. Beresna, and P. G. Kazansky, “Dependence of the femtosecond laser refractive index change thresholds on the chemical composition of doped silica glasses,” Opt. Mater. Express 1(4), 711–723 (2011). [CrossRef]
4. S. S. Fedotov, R. Drevinskas, S. V. Lotarev, A. S. Lipatiev, M. Beresna, A. Cerkauskaite, V. N. Sigaev, and P. G. Kazansky, “Direct writing of birefringent elements by ultrafast laser nanostructuring in multicomponent glass,” Appl. Phys. Lett. 108(7), 071905 (2016). [CrossRef]
5. F. Chen and J. R. Vazquez de Aldana, “Optical waveguides in crystalline dielectric materials produced by femtosecond-laser micromachining,” Laser Photonics Rev. 8(2), 251–275 (2014). [CrossRef]
6. S. Eaton, H. Zhang, P. Herman, F. Yoshino, L. Shah, J. Bovatsek, and A. Arai, “Heat accumulation effects in femtosecond laser-written waveguides with variable repetition rate,” Opt. Express 13(12), 4708–4716 (2005). [CrossRef] [PubMed]
8. D. J. Little, M. Ams, S. Gross, P. Dekker, C. T. Miese, A. Fuerbach, and M. J. Withford, “Structural changes in BK7 glass upon exposure to femtosecond laser pulses,” J. Raman Spectrosc. 42(4), 715–718 (2011). [CrossRef]
9. R. R. Thomson, S. Campbell, I. J. Blewett, A. K. Kar, D. T. Reid, S. Shen, and A. Jha, “Active waveguide fabrication in erbium-doped oxyfluoride silicate glass using femtosecond pulses,” Appl. Phys. Lett. 87(12), 121102 (2005). [CrossRef]
10. J. Lapointe, Y. Ledemi, S. Loranger, V. L. Iezzi, E. Soares de Lima Filho, F. Parent, S. Morency, Y. Messaddeq, and R. Kashyap, “Fabrication of ultrafast laser written low-loss waveguides in flexible As2S3 chalcogenide glass tape,” Opt. Lett. 41(2), 203–206 (2016). [CrossRef] [PubMed]
11. M. Hughes, W. Yang, and D. Hewak, “Fabrication and characterization of femtosecond laser written waveguides in chalcogenide glass,” Appl. Phys. Lett. 90(13), 131113 (2007). [CrossRef]
12. S. L. Li, P. Han, M. Shi, Y. Yao, B. Hu, M. Wang, and X. Zhu, “Low-loss channel optical waveguide fabrication in Nd3+-doped silicate glasses by femtosecond laser direct writing,” Opt. Express 19(24), 23958–23964 (2011). [CrossRef] [PubMed]
13. W.-H. Yuan, J.-M. Lv, X.-T. Hao, and F. Chen, “Optimization of waveguide structures for beam splitters fabricated in fused silica by direct femtosecond laser inscription,” Opt. Laser Technol. 74, 60–64 (2015). [CrossRef]
14. L. Tong, R. R. Gattass, I. Maxwell, J. B. Ashcom, and E. Mazur, “Optical loss measurements in femtosecond laser written waveguides in glass,” Opt. Commun. 259(2), 626–630 (2006). [CrossRef]
15. B. H. Babu, M. Niu, X. Yang, Y. Wang, L. Feng, W. Qin, and X.-T. Hao, “Systematic control of optical features in aluminosilicate glass waveguides using direct femtosecond laser writing,” Opt. Mater. 72, 501–507 (2017). [CrossRef]
16. T. Toney Fernandez, P. Haro-González, B. Sotillo, M. Hernandez, D. Jaque, P. Fernandez, C. Domingo, J. Siegel, and J. Solis, “Ion migration assisted inscription of high refractive index contrast waveguides by femtosecond laser pulses in phosphate glass,” Opt. Lett. 38(24), 5248–5251 (2013). [CrossRef] [PubMed]
17. J. A. Dharmadhikari, A. K. Dharmadhikari, A. Bhatnagar, A. Mallik, P. C. Singh, R. K. Dhaman, K. Chalapathi, and D. Mathur, “Writing low-loss waveguides in borosilicate (BK7) glass with a low-repetition rate femtosecond laser,” Opt. Commun. 284(2), 630–634 (2011). [CrossRef]
18. W. Yang, C. Corbari, P. G. Kazansky, K. Sakaguchi, and I. C. S. Carvalho, “Low loss photonic components in high index bismuth borate glass by femtosecond laser direct writing,” Opt. Express 16(20), 16215–16226 (2008). [CrossRef] [PubMed]
19. W.-H. Yuan, J.-M. Lv, C. Cheng, X.-T. Hao, and F. Chen, “Waveguides and proportional beam splitters in bulk poly(methyl methacrylate) produced by direct femtosecond laser inscription,” Opt. Mater. 49, 110–115 (2015). [CrossRef]
21. W. Nie, Y. Jia, J. R. Vázquez de Aldana, and F. Chen, “Efficient second harmonic generation in 3D non-linear optical lattice-like cladding waveguide splitters by femtosecond laser inscription,” Sci. Rep. 6(1), 22310 (2016). [CrossRef] [PubMed]
22. J. Lv, X. Hao, and F. Chen, “Green up-conversion and near-infrared luminescence of femtosecond-laser-written waveguides in Er3+, MgO co-doped nearly stoichiometric LiNbO3 crystal,” Opt. Express 24(22), 25482–25490 (2016). [CrossRef] [PubMed]
23. C. Zhang, N. Dong, J. Yang, F. Chen, J. R. Vázquez de Aldana, and Q. Lu, “Channel waveguide lasers in Nd:GGG crystals fabricated by femtosecond laser inscription,” Opt. Express 19(13), 12503–12508 (2011). [CrossRef] [PubMed]
24. W. Nie, R. He, C. Cheng, U. Rocha, J. Rodríguez Vázquez de Aldana, D. Jaque, and F. Chen, “Optical lattice-like cladding waveguides by direct laser writing: fabrication, luminescence, and lasing,” Opt. Lett. 41(10), 2169–2172 (2016). [CrossRef] [PubMed]
26. S. M. Eaton, M. L. Ng, J. Bonse, A. M. -Blondin, H. Zhang, A. Rosenfeld, and P. R. Herman, “Low-loss waveguides fabricated in BK7 glass by high repetition rate femtosecond fiber laser,” Appl. Opt. 47(12), 2098–2102 (2008).
27. A. Paleari, E. Franchina, N. Chiodini, A. Lauria, E. Bricchi, and P. G. Kazansky, “SnO2 nanoparticles in silica: Nanosized tools for femtosecond-laser machining of refractive index patterns,” Appl. Phys. Lett. 88(13), 131912 (2006). [CrossRef]
28. B. Hari Babu, M. Lancry, N. Ollier, H. El Hamzaoui, M. Bouazoui, and B. Poumellec, “Radiation hardening of sol-gel derived silica fiber performs through fictive temperature reduction,” Appl. Opt. 55(27), 7455–7461 (2016). [CrossRef] [PubMed]
29. M. H. Manghnani, A. Hushur, T. Sekine, J. Wu, J. F. Stebbins, and Q. Williams, “Raman, Brillouin and nuclear magnetic resonance spectroscopic studies on shocked borosilicate glass,” J. Appl. Phys. 109(11), 113509 (2011). [CrossRef]
30. A. K. Yadav and P. Singh, “A review of the structures of oxide glasses by Raman spectroscopy,” RSC Advances 5(83), 67583–67609 (2015). [CrossRef]
31. T. Seuthe, M. Grehn, A. Mermillod-Blondin, H. J. Eichler, J. Bonse, and M. Eberstein, “Structural modifications of binary lithium silicate glasses upon femtosecond laser pulse irradiation probed by micro-Raman spectroscopy,” Opt. Mater. Express 3(6), 755–764 (2013). [CrossRef]
32. W. L. Konijnendijk and J. M. Stevels, “The structure of borosilicate glasses studied by Raman scattering,” J. Non-Cryst. Solids 20(2), 193–224 (1976). [CrossRef]
33. D. Maniu, T. Iliescu, I. Ardelean, S. Cinta-Pinzaru, N. Tarcea, and W. Kiefer, “Raman study on B2O3-CaO glasses,” J. Mol. Struct. 651–653, 485–488 (2003). [CrossRef]
34. B. Hari Babu and V. V. Ravi Kanth Kumar, “Warm white light generation of γ-irradiated Dy3+, Eu3+ codoped sodium aluminoborate glasses,” J. Lumin. 169, 16–23 (2016). [CrossRef]
35. M. Lancry, B. Hari Babu, N. Ollier, and B. Poumellec, “Radiation hardening of silica glass through fictive temperature reduction,” Int. J. Appl. Glass Sci. 8(3), 285–290 (2017).