Here, single-mode waveguides are written in silica glass by femtosecond (fs) laser. The waveguides exhibit a low propagation loss of 0.15 dB/cm and a symmetrical mode field. The loss performance and the mode field symmetry are demonstrated to strongly depend on the pulse repetition and polarization of fs laser, respectively. Raman and photoluminescence properties indicate that an increase of refractive index originates from an increment in the total density of glass networks consisting of small and large rings, but only increasing the number of small rings does not necessarily lead to an increase in refractive index. The long working distance of the objective lens promises to fabricate low-loss waveguides in a large depth of glass, which would inspire manufacturing integrated photonic devices by fs laser writing. Clarifying the mechanism of refractive index change offers a new perspective to understand the fs laser-matter interaction.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Embedding optical elements three-dimensionally in the transparent glasses and crystals involving controlled structure modifications and optical properties is of highly scientific and technological significance, but it remains a tremendous challenge for most micro-fabrication techniques [1–8]. In the past decades, fs laser inducing refractive index change as well as writing waveguides has evolved to be a robust technique for rapidly modifying the local structures and integrating photonic circuits in a variety of glasses [2,3,9,10]. Up to now, a plethora of researches about fs laser writing waveguides and their applications in quantum photonics and telecommunications have been demonstrated. However, the effects of spherical aberration, self-focusing and nonlinear absorption usually result in formation of an asymmetrical structure and refractive index distribution within the waveguide, which could lead to an asymmetrical mode field (MF) pattern of guided light [1,3,11–16]. The small heat accumulation effect makes this symmetry even worse with ellipticity typically smaller than 0.8 for the waveguides in the silica glass written by fs laser with various repetition rates from 1 kHz to several MHz [11–15,17]. Recently, complex beam-shaping techniques or post-annealing has been adopted to modify the morphology of the fabricated waveguides and improve the quality of MF [14,15,18–22]. Unfortunately, obvious asymmetry in the MF pattern of waveguides in silica still appears. Furthermore, the asymmetry of MF pattern prevents the reduction of propagation loss, which is usually higher than 0.3 dB/cm in the silica glasses.
Besides the efforts to mitigate MF asymmetry and propagation loss, the underlying physics resulting in the refractive index change is also a subject of intense study and debate. Until now, several mechanisms, such as an increase in the fictive temperature, densification and/or defects have been claimed to be responsible for the observed increase in refractive index in the fs laser-irradiated glass, but obvious inconsistence exists in the studies from different authors [3,23–25]. Therefore, uncovering the mechanisms of fs laser-induced refractive index modification is very urgent, which may not only facilitate writing of waveguides with high MF ellipticity and low propagation loss, as well as construction of desirable complex 3D photonic structures in arbitrary dimensions, but also benefits fundamental study and the development of conceptually new photonic devices [4,20,26–28].
Here, we report writing waveguides in silica with highly symmetrical MF by using fs laser, which exhibit a low propagation loss of 0.15 dB/cm at 1550 nm. We demonstrate that the ellipticity can be tuned by controlling the polarization of the writing beam and the obtained highest ellipticity is nearly 1. Furthermore, we reveal that densification resulting from the increase of total matter density is responsible for the increase of refractive index using Raman and photoluminescence spectroscopy. Our discussions indicate that increase of small rings do not necessarily lead to higher refractive index, in contrast to previous proposals. Our work will be important for designing and fabricating three dimensional integrated photonic devices more efficiently.
2. Experimental methods
A femtosecond laser operating at λ = 515 nm with a repetition rate of 200-1000 kHz and an average power of 100-700 mW was adopted in the experiments. The laser pulse duration was kept at 200 fs. A 50x, 0.42-NA microscope objective was utilized to focus the laser beam into the silica glass at a depth of approximately 220 μm. Waveguides were written by translating the samples with scan rates in the range of 50-1000 μm/s. Circular polarization and linear polarizations parallel and perpendicular to the scan direction were controlled by a quarter and half wave plate, respectively. After waveguide writing, the end facets of samples were ground and polished. Butt-coupling was used to characterize the waveguide insertion loss (Li), propagation loss, and mode profile with a 1550 nm laser source, fed by a single mode optical fiber. The insertion loss was measured from the power transmitted through the waveguide between two butt-coupling SMF fibers at the input and the output side, and compared to the power propagated by directly butt-coupled input and output fibers. The mode field profiles were recorded by imaging the light intensity at the output facet of the waveguide samples with a CCD camera.
3. Results and discussion
As has extensively studied and widely used, silica glass is one of the most attractive matrices for photonic circuit formation because of the low cost, large productivity and high compatibility with the silicon photonic devices. Using silica glass as a model platform, we fabricated waveguides with low-loss and symmetrical MF patterns.
Typical waveguides written by fs laser with various powers in the silica glass are shown in Figs. 1(a)–1(e), which exhibit smooth morphology. We can observe significantly elongated structures with two different zones, denoted as dark and bright regions in the cross-section of the waveguides (front view in Fig. 1). Generally, the dark color indicates a negative refractive index change, resulting in no light guiding performance in the dark region, and the bright region corresponds to a positive refractive index change, which acts as the waveguide core [17,29–32]. When the fs laser irradiates the glass with sufficiently high pulse energy, the effects of spherical aberration, self-focusing and nonlinear absorption gives rise to the redistribution of the laser energy along the beam propagation direction. A plasma generation before the geometrical focus with an elongated shape can be expected in the irradiated region [33,34]. When the temperature reaches the softening point of silica in this pre-focal zone, thermal expansion will be triggered. The resultant rarefaction leads to a negative density change accompanied with a decrease in refractive index. The bright region is usually proposed to originate from material densification after fs laser induced reorganization of the structural networks with an increase in the numbers of 3- and 4-ring structures and a reduction of the average Si-O-Si angle, as discussed in more detail below.
By controlling the exposure conditions (repetition rate, scan speed, average power, and polarization), we can tune the structures and refractive index change of the waveguides to optimize the performance. For example, thermal diffusion and heat accumulation is enhanced by increasing the repetition rate from 1 kHz to 1.5 MHz, which significantly improves the quality of the MF pattern and the insertion loss of waveguides in the borosilicate and multicomponent glasses [35,36]. Figures 2(a) and 2(b) show the waveguides inscribed by fs laser with 200 kHz and 1 MHz repetition rate, respectively. Waveguides fabricated at 200 kHz exhibit very similar features with that fabricated at 500 kHz (Fig. 1), but the ratio between the height of dark and bright zone is larger in the 200 kHz case. In contrast, the dark region is very small using 1 MHz pulses and the contrast between the bright zone and glass is also small, which is indicative of a small change in refractive index. Figure 2(d) illustrates that the ratio between the height of the dark and bright zone drastically decreases with increase of repetition rates and this is attributed to the increase of thermal effect [11,35]. We measured the input and output powers with and without waveguides by the traditional butt-coupling method to determine the insertion loss, which includes the contribution of propagation loss, total fiber coupling loss and Fresnel loss (Lf ∼ 0.36 dB). Figure 2(d) shows that the insertion loss for waveguides generated with 1 MHz fs laser is much higher than that of the waveguides obtained at 200 and 500 kHz, which is consistent with the refractive index change. In addition, the minimum insertion loss at telecom wavelength of 1550 nm using 500 kHz pulses (Li = 1.28 dB) is smaller than that using 200 kHz (Li = 1.55 dB) and 1 MHz (Li = 5.19 dB) in the current study, which is different from the previous reported waveguides fabricated in the silica and borosilicate glasses with the optimal repetition rates of 1∼1.5 MHz [11–14,35,36]. In the case of 200 kHz, 500 kHz, and 1 MHz, the optimal laser power for obtaining the lowest insertion loss is about 105, 200 and 600 mW, respectively. The difference in the optimal energy per pulse may be attributed to the influence of thermal accumulation effect on the writing with different laser repetition rate. Furthermore, strong thermal effect in the waveguide can be identified using 1 MHz fs laser and this is also in contrast to the previous proposal that thermal effect was small or negligible in the writing waveguides in the silica due to the high softening temperature and fast heat diffusion rate [11–14,35,36]. This difference may originate from the different experimental conditions, such as the longer working distance and smaller NA here, which allows for thermal accumulation in a larger area. However, our work also implies that too stronger thermal accumulation may be detrimental to the performance of waveguides in the silica due to the small change in the refractive index.
Refractive index change as well as waveguide inscription in glass is driven by the absorption of optical energy from the fs laser, initially owing to photo-ionization (such as multi-photon ionization and tunneling ionization processes), which is dependent on the polarization of the writing beam . This polarization-dependence of photo-ionization promises the potential to modify the distribution of refractive index change and the performance of waveguides by tuning the polarization of fs laser [13,14,37]. We measured the insertion loss of waveguides generated by fs laser with polarization parallel [0°, Fig. 3(b)] to the scan direction. Figure 3(b) demonstrates that the insertion loss strongly depends on the laser power and scan speed, and similar behavior is also found for the circular and perpendicular linearly polarized cases, as shown in Figs. 3(a) and 3(c), respectively. The obtained minimum insertion loss for the waveguides inscribed by circular (denoted as 180°), parallel (0°) and perpendicular (90°) linearly polarized light is 1.24, 1.14 and 1.36 dB, respectively, as shown in Fig. 3(d) with the errors as the measurement uncertainties. It is worth noting that polarization direction (0° and 90°) of the linearly polarized beam has notable influence on the performance of the waveguides, which has been overlooked in the previous reports. These imply that the difference in the waveguides written by circularly and linearly polarized light may not be, as proposed previously, associated with the formation of nanograting structures [13,14]. The lowest propagation loss is derived to be 0.15 dB/cm at 1550-nm wavelength by measuring the insertion loss of waveguides with different lengths, which is lower than most of the reported values [3,11–15,17]. Although the multiscan technique may reduce the propagation loss to be 0.12 dB/cm, but the effective scanning speed of 0.5 μm/s is much lower than that of 100 μm/s in the current work .
The elongated morphology and refractive index distribution in the waveguides usually give rise to highly asymmetrical mode field distribution. We consider that as the photo-ionization rate is higher along the polarization direction of the writing beam, the 90° polarized light may enhance the photo-ionization rate perpendicular [x in Fig. 3(e)] to the scan direction, which could modify the refractive index distribution in the respective direction and improve the symmetry of model field pattern [13,14,37]. To confirm this idea experimentally, we record the MF images of single-mode waveguides written with (e) circular, (f) 0° and (g) 90° polarized fs laser, as shown in Fig. 3. Although the cross-sections of the waveguide exhibit obvious elongated shape, the mode fields remain high symmetrical and the typical diameter of the mode field is approximately 14 μm. To characterize the symmetry, we define the MF ellipticity (E) as the ration between the diameter along x (dx) and y (dy) direction, E = dx/dy. E is determined to be 0.977, 0.931 and 0.991 in the cases of circular (Ecir), 0° (E0) and 90° (E90) linearly polarized light, respectively, which indicates that the obtained MF patterns possess much higher symmetry than the previous reported waveguides in the silica [1,3,11–15,17]. The Ecir and E90 are comparable or even larger than that of single-mode waveguides in the borosilicate glass written by 1 MHz fs laser with a beam-shaping technique [18,20]. In this case, to measure dx and dy correctly and decrease the error as much as possible, we recorded the images of propagating light out of the waveguides by a CCD camera with controlled distances between the waveguide end-fact and the CCD camera. With optimized coupling, dx and dy could be obtained with high quality fitting of the data points measured at various positions. As a result, we can minimize the errors for the measurement of dx and dy as well as the calculated ellipticity. In addition, our previous study demonstrates that the insertion loss of waveguides strongly depends on the size of the guiding core . Unfortunately, the width of the bright zone corresponding to the waveguide core is approximately 2.4 μm for all the waveguides in the silica glass writing with various polarizations of fs laser, and it is difficult to distinguish the small difference in the waveguide size under the current conditions, as shown in Figs. 3(h)–3(j). Scanning electron microscope may be helpful for determining the exact size of waveguides. More investigations are needed to clarify the intrinsic mechanism about the influence of polarization on the properties of waveguides written in silica glass by fs laser.
Therefore, one of the key observations is that we can produce waveguides with highly symmetrical MF meanwhile keeping the propagation loss as low as 0.15 dB/cm. As far as we know, this is the first time to report the possibility to control the ellipticity of the MF pattern by adjusting the polarization of fs laser. The 50× objective lens used in this study has a working distance of 17 mm in air, which is much larger than that in the previous reports. This method has important implications for the fabrication of three dimensional integrated photonic devices over a large depth.
In order to reveal the structures of the fabricated waveguides, we measured the Raman spectra and mapping on the cross-section of the waveguide. The network of fused silica typically consists of predominantly large 5- and 8-fold ring structures. The pronounced Raman peak at around 430 cm-1 (ω1) in Fig. 4(a) is referred to the Si–O–Si bond rocking and bending vibration in the SiO4 tetrahedra. The two smaller bands at 487 (D1) and 604 cm-1 (D2) are attributed to the symmetrical stretching mode of bridging oxygens in 4- and 3-membered silica rings, respectively [38,39]. Compared to the glass matrix, there is a large increase in Raman intensity related with the 4- and 3-fold ring structure in the bright region, which is consistent with the Raman mapping at the D1 and D2 peaks, as shown in Figs. 4(c) and 4(d). In contrast, compared to the silica glass, only the D2 peak shows enhanced intensity in the dark region and no obvious change is observed for the D1 peak. Additionally, as the Raman peaks at 487 and 604 cm-1 are far away from the luminescence region with wavenumber larger than 1600 cm-1, we do not consider the influence of luminescence on the Raman mapping at 487 and 604 cm-1. Generally, an increase in the population of smaller 3- and 4-fold ring structures is indicative of a decrease in the overall bond angle and the densification of the glass [24,38,40]. Furthermore, Fig. 5(a) shows that all the ω1, D1 and D2 peaks shift to higher frequencies in both modified zones and this frequency shift is usually suggested to be produced by slight buckling of smaller rings with densification as well as a decrease in the mean Si–O–Si angle . Consequently, the refractive index change induced by fs laser irradiation is usually interpreted with respect to the densification of the glass. However, the previous explanations cannot account for the decrease of refractive index in the dark region, as the corresponding population of 4-membered ring structures also increases and the peak shift at ω1 and D1 peaks in the dark zone is larger than that in the bright zone. In addition, Figs. 5(b) and 5(c) demonstrate that the width of ω1 increases in both the laser modified regions, which indicates that refractive index change induced by fs laser in the silica glass is not caused by the fictive temperature effect . To better compare the Raman spectra of the glass and waveguides, we then normalize the Raman intensity to the ω1 peak [Fig. 5(b)] or that at around 800 cm-1 [ω3, which is attributed to vibration in the random network of Si-O bonds, Fig. 5(c)], as done in the previous reports [13,38,39,41]. We find that the normalized Raman spectra are very similar to the reported results. However, it is worth to note that a larger increase in the intensity of D2 is identified in the dark region than that in the bright region, and this is inconsistent with the related negative refractive index change. In the previous studies, most of the discussions on the mechanism of refractive index change have been focused on the bright light-guiding area, but almost no attention has been paid to the structures in the dark zone. Our results show that the intrinsic mechanism is far from fully understood and more work is needed to clarify the possible mechanism for the refractive index change induced by fs laser.
Exposure of the glass to fs laser results in the formation of plasma with high temperature and high pressure, which generates many non-bridging oxygen hole centers (NBOHCs), consisting of Si–O groups with holes trapped on the oxygen atom [1,3]. As a consequence, defect electronic states are induced in the bandgap of silica, and enhanced NBOHC related photoluminescence (PL) can be expected. The PL spectra in Fig. 4(d) show that the emission intensity increases drastically from initial glass to the dark region to the bright region with the peak keeping nearly constant at around 710 nm. Furthermore, two PL shoulders appear with the peak positions at about 680 and 650 nm, which are absent for the silica glass. The PL behavior indicates that fs laser produce more NBOHCs that create new electronic states within the bandgap. The concentration increase of NBOHCs in the whole modified area implies that the generation of NBOHCs may not be responsible for the refractive index change [25,42,43].
As the refractive index change scales linearly with the density change of the glass, as mentioned previously, many previous reports suggested that the refractive index increase is associated with the densification induced by fs laser irradiation in the glass, which is inconsistent with our results [24,34]. Firstly, the increase in the number of 3- and 4-membered ring structures cannot be always connected with densification accompanied with refractive index increase in the waveguides, as the change of refractive index is negative in the dark area. Secondly, there is also obvious contradiction in the normalized relative Raman intensity in the spectra (Fig. 5) and Raman mapping [Figs. 4(b) and 4(c)] at the D1 peak for glass and the modified region, which implies that analysis of the Raman spectra after a usual normalization process may be not valid. Thirdly, it should be noted that the intensity of ω1 peak in the bright zone is larger, but in the dark zone it is smaller than that of the silica glass, which is associated with the relative population of 5-8 membered ring structures in different regions [13,38–41]. As there is an instant thermo-mechanical expansion of the material and propagation of pressure wave in the fs laser exposure zone, besides the rearrangement of the atoms and broken bonds, matter immigration also occurs from the higher temperature part to the low temperature part [3,30,34,44]. Therefore, we suggest that the matter-expansion-assisted densification should be considered as a result of increase of the total density of the networks including small and large rings. In the dark zone, the decrease of 5-8 membered ring structures is responsible for the lower density, although the population of 3-membered ring structure indeed becomes larger. Our results indicate that more small rings do not necessarily lead to higher refractive index, which is different from the previous proposals. Furthermore, we have confirmed that there is no birefringence signal from the fabricated waveguides both in the top view and the front view under crossed polarizers as did in the previous studies, which indicates that the influence of the birefringence on the performance of waveguides is negligible [44,45]. This is reasonable due to the fact that in most of the cases, the typical repetition rate for writing waveguides is 500 kHz and is not favorable for the formation of periodic nanogratings due to the heat accumulation [35,46]. In addition, the net fluence used in the current work is typically smaller than 2.5 mJ/μm2, and a higher net fluence with higher pulse energy and/or smaller scanning speed may be helpful to generate and control the birefringence for applications in certain integrated optics [32,47,48].
A series of waveguides are fabricated by using fs laser writing with controlling repetition rate, polarization, power and scan speed. We found that the performance of the waveguides written by 200 and 500 kHz repetition is better than that inscribed by 1 MHz, which originates from the larger refractive index change in the former cases. A high symmetrical MF pattern and a low propagation loss of 0.15 dB/cm are obtained. The ellipticity can be modified by controlling the polarization of the writing beam and the obtained highest ellipticity is nearly 1, when the polarization of the fs laser is perpendicular to the scanning direction, which is assigned to the enhanced photo-ionization rate along the light polarization direction. Raman and photoluminescence spectra indicate that densification resulting from the increase of total density of the glass network consisting of small (3-4 membered) and large (5-8 membered) rings accompanied with the matter-expansion and network rearrangement is responsible for the increase of refractive index. Only increasing the amount of small rings does not necessarily raise the refractive index. The long working distance of the focusing lens promises fabricating low-loss waveguides with highly symmetrical MF in a large depth, which will be significant for integrating photonic circuits in three-dimension, and revealing the mechanism of refractive index change is also critical for understanding the fs laser-matter interaction.
National Natural Science Foundation of China (U20A20211, 51902286, 51772270, , 61775192, 61905215); State Key Laboratory of High Field Laser Physics (Open Fund); Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences (Open Fund); Fundamental Research Funds for the Central Universities (Open Fund).
The authors declare no conflicts of interest.
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