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Abstract
We report the fabrication of an optical waveguide with a propagation loss below 1 dB/cm in fused silica using the fundamental beam of an Yb:KGW femtosecond laser. By selecting the pulse energy and pulse duration, we obtained a propagation loss comparable with that of a waveguide made using a Ti:Sapphire laser. We demonstrate that the nano-wrinkles and nano-pores, which dominate the losses, are not formed in the modified silica if the pulse energy is below 0.80 μJ and the pulses are shorter than 300 fs.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Femtosecond micromachining of transparent materials has been studied extensively over the past few decades [1,2]. In particular a lot of interest is in the fabrication of photonic components and devices including waveguides [3,4], lenses [5,6], beam splitters [7], bulk [8,9] and fiber [10,11] gratings, waveplates [12], and polarization beam splitters [13]. These devices can be made through changing the refractive index with micro- and nanometer spatial resolution during irradiation with femtosecond laser pulses [14]. This fabrication technique is referred to as laser direct writing (LDW).
Among a variety of transparent materials, fused silica, which is one of the most common types of optical glass, is frequently used in LDW. In particular, irradiation with a tightly focused beam from a femtosecond Ti:Sapphire laser, at a wavelength of around 800 nm, allows one to form a waveguide inside the fused silica with a propagation loss as low as 1 dB/cm [15]. Such a waveguide is suitable for integrated photonic circuits for, for example, quantum optics [12] or fan-out devices for multicore fibers [16], which are several centimeters in size. However, using Ti:Sapphire lasers for LDW is not always convenient because these lasers are bulky and may have stability issues. In contrast, Yb-based femtosecond lasers at a wavelength of around 1 μm, are free from these shortcomings, and are very attractive for femtosecond micromachining and LDW. In particular, Yb:KGW and Yb:KYW crystals with direct laser diode pumping allow these lasers to outperform Ti:Sapphire lasers in terms of footprint, stability, and cost. However, an exact set of laser parameters for fabricating low-loss waveguides still need to be determined. The waveguides fabricated by LDW in fused silica using the fundamental beam of Yb-based lasers is reported to have large propagation loss [17,18], which makes them useless for photonic applications. This large propagation loss is caused by nano-wrinkles and nano-sized pores [18], which are formed in the silica, which is irradiated with the fundamental beam of an Yb-based laser. The fabrication of waveguides with infrared (IR) wavelength has already been reported [19–21], but the necessary conditions for suppressing the nano-sized structures remain unclear in the case of the fundamental beam of Yb-based lasers. In the case of fabricating waveguides with Ti:Sapphire lasers, a refractive index change without nano-sized structures, which is often called a “Regime I” modification [22], can be induced when the duration of the laser pulse is less than 150 fs [22,23]. However, the pulse durations of Yb-based lasers are normally longer than 150 fs [17,18]. Thus, fabricating a low-loss waveguide with a fundamental beam of Yb-based lasers is not feasible from this point of view. The formation of nano-sized structures can be suppressed using a second harmonic (SH) beam [17,24]; however, this results in the fabrication being more costly and less reliable.
In this work, we report a detailed investigation of fused silica modification under irradiation with a tightly focused fundamental beam from an Yb:KGW laser. By studying the modified silica’s morphological and optical property dependence on laser pulse energy and pulse duration, we reveal how to avoid the formation of nano-wrinkles and nano-sized pores with longer laser pulses compared with those of a Ti:Sapphire laser, and we demonstrate a technique that enables fabricating low-loss waveguides in fused silica using a fundamental beam of an Yb:KGW laser.
2. Experimental methods
In the LDW process, we utilized an Yb:KGW laser with a 1028-nm wavelength (PHAROS, Light Conversion Ltd.). The duration of the laser pulse in the femtosecond laser system was changed from 200 to 600 fs with a compressor built in the laser. We set the repetition rate of the laser pulse to 100 kHz. The pulse energy for waveguide fabrication was changed from 0.71 to 0.97 μJ. The spatial profile of the laser beam was modified with an optical slit to fabricate waveguides with a circular cross-section [25]. The beam was focused to a 23 μm2 spot using an objective lens that had a numerical aperture (NA) of 0.42. Here, the size of the spot was determined from the position where the intensity was 1/e2 of the center of the spot. The size of the laser spot was calculated using the formula in Ref. [25], in which the size of the laser spot was calculated based on Gaussian optics. Nonlinear propagation was not considered in estimating the size of the spot. The profile of the fabricated waveguides was actually affected by the nonlinear propagation effect of a femtosecond laser beam [26]. However, the effect was found to be significant at pulse energies of more than 5 μJ in the case of 120-fs pulse duration in a previous report [27], where similar focusing conditions were used. Because we used a pulse energy of less than 1 μJ and a pulse duration of more than 200 fs, we assumed that nonlinear propagation can be negligible. With our condition, the peak intensities of fabrication laser pulses were 0.51–2.1 × 1017 W/m2. The laser beam was focused 400 μm under the surface of a fused silica plate, which was positioned by auto-stages M-511 & M-451 (Physik Instrumente). The polarization of laser light was set to be parallel to the scanning direction. The number of irradiated pulses per unit length was changed from 50 to 1500 pulses/μm by changing the beam scan speed. Here, we calculated the number of irradiated pulses per unit length using dividing the repetition rate of the femtosecond laser by the speed of scanning of our auto-stage. All of these experimental parameters are summarized in Table 1.
Table 1. Experimental parameters for waveguide fabrication.
The propagation loss was evaluated by measuring the optical loss in two waveguides with lengths of 8.7 and 18.7 mm. We fabricated each waveguide in different glass plates, the length of which was 10 and 20 mm, and we measured the propagation loss using the difference in output power from the two waveguides. We used a continuous-wave laser with a 780-nm wavelength for evaluating the propagation loss. The laser beam was focused to the edge of a waveguide having an objective lens with a NA of 0.25. Laser light propagated in the waveguide was collected with another objective lens with a NA of 0.40 and measured with a power meter. We used the same laser beam to estimate the amount of change in the refractive index.
For observing the shape of the fabricated waveguides, we used an optical microscope (VHX-100, Keyence). We cut the waveguide-fabricated fused silica plate with a dicing saw in the direction perpendicular to the waveguide to observe the internal structure of the fabricated region. The cut surface was polished using an ion milling machine (IM 4000, Hitachi High-Technologies) and observed with a scanning electron microscopy (SEM) (S-4800, Hitachi High-Technologies).
3. Results and discussion
The obtained optical microscope images of the LDW waveguides fabricated at pulse energies from 0.75 to 0.97 μJ and at pulse durations from 200 to 430 fs are shown in Fig. 1. At this repetition rate, heat accumulation has been found not to occur in fused silica [28]. The beam scan speed was 2 mm/s, which corresponds to the number of irradiated pulses per unit length of 50 pulses/μm. When the pulse energy is larger than 0.84 μJ, bright and dark dots appear in the waveguide. The longer the pulse, the higher the density of these dots. At the pulse energy below 0.80 μJ, the fabricated waveguides are smooth, and no dots are observed.
Fig. 1. Optical microscope images of LDW waveguides fabricated with different pulse energies and pulse durations. The images were observed from the same direction of irradiation of the femtosecond laser.
The influence of the number of irradiated pulses per unit length was examined at 50 to 1500 pulses/μm by changing the beam scan speed. Here, we applied a pulse duration of 250 fs. The repetition rate of the laser was set to 150 kHz for this experiment. The same types of modification as those in Fig. 1, bright and dark dots at higher pulse energy or smooth modification at lower pulse energy, were observed in all conditions, as shown in Fig. 2(a). However, the threshold pulse energy between the two types of modification is dependent on the number of irradiated pulses per unit length. The threshold pulse energies are shown in Fig. 2(b). The threshold decreased when the number of irradiated pulses increased, and the difference in the threshold between 50 pulses/μm and 1500 pulses/μm was more than 0.2 μJ. In previous research, the influence of the number of irradiated pulses was attributed to the accumulation of semi-permanent defects [29,30]. In the following experiments, we set the number of irradiated pulses per unit length to 50 pulses/μm, which is the same condition as the data shown in Fig. 1.
Fig. 2. (a) Optical microscope images of LDW waveguides fabricated with different number of irradiated pulses per unit length and pulse energies. (b) Threshold pulse energy between two types of modification. The pulse duration was set to 250 fs. The number of irradiated pulses was tuned from 50 to 1500 pulses/μm. The thresholds were measured on the basis of optical microscope images of fabricated waveguides.
To investigate the bright and dark dots in more detail, we took cross-sectional SEM images of LDW waveguides fabricated using 0.89-μJ pulse energy. One can observe from Fig. 3(a) that at this pulse energy, nano-wrinkles were formed in the waveguide at pulse durations from 200 to 430 fs. The wrinkle-like structure was assumed to comprise a different structural configuration of nano-gratings [18]. When the polarization was parallel to the scanning direction, nano-wrinkles were formed in a waveguide, and when the polarization was perpendicular to the scanning direction, a nano-grating structure was formed. The number of wrinkles increased when the pulse duration increased. Because the number of bright and dark dots in Fig. 1 also increased when the pulse duration increased, one may conclude that the dots in the microscope image represent the wrinkles shown in Fig. 3(a).
Fig. 3. (a) Cross-sectional SEM images of rough-patterned waveguides fabricated at different pulse durations. The pulse energy for fabrication was 0.89 μJ. The blue arrow at the upper right of the figure shows the direction of the femtosecond laser beam for fabricating the waveguides. (b) Cross-sectional SEM images of smooth-patterned waveguides fabricated at different pulse durations. The images in the square balloons are higher-magnification observations. The pulse energy for fabrication was 0.71 µJ. Because the refractive index change was caused by changes in chemical bonds [24,25], no structure was evident in the SEM images of 200-, 250-, and 300-fs pulse durations.
Figure 4 shows an optical microscope image of the waveguide fabricated at 0.89-µJ pulse energy and 200-fs pulse duration when a laser with a wavelength of 635 nm propagated through it. The strong scattering, presumably caused by nano-wrinkles, generated the high propagation loss of the fabricated waveguide. Therefore, the data shown in Fig. 1 indicates that the pulse energy of the fundamental beam of the Yb-based laser should be set to less than 0.80 µJ in order to fabricate a low-loss wrinkle-free LDW waveguide.
Fig. 4. Microscope image of rough-patterned waveguide. The pulse energy and pulse duration of the fabrication were 0.89 µJ and 200 fs, respectively. 635-nm laser light propagate through the waveguide. The red points shown in the image are scattered light in the waveguide
Figure 3(b) shows cross-sectional SEM images of waveguides fabricated at the pulse energy of 0.71 µJ. One can observe that in contrast to the rough-patterned waveguides shown in Fig. 3(b), these smooth-patterned waveguides display no nano-wrinkles. However, if the pulse duration was longer than 300 fs, the waveguide cross-section contained nano-pores, which were uniformly distributed over the waveguide cross-section, and the density increased when the pulse duration increased. It is worth noting that they vanished when the pulse duration was less than 300 fs and that only the refractive index change occurred at those pulse durations. Because the refractive index change was caused by changes in the chemical bonds [29,30], no structure is evident in the SEM images of 200-, 250-, and 300-fs pulse durations in Fig. 3(b).
To evaluate the quality of the fabricated waveguides, we measured the propagation loss of the smooth-patterned waveguides fabricated with 0.75-µJ pulse energy, as shown in Fig. 5(a). We found that, when the pulse duration was shorter than 400 fs, the propagation loss was less than 1 dB/cm, which is on the same order as the reported value of the LDW waveguide fabricated using a Ti:Sapphire laser [15]. These results demonstrate that pulse durations of less than 400 fs are required to fabricate a low-loss LDW waveguide.
Fig. 5. (a) Measured propagation loss of waveguides fabricated at different pulse durations. The Pulse energy for fabrication was 0.75 µJ. The accuracy of the measurement was ±0.3 dB/cm. The propagation loss was smaller than the detection limit of our evaluation setup at 400 fs. (b) Example of mode profile of propagating light. The image was obtained by observing the waveguide edge with an optical microscope. The pulse energy and pulse duration for fabrication were 0.75 µJ and 200 fs. (c) Relationship between FWHM of LP01 mode and refractive index change ($\Delta {\textrm{n}}$). The waveguide radius was set to 5 µm. (d) Estimated refractive index change in waveguide fabricated at different pulse durations. The mode profiles observed in the waveguide fabricated using each pulse duration are shown above the graph. The pulse energy for fabrication was 0.75 µJ.
We also evaluated the refractive index change (Δn) of the fabricated waveguide from the spatial profiles of light propagating in the waveguides. Figure 5(b) shows the spatial profile of propagating light at the edge of a waveguide measured with an optical microscope. For the analysis, we assumed that the fabricated waveguides have a step-index profile of the refractive index. The fundamental mode of a circular and step-index waveguide is the lowest order of a linearly polarized (LP) mode, called an LP01 mode [31]. The intensity profile of the LP01 mode can be described using the Bessel and modified Bessel functions, ${\textrm{J}_\textrm{n}}(x )$ and ${\textrm{K}_\textrm{n}}(x )$, as [29]:
Equations (1)–(3) give the relationship between the LP01 mode size and the refractive index change ${\Delta \textrm{n}}$ as shown in Fig. 5(c). Using this curve, we can estimate the refractive index of a fabricated waveguide from a measured profile of propagating light as shown in Fig. 5(d). ${\Delta \textrm{n}}$ is on the order of 10−4–10−3, and the shorter the pulse, the larger the ${\Delta \textrm{n}}$. Because the pulse energy was kept constant in this experiment, a laser pulse had a higher peak intensity when the pulse duration was shorter. This would be a cause of higher ${\Delta \textrm{n}}$ at the shorter pulse duration. The results in Fig. 5(c) are consistent with previous research [32], in which higher ${\Delta \textrm{n}}$ compared with those of the usual Regime I modifications was obtained with few-cycle laser pulses.
Figures 5(a) and 5(d) indicate that both propagation loss and ${\Delta \textrm{n}}$ increased when the pulse duration was shorter. The larger propagation loss can be attributed to defects called non-bridging oxygen hole centers (NBOHC) [29]. Also, changes in Δn may be related to NBOHC and densification due to restructuring of Si-O networks [30]. Because the shorter pulse duration implies higher peak intensity, one may expect the defects’ density to be increased and a change that is more pronounced in the Si-O networks to be achieved. Further investigations are necessary to visualize and quantify the underlying physical mechanisms of the light-induced modification of the silica structure.
The observed decrease in ${\Delta \textrm{n}}$ when the pulse duration increased, as shown in Fig. 5(d), implies that ${\Delta \textrm{n}}$ became negative when the pulse duration was longer than 500 fs. We did not observe a waveguiding effect for fused silica modified with such pulses. To determine if ${\Delta \textrm{n}}$ is negative, we compared the optical microscope images obtained from the same direction to the femtosecond laser beam for fabricating the waveguides.
The optical microscope images of waveguides fabricated at pulse durations of 200 and 600 fs are shown in Fig. 6(a). The pulse energy for fabrication was set to 0.71 µJ. The focal points of the images in Fig. 6(a) were set to 6 µm above or below the center of the fabricated waveguides. It can be clearly seen that the contrasts of the images are inverted depending on the direction of the offset and the pulse durations.
Fig. 6. (a) Optical microscope images of waveguides fabricated using 200-fs and 600-fs pulses. The pulse energy for fabrication was 0.71 µJ. The focal points of the optical microscope were set to 6 µm above or below the center of the fabricated waveguides. When the waveguide was placed more closely to the imaging lens than the focal plane, we defined the sign of the focus offset as being plus. (b) Model used for explanation of contrast inversion shown in (a).
The contrast in a transparent object is known to change depending on the sign of defocusing [33]. The mathematical formulation of the effect is shown in Ref. [33]. The contrast of a transparent object, $C({x,y} )$, can be expressed using the following equation.
$\mathrm{\Delta }f$, k, and $\phi $ are the amount of defocusing, wavenumber, and phase difference introduced by the object, respectively. The positive sign of $\mathrm{\Delta }f$ means that the fabricated waveguides approach an imaging lens [same as in Fig. 6(a)]. Here, we assume that the fabricated waveguides have a step-index profile of the refractive index [a model for calculation is shown in Fig. 6(b)]. In this case, $\; \phi $ becomes and $C({x,y} )$ isTherefore the contrast of a waveguide is proportional to $- \mathrm{\Delta }f\mathrm{\Delta }n$, which means that the sign of contrast of a fabricated waveguide depends on the signs of defocusing and the change in the refractive index.
Because the refractive index change was positive when a waveguide was fabricated with a 200-fs laser pulse, the inversion of contrast of a waveguide fabricated with a 600-fs laser pulse means the sign of refractive index change was negative. We suggest that the negative ${\Delta \textrm{n}}$ was caused by the increased number of the nano-sized pores for longer pulses.
Several mechanisms have been proposed about the formation of nano-wrinkles and nano-pores, for example, the effects of nanometric inhomogeneities of glass [34] and Colombian nano-explosion for nano-sized pores [35]. However, it is worth noting that this phenomenon is still not fully understood, and revealing the underlying physical mechanisms is challenging.
Table 2 shows a comparison of our results on the fabrication of low-loss waveguides and those of previous reports using Ti:Sapphire lasers. Here, we picked up two previous reports in which the beam profile was controlled to fabricate circular waveguides and in which the fabrication conditions are similar to ours. One remarkable difference other than the wavelength was the pulse duration. As aforementioned, the pulse duration should be less than 150 fs to make Regime I [22,23] using Ti:Sapphire lasers. In our case, we found a refractive index change without nano-sized structures can be fabricated at up to a 300-fs pulse duration. Nano-sized pores were formed when the pulse duration was more than 300 fs; however, we could obtain positive ${\Delta \textrm{n}}$ up to 430 fs without nano-wrinkles (or nano-gratings) forming, which causes significant propagation loss. This result is important for fabricating waveguides with Yb-based lasers because their pulse durations are often longer than 150 fs [17,18]. In other points, our results were similar to those of previous research using Ti:Sapphire lasers. The obtained value of ${\Delta \textrm{n}}$, 1.7 × 10−3 was on the same order as that of waveguides fabricated with Ti:Sapphire lasers, which is typically on the order of 10−4–10−3 [27,36]. The measured propagation losses were also in the same order to those in Ref. [37]. In Ref. [ 27], a higher propagation loss of 1.75 dB/cm was observed, but this would be a result of nano-sized structures forming, owing to 3.5-µJ pulse energy, as mentioned by the authors in Ref. [27].
Table 2. Comparison between our results and those of previous reports using Ti:Sapphire lasers.
4. Conclusion
In conclusion, we investigated fused silica modification under irradiation with a tightly focused fundamental beam from an Yb:KGW laser, and we demonstrated a laser direct writing method for low-loss waveguides using the fundamental beam of an Yb-based femtosecond laser. We successfully fabricated waveguides with a propagation loss of less than 1 dB/cm, which is on the same order as that of waveguides fabricated using a Ti:Sapphire laser; when the number of irradiated pulses per unit length was 50 pulses/µm, the pulse energy was less than 0.80 µJ, and the pulse duration was less than 400 fs. These conditions suppress the formation of nanostructures such as nano-wrinkles and nano-sized pores, which result in large optical loss and smaller ${\Delta \textrm{n}}$. When the pulse duration is shorter, ${\Delta \textrm{n}}$ become higher presumably because of the higher peak intensity of a laser pulse. We also clarified that ${\Delta \textrm{n}}$ is negative when the pulse duration is longer than 500 fs by comparing the inversion in contrast with the optical microscope images of fabricated waveguides. Because the Yb-based femtosecond laser is one of the most common types of ultrafast lasers used in the industry, the fabrication of waveguides using the fundamental light of the Yb-based femtosecond laser shows great promise for the practical usage of LDW technology.
Funding
Hitachi; Japan Science and Technology Agency.
Acknowledgments
We thank Dr. Hiroyuki Minemura, Mr. Kengo Asai and Mr. Takafumi Miwa of Hitachi Ltd., for their helpful advice and experimental support regarding the SEM observations. This work was partially supported by the Center of Innovation Program from the Japan Science and Technology Agency.
Disclosures
Hitachi, Ltd. (F,E)
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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