A thorough study of the femtosecond photo-inscription of optical waveguides in fused silica is presented. Quantitative phase microscopy was used to study the variation of the index contrast of the waveguides as a function of the writing conditions. It is revealed that waveguides based exclusively on Type I refractive index modifications are difficult to form for pulses longer than 300 fs. We show that this limitation can be circumvented by scanning the laser beam multiple times at low pulse energy. We also demonstrate that by equally multiplying the scan speed and the number of passes, the index contrast can be increased, which was not expected for the low-repetition-rate regime. Based on the nonlinear ionization memory, we propose an explanation for this phenomenon. For shorter pulses, multiple passes of the beam allowed for the formation of waveguides with an enhanced index contrast while preserving the morphology and uniformity of Type I modifications. Index contrasts up to 9 × 10−3 in Heraeus F300 fused silica are reported. Using this method, waveguides that exhibits single mode operation at wavelengths of 405, 633, 980 and 1550 nm were successfully inscribed.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Femtosecond laser direct inscription of optical waveguides has evolved to a mature and enabling technology for the fabrication and enhancement of integrated photonic devices [1,2]. In addition to its unique capability for three-dimensional processing, one of the most important advantages of femtosecond laser inscription is that it can be applied to practically every transparent material. Indeed, low-loss optical waveguides were successfully inscribed in a wide range of glasses [3–8], crystals [9,10] and polymers [11,12]. Still, very few optical materials have proven to be eligible for practical applications. Either the characteristics of photo-induced waveguides are lacking, or the physical properties of the material impose trade-offs and limitations.
Given its low-cost, high purity as well as appealing optical and mechanical properties, fused silica is extensively used in the photonic industry. For instance, most bulk optical component and low-loss optical fibers are made of silica-based glasses. Consequently, considerable efforts were deployed to form buried optical waveguides in fused silica ever since the first demonstration of femtosecond laser direct inscription . Moreover, beam-engineering techniques have been developed to improve the shape of the laser written waveguides, such as the slit technique , using cylindrical lenses , the dual-beam technique , using a nondiffractive Bessel beam , the simultaneous spatial and temporal focusing (SSTF) technique , using a spatial light modulator (SLM)  and using an objective with a correction collar . However, it is shown that waveguide inscription in fused silica proves more difficult as compared to other materials, especially glasses prone to heat accumulation . Indeed, waveguides inscribed in fused silica generally exhibit a weak index contrast (3 × 10−3) and a diameter which is limited to the size of the plasma formed at the focus of the laser beam. Moreover, so-called Type II (nanogratings ) and Type III (voids, ablation, damage [22,23]) modifications appears under relatively low laser fluence thus restraining the range of irradiation parameters that allows for the formation of smooth and homogenous Type I modifications.
In the last decade, strategies such as frequency doubling , high repetition rates , static exposure , multiple scans of the laser beam  and hydrogen loading  were employed in order to increase the index contrast in fused silica. Even though index changes close to 0.03 are reported , the photo-induced structures are highly asymmetric [25,26], composed of a mix of Type I and III modifications [24,27] or unusable as waveguides . Moreover, except for Ref. 25, the refractive index change was evaluated through indirect measurement methods. To our knowledge, only Eaton et al.  successfully inscribed a waveguide which exhibits both moderate propagation losses (0.55 dB/cm) and a high index contrast (Δn = 0.016) using a frequency-doubled MHz laser system. However, the cross section of the waveguide is highly asymmetric and the index profile consists in a complex arrangement of small regions of positive and negative index change. Note that the highest index contrast achieved using beam-engineering techniques to improve the waveguide symmetry was estimated to be approximately 3-5 × 10−3 in fused silica . Also, it is relevant to note an interesting work by Liu et al. in which the asymmetry of the waveguide shape was partially corrected through multiple scans separated by 1 or 2 μm . Using this method, a maximum refractive index change of 7.0-7.5 × 10−3 has been reached. However, the minimum size of the waveguide is limited by at least the number of scans times 1-2 μm.
While the underlying mechanisms are not fully understood, there is strong experimental evidence that high laser fluence is necessary to increase the refractive index change in silica beyond the 2 × 10−3 mark [25,26]. While Ti:Saphire femtosecond laser chains can provide mJ pulses, they do so at a repetition rate of a few kHz which limits the processing speed to the µm/s range. As such, fiber-based femtosecond lasers developed in the last decade are particularly suited to waveguide inscription since they deliver µ-Joules pulses at repetition rates in the MHz range. However, owing to the highly non-linear phase accumulated through the fiber oscillator the pulse duration of such laser source can generally not be compressed below 300 fs. While this does not impact the inscription process significantly for the vast majority of glasses, it is shown that the energy threshold for the formation of Type III modifications in fused silica decreases sharply as pulse duration surpasses 150 fs . This prevents the formation of optical waveguides with high repetition rate lasers unless external means are used to shorten the pulse or modify the wavelength. In fact, the inscription of waveguides in fused silica based solely on Type I modifications at the fundamental wavelength of a MHz laser has yet to be demonstrated.
Note that a new phenomenon allowing a significant increase of the refractive index contrast for wavelengths approaching the electronic resonance of any transparent material was recently reported . It was shown that a fs-laser-induced band gap shift (FLIBGS) occurs in any Type I waveguide photo-inscribed. Using the FLIBGS approach, waveguides with submillimeter bend radii have been demonstrated for the first time. The FLIBGS can also invert a negative index contrast to positive, allowing direct waveguide inscription in crystals and certain glasses. However, the phenomenon is only practicable in a narrow wavelength window, which limits the applications.
In this paper, we present a thorough study of photo-induced refractive index modifications in fused silica using a femtosecond laser. First, the evolution of the pulse energy window for the formation of Type I modification as a function of increasing pulse duration was examined. It is shown that inscribing waveguides based exclusively on Type I index modifications for pulse duration of 300 fs or above is very challenging. We show that this limitation can be avoided by scanning the laser beam multiple times at low pulse energy. Using this method, an optical waveguide was successfully formed at long pulse duration but at the expense of an endless inscription process.
Next, quantitative phase microscopy was used to study the variation of the induced index contrast of waveguides inscribed using 65 fs pulses as a function of pulse energy, translation speed and the number of successive laser passes. In every case, a saturation of the index contrast is observed when the diameter of the waveguide exceeds the size of the photo-induced micro-plasma.
Use of multiple passes of the beam has proven advantageous as it allowed for a significant increase of the index contrast. Indeed, smooth waveguides with an index contrast up to 9×10−3 were formed in Heraeus F300 fused silica. Moreover, this method provides a precise control over the index contrast while preserving the morphology and uniformity of Type I waveguides. As a proof of concept, high-index contrast waveguides that exhibits single mode operation at wavelengths covering the visible range up to the telecom C-band were successfully inscribed. Our results open a new pathway toward the fast processing of tridimensional photonic devices that necessitate strong light confinement.
2. Experimental method
Waveguides were inscribed in a Heraeus F300 fused silica glass sample (20 × 40 × 4.5 mm3, OH content below 1 ppm) using a chirped pulse amplification (CPA) system (Coherent RegA), operated at a repetition rate of 250 kHz and at a wavelength of 795 nm. To stretch the pulse from 65 fs to 355 fs (measured before the objective), the pulse was negatively chirped by changing the position of the gratings in the CPA system. Note that the chirp sign can influence the photo-induced modification . Pulses were focused 150 µm beneath the surface of the glass sample using a 50× (Edmunds, f = 4 mm, 0.55 NA) and a 100× (Nikon, f = 2 mm, 0.8 NA) microscope objectives. Glass samples were translated in a direction perpendicular to the laser beam, at a translation speed vx of 10 mm·s-1, using motorized mechanical stages (Newport XML210, XMS160 and GTS30V) to form 20 mm long waveguides. A quarter-wave plate was used to induce a circular polarization state of the beam in order to prevent directional effects and inhibit the formation of nanogratings . A cylindrical lens telescope was used to produce an astigmatic beam and shape the focal volume so as to form waveguides with quasi-circular cross sections . The photo-induced plasma was imaged using a 50× objective (Mitutoyo, f = 4 mm, 0.5 NA) mounted on a tube lens and a CCD camera. The sizes of the photo-induced plasma volumes were evaluated to 9.5 μm (2wy) by 7.7 μm (2zRx) by 1 μm (2wx) for the 50× objective and 4 μm (2wy) by 4.5 μm (2zRx) by 0.7 μm (2wx) in the case of the 100× objective.
After the inscription process, the end faces of the samples were cut and polished. To measure the photo-induced refractive index modifications, structures were examined using a bright field microscope (Olympus IX71) and a camera equipped with a bi-dimensional Hartmann grating (Phasics SID4Bio). The camera is a wave-front analyser that uses lateral shearing interferometry (QWLSI) to yield a quantitative phase image of transparent objects . The methodology described in detail in Ref.  was carried out to recover the index contrast (Δn) of the waveguides from the phase image. Accordingly, Δn measurements are considered exact within a 2% error margin or better.
3.1 Pulse energy window for the formation of Type I modifications
For given irradiation conditions, the maximum index contrast of Type I waveguides that can be induced is ultimately limited by the energy threshold for the onset of photo-induced damage. Determination of the threshold is paramount for waveguide processing and it has been shown that it depends on many inscription parameters . Among those, pulse duration has a critical influence over the inscription process and consequently, implications toward the choice of fs laser for waveguide processing. As such, we seek to get a detailed assessment of the effect of pulse duration on the photosensitivity of fused silica.
In order to do so, we devised an experiment similar to Hnatovsky et al.  with the difference that a circularly polarized beam was used to prevent the formation of Type II modifications. Circularly polarized beam is also preferred for 3D devices fabrication since the inscription is independent of the writing direction. Sets of waveguides were inscribed while varying both pulse energy and duration in order to determine the window for waveguide inscription delimited by the thresholds for the formation of Type I and Type III modifications. Then, the Δn of the photo-induced waveguides in the Type I processing window was measured. Results are shown in Fig. 1.
We see that irradiation with fs pulses yields a weak increase of the refractive index so that waveguiding is observed in the cases of every Type I modifications. It should be noted that Type III structures inscribed at higher pulse energy may, in whole or in part, guide light as demonstrated by other groups . Also, even though circular polarization was used, it is readily possible that Type II modifications may be formed for energy close to the Type III threshold. No such modifications were observed but only a conventional microscope was used and no further investigations were made to detect Type II structures. For the remainder of the manuscript however, we will focus on the characterization of waveguides based solely on Type I modifications. Note that the pattern formed for longer pulse length (easily visible for laser pulses of 280 fs and 410 nJ) could be induced by a similar mechanism as shown in previous work [35,36].
Our results are in good agreement with the observations reported by Hnatovsky et al., which means that the fs laser-induced modification follows the same behavior no matter the polarization. Indeed, it is shown that the pulse energy window for Type I modification narrows down with increasing pulse widths and eventually vanishes. At pulse duration of 65 fs the processing window is essentially spanning over 200 nJ (i.e. from 310 to 485 nJ) while showing a maximum Δn is of 1.4 × 10−3. Then the threshold for the onset of Type III modifications decreases sharply at 100 fs and remains relatively stable afterward. In opposition, the Type I threshold increases slowly and catches up with the Type III onset at 280 fs. Beyond this pulse duration value waveguide inscription of Type I is no longer possible under such one-pass writing conditions.
Still, we show that this limitation to Type I inscription can be avoided without resorting to external means. Indeed, it is possible to inscribe a Type I waveguide by fixing the pulse energy just below the Type III threshold and scan the laser beam multiple times. Images of the longitudinal and cross sections of such a waveguide are shown in Fig. 2.
A Type I waveguide is inscribed after 100 successive passes at a pulse energy of 395 nJ, which corresponds to 2.5% below the pulse energy threshold for Type III modifications. The refractive index change is modest (8.5 × 10−4) but could be increased considerably by increasing the number of laser passes and/or reducing scan speed. Moreover, the multiple passes process has a beneficial effect on the uniformity of the waveguide, which is desirable for low loss propagation. Indeed, as shown in Fig. 1, formation of wobbles occurs with long laser pulses (the mechanism is still under investigation). Using the multiple passes process, the non-uniformities are averaged, which contributes to smooth the waveguide. Besides the refractive index changes accumulated for these multiple passes waveguides (undetectable for a few passes), we suggest that nonlinear ionization memory  may play an important role in the process. A shot-to-shot reduction in the threshold laser intensity for ionization has been demonstrated. Thus, the multiple passes process increases the ionization efficiency without the need to increase the intensity, which results in an enhanced Type I modification rather than Type III. Still, the processing time is significantly increased (200s for 20 mm long waveguides) and inscription using pulse energy as close to the threshold is tedious since defects in the material or small variation of the laser power may initiate the formation of Type III modifications. In all, our observations confirm the difficulty of waveguide inscription in fused silica using longer fs pulses.
3.2 Enhancement of the photo-induced index contrast
Next, we studied the variation of the photo-induced index contrast of Type I modifications for short pulses (65 fs). Prior to waveguide inscriptions, the micro-plasma formed at the focus of the femtosecond laser beam was imaged using an approach similar to Ferrer et al. . The beam focus is positioned 150 µm beneath the surface of the bulk material and the plasma is imaged from the side using a tube lens and a CCD camera. As the pulse energy is slowly increased, the plasma grows then reaches and maintains a maximum size. Then, a further increase of the pulse energy or prolonged exposure yield optical damage akin to Type III modifications. An image of the full-sized plasma (pulse energy just below the damage threshold) along with the cross section of the resulting waveguide inscribed with a 50× objective are shown in Fig. 3.
The intensity profile of light emitted from the plasma is directly linked to the electron density distribution, and thus provides an a priori assessment of the morphology of the photo-induced modifications. Owing to the high thermal conductivity of fused silica, heat is diffused out of the focal volume between each consecutive pulse for repetition rates below the 1 MHz level . As such, no sustained melting takes place outside the focal volume and the extent of the photo-induced modification is dictated by the electron density distribution. Indeed, we can see from Fig. 3 that the shape of the photo-induced waveguides fits very closely with the intensity profile of light emitted by the plasma. In the case of the 100× objective, the plasma at the focus was measured to be of zRx = 4.5 µm by wy = 4 µm (not shown).
Then, waveguides were inscribed using the 50 and 100× microscope objectives for increasing pulse energy. The Δn measured using QWLSI and the size of the waveguides along the z axis are shown in Fig. 4.
We can see that both the intensity of the focused laser beam and translation speed have a significant impact on the magnitude of the photo-induced index change. In fact, laser fluence is the relevant parameter as it incorporates both beam intensity and number of overlapping laser pulses. Our results show that stronger index changes are induced under high fluence (low speed and/or intense focusing).
In the case of the 100× objective, Type I modifications are formed for a pulse energy above 275 nJ. Then, the index change increases sharply, saturates at 2.2 × 10−3 for a pulse energy of 450 nJ and decreases following a further energy increase. The saturation point of the index change occurs when the size of the waveguides surpasses the dimension of the (full-sized) plasma. Finally, Type III modifications appear for pulse energy above 700 nJ. In the case of the 50× objective, it is observed that the threshold for the formation of Type I modifications is considerably higher (≈1µJ) and that the index change does not go beyond 1.1 × 10−3. Also, there is no saturation of the Δn or onset of Type III modifications up to the maximum energy that can be delivered by the laser. Coincidentally, the size of the waveguides is kept smaller than the plasma. Note that Type II modifications, which typically produce circular nanogratings using circular polarization , have not been observed in our experiment. It is possible that the linear inscription produces a disorder in the circular nanostructures and smooth the modifications induced along the waveguide, which would make them difficult to observe. In this case, Type II modifications could explain the decrease of index contrast induced by higher pulse energies.
Varying the translation speeds also allows for a substantial increase of the index change. A Δn of 3.5 × 10−3 is reached using a translation speed of 1 mm/s while a maximum of 2.2 × 10−3 and 2 × 10−3 were observed for speeds of 10 and 50 mm/s respectively. While those values are adequate for most waveguiding applications, a further increase of the Δn would involve an important trade-off on processing time. Also, it is seen that the Type I windows do not change much. Increasing speeds over more than an order of magnitude results only in small variations on the pulse energy onset for Type I and III modifications. This implies that the occurrence of Type III modifications is mainly dictated by pulse energy and that other writing parameters must be adjusted in order to reach the fluence level necessary to induce an appreciable index change.
Even though moderate index changes were successfully induced under high laser fluence, the Δn is still an order of magnitude below the 0.03 index difference that can be attained using photolithography and the 0.022 mark reported in ref. 25. To reach a further increase of the index change, waveguides were inscribed using multiple passes of the laser beam. Results are presented in Fig. 5.
First, we see that multiple passes of the laser beam allow for a considerable increase of the photo-induced Δn. Even 5 passes are sufficient to gain a two-fold increase of the Δn over waveguides inscribed with a single pass. Moreover, the resulting high-Δn waveguides still preserve the smooth and homogenous index profile akin to Type I modifications.
We show that Type I modifications with a Δn as high as of 9.4 × 10−3 can be reached using the 100× objective and 500 successive laser passes. The refractive index profile is shown in Fig. 5(c)). For ten passes, Δn of 5.8 × 10−3 and 3.6 × 10−3 can reached for pulse energy of 460 and 350 nJ respectively. The Δn of waveguides inscribed using the 50× objective follow the same trend. A maximum Δn of 3.5 × 10−3 is reached for a pulse energy of 1.35 µJ and 100 passes. Remarkably, the photo-induced Δn still peaks and decreases when the waveguide size grows past the dimensions of the plasma.
An interesting result stands out. Intuitively one would think that, for a given number of passes, increasing pulse energy would yield stronger modifications. However, we show that using lower pulse energy is preferable for the formation of waveguides with a high Δn. Indeed, the Δn reaches a saturation point as the modification size grows equal to the size of the plasma. At high energy, a single pass of the laser beam yields waveguide which size approaches that of the plasma. Consequently, the saturation point is attained quickly after a few passes and a similar trend was observed with other glasses [34,39].
Another significant observation can be made. For a given number of impinging pulses, the photo-induced contrast is higher using multiple passes of the beam than by decreasing the speed accordingly. Indeed, the Δn is higher for a waveguide inscribed with 10 passes of the laser beam at a speed of 10 mm/s (Δn = 5.8 × 10−3) than for one induced by a single pass at a speed of 1 mm/s (Δn = 3.5 × 10−3). This is surprising since the photo-inscription are conducted using a low repetition rate and there is no heat accumulation effect . The phenomenon is not fully understood at this point. However, nonlinear ionization memory  may play an important role in the process. It has been demonstrated that the nonlinear absorption efficiency increases significantly when the laser focalization occurs in an already photo-inscribed region compared to an unaffected one. Since a portion (although quite small) of every single laser shot interacts with an unaffected zone in a single pass process, the nonlinear absorption slightly decreases. On the other hand, every single shot of all the next passes will interact with a fully affected zone. The fact that this phenomenon is more significant at higher intensity supports our hypothesis since nonlinear ionization memory follows the same trend.
So far, we demonstrated that the Δn of photo-induced waveguides in fused silica can be significantly increased using multiple passes of the laser beam. Hereafter, we show that this method can be used to achieve precise control over the Δn and hence the guiding properties of the waveguides. As a proof of concept, waveguides were inscribed using incremented number of laser passes while retaining all other writing parameters fixed. After the inscription process, visible and NIR light was injected in the waveguides using a single-mode fiber and the output face was imaged onto a camera using a 50× objective. The intensity profiles of transmitted light imaged at the output face of the waveguides are presented in Fig. 6.
It is shown that varying the number of passes allows for the inscription of waveguides with predetermined characteristics. Since the Δn increases rapidly with successive passes, only a few passes are necessary to form single-mode waveguides operating at wavelengths covering the visible up to the telecom C-band. In order to demonstrate that each single-mode waveguide operates close to the cutoff wavelength, the bottom images in Fig. 6 show that the same waveguide (with the same number of passes) is multimode at a slightly shorter wavelength. These results are consistent with the V numbers calculated: V = 2.0 at 633 nm for 5 passes, V = 3.12 at 405 nm for 5 passes, V = 1.56 at 980 nm for 10 passes, V = 2.42 at 633 nm for 10 passes, V = 1.31 at 1550 nm for 20 passes and V = 2.08 at 980 nm for 20 passes. Moreover, the Δn can be tailored as to reach a relatively strong confinement while restraining the diameter of the structure. As a consequence, single-mode field areas of ∼3 × 4 µm2 or less are reported for wavelengths of 405, 633, 980 and 1550 nm.
We presented a detailed study of photo-induced refractive index modifications in fused silica using a femtosecond laser. First, we confirmed that pulse duration has a significant impact over the processing of optical waveguides. The pulse energy window for the formation of Type I modification as function of increasing pulse duration was examined. It is shown that inscribing waveguides based exclusively on Type I index modifications for pulse duration above 300 fs is impractical. This limitation can be circumvented, however, by scanning the laser beam multiple times at low pulse energy. Using this technique, an optical waveguide was successfully inscribed with 355-fs pulses.
Next, quantitative phase microscopy was used to investigate the photo-induced index contrast of waveguides inscribed using 65 fs pulses as a function of pulse energy, translation speed, numerical aperture of the objective and the number of successive laser passes. In every case, a saturation of the index contrast is observed as the waveguide diameter grows past the size of the photo-induced micro-plasma.
Finally, we demonstrated that uses of multiple passes of the beam allowed for a significant increase of the index contrast. Indeed, smooth waveguides with an index contrast up to 9 × 10−3 were formed in Heraeus F300 fused silica. Furthermore, this method provides a precise control over the index contrast while preserving the morphology and uniformity of Type I waveguides. As a proof of concept, high-index contrast waveguides that transmit a single mode with a small field area of ∼10 µm2 at wavelengths of 405, 633, 980 and 1550 nm were successfully inscribed. Our results open a new pathway toward the fast processing of tridimensional photonic devices that necessitate strong light confinement.
Canada First Research Excellence Fund (Sentinel North); Canada Foundation for Innovation (34265, 37422); Natural Sciences and Engineering Research Council of Canada (IRCPJ469414-13).
This research was supported by the Sentinel North program of Université Laval, made possible, in part, thanks to funding from the Canada First Research Excellence Fund.
The authors declare no conflicts of interest.
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