We demonstrate the inscription of embedded waveguides, anti-waveguides and Bragg gratings by use of intense femtosecond (fs) UV laser pulses at 266nm in pure fused silica, and for the first time, in bulk fused quartz and ZBLAN glasses. The magnitude of induced index changes, depends, besides pulse energy and translation speed, largely on writing depth and varies from ~10−4 for smooth modifications to ~10−3 for damaged structures. The obtained results are promising as they present the feasibility of fabrication of short (< 0.2μm) period first-order fiber Bragg gratings (FBGs) for applications such as in realization of all-fiber lasers operating at short wavelengths.
©2012 Optical Society of America
The photosensitivity of Germanium (Ge) doped silica glass associated to low-intensity (kW/cm2 to MW/cm2) standard UV laser irradiation has been extensively investigated in the past for the inscription of index-modified structures, in particular one and two-dimensional waveguides, fiber Bragg gratings (FBGs) and long-period fiber gratings (LPFGs) [1,2]. These gratings are typically written with 244 and 248nm laser sources which coincide with the maximum of the absorption band of defects in germanosilicate glass around ~5 eV resulting in typical index changes of the order of 1-5 x 10−4. Short-wavelength pulsed lasers mostly at 193nm and 157nm were also used by which somewhat higher index changes were obtained [3,4]. The index modifications induced by all these laser sources rely on either single-photon or two-step photon absorption processes and comprehensively described based on the color center and densification models [5,6]. In contrast, non-doped pure silica glasses (band-gap ~9.3eV) exhibit rather poor photosensitivity when exposed to such UV lasers and relatively weak index changes (~10−5-10−4) could be induced only after prolonged exposures which are not actually useful for many FBG applications .
The photosensitivity of non-doped and rare-earth doped fluoride-based glasses (especially ZBLAN) upon irradiation with the standard UV lasers were also studied particularly for the realization of infrared (IR) waveguides, amplifiers and up-conversion fiber lasers owing to their low phonon energy and high IR transparency [8–10].
There has been a significant research effort since 1996 beginning by Davis et al. in the use of very high intensity (~GW/cm2) focused infrared femtosecond (fs) laser pulses for the inscription of three-dimensional embedded waveguide structures and Bragg and long-period fiber gratings inside pure and doped silica as well as in other optical materials such as fluoride-based glasses [11–15]. Mihailov et al. reported the first FBG fabrication by use of intense 800nm fs laser pulses and a phase mask in standard SMF-28  and all-silica core fibers  and could achieve thermally-stable index modulations as high as 1.9 x 10−3 in both fibers at a laser intensity of 1.2 x 1013 W/cm2 showing a significant improvement as compared to those written with the standard UV lasers. Recently, the first FBGs in both Tm-doped and non-doped ZBLAN fibers were demonstrated  using 800nm fs laser pulses originating from a negative index change of the order of 10−3 resulting from the fs-laser-induced structural expansion .
The first FBG written in an hydrogen-loaded SMF-28 fiber by use of intense UV fs laser pulses at 264nm based on two-photon absorption (2PA) process was reported by Nikogosyan et al. where index changes up to 1.92 x 10−3 were obtained under a much lower irradiation intensity of 4.7 x 1010 W/cm2 as compared to the IR fs written FBGs . FBGs were also written in doped- and pure-silica core standard and photonic crystal fibers (PCFs) with UV fs pulses using different writing configurations [19–22]. The nanostructures with periods down to 250nm were realized in pure silica by point-by-point technique using tightly focused 267nm fs pulses . This actually represents the great advantage of the fs UV pulses for 3D laser nano-structuring of optical materials down to ultra-small sizes not achievable by use of IR fs lasers.
In this paper, we present the experimental results on the inscription of waveguides, anti-waveguides and Bragg gratings inside the bulk of pure fused silica, fused quartz and ZBLAN glasses by use of intense 1-kHz fs laser pulses at 266nm. This is, to the best of our knowledge, the first demonstration of fs UV laser-induced index modification in fused quartz and ZBLAN glass. The refractive index changes are characterized in terms of pulse energy, translation speed and writing depth and their physical formation mechanisms are discussed. We present the achievement of large index modulations (~10−3) for the gratings written in these bulk samples which for pure fused silica it exceeds the values reported in prior for its fiber counterpart. With the support of these results, we propose the feasibility of up-coming realization of short pitch (< 0.2μm) first-order FBGs in doped and non-doped silica- and fluoride-based glasses using our fs UV laser pulses.
We used a Ti-Sapphire regenerative amplifier system (Legend Elite HE USP, Coherent Inc.) to produce ‘p’ polarized 3.5mJ pulses at 800nm and 1-kHz repetition rate. The pulse duration was measured by a single-shot autocorrelator (SSA) to be 40fs. A 0.2mm thick nonlinear BBO crystal (Eksma-Optics) was used to generate ‘s’ polarized second harmonics (SH) pulses up to 1-mJ at 400 nm. By frequency mixing of the fundamental and SH pulses collinearly in another BBO crystal inside our home-built fs tripler, ‘p’ polarized third harmonics (TH) pulses at 266nm with up to 600μJ/pulse were generated. The pulse duration of the TH beam was evaluated through its cross correlation with the fundamental 800nm beam inside a difference-frequency (DF) BBO crystal using a high-resolution (1-μm) motorized optical delay line. By measuring the temporal FWHM of the 400nm cross-correlated signal and 800nm pulses inside the DF crystal, the TH pulse duration was estimated to be ~160fs.
Two separate sets of experiments were conducted to write straight embedded tracks and Bragg gratings in our three different types of glasses. The schematic layouts of the experimental setups are shown in Fig. 1 . In the first experiment, the D = 10mm diameter TH beam was focused by use of a reflective microscope objective (NA = 0.28, 15x, Edmund-Optics) from 100 to 800μm in depth of the polished non-doped bulk samples (5mm x 5-6mm x 20mm) of fused quartz (GE-124, GM Associates Inc.), fused silica (HPFS Corning 7980, Glass Fab Inc.) and ZBLAN (mol.%: ZrF4:56.2, BaF2:20.8, LaF3:4.1, AlF3:3.1, NaF:15.6, INO Inc.). The fs beam spot-size was estimated to be 0.6 μm by assuming Gaussian beam optics and neglecting the objective’s distortion and aberration effects. The pulse energy was controlled using a half-wave plate and a polarizer and measured before the objective lens with ~35% insertion loss.
A motorized translation stage (Nanomover, Melles-Griot) was utilized to move the samples perpendicular to the beam propagation direction with a speed ranging from 10 to 500μm/s to inscribe elliptic 5mm-long tracks 200μm apart at different pulse energies and scan speeds. After the inscription, the microscope images of the written tracks were obtained with a phase-contrast optical microscope (Olympus IX71). By using the quantitative phase microscopy (QPM) method through its commercial software (Iatia ltd.), we retrieved the radial refractive index change profiles of the written smooth tracks from their measured slightly defocused bright-field optical microscope images .
In the second experiment, the standard FBG writing setup using holographic phase mask was employed to inscribe Bragg gratings inside the bulk glasses. A UV-grade fused silica cylindrical lens (f = 150mm) with an estimated focal line of 2w0 = 5μm and a depth of focus of ~150μm was used to focus the beam into the samples through a phase mask. The laser beam’s focal area was estimated as ~2w0 x D = 5 x 10−4 cm2. We fabricated our Λm = 1.07 and 6.0 μm period phase masks on 1.5mm-thick fused quartz (GM. Associates) and fused silica (Esco Products) substrates (40mm x 10mm) by a holographic lithography technique. The groove depths of both masks were optimized for 266nm irradiation by rigorous coupled-wave analysis to be ~280nm. This produced a maximum diffraction efficiency of 38% for each of + 1 and −1 diffracted orders and zero-order efficiency as low as 2-3%. The measured diffraction efficiencies of the higher orders were too weak to affect the interference pattern formed by the two + 1 and −1 orders. The distance between the phase masks and the samples was adjusted to be approximately 2.5mm. The gratings were written in two cases for which either the samples were isolated or glued to the phase mask (keeping the same 2.5mm distance) to reduce their possible small relative movements and thus to improve the quality of the inscribed interference fringes. The masks were moved vertically to a new location after each laser irradiation to ensure that the subsequent exposure would be through a fresh area. The pulse energy was measured before the cylindrical lens varying from 10 to 400μJ but inevitably suffered from a significant amount of loss while transferring to the sample due to large two-photon absorption (2PA) of the mask substrate (see later). The Λg = 0.5 and 3.0μm pitch gratings with nearly 200μm length measured along the fs beam propagation were written at different depths for various pulse energies and exposure times from 1 min. up to 1 hour.
To verify the actual formation of gratings and to evaluate their index modulation (Δnmod) a He-Ne laser beam (λR = 633nm) was focused by a 6.3x objective lens into each grating at the Bragg angles of θB ~39° and ~6° (sin θB = λR/2.Λg), associated with 535nm and 3.0μm pitches measured in air, respectively.
The fs-irradiated fused quartz was specifically treated by rapid thermal annealing (RTA) at 300°C for 10 min. A photo-spectrometer (Cary 500 Scan, Varian) was utilized to measure the transmission spectra of the exposed and unexposed regions before and after the annealing. The samples with embedded 3.0μm pitch gratings written in different depths inside fused silica were polished down to 500μm and then etched in 10 and 20% HF acid for 10 to 30 min. in an ultrasonic bath. A scanning electron microscope (SEM, Quanta 3D FEG, FEI) was subsequently employed to visualize the periodic surface structures.
Results and discussions
Waveguides and anti-waveguides
Typical refractive index change profiles for the tracks written in a depth of 700μm in fused silica, fused quartz and ZBLAN are illustrated in Fig. 2 .
The positive index changes induced in pure silica and fused quartz are suggesting the formation of structures that could function as optical waveguides if written under a limited range of pulse energy and scan speed. The optical microscope pictures of the smooth tracks with elliptical cross sections written in these samples are also shown in Fig. 3 . We found out that the morphology of the written tracks strongly depends on the writing depth under the same inscription conditions. That is, the tracks written closer to the surface (~100-200μm) were damaged as opposed to those written deeper in the glass (e.g. 700μm). This is indeed due to reaching higher laser peak intensities exceeding the glass damage threshold in the case of shallow writings. In contrast, for sufficiently deep writings the light intensity at the focal area decreases resulting from an increased spot-size caused by the objective’s aberration and/or significant two-photon nonlinear absorption of high-intensity 266nm fs pulses.
To demonstrate the significant role of writing depth in laser-induced modifications, Fig. 3(e) shows the picture of a damaged track written in silica in a depth of 200μm at 1.0μJ and 20μm/s. This actually suggests the potential of our UV fs laser source to produce index changes associated with the uniform and damaged tracks in pure silica which are on the same order (~10−4) and even larger (~10−3) than those in the type-I gratings and type-II nanostructures produced by other UV low-intensity and fs lasers [7,19–21,23], while they are comparable to the type-II index changes induced by fs IR pulses [11,15].
It appears that the index modifications measured in fused quartz under similar writing conditions are of the same order and even slightly larger than those in silica glass (see Fig. 2). Their microscope pictures also look somewhat darker with a higher image contrast. For instance, we could damage fused quartz at 0.42μJ and 20μm/s at 200μm writing depth whereas only smooth index modifications were induced in silica. Also, fused quartz was partially damaged at 1.0μJ and 100μm/s whereas no void-like morphology appeared in silica glass. Later on when presenting the results on writing Bragg gratings we will address this issue in more details.
As for the onset of index modifications, at 700μm depth the written tracks were visible at very low pulse energy of 18nJ in fused quartz and 30nJ in fused silica for which we had observed no index change produced by 800nm fs irradiation under similar focusing conditions . This indicates a lower intensity threshold required for the onset of structural modifications in glass as compared to the 800nm fs case, which is indeed resulting from a lower order of nonlinear absorption at 266nm (2 photons against 6 photons) and accordingly a higher nonlinear ionization efficiency. Bennion et al. have reported a threshold of 30nJ for index modifications at an optimal depth of 170μm in silica for their 300fs UV pulses .
In contrast to fused quartz and silica, all the UV fs tracks written in ZBLAN exhibited a negative index change across their irradiated regions. The observed positive index change lobes surrounding the central negative area in Fig. 2(c) are a consequence of the UV fs laser-induced structural expansion which is accompanied by local compaction of the unexposed peripheral regions. This result is consistent with the observed expansion of ZBLAN glasses induced by 193nm nanosecond and low-rep. rate 800nm fs laser pulses [16,17] and demonstrates the so called ‘anti-waveguide’ formed by use of UV fs laser irradiation in this glass. It should be noted, however, that waveguide formation in ZBLAN glasses was originally demonstrated using a 200 kHz 800nm fs laser where accumulated thermal effects were likely involved in the induced positive index changes . We observed no track visible for pulse energies almost below 20nJ.
The variation of UV fs laser-induced refractive index change versus scan speed for three incident pulse energies in fused silica for a writing depth of 700μm is depicted in Fig. 4 . As expected, the index change increases for lower scan speeds and higher pulse energies as the total laser fluence incident on each irradiated spot is increased.
Although we observed no white-light generation during the inscription, it should be mentioned that the critical power (Pc ∝ λ2) for self-focusing in silica at 266nm is ~150 kW which is approximately 15-fold lower than that at 800nm (~2300 kW). This corresponds to an input energy of 24nJ for our 160fs pulses. Thus, it is expected that nonlinear propagation effects and self-focusing somehow should have contribution in our experimental results [25,26].
To inscribe Bragg gratings using a phase mask the major drawback limiting the delivery of UV fs pulse energy to the sample is significant two-photon absorption (2PA) taking place in the mask substrate which leads to its degradation and early optical damage. To investigate this issue, we characterized two 1.5mm-thick fused quartz and fused silica substrates by measuring their transmission with respect to distance from the focal plane of the cylindrical lens for different incident laser powers, as illustrated for the silica substrate at 100mW (Fig. 5 ). During the grating writing process we inserted the mask nearly 2.5mm away from the focus which thus corresponds to 18% nonlinear absorption loss.
The grating writing in silica glass was accompanied by a strong red luminescence observed during irradiation which is attributed mostly to the known NBOHC color centers . Due to the lack of sufficient contrast, we were unable to observe all the written gratings with the optical microscope. Figure 6(a) shows the microscope image of a grating with un-resolved lines having a width of 7μm exposed during 15 min. at 100mW measured before the lens which corresponds to an estimated interference fringe intensity of 1.4 x 1012 W/cm2 on the sample. By using the SEM we could obtain a well-resolved picture of the grating with sufficient contrast (Fig. 6(b)).
As opposed to silica, the gratings written in fused quartz appeared too dark after irradiation while their exposed regions extended over much larger areas beyond the focal volume reaching even the sample’s thickness (Fig. 6(c)). To investigate the origin of such laser-induced darkening, the transmission spectra of the exposed and unexposed regions before and after thermal annealing were measured (Fig. 7 ).
A broad fs laser-induced absorption band around 540nm appears which is very similar to the optical band induced in fused quartz exposed by other radiation sources . Such absorption band associated to the intrinsic glass impurities is indeed responsible for the laser-induced darkening and it was almost completely erased after annealing when the glass was restored to its natural appearance. The gratings written in ZBLAN were even less visible and experienced no laser-induced coloration as for the case of fused silica.
We observed the Bragg diffracted lights by coupling the He-Ne laser beam into the embedded gratings in all the samples. The diffraction efficiencies up to 0.1% were measured corresponding to an index modulation (Δnmod) of the order of 1.6 x 10−3 for the 200μm long gratings written in silica (210mW, 2.9 x 1012 W/cm2, 20 min.) and in ZBLAN (220mW, 3.0 x 1012 W/cm2, 15 min). The variation of index modulation as a function of laser exposition time in ZBLAN is illustrated in Fig. 8 under two incident pulse energies.
Note that in order to obtain this plot seven individual gratings were written under different exposure times before measuring their static diffraction efficiencies under the same read-out conditions. The degradation of the grating’s quality at sufficiently higher exposures, likely due to severe thermal effects, is responsible for the decreased index modulation.
Our results present an improved UV fs laser-induced index modulation obtained for an untreated pure silica glass compared to the values of the order of 10−4 achieved in silica-core fibers [19,21]. It should be emphasized that we have demonstrated in this work, for the first time, the inscription of Bragg gratings using UV fs laser pulses in bulk ZBLAN and fused quartz. Our promising results bring the evidence on the eventual feasibility of writing first-order short-pitch (down to ~150nm) FBGs with large reflectivity in non-doped non-photosensitive silica and fluoride-based glasses which could provide many applications such as for the fabrication of compact up-conversion fluoride-based visible fiber lasers. Our first attempt to inscribe Bragg gratings in standard telecom fibers using 266nm fs pulses by employing a new writing scheme has been satisfactory. Figure 9 shows a preliminary result of the transmission spectral response for an FBG written by our fs UV laser in a hydrogen-free SMF-28 fiber.
Considering similar optical band-gaps of the glasses used in this work (fused silica ~9.3eV, fused quartz ~9.0eV, ZBLAN ~8.5eV), we believe that the common primary mechanism for the induced index changes in both waveguide and grating writing regimes results from the two-photon absorption of 266nm (4.66eV) pulses and subsequent nonlinear ionization and plasma formation in the focal volume. It appears that in fused quartz there is a more important contribution from color/defect centers in the observed UV fs laser-induced index modifications which leads to a larger total index change than in fused silica. Silica and ZBLAN glasses show opposite photo-induced structural responses as compaction and expansion against low-repetition rate UV fs laser irradiation, respectively. Such structural transformations have been similarly observed in prior in these glasses upon their irradiation by 1-kHz 800nm fs pulses [16,29]. Thermal annealing investigations are required in order to identify the relative contributions of laser-induced color centers and structural transformations in the measured index changes in these glasses.
The fabrication of waveguides, anti-waveguides and Bragg gratings using 266nm fs pulses in fused silica, fused quartz and ZBLAN glasses was demonstrated. The magnitude of index modifications strongly depends on the writing depth by which transition from smooth to void-like damaged structures could occur under the same other writing parameters. The lower threshold energy required for the onset of index change formation at 266nm as opposed to the 800nm fs case is evidently due to its higher two-photon nature nonlinear absorption efficiency. ZBLAN exhibits similar negative index change behavior as observed in prior by 1-kHz 800nm fs irradiation. Besides laser-induced structural changes, color/defect centers contribute to index modifications in fused quartz more importantly than in silica. The obtained large index modulations are promising for the realization of first-order short period fiber Bragg gratings in silica- and fluoride-based glasses.
This research was supported by the Canadian Institute for Photonic Innovations (CIPI), the Fonds de recherche du Québec – Nature et technologies (FQRNT), the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Canada Foundation for Innovation (CFI).
References and links
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