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Spectroscopic characterization of differentfemtosecond laser modification regimes

in fused silica

Wilbur J. Reichman,1,2 James W. Chan,2 Christopher W. Smelser,3 Stephen J. Mihailov,3 and Denise M. Krol1,*1Department of Applied Science, University of California, Davis, California 95616, USA

2Lawrence Livermore National Laboratory, Livermore, California 94550, USA3Communications Research Centre Canada, Ottawa, Ontario ON K2H 8S2, Canada

*Corresponding author: dmkrol@ucdavis.edu

Received January 22, 2007; accepted February 24, 2007;posted March 20, 2007 (Doc. ID 79281); published June 15, 2007

Structural changes associated with femtosecond laser fabrication of waveguides and Bragg gratings in fusedsilica were analyzed using optical microscopy and laser spectroscopy. Using 800 nm femtosecond lasers with akilohertz repetition rate and various pulse energies, both smooth and rough modifications were induced. Thedifferent modification regimes were characterized by measuring the spectra of the light emitted during writingwith the femtosecond laser and collecting fluorescence spectra after femtosecond writing using a low power488 nm laser as an excitation source. The spectral features observed during and after writing can be used todistinguish the smooth and rough modification regimes, and they assist in understanding the underlying modi-fication mechanisms.. © 2007 Optical Society of America

OCIS codes: 350.3390, 170.2520, 160.2750.

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. INTRODUCTIONemtosecond lasers have been used to fabricate a wide va-iety of optical devices inside bulk glasses and optical fi-ers. These include passive devices such as waveguides,plitters, couplers, and Bragg gratings, and active devicesuch as amplifiers and lasers [1–9]. The femtosecond-aser fabrication technique makes use of the fact that aocused femtosecond laser can locally modify the refrac-ive index of a glass with micrometer-size precision.

It is widely accepted that the modification process isnitiated by the rapid absorption of laser energy through aombination of multiphoton absorption and avalanchehotoionization [10–14]. For high enough pulse energies,his eventually leads to plasma formation. Immediatelyfter absorption of the very-short-laser-pulse, all the ab-orbed energy is in the electrons, but it is the subsequentissipation of this energy into the lattice that causes per-anent changes within the glass.While fabricating devices, several modification re-

imes, in which different morphological responses resultrom the femtosecond-laser modification process, haveeen observed [9,15–17]. These regimes are distinguishedy the laser pulse fluence (energy and area) and pulse du-ation. For a constant pulse duration of �100 fs, there is aulse fluence threshold below which no modification oc-urs. Slightly higher pulse fluences produce a smoothodification with a different refractive index than the

ulk glass; this regime can be used to create waveguides.ven greater pulse fluences produce a macroscopicallyough modification, with ordered microscopic nanograt-ngs at the low end of this regime [16]. The macroscopic

odification regimes are graphically depicted for fusedilica in Fig. 1. It is important to understand the nature of

0740-3224/07/071627-6/$15.00 © 2

hese modification regimes and the conditions that pro-uce them, because for most optical devices fabricatedith the femtosecond-laser writing technique, low loss isesirable, and thus, it is necessary to work in the smoothodification regime.In addition to characterizing the morphology of the

aser-induced changes, it is useful to investigate thetomic-scale changes associated with femtosecond-laserodification, i.e, how the short- and intermediate-range

onding in the glass is changed. Confocal fluorescencend Raman spectroscopy are two techniques capable ofetecting changes in the atomistic molecular structureithin the microscopic laser-modified volumes [17]. Fluo-

escence spectroscopy is very sensitive to the presence ofefects within fused silica, while Raman spectroscopy de-ects changes in the types of silicon–oxygen networktructures within the silica glass.

In this paper, we report on spectroscopic studies ofaveguides and Bragg gratings fabricated in fused silicaith femtosecond-laser pulses. We show that this spectro-

copic information can be used to distinguish the smoothnd rough types of modification.

. EXPERIMENTAL PROCEDUREe studied two types of modified structures: (1) lines andaveguides written in bulk fused silica and (2) Braggratings written in single-mode all-silica core fluorine-oped cladding optical fiber (Sumitomo Z fiber).Waveguides were directly written in fused silica (Corn-

ng 7940) using an in-house amplified Ti:sapphire femto-econd laser (Spectra-Physics Tsunami/Spitfire laser sys-em). The laser was operated at 800 nm with �125 fs

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1628 J. Opt. Soc. Am. B/Vol. 24, No. 7 /July 2007 Reichman et al.

ulses, a repetition rate of 1 kHz, and 0–5 �J pulse ener-ies. It was focused into silica cubes using a 50�0.55 NAicroscope objective, and the sample was moved either

ongitudinally or transversely at 20 �m/s. In addition, forome of the experiments, large regions consisting of three00 �m�100 �m planes spaced 2 �m apart along theeam axis were constructed to improve the signal-to-noiseatio for some of the spectroscopic measurements. Eachlane consisted of 128 waveguides, 100 �m long, spaced�m apart [17].The Bragg grating inscription was performed using a

econd Ti:sapphire amplification system operating at aavelength of 800 nm (FWHM of 10 nm) and 125 fs pulseuration. A beam with a Gaussian profile and 6.4 mm di-meter was focused with a 30 mm focal length cylindricalens through a 4.284 �m pitch phase mask into the core ofhe single-mode fiber. For the formation of smooth modi-cation, the fiber was placed 6 mm behind the phaseask to create a two-beam interference pattern, resulting

n a grating periodicity of 2.142 �m [18]. For rough modi-cation, the fiber is placed flush with the phase mask re-ulting in a multiple-beam interference pattern that in-reases the peak intensity and creates a grating with aeriodicity identical to the pitch of the phase mask [19]. Ahase-mask pitch of 4.284 �m was selected in order toreate the spatially modulated exposure that could beasily probed using optical microscopy and laser spectros-opy. During grating fabrication, the focused laser beams scanned vertically ±20 �m every 10 s using a piezo-riven translation stage. As the focal spot size is quitemall ��5 �m�, this allows for complete coverage of theore region of the fiber. For smooth modification, the fiberas irradiated with approximately 10 000 pulses of200 �J. The two-beam interference pattern produces anntensity of 4.5�1013 W/cm2, slightly below the thresholdor the two-beam damage transition in Ge-doped silicaelecommunication fibers [9]. It should be noted that thehreshold behaviors for Ge-doped silica and all-silica coreber are very similar. For rough modification, the fiberas irradiated with as few as five to ten 950 �J pulses. Aultiple-beam interference model indicates that these

ulse intensities are likely two to three times �9−161013 W/cm2� that used to fabricate the smooth gratings,

nd are well above the damage threshold [9,19].White-light images and fluorescence images were col-

ected for all of the waveguides and Bragg gratings.hite-light transmission images were collected using a

ony XC-75 CCD camera. The fluorescence spectra werebtained by focusing a 50 mW, 488 nm Argon ion �Ar+� la-

ig. 1. Laser modification with a 1 kHz pulse repetition rateemtosecond laser in fused silica as a function of pulse energy.he white region has a higher refractive index than the bulklass, allowing it to function as waveguides. The black region hasacroscopic damage, which causes it to scatter light.

er into the samples using a 100�0.73 NA microscope ob-ective; the back-emitted fluorescence was collected usinghe same objective and a long-pass filter (Croma 488 nmQLP). The laser power at the sample was �30 mW. Theuorescence spectrum was analyzed with a 0.25 m spec-rometer (Oriel MS-257) using a 300 groove mm−1 gratingnd a liquid-nitrogen-cooled CCD array detector (Roperpec-10), with a 1–10 s acquisition time for these experi-ents. Spatial fluorescence profiles of select waveguides

nd of the fiber Bragg gratings were measured by scan-ing the sample with respect to the microscope objectiveocus with a 1 �m step size traveling perpendicular toacross) the waveguides and a 3 �m step size travelinglong the waveguides. Images of the fiber Bragg gratingsere generated by collecting the fluorescence using a

ong-pass filter and a PerkinElmer MP-942 PMT with aollection time of 0.1 s per point and scanning the sampleith 161 steps of 1 �m across the fiber and 41 steps of.5 �m along the fiber. For imaging, the total fluorescencehe long-pass filter was a Croma 488 nm HQLP filter, andor imaging the fluorescence at wavelengths longer than00 nm, a 600LP filter was used.For the waveguides, we also collected spectra of the

ight emitted during exposure to the femtosecond laser,.e., during waveguide writing. The emitted light was col-ected using the same spectrometer used for the fluores-ence spectra. In this case, there was no filter; there wasnly a broadband dielectric beam splitter in the emissioneam path. The acquisition time was 1 s per spectrum.

. RESULTShe first step in our analysis of the different femtosecond-

aser modification regimes was to collect white-light im-ges for waveguides written using different laser pulsenergies and constant focusing conditions. These imagesere used to categorize the modification as unmodifiedlass, smooth waveguides, or rough lines. It was deter-ined that when using a 50�0.55 NA objective, 0.15 �Jas required to create a smooth waveguide, and that the

ransition between creating smooth waveguides andough lines occurs between 0.3 and 0.53 �J. Sample pic-ures of a smooth and a rough line are shown in Figs. 2(a)nd 2(b), respectively.The fluorescence spectra [Figs. 2(c) and 2(d)] measured

fter the modification process show different featuresithin the different modification regimes. No fluorescenceas observed in the unmodified fused silica. A weak fluo-

escence peak centered at �530 nm was observed for themooth waveguides, as shown in Fig. 2(c). A strong fluo-escence peak centered at �630 nm was observed for theough lines, as shown in Fig. 2(d). Notice that the totaluorescence intensity is much smaller for the smoothaveguide. Figure 3(a) displays spectra collected foraveguides written using different pulse energies, nor-alized to their intensity at 530 nm. From this figure, it

an be seen that the fluorescence peak at 530 nm isresent for both the smooth waveguides and the roughines, however, in the rough lines it is overshadowed byhe fluorescence peak at 630 nm.

An analogous transition was observed for the emissionpectra collected during the modification process. No

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Reichman et al. Vol. 24, No. 7 /July 2007 /J. Opt. Soc. Am. B 1629

mission was observed for pulse energies that did notodify the glass. A “blue” emission centered at �475 nmas observed while writing smooth waveguides, as shown

n Fig. 2(e). A “white” emission centered at �540 nm wasbserved while writing rough lines, as shown in Fig. 2(f).igure 3(b) displays emission spectra collected foraveguides written using different pulse energies, nor-alized to their intensity at 475 nm. It is apparent that

he blue emission peak is present for all of the modifiedines, although for the rough lines, it is overwhelmed byhe white emission.

The next phase of our analysis involved observing fiberratings that had been fabricated using an interferenceattern. These gratings showed similar trends to thosebserved for directly written waveguides. Relatively lowulse energies corresponding to peak intensities of ap-

ig. 2. (Color online) (a),(b) White-light transmission images; (mission spectra collected during laser modification for smooth a

roximately 4.5*1013 W/cm2 produced smooth gratingines, as shown in Fig. 4(a). These gratings exhibit both ofhe fluorescence peaks seen in the directly writtenaveguides, with a moderate intensity peak at 530 nmnd a weaker peak at 630 nm, as shown in Fig. 4(c). Fluo-escence imaging reveals the grating pattern and period-city (cf., Fig. 5) indicating that the fluorescence is indeedssociated with the formation of the grating structures.hough not collected, a “blue” emission was observed byye while fabricating this grating. Greater pulse energiesorresponding to peak intensities of 12–16*1013 W/cm2

roduced the lines shown in Fig. 4(b). These lines have auorescence peak at �630 nm, as shown in Fig. 4(d), andave off a white emission while being fabricated.

The fluorescence photobleaching rates of the two fluo-escence peaks were measured by continuously exposing

uorescence spectra collected after laser modification; and (e),(f)gh femtosecond-laser modified lines.

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1630 J. Opt. Soc. Am. B/Vol. 24, No. 7 /July 2007 Reichman et al.

he smooth grating to the Ar+ laser for half an hour, ashown in Fig. 6. Exponential curve fits were used to cal-ulate bleaching rates of 5.0±0.6 and 6.2±0.5 min for the30 and 630 nm peaks, respectively.

. DISCUSSIONo compare the results obtained from the waveguides andratings quantitatively, it is necessary to calculate the la-er peak intensities for both cases. For the waveguides,he peak intensities are calculated using the following ex-ressions:

ig. 3. (Color online) (a) Fluorescence spectra normalized to the30 nm band and (b) emission spectra observed during laserodification normalized to the 475 nm band as a function of

emtosecond-laser pulse energy.

ig. 4. (Color online) (a),(b) White-light transmission imagesnd (c),(d) fluorescence spectra for smooth and roughemtosecond-laser-written Bragg gratings.

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With the parameters used in our experiments (N0.55, M=1.5, n=1.45, and �=800 nm), the laser peak in-

ensity required to cause a noticeable, smooth modifica-ion with direct writing is �5.2*1013 W/cm2 (correspond-ng to a pulse energy of 0.15 �J) and an intensity of1.7*1014 W/cm2 (pulse energy =0.5 �J) is required to

ause rough damage. These values are slightly higherhan the values 4.5*1013 W/cm2 and 12–16*1013 W/cm2

alculated for the interference pattern writing. This dis-repancy is most likely the result of the value of M=1.5hat we used in our calculations. This M value is theominal specification, but the actual value for our laseream in the waveguide writing experiments might haveeen higher. Nevertheless, the main point is that smoothnd rough modification in waveguides and fiber gratingsccur at roughly the same femtosecond-laser intensities.

The fluorescence peaks that we observe after modifica-ion originate from defects that are formed in the glass as

result of the femtosecond-laser exposure. The fluores-ence peak centered at �630 nm matches the characteris-ics of nonbridging oxygen hole center (NBOHC) defects17]. The peak centered at �540 nm matches the charac-eristics of self-trapped exciton �E�� defects from verymall silicon nanoclusters that several groups have re-orted seeing in �-irradiated fused silica [20,21]. The fluo-escence photobleaching rate can be used to further dis-inguish the two types of defects, because the E� defectsleach �20% faster than the NBOHC defects as shown inig. 6.

ig. 5. Fluorescence images of the fiber Bragg grating shown inig. 4(a). (a) the total fluorescence, and (b) the fluorescence fromBOHC defects.

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The origins of the emission bands at 475 and 540 nmbserved during writing are less certain. Carr et al. [22]ave measured the emission generated duringemtosecond-laser exposure under slightly different con-itions. They suggested three potential emission mecha-isms to explain their results: second-harmonic genera-ion, supercontinuum generation, and plasma emissions.he emission bands that they observed had different peakositions than those seen in our results, which indicatehat our emissions may be produced by a slightly differentrocess. The band at 540 nm is only observed at higherulse energies where other experiments have shownlasma forms [14], thus we believe that it relates tolasma emission.Our results clearly show that different morphological

hanges are associated with different spectroscopic fea-ures, during and after writing. The smooth modificationegime is characterized by a blue emission during writingnd the presence of a E�� defect after writing. The roughodification (damage) regime has a white emission whileriting and the presence of NBOHC defects after writing.his suggests that different modification mechanisms areesponsible for the morphological changes observedithin the two modification regimes. For the high-

ntensity regime in which rough macroscopic changes arebserved, the white emissions while writing are probablyhe result of plasma formation. The formation of plasmaeads to a large charge separation within the exposed re-ion. This charge separation may be sufficient to cause aoulomb explosion, which would generate a shock wave

23]. The shock wave would then carry matter and energyway from the focal volume, compressing the surroundingaterial and leaving a rarified central region. The con-

ention that shock waves exist for femtosecond modifica-ion with high pulse energies is further supported by theetection of acoustic waves originating from the focaloint [24], and by scanning electron microscope (SEM)nd atomic force microscope (AFM) images of polishedurfaces and fracture planes that show the presence ofoids [25,26].

It is unlikely that plasma forms within the low-ntensity regime, which is characterized by blue emis-

ig. 6. (Color online) Fluorescence photobleaching of fiber gratinng, and (b) shows the normalized peak intensities as a function

ions while writing, and the formation of good, smoothaveguides. In this case, there would not be sufficient

harge separation to generate a shock wave, thus anothernergy dissipation mechanism is required to account forhe modification. It is our belief that the energy primarilyissipates through thermal diffusion within the glass.he high thermal gradient (potentially thousands of de-rees temperature difference over a couple of microme-ers) and the high surface-to-volume ratio of the laser fo-us would result in a very fast cooling rate. Given that theefractive index of fused silica increases with increasingooling rates [27,28], this would account for the observedncrease in the local refractive index. Similar results forther glasses [29–32] that have increased or decreased re-ractive indices with increasing cooling rates also supporthe idea that the relationship between the refractive in-ex of a glass and the cooling rate used to produce thelass is critical for determining the effects of theemtosecond-laser writing process.

. CONCLUSIONSistinct spectroscopic emissions can be used to differenti-te the macroscopically smooth and rough femtosecond-aser modification regimes, during and after fabrication.he smooth modification produced when using relatively

ow laser intensities is characterized by blue emissionhile writing and the presence of E�� defects (540 nm fluo-

escence) after writing. The rough modification producedhen using higher laser intensities is characterized byhite emission while writing and the presence of NBOHCefects (650 nm fluorescence) after writing.

CKNOWLEDGMENTShe authors acknowledge financial support through theational Science Foundation grant DMR—0307002.ork at Lawrence Livermore National Laboratory was

erformed under the auspices of the U.S. Department ofnergy by the University of California, Lawrence Liver-ore National Laboratory, under contract W-7405-Eng-

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1632 J. Opt. Soc. Am. B/Vol. 24, No. 7 /July 2007 Reichman et al.

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1. R. R. Thomson, S. Campbell, I. J. Blewett, A. K. Kar, andD. T. Reid, “Optical waveguide fabrication in z-cut lithiumniobate �LiNbO3� using femtosecond pulses in the lowrepetition rate regime,” Appl. Phys. Lett. 88, 111109 (2006).

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