femtosecond laser writing of optical edge ﬁlters in fused silica ... · femtosecond laser writing...

Femtosecond laser writing of opticaledge filters in fused silica optical

waveguides

Jason R. Grenier,1,∗ Luı́s A. Fernandes,1,2 and Peter R. Herman1

1Institute for Optical Sciences, and the Department of Electrical and Computer Engineering,University of Toronto, 10 Kings College Rd., Toronto, Ontario, M5S 3G4, Canada

2INESC-Porto, Departamento de Fı́sica e Astronomia da Universidade do Porto, Rua doCampo Alegre 687, 4169-007 Porto, Portugal

∗[email protected]

Abstract: The positional alignment of femtosecond laser written Bragggrating waveguides within standard and coreless optical fiber has been ex-ploited to vary symmetry and open strong optical coupling to a high densityof asymmetric cladding modes. This coupling was further intensified withtight focusing of the laser pulses through an oil-immersion lens to controlmode size against an asymmetric refractive index profile. By extendingthis Bragg grating waveguide writing into bulk fused silica glass, strongcoupling to a continuum of radiation-like modes facilitated a significantbroadening to over hundreds of nanometers bandwidth that blended intothe narrow Bragg resonance to form into a strongly isolating (43 dB)optical edge filter. This Bragg resonance defined exceptionally steep edgeslopes of 136 dB/nm and 185 dB/nm for unpolarized and linearly polarizedlight, respectively, that were tunable through the 1450 nm to 1550 nmtelecommunication band.

© 2013 Optical Society of America

OCIS codes: (230.7408) Wavelength filtering devices; (230.1480) Bragg reflectors; (230.3120)Integrated optics devices; (130.2755) Glass waveguides; (140.3390) Laser materials process-ing.

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21. R. Osellame, G. Cerullo, and R. Ramponi, Femtosecond Laser Micromachining (Springer-Verlag, 2012).22. S. M. Eaton, M. L. Ng, R. Osellame, and P. R. Herman, “High refractive index contrast in fused silica waveguides

by tightly focused, high-repetition rate femtosecond laser,” J. Non-Cryst. Solids 357, 2387–2391 (2011).23. H. Zhang, S. M. Eaton, J. Li, A. H. Nejadmalayeri, and P. R. Herman, “Type ii high-strength bragg grating

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externally modulated femtosecond fiber laser,” Opt. Lett. 32, 2559–2561 (2007).26. J. U. Thomas, N. Jovanovic, R. G. Krämer, G. D. Marshall, M. J. Withford, A. Tünnermann, S. Nolte, and M. J.

Steel, “Cladding mode coupling in highly localized fiber bragg gratings ii: complete vectorial analysis,” Opt.Express 20, 21434–21449 (2012).

27. J. R. Grenier, L. A. Fernandes, P. V. S. Marques, J. S. Aitchison, and P. R. Herman, “Optical circuits in fibercladding: Femtosecond laser-written bragg grating waveguides,” in CLEO - Laser Applications to Photonic Ap-plications, (Optical Society of America, 2011), p. CMZ1.

28. W. Yang, P. Kazansky, and Y. Svirko, “Non-reciprocal ultrafast laser writing,” Nature Photon. 2, 99–104 (2008).29. J. Li, S. Ho, M. Haque, and P. Herman, “Nanograting bragg responses of femtosecond laser written optical

waveguides in fused silica glass,” Opt. Mater. Express 2, 1562–1570 (2012).30. J. R. Grenier, L. A. Fernandes, J. S. Aitchison, P. V. S. Marques, and P. R. Herman, “Femtosecond laser fabrication

of phase-shifted bragg grating waveguides in fused silica,” Opt. Lett. 37, 2289–2291 (2012).31. L. A. Fernandes, J. R. Grenier, P. R. Herman, J. S. Aitchison, and P. V. S. Marques, “Femtosecond laser writing

of waveguide retarders in fused silica for polarization control in optical circuits,” Opt. Express 19, 18294–18301(2011).

32. V. Bhardwaj, P. Corkum, D. Rayner, C. Hnatovsky, E. Simova, and R. Taylor, “Stress in femtosecond-laser-written waveguides in fused silica,” Opt. Lett. 29, 1312–1314 (2004).

33. L. A. Fernandes, J. R. Grenier, P. R. Herman, J. S. Aitchison, and P. V. S. Marques, “Stress induced birefringencetuning in femtosecond laser fabricated waveguides in fused silica,” Opt. Express 20, 24103–24114 (2012).

1. Introduction

Laser writing of finely pitched gratings within single mode optical waveguides such as in op-tical fibers and planar optical circuits have offered substantial opportunity of creating spectralfilters for diverse application in optical communications, fiber lasers, and sensing [1-3]. FiberBragg gratings (FBG) are typically defined by a weak periodic modulation in the effective re-fractive index along the length of the pre-existing fiber core, which can be fabricated throughphase masks or point-by-point direct writing by exposure with either UV [4, 5] or femtosecond[6-8] laser light. For the most part, a uniform transverse refractive index modification is desir-


able in the majority of these applications that strictly limits coupling to only between forwardand backward propagating modes at the Bragg resonance. However, differing photosensitivityresponses in the fiber core and cladding can break this symmetry to permit grating coupling ofthe fundamental LP01 core mode to backward-propagating LP0n cladding modes that manifestin an unwanted series of sharp resonance loss lines that appear blue-shifted from the Braggresonance in the transmission spectrum [9].

While cladding modes are generally undesirable, coupling to such modes offer opportunitiesfor extending the tuning range available for spectral shaping as well as new types of sensingsuch as at the outer cladding surface [10]. Many groups have therefore endeavored to enhancethe asymmetry of the refractive index profile and expand the Bragg coupling to odd symmetricLP1n modes in the cladding, for example, through tilting of the grating planes [11, 12], type IIlaser writing of gratings at the core-cladding interface [13, 14], and transversely offsetting therefractive index modification by focusing outside the axial center of the waveguide in point-by-point writing [15]. In this way, tilted FBGs have been used for wideband gain flattening of anerbium fiber amplifier [16], refractometry of liquids outside of the cladding [10], temperature-insensitive gauging of non-uniform axial strain [17] and acceleration sensing [18].

Tilted chirped FBG have also been tuned to serve as in-fiber edge filters [19, 20]. While thesefiber edge filters have the advantage of integration into compact optical fibers, their spectralisolation has been less favorable than that of traditional larger bulk devices such as dielectricthin-film stacks, doped glasses, and dyes where high optical density (OD > 5), low loss (< 5%)and moderate edge steepness (10 dB/nm) are readily available. FBG-based edge filters havegenerally provided only weak contrast with shallow edge slopes and narrow wavelength range,and tilted-grating edge filters have not been exploited for on-chip integration.

In recent years there has been significant progress in applying femtosecond lasers to di-rectly write buried optical waveguides and other optical devices in bulk glasses for flexibleintegration into three-dimensional (3D) photonic circuits [21]. This facile approach offers flex-ible manipulation of the refractive index profile to manipulate waveguide loss, mode size andbirefringence that with strongly focused oil-immersion lenses can drive strong refractive in-dex contrast (Δn = 0.022) and confine waveguiding to mode field diameters (MFDs) as smallas 7.1 µm [22]. Axial modulating during femtosecond laser waveguide writing with point-by-point single pulses [23, 24] or burst-trains of pulses [25] have further opened new directions forspectral filtering and sensing with 3D optical circuits in bulk glass without the requirement fora pre-existing guiding core. This flexible direct-write positioning of laser modification trackswas only recently exploited by Thomas et al. [15, 26] to asymmetrically shift the Bragg gratingstructure within the pre-existing guiding core of standard optical fiber and facilitate stronglyenhanced coupling to higher azimuthal order cladding modes.

In this paper, we build on our prior demonstration [27] of oil-immersion lens focusing towrite Bragg grating waveguides (BGWs) flexibly positioned within either the core or claddingof standard optical fiber and thereby enhance the core-to-cladding mode coupling. A strongbroadening to a near-continuum loss spectrum is reported as the laser modification track movesfrom inside the doped fiber core to the all-silica cladding, revealing a favorable merging of thecladding mode spectrum with the sharp Bragg resonance. With further engineering to manip-ulate the mode size relative to the formation of a highly asymmetric refractive index profile, adramatically amplified coupling was demonstrated. In extending the BGW writing from stan-dard to coreless optical fiber, and then to bulk glass, this asymmetry gave way to a dense contin-uum of radiation-like modes, defining a uniquely strong and broadband edge filter with a steepedge that was shaped by the narrow Bragg grating response. In this way, direct femtosecondlaser writing of grating waveguides promises new filtering devices for in-fiber or bulk glassoptical circuits.


2. Waveguide fabrication and characterization

A femtosecond fiber laser (IMRA America; µJewel D-400-VR) of 420 fs pulse duration and500 kHz pulse repetition rate was frequency-doubled to 522 nm and directed into an acousto-optic modulator (AOM; NEOS 23080-3-1.06-LTD) controlled by a 500 Hz, 60% duty cycle,square-wave to create burst trains of 600 laser pulses as previously reported [25]. As shown inFig. 1, the laser burst trains were focused 50 µm below the surface of a fused silica substrate(Corning 7980; 50.8 mm x 25.4 mm x 1 mm with all faces optically polished) with a 100×,1.25 NA oil-immersion objective lens to form a segmented waveguide consisting of an array ofpartially overlapping refractive index voxels, resulting in a first order BGW. The Bragg wave-length is governed by the Bragg relation: λB = 2neffΛ, where neff is the effective index (1.445)of the waveguide mode and Λ is the grating period, which was controlled by the ratio of the scanspeed to the AOM modulation frequency. For optical fiber, the oil-immersion focusing avoidedthe otherwise strong astigmatic optical aberration of an air-cladding interface and enabled pre-cise positioning of the BGW anywhere in the core or cladding of Corning SMF-28 fibers aspreviously demonstrated [27]. BGWs were written in coreless fused silica (Suprasil F100) fiberwith 125 µm diameter to reduce refractive effects from the Δn = 0.36% index contrast betweenthe fiber core and cladding. Optical fibers were stripped of their polymer buffer and held taughtwith ±1 µm accuracy to the laser scanning direction over a distance of 10 cm. Motion con-trol stages (Aerotech ABL1000 with 2.5 nm resolution and 50 nm repeatability) translated thesample with respect to the laser beam, which was linearly polarized parallel to the direction ofmotion, at a speed of 0.268 mm/s to form BGWs of ~25 mm length. AOM modulation frequen-cies from 534.4 Hz to 500 Hz served to tune the Bragg resonance wavelength from 1450 nm to1550 nm. Pulse energies over the range from 80 nJ to 140 nJ were explored to manipulate therefractive index profile and MFD.

Oil

Scan Direction

100X1.25 NA

y

z

x

10 μm

ᴧ

Fig. 1. Burst trains of femtosecond laser pulses focused into a traversing fused silica glassby a 100×, 1.25 NA oil-immersion lens to form a buried BGW. Inset, an optical micrographof the end-facet of the resulting laser formed BGW.


The transmission and reflection spectra of the BGWs were recorded by coupling unpolar-ized light from broadband infrared light sources (Thorlabs; ASE-FL7002, 1520 nm to 1610 nmor Agilent 83437A, 1200 nm to 1700 nm) through single mode fibers (SMF) and an opticalfiber circulator and end-coupled to the BGW devices. Index matching oil was applied at allglass-fiber interfaces to reduce the Fresnel reflections and Fabry-Perot effects. All spectra werenormalized relative to direct fiber-to-fiber transmission and recorded with an optical spectrumanalyzer (OSA; Ando AQ6317B, with 0.01 nm resolution). To probe the birefringence of theBGWs, the infrared light source was coupled to free space and passed through a broadbandpolarizer (Thorlabs LPNIR) and a 10× (0.16 NA) lens to excite either horizontally or verticallypolarized eigenmodes with electric fields aligned with the y- or z-axis (Fig. 1), respectively.The intensity profiles of the BGW modes were captured by launching tunable laser light (Pho-tonetics Tunics-BT, 1 pm resolution) into the BGW and imaging the end facet onto a phosphorcoated CCD camera (Spiricon SP-1550M) with a 60× (0.65 NA) lens.

3. Bragg grating waveguide spectra and discussion

BGWs were fabricated in the center of single mode and coreless optical fibers to explore thesymmetric coupling to cladding modes from a pre-existing germanium-doped and a femtosec-ond laser formed core, respectively. BGWs were also positioned off-center in the cladding ofSMF to enhance the coupling to higher azimuthal order cladding modes and in bulk glass wherecoupling to a broad continuum of radiation-like modes was explored.

3.1. Cladding and radiation mode coupling

The transmission spectra of BGWs written with nearly identical exposures (120 nJ to 130 nJ)are shown in Figs. 2(a) - 2(d) for the respective cases of positioning the BGW near-center in thepre-existing core of SMF, the center of a coreless optical fiber, off-center (Δr = 30 µm) in thecladding of a SMF and 50 µm deep in bulk fused silica glass. The inset images of waveguideend-facets and their magnified views reveal similar attributes of a dual modification zone ofpositive increase in refractive index of a larger diameter (3.6 µm) white zone under a smallerdiameter zone (2.1 µm) of negative refractive index change. The spectra are similar in providinga narrow Bragg resonance (Δλ = 0.3 nm) near λB = 1550 nm, but differ significantly in termsof the spectral density and the strength of coupling to the permitted cladding modes or theradiation-like modes in the bulk glass.

For BGWs centered in both the core of the SMF (Fig. 2(a)) and the coreless fiber (Fig. 2(b)),a very open spectrum is dominated by strong coupling to the discrete LP0n cladding modes thatare seen to match well with the stick spectra as shown in the SMF case (Fig. 2(a)), calculatedaccording to [9]:

δλS � 3.1(

λL2πnclacl

)√λSλL. (1)

Here, the spacing between adjacent cladding modes (δλS) is related to the wavelength offset(λS) of the cladding mode from the first cladding mode (λL at LP01), the cladding refractiveindex (ncl) and the cladding radius (acl). This well matched stick spectra was calculated withthe measured values of λL = 1549.49 nm and the refractive index of fused silica at 1550 nm(ncl = 1.44402) while adjusting the cladding radius to acl = 64.25 µm, which slightly exceededthe expected fiber radius of 62.5 ± 0.3 µm.

In the case of a BGW fabricated in the core of an SMF (Fig. 2(a)), the strong couplingobserved to the symmetric LP0n cladding modes can be anticipated from the large asymmet-rical mismatch in the size of the guiding structure (5.2 µm x 2.16 µm) and the mode profile


Fig. 2. Transmission spectra of ~25 mm long BGWs fabricated in (a) the core of a SMFusing 130 nJ, (b) the center of a coreless fiber using 130 nJ, (c) the cladding of a SMF using120 nJ and (d) bulk fused silica glass using 120 nJ pulse energy. Insets, optical micrographsof BGW end-facets (writing laser was from the top) with expanded views (image diameteris 12 µm) of the waveguide zone overlain on the top-left, and mode profile pictures (imagewidth is 10 µm).

(7.4 µm x 6.9 µm MFD). A moderately strong coupling is also observed to a higher order az-imuthal cladding mode (LP1n) that may arise from imperfect centering of the BGW in the fibersas reported by [15, 26], as well as from an asymmetric refractive index profile as seen in thehigher resolution images inset in Fig. 2. Coupling to an increasing density of azimuthal claddingmodes was observed as the BGW position was increasingly offset from the center axis, leadingto a broadened but weakened coupling that in the case of Fig. 2(c) shows a coarse continuumof 7.3 dB loss when the BGW was offset by 30 µm from the fiber center. Similar broadeningof the cladding modes is expected for a BGW positioned in the cladding of coreless opticalfiber. Extending further to position the BGW inside the much larger glass plate (Fig. 2(d)) wassignificant in opening the coupling to a much higher density of radiation-like modes, producinga smooth continuous rejection-band that is 9.4 dB deeper than for wavelengths longer than theBragg wavelength.

For BGWs positioned within the SMF core (Fig. 2(a)), the cladding mode spectrum is iso-lated from the the Bragg resonances according to wavelength offset, λoff, given by [9]:

λoff =λB2

(1 − ncl

neff

). (2)

The strong refractive index contrast (ncore − ncl = 0.0052) of the doped core over the fibercladding therefore predicts a large λoff = 2.78 nm, which aligns well with the λoff = 1.88 nmoffset seen in Fig. 2(a). On the other hand, the small refractive index contrast expected betweenthe waveguide (neff = 1.44499 measured from the Bragg relationship) written in the fused silicaand the background glass (ncl) predicts a small λoff = 0.52 nm offset, which then leads to afavorable closing of the cladding-mode and Bragg gap for the cases of BGWs written in theSMF cladding (Fig. 2(c)), the coreless fiber (Fig. 2(b)) and the bulk glass (Fig. 2(d)). Hence,


Fig. 2 demonstrates how the positioning of BGWs in the core and cladding of optical fibers andin bulk glass further expands the control over cladding and radiation mode coupling to openinto a broad continuum that can blend into the Bragg resonance and open an opportunity tocreate a strong and broad rejection-band optical filter within the waveguide.

3.2. Optical edge filter waveguide

To manipulate the waveguide mode confinement and further optimize the coupling to the radi-ation modes, BGWs were fabricated over a range of pulse energies from 80 nJ to 140 nJ at adepth of 50 µm below the surface of bulk fused silica glass. Scan directions were also reversed(±x) along directions that were perpendicular to the grating compressor axis of the laser in or-der to minimize the effect of pulse front tilt on non-reciprocal writing (i.e. “quill” effect) [28].The transmission spectra of unpolarized light guided through BGWs having Bragg resonancesat 1550 nm are shown in Figs. 3(a) and 3(b) for the -x and +x scanning direction, respectively.At a maximum available pulse energy of 140 nJ (entering the glass), a strong grating strengthof 39.5 dB, a 3 dB bandwidth of 0.2 nm, a 5 dB radiation mode loss, and a 5 dB total insertionloss (1.9 dB/cm) are reported for -x scanning direction (Fig. 3(a)), confirming that femtosecondlaser writing of strong, first-order BGW can be extended from low NA writing [25] to high NAoil-immersion writing [27]. However, as the pulse energy was decreased from 140 nJ to 80 nJ,the radiation mode loss increased dramatically to 36 dB and opened into the Bragg resonance(λoff ≈ 0), resulting in the formation of an optical edge filter waveguide.

Fig. 3. Unpolarized transmission spectra for Bragg grating waveguides fabricated in bulkglass with pulse energy from 80 nJ to 140 nJ, with scanning in the (a) -x and (b) +x direction.

BGW response for +x scanning direction (Fig. 3(b)) yielded a similar BGW strength devicebut with broader 3 dB bandwidth (0.4 nm) and stronger (21 dB) radiation mode loss. The trendto lower pulse energy generated substantially stronger radiation mode loss (43 dB) comparedwith the BGWs fabricated in the -x direction, reinforcing a non-reciprocal writing effect in theunexpected direction orthogonal to the pulse front tilt as reported by [29]. Such large couplingto the radiation-like modes, which was not seen in BGWs previously written with a weaker0.55 NA lens [25], is due to the smaller refractive index structure (4 µm x 10 µm) formed withthe 1.25 NA oil-immersion writing process relative to the guided mode sizes. This is visualizedin Fig. 4 where the top row shows optical micrographs of the BGW end-facets and the bottomrow shows the guided mode profiles at 1560 nm, for the BGWs of Fig. 3(b). The measuredMFD values defined the ellipses (dotted lines) that were superimposed over the optical micro-graphs to indicate the size and approximate position of the mode relative to the positive (white)


and negative (black) refractive index zones of the BGW end-facets. As the pulse energy was de-creased from 140 nJ to 80 nJ, the MFD increased from 10.5 µm x 10 µm to 16.4 µm x 13.9 µm,extending the mode partially into the negative index region. This lower energy exposure there-fore defines a highly asymmetric grating index profile that breaks the symmetry to permit strongcoupling to the continuum of available radiation modes that is especially strongest (43 dB) forthe 100 nJ case in Fig. 3(b) and diminishes for the case of 80 nJ when the asymmetric negativezone is fully encapsulated by the larger mode profile.

Fig. 4. Optical micrographs of the end-facets (top row) and mode profile images (bottomrow) of the BGWs written with pulse energies of (a) 140 nJ, (b) 120 nJ, (c) 100 nJ and (d)80 nJ. The dotted ellipses indicate the size and approximate position of the mode relativeto the guiding zone of the waveguide (white zones in top row).

The strong coupling of light into the continuum of radiation modes in bulk glass reveals anopportunity for very sharp and potentially broadband edge filters. The transmission and reflec-tion spectra for the BGW written in the +x direction with a pulse energy of 100 nJ (Fig. 5(a))show a strong and narrow Bragg response seen only in reflection while the strong cladding modecoupling is seen only in transmission. The 3 dB bandwidth of 0.56 nm as seen in reflection is2.8× larger than that reported for BGWs fabricated with a 0.55 NA lens [25, 30]. This broaden-ing may be due to an increased grating strength with higher resolution focusing, but waveguidebirefringence is also a contributing factor. The inset in Fig. 5(a) reveals a 114 ± 3 pm shift in theedge wavelengths when the BGW was probed with vertical and horizontal polarization, lead-ing to an inferred waveguide birefringence of (1.06±0.03)×10−4. This birefringence is 2-foldlarger in comparison to waveguides written with weaker focusing of 0.55 NA in air at the samepulse energy [31] and may arise from increased asymmetric stress [32, 33] for focusing withthe 1.25 NA oil immersion lens.

The extinction ratio between the passband and rejection-band in Fig. 5(a) was measured tobe 43 dB, with an edge steepness of 136 dB/nm when probed with unpolarized light from3 dB to the bottom of the rejection-band (43 dB). When probing with linearly polarized light(Fig. 5(a) inset) this edge steepness increases to 185 dB/nm, which is approximately two ordersof magnitude higher than the 0.7 dB/nm to 13 dB/nm values reported in FBG-based edge filters[19, 20]. A broader spectral examination of two edge filter waveguides (Fig. 5(b)) shows theedge filter response to extend from the Bragg wavelengths (1450 nm and 1550 nm) to the limitof our source (1250 nm). The 1550 nm edge filter waveguide maintains an extinction ratio


1535 1540 1545 1550 1555 1560Wavelength (nm)

−50

−40

−30

−20

−10

0

Tran

smis

sion

(dB

)

(a)−35

−30

−25

−20

−15

−10

−5

0

Refl

ectio

n(d

B)

1250 1350 1450 1550 1650Wavelength (nm)

−50

−40

−30

−20

−10

0

Tran

smis

sion

(dB

)

(b)

1550.0 1550.3 1550.6

Δλ = 114 ± 3 pm

Fig. 5. (a) Transmission (solid blue curve) and reflection (dashed green curve) spectrarecorded through a BGW with a large radiation mode loss for unpolarized probing lightand transmission spectra (inset) with vertical (solid red curve) and horizontal (dashed blackcurve) polarized light. (b) Transmission spectra for edge filter waveguides fabricated withedge wavelengths of 1450 nm (dashed green curve) and 1550 nm (solid blue curve).

of 43 dB until 1520 nm, diminishing to 18 dB at 1370 nm before increasing again to 30 dBat 1250 nm. These edge filters waveguides are therefore 15 times broader than the 2 nm to20 nm bandwidths reported for tilted FBG [19] and further represent the highest attenuatingand steepest edge filter responses reported to date for a single mode waveguide.

Figure 5(b) shows the transmission spectra for an edge filter waveguide with an edge wave-length of 1450 nm that was tuned by changing the AOM modulation frequency. In this way, theBragg wavelength and hence the edge filter wavelength can be tuned for applications requir-ing filtering across the 1200 nm to 1700 nm spectrum tested here. The weaker extinction ratiofor this shorter wavelength edge filter can be recovered by increasing the laser pulse energy tocompensate for the smaller mode size expected at this wavelength.

The 7 dB insertion loss for the edge filter in Fig. 5(a) mostly consists of waveguide propaga-tion loss, found here to be 2.2 dB/cm, since only a small 0.13 dB modal mismatch is expectedat 1560 nm for coupling between SMF (MFD = 10.4 µm) and BGW mode (13.3 µm × 10.8 µm)shown in Fig. 4(c). This propagation loss is comparable to the lowest loss (2.27 dB/cm) for thewaveguide with the sharp Bragg resonance in Fig. 3(a), but is 3.6 times higher than the lowestlosses reported for BGW fabrication with weaker 0.55 NA air lens focusing [25]. Edge filterdevices with lengths shorter than the 25 mm presented here may be considered to reduce thetotal insertion loss, with a trade off of weaker extinction ratio.

Despite the moderate propagation loss, the reported edge filter waveguide devices offeredvery steep slopes (185 dB/nm) for linearly polarized light, a large attenuation (OD 4) over a30 nm wavelength range, and approximately OD 2 attenuation over 300 nm bandwidth. Thesedevices are attractive for inserting into 3D bulk optical circuits or planar waveguide circuitswhich cannot be achieved with the traditional, lower loss, bulk optical edge filters. A major ad-vantage of the femtosecond laser written edge filter waveguides is the steep edge cut off that isdefined by the sharp resonance of the Bragg grating edge together with the strong coupling lightto the continuum of radiation modes in bulk glass. A similarly dense range of cladding modescould also be made available for BGWs written asymmetrically in optical fiber by adopting anindex-matched coating around the fiber [9] and thus broaden the spectrum reported in Fig. 2(a).However, writing in the fiber cladding or in coreless fiber is preferred to close the λoff gap and


merge the cladding mode spectrum with the sharp Bragg resonance. Further exploration of laserbeam shaping and focusing techniques would be attractive to generate more asymmetric refrac-tive index profiles and couple even more strongly to higher order cladding modes to broadenedthe spectral control of Bragg grating and edge filter waveguide devices in bulk and fiber glasses.

4. Conclusion

Strong coupling to a high density of asymmetric cladding and radiation modes was exploredby controlling the position of BGWs written into standard and coreless optical fiber and inbulk glass with a femtosecond laser. Strong focusing with an oil-immersion lens further offeredstrongly asymmetric refractive index profiles that together with manipulation of the MFD couldcontrol the modal overlap for strong coupling to a continuum of radiation modes in bulk glass.In this way, the first demonstration of strongly isolating (43 dB) optical edge filter waveguideswith exceptionally steep edge slope of 185 dB/nm was reported for linearly polarized light.These edge-filter waveguides extend the spectral filtering functions available in optical fiberand 3D integrated optical circuits that are attractive for facile monolithic integration with otheroptical and optofluidic devices for broad based applications in telecommunications, fiber lasers,and micro-total analysis systems, optical sensing, and spectroscopy.

Acknowledgments

This work was supported by the National Science and Engineering Research Council ofCanada, the Canadian Institute for Photonic Innovation and the Portuguese Fundação paraa Ciência e Tecnologia.


femtosecond laser writing of optical edge ﬁlters in fused silica ... · femtosecond laser writing...

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