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Investigation of ultrafast laser–photonicmaterial interactions: challenges for

directly written glass photonicsM. Ams, G. D. Marshall, P. Dekker, M. Dubov, V. K. Mezentsev, I. Bennion and M. J. Withford, Member, OSA

(Invited Paper)

Abstract—Currently, direct-write waveguide fabrication isprobably the most widely studied application of femtosecondlaser micromachining in transparent dielectrics. Devices such asburied waveguides, power splitters, couplers, gratings and opticalamplifiers have all been demonstrated. Waveguide propertiesdepend critically on the sample material properties and writinglaser characteristics. In this paper we discuss the challengesfacing researchers using the femtosecond laser direct-write tech-nique with specific emphasis being placed on the suitability offused silica and phosphate glass as device hosts for differentapplications.

Index Terms—Laser machining, laser materials-processing ap-plications, optical glass, optical waveguides.

I. INTRODUCTION

IN 1996, it was shown that tightly focussed femtosecondinfrared laser pulses can create a permanent refractive index

modification inside bulk glass materials [1], [2]. Although in-vestigations into understanding the nature of this modificationand the conditions that produce it are ongoing, it is widelyaccepted that the modification process is initiated by therapid absorption of laser energy through nonlinear excitationmechanisms [3]. The subsequent dissipation of this energyinto the lattice causes the modification inside the glass. Thisresult enables the direct-write fabrication of optical devices,active and passive, in a variety of dielectric optical materialsincluding amorphous glasses, crystalline materials and opticalpolymers simply by moving the glass sample through the focusof a femtosecond laser beam. The material surrounding thefocal volume remains largely unaffected by the writing beampassing through it, allowing structures to be written at arbitrarydepths and in a three-dimensional fashion.

The femtosecond laser direct-write technique has been usedto fabricate buried waveguides [1], [4], [5], power splitters[6]–[8], couplers [9]–[12], gratings [13]–[18] and optical am-plifiers [19]–[22]. These devices have been produced using(i) regeneratively amplified Ti:Sapphire laser systems thatprovide high pulse energies (µJ-mJ) at kHz repetition rates,

Manuscript received February 11, 2008; revised February 11, 2008.M. Ams (mams@physics.mq.edu.au), G. D. Marshall, P. Dekker

and M. J. Withford are with the MQ Photonics Research Centre, the Centrefor Ultrahigh bandwidth Devices for Optical Systems (CUDOS) and theDepartment of Physics, Macquarie University, North Ryde, NSW 2109,Australia.

M. Dubov and I. Bennion are with the Photonics Research Group, Elec-tronic Engineering, Aston University, Aston Triangle B4 7ET Birmingham,United Kingdom.

(ii) oscillator-only Ti:Sapphire systems with low energy (nJ)and high repetition rates (MHz), (iii) high pulse energy (nJ-µJ)ytterbium-doped fibre lasers at high repetition rates (100 kHz-MHz) as well as cavity dumped Yb:KYW laser oscillators.While all of the systems described above are effective atmodifying transparent dielectrics significant differences existbetween the mechanism underlying the modification and there-fore also the strength of the modification, level of damage (ifany) and most importantly in terms of waveguides whether theindex change is positive or negative. Key parameters whichaffect the writing properties include the sample translationspeed, focussed beam shape, beam polarisation, pulse energy,pulse repetition rate, wavelength and pulse duration. Otherproperties that dictate the type of material modification in-clude, for example, bandgap energy, whether the sample iscrystalline or amorphous, thermal characteristics and fracturestrength. In this paper we review work in the area of direct-write femtosecond laser modification of photonic materialswith an emphasis on fabricating waveguides devices in silicaand doped phosphate glass using a high energy, low repetitionrate Ti:sapphire laser amplifier and for comparison, a highrepetition rate, low energy oscillator-only laser system.

II. MATERIALS

The materials interaction processes at play within the laserfocus are strongly dependant on both the material and thelaser parameters, and it is common to observe both positiveand negative changes in the material refractive index underdifferent laser processing conditions or even within the sameinteraction region. Most studies into the fundamental physicalprocesses that occur at the laser focus have been conductedin fused silica. In comparison with other optical materials,fused silica can be processed under a wide range of laserpulse frequencies, durations and energies, wavelengths andsample translation speeds. Furthermore, fused silica is easy toobtain in high purity forms by virtue of it being a popular UVoptical material. Borosilicate glass has also been extensivelystudied. The most important property of borosilicate glassesis, however, the response of the glass to cumulative heatingresulting from high (>500 kHz) pulse repetition frequencylaser exposure. Borosilicate glasses (in contrast to fused silica)have been demonstrated to exhibit controlled growth of theheat affected zone centred at the laser focus within thematerial thus controlling the dimensions of the written optical

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waveguide [23]. This facile control of the heat affected zonethrough judicious selection of the laser processing parametersis an example of how the combination of the correct materialand laser processing parameters can be used to great effect inthe creation of arbitrary waveguide designs.

Fused silica and borosilicate glasses provide an excellentplatform in which to create passive optical devices. Thesolubility of active, rare-earth, ions in these glasses is lowand despite the extensive use of rare-earth ion doped silicaglasses in optical fibre devices, the relatively low gain-per-unitlength value (0.3 dB/cm) of these materials makes it difficultto realise high-gain devices in a typical few centimetres longdirectly-written device. Consequently there has been a greatdeal of interest in phosphate glass hosts in which tens ofpercent by weight of rare-earth ions can be held in solutionoffering a higher gain-per-unit length value (4 dB/cm) withoutdetrimental effects such as ion-clustering. Erbium and Ytter-bium co-doped phosphate glass hosts have been successfullylaser processed and used to create optical amplifiers and,with the addition of external reflectors, optically pumpedwaveguide-lasers (WGL) [24].

Apart from the passive and active glass materials typicallyused for directly-written devices, the femtosecond laser direct-write technique has also been applied to common crystallinematerials such as LiNbO3 [25], YAG [26], [27], LiF [28]and Ti:Sapphire [29]. The dominant material change, whenusing a low repetition rate femtosecond laser, in most of thesematerials is a negative refractive index change, however, useof suppressed cladding arrangements or induced stress fieldsallowed waveguiding regions to be realised.

III. MATERIALS INTERACTION PROCESSES

A. Nonlinear Excitation Mechanisms

In non-metallic materials, the valence band is the highestoccupied energy level where electrons are normally presentat absolute zero, i.e. the lowest band of allowed states. Sinceelectrons have a tendency to fill the lowest available energystates, the valence band is always nearly completely filled withelectrons. An energy gap, Eg , separates the valence band fromthe conduction band; the lowest unoccupied energy level in thematerial. When valence electrons gain enough energy, from aradiation field for example, they can leave the valence band torise up to the conduction band and become free electrons.To achieve such a promotion, the radiation field’s photonenergy must exceed the bandgap energy, Eg . Typically, a singlephoton of visible light does not possess enough energy toexceed the bandgap energy of typical optical materials. In thiscase, nonlinear absorption processes are required to promotevalence electrons to the conduction band. This can be accom-plished through photoionisation and avalanche ionisation [3].Photoionisation refers to the direct excitation of electrons bythe laser field and can be broken down into two differentregimes which are dependant on laser frequency and intensity:multiphoton ionisation (MPI) and tunnelling ionisation.

1) Tunnelling Ionisation: Upon radiation, the band struc-ture of a dielectric can be distorted due to the presence of anelectromagnetic (EM) field. This field suppresses the potential

Fig. 1. Nonlinear photoionisation processes underlying femtosecond lasermachining. (a) Tunnelling Ionisation, (b) Multiphoton Ionisation and (c)Avalanche Ionisation: Free carrier absorption followed by impact ionisation.VB - Valence Band, CB - Conduction Band.

that binds a valence electron to its parent atom. If the laser fieldis strong enough, band to band transitions can occur wherebya bound electron tunnels out to become a free electron. This isillustrated in Fig. 1(a). Tunnelling ionisation is the dominantnonlinear ionisation regime for strong laser fields and low laserfrequencies [3].

2) Multiphoton Ionisation (MPI): MPI occurs due to thesimultaneous absorption of multiple photons by a single elec-tron in the valence band. In order for an interband transitionto occur, the total absorbed energy of all the n photons thatinteract with the single electron must have an energy thatexceeds the bandgap energy Eg , i.e. nh̄ω ≥ Eg where h̄ isPlanck’s constant and ω the laser frequency. In fused silica,which has a bandgap energy of 9 eV, at least 6 photons arerequired to be absorbed by a single valence electron to drivean interband transition. MPI is illustrated in Fig. 1(b). MPIis typically associated with high laser frequencies (still belowthat required for linear absorption) [3].

3) Avalanche Ionisation: An electron already in the con-duction band can also sequentially absorb several laser photonsuntil its energy exceeds the conduction band minimum bymore than the bandgap energy Eg (see Fig. 1(c)). This electroncan then collisionally ionise (via impact ionisation) anotherelectron from the valence band, resulting in two electronsin the conduction band’s lowest available energy state [3],[30]. This process will repeat as long as the laser fieldis present causing the conduction band electron density toincrease exponentially. Kaiser et al. showed that avalancheionisation typically develops for pulse durations greater than200 fs [31].

B. Energy Transfer

The ionisation process results in a transfer of energy fromthe radiation field to the dielectric’s electrons creating a free

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Fig. 2. Material modification regimes of (most) transparent glasses inducedby the femtosecond laser direct-write technique.

electron gas. Eventually this deposited energy is redistributedover the various energy states of the system, i.e. the energy isthen transferred from the electrons to the lattice via electron-phonon coupling. It is important to note that femtosecond non-linear absorption occurs on a time scale that is short comparedto the time scale of energy transfer within the system; thetime it takes for the electrons to transfer their energy to thelattice. As a consequence the absorption and lattice heatingprocesses can be decoupled and treated separately [30]. Inessence, a femtosecond laser pulse produces a strong non-equilibrium condition in a material with electron temperaturesmuch higher than lattice temperatures. In other words, atthe end of the femtosecond laser pulse there are many hotelectrons within a cold lattice. How a system reacts to thesestrong non-equilibrium conditions determines the process ofenergy relaxation and the types of structural changes that canbe produced inside a material.

IV. MODIFICATION REGIMES

Three different types of material modification have beeninduced in the bulk of transparent materials using the fem-tosecond laser direct-write technique; a smooth isotropic re-fractive index change, a birefringent refractive index changeand a void. Fig. 2 illustrates these modification regimes forfused silica (and most other transparent glasses) induced byfemtosecond laser pulses.

A. Smooth Refractive Index Change

1) Thermal Model: Although the explicit mechanism thatmay contribute to refractive index change is not known, itseems likely that energy deposited into the focal volumeof a material by near-threshold femtosecond laser irradiationleads to local rapid heating and modification of a smallvolume of the glass at the focal spot [32], [33]. Becausethe thermal gradient achieved by this process is localised tothe focal point, a very small fraction of the whole lattice,the glass subsequently cools very rapidly. In fused silica,the density increases if the glass is quenched from a hightemperature [34]–[36], explaining the higher refractive index(typically on the order of 10−3) observed in femtosecond laserirradiated fused silica [1], [4], [37]. Similar results for otherglasses [38]–[42] show either an increased or decreased1 re-fractive index change with increasing cooling rates confirmingthat the induced index change is related to a thermal process.However, a model of a thermal origin for the index change

1Note that even in glasses where irradiation leads to a refractive indexdecrease in the irradiated volume, there is often a refractive index increasejust outside this volume, likely due to compression and densification of thematerial that surrounds the, now less dense, irradiated volume [39].

showed that low energy infrared oscillator exposure and highenergy infrared amplifier exposure do not achieve the sametemperatures during fabrication, yet they induce similar indexchanges [43]. This result suggests that thermal heating is notthe only mechanism that can lead to a smooth refractive indexchange.

2) Colour Centre Model: It has been suggested that theeffect of radiation produces colour centres [1], [5], [33], [38],[44] in sufficient numbers and strength to alter the refractiveindex through a Kramers-Kronig mechanism [43]. This theoryhas been a proposed mechanism for the index change producedby deep-UV excitation of Ge-doped silica fibres that result infibre Bragg gratings [45]. A high electron density, produced bythe nonlinear absorption mechanisms outlined above, leads to asufficient trapped species (colour centres) concentration in theexposed region resulting in different types of substrate defectsbeing formed. Confocal fluorescence spectroscopy at 488 nmhas been used by a number of different research groups todetect changes in the molecular structure within femtosecondlaser irradiated regions [1], [33], [44], [46]. According to stan-dard electron spin resonance (ESR) investigations of irradiatedfused silica glass2, a fluorescence peak at 630 nm due to non-bridging oxygen hole centre (NBOHC) defects is produced aswell as a peak centred at 540 nm, characteristic of self-trappedexciton SiE’ defects from small silicon nanoclusters. Thisdirect evidence of colour centre formation in a femtosecondlaser modified region may contribute to the refractive indexchanges also associated with femtosecond laser modification.These colour centres, however, do not produce the majority ofthe induced refractive index change because eliminating themby annealing (photobleaching) does not recover the originalindex [37], [43], [47].

3) Structural Change Model: Poumellec et al. [48] showedthat densification and strain in the glass due to femtosecondlaser radiation may also account for changes in the indexof refraction. In order to detect changes in the types ofnetwork structures within a glass material, researchers usedRaman spectroscopy [33], [46], [49]. Fused silica typicallyhas large 5- and 6- fold ring structures dominant in itsnetwork [50], [51]. However, scattering from a femtosecondlaser modified region of fused silica resulted in Raman peakscentred at 490 cm−1 and 605 cm−1, which were attributed tothe breathing modes of 4- and 3- membered ring structures inthe silica network respectfully [50], [52]. These low rank ringsare a sign of elevated energy in the silica structure consistentwith the nonlinear absorption mechanisms mentioned earlier.An increase in the 3- and 4- fold ring structures (and anassociated decrease in the number of 5- and 6- fold ringstructures) present in femtosecond laser modified regions leadsto a decrease in the overall bond angle in the silica networkand a densification of the glass [5], [15], [32], [51], [53].It has also been shown that the refractive index and theabundance of these low rank ring structures in femtosecondlaser exposed regions increase in the same way. Furthermore,Hirao et al. examined the laser modified region by an atomic

2Fused silica has been outlined here as an example and because it has beenactively researched. Other defects associated with other glasses do exist andcan be found in the literature, for example [5], [38].

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force microscope (AFM) and showed that a refractive indexchange is related to this densification process [54]. In contrastto colour centre photobleaching, the Raman changes observedin the network structure remain permanent.

A common feature of laser induced densification is the stressthat is produced in the surrounding unexposed medium inresponse to volume changes produced in the exposed region.These stresses manifest themselves as birefringence [4], [43].Assuming a uniform densification within the femtosecondlaser modified region, the relative magnification of the in-duced density change can be calculated from the measuredbirefringence [4], [43]. Such measurements have indicated thatdensification alone cannot account for the entire change in theindex of refraction [4], [43], [55].

4) Summary: Smooth refractive index change induced byfemtosecond laser radiation is likely due to a contribution ofall the effects outlined above, i.e. colour centre formation,densification (structural change) and thermal treatment (melt-ing) of the glass. Optical waveguide devices are fabricated inmaterials using design parameters that give rise to this regimeof modification.

B. Birefringent Refractive Index ChangeUnder slightly different parameters, it has been shown

that modified regions in fused silica using the direct-writetechnique contain nanoporous structures that are dependent onthe polarisation of the femtosecond laser writing beam [56]–[58]. These nanostructures were found to be self-orderedand periodic (with a size and period as low as 20 nm and140 nm respectively) while being orientated in a directionperpendicular to the electric field vector of a linearly polarisedfemtosecond laser writing beam.

Using a scanning electron microscope (SEM) and selectivechemical etching, researchers were able to show that thenanostructures consist of alternating regions of material withslightly increased density and slightly decreased density. Thisperiodic varying material composition found in the irradiatedvolume gives rise to birefringent refractive index changes [14],[47]. Furthermore, Auger electron spectroscopy of the sameregions revealed that the concentration of oxygen varies acrossthe irradiated area [56], [59]. These results indicated thatthe periodic nanostructures consist of periodically distributedoxygen deficient regions.

Two explanations for the formation of these nanostructuresor ‘nanogratings’ have been postulated. Shimotsuma et al.argue that the interference between the incident light field andthe electric field of the bulk electron plasma wave, inducedvia nonlinear absorption, results in a periodic modulationof electron plasma concentration and permanent structuralchanges in the glass network [56], [59]. This theory wouldserve as the first direct evidence of interference betweenlight and electron density waves. Hnatovsky et al. suggestthe evolution of nanoplasmas into disc shaped structures dueto high nonlinear ionisation creates the nanostructures [57].The observed nanostructures represent the smallest embeddedstructures created to date using light. Thus far, this induced‘form birefringence’ has been shown to only exist in fusedsilica.

Fig. 3. Writing setup used to fabricate optical waveguide devices.

C. Void

At extremely high intensities, the region of modificationis characterised by material damage or void formation. Dueto avalanche ionisation and continuous impact ionisation, alocalised plasma is formed in the focal region [2], [60]. Asthe temperature increases in the exposed region, the plasmacauses a large charge separation resulting in high pressures.This charge separation is sufficient enough to cause a Coulombexplosion (microexplosion) generating a shock wave [60]–[62]. Because this explosion or expansion occurs within thebulk of a material, the shock wave carries matter and energyaway from the focal volume, compressing the surroundingmaterial and leaving a rarified (less dense or hollow core)central region termed a void [63], [64]. The contention thatshock waves exist during femtosecond laser modification withhigh pulse energies is supported by the detection of acousticor pressure waves originating from the focal point [65]–[67].Voids have been used in for the fabrication of optical memorydevices [68], fibre Bragg gratings [69] and 2D waveguidearrays [70].

V. EXPERIMENT

A. Fabrication

In this paper, optical waveguide devices were manufacturedusing either a regeneratively amplified Ti:sapphire SpectraPhysics Hurricane laser (pulse length 120 fs, repetition rate1 kHz) or a Ti:sapphire Femtolaser XL oscillator (pulselength 60 fs, repetition rate 11 MHz) together with the setupshown in Fig. 3. The 800 nm beam exiting the femtosecondlaser passed through a computerised rotatable 1/2-wave plateand linear polariser setup allowing fine control of the pulseenergy to be achieved. The femtosecond laser pulses were thenfocussed into the glass sample using a microscope objective.A variety of objectives with different numerical aperture (NA)and working distances were used so that the size and shapeof the fabricated structures could be tailored to a certaindegree. Typically, high NA objectives were used in conjunctionwith the XL oscillator system as a tight focus was requiredto reach an intensity in order to modify the glass substrate.Such a tight focus was not required when using the amplifiedHurricane system. The microscope objectives we used areshown in Table I. Before entering the microscope objective,the polarisation of the laser beam could be adjusted using apolarisation compensator (New Focus Model 5540). When theHurricane laser was used, the physical shape of the laser pulses

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TABLE IMICROSCOPE OBJECTIVES.

Mag. Type Supplier NA WorkingDistance (WD)

20× UMPlanFL Olympus 0.46 3.1 mm50× UMPlanFL Olympus 0.8 660 µm60× 0.17 mm cover Nikon 0.85 330 µm

slip corrected

were modified by a horizontal slit aperture positioned beforethe microscope objective. The slit (which was orientated withits long dimension in the direction of sample translation)served to expand the laser focus in the direction normal to thelaser beam propagation and sample translation. This enabledwaveguides with circular symmetry to be written using a lowmagnification, long working distance objective [71], [72]. TheHurricane’s amplified laser output could also be square-wavemodulated in intensity using an external frequency source,thereby creating a waveguide structure formed by segmentsof exposed glass with a desired period. This technique wasused, for example, to fabricate waveguide Bragg gratings [73].Pulse energies ranging from 0.005-10 µJ, measured beforefocussing (and after passing through the slit if used), wereused in the formation of optical waveguide devices. Usingan air-bearing computer controlled XYZ stage (Aerotech),glass samples were scanned in a direction perpendicular tothe direction of beam propagation, at speeds ranging between25 µm/s and 10 mm/s.

B. Materials & Material Processing

The two glass materials used in this paper were high qualitygrade fused silica from Schott AG (Lithosil Q0, n800 ≈ 1.454)and a custom Er/Yb co-doped phosphate glass melt fromKigre Inc. (QX 2% wt Er, 4% wt Yb, n800 ≈ 1.52 ).Glass samples were cut to size (diamond disc blade) andground and polished (Logitech PM4) before device fabrication.A high grade optical-polished surface is typically requiredsince defects in the surface through which the writing-laseris transmitted can cause waveguide-defects that contribute topropagation losses. After device fabrication, both the input andoutput faces of the device were ground and polished back byapproximately 150 µm so that clean and uniform entry and exitpoints of the device could be accessed for characterisation. Allthe glass samples used were not thermally treated before orafter fabrication.

C. Characterisation

Typically, all waveguide devices were characterised interms of their transmission and reflection data, near and far-field mode distributions, insertion, coupling, propagation andpolarisation-dependent losses, induced refractive index con-trasts and finally device gain. The experimental setup used totake transmission and reflection measurements from the deviceunder test (DUT) is shown in Fig. 4. Light sources including635 nm, 976 nm, 980 nm and tunable C-band laser diodes wereused to probe fabricated devices. Optical spectrum analysers

Fig. 4. Experimental setups used to take various transmission, reflection andpropagation loss measurements. DUT - Device Under Test.

(OSAs), power meters and CCD cameras were used to analysedevice properties. Characterisation fibres were aligned to theDUT using 6-axis flexure stages. Index matching gel was usedto reduce losses whenever optical fibres were used to eitherpump light into or collect light from the DUT.

A computational method [74] was used to estimate thepeak refractive index change between the bulk material andthe waveguide structures. Transverse and end-on images offabricated waveguide devices were taken in both reflection andtransmission with Olympus differential interference contrast(DIC) microscopes.

The insertion loss (IL) of a fabricated device was taken to bethe ratio of the measured transmitted powers with and withoutthe DUT in the setup shown in Fig. 4 and included cou-pling, propagation and absorption losses. The coupling losseswere estimated using the technique described in [41] whilstabsorption losses are material specific and can be measuredusing a spectrometer. The propagation loss was determinedby taking the difference between the IL in reflection when thecollecting fibre in Fig. 4 is replaced with a highly reflectingmirror aligned square to the device’s output and the IL intransmission without the mirror.

The setup shown in Fig. 4 was slightly modified for activewaveguide characterisation in that wavelength division mul-tiplexers (WDMs) were inserted at the device’s outputs sothat both a signal source and pump source could co-propagatealong the device in a bidirectional configuration.

VI. DESIGN CONSIDERATIONS

A. Laser Repetition Rate

Already mentioned was the fact that when using highrepetition rate femtosecond laser systems hundreds of pulsesaccumulate to heat the focal volume constituting a point sourceof heat within the bulk of the material. Longer exposure of thematerial to this heat source gives rise to higher temperaturesat focus resulting in a larger affected region [23], [75]. Dueto symmetric thermal diffusion outside of the focal volume,a spherically shaped modified region is produced. In contrast,when using a low repetition rate femtosecond laser system thefocal volume returns to room temperature before the arrivalof the next pulse resulting in the same region of the materialbeing heated and cooled many times by successive pulses.This repetitive type of machining means that the structuralmodification of the material is confined to the focal volumealone. If a low NA objective (<0.5 NA) is used in conjunctionwith a low repetition rate femtosecond laser, the focal volume

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Fig. 5. Effects of spherical aberration on sub-surface focussing. Waveguidecross-sections become more asymmetric with deeper material penetration. Apulse energy of 1.6 µJ and a translation speed of 25 µm/s were used tofabricate these waveguides.

(and hence the modified material volume) becomes asymmet-ric [71]. Beam shaping techniques, such as the slit methodpreviously outlined, or multiple fabrication raster scans of thewriting beam [41], [76], [77] need to be employed to matchthe Rayleigh length with the focal spot diameter (confocalparameter).

B. Spherical Aberration

Most fabricators of waveguides use microscope objectivesgiven that they are well corrected with high numerical aper-tures. However, directly written waveguides are created belowthe surface of a substrate and accordingly it is important toconsider the effect of spherical aberration that sub-surfacefocusing causes. Fig. 5 provides an example of the effectof spherical aberration on sub-surface focussing using a lowrepetition rate femtosecond laser. An objective designed forsurface observations was used to create an array of waveguidesat decreasing depths. The waveguide cross-sectional shapechanges from an aberrated and triangular one deep under thesurface (aspect ratio of 0.64) to a more circular section closerto the surface of the glass (aspect ratio of 1.1). Sphericalaberration and its effects on the waveguide cross section canbe controlled using objectives that are corrected for focusingthrough a fixed depth of material (for example a cover-slipcorrected objective) however this limits the 3-dimensionalcapabilities of the writing platform. A more suitable solution isto use oil-immersion focusing objectives that are not sensitiveto the depth of focus in the material since all optical pathlengths to the focus remain constant3.

VII. RESULTS & DISCUSSION

To date our research program has focused on the develop-ment of processing methodologies enabling the fabrication of

3A suitable readjustment of the slit width (if the slit method is used) canalso reduce the effects spherical aberration.

Fig. 6. Top-view DIC microscope images of waveguides fabricated with anintensity at focus of 2.8×1014 W/cm2 and a translation speed of 25 µm/s infused silica (a) without a slit and (b) with a 500 µm slit positioned before thefocussing objective. The insets show end-on white light transmission imagesof the respective waveguides.

the key building blocks of photonic circuitry, namely passiveand active devices including low-loss waveguides, splitters,gratings, amplifiers and lasers for use in optical telecommuni-cation systems.

A. Passive Devices

In most optical materials the limit to the maximum refractiveindex change that can be induced using the direct-writetechnique is determined in-part by the maximum intensity oflaser exposure that the material can tolerate without sufferingdestructive damage. At the other extreme, an initial change inthe index of refraction occurs when the laser intensity at focusreaches a level to initiate the onset of nonlinear absorption pro-cesses in the material. In fused silica, a refractive index changecan first be seen with a writing intensity of 2×1013 W/cm2

at 100 fs, 1 kHz. The threshold for damage in fused silicaoccurs at laser intensities around 1×1016 W/cm2. Waveguideswritten with intensities greater than this value develop void-like inclusions and the propagation losses in the waveguidesrapidly increase to unacceptable levels with writing intensity.In phosphate glass hosts, a refractive index change beginsto occur at intensities corresponding to 6.5×1013 W/cm2

at 100 fs, 1 kHz and 1×1013 W/cm2 at 60 fs, 11 MHzrespectively. At intensities above 2.5×1014 W/cm2 at 100 fs,1 kHz and 2.6×1013 W/cm2 at 60 fs, 11 MHz [78], waveguidestructures contain a significant number of voids. It can be seenfrom these values that the fabrication window for creatingrefractive index changes in bulk fused silica is much widerthan the corresponding window for phosphate glass.

Fig. 6(a) shows the top-view of a linear structure and its end-on cross-section fabricated with the low repetition rate systemin fused silica without using any beam shaping techniques.Because of its elliptical cross-section, a guided circular modecould not be sustained along this structure. The waveguideshown in Fig. 6(b) was fabricated in the same sample usingthe same focussing objective and translation speed albeit with a500 µm slit aperture positioned before the focussing objectivealigned parallel to the direction of sample translation. Togenerate the same intensity at focus (2.8×1014 W/cm2), aswas used to create the waveguide shown in Fig. 6(a), thepulse energy was adjusted after passing through the slit. White

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Fig. 7. End-on white light transmission micrographs of waveguides writtenin phosphate glass using different writing conditions: (Left) pulse energy –0.25 µJ, translation speed – 25 µm/s, slit width – 900 µm and (Right) pulseenergy – 19 nJ, translation speed – 1 mm/s.

light transmitted through the core of the waveguide fabricatedusing the slit method is clearly shown in the inset to havea circular diameter less than 13 µm. The induced refractiveindex change of this waveguide was estimated to be 5.2×10−4.Typically, the propagation loss of fused silica waveguidesat 1550 nm were measured to be 0.83 dB/cm. A similarstudy was conducted in phosphate glass at an intensity of2.1×1014 W/cm2 revealing waveguides that guide a circularmode can be fabricated with propagation losses as low as0.39 dB/cm [71]. As stated before, due to cumulative heatingeffects, the cross-section of waveguides written using a highrepetition rate femtosecond laser oscillator are intrinsicallycircularly symmetric. The end-on white light transmission DICimage of such a waveguide, written in phosphate glass, isshown in Fig. 7.

Whilst characterising the waveguides written in phosphateglass (outlined above), an additional loss mechanism wasfound after wet polishing due to cracking of the bulk materialat device end facets. This phenomenon is thought to be causedby local stress relief as it is specific to the first 1-2 µm of thewaveguide length. These cracks may be eliminated by fabri-cating waveguides at intensities below 1.9×1014 W/cm2 or bydry polishing the material. However, these stress cracks havebeen noted as revealing information regarding asymmetries inthe waveguide form whilst also offering useful insights intothe spatial nature of the embedded stress field. With increasingspherical aberration caused by sub-surface waveguide writingwithout a corrected objective, the end facet cracks are observedto increase in their deviation from a symmetric three radial120◦ separated fracture. Fig. 8 shows an excerpt from Fig. 5in which the end facet cracks from a circular waveguidewritten at a shallow depth are compared alongside a moredeeply written and asymmetric waveguide. It could be arguedthat the asymmetric nature of the stress field for the deeperwritten waveguide will introduce stress related birefringencenot observed in the shallow written waveguide.

Phosphate glass also displayed an interesting property inthat not just the magnitude but also the sign of the net refrac-tive index change induced by the writing laser is a function

Fig. 8. End facet cracks from (Left) a symmetric waveguide written at ashallow depth and (Right) an asymmetric waveguide written more deeply.Image modified from Fig. 5.

Fig. 9. Linear structures written with a translation speed of 25 µm/s inphosphate glass using the 50× (0.8 NA) microscope objective and variouspulse energies measured after a 900 µm slit. The top two structures guidelight indicating a positive index change. The bottom two structures do notguide light representing a negative index change. The structure in the middleshows the transition between a positive and negative index change.

of pulse energy. This effect is observed in high numericalaperture (≥0.5 NA) focusing arrangements with the transi-tion between positive and negative refractive index changeoccurring at intensities of approximately 3.6×1014 W/cm2

at 100 fs, 1 kHz and 3.1×1013 W/cm2 at 60 fs, 11 MHz.Increasing the laser intensity in these high numerical aper-ture focusing arrangements creates a greater magnitude ofnegative refractive index change. Fig. 9 clearly shows thistransition where positive index waveguides appear in differentcontrast to negative index structures when viewed using DICmicroscopy techniques. This manner of change was observedin both high repetition rate (11 MHz) and low repetition rate(1 kHz) waveguide writing configurations, using a number ofmicroscope objectives, which rely on different photoionisationmechanisms to produce the refractive index modification.This result effectively indicates that the range of writingintensities available for producing positive index waveguidedevices in phosphate glass is reduced to 6.5×1013 W/cm2-3.6×1014 W/cm2 at 100 fs, 1 kHz and 2.4×1013 W/cm2-3.1×1013 W/cm2 at 60 fs, 11 MHz.

We also compared and contrasted the optical transmissionproperties of straight and curved waveguides written with lin-early and circularly polarised light (at 100 fs, 1 kHz) in fusedsilica glass, and showed an increase in transmission throughwaveguides written using circularly polarised light [79]. This

8

increase in light transmission is still under investigation butmay be explained by a modification of the periodic alignednanostructures that accompany devices fabricated with linearlypolarised radiation [56], [57]. Waveguides fabricated in fusedsilica using the slit method and a circularly polarised writingbeam possess a propagation loss of approximately 0.83 dB/cm.The use of a circularly polarised writing beam has also beenfound beneficial in fabricating low-loss waveguides in LiNbO3

samples [80].It has been shown that the refractive index contrast of a

modified region can be increased by overwriting a waveguidewith more than one pass of a low repetition rate femtosecondlaser beam in a multiple fabrication scan fashion [1], [8],[54]. We conducted a similar study and found that waveguideswritten in fused silica with 8 multiple passes exhibit an un-saturated propagation loss of approximately 0.36 dB/cm. Thediameter of these waveguides did not change with increasinglaser scans. This result is consistent with other reports thatattribute increased refractive index changes with the numberof fabrication passes [54]. Another possible explanation maybe that waveguides become more ‘smooth’ due to consecutivelaser scans correcting waveguide imperfections produced byearlier scans. In the case of waveguides fabricated in phosphateglass, it was found that after 7 multiple passes of the writingbeam, the index change became negative. We also found thatas the translation speed decreases, the propagation loss alsodecreases. Unlike the multiple pass scheme, however, whichshowed that the propagation loss decreased linearly with thenumber of passes, the propagation loss associated with areduction in translation speed seemed to saturate beyond anon target energy density of 0.06 µJ/µm3, a value that wouldcorrespond in energy deposition to a waveguide being writtenwith 6 multiple passes.

Typically, we found that all waveguide devices fabricatedin fused silica remained stable up to a temperature of 600◦C.Devices fabricated in phosphate glass, however, were onlystable up to a temperature of 350◦C. Beyond this value, thepositive index change associated with the fabricated devicesinverted and became negative. Investigations into the thermalcharacteristics of waveguide devices written in phosphate glassare ongoing. These results show that there are additionalcomplications with phosphate glass whilst also underlining theimportance of matching the correct host material to the targetapplication.

B. Active Devices

Earlier is was pointed out that doped silica glasses have alow gain-per-unit length value (0.3 dB/cm) which is compara-ble in size to the typical propagation losses (≈0.2 dB/cm) as-sociated with femtosecond laser written waveguides fabricatedin such glass materials [54]. Waveguides written in dopedsilicate glasses possess similar propagation losses, however,the gain-per-unit length of these waveguides can reach valuesnear 2 dB/cm [22]. There have been several reports of C-bandamplifying waveguide devices created in Erbium/Ytterbiumco-doped phosphate glass hosts [20], [21], [81], [82]. Thesedevices typically exhibit between 2-4 dB/cm of internal gain

Fig. 10. Absorption spectra of Kigre co-doped QX glass. (a) Absorption dueto the Yb3+ ions (pump wavelength) and (b) Absorption due to the Er3+ ions(signal wavelength).

and less than 0.5 dB/cm of propagation loss, demonstratingthat doped phosphate glass hosts may be more suited to activedevice fabrication than silica based materials.

Example absorption spectra of the Ytterbium and Erbiumions in a commonly employed phosphate glass host are shownin Fig. 10(a) and (b) respectively. The Ytterbium absorptionspectrum is characterised by a single dominant peak at 975 nmdue to the 2F7/2 to 2F5/2 transition of the Yb3+ ion whichis used to optically pump the waveguide device. The Er3+

absorption spectrum displays two broad absorption curvesthat are the result of the many host-field Stark split 4I13/2

to 4I15/2 transitions. Linear waveguides were written in anEr/Yb co-doped phosphate glass sample with an intensityof 1.9×1014 W/cm2 and translation speed of 25 µm/s. Bysupplying the maximum amount of available pump power tothese waveguides, an internal gain (at approximately 1534 nm)of 2.7 dB/cm was obtained and optical amplification wasshown to exist over the entire C-band (see Fig. 11). This resultcompares well with previous reports in the literature [41], [83].There is still also potential for improvement of the internalgain figure. By optimising the amplifier’s physical length andfinding the optimal rare-earth ion doping concentrations, theoverall internal gain is expected to increase.

Because the waveguide devices fabricated in the co-doped

9

Fig. 11. (Top) Signal enhancement at 1534.6 nm versus total pump power.(Bottom) Internal gain of a waveguide amplifier fabricated in Er/Yb co-dopedphosphate glass.

phosphate glass sample exhibit an internal gain, they canbe used to create a laser oscillator. A waveguide resonatoris formed by distributing the optical feedback over the en-tire length of the waveguide amplifier [84], [85]. Severalexperimental techniques have been reported that enable therealisation of Bragg grating structures inside femtosecond laserwritten waveguides to create such a cavity [69], [86]. Bysquare-wave modulating the low repetition rate femtosecondlaser’s output, a first order waveguide Bragg grating (WBG)was fabricated in Er/Yb co-doped phosphate glass [73]. Usinga combined bidirectional pump power of 710 mW, a waveguidevariant of a distributed feedback (DFB) laser was demonstratedusing an external point source of heat to create the requiredπ/2 phase shift mid-grating [87]. The total output power ofthis laser measured 0.37 mW and had a linewidth <4 pm.Although it is known that the WBG structure contributes toan increase in the propagation loss of the amplifier device [86],clearly the WBG is of a high enough quality that the internalgain in the system still exceeds this increase.

VIII. CONCLUSION

We reviewed the femtosecond laser direct-write techniqueas a technology capable of producing optical waveguide de-vices inside bulk transparent materials without the need for

lithography, etching, a controlled environment or much samplepreparation. A number of investigations into the challengesfacing researchers using the femtosecond laser direct-writetechnique were undertaken. Most importantly, it was foundthat specific consideration of the pulse repetition rate andenergy, writing beam polarisation, sample translation speed,number of fabrication scans, spherical aberration, polishingtechniques and material preparation must be taken into accountin order to fabricate low-loss positive index guiding waveguidedevices in a specific type of glass. Our results highlight thecomplexities associated with the application of the femtosec-ond laser direct-write technique to phosphate glass hosts.In particular, phosphate glass, compared to fused silica, hastighter fabrication constraints with respect to pulse energy,wet polishing and thermal treatment. Nonetheless, the devicesfabricated in both glass types outlined in this paper raise theprospect of creating optical devices for the use in aiding all-optical access communication networks.

ACKNOWLEDGMENT

This work was produced with the assistance of the Aus-tralian Research Council under the ARC Centres of Excellence& LIEF programs. Graham Marshall would like to thank theAustralian Academy of Science for their financial assistancethrough the Scientific Visits to Europe scheme. M. Dubov,V. K. Mezentsev & I. Bennion also acknowledge the financialsupport of the Engineering & Physical Sciences ResearchCouncil in carrying out this work.

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