Integrated Photonics on Glass: a Review of the Ion-Exchange Technology Achievements

applied sciences

Review Integrated Photonics on Glass: A Review of the Ion-Exchange Technology Achievements

Jean-Emmanuel Broquin 1,* and Seppo Honkanen 2

1 IMEP-LAHC, University Savoie Mont Blanc, University Grenoble Alpes, CNRS, Grenoble INP, 38000 Grenoble, France 2 Institute of Photonics, University of Eastern Finland, 80100 Joensuu, Finland; seppo.honkanen@uef.ﬁ * Correspondence: [email protected]

Featured Application: ion-exchange on glass has been extensively studied for the realization of Planar Lightwave Circuits. Monolithically integrated on a single glass wafer, these devices have been successfully employed in optical communication systems as well as in sensing.

Abstract: Ion-exchange on glass is one of the major technological platforms that are available to manufacture low-cost, high performance Planar Lightwave Circuits (PLC). In this paper, the principle of ion-exchanged waveguide realization is presented. Then a review of the main achievements observed over the last 30 years will be given. The focus is ﬁrst made on devices for telecommunications (passive and active ones) before the application of ion-exchanged waveguides to sensors is addressed.

Keywords: integrated photonics; glass photonics; optical sensors; waveguides; lasers

Citation: Broquin, J.-E.; Honkanen, S. Integrated Photonics on Glass: A 1. Introduction Review of the Ion-Exchange Unlike microelectronics where the CMOS technology emerged as the dominant plat- Technology Achievements. Appl. Sci. form, integrated optics or, as it is called nowadays, integrated photonics, does not rely 2021, 11, 4472. https://doi.org/ on one single technological platform. Indeed, silicon photonics, III-V photonics, polymer 10.3390/app11104472 photonics, LiNbO3 photonics, and, last but not least, glass photonics co-exist in parallel, each of them presenting their own drawbacks and advantages. Academic Editor: As for ion-exchange on glass, also called glass integrated optics, it is based on a mate- Alessandro Belardini rial that has been known and used for centuries. Glass is easily available and can be easily recycled. The ion-exchange technique, although it is based on using microfabrication tools, Received: 25 April 2021 can be considered as a relatively low-cost approach, which allows realizing waveguides Accepted: 11 May 2021 Published: 14 May 2021 with low propagation losses and a high compatibility with optical ﬁbers. Glass photonics is not a platform that has been developed for a speciﬁc application. Therefore, Planar

Publisher’s Note: MDPI stays neutral Lightwave Circuits (PLCs) realized by ion-exchange on glass are found in many ﬁelds with with regard to jurisdictional claims in a wide range of applications. published maps and institutional afﬁl- From its very beginning in 1972 [1], to products currently on the markets, thousands iations. of papers have been published on this vivid topic. For this reason, making an extensive review of this technology is a cumbersome task. However, since excellent reviews have already been published in the past years [2–9], we can skip the pioneering years when the basis of the technology was set by testing several glasses and ions and making multimode waveguides. In this paper, we will hence focus on devices made by ion-exchange on glass, Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. their performances, and their applications. This article is an open access article After a presentation of ion-exchanged waveguides, their realization process, their distributed under the terms and modelling, and their main characteristics, we will review devices made for telecommu- conditions of the Creative Commons nication purpose. Then, we will review the use of ion-exchanged waveguides for the Attribution (CC BY) license (https:// fabrication of optical sensors since these types of applications are taking a growing place in creativecommons.org/licenses/by/ integrated photonics. 4.0/).

Appl. Sci. 2021, 11, 4472. https://doi.org/10.3390/app11104472 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, 4472 2 of 18

2. Ion-Exchanged Waveguides 2.1. Principle and Technology Typically, an optical glass is an amorphous material composed by several types of oxides mixed together. According to Zachariasen [10], theses oxides can be sorted in three main categories: network formers like SiO2, GeO2, or P2O5 that can create a glass on their own; intermediate network formers (Al2O3, TiO2, ... ) that can hardly create a glass alone but can be combined with network formers; ﬁnally network modiﬁer oxides like Na2O, K2O, CaO, or BaO that can be inserted in a matrix made by glass formers but are weakly linked to it because of a mismatch between their respective molecular binding structures. The refractive index of a glass depends on its composition through an empirical relation [11]: a N R n = 1 + m m = 1 + 0 , (1) ∑ V V m 0 0

where am is the “refractivity constant” of the chemical element “m”, Nm the number of chemical element “m” by atom of oxygen, V0 and R0 are the glass volume and refractivity by atom of oxygen, respectively. A replacement of a portion of one of the glass components by another one with the same coordination can therefore entail a change of refractive index. Providing that this exchange does not create strong mechanical stresses and does not strongly change the nature of the glass, (1) can be used to link the induced variation of the refractive index to the fraction c of substituting ions as follows:

c ∆VR ∆n = ∆R − 0 , (2) V0 V0

∆R and ∆V are the variation of R0 and V0, respectively, caused by the substitution. From (2), it can easily be deduced that a local change of the glass composition is creating a localized change of refractive index, which can be used to create a waveguide. Since alkali ions are weakly linked to the glass matrix, they are natural candidates for such a process. Indeed, when alkali ions react with silica to form a multicomponent glass, the silica network is maintained because each silicon-oxygen tetrahedron remains linked to at least three other tetrahedra [12]. Therefore, one can exchange one alkali ion to another one without damaging the original glass. Throughout the years, several ion-exchanges have been demonstrated [13,14] but the topic of this article being integrated glass photonics, we will restrain ourselves on the few ones that have enabled realizing efficient devices. In this case, the ion that is present in the glass is usually Na+ (sometimes K+). It is nowadays mostly exchanged with silver (Ag+), more rarely with potassium (K+) or thallium (Tl+). The ion source that allows creating the higher refractive index waveguide’s core can be either liquid or solid. The simplest way of performing an ion-exchange is described on Figure1a. It consists in dipping the glass wafer in a molten salt containing a mixture of both the doping ions B+ and the glass ones A+. The salt is usually a nitrate, but sulfates are sometimes used when a temperature higher than 450 ◦C is required for the exchange. Although the principle of the process is very simple, it must be kept in mind that ionic diffusion is a process that strongly depends on the temperature; this parameter should hence be homogeneous all other the wafer and consequently in the molten salt. In order to define the parts of the wafer that will be ion-exchanged, a thin-film has previously been deposited and patterned in a clean room environment to define the diffusion apertures. Once the ion-exchange is completed, the masking layer is removed and diffused surface waveguides are obtained. If a more step-like refractive index profile is required, an electric field can be applied to push the doping ions inside the glass, as described in Figure1b [ 1]. Nonetheless, this complicates the set-up and might also induce the reduction of the doping ions into metallic clusters that dramatically increase the propagation losses (specifically when silver is involved). The use of a silver thin film has also been employed successfully for the creation of the waveguide’s core [15]. The thin film can be either deposited on an Appl. Sci. 2021, 11, x FOR PEER REVIEW 3 of 18

can be applied to push the doping ions inside the glass, as described in Figure 1b [1]. Nonetheless, this complicates the set-up and might also induce the reduction of the dop- Appl. Sci. 2021, 11, 4472 ing ions into metallic clusters that dramatically increase the propagation losses (specifi-3 of 18 cally when silver is involved). The use of a silver thin film has also been employed successfully for the creation of the waveguide’s core [15]. The thin film can be either depos- itedexisting on an mask, existing as depicted mask, as ondepicted Figure 1onc, orFigure patterned 1c, or patterned directly on directly the glass onsubstrate the glass [sub-16]. strateAn applied [16]. An electric applied field electric ensures field an ensures efficient an electrolysis efficient electrolysis of Ag+ ions of Ag into+ ions the glassinto the by glassthe consumption by the consumption of the silver of the film silver anode. film These anode. three These different three different processes processes allow realizing allow realizingwaveguides waveguides whose core whose is placed core is at placed the surface at the of surface the glass of the wafer glass and wafer whose and shape whose is, shapedepending is, depending on the process on the parameters, process parameters, semi-elliptical semi-elliptical with a step with refractive a step refractive index change index at changetheir surface at their and surface diffused and interfaces diffused interfaces inside the inside glass. the Intrinsically, glass. Intrinsically, such waveguides such wave- are guidessupporting are supporting modes that modes are prone that to are interact prone with to interact the elements with the present elements on the present wafer surface.on the waferInteresting surface. and Interesting even maximized and even for maximized the realization for the of realization sensors, this of interactionsensors, this is interac- often a tiondrawback is often when a drawback dealing when with telecomdealing deviceswith telecom where devices the preservation where the ofpreservation the quality of the qualityoptical of signal the optical is a key signal factor. is a For key this factor. reason, For ion-exchangedthis reason, ion-exchanged waveguide waveguide cores are usually cores areburied usually inside buried the glass. inside the glass.

V V app app

BNO +ANO BNO3+ANO3 3 3 B+ B+ Ag Ag+ Na+ A+ A+

ANO3

(a) (b) (c)

Figure 1. Three main processes used to realize surface waveguides by an ion-exchange on glass. + Figure+ 1. Three+ main processes used to realize surface waveguides by an ion-exchange on glass. A A and+ B represent the ions contained in the glass and the ones replacing them, respectively. (a) and B represent the ions contained in the glass and the ones+ replacing them, respectively. (a) the glassthe glass wafer wafer is dipped is dipped into intoa molten a molten salt containing salt containing B+ ions B entailingions entailing a thermal a thermal diffusion diffusion on the on ex- the changeexchange ions ions through through a diffusion a diffusion aperture; aperture; (b) ( bthe) the diffusion diffusion process process is assisted is assisted by byan anelectric electric field; ﬁeld; + ((cc)) an electrolysis ofof aa silversilver thinthin ﬁlmfilm is is used used to to generate generate Ag Ag+ions ions that that are are migrating migrating by by diffusion diffusion and andconduction conduction inside inside the glass.the glass.

FigureFigure 22 depictsdepicts thethe twotwo mainmain processesprocesses thatthat cancan bebe used:used: thethe firstfirst oneone consistsconsists ofof plunging the wafer containing surface cores in a molten salt containing only the ions that were originally present in the glass. A reverse ion-exchange is then occurring, removing dopingdoping ions ions from from the the surface surface of of the the glass glass [17]. [17 ].This This process process entails entails a quite a quite important important de- creasedecrease of the of the refractive refractive index index change change and and an increase an increase of the of thewaveguide’s waveguide’s dimension dimension be- causebecause of thermal of thermal diffusion, diffusion, which which practically practically limits limits the thedepth depth of the of burying the burying to one to to one two to micrometers.two micrometers. In order In order to reach to reach a deeper a deeper depth depth and andensure ensure a good a good optical optical insulation insulation of the of guidedthe guided mode, mode, the thereverse reverse ion-exchange ion-exchange is isquit quitee often often assisted assisted by by an an electric electric field field that forcesforces the the migration migration of of the the core core inside inside the the glas glasss preventing preventing hence a loss loss of of refractive refractive index variation. Moreover, by a proper tuning of the process parameters, circular waveguide variation. Moreover, by a proper tuning of the process parameters, circular waveguide cores can be obtained in order to maximize the coupling efficiency with optical fibers. cores can be obtained in order to maximize the coupling efficiency with optical fibers. Nonetheless, it must be noticed that the applied voltage can be close to 1 kV, which requires Nonetheless, it must be noticed that the applied voltage can be close to 1 kV, which re- on one hand, a proper and well secured dedicated set-up, and on the other hand, an quires on one hand, a proper and well secured dedicated set-up, and on the other hand, excellent quality of the glass wafer in order to prevent percolation path formation and short an excellent quality of the glass wafer in order to prevent percolation path formation and circuits. Figure3 depicts an optical image of a buried optical waveguide realized on a Teem short circuits. Figure 3 depicts an optical image of a buried optical waveguide realized on Photonics GO14 glass by a silver-sodium ion-exchange. Burying depth as high as 47 µm have been realized, as shown in Figure4, but such extreme values are rarely required in practical devices where the burying depth is of the order of 10 µm. Appl. Sci. 2021, 11, x FOR PEER REVIEW 4 of 18 Appl. Sci. 2021, 11, x FOR PEER REVIEW 4 of 18 Appl. Sci. 2021, 11, x FOR PEER REVIEW 4 of 18

a Teem Photonics GO14 glass by a silver-sodium ion-exchange. Burying depth as high as Appl. Sci. 2021, 11, 4472 a Teem Photonics GO14 glass by a silver-sodium ion-exchange. Burying depth as high4 of 18as 47 µm have been realized,a Teem as shown Photonics in Figure GO14 4, glass but suchby a silver-sodiumextreme values ion-exchange. are rarely re- Burying depth as high as 47 µm have been realized, as shown in Figure 4, but such extreme values are rarely required in practical devices47 where µm have the buryingbeen realized, depth asis ofshown the order in Figure of 10 µm.4, but such extreme values are rarely required in practical devices where the burying depth is of the order of 10 µm. quired in practical devices where the burying depth is of the order of 10 µm. V app V app V app

ANO3 + ANO3 A + ANO3 + A ANO3 A ANO3 + ANO3 A+ A A+ B+ B+ B+ ANO3 ANO3 ANO3

(a) (b ) (a) (b) Figure 2. (a) Thermal Figureburying 2. of (aa )waveguide’s Thermal burying core; (b of) electrically a waveguide’s(a) assisted core; burying (b) electrically of the wave- assisted(b) burying of the Figure 2. (a) Thermal burying of a waveguide’s core; (b) electrically assisted burying of the waveguide’s core. The competitionwaveguide’sFigure between 2. core. (a )ionic Thermal The diffusi competition buryingon and ofbetween transport a waveguide’s ionic allows diffusion core;obtaining (b and) electrically quasi transport circu- assisted allows burying obtaining of quasithe waveguide’s core. The competition between ionic diffusion and transport allows obtaining quasi circular profiles. circularguide’s proﬁles. core. The competition between ionic diffusion and transport allows obtaining quasi circular profiles. lar profiles.

Figure 3. Image of a quasi-circular waveguide observed with an optical microscope, the glass is in Figure 3. Image of a quasi-circular waveguide observed with an optical microscope, the glass is in light blue, the core is inFigure pink,Figure 3. airImage is 3.in Image dark of a quasi-circularblue. of a quasi-circular waveguide waveguide observed observed with anwith optical an optical microscope, microscope, the glass the glass is in is in light blue, the core is in pink, air is in dark blue. light blue,light theblue, core the is core in pink, is in airpink, is in air dark is in blue. dark blue. Intensity (A.U.) Intensity (A.U.) Intensity (A.U.)

Distance (µm) Distance (µm) Distance (µm) (a) (b) (a) (b) Figure 4. Realization of deeply buried waveguides by an Ag(+a/Na) + ion-exchange on a GO14 TeemPhotonics glass.(b) The Figure 4. Realization of deeply buried waveguides by an Ag+/Na+ ion-exchange on a GO14 TeemPhotonics glass. The applied electric field duringFigure the burying 4. Realization process wasof deeply 650 kV/m; buried (a) waveguidesimage of the byoutput an Ag of+ /Nathe +waveguide ion-exchange observed +on a +GO14 with TeemPhotonics glass. The applied electric field duringFigure the burying 4. Realization process ofwas deeply 650 kV/m; buried (a waveguides) image of the by output an Ag of/Na the ion-exchangewaveguide observed on a GO14 with Teem- an InGaAs Camera at λ = 1.5applied µm; (b )electric vertical field cut ofduring the measured the burying intensity process show wasing 650 the kV/m; position (a) image of the ofmode the outputwith respect of the waveguide observed with an InGaAs Camera at λ = 1.5Photonics µm; (b) vertical glass. The cut appliedof the measured electric ﬁeld intensity during show theing burying the position process of was the 650mode kV/m; with (respecta) image of to the glass wafer substrate.an InGaAs Camera at λ = 1.5 µm; (b) vertical cut of the measured intensity showing the position of the mode with respect to the glass wafer substrate.the output of the waveguide observed with an InGaAs Camera at λ = 1.5 µm; (b) vertical cut of the to the glass wafer substrate. measured intensity showing the position of the mode with respect to the glass wafer substrate.

2.2. Modelling Ion-Exchanged Waveguides Extensive work has been carried-out throughout the years to characterize and model ion-exchanges processes [18–22]. In this article, we will focus on a relatively simple description since it occurred to be reliable enough to allow us designing waveguides and predicting their optical behavior efﬁciently. Ion-exchange can be seen as a two-step process: ﬁrst the exchange itself that occurs at the surface of the glass and creates a normalized Appl. Sci. 2021, 11, x FOR PEER REVIEW 5 of 18

2.2. Modelling Ion-Exchanged Waveguides Appl. Sci. 2021, 11, 4472 Extensive work has been carried-out throughout the years to characterize and model5 of 18 ion-exchanges processes [18–22]. In this article, we will focus on a relatively simple description since it occurred to be reliable enough to allow us designing waveguides and predicting their optical behavior efficiently. Ion-exchange can be seen as a two-step pro- concentration c of doping ions. For thin film sources, this concentration is linked to the cess: first the exchanges itself that occurs at the surface of the glass and creates a normalized applied current by the following relation: concentration 𝑐 of doping ions. For thin film sources, this concentration is linked to the applied current by the following relation:∂c J (c − 1) s = 0 s , (3) ∂𝜕𝑐x 𝐽D𝑐Ag−1 = , (3) 𝜕𝑥 𝐷 where J0 is the ion flux created by the electrolysis, x is the direction normal to the surface, whereand D 𝐽Ag isis the diffusionion flux created coefficient by the of silverelectrolysis, in the glass. 𝑥 is the direction normal to the sur- + + face, andFor liquid𝐷 is sourcesthe diffusion made coefficien of a mixturet of silver of molten in the salts glass. containing B and A ions in orderFor to liquid replace sources A+ ions made of the of glass, a mixture an equilibrium of molten at salts the glasscontaining surface B is+ and usually A+ ions rapidly in orderreached, to replace according A+ ions to the of chemicalthe glass, reaction:an equilibrium at the glass surface is usually rapidly reached, according to the chemical reaction: A+ + B+ ⇔ B+ + A+ , (4) salt glass salt glass 𝐴 +𝐵 ⇔𝐵 + 𝐴, (4) Considering that the amount of ions in the molten salt is much bigger than the one of Considering that the amount of ions in the molten salt is much bigger than the one the glass, the ion concentrations in the liquid source can be considered as constant, which of the glass, the ion concentrations in the liquid source can be considered as constant, allows deriving the relative concentration at the surface: which allows deriving the relative concentration at the surface:

Kx𝐾𝑥B cs = , (5) 𝑐 =1 + x (K − 1) , (5) 1+𝑥B𝐾−1

𝐾 being the equilibrium constant of the chemical reaction salt(4) andsalt 𝑥salt= K being the equilibrium constant of the chemical reaction (4) and xB = CB / CB + CA + 𝐶is the⁄ molar𝐶 +𝐶 fraction is of the doping molar ions fraction B+ in of the doping molten ions salt. B in the molten salt. SinceSince the the refractive refractive index is proportionalproportional toto the the relative relative concentration, concentration, according according to (2),to (2),it is it easyis easy to ﬁxto fix the the refractive refractive index index change change at the at the glass glass surface surface by setting by setting the ratiothe ratio of B +ofions B+ ionsin the in liquidthe liquid source. source. Figure Figure5 shows 5 shows an experimental an experimental determination determination of this of dependence this depend- for encea silver/sodium for a silver/sodium ion-exchange ion-exchange on a Schott-BF33 on a Schott-BF33 glass. These glass. data These have data been have obtained been ob- by tainedrealizing by realizing highly multimode highly multimode slab waveguides slab waveguides and retrieving and retrieving their refractive their refractive index proﬁle in- dexthrough profile m-lines through measurements m-lines measurements [23] and the [23] Inv-WKB and the procedureInv-WKB procedure [24,25]. [24,25].

0.025 λ = 632.8 nm 0.02 T= 353 °C

0.015 surface

∆n 0.01 K = 3.32 ± 0.58 –4 ∆nmax= 0.018 ± 7 x 10 0.005

0 0 0.2 0.4 0.6 0.8 1 xB

FigureFigure 5. 5. RefractiveRefractive index index change change measured measured at the at su therface surface of a Schott-BF33 of a Schott-BF33 glass for glass different for different x AgNO + (1 − x )NaNO molten salts at a temperature of 353 ◦C. 𝑥BAgNO +3 1−𝑥NaNOB molten3 salts at a temperature of 353 °C. The ions exchanged at the glass surface entail a gradient of concentration inside the The ions exchanged at the glass surface entail a gradient of concentration inside the glass. Hence, B+ ions migrate inside the glass while A+ ions are moving towards the glass. Hence, B+ ions migrate inside the glass while A+ ions are moving towards the sur- surface. Since the two species of ions have different mobilities, an internal electrical field⃗ face.−→ Since the two species of ions have different mobilities, an internal electrical field−→ 𝐸 isE createdint is created during during the diffusion the diffusion process. process. To this To field this fieldan external an external applied applied field field𝐸⃗E canapp canbe Appl. Sci. 2021, 11, 4472 6 of 18

−→ −→ + + be added, which results in ions ﬂuxes JA and JB , for A and B , respectively, which are determined by the Nernst–Einstein equation:

−→ → −→ −→ e J = −D ∇C − C E + Eapp A A A H kBT A int −→ → −→ −→ , (6) e J = −D ∇C − C E + Eapp B B B H kBT B int

where Di is the diffusion coefficient of the ion i, Ci its concentration, e is the electron charge, kB the Boltzmann constant, T the temperature and H the Haven coefficient. Assuming that all the sites left by ions A+ are filled by ions B+, it can be written that at any position in + the glass the relation CA + CB = CA0, where CA0 is the concentration of A ions before the exchange, is always valid. With this relation and Equation (6), the total ionic flux can be expressed as: → −→ −→ → e −→ −→ J = JA + JB = −DACA0 α∇c − (1 − αc) Eint + Eapp , (7) HkBT

where the Steward coefficient α = 1 − DB/DA and the normalized concentration c = CB/CA0 have been introduced. If no electric ﬁeld is applied, then the total current is null, which allows determining −→ easily Eint: → −→ Hk T α∇c E = − B (8) int e 1 − αc The second Fick’s law implies that:

∂C −→ B = −∇ J . (9) ∂t B Combining (6), (8) and (9), the equation that governs the evolution of the relative concentration as a function of time is obtained: → → −→ ∂c DB eDB = ∇ ∇c − cEapp . (10) ∂t 1 − αc HkBT

Equation (10) can be solved numerically by Finite Difference or Finite Element schemes but for accurate modelling, the dependence of ionic mobility and diffusion on the concentration should not be neglected. The so-called mixed alkali effect plays indeed a significant role in ion-exchanges where a high doping concentration is required [26,27]. It must also be noticed that ion-exchange modifies the conductivity of the glass, which in turn, modifies −→ the field distribution of Eapp. Therefore, solving Equation (10) is actually much less obvious than it might appear and handling these problems has been the subject of a quite abundant literature [28–31]. Figure6 displays typical refractive index profiles that have been obtained considering mixed alkali effect and the coupling between the ion-exchange and the applied electric field. Simulations have been done with an in-house software based on a finite difference scheme. It can be clearly seen how a proper choice of the experimental parameters can lead to circular waveguides. However, the maximum refractive index change is dropping from almost 0.1 to 10−2 during the burial process because of the spreading of doping ions caused by thermal diffusion. Appl. Appl.Sci. 2021 Sci., 202111, x, FOR11, 4472 PEER REVIEW 7 of 187 of 18

(a) (b)

FigureFigure 6. (a) 6.Refractive(a) Refractive index indexdistribution distribution of a thermally of a thermally diffused diffused waveguide, waveguide, diffusion diffusion aperture aperture width is 2 µm, exchange time is 2 min, DB = 0.8 µm2/min;2 (b) refractive index profile of the wave- width is 2 µm, exchange time is 2 min, DB = 0.8 µm /min; (b) refractive index proﬁle of the waveguide guide (a) after an electrically assisted burying in a pure NaNO3 molten salt, process duration is 1 (a) after an electrically assisted burying in a pure NaNO3 molten salt, process duration is 1 h30 for an h30 for an applied electric field of 180 kV/m. applied electric ﬁeld of 180 kV/m.

2.3. Waveguide’s2.3. Waveguide’s Performances Performances The mainThe main characteristics characteristics when when dealing dealing withwith integrated integrated optics optics waveguides waveguides are their are their spectralspectral operation operation range, range, their theirlosses losses that thatcan be can split be splitbetween between coupling coupling and andpropagation propagation losses,losses, and their and their behavior behavior with withrespect respect to light to light polarization. polarization.

2.3.1.2.3.1. Passive Passive Glasses Glasses SinceSince the first the firstwaveguides waveguides demonstrated demonstrated by Izawa by Izawa and andNakagome Nakagome [1], huge [1], huge efforts efforts havehave been been made made to reduce to reduce the the losses losses of of the the waveguides. waveguides. Historically, Historically, scatteringscattering represented repre- sentedthe the main main source source of of losses. losses. Indeed, Indeed, the the quality quality of of the the photolithography photolithography usedused forfor thethe real- realizationization of of the the masking masking layer layer before before the the ion-exchange ion-exchange was was an an issue issue as as well well as as scratches scratches or or dirtdirt deposited deposited on onthe the glass glass surface surface or refractive or refractive index index inhomogeneities, inhomogeneities, such such as bubbles. as bubbles. TheseThese problems problems are typical are typical optical optical glass glass issues issues that are that encountered are encountered when when a custom-made a custom-made glassglass is realized is realized for the for first the firsttime timein small in small volumes, volumes, but they but theyare easily are easily handled handled by glass by glass manufacturersmanufacturers when when a higher a higher volume volume of glass of is glass produced. is produced. Therefore, Therefore, state-of-the-art state-of-the-art ion- exchangedion-exchanged waveguides waveguides are nowadays are nowadays based on based glass on wafers glass wafers specifically specifically developed developed for for this applicationthis application or at orleast at least for microtechno for microtechnologies.logies. Among Among them, them, the more the more used used are BF33 are BF33 by Schottby Schott because because of its of compatibility its compatibility with withMEMS MEMS process, process, GO14 GO14 by TeemPhotonics by TeemPhotonics SA SA and BGG31 by Schott [32], which have both been developed specifically for silver-sodium and BGG31 by Schott [32], which have both been developed specifically for silver-sodium ion-exchanges. The interest of silver-sodium ion-exchange is that it allows the realization ion-exchanges. The interest of silver-sodium ion-exchange is that it allows the realization of buried waveguides solving, hence the problem of scattering due to surface defects or of buried waveguides solving, hence the problem of scattering due to surface defects or contaminations while dramatically improving the coupling efficiency with optical fibers. contaminations while dramatically improving the coupling efficiency with optical fibers. Nonetheless, silver-based technologies present also challenges since Ag+ has a strong Nonetheless, silver-based technologies present also challenges since Ag+ has a strong ten- tendency to reduce into metallic Ag creating metallic clusters that are absorbing the optical dency to reduce into metallic Ag creating metallic clusters that are absorbing the optical signals. The glass composition should therefore be adapted not only to remove reducing signals. The glass composition should therefore be adapted not only to remove reducing elements like Fe, As, or Sb, but also to create a glass matrix where Na+ ions are not linked elements like Fe, As, or Sb, but also to create a glass matrix where Na+ ions are not linked to non-bridging oxygens [33]. The choice of the material for the masking layer should also to non-bridging oxygens [33]. The choice of the material for the masking layer should also be made with caution because the use of metallic mask can also induce the formation of be madeAg nanoparticles with caution atbecause the vicinity the use of theof metallic diffusion mask apertures can also [34 induce]. Therefore, the formation the use of of Al or Ag nanoparticles at the vicinity of the diffusion apertures [34]. Therefore, the use of Al or Ti mask is now often replaced by Al2O3 [35,36], SiO2, or SiN [37] ones. Ti mask isTable now1 oftenpresents replaced the main by Al characteristics2O3 [35,36], SiO of single2, or SiN mode [37]ones. waveguides realized on GO14, BGG31,Table 1 andpresents BF33, the respectively. main characteristics GO14 and BGG31of single that mode have wave beenguides optimized realized for telecomon GO14,applications BGG31, and and BF33, ion-exchange respectively. present GO14 very and low BGG31 propagation that have losses been and optimized birefringence for that telecomare keyapplications characteristics and ion-exchange for data transmission. present very BF33 low is notpropagation a glass that losses has beenand birefrin- designed for genceion-exchange that are key butcharacteristics it is a relatively for data low-cost transmission. glass that presentsBF33 is not a quite a glass good that refractive has been index designed for ion-exchange but it is a relatively low-cost glass that presents a quite good Appl. Sci. 2021, 11, 4472 8 of 18

change and that is speciﬁcally indicated by its manufacturer for MEMS and microtechnol- ogy applications. Therefore, it is an excellent candidate for sensor realization and is mainly used for that. The relatively high propagation losses observed in BF33 is mainly due to the fact that this parameter is not very important in sensors and has, hence, neither been optimized nor measured accurately.

Table 1. Main characteristics of single mode waveguides realized on three different glasses.

Glass Type GO14 BGG31 BF33 Losses <0.05 dB/cm [7] <0.1 dB/cm <1 dB/cm (@780 nm) ∆n max 8 × 10−2 [7] 3.2 × 10−2 [38] 1.8 × 10−2 [35] Birefringence <5 × 10−4 [39] <2 × 10−5 [40] N.A. Burying depth ~10 µm (50 µm max) ~10 µm [38] ~5 µm [35]

We deliberately did not mention Tl+/K+ ion-exchanged waveguides although the process is indeed the ﬁrst one that has been used and the ﬁrst one to be tentatively implemented in a production line. However, the advantages of a Tl+/K+ ion-exchange, namely a high refractive index change and the absence of clustering and absorption, are strongly counterbalanced by its toxicity, which implies dedicated safety procedures and waste treatments. It is therefore very scarcely used.

2.3.2. Active Glasses The possibility of performing ion-exchange on rare-earth doped glasses was identified quite early. However, it was only in the 1990s with the development of WDM telecommunication that a lot of work was carried-out on the realization of efficient optical amplifiers and lasers. Because the solubility of rare earths into silicate glasses is quite low, which entails quenching due to clustering and reduces the amplifier efficiency, phosphate glasses rapidly emerged as the most efficient solution for obtaining high gain with compact devices. Among phosphate glasses, two specific references set the state of the art: they were the IOG 1 by Schott [41] and a proprietary glass referred as P1 by TeemPhotonics [42]. These two glasses succeeded in obtaining a high doping level without rare-earth clustering while being chemically resistant enough to withstand clean room processes and ion-exchange. The competition in the field of rare earth doped waveguides having been very hard, the characteristics of the different waveguides obtained in these glasses are difficult to find in the literature since the emphasis was mostly put on the active device performances, as will be detailed later.

2.3.3. Exotic Substrates Some exotic glasses like fluoride glasses [43] or germanate glasses [44,45] have also been used for the realization of ion-exchanged waveguides but the difficulty in making sufficiently good wafers available at a reasonable cost, strongly limited the research in these directions.

3. Telecom Devices 3.1. Context and Historical Overview Optical Telecommunications was originally the reason why Miller introduced the concept of integrated optics in 1969 [46]. Therefore, the pioneering work of integrated photonics on glass has been mainly devoted to telecommunication devices pushing steadily towards the development of not only ion-exchange processes but also of a full technology starting from the wafer fabrication and ending with the packaging of the manufactured Planar Lightwave Circuits. Figure7 shows this evolution by displaying on one side one of the ﬁrst demonstrations of a 1 to 8 power splitter made by cascading multimode Y- junctions [47] and, on the other side, its 2006 commercially available counterpart, single mode and Telcordia 1209 and 1221 compliant [7,48]. Appl. Sci. 2021, 11, x FOR PEER REVIEW 9 of 18

Appl. Sci. 2021, 11, 4472 9 of 18 junctions [47] and, on the other side, its 2006 commercially available counterpart, single mode and Telcordia 1209 and 1221 compliant [7,48].

(a) (b)

Figure 7. 1 toFigure 8 power 7. 1 splitterto 8 power made splitter by ion-exchange made by ion-exchange on glass (a) on early glass demonstration (a) early demonstration in 1986; (b) in qualiﬁed 1986; (b pigtailed) and packaged commerciallyqualified pigtailed available and product. packaged commercially available product.

Once elementaryOnce functions, elementary such functions, as Y-junctions such as Y-junctions and directional and directional couplers couplers were were demon- demonstrated, strated,studies were studies oriented were orientedtowards towardsall the functions all the functionsthat could that be required could be for required for op- optical fiber communicationstical ﬁber communications like thermo-optic like thermo-optic switches [49], switches Mach–Zehnder [49], Mach–Zehnder interferom- interferome- eters [50,51] andters Multimode [50,51] and Mode Multimode Interferen Modece (MMI) Interference couplers (MMI) [52–55]. couplers These [52buildings–55]. These buildings blocks have thenblocks been have optimized then been and/or optimized combin and/ored on a combinedsingle chip on to aprovide single chip more to func- provide more func- tionality. In thetionality. next sections, In the we next will sections, review we some will of review them and some put of the them emphasis and put on the the emphasis on the specificity broughtspeciﬁcity by the brought use of ion-exchange by the use of on ion-exchange glass. on glass.

3.2. Wavelength3.2. Multiplexers Wavelength Multiplexers A five-channelA wavelength ﬁve-channel demultiplexer-multiplexer wavelength demultiplexer-multiplexer has been demonstrated has been demonstrated as as early early as 1982 byas Suhara 1982 by et al. Suhara using et silver al. usingmultimode silver waveguides multimode combined waveguides with combined a Bragg with a Bragg grating [56]. Moregrating advanced [56]. More devices advanced using single devices mode using waveguides single mode include waveguides Arrayed- include Arrayed- Waveguide GratingWaveguide (AWG) Grating multiplexers, (AWG) whose multiplexers, quite large whose footprint quite largeis compensated footprint is by compensated by their low sensitivitytheir to low the sensitivity light polarization to the light thanks polarization to the use of thanks silver tobased the buried use of wave- silver based buried guides [38]. A waveguidesgood thermal [38 stability]. A good provided thermal by stability the thickness provided of by the the glass thickness substrate of the is glass substrate is also reported but a ﬁne thermal tuning of the AWG’s response remained possible [57]. also reported but a fine thermal tuning of the AWG’s response remained possible [57]. Add and drop multiplexing has been achieved by combining Bragg gratings with Mach– Add and drop multiplexing has been achieved by combining Bragg gratings with Mach– Zehnder interferometers or more originally with a bimodal waveguide sandwiched by Zehnder interferometers or more originally with a bimodal waveguide sandwiched by two asymmetric Y-branches [58]. Bragg grating can be integrated on glass by etching [59], two asymmetric Y-branches [58]. Bragg grating can be integrated on glass by etching [59], wafer bonding [60], or photowriting [61–63]. wafer bonding [60], or photowriting [61–63]. Asymmetric Y-junctions are very interesting adiabatic devices that are well adapted to Asymmetric Y-junctions are very interesting adiabatic devices that are well adapted the smooth transitions between waveguides obtained by ion-exchange processes. Therefore, to the smooth transitions between waveguides obtained by ion-exchange processes. asymmetric Y-junctions have been used as stand-alone broadband wavelength multiplexers. Therefore, asymmetric Y-junctions have been used as stand-alone broadband wavelength For this type of applications, the asymmetry of the branches is obtained by a difference of multiplexers. For this type of applications, the asymmetry of the branches is obtained by the waveguide dimensions and a difference in their refractive index. Tailoring the refractive a difference of the waveguide dimensions and a difference in their refractive index. Tai- index of ion-exchanged waveguides can be achieved by segmenting the waveguide as loring the refractivedemonstrated index of byion-exchanged Bucci et al. [64 waveguides]. As can be can seen be on achieved Figure8 ,by using segmenting vertical integration of the waveguidedeeply as demonstrated buried waveguides by Bucci with et al selectively. [64]. As can buried be seen waveguides on Figure allowed 8, using obtaining a very vertical integrationbroadband of deeply duplexing buried behaviorwaveguides while with maintaining selectively a buried relatively waveguides small surface al- footprint [36]. lowed obtaining a very broadband duplexing behavior while maintaining a relatively small surface footprint [36]. Appl. Sci. 2021, 11, 4472 10 of 18 Appl. Sci. 2021, 11, x FOR PEER REVIEW 10 of 18

(a) (b)

Figure 8. FigureVertically 8. Vertically integrated integrated broadband broadband duplexer duplexer(a) fabrication (a) fabrication steps; (b) steps; measured (b) measured transmis- transmission sion (the (theinsets insets display display the observed the observed device device output’s output’s mode modeat specific at speciﬁc wavelengths). wavelengths). Top and Top bot- and bottom tom branchesbranches are separated are separated by 28 by µm 28 [36].µm [36].

3.3. Waveguide3.3. Waveguide Amplifiers Amplifiers and Lasers and Lasers Active devicesActive have devices been have linked been to linked the dev toelopment the development of ion-exchanged of ion-exchanged devices devicessince since the beginningthe beginning of this technology. of this technology. Indeed, Indeed,Saruwatari Saruwatari et al. demonstrated et al. demonstrated in 1973 a in laser 1973 a laser made withmade an withoptical an amplifier optical amplifier based on based a buried on a multimode buried multimode ion-exchanged ion-exchanged waveguide waveguide realized realizedin a neodymium-doped in a neodymium-doped borosilicate borosilicate glass [65]. glass However, [65]. However, research research on active on active de- devices vices reallyreally became became a major a major field field of ofresearch research with with a strong a strong competition competition atat the the beginning beginning of the of the 1990s1990s when when a alot lot of of studies studies were were ca carried-out.rried-out. Work Work was was first first concentrated concentrated on on Nd- Nd-doped doped amplifiersamplifiers and and lasers lasers emitting atat 1.061.06 µµmm since since the the four four energy energy levels levels pumping pumping scheme of scheme ofthis this transition transition made made it easier it easier to achieve to achieve a net gaina net with gain the with 800 the nm 800 pumping nm pumping diodes available diodes availableat the moment at the [moment66–70]. With[66–70]. the riseWith of the Wavelength rise of Wavelength Division Multiplexing Division Multiplex- systems, optical ing systems,amplifiers optical and amplifiers sources and operating sources in op theerating C+L in band the (fromC+L band 1525 (from nm to 15251610 nm nm to) became 1610 nm)key became devices key and devices research and on rare-earthresearch on doped rare-earth integrated doped devices integrated switched devices to the use of 4 4 erbium ions whose transitions from the I 13/2 level to the I 15/2 one is broad enough to switched to the use of erbium ions whose transitions from the 𝐼/ level to the 𝐼/ cover this wavelength range. Dealing with Er3+ active ions, the main issue was to realize one is broad enough to cover this wavelength range. Dealing with Er3+ active ions, the waveguides with low-losses and a good overlap of the pump and signal modes. Indeed, main issue was to realize waveguides with low-losses and a good overlap of the pump the pumping scheme of this rare earth being a three levels one, the 4 I ground state and signal modes. Indeed, the pumping scheme of this rare earth being a three levels15/2 one, absorbs the optical signal when it is not sufficiently pumped. Barbier et al. managed to the 𝐼 ground state absorbs the optical signal when it is not sufficiently pumped. /solve this problem by developing a silver-sodium ion-exchange in their Er/Yb co-doped Barbier et al. managed to solve this problem by developing a silver-sodium ion-exchange P1 glass [42]. 41 mm-long buried waveguides achieved 7 dB of net gain in a double pass in their Er/Yb co-doped P1 glass [42]. 41 mm-long buried waveguides achieved 7 dB of configuration. This work has been followed by the demonstration of an amplifying four net gain in a double pass configuration. This work has been followed by the demonstra- wavelength combiner [71] and the qualification of Erbium Doped Waveguide Amplifiers tion of an amplifying four wavelength combiner [71] and the qualification of Erbium (EDWAs) in a 160 km-long WDM metro network [72]. This work has been completed by Doped Waveguidepackaging andAmplifiers qualification (EDWAs) developments in a 160 km-long in order WDM to create metro a product network line [72]. commercialized This work hasby been TeemPhotonics. completed by packaging and qualification developments in order to create a product lineMeanwhile commercialized the phosphate by TeemPhotonics. glasses developed by Schott also gained a lot of attention. MeanwhilePatel et al.the achieved phosphate a record glasses high developed gain of 13.7by Schott dB/cm also in agained 3 mm-long a lot of waveguide attention. realized Patel et al.by achieved a silver film a record ion-exchange high gain [73 of]. 13.7 Such dB/cm a gain in per a length3 mm-long unit waswaveguide made possible realized by a high by a silverdoping film ion-exchange level of the glass [73]. in Such Er (8a gain wt. %) per and length Yb (12unit wt. was %). made possible by a high doping level Er-dopedof the glass waveguide in Er (8 wt. amplifiers %) and Yb being (12 wt. available, %). Er-doped lasers followed. Actually, Er-dopedthe first waveguide proof of concept amplifiers of an being ion-exchanged available, waveguideEr-doped lasers laser wasfollowed. obtained Actually, on a modified the first BK7-silicateproof of concept glass of containing an ion-exchanged 0.5 wt. % ofwaveguide Er, with a laser potassium was obtained ion-exchange on a modi- and two thin- fied BK7-silicatefilm dielectric glass containing mirrors bonded 0.5 wt. to % the of waveguide’s Er, with a potassium facets forming ion-exchange a Fabry-Perot and two cavity [74]. thin-filmNonetheless, dielectric mirrors from abonded strict point to the of wave view,guide’s this device facets was forming not a fullya Fabry-Perot integrated cavity laser because [74]. Nonetheless,the mirrors from were a not strict integrated point of on view, the chip.this device Therefore, was thenot next a fully generation integrated of Er-laser laser relied because onthe the mirrors use of were Bragg not gratings integrated as mirrors. on the chip. In Distributed Therefore, FeedBack the next (DFB)generation or Distribute of Er- Bragg Appl. Sci. 2021, 11, x FOR PEER REVIEW 11 of 18

Appl. Sci. 2021, 11, 4472 11 of 18 laser relied on the use of Bragg gratings as mirrors. In Distributed FeedBack (DFB) or Dis- tribute Bragg Reflectors configurations, these lasers presented a single frequency emission Reflectorscompatible configurations, with their use theseas transmitters lasers presented in WDM a singlesystems. frequency Similar for emission waveguide compatible ampli- withfiers, theirthe use use of as phosphate transmitters glass in entailed WDM systems. a major breakthrough Similar for waveguide in the performances. amplifiers, DBR the uselasers of phosphatewere demonstrated glass entailed by Veasey a major et breakthroughal. using a potassium in the performances. ion-exchangeDBR [41], laserswhile wereMadasamy demonstrated et al. manufactured by Veasey et al. similar using adevices potassium with ion-exchange a silver thin [41 film], while [75]. Madasamy These ap- etproaches al. manufactured allowed integrating similar devices several with lasers a silver on thina single film chip [75]. to These provide approaches arrays of allowed multi- integratingwavelength several sources lasers with one on a single single grating, chip to the provide wavelength arrays ofselection multiwavelength being made sources by tun- withing the one effective single grating, indices the ofwavelength the waveguides selection through being their made dimensions. by tuning the Thanks effective to indicesthe use ofof thehighly waveguides concentrated through molten their salt dimensions. of silver nitrate Thanks and to a theDFB use configuration, of highly concentrated Blaize et al. moltensucceeded salt in of creating silver nitrate a comb and of a15 DFB lasers configuration, with one single Blaize Bragg et al. grating succeeded [76]. The in creating emitters’ a combwavelengths of 15 lasers were with spaced one single by 25 Bragg GHz gratingand 100 [ 76GHz]. The and emitters’ set to be wavelengths on the Dense were WDM spaced In- byternational 25 GHz and Telecommunication 100 GHz and set to Union be on the(ITU) Dense grid. WDM TheInternational output power Telecommunication of these devices Unioncould be (ITU) as high grid. as The 80 mW output for a power 350 mW of thesecoupled devices pump could power be [41], as high while as a 80 linewidth mW for of a 350only mW 3 kHz coupled has been pump reported power by [41 Bastard], while et a al. linewidth on their ofDFB only lasers 3 kHz [77]. has Figure been 9 reported displays bya picture Bastard of et such al. on a theirDFB DFBlaser laserspigtailed [77]. to Figure HI10609 displays single mode a picture fibers. of suchThe astability DFB laser and pigtailedpurity of tothe HI1060 emission single of erbium mode fibers. doped The waveguide stability andlasers purity has been of the recently emission used of erbiumto gen- dopederate a waveguide Radio Frequency lasers has signal been and recently successfully used to transmit generate data a Radio at a Frequencyfrequency signalof 60 GHz and successfully[78]. transmit data at a frequency of 60 GHz [78].

FigureFigure 9.9. PicturePicture ofof aa DFBDFB laserlaser realizedrealized by by silver-sodium silver-sodium ion-exchange ion-exchange on on P1 P1 phosphate phosphate glass glass at at the IMEP-LaHCthe IMEP-LaHC (device (device similar similar to [77 to]). [77]).

BraggBragg gratingsgratings on on phosphate phosphate glass glass can can be be made made by by photolithography photolithography steps steps and and etching etch- likeing inlike [41 in,76 [41,76,77],77] or by or direct by direct UV inscription UV inscription like in like [79, 80in] [79,80] and on and IOG1. on TheIOG1. use The of a use hybrid of a un-doped/dopedhybrid un-doped/doped IOG1 substrate IOG1 substrate allowed allowed Yliniemi Yliniemi et al. [80 et] al. to realize[80] to realize UV-written UV-written Bragg gratingsBragg gratings with high with reﬂectance high reflectance and selectivity, and selectivity, demonstrating demonstrating hence ahence single a frequencysingle fre- emissionquency emission with an outputwith an power output of power 9 mW of and 9 mW a slope and efﬁciency a slope efficiency of 13.9%. of 13.9%.

3.4.3.4. HybridHybrid DevicesDevices Ion-exchangedIon-exchanged waveguideswaveguides beingbeing mademade insideinside thethe glassglass wafer,wafer, theythey leaveleave itsits surfacesurface planeplane andand available available for for the the integration integration of of other other materials materials or or technologies. technologies. The The realization realization of deeplyof deeply buried buried waveguides waveguides [81 [81]] and and selectively selectively buried buried waveguides waveguides [ 82[82]] acting acting as as optical optical viasvias betweenbetween twotwo differentdifferent layerslayers increasedincreased furthermorefurthermore thethe possibilitypossibility ofof 3D3D integration.integration. InIn orderorder toto overcomeovercome thethe quitequite weakweak chemicalchemical durabilitydurability ofof anan Yb-ErYb-Er dopeddoped phosphatephosphate glass,glass, GardillouGardillou etet al.al. [[83]83] waferwafer bondedbonded itit onon aa silicatesilicate glassglass substratesubstrate containingcontaining surfacesurface TlTl ion-exchangedion-exchanged strips.strips. TheThe higherhigher refractiverefractive indexindex activeactive glassglass waswas thenthen thinnedthinned byby anan appropriateappropriate polishingpolishing processprocess toto becomebecome aa singlesingle modemode planarplanar waveguide.waveguide. AtAt thethe placeplace wherewhere thethe planar planar waveguide waveguide was was in in contact contact with with the the ion-exchanged ion-exchanged strips, strips, the the variation variation of refractiveof refractive index index provided provided the the lateral lateral conﬁnement confinement creating creating hence hence a hybrid a hybrid waveguide. waveguide. A gain of 4.25 dB/cm has been measured with this device. This approach has been pursued A gain of 4.25 dB/cm has been measured with this device. This approach has been pursued by Casale et al. [59] who realized a hybrid DFB laser combining a planar ion-exchanged by Casale et al. [59] who realized a hybrid DFB laser combining a planar ion-exchanged waveguide made on IOG1 with a passive ion-exchanged channel waveguide realized on waveguide made on IOG1 with a passive ion-exchanged channel waveguide realized on GO14. The Bragg grating was etched on the passive glass and encapsulated between the GO14. The Bragg grating was etched on the passive glass and encapsulated between the two wafers. two wafers. Polymers have also been used to functionalize an ion-exchanged waveguide. As an example, a thin ﬁlm of BDN-doped cellulose acetate deposited on the surface of ion- exchanged waveguide lasers allowed the realization of passively Q-switched lasers on Appl. Sci. 2021, 11, 4472 12 of 18

Nd-doped [84] and Yb doped [85] IOG1 substrates. A peak power of 1 kW for pulses of 1.3 ns and a repetition rate of 28 kHz has been reported by Charlet et al. [86] and used successfully to pump a photonic crystal fiber and generate a supercontinuum [87]. Recently, a proof of concept of LiNbO3 thin films hybridized on ion-exchanged waveguides have been reported [88]. The combination of these two well-known technological platforms for integrated photonics opens the route towards efficient low-loss non-linear integrated devices including electro-optic modulators. Hybrid integration of semiconductor devices on glass wafers containing ion-exchanged waveguides have been reported for the first time in 1987, by MacDonald et al. [89] They bonded GaAs photodiodes on a metallic layer previously deposited and patterned on the glass wafer. Waveguides were done by a silver thin film dry process. Silicon [90] and germanium [91] photodetectors have been produced on potassium waveguides, while Yi-Yan et al. proposed a lift-off approach to bound thin III-V semiconductor membranes on the surface of a glass wafer containing ion-exchanged waveguide and realize Metal– Semiconductor–Metal (MSM) photodetectors [92].

4. Sensors Integrated photonics is intrinsically interesting for the realization of optical sensors because it provides compact and reliable self-aligned devices that can be easily deported when pigtailed to optical fibers. Glass is a material that is chemically inert, bio-compatible, and mechanically stable. Therefore, making optical sensors on glass wafers or integrating optical glass chips into complex set-ups have encountered a huge interest. We will detail here a selection of ion-exchanged based glass sensors as examples of possible applications. Although AWGs used in telecom are actually integrated spectrometers, they are not well adapted to the rapid measurement of full spectra. For this reason, a Stationary-Wave In- tegrated Fourier-Transform Spectrometer (SWIFTS) has been proposed and developed [93]. It is a static Fourier Spectrometer that measures directly the intensity of a standing wave with nanoprobe placed on a waveguide. In the instrument reported by Thomas et al. [94], the waveguide is made by a silver ion-exchange on a silicate glass and the nanoprobes are gold nanodots. The interaction of gold nano-antennas with an ion-exchanged waveguide has been studied by Arnaud et al. [95]. This spectrometer has a spectral measurement range that starts at 630 nm and ends at 1080 nm with a spectral resolution better than 14 pm. SWIFTS interferometers are currently integrated in the product line commercialized by Resolution Spectra Systems [96]. Displacement sensors allow measuring accurately the change of position of an object through interferometry. Helleso et al. [97] implemented a double Michelson interferometer on a glass substrate using potassium ion-exchange; the device provided two de-phased outputs in order to give access not only to the distance of the displacement but also its direction. However, having only two interferometric signals is not sufficient to prevent the measure from being affected by unexpected signal variations. For this reason, Lang et al. [98] proposed a new design for the interferometric head that provided four quadrature phase shifted outputs. The device made by potassium ion-exchange demonstrated a measurement accuracy of 79 nm over a measurement range of several meters when used with an HeNe laser as a source. After technological improvements and the use of a silver- sodium ion-exchange on GO14 glass, an evolution of this sensor is now commercialized by TeemPhotonics and presents a resolution of 10 pm for a 1530 nm–1560 nm operating wavelength range [48]. Measuring speed is also something that can be of major importance, specifically in the case of aircrafts where their True Air Speed (TAS), which is their speed with respect to the air surrounding them, conditions their lift. Airborne LIDARs have hence been developed as a backup to Pitot gauges in order to increase the safety of flight by providing a redundant accurate measurement of the aircraft TAS. The operation principle of an airborne LIDAR is based on the Doppler frequency shift measured on a laser signal reflected on the dust particles of the atmosphere. This shift being quite low and presenting a low amplitude Appl. Sci. 2021, 11, x FOR PEER REVIEW 13 of 18

Appl. Sci. 2021, 11, 4472 a redundant accurate measurement of the aircraft TAS. The operation principle of an13 ofair- 18 borne LIDAR is based on the Doppler frequency shift measured on a laser signal reflected on the dust particles of the atmosphere. This shift being quite low and presenting a low amplitude when compared to the emitted signal, a laser source that presents a narrow linewidth,when compared a low Relative to the emitted Intensity signal, Noise a and laser that source is resilient that presents to mechanical a narrow vibrations linewidth, is required.a low Relative Bastard Intensity et al. [99] Noise realized and that such is a resilient laser source to mechanical on an Er/Yb vibrations doped isphosphate required. Bastard et al. [99] realized such a laser source on an Er/Yb doped phosphate glass with glass with silver ion-exchanged waveguides and a DFB structure. This laser presented a silver ion-exchanged waveguides and a DFB structure. This laser presented a fiber coupled fiber coupled output power of 2.5 mW, a linewidth of 2.5 kHz, and a RIN that was 6 dB output power of 2.5 mW, a linewidth of 2.5 kHz, and a RIN that was 6 dB lower than the lower than the specification limit. The device has then been successfully implemented in specification limit. The device has then been successfully implemented in the LIDAR set-up the LIDAR set-up and validated in flight [100]. and validated in flight [100]. Astrophysical research programs rely on telescopes with always higher resolution to Astrophysical research programs rely on telescopes with always higher resolution to detect exoplanets, young star accretion disks, etc. Optical long baseline instruments, detect exoplanets, young star accretion disks, etc. Optical long baseline instruments, which which interferometrically combine the signal collected by different telescope have been interferometrically combine the signal collected by different telescope have been developed developed for this purpose. Such complex interferometers are very sensitive to misalign- for this purpose. Such complex interferometers are very sensitive to misalignment and ment and vibrations, therefore the use of integrated optics as telescope recombiners have vibrations, therefore the use of integrated optics as telescope recombiners have been studied. been studied. Haguenauer et al. [101] used a silver-sodium ion-exchange on a silicate glass Haguenauer et al. [101] used a silver-sodium ion-exchange on a silicate glass to realize a totwo realize telescope a two beam telescope combiner beam operating combiner on operating the H atmospheric on the H atmospheric band (from bandλ = 1.43 (fromµm λ to= 1.43λ = 1.77µm µtom λ). Consisting= 1.77 µm). of Consisting a proper arrangement of a proper ofarrangement three Y-junctions, of three the Y-junctions, device had twothe devicephotometric had two and photometric one interferometric and one outputs.interferomet Theric fringe outputs. contrast The obtained fringe contrast in the laboratory obtained inwas the 92% laboratory and the devicewas 92% was and included the device in the was Integrated included Optic in the Near Integrated infrared Optic Interferometric Near in- fraredCamera Interferometric (IONIC) put into Camera a cryostat (IONIC) and put successfully into a cryostat qualified and on successfully the sky [102 qualified]. Figure on10 theshows sky the[102]. MAFL Figure chip 10 [103 shows] that the was MAFL developed chip [103] for the that interferometric was developed combination for the interfero- of three metrictelescopes. combination The pigtailed of three instrument telescopes. contained The pigtailed not only instrument the science contained interferometers not only butthe sciencealso three interferometers other ones dedicated but also three to metrology, other ones which dedicated permitted to metrology, measuring which of the permitted different measuringoptical paths. of the The different functions optical multiplexing paths. The and demultiplexingfunctions multiplexing the metrology and demultiplexing signal and the thescience metrology ones were signal also and implemented the science ones on the were chip. also implemented on the chip.

Figure 10. Figure 10. PicturePicture of of the the MAFL combining module. TheThe opticaloptical chipchip containscontains waveguides waveguides made made by by a asilver silver sodium sodium ion-exchange. ion-exchange. The chemical durability of silicate glasses is a major advantage when a use in harsh The chemical durability of silicate glasses is a major advantage when a use in harsh environment is required. The opto-ﬂuidic sensor developed by Allenet et al. [104] repre- environment is required. The opto-fluidic sensor developed by Allenet et al. [104] repre- sents a quite extreme example of this. Indeed, the ion-exchange technology developed by sents a quite extreme example of this. Indeed, the ion-exchange technology developed by Schimpf et al. [35] on BF33 glass has been employed to realize a sensor for the detection of Schimpf et al. [35] on BF33 glass has been employed to realize a sensor for the detection plutonium in a nuclear plant environment. The fully pigtailed and packaged device that is of plutonium in a nuclear plant environment. The fully pigtailed and packaged device depicted on Figure 11, has been successfully tested in a nuclearized glove box, detecting that is depicted on Figure 11, has been successfully tested in a nuclearized glove box, de- plutonium dissolved in 2 Mol nitric acid without a failure over a period of one month. tecting plutonium dissolved in 2 Mol nitric acid without a failure over a period of one Such a reliability was achieved by co-integrating microﬂuidic channels fabricated by HF month. Such a reliability was achieved by co-integrating microfluidic channels fabricated wet etching on one BF33 wafer with silver ion-exchanged waveguides realized on another by HF wet etching on one BF33 wafer with silver ion-exchanged waveguides realized on wafer. The two wafers have been assembled by molecular adherence avoiding hence the use of radiation sensitive epoxy glues. Appl. Sci. 2021, 11, x FOR PEER REVIEW 14 of 18

Appl. Sci. 2021, 11, 4472 14 of 18 another wafer. The two wafers have been assembled by molecular adherence avoiding hence the use of radiation sensitive epoxy glues.

Interface Microchannel + connectors V = 21 μL

Capillaries

1 cm

Figure 11. PicturePicture of of an an optofluidic optoﬂuidic sensor sensor realized realized on on BF33 BF33 glass glass wafer wafer for forthe themeasurement measurement of of radioactiveradioactive elementselements diluteddiluted inin highlyhighly concentratedconcentrated nitricnitric acid.acid.

5. Conclusions InIn thisthis paper,paper, wewe reviewedreviewed overover thirtythirty yearsyears ofof activitiesactivities inin glassglass photonics.photonics. The ion- exchange realizationrealization processprocess asas wellwell asas itsits modellingmodelling hashas beenbeen exposed.exposed. PassivePassive andand activeactive devices forfor telecommunicationtelecommunication applicationsapplications havehave thenthen beenbeen presentedpresented withwith thethe emphasisemphasis on thethe majormajor breakthroughsbreakthroughs ofof thisthis ﬁeld.field. TheThe sectionsection dedicateddedicated toto sensorssensors underlinesunderlines thethe evolution of thethe ion-exchangeion-exchange technology,technology, whichwhich isis movingmoving fromfrom quitequite simple,simple, thoughthough extremely performantperformant functions,functions, toto moremore complexcomplex integratedintegrated opticaloptical microsystems.microsystems. TheThe authors hopehope thatthat the the picture picture of of glass glass photonics photonic thats that they they presented presented will wi soonll soon be outdatedbe outdated by theby the new new results results that that are currentlyare currently being being elaborated elaborated in the in manythe many laboratories laboratories of universities of univer- andsities companies and companies involved involved in this in ﬁeld this throughout field throughout the world. the world.

Funding: TheThe visit visit of of Pr Pr Broquin Broquin at atthe the University University of Eastern of Eastern Finland Finland is fund ised funded by the by Nokia theNokia Foun- Foundationdation within within the theframe frame of the of the Institut Institut França Françaisis de de Finlande—Nokia Finlande—Nokia Foundation DistinguishedDistinguished Chair. ThisThis workwork isis alsoalso partpart ofof thethe AcademyAcademy ofof FinlandFinland FlagshipFlagship Programme,Programme, PhotonicsPhotonics ResearchResearch andand InnovationInnovation (PREIN),(PREIN), decisiondecision 320166.320166. InstitutionalInstitutional ReviewReview BoardBoard Statement:Statement: Not applicable.applicable. InformedInformed ConsentConsent Statement:Statement: Not applicable. ConﬂictsConflicts ofof Interest:Interest: TheThe authorsauthors declaredeclare nono conﬂictconflict ofof interest.interest.

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