Waveguide lasers based on dielectric materials

Optical Materials 34 (2012) 555–571

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Optical Materials journal homepage: www.elsevier.com/locate/optmat

Waveguide lasers based on dielectric materials E. Cantelar, D. Jaque, G. Lifante ⇑ Dept. de Física de Materiales, C-04, Facultad de Ciencias, Universidad Autónoma de Madrid, 28049 Madrid, Spain

a r t i c l e

i n f o

Article history: Received 15 November 2010 Received in revised form 6 May 2011 Accepted 9 May 2011 Available online 8 June 2011 Keywords: Optical waveguides Rare earths Waveguide lasers

a b s t r a c t Fifty years since the invention of the laser have been witness of the development of many different laser systems and designs. Among them, miniaturized versions of solid sate lasers based on rare-earth-doped dielectric materials have been proposed and demonstrated during the last 20 years. They are based on confined radiation provided by optical waveguide structures. Although many materials and techniques have been studied for producing planar and channel waveguides, only a few of them have shown to be adequate routes for fabricating waveguide lasers. Here we summarize the theory and specific technologies developed for characterizing waveguide structures, and we present some common fabrication techniques already successfully applied to fabricate dielectric waveguide lasers, where relevant examples of demonstrated working devices are outlined. Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction Since the invention of the laser, many systems have been proposed and demonstrated to be adequate for performing optical oscillation, and their characteristics have diversified to match different development areas. The research in laser systems includes new optically active centers, artificial materials to be activated by the active centers, and novel configurations and designs. Several objectives have been pursued in the research of laser systems: span the electromagnetic range covered by the laser radiation; increase the power delivered by the lasers; shorten the laser pulse duration; miniaturization of laser devices; and incorporation of several photonic component into the laser system. Research on waveguide lasers based on dielectric materials is one clear example of these tendencies. Although the evolution of waveguide lasers has followed somewhat similar trends, they exhibit certain peculiarities that make them worthy of especial attention. Optically pumped waveguide lasers have an inherent advantage over conventional bulk lasers in that both the pump light and the laser radiation are constrained to propagate together in the small cross section of the waveguide, and thus the overlap between both beams is necessarily quite high for unlimited propagation distances. A scheme of conventional bulk laser and channel waveguide laser is presented in Fig. 1. The small spot sizes lead to high intensities for relative low power. This is already true for planar waveguide lasers, but particularly in channel waveguides, which provide two-dimensional light confinement. If the propagation loss of the waveguide can be maintained low, a high

⇑ Corresponding author. Tel.: +34 91 4974783; fax: +34 91 4978579. E-mail address: [email protected] (G. Lifante). 0925-3467/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2011.05.012

gain per unit pump power and a low laser threshold can be achieved. As the device length can be very long, low RE concentrations are required, allowing to avoid up-conversion, cross-relaxation and other mechanisms that would otherwise reduce laser performance. Channel waveguides lasers can be integrated with other elements to achieve a full compact and functional integrated optic device, that is, easy on-chip integration with other optical components. Typical in-depth direction of the order of microns facilitates microstructuring, for example, to integrate DBR to provide laser feedback. In addition, taking advantage of the combination of high laser gain and electrooptic, acoustooptic, or non-linear effects in the same medium is particularly interesting in a single-mode waveguide configuration, where the high optical energy confinement increases the laser gain, the efficiency of nonlinear interactions, and allows for the realization of high speed electrooptic components with low drive power [1]. In channel waveguide devices modulators may be readily monolithically integrated [2], complex multiple-cavity devices may be photolithographically defined [3] and waveguide geometries may be varied along a device for efficient interfacing to external components [4]. Finally, waveguide devices are small, stable, and easy to handle, providing high potential for miniaturization. The article is structured as follows: Section 2 is an overview of the waveguide theory and numerical techniques developed for the performance simulation of passive as well as active waveguides, starting from very basic theory of planar waveguides up to the simulation techniques implemented to model waveguide lasers. Section 3 is devoted to the description of general techniques for characterizing active waveguides, from index profiles to spectroscopic techniques, including micro-Raman studies. Examples of demonstrated waveguides lasers are presented in Section 4, where the two main routes to fabricate waveguide structures are reviewed: deposition techniques and modification techniques. Along

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Fig. 1. Left: conventional bulk laser scheme. Right: scheme of channel waveguide lasers.

the different sections, relevant references are given which represent significant advances in the research topic, or that indicate new materials, fabrication processes or novel strategies for further developments. Finally, in Section 5 some conclusions are drawn, which try to summarize the research in the field and the future trends pursued by the investigation in the arena of waveguide laser technology. 2. Passive and active waveguide modeling Device modeling plays significant role in the advancement of optical components, including optimization of current designs and evaluation of new device concepts. This is particularly true for waveguide lasers where modal analysis, beam propagation and interaction of light beams with the active material conform a scenario where theoretical modeling and numerical simulation are highly valuable, and here we summarize some of them. 2.1. Basic waveguide structures Depending on the number of dimensions of light confinement in a structure, the optical waveguides can be classified as planar waveguides (one-dimension confinement) or channel waveguides (two-dimensional confinement). In general, for waveguide lasers delivering high power, multimode planar waveguides are preferred [5], as the total laser power is proportional to the active volume. On the other hand, monomode channel structures are desirable for waveguide laser devices which require further control of the radiation (via EO or AO effects) [6], which usually include the integration of extra elements (DBR, couplers, Y-junctions, phase shifters, etc.) with the active resonant cavity. While both types of guides are compatible with semiconductor laser pumping, channel waveguides have the advantage of further miniaturization and compatibility for fiber coupling. A waveguide is an inherent inhomogeneous structure that can support EM fields without suffering diffraction in one direction (planar waveguides) or two directions (channel waveguides). The simplest waveguide geometry is the step-index planar waveguide

(Fig. 2a), where a homogeneous region (core) is sandwiched by two semi-infinite regions, cover (usually air) and substrate, with lower refractive index than the film. This structure is formed when the waveguide is fabricated by deposition techniques (MBE, LPE, etc.). If the fabrication method involves index increase in a given substrate, usually the planar waveguide shows a graded index profile (Fig. 2b). These profiles are common in fabrication techniques such as ionic or metallic diffusion, or ion implantation. While in planar waveguides the light is confined in one direction, thus suffering in-plane diffraction, channel waveguides structures also provide lateral confinement, allowing full diffractionless propagation through arbitrarily long distances. In channel waveguide lasers the additional lateral confinement of both the pump and laser beams means that lower threshold devices are possible while maintaining good slope efficiencies. Standard photolithographic techniques are used in combination with film deposition giving rise to rib or stripe structures (Fig. 2c); or in combination with surface modification techniques to obtain diffused channel waveguides (Fig. 2d). In any case, the channel waveguide is defined by its transversal index profile n(x, y). Although the active medium in a waveguide laser, as well as in a bulk laser, is responsible for the amplification of the laser radiation, the optical resonator also plays an essential role. The laser cavity must filter the radiation and must provide the optical feedback required to induce the optical amplification in the active medium. A broad variety of optical cavities with diverse geometries has been employed to obtain laser oscillation in guiding configuration. Depending on the substrate, it is also possible to find extra elements to control the properties of the output radiation. A schematic illustration of the optical resonators commonly used in channel waveguide lasers is presented in Fig. 3, where some of these optical resonators are also compatible with planar waveguide structures. The laser oscillation in CW-regime can be obtained by designing a Fabry–Perot laser cavity. The simplest optical resonator (Fig. 3a), consists in an active medium (that can be a planar waveguide [7]) and two dielectric mirrors attached on the waveguide facets to provide the optical feedback. The reflectivity of the output coupler to the pumping wavelength will determine the single- or

(a)

(b)

(c)

(d)

Fig. 2. Basic waveguide geometries: (a) Step index planar waveguide; (b) graded index planar waveguide; (c) rib channel waveguide; and (d) diffused channel waveguide.

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Fig. 3. Optical resonators commonly used in channel waveguide lasers: (a–c) CW regime, (d) and (e) pulsed lasers.

double-pass characteristic of the cavity as well as the width of the output laser radiation. The dielectric mirrors can be deposited directly on the polished end-faces [8–10], held in place by spring clips [11], butted to the end-faces [12–15], by using index matching oil or gel, or even bonded using a UV-curing epoxy [16]. A more complex cavity can be used to obtain a narrow linewidth laser radiation of a few GHz by substituting the dielectric mirrors by Bragg-reflectors at the waveguide ends [17,18] (Fig. 3b). In this case, the laser FWHM is fixed by the reflectivity and line width of the output coupler grating (obviously, laser output spectrum is also affected by the mirror at the other end of the cavity). It is has also been demonstrated that narrow line-width laser radiation can be attained by including a grating inside the optical resonator formed by using dielectric mirrors [19]. Another possibility is to design an integrated ring resonator (Fig. 3c). Here an optical circuit, consisting of a ring and two straight tangential waveguides, forms two directional couplers, which allow coupling the excitation clockwise and counter-clockwise inside the ring. The output laser radiation can be observed via both outputs of the straight channels [20,21]. The electrooptical properties of LiNbO3 have been used to develop mode-locked and Q-switched lasers in Nd3+ [2,22,23] and Er3+ doped substrates [24–27]. The scheme of a mode-locked waveguide laser is shown in Fig. 3d. Over the channel waveguide two electrodes are deposited playing the role of a broad band traveling wave modulator, which allows efficient phase modulation at different harmonics of the axial mode frequency spacing [28]. Another possible way to fabricate a pulsed laser is to design the optical circuit (active medium) as a Mach–Zehnder interferometer (Fig. 3e). Once again, the electrooptical properties of the host are exploited to control the cavity losses via an intracavity modulator, which is the basis of Q-switched operation [28]. It is important to remark that also the acoustooptical properties of LiNbO3 have been used to developed waveguide tuneable lasers. Although the design is not shown in Fig. 3, it involves optical circuits with more complex geometries including extra-intracavity elements [6].

2.2. Modal analysis The essence of waveguide lasers relies on the characteristics of the pump and laser beams inside the resonant cavity. At variance with laser beam propagation in homogeneous media, where in general the light propagates as Gaussian beams suffering diffraction, the inhomogeneous structure provided by the waveguide structures gives rise to particular guided modes. The waveguide modes, which

are defined as the solution of the EM wave equation in z-invariant inhomogeneous structures, have the property of avoiding diffraction, thus the intensity of the beams (both pump and laser beams) are highly confined along the whole cavity resonator. The mode fields are expressed as Eðx; y; z; tÞ ¼ Wðx; yÞeiðxtbzÞ , where x is the angular frequency of the monochromatic radiation, W is the transversal field distribution, and b represents the propagation constant [29]. For lossy or amplifying waveguides, the propagation constant becomes complex, hence in addition to phase changes, there will also be attenuation/amplification of the fields. The index profile of a given optical waveguide will determine the number of guided modes supported by the structure, their propagation constants and their transversal field distribution. Modal analysis is then fundamental to simulate the performance of waveguide laser devices, for instance to determine: overlap between pump and signal beams [30]; coupling in Q-switching lasers [22]; performance in MZ structures [28]; interaction of guided modes with absorbing cladding for Q-switching operation [31]; or resonant wavelengths in multichannel waveguide lasers [11]. The guided modes of a given planar waveguide, defined by its 1D-index distribution (Fig. 2a and b), and assuming monochromatic waves in a z-invariant structure eiðxtbzÞ , can be calculated with the desired degree of accuracy by different methods, and can be classified as TE or TM modes, dependent on their polarization. The multilayer method to calculate the propagation constants b and field distribution of the modes (either TE or TM) consists on approximating the index profile by a high enough number of layers of constant refractive index. For TE-modes, within each layer of constant index ni, the only non-vanishing component of the electric field Ey fulfils the wave equation: 2

d Ey ðxÞ dx

2

2

þ ½k0 n2i  b2 Ey ðxÞ ¼ 0

where the solution is either a sinusoidal or an exponential function. By setting the appropriate continuity of the fields components at each interface, guided modes are obtained for values of the propagation constants b that give exponential decay fields at the cover and substrate regions [32,33] (Fig. 4, left). Whereas the modes in planar waveguides are pure TE or TM modes, in 3D waveguides (channel waveguides) the modes are always hybrid, where more than one component of the electric and magnetic fields are present. One powerful numerical method for mode solving in 3D structures is based in the finite difference method. It consists on discretizing the region of interest in a rectangular grid, where the refractive index is assumed to be constant within

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Fig. 4. Left: Modes on a planar waveguide. Right: Fundamental mode for the graded-index channel waveguide (shown in the middle).

each cell. From Maxwell’s equations, the wave equation in a uniform region of space is of the form:

r2 E þ n2 ðx; yÞk20 E ¼ 0 and a similar equation for the magnetic field, with the appropriate boundary conditions between adjacent cells. Efficient methods have been proposed which provide scalar, semi-vectorial and vectorial solutions [34,35]. Also, algorithms based on propagation along the imaginary z-axis have proved to be very efficient numerical techniques to obtain the modal properties of channel waveguides [36] even with structures showing high index discontinuities. Fig 4, right) presents the mode intensity profile of the refractive index map shown in the center, using a scalar approach. 2.3. Beam propagation method Many photonic devices involve propagation in waveguides that are non-uniform and, therefore, modal analysis is not enough for a complete modeling of the devices. The most powerful method to simulate the optical propagation in waveguides is the so-called beam propagation method (BPM), and nowadays most of them are based on finite difference methods [29,37]. The BPM relies on considering that the wave propagation is primarily along the positive z direction (paraxial propagation), and that the refractive index changes slowly along this direction. Under these circumstances, it is beneficial to factor out the problem by introducing a slowly varying field Wt associated with the transverse electric field, defined through Et ¼ Wt ein0 k0 z , where n0 represents the refractive index of a reference medium. Assuming that the optical variation is slow in the propagation direction, one obtains the basic vectorial BPM equation:

2in0 k0

@ Wt ^ Wt ¼H @z

^ represents a differential operator [38]. This paraxial equawhere H tion can be solved directly in the spatial domain, and powerful methods based in finite differences of the vectorial wave equation have been developed [38]. This numerical technique allows the simulation of strong guiding structures that also can vary in the propagation direction. If the coupling between the two polarizations is weak, a semivectorial treatment gives accurate results, and this approach is usually the adequate option for modeling of dielectric waveguide lasers, where the coupling terms between the transversal components of the fields can be neglected [39]. One important aspect of the BPM relies on the fact that it is a very flexible and extensible technique, allowing inclusion of most effects of interest, such as nonlinearities or electrooptic effect, and treating anisotropic media or lossy/gain media, which is of particular interest for modeling waveguide laser structures. In this case, iterative algorithm should be implemented to take account

and link the forward and backward fields for both the signal and pump beams [40]. BPM also allows the propagating modes of a z-invariant structure to be obtained by means of propagation along the imaginary z-axis, known as the imaginary distance BPM [36]. The idea of the method is to launch an arbitrary field and propagate it along the imaginary axis, where the field becomes exponential growth, with a growth rate of each mode being equal to its real propagation constant. Since the fundamental mode has the highest propagation constant, its contribution to the field will have the highest growth rate and will dominate all other modes after a certain distance. Higher modes can be obtained by using an orthonormalization procedure to subtract contributions from lower order modes while performing the propagation.

2.4. Modeling of waveguide lasers The performance of waveguide lasers operating in continuous wave regime (CW) based on RE3+-doped materials can be modeled, within a semi-classical approach by using the overlapping integrals method (OIM) [41–43] and rate equation formalism to describe the population dynamics of the different RE3+ levels [43–45]. The model involves two types of coupled differential equations: those describing RE3+ population dynamics and those describing the forward and backward propagation of the optical beams. The power evolution of pump and signal beams inside the waveguide using the overlapping integrals method [30,42,43] starts by considering both beams as monochromatic waves with fixed normalized transversal distributions associated with each propagating mode. This modal distribution depends only on the refractive index profile, and can be calculated by either the effective index method [33] or by the imaginary-distance BPM [36]. On the other hand, the overlap integrals represent the overlap of the steady state of the level population density with the normalized modal intensity distribution as [30,43]. The model also considers the stimulated emission and absorption rates, which are dependent on the power distribution of pump and signal beams, as well as on the longitudinal and transversal coordinates. The OIM results on a set of coupled differential equations, which are solved based on a finite discretization of the active medium along both the transversal and longitudinal dimensions. At each point in the transversal plane the stimulated transition probabilities are evaluated taking into account the modal intensity distributions. Then the rate equations are solved, obtaining the different steady-state populations. When these populations are known, the contribution of each point in the transverse plane to the overlapping integrals is evaluated. Once the transversal plane is solved, the pump and signal beams are propagated one longitudinal step along the propagation direction. In waveguide laser modeling, the forward and backward components are linked through the boundary conditions at the input and output

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Fig. 5. Left: Evolution of the forward and backward powers along the cavity position of a Fabry–Perot channel waveguide laser. Right: Results of the waveguide laser modeling for the output power as function of the cavity length, for various pump powers [30].

end-faces, taking into account the reflectances of the two endface mirrors at the pump and signal wavelengths. At t = 0 and z = 0 the active medium is supposed to be at rest, with all the ions in the ground state. Then the pump power is injected and energy level populations and cavity powers evolve according to the dynamic equations. Initially the forward component of the signal beam in z = 0 is initialised at an arbitrarily low value, from where it develops independently of this initial condition. The procedure iterates until a consistent solution is reached for the forward and backward components (Fig. 5, left). This method allows, for example, the evaluation of the laser characteristics of channel waveguides as function of the parameters’ device, thus providing routes for the device optimization (Fig. 5, right). Alternatively, waveguide laser modeling can be accomplished by using BPM algorithms [40,46] based on field amplitudes. To simulate the optical attenuation/amplification in waveguide structures, the propagations of the pump and signal beams are performed simultaneously. In the BPM method, the energy variation experienced by each beam is controlled by considering a complex refractive index, which includes both gain and losses. The imaginary part of the refractive index distribution at a particular transversal plane of the propagation, corresponding to the signal and pump wavelengths, depends on the population density of the levels. BPM is used to propagate the signal and pump fields simultaneously, forward and backward iteratively. The boundary conditions at the end-faces, which link the forward and backward fields, are given by the mirrors’ reflectances [40]. The clear advantage of using BPM method instead of the OIM is that the fields are allowed to evolve along the propagation. This implies that the method can simulate waveguide structures beyond straight waveguides lasers, such as Y-splitters or Mach–Zehnder interferometers, and also can deal with multimode waveguides in a natural way. In addition, the BPM method can account for interference effects between the forward and backward light waves. The obvious disadvantage is that this method is numerically more intensive than the OIM. 3. Active waveguides The active waveguide represents the essential element in an integrated laser because it constitutes the active region in which the optical amplification by stimulated emission will take place. Also, the waveguide should adequately confine pump and signal beams. Simultaneously the propagation losses at both wavelengths

should be low enough to reach optical gain at the laser wavelength with moderate pump powers. On the other hand, waveguide fabrication techniques must preserve the spectroscopic properties of the active ions. These aspects, and other requirements, which must be satisfied to obtain efficient optical amplification in the active medium, are reviewed in next paragraph. Finally, experimental techniques often used to characterize active waveguides are presented. 3.1. Material requirements The selection of the active medium is a fundamental step in the development of an integrated optical device. For instance, low phonon materials such as fluoride crystals and glasses have demonstrated most effectively to produce mid-infrared and upconversion lasers [47]. In addition, the host should provide interesting characteristics beyond the spectroscopic properties of the active ions, making possible a multifunctional operation. Such is the case of integrated lasers based on rare-earth doped lithium niobate (LiNbO3) in which the electro-optical or acousto-optical properties of the lattice are exploited to pulse the laser radiation [28] or to obtain tuneable lasers [6]. Optical waveguides can be fabricated in the active medium (crystalline or vitreous) following a wide variety of strategies, all of them based on the production of a high refractive index layer over the initial wafer to support the optical confinement. In general, such kind of structures are obtained following two basic schemes: by direct deposition of a thin film with a higher refractive index than the substrate or, by adding or exchanging ions giving rise to an adequate modification of the bulk refractive index. Therefore it could be said that, in principle, any physical or chemical procedure that generates these structures is a valid technique to produce optical waveguides. Nevertheless, there are other important aspects that must be taken into account to successfully sustain the guided-wave laser operation: (i) The guiding region should have a high optical damage threshold to support the power of the pump and signal beams. (ii) The fabrication technique should preserve not only the original surface quality of the wafer, in order to do not cause scattering losses, but also the spectroscopic properties of the active ions. Simultaneously, the induced refractive index change should be stable and permanent to assure the device operation over long times.

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(iii) If the active ions are incorporated to the wafer through a diffusion process, their distribution in the transversal plane should overlap with the waveguide mode. (iv) In the case of direct deposition of the thin films, it is also needed a good adhesion between the layer and the wafer. (v) In general, the selected technique must have a high level of reproducibility that allows to produce similar devices by controlling the fabrication parameters. (vi) Finally, in relation to channel waveguides, it is interesting that the structure provides a modal spatial output compatible for low-loss coupling to fiber optic components. 3.2. Waveguide characterization The characterization of waveguide lasers includes the study of basic parameters of the passive waveguide, such as index profile, modal field distributions and losses, and the spectroscopic properties of the active ions in the structure, as well as the structural changes induced in the material during the waveguide fabrication or during the doping step. The techniques used to carry out such characterization are briefly summarized next. As the guiding properties of a waveguide depends on the refractive index profile of the structure (at a given wavelength), it is clear that the most important characterization of an integrated optical device concerns to its index distribution. Although many integrated optical devices (f.i. waveguide lasers) are based on channel waveguides, the first step in characterizing waveguides often starts from the study of planar waveguides, especially when they are fabricated by means of diffusion processes. The determination of the refractive index profile of a planar waveguide is carried out by the measurement of the propagation constants of the confined modes (effective indices). The effective indices of the guided modes are obtained by using the dark-mode method (Fig. 6). The essence of the method consists of injecting polarized light into the waveguide by means of a high index prism. When the light is coupled to a synchronous angle corresponding to a propagating mode, the light is efficiently injected to the waveguide, and the reflectivity shows a sharp dip. By measuring the angular positions of the dips, the effective refractive indices of the propagating modes are calculated. If the number of modes in the dark mode spectra is sufficiently high, a precise index profile for the planar waveguide can be determined by using algorithms such as the IWKB method [48] or the multilayer approach [32].

Although the guided modes in channel waveguides can also be examined by prism coupling, this technique is scarcely used in practice for characterizing channel waveguides. In addition, these structures are usually designed to be monomode at the operation wavelength. The method used to determine their transversal index distribution should be compatible with this circumstance, and one of the methods which gives good results is based on the analysis of near-field measurements, using the set-up shown in Fig. 7. From the measure of the intensity profile of the fundamental mode Im, the electric field distribution Em can be calculated from Em = Im1/2, where for the fundamental mode the electric field does not vanish anywhere and has a definite sign. Once Em(x, y) is determined from the near-field profile map, the index distribution of the channel waveguide can be reconstructed by using numerical algorithms based on finite difference methods [49]. Supporting guided modes is not enough for a given a structure to act as useful waveguide. An additional characteristic of the waveguide is necessary for practical purposes: low propagation losses. In particular, this parameter is of crucial importance in waveguide lasers, as high losses will result in low performance devices. Although the cut-back method is an adequate procedure to estimate losses in fibers, it is not applicable to waveguides, due to the short length involved (few centimeters). In general, measurement of the scattered light along the waveguide provides a good estimation of the losses, although this method is only valid for losses above 1 dB/cm. For lower values of losses, the Fabry– Perot method is an adequate technique [50]. This method, which requires monomode channel waveguides and a laser source with high coherence length, measures the FP fringes recorded as function of the sample temperature (Fig. 8), using the set-up shown in Fig. 7, but using a detector instead the CCD camera. The contrast of the fringes provides the value of losses in the channel waveguide, and is suitable for losses in the range 1–0.01 dB/cm. Thus, when possible, this is the right choice for a correct evaluation of losses in channel waveguides. In addition to that, the losses can also be evaluated by the output characteristics of the waveguide laser running for different values of the output coupler transmission [51]. Many waveguide fabrication methods imply the artificial and controlled modification of the original material. This modification not only affects the local refractive index but also the luminescence properties of the active ions, since any change in the original network leads to a modification of the crystal field (CF) that

Fig. 6. Dark-mode method for measuring the effective indices of a planar waveguide. Below is a blow-up for the system high index prism-waveguide.

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Fig. 7. Set-up for measuring modal intensity profiles in channel waveguides, using the technique of end-fire coupling.

Fig. 8. Transmitted intensity of a monomode channel waveguide recorded during cooling/heating the sample. From the contrast, losses of the waveguide is evaluated (0.3 dB/cm on a 20 lm channel LiNbO3:Zn waveguide).

Fig. 9. Confocal fluorescence images of a proton implanted channel waveguide fabricated in a Nd:YAG crystal. (a–c) correspond to the images obtained in terms of the spatial variation of the intensity, induced shift and width of the Nd fluorescence lines. Dashed line indicates the location of the nuclear damaged region [54].

determines their fluorescence properties [52]. In this sense, confocal micro-Photoluminescence (l-PL) microscopy has emerged as a powerful characterization technique. It allows for the acquisition of the luminescence spectra generated at each point of the waveguide [53], from which a map of the main parameters describing the fluorescence lines can be obtained. In order to illustrate the potential of this technique we show in Fig. 9 the fluorescence image of a channel waveguide fabricated by ion implantation in a Nd:YAG substrate [54]. The observed decrease in the fluorescence intensity (see Fig. 9a) denotes the presence of some damaged defects at the active volume. In addition the fluorescence image obtained in terms of the fluorescence line position (Fig. 9b) also provides a large contrast at the active volume. This means that the crystal field affecting luminescence ions has been modified at active

volume as a consequence of a lattice distortion [52]. Finally, fluorescence images can be obtained also from the spatial variation of the fluorescence width, revealing those areas in which the lattice has been disordered (see Fig. 9c in which disorder has been found at the electronic damage region). Thus, confocal fluorescence imaging is a powerful tool, not only for the evaluation of fabricated waveguides as integrated laser sources from the intensity maps, but also for determining the nature and extension of the micro-structural modifications induced in the original network, which are at the basis of refractive index modification. The technique has been recently successfully used in the characterization of laser waveguides including rare earth and metal transition doped transparent ceramics [55] and crystals [56]. A complete understanding of the fluorescence images requires, sometimes, additional measurements such as micro-Raman measurements and time resolved measurements. A good example of this can be found in the luminescence behavior of proton exchange channel waveguides fabricated in neodymium doped LiNbO3 crystals, where refractive index increment in the matrix was accompanied by a relevant luminescence reduction [57]. Time resolved measurements showed a lifetime reduction at waveguide’s volume, indicating that proton exchange has increased the nonradiative probability of Neodymium ions, this being a drawback for laser applications. The conclusions and results obtained from l-PL measurement can be only well understood when combined with Raman measurements. l-Raman spectra are usually constituted by a series of modes that correspond to specific vibrations of the lattice, each of one involving a particular group within the constituting atoms [58]. If the assignation of these modes and the corresponding vibration modes are well determined, then it is possible to elucidate with great detail in which environment of each particular constituent atom has been modified. The case of ultrafast laser inscribed waveguides in Neodymium doped LiNbO3 constitutes a good example of how l-Raman measurements could provide complementary information to that provided by micro-luminescence experiments [59]. In this case, l-Raman detected the presence of anisotropy in the lattice distortions that simultaneously leads to Nd3+ fluorescence shifts and refractive index increments. The use of l-Raman in the understanding of the structural changes at the origin of refractive index changes has not only been applied to crystals but also to glasses. In those cases, and due to the large broadening of Raman modes, the information about the presence of local structural modifications cannot be only obtained from induced changes in the vibration energies but also from the appearance of new vibration lines, that can help to understand the mechanisms governing the refractive index changes appearing after ultrafast laser waveguide inscription [60–63].

4. Examples of waveguide lasers Although it is a difficult task to give a complete review of the techniques and materials that have been used to fabricate lasers in waveguide configuration based on dielectric media, here some

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Fig. 10. Sequence of channel fabrication using standard photolithographic techniques, applied in waveguide fabrication methods based on ion diffusion.

representative examples are presented. All the techniques pursue to have a high index active region surrounded by low index media, and many physical or chemical methods have been developed for that purpose. Some of them have demonstrated to be a valid route for develop waveguide lasers, while other promising techniques are under intense research [64–66]. Usually the validity of a duo technique/material for developing waveguide laser is first demonstrated in planar configuration, and later patterning techniques are applied to define channel structures. This is performed by using standard photolithographic techniques compatible with the specific waveguide fabrication process, and Fig. 10 shows an example of such procedure when using ion diffusion methods. Sometimes it is useful to further deposit a top cladding layer onto the channel or planar waveguide to provide mechanical protection, and serves also to symmetrise the mode profile [67]. In the following we present some waveguide fabrication techniques which have already been successfully applied to develop waveguide lasers, including deposition techniques of crystalline and amorphous materials, and techniques that involve index modification of the substrate.

4.1. Deposition techniques Here we will discuss results of waveguide lasers fabricated by some deposition techniques applied to crystalline materials (MBE, LPE) and vitreous matrices (sol–gel), although other techniques have also proved to be an adequate route to develop waveguide lasers, such as direct diffusion bonding [68], pulsed laser deposition [69] or flame-hydrolysis deposition [70]. Molecular beam epitaxy (MBE) is a widely used deposition technique for epitaxial growth on oriented crystalline substrates with accurate control of thickness and composition. Initially developed for semiconductor devices, its use has been successfully reported in fabricating active optical waveguides in dielectric materials. The thermodynamical conditions imposed during MBE growth (low temperature and growth rate) can favorably modify the incorporation of rare-earth ions compared to bulk crystals, allowing a significant increase in the optically active doping level [71]. This kind of control can be useful to realize active waveguide components as it allows both refractive index and active layer engineering [72]. The MBE chamber (Fig. 11, left) used for epitaxial growth of active dielectric materials is equipped with several Knudsen effusion cells containing the species to be deposited. The effusion cells are calibrated separately by measuring the deposited layer thickness as function of the cell temperature. This calibration is used to control the thickness and the composition of the different layers constituting the final waveguide structure. Typical growing speed ranges from 0.1 to 1 lm/h. The high vacuum of the chamber allows

it to perform in situ reflection high-energy diffraction (RHEED) pattern to assure the crystalline quality of the deposited films. First experiments in MBE growth of crystalline active materials were performed on oriented CaF2 substrates, where epitaxial films of CaF2:Er and CaF2:Pr were grown with high crystallinity and good spectroscopic properties [72]. Besides the preservation of the spectroscopic characteristics of the incorporated rare-earths, the waveguiding properties of the films were established (Fig. 11, right). The first demonstration of waveguides lasers using dielectric crystals by MBE was reported in Nd3+-doped LaF3 thin film grown on a CaF2 substrates, using two separated effusion cells filled with LaF3 and NdF3. Owing to the refractive index difference between CaF2 and LaF3 crystals, the growth of doped-LaF3 layers on CaF2 substrates produced an optically active waveguide, without posttreatment requirements. The layers were found to be free of cracks and exhibited a featureless surface under optical microscopy. For the planar waveguide laser, the thickness of the doped layer was 3.6 lm, and a cladding layer 0.5 lm thick of pure CaF2 was grown on top of the Nd:LaF3 film, giving thus a guide with a symmetric step-index profile [73]. Channel waveguides were also fabricated by either ion-milling techniques or by using a BCB polymer overlay, with estimated losses of around 1 dB/cm [74]. The TM-laser emission at 1.064 lm showed a threshold of 17 mW, with a slope efficiency of 14%, and a maximum power output of 32 mW. Also, laser emission at 1.3 lm was reported using these technologies [75]. Liquid phase epitaxy (LPE) is a technology for depositing epitaxial layers onto high-quality polished, oriented and singlecrystalline substrates. The grown films, from high-temperature solutions, are formed by supersaturation or cooling, so that many principles, choice of solvents, and technological experience from growth of bulk crystals can be transferred to LPE. The epitaxial deposition can be done from diluted solutions at low temperatures, from concentrated solutions at higher temperature and even from melts near the high melting point. Successful LPE relies on uniform, clean and damage-free substrates with very low dislocation densities. One of the attractive features of LPE is its low capital equipment and operating costs, especially in comparison to other epitaxy technologies such as MBE. Although LPE has been a traditional technique to grow epitaxial layers of semiconductors materials, it can also be successfully applied to obtain high quality films on dielectric crystals for optical waveguide structures [76]. Due to its excellent laser characteristics, LPE films of Nd:YAG was one of the first systems to be investigated. Although multimode, a planar waveguide laser of LPE Nd:YAG on pure YAG substrates showed a nearly 40% of slope efficiency, with a threshold as low as 0.7 mW, owing the low losses of the planar waveguides (<0.05 dB/cm) [77]. Also, Tm-doped YAG planar waveguide grown by liquid phase epitaxy with good laser characteristics has been reported, with

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Fig. 11. Left: MBE chamber for epitaxial growth of fluoride crystals. Right: Dark mode spectra of a homoepitaxial CaF2/CaF2:Er waveguide, showing the guided modes, as well as the resonances (below the critical angle).

640 mW of threshold power and a P 68% of slope efficiency [78]. Single mode-rib-channel waveguide lasers in Nd:GGG prepared by liquid phase epitaxy, with losses down to 0.25 dB/cm, and patterned by ion-beam etching, demonstrated also very low thresholds and high slope efficiencies (50%) [79]. More recently, double tungstates based on the KY(WO4)2 family have received much attraction for developing compact active devices, and laser operation was demonstrated in a thin Yb:KYW layer grown by LPE technique [80] as well as a thin-disk Yb:KLu(WO4)2 laser [81]. CW planar waveguide lasers fabricated by LPE have been reported in Yb:KY(WO4)2, Tm:KY(WO4)2, and Gd/Lu co-doped Yb:KY(WO4)2 [82–84], as well as Q-switched operation of a diodepumped Yb3+:KY(WO4)2 [85]. Ar beam milling has been used to fabricate ridge channel waveguides on LPE grown Gd/Lu co-doped Yb:KY(WO4)2, with low power threshold and high slope efficiency, and up to 76 mW of output power at 1025 nm [86]. Recently, mirrorless laser action was generated on a buried rib waveguide of KY0.58Gd0.22Lu0.17Tm0.03(WO4)2 LPE grown on an Ar milled b-oriented KY(WO4)2 substrate, operating at 1.84 lm with a 13% of slope efficiency (Fig. 12) [87]. Table 1 presents data of some relevant waveguide lasers fabricated by deposition techniques on dielectric crystals, excluding ferroelectric crystals. The sol–gel method is among the simplest and cheapest techniques to fabricate doped optical waveguides, as liquid precursors are mixed and spin coated on a low-index substrate [88,89]. The main advantages of the sol–gel chemical processes are the high control of chemical homogeneity and purity, low temperature of

synthesis, easy incorporation of active rare-earth ions or dyes, and the possibility to cover large substrates areas by dip- or spin-coating techniques [90]. Also, it is suitable for the preparation of multicomponents systems since the use of different components at the molecular level can be easily achieved from solutions. Alkoxides and salts are typically involved as precursors, where their hydrolysis must be carefully controlled. Although high gain is relatively easy to achieve from dye doping, sol–gel waveguide laser dyes are unstable with time and operation. By contrast, rare-earth ions do not have such limitations, and the first RE-waveguide laser using sol–gel technology was demonstrated in 2008 [91]. The reported Nd-doped silica-hafnia sol–gel structures consisted of tapered rib waveguides deposited by spin-coating on silica-on-silicon substrates. As the sol–gel guiding layer thickness was only 600 nm, two integrated input/output grating coupler for the pump and signal light were implemented. The rib waveguides were defined by photolithography followed by CHF3 reactive ion etching (RIE), and ZEP-mediated electron beam lithography was carried out by the same RIE process in order to etch the gratings into the rib waveguide. Although losses of 1.65 dB/cm were measured, CW laser oscillation was observed, with a pump threshold of 20 mW and 1.2% of slope efficiency. Up to 2.4 mW of output power was achieved from a 3 cm long device. Also, using the sol–gel route, amorphous (glassy) Er: YAl3 (BO3)4 (Er:YAB) thin films have been reported [90]. The layers, spin-coated on silica substrates, exhibited high surface quality, and waveguide propagation losses of 0.8 dB/cm were measured

Fig. 12. Left: Buried channel waveguide of Tm-doped double tungstate fabricated by LPE. Right: laser characteristics at 1.84 lm [87].

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Table 1 Characteristics of some waveguide lasers on crystals, excluding ferroelectric crystals. Material GGG

Active ion Nd

3+

Fabrication technique

Regime

Operating wavelength (lm)

Losses (dB/cm)

Pump threshold (mW)

Slope eff. (%)

Ref.

LPE Ion Impl.

CW CW

1.062 1.06

0.25 1.6

10

50 27

[79] [140]

(Gd, Lu)2O3

Nd3+

PLD

LaF3

Nd3+

MBE

LiNdP4O12

Nd3+

Glass clad.

KY(WO4)2

Tm3+ Yb3+

LPE LPE

p

p

p

p

CW

1.079

4.8

0.8

0.5

[69]

CW

1.063 1.05 1.06 1.30

1 1

140 17

11 14

[73] [74]

CW

1.047

6.7

15

[141]

CW CW

1.965 1.025 1.025 1.028 1.040 1.037

75 80 18 5 40 100

11.5 68 82.3 62 62 14

[83] [82] [84] [86] [85] [130]

1.64

26

[142]

21

[67]

9 13 5.9 29 54 12 60 68 43 >77 30 13

[77] [118] [7] [121] [144] [139] [145] [136] [78] [68] [146] [147] [148]

38.7 19 75

[137] [149] [138]

CW and Q-switched CW

ULI

0.34 0.06 1.9

Sapphire

Ti4+

Laser Ablat.

Quasi – CW

0.802

SiO2

ZnO

Sol–gel

Pulsed random

0.388

Ta2O5

Nd3+

RF-Sputt.

CW

1.060

0.22

34

YAG

Nd3+

LPEp Ion Impl.

CW

<0.05 0.6

40 22 62 26 3 63 30 68 <40

1.8

<43 8  103 250

1 2 1.3

14 30 245

3+

Yb

3+

Yb /Nd YVO4

Nd3+ Yb3+

3+

0.5 (MW/cm2)

LPEp Diffusion bonding LPE p Diffusion bonding IE

CW CW CW

ULI Ion. Impl. ULI

CW CW CW

1.06 1.03 1.03

CW

(*)

Tm3+

1.8

0.7 1.064 1.338 1.064 1.064 1.064 1.064 1.064 2.012 2.020 1.03 and 1.05 1.032 1.03

p

ULI

*

0.08

CW

4 1.6 <1 0.6

[143]

Ceramic YAG. Planar waveguide.

p

at 633 nm after post treatment involving drying and annealing. Thus, the sol–gel method is envisaged as a promising route for waveguide laser technology. 4.2. Modification techniques Optical waveguides can also be fabricated by using chemical or physical methods to induce a local modification in the refractive index of the initial wafer. In general these transformations can involve ionic exchange from solutions, diffusion of some species from external sources, or even slight amorphization of the material using high energetic ions or short laser pulses. When the process is accompanied by a refractive index increase, then the modified region can directly sustain the optical confinement. If a decrease of the refractive index is obtained beneath the surface, an optical barrier is formed, which conveniently located, can also act as an optical waveguide. In the following the main modification techniques for waveguide fabrication which have demonstrated their ability to develop optical waveguide lasers on vitreous and crystalline substrates are summarized. The ion exchange (IE) can be considered as the most extended waveguide fabrication technique in glasses [92,93] due to its advantages over more complex technologies, which includes low propagation losses, ease of fabrication, compatibility with optical fibers and low material cost. The technique was firstly reported in the seventies using Tl+ exchange to fabricate waveguides in silicate glass containing sodium and potassium oxides [94]. The IE method is based in the exchange of monovalent cations of the glass, usually Na+ present in

the glass as Na2O, by other monovalent cations with different polarizabilities and sizes such as K+, Ag+, Tl+, Rb+, Cs+ or Li+. This cation exchange can be considered as a diffusion process, sometimes accelerated by the presence of an electric field, that produces a local increase in the refractive index of the glass [95,96]. The basic experimental set-up used to perform the ion exchange in glasses from molten salts is sketched in Fig. 13, left). The melt, consisting of the appropriate salt (source of the diffusing ions), is held in a crucible which is kept at a furnace with automatic temperature control. Nitrate salts have been extensively used as ionic sources because of their reasonable stability and low melting temperatures. The ion exchange takes place inside the crucible where the sample is immersed in the molten salt. The fabrication parameters that control the final graded refractive index profile are those associated with a diffusion process, that is, temperature and treatment time [95,96]. Although typical temperature values range from 210 to 450 °C and times from 0.5 to 74 h it must be taken into account that the ionic exchange is quite sensitive to the glass and the exchanged cation [92]. For instance, Tl+ and Ag+ produce a large surface index change (Dn  0.1) leading to multimode waveguides, thus diluted melts are needed to fabricate single-mode waveguides. On the opposite limit, K+ is easily incorporated in glasses but the index change is one order of magnitude lower (Dn  0.01), thus large diffusion times are required for suitable waveguiding. The compatibility with optical fibers can be improved by a second ionic exchange process. Now the monovalent cations, firstly exchanged, are substituted by Na+ cations giving rise to a buried

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Fig. 13. Left: Experimental set-up used to perform the IE process. Right: DBR waveguide laser array fabricated in Yb3+/Er3+ codoped phosphate glass [11].

Table 2 Characteristics of some waveguide lasers on glasses. Material

Active ion

Fabrication technique

Regime

Operating wavelength (lm)

Losses (dB/cm)

Pump threshold (mW)

Slope eff. (%)

Ref.

BK-7

Nd3+

IE

CW

0.8 0.1–1

Er3+ Er3+/Yb3

IE IE

Q-switch CW CW

1.058 1.060 1.060 1.540 1.544

0.5 0.2

7.5 5 35 150 5

6 42 15 0.55 2

[8] [4] [3] [16] [164]

Chalcogenide

Nd3+

ULI

CW

1.075

<0.5

40

17

[150]

Silica hafnia

Nd3+

Sol–gel

CW

1.062

1.6

20

1.2

[91]

Silicate

Nd3+

CW – Ring CW CW

1.60 1.064 1.020

0.013 0.17

110 360 50

15 5

[21] [151] [152]

p

Yb

IE ULI IE

PMMA

ZnO-nc

Spin-coating

Pulsed

0.590

Phosphate

Nd3+ Yb3+

IE IE ULI IE

CW and Q–switch CW-Tuneable CW CW-Tuneable CW CW-DBR CW

1.054 0.986–1.05 1.033 1.525–1.564 1.533–1.542 1.534 1.5 1.535 1.537 1.5

3+

Er3+/Yb3+

p

ULI

Mode-locked p

0.03 (MW/cm2) 1.2 0.1–0.2 0.1–0.2

0.4 3.1 0.4

60 18 120 23 14 135 120 230 639 450

[153] 4.5 67 17 28 5.7 13.9 21 3

[31] [11] [154] [11] [18] [93] [155] [156] [135] [157]

Planar waveguide.

waveguide. In this way the symmetry of the index profile is improved for fiber-waveguide coupling while simultaneously the scattering losses due to the proximity of the surface are reduced. Using this technique Nd3+-doped BK-7 laser waveguides have been reported [3]; the first step involves the K+–Na+ ion exchange, and the waveguide was buried by a second exchange by immersion in molten sodium nitrate. Laser oscillation in potassium ion-exchanged planar waveguides has been also demonstrated in Er3+-doped BK-7 by using optical cavities with dielectric mirrors [16]. More complex resonators (Fig. 13, right) have been also used in Yb3+-sensitized systems [11,18]. Such is the case of channel waveguide lasers based on Yb3+/Er3+-codoped phosphate glasses which have been fabricated by K+–Na+ [11] and Ag+–Na+ [18,93] ion exchange treatments. These and other relevant examples of IE in glasses have been included in Table 2 where the laser characteristics of waveguide lasers made in glasses are presented. Ion exchange techniques have also been applied to ferroelectric crystals such as LiNbO3 and LiTaO3 to produce waveguide lasers. In these ferroelectric crystals, the optical confinement is generated by the substitution of Li+ cations by protons. The technique is known as proton exchanged (PE), and it was firstly used in LiNbO3 crystals [97]. Several proton sources, organic or inorganic acids with a low boiling point and vapor pressure, can be used to prepare optical waveguides by this approach [98].

The presence of protons in the crystal structure of these ferroelectric crystals increases the extraordinary index, with a step-like profile, while the ordinary one is slightly reduced. Therefore, only light with the electric field parallel to the optical axis can be confined in PE-waveguides. In spite of this drawback it must be remarked that the first LiNbO3:Nd3+ laser in guiding configuration was demonstrated in PE-channel waveguides [99] and then in LiTaO3:Nd3+ [10]. There also exist other possibilities to produce ion exchange in these ferroelectric crystals, such as in the case of annealed proton exchange (APE) and reverse proton exchange (RPE), which allow the design of new refractive index profiles. APE techniques include an additional annealing step after the proton exchange process. This method is simple and has been used to produce stable graded index low-loss Fabry–Perot lasers in the extraordinary polarization, mode-locked and Q-switched waveguide lasers [2,22,23]. At variance with APE waveguides, RPE-waveguides consist of two different waveguides that confine orthogonal polarizations [100,101]. In the case of z-cut samples, the ordinary waveguide (TE propagation modes) is formed at the surface of the wafer while the extraordinary waveguide (TM propagation modes) is buried. This spatial separation also originates the separation of the optical fields that only partially overlap. In spite of this field separation it has been demonstrated laser action of Nd3+ ions at 1.08 lm, confined in the buried waveguide either for TE or TM-pumping [102].

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Table 3 Characteristics of some waveguide lasers on ferroelectric crystals. Material

Active ion

Fabrication technique

Regime

Operating wavelength (lm)

Losses (dB/cm)

Pump threshold (mW)

Slope eff. (%)

Ref.

LiNbO3

Nd3+

PE

CW

0.6

1.5 4.0 17 2.1 13 68 27 1.25 64 2.2 5–7 15 3 8 13 25 10.5 74 70 175

14 14.2

[99] [9] [144] [158] [159] [105] [106] [13] [14] [2] [22] [23] [102] [160] [161]

Tm3+

Ti in-diff.

AO – tuneable Q – switched CW

Yb3+

Zn in-diff. Ti in-diff.

CW CW

APE

CW

1.084 1.084 1.084 1.085 1.084 1.093 1.085 1.084 1.374 1.085 1.085 1.085 1.084 1.532 1.563 1.576 1.531 1.53 1.561 1.531 1.603 1.532 1.562 1.53–1.57 1.562 1.81 1.85 1.762 1.008 1.030 1.060 1.061

Nd3+

APE

CW

1.08

Ion Impl. Ti in-diff.

Er3+

p

CW CW

Zn in-diff.

CW

APE

Mode-locked Q – switched

RPE Ti in-diff.

CW CW

CW – DBR CW – DFB CW – Ring Mode-locked

LiTaO3

1 1.4 1 1 0.9 0.3–0.5

0.45 0.3 0.5 0.3 0.1 0.5 0.2 0.15

16 15 40 0.14 20 0.5 31 20–30 13 2.8 6 3 8.5 1.3 2

1

[105] [107] [17] [19] [20] [24] [27] [6] [26] [12]

1 16

[15] [162]

70 0.23 0.1 0.2 0.5 0.25

40 74 54 90 42 37 15

12 0.7 @ 1.561

40

0.003

[163]

2.9

33

[10]

In the case of pulsed lasers, the maximum power corresponds to the peak power. p Planar waveguide.

Although proton exchange techniques have succeeded in the fabrication of Nd3+ and Yb3+-based waveguide lasers, as it is summarized in Table 3. it has been proved that they are not adequate in Er3+-activated materials. This fact is due to the dramatic fluorescence quenching induced by the coupling to OH-phonons [103,104]. Thermal in-diffusion of transition metal ions has been established as a standard method in the fabrication of low-loss optical waveguides in LiNbO3. In particular, it has been extensively proved that the diffusion of Ti or Zn ions into this crystal follows the Fick’s law producing a modified region with a profile close to a Gaussian distribution that increases the ordinary and extraordinary refractive indices. Therefore, the resulting waveguides provide guiding in both TM- and TE-polarizations. An additional advantage is that these metal in-diffusion techniques preserve the spectroscopic properties of the active ions, this characteristic being essential in the construction of waveguide lasers. Ti-indiffusion represents the most common method to fabricate optical waveguides in rare-earth doped LiNbO3. Ti ions are diffused from a thin metal layer previously deposited on the wafer surface (between 30 and 60 nm thickness) or from a stripe (photolithographically defined) depending if a planar or channel waveguide is desired. The diffusion process itself consists of thermal treatment at temperatures ranging from 1030 °C to 1070 °C with a duration between 7.5 and 20 h. The Ti in-diffusion is usually performed in wet O2 atmosphere [105,106] to prevent Li out-diffusion from the substrate. An inherent problem of this kind of waveguides, which can be partially attenuated by using Mg-doped LiNbO3 substrates, is the instability at visible and near-infrared wavelengths as result of the photorefractive damage induced by the high power densities reached in a guided-wave structure. De-

spite of the photorefractive effect, Ti in-diffusion has been extensively used to fabricate integrated lasers in LiNbO3. In fact, a great variety of CW waveguide lasers operating at different wavelengths by using cavities of different geometries has been reported, for instance: doped with Nd3+ [105,106], Er3+ [105,17,19,107,20], and Tm3+ [12]. Also based on LiNbO3:Er3+, successful devices have been published such as mode-locked lasers [24,25], Q-switched lasers [26], and tuneable waveguide lasers [6]. The performance of these advanced waveguide lasers takes advantage of the excellent electrooptic and acoustooptic properties of the LiNbO3 crystal. The diffusion of Zn ions was initially proposed as an alternative method to produce waveguides with a higher optical damage threshold than Ti in-diffused waveguides, exploiting the capabilities of Zn ions to reduce the photorefractive effect in LiNbO3 [108]. Zn-diffused waveguides have been fabricated following either a similar procedure to Ti-diffusion [109] as well as from the vapor phase in a two step procedure [110]. The experimental set-up used in the Zn in-diffusion from the vapor phase is shown in Fig. 14. In this fabrication method, substrates were placed in a pipe reactor, where metallic zinc is deposited on the bottom of the tube. The system is fed with argon at a pressure of 550 mbar, and then the tube is heated at a temperature of 550 °C. This first step in Zn vapor atmosphere gives rise to a Zn-rich region at the sample surface [111,112]. Although this step gives rise to an index increase in the Zn-rich layer, the depth is too small to confine the optical radiation, besides that after this treatment the sample appears dark. Therefore, a second step is necessary, which consists on an annealing of the sample performed in open atmosphere at higher temperatures, typically in the range 800–900 °C. The annealing step has a twofold purpose. First, Zn ions diffuse further in the crystal, thus a deeper waveguide is obtained. On the other

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Fig. 14. Left: System for fabricating optical waveguides in LiNbO3 using Zn-diffusion from the vapor phase. Right: Photograph of a Zn in-diffused LiNbO3:Tm3+ channel waveguide laser.

hand, the sample recovers the initial transparency due to a re-oxidation of the matrix. In both techniques of Zn in-diffusion, metal or from the vapor phase, the temperatures involved in the thermal treatments (between 550 and 900 °C) are lower than for Ti in-diffusion avoiding the problems of Li out-diffusion. In the literature it is possible to find some CW waveguide lasers fabricated by Zn indiffusion in LiNbO3 doped with Nd3+ [13,14] and doped with Tm3+ involving ion-ion interactions [15] (Fig. 14, right). All of them, although pumped in the near infrared region (around 800 nm), have demonstrated a good temporal stability indicating that the photorefractive damage is highly reduced [113]. Ion implantation has demonstrated to be one of the most versatile routes for the fabrication of waveguides in optically active media. It is based on the refractive index changes caused by the interaction between accelerated ions and the atoms in a solid [114,115]. When an ion penetrates in a material it starts to lose energy until it is finally stopped. During this process the ions first partially transfer their energy through electronic interactions (at the electronic damage region, EDR) until they finally undergo nuclear collision against the constituent atoms (at the Nuclear Damage Region, NDR). The relative magnitude of the structural modifications, and hence of the induced refractive index changes, caused in both the EDR and NDR strongly depends of the irradiation conditions, such as implantation dose, ion mass and ion energy. The first waveguide lasers fabricated by light ion implantation were fabricated by achieving total implantation doses lying in the 1016–1017 ions/cm2 range. It was stated that in the waveguides of this type, the refractive index profile essentially matches the

profile of energy deposited by nuclear process. This is the case, for example, of proton implanted waveguide lasers fabricated in Neodymium doped YAG crystals [116]. Nowadays, the research on ion implanted laser waveguides is developed along three different lines: (i) Heavy ion implanted laser waveguides. In these cases, the fluences needed for the waveguide fabrication are remarkably reduced down to 1014 ions/cm2, thus reducing the manufacturing costs and time. Waveguide formation by heavy ion implantation has been demonstrated only in a few crystalline materials. Among all of them only laser emission has been demonstrated in Nd:YAG waveguides fabricated by carbon implantation [117,118]. The laser performance of these waveguides is summarized in Fig. 15. (ii) Swift ion implantation. Waveguide fabrication by swift ion (typically 0.5–1 MeV/amu) irradiation was firstly demonstrated in LiNbO3 crystals [119]. In this case, it has been confirmed that the electronic excitations (instead of the nuclear collisions) induced by the swift ions determine the refractive index changes (electronic interactions could even drive the original network to an amorphous state). Waveguide formation by swift ion irradiation was later demonstrated in laser gain crystals such as KGW and YAG [120,121]. The case of Nd:YAG is of special relevance since is the only system in which laser action has been demonstrated [121]. (iii) Proton Beam Written (PBW) waveguide lasers. In all of the previous described techniques ion irradiation was performed over a determined irradiation area thus yielding to the for-

Fig. 15. Left: Refractive index profile of a Carbon-implanted Nd:YAG crystal. Right: Laser curve at 1.06 lm obtained from the Nd:YAG planar waveguide under 808 nm pumping [118].

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Fig. 16. Left: Schematic diagram of the experimental set-up used for the fabrication of buried waveguides by the PBW technique. Middle: Optical transmission image of a PBW buried waveguide fabricated in a Nd:YAG crystal. Right: Laser spectrum and laser mode (inset) obtained from the Nd:YAG waveguide.

mation of 2D planar waveguides. The use of a focused ion (proton) beam overcomes these drawbacks so that direct fabrication of 3D waveguides becomes possible. Fig. 16, left) shows a schematic diagram of the waveguide fabrication process by PBW. Fig. 16, center) shows, as an example, the optical microscope transmission image of a PBW waveguide in a Nd:YAG crystal [122]. The fabricated waveguides showed low propagation losses as well as highly symmetrical propagation modes, which favors the input and output coupling into fibers. Fig. 16, right) shows the laser mode and spectrum corresponding to a PBW Nd:YAG laser waveguide operating under 808 nm pumping [123]. During the last years, Ultrafast Laser Inscription (ULI) has emerged as a powerful technique for the fabrication of mask-less channel waveguides [124]. This technique is based on the focusing of ultrafast laser pulses (duration in the femtosecond–picosecond range) inside the material to be micro-structured. ULI relies on the structural changes appearing after the complete relaxation of the excited electrons at focus. These changes may manifest themselves through a refractive index modification, which can be used to directly inscribe optical channel waveguides by the translation

of the irradiated sample (see Fig. 16, left). The nature, magnitude and origin of the refractive index modification depend on the inscription parameters: pulse energy, pulse duration, laser polarization, laser wavelength, scan speed, pulse repetition rate and focusing geometry, as well as on the particular optical response of the irradiated material. The mechanisms at the basis of the refractive index modifications have attracted significant attention over recent years, concluding that refractive index modification arises as the result of a complex combination of different phenomena such as lattice damage, stress-induced lattice distortions, compositional changes, and photochemical reactions [125–127]. The use of ULI for fabricating optical waveguides was firstly demonstrated in glasses [128], and since then, ULI channel waveguides have been fabricated in a great variety of materials (including laser glasses and crystals) [129,130]. The morphology and properties of the ULI waveguides depend on a great variety of parameters, and as consequence, a large variety of waveguides with different properties have been demonstrated up to now. As an example Fig. 17 shows several cross-sectional optical transmission images of different waveguides fabricated in different materials under very different writing conditions [131,59,132].

Fig. 17. Left: Schematic diagram of the ULI procedure used for waveguide fabrication. Right: Optical transmission cross-sectional images and corresponding propagation modes of different waveguides fabricated by ULI in different laser materials.

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materials will be used for further optical integration in order to achieve on-chip devices, which will combine multifunctional capabilities to enhance the scope of applications in integrated optics. Acknowledgements Work partially supported by Ministerio de Ciencia e Innovación (projects MAT2009-14102, MAT2010-16161 and TEC2010-21574C02-01) and Comunidad de Madrid (P2009/TIC-1476), Spain. Authors would like to thank Andrew Johnston for his cooperation in the English style revision. References

Fig. 18. Laser curve at 1.06 lm generated from a ULI Nd:YAG waveguide under 808 nm pumping [136].

The first ULI channel waveguide lasers were demonstrated in rare earth doped glasses, from which single mode laser light generation in the telecom band was reported [133,134]. Since then, the laser performance of rare earth doped glasses has been improved. By means of a controlled modulation of the writing pulses, a distributed feedback waveguide-Bragg grating was defined, which led to a monolithic waveguide laser design [135]. This technique has been successfully extended during the last years from glasses to laser crystals. An adequate choice of the writing parameters leads to low loss waveguides (below 1 dB/cm) in laser crystals, ensuring low threshold and efficient laser operation [126]. Up to know, efficient ULI crystal waveguide lasers have been reported in Nd:YAG, Yb:KGW, Yb:YAG, Nd:YVO and Nd:GdVO systems [136,130,137,138] (Fig. 18). Very recently, slope efficiencies as high as 60% have been demonstrated from a double-line ULI Nd:YAG waveguide laser, showing output powers in excess of 1 W [139]. 5. Conclusions In this review, the progress realized so far with dielectric materials for miniaturized integrated lasers has been summarized. There has been a large effort over the last years to combine the attributes of waveguide geometries with the optical material properties of dielectric media for the development of active waveguide devices. Waveguide structures on these materials have shown to be promising routes to develop miniature versions of solid state lasers, based mainly on rare earths as active centers. Several waveguide fabrication techniques have proven to be effective and reliable routes to fabricate active waveguides, both in planar and channel geometries. The techniques, either by film deposition or by index modification of the substrate, have demonstrated to produce waveguides with low propagation losses, while preserving the spectroscopic properties of the active centers. The efforts on the development of methods for the fabrication of waveguide on different materials have led to different types of waveguide lasers. Additionally, numerical techniques have been implemented to model both passive and active waveguide structures, which besides specific characterization techniques, constitute powerful tools for the rapid advance, optimization and simulation performance of these miniaturize devices. These facts have given rise to the demonstration of high performance waveguide lasers with low threshold and high slope efficiencies. Also, when combing with electrooptic or acoustooptic properties of the substrate, high versatile and functional integrated optical devices have been demonstrated. Therefore, it can be foreseen than in the future dielectric

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