We review the history and current status of ion exchanged glass waveguide technology. The background of ion exchange in glass and key developments in the first years of research are briefly described. An overview of fabrication, characterization and modeling of waveguides is given and the most important waveguide devices and their applications are discussed. Ion exchanged waveguide technology has served as an available platform for studies of general waveguide properties, integrated optics structures and devices, as well as applications. It is also a commercial fabrication technology for both passive and active wave-guide components.
Highly nonlinear waveguides are essential components for all-optical signal processing. Many promising nonlinear
waveguides utilize the Kerr nonlinearity, the strength of which is determined not only by the material properties,
but also by geometrical factors, quantified by the waveguide's nonlinear effective area A<sub>eff</sub>. In an all-optical
switch, the switching threshold power is proportional to A<sub>eff</sub>, so optimization of the nonlinear waveguide is
equivalent to minimization of A<sub>eff</sub>. Recent studies have shown that dielectric slot waveguides can confine optical
energy far below the diffraction limit, with nonlinear effective areas considerably less than those attainable in
total internal reflection waveguides.
In this work, we instead consider the use of a gap plasmonic waveguide (GPW) for deep sub-wavelength optical
confinement. Using finite element methods, we compare optimized slot waveguides with GPWs of identical
geometry. We show that the GPW achieves a nonlinearity more than an order of magnitude superior to the
corresponding dielectric slot waveguide, and that a further optimization of the GPW is possible.
The Bragg reflection waveguide (BRW), or one-dimensional photonic crystal waveguide, has recently been proposed
for a wide spectrum of applications ranging from particle acceleration to nonlinear frequency conversion. In this work,
we conduct a thorough analytical investigation of the quarter-wave BRW, in which the transverse wavevector has a
phase thickness of &pgr;/2 in each layer of the resonant cladding. For this case, an analytical solution to the mode dispersion
equation is derived, and it is shown that the quarter-wave BRW is polarization degenerate, although the TE and TM
mode profiles differ significantly as the external Brewster angle condition in the cladding is approached. Analytical
expressions for waveguide properties such as the modal normalization constants, propagation loss, and overlap factors
between the mode and each waveguide layer are derived. Finally, a perturbation theory is developed to calculate
dispersion and tuning curves for the waveguide.
The Bragg reflection waveguide (BRW), or one-dimensional photonic bandgap waveguide, has recently received much
interest for applications such as nonlinear frequency conversion, mechanically tunable air-core filters, and electron
accelerators. One variation of this waveguide is the quarter-wave BRW (QtW-BRW), in which all cladding layers have
a phase thickness of π/2. This places the mode in the center of the cladding stopband, ensuring the strongest possible
confinement for a given pair of cladding materials. In addition, operating at the quarter-wave point permits the effective
index of the guided modes to be given by a simple closed-form expression.
For many applications of BRWs, the dispersion of effective index with frequency is of primary concern. In this
work, we use a perturbation approach to derive analytical expressions for the dispersion of a QtW-BRW, and compare
the results to numerical simulations to demonstrate accuracy. Several interesting properties of these waveguides are
developed. The birefringence of the guides changes sign at the quarter-wave point. For fundamental modes of even
symmetry, the first-order dispersion is always normal if the material dispersion is normal. It is shown that for certain
QtW-BRW designs, group index and group velocity dispersion (GVD) can be orders of magnitude higher than is the
case for their constituent materials, or on the other hand, very small or zero values of GVD can be attained. We will
conclude with a discussion of the applications of such waveguides.
Modeling the process of ion exchange in glass requires accurate knowledge of the self-diffusion coefficients of the incoming and outgoing ions. Furthermore, correlating the concentration profile of the incoming ions to a change in refractive index requires knowledge of the correlation coefficient. We present a method by which these three parameters can be quickly determined experimentally, using a genetic algorithm. Comparison with published data is presented.
In this invited paper, we will discuss the use of quantum dots as
nonlinear optical elements in fiber laser sources. Furthemore, a
review of the fabrication of the first low-loss (< 0.5 dB/cm)
ion-exchanged waveguides in a quantum-dot-doped glass will be
presented. We will discuss the coupling, propagation, absorption,
and scattering losses in these waveguides. The near-field mode
profile along with the refractive index profile of these waveguides will be presented. This PbS quantum-dot-doped glass was chosen due to its attractive optical gain and bleaching characteristics at wavelengths throughout the near infrared. This bleaching of the ground-state optical transition has been utilized for passive modelocking of a variety of lasers in the near infrared. In addition, we will discuss some of the potential integrated and fiber optics applications of our quantum-dot-doped waveguides.
Optical add/drop multiplexers (OADM) based on asymmetric y-branches and tilted gratings can be easily fabricated using ion exchange techniques and photosensitive glasses. These devices offer excellent operating characteristics. However, optimum OADM performance depends critically on the angle of the tilted grating. In this paper, results from fabrication and modeling are compared for the ion exchange process using four different angles of the tilted grating. The transmission spectra for the fabricated and simulated OADMs show an excellent agreement.
Glass waveguide devices fabricated by ion exchange have evolved to the point where conventional assumptions of waveguide symmetry and mutual independence are no longer valid. The modeling of ion-exchanged waveguide devices is far more complicated compared to, e.g., silica on Si waveguide devices. For example, during field-assisted ion exchange processes, the nonhomogeneity of ionic conductivity in the vicinity of the waveguide results in a time-dependent perturbation of the electric field. Previous studies have shown that the depth and vertical symmetry of buried waveguides are affected by the field perturbation.
In this work, we describe an advanced modeling tool for guided-wave devices based on ion-exchanged glass waveguides. The effect of field perturbation, due not only to the conductivity profile, but also to the proximity of adjacent waveguides or partial masking during a field-assisted burial are accounted for. A semivectorial finite difference method is then employed to determine the modal properties of the waveguide structures.
Optical communications networks require integrated photonic components with negligible polarization dependence, which typically means that the waveguides must feature very low birefringence. Recent studies have shown that waveguides with low birefringence can be obtained, e.g., by using silica on Si waveguides and by buried ion-exchanged glass waveguides. However, many integrated photonic circuits consist of waveguides with varying widths. Therefore, low birefringence is consequently required for waveguides having different widths. This is a difficult task for most waveguide fabrication technologies. In this paper we present theoretical and experimental results on waveguide birefringence for buried silver ion-exchanged glass waveguides. We show that the waveguide birefringence is on the order of 10-6 for waveguide mask opening widths ranging from 2 to 9 μm. The measured values are in good agreement with the values calculated with our modeling software for ion-exchanged glass waveguides. This unique feature of ion-exchanged waveguides may be of significant importance in a wide variety of integrated photonic circuits requiring polarization independent operation.