All-optical modulation at 1.3 μm wavelength in the inGaAsP/InP system

All-optical, normal-to-surface modulation in InGaAsP epitaxial layers, lattice matched to InP, is investigated. Close to the gap wavelength of 1.3 m a transmission increase under optical excitation is observed. A modulation depth of 34% is achieved for 0.8 mW pump power at 790 nm wavelength. The modulation frequency is limited by excess carrier lifetime. A 3 dB frequency of 80 MHz is achieved with a 10 dB decrease at 400 MHz. A lateral electric field enlarges the bandwidth but decreases the modulation depth. For weak excitation the experiments are well described in terms of direct electronic transitions between parabolic bands or in terms of simple band filling. The devices are well suited for parallel optical data processing.     

Electromagnetic propagation at interfaces and in waveguides in uniaxial crystals

The use of surface-impedance and surface-admittance concepts for analyzing reflection and refraction at an isotropic dielectric interface (first developed about 1938) is extended to include an interface between uniaxial birefringent dielectrics. Total internal reflection and the polarizing (Brewster) angle at an anisotropic interface are shown to be naturally explainable in terms of surface impedance (for TM polarization) and surface admittance (for TE polarization). The allowable modes in an integrated optical uniaxial asymmetric dielectric slab waveguide are also shown to be directly obtainable using the surface impedance/admittance approach. Numerical examples are presented.     

Second harmonic generation in planar optical waveguides

We study optical second harmonic generation (SHG) in planar waveguide structures composed of several layers with different dielectric constants. We develop a general formalism for the calculation of mode generation by a planar antenna embedded in the waveguide. As an application we consider a monolayer of high second-order susceptibility adsorbed at the interface between two layers of the waveguide structure. Periodic modulation of the nonlinear susceptibility allows phase matching leading to dramatically enhanced second harmonic intensities. We investigate the SHG-efficiency of various experimentally realizable geometries.     

Ti:LiNbO3 optical waveguides

Various experiments on Ti diffused optical waveguides in LiNbO₃ have been carried out in order to determine precisely the character of the diffusion process. The required guide parameters and the effective mode indices could be controlled by adjusting only the diffusion time under fixed temperature and film thickness. Therefore the dependence of the guide characteristics on the diffusion time has been investigated in detail. On the basis of the data obtained, a two-stage diffusion model is proposed. In the first stage, the Ti diffusant profile is described by a erfc-function, and the second stage is characterized by a modified Gaussian form.     

Incoherent optical switching of semiconductor resonator solitons

We demonstrate experimentally the bistable nature of the bright resonator solitons in a semiconductor microresonator with mixed absorptive/defocusing nonlinearity and show that they can be written and erased by incoherent local optical injection. Received: 22 February 2001 / Published online: 27 April 2001     

Nonlinear optical studies of surfaces

A brief overview on second-harmonic generation and sum-frequency generation as surface analytical tools is given with a discussion on the intrinsic limitations of the techniques often encountered in the applications. The possibly of using nonlinear optical processes for surface studies has attracted increasing attention in recent years. Optical Second-Harmonic Generation (SHG) and Sum-Frequency Generation (SFG), in particular, have been well accepted as viable surface probes [1]. They have many advantages over the conventional techniques. By nature, they are highly surface-specific and has a submonolayer sensitivity. As coherent optical processes, they are capable of in-situ probing of surface in hostile environment as well as applicable to all interfaces accessible by light. With ultrafast pump-laser pulses, they can be employed to study surface-dynamic processes with a subpicosecond time resolution. These advantages have opened the door to many exciting research opportunities in surface science and technology. This paper gives a brief overview of this fast-growing new area of research.Paper presented at the 129th WE-Heraeus-Seminar on Surface Studies by Nonlinear Laser Spectroscopies, Kassel, Germany, May 30 to June 1, 1994     

Impact of misfit dislocations on wavefront distortion in Si/SiGe/Si optical waveguides

Silicon-rich SiGe alloys represent a promising platform for the development of large-area single-mode optical waveguides to be integrated in silicon-based optical circuits. We find that SiGe layers epitaxially grown on Si successfully guide radiation with a 1.55 μm wavelength, but, beyond a critical core thickness, their optical properties are strongly affected by the clustering of misfit dislocations at the interface between Si and SiGe, leading to a significant perturbation of the local refractive index. Transmission electron microscopy and micro-Raman spectroscopy, together with finite-element simulations, provide a complete analysis of the impact of dislocations on optical propagation.     

Second-order optical non-linearity of proton exchanged lithium tantalate waveguides

A detailed correlation between the fabrication conditions, crystallographic phase state of H_xLi_1-xTaO₃ waveguides and second-order optical non-linearity has been investigated by using reflected SHG measurements from the polished waveguide end face. The non-linearity, strongly reduced after the initial proton exchange, is found to be restored and even increased after annealing. However, this apparent increase in the non-linearity is accompanied by a strong degradation of the quality of the SHG reflected beam in the region of the initial as-exchanged waveguide due to beam scattering. The high temperature proton exchange technique has been shown to produce high-quality α-phase waveguides with essentially undegraded non-linear optical properties. There is no phase transition when the α-phase waveguides are fabricated by direct exchange. This phase presents the same crystalline structure as that of LiTaO₃ and maintains the excellent non-linear properties of the bulk material. The results obtained are important for the design, fabrication and optimization of guided-wave non-linear optical devices in LiTaO₃. Received: 21 May 2001 / Published online: 23 October 2001     

Low-loss, compact waveguiding with TE mode in metal/dielectric waveguides for planar lightwave circuit

We propose a low-loss metal/dielectric waveguide for compact planar lightwave circuit. The basic waveguide structure is a metal-defined high-index-contrast strip waveguide based on silicon/silica. As the guide is designed for TE single mode waveguiding, extremely low propagation loss (e.g. <0.04 dB/cm), very low bend loss (e.g. 0.0043 dB/90°-turn) and small waveguide pitch of zero-crosstalk are theoretically achievable, and can be further improved by compromising with component size and density. Examples of multi-bends and device integration are demonstrated with numerical simulations. The proposal is compatible with silicon technology and appealing for development of silicon-based planar lightwave circuit.     

Tapered microstructured fibers for efficient coupling to optical waveguides: a numerical study

We report the possibility of using tapered microstructured fibers to improve the coupling efficiency from a standard single-mode fiber to a photonic crystal waveguide. The tapered microstructured fiber allows for the reduction of the mode mismatch between the output of the standard fiber and the input of the waveguide while maintaining single-mode guidance, which results in an enhanced coupling efficiency. Numerical simulations are conducted in order to optimize the cross section of the microstructured fiber as well as the taper profile. An improvement of more than 4 dB is obtained compared to non-tapered fibers. For further improvement, an elliptical-core tapered microstructured fiber is analyzed. The effect of misalignment between the tapered microstructured fiber and the waveguide is also studied.     

Strong optical non-linearities for efficient all-optical switching in GaAlAs waveguides

The light intensity transmission of GaAlAs strip waveguides is sensitively dependent on the strength of the electric field inside the waveguide, when the photon energy is close to the band gap (Franz-Keldysh effect). In a waveguide embedded in a pn-junction the transmitted light itself induces the field changes through the photoelectric effect. This photo-induced Franz-Keldysh effect causes a non-linear intensity transmission of the waveguide. Light power levels far below 1 mW are sufficient to give strong non-linearities. Possible application schemes for modulation and all-optical switching in integrated optics and optoelectronics are discussed.     

Femtosecond Kerr ellipsometry in polydiacetylene solutions: Two-photon effects

We describe a pump-probe femtosecond Kerr experiment in polydiacetylene solutions (4BCMU, yellow form). The Kerr ellipsometry technique permits a separate determination of the real and of the imaginary part of anisotropic index changes. The wavelength (620 nm) of the pump pulses lies in the transparency region of polydiacetylene while the probe pulses have a broad white spectrum. The largest contribution to the Kerr signal is a quasi-resonant stimulated anti-Stokes Raman absorption. A flat two-photon transition spectrum with 4×10^–3 cm/MW is observed close to 4 eV. The two-photon excited states have a life time close to 12 ps and they present a broad absorption band in the 2 eV range.     

Tunable optical switch based on two orthogonal line defect waveguides

The transmission characteristics of a 2-dimensional (2D) square lattice photonic crystal waveguide (PCW) with an additional perpendicular line defect are investigated by the plane wave expansion method (PWM) and the finite difference time domain (FDTD) method. From simulation results and theoretical analysis, it is found that changing the refractive index of the dielectric columns in the additional line defect can effectively adjust the light transmission through the PCW. The working mechanism can be boiled down to either the excitation of a single defect mode or the contra-directional coupling of the two defect modes. And this mechanism can be used to design other tunable optical devices.     

CdTe epilayers for uses in optical waveguides

CdTe epilayers have been grown by vapor phase epitaxy (VPE) on glass, MgO, sapphire, LiNbO₃ and mica substrates. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) studies show the good structural quality of the epilayers. In these epilayers, a few optical modes were excited with a 1.33-μm laser. The measured propagation losses were in the range between 5 dB/cm and less than 0.5 dB/cm. From dark-mode m-lines, the epilayer thickness was found to be in the 1–3 μm range, in good accord with that obtained by SEM measurements. The refractive index obtained from the fitting is also in good accord with that of bulk CdTe. Received: 7 October 1999 / Accepted: 13 March 2000 / Published online: 5 July 2000     

Significant Enhancement of Unidirectional Transmission in Asymmetrically Confined Photonic Crystal Defect Pairs

By carrying out the two ideas of asymmetrical confinement and asymmetrical response into the photonic crystal (PC) structures that contain two or more nonlinear defects, we find that significantly unidirectional transmission can beachieved while the transmission for the positive launch direction maintains at large values. Our analyses are supported by the simulation results based on the finite-difference time-domain technique.     

Correct determination of net gain in Er-doped optical waveguide amplifier from pump-on/off measurement

Starting with evolution equation of signal optical power in an Er-doped channel waveguide, rigorous theoretical expressions used for correct determination of net gain from signal enhancement measured by using pump-on/off method are derived. These expressions allow to clarify the argument on relationships between net gain, internal gain and signal enhancement in some earlier literatures. Physical implications of net gain, internal gain and signal enhancement expressions are analyzed. Standardized definitions for the three physical quantities are proposed. The definition for net gain is proposed on the basis of practical application of an EDWA in optical fiber communication and the definitions for the latter two are suggested based on the physical implication analysis of their expressions.     

Second-harmonic generation in Zn-diffused periodically poled LiNbO3 channel waveguides

In this work second-harmonic generation by quasi-phase matching (QPM) in Zn-diffused periodically poled lithium niobate channel waveguides is presented. A stable TM?TE conversion by QPM has been found. The results are in good accordance with theoretical estimations obtained by the phase-matching condition, either for the polarisation character of the second-harmonic wave as well as for the spectral range, taking into account the periodicity of the domains. Received: 16 May 2001 / Revised version: 7 September 2001 / Published online: 30 October 2001     

Temperature distribution in a film heated with a laser spot: Theory and measurement

For the measurement of the thermal capacity and the thermal conductivity of films, the thermal excitation of the sample is commonly performed by the absorption of light. This results in a spatial and temporal temperature distribution within the film. With a variety of methods static or dynamic temperature recordings are performed.Two problems with these methods are discussed, the calculation of the temperature distribution in the film and the measurement of the mean surface temperature of the film. An analytical solution of the heat conduction problem for a cylindrical geometry with any radial distribution of the absorbed light is given. Resistive bolometers are introduced for the measurement of the mean surface temperature of the film within a circular area. Experiments with a 25 m thick PVDF film give excellent agreement with the theoretical calculations within the modulation frequency range 10^–3 Hz to 10³ Hz, thus allowing a determination of various thermal parameters of the investigated film.