F. Grillot

Size influence on the propagation loss induced by sidewall roughness in ultrasmall SOI waveguides

Silicon-on-insulator (SOI) optical waveguides with high electromagnetic field confinement suffer from sidewall roughness which is responsible for strong scattering effects. This letter reports a numerical investigation on the size influence of ultrasmall SOI waveguides on the propagation loss due to sidewall roughness. It is shown that for a size smaller than 260 /spl times/ 260 nm the roughness-induced propagation loss decreases. As the optical mode confinement is reduced, a very low loss light coupling from and to a single-mode fiber can be achieved with propagation loss as low as 0.5 dB/cm for a 150 /spl times/ 150 nm cross-sectional waveguide.

Light injection in SOI microwaveguides using high-efficiency grating couplers

An experimental characterization of the grating couplers for sub-micrometer silicon-on-insulator (SOI) waveguides is presented. The grating couplers have been designed, realized, and characterized for the +1 diffraction order at an operating wavelength of 1.31 mum for TE polarization. At the resonant angle, a coupling efficiency higher than 55% has been measured. The angular coupling range and the wavelength tolerance have been evaluated to 3deg and 20 nm, respectively. The grating coupler is followed by a taper, and about 50% of the input power at 1.31 mum is coupled into sub-micrometer rib and strip SOI waveguides. The ration between light power decoupled toward the cladding and light power decoupled toward the substrate is about three

Gain Compression and Above-Threshold Linewidth Enhancement Factor in 1.3-

Quantum-dot (QD) lasers exhibit many useful properties such as low threshold current, temperature and feedback insensitivity, chirpless behavior, and low linewidth enhancement factor (alphaH-factor). Although many breakthroughs have been demonstrated, the maximum modulation bandwidth remains limited in QD devices, and a strong damping of the modulation response is usually observed pointing out the role of gain compression. This paper investigates the influence of the gain compression in a 1.3-mum InAs-GaAs QD laser and its consequences on the above-threshold alphaH-factor. A model is used to explain the dependence of the alphaH-factor with the injected current and is compared with AM/FM experiments. Finally, it is shown that the higher the maximum gain, the lower the effects of gain compression and the lower the alphaH-factor. This analysis can be useful for designing chirpless QD lasers with improved modulation bandwidth as well as for isolator-free transmission under direct modulation.

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Silicon-on-insulator (SOI) optical waveguides insure high electromagnetic field confinement but suffer both from sidewall roughness responsible of scattering effects and from leakage toward the silicon substrate. These two mechanisms are the main sources of loss in such optical waveguides. Considering the case of single-mode ultrasmall square SOI waveguides, propagation loss is calculated at telecommunication wavelengths taking into account these two loss contributions. Leakage toward the substrate and scattering effects strongly depend on the waveguide size as well as on the operating wavelength.

InAs–GaAs Quantum-Dot Lasers

Using the conventional rate equations describing an injection-locked system, a novel modulation response function is derived, which implicitly incorporates nonlinear gain through the free-running relaxation oscillation frequency and damping rate of the slave laser. In this paper, it is shown that the model presented can be used to extract the characteristic parameters of the coupled system from experimental data. The number of fitting parameters in the model is reduced by determining the fundamental slave parameters through the conventional free-running response function; these parameters are considered to be constant during the curve-fitting of the injection-locked system. Furthermore, in order to reduce the number of possible solutions generated during the least-squares-fitting process, the remaining fitting parameters are tightly constrained based on the physical limits of the coupled system. By reducing the number of unknown fitting parameters and constraining the remaining terms, a stronger confidence in the extracted parameters is achieved. Using a series of response curves measured from an injection-locked quantum dash laser, characteristic parameters of the system are extracted and validity of the model is examined. The verified model is used to analyze the impact of the linewidth enhancement factor on the characteristics of the response function in the microwave domain.

Propagation loss in single-mode ultrasmall square silicon-on-insulator optical waveguides

In this paper, a technique based on the use of a Mach-Zehnder (MZ) interferometer is proposed to evaluate chirp properties, as well as the linewidth enhancement factor (αH-factor) of optoelectronic devices. When the device is modulated, this experimental setup allows the extraction of the components response of amplitude modulation (AM) and frequency modulation (FM) that can be used to obtain the value of the αH-factor. As compared with other techniques, the proposed method gives also the sign of the αH-factor without requiring any fitting parameters and, thus, is a reliable tool, which can be used for the characterization of high-speed properties of semiconductor diode lasers and electroabsorption modulators. A comparison with the widely accepted fiber transfer function method is also performed with very good agreement.

Modeling the Injection-Locked Behavior of a Quantum Dash Semiconductor Laser

In this paper, a theoretical model is used to investigate the lasing spectrum properties of InAs-InP(113)B quantum dot (QD) lasers emitting at 1.55 mum. The numerical model is based on a multipopulation rate equations analysis. Calculations take into account the QD size dispersion as well as the temperature dependence through both the inhomogeneous and the homogeneous broadenings. This paper demonstrates that the model is capable of reproducing the spectral behavior of InAs-InP QD lasers. Especially, this study aims to highlight the transition of the lasing wavelength from the ground state (GS) to the excited state (ES). In order to understand how the QD laser turns on, calculated optical spectra are determined for different cavity lengths and compared to experimental ones. Unlike InAs-GaAs QD lasers emitting at 1.3 mum, it is shown that a continuous transition from the GS to the ES is exhibited because of the large inhomogeneous broadening comparable to the GS and ES lasing energy difference.

Measuring the Chirp and the Linewidth Enhancement Factor of Optoelectronic Devices with a Mach–Zehnder Interferometer

A dual-wavelength emission source is realized by asymmetrically pumping a two-section quantum-dot distributed feedback laser. It is found that under asymmetric bias conditions, the powers between the ground-state and excited-state modes of the two-section device can be equalized, which is mainly attributed to the unique carrier dynamics of the quantum-dot gain medium. As a result, a two-color emission with an 8-THz frequency difference is realized that has potential as a compact THz source. It is also shown that the combination of significant inhomogeneous broadening and excited-state coupled mode operation allows the manipulation of the quantum-dot states through external optical stabilization.

Spectral Analysis of 1.55-

The effect of external optical feedback on an InAs/GaAs quantum dot passively mode-locked laser is investigated. The rf linewidth narrows from 8 KHz in the free-running situation to a value as low as 350 Hz under relatively low feedback. The rf linewidth characterization under resonant feedback at a multiple of the laser cavity length validates the prediction of a previous numerical simulation. It is also confirmed that the integrated rms timing jitter varies as the square root of the rf linewidth. The results are promising for the development of compact, monolithic semiconductor mode-locked lasers as low noise optoelectronic oscillators.

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The pulse-to-pulse rms timing jitter of a 5.25-GHz quantum-dot (QD) two-section passively mode-locked laser is characterized through an all-microwave technique. The experimental phase noise spectra at different harmonics are in good agreement with previous diffusion-based theory. This theory is validated for a QD mode-locked laser device for the first time. This measurement technique provides a simple way to characterize the noise performance of a passively mode-locked laser. Furthermore, the average pulse-to-pulse rms timing jitter reduces from 295 to 32 fs/cycle via external optical feedback.