The objective of this paper is to present an overview of topics related to one of the fundamental parameters for semiconductor lasers-the linewidth broadening factor α that describes the coupling between carrier-concentration-induced variations of real and imaginary parts of susceptibility. After introducing the definition of α and discussing its dependence on carrier concentration, photon energy, and temperature, we give a short historical summary on how the concept of α evolved over the past two decades. This is followed by a discussion of α dependence on device structure in gain-guided and subdimensional lasers (quantum wells and quantum wires). The bulk of the paper is devoted to a detailed review of laser properties influenced by α and of associated methods of estimating the value of α. Results of measurements reported to date are collected and the most reliable methods are indicated.

Linewidth broadening factor in semiconductor lasers--An overview

Polarization-maintaining fibers and their applications are reviewed. The classification of high-birefringent fibers and low-birefringent fibers and their fabrication methods and characteristics are discussed in Section II. Analytical methods and numerical methods for fiber design on the birefringence are presented in Section III. Degradation factors of polarization maintenance expressed as crosstalk or mode-coupling parameters caused by internal origins such as structural imperfections, wavelength, and nonlinear effects, and by external origins such as temperature fluctuations, mechanical perturbations, and electromagnetic effects, are discussed in Section IV. Characterization methods on beat length, mode coupling, stress distribution, and mechanical strength are presented in Section V. Applications to the fiber devices and nonlinear effects, and splicing methods for the polarization-maintaining fibers are described in Sections VI and VII.

Polarization-maintaining fibers and their applications

Line broadening up to 25 GHz in a single-mode semiconductor laser with relatively strong optical feedback is reported and theoretically analyzed. Measurements of the coherence function were performed using a Michelson interferometer and demonstrate that the coherence length decreases by a factor 1000 (to approximately 10 mm) due to optical feedback. A self-consistent theoretical description is given, which is based on the view that coherence collapse is maintained due to optical-feedback-delay effects, in which quantum fluctuations play no role of importance. A connection with recently suggested chaotic behavior is made. The theoretical results obtained are in good qualitative and reasonable quantitative agreement with measurements.

Coherence collapse in single-mode semiconductor lasers due to optical feedback

The dynamical complexity of optically injected semiconductor lasers

To meet the increasing demand for higher capacity in optical communications, signal transmission at higher modulation rates and over a broader wavelength range will be required. Signal degradation in the optical channel caused by dispersion, nonlinearity and noise becomes a critical issue as data rates increase. Thus, it is highly desirable to develop broadband, high-speed regeneration devices1. Recent advances in silicon-on-insulator photonic devices offer the potential for highly integrated, robust opto–electronic architectures, and optical processes such as amplification2,3,4, wavelength conversion5,6,7 and amplitude modulation8,9 have already been demonstrated in such structures. In this work, we demonstrate two regeneration schemes using low-power four-wave mixing in a silicon nanowaveguide and compensate for the effects of poor extinction ratio, dispersive broadening and timing jitter. This capability further expands the range of optical functions that can be incorporated into silicon-compatible photonic devices offering a broadband and integrated solution for regeneration.

Signal regeneration using low-power four-wave mixing on silicon chip

Experimental results concerning the study of phase noise in single-mode semiconductor lasers are reported, which show a strict connection between phase and intensity noise. In particular, phase-noise spectrum is found to present a sharp peak at the same peak frequency of intensity-noise spectrum, a fact which is proven to be responsible for the appearance of satellite peaks in the emission line shape. Direct measurements of the line shape, performed by means of a Fabry-Perot interferometer, are in agreement with the line shape evaluated by using phase-noise spectrum measurements.

Phase noise and spectral line shape in semiconductor lasers

Injection locking properties of distributed feedback semiconductor lasers are studied systematically. Due to the high side mode suppression, these devices show different locking properties when compared to lasers with Fabry-Perot structures. The main result is the identification of four regimes for different injection levels. In particular, a symmetrical locking band at low optical injection level is confirmed. The presence of this symmetrical band can be exploited in some applications. As examples, the measurement of the linewidth enhancement factor and the phase-shift-keying modulation capability are reported. >

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Injection locking in distributed feedback semiconductor lasers

The traveling-wave equations of four-wave mixing in semiconductor amplifiers are solved analytically including saturation of the amplifier. Excellent agreement is obtained with numerical solutions of the traveling-wave equations. The analytical model is used to analyze a highly nondegenerate four-wave mixing experiment in a bulk semiconductor amplifier. >

Four-wave mixing in traveling-wave semiconductor amplifiers

We derive analytical expressions for the power spectral densities of intensity and frequency noise of single mode semiconductor lasers in the presence of an optical feedback. By explicitly taking into account the spontaneous emission processes into the laser mode, we obtain a behavior which, at high frequencies significantly differs from the usual one, and is in good agreement with recent experimental results. In particular, we are able to show in which way the intensity and frequency noise, besides being influenced by the external cavity length, are affected by the presence of the well known resonant peaks in the noise spectra of the solitary laser and how a substantial lowering and flattening in the noise spectra can be obtained with external cavity round trip times shorter than the inverse of the peak resonant frequency. We also present experimental results in good agreement with the theoretical ones.

Theory of noise in semiconductor lasers in the presence of optical feedback

All optical frequency converters are key devices for new-conception optical networks A theoretical model is presented to evaluate performances of frequencies converters based on highly nondegenerate four-wave mixing (FWM) in semiconductor amplifiers Conversion efficiency and noise performances are evaluated in generic saturation conditions and compared with experimental results >

Paolo Spano

Papers

Phase noise and spectral line shape in semiconductor lasers

Injection locking in distributed feedback semiconductor lasers

Four-wave mixing in traveling-wave semiconductor amplifiers

Theory of noise in semiconductor lasers in the presence of optical feedback

Efficiency and noise performance of wavelength converters based on FWM in semiconductor optical amplifiers