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

We discuss a number of theoretical and experimental issues in quantum well lasers with emphasis on the basic behavior of the gain, the field spectrum, and the modulation dynamics It is revealed that the use of quantum well structures results in improvement of these properties and brings several new concepts to optical semiconductor devices

/pdf/quantum-well-lasers-gain-spectra-dynamics-3gftmp6esb.pdf

Quantum well lasers--Gain, spectra, dynamics

We investigate theoretically a number of important issues related to the performance of AlGaAs quantum well (QW) semiconductor lasers. These include a basic derivation of the laser gain, the linewidth enhancement factor α, and the differential gain constant in single and multiple QW structures. The results reveal the existence of gain saturation with current in structures with a small number of wells. They also point to a possible two-fold increase in modulation bandwidth and a ten-fold decrease in the spectral laser linewidth in a thin QW laser compared to a conventional double heterostructure laser.

/pdf/theory-of-gain-modulation-response-and-spectral-linewidth-in-52vc7uds64.pdf

Theory of gain, modulation response, and spectral linewidth in AlGaAs quantum well lasers

A detailed theoretical analysis of stability is presented for a semiconductor laser in an external cavity. The limits of stable operation are determined as a function of the external cavity parameters and the linewidth enhancement factor α. Instability is related to jumps of the laser frequency between external cavity modes (frequency bistability) or to feedback-induced intensity pulsations due to the carrier density dependence of the refractive index. The limit of bistability is derived from the steady-state solutions of the rate equations and the intensity pulsation limit is obtained from a small-signal analysis. This analysis also gives the location of zeros in the system determinant and the resulting FM noise spectrum. For practical applications we emphasize the determination of the stable tuning range for the phase in the external cavity and the classification of the possible types of instability for various feedback levels.

Stability analysis for a semiconductor laser in an external cavity

We calculate the relaxation oscillation corner frequency fr and the linewidth enhancement factor alpha for both a quantum well and a quantum wire semiconductor laser. A comparison of the results to those of a conventional double heterostructure device indicates that fr can be enhanced by 2× in the quantum well case and 3× in the quantum wire case while alpha is reduced in both cases.

/pdf/quantum-noise-and-dynamics-in-quantum-well-and-quantum-wire-wtpib3akpa.pdf

Quantum noise and dynamics in quantum well and quantum wire lasers

A simple model for the linewidth enhancement factor α and its frequency dependence in semiconductor lasers is presented. Calculations based on this model are in reasonable agreement with experimental results.

S. Margalit

Papers

On the linewidth enhancement factor α in semiconductor injection lasers