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

Fibre-optic communications systems have traditionally carried data using binary (on-off) encoding of the light amplitude. However, next-generation systems will use both the amplitude and phase of the optical carrier to achieve higher spectral efficiencies and thus higher overall data capacities(1,2). Although this approach requires highly complex transmitters and receivers, the increased capacity and many further practical benefits that accrue from a full knowledge of the amplitude and phase of the optical field(3) more than outweigh this additional hardware complexity and can greatly simplify optical network design. However, use of the complex optical field gives rise to a new dominant limitation to system performance-nonlinear phase noise(4,5). Developing a device to remove this noise is therefore of great technical importance. Here, we report the development of the first practical ('black-box') all-optical regenerator capable of removing both phase and amplitude noise from binary phase-encoded optical communications signals.

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All-optical phase and amplitude regenerator for next-generation telecommunications systems

We present a theoretical and experimental investigation of injection locking of semiconductor lasers. The theoretical analysis takes into account the dependence of refractive index on the carrier density expressed by the linewidth enhancement factor α. Locking conditions and dynamic stability are analyzed. The nonzero value of α results in an increased locking bandwidth, where only part of the range corresponds to a dynamically stable state. Asymmetric characteristics are obtained for the locked power and phase as a function of frequency detuning between the master and slave laser. Outside the stable range, light injection gives rise to beat phenomena and intensity pulsations. The theoretical results were confirmed by experiments on 830 nm CSP lasers and 1.3 μm BH lasers. The experiments include the first measurements of locking bandwidth characteristics reported for 1.3 μm lasers. Power spectra are recorded under locked and near-locked conditions and compared with theory. The 1.3 μm lasers are found to have a better dynamic stability than 830 nm lasers. Even so, the stability problems may exclude the particular application of injection locking where phase modulation is generated for coherent transmission.

Locking conditions and stability properties for a semiconductor laser with external light injection

The dynamical complexity of optically injected semiconductor lasers

The subject of phase noise in semiconductor lasers is reviewed. The description of noise in lasers and those aspects of phase noise that are relevant to optical communications are emphasized. The topics covered include: Langevin forces; laser linewidth above threshold and below threshold; line structure due to relaxation oscillations; phase fluctuations; line narrowing by a passive cavity section and by external feedback; coherence collapse due to optical feedback; the shot noise limits of several schemes of coherent optical communication, and the linewidth required to approach these ideal limits.

Phase noise in semiconductor lasers

An expression is derived for the FM noise spectrum of an injection-locked semiconductor laser. For increasing injection level the FM noise spectrum and the lineshape change gradually from those of the slave laser to those of the master laser. The ultimate linewidth reduction is obtained if the locking bandwidth is large enough to accommodate the frequency fluctuations and if the detuning is controlled carefully.

FM noise suppression and linewidth reduction in an injection-locked semiconductor laser

Light intensity pulsations in an injection locked semiconductor laser

We present an analytical expression for the FM noise spectrum of an injection-locked semiconductor laser, taking the α-parameter into account. Under the phase-matched condition the FM noise is shown to be suppressed in an effective bandwidth equal to the locking half-width ΔωL0 for α=0. The effective bandwidth can be made larger by increasing the phase delay of the slave laser, but at the expense of a higher resonance peak.

Effective bandwidth for FM noise suppression in an injection-locked semiconductor laser

140 and 70 Mbit/s FSK heterodyne system experiments are reported using single-filter detection with transmitter laser linewidths of 30 and 60 MHz. The system requirements that must be satisfied to avoid large performance penalties are considered.

F. Mogensen

Papers

Locking conditions and stability properties for a semiconductor laser with external light injection

FM noise suppression and linewidth reduction in an injection-locked semiconductor laser

Light intensity pulsations in an injection locked semiconductor laser

Effective bandwidth for FM noise suppression in an injection-locked semiconductor laser

FSK heterodyne system experiments at 1.5 μm using a DFB laser transmitter