Effects In Semiconductor Lasers Research Articles

正弦波変調による半導体レーザの自己結合効果を利用した距離計

正弦波変調による半導体レーザの自己結合効果を利用した距離計

Theoretical analysis of noise-reduction effect in semiconductor lasers with help of self-sustained pulsation phenomena

The excess noise of semiconductor lasers has been reduced mostly with the help of the self-sustained pulsation in experiments. A theoretical analysis is given for mechanisms of the noise generation and the reduction method basing on the mode competition theory. Basic equations of these dynamics are given in the form of rate equations, where the nonlinear gain terms and Langevin noise sources are included. Effect of the optical feedback is introduced to give changes in the threshold gain level and by building of external cavity modes formed by the facet of the laser and the reflecting surface. The self-sustained pulsation phenomena are examined in the narrow stripe structure geometries having the saturable absorbing region beside the active region. The amount of noise is expressed relating to time averaged values of the photons and the carrier densities in laser, whose levels are modified by the pulsation. It is shown that the excess noise due to mode competition among the internal cavity modes is well suppressed by the self-sustained pulsation, while noise due to mode competition among the external cavity modes is mostly suppressed but sometimes not suppressed. More general discussions for the mechanisms are also given qualitatively by help of topological charts for dynamics. Three types of mode competitions are indicated. The first one is with a bistable state due to the symmetric profile of gain suppression for wavelength distribution. The second one is with an elliptic eddy motion of dynamics due to weak asymmetric property of the gain suppression. The third one is with a circular eddy motion of dynamics due to the strong asymmetric property of the gain suppression. Some difficulty in the noise reduction in the case of the strong asymmetric property of the gain suppression is mentioned.

On distributed microwave effects in semiconductor lasers and their practical implications

Recently, it has been shown that when a semiconductor laser is directly modulated at high frequencies, the modulation signals suffer substantial loss and phase shift propagating from the wire feed point along the length of the laser. It was suggested that these distributed microwave effects lead to a further bandwidth degradation in electrical current injection over the single-pole roll-off predicted by a lumped RC model. We show, however, that this degradation is significant only when the laser is driven directly by a voltage source. In contrast, when the laser is driven, commonly, through a 50 Ω transmission line the degradation is minimal, and the total injection current is still RC limited.

Sisyphus effect in semiconductor lasers with optical feedback

We identify the various physical mechanisms in low frequency fluctuations, which occur when a semiconductor laser is subject to moderate optical feedback while operating close to its solitary laser threshold. In attempting to reach the maximum gain mode, which often is stable, the system forms short mode-locked pulses. In between pulses mode-slipping can occur, generally in the direction of maximum gain. Inevitably, the trajectory passes too close to one of the many saddle points, which will take the system back to the solitary laser state.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A Modified BandwidthKFactor Relationship Due to Parasitic Effects in Semiconductor Lasers

Abstract A novel relationship between the maximum bandwidth and the bandwidth K factor has been obtained to describe the frequency response of laser diodes under the influence of parasitic effects. The relationship is applicable to the evaluation of the intrinsic bandwidth of quantum well lasers subject to carrier transport effects.

Optical feedback induced mesoscopic effects in semiconductor lasers

By quantum field theory techniques it is shown that even very tiny optical feedback in semiconductor lasers may trigger breakdown of the front-rear facet symmetry. Spatial homogeneity of the carrier distribution inside the active cavity is affected on a scale intermediate between the emitted wavelength and the device length. Finite volume effects set a threshold for the feedback field above which symmetry breakdown appears. Agreement with experimental observations is discussed.

Oscillation wavelength and laser structure dependence of nonlinear damping effect in semiconductor lasers

For the first time, the oscillation wavelength and the laser structure dependence of the K factor and of the nonlinear gain suppression coefficient ε in semiconductor lasers were systematically investigated. As a result, we suggest that the nonlinear gain effect can be explained by the spectral hole burning theory.

Theory of self-locking FM operation in semiconductor lasers

Gain saturation and multiwave mixing are shown to give rise to self-locking effects in semiconductor lasers resulting in FM operation. The analysis is shown to be in agreement with recently reported experimental observations of this phenomenon. >

The effects of electron-hole Coulomb interaction in semiconductor lasers

A recent many-body theory is applied to investigate the corrections to semiconductor laser gain and carrier-induced refractive index. The results show nonnegligible modifications to these quantities and demonstrate the importance of band-gap renormalization and Coulomb enhancement. The many-body Coulomb corrections also result in a different prediction of filamentation effects in semiconductor lasers.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Theory of hot carrier effects on nonlinear gain in GaAs-GaAlAs lasers and amplifiers

A concise and straightforward model of nonlinear grain based on the carrier heating effect in semiconductor lasers is presented. The problem is formulated using the density matrix approach and includes a priori the effect of free-carrier absorption. Coupled field-medium equations involving photon densities, carrier densities, and carrier temperatures are derived using the results of the density matrix method. The propagation of ultrashort pulses in laser amplifiers is studied and a qualitatively new model along with results on the transient gain recovery dynamics are presented. The model accounts for the wavelength dependence of the asymmetric part of the nonlinear gain observed in direct mixing experiments observed in semiconductor lasers. >

Simulation of inhomogeneous broadening and mode-beating effects in semiconductor lasers

A model is proposed in which the conduction band is discretised into energy intervals corresponding to longitudinal-mode separations for the simulation of inhomogeneous broadening and mode beating effects in semiconductor lasers. Four first-order relaxation processes are proposed to take account of intraband relaxations, and mode beating is included as a separate nonlinear gain term in the rate equations. In addition to the usual results of spectral-hole burning, single longitudinal-mode stabilisation and increased damping of relaxation oscillations, the model also accounts for the excitation of nonadjacent longitudinal modes experimentally observed at high bias in narrow planar-stripe lasers and the temperature dependence of spectral behaviour in CSP lasers.

Saturation effects in semiconductor lasers

This paper describes a theory for a semiconductor active medium interacting with a laser field. In a semiconductor laser, the charge carrier transitions are inhomogenously broadened, and electron-electron and electron-phonon collisions tend to dephase the laser transitions and maintain thermal equilibrium among the carriers. These properties cause semiconductor lasers to frequency tune as though they are inhomogeneously broadened and to saturate as though they are homogeneously broadened. A theory that contains these two aspects of semiconductor laser behavior is presented. From it, we are able to calculate the loaded gain, efficiency, intensity, and carrier-induced refractive index of a semiconductor active medium.

Longitudinal analysis of semiconductor lasers with low reflectivity facets

An analysis is made of longitudinal effects in semiconductor lasers with low facet reflectivities. For this purpose, a self-consistent model is used based on the beam propagation method, which takes into account both the lateral and longitudinal dimension. The calculations show that longitudinal effects have a significant influence on the output fields in the laser.

Minimizing pattern effects in semiconductor lasers at high rate pulse modulation

This paper deals with analytical and experimental work related to modulation of a semiconductor laser used in high bit rate communication. The approach is based upon minimizing the charge storage effect by a proper choice of the area of the modulating pulses and the bias current. The concept of using additional current pulses to probe for variations in electron density between pulses is investigated. The primary limitation on bit rate is found to be the ability to generate laser drive pulses free of ringing or similar transients. This and the 300 ps pulsewidth, an experimental constraint, limit the bit rate to about 1 Gbit/s. However, by using this approach it appears that bit rates considerably higher than 2 Gbits/s could be reached with sufficiently accurate control of drive pulse shape. It is found that the laser bias and the current pulse area had to be controlled within 1 percent and 10 percent, respectively.

Detailed calculations of transient effects in semiconductor injection lasers

Detailed calculations have been carried out of time delay and Q -switching effects in semiconductor lasers using a model described previously, incorporating saturable absorption and loss of optical confinement. The model has been extended to include the high-current limit on the Q -switching region and also the wavelength dependence of threshold current in grating-controlled lasers. In addition, the full rate equations for the electron concentrations in the lasing and absorbing states and for the photon concentration in the lasing mode have been solved by integration in the time domain using a Runge-Kutta numerical procedure. The results illustrate in a relatively simple form the complicated processes occurring during long time delays, abnormal time delays, and Q -switching.

New theory of internal Q switching in semiconductor lasers

By considering optical-confinement effects in semiconductor lasers, a new theory is presented of transient phenomena in these devices. It is shown that a partial loss of confinement at high injection levels, when combined with the effects of saturable absorption, can satisfactorily account for Q switching.