Semiconductor Lasers with Improved Lasing Characteristics

We consider research results on the effect of the active medium temperature, which varies with the operation of semiconductor and solid-state (DPSS) lasers of the visible range, on the output spectral characteristics. The paper presents the study results of the spectral-optical radiation parameters of semiconductor lasers, their coherence lengths, and the dependence of the spectral maximum position on temperature. This study is necessary for selection of the most optimal laser, which in the future is planned to be used in medical ophthalmologic diagnostics. The experiment was carried out by solid-state (DPSS) and semiconductor laser modules based on a laser diode. Spectral dependences on the active medium temperature of lasers were obtained, ranging from 300 to 370 K. The spectra were recorded with the use of an automated spectral complex based on the MDR-23 monochromator. We show determination possibility of the internal stabilization damage of laser modules without mechanical intervention but only by applying their spectral characteristics. The obtained data give the possibility to take into account the temperature characteristics and perform further optimization for parameters of such lasers

In order to improve the output spectral characteristics of 980 nm semiconductor laser,the expression of transmittivity of dual fiber Bragg grating was deduced by the transfer matrix theory.The influence of fiber Bragg grating length and cavity length on output spectrum was conducted by simulation,the result shows that the fiber Bragg grating length′s influence on output spectrum is greater than fiber Bragg grating cavity length′s,and the width of output spectrum can be reduced by lengthening the fiber Bragg grating length.The dual fiber Bragg grating was fabricated on the tail fiber of 980 nm semiconductor laser to reduce the output spectral width of laser and to improve the stability of the laser output spectrum.The experimental results show that the center wavelength of 980 nm semiconductor laser changes only 0.06 nm when the environment temperature changes within the range of 0℃to 75℃.

In this paper, we present a rigorous approach based on split-step time-domain dynamic modeling (SS-TDM) algorithm using to solve the time-dependent coupled wave equations of the time-domain traveling-wave (TDTW) model to investigate the effect of facets reflectivity on the Light-Current characteristics of (DFB) semiconductor laser for different values the coupling coefficient. The simulation results show that using high reflection coating on the front facet and low reflection coating on the back facet with a proper diffraction grating can significantly improve the power-conversion efficiency of the (DFB) semiconductor laser.

Monte Carlo markovian models of a dual-mode semiconductor laser with quantum well (QW) or quantum dot (QD) active regions are proposed. Accounting for carriers and photons as particles that may exchange energy in the course of time allows an ab initio description of laser dynamics such as the mode competition and intrinsic laser noise. We used these models to evaluate the stability of the dual-mode regime when laser characteristics are varied: mode gains and losses, non-radiative recombination rates, intraband relaxation time, capture time in QD, transfer of excitation between QD via the wetting layer... As a major result, a possible steady-state dual-mode regime is predicted for specially designed QD semiconductor lasers thereby acting as a CW microwave or terahertz-beating source whereas it does not occur for QW lasers.

The injection locking characteristics of a multi-mode semiconductor laser are considered. A formalism is developed to investigate the stability properties of an arbitrary laser mode subject to optical injection. The formalism is used to show that the relaxation oscillation frequency (ROF) in a semiconductor laser subject to optical injection is increased relative to that of the free running laser diode. Methods of utilizing positive detuning to determine the best approach of increasing the ROF of a semiconductor laser via injection locking are considered.

The finite intraband relaxation time in semiconductor lasers leads to gain saturation at high laser powers. The nonperturbative solution of the single-mode density-matrix equations shows that both the optical gain and the refractive index become intensity dependent as a result of intraband relaxation dynamics. Gain and index nonlinearities are included in the rate equations, and how the modulation response and noise characteristics of semiconductor lasers are affected by such nonlinearities is studied. The intensity dependence of the frequency and the damping rate of relaxation oscillations leads to a fundamental limit imposed on the small-signal modulation bandwidth; the analysis provides an expression for the ultimate modulation bandwidth in terms of the material parameters. >

Physics of Semiconductor Lasers

The spatio-temporal dynamics of high-power semiconductor lasers are studied theoretically on the basis of space-dependent microscopic semiconductor-Maxwell-Bloch model equations. In the free-running broad-area and multi-stripe semiconductor lasers, complex spatio-temporal patterns are observed as a result of self-focusing, filamentation and transverse modulational instabilities in space-time images of the near-field output intensity. It is the irregular, possibly chaotic, dynamics of the filaments which are responsible for the generally much inferior beam quality and spectral characteristics of the high-power broad-area lasers when compared to common semiconductor lasers with moderate output power. The inherent coupling of optical diffraction with carrier transport processes leads to the formation of the spatio-temporal patterns. The simultaneous relevance of spectral and spatial hole-burning phenomena is manifested in the space- and time-dependence of the optical near-field showing, at the same time, spatio-temporal and spectral-temporal variations in the sub-ps range.

There are several equation sets that are used to characterize of the performance parameters of a quantum well (QW) lasers. One of simple and important equations groups which are used to study of laser characteristics in the almost powerful and expensive simulation tools such as Crosslight, ISE TCAD and SILVACO is of rate equations. A numerical analysis of rate equations is a powerful tool to study of carriers and photons behavior in the semiconductor lasers. The differential rate equations are used to estimate simulated and spontaneous emissions of laser in the all semiconductor simulation softwares [1-7]. A new visual and analytical model to study effects of structure parameters on laser performance of InGaN single quantum well lasers based on solving rate equations by using user friendly programming software, Delphi, is presented. This model has provided a graphical user interface (GUI) for researchers by which one can work directly with all laser parameters throughout the simulation work and analysis without the need to access source code[1]. We have investigated effects of different parameters such as quantum well thickness, separate confinement heterostructure (SCH) thickness and cavity length on the principle characteristics of the laser which include laser time response (P-t), and output power-current (P-I) characteristics. Meanwhile related features such as turn-on delay time of lasing, threshold current and slope efficiency have been investigated. Figure 2 shows the graphical user interface (GUI) of the main program, which uses to input parameters such as QW and SCH thickness and etc. Simulation results of P-I characteristics of InGaN SQW laser with our software has shown in figure 2. Also figure 3 shows the P-t characteristics of the simulated InGaN SQW laser. In conclusion, the presented work demonstrates a new visual, open source and analytical model based on solving rate equations by user friendly Delphi programming software. Our model in comparison with other analytical software and expensive simulators provides directly access to the parameters of lasers throughout the simulation and analysis.

A device has been demonstrated which for the first time incorporates a short two-dimensional grating in the ridge of a ridge-waveguide semiconductor laser. The device used for this work is a standard Fabry-Perot ridge-waveguide laser diode operating at 1.3 /spl mu/m. The active layer contains seven InGaAsP quantum wells. By positioning the grating in the ridge of the laser there is a strong interaction with the lasing mode and only a short grating length is required to dramatically alter the performance of the laser. The output spectrum of the device shows it is lasing on a single longitudinal mode at a considerably shorter wavelength than before the grating was incorporated. Additional structure is observed from the grating and the temperature dependence of the lasing mode is reduced to that expected from a distributed-feedback semiconductor laser. This shows the dominance of the grating in determining the laser characteristics. Incorporating structures into the cavity in this way may allow enhanced functionality in future devices.

The chaos characteristics of a DFB semiconductor laser subjected to single and double external cavity optical feedback are analyzed experimentally. The results show that the correlation dimension, bandwidth of the chaos generated by one external cavity optical feedback can be expanded by using double external cavity optical feedback. When the laser is subjected to double external cavity optical feedback, the bandwidth with one external cavity feedback is increased from 5.5 GHz to 11 GHz. Correspondingly, the correlation dimension is increased from 2.8 to 3.6. The information of the external cavity length of the chaos generated by double external cavity optical feedback can be masked in the autocorrelation function.

Theoretical investigation into spectral characteristics of a semiconductor laser with dual-FBG external cavity

The characteristics of high-power semiconductor lasers with an 800 μm emitting aperture based on tunnel-coupled InGaAs/AlGaAs/GaAs heterostructures with three optically uncoupled laser sections are studied. The maximum power achieved under pumping by current pulses with an amplitude of 47 A and a duration of 1 μs is 110 W with the maximum active region heating not exceeding 4.7 °C. At a pulse duration of 860 μs, the maximum optical power is 22.6 W, and the decrease in the optical power to the pulse end reaches 6.7 %. A decrease in the laser pulse duration to 85 μs leads to an increase in the peak laser power to 41.4 W at a pump current amplitude of 20 A.

Experimental evidence of a physical phenomenon characteristic of semiconductor lasers, i.e., cooling of the electrons above the threshold for stimulated emission, is reported. We show that this effect is directly related with the internal quantum efficiency (ηint), which is one of the central physical quantities in the theory of semiconductor lasers. As a model system we selected the terahertz quantum-cascade laser that is particularly suitable for the investigation of nonequilibrium electronic ensembles. The reported procedure for the assessment of ηint can be easily extended to other model systems, enlightening the relevance of including hot-electron distributions in semiconductor laser modeling.

The LaserCADIII is software that has been developed for high speed simulation of base characteristics of bulk and quantum-well semiconductor lasers. The algorithm of the software is based on modern theory of semiconductor laser including rate equation, band structure theory, gain theory and semiconductor alloy engineering. LaserCADIII can be applied both for scientific simulation and education aims.