Continuous-wave lasing of hybrid lasers

The continuous-wave (CW) lasing of hybrid lasers, which contain a broad-band inhomogeneously broadened laser medium and a narrow-band homogeneously broadened laser medium in a single cavity, is analyzed theoretically. The interactions of the laser modes and two gain media are solved self-consistently using the coupled rate equations. The spectral, gain, and power characteristics under different gain conditions are simulated. It is shown that a small gain from a second narrowband laser medium can effectively shape the lasing spectrum and improve the spectral concentration of the hybrid laser. The total saturated gain profile of the hybrid laser is relatively smooth, due to saturation of the gain of the narrow-band medium to a smaller and below-threshold level     

Two-band lasing in epitaxially stacked tunnel-junction semiconductor lasers

Epitaxially stacked tunnel-junction laser hetero structures were grown by hydride metalorganic vapor-phase epitaxy in the system of AlGaAs/GaAs/In GaAs alloys. Based on such structures, mesa stripe lasers with an aperture of 150 s- 7 m were fabricated. The possibility of controlling the lasing wavelength by varying the active region thickness in each tunnel-junction laser structure was demonstrated. Independent two-band lasing at wavelengths of 914 and 925 nm (the difference frequency is 2.3 THz) was achieved at a maximum optical radiation power of 20 W in each band of the epitaxially stacked tunnel-junction semiconductor laser.     

Practical aspects of lasing without inversion in various media

A rigorous density-matrix-based theory of lasing based on optically induced or autoionization-induced coherence between the levels is developed. The balance equations are obtained, and the conditions for lasing threshold are derived as a function of pumping strength and relaxation rates. Connection between the lasing without inversion and Raman and parametric processes is established. The main conclusion is that true CW lasing without inversion can indeed be obtained but only in a system where pumping and relaxation rates are favorable for attaining conventional lasing with population inversion. Practical implications of it, especially for the intersubband lasers, are discussed     

Lateral waveguiding in stripe-geometry double-heterostructure lasers below the lasing threshold

Structure in the far-field pattern of current-confined stripe-geometry double-heterostructure lasers in the angle rangepm 20degis observed at current levels of less thanfrac{1}{4}of threshold value. The behavior is explained by leaky-mode guiding due to a gain maximum under the stripe and to a negative refractive index step on the order of -0.01.     

Water soluble blue-green lasing dyes for flashlamp-pumped dye lasers

2-(4-Pyridyl)-5-phenyloxazole (4PyPO) and its pyridinium salts have been investigated for laser action in the violet and blue-green spectral regions, respectively. The phenyloxazolyl pyridinium salts show good lasing characteristics, broad tuning ranges, and superior photochemical stability when compared with coumarin 175 in water. Recommendations were made on how the lasing maxima could be blue or red shifted and how the output power and photochemical stability of the new classes of laser dyes could be made to approach that of C8F, which is one of the most photochemically stable lasing dyes reported to date. Because of the reactivity of the nonbonding electrons on the pyridyl group of 4PyPO, hundreds of new laser dyes are possible from the parent dye. A new generation of water soluble, low threshold multifluorophoric laser dyes capable of intramolecular energy transfer and intramolecular triplet state quenching was proposed.     

Features of simultaneous ground- and excited-state lasing in quantum dot lasers

The lasing spectra and light-current (L-I) characteristics of an InAs/InGaAs quantum dot laser emitting in the simultaneous lasing mode at the ground- and excited-state optical transitions are studied. Lasing and spontaneous emission spectra are compared. It is shown that ground-state quenching of lasing is observed even in the absence of active region self-heating or an increase in homogeneous broadening with growth in the current density. It is found that the intensities of both lasing and spontaneous emission at the ground-state transition begin to decrease at a pump intensity that significantly exceeds the two-level lasing threshold. It is also found that different groups of quantum dots are involved in ground- and excited-state lasing.     

Minority carrier lifetimes and lasing thresholds of PbSnTe heterostructure lasers

A new calculation of radiative minority carrier lifetime in PbSnTe is presented which yields lifetimes an order of magnitude greater than previously accepted values. These results are used in combination with recent measurements of minority carrier lifetime and interface recombination velocity in PbSnTe double heterojunction laser structures to calculate DH laser thresholds. It is demonstrated that radiative recombination is a relatively inefficient process in present PbSnTe lasers, and that when this inefficiency is taken into account the magnitude of the experimentally observed threshold and its variation with active region width and temperature can be accurately predicted, even at low temperatures where all previous models have failed.     

Two-dimensional numerical analysis of lasing characteristics forself-aligned structure semiconductor lasers

Lasing characteristics in 0.78 μm AlGaAs-GaAs self-aligned structure (SAS) lasers are calculated on the basis of a newly developed two-dimensional analytical method. The calculated results are compared in detail with experimental results for MOVPE (metalorganic vapor phase epitaxial) grown SAS lasers. It is shown that calculated results agree well with experimental results, and that the newly developed two-dimensional simulator is very effective in calculating actual lasing characteristics accurately. The optimum design conditions for AlGaAs-GaAs SAS lasers obtained from experimental and calculated results are discussed     

Analysis of quenching conditions of Fabry-Perot mode lasing in semiconductor stripe-contact lasers

Radiative characteristics of semiconductor stripe-contact lasers operating under quenching conditions of Fabry-Perot-mode lasing are studied. It is found that reversible turning off of Fabry-Perot-mode lasing is caused by switching to closed-mode lasing. Radiative characteristics of the closed mode are controlled by the mode structure with the close-to-zero loss for radiation output, which covers the entire crystal. The main threshold conditions of closed-mode lasing are a decrease in interband absorption in the passive region and an increase in the modal gain of the closed-mode lasing line. It is shown that a decrease in interband absorption in the passive region can be provided by both spontaneous emission from the injection region and lasing-mode photons. An increase in the modal gain of the closed-mode lasing line is provided by shifting the energy minima of the conduction band and maxima of the valence band of the injection region with respect to the energy bands of the passive region.     

Long delay time for lasing in very narrow graded barrier single-quantum-well lasers

We have observed the presence of very long delay times (~ 1 ?s) for lasing in graded barrier single-quantum-well (GB-SQW) lasers for active layers thinner than 150 A? and stripe widths below approximately 10 ?m. It is shown that an explanation of this phenomenon based on the dynamic evolution of the waveguide properties of these lasers gives good agreement with experimental results.     

A multimode self-consistent model of the lasing characteristics ofburied heterostructure lasers

Constricted mesa and buried heterostructure lasers have extremely wide modulation bandwidths due to their low parasitic capacitance and resistance and-because the structure provides very high confinement of both the carriers and photons in the active layer. The modulation bandwidth of these lasers is limited by current leakage effects and multilateral mode operation which both conspire to reduce the effective optical gain of the laser for a given operating current. These two problems are addressed and a fully self-consistent model of the lateral modal behavior and the current leakage problem in buried heterostructure lasers is addressed. The optical problem is solved using a modified version of the weighted index method, which provides a two-dimensional solution for optically active material with complex permittivity, and the onset of multimode behavior, where there is a superposition of two or more lateral modes, is also examined     

Effect of active-region modulation doping on simultaneous ground-state and excited-state lasing in quantum-dot lasers

The lasing spectra and light-power characteristics of lasers based on InAs/InGaAs quantum dots with p-type modulation doping are studied over a broad range of pump currents. It is shown that p-type doping leads to a significant increase in the threshold current for the onset of lasing at the excited-state transition and makes it possible to attain higher output powers for lasing at the ground-state transition as compared to lasers with an undoped active region. An explanation for the observed features of two-level oscillation in quantum-dot lasers is suggested.     

Effect of carrier dynamics and temperature on two-state lasing in semiconductor quantum dot lasers

It is analytically shown that the both the charge carrier dynamics in quantum dots and their capture into the quantum dots from the matrix material have a significant effect on two-state lasing phenomenon in quantum dot lasers. In particular, the consideration of desynchronization in electron and hole capture into quantum dots allows one to describe the quenching of ground-state lasing observed at high injection currents both qualitatevely and quantitatively. At the same time, an analysis of the charge carrier dynamics in a single quantum dot allowed us to describe the temperature dependences of the emission power via the ground- and excited-state optical transitions of quantum dots.     

Far- and near-field investigations on the lasing modes intwo-dimensional photonic crystal slab lasers

We present experimental investigations on the lasing modes in two-dimensional (2-D) hexagonal cavities defined by photonic crystals on slab waveguide structures. The far-field emission patterns and near-field intensity distributions of the lasing modes are analyzed in polarization-resolved 2-D angular distribution measurements and spectrally resolved near-field scanning optical microscopy. The far- and near-field analyses result in identification of the various lasing modes and their subsequent classification into one- and two-dimensional modes. In the one-dimensional modes oscillating between two parallel boundaries of the cavities, longitudinal and transverse modes are identified and found to have transverse-electric polarization. In the two-dimensional modes oscillating two-dimensionally in the cavities, various modes including whispering-gallery-like modes are observed and found to exhibit various polarization states     

Synchronization of dual-line lasing and subsequent gigahertzmodulation of iodine lasers through mode locking

Dual hyperfine line lasing of a Zeeman-tuned photolytic iodine laser resulting in 14-GHz modulated laser radiation was studied experimentally and theoretically as a function of the applied magnetic field. Synchronization of the phases of the two hyperfine lines necessary for optimum gigahertz modulation was achieved by mode locking. The resulting pulse durations of 740 ps were shorter than those previously observed without a magnetic field. Short pulses, together with the possibility of scaling the results to high-power chemical-oxygen iodine lasers, make this concept attractive for a laser-driven ultrawideband microwave source     

Increased threshold for the first-order lateral mode lasing in low-ridge waveguide high power QW lasers

A self-consistent two-dimensional model is extended to include two-mode operation and is applied to study the characteristics of ridge-waveguided high power InGaAs-GaAs quantum-well lasers. This work provides physical insight into the most recent experimental result that use low-ridges and thin p-claddings to give high single lateral mode output and a low threshold.     

High-power multiple-stripe injection lasers with channel guides

A new type of injection laser, the channel-guide (CG) structure, has been developed for use in multiple-stripe laser configurations. Leaky-mode operation of single-stripe CG lasers has been observed, and these devices produce "kink-free" outputs to pulsed powers of 180 mW (40 ns). Ten-stripe lasers fabricated using the channel-guide structure show strong leaky-mode coupling between stripes. Linear outputs to 2.8 W (40 ns) with stable far fields have been measured. Our results indicate that a real index antiguide is formed by the crowding of carriers into the channel region. The simple fabrication procedure makes the CG structure desirable for use in multiple-stripe high-power lasers.     

Second quantized state lasing and gain spectra measurements inn-type modulation doped GaAs-AlGaAs quantum-well lasers

The emission characteristics of n-type modulation doped GaAs-AlGaAs quantum-well lasers are studied for constant doping density and stepped doping density laser cores. Constant doping density cores are found to have a shift to shorter wavelength with increasing doping density but suffer from a corresponding large increase in threshold current density. Stepped doping density cores exhibit clear wavelength shifting from the first to second quantized state transitions with increased doping near the quantum well while maintaining low threshold current densities. Threshold current densities of 440 A/cm² are measured for second quantized state lasing in stepped core lasers. Gain spectra are measured for the stepped doping density core devices and modulation doping is shown to improve the gain bandwidth by 50% over undoped devices     

Analysis of the emission efficiency of powerful semiconductor lasers under closed-mode threshold lasing conditions

A new model describing the decrease in the emission efficiency and optical output power of a semiconductor laser above the lasing threshold of the Fabry-Perot mode is suggested. The mechanism of deterioration of the output-power characteristics is described in the suggested model in terms of the achievement of closed-mode threshold conditions. Rate equations are used to analyze how the closed-mode threshold conditions are satisfied in semiconductor lasers.