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     

Influence of inhomogeneous broadening and deliberately introduced disorder on the width of the lasing spectrum of a quantum dot laser

Analytical expressions for the shape and width of the lasing spectra of a quantum-dot (QD) laser in the case of a small (in comparison with the spectrum width) homogeneous broadening of the QD energy levels have been obtained. It is shown that the dependence of the lasing spectrum width on the output power at room temperature is determined by two dimensionless parameters: the width of QD distribution over the optical-transition energy, normalized to temperature, and the ratio of the optical loss to the maximum gain. The optimal dimensions of the laser active region have been found to obtain a specified width of the emission spectrum at a minimum pump current. The possibility of using multilayer structures with QDs to increase the lasing spectrum’s width has been analyzed. It is shown that the use of several arrays of QDs with deliberately variable optical-transition energies leads to broadening of the lasing spectra; some numerical estimates are presented.     

Spectral characteristics of external-cavity controlled semiconductor lasers

Experiments with an external-cavity semiconductor laser indicate that single-frequency oscillation may be obtained in an injection laser when the gain is temporally and spatially stabilized within the active region. Using an originally multimode laser diode as the gain medium, the external-cavity laser oscillates stably in the fundamental spatial mode and in a single longitudinal mode with a frequency which is tunable over a 10 nm spectral range. The rms frequency jitter of the unstabilized laser is 500 kHz. These mode characteristics suggest that spatially inhomogeneous gain saturation is significant only in the lateral direction in the active region of a stripe-geometry double-heterostructure laser diode. A quantitative analysis of the spontaneous emission in the spectral vicinity of the lasing mode shows no evidence of spectral hole burning, with a 0.5 percent upper limit to the depth of the hole burned in the gain spectrum by a lasing mode power of 5 mW.     

Spectral dependence of differential gain, mode shift, and linewidthenhancement factor in a InGaAs-GaAs strained-layer single-quantum-welllaser operated under high-injection conditions

Strained-layer quantum-well lasers, when operated at moderate injection levels, are known to exhibit a reduced linewidth enhancement factor, α, an important parameter in the design of semiconductor lasers. Under increased loss conditions, when spill-over of carriers into the barrier becomes significant, they can also be operated in a quasi-double-heterostructure mode at the barrier wavelength. Thus, by proper control of loss, lasing at different wavelengths is possible, at least in principle. An investigation has been carried out to determine explicitly the spectral dependence of the differential gain, spectral shift, and α of a narrow-stripe InGaAs-GaAs separate-confinement strained-layer single-quantum-well laser operated near the wavelength of the GaAs barrier under extremely high injection. Despite the carrier spill-over, α was found to be remarkably low, about 1.7 at the spectral gain peak (877 nm), which was also the lasing wavelength     

A unified theory of magnetic bremsstrahlung, electrostatic bremsstrahlung, Compton-Raman scattering, and Cerenkov-Smith-Purcell free-electron lasers

This paper discusses in a comparative way the main operating parameters of various free-electron lasers (FEL's), providing a useful tool for laser design and a comparative evaluation of the various lasers. We show that the various kinds of FEL's satisfy the same gain-dispersion relation and differ only in a single coupling parameterk. The different gain regimes which are common to all FEL's are delineated. We find the small signal gain in all the gain regimes (warm and cold beam, low- or high-gain, single electron, collective or strong coupling interaction). The laser gain parameter, radiation extraction efficiency, maximum power generation, and spectral width are given and compared in the various kinds of FEL's and gain regimes. The maximum power generation of all FEL's (except Compton-Raman scattering) is shown to be limited by an interaction region width parameter. This parameter and, consequently, the laser power are larger in the highly relativistic limit by a factorsim gamma_{0}in all bremsstrahlung FEL's, in comparison to Cerenkov-Smith-Purcell FEL's. Some expressions which were derived earlier for the magnetic bremsstrahlung FEL, like the expression for gain in the low-gain regime with the space charge effect correction and the low-gain expression for efficiency, are shown to be special cases of more general expressions.     

Longitudinal mode spectrum of semiconductor lasers under high-speed modulation

Experimental observations of the lasing spectrum of a single-mode semiconductor laser under continuous microwave modulation reveal that the lasing spectrum is apparently locked to a single-longitudinal mode for optical modulation depths up to ∼ 80 percent, beyond which the lasing spectrum breaks into multimode oscillation. The width of the envelope of the multimode spectrum increases very rapidly with further increase in modulation depth. These results are satisfactorily explained by a theoretical treatment which gives simple analytic results for the time evolution of the individual longitudinal modes. It also yields considerable insight into spectral dynamics, and enables one to predict the dynamic lasing spectrum of a laser from its CW lasing spectra at various output powers. The results can also be used to predict the amount of spectral envelope broadening under single or pseudo-random pulse modulation.     

Detuning adjustable multiwavelength MQW-DFB laser array grown byeffective index/quantum energy control selective area MOVPE

A multiwavelength MQW DFB laser array with novel structure Is described. Oscillation wavelength and gain peak wavelength were simultaneously controlled on the same epitaxial wafer by using modulated grown thicknesses of selectively grown InGaAs/InGaAsP/InP MQW active waveguides. The laser array with constant-pitch built-in corrugation fabricated by a simple DFB laser process demonstrated 10.1 nm controllable range for lasing wavelength and 45 nm for gain peak wavelengths, with uniform lasing properties and narrow spectral linewidths. The technique is attractive for light sources used in WDM/FDM applications     

Intrinsic gigahertz modulation of photolytic iodine lasers

A photolytic iodine laser is shown to have a 13.59-GHz modulation in the laser radiation when simultaneous lasing of two hyperfine transitions is obtained. The necessary gain tuning required to achieve dual-line lasing is accomplished through the use of a magnetic field. In addition, we investigate the possibility of using a nonlinear mirror to increase the gain coupling between the two transitions     

Behavioral Investigations of an Erbium-Doped Fiber Ring Laser through Numerical Simulations

An erbium-doped fiber ring laser is modeled, using the standard propagation and rate equations of a homogeneous, two-level medium. The numerical model has been obtained by developing a spectral analysis of erbium-doped fiber amplifiers. This approach is modified to account for the cyclical propagation of the field in the ring laser. The numerical simulations provide interesting information on the spectral behavior as well as important characteristics of the cavity, mainly the lasing wavelength and output power. The model also permits the study of competing modes' gain evolution during the transient state of a ring laser cavity, which enables cavity lasing wavelength predictions. To our knowledge, this study is the first of its kind.     

Effect of excited-state transitions on the thresholdcharacteristics of a quantum dot laser

The general relationship between the gain and spontaneous emission spectra of a quantum dot (QD) laser is shown to hold for an arbitrary number of radiative transitions and an arbitrary QD-size distribution. The effect of microscopic parameters (the degeneracy factor and the overlap integral for a transition) on the gain is discussed. We calculate the threshold current density and lasing wavelength as a function of losses. The conditions for a smooth or step-like change in the lasing wavelength are described. We have simulated the threshold characteristics of a laser based on self-assembled pyramidal InAs QDs in the GaAs matrix and obtained; a small overlap integral for transitions in the QDs and a large spontaneous radiative lifetime. These are shown to be a possible reason for the low single-layer modal gain, which limits lasing via the ground-state transition for short (several hundreds of micrometers) cavity lengths     

Analysis of threshold conditions for generation of a closed mode in a Fabry-Perot semiconductor laser

Threshold conditions for generation of a closed mode in the crystal of the Fabry-Perot semiconductor laser with a quantum-well active region are analyzed. It is found that main parameters affecting the closed mode lasing threshold for the chosen laser heterostructure are as follows: the optical loss in the passive region, the optical confinement factor of the closed mode in the gain region, and material gain detuning. The relations defining the threshold conditions for closed mode lasing in terms of optical and geometrical characteristics of the semiconductor laser are derived. It is shown that the threshold conditions can be satisfied at a lower material gain in comparison with the Fabry-Perot cavity mode due to zero output loss for the closed mode.     

Self-mixing interference inside a single-mode diode laser foroptical sensing applications

This paper presents a theoretical analysis and a comparison with experimental results on self-mixing interference inside a single-longitudinal-mode diode laser. A theoretical model, based on the steady-state equations of the lasing condition in a Fabry-Perot type laser cavity, is described, and through it a satisfactory analysis of self-mixing interference for optical sensing applications is given. In this work, the self-mixing interference produced by an external optical feedback is found to be due to the variations in the threshold gain and in the spectral distribution of the laser output. The gain variation results in an optical intensity modulation, and the spectral variation determines both the modulation waveform and the coherence properties of the interference. The theoretical analysis of the self-mixing interference is seen to yield a simulation of the laser power modulation which is in good agreement with the experiment results reported     

Linewidth reduction in DFB laser by detuning effect

The detuning effect on the spectral linewidth in the DFB laser has been investigated. When we set the lasing wavelength of the DFB laser at the shorter side of the gain peak by 10 nm, a reduction in spectral linewidth of 50% was obtained experimentally. This result agreed with the theoretical prediction.     

Compound cavity gain of tandem-electrode multiple-quantum-wellAlGaAs laser diodes

The compound cavity gain of multiple-quantum well AlGaAs semiconductor lasers with segmented contacts was measured in detail. A second peak of the gain curve is observed. We discuss the impact of the individual features of gain and absorption on the characteristics of the laser diode and demonstrate that, by changing the bias conditions, these features can be used to produce spectral switching of the lasing wavelength by more than 10 nm     

Second-harmonic generation with the GaAs laser

The second harmonic of pulsed GaAs injection lasers was generated with the nonlinear crystal α-iodic acid (HIO3) phase matched by angular tuning. The injection laser was operated with an external cavity, and the HIO3crystal was placed inside the cavity to increase the conversion efficiency. The maximum harmonic power observed was 15 μW with a fundamental power of 4.5 watts inside the cavity. Since the lasing bandwidth was typically 30 Å and the calculated bandwidth for collinear harmonic generation was only 6.3 Å, a grating was used to narrow the lasing bandwidth to 3 Å. The expected large increase in harmonic power did not occur. It is shown that this is due to sum-frequency generation by laser modes lying outside the harmonic bandwidth. It is also found that the spectral width of the harmonic becomes bigger than the collinear bandwidth when a divergent laser beam is used.     

量子线和量子箱激光器—下一代高性能半导体激光器

本文讨论了量子线和量子箱激光器的特性,如呈现非常低的阈值电流、展宽的调制带宽和窄的谱线宽度。还介绍了量子尺寸结构的制造工艺、尺寸起伏和非线性增益对激射特性的影响。最后给出量子微腔激光器的新概念。     

Temperature dependent lasing characteristics of InAs/InP(100) quantum dot laser

We report on the lasing characteristics of InAs/InP(100) quantum dots laser through changing the temperature under continuous-wave mode. Three lasing peaks are simultaneously observed at temperature of 80 K and the lasing order of each peak is unrelated with each other when injection current increases. Laser spectra obtained under fixed current for different temperatures show a drastic influence on their shape. A large spectral broadening is observed at low temperature, while the width of lasing spectra gradually narrows when the operating temperature increased. The lasing process of quantum dot laser is obviously different from that of a reference quantum well laser in the same wavelength region. In addition, very high wavelength stability of 0.088 nm/K in the temperature range of 80–300 K is obtained, which is 6.2 times better than that of reference quantum well laser.     

A temperature-stable semiconductor laser based on coupled waveguides

The design of a stripe semiconductor laser with a composite waveguide formed of two tunneling-coupled waveguides, one of which is narrow and contains the active medium and the other of which is wide and forms the optical mode is considered. It is shown that temperature stabilization of the lasing wavelength can be attained with such a semiconductor laser design. Stabilization of the wavelength can be brought about by lasing in antiresonance conditions of coupling of two waveguides; these conditions arise in a narrow range of wavelengths that depends on temperature only slightly. Calculations performed at the parameters characteristic of the InAlGaAsP laser system show that it is possible to reduce the temperature dependence of the lasing wavelength by a factor of 3 in the temperature range with the width of 60 K.     

Tunable grating-coupled laser oscillation and spectral hole burningin an InAs quantum-dot laser diode

Emission spectra are investigated of a low-threshold InAs quantum-dot laser of the “dots-in-a-well” (DWELL) type operating near 1230 nm. An external dispersion cavity with a diffraction grating is coupled to the laser diode to suppress the subsidiary modes and to tune the central wavelength. A wavelength-dependent competition between the grating-coupled mode and the internal Fabry-Perot modes of the laser suggests that a hole burning in the spectral density of a DWELL laser occurs with a characteristic spectral half width of ~13 nm (10.5 meV). Simple models of spectral “flattening” and spectral hole burning are presented to explain the broad free-running and grating-coupled lasing spectra of the DWELL device     

Compensation of line-broadening factor in twin-stripe semiconductor lasers and improved modulation potential

A concept for a twin-stripe semiconductor laser is presented in which the simultaneous gain and refractive index variation induced by carriers injected into one stripe can be converted into either a pure amplitude or a pure frequency variation in the lasing supermode. Applications include high-frequency chirp-free amplitude modulation, reduced line width and pure phase modulation.