Spectral line broadening in quantum wells due to Coulomb interaction of current carriers

2010

J Appl Spectrosc

Numerical modelling is used to study the effect of tuning the laser output over the gain bandwidth on the modulation response of GaInAs-GaInAsP quantum-well heterolasers for different modulation frequencies of the pump current. It is found that the maximum frequency bandwidth of the response band and the greatest feasibility of high speed modulation for transmission of signals in information systems are achieved in the center of the gain band. Raising the dc component of the pump current increases the response bandwidth. For typical parameters of this system (near 1.5 μm) the maximum response bandwidth can approach ≈40 GHz.For certain parameters, the amplitude-frequency characteristics of the heterolasers have two local maxima: one at low frequency corresponding usually to a resonance for the 1/2 subharmonic and one at high frequency, for the fundamental resonance.Introduction. Solving a number of scientific and practical problems requires single-mode (single frequency) semiconductor lasers with a narrow and stable emission line. Furthermore, in a number of cases the frequency of the laser radiation must be tuned over a certain range within the confines of the gain band. As a rule, single frequency operation is obtained using an external dispersive cavity. For example, narrow band cw lasing (Δλ ≤ 0.15 nm) at an output power of up to 20 mW tuneable over an interval of ≈10 nm around the central wavelength of 884 nm has been obtained [1] in a GaAs-AlGaAs heterolaser with an external dispersive cavity at room temperature. Single frequency emission from a laser with stressed active layers (λ = 975 nm) and a cw power of >100 mW has been obtained [2] with suppression of side longitudinal modes at a level of >30 dB. It has been pointed out [3] that with the right position for the fixed axis of rotation of an external mirror, an injection laser can essentially be tuned continuously over its entire gain band. An external waveguide grating mirror has been used to obtain a significant narrowing (to 0.1 nm) of the output spectrum and it was shown that the output wavelength of a semiconductor laser can be tuned smoothly over a range of 10-18 nm [4].It has been noted [5] that one of the most important problems in fiber optic communications lines with channel spectral densification is the creation of laser sources with a narrow and stable spectrum (no more than 0.1 nm) that must be maintained in a dynamic regime with modulation of an information signal at frequencies up to 10 GHz. These requirements may correspond best to semiconductor lasers with external fiber Bragg gratings on single-mode fiber light guides. For GaInAsP-InP lasers the half width of the laser output spectrum with a fiber Bragg grating is no more than 0.1 nm with pulsed modulation of the pump current at a frequency of up to ≈5 GHz. Other features of the output parameters of single frequency semiconductor lasers are discussed in [6][7][8][9][10][11]. Methods of creating dynamic single-frequency semiconductor lasers that operate stably to generate a single longit...

mentioning

confidence: 99%

Modulation response of dynamic single-mode quantum-well heterolasers in the 1.5 μm range

Кунцевич

2010

J Appl Spectrosc

“…In this instance the gain spectrum is bell-shaped, which in essence corresponds to the spectral broadening effect [11].…”

mentioning

confidence: 99%

Influence of changing the forbidden-band gap on radiation frequency sweeping of quantum-well heterolasers

Кунцевич

2009

J Appl Spectrosc

The principal features of changing (sweeping) the "instantaneous" emission frequency of quantum-well heterolasers as functions of the modulation frequency and depth and the constant component of the pump current when tuning the lasing frequency within the gain band are established using numerical modeling. The active medium is described in the framework of a two-band model with similar distributions of levels in subbands of electrons and holes assuming transitions between ground subbands with no k-selection rule. Emission frequency sweeping occurs because of changes in both the index of refraction of the active medium due to a variation in the concentration of non-equilibrium charge carriers and the forbidden-band gap. Emission frequency sweeping does not occur at low frequencies of current modulation that correspond with quasi-stationary lasing regimes. The modulation depth of the output radiation and, therefore, the emission frequency sweeping, also approach zero for the other limiting case of relatively high current modulation frequencies.The sweeping is greatest at intermediate current modulation frequencies. The sweeping value is approximately halved in certain instances by taking into account the change of forbidden-band gap of the semiconductor. In general, the sweeping value is determined by the combined effect of the modulation frequency and depth, the constant component of the pump current, and the spectral position of the laser emission frequency within the gain band.Introduction. Semiconducting lasers with narrow and stable spectral emission lines are required for fiber optic communications, high-resolution spectroscopy, measurement technology, etc. Therefore, so-called dynamic single-frequency (SF) semiconducting lasers that operate stably in single-logitudinal generation mode with high-frequency pump current modulation have been developed [1]. However, the "instantaneous" emission frequency can change even in this regime with current modulation (so-called sweep, chirp, dynamic shift, etc.) [2]. This is explained by a change of the index of refraction n of the active medium and, therefore, the instantaneous longitudinal frequencies of resonator modes and the generated radiation.Studies of the features of emission frequency sweeping are of definite practical interest. For example, certain features of the change of emission frequency in vertical cavity surface-emitting lasers [3,4] and of actively and passively mode-locked semiconducting lasers [5] have been examined. The effect of frequency chirp on the dynamics of recirculation in a closed contour and on an injection laser in high-velocity code modulation has been studied [6,7]. Features of frequency sweeping of emission from heterojunction quantum lasers with pump current modulation have been studied for certain special cases in recently published papers [8,9].Herein we report a systematic study of the features of quantum-well heterolaser sweeping as a function of modulation depth and frequency and the value of the constant component of pump current...

“…Two limit cases for analysis of the dispersion of the nonlinear gain can be took into consideration, i. e., the direct optical transitions and transitions with no the k-selection rule. The last approach is attributed qualitatively to spectral broadening effects [7]. For modeling the optical transitions in QWs and evaluating the nonlinearity parameter, it is possible to use the self-consistent two-band model [8,9].…”

mentioning

confidence: 99%

Nonlinear gain and output power characteristics of quantum-well sources

2008 4th International Conference on Advanced Optoelectronics and Lasers

2008

The gain saturation in quantum-well heterostructures under the change in the populations of the subband levels is described in detail. The nonlinearity parameters are introduced and their change with the pump, radiation frequency, temperature, polarization, and the quantum-well level distribution is examined. The main attention is given to quantum-well heterostructure sources based on III-V compounds, including GaInAsSb, GaInAs, GaInAsP, GaAs, and GaInN, which emit in the spectral interval from the mid-infrared to visible light.For description of the emission dynamics in different quantum-well (QW) heterostructure systems (lasers, amplifiers, superluminescent diodes, photonic crystals, etc.) it is necessary to take into account saturation effects of the gain in the active region. The nonlinear gain is accompanied by nonlinear refraction and hole burning processes [1][2][3][4][5].In the paper, the gain saturation in QW heterostructures under the change in the populations of the subband levels is described in detail. The nonlinearity parameters are introduced and their change with the pump, radiation frequency, light intensity and polarization, temperature, and QW level distribution is examined. The main attention is given to QW heterostructure sources based on III-V compounds, which emit in the spectral interval from the mid-infrared to visible light.As a rule, the nonlinear gain is described in the phenomenological manner [6] and dependence of the nonlinearity parameter on the pump, frequency, temperature, and electron state structure is not considered. Generally, the gain saturation in QW heterostructures connected with the change in the population of the subbands depends on the semiconductor energy gap, temperature, and QW width in the active region. Two limit cases for analysis of the dispersion of the nonlinear gain can be took into consideration, i. e., the direct optical transitions and transitions with no the k-selection rule. The last approach is attributed qualitatively to spectral broadening effects [7]. For modeling the optical transitions in QWs and evaluating the nonlinearity parameter, it is possible to use the self-consistent two-band model [8,9].In the general case, the gain coefficient follows, in dependence on the radiation flux in the active region, the law [10] k = k 0 /(1+αS), where k 0 is the initial gain (including the light confinement in the QW source waveguide), α is the nonlinearity parameter, which depends on the frequency ν, quasi-Fermi level difference ΔF 0 , and sheet photon density S. There are three regions of the gain saturation. At low fluxes, k ≈ k 0 (1-α 0 S), where the initial nonlinearity parameter is α 0 . When the gain varies by a factor of two (k ≈ k 0 /2), it is worthwhile to use a middle value α 1/2 . At high powerful radiation the gain drops as k ≈ k 0 /α ∞ S, where the nonlinearity parameter is equal to α ∞ . Generally, the relation α 0 < α 1/2 < α ∞ is fulfillment. For the absorption saturation, the inverse relation between the parameters α 0 , α 1/2 , and α ∞ exists...