J. LaCourse

Measurement of the carrier dependence of differential gain, refractive index, and linewidth enhancement factor in strained‐layer quantum well lasers

Measurements of the variation of differential gain, refractive index, and linewidth enhancement factor with carrier density in InGaAs‐GaAs strained‐layer quantum well lasers are presented for the first time. These results verify predictions of improvement over unstrained bulk or quantum well lasers, but only at certain carrier densities. Differential gain (dg/dN) is found to vary from 7.0×10−16 to 2.5×10−16 cm2 over the range of carrier densities studied, while the carrier dependence of the real part of the refractive index (dn/dN) ranges from a peak of −2.8×10−20 down to −7.0×10−21 cm3. From these measurements the resulting linewidth enhancement factor (α) is found to vary from 5 to a minimum of 1.7. This information is critical to successfully exploiting the potential advantages of strained‐layer lasers for such devices as high‐frequency or narrow linewidth lasers.

Anomalously high damping in strained InGaAs-GaAs single quantum well lasers

Measurements of the relative intensity noise spectra of strained, single-quantum-well, separate-confinement-heterostructure (SCH) InGaAs-GaAs lasers indicate that their frequency response is strongly damped. The ratio of the damping rate to the square of the resonance frequency is k=2.4 ns. This intrinsically limits the 3-dB modulation bandwidths of these lasers to about 4 GHz, negating the predicted increase in modulation bandwidth due to the large differential gain often associated with quantum-well devices. The damping behavior of these lasers is inconsistent with previous predictions of damping in bulk lasers due to spectral hole burning. A structure-dependent damping mechanism is proposed for quantum-well lasers.<<ETX>>

Characterization of the dynamics of semiconductor lasers using optical modulation

An optical modulation technique for measuring the intrinsic frequency response of semiconductor lasers is described. This technique, which uses an RF-modulated pump laser to create an optical modulation signal to inject into a DC-biased probed laser, offers significant advantages over previous methods such as being affected by electrical parasitics of either the laser to be characterized or the photodetector. The method allows extremely accurate measurements of many important dynamic parameters, including the nonlinear gain coefficients, the amount of spontaneous emission into the guided modes, and the differential carrier lifetime at lasing threshold. >

Simultaneous measurement of spontaneous emission rate, nonlinear gain coefficient, and carrier lifetime in semiconductor lasers using a parasitic‐free optical modulation technique

An optical modulation technique is used to determine three important parameters for 1.3 μm InGaAsP diode lasers: the rate of spontaneous emission into the guided modes, the nonlinear gain coefficient, and the carrier lifetime at threshold. These results are unaffected by electrical parasitics, and are essential to understanding the noise and modulation properties of diode lasers.

The relation of doping level to K factor and the effect on ultimate modulation performance of semiconductor lasers

For the first time, K, which is the ratio of the damping factor gamma to the square of the resonance frequency f/sub 0//sup 2/, is shown to depend on the doping level for bulk semiconductor lasers. Since the differential gain is known to depend on the doping level in the active layer, K also depends on the differential gain. The results presented strongly suggest that an effective means to decrease the damping is by increasing the doping level of the active region of the semiconductor laser. Since damping must be reduced in order to increase the maximum damping-limited bandwidth, this result may have important implications for improving the modulation bandwidths of bulk lasers and may be equally significant with respect to damping and the ultimate achievable bandwidth in quantum-well lasers.<<ETX>>

Determination of the gain nonlinearity time constant in 1.3 μm semiconductor lasers

By comparison of the measured K factors (ratio of the damping factor to the square of the resonance frequency) of distributed feedback and Fabry–Perot lasers, it is found that the relaxation time associated with nonlinear gain for 1.3 μm InGaAsP lasers is about 0.1 ps. This short time constant is consistent with spectral hole burning being the dominant process responsible for the nonlinear gain.

Relative intensity noise in semiconductor optical amplifiers

The spontaneous noise spectrum of high-gain semiconductor optical amplifiers is normally assumed to be dominated by spontaneous-spontaneous and signal-spontaneous beat noise, which is white over the frequency range important to fiber-optic systems. Recent measurements have shown that a strong resonance peak in the spontaneous noise spectrum appears well below the threshold current, indicating the existence of relative intensity noise. This noise term has important implications for system design, and its effect on several transmission systems is described. Relative intensity noise in semiconductor optical amplifiers is compared to the similar relative intensity noise found in semiconductor lasers.<<ETX>>

Effective differential gain and modulation bandwidth reduction in quantum well lasers due to confinement factor modulation

A mechanism that may reduce the effective differential gain due to the modulation of the confinement factor with carrier density in quantum-well lasers is described. This mechanism may limit modulation bandwidth for quantum-well lasers with high threshold carrier density and narrow confining layer.<<ETX>>

Reduced effective differential gain in diode lasers due to confinement factor modulation

A reduced effective differential gain is shown to arise in diode lasers by including the modulation of the confinement factor with carrier density. This effective differential gain, not the material gain, is the parameter determined from conventional measurements of the differential gain. This term is in addition to the static reduction in confinement factor with carrier density, and can significantly reduce the resonance frequency and modulation bandwidth for lasers with short cavities and thin active layers. >

Structure-dependent damping in quantum-well lasers

The maximum 3 dB modulation bandwidth of a semiconductor laser is determined, if not by RC or power limits, by damping that arises from photon-dependent suppression of the optical gain. In bulk lasers this damping limit is found, both experimentally and analytically, to be relatively constant at 25 - 45 GHz, independent of device design. In contrast, the damping limit is found to vary widely for quantum well lasers. In this paper we will describe experimental results showing the structure dependence of the damping, and we will present evidence for a new model explaining the structure dependence as a result of well-barrier hole burning. This hole burning arises from a buildup of carriers in the barrier layers due to the nonzero carrier capture times of the wells, causing a spatial hole to be burned perpendicular to the active region. This hole can behave like a photon dependent gain suppression, leading to a larger nonlinear gain parameters and a lower effective differential gain. We also suggest ways to optimize quantum well laser structures for maximum modulation bandwidth.