W. Rideout

Well-barrier hole burning in quantum well lasers

The reported wide variations in the damping behavior of quantum well lasers are explained by a novel theory of nonlinear gain, well-barrier hole burning. In the model a spatial hole develops perpendicular to the active region involving carriers moving between the wells and the barrier/confinement layers. The modified rate equations describing well-barrier hole burning are presented. An analytical approximation for the nonlinear gain coefficient epsilon , valid only under certain conditions, is given. A numerical solution is given for the case of high photon densities and large capture-times. It is shown how well-barrier hole burning explains the measurements of the increased spontaneous emission from the barrier/confinement region above threshold. Various higher-than-expected damping rates reported in some quantum well lasers are shown to be consistent with the model.<<ETX>>

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. >

Effect of Auger recombination and differential gain on the temperature sensitivity of 1.5 μm quantum well lasers

The temperature sensitivity of both strained and lattice‐matched 1.5 μm quantum well lasers has been studied. From a complete experimental investigation of the temperature behavior of carrier lifetime, gain, and internal loss, it is found that Auger recombination is not the dominant factor in affecting the temperature sensitivity of threshold currents in 1.5 μm lasers. Instead, the dominant contribution to the temperature dependence of threshold currents in 1.5 μm lasers is the change in differential gain with temperature—a characteristic not improved by strain.

Wide-bandwidth receiver photodetector frequency response measurements using amplified spontaneous emission from a semiconductor optical amplifier

The white optical noise (spontaneous-spontaneous beat noise) generated by amplified spontaneous emission from a semiconductor-optical amplifier is used to measure the frequency response of over-wide-bandwidth photodetectors and optical receivers. This technique can be used to characterize optoelectronic components of arbitrarily wide bandwidths. >

Experimental verification of strain benefits in 1.5- mu m semiconductor lasers by carrier lifetime and gain measurements

Recombination processes, gain, and loss have been comparatively studied in both strained and lattice-matched 1.5- mu m semiconductor quantum-well lasers using differential carrier lifetime techniques and other measurements. For the first time, some predicted strain benefits to 1.5- mu m semiconductor lasers have been verified, including (i) the reduction of the Auger recombination rate in devices with both 0.9% and 1.8% compressive strain; and (ii) a 33% reduction of transparency carrier density in lasers with 0.9% strain compared to lattice-matched lasers. The authors, however, did not observe an increase of the differential gain in strained devices as predicted.<<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.

Heterodyne video distribution systems sharing transmitter and local oscillator lasers

The authors present theoretical and experimental results for coherent subcarrier multiplexed (SCM) systems using a novel architecture that shares both the transmitter and local oscillator (LO) laser among multiple optoelectronic receivers. The ability to share both lasers significantly reduces the cost and complexity compared to a multichannel coherent frequency division multiplexed (FDM) system. Experimental results confirm that the system performance can be greatly enhanced by inserting an inline optical amplifier so that many receivers can share one transmitter and LO laser. For example, with an amplifier chip gain of 24 dB, increasing the optical power at the input to the amplifier by 3 dB from -27.6 to -24.5 dBm, the number of receivers can be increased from 2 to 32. >

Competing effects of well-barrier hole burning and nonlinear gain on the resonance characteristics of quantum-well lasers

The effects of the quantum capture and release of carriers from quantum wells (QWs) on the resonance response of QW lasers are investigated from a model of well-barrier hole burning with built-in nonlinear gain. Significant similarities and contrasts with the conventional single-mode model are noted in both the large-signal transient behavior and in the small-signal resonance characteristics. The competition between well-barrier hole burning and nonlinear gain is explored by studying of time responses, phase portraits, frequency transfer functions; and contour maps of constant resonance frequency, damping rate, and 3-dB bandwidth in the parameter spaces defined by the nonlinear gain coefficient versus the ratio of relaxation times for capture and release of carriers by the wells. A systematic treatment of the well-barrier model is presented along with these predicted dynamical trends. >