S. H. Pyun

Continuous-wave operation of 1.5μm InGaAs∕InGaAsP∕InP quantum dot lasers at room temperature

Continuous-wave operation at room temperature from InGaAs∕InGaAsP∕InP quantum dot (QD) laser diodes (LD) has been achieved. A ridge waveguide QD LD with 7 QD-stacks in the active region lases at 1.503μm at 20°C and that with 5 QD-stacks lases at 1.445μm at room temperature. The shift in lasing wavelength is believed to be due to the difference in the quantized energy states involved in producing gain for lasing. With smaller number of QD stacks and shorter cavity length, the lasing wavelength shifts to shorter wavelength indicating that more of higher excited states are involved in producing gain. By increasing the number of QD stacks to 15, lasing at 1.56μm has been achieved under pulsed mode.

Room temperature operation of InGaAs∕InGaAsP∕InP quantum dot lasers

The growth conditions for InGaAs∕InGaAsP∕InP quantum dots (QDs) have been optimized and QDs of high luminescence efficiency and the room temperature operation of QD lasers emitting at ∼1.5μm have been demonstrated. Lattice-matched InGaAsP (λg=1.0–1.1μm) was used as a barrier layer for the InGaAs QDs and the emission wavelength was controlled by the QD composition. High-density InGaAs QDs with an areal density as high as 1.13×1011cm−2 have been grown. The integrated and peak intensity of the photoluminescence (PL) spectra at room temperature are as high as 25% and 10% of those at 10K, respectively. The room temperature PL peak intensity is about 50% that of a high-quality InGaAs∕InP quantum well. Room temperature, pulsed operation at ∼1.5μm has been achieved from broad area lasers with a 1mm cavity length. Threshold current density per QD stack of ∼430A∕cm2 is measured for the five-, seven-, and ten-stack QD lasers.

High-speed wavelength conversion in quantum dot and quantum well semiconductor optical amplifiers

We experimentally investigate wavelength conversion in quantum dot and quantum well optical amplifiers via four-wave mixing. Our results show superior conversion efficiency in a quantum dot device compared to a quantum well device with identical gain. Furthermore, a small-signal modulation bandwidth >25GHz was measured with greater than 100% efficiency. Cross talk between two simultaneously input beams was found to be 20dB below the signal power demonstrating the possibility for high-speed, multichannel performance. Cross-gain modulation measurements were performed as well and show a much smaller bandwidth of 1GHz indicating that four-wave mixing is superior for high-speed signals.

Electrically tunable slow and fast lights in a quantum-dot semiconductor optical amplifier near 1.55 μm

We have demonstrated both slow light in the absorption regime and fast light in the gain regime of a 1.55 microm quantum-dot semiconductor optical amplifier at room temperature. The theory with coherent population oscillations and four-wave mixing effects agrees well with the experimental results. We have observed a larger phase delay at the excited state than that at the ground state transition, likely due to the higher gain and smaller saturation power of the excited state.

Gain characteristics of InAs∕InGaAsP quantum dot semiconductor optical amplifiers at 1.5μm

The authors have fabricated ridge waveguide quantum dot (QD) semiconductor optical amplifiers (SOAs) on InP substrates that operate in the 1.5μm region. The active layer consists of InAs∕InGaAsP QD layers with a high dot density, but which still have good isolation between dots in the lateral and vertical directions, as confirmed by time-resolved photoluminescence measurements. One of these QD SOAs exhibited a fiber-to-fiber gain of 22.5dB and a chip gain of 37dB at 1.51μm. The spectral gain shape was found to be maintained for variations of the peak gain from 12to22dB, reflecting the zero-dimensional density of states at room temperature.

Photoluminescence and lasing characteristics of InGaAs∕InGaAsP∕InP quantum dots

The InGaAs quantum dots (QDs) were grown with InGaAsP(λg=1.0–1.1μm) barrier, and the emission wavelength was controlled by the composition of InGaAs QD material in the range between 1.35 and 1.65μm. It is observed that the lateral size increases and the height of the QDs decreases with the increase in relative concentration of trimethylgallium to trimethylindium supplied during InGaAs QD growth. It is seen that the higher concentration of group III alkyl supply per unit time leads to higher QD areal density, indicating that the higher concentration causes more QDs to nucleate. By optimizing the growth conditions, the QDs emitting at around 1.55μm were grown with an areal density as high as 8×1010cm−2. The lasing action between the first excited subband states at the wavelength of 1.488μm has been observed from the ridge waveguide lasers with five QD stacks up to 260K. The threshold current density of 3.3kA∕cm2 at 200K and a characteristic temperature of 118K were measured.

Gain dynamics of an InAs/InGaAsP quantum dot semiconductor optical amplifier operating at 1.5 μm

We have studied gain dynamics of InAs/InGaAsP quantum dot semiconductor optical amplifier. The recovery time at the ground state was ~3 ps, much faster than the previously reported values at 1.5 um.

Gain recovery in a quantum dot semiconductor optical amplifier and corresponding pattern effects in amplified optical signals at 1.5 μm

Fast gain recovery observed in quantum-dot semiconductor-optical-amplifiers (QDSOAs) is useful for amplifying high-speed optical signals. The small but finite slow recovery component can deteriorate the signal amplification due to the accumulation of gain saturation during 10 Gb/s operation. A study of the gain recoveries and pattern effects in signals amplified using a 1.5 μm InAs/InGaAsP QDSOA reveals that the gain recovery is always fast, and pattern-effect-free amplification is observed at the ground state. However, at the excited state, the slow component increases with the current, and significant pattern effects are observed. Simulations of the pattern effects agreed with the observed experimental trends.

Unambiguous observation of electronic couplings between InGaAs∕InGaAsP quantum dots emitting at 1.5μm

We have unambiguously estimated the vertical and lateral electronic couplings between quantum dots (QDs) by comparing the carrier lifetimes at different energy positions inside the ground state band. InGaAs∕InGaAsP QDs on InP(100) substrate give photoluminescence around 1.55μm and have the dot density over 1011∕cm2. The measured carrier lifetimes are almost the same across the entire photoluminescence band, indicating negligible lateral electronic coupling between QDs at this high dot density. However, for a QD sample with the 15nm barrier spacing between QD layers the lifetime increases with increasing wavelength, clearly indicating the significant vertical electronic coupling between QDs.

Effects of band-offset on the carrier lifetime in InAs quantum dots on InP substrates

The carrier lifetime of an InAs/InGaAsP quantum dot (QD) on an InP substrate is measured to be twice that of an InAs/InAlGaAs QD on the same substrate, although the ground-state energy levels and barrier heights of these QDs are comparable. These differences are interpreted in terms of the smaller conduction band-offset in InAs/InGaAsP QDs compared to InAs/InAlGaAs QDs.