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sizes, with the QD layers being separated by spacer layers with thicknesses of 20 nm and 100 nm. In the DLTS measurement for the QD samples, two signals obtained from discrete energy levels of the QDs were analyzed. For the 100-nm-spacer layered QD system, the activation energies of the signals were 0.35 eV and 0.42 eV while for the 20-nm-spacer layered QD system, the activation energies of the signals were 0.36 eV and 0.43 eV, respectively. These activation energies of the QD signals could be varied by changing applied voltage during DLTS measurement.

The growth condition for In(Ga)As/InGaAsP/InP quantum dots (QDs) has been optimized and room temperature lasing has been obtained from QD laser diodes (LDs) emitting at 1.55 m. However, it is observed that many new lasing modes keep appearing at longer wavelength with increase in injection current, and the output power of the new modes at longer wavelength gets much stronger than that of the mode that started to lase rst. This behavior is explained as having been caused both by an inhomogeneously broadened gain of QD LDs where the gains at di erent wavelengths keep increasing even though the gain of the primary mode is clamped at the threshold value and by a Gaussian-like distribution of the magnitudes of density of states due to a statistical distribution of the sizes of the grown QDs.

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.

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.

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 mu m 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. (C) 2012 Optical Society of America

We experimentally investigate wavelength conversion in quantum-dot semiconductor optical amplifiers via four-wave mixing. Using four 40-GHz probes we demonstrate greater than 100% conversion over 160 GHz with a large signal-to-noise ratio.

High performance quantum dot Semiconductor Optical Amplifiers with penalty free transmission of 10 Gb/s signal with gain of ∼8.2 dB and 40 Gb/s transmission with pattern-effect-free, wide open eye have been demonstrated.

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.

Small gain peak shift of 5 nm and broadening of gain spectrum are observed for InAs/InGaAsP quantum dot laser diodes at 15 and 300 K. This observation is likely due to inhomogeneous characteristics of quantum dot laser diodes.

InGaAs/InGaAsP/InP quantum dots (QDs) semiconductor optical amplifiers were fabricated and high performance was demonstrated. The fiber-to-fiber small signal gain of 22.5 dB and the saturation output power of 10 dBm were measured with a facet coupling loss of 7.2 dB.

By utilizing the four-wave mixing effect in a quantum dot semiconductor optical amplifier, we have demonstrated multi-channel wavelength conversion with greater than 100% efficiency and small signal modulation bandwidths in excess of 25 GHz.

Slow light in the absorption regime and fast light in the gain regime are demonstrated in 1.55 μm quantum-dot semiconductor optical amplifiers at room temperature for both ground state and excited state.

We have measured I-V, L-I curves and electroluminescence spectra from InAs/InGaAsP quantum dot (QD) laser diodes (LDs) to investigate how to optimize QD LDs for high output power. The slope of an L-I curve, which is proportional to the differential quantum efficiency, decreased rapidly after lasing due to heat in cw mode. Since the heat problem is not significant in pulse mode, the efficiency is constant up to a rather high current level. In spite of the heat problem, the maximum output power is over 79 mW from a single facet in cw mode at 20 °C. At the same temperature, the lowest threshold current is 132 mA with cavity length, width and QD layers of 500 um, 5 um and 7 stacks, respectively. The characteristic temperatures of QD LD are 188 K and 111 K under pulse and cw mode, respectively. Typical lasing wavelength is around 1.55 um. The slope efficiency, internal loss and gain are 0.368 W/A, 5.2 cm-1 and 15 cm-1, respectively.

Round and dome-shaped InAs/InGaAsP QDs on InP (100) substrate were used for semiconductor optical amplifier. The diameter, height and density of QDs were 32 nm, 3.4 nm and 1.1x10 11 cm -2 , respectively. The PL peak was controlled from 1.43 to 1.57 μm and the room temperature PL yield was quite high, about 25% of the low temperature value. We have fabricated SOAs with 2 ~ 4 μm ridge width using the above mentioned QDs. The QDSOAs had a barrier of 1.1 μm emitting InGaAsP and a simple separate confinement hetero-structure with InP cladding layer. The spacing between adjacent QD layers was 40 nm to ensure no vertical electronic coupling, confirmed by time resolved PL measurements. The small signal gain was about 20 dB and the typical gain peak was around 1.54 μm, which matches well with the optical communication band. We have also measured 3-dB gain bandwidth by using a broadband light source with 200 nm flat band and found that it was around 40 nm.

We have found that the carrier lifetime of an InAs/InGaAsP quantum dot (QD) on an InP substrate is 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. The carrier lifetime of the InAs/InAlGaAs QD within the ground state PL band is shorter as the detection wavelength is longer. On the contrary, it shows the same carrier lifetime for the InAs/InGaAsP QD. The difference is interpreted in terms of the smaller conduction band‐offset in InAs/InGaAsP QDs compared to InAs/InAlGaAs QDs.

We investigate optical properties of In(Ga)As/InGaAsP/InP quantum dots (QDs) operating at 1.5 μm and device characteristics of laser diodes (LDs) made of this QD system. Round and dome-shaped QDs are formed and emission wavelength can be adjusted from 1.4 μm to 1.6 μm by changing QD growth conditions with the same InGaAsP barrier. Even though relatively large QDs were formed, the areal density of QDs is quite high (1.1×10 11 /cm 2 ) compared to other QD systems. Time-resolved photoluminescence measurements were carried out at low temperature and revealed no evidence of electronic coupling between QDs in spite of this high QD density. We also fabricate LDs with these QDs as gain medium. LDs are operated in continuous-wave mode over 40°C. The lasing spectrum shows strong inhomogeneous characteristics at room temperature. This InP based QD system would be appropriate for multi-wavelength device applications such as semiconductor optical amplifier for optical communications.

CW operation of QD lasers emitting at /spl sim/1.5 /spl mu/m at room temperature have been demonstrated. J/sub th/ per QD stack of /spl sim/430 A/cm/sup 2/ is measured for broad area lasers with 5, 7, and 10 QD stacks.

The gain and phase dynamics of a high quality quantum dot semiconductor optical amplifier were measured at various wavelengths. In the ground state (GS), the amplitude of the slow component was negligible and the fast dominant gain recovery time was 0.7 ps. In the excited state (ES), the slow component was not negligible although small. The time required to recover from 90% to 10% was 2.0 ps in the GS but gradually increased to 31 ps in the ES. This finding predicts no pattern effects in the GS, but finite pattern effects in the ES.

Selective area growth was adopted to grow high-quality quantum dots (QDs) of different energy levels on the same plane at 1.5 μm. At room temperature, the photoluminescence (PL) peak of InAs/InGaAsP QDs on InP substrate was shifted from 1445 to 1570 nm for sample 1 (from 1385 to 1485 nm for sample 2) in a plane, with a PL intensity comparable to those of regular samples grown without dielectric patterns. The dot shape was a round dome, with the density reduced by 28% and the height increased by 17%. Time-resolved PL indicated that the selectively grown QDs behaved similarly to regular QDs. These results open up a practical method for in-plane integration of QD devices.

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.

Long-wavelength emission at 1.5 m at room temperature has been achieved from InGaAs/GaAs quantum dots (QDs) grown by metal-organic chemical vapor deposition. The luminescence intensity of the InGaAs QDs was enhanced by optimizing the growth interruption duration without AsH3 supply just after the InGaAs QD layer had been formed. The use of InxGa1 xAs strain reducing layer to cover the InGaAs QDs shifted the QD emission wavelength to longer wavelength and made the PL wavelength reach 1.5 m at room temperature. The luminescence from the ground state and those from excited states were seen and this coexistence, and the change in the luminescence intensity with excitation power were explained by the coexistence of unoccupied ground states and excited states in di erent QDs that are electrically isolated from each other in a QD ensemble.

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.

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.

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.

Nitridized quantum dots (QDs) were prepared by metal-organic chemical vapor deposition. These QDs all showed strong photoluminescence (PL) emission at 1.3μm at room temperature, narrow spectral widths of 30meV, and large separations of 98meV between the ground and first excited states. Interestingly, the PL peak positions of the nitridized QDs were all around 1.3μm, despite the QDs having been prepared using significantly different amounts of nitrogen. Time-resolved PL revealed no electronic coupling between the QDs. These properties could potentially make these nitridized QDs very useful candidates for the fabrication of devices emitting at 1.3μm.

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.

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.

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.