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<sup>11</sup> cm<sup>-2</sup>, 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 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,
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<sup>11</sup>/cm<sup>2</sup>) 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.