736 CHINESE OPTICS LETTERS / Vol. 6, No. 10 / October 10, 2008
Slow and fast light in quantum-well and quantum-dot
semiconductor optical amplifiers
Invited Paper
Piotr Konrad Kondratko, Akira Matsudaira, Shu-Wei Chang, and Shun Lien Chuang
Department of Electrical and Computer Engineering, University of Illinois at Urbana-Champaign, Urbana IL, 61801, USA
Received June 16, 2008
Slow and fast light in quantum-well (QW) and quantum-dot (QD) semiconductor optical amplifiers (SOAs)
using nonlinear quantum optical effects are presented. We demonstrate electrical and optical controls of
fast light using the coherent p opulation oscillation (CPO) and four wave mixing (FWM) in the gain regime
of QW SOAs. We then consider the dependence on the wavelength and modal gain of the pump in Q W
SOAs. To enhance the tunable photonic delay of a single QW SOA, we explore a serial cascade of multiple
amplifiers. A model for the number of QW SOAs in series with variable optical attenuation is developed
and matched to the experimental data. We demonstrate the scaling law and the b andwidth control by
using the serial cascade of multiple QW SOAs. Experimentally, we achieve a phase change of 160
◦
and
a scaling factor of four at 1 GHz using the cascade of four QW SOAs. Finally, we investigate CPO and
FWM slow and fast light of QD SOAs. The experiment shows that the bandwidth of the time delay as a
function of the modulation frequency changes in the absorption and gain regimes due to the carrier-lifetime
variation. The tunable phase shift in QD SOA is compared between the ground- and first excited-state
transitions with different modal gains.
OCIS codes: 230.4320, 270.1670, 250.5980.
doi: 10.3788/COL20080610.0736.
1. Introduction
A compact, electrically a nd optically tunable optical
buffer which o perates at room temperatur e is one of the
essential elements in the future optical communication
system. An all optical buffer eliminates the need for
electrical-to-optical and optical-to-electrical conversions
of the transmitted signal. It can provide data s torage to
buffer optical signals during the traffic jam of informa-
tion flow. These requirements stimulate the investigation
of the slow and fast light. Since most of the slow- and
fast-light phenomena are based on the dispersion and
phase tuning from materials or des igned spatial struc-
tures, they can also be applied to other areas such as
dispersion compensation (pulse shaping) and the o ptical
control of microwave a ntenna arrays.
There are several approaches to implement slow a nd
fast light. The conventional one is to switch the sig-
nal pulse into a fib e r loop
[1]
. This scheme can provide
high bandwidth and multi-bit delay but requires phys-
ical switching of the signal into the fiber loop. Since
the length of the fiber is fixed, the time delay is fixed.
Thus, only discrete rather than continuous tuning is pos-
sible by using different lengths of fiber s. The second
type is based on the exotic quantum or nonlinear optical
interactions between light and matter such as e le c tro-
magnetically induced transparency (EIT) and coherent
population oscillation (CPO)
[2−4]
. A large slowdown
factor is the most spectacular feature of this approach.
However, some examples in this category require ex-
treme operation conditions such as low temperature or
bulky experimental setup. Also, the bandwidth is lim-
ited by the co rresponding dynamics of the system, and in
many cases is too narrow to be used in real applications.
The third type is to utilize the filter effect from the
designed spa tial structure such as ring resonators
[5]
or
photonic-crystal waveguides
[6]
. This approach can pro-
vide the necessary tunability and bandwidth engineering.
However, the shortcoming from dispersion still requires
improvement.
For c ompact integration with active optoelectronic de-
vices, it is desirable to implement optical buffers on
semiconductors, which can als o provide electrical and
optical control of slow and fast light. The basic working
principle of slow or fast light is to manipulate the group
velocity of the wave packet by the engineering of linear
dispersion or nonlinear propagation. An example for the
first category which is commonly used in semiconduc-
tors is CPO, and an instance of the second type is the
four wave mixing (FWM), which is usually a ccompanied
with the presence of CPO. Compared with other control
schemes, semiconductor slow-light devices can offer not
only optica l
[7,8]
but also electrical control based on the
forward current injection or reverse voltage bias
[9−16]
.
This featur e makes the semiconductor unique among
other slow- a nd fa st-light schemes.
In this letter, we first discuss the fas t light based on
CPO and FWM in quantum wells (QWs). The exper-
iments of electrical and optical control of the fast-lig ht
system are then presented with new data. Wavelength
and modal gain dependences of the pump laser are ob-
served and modeled. We show that our theoretical re-
sults agree very well with experiments. Furthermore,
we demonstrate the scaling law by using casc aded QW
semiconductor optical amplifiers (SOAs) operating above
transparency. Finally, slow and fast light in quantum-dot
(QD) SOAs are investigated using absorption and gain
by changing the bias current.
2. Slow and fast light via CPO and FWM
Coherent population osc illation is the bea ting of the
carrier density induced by an intense pump and pro be
signal. Usually, the pump is a continuous-wave (CW)
1671-7694/2008/100736-07
c
2008 Chinese Optics Letters