FEMTOSECOND SPECTRAL-TEMPORAL CHARACTERISATION OF
OPTOELECTRONIC DEVICES USING FREQUENCY-RESOLVED OPTICAL GATING
D K Baxter and J Allam
Advanced Technology Institute, School of Electronics and Physical Sciences, University of Surrey, Guildford GU2 7XH
Key words to describe the work: ultrafast optics autocorrelation semiconductor laser dynamics
Key Results: The temporal symmetry inherent in second-harmonic frequency-resolved optical gating is
removed by the simple addition of a dispersive element in one of the beams, allowing determination of the
sign of the chirp of an ultrashort optical pulse.
How does the work advance the state-of-the-art?: The modified method allows unambiguous extraction of the
electric field evolution (i.e. the amplitude AND phase) of an ultrashort optical pulse.
Motivation (problems addressed): The modified FROG measurement allows a complete determination of the
spectral-temporal dynamics in semiconductor optoelectronic devices such as laser diodes, modulators and
optical switches for Terabit/s optical communication systems.
Introduction:
Semiconductor optoelectronic devices such as laser
diodes, modulators and optical switches are
important components for optical communications.
The communication bandwidth is limited by the
switching and recovery times, i.e. by the dynamics
of internal processes (eg. energy relaxation) in the
semiconductor. These may occur on picosecond or
shorter timescales, and therefore must be
investigated using ‘ultrafast’ methods such as
autocorrelation or pump-probe, using pulses from a
femtosecond mode-locked laser. The optical phase
dynamics are also important (e.g. in interferometric
optical switches). In addition, ‘chirp’ (variation of
the output or transmitted central wavelength 0 as a
function of time) causes cross-talk in multiplexed
systems which also limits the bandwidth. However,
conventional intensity autocorrelation methods do
not yield the phase information and hence the chirp.
Frequency-resolved Optical Gating:
Frequency-resolved optical gating1 (FROG)
measurements
involve
the
simultaneous
measurement of the temporal and spectral evolution
of ultrashort pulses. The simplest implementation is
second-harmonic FROG (SH-FROG) shown in
Figure 1, where the frequency doubled light (2)
from second harmonic generation (SHG) in a
conventional autocorrelator is spectrally resolved.
The FROG measurement yields the detected light
intensity I() as a function of wavelength and
time delay  (the path difference). From this
information the evolution of the amplitude AND
PHASE of the optical pulse (i.e. the complete
electric field) can be extracted. However the SHFROG traces are symmetric in time due to the
beam splitter
time
delay
A
B
SHG
crystal
lens
dispersive plate
2
Spectro
-meter
filter
Fig. 1. Schematic
of
modified
SH-FROG
measurement. The dispersive plate is
positioned in either beam A or B to distinguish
the direction of chirp.
indistinguishability of the two beams A and B, and
hence the direction of chirp cannot be determined.
We have investigated a simple method for
identifying the direction of chirp in SH-FROG. A
thin silica plate is inserted into one of the optical
beams A or B. Optical dispersion induces a chirp in
the optical pulse passing through the plate, which is
detected as a shift in the central wavelength of the
frequency-doubled spectrum depending on the time
delay . The direction of the shift is reversed when
the plate is shifted from beam A to beam B, due to
the effective time reversal.
The method is a little different from XFROG2, in
which the optical properties of the plate in beam A
would be measured by cross-correlation with a
previously-characterised reference beam B. SHFROG characterisation of the reference beam is of
course still subject to time-reversal ambiguity. This
method is also related to the POLKADOT FROG
method3 in which an etalon is used to create a
double pulse into the autocorrelator.
Experimental results:
The FROG trace of a ~80fs optical pulse with a
centre wavelength of 780nm from a SpectraPhysics Tsunami laser is shown in Fig. 2(a). The
false colour indicates the detected intensity I().
The autocorrelation width is 120 fs and the spectral
width is 4.7nm. A small tilt in the I() trace
indicates chirp in beam A resulting from optical
dispersion induced by the beamsplitter (~8.5mm
path length).
(a)
(b)
dispersion prism-pair ahead of the autocorrelator.
Removal of the dispersive plate then induces a
known chirp as in 2(c). The plate also serves to
calibrate the time axis of the autocorrelator.
Future work:
We are interested in the spectral-temporal dynamics
of semiconductor optoelectronic devices such as
laser diodes, optical amplifiers, and modulators. Of
particular interest are devices working at 1.3μm and
1.5μm made of new materials such as quantum
dots, and “dilute nitride” semiconductors.
beam splitter
time
delay
2
nm
SHG
crystal
A
O1 S
100 fs
(c)
(d)
2
O2
lens

time
Fig. 2. FROG traces (second harmonic light intensity
as a function of wavelength and time delay) with
different locations of dispersive plate. (a) no plate, (b)
plate located ahead of initial beamsplitter, (c) plate in
beam A, (d) plate in beam B.
Fig. 2(b) shows the FROG trace when a 13mm
thick silica plate is located ahead of the
beamsplitter. The pulse is slightly broadened
(autocorrelation width = 142 fs) by dispersion
(wavelength-dependent velocity of light). However
the tilt is unchanged.
When the dispersive plate is located in either beam
A or B alone, the autocorrelation width is ~130fs,
intermediate between the above two cases. Fig. 2(c)
shows the FROG trace when the dispersive plate is
in beam A. The dispersion adds to that caused by
the beam splitter and the tilt (chirping of pulse in
beam A) is visibly enhanced. On the other hand,
when the plate is located in beam B, the dispersion
in both beams becomes almost identical, and the tilt
is diminished. A modified FROG extraction
algorithm would allow the extraction of the electric
field of the optical pulse without ambiguity in the
time axis. Matching the dispersion in the plate to
that in the beam splitter would allow their effect to
be simultaneously cancelled using a negative-
Spec.
B
filter
Fig. 3. Schematic of XFROG (spectrally-resolved
cross-correlation) measurement. Microscope
objectives O1 and O2 focus (and subsequently
recollimate) the light into a sample S, such as a
semiconductor laser diode.
An XFROG apparatus has been constructed (Fig. 3)
in which one (weaker) beam A is focussed onto the
facet of a waveguide device using a microscope
objective. The propagated pulse is recollimated by
a second objective before being focussed onto the
nonlinear SHG crystal. A time-delayed strong
reference pulse is spatially- and temporallyoverlapped to generate a cross-correlation signal at
2. A similar method has been used4 to observe
pulse propagation effects in a semiconductor
optical amplifier. Intensity cross-correlation
measurements of pulse propagation in laser diodes5
revealed interesting phenomena including longlived ‘dark pulses’. The spectrally-resolved
measurements should give useful information on
the origin of such effects.
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