Time delay enhancement in stimulated-Brillouin-scattering-based slow-light systems.
ABSTRACT We show a simple method of time delay enhancement in slow-light systems based on the effect of stimulated Brillouin scattering. The method is based on the reduction of the absolute Brillouin gain by a loss produced by an additional pump laser. With this method we achieved pulse delays of nearly 100 ns in a standard single-mode fiber. In the presented approach the delay or acceleration of optical signals is decoupled from their amplification or attenuation, which allows the adaptation of the pulse amplitudes to the given application.
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Time delay enhancement in stimulated-Brillouin-
scattering-based slow-light systems
Thomas Schneider, Markus Junker, and Kai-Uwe Lauterbach
Deutsche Telekom AG, Fachhochschule Leipzig, Gustav-Freytag-Strasse 43-45, D-04277 Leipzig, Germany
Received October 5, 2006; accepted October 18, 2006;
posted November 6, 2006 (Doc. ID 75802); published January 12, 2007
We show a simple method of time delay enhancement in slow-light systems based on the effect of stimulated
Brillouin scattering. The method is based on the reduction of the absolute Brillouin gain by a loss produced
by an additional pump laser. With this method we achieved pulse delays of nearly 100 ns in a standard
single-mode fiber. In the presented approach the delay or acceleration of optical signals is decoupled from
their amplification or attenuation, which allows the adaptation of the pulse amplitudes to the given
application. © 2007 Optical Society of America
OCIS codes: 060.4370, 290.5900, 190.0190, 190.5890.
The concept of slow and fast light based on stimu-
lated Brillouin scattering (SBS) has attracted much
recent interest since it offers the possibility of devel-
oping practical devices such as optical delay lines, op-
tical buffers, and optical equalizers for telecommuni-
cations systems.1The exploitation of SBS in optical
fibers has several advantages over other slow-light
methods: (i) the SBS requires—at least in long
fibers—only a small pump power for high delays, (ii)
Brillouin scattering works in all fiber types and over
the entire transparency range of the fiber, and (iii)
off-the-shelf telecom equipment can be used for slow-
light delay lines. With SBS delay lines the group ve-
locity can be controlled over a very large range. For
instance, in a short optical fiber the group velocity
was changed from 71,000 km/s to more than the
speed of light in vacuum.2
Most of the SBS delay lines presented so far can be
seen as Brillouin amplifiers. Therefore, all delayed
pulses will be amplified, whereas all accelerated
pulses will be attenuated. Brillouin amplifiers have
three important disadvantages: (1) the amplified
spontaneous emmission noise can be about 20 dB
higher than that of an ideal amplifier,3(2) the natu-
ral bandwidth of Brillouin scattering is only about
30 MHz, and (3) the maximum pulse delay is limited
by the pump depletion. The first and second disad-
vantages can be circumvented since, under particular
circumstances, the amplified spontaneous emission
can be reduced significantly4and the Brillouin band-
width can be enhanced drastically.5But the maxi-
mum pulse delay is in most experiments only around
30 ns for the natural Brillouin bandwidth in a stan-
kilometers.1,6If four delay lines are cascaded, this
maximum delay can be enhanced to 152 ns.7On the
other hand, this approach is rather complicated,
since it requires additional equipment and the Bril-
louin properties of all fiber segments must be identi-
In Ref. 8 a SBS-based slow-light system that is not
a Brillouin amplifier was presented. In this approach
two widely separated absorption resonances were
generated by SBS, which led to a delay for pulses
whose frequency lies in between the two lines. With
this approach they achieved a pulse delay of around
Here we present for the first time, to the best of our
knowledge, a pulse delay of around 100 ns in just one
fiber spool. The amplitudes of the delayed or acceler-
ated pulses do not depend on the delay or accelera-
tion time and can be adapted to the given application.
Our concept is based on the fact that the amplifi-
cation of the pulse and therefore the pump depletion
depends on the absolute height of the Brillouin gain
in its line center, whereas the pulse delay is a func-
tion of its shape in the frequency domain. So, if the
shape remains the same and only the height is re-
duced, the amplitude of the delayed signal will be re-
duced, but its delay time will be unchanged. Because
the pump depletion can be reduced as well, higher
maximum delay times will be possible with this con-
In the insets of Fig. 1 two different Brillouin gains
can be seen; their shapes are identical in the center
region only. The right-hand inset shows the Lorentz-
ian shape of the natural Brillouin gain. It starts from
zero and is very high in the center, which would re-
ferent shapes of the Brillouin gain, shown in the insets (fB
is the Brillouin frequency shift and ?fBis the bandwidth of
the Brillouin gain).
Alteration of the group velocity index for two dif-
OPTICS LETTERS / Vol. 32, No. 3 / February 1, 2007
0146-9592/07/030220-3/$15.00 © 2007 Optical Society of America
sult in a high amplification of the pulses. The gain in
the left-hand inset is a loss, in fact; only in the center
region does it have an absolute value of zero, i.e., the
amplitude of the delayed pulse will not be altered
during the delay process. Nevertheless, the change of
the group velocity index ?ngand therefore the ex-
pected pulse delay is nearly equal for both Brillouin
A Brillouin gain shape like that in the left-hand in-
set of Fig. 1 can be generated with two pump lasers
as shown in Fig. 2. One pump laser at an optical fre-
quency of fPgenerates a gain (Stokes) at a frequency
region fP−fBand a loss (anti Stokes) at a frequency
region fP+fB(dashed curve in Fig. 2), with fBas the
Brillouin shift in the fiber. In standard single-mode
fibers this Brillouin shift is around 11 GHz. The gain
produced by the pump is broadened by a modulation
of the pump laser. A second pump laser has a fre-
quency of fP2=fP+2fB and a narrower spectrum.
Again, it generates a loss at fP2+fBand a gain at
fP2−fB(dashed–dotted curve in Fig. 2). The center of
the loss from fPcoincides with the gain center from
fP2, since fP2−fB=fP+fB. The superposition between
both spectra shows the expected shape (solid curve in
To test the properties of this idea we used the ex-
perimental setup shown in Fig. 3. A noise source with
a bandwidth of 24 MHz modulates the control cur-
rent of two laser diodes directly. The bandwidth of
the laser diodes—and the bandwidth of the generated
Brillouin gain—is determined by the modulation de-
gree of the current. This can be adjusted for each la-
ser diode independently by a potentiometer. The re-
lation between the optical powers of the two pump
lasers can be controlled with a tunable coupler. The
amplified pump waves are coupled into a 50 km stan-
dard single-mode fiber (SSMF) via the 1→2 port of a
circulator. We used such a long fiber to reduce the
pump power requirements. From the other side the
very narrow wave ??1 MHz? of a fiber laser was
coupled into the same fiber via a Mach–Zehnder
modulator (MZM). The MZM was driven by a wave-
form generator, and the signal was coupled out by
circulator port 2→3, detected by a photodiode, and
interpreted by an oscilloscope.
First we measured the spectra of the generated
Brillouin gains in the fiber. To do this we drove the
MZM in a suppressed carrier regime with a sinu-
soidal signal generated by the waveform generator.
Due to the suppressed carrier amplitude modulation,
two sidebands will be generated in the output of the
MZM. By changing the frequency of the sinus we
were able to scan one of the sidebands through the
Brillouin gain spectrum. In the spectrum the side-
band was amplified depending on the Brillouin gain
generated by the counterpropagating pump waves.
The output power of the sideband as a function of the
frequency is a measure for the unknowm spectrum.
For convenience we used an optical powermeter in-
stead of the photodiode and the oscilloscope in Fig. 3.
The results can be seen in Fig. 4.
The shape of the superimposed spectra depends on
the relations between the optical powers and the
bandwidths of the pump lasers. Therefore, it can be
controlled by the tunable coupler and the potentiom-
eter in the setup. On the left-hand side of Fig. 4, a
small gain was generated in a broad loss spectrum,
which is useful for the delay of optical signals,
whereas on the right-hand side a small loss was gen-
erated in a broad gain, which is useful for their accel-
laser diodes at optical frequencies of fpand fp2=fp1+2fB.
Brillouin gain and loss spectra generated by two
SSMF, standard single-mode fiber; PD, photodiode; FL, fi-
ber laser; LD1, LD2, laser diodes; EDFA, erbium-doped fi-
ber amplifier; C, circulator; P, potentiometer.
Principal setup (required isolators are not shown).
optical powers and bandwidths of the pump lasers.
Measured Brillouin gain spectra for two different
February 1, 2007 / Vol. 32, No. 3 / OPTICS LETTERS
With a Brillouin spectrum similar to that of the
left-hand side of Fig. 4, we delayed optical pulses that
were generated by the waveform generator. The
pulses had a temporal width of around 34 ns. For de-
tection of the pulse delay we used the setup shown in
Fig. 3. The result is presented in Fig. 5.
First we delayed the pulses in a conventional man-
ner without an additional loss spectrum. For this
measurement the power of the gain laser at the
erbium-doped fiber amplifier output was 12.4 dBm.
The pulse delay was around 45 ns (dashed curve in
Fig. 5). The amplification gain ?10 log?AD/AR??, with
ADas the amplitude of the delayed pulse and ARas
the amplitude of the reference pulse, both measured
with the photodiode, was around 16 dB (dashed curve
in the inset of Fig. 5). Then we generated an addi-
tional loss spectrum of around 15.8 dBm and ad-
justed the power of the gain laser ?16.7 dBm? so that
the amplification gain was the same as without the
loss spectrum. Because the absolute gain remains the
same while the group velocity index is increased, we
achieve a much higher pulse delay of 97 ns (dashed–
dotted curve in Fig. 5). To the best of our knowledge
this is the highest pulse delay ever reported in just
one fiber spool.
We believe that with a higher loss the change of the
group velocity index can be further increased, which
would result in higher pulse delays. In principle, the
limit up to which the loss can be increased is the Bril-
louin threshold of the pump laser fP. In this case the
change of the group velocity index, which is produced
by fP2, can be nearly doubled. The threshold of fPwill
be determined by the gain at fP−fB. With a third la-
ser at a frequency fP−2fB, the gain at fP−fBcan be
decreased, which allows a higher loss at fP+fBand
therefore a further increase of the slow-light delay.
In conclusion, we have shown a SBS slow-light con-
cept in which the amplitudes of the delayed or accel-
erated pulses do not depend on the delay or accelera-
application. We have shown that with this approach
the pump depletion can be reduced significantly,
leading to a drastic enhancement of the maximum
The authors are very thankful for the help of J.
Klinger and R. Henker of the Fachhochschule
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with a conventional Brillouin gain spectrum (dashed curve)
and with a gain spectrum similar to that shown in the left-
hand side of Fig. 4 (dashed–dotted curve). The solid curve
shows the reference signal, and in the inset the pulse am-
plitudes are shown on a logarithmic scale.
Normalized amplitudes of optical pulses delayed
OPTICS LETTERS / Vol. 32, No. 3 / February 1, 2007