Photonic scanning receiver using an electrically
tuned fiber Bragg grating
P. Rugeland,1Z. Yu,1C. Sterner,1O. Tarasenko,1G. Tengstrand,2and W. Margulis1,*
1Department of Fiber Photonics, Acreo AB, Electrum 229, 164 40 Stockholm, Sweden
2Saab Avitronics, Nettovägen 6, 17588 Stockholm, Sweden
* Corresponding author: Walter.Margulis@acreo.se
Received September 8, 2009; revised October 20, 2009; accepted October 30, 2009;
posted November 3, 2009 (Doc. ID 116796); published December 4, 2009
A 5-cm-long electrically tuned fiber Bragg grating is used to filter a microwave signal on an optical carrier
at 1.55 ?m. A chirped distributed-feedback structure is employed, with a transmission bandwidth of
54 MHz and relative optical carrier rejection of ?30 dB for rf frequencies ?2 GHz. The rapid monotonic
sweep of the Bragg wavelength is translated into a fast-frequency sweep for rf analysis. © 2009 Optical So-
ciety of America
OCIS codes: 060.4005, 060.7140.
Microwave photonics [1,2] is one area in which tech-
nologies developed for optical communications can
assist rf techniques. Since the optical carrier has very
high frequency, even limited tuning in the optical do-
main corresponds to a large frequency sweep in the
microwave domain. The rf filtering with photonics
[3–7] can be compared to traditional rf filtering, e.g.
[8–10]. Recently, a fiber component was presented
, consisting of a fiber Bragg grating (FBG) written
in a microstructured fiber with internal electrodes.
When driven electrically, the refractive index is
changed and the Bragg wavelength adjusted owing to
the energy deposited in the metal. The tunability in
the optical domain is modest ??0.1 nm?, but a sweep
time as short as a few nanoseconds can be achieved.
FBGs with distributed-feedback structure (DFB) can
have narrow transmission peaks ??100 MHz? and
therefore be used for selecting an rf sideband from an
optical carrier lying only a few gigahertz away. By us-
ing DFB FBG in electrically controlled fibers, it is
possible to scan the passband and create a rapidly
sweeping transmission filter that is all in-fiber. In the
present Letter, an electrically driven 5-cm-long DFB
FBG is demonstrated to select a single sideband out
of an optical signal at ?1545 nm modulated at a few
gigahertz with rejection of ?30 dB. The transmitted
wavelength sweeps monotonically in time when the
internal electrode in the fiber is driven. Relatively
high-speed spectral analysis in the rf domain is thus
achieved, with potential use in radar warning sys-
The measurement is performed with the experi-
mental setup illustrated in Fig. 1, similar to that
used in microwave signal processing . The cw op-
tical signal is generated by an Ando AQ4321A tun-
able light source set to near ?0?1545 nm. Light is
then modulated by an incoming rf signal in a 10 GHz
LiNbO3 Mach–Zehnder modulator, filtered by the
tunable FBG, so that a single sideband is transmit-
ted and detected. The unknown rf signal to be mea-
sured in radar warning applications has frequency in
the range 2–18 GHz. Although 40 GHz modulators
can be purchased, this experiment is limited to the
range ?2–9 GHz by the available modulator. The
modulated optical spectrum consists of two sym-
metrical sidebands around the optical carrier. The fil-
ter’s frequency sweep is achieved by heating the in-
ternal electrodes of the fiber with a short electrical
pulse that shifts the Bragg wavelength of the grating
to longer wavelengths. Here, the lower sideband is
chosen, and the transmission peak of the FBG is
initially set to the wavelength that corresponds to the
?135 pm? away from the carrier. The deposited heat
shifts the grating closer to the carrier wavelength,
making the filter sweep modulation frequencies from
the highest to the lowest. During the sweep, each fre-
quency is transmitted after a particular delay. The
optical signal is then detected by an InGaAs photodi-
ode connected to an oscilloscope that registers any in-
coming signal. The photodiode monitors only the
presence of light, which translates to the presence of
a particular rf frequency, and does not follow the rf
cycles. The oscilloscope is externally triggered by the
electrical pulse generator that drives the filter, so
that the time sweep is translated into a frequency
sweep. The pulse generator yields a 20 V short elec-
trical pulse that feeds the internal electrodes of the
FBG. The entire frequency range of interest can be
covered by controlling the total energy deposited onto
the electrodes. After the current pulse, the fiber cools
back to the initial temperature by dissipating heat
into the substrate. A second (backwards) sweep re-
sults, much slower than the one associated with the
deposition of heat. Blanking can be used, so that a
single fast sweep is shown by the oscilloscope. The
repetition rate of the pulse generator is kept well be-
low 200 Hz to ensure that the deposited heat is fully
dissipated between the pulses.
filtering and rf measurement. In the experimental demon-
stration here the rf is provided by a microwave generator.
(Color online) Schematic for rapid single side-band
OPTICS LETTERS / Vol. 34, No. 24 / December 15, 2009
0146-9592/09/243794-3/$15.00© 2009 Optical Society of America
The filter fabrication starts with a silicate fiber
with diameter 125 ?m and 8.3 ?m core, drawn with
two 25 ?m holes symmetrically placed in the clad-
ding, separated from each other by 32 ?m (edge-to-
edge distance).An ?20-cm-long section of fiber is pro-
vided with internal BiSn electrodes, according to the
procedure described in . The fiber is spliced on
both sides to standard Corning SMF28 fiber, and hy-
drogen loaded at 150 atm for two weeks at room tem-
perature. For simpler thermal stabilization and
packaging, a shorter DFB grating (details below) is
chosen here, instead of the 10–20-cm-long strong
FBG reported in the past [13,14]. After recording the
grating, the piece of fiber is mounted on an alumi-
num base and covered with heat conductive but elec-
trically isolating epoxy. Contacts to the electronics
are provided by side-polishing the fiber to expose one
of the internal electrodes and bonding a 20-?m-thick
Au-coated tungsten wire . Current pulses of 0.4 A
and duration 250 ?s are applied to the 50 ? fiber
electrode. The heat deposition time is kept suffi-
ciently short so as not to melt the alloy and damage
the fiber device. The external coaxial cables are con-
nected to the electrode via an SMA contact provided
on the package.
The FBG filter consists of a 5-cm-long FBG with a
?-phase shift at its midpoint. The grating is recorded
in the center of the section of the fiber with internal
electrodes. UV radiation (25 mW at 244 nm wave-
length) is used in an interferometer that allows the
recording of an arbitrary FBG profile. To ensure that
the carrier and the unwanted sideband remain
blocked by the filter even when the transmission
peak is shifted, the stopband of the grating is made
spectrally broad by designing it with a 1 nm chirp.
Super-Gaussian apodization is used to reduce the
sidelobes. After recording, the grating is annealed at
100°C for 12 h, to remove the remaining H2. The re-
sulting index modulation is as high as ?3?10−4. The
profile of the DFB FBG fabricated in the twin-hole fi-
ber with metal electrodes and measured with a tun-
able light source and optical spectrum analyzer is
shown in Fig. 2, together with the simulated profile.
The width of the transmission peak is B=54 MHz
measured by scanning a Photonetics 3642 PY SC
tunable external cavity laser, assuming that the mea-
surement is not limited by the bandwidth of the laser
source, believed to be ?20 MHz. The dual peaks of
the transmission spectra seen in Fig. 2 occur because
the grating is birefringent, owing to the internal elec-
trodes. By adjusting the input polarization state it is
possible to adjust the relative strength of the two
peaks and eliminate the unwanted transmission win-
dow. The filter fabricated had a very narrow trans-
mission peak and sharply increased reflection in the
entire band 15–270 pm from the peak. The measured
relative rejection of the filter 15 pm ?2 GHz? away
from the peak is 33 dB in the optical domain, increas-
ing for larger spectral separation. The good rejection
of the optical carrier and unwanted sideband is illus-
trated in the spectral trace shown in Fig. 3. Note
that, in the laboratory experiments here, limited care
was taken to prevent the light source and the grating
from drifting due to temperature fluctuations. Stabi-
lization would be required in a hands-free system.
When the electrical pulse is initiated, the fre-
quency filter sweeps the spectrum from approxi-
mately 9–2 GHz owing to the heat deposited by the
pulse. The rapid sweep and frequency analysis can be
seen in Fig. 4(a). The amplitude variation observed
for various frequency values can be attributed to the
variation in efficiency of the modulator, which de-
creases for higher frequencies. The signal-to-noise ra-
tio (SNR) measured at every peak is better than 10:1
in the entire range, again getting worse at the higher
frequencies measured because of the modulator. The
frequency sweep measured here is monotonic. Be-
sides, as observed in Fig. 4(b), the sweeping of the
spectrum is nearly linear in time, apart from the last
region where the filter approaches 2 GHz. As the fil-
ter reaches its maximum temperature, it starts cool-
ing off and the sweep reverses. Although linearity in
time is not a necessary condition, it simplifies the
treatment of the data. The time dependence of the
peaks in the cooling phase has also been studied (not
shown) and is approximately of the form ?=?0+??
?0.87 ms. As the process is given by heat conduction,
it is reasonable to assume that the time dependence
is dominated by an exponential, as seen experimen-
tally. It is found that the FWHM of the signals mea-
sured when sweeping becomes broader than in the
steady-state mode. For example, in Fig. 4(a) the
width of the peaks is ?130 MHz as compared with
54 MHz in the static case. When sweeping during
cooling, the FWHM measured is 83 MHz. Neverthe-
(solid) spectra of DFB FBG. The secondary peak on the
right is created by birefringence of the fiber component and
is eliminated by adjusting the input polarization. Inset,
broader view of the stopband spectrum.
(Color online) Simulated (dotted) and measured
phase-shifted chirped FBG shown in Fig. 2.
Modulated signal before and after filtering by
December 15, 2009 / Vol. 34, No. 24 / OPTICS LETTERS
less, the incoming frequency can be determined from
the peak value measured to better than ?5 MHz.
In conclusion, a method is described for rapid mi-
crowave frequency measurement in the optical do-
main using an electrically tunable FBG. Here, it was
not possible to measure the entire rf spectrum of
2–18 GHz owing to limitations in the LiNbO3modu-
lator available. Nevertheless, the DFB FBG filter em-
ployed has all necessary characteristics for its use
over the entire frequency range of interest in radar
warning systems. In the experiments carried out, the
voltage pulse applied to the fiber electrode was lim-
ited to 20 V (i.e., 0.4 A into 50 ?). It has been shown
in other applications of fibers with internal electrodes
 that higher voltage can be applied for a shorter
time interval, provided that the energy deposited is
the same. The risetime can thus be made signifi-
cantly shorter (submicroseconds, if needed). How-
ever, an improved optical system would be required if
a good SNR is to be maintained, since less light
reaches the detector when the frequency sweep is
faster. Assuming a minimum signal-to-noise detect-
able ratio of 14 dB, a noise figure?loss=10 dB, and
B?50 MHz, one calculates a detection sensitivity
Pmin=−73 dBm. Also, in a final system implementa-
tion, the vibrational and temperature stability of the
FBG should deserve attention. The repetition rate of
the system can be increased to above 200 Hz by in-
troducing active cooling. Care is needed then to en-
sure that the carrier frequency remains at a stable
wavelength and does not wander in relation to the
The authors thank F. Laurell (Royal Institute of
Technology, Stockholm, Sweden) for support. This
work was carried out within Acreo Fiber Optic
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modulation frequencies during rapid sweep of metal-filled
FBG filter. (b) Time dependence of peaks from (a).
(a) Averaged detected single sideband at different
OPTICS LETTERS / Vol. 34, No. 24 / December 15, 2009