1. Introduction
The widely used Brillouin Optical time Domain Analysis (BOTDA) technique is based on
stimulated Brillouin scattering (SBS), where two counter-propagating light waves, most often a
pulsed pump and a CW probe, interact along a sensing fiber. At each point in time, the probe
wave at a specific location may be amplified by the traveling pump pulse, depending on the
frequency difference between these two light waves. By scanning the optical frequency of either
wave with respect to the other, the narrow (30MHz) Brillouin Gain Spectrum (BGS) is recovered,
and the frequency difference, gauged by the location of the peak gain, can be translated to
strain or temperature at each point along the sensing fiber. Although a very useful technique,
its currently common implementations are relatively slow, on the orders of seconds to minutes.
Recently, a technique based on Brillouin Optical Correlation Domain Analysis (BOCDA), has
experimentally achieved the true distributed measurement of a 1.3Hz fiber vibration, acquired at
a repetition rate of 20Hz at a spatial resolution of 80cm over a 100m fiber [
1K. Y. Song, M. Kishi, Z. He, and K. Hotate, “High-repetition-rate distributed Brillouin sensor based
on optical correlation-domain analysis with differential frequency
modulation,” Opt. Lett.
36(11), 2062–2064
(2011). [CrossRef] [PubMed]
]. A different approach proposes to use multiple pumps and
multiple probes to avoid the time-consuming frequency sweeping time required by the classical
BOTDA technique. Measurement speed will potentially increase but at the expense of frequency
granularity [
2A. Voskoboinik, J. Wang, B. Shamee, S. R. Nuccio, L. Zhang, M. Chitgarha, A. E. Willner, and M. Tur, “SBS-Based fiber optical sensing using frequency-domain
simultaneous tone interrogation,” J. Lightwave
Technol.
29(11), 1729–1735
(2011). [CrossRef]
]. A recently demonstrated BOTDA based
method, called Slope Assisted (SA) BOTDA, probes the fiber with a single frequency, located at
the middle of the slope of the local BGS, allowing a single pump pulse to sample fast strain
variations along the full length of the fiber [
3Y. Peled, A. Motil, L. Yaron, and M. Tur, “Slope-assisted fast distributed sensing in optical
fibers with arbitrary Brillouin profile,” Opt. Express
19(21), 19845–19854
(2011). [CrossRef] [PubMed]
,
4Y. Peled, A. Motil, L. Yaron, and M. Tur, “Distributed and dynamical Brillouin sensing in optical
fibers,” Proc. SPIE
7753, 775323, 775323-4
(2011). [CrossRef]
]. By using a specially synthesized and adaptable probe wave,
a fiber with an arbitrary distribution of the Brillouin frequency shift (BFS) can be
interrogated. Strain vibrations of up to 400Hz were demonstrated, simultaneously measured on two
different sections of an 85m long fiber, having different static Brillouin shifts and with a
spatial resolution of 1.5m. Although fully distributed and very fast, the measured strain
vibration amplitude is limited to the extent of the linear section of the BGS slope (~600
µε @ a 10ns pump pulse).
Four main factors control the sensing speed of a BOTDA setup [
5Y. Peled, A. Motil, and M. Tur, “Fast
microwave-photonics frequency sweeping for Brillouin ranging of strain or temperature,”
in Proceedings of IEEE Conference on Microwaves, Communications, Antennas and
Electronics Systems, (IEEE, 2011).
]:
1) Time of Flight: the repetition rate of the pump pulses should not exceed
, where is the group velocity speed of light traveling inside the fiber
and L is the fiber's length.
2) Averaging over ( = 10 to thousands) pump pulses is required to achieve
satisfactory signal to noise ratio (SNR), especially over long fibers.
3) Scanning granularity: in order to precisely map the Brillouin gain within the expected
dynamic range of strain/temperature variations, = 100 to 200 different frequencies should be probed.
4) Optical frequency switching speed of the sweep mechanism requires a finite time, depending
on the actual implementations: on the order of milliseconds or longer, inclusive of
stabilization.
The first length dependent factor cannot be improved. The second solely depends on the
available SNR from a single pump pulse, which in turn, depends on system design
and nonlinear effects in the fiber under test (FUT). The third factor is determined by the
required strain/temperature resolution. The first two factors are dominant for long (tens of
kilometers) sensing fibers, resulting in long acquisition times on the order of minutes. On the
other hand, when dealing with a relatively short (less than 1km) fiber, the fourth factor,
frequency switching speed, becomes dominant. The technique proposed and demonstrated in this
paper enables almost an instantaneous frequency transition (on the order of nanoseconds).
Furthermore, when dealing with a short sensing fiber, a stronger pump pulse can be launched into
the fiber without giving rise to undesired nonlinear effects, enabling a much better
SNR (per a single pump pulse), so that much fewer averages are required,
thereby reducing the weight of the second factor. Thus, if we ignore the frequency switching
time, the time required for a complete scan of the BGS, including averages and frequency steps, is given by:
In this work we demonstrate one complete distributed (1.3m spatial resolution) Brillouin scan
of a 100m long fiber using 100 optical frequencies in 120 microseconds. With 10 averages (i.e.,
in 1.2ms), a BGS was reconstructed with a BFS standard error of less than 0.25MHz.
2. Method
Here we use the well-known approach [
6M. Nikles, L. Thevenaz, and P. A. Robert, “Brillouin gain spectrum characterization in single-mode
optical fibers,” J. Lightwave Technol.
15(10), 1842–1851
(1997). [CrossRef]
] to generate both
the pump and probe waves from the same highly coherent laser by using an electro-optic modulator
to create a frequency difference between the pump (directly derived from the laser) and probe
(down shifted in frequency from that of the laser by a controllable difference on the order of
the BFS~11GHz). The modulator is normally driven by a relatively slowly sweeping YIG- or
VCO-based electronic synthesizer, whose frequency is scanned at a rate on the order of 1ms per
frequency step (or slower), to cover the frequency span of interest (100’s of MHz).
Instead, we propose the use of an arbitrary waveform generator (AWG). While the pump pulse
frequency is maintained at a fixed value, the probe frequency can be changed every
(for a 100m long fiber). This very fast change is obtained by first
writing into the deep memory of the AWG a numerical description of 1µs long sine wave of
the first frequency, followed by the numerical description of 1µs long sine wave of the
second frequency, until all
frequencies have been recorded. Then, in synchronization with the
first pump pulse, the digital to analog output stage of the AWG emits a 1µs long analog
sine wave of the first frequency, followed by a 1µs long analog sine wave of the second
frequency, destined to meet the second pump pulse, and so on and so forth until the last
frequency has been launched. This sequence of
scans defines a temporal frame of length
,
Fig. 1(a)
. For averaging and
SNR improvement, the process repeats itself for
times. Clearly, the same fast scanning idea can be equally
implemented by keeping a fixed probe frequency while fast changing the frequency of the pump
pulse,
Fig. 1(b). In either case, one could perform the
averages per frequency before switching to the next frequency.
Clearly, this mode is less demanding on the frequency switching speed but compromises the
sampling rate of the phenomenon to be sensed. Additionally, the first type of data acquisition
allows a more flexible post-processing, e.g., performing different amounts of averaging on
different fiber segments.
Fig. 1 An example of F-BOTDA fast sweep assembled from three frequencies. Normally, such a sweep
comprises between 100 and 200 different frequencies. (a) A sequence of fixed frequency pump
pulses meet probe waves of different frequencies. (b) The probe frequency is fixed while the
frequency of the pump pulse changes from one pulse to the other.
The main characteristics required from the AWG are: (i) wide bandwidth of at
least a few hundred MHz to be able to cope with the dynamic range spanned by the varying strain
and temperature along the fiber; and (ii) deep enough memory to contain all
waveforms representing the planned scan frequencies. While these two requirements are met in
full by a few currently available AWGs, it is very difficult to find an AWG which can directly
synthesize frequencies around the required center frequency of ~11GHz. Instead, as shown in Sec.
3, frequency upconversion can lift the output of ~1GHz AWGs to the Brillouin regime. We call
this variant of BOTDA: Fast BOTDA, or F-BOTDA for short.
Finally, it should be noted that a lot of data is involved in fast repeated acquisition of the
full Brillouin distance-frequency map. One method to collect, process and present the measured
data of F-BOTDA will be discussed in the following section.
3. Experiment
A highly coherent 1550nm DFB laser diode (DFB-LD), with a linewidth of 10kHz, is split into
pump and probe channels,
Fig. 2
. A complex waveform, of the shape of
Fig. 1, to be
described in more details below, feeds the probe channel Mach-Zehnder modulator (EOM1), which is
biased at its zero transmission point to generate two sidebands, the lower one for the probe
wave and the upper one to be discarded later by the fiber Bragg grating (FBG) filter. The EOM1
output is then amplified by an Erbium doped fiber amplifier (EDFA1), optionally scrambled by a
polarization scrambler (PS), and launched into one side of the fiber under test (FUT),
Fig. 3
, through an attenuator (ATT). Modulator EOM2 forms a 13ns pump pulse, which is amplified
by EDFA2 and launched into the other side of the FUT through a circulator (CIR1). The
Brillouin-amplified probe wave is finally routed to a fast photodiode (PD) by CIR1 and CIR2. A
narrow bandwidth fiber Bragg grating (FBG) filters out pump backscattering, as well as the upper
sideband generated by EOM1. Finally, the output of the photodiode is sampled at
1 GSamples/s by a real-time oscilloscope with deep memory.
Fig. 2 Experimental setup: AWG: arbitrary waveform generator, EOM: electro-optic modulator, EDFA:
Erbium-doped fiber amplifier, CIR: circulator, FBG: fiber Bragg grating, PS: polarization
scrambler, IS: isolator, ATT: attenuator, FUT: fiber under test, PD: photodiode.
Fig. 3 The 100m FUT comprising five sections of SMF fiber. The two sections of 0.9m (a) and 1.4m
(b) are mounted on manually stretching stages, making it possible to adjust their static
Brillouin frequency shifts. Additionally, audio speakers are physically attached to these two
sections in order to induce fast strain variations of various frequencies and magnitudes.
Segment c is loosed as the rest of the fiber.
Obtained from the lower sideband of the modulator output, the optical frequency of the probe
signal is given by , where is the frequency of the RF signal at the modulator input. In
standard single mode fibers at 1550nm, the BFS is around 11GHz and quite a wide range of
temperature and strain variations can be monitored by scanning the frequency
over a range of a few hundred MHz around 11GHz with a specified
granularity. Our method calls for an essentially instantaneous switching of the scanning
frequency, a task which cannot be currently performed by commercial frequency synthesizers. The
required fast switching (~1ns) can be achieved by high-speed arbitrary waveform generators
(AWGs), most of which are still limited to a few GHz. In this experiment we used a combination
of a dual-channel 500MHz AWG together with a microwave vector signal generator with I/Q inputs
to achieve a clean RF modulating signal, having a fast changing frequency, covering up to 1GHz
around 11GHz.
To generate a sequence of signals of frequencies:, the microwave signal generator frequency,
fc, was set to a fixed value around 11GHz and the two channels of
the AWG, and were programmed to synthesize cosine and sine waveforms at baseband
frequencies :
A frequency synthesizer with I/Q modulation capabilities mixes its I
() and Q () inputs with a high frequency carrier, generated by the
synthesizer at frequency , to produce a clean RF output of the required form:
Ideally, this RF signal instantaneously switches between consecutive frequencies. In practice
the switching speed is limited by the analog bandwidth of the AWG to less than 1ns. Unlike a
simple RF mixer, the I/Q modulator highly suppresses the carrier and unwanted images.
In this experiment the probe wave optical frequency was swept between
fstart = 10.8GHz and
fend = 10.998GHz
below that of the optical frequency of the pump, in
= 100 frequency steps of
fstep = 2MHz,
the duration of which should be at least
Tround_trip long. Due to
the technical specifications of the AWG, the duration of each frequency was set to1.2μs
(instead of 1μs for the 100m FUT), which is long enough for a single pump pulse to
interrogate a 120m long fiber. Using
T = 1.2μs and
= 100 leads to
. Thus, the acquisition rate was ~8.3kHz, enabling the measurement
of vibrations as fast as 4kHz. Polarization considerations may slow this fast sampling rate.
Indeed, to mitigate the dependence of the Brillouin gain on the fiber birefringence, some kind
of polarization scrambling must be invoked. Switching the pump or probe polarization between two
orthogonal states [
7K. Hotate, K. Abe, and K. Y. Song, “Suppression of signal fluctuation in Brillouin optical
correlation domain analysis system using polarization diversity
scheme,” IEEE Photon. Technol. Lett.
18(24), 2653–2655
(2006). [CrossRef]
] will reduce the sampling speed by
half. Random scrambling can also be applied with minimum impact if averaging is used to improve
the
SNR of the Brillouin signal. The reported measurements were taken without a
polarization scrambler. Instead, the polarization of the probe wave was adjusted by a
polarization controller (PC) so that both stretched sections (
a and
b
in
Fig. 3) experienced the same Brillouin gain.
The measurement ended with one long vector of 50 million samples, taken over 50ms. For
analysis, the sampled vector was segmented into shorter vectors of length
, and then to groups of short vectors each, which could then be stored as an
(1200X100) array. Each array, to be referred to later as a
temporal frame, holds a complete distance-frequency map of the distributed BGS measurement of
the fiber over frequencies and spatial points. For a measurement carried out along an interval of
seconds, such temporal frames can be assembled into a three dimensional
(distance, frequency, time) matrix, , which contains the time evolution of the BGS of every spatial
point along the fiber. A moving average of order along the axis of can now be used to improve the signal to noise ratio and to
accommodate random polarization scrambling.
4. Results
Figure 4
shows the last 20m of the 100m FUT of
Fig. 3,
having the two sections of length 90cm (
a) and 140cm (
b) statically
stretched to the same strain (~800με), corresponding to a BFS of 40MHz higher than
that of the loose fiber. Ten sequential temporal frames were averaged (
10,
1.2ms) to produce the figure. No further processing was applied.
The observed spatial resolution (~1m) is that expected from the 13ns pump pulse. Then, segments
a and
b were vibrated at 100Hz and 80Hz, respectively, using the
physically attached audio speakers. An in-between, non-vibrating fixed segment (
c)
was chosen as a static reference. With a pump pulse repetition rate of 833kHz, a continuous
measurement of the Brillouin signal was taken at a sampling rate of 1GHz for 50ms, resulting in
a 50M samples vector. With 100 scanning frequencies, the
was 120µs long, representing an effective vibrations
sampling rate of 8.33kHz. As described above, the data were arranged in the matrix
, whose three dimensions, respectively represent, distance,
frequency and time evolution.
Fig. 4 A zoom in on the last 20m of the 100m fiber, having two sections of 90cm and 140cm long
stretched to a static strain of ~800με. The interrogation of the whole 100m
fiber took 1.2ms, including 10 averages.
When averaging was required,
sequential temporal frames were averaged, effectively reducing the
vibrations sampling rate by
(833Hz for
10). Slicing
at a certain distance
z1 (i.e., along
its first dimension,
) gives a 2D, frequency-time matrix, which describes the time
evolution of the BGS of the spatial resolution cell
z1.
Figure 5
shows 3 different such slices along the FUT at
,
and
. The vibrating dynamic strains, at 100Hz and 80Hz are clearly
observed at segments
a and
b, while the BGS of segment
c
remains static. Quantitative analysis of the raw data of
Fig.
5 was performed by fitting a Lorentzian curve to the measured BGS of each time slot.
Fig. 5 On the right: Top view of
Fig. 4, indicating the
locations of the vibrating (
a and
b) and non-vibrating
(
c) segments of the FUT. On the left: the measured frequency distribution of
the BGS as a function of time at three different segments of the FUT
(
,
and
).
10. Vibrations of 100Hz and 80Hz are clearly observed at
segments
a and
b, while segment
c is static.
The calculated BGSs peaks, describing the time dependent BFS at segments
a and
b, are shown in
Fig. 6
. A movie, assembled from part of matrix
, zooming on the three sections of interest can be viewed at
Fig. 7
(
Media
1).
Fig. 6 The time dependent BFS of the two vibrating segments, as determined from the peaks of
Lorentzian fits to the BGSs of each time slot.
Fig. 7 Single-frame excerpts from a video assembled from matrix M, zooming on the last 20m of the
FUT. Vibrations of segments
a and
b are observed (
Media
1).
The measurement frequency noise was evaluated by determining the standard deviation
(std) of the BFS (after 10 averages and a Lorentzian fit) at the static
segment c, and found to be as low as 0.25MHz (equivalent to ~5
µε).
While the
full contents of the
matrix is required for the F-BOTDA technique, a different cut
through the matrix provides information on the time dependence of the Brillouin gain along the
entire fiber at
one selected frequency. Specifically, choosing this frequency
to be at the middle of the slope of the time-averaged,
Fig.
8(a)
, we obtain,
Fig. 8(b), a Slope-Assisted BOTDA
[
3Y. Peled, A. Motil, L. Yaron, and M. Tur, “Slope-assisted fast distributed sensing in optical
fibers with arbitrary Brillouin profile,” Opt. Express
19(21), 19845–19854
(2011). [CrossRef] [PubMed]
] measurement of both segments
a and
b, as well as the 15m non-vibrating in-between segment. Needless to say, this
SA-BOTDA measurement, requiring only one frequency (
) could have been obtained in only
of the time required for the F-BOTDA technique, although only one
gain point is measured.
Fig. 8 (a) A 2D cut through the 3D
matrix at
z =
zb,
describing the measured BGS of segment
b 80Hz vibrations as a function of time.
The black line is an additional cut through the plotted 2D at frequency
f =
f3dB = 10.93GHz, showing the Brillouin gain variations as a
function of time, at
z =
zb at a frequency
located at the middle of the BGS slope. Thus, if one cuts the 3D matrix at
f
=
f3dB, a full Distance-Time Gain picture for the whole fiber is
obtained from a single frequency probing. (b) A 2D cut from the 3D
matrix at
f =
f3dB,
describing the Brillouin gain as a function of time at the frequency
f3dB along the entire FUT.
f3dB
designates the frequency at the middle of the slope of the averaged BGS. This technique is
called SA-BOTDA [
3Y. Peled, A. Motil, L. Yaron, and M. Tur, “Slope-assisted fast distributed sensing in optical
fibers with arbitrary Brillouin profile,” Opt. Express
19(21), 19845–19854
(2011). [CrossRef] [PubMed]
].
5. Discussion and summary
We have proposed and demonstrated a new technique, F-BOTDA, for a very fast implementation of
BOTDA, whose speed is basically limited only by the fiber length and the number of required
averages. Fast switching of the optical frequency was achieved using an electronic arbitrary
waveform generator, although it could be equally accomplished by customized and much cheaper
electronics. This technique carries the classical BOTDA method from the static domain to the
dynamic one. The proposed technique is useful mainly for short fiber sensors, on the order of a
few hundred meters or less. As the fiber gets longer, it is the need for longer averaging, which
starts to limit the acquisition rate. This F-BOTDA method was demonstrated on a 100m long fiber,
having two vibrating segments of 90cm and 140cm towards its end. Full scans of the BGS, using
100 frequencies, were done at a rate of 8.3kHz, easily capturing the 80Hz and 100Hz induced
vibrations. With only ten averages, a fairly low standard deviation of 0.25MHz was obtained,
corresponding to ~5 µε. Since this technique is based on classical BOTDA, most
BOTDA recently introduced methods for high spatial resolution [
8A. W. Brown, B. G. Colpitts, and K. Brown, “Dark-pulse Brillouin optical time-domain sensor with
20-mm spatial resolution,” J. Lightwave Technol.
25(1), 381–386
(2007). [CrossRef]
–
10S. M. Foaleng, M. Tur, J. C. Beugnot, and L. Thevenaz, “High spatial and spectral resolution long-range sensing
using brillouin echoes,” J. Lightwave Technol.
28(20), 2993–3003
(2010). [CrossRef]
], can be employed to achieve fast
dynamical and distributed Brillouin sensing with high spatial resolution. Finally, although real
time implementations of the technique would involve the recording and processing of large amount
of data, modern data acquisition and signal processing circuitry can handle the challenge at a
post averaging sampling rate of hundreds of hertz.