We present an enhanced performance Brillouin optical time-domain analysis sensor that uses dual probes waves with optical frequency modulation and cyclic coding. The frequency modulation serves to increase the probe power that can be injected in the fiber before the onset of non-local effects and noise generated by spontaneous Brillouin scattering. This leads to higher detected signal-to-noise ratio (SNR), which is further increased by the coding gain. The enhanced SNR translates to extended range for the sensor, with experiments demonstrating 1-m spatial resolution over a 164 km fiber loop with a 3-MHz Brillouin frequency shift measurement precision at the worst contrast position. In addition, we introduce a study of the power limits that can be injected in the fiber with cyclic coding before the appearance of distortions in the decoded signal.
KEYWORDS: Temperature metrology, Modulation, Signal processing, Raman spectroscopy, Signal detection, Signal to noise ratio, Sensors, Signal attenuation, Optical filters, Spatial resolution
We demonstrate a simple technique to enhance the signal-to-noise ratio (SNR) in Brillouin optical time-domain analysis sensors by the addition of gain and loss processes. The technique is based on the shift of the pump pulse optical frequency in a double-sideband probe system, so that the gain and loss processes take place at different frequencies. In this manner, the loss and the gain do not cancel each other out, and it makes possible to take advantage of both informations at the same time, obtaining an improvement of 3 dB on the SNR. Furthermore, the technique does not need an optical filtering, so that larger improvement on SNR and a simplification of the setup are obtained. The method is experimentally demonstrated in a 101 km fiber spool, obtaining a measurement uncertainty of 2.6 MHz (2σ) at the worst-contrast position for 2 m spatial resolution. This leads, to the best of our knowledge, to the highest figure-of-merit in a BOTDA without using coding or raman amplification.
This paper presents new techniques designed to improve the performances of a BOTDR. The first one introduces a second pump to the sensor, thus doubling the Brillouin signal on the receiver. The second one uses image processing with a two-dimensional Gaussian filter whose parameters are defined. The last technique explores the possibilities offered by colour codes. The benefits of each, in terms of signal-to-noise ratio, is presented by comparing measurements over a distance range of 50km with a spatial resolution of 5m. These techniques can easily be combined and the global improvement is estimated at 10dB, compared to conventional sensors.
In this communication, a novel compact fibre Bragg grating-based thermometer for on-line temperature monitoring of drill bits is reported. Our proposed technique can potentially be used to optimize any drilling process, requiring the use of small drill bits, through direct temperature measurement at the drill bit instead of relying on indirect parameters (speed of rotation, applied force) in order to avoid an overheating as it is currently done nowadays.
A distributed fiber sensor based on a differential chirped-pulse pair Brillouin optical time domain analysis (DCPBOTDA) is proposed for sub-meter spatial resolution sensing. The technique is based on the subtraction of two measurements made with the same pump pulse widths, but differing in the final short section of the pulse by a positive or negative frequency chirp, respectively. Experimental results are compared with a precise theoretical modeling, validating the sub-meter sensing capabilities of the technique.
KEYWORDS: Sensors, Fiber optics sensors, Distance measurement, Current controlled current source, 3D metrology, Signal to noise ratio, Time metrology, Binary data, Matrices, Optical fibers
For the first time, a colour cyclic coding (CCC) is theoretically and experimentally demonstrated for Brillouin optical time-domain analysis (BOTDA) distributed sensors. Compared to traditional intensity-modulated cyclic codes, the code presents an additional gain of √2 while keeping the same number of sequences as for a colour coding. A comparison with a standard BOTDA sensor is realized and validates the theoretical coding gain.
In this paper, we propose a new Brillouin Optical Time Domain Analysis (BOTDA) set-up that combines simultaneous Brillouin gain/loss measurements with colour coding. This technique gives the advantage that the pump power can greatly be increased, compared to other coding schemes, thus increasing the sensing range. A measurement over a 200 km fiber-loop is performed, with a 3 meter spatial resolution and an accuracy of ± 3 MHz (2σ) at the end of the sensing fiber. To the best of our knowledge, this is the best result obtained with a Brillouin sensor without Raman amplification.
The possibility to customize Simplex coding for long range Brillouin Optical Time Domain Analysis is demonstrated by
“colouring” the sequences in the frequency domain. The coding gain is identical to the traditional intensity-modulated
Simplex code, though with much simplified series of sequences. The frequency-hopping pulses in return-to-zero
intensity-modulated format are generated with a Direct-Digital Synthesizer. The proof-of-concept is experimentally
demonstrated with measurements over a 50 km range (100 km fibre-loop) and a 2 meter spatial resolution.
We propose and experimentally demonstrate the possibility to use a pump signal based on bipolar pulse sequences using
single-sideband suppressed-carrier (SSB-SC) modulation in Brillouin optical time-domain analysis (BOTDA) sensors.
The SSB-SC modulated pump makes a sequential use of the Brillouin gain and loss spectra, increasing the intensity
contrast of the measurements. The method is demonstrated using bipolar Golay codes along a 50 km sensing fiber and
2 m spatial resolution. Results indicate that the use of bipolar sequences provides a higher SNR enhancement and
stronger robustness to pump depletion in comparison to BOTDA systems employing conventional unipolar sequences.
In this paper, we propose a novel coding for long range Brillouin Optical Time Analysis (BOTDA) distributed sensors
based on a combination of time and frequency pulses, resulting in an additional coding gain of √2 with respect to
traditional intensity-modulated codes. The generation of frequency-chirped pseudo-arbitrary pulses in return-to-zero
(RZ) format with a Direct-Digital Synthesizer (DDS) is presented and the coding gain is experimentally verified,
perfectly matching its theoretical value.
A simple technique to generate an optical frequency comb, based on a conventional dual-drive Mach-Zehnder intensity
modulator, has been used as optical source for a high accuracy distance measurement in an interferometric set-up. The
modulator has been driven by a direct-digital synthesizer that is able to deliver a pure ramp in frequency between 13
GHz and 14 GHz. We have obtained about 15 modes, corresponding to a spectral span of 200 GHz. This optical signal,
launched in a Michelson interferometric set-up, allowed performing absolute distance measurement by sweeping the
radio-frequency of the direct digital synthesizer. Measurements have been compared to a standard, which was a mode-locked
femtosecond laser along with a counting interferometer. Absolute distance measurements over a range of 1 to 24
meters gave an accuracy of about ± 10 microns, with a repeatability of ± 5 microns, corresponding to a sub-ppm absolute
distance measurement.
A new low-cost superheterodyne configuration, without acousto-optic modulator, is applied to the two-wavelength
interferometry for absolute distance measurement. The principle relies on a synchronized frequency sweep of two optical
signals, but with different frequency excursions. The frequency difference between the two optical waves is highly
accurate. This is realized by injecting a frequency modulated laser signal in an intensity modulator that is biased at halfwave
voltage and driven by a digitally swept radio-frequency signal between 13 and 15 GHz. This latter is a continuous
up and down ramp. The two synchronized optical signals emerging from the modulator produce in a Michelson
interferometer a distance dependent superheterodyne signal, with a variable synthetic wavelength of about 10 mm. The
superheterodyne frequency depends linearly on distance and on the radio-frequency excursion. The integration time for a
distance measurement point corresponds to the duration of single sweep (i.e. one millisecond in our case). Absolute
distance measurements from 1 to 15 meters yield an accuracy of ±50 μm, showing the validity of the technique.
Injection locking of two DFB semiconductors opens new possibilities to generate effective signals for optical sensing, in order to reach better performances. Pure waveforms can be generated with qualities exceeding those obtained using external modulators.
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