Sébastien Le Floch

High-accuracy absolute distance measurement using frequency comb referenced multiwavelength source

We propose a new approach to multiple-wavelength interferometry, targeted to high bandwidth absolute distance measurement, with nanometer accuracy over long distances. Two cw lasers are stabilized over a wide range of frequency intervals defined by an optical frequency comb, thus offering an unprecedented large choice of synthetic wavelengths. By applying a superheterodyne detection technique, we demonstrated experimentally an accuracy of 8 nm over 800 mm for target velocities up to 50 mm/s.

Bipolar optical pulse coding for performance enhancement in BOTDA sensors

A pump signal based on bipolar pulse coding and single-sideband suppressed-carried (SSB-SC) modulation is proposed for Brillouin optical time-domain analysis (BOTDA) sensors. Making a sequential use of the Brillouin gain and loss spectra, the technique is experimentally validated using bipolar complementary-correlation Golay codes along a 100 km-long fiber and 2 m spatial resolution, fully resolving a 2 m hot-spot at the end of the sensing fiber with no distortion introduced by the decoding algorithm. Experimental results, in good agreement with the theory, indicate that bipolar Golay codes provide a higher signal-to-noise ratio enhancement and stronger robustness to pump depletion in comparison to optimum unipolar pulse codes known for BOTDA sensing.

Colour simplex coding for brillouin distributed sensors

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.

Time/frequency coding for Brillouin distributed sensors

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.

Radio frequency controlled synthetic wavelength sweep for absolute distance measurement by optical interferometry

We present a new technique applied to the variable optical synthetic wavelength generation in optical interferometry. It consists of a chain of optical injection locking among three lasers: first a distributed-feedback laser is used as a master to injection lock an intensity-modulated laser that is directly modulated around 15 GHz by a radio frequency generator on a sideband. A second distributed-feedback laser is injection locked on another sideband of the intensity-modulated laser. The variable synthetic wavelength for absolute distance measurement is simply generated by sweeping the radio frequency over a range of several hundred megahertz, which corresponds to the locking range of the two slave lasers. In this condition, the uncertainty of the variable synthetic wavelength is equivalent to the radio frequency uncertainty. This latter has a relative accuracy of 10(-7) or better, resulting in a resolution of +/-25 microm for distances exceeding tens of meters. The radio frequency generator produces a linear frequency sweep of 1 ms duration (i.e., exactly equal to one absolute distance measurement acquisition time), with frequency steps of about 1 MHz. Finally, results of absolute distance measurements for ranges up to 10 m are presented.

Novel Brillouin Optical Time-Domain Analyzer for Extreme Sensing Range Using High-Power Flat Frequency-Coded Pump Pulses

In this paper, we propose a novel Brillouin optical time-domain analysis setup 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 first measurement over a 200-km fiber loop is performed, with a 3-m spatial resolution and an accuracy of ±3 MHz (2σ) at the end of the sensing fiber. In a second setup, high-power flat pump pulses are generated by applying an arbitrary waveform signal on a frequency shifter; thus, further increasing the performance of the novel Brillouin sensor. To the best of our knowledge, these are the best results obtained with a Brillouin sensor without Raman amplification.

Superheterodyne configuration for two-wavelength interferometry applied to absolute distance measurement

We present a new superheterodyne technique for long-distance measurements by two-wavelength interferometry (TWI). While conventional systems use two acousto-optic modulators to generate two different heterodyne frequencies, here the two frequencies result from synchronized sweeps of optical and radio frequencies. A distributed feedback laser source is injected in an intensity modulator that is driven at the half-wave voltage mode. A radio-frequency signal is applied to this intensity modulator to generate two optical sidebands around the optical carrier. This applied radio frequency consists of a digital ramp between 13 and 15 GHz, with 1 ms duration and with an accuracy of better than 1 ppm. Simultaneously, the laser source is frequency modulated by a current modulation that is synchronized on the radio-frequency ramp as well as on a triangle waveform. These two frequency-swept optical signals at the output of the modulator illuminate a Michelson interferometer and create two distinct distance-dependent heterodyne frequencies on the photodetector. The superheterodyne signal is then detected and bandpass filtered to retrieve the absolute distance measurement. Experiments between 1 and 15 m confirm the validity of this new concept, leading to a distance accuracy of +/- 50 microm for a 1 ms acquisition time.

Cyclic coding for Brillouin optical time-domain analyzers using probe dithering

We study the performance limits of mono-color cyclic coding applied to Brillouin optical time-domain analysis (BOTDA) sensors that use probe wave dithering. BOTDA analyzers with dithering of the probe use a dual-probe-sideband setup in which an optical frequency modulation of the probe waves along the fiber is introduced. This avoids non-local effects while keeping the Brillouin threshold at its highest level, thus preventing the spontaneous Brillouin scattering from generating noise in the deployed sensing fiber. In these conditions, it is possible to introduce an unprecedented high probe power into the sensing fiber, which leads to an enhancement of the signal-to-noise ratio (SNR) and consequently to a performance improvement of the analyzer. The addition of cyclic coding in these set-ups can further increase the SNR and accordingly enhance the performance. However, this unprecedented probe power levels that can be employed result in the appearance of detrimental effects in the measurement that had not previously been observed in other BOTDA set-ups. In this work, we analyze the distortion in the decoding process and the errors in the measurement that this distortion causes, due to three factors: the power difference of the successive pulses of a code sequence, the appearance of first-order non-local effects and the non-linear amplification of the probe wave that results when using mono-color cyclic coding of the pump pulses. We apply the results of this study to demonstrate the performance enhancement that can be achieved in a long-range dithered dual-probe BOTDA. A 164-km fiber-loop is measured with 1-m spatial resolution, obtaining 3-MHz Brillouin frequency shift measurement precision at the worst contrast location. To the best of our knowledge, this is the longest sensing distance achieved with a BOTDA sensor using mono-color cyclic coding.

Liquid-air based Fabry-Pérot cavity on fiber tip sensor

This paper presents a Fabry-Perot fiber tip sensor based on an air-liquid filled cavity. The cavity is sealed off by a thin gold coated membrane of parylene C, between 300 and 350 nm, creating a particularly flexible diaphragm. In order to retrieve and track the cavity of interest from other cavities formed within the sensor tip, a signal processing of the feedback signal is performed by inverse fast Fourier transform. The experimental sensor has been manufactured and tested for temperature, giving cavity length sensitivities of 6.1 nm/°C and 9.6 nm/°C for temperature increase and decrease respectively. The external gas pressure response gives a sensitivity of 15 nm/kPa. The fiber sensor has also been adapted for force sensing after silicone embedment and has shown a sensitivity of about 8.7 nm/mN. Finally, the sensor has been tested on insertion into a human temporal bone, proving that it could be an interesting candidate for insertion force monitoring for robotic cochlear implantation.

Bipolar pulse coding for enhanced performance in Brillouin distributed optical fiber sensors

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.