A four-wavelength low-power continuous-wave frequency laser reference system has been realised in the 935.4-nm range for water vapour differential absorption lidar (DIAL) applications. The system is built around laboratory extended-cavity and DFB diode lasers. Three lasers are directly locked to three water vapour absorption lines of different strength, whereas the wavelength of the fourth laser lies out of any absorption line (offline). On-line stabilisation is performed by wavelength modulation spectroscopy technique, while precise offline stabilisation is realised by an offset locking at 18.8 GHz. Offset frequency larger than 320 GHz has also been demonstrated at 1.55 μm, based on an all-fibre optical frequency comb. First steps towards the use of a photonic crystal fibre as ultra compact reference cell with long optical pathlength were realised. The developed techniques for direct and offset-lock laser stabilisation can also be applied to other gases and wavelengths, provided the required optical components are available for the laser wavelength considered.
A novel approach for fibre distributed sensing is proposed, conceived to match as closely as possible to an ideally responding distributed sensor. It is demonstrated that it can be actually realized using fibre Bragg gratings of extremely low index contrast and continuously printed over the entire fibre length. The concept is experimentally validated over a restricted distance range that proves the huge potentialities of the technique in terms of response and precision.
A novel scheme is proposed to extend the sensing range of Brillouin optical time-domain analyzers (BOTDA).
Specially-designed erbium doped fiber amplifier (EDFA) repeaters are located every 65km fiber along the sensing cable
to achieve a total sensing length of 325km, corresponding to a 650km loop. At the end of the sensing fibre, we
experimentally demonstrated a measurement repeatability of 2°C (2σ) using a three meters spatial resolution.
In this paper we combine the use of optical pulse coding and seeded second-order Raman amplification to extend the sensing distance of Brillouin optical time-domain analysis (BOTDA) sensors. Using 255-bit Simplex coding, the power levels of the Raman pumps and the Brillouin pump and probe signals were adjusted in order to extend the real physical sensing distance of a BOTDA sensor up to 120 km away from the sensor interrogation unit, employing a 240-km long loop of standard single-mode fiber (SSMF) with no repeater. To the best of our knowledge, this is the first time that distributed measurements are carried out over such a long distance with no active device inserted into the entire sensing loop, constituting a considerable breakthrough in the field.
An analytical model is presented to describe the behavior of the acoustic wave, probe signal and Brillouin gain in
double-pulse Brillouin optical time-domain analysis (DP-BOTDA) sensors. The proposed model is a tool that provides a
full physical insight into the Brillouin interaction occurring in this double-pulse configuration, and allows the
optimization and complete analysis of the system. The proposed solution is experimentally validated in a long-range
system, which is optimized to demonstrate experimentally, for the first time, the capability of DP-BOTDA to achieve a
11 km sensing distance with 20 cm spatial resolution and a frequency resolution of 0.5 MHz.
A couple of experiments are here presented to clarify the impact of slow light on light-matter interaction. The
experiments are designed, so that the process generating slow light and the probed light-matter interaction only present a
marginal cross-effect. The impact of slow light on simple molecular absorption could be separately evaluated under
either material or structural slow light propagation in the same medium and led to an entirely different response.
Kerr effect accounts for the change in refractive index of a material with the light intensity and appears in all known
optical materials. In this work we analyze Kerr effect in structured superluminal media (e.g some specific types of
resonators). We show that Kerr effect in these structures can be cancelled or even reversed (in comparison with the Kerr
effect of the material composing the structure) depending on the group index of the structure. We also discuss some
possible realizations of structured superluminal media.
Stimulated Brillouin scattering (SBS) amplification of probe signals is highly polarization dependent. Maximum and
minimum gain values are associated with a pair of orthogonal states of polarization (SOP) at the fiber output. Since the
maximum gain is much higher than the minimum, the output probe SOP is pulled towards that of the maximum
amplification. Polarization pulling is restricted, however, by pump depletion. In this work, we propose, analyze and
demonstrate a method for enhanced SBS polarization pulling, using two orthogonal pumps: the one amplifies the probe
wave whereas the other attenuates it. The method provides the same polarization pulling as that of a single amplifying
pump, however it is considerably more tolerant to depletion.
Slow light systems are particularly attractive for analog signal processing, since their inherent limitation to a delay-bandwidth
product of 1 is less critical for analog systems such as those used in microwave photonics. We present here
the implementation of two basic functions - phase shifting and true time delaying - fully optically controlled using
stimulated Brillouin scattering in optical fibers. The combination of these two functions makes possible the
implementation of true time delays without limitation on the microwave carrier frequency using the separate carrier
tuning technique. This is illustrated by the implementation of the delaying system for the realization of a microwave
tunable notch filter.
Optical fibre sensors based on stimulated Brillouin scattering have now clearly demonstrated their excellent capability
for long-range distributed strain and temperature measurements. The fibre is used as sensing element and a value for
temperature and/or strain can be obtained from any point along the fibre. While classical configurations have practically
a spatial resolution limited by the phonon lifetime to 1 meter, novel approaches have been demonstrated these past years
that can overcome this limit. This can be achieved either by the prior activation of the acoustic wave by a long lasting
pre-pumping signal, leading to the optimized configuration using Brillouin echoes, or by probing a classically generated
steady acoustic wave using a ultra-short pulse propagating in the orthogonal polarization of a highly birefringent fibre.
These novel configurations can offer spatial resolutions in the centimetre range, while preserving the full accuracy on the
determination of temperature and strain.
We propose and experimentally demonstrate the highest-resolution BOTDA system ever
reported using Brillouin dynamic grating in a polarization-maintaining fiber (PMF). Acoustic
waves containing the information of local Brillouin frequency are generated by a long pump
pulse in one polarization, and read out by a short probe pulse in the orthogonal polarization at
a clearly distinct optical frequency from the pump. In the experiment, a distributed strain
measurement with 1 cm spatial resolution is performed over a 20 m fiber.
The absorption of light by a gas molecule has been measured comparatively using light propagating in normal conditions
and in a slow light regime. The experiment is designed to make the 2 measurements possible without modifying the
interaction conditions, so that the sole effect of slow light is unambiguously observed. A 26% group velocity reduction
induced by stimulated Brillouin scattering in a gas-filled microstructured fiber caused no observable change in the
measured absorption, so that it is proved that material slow light does not enhance Beer-Lambert absorption and has a
null impact on gas sensing or spectroscopic applications.
The ubiquitous role of optical fibers in modern photonic systems has stimulated research to realize slow and fast light
devices directly in this close-to-perfect transmission line. Recent progress in developing optically-controlled delays in
optical fibers, operating under normal environmental conditions and at telecommunications wavelengths, has paved the
way towards real applications for slow and fast light. Advanced schemes can be realized thanks to the extremely flexible
possibility to shape the gain spectrum to make it optimized for applications. Ultra wide bandwidth, delaying with flat
amplitude response and lower distortion were successfully demonstrated this way.