A broadband SWIR/MWIR spectroscopic lidar for detection of gaseous pollutants in air is presented for doing
differential optical absorption spectroscopy (DOAS). One of the distinctive parts of the lidar is the use of a picosecond
PPMgO:LN OPG (optical parametric generator) capable of generating broadband (10 to <100 nm FWHM) and tunable
(1.5 to 3.9 μm) SWIR/MWIR light. The optical source layout and properties are presented, along with a description of
the lidar breadboard. Results from indoor simulated typical operation of the lidar will be discussed, the operation
consisting in emitting the broadband coherent light along a line of sight (LOS) and measuring the back-scattering returns from of a topographic feature or aerosols. A second distinctive part is the gated MCT-APD focal plane array used in the output plane of the grating spectrograph of the lidar system. The whole of the returned spectra is measured, within a very short time gate, at every pulse and at a resolution of a few tenths to a few nm. Light is collected by a telescope with variable focus for maximum coupling of the return to the spectrograph. Since all wavelengths are emitted and received simultaneously, the atmosphere is “frozen” during the path integrated measurement and hopefully reduces the baseline drift problem encountered in many broadband scanning approaches. The resulting path integrated gas concentrations are retrieved by fitting the molecular absorption features present in the measured spectra. The use of broadband pulses of light and of DOAS fitting procedures make it also possible to measure more than one gas at a time, including interferents. The OPG approach enables the generation of moderate FWHM continua with high spectral energy density and tunable to absorption features of a great number of molecules. Proposed follow-on work and applications will also be presented.
Remote sensing or stand-off detection using controlled light sources is a well known and often used technique for
atmospheric and surface spatial mapping. Today, ground based, vehicle-borne and airborne systems are able to cover
large areas with high accuracy and good reliability. This kind of detection based on LiDAR (Light Detection and
Ranging) or active Differential Optical Absorption Spectroscopy (DOAS) technologies, measures optical responses from
controlled illumination of targets. Properties that can be recorded include volume back-scattering, surface reflectivity,
molecular absorption, induced fluorescence and Raman scattering. The various elastic and inelastic backscattering
responses allow the identification or characterization of content of the target volumes or surfaces. INO has developed
instrumentations to measure distance to solid targets and monitor particles suspended in the air or in water in real time.
Our full waveform LiDAR system is designed for use in numerous applications in environmental or process monitoring
such as dust detection systems, aerosol (pesticide) drift monitoring, liquid level sensing or underwater bathymetric
LiDARs. Our gated imaging developments are used as aids in visibility enhancement or in remote sensing spectroscopy.
Furthermore, when coupled with a spectrograph having a large number of channels, the technique becomes active
multispectral/hyperspectral detection or imaging allowing measurement of ultra-violet laser induced fluorescence (UV
LIF), time resolved fluorescence (in the ns to ms range) as well as gated Raman spectroscopy. These latter techniques
make possible the stand-off detection of bio-aerosols, drugs, explosives as well as the identification of mineral content
for geological survey. This paper reviews the latest technology developments in active remote sensing at INO and
presents on-going projects conducted to address future applications in environmental monitoring.
A SWIR/MWIR spectroscopic lidar is proposed for standoff bio-agent cloud detection using simultaneous broadband
differential scattering (DISC). Measurements and/or modeling of DISC spectra of simulants are revisited and the rational
of the SWIR/MWIR DISC approach is explained, especially in light of the LWIR DISC experiments and conclusions
done elsewhere. Preliminary results on the construction of a low power non-linear broadband source in the SWIR/MWIR
are presented. Light from a 1064-nm pump laser is passed through a period and temperature tunable PPMgO:LN Optical
Parametric Generator (OPG) to generate broadband light with a full width at half maximum (FWHM) of 10 to >100 nm
in the SWIR/MWIR between 1.5 and 3.9 μm. Broadband coherent light from this source is to be emitted towards a cloud
that generates back-scattering. This source is being used in a short-range chemical remote detection breadboard, showing
the possible dual use of the setup. Light collected by the receiver telescope is coupled to a grating spectrometer and the
return signal (DISC in the proposed setup) is detected using a gated MCT-APD array in much the same way clouds are
interrogated using UV-LIF. A programmable volume of space along the laser beam path is imaged at the entrance of the
spectrometer and 320 spectral channels can be measured simultaneously, attenuating the effects of atmospheric
instabilities on DISC measurements. Proposed follow-on work will be presented.
We have developed a small, relatively lightweight and efficient lidar instrument for remotely detecting and classifying
minerals. The system is based on a pulsed, eye-safe, diode pumped Nd:YAG laser, tripled (355nm) or quadrupled
(266nm), for UV excitation of minerals, which then fluoresce with a typical spectrum and lifetime. Fluorescence is
detected through a telescope / filter / fiber bundle / spectrograph / multi-channel detector system capable of photon
counting. Transmission and detection efficiency have been optimized to reduce the need for high optical excitation
energy. Detection electronics are based on gated charge integration using a multi-anode photomultiplier tube. Spectra
shown are measured in the 420 to 720 nm visible range with 355 nm laser excitation. Results show that it is relatively
easy to distinguish between vegetation and non-vegetation spectra using lifetime data. Lifetime of vegetation is
relatively short when compared to the mineral samples investigated. Although results shown are measured in a
controlled environment on the ground, the system is being developed for eventual use in a low altitude airborne
application. System parameters are presented and upgrade paths are discussed.
Periodically Poled Lithium Niobate (PPLN) components are very promising for the development of active photonic
devices. Most of the PPLN devices currently fabricated are based on the use of z-cut wafers. We report on the
fabrication of Periodically Poled Lithium Niobate on x-cut substrates by the application of a high electric pulse. The
technique is taking advantage of the use of high voltage amplifier to generate the needed high voltage waveform. The
shape of the pulse is controlled by feedback to maintain the poling charge constant. This approach makes the total charge
independent from the electric field amplitude and the pulse duration. Using this system we show evidence that a long
poling pulse improves the domains wall propagation in the forward direction e.g. from Z+ to the Z- side. We also
observed an improvement of the quality of the inverted domains when controlling the poling current to a low constant
value. High nucleation spike was also critical to obtain uniform inversion and repeatable poling curves for different
samples. Using adequate pulse duration, nucleation spike and charges amount, uniform 1 micrometer deep PPLN were
successfully fabricated on x-cut substrates.
A quasi-distributed fibre optics polarimetric sensor has been studied both theoretically and experimentally. Theoretical results demonstrate the feasibility of such a sensor when a polarization-maintaining optical fibre has been properly designed. Two designs are proposed. Distributed detection of water has been demonstrated with a standard polarization-maintaining fibre whose cladding was locally removed by chemical etching.
We present experimental results demonstrating the possibility of obtaining low-loss splices of microstructured optical fibers (MOFs) by using conventional electric-arc splicers. We show evidence of the effectiveness of the method by splicing two MOFs together as well as a MOF with a standard single mode fiber (SMF). The results are presented in terms of fusion losses and tensile strength. Theoretical calculations of the losses attributable to mode mismatch between the MOF and the SMF suggest that the splicing losses could be further reduced by optimizing the MOF design parameters. For the case of a MOF-MOF splicing, the loss that could be due to a possible rotational misalignment that comes with the non-cylindrical symmetry of the modal distribution is also evaluated.
We describe the fabrication process of silicon nitride (Si<sub>3</sub>N<sub>4</sub>) based two-dimensional photonic crystals. The fabrication process mainly involves e-beam direct-write lithography and reactive ion etching. The concerned photonic crystal structures consist of a periodic arrangement of sub-micrometric holes transferred into a suspended Si<sub>3</sub>N<sub>4</sub> membrane using a poly-methylmethacrylate resist layer as a mask. Numerical simulations based on a plane wave expansion method for 2D photonic band gap approximation were conducted to determine the design parameters of the photonic crystal membranes. Flat and stress free photonic crystal membranes were achieved with very good control in sidewall profile and feature shape.
Self-guided waves that can be excited in quadratic nonlinear media have been extensively studied for their potential applications in ultra-fast all-optical processing. We have previously reported the use of solitary waves collision in a KTP crystal to experimentally demonstrate all-optical switching of IR picosecond pulses. Up to now, the intensity required to obtain self-trapping of a beam remained at a high level. This has been due to the lack of nonlinear crystals which combine the attributes of a large nonlinearity and phase-matching capability at experimentally convenient wavelengths. The availability of Periodically Poled Lithium Niobate can circumvent this difficulty. 2D spatial solitary waves in PPLN have been predicted theoretically and simulated numerically. In this communication we will report their experimental observation and for the first time their interaction in a 15mm long crystal. Then we will compare solitary wave behavior in KTP and PPLN, in particular self-trapping intensity threshold versus phase mismatch. We will also compare experimental data with the reslut of our computations modeling. In a last part we will show our first experimental result about 2D quadratic soliton collision in a PPLN crystal. Finally we will discuss the advantages of choosing PPLN to realize all- optical devices using solitary wave interactions.
We review recent experimental, theoretical and numerical results dealing with the generation and the interaction of solitary waves in second-order nonlinear crystals. Particular emphasis is devoted to the issues related to the collisions between two-dimensional type II second-order solitons of orthogonal polarization in a KTP crystal. We report the experimental evidence that both quasi elastic and inelastic collision (when two solitons at input merge into a single one at output) are feasible, depending on the relative transverse velocity of the interacting beams.