The diffraction limit is one of the main difficulties in order to achieve nano-resolution. An evolution in this domain can allow a significant advance in the field of photonic circuit fabrication. Surpassing the diffraction limit can be achieved by employing significant optical nonlinearities (nonlinear refraction and/or nonlinear absorption). In this direction we investigate the nonlinear absorption efficiency of annealed Sb2Te3 chalcogenide thin films (20 nm thickness). The studies have been performed using the Z-scan technique, employing 11 ns duration pulses at 1064 nm. The Z-scan technique has been chosen as it allows the simultaneous determination of the nonlinear refraction and the nonlinear absorption of a material, under certain conditions. Our results indicate a significant nonlinear absorption, while the nonlinear refraction is at least one order of magnitude lower. The nonlinear absorption parameter has been found to be in the order of -10-3 m/W under infrared irradiation showing the importance of the prepared materials for laser photoinscription applications. Our values are compared with previous studies, which have been carried out in the visible part of the spectrum. The possibility to perform photonic component fabrication by local modification of the linear or the nonlinear properties of the materials is also discussed.
This paper, “Broad-band absorbers for reduction of parasitic light: two alternative solutions," was presented as part of International Conference on Space Optics—ICSO 1997, held in Toulouse, France.
This paper proposes a method for improving the localization and the quantification of the optical parameters in photoacoustic (PA) tomography of biological tissues that are intrinsically heterogeneous in both optical and acoustic properties. It is based on the exploitation of both the PA signal, generated by the heterogeneous optical structures, and the secondary acoustic echoes due to the interaction between a primary PA wave generated near the tissue surface and the heterogeneous acoustic structures. These secondary echoes can also be collected through proper measurements of the PA signals. The experimental procedure is presented along with the method to filter the signal and the reconstruction algorithm that includes the account of the acoustic information.
Photoacoustic offers promising perspectives in probing and imaging subsurface optically absorbing structures in biological tissues. The optical uence absorbed is partly dissipated into heat accompanied with microdilatations that generate acoustic pressure waves, the intensity which is related to the amount of fluuence absorbed. Hence the photoacoustic signal measured offers access, at least potentially, to a local monitoring of the absorption coefficient, in 3D if tomographic measurements are considered. However, due to both the diffusing and absorbing nature of the surrounding tissues, the major part of the uence is deposited locally at the periphery of the tissue, generating an intense acoustic pressure wave that may hide relevant photoacoustic signals. Experimental strategies have been developed in order to measure exclusively the photoacoustic waves generated by the structure of interest (orthogonal illumination and detection). Temporal or more sophisticated filters (wavelets) can also be applied. However, the measurement of this primary acoustic wave carries a lot of information about the acoustically inhomogeneous nature of the medium. We propose a protocol that includes the processing of this primary intense acoustic wave, leading to the quantification of the surrounding medium sound speed, and, if appropriate to an acoustical parametric image of the heterogeneities. This information is then included as prior knowledge in the photoacoustic reconstruction scheme to improve the localization and quantification.
Multipulse laser-induced damage is an important topic for many applications of nonlinear crystals. We studied multipulse damage in X-cut KTiOPO4. Using a 6-ns Nd:YAG laser with a weakly focused beam, a fatigue phenomenon was observed. We addressed whether this phenomenon necessarily implies material modifications. Two possible models were tested, both of them predicting increasing damage probability with increasing pulse number while all material properties are kept constant. The first model, pulse energy fluctuations and depointing, increases the probed volume during multiple pulse experiments. The probability to cause damage thus increases with increasing pulse number; however, this effect is too small to explain the observed fatigue. The second model assumes a constant single-shot damage probability p1, so a multipulse experiment can be described by statistically independent resampling of the material. Very good agreement was found between the 2000-on-1 volume damage data and this statistical multipulse model. Additionally, the spot size dependency of the damage probability is well described by a precursor presence model. Supposing that laser damage precursors are transient, the presented data explain the experimental results without supposing material modifications.
Multipulse laser induced damage optical materials is an important topic for many applications of nonlinear crystals. We
studied multi pulse damage in X-cut KTiOPO4. A 6ns Nd:YAG laser has been used with a weakly focused beam. A
fatigue phenomenon has been observed and we try to clarify the question whether or not this phenomenon necessarily
implies material modifications. Two possible models have been checked, both of them predicting increasing damage
probability with increasing pulse number while all material properties are kept constant: (i) Pulse energy fluctuations and
depointing increase the probed volume during multiple pulse experiments. The probability to cause damage thus
increases with increasing pulse number. However, this effect turned out to be too small to explain the observed fatigue.
(ii) Assuming a constant single shot damage probability p1 a multipulse experiment can be described by statistically
independent resampling of the material. Very good agreement has been found between the 2000-on-1 volume damage
data and the statistical multipulse model. Additionaly the spot size dependency of the damage probability is well
described by a precursor presence model. Supposing that laser damage precursors are either transient or, if they are
permanent, irradiation of the precursor above its threshold only causes damage with a small probability, the presented
data can be interpreted without supposing material modifications.
Accurate laser damage measurements are more difficult to perform in nonlinear optical crystals than in glasses due to several effects proper to these materials or greatly enhanced in these materials. Before discussing these effects, we address the topic of error bar determination for probability measurements. Error bars for the measured damage probabilities are especially important when testing small and expensive samples like nonlinear crystals, where only few sites are used for each measurement. The mathematical basics for the numerical calculation of probability error bars corresponding to a chosen confidence level are presented. Effects that possibly modify the maximum light intensity obtained by focusing into a biaxial nonlinear crystal are mainly the focusing aberrations and self-focusing. Depending on focusing conditions, position of the focal point in the crystal, beam propagation direction, and polarization, strong aberrations may change the beam profile and drastically decrease the maximum intensity in the crystal. A correction factor based on former theoretical work is proposed for this effect. The characteristics of self-focusing are quickly reviewed for the sake of completeness, and a note on parasitic second harmonic generation is added at the end.
Laser damage measurements in nonlinear optical crystals, in particular in biaxial crystals, may be influenced by several effects proper to these materials or greatly enhanced in these materials. Before discussion of these effects, we address the topic of error bar determination for probability measurements. Error bars for the damage probabilities are important because nonlinear crystals are often small and expensive, thus only few sites are used for a single damage probability measurement. We present the mathematical basics and a flow diagram for the numerical calculation of error bars for probability measurements that correspond to a chosen confidence level. Effects that possibly modify the maximum intensity in a biaxial nonlinear crystal are: focusing aberration, walk-off and self-focusing. Depending on focusing conditions, propagation direction, polarization of the light and the position of the focus point in the crystal, strong aberrations may change the beam profile and drastically decrease the maximum intensity in the crystal. A correction factor for this effect is proposed, but quantitative corrections are not possible without taking into account the experimental beam profile after the focusing lens. The characteristics of walk-off and self-focusing have quickly been reviewed for the sake of completeness of this article. Finally, parasitic second harmonic generation may influence the laser damage behavior of crystals. The important point for laser damage measurements is that the amount of externally observed SHG after the crystal does not correspond to the maximum amount of second harmonic light inside the crystal.
We present the results of Z-scan studies on a new setup in the sub-picosecond regime (central wavelength 800nm) carried out on solid and liquid materials such as pure water and silica. These measurements are made possible thanks to a high sensitivity setting up of our Z-scan method and in-situ characterizations of the spatio-temporal parameters of the beam. Besides, with the use of a newly adapted numerical simulation, only calibration errors of measurement devices are significant. These measurements are then used to separate the different contributions to the nonlinear refractive index from nanosecond scale mechanisms like electrostriction and/or thermal relaxation.
The Z-scan method has been widely used for the estimation of the nonlinear refractive index and the nonlinear absorption coefficient of various materials which usually show quite important nonlinear behaviors. However, it still remains difficult to perform accurate measurements of small nonlinear phase shifts since the major drawbacks of the method are an important multiplicative noise and a great sensitivity to the incident beam spatial quality and to the pulses temporal profile. In order to measure accurately the nonlinear refractive index of optical glasses in the nanosecond regime we had to improve the Z-scan method sensitivity and to reduce
drastically the numerous possible errors. We have developed and optimized a Z-scan experimental setup which is well-adapted for the metrology of the nonlinear refractive index.
Self-focusing is one of the dramatic phenomena that may occur during the propagation of a high power laser beam in a nonlinear material. This phenomenon leads to a degradation of the wave front and may also lead to a photoinduced damage of the material. Realistic simulations of the propagation of high power laser beams require an accurate knowledge of the nonlinear refractive index γ. In the particular case of fused silica and in the nanosecond regime, it seems that electronic mechanisms as well as electrostriction and thermal effects can lead to a significant refractive index variation. Compared to the different methods used to measure this parmeter, the Z-scan method is simple, offers a good sensitivity and may give absolute measurements if the incident beam is accurately studied. However, this method requires a very good knowledge of the incident beam and of its propagation inside a nonlinear sample. We used a split-step propagation algorithm to simlate Z-scan curves for arbitrary beam shape, sample thickness and nonlinear phase shift. According to our simulations and a rigorous analysis of the Z-scan measured signal, it appears that some abusive approximations lead to very important errors. Thus, by reducing possible errors on the interpretation of Z-scan experimental studies, we performed accurate measurements of the nonlinear refractive index of fused silica that show the significant contribution of nanosecond mechanisms.
We present here a Z-scan based experimental setup and an adapted numerical simulation to perform absolute measurements of small nonlinear refractive indexes in the nanosecond regime, where bound electronic, as well as electrostriction and thermal effects, can occur. In order to have a reliable and stable experimental setup and a
better sensitivity, a trimmed Airy beam has been used. An accurate study of the spatio-temporal parameters of the beam allows us to take into account the real nature of the beam in the nonlinear refractive index estimation. In these conditions, measurements have been performed in different types of fused silica at 1064nm and 532nm.
A nonlinear refractive index of 5.2x10-20m2/W has been found at 1064nm and 3.5x10-20 m2/W at 532nm.
The experimental setup developed in Marseille for the laser damage testing allows a localized study. Indeed the use of a 25 micrometers for the waist of the focused beam, permits to de-correlate the extrinsic damage due to the micronic defects (visible under microscope) for the intrinsic ones (non-detectable before damage with conventional imaging systems). The probability of damage versus incident fluence is an S curve given in the range of two thresholds, SL and SH, the low and high damage thresholds. Most often the shape of probability damage curves are different between the intrinsic and the extrinsic cases. In our arrangement the beam size and the extrinsic defect size are in the same range, so by pointing at these visible defects it is possible to determine their specific threshold, and the density of defect is directly obtained from the optical image. Therefore a specific study of the intrinsic zones by pointing the beam at a zone free of extrinsic point, allows us to focus our attention only on these invisible defects. These particles are supposed to be nano-sized. The highlight and the identification of these nono-precursors is the aim of this paper.
To understand laser damage mechanisms using nano-second pulsed laser, different tools have been developed. Indeed, one challenge today is to exhibit nano-absorbing centers, supposed to be the main precursors of damage. In general, for practical reasons these tools involve a shopped CW laser, associated with a lock-in amplifier in order to exhibit local absorption in materials. Most often, no evident correlation appears between the zone revealed by the CW pump and the damage site created with pulsed laser shoot. The aim of this paper is to investigate this point, by similar experiments using both CW and pulsed laser. We will show results obtained on materials with different absorption level, using standard CW/pulsed photothermal techniques and atomic force microscopy morphology studies. A direct consequence of this study is also to evaluate the contribution of thermal effect in the laser damage process, to highlight if the case arises with other kind of mechanisms.
In order to demodulate optical fiber interferometric sensors in the coherence multiplexing scheme it is worthwhile using static interferomers. Measurement of a physical parameter is obtained by matching the optical path difference (OPD) of each interferometric sensor with a receiver. An attractive solution consists in using grating interferometer as a demodulator. Indeed, it permits one to record correlation peaks (each one corresponding to a sensor) on a photodiode array, thus making demultiplexing easier. On the other hand, it is possible to record only the peak envelope if the grating interferometer is adjusted in a well defined configuration. In this paper we concern ourselves with the theoretical determination of the conditions needed to simplify calculations and interpretation of the experimental results.
We show that a grating interferometer can be used as a receiver for quasi-distributed fibre optic sensors using white light interferometry. We report the results obtained when the sensor is based on an embedded fibre optic polarimetric interferometer for strain measurement of carbon/epoxy composites.
A perturbation method for studying the bistable behavior of a prism or grating coupler filled with a Kerr nonlinear medium is developed. A field representation limited to its first space harmonic already gives a satisfactory agreement with the much more complicated rigorous differential method. A better accuracy can be obtained by the use of the two first space harmonics. We then obtain an almost perfect agreement with rigorous theory, always with much lower computation times. All the relevant parameters useful to characterize the bistable behavior are given by this perturbation method.
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