Showing papers on "Pulse duration published in 1994"

Short-pulse terahertz radiation from high-intensity-laser-produced plasmas.

Abstract: The interaction between high-intensity, ultrashort laser pulses and plasmas leads to the emission of coherent, short-pulse radiation at terahertz frequencies. In this work we discuss a model for this effect and its experimental realization. Our measurements constitute the direct observation of laser-induced wake fields. From gas-density targets, resonant enhancement of the terahertz emission is observed if the plasma frequency is close to the inverse pulse length of the exciting laser pulse. At higher plasma densities, the emission of subpicosecond, unipolar electromagnetic pulses is observed. With the use of solid density targets, emission of more than 0.5 \ensuremath{\mu}J of far-infrared-radiation energy was measured. Simultaneous emission of MeV x rays and 0.6-MeV electrons was observed and correlated with the terahertz emission. This indicates that the radiative processes in such plasmas are driven by ponderomotively induced space-charge fields in excess of ${10}^{8}$ V/cm.

Experimental study of sideband generation in femtosecond fiber lasers

Abstract: We present a thorough experimental investigation of the phenomenon of soliton resonance sideband generation in femtosecond fiber lasers. The dependence of the sideband wavelengths on the dispersion and pulse length is confirmed. Third order dispersion is found to play a significant role in determining the sideband spectrum. We show that the minimum pulse length obtained in a fiber laser is determined by the cavity dispersion and relate this to loss into the sidebands. We show how the sideband spectrum can be used as a diagnostic of the fiber parameters and of the formation of ultrashort pulses in the laser. >

Pulse evolution in a broad-bandwidth Ti:sapphire laser.

Abstract: We demonstrate that by operating near the zero second- and third-order dispersion point in a self-mode-locked Ti:sapphire laser we can generate sub-10-fs pulses. Our numerical simulations show that the pulse duration is limited by fourth-order dispersion and that shorter pulses will be possible if this can be reduced. Also, by inserting a pellicle in various positions in a Ti:sapphire cavity, we have measured the intracavity pulse duration and chirp of the circulating pulse in the laser. Our results demonstrate that the pulse is shortest near the middle of the laser crystal, in one direction of propagation. In the other direction of propagation, the pulse is positively chirped and several times longer.

Mechanisms of intraocular photodisruption with picosecond and nanosecond laser pulses

Abstract: Nd:YAG laser photodisruption with nanosecond (ns) pulses is an established method for intraocular surgery. In order to assess whether an increased precision can be achieved by the use of picosecond (ps) pulses, the plasma size, the shock wave characteristics, and the cavitation bubble expansion after optical breakdown with ps- and ns-laser pulses were investigated by time-resolved photography and acoustic measurements. Nd:YAG laser pulses with a duration of 30 ps and 6 ns, respectively, were focused into a water-filled glass cuvette. Frequency doubled light from the same laser pulses was optically delayed between 2 ns and 136 ns and used as illumination light source for photography. Since the individual events were well reproducible, the shock wave and bubble wall position could be determined as a function of time. From the slope of these r(t) curves, the shock wave and bubble wall velocities were determined, and the shock wave pressure was calculated from the shock velocity. The plasma size at various laser pulse energies was measured from photographs of the plasma radiation. The breakdown thresholds at 30 ps and 6 ns pulse duration were found to be 15 μJ and 200 μJ, respectively. At threshold, ps-plasmas are shorter than ns-plasmas, but at the same pulse energy they are always ∼2.5 times longer. The initial shock pressures were 17 kbar after ps-pulses with an energy of 50 μJ, and 21 kbar after 1 mJ ns-pulses. The pressure amplitude decayed much faster after the ps-pulses. The maximum expansion velocity of the cavitation bubble was 350 m/s after a 50 μJ ps-pulse, but 1,600 m/s after a 1 mJ ns-pulse. The side effects of intraocular microsurgery associated with shock wave emission and cavitation bubble expansion can be considerably reduced by the use of ps-pulses, and new applications of photodisruption may become possible. © 1994 Wiley-Liss, Inc.

Method and apparatus for testing a communicating line using time domain reflectometry

Abstract: A device for testing an electrical line by time domain reflectometry, using a pulse generator to generate a series of test pulses, a programmable delay generator which provides a delay in the transmission of the test pulses, and an analog to digital converter which samples the reflected signals from the line and sends them to memory. The test pulse is transmitted to the line and a number of digital sample values are produced sequentially at a defined sampling rate by the delay generator, which is incremented or decremented to advance or retard the transmission of the test pulse by 1/n times the sampling period, and sample values from each pulse obtained. The above steps are repeated until transmission of the test pulse has been retarded or advanced by at least (n-1)/n times the sampling period and sample values from each pulse obtained, with the sampled values arranged in order of increasing time difference between transmission of the test pulse and sampling.

Theory of Kerr-lens mode locking: role of self-focusing and radially varying gain

Abstract: The ABCD-matrix formalism is generalized for the description of self-focusing and radially varying pump power in a nonparabolic approximation. This extended formalism is used for the investigation of the operation and for the optimization of the saturable aperture loss, the power-dependent average gain, and the resonator magnification of Kerr-lens mode-locked lasers with as well as without an internal aperture. The calculated amplitude modulation parameters are compared with the necessary condition for self-starting and for the shortest possible pulse duration limit.

Transdermal delivery of metoprolol by electroporation

Abstract: Electroporation, i.e., the creation of transient "pores" in lipid membranes leading to increased permeability, could be used to promote transdermal drug delivery. We have evaluated metoprolol permeation through full thickness hairless rat skin in vitro following electroporation with an exponentially decaying pulse. Application of electric pulses increased metoprolol permeation as compared to diffusion through untreated skin. Raising the number of twin pulses (300 V, 3 ms; followed after 1 s by 100 V, 620 ms) from 1 to 20 increased drug transport. Single pulse (100 V, 620 ms) was as effective as twin pulse application (2200 V, 1100 V or 300 V, 3 ms; followed after 1 s by 100 V, 620 ms). In order to investigate the effect of pulse voltage on metoprolol permeation, 5 single pulses (each separated by 1 min) were applied at varying voltages from 24 to 450 V (pulse time 620 ms). A linear correlation between pulse voltage and cumulative metoprolol transported after 4 h suggested that voltage controls the quantity of drug delivered. Then, the effect of pulse time on metoprolol permeation was studied by varying pulse duration of 5 single 100 V pulses from 80 to 710 ms (each pulse also separated by 1 min). Cumulative metoprolol transported after 4 h increased linearly with the pulse time. Therefore, pulse time was also a control factor of the quantity of drug delivered but to a lesser extent than the voltage at least at 100 V.(ABSTRACT TRUNCATED AT 250 WORDS)

Method of verifying capture of the heart by a pacemaker

Abstract: A method of verifying cardiac capture. A cardiac signal evoked in response to a cardiac stimulation pulse is sensed via an electrode. The sensed signal is lowpass filtered to remove noise and to pass frequencies characteristic of the evoked cardiac signal. The filtered signal is processed to render a waveform signal representing the second derivative of said filtered signal and the second derivative signal is further analyzed to detect a minimum and a maximum amplitude excursion during a selected window of time beginning at a selected time delay following delivery of the cardiac stimulation pulse. The amplitude difference between the minimum and the maximum is measured and compared to a first reference value. The amplitude of the second derivative is measured during a second selected window of time beginning at a selected time delay following delivery of cardiac stimulation pulse, and compared to a second reference value. A capture detect signal is generated if the amplitute difference exceeds the first reference value, but the amplitude does not exceed the second reference value.

Optical pulse propagation at negative group velocities due to a nearby gain line

Abstract: For a pulse with carrier frequency detuned less than an atomic plasma frequency but outside a narrow Lorentzian gain line, the group velocity will be negative. Unlike propagation at the center of an absorption line, the energy velocity is approximately equal to the group velocity, and is also negative. We show that a classical Gaussian pulse with such a carrier frequency will propagate at the negative group velocity for many atomic plasma wavelengths, before dispersion deforms the pulse shape. The peak of the transmitted pulse leaves the gain medium before the peak of the incident pulse enters, i.e., the pulse is transmitted superluminally. For a sufficiently long medium the exit pulse is well resolved from a comparison pulse traveling through an equal distance of vacuum. There is no conflict with causality, as numerical simulations with a switched-on Gaussian demonstrate. We propose an experiment to observe this kind of propagation by sending a pulsed probe beam through a Xe gas cell pumped to achieve inversion.

Soliton fiber ring laser stabilization and tuning with a broad intracavity filter

Abstract: The periodic perturbations to the soliton in passively mode-locked fiber soliton lasers cause dispersive wave shedding, which lead to sharp spectral sidebands that limit pulse duration. By using a broad intracavity birefringent plate filter, the side-bands are greatly reduced, and the pulse duration is shortened. The filter also allows wavelength tuning, and a 43 nm continuous tuning range is demonstrated for pulses of 311-357 is duration in a fully self-starting diode pumped system. >

Highly Selective and Highly Anisotropic SiO2 Etching in Pulse-Time Modulated Electron Cyclotron Resonance Plasma

Abstract: A 10-100 μs modulated electron cyclotron resonance (ECR) plasma is discharged to control the generation of reactive species in high-density, low-pressure plasma. The density ratio of CF 2 radicals to F atoms in the CHF 3 plasma correlates well with the pulse duration. This is because the generation of reactive species in the ECR plasma depends on time (10-100 μs). Moreover, we found that a collimated ion flux was generated in the pulsed plasma. This method achieves a high ratio of SiO 2 etching selectivity to Si etching and eliminates microloading effects during SiO 2 contact hole etching

Lund high-power laser facility - systems and first results

Abstract: A high-power laser facility has been established at the Lund Institute of Technology, Sweden, available to national groups and their international collaboration partners. The laboratory incorporates three major 10Hz laser systems, that can be operated individually or synchronized in combined use. Each system represents extreme optical powers in a certain sense. The main system is a terawatt laser based on chirped pulse amplification in titanium doped sapphire. It generates radiation tunable in the 760-840 nm region with a maximum power of 1.5 TW in 150 fs long pulses. A further system is set up to generate nanosecond pulses of extremely bright VUV radiation (down to 120 nm) by sum-difference four-wave mixing in krypton gas. Finally, a third system generates widely tunable radiation (200-900 nm) of a pulse duration of 10's of picoseconds. The experimental areas incorporate target chambers, spectrometers and fast electronics. The systems have been used in a number of experiments requiring high optical powers, and a brief account of the initial activities is given. Detailed studies of high-harmonic generation in inert gases are reported as well as the generation and application of hard X-rays from microplasmas. Intense whitelight generation by self-phase modulation in water has been used for biological tissue studies and for chemical pump/probe experiments. VUV radiation has been used for studies of radiative properties of excited states.

Subpicosecond pulse amplification in semiconductor laser amplifiers: theory and experiment

Abstract: We present a phenomenological model of subpicosecond pulse evolution in semiconductor laser amplifiers, which includes carrier heating effect, gain dispersion, gain saturation, and three kinds of self-phase modulations (SPM). The results obtained from this model are applied to and found to agree very well with experimental measurements of spectral distortions and time-resolved gain on a semiconductor laser amplifier with 460-fs and 2-ps input pulses. The device parameters that are used to match the experimental results are within a reasonable range, either suggested by previous experiments or by published calculations, The results show that the evolution of the pulses and their spectra is sensitively dependent on the input pulse shape and on a variety of time domain and frequency domain amplitude and phase shaping effects. Matching the theory to the experimental data suggests that among the possible causes of the carrier heating two-photon absorption (TPA) is the most important. This effect of TPA needs to be included to properly account for the observed dependence of the carrier heating gain reduction on the pulse length. By examining separately the contribution of the corresponding terms, we also prove the importance of SPM due to the carrier heating and to the instantaneous nonlinear index in shaping the output spectrum. We compare our model with Agrawal's theory, which is valid for pulses of tens of picosecond duration, and find large differences for subpicosecond pulses. For 2-ps pulses, on the other hand, the differences are fewer and less dramatic. The good agreement of the new model with experiments will now allow diagnostics and predictions of pulse evolution in semiconductor optical amplifiers from the picosecond to the subpicosecond range. >

Effects of pulsed ultrasound on the mouse neonate: hind limb paralysis and lung hemorrhage

Abstract: Exposure conditions were determined for hind limb paralysis and lung hemorrhage of neonatal mice due to pulsed exposure (10 microsecond pulse duration) to 1 MHz focused ultrasound. Spatial peak pulse average intensity and peak rarefactional pressure levels for paralysis in 50% of specimens sonicated were determined for pulse repetition frequencies of 1, 5 and 50 kHz at 10 degrees C and 2.4 s exposure duration. The results suggest that cavitation was involved in the paralysis at a pulse repetition frequency (PRF) of 50 kHz, but that cavitation took place in the coupling medium and probably not within the specimen during exposures at a PRF of 5 kHz. The results show an inverse relation between spatial peak pulse average intensity, or peak rarefactional pressure and sound on-time. Exposure conditions for lung hemorrhage were determined for a pulse duration of 10 microseconds at 10 degrees C and exposure durations of 2.4 and 180 s. The results show that the threshold exposure conditions for lung hemorrhage are much less than the conditions for cavitational or other effects reported for tissues that do not contain well defined gas bodies. In addition, the results show an inverse relation between exposure level and either exposure duration or sound on-time, suggesting that time is an important parameter associated with bubble effects.

Dynamics of laser-induced channel formation in water and influence of pulse duration on the ablation of biotissue under water with pulsed erbium-laser radiation

Abstract: The ability to use fiber-delivered erbium-laser radiation for non-contact arthroscopic meniscectomy in a liquid environment was studied. The laser radiation is transmitted through a water-vapor channel created by the leading part of the laser pulse. The dynamics of the channel formation around a submerged fiber tip was investigated with time-resolved flash photography. Strong pressure transients with amplitudes up to a few hundreds of bars measured with a needle hydrophone were found to accompany the channel formation process. Additional pressure transients in the range of kbars were observed after the laser pulse associated with the collapse of the vapor channel. Transmission measurements revealed that the duration the laser-induced channel stays open, and therefore the energy transmittable through it, is substantially determined by the laser pulse duration. The optimum pulse duration was found to be in the range between 250 and 350 µS. This was confirmed by histological evaluations of the laser incisions in meniscus: Increasing the pulse duration from 300 to 800 µs leads to a decrease in the crater depth from 1600 to 300 µm. A comparison of the histological examination after laser treatment through air and through water gave information on the influence of the vapor channel on the ablation efficiency, the cutting quality and the induced thermal damage in the adjacent tissue. The study shows that the erbium laser combined with an adequate fiber delivery system represents an effective surgical instrument liable to become increasingly accepted in orthopedic surgery.

Non-contact photothermal method for measuring thermal diffusivity and electronic defect properties of solids

Abstract: There is provided a method of measuring thermal diffusivity of solids and electronic lifetimes and defect properties of semiconductors useful for in-situ, non-destructive monitoring of engineered materials and electronic substrates. The method, termed photothermal rate window method, involves irradiating a sample with a repetitive square laser pulse of duration Tp and period T0 and monitoring the temperature profile by measuring the photothermal signal emitted from the sample. The period T0 of the repetitive heating pulse is maintained constant and the pulse duration Tp is varied in the range between 0 and T0 with the temperature measured at each value of Tp. The method of measuring semiconductor recombination lifetimes involves irradiating a sample and scanning one of either the period T0 and the pulse duration Tp of the repetitive laser pulse with the other held constant. The photothermal signal emitted from the surface is measured. Defect energy states in semiconductors are measured by irradiating the sample with a repetitive laser pulse of duration Tp and period T0 (both fixed) and monitoring the photothermal signal as the sample temperature is scanned. Defect levels are correlated with extremum in the profile. The photothermal signal in all the foregoing methods is input into a lock-in amplifier which measures the fundamental Fourier component of the signal. The output of the lock-in amplifier is fitted to a theoretical model of the photothermal response of a repetitively irradiated sample to obtain the thermal diffusivity, or the recombination lifetime.

500 GHz optical short pulse generation from a monolithic passively mode‐locked distributed Bragg reflector laser diode

Abstract: A 500 GHz optical short pulse train was generated from a passively mode‐locked distributed Bragg reflector laser diode (DBR‐LD). The repetition frequency was thirteen times the fundamental round trip frequency in the LD, and harmonic mode‐locking occurred. The pulse duration was 700 fs and the time‐bandwidth product was 0.51, very close to the transform‐limited value of a Gaussian waveform, 0.44. To our knowledge, this is the first report on the harmonic mode locking of a monolithic passively mode‐locked laser diode.

Optical disk recording device

Abstract: EFM input signal is received via an EFM input interface. Recording-timing adjustment data is received via a computer interface from a computer and stored in a register. To obtain EFM output signal having its pulse duration controlled on the basis of the EFM input signal and the record-timing adjustment data, there are further provided a clock signal generation circuit for generating clock signal by a phase-locked-loop arrangement based on a crystal oscillator, a first pair of counter and comparator driven by the clock signal to determine the pulse duration of a signal to be written, a second pair of counter and comparator that is driven by the clock signal and responsive to the output from the comparator of the first pair to start counting so as to determine a pulse duration to be adjusted, and a flip flop that is reset by the output from the comparator to provide an EFM output signal having a controlled pulse duration.

High repetition rate traveling wave optical parametric generator producing nearly bandwidth limited 50 fs infrared light pulses

Abstract: A 1 kHz Ti:sapphire femtosecond regenerative amplifier system is used to pump a lithium triborate optical parametric generator in a type II noncritical phase matching configuration and a β‐barium borate parametric amplifier. Infrared pulses near 1460 nm as short as 50 fs are achieved with a pulse duration/bandwidth product of only 0.44.

Pulse meter with pedometer function

Abstract: An electronic combined pulse meter and pedometer may be provided with only a single sensor used for determining walking pace and pulse rate. A pulse wave detector detects a pulse wave of the user and outputs a corresponding pulse wave signal to a calculating circuit and a walking state detector. The walking state detector compares the detected pulse wave with a reference level stored in a pulse wave level memory and outputs a walking state signal if the level of the detected pulse wave exceeds the reference level. Calculation control circuitry selects various constant values pre-stored in a constant value memory based upon the walking state signal and outputs the selected constant values to the calculating circuit and a display device. The calculating circuit calculates the time interval between successive pulses of the detected pulse wave signal in accordance with a clock signal and the selected constant values, and outputs the result to the display device.

Comparison of self-heating effects in bulk-silicon and SOI high-voltage devices

Abstract: A study of the self-heating effect in SOI and bulk-Si power devices which were subjected to large transient power overloads is described. The time-dependent temperature rise and decay was measured and compared in the two device types by relating the transient on-resistance of the device to its temperature. The temperature rise in SOI devices is shown to be more rapid than in bulk-Si devices in the initial stage of the power pulse. With extended pulse duration the difference between the temperature rise in SOI and bulk-Si devices converges to a constant value which is proportional to the thickness of the buried oxide. This difference is relatively small in comparison to the overall temperature rise. >

Sub-100-fs optical parametric generator pumped by a high-repetition-rate Ti:sapphire regenerative amplifier system.

Abstract: Infrared light pulses as short as 70 fs that are nearly bandwidth limited are obtained in a simple double-pass optical parametric generator with a single β-barium borate nonlinear crystal. Tunability in the 1.2–1.3-μm range is demonstrated with sub-100-fs pulse duration, microjoule pulse energy, and 1-kHz repetition rate.

Nonlinear mirror mode locking of a cw Nd:YLF laser

Abstract: Passive mode locking of a cw lamp-pumped Nd:YLF laser with the nonlinear mirror technique is reported. Nearly transform-limited pulses of 13-ps duration and 1.5-W average power at 1.047 μm have been obtained. The nonlinear mirror consists of a lithium triborate frequency-doubling crystal and a dichroic mirror with high reflectivity for the second harmonic and lower reflectivity for the fundamental frequency. The mode-locking process is self-starting, with pulse duration and stability strongly dependent on the cavity parameters.

Plasma-mediated ablation of brain tissue with picosecond laser pulses

Abstract: Plasma-mediated ablations of brain tissue have been performed using picosecond laser pulses obtained from a Nd:YLF oscillator/regenerative amplifier system. The laser pulses had a pulse duration of 35 ps at a wavelength of 1.053 µm. The pulse energy varied from 90 µJ to 550 µJ at a repetition rate of 400 Hz. The energy density at the ablation threshold was measured to be 20 J/cm2. Comparisons have been made to 19 ps laser pulses at 1.68 µm and 2.92 µm from an OPG/OPA system and to microsecond pulse trains at 2.94 µm from a free running Er:YAG laser. Light microscopy and scanning electron microscopy were performed to judge the depth and the quality of the ablated cavities. No thermal damage was induced by either of the picosecond laser systems. The Er:YAG laser, on the other hand, showed 20 µm wide lateral damage zones due to the longer pulse durations and the higher pulse energies.

The effect of laser parameters on the zone of thermal injury produced by laser ablation of biological tissue.

Abstract: A thermal model to predict the effect of laser parameters on the zone of thermal injury produced by laser ablation of biological tissue is presented. The model suggests that the Peclet number based on the optical penetration depth of laser radiation is the key parameter in determining the resulting zone of thermal injury. We show that the zone of thermal injury is minimized for Peclet numbers greater than one since the transport of energy via conduction beyond the ablation front is minimized. We also show that for Peclet numbers less than one, larger zones of thermal damage are unavoidable regardless of the laser pulse duration. The predictions of the model are compared with data available in the literature. Deviations between the model predictions and published data are discussed and the potential effects of the model assumptions, optical scattering, pyrolysis, temporal pulse shape, pulse duration, irradiance and pulse repetition rate are explored.

Investigation of the bulk laser damage of lithium triborate, LiB3O5, single crystals

Abstract: The bulk laser damage threshold of lithium triborate (LiB3O5) single crystals were determined to be 45.0 GW/cm2 at 1.064 μm, 1.1 ns pulse width. The damage threshold was dependent on pulse duration and wavelength: at 1.064 μm, 25 ns the damage threshold was 11.2 GW/cm2, at 0.532 μm, 1.0 ns the damage threshold was 26 GW/cm2; and at 0.355 μm, 0.9 ns the damage threshold was 21.9 GW/cm2. To the best of our knowledge this is the highest value reported among inorganic frequency conversion crystals, and approximately 1.8 times higher than that of fused silica.

Medication delivery control and pulse wave detection apparatus.

Abstract: A microcomputer performs a frequency analysis of pulse waves which are taken at regular time intervals from a patient through a pulse wave detection section. Then, if the amplitudes and phases of the spectra of the pulse waves fulfill some set conditions, either α-blocker or β-blocker is given to the patient, and the patient's circulatory activity is maintained in a stable condition.

Femtosecond pulse broadening in the focal region of a two‐photon fluorescence microscope

Abstract: We have determined the broadening of 130 fs and 80 fs pulses in the focus of a high numerical aperture microscope. The focal pulse length has been measured by cross-correlating two counter-propagating pulses at the focus of a 4Pi confocal microscope. This method allowed us to determine the pulse length directly in the sample. Focusing through refractive index interfaces leads to pulse broadening depending on the change in refractive index. Our results are relevant to the field of two-photon fluorescence microscopy and studies of nonlinear phenomena with high spatial resolution.

Early detection of thermal contrast in pulsed stimulated thermography

Abstract: Stimulated Infrared Thermography (TIS) is a fast and global method for NonDestructive Evaluation (NDE). Among the recently emerging NDE methods, it is probably the less intrusive one since it really needs no contact at all with the tested structure. A new inversion technique, using an early detection of the contrast, is presented to demonstrate how it recovers the depth of the defects with accuracy and partially removes the effects produced by the lateral heat diffusion. l . INTRODUCTION The main ONERAfL3C contribution in the Non Destructive field lies in the techniques of the Stimulated Infrared Thermography (TIS) which consist in a pulse uniform illumination of the surface of the studied specimen, followed by the analysis of its InfraRed (IR) emission as a function of time /I/. Section 2 of this paper is devoted to the recent improvements brought by our group to this analysis and which allows to strongly decrease the effects of the lateral heat diffusion (3-D effects). As a consequence, the method leads to a high accuracy as regards the identified in-depth position of the defects, and to a better identified defect shape (see section 3). The advantages of this new inversion will be assessed, by performing it on artificial and natural impact defects present in composite materials. 2. TIS EARLY DETECTION 2.1. Industrial set up The set-up used in the experiments described in this paper, corresponds to the front surface configuration, i.e. illumination and detection on the same surface. The pulsed sources available in our system can be either flashes (pulse duration 4 ms) or continuous IR lamps (48 kW), the pulse being achieved, in that case, by the opening/closing of mechanical shutters electromechanically driven (the smallest pulse duration obtained with this system is 200 ms). An AGEMA 880LW records the surface temperature as a function of time during periods of times up .to 10 mn after the pulse illumination. For instance, when placed at 1.2 m from the sample surface, the system allows to obtain a field of view of 20 cm and a spatial resolution of 2 mm with the objective of 12' used for these experiments. 2..2. Inversion using the early detection The detection and the characterization of the resistive subsurface defects is achieved by seeking local emergence of thermal contrast after the pulse illumination, i.e. an increase of the local temperature above the defect with respect to the temperature in a sound region, (figure la). Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:1994712 C7-48 JOURNAL DE PHYSIQUE IV Generally, the depth and the resistance of the defect are deduced from the value and the time of the maximum contrast /l/. This search is made pixel by pixel and finally leads to two synthetic images, respectively for the local depths and resistances of the subsurface defects. An alternative consists in the detection of the time corresponding to half the development of the contrast 121. The deeper the defects are, the longer these characteristic times are, with an important lateral heat diffusion as a consequence. Under these conditions, the inversion provides an underestimation of both the depths and the thermal resistances of the detected defects. a) Schematic explanations of the two detections b) Definition of the detection threshold, dT being the noise