Laser Intensity Modulation Research Articles

Nanomechanical property evaluation of tungsten thin film via frequency-domain photoacoustic microscopy

This study investigates the reduction in nanomechanical properties of a tungsten (W) thin film measured using a laser ultrasonic technique. A 96 nm thick W film was prepared via direct-current magnetron sputtering and inspected using scanning electron microscopy with energy dispersive X-ray spectroscopy, X-ray diffraction, and X-ray reflectivity techniques. Moreover, frequency-domain photoacoustic microscopy was employed to evaluate the Young's modulus (EW) of W. The velocity dispersion of a surface acoustic wave excited by an intensity-modulated laser was measured and curve-fitted using a computational finite element model to estimate EW. In addition, a high-frequency microcantilever resonance test was performed to verify the results. The results show that EW of the film is 45.3% lower than that of the macroscopic property because of the predominant β-W phase. The Young's moduli obtained from the two different techniques agree well with each other, with a relative error of 13.3%. The hardness-to-EW ratio obtained using a nanoindenter was calculated to be 0.025±0.004.

Long range dynamic displacement: precision PGC with sub-nanometer resolution in an LWSM interferometer

We propose a precision phase-generated-carrier (PGC) demodulation method with sub-nanometer resolution that avoids nonlinear errors in a laser wavelength sinusoidal modulation fiber-optic interferometer for long range dynamic displacement sensing. Using orthogonal detection and an AC-DC component extraction scheme, the PGC carrier phase delay (CPD) and laser intensity modulation phase delay can be obtained simultaneously to eliminate the nonlinear error from accompanied optical intensity modulation and CPD. Further, to realize long range displacement sensing, PGC phase modulation depth (PMD), determined by the laser wavelength modulation amplitude and the working distance of the interferometer, is required to maintain an optimal value during measurement, including initial position and dynamic movement. By combining frequency sweeping interference and modified PGC-arctan demodulation to measure real-time working distance, adaptive PMD technology is realized based on proportion control. We construct a fiber-optic Michelson and SIOS commercial interferometer for comparison and perform experiments to verify the feasibility of the proposed method. Experimental results demonstrate that an interferometer with sub-nanometer resolution and nanometer precision over a large range of 400 mm can be realized.

Effect of laser-induced ultrasound treatment on material structure in laser surface treatment for selective laser melting applications

A new mechanism for controlling the microstructure of products in manufacturing processes based on selective laser melting is proposed. The mechanism relies on generation of high-intensity ultrasonic waves in the melt pool by complex intensity-modulated laser irradiation. The experimental study and numerical modeling suggest that this control mechanism is technically feasible and can be effectively integrated into the design of modern selective laser melting machines.

Simultaneous thermal and optical characterization of semiconductor materials exhibiting high optical absorption by photopyroelectric spectroscopy

This work presents the results of the investigations on the thermal and optical properties of materials exhibiting high optical absorption. As test samples, some II-VI binary semiconductors were grown by using the Bridgman-Stockbarger method. The photopyroelectric (PPE) method, in two configurations (calorimetric and spectroscopic), was applied for both thermal and optical characterization of the specimens. The thermal diffusivity was obtained in the back detection configuration (BPPE) with an intensity-modulated laser as an excitation source. The same detection configuration, but with a tunable light source, provided information about the energy gap, the absorption coefficient, and the investigated sample's thermal diffusivity. The results obtained with these two methods were also compared with the data collected by conventional transmission and luminescence spectroscopies (absorption coefficient, energy gap). The main conclusion is the photopyroelectric spectroscopy (PPES) can provide in one measurement good quality information about the thermal and optical properties of the specimen.

In-situ thermal transport measurement of flowing fluid using modulated photothermal radiometry

In situ thermal transport measurement of flowing fluid could be useful for the characterization and diagnosis of practical thermal systems such as fluid heat exchangers and thermal energy storage systems. Despite abundant reports on the ex-situ thermal conductivity measurement of stagnant fluids, a suitable technique for the thermal conductivity measurement of flowing fluid has been rarely reported. This paper presents the thermal conductivity measurement of flowing fluid within a pipe using a non-contact modulated photothermal radiometry (MPR) technique, where the surface of the pipe is heated by an intensity-modulated laser and the heat diffuses into the fluid with suitable modulation frequency. We design a tube section with small wall thickness suitable for the MPR measurements to maximize the sensitivity of the thermal response to the fluid properties while minimizing the lateral heat spreading effect. Intrinsic thermal conductivity of different fluids was obtained within a proper range of frequency and flow velocity where the forced convection effect is negligible. The forced convection effect became prominent at high flowing velocity and at low modulation frequency, leading to higher effective thermal conductivity of the fluid. It is found that the intrinsic thermal conductivity could be obtained when the flow velocity is less than 100 mm/sec and ReD1/2Pr1/3 < 100 for DI water and Xceltherm oil under the specified experimental conditions, where ReD is the Reynolds number and Pr is the Prandtl number.

Space charge effect and application in solid insulation dielectrics

<p indent="0mm">Space charge effect and its application in solid insulation dielectrics are long-term issues with continuous development. The study of space charge effect is of great significance to high-voltage direct current (HVDC) transmission. This paper summarizes the development history and research progress of space charges in solid dielectrics, from the aspects of measurement technologies, damage, suppression, application, and simulation, etc. Currently, the most widely used space charge distribution measurement technologies can be roughly divided into two categories, acoustic effect technique and thermal effect technique, whose spatial resolution and application range are different. For the acoustic effect technique, pressure wave propagation method and pulsed electro-acoustic method are most commonly used with proven technique. The spatial resolutions of these two are similar, both can reach an average of <sc>10 μm.</sc> With extremely narrow pulses, the highest spatial resolution can reach about <sc>1 μm.</sc> The thermal effect techniques, such as thermal pulse method and laser intensity modulation method, have more advantages in spatial resolution and practicability, especially for micron-thick film detection. Future research may focus on the more widely application of space charge distribution measurement technologies to electrical equipment in operation. For the trap parameters and charge storage properties of insulation dielectrics, there is still a lack of reliable and practical measurement technologies. The results of the isothermal decay method are more reliable and credible, but the required measurement time is too long. Therefore, it has been rarely used in practice in recent years. The most commonly used methods, thermal stimulated discharge current method and photo stimulated discharge method, each has its own disadvantages. And their results for the same sample are quite different for mutual corroboration. In recent years, some researchers are more focused on piezo-stimulated current method, but this method is still in a preliminary stage of experiment and application. In general, the space charge effect has pros and cons. On one hand, space charge accumulation in solid insulation dielectrics may cause electric field distortion and discharge damage, eventually lead to breakdown and destruction of insulation. In order to prevent these damages, many effective methods have been proposed, such as surface modification of insulation dielectrics and material modification by additives. However, these methods have not yet been practically used in industrial applications. On the other hand, space charge effect also has many efficient applications. One of the most important applications is electret, a dielectric material that quasi-permanently stores real charges in surface or volume traps. A large number of significant contributions have been published on preparation process, charge characteristics and applications of electrets. The simulation research of space charges can mainly be divided into physical model research of space charge behavior and data processing of experimental results. The development of space charge simulation is relatively backward compared to other research fields as the physical model of space charge behavior has not yet been built completely. The issues of space charge effect and its application are involved with multiple factors. Although some technical and theoretical developments have been achieved in recent years, there are still some uncertain questions remained to be explored.

Sub-micrometer pyroelectric tomography of AlScN films

We report on one- to three-dimensional characterization of the pyroelectric properties of aluminum scandium nitride. By means of the laser intensity modulation method, we reconstructed the in-depth distribution of the spontaneous polarization with sub-micrometer resolution. The reconstructed profiles of the spontaneous polarization indicate that the thermal diffusivity and its temperature-dependence differ significantly from what is reported for pure aluminum nitride, which we attribute to the dominant role of phonon-alloy scattering for the heat transfer.

Simple carrier-envelope phase control and stabilization scheme for difference frequency generation-based systems.

We report about a setup for carrier-envelope phase (CEP) control and stabilization in passive systems based on difference frequency generation (DFG). The principle of this approach relies on the amplitude to phase modulation transfer in the white-light generation process. A small modulation of the pump laser intensity is used to obtain a DFG output modulated in CEP. This technique is demonstrated in a CEP-stable system pumped by an Yb-doped fiber amplifier. It is first characterized by measuring CEP modulations produced by applying arbitrary waveforms. The CEP actuator is then used for slow drifts correction in a feedback loop. The results show the capability of this simple approach for OPA/OPCPA CEP-stabilized setups.

Thermal diffusivity measurement of microscale slabs by rear-surface detection thermoreflectance technique

A new approach to measure the cross-plane thermal diffusivity of a microscale slab sample, which can be fabricated by the focused ion beam and attached to a substrate, is proposed. An intensity-modulated pump laser is applied to heat the front surface of the sample uniformly, and the thermoreflectance signal is observed at the rear surface to evaluate thermal wave transport in the material. The thermal diffusivity can be obtained by fitting the phase lags of the experimental data with a theoretical model. The model was developed for the sample with thin-film coatings and heat transfer to the substrate. Although the absorbed heat can cause a significant DC temperature increase in the microscale sample, a thin-film coating with high thermal conductivity can effectively reduce the DC temperature increase within low thermal conductivity samples. To validate the method, we conducted measurements of a fused silica sample of 2.16 µm thickness, coated with 95 nm Ti film on the front surface and 120 nm Au film on the rear surface. The measured thermal diffusivity is in good agreement with the literature value. The uncertainty analysis shows that the measurement uncertainty is within 6%. This proposed approach, designed for microscale samples, offers a unique option for thermal property measurements of special materials, such as irradiated nuclear fuel or other irradiated materials, to enable microscale property determination while minimizing sample radioactivity.

Measurement of High-temperature Thermophysical Properties of Bulk and Coatings Using Modulated Photothermal Radiometry

This paper presents the development of instrumentation for the measurement of high-temperature thermal conductivity of bulk and coatings using a modulated photothermal radiometry (MPR) method, where a sample is heated by an intensity-modulated laser to probe into different layers of the sample. While MPR has been previously established, most of the prior studies only focus on the measurement at room temperature. The MPR has not been well studied for measurements of bulk and coating materials at high temperatures, which are increasingly important for a multitude of applications, such as materials used in the concentrating solar power (CSP) plants and the nuclear reactors. MPR is a non-contact technique that utilizes the intrinsic thermal emission from the specimens for thermometry, which is favorable for measurements at high temperatures in harsh environment. The authors designed and utilized a sample holder suitable for high temperature measurement up to 973 K with good temperature uniformity within the sample. The high-temperature MPR setup was validated by measuring bulk materials with known thermal conductivity. The setup and technique were then extended to the measurement of black solar-absorbing coatings of 10 to 50 μm thick on various substrates by modulating the frequency of the laser heating beam and the thermal penetration depth. The studies showed that thermal conductivities of typical solar-absorbing coatings are 0.4 ~ 0.8 W m−1 K−1, indicating a possibly large temperature drop within the coating under high solar irradiation flux, such as over 1000-sun for central solar towers in CSP plants.

Intensity equations for birefringent spin lasers

Semiconductor spin lasers are distinguished from their conventional counterparts by the presence of spin-polarized carriers. The transfer of angular momentum of the spin-polarized carriers to photons provides important opportunities for the operation of lasers. With the injection of spin-polarized carriers, which lead to the circularly polarized light, the polarization of the emitted light can be changed an order of magnitude faster than its intensity. This ultrafast operation of spin lasers relies on a large birefringence, usually viewed as detrimental in spin and conventional lasers. We introduce a transparent description of spin lasers using intensity equations, which elucidate the influence of birefringence on the intensity and polarization modulation of lasers. While intensity modulation is independent of birefringence, for polarization modulation an increase in birefringence directly increases the resonant frequency. Our results for dynamical operation of lasers provide a guide for their spin-dependent response and spintronic applications beyond magnetoresistance.

Narrow-linewidth coherent population oscillation spectroscopy of room-temperature cesium atomic ensemble

Coherent population oscillations spectroscopy, which is based on the interaction between atoms and the phase locked laser, is a kind of atomic population modulation spectroscopy. When the laser frequency difference is less than natural width of energy level, the coherent oscillation of atomic population will be induced by laser intensity modulation so that the probe laser transmission with narrow bandwidth can be realized. For a closed two-level system (TLS), the spectral line-width is limited mainly by the spontaneous emission lifetime of the upper atomic energy level. As for a three-level atomic system of Λ configuration, the two linearly polarized beams with both σ+ and σ- polarization component, the laser-atom interaction satisfies the selection rule. The spectral line-width mainly depends on the ground-state relaxation time, and the dependence on the line-width of spontaneous radiation is eliminated. In this paper, the laser from a external-cavity diode laser has its frequency locked to Cesium &lt;inline-formula&gt;&lt;tex-math id="M1"&gt;\begin{document}$6{{\rm{S}}_{1/2}}\left( {F = 3} \right) \to 6{{\rm{P}}_{3/2}}\left( {F' = 3} \right)$\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20210405_M1.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20210405_M1.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt; transition. The frequencies of the two beams are shifted down by two independent double-passed acousto-optic modulators (AOM) to nearly resonate to Cesium &lt;inline-formula&gt;&lt;tex-math id="M2"&gt;\begin{document}$6{{\rm{S}}_{1/2}}\left( {F = 3} \right) \to 6{{\rm{P}}_{3/2}}\left( {F' = 2} \right)$\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20210405_M2.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20210405_M2.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt; transition. The probe beam and the coupling beam are superposed at polarization beam splitter (PBS) cube and transmitted through the magnetically shielded cesium vapor cell in the same direction. The two beams have approximately the same Gaussian diameter of 6.6 mm. The beams are separated by another PBS behind the vapor cell, and the probe beam is detected by a photodiode. We realize the coherent population oscillation spectroscopy through the Cesium vapor cell at room temperature without buffer gas. The spectral linewidth is typically less than 50 kHz which is far below the spontaneous radiation linewidth(~5.2 MHz). The linewidth of coherent population oscillation spectroscopy of the Λ-type atomic energy level structure depends only on the population associated with the oscillation of multiple degenerate level systems except phase correlations of atomic states. Coherent population oscillation is beneficial to the obtaining of the narrow linewidth spectroscopy through the Rydberg atomic system with long excited state lifetime. Considering the importance of electric field measurement using Rydberg atoms, the method of coherent population oscillation can be used to improve the sensitivity of precise measurements based on Rydberg atoms.

The Influence of Diode Laser Intensity Modulation on Photoacoustic Image Quality for Oral Soft Tissue Imaging.

Introduction: Imaging technologies have been developed to assist physicians and dentists in detecting various diseases. Photoacoustic imaging (PAI) is a new technique that shows great applicability to soft tissues. This study aimed to investigate the effect of diode laser intensity modulation on photoacoustic (PA) image quality. Methods: The prototype of the PAI system in this study utilized a non-ionizing 532 nm continuouswave (CW) diode laser illumination. Samples in this study were oral soft tissues of Sprague-Dawley rats fixed in 10% formalin solution. PA images were taken ex vivo by using the PAI system. The laser exposure for oral soft tissue imaging was set in various duty cycles (16%, 24%, 31%, 39%, and 47%). The samples were embedded in paraffin, and PA images were taken from the paraffinembedded tissue blocks in a similar method by using duty cycles of 40%, 45%, 50%, 55%, 60% respectively to reveal the influence of the laser duty cycle on PA image quality. Results: The oral soft tissue is clearly shown as a yellow to red area in PA images, whereas the nonbiological material appears as a blue background. The color of the PA image is determined by the PA intensity. Hence, the PA intensity of oral soft tissue was generally higher than that of the nonbiological material around it. The Kruskal-Wallis test followed by Mann-Whitney post-hoc analysis revealed significant differences (P<0.05) in the quality of PA images produced by using a 16%-47% duty cycle of laser intensity modulation for direct imaging of oral soft tissue fixed in 10% formalin solution. The PA image quality of paraffin-embedded tissue was higher than that of direct oral soft tissue images, but no significant differences in PA image quality were found between the groups. Conclusion: The PAI system built in this study can image oral soft tissue. The sample preparation and the diode laser intensity modulation may influence the PA image quality for oral soft tissue imaging. Nonetheless, the influence of diode laser intensity modulation is not significant for the PA image quality of paraffin-embedded tissue.

Microwave frequency comb generation through optical double-locked semiconductor lasers

A microwave frequency comb (MFC) consists of equally spaced discrete microwave frequency components that have small magnitude variations across its bandwidth. In this work, a scheme comprising of a slave laser that is optically injected by a strongly intensity-modulated master laser is proposed and numerically investigated to generate MFCs. Based on the double-locking characteristics of the periodic oscillations induced by strong intensity-modulated optical injection, locked comb structures are invoked when the modulation frequency is at a submultiple frequency of the period-one oscillation frequency. Furthermore, broad microwave frequency comb bandwidths are achieved through the bandwidth enhancement effect of optically injected lasers at appropriate operating conditions. The demonstrated MFC source has a continuously tunable comb-line spacing between 1–10 GHz reaching bandwidths that are six times the relaxation resonance frequency of the free-running semiconductor laser.

A Device for Noncontact Determination of the Photosensitivity Distribution on Areas of n+–p(n)–p+-Type Silicon Structures

A device is described that allows one to measure the photosensitivity contrast over the area of n+–p(n)–p+ silicon structures without contacts. The structure is placed between the capacitor plates and is locally illuminated on one side by two intensity-modulated lasers. The laser wavelengths are 1064 and 808 nm. The radiations of the first and second lasers are absorbed in the volume of the base region and only near its illuminated surface, respectively. The local photosensitivity is determined by the ratio of the amplitudes of the modulations, at which the total variable photo-emf vanishes. This compensation allows one to avoid an error that is associated with shunting of the illuminated area of the structure by the rest of it due to currents in the n+ and p+ layers. The photosensitivity contrasts measured by the proposed compensation method and the standard method based on short-circuit current measurements were compared on n+–p–p+ single-crystal silicon structures. The difference was no more than 6%, which is consistent with the calculations.

High-Power Broadband Frequency Chirped Intensity-Modulated Single-Frequency 1064-nm Laser

Optical carried microwave radar (OCMR), combining the advantages of lidar and microwave radar, uses an intensity-modulated laser beam as the medium to detect the distance and velocity of objects. In order to achieve a high ranging resolution at long distance, a high-power frequency chirped intensity-modulated light source at a 1064-nm wavelength was developed. A low-power narrow-linewidth 1064-nm laser beam was input to a Mach-Zehnder electro-optic modulator (EOM), and the frequency of the EOM was tuned from 10 MHz to 2.1 GHz. The output from the EOM was coupled to a laser-diode-pumped ytterbium-doped fiber amplifier (YDFA). A maximum output power of 29.5 W was achieved from the YDFA. We measured the frequency characteristics of the modulated laser beams before and after amplification, respectively. The amplification process did not bring any obvious degeneration of the modulation in terms of the bandwidth, frequency stability, chirping linearity, and modulation depth. Our research presents a new approach of obtaining a high-power broad-band intensity-modulated light source for OCMR.

Magneto-optical spectroscopy with arbitrarily polarized intensity-modulated light in He4 atoms

We present both a theoretical and experimental study of the magneto-optical spectra induced with intensity-modulated resonant laser light that synchronously pumps and detects the metastable $^{4}\mathrm{He}$ atoms in the presence of a quasistatic magnetic field. We extend previous work and derive complete analytical expressions for the resonance signals by taking into account arbitrary magnetic-field directions and arbitrary light polarizations. The analytical results are derived by solving the Liouville equation using the irreducible tensor formalism on the assumption of low light power. We discuss the potential application of the derived analytical results in constructing and optimizing a dead-zone-free all-optical atomic magnetometer.

Active modulation of intracavity laser intensity with the Pound-Drever-Hall locking for photoacoustic spectroscopy.

Here we report a novel, to the best of our knowledge, method of active intracavity intensity modulation for cavity-enhanced photoacoustic spectroscopy (PAS) without the need for any external optical modulators. Based on the Pound-Drever-Hall (PDH) locking technique, a dither is added to the PDH error signal to periodically vary the locking point between the laser frequency and optical cavity within a sub-MHz frequency range. While significantly enhancing the intracavity laser intensity, the optical cavity also acts as an intensity modulator. As a proof-of-principle, we demonstrated the PAS of ${{\rm C}_2}{{\rm H}_2}$C2H2 by placing a photoacoustic cell ($Q$Q-factor $\sim{10}$∼10) inside a Fabry-Perot cavity (finesse $\sim{628}$∼628) and adopting the proposed intracavity intensity modulation scheme. By detecting the weak ${{\rm C}_2}{{\rm H}_2}$C2H2 line at ${6412.73}\;{{\rm cm}^{ - 1}}$6412.73cm-1, the sensor achieves a normalized noise equivalent absorption (NNEA) coefficient of ${1.5} \times {{10}^{ - 11}}\;{{\rm cm}^{ - 1}}{{\rm WHz}^{ - 1/2}}$1.5×10-11cm-1WHz-1/2. This method enables the continuous locking of laser frequency and optical cavity, and it achieves the intracavity intensity modulation with an adjustable modulation depth as well.

On acoustical registration of irradiated surface displacement during nanosecond laser-metal interaction and metal–nonmetal transition effect

Laser ablation of mercury is investigated with the help of acoustical monitoring method which uses pulsed modulated laser intensity and piezo-transducer under the irradiated target. Two different trains consisting of 6 and ~70 short peaks (60 ps each) divided with 8 ns intervals are used. The method permits to detect effective displacement of the irradiated surface provided some necessary conditions are fulfilled. The paper purpose is to study influence of possible metal-nonmetal transition on the considered approach. It is found that the observed values of the effective displacement cannot be explained in the framework of a simple surface vaporization model. Another interpretation of the results which uses assumption about metal-nonmetal transition is proposed. The assumption is in accordance with our main results: observation of significant acoustic signal shift (70 ns) to earlier times and the signal widening at higher laser fluences (7 J/cm2) for the case of relatively smooth 30 ns laser pulse. Initial part (10 ns) of the shift is observed as well for the train consisting of 6 peaks. The obtained new experimental results can be considered as the first demonstration of metal-nonmetal transition during nanosecond laser ablation.

Wavelength modulation-direct absorption spectroscopy combined with improved experimental strategy for measuring spectroscopic parameters of H2O transitions near 1.39 μm

The wavelength modulation-direct absorption spectroscopy (WM-DAS) was used to measure the spectroscopic parameters for 8 H2O absorption transitions near 1.39 μm at 300–900 K. A reference light path was introduced to eliminate the effect of ambient gas and inherent fluctuation of laser intensity. Experimental results at high temperatures for the transition at 7185.597 cm−1 showed that the WM-DAS enhanced noise rejection by extracting the characteristic frequencies of modulated laser intensity. The SNR was enhanced to 1800 for the peak absorbance of 20% and remained 380 around the peak absorbance of only 0.5% by using this method, showing its huge potential in situ gas sensing in harsh engineering fields. The Voigt, Rautian, Galatry and quadratic speed-dependent Voigt profiles were used to recover the absorbance and determine the best-fit spectroscopic parameters and their temperature dependence. The measured N2-broadening coefficients compare well with the available values listed in the HITRAN/HITEMP database. The Dicke narrowing and speed-dependent coefficients were also determined with analyses of the uncertainties in these measured parameters.