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In this paper, free vibration and transmission response for the torsional vibration of circular annular plate are presented. To the author’s knowledge, few studies can be found for the torsional vibration from wave standpoint. For this purpose, in this study, natural frequencies for the torsional vibration of annular plate with clamped–clamped and free–free boundaries are calculated. The natural frequencies obtained by wave approach are compared with those derived by the classical method. Furthermore, transmissibility curves of the periodic annular model and Fibonacci annular model are analyzed. The finite element simulations are carried out to verify the theoretical results. Finally, the influence of inner radius and length ratio on the transmission response is also discussed. The obtained results are useful for the torsional vibration reduction of machinery structures.

A Long Period Grating (LPG) is a structure inscribed in an optical fiber that is of great interest in the field of communications and sensors. At Centro de Investigaciones Ópticas (CIOp), an LPG manufacturing process was implemented. It is based on use of electric arcs produced with an optical fiber splicer to generate a series of physical disturbances that modify optical fiber behavior as a light transmission guide. Since transmission response of LPG is sensitive to fiber environment conditions, it can be used to detect changes in certain physical and chemical parameters. In particular, it enables the implementation of optical sensors sensitive to temperature, surrounding medium refractive index and axial deformation. Characterization of locally generated LPGs is essential for future design of sensors applicable in many engineering areas. In this article, typical LPG sensitivities to the three mentioned parameters are reported Fil: Alustiza, Diego Horacio. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Investigaciones Ópticas. Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas. Centro de Investigaciones Ópticas. Universidad Nacional de La Plata. Centro de Investigaciones Ópticas; Argentina. Universidad Tecnológica Nacional. Facultad Regional La Plata. Departamento de Ciencias Básicas; Argentina Fil: Mineo, Marcos. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Investigaciones Ópticas. Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas. Centro de Investigaciones Ópticas. Universidad Nacional de La Plata. Centro de Investigaciones Ópticas; Argentina Fil: Russo, Nelida Araceli. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Investigaciones Ópticas. Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas. Centro de Investigaciones Ópticas. Universidad Nacional de La Plata. Centro de Investigaciones Ópticas; Argentina

This paper presents a microwave sensor for simultaneous measurement of permittivity and thickness in dielectric materials. The sensor is made of a dual-mode magnetic- $LC$ resonator coupled to a microstrip transmission line. Due to a dual-mode resonance of the magnetic- $LC$ resonator, the transmission response of the sensor shows two zero frequencies. By applying dielectric materials of various permittivities and thicknesses to the resonator, both of the transmission zero frequencies are modified. The alterations of the two transmission zero frequencies are used to characterize the permittivity and thickness of the dielectric samples under test. A prototype of the sensor is fabricated and measured for verifying the proposed sensing principle.

The autocorrelation of the seismic transmission response of a layered medium (autocorrelogram), in the presence of a free surface, corresponds to the reflection response.Despite many studies on the...

An in-plane two-layer meandered wire resonator structure is utilized to enable transmission through a subwavelength aperture. The structure exhibits effectively 100% transmission at approximately 1.94 GHz through a λ 0/33 square aperture. The extremely high Q resonance presents a transmission greater than –10 dB over a bandwidth of 0.007%. A complementary subwavelength resonator is also proposed, which exhibits a narrow band transmission response operating at 1.92 GHz with a fractional bandwidth of 0.04% at an overall electrical size of λ0/43. A good agreement between measured and simulated results is achieved for both designs. The subwavelength structures show a slight sensitivity to fabrication tolerances, but are realizable with modern printed circuit fabrication techniques due to their planar nature.

The tuning of transmission response of an optical microring resonator (MRR) always associates with significant shift of the resonant wavelength, which leads to serious problems for most of applications. In this letter, we demonstrate that the Mach-Zehnder-interferometer (MZI)-coupled microring resonators with asymmetric directional couplers in the MZIs could still have large magnitude and phase tuning range, while maintaining the resonant wavelength unchanged. An add-drop MZI-coupled microring resonator with asymmetric directional couplers is fabricated on a silicon-on-insulator chip, and the measurement agrees well with the theoretical prediction. Our study would be interesting to photonic integrated circuits, which require tunable MRRs.

Z domain model of a single cavity structure as well as double cavity structure composed of fiber Bragg gratings (FBG) is developed and their performance analysis is presented in this article. A generalized approach for determination of transmittance in Z domain of FBG based cavities is introduced. Spectral characteristics in terms of peak transmittance, side lobes formation and bandwidth are determined for a single cavity structure of length 4.5 mm. The results are compared with previously published measured data of the same configuration and are found to be in very close agreement. An ultrashort single cavity structure is proposed and its transmission response is carried out in the article. The spectral characteristics of both symmetrical and asymmetrical double cavity structure of same length as that of single cavity FBG are determined. Performance parameters like FWHM, suppression of peaks, wavelength spacing are analyzed for the different configurations in Z domain.

This chapter mainly focuses on various sensing technologies that have been employed to detect food and water contamination. It has been found that these conventional sensing technologies appear to be unfeasible and impractical to meet with the challenging growth of population. In this aspect, THz sensing is discussed in detail and deemed to be more effective due to its strong penetration feature, high resolution, and sensitivity to monitor the molecular changes in fruits. This chapter also introduces a novel technique of fruits contamination detection by monitoring MC and observe the transmission and path loss response of fruits. It also investigates an important parameter such as the absorption coefficient and shows some significant results and correlation of MC with transmission response and absorption coefficient. Upon close analysis, these results give meaningful information about the composites present in fruits such as carbohydrates and proteins. Toward the end, this chapter emphasizes on the advancement and development of terahertz technology applications and found that the THz sensing is a promising candidate and has a potential to change a paradigm in the plant science sector.

This paper presents preliminary results obtained through the measurements of the distinguished method for sensing the contaminants in the water using terahertz (THz) wave leveraging a commercially available THz material characterization kit (MCK) based on vector network analyzer (VNA) techniques and Swissto 12 system. The primary goal of this work is to highlight transmission response of THz wave through the frozen sugar-water and salt-water over the frequency range 0.75 THz to 1.1 THz. The different concentrations of mixed solution were frozen and measured to observe the transmission responses of corresponding solutions at different frequency regions since each additive in water induces a unique transmission response that can be examined to detect specific substances that caused water contamination. The results presented in this paper pave the way for applicability of THz technology for sensing the water contamination detection.

In this work, we demonstrate theoretically that a planar metamaterial can achieve electromagnetically induced transparency (EIT), in which insensitive to the incident angle and polarization of electromagnetic waves. We describe simple metamaterial designs that able to support exciting the high Q-value antisymmetric trapped-mode. These features enable the proposed metamaterials to achieve potential applications in filtering, data storing, switching, and sensing. Finally, we have realized analog of electromagnetically induced absorption (EIA) by its complementary of this metamaterials structure.

This work is supported by the projects DPT-HAMIT, NATO-SET-193, TUBITAK-113E331, TUBITAK-114E374 and Research Fund of Abdullah Gul University Project Number FAB-2015-2. The authors (E.O. and H.C.) also acknowledges partial support from the Turkish Academy of Sciences. One of the authors (H.C.) also acknowledges partial support from "For Women in Science" fellowship by L'Oreal-Unesco Turkey. Graphene is a strong candidate for active optoelectronic devices because of its electrostatically tunable optical response. Current substrate back-gating methods are unable to sustain high fields through graphene unless a high gate voltage is applied. In order to solve this problem, ionic liquid gating is used which allows substrate front side gating, thus eliminating the major loss factors such as a dielectric layer and a thick substrate layer. On the other hand, due to its two dimensional nature, graphene interacts weakly with light and this interaction limits its efficiency in optoelectronic devices. However, V-shaped plasmonic antennas can be used to enhance the incident electric field intensity and confine the electric field near graphene thus allowing further interaction with graphene. Combining V-shaped nanoantennas with the tunable response of graphene, the operation wavelength of the devices that utilize V-shaped antennas can be tuned in situ. In the present paper, we demonstrate a graphene-based device with ionic liquid gating and V-shaped plasmonic antennas to both enhance and more effectively tune the total optical response. We are able to tune the transmission response of the device for up to 389 nm by changing the gate voltage by 3.8 V in the mid-infrared regime. (C) 2016 Elsevier Ltd. All rights reserved. Turkiye Cumhuriyeti Kalkinma Bakanligi Abdullah Gul University FAB-2015-2 Turkish Academy of Sciences "For Women in Science" fellowship by L'Oreal-Unesco Turkey NATO-SET-193 TUBITAK-113E331 TUBITAK-114E374

This paper presents the results of a study of polyethylene terephthalate (PET) films irradiated with Ar and Kr ions at both normal orientation and an angle of 40&deg, to the normal. Normal irradiation was performed using Ar8+ and Kr15+ ions with an energy of 1.75 MeV/au and fluences in the range (2&ndash, 500) &times, 1010 cm&minus, 2 for Ar8+ ions and (1.6 &minus, 6.5) &times, 2 for Kr15+ ions. Kr ions with an energy of 1.2 MeV/au and charges of 13+, 14+, and 15+ were used for angled irradiation. For each Kr ion charge value, three fluence values were used: 5 &times, 1010, 1 &times, 1011, and 2.5 &times, 1011 cm&minus, 2. It is well known that irradiation of PET films by swift heavy ions results in a red shift of the UV-vis transmission spectra absorption edge. The experimental transmission spectra exhibit well-defined interference fringes, which obscure the underlying transmission response. Using an existing technique to obtain interference-free transmission curves T&alpha, (&lambda, ) for both pristine and irradiated PET film samples, we found that S, the total radiation-induced absorption of light by the PET film, is proportional to the logarithm of the fluence F. In addition to this dependence on the irradiating fluence, we also found that the charge of the irradiating ion has a significant influence on the position of the absorption edge in the UV-vis spectra. This provides experimentally independent evidence to confirm our previous results showing that ion charge has an effect on the post-irradiation state of PET films. We present a physical interpretation of the observed absorption edge red shift in irradiated PET films as being due to the growth of extended conjugated systems via the formation of intermolecular helical structures. Our investigations into the stability of irradiation-induced effects in PET films show that comparison of UV-vis transmission spectra before and after annealing can provide information about the structure of deep traps in PET.

A miniaturised frequency selective surface (FSS) with λ 0 /7 × λ 0 /7 unit cell at the pass band centre frequency is presented for millimetre wave remote sensing. A resonant corner convoluted square ring (CCSR) is loaded with non-resonant cross dipole (CD) for tri-band spectral response. Rectangular patches integrated in the CCSR are used to optimise the coupling between the CD and CCSR to reject 50-60 GHz (B 1 ) and 170-195 GHz (B 3 ), and transmit 87-91 GHz (B 2 ). The FSS fabricated on 175 μm thick quartz with 3721 unit cells indicates polarisation and angle stable transmission response.

Fabry–Perot fiber etalons (FPEs) built from three or more reflectors are attractive for a variety of applications, including communication and sensing. In order to realize a desired transmission profile, usually multistage FPEs are designed with equal cavity lengths followed by a determination of the required reflectivities. As seen in previous works, however, fabricated reflectors may slightly differ from the designs, leading to a departure from the desired transmission profile of the FPE. Here, we show a novel digital synthesis of multistage etalons with off-the-shelf reflectors and unequal lengths of involved cavities. We find that, in contrast to equal cavity lengths, unequal lengths provide more poles in the z domain and hence more flexibility to achieve a desired multicavity FPE transmission response. For given reflectivities and by determining correct unequal lengths of a multicavity FPE with our synthesis technique, we demonstrate two design examples involving free spectral range enhancement and passband shaping. This work is generalizable to ring resonators and mirror- and fiber-Bragg-grating-based cavities, enabling the design and optimization of cavity systems for a wide range of applications, including lasers, sensors, and filters.

We propose a reconfigurable multiband notch filter in the FarIR band of 7 to 17 μm (40–18 THz) utilizing phase change material. The use of the phase change material, VO 2 , allows the cross shaped FSS to effectively change its resonant frequencies. The FSS unit cell is divided into four quadrants containing two different dimensions of cross dipoles. Dividing the unit cell in this method allows for multiple notches in the transmission response.

In this paper, a compact defected ground mushroom electromagnetic bandgap (DG-MEBG) structure is presented. Compactness is achieved by employing a DG in the conventional MEBG. DG-MEBG exhibits a low pass transmission response with a cut-off frequency of 4.2 GHz and wide stop band up to 17 GHz as against a cut-off frequency of 10.4 GHz observed in a conventional MEBG. Thus the proposed DG-MEBG is 60% compact in comparison with a conventional MEBG structure. The measured results show a very good agreement with the simulated ones.

In this paper, we analyze the performance of the birefringent fiber loop (BFL) when the settings of the components that construct this module are not perfect. The BFL is employed for suppressing the pattern effect on directly amplified data in a semiconductor optical amplifier (SOA). The analysis is conducted by describing the non-optimum BFL transmission response using Jones matrix method. This allows to formulate a comprehensive model, which is validated by comparing it to the experiment. Then we investigate and specify how sensitive the BFL is to imperfections of its building components by assessing its performance against the output amplitude modulation, power penalty, and crosstalk. For each critical operating parameter, we derive the tolerance range within which these performance metrics are acceptable. The obtained results suggest that the BFL can efficiently mitigate the SOA pattern effect even if it is built from non-ideal components. This can be achieved provided that these components are designed according to their extracted operating conditions, which are practically satisfiable.

Here, a general circuit model-based formalism is established to quantify the interaction of electromagnetic waves (both propagating and evanescent waves) with mushroom-type structures. A thickness-dependent local permittivity is exploited in order to characterize the isolated and bounded wire medium (WM) topologies which can take into account the spatial dispersion and the effect of boundaries. The obtained robust method can be a promising alternative for the previously developed nonlocal homogenization approach and it can also be useful to simplify the formulation for the near-field problems in particular subwavelength imaging applications. The proposed circuit model can accurately evaluate the amplification of the evanescent modes which is the physical mechanism behind the subwavelength imaging. The analytical results which are achieved in terms of the transmission response versus excited evanescent modes and the image profile demonstrate a good agreement between the local thickness-dependent and the nonlocal homogenization models.

The electromagnetically-induced transparency (EIT)-like effects in planar and non-planar metamaterials (MMs) were investigated for microwave (GHz) frequencies. The specific MMs used in this study consisted of a cut-wire resonator and a ring resonator, where were placed on the top and the bottom layers, respectively. A transmission window was produced, due to the interference between bright- and bright-mode coupling. Using the numerical and the experimental results, we demonstrate that the bending of MM leads to enhanced transmission and bandwidth, as well as an additional EIT-like peak. This provides an effective way of realizing the tunable devices, including the switching sensors.

A novel structure of three dimensional Cylindrical Shaped Frequency Selective Surface (CSFSS) has been structured and its polarization sensitivity has been analyzed. The effect of oblique angle of incident electromagnetic wave on the CSFSS by varying the circumference of the outer ring has been observed. The analysis has been made for oblique incident angles from 0° to 60°. The resultant frequency response reveals that the CSFSS structure has been found highly polarization insensitive and obtained a stable transmission response with respect to incident EM wave. Moreover, an analysis has been made to find out the effect of variation of the circumference on the resonant frequency and result shows a highly sensitive and interesting relation between them.

In this paper, propagation of flexural vibration in phononic crystal thin plates with straight, bending or branching linear defects are explored using finite element method. The plate is composed of an array of circular crystalline Al2O3 cylinders embedded periodically in the epoxy matrix with a square lattice. The numerical results showed that accurate band structures and transmission response curves could be obtained by finite element method compared with that of improved plane wave expansion method. The exploration indicated that finite element method is efficient and suitable in dealing with the wave propagation in phononic crystal, and displays potential abilities in dealing with complex structures.

A novel double layer Dual Band Frequency Selective Surface (FSS)with miniaturized element is presented, in which each periodic cell consists of parallel metallic patch of two different sizes. The FSS with such structure is designed, The simulated results show that the FSS dimension is miniaturized to 0.06 λ (λ refers to the resonant wavelength of 2GHz). In addition, it also has good stability to oblique incidence waves for both TE and TM polarization, and there is no other resonance frequency up to 10 GHz. The influence of dimension parameters on FSS performances has been investigated. Then, dual-band FSS operated at 2GHz and 3.45GHz is designed. Simulated results indicate that the miniaturized FSS also provides good transmission response.

The Parity-Time pt symmetry properties of natural and perfect dispersive media are compared. It is shown that perfect dispersive media, characterized by a flat magnitude transmission response along with an arbitrary phase (dispersion) response, are inherently pt symmetric, whereas all naturally dispersive media are t asymmetric.

In this paper, a metamaterial-inspired miniaturization method for waveguide filter design is proposed. The transmission response and effective medium parameters of I-shaped resonator in metallic re ...

SUMMARY One of the key challenges in the context of local site effect studies is the determination of frequencies where the shakeability of the ground is enhanced. In this context, the H/V technique has become increasingly popular and peak frequencies of H/V spectral ratio are sometimes interpreted as resonance frequencies of the transmission response. In the present study, assuming that Rayleigh surface wave is dominant in H/V spectral ratio, we analyse theoretically under which conditions this may be justified and when not. We focus on ‘layer over half-space’ models which, although seemingly simple, capture many aspects of local site effects in real sedimentary structures. Our starting point is the ellipticity of Rayleigh waves. We use the exact formula of the H/V-ratio presented by Malischewsky & Scherbaum (2004) to investigate the main characteristics of peak and trough frequencies. We present a simple formula illustrating if and where H/V-ratio curves have sharp peaks in dependence of model parameters. In addition, we have constructed a map, which demonstrates the relation between the H/V-peak frequency and the peak frequency of the transmission response in the domain of the layer's Poisson ratio and the impedance contrast. Finally, we have derived maps showing the relationship between the H/V-peak and trough frequency and key parameters of the model such as impedance contrast. These maps are seen as diagnostic tools, which can help to guide the interpretation of H/V spectral ratio diagrams in the context of site effect studies.

This work proposes the use of Non-Bianisotropic Split Ring Resonators (NB-SRRs) as building blocks for dual-band response fllters. Design parameters will be evaluated in order to characterize coupling mechanisms for both the particle and fllter structure. A ready-to-use dual-band fllter for a multiconstellation Galileo/GPS global positioning receiver is manufactured in order to validate and test the proposed design. The device transmission response measurement agrees with both circuital and electromagnetic simulations, as well as with theoretical insertion losses, which are 2.4dB and 3.5dB at center passbands for L5 and L1 bands respectively.

A new band-notched ultra-wideband (UWB) bandpass filter (BPF) with an improved upper stopband is designed using a multi-mode resonator (MMR). The MMR, being a modified version of a common tri-section stepped-impedance resonator (TSSIR), is split at its two open ends with the input/output feed lines sandwiched in between to form an interdigital parallel-coupled configuration that results in stronger coupling. In addition, an extended coupling length in the second section of the TSSIR is featured to provide further coupling and impedance-matching tuning. With such a design strategy, coupling gaps of no smaller than 0.1 mm was achieved to obtain the required UWB response. In this design, the first five resonant modes of the MMR and the coupling peaks of the input/output parallel-coupled lines are properly located in the UWB passband to achieve a uniform in-band transmission response. Also, the spur lines and open-circuited stubs are implemented in the MMR to create a 5-GHz notched band and to broaden the upp...

This paper presents a microwave microfluidic sensor based on a complementary electric-LC resonator loaded on a microstrip line. By flowing water-ethanol solutions with different water volume fractions, the resonance frequency and the notch depth in the transmission response of the sensor are modified. These changes are used for determining the complex permittivity of the solution. A sensor prototype has been fabricated and measurements are used for verification of the sensing principle.

International audience; We study a polarizer-analyzer mounting for the terahertz regime with perfectly conducting metallic polarizers made of a periodic subwavelength pattern. With a renewed Jones formalism, we analytically investigate the influence of the multiple reflections, which occur between the polarizer and the analyzer, on the transmission response. We demonstrate that this interaction leads to a modified transmission response: the extended Malus law. In addition, we show that the transmission response can be controlled by the distance between the polarizer and the analyzer. For particular setups, the mounting exhibits extremely sensitive transmission responses. This interesting feature can be employed for high-precision sensing and characterization applications. We specifically propose a general design for measuring the electro-optical response of materials in the terahertz domain allowing detection of refractive index variations as small as 10−5.

En este artículo se presenta un estudio del efecto del plano de masa en filtros pasa banda mediante la utilización de celdas meta material en estructura de resonadores abiertos en anillo dividido OSRR (Open Split Ring Resonator) sobre sustratos microstrip. Se plantean tres tipos de configuraciones, a saber: eliminar parcialmente el plano de masa en la parte posterior a las celdas OSRR, diseñar ventanas sobre el plano de masa al respaldo de cada celda y colocar el plano de masa completo. El análisis llevado a cabo muestra que la respuesta de transmisión del filtro con ventanas en el plano de masa resulta ser la más plana de los tres casos y de igual manera con un ancho de banda intermedio. Por otra parte, el desempeño de este tipo de filtros es similar al de un filtro convencional en tecnología microstrip de tres polos, sin embargo su tamaño resulta ser un 60% más pequeño. This paper presents the study of the ground plane effect in passband filters using metamaterial cells in Open Split Ring Resonators (OSRR) structures on microstrip substrates. Three different configurations have been proposed, namely: by removing partially the ground plane of the OSRR cell back end, by windows design over the ground plane at the back end of each cell and by placing a full ground plane. The conducted analysis shows that the filter transmission response featuring a ground plane including windows proved to be the most flattened with the middle bandwidth of the three configurations. On the other hand, the performance of these filters is similar to a conventional filter based on three-pole microstrip technology but with a considerable reduction in size of about 60%.

A planar four-pole dual-mode filter with symmetric transmission response based on substrate integrated waveguide (SIW) is proposed. The filter centered at 10 GHz, consisting of two TE102–TE201 dual-mode SIW cavities, is implemented in a single layer substrate by PCB process. A linear microstrip taper is used to implement the transition between 50 Ω microstrip and SIW. The measured results are in agreement with simulated results.

A simple, low-cost frequency selective (FS) wall which filters out a particular frequency with about 30 dB attenuation is constructed by covering an existing wall with a custom designed Frequency Selective Surface (FSS). With 10 mm air spacing between the FSS cover and the wall surface, independence between the two can be achieved. Under such a scenario, performance of the FS wall can be evaluated by cascading the FSS and the wall responses individually. Good agreement between the cascaded evaluation and the actual measured transmission response was found

The shaft misalignment usually causes the severe vibration of rotating machine. In this paper, a three-dimensional dynamic model of a coupled gear-rotor system is developed to simulate this phenomenon. The numerical simulation of the dynamic characteristics is performed to analyze the effects of both internal gear meshing and external shaft misalignment related dynamic excitations on the gearbox transmission response. This study provides useful theoretical guideline to the helical gear system design. Keywords-helical gear; dynamic model;coupling misalignment; vibration analysis

Wave-induced runup and wave transmission due to overtopping are important variables in coastal structure design. Many studies have been performed on different types of armor layers, but there is no generalized design guidance available on CORE-LOC® armor layer performance with respect to wave runup and transmission. An experimental study was performed to investigate the runup and transmission response of a CORE-LOC® armor layer. Wave runup and transmitted wave heights were measured for a wide range of wave, water level, and structure conditions in order to develop predictive tools for CORE-LOC® layer response. A new empirical model to predict runup levels was developed based on an existing model for rock revetments. Further, an analysis of transmission data was conducted to give insight and guidance for design purposes. Reflection coefficients were also analyzed.

The influence of the waveguide spacing on transmission response for parallel-plate waveguide with single deep groove has been experimentally studied. The results indicate that the decrease of spacing can efficiently excite the higher order cavity modes in very deep groove. The relationship between the spacing and the channel number presents the possibility of a notch filter to mechanically select the channel number. Experimental data and simulations results verify this channel number tunibility, which may be of great interest for communication applications.

Traveling waves are used not only in exploration geophysics but also in other disciplines faced with remote detection problems. A physical system may be described in terms of the input (the source), the medium, and the output (the received signal). The received signal can be made up of either transmitted waves or reflected waves. Two types of inverse problems can be considered, namely, the inverse source problem and the inverse medium problem. In the inverse source problem, the objective is to determine the source. In the inverse medium problem, the objective is to determine the medium. Thus, in terms of this general classification, four types of problems can be encountered, namely, an inverse source problem with transmitted waves, an inverse source problem with reflected waves, an inverse medium problem with transmitted waves, and an inverse medium problem with reflected waves. Let us look at nature. Twinkle, twinkle, little star. The transmission of starlight though the atmosphere makes the star twinkle. A better image of the star can be obtained by solving an inverse source problem using the transmitted starlight. In the typical inverse source problem, the source of energy is remote, the medium transmits the source signal, and the received data are the transmitted waves. Examples are classical earthquake seismology, radio transmission, and passive sonar. Shakespeare said; “For the eye sees not by itself, but by reflection.” Thus the miracle of eyesight solves an inverse medium problem that uses reflected waves. In the typical inverse medium problem, the source of energy is local and often man‐made, the medium reflects the source signal, and the received data are the reflected waves. Examples are reflection seismology, radar, and active sonar. Thus, the two principle types of inverse problems encountered in nature are the inverse source problem with transmitted waves and the inverse medium problem with reflected waves.