Abstract-Boeing : Patent Issued for Terahertz Material Evaluation and Characterization via Material Difference Frequency Generation (USPTO 9557263)

http://www.4-traders.com/BOEING-CO-4816/news/Boeing-Patent-Issued-for-Terahertz-Material-Evaluation-and-Characterization-via-Material-Differenc-23848925/

By a News Reporter-Staff News Editor at Journal of Engineering -- THE BOEING COMPANY (Chicago, IL) has been issued patent number 9557263, according to news reporting originating out of Alexandria, Virginia, by VerticalNews editors.

The patent's inventors are Hunt, Jeffrey H. (Thousand Oaks, CA); Belk, John H. (St. Louis, MO).

This patent was filed on October 22, 2014 and was published online on January 31, 2017.

From the background information supplied by the inventors, news correspondents obtained the following quote: "In non-linear optical, or wave mixing processes, outputs are produced at sum, difference or harmonic frequencies of the input(s). Second order non-linear optics, or three wave mixing involves combining two inputs to produce one output at one of the combined frequencies. The use of second order non-linear optical surface spectroscopy to examine physical properties of a material surface is known. However, practical constraints on such known methods have impeded progress on the material evaluation and characterization beneath the surface of a material under inspection.

"The Terahertz (hereinafter 'THz') range refers to electromagnetic waves with frequencies between 100 GHz and 10 THz, or wavelengths between 3 mm and 30 .mu.m, existing in the radiation spectrum between the microwave and infrared region. Terahertz waves are known to pass through a variety of amorphous and opaque substances. In addition, many biomolecules, proteins, explosives or narcotics also feature characteristic absorption lines, so-called spectral 'fingerprints', at frequencies between 0.3 and 3 THz. The two main advantages of THz radiation are thus the penetration of conventionally opaque materials on one hand, and a high chemical selectivity on the other hand. Terahertz imaging is therefore thought to be a non-destructive technique for interrogating dielectric (non-conducting) materials. The use of terahertz waves for non-destructive evaluation of materials enables inspection of multi-layered structures and can therefore identify abnormalities from foreign material inclusions (contamination), delamination, heat damage, etc.

"The spectroscopic frequency band of a 0.1 to 10 THz is not easily accessible. Electronic sources like Gunn or Schottky diodes with subsequent frequency multipliers, provide high output levels (mW range), up to some 100 GHz, yet become inefficient in the sub-millimeter range. Direct optical sources, like quantum cascade lasers, are usually limited to frequencies greater than 5 THz, even when operated at cryogenic temperatures.

"Optoelectronic THz generation is an expression for indirect methods, where near-infrared laser light illuminates a metal-semiconductor-metal structure, generating a photocurrent that becomes the source of a THz wave. Both pulsed and continuous-wave (CW) techniques have been realized, and both have their advantages and limitations. Pulsed THz radiation offers a higher bandwidth (typically from about 0.1 to about 10 THz) and permits very fast measurements (a spectrum can be acquired within milliseconds). On the other hand, the frequency resolution is limited to several GHz. Vice versa, a CW system features a somewhat lower bandwidth (typically from about 0.1 to about 2 THz) and requires longer measurement times (acquiring a spectrum takes several minutes), yet the frequency can be controlled with extreme precision (down to single MHz).

"Composite materials such as fiberglass, Kevlar and carbon fiber are increasingly being used as structural components in aircraft because of their high strength to weight ratios, improved performance, reduced corrosion, etc., compared with other known structural materials. However, composites can be weakened by various defects and stress during their life cycle. Further, routine maintenance of composites requires complicated inspection and repair techniques.

"Terahertz radiation offers a non-invasive, non-contract, non-ionizing method of assessing composite part condition. However, THz sources have generally been difficult to produce. While there has been recent development using quantum cascade lasers, such devices remain largely in the developmental stage, and are intrinsically low-power devices. This limits their application to selected industrial manufacturing environments that require ease of operation and speed of data acquisition. In addition, use of THz surface of a substrate surface using presently available technology would still lack the degree of surface specificity required for interrogating composite and other opaque surfaces.

"The use of composite materials in modern manufacturing scenarios requires the existence of diagnostics that can reliably interrogate surface and subsurface composite characteristics. Such interrogation developments were not required for interrogating previous metallic-based manufacturing, since such systems were developed based on several centuries of metal manufacturing experience. While x-ray technologies were adequate to perform subsurface measurements with metal substrates, concerns regarding health and safety of personnel have properly precluded their use and adaptation in all but the most carefully controlled environments, such as medical facilities."

Supplementing the background information on this patent, VerticalNews reporters also obtained the inventors' summary information for this patent: "According to one aspect, the present disclosure is directed to methods, systems and apparatuses for evaluating and characterizing substrate material surfaces and sub-surfaces in the THz regime by using a plurality of optical source input radiation (non-THz regime sources). According to another aspect, the optical sources undergo second order mixing, specifically difference frequency generation (DFG). The present methods, systems and apparatuses require only optical sources, but achieve the effect of using THz regime radiation. The optical sources of inherent interest exist in abundance, can be operated for long-duration with reduced maintenance, have a comparatively easy set-up, and can be easily inspected.

"According to the methods, systems and apparatuses disclosed herein, it has now been demonstrated that THz regime investigation can be performed using only optical input sources that are in the optical regime. Desired effects in the THz regime are achieved via the use of non-linear optical interaction of optical source input beams at a material surface and/or sub-surface of a material being interrogated, by obtaining a difference frequency generation (DFG) between the two optical source inputs.

"According to a further aspect, one or more properties of the substrate surface are measured based on the output beam received by the detector. A first optical input beam from a first optical source having a first frequency, for example, in the optical range of from about 10,000 cm.sup.-1 to about 30,000 cm.sup.-1, is directed onto a predetermined region of a substrate material surface. A second optical input beam from a second optical source having a second frequency different from the first source frequency, but, for example, between from about 10,000 cm.sup.-1 to about 30,000 cm.sup.-1 is directed onto a predetermined region of a substrate material surface. While the frequency of the first optical source is largely arbitrary (but within the stated range), the frequency of the second optical source differs from the first optical source by the amount of the desired THz frequency to be generated. The first and second input beams are mixed at or beneath the surface of the substrate material surface to provide an output beam having a THz frequency of from about 0.1 THz to about 100 THz. The output beam is received by a terahertz detector. One or more properties of the substrate material surface are measured based on the output beam received by the THz detector.

"Therefore, according to one aspect, the present disclosure contemplates evaluating a substrate material surface comprising the steps of directing a first optical input beam from a first optical source having a first frequency onto a region of a substrate material surface; and directing a second optical input beam from a second optical source having a second frequency onto the region of the substrate material surface, with the first frequency of the first optical input beam differing from the second frequency of the second optical input beam. The first optical input beam and the second optical input beam are mixed at or beneath the substrate material surface to provide an output beam having a THz frequency. The output beam is directed to and received by a THz detector, and one or more properties of the substrate material surface are measured based on the output beam received by the detector.

"According to aspect of the present disclosure, the disclosed methods, systems and apparatuses can measure changes in a substrate material surface and subsurface, or properties and characteristics of any substrate material. For example, such characteristics include, mechanical defects, surface roughness, molecular contamination by non-native species, surface molecular orientation, etc. and combinations thereof. According to an aspect of the present disclosure, while the achievable absolute sensitivity depends on the particular parameters of the optical sources, and detectors selected (such as, for example, signal-to-noise ratio, etc.), the resulting signal changes in an output beam from as little as a tenth of a molecular monolayer at the material substrate surface will be detected.

"According to still further aspects, the difference between the first frequency of the first optical input beam and the second frequency of the second optical input beam is a THz frequency.

"In a further aspect, the first optical input beam and the second optical input beam are both emitted from a single emitting device, and the emitting device may be a laser, and, more particularly, the emitting device is not a THz frequency emitting source.

"According to still further aspects, the first optical input beam and the second optical input beam are emitted from differing emitting devices, and the emitting devices may be lasers, but neither optical input beam is emitted from a THz emitting source.

"According to a further aspect, the output beam is non-co-linear with the first optical input beam and/or second optical input beam.

"Still further, according to one aspect, the first optical input beam and the second optical input beam are directed onto the region of the substrate surface simultaneously or substantially simultaneously.

"According to another aspect, the present disclosure is directed to a system for characterizing a substrate surface. The system comprises one or more one optical emitting devices for emitting a first optical input beam having a first frequency and a second optical input beam having a second frequency and a THz detector, wherein the one or more optical emitting devices is positioned to direct the first optical input beam and the second optical input beam to a region of the substrate surface, wherein the first frequency of the first optical input beam differs from the second frequency of the second optical input beam, and wherein the THz detector is positioned to receive a THz output beam formed from a mixing of the first optical input beam and the second optical input beam at or beneath the substrate surface. The THz detector is in communication with computer software and/or hardware configured to measure one or more properties of the substrate material surface based on the THz output beam received by the detector.

"In a further aspect, the first optical input beam and the second optical input beam are both emitted from a single emitting device that is not a THz frequency emitting source. In another aspect, the single emitting device is a laser.

"In a still further aspect, the system comprises a plurality of optical emitting devices and the first optical input beam and the second optical input beam are emitted from differing emitting devices, and wherein neither the first optical input beam nor the second optical input beam is emitted from a THz emitting source.

"In a further aspect, the first optical input beam and the second optical input beam are each emitted from a laser.

"In another aspect, the output beam is non-co-linear with the first optical input beam and/or second optical input beam.

"In a still further aspect, the systems of the present disclosure further comprise computer software and/or hardware configured to measure one or more properties of the substrate surface based on the output beam received by the detector.

"According to another aspect of the systems of present disclosure, one or more optical emitting devices comprises one or more controls selected from a group comprising an optical input beam frequency control, an optical input beam intensity control, an optical input beam bandwidth control, and combinations thereof.

"In a further aspect, the systems of the present disclosure comprise emitting devices having one or more controls that produce a frequency difference between the first optical input beam and the second optical input beam that is a THz frequency.

"Still further aspects are directed to apparatuses comprising a system for characterizing a substrate surface. The apparatuses comprise one or more one optical emitting devices for emitting a first optical input beam having a first frequency and a second optical input beam having a second frequency and a THz detector, wherein the one or more optical emitting devices is positioned to direct the first optical input beam and the second optical input beam to a region of the substrate surface, wherein the first frequency of the first optical input beam differs from the second frequency of the second optical input beam, and wherein the THz detector is positioned to receive a THz output beam formed from a mixing of the first optical input beam and the second optical input beam at or beneath the substrate surface.

"In a still further aspect, the methods, systems and apparatuses of the present disclosure are directed to a vehicle comprising a substrate material surface, wherein at least a portion of the substrate material surface is interrogated by directing a first optical input beam from a first optical source having a first frequency onto a region of a substrate material surface. A second optical input beam from a second optical source having a second frequency is directed onto the region of the substrate material surface, with the first frequency of the first optical input beam differing from the second frequency of the second optical input beam. The first and second optical input beams are mixed at or beneath the substrate material surface to provide an output beam having a THz frequency. The output beam is directed to and received by a THz detector, and one or more properties of the substrate material surface are measured based on the output beam received by the detector.

"In a further aspect the vehicle includes, but is not limited to, manned or unmanned objects and structures in an atmospheric or space environment. Contemplated objects include vehicles, such as, for example, aircraft, satellites, rockets, missiles, etc., and therefor include manned and unmanned aircraft, spacecraft, terrestrial vehicles, non-terrestrial vehicles and even surface and sub-surface water-borne marine vehicles, objects and structures.

"For the purpose of this disclosure, the terms 'area', 'location' and 'region' are used interchangeably and have equivalent meaning when referring to the substrate material.

"For the purpose of this disclosure, the terms 'beam' and 'signal' may be used interchangeably and have equivalent meaning when referring to the substrate material.

"For the purpose of this disclosure, the terms 'interrogation' and 'characterization' are used interchangeably and have equivalent meaning when referring to the substrate material.

"The methods, systems and apparatuses of the present disclosure provide solutions to the problems of accurately and cost-effectively interrogating and evaluating substrate material surface and sub-surface characteristics including, but not limited to the interrogation and evaluation of chemical composition, homogeneity, heterogeneity, mechanical surface irregularities (including, but not limited to, defects and contamination), crystallographic defects, surface, etc., and combinations thereof."