A FAMILY OF MICROINSTRUMENTS FOR SMART MATERIALS, ENERGY MANAGEMENT,

AND BIOMEDICINE IN SPACE MISSIONS

1M. Dudziak, PhD, Silicon Dominion Computing, Inc. (USA) and Dept. of Physics, Moscow State University (Adjunct Faculty)

2A. Chervonenkis, PhD, MODIS Corporation (USA)

ABSTRACT

Using a family of Fe-Ga based thin films fabricated epitaxially to have a range of magneto-optic (Faraday effect) properties, we have designed a class of instruments that can be used for a variety of challenging tasks pertaining to the maintenance and performance of spacecraft as well as astronaut crews on long-term missions into deep space. The principle and the generic design of this class of instruments applies to four diverse areas of utility and interest to the space exploration community:

Smart surface sensing and non-destructive testing for spacecraft components, particularly protective shield, hull, and structural assembly parts

Detection and mapping of electromagnetic field activity in and around the spacecraft, including at substantial (> 10,000 km) distances, for use in safety management and also possible novel energy production and propulsion techniques

Monitoring and mapping of biomagnetic fields detectable from human crew members as a remote-sensing / multi-spectral technique in diagnostic and preventative medicine

High-speed spatial light modulators for optical signal processing and optical switching including application to neural-network-like pattern processing.

Each device is based upon the operation of a MODE sensor designed from the thin-film material with its uniaxial anisotropic properties of magnetic domain transfer and reorganization in response to external proximate magnetic fields of varying strength, intensity, and duration. The sensor is coupled with a polarizing light source fed to the sensor via fiber optic channel, and a spatial light modulator switching element that operates in response to the changes effected in the sensor. The switching element provides input via fiber optic to an electro-optics element that is interfaced with digital logic for activating a response based upon one or several of the output configurations from the spatial light modulator.

1 CEO and Director of Research & Development, Silicon Dominion Computing, Inc., 3413 Hawthorne Avenue, Richmond, VA 23222-1821, 804-329-8704, 804-329-1454 fax, mdudziak@silicond.com2 Exec Vice President and Principal Scientist, MODIS Corporation, 1318 Pavilion Club Way, Reston, VA 20194, (703) 281-2100, (703) 281-2131 fax, arsen4@orc.ru

1. INTRODUCTION

The goal of the MicroMAG project, conducted as a joint venture of Silicon Dominion Computing, Inc. and MODIS Corporation, has been to develop a highly compact and modular set of devices for magnetic imaging and control functions based upon Faraday-effect magneto-optics, using a proprietary class of thin-film sensors with extremely sensitive (< 10-7 Oe) properties. The initial phase of the project has been to refine the sensor materials and to design a class of instrumentation that will meet the requirements of several functions for aerospace-based applications that benefit from magneto-optic methods in non-destructive defect detection and other EMF intensity measurements for structural, engineering, and bioengineering applications. Modularity, lightweight composition, compactness, and robustness for unmanned or low-maintenance applications have been goals of the project in order to provide for systems that are consistent with earth-orbit and deep space missions.

Surface fatigue, stress, crack, and other defect testing operations can be performed by introducing an array of sensor apparatus throughout the spacecraft structure, or by a member of the crew (or a robot) using a portable inspection unit. Generation of a “flash” eddy current in the metal structure is required but can be effected by either a built-in or portable apparatus including permanent magnet units that can be incorporated into the sensor device.

EMF activity can be observed and measured through modification of the MODE sensor thin film for increasing sensitivity at some cost in field effect intensity or duration (memory) within the sensor. The possible applications of this technology for energy generation have been discussed in a previous paper, “Design of Magneto-Optic Wide-Area Arrays for Deep Space EMF Studies and Power System Control”[Dudziak, 1998a].

For biomedical applications, the principle is similar to that which would be employed in detection of EMF outside the ship or in deep space. The instrumentation is intended principally for external measurements, using a band-aid strip device that can be taped to different locations on the body, with fiber-optic links to and from the sensor.

Spatial light modulators (SLM) for optical switching and image processing have been designed using magneto-optic media with typical switching times of 1 s but with recent new methods it is possible to attain reduction in size and increase of speed to approx. 0.1 s and optical contrast exceeding 1000:1. Such characteristics enable SLMs to be considered as a competitive solution for massively parallel or widely-dispersed network applications onboard spacecraft including embedded devices in smart structural materials.

All of these applications involve a fundamental common technology, using Fe-Ga based magneto-optically sensitive thin films of variable composition and sensitivity, and known as MODE (Magneto-Optic Detection and Encoding).

2. MODE THIN-FILM AND ITS PROPERTIES

The MODE technology is based upon a field visualizing film (FVF). It consists of a transparent ferromagnetic layer of Bi-substituted iron-garnet grown by LPE technique on a non-magnetic substrate. The composition of the FVF is characterized by the formula (R Bi)3 (M Fe)5012, where R is a rare-earth ion (Y, Lu, Tm, Gd, Ho, Dy, Tb, Eu for example) and M is

generally Ga or Al. Magnetic and magneto-optic properties of the FVF are controlled by composition, growth conditions and post-epitaxial treatment [Randoshin, 1990]. The specific Faraday rotation of 10^4 deg/sm and absorption coefficient less then 10^3 cm-1 are available in a generic composition (Tm Bi)3 (Fe Ga)5012. High contrast domain structures can be easily observed using a polarizing microscope. Figures 1 and 2 illustrate four sample images obtained with the MODE technology, all laboratory images made in ambient environments using sample materials (microprocessor chip circuitry (pads) and steel plates with defects) such as may be encountered on space vehicles and satellite assemblies.

Figure 1 MODE imaging of 16-bit microprocessor lead pads

Figure 2 Digital imaging of steel plate by MODE (left) and ordinary light (right)

The magneto-optic layer or FVF is created by growing the epitaxial layer on the garnet substrate, deposited in a supercooled flux, containing a solvent of composition Bi203-PbO-B203 as well as garnet-formed oxides at a temperature range of 940K to 1108K. By introducing a high level of Bi3+ ion substitution into the FVF a high MO figure of merit can be achieved, s.t. = 2F / > 10 grad/dB. An important feature of the FVF of value for possible deep space magnetic anomaly and variation studies is the high domain wall velocity (> 1000m/s) obtained in four types of films:

(i) high-anisotropic-oriented films with Y and Lu composition, in the presence only of in-plane magnetic fields, (ii) films with Gd and Tm, with angular momentum compensation (AMC), (iii) films with Y, Lu, and Pr (orthorhombical magnetic anisotropy (ORMA), and (iv) films with Gd and Eu (both AMC and ORMA).

Figure 3 illustrates saturation magnetization properties of the MODE film [B(G)] and an iron platelet [B(Fe)] - the ratio of the anisotropy field H / B(G) increases over the normal distance z.

Figure 3 MODE Saturation Magnetization Levels

Figure 4 provides a schematic of the basic operation of magneto-optic imaging using a MODE thin film crystal sensor. By incorporating the polarized light source into a fiberoptic delivery system, the packaging of a sensor unit can be sized down to a chip set incorporating CCD and control logic in one device and optics in a second hybrid device.

Figure 4 Basic operation of MODE Imaging

3. MagVision PROTOTYPE SCANNERAn early laboratory workbench scanner has been produced which generates NTSC or PAL compatible video output from a magneto-optic imaging apparatus. The basic design is illustrated in Figure 5 below. The “rotatable analyzer” is replaceable with a micro videocam assembly and can be adapted with an objective lens for a microscope. The solid housing can be a permanent magnet of varying strength (typically 5G) for enhancing the magnetic field of the sample as in imaging applications where the magnetic field of interest is affixed to a nonmagnetizable surface such as plastic or some insulator. The minimal detection of the current scanner is @ 0.1 Oe but the theoretical limit of the thin film extends to 10 -8 Oe. A yellow-orange halogen lamp is used with a thin-film polarizer for the light source.

Figure 5 Schematic of MagVision Prototype Scanner

4. MODE DEFECT AND STRESS DETECTION

The MODE technology has been tested in several experiments for use in detecting microcracks and other defects in metallic surfaces. These tests have been performed using a variety of materials, both magnetized and non-magnetized but ferromagnetic. The objective has been to refine methods for magnetic imaging that can be implemented without the use of an external eddy current applied to the sample, either as a steady low-amp current or using a “flash” technique (approx. 10-15 kA for 10-20 ms).

Real-time images of defects are obtained in both magnetic and nonmagnetic metal parts as well as in magnetic ceramics (ferrites) [Randoshin, 1990]. Magnetic anomalies caused by these defects are sensed by the MODE device. In ferromagnetic metals such as steel, the instrument senses and displays an image of the flux image leakage associated with the flaw in a previously magnetized test piece, while in nonferromagnetic metals, particularly in the case of Al, the instrument excites in the test piece eddy currents in order to provide a direct image of defects resulting from the magnetic fields with the flow of eddy currents.

The whole FVF region spontaneously divides into a labyrinthine domain space (DS), the magnetization vector M being strictly normal to the FVF surface. Domain walls (DWs) are taken

to be infinitely thin and thus M direction changes sharply to the opposing vector coming across the DW. The DS varies due to DW displacement when field Ho normal to FVF plane is applied. If M in the domain and Ho directions coincide, the domain increases in size; otherwise it decreases and inevitably collapses at the saturation field Hs. The primary FVF parameters (i.e. uniaxial anisotropy constant Ku, saturation magnetization Ms, coercive field Hc and thickness h) characterizing the DS behavior under the influence of inhomogeneous field, are customized through the composition process to satisfy the specific requirements. If a purely uniaxial anisotropy and zero Hc could ever be reached, the DS would be in unique conformity with the external field. It is extremely important that the FVF possess a very high uniaxial anisotropy field Hk=2Ku/Ms At a fixed Ms the desired Hk > 103 kA/m is attained by increasing Bi content. If the constants of non-uniaxial anisotropy and Ku are of same order then in the presence of some invariable inhomogeneous field the DS patterns would differ when rotating the film in its plane.

The requirements on the quality of the MODE FVF coincide with those for classical bubble memory devices. It is essential that the FVF possess a very low concentration of magnetic defects (i.e. single dislocations, inclusions, scratches) which impede a DW movement. All magnetic parameters and thickness are to be held constant to 1% over FVF region. Under such conditions homogeneously distributed coercivity always exist. Taking this into account, if the MODE defect detection sensor should possess "memory" capability, than it is necessary to induce elevated and uniform Hc of the order of Ms. There are several technological methods to increase Hc either during the epitaxial process or by special post-epitaxial treatment of the film.

4.1 Defect Detection Operating PrinciplesA magnetically soft object is magnetized up to saturation in its plane by an external field, created for example by a pair of permanent magnets such as are affixed to the sensing device. MODE film (FVF) grown on a transparent substrate is brought into contact with the object surface. If within the object thickness under the MO film a defect is present, the magnetic flux formed by the permanent magnets will be distorted in such a way that a portion of the flux force lines will emerge from the object surface and penetrate into the sensitive element, thereby rearranging its equilibrium domain structure. The changes in the DS may be visualized by the help of Faraday effect. The light from a polarized light-source by the help of a light-dividing cube is directed on the MODE film, which is closely attached to the object surface. Passing twice through the sensing film (due to reflection from a mirror layer) the light after the objective comes onto the screen where the contrast image of the domain structure is formed.

In the absence of the defect a uniform ("gray") DS will be seen on the screen. This refers to the demagnetized state of the film. However in the presence of the defect the DS in the film will be distorted under the influence of the magnetic flux force lines going out of the object surface. These distortions will be seen on the screen and thereby identify the position of the defect. Computer modeling in agreement with the experiment has shown, that the visualized picture of the defect substantially depends on the defect dimensions and the deepness of its position.

When the inspection subject is a nonmagnetic metal the principle scheme of the detector is similar to the fore-mentioned (see for example [Fitzpatrick, 1993]) with the exception that excitation of stray magnetic fields by means of eddy currents are additionally introduced. Both sinusoidal and pulse power sources may be used. The excitation of high magnetic fields by high power unique current pulses has significant advantage over traditional excitation by high frequency eddy currents/ In the former case, due to thermal limitations, high values of currents are prohibited, hence the induced magnetic fields are small. As a result the sensitivity and spatial

resolution of the sensor is limited. The use of unique current pulses eliminates these limitations, however, due to the short duration of the pulses (several ms), and dictates the introduction of MODE pattern "memory" capability.

Two mechanisms are proposed for MicroMAG implementation. In one case the sensitive MO element is produced with artificially induced and elevated DW coercivity. This provides image-memory of the rearranged DS by the element. In the second case one uses an intermediate flexible magnetic carrier (a kind of magnetic tape) that “remembers” or “captures” the structure of the stray magnetic field above the object surface in the case of close attachment of the tape to the object surface. After that step the visualization of the formed magnetic pattern is conducted by the use of a standard sensitive MODE detecting element. The second mechanism provides an additional advantage connected with the opportunity to reveal hidden defects in the objects with nonflat surface (welds, tubes etc.) since, unlike the fixed-element sensor unit (cf. Figure 5), magnetic resin tape can easily replicate the surface of any arbitrary object.

4.2 Summary of Advantages

MODE defect detection has a number of advantages over existing eddy-current instruments and techniques. The system is capable of providing direct, real time images of cracks, corrosion and other anomalies in inspected areas, as well as high resolution, with low probabilities of false calls (indicating a defect when none is present). Direct visual images, which can be directly recorded on video-tape for complete documentation of an examination, are intuitively understandable and easy to interpret and require no preprocessing. In contrast, interpretation of the impedance plane displays typically provided by eddy-current instruments are often difficult to stabilize and tune and require a highly-trained operator. The MODE approach lends itself to operation by the non-NDT-specialist and to automated expert-system enhanced operation.

Figures 6 through 9 below illustrate several representative images.

Figure 6 MODE picture of nonmagnetic inclusion (round spot) in a platelet of conventional stainless steel. The black area to the right is the edge of the platelet.

Figure 7 MODE picture of an internal microcrack in a conventional steel alloy platelet. The depth and dimensions of the crack may be evaluatable through translation of the

Figure 8 MODE image (left) of steel platelet0.8mm thick (upper surface) with defects on lower surface ranging from 0.1mm to 0.2mm deep (shown in optical image (right)

Figure 9 (Upper) Optical-only image of a steel blade covered by a thin (0.5 – 2 mm) layer of ice. (Lower) MODE image showing transverse crack in the steel and visible through the ice.

5. MODE APPLIED TO EMF DETECTION THROUGH NONMAGNETIC, NONCONDUCTIVE SURFACES

Early tests with MODE thin-films using the laboratory sensor device illustrated in Figure 5 have been performed with a number of materials including paper and plastic. Figure 10 below illustrates some of the results. Imaging is performed with the assistance of a peripheral permanent magnet, a hexaferrite composition (BaFe12O19), measuring 20 x 15 x 7mm, with 6000 G magnetic saturation and uniaxial anisotropy normal to the flat sample surface.

Figure 10 (Left) Image (10X) of magnetic code layered on plastic; (Right) Image (10X) of magnetic ink printed onto paper

There are several possibilities for how MODE can be employed in spacecraft operations through the detection of EMF and for the measurement of anomalies that may be the result of ferromagnetic artifacts or inclusions within asteroids, planets, and other natural bodies encountered in space. While much of the following is hypothetical and speculative, it is based upon experiments indicating that the sensitivity of the MODE films can serve a wider purpose than the traditional non-destructive testing and evaluation for metallic components which itself can be useful for spacecraft and other man-made devices.

An array of sensors equipped with fiber optic light sources and return channels could be distributed throughout critical parts, sections, and components of a spacecraft, or in a deployed external array, for detection of changes in magnetic fields. Such field variations might be indicate systemic flaws within portions of the spacecraft structure, particularly in regions of a craft that may be powered by ionic or other nuclear-reaction based propulsion. Alternatively such EMF fluxes might serve as predictable behaviors, a type of EMF barometer, indicative of changing conditions in the external environment, much as the MODE sensor has been applied to measurement of EMF leakage from high-voltage power lines. The deployment of such an external array could be extended for literally millions of kilometers provided that there were minimal-energy triggered transmitters that could in turn relay useful data upon detection to a receiving array, either on a spacecraft or in an intermediate location such as a geo(planet)-stationary orbital platform.

The possibility that such a wide-area array could be applied to novel “breakthrough physics” propulsion systems such as the extraction of useful energy from coherent vacuum currents has been explored in [Dudziak, 1998b]. It would necessitate a similar type of simple, small, and

fault-tolerant (through parallelism) device such as would play a role in the non-destructive testing and evaluation applications, leading to the MicroMAG architecture (cf. Section 8.)

6. MODE APPLIED TO BIOENGINEERING

A third application for MODE is in the measurement of biomagnetic fields that may be emitted either naturally by the human body or induced artificially through the introduction of magnetic tags into pharmaceutical agents. The latter approach has been investigated by Davis et al [Wagreich, 1996a; Wagreich, 1996b] using Mach-Zehnder and Fabry-Perot interferometers with promising results. It is possible that the increased sensitivity of the MODE films can accomplish in a smaller and simpler instrumentation the same level of accuracy (low pT/Hz 1/2) over several cubic cm or more in spatial resolution.

Figure 11 shows the results of very preliminary studies using the MODE sensor to reproduce the magnetic field structure above test samples of thin film permanent magnets (1 cm in diameter and 100 micron thick). The top row has images made with the thin film directly on the surface, the lower row with the sensor positioned 0.5 mm higher. There are different levels of bias field and clearly visible asymmetry caused by imperfect technology of SmCo5 sputtering.

Figure 11 MODE imaging of permanent magnet thin films

By reducing the size of the MODE sensor element it is possible that an array of such sensors can be positioned in a configuration that is analogous to an array of EEG or MEG sensors. Experiments are underway to develop an improved technique of affixing the thin film element to flexible materials which include embedded optical fibers woven into the fabric. In such manner a multitude of geometries can be handled for measurement tasks, with the optical outputs from the sensor array feeding into a spatial light modular device (SLM) as described briefly in the next section.

7. MODE SPATIAL LIGHT MODULATORS

A fourth projected use for a modular microscale class of MODE elements is for spatial filtering and optical correlation and switching that can be employed not only in fast binary image comparison and recognition but in other pattern recognition and matching tasks. Such processing may not be derived from images per se but can originate with data sets that are measured and stored as optical patterns. Applications in space of this nature could include measurement of vibration data, for instance, where similar to atomic force microscopy principles, laser beam reflections are routed to a photodiode array. The foremost use may be in optical computing where operations such as Fourier and Gabor transforms could be rapidly executed through a MOSLM (magneto-optic spatial light modulator).

What makes it possible to have a practical and small scale SLM is the typical switching time of ~ 0.1 s while at the same time attaining optical contrast exceeding 1000:1 and optical efficiency of 0.15 at = 0.63 m, with a specific Faraday rotation at that of = 2.0 0.1 / m with optical absorption coefficient of =0.35 – 0.4 dB/m. These properties apply to (Rbi)3(FeGa)5O12 films grown on (GdCa)3(ZrMgGa)5O12 substrate with crystalline lattice parameter of a=12.495 . In the diffusion heat process, employing an Si mask allows the formation of a domain structure that can have considerable variation in geometry and the areas of decreased magnetization and single domain structures provide for light shutters with typical cell structure @ 100 m2.

By switching from a (111) orientation to an in-plane field the domain wall velocity Vs can be increased to @ 1000 m/s and particularly by composition with high g-factor and high orthorhombic anisotropy to @ 3000 m/s as an upper limit. At Vs = 3000 m/s and with MOSLM dimensions of 100 m2 the switching time is approx. 0.03 s.

An experimental platform for a high-speed optical correlator based upon the MODE thin film was developed and is described in [Chervonenkis, 1992]. This was a large scale laboratory device using two MOSLMs in series separated by a polarizer and Fourier processing element. Test images consisted of 16 x 16 elements and comparisons were made between input and control images in order to filter out the differences between patterns.

Reduction of the MOSLM could be attained to a degree limited by the construction of the Si mask and the limits of domain wall distinguishability. Reducing the cell structure size is a topic of current research. The proposed approach is to create the cells in the form of pits or indentations in the background on the initial film thickness such that the thickness within the pit does not exceed 1 m. Single-domain states for each cell can be maintained even in high saturation magnetization (e.g., @ 300 G). Ion implantation can create a uniaxial anisotropic gradient that is normal to the plane of the film and thereby the remagnetization for cell switching occurs not as a result of vertical domain wall propagation inwards from the edges to the centers of the cells but instead by vertical propagation of the horizontal domain walls themselves. even at a lower Vs = 1000 m/s the switching time can be estimated at 1 ns maximum. The object for doing so is to reduce not only the performance time for such a MOSLM but to enable, once again, the deployment of a potentially massive and distributed number of simple and small sensing arrays across arbitrarily large surfaces such as for smart materials applications.

8. MicroMAG MODULAR ARCHITECTURE: FUTURE DIRECTIONS

A truly microscalar MicroMAG device has not yet been constructed, only simulated through larger apparatus with the theoretical limits and constraints of the MODE thin film providing the supporting evidence that the scale can be miniaturized. Scale however is not the only issue governing the utility of this technology for the multiple applications indicated above. For near-term space applications most technical implementation will effectively rest upon fabrication performed on Earth or in limited geo-stationary or lunar bases. Developing an apparatus that can be easily reproduced and introduced into a number of different structures will be a step forward from having very complex and unique instrumentation that cannot be interchangeable. The MicroMAG project aims at creating the equivalent of the simple diode or transistor in terms of functional simplicity and ubiquity. The goal is a basic magneto-optic component that can be incorporated into a variety of “circuits” so to speak just like a simple capacitor or resistor can be used in a variety of tasks and in a variety of physical configurations.

One area of current attention by the MODIS research team is on the development of a common interface for optic channel devices to MODE sensors that may require variable lines for input and output depending upon the nature of how many individual sensors are employed. This is the equivalent of designing a crossbar switch to serve a variable number of line-in and line-out signals and where the actual switching element can be replaced depending upon the task. In this way a common hardware component can be designed which will enhance the production and fabrication of optical processing networks such as MONA (magneto-optic neural array), an extension of earlier work by one of the authors and others at using MOSLM technology for neural-like distorted image recognition and correction [Nikerov, 1991].

This MODE element may never be reducible to the nanoscale level due to the constraints of domain size and domain wall behavior. However, some type of nanostructured material, embedded with ferromagnetic atoms in a prescribed geometry, perhaps through a controlled AFM nanofabrication process, could give be employed to enhance sensitivity to a MODE sensor embedded within or placed on the immediate surface of the material. The closest analogy seems to be that of the use of magnetic ink or fibers within paper or plastic (cf. Figure 10 above). The nonmagnetized, non-conductive material, perhaps part of the structure of a spacecraft or the protective suit of an astronaut, would be difficult to test for cracks, leaks, and other micro-damage. Embedding a very thin and distributed layering of ferromagnetic atoms, either through a physical spray process or as fibers, could enable a MODE apparatus to sense breaks indicative of the embedding material failure. What might be detected are breaks like those irregularities in Figure 10 (right) or Figure 11.

No doubt structural testing and integrity measurement is the dominant and leading application of MODE technology in space. SLM-based computing and biomedical applications, much less deep space energy extraction, are quite futuristic and hypothetical. However it seems to be sensible to start from the outset to address both that which is known and predictable and that which is hypothetical so that what emerges is something that can hopefully serve both should the hypothetical, as remote as it may seem today, turn out to be quite concrete and definite. Space travel and colonization itself seemed to be relegated by many to the domain of Jules Verne until some four decades ago, and despite the hopefully brief hiatus in major space-based undertakings, the next decade or two may yield more surprises and changes of thought than those brought on by the likes of Gagarin and Glenn.

REFERENCES

[Chervonenkis, 1992] Chervonenkis, A. Ya., Kirukhin, N. N., Randoshkin, V. V., & Ayrapetov, A. A., High Speed Magnetooptical Spatial Light Modulators, Advanced in Magneto-Optics II, Proc. 2nd Int’l. Symp. Magneto-Optics, Fiz. Nizk. Temp., Vol. 18, Supplement No. S1 (1992), 435-438

[Dudziak, 1998a] Dudziak, M. J. & Chervonenkis, A. Ya., Design of Magneto-Optic Wide-Area Arrays for Deep Space EMF Studies and Power System Control , 2nd International Academy of Astronautics Symposium on Realistic Near-Term Advanced Scientific Space Missions, Aostia, IT, June 1998, 155-161

[Dudziak, 1998b] Dudziak, M. J. & Pitkänen, M., How Topological Condensation of Photons Could Make Possible Energy Extraction in Deep Space, 2nd International Academy of Astronautics Symposium on Realistic Near-Term Advanced Scientific Space Missions, Aostia, IT, June 1998, 129-138

[Fitzpatrick, 1993] Fitzpatrick G. L., Novel eddy current field modulation if magneto-optic films for real time imaging of fatigue cracks and hidden corrosion, SPIE Proceedings, Vol. 2001, 210-222, 1993.

[Nikerov, 1991] Nikerov, V. A., Polyakova, J., Chervonenkis, A. Ya., Double MOSLM neural-like coherent optical processor for distorted image recognition, Optics, Illumination, and image sensing for Machine Vision VI, SPIE Vol. 1614 (1991)

[Randoshkin, 1990] Randoshkin V.V. & Chervonenkis A. Ya., Applied Magnetooptics, Energoatomizdat, M., 1990 (in Russian)

[Wagreich, 1996a] Wagreich, R. B. & Davis, C. C., Magnetic Field Detection Enhancement in an External Cavity Fiber Fabry-Perot Sensor, Journal of Lightwave Technology, Vol. 14, pp. 2246-2249, 1996.

[Wagreich, 1996b] Wagreich, R. B. & Davis, C. C, Performance Enhancement of a Fiber-Optic Magnetic Field Sensor Incorporating an Extrinsic Fabry-Perot Interferometer, Proc. Photonics, Beijing, China, November, 1996