Valery Petrov

Electronic speckle pattern interferometry with a holographically generated reference wave

A new method of electronic speckle pattern interferometry (ESPI) is introduced. It is a two-stage process. In the first stage a holographic optical element (HOE) is recorded in a usual holographic arrangement. This HOE illuminated later reconstructs an object wave that serves as an ESPI reference wave. Well suited HOEs are produced on photothermoplastic or silver halide media. In the case of silver halide media, both recording and momental monobath photoprocessing of the HOE are performed in presence of an intense polychromatic illumination to meet industrial requirements. Due to the introduction of HOE the second stage, ESPI implementation can be performed with an extremely simple optical setup. The number of optical elements, apart from the video camera, can be drastically reduced to only two elements: a plane mirror and the HOE. High accuracy alignment of the optical elements is not necessary. Our method also enables HOE recording in the ESPI setup practically without modifications. Both experimental results and the ready device are presented. Our optical setup is compact and needs no sophisticated vibration insulation and no dark room, thus it is ideally suited for industrial applications.

ESPI with holographically stored waves and other innovative ESPI methods used for real-time monitoring of dynamic thermal deformations in a practical industrial environment

Electronic speckle pattern interferometry (ESPI) is a rapidly developing optoelectronic method of nondestructive laser metrology supported with computer evaluations. It permits measurement of deformations in the micrometer and submicrometer ranges produced by an electrical signal, heating, mechanical stress or another load. Though the method seems very friendly for industrial checks it has several drawbacks which prevent its application in real industrial environment: complexity, bulkiness and high costs of optical setups, difficulties in aligning of the optical elements. There are problems in working outside the laboratory especially due to high sensitivity of ESPI devices against environmental vibrations and daylight. The method of ESPI with holographically stored waves was introduced by these authors in 1995. It permitted to avoid all drawbacks mentioned above and to build an elegant, portable ESPI device with 2 optical components only: a HOE and a plane mirror. The device works very well with different test specimens subjected to deformations and vibrations. It was found that the method and the device also suit well for quasi-real time monitoring of dynamic thermal deformations. Such a novel possibility of ESPI to monitor temporal development of deformations due to electrical heating of electronic components populating a printed circuit board (PCB) and to locate a component subjected to excessive heating is presented in this work for the first time. Temporal development of deformations of the same PCB were monitored in quasi-real time in other innovative ESPI devices also originating from our laboratory. These portable, compact devices utilize alternative physical principles for combining of object and reference ESPI waves and guiding them to the sensor of the CCD camera. Such novel devices also ensure proper acquisition and documentation of temporal development of deformations and dynamics of propagation of thermal waves. Vast experimental data properly illustrating the possibilities of the novel methods are presented.

Compact and simple ESPI devices for testing of materials and components in microelectronic manufacturing

With the increasing packaging density of electronic components in the integrated circuits (IC) as well as on the printed circuit boards the problems with heat dissipation also increase. Electronic speckle pattern interferometry (ESPI) is a rapidly developing optoelectronic method of nondestructive laser metrology supported with computer evaluations. It permits measurements of deformations in the micrometer and submicrometer ranges produced by an electrical signal, heating, mechanical stress or another load. Though these method seems very friendly for testing microelectronic components, composite materials, miniature devices, circuit boards, wafers etc., it has several drawbacks which prevent its application in real industrial environment: complexity, bulkiness and high costs of optical setups, difficulties in aligning of the optical elements. There are problems in working outside the laboratory especially due to high sensitivity of ESPI devices against environmental vibrations and daylight. We present several methods ESPI based on which a family of devices was built. Their unique properties permit to avoid all drawbacks mentioned above. These devices are extremely simple and compact. They are easily aligned and operated even by persons who are not skilled in optics. These devices are ideally suited for working outside the laboratory even by persons who are not skilled in optics. These devices are ideally suited for working outside the laboratory for example in a well-lit industrial environment. The innovative devices permit to measure deformations both in the plane of the object surface and perpendicular to it. A detailed description of the methods and devices given. Multiple examples of ESPI computer evaluations obtained in the novel devices with different small objects under test are presented. However these devices permit to work with much bigger objects if required. Our methods and devices are also suited for qualitative as well as quantitative analysis.

Electronic speckle pattern interferometry with thin beam illumination of miniature reflection and transmission speckling elements for in-plane deformation measurements

A new method of electronic speckle pattern interferometry (ESPI) for in-plane deformation measurements is presented. Unlike usual ESPI methods used for in-plane deformation measurements with two symmetrical smooth illuminating waves, we introduce miniature speckling elements and thin laser beam illumination of them. The minia- ture elements serve for generating two symmetrical speckled waves. These miniature speckling elements are produced as reflection or trans- mission holograms. Nonholographic speckling elements were also tested. We present a family of flexible electronic speckle pattern interfer- ometers for in-plane deformation analysis based on our method. Due to their simplicity, compactness, and low cost, the devices are ideally suited for industrial automated inspection. Experimental results obtained with the novel ESPI devices are given.

Application of combined method of electronic speckle pattern interferometry and holography to vibration analysis

Electronic speckle pattern interferometry (ESPI) and holographic interferometry are powerful tools for vibration analysis. They allow to measure the spatial distribution of a vibration amplitude in the micrometer or submicrometer range for the purpose of modal analysis for nondestructive testing. As for ESPI, this method principally is very suited for large-scale industrial optical inspections because the interferograms are recorded with a video camera and evaluated in quasi real time with a computer and no media costs for the measurements arise. However current speckle pattern interferometers have rather complicated and expensive optical setups whose elements are aligned with difficulty. On the other hand, holographic interferometry which can provide interferograms with less noise and higher resolution requires an optical setup of at least similar complexity, and in addition a holographic recording medium such as silver halide material or photothermoplastic film. The negative influence of all these drawbacks are drastically reduced in a method which combines positive features of ESPI and holography. This method has already proved its validity for different objects loaded in statics. It can be successfully applied to vibrating objects.

Family of novel compact and very simple electronic speckle pattern interferometers for out-of-plane and in-plane deformation measurements

Electronic speckle pattern interferometry (ESPI) is a powerful tool for nondestructive testing of materials and products. Like holographic interferometry it allows to measure deformations and vibrations in the micrometer and submicrometer range. However current speckle pattern interferometers have rather complicated and expensive optical setups whose elements are aligned with difficulty. Moreover commercial ESPI devices lack flexibility in their optical setups. We present a family of flexible electronic speckle pattern interferometers for out-of-plane and in- plane deformation and vibration analysis which were quite recently developed at Laboratory of Technical Optics, Laser Techniques und Optoelectronics of Fachhochschule Ulm. Their common properties are: extreme simplicity and compactness of the optical setups due to the use of transmission or reflection HOEs or other very small, simple diffusers and a compact laser; very effective usage of laser radiation; no need of sophisticated vibration insulation; alignment easily performed without requiring high accuracy; simple intensity matching of ESPI object and reference waves; optically skulled personnel is not required for ESPI operation and device maintenance; and very low costs of the optical setups. Due to simplicity, compactness and low costs the introduced devices are ideally suited for industrial automated inspections. Extensive experimental results are given which were obtained with the novel ESPI devices.

Holographic data acquisition and display in real and quasi-real time: friendly for industrial inspections

Quick, reliable and inexpensive testing of products and components is needed in industry more often than not. Holography due to its intrinsic limitations and reasonably high costs yet is a method which is used comparatively rarely for industrial optical inspections. In the Laboratory of Technical Optics, Laser Technologies and Optoelectronics we produce holograms and interferograms on silver halide media in extremely unpromising conditions intentionally introduced to fit industrial ones. The main features of our processes are: (1) high speed of the whole process of data acquisition, storage and final presentation; (2) possibility to perform all these operations in strong daylight corresponding to industrial illumination levels or even exceeding recommended levels of illumination of industrial working places; (3) possibility to avoid any dark rooms; (4) high diffraction efficiency of holograms measured at the wavelength of recording, i.e. any wavelength shifts of holographic reconstruction are avoided for any types of holograms including reflection ones usually suffering from such shifts; (5) low costs due to low labor and equipment costs; and (6) easiness of performing quick checks in industrial environment. In our experiments we investigated various optical setups for recording holograms, double exposure and real time interferograms, in situ among them, and different types of holographic optical elements. We used different types of lasers: helium-neon (633 nm), argon (488 nm), semiconductor (675 and 685 nm), ruby (693 nm). Various silver halide media were used: Agfa Gevaert 8 E 75 HD films and plates, Russian PFG-03 and PFG-03 C (color).

Bridging the gap between electronic speckle pattern interferometry and holography

Both electronic speckle pattern interferometry (ESPI) and holography are rather powerful tools for nondestructive testing of materials and products. ESPI and holographic interferometry allow to measure deformations and vibrations in the micrometer and submicrometer ranges. In spite of definite similarities they differ chiefly in the methods of acquiring and presenting optical data. Each method has its special advantages and drawbacks. The latter are severely limiting their applications especially in industry. It would be ideal to combine the advantages of ESPI and holography and to reduce influence of their drawbacks. As for ESPI, this method is very suited for large scale industrial optical inspections because the interferograms are acquired with a CCD camera and evaluated in quasi real time with a computer thus no media costs for the measurements arise. However current speckle pattern interferometers have rather complicated and expensive optical setups whose elements are aligned with difficulty. Moreover commercial ESPI devices lack flexibility in their optical setups. The negative influences of these drawbacks are drastically reduced in our method which is based on a very simple principle. We offer to use an unusual ESPI reference wave which is stored in a holographic optical element (HOE) as holographic object wave. In other words, a new method of ESPI is introduced which is a two stage process. At the first stage a HOE is recorded in a usual holographic arrangement. This HOE being illuminated later reconstructs an object wave which serves as ESPI reference wave. Well suited HOEs are produced on photothermoplastic or silver halide media. The latter are easily produced in an industrial environment. Experimental results and ready compact and inexpensive device are presented.