Optical Coherence Tomography (OCT) for Industrial NDT (Non-Destructive Testing)

By AZoM Staff Writers

Topics Covered

Introduction
Working of OCT
Line Sensor Camera
2D OCT Camera
Test Object
SWIR Line Sensors in InGaAs Technology
Flexible Evaluation Circuitry
Conclusion
About Xenics

Introduction

Optical coherence tomography (OCT) has evolved to a non-invasive standard biomedical analysis technique yielding a detail-rich cross-sectional image of living tissue. As a high frequency analog to and as a possible replacement of ultrasound examination techniques, optical coherence tomography delivers images of a much higher resolution well into the micrometer realm.

The penetration depth possible with OCT is determined by the type of material under test and the wavelength used as shown in Figure 1. In medical applications, for human tissue investigations, and when using light sources in the short wave infrared, OCT penetration can reach 6mm and more (much larger depths than with visible or near infrared light sources)Hence OCT can bridge the realms of confocal microscopy and ultrasound, as well as other computer tomography (CT) methods such as magnetic resonance imaging (MRI).

Figure 1. Examination methods for analyzing depth structures, penetration depth and resolution

Working Principle of OCT

Using OCT, cross sectional images are obtained from within material objects without the need of touching them or using destructive procedures. Figure 2 shows several cross sections of a MEMS (micro electro-mechanical system) pressure sensor at various depth levels achieved by penetrating it through its membrane.

Hence, OCT is enabling novel ways for process control and industrial quality assurance. Real-time OCT can be used to monitor continuous manufacturing and assembly processes.

Figure 2. Penetrating the membrane of an MEMS pressure sensor (above left), OCT yields cross-sectional images at various depth levels to unveil details of its internal structure.

The basics of optical coherence tomography are delineated in Figure 3. It is based on the interference of a direct light beam from a light source striking the object under test with the light beam reflected by the object through a semi-transmissive mirror.

Line Sensor Camera

In the upper section of Figure 3, a broadband light source radiates light of a low coherence length through a semi-transmissive mirror onto the object under test. A portion of the beam is reflected by the beam splitter and guided to a reference mirror, which feeds it via the semi-transparent mirror to the input of a spectrum analyzer.

Figure 3. Two concepts of optical coherence tomography (OCT): high-resolution line camera Lynx and very fast 2D camera Cheetah-640 CL, used for a cross-sectional examination of a plastic, three-layer container wall.

There the light reflected by the test object and the reference beam interfere. As a result, the spectrum analyzer outputs the spectrum of this interference pattern, which is then converted by an optoelectronic line sensor to an electrical intensity signal.

2D OCT Camera

The lower section of Figure 3 shows an OCT procedure. It makes use of a 2D camera with InGaAs sensor technology. This 2D camera setup works without a depth scan mechanism.

The relatively small-band light beam of superluminescent diode is led through a beam splitter yielding a reference beam going to the left, and a test beam going downwards. The test beam is widened by a cylindrical lens to form a line on the test object.

The diffraction grating deflects the reference beam, which is positioned in a way that a higher-order diffraction image is directed towards the camera. Hence the 2D camera observes an interference pattern generated by the reference and test beams.

By studying several interference images, which show different phase relations, the object under test is optically examined. Diffraction gratings with micro actors made of a piezo ceramic, enable a mirror small travel range in the realm of a few micrometers.

Test Object

A three-layer plastic part, about 2.3mm thick, serves as the wall of liquids container was chosen a test object for an OCT demonstration. It includes a high-density poly-ethylene material plus a 100µm thin EVOH (ethyl vinyl alcohol) layer as diffusion barrier.

The Fraunhofer-Institute studied the properties of this structure in the context of a project that is to explore resource-saving manufacturing methods in the plastics industry.

The objective was the development of an Interferometric Inline Control System in the Production of Multi-layer Plastic Foils. OCT is used to determine the thickness and uniformity of individual thin layers in real time during a production run.

SWIR Line Sensors in InGaAs Technology

For the measurement of these short wave  infrared spectra the usual CMOS or CCD image sensors are not appropriate since their sensitivity is limited to the visible realm. A significantly higher SWIR sensitivity is offered by sensors with InGaAs technology as shown in Figure 4. They are currently available as linear detectors (Xenics Lynx) with up to 2048 pixels and as a 2D camera (Xenics Cheetah-640CL) featuring 640x512 pixels.

 

Figure 4. An InGaAs image sensor can operate in the near infrared realm.

Flexible Evaluation Circuitry

Though InGaAs is a perfect choice for SWIR-sensitive photodiodes in line sensors, it is not really suited to integrate the read-out circuitry on the same chip. Therefore, an external read-out IC (ROIC) based on CMOS technology serves as evaluation circuitry for the InGaAs line sensor.

The analog ROIC front-ends can be optimized to function as detector interfaces, and their parameter values are software settable in wide margins to cover various pixel sizes and application demands.

Figure 5 provides a simplified block diagram of the ROIC's analog functionality needed for the multi-stage pre-processing of the sensor signal. The InGaAs photodiode, featuring n-well-capacities of 0.7 to 2 million electrons, is held at a constant voltage level by the current-to-voltage converter.

Figure 5. Pre-processing the sensor signal of a pixel with various software-settable parameters accommodates the camera to different applications.

The subsequent stage performs a correlated double sampling (CDS), thereby compensating the offset variations of the current/voltage converter and also eliminating its reset noise in line is the sample/hold stage.

It decouples integration and read-out. Hence the charge of the actual frame exposure can be integrated while the preceding frame is being read out simultaneously. Finally, an analog multiplexer and pad driver transfer all pixel values via the IC output to an external analog/digital converter.

Conclusion

With a range of sensor types to choose from, ROICs with various parameter settings and multi-stage thermoelectric cooling, today's InGaAs cameras are scalable OCT platforms. They are suited for a number of industrial, scientific and medical applications enabling direct imaging and spectroscopy.

Most important, they include sensors that deliver a resolution of or below one micrometer, at a penetration depth of more than the current standard of 6 mm. This empowers the system designer to implement OCT in the short wave infrared, as a meaningful and detailed analysis tool for hidden structures to increase quality, throughput and yield in manufacturing processes.

About Xenics

Xenics is a well established and rapidly growing high-tech company about to enter exciting new fields of IR products and applications, serving the markets with excellent products and a strong technology background. This enables us to do custom designed chips and add functionality as requested, such as specific read-out capability.

Uncooled InGaAs-based devices will revolutionize the global markets for short wave infrared spectroscopy, imaging and non-contact temperature measurement. Uncooled bolometer-based products will revolutionize the markets for thermal imaging and thermography. While advanced cooled products will continue to grow in the markets that require the ultimate performance level or specialty features.

You will see many exciting new applications – commercial, industrial, medical, security-related, scientific, and many others - over the coming years. That's exactly where Xenics is positioned. Thus, Xenics will play a leading role in developing these exciting markets.

Xenics offers products covering the most active IR wavelength areas from 1 up to 14 micrometer. They come as line-scan as well as two-dimensional devices. Xenics also delivers custom detectors, cameras or electro-optic instrumentation solutions according to our customers' agreed specifications and project setups.

This information has been sourced, reviewed and adapted from materials provided by Xenics.

For more information on this source, please visit Xenics.

Date Added: Nov 21, 2013 | Updated: Dec 12, 2013
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