Fluctuations in the output intensity and wavelength of an external cavity diode laser can introduce significant error to wavelength-tuned interferometric measurement. However, a robust phase-retrieval algorithm can compensate for these nonlinearities. Employing an inexpensive phosphor-coated charge-coupled device camera sensitive to C-band infrared, full-field interferometric phase retrieval utilizing wavelength tuning of a 1555 nm external cavity diode laser is reported. Phase measurement of a tilted mirror is presented with an estimated accuracy within 7 nm.
Digital stepping is desirable in optical metrology--operation is simple, absolute position is known, and random regions of interest can be skipped to, rapidly and accurately. However, in white-light interferometry, analog scanning has traditionally been employed because, in one operation, it achieves depth scanning of a sample and an electronically detectable optical carrier through a Doppler shift. This is not obligatory nor efficient in functional machine vision, especially if approximate preknowledge of the sample exists. Two methods, utilizing digital depth stepping and a superluminescent diode, are presented to decouple optical carrier generation from depth scanning in full-field white-light interferometry. One technique employs a complementary metal-oxide semiconductor camera and acousto-optic modulation to generate a frequency difference between two arms of a Mach--Zehnder interferometer. The other technique uses a Michelson interferometer with a piezoelectric transducer integrated to the digital stepper motor to facilitate 2λ analog scanning and an optical carrier of 4 periods, sampled with a standard charge-coupled device camera. In the former case, random depth access measurement of an engineering gauge block calibration sample is presented, while the latter demonstrates the application of the random depth access full-field white-light interferometry to a small punch test. A further benefit of these techniques is the possibility of interferometric phase retrieval on condition of path length matching; this is proven by the implementation of a heterodyne phase retrieval algorithm in the gauge block measurement. Both techniques represent an advance in optical metrology, offering an inexpensive and functional solution to machine vision and industrial measurement applications.
KEYWORDS: Cameras, Optical coherence tomography, Digital signal processing, Signal processing, Signal to noise ratio, Semiconductors, Optical signal processing, Optical filters, Light sources, 3D image processing
Full-field optical coherence tomography (OCT) using a complementary metal-oxide semiconductor (CMOS) camera with an integrated a digital signal processor (DSP) is demonstrated. The CMOS-DSP camera employed is typically used in machine vision applications and is based on an array of 1024×1024 direct readout pixels that are randomly addressable in space and time. These characteristics enable the camera to be used as a fast full-field detector in carrier-based optical metrology systems. The integrated DSP facilitates basic signal processing including real-time filtering and undersampling. The optical setup used to implement this OCT method is composed of a free-space Michelson interferometer and a superluminescent diode (SLD) light source, with an electromechanical shaker for depth scanning. Unlike classical OCT approaches, however, the setup does not require any electromechanical device for lateral scanning. A 64×30 pixel region of interest was imaged at 235 frames/s and sampled in depth, corresponding to a volumetric measurement of 875×410×150 µm. Measurements carried out on a simple calibration specimen indicated lateral and axial resolutions of 14 and 22 µm, respectively. The presented approach offers an inexpensive and versatile alternative to traditional OCT systems and provides the basis for a functional machine vision system suitable for industrial applications.
Presented is a comprehensive characterisation of a complementary metal-oxide semiconductor (CMOS) and digital signal processor (DSP) camera, and its implementation as an imaging tool in full-field optical coherence tomography (OCT). The camera operates as a stand-alone imaging device, with the CMOS sensor, analogue-to-digital converter, DSP, digital input/output and random access memory all integrated into one device, autonomous machine vision being its intended application. The 1024x1024 pixels of the CMOS sensor function as a two-dimensional photodiode array, being randomly addressable in space and time and producing a continuous logarithmic voltage proportional to light intensity. Combined with its 120dB logarithmic response range and fast frame rates on small regions of interest, these characteristics allow the camera to be used as a fast full-field detector in carrier based optical metrology. Utilising the camera in an OCT setup, three-dimensional imaging of a typical industrial sample is demonstrated with lateral and axial resolutions of 14μm and 22μm, respectively. By electronically sampling a 64x30 pixel two-dimensional region of interest on the sensor at 235 frames per second as the sample was scanned in depth a volumetric measurement of 875μm x 410μm x 150μm was achieved without electromechanical lateral scanning. The approach presented here offers an inexpensive and versatile alternative to traditional OCT systems and provides the basis for a functional machine vision system suitable for industrial applications.
A comprehensive characterisation of a complementary metal-oxide semiconductor (CMOS) and digital signal processor (DSP) camera, used typically in machine vision applications, is presented in this paper. The camera consists of a direct read-out CMOS sensor, each pixel giving a direct analogue voltage output related to light intensity, with an analogue-to-digital converter and digital signal processor on the back-end. The camera operates as a stand-alone device using a VGA display; code being pre-programmed to the onboard random access memory of the DSP. High detection rates (kHz) on multiple pixels were achieved, and the relationship between pixel response time and light intensity was quantified. The CMOS sensor, with 1024x1024 pixels randomly addressable in space and time, demonstrated a dynamic logarithmic light intensity sensitivity range of 120dB. Integrating the CMOS camera with a low coherence Michelson interferometer, optical coherence tomography (OCT) axial depth scans have been acquired. The intended application is an imaging device for simple yet functional full-field optical coherence tomography. The advantages of the CMOS sensor are the potential for carrier-based detection, through the very fast pixel response with under-sampling, and the elimination of the electromechanical lateral scanning of conventional OCT by replacing it with electronic pixel scanning.