We describe simultaneous determination of surface height profile and material distribution in the surface by imaging polarization detection. Starting from the single-beam case, the measurement principle based on the Stokes-Mueller formalism is deduced. This algorithm implies waviness detection of smooth as well as of rough surfaces. The basic concept of an imaging ellipsometer and its system configuration are discussed in the Stokes-Mueller formalism. Measuring the local slopes of the surface applying Fresnel's equations, the height image is deduced. Furthermore the imaging ellipsometer delivers the local complex refractive index from which the image of material distribution can be derived by a look-up table.
KEYWORDS: Modulation, Semiconductor lasers, Sensors, Frequency modulation, Linear filtering, Phase shift keying, Interferometry, Interferometers, Signal detection, Signal processing
We present a coherent-optical measurement procedure for contactless distance sensing. The radiation source is a laser diode whose optical wavelength is randomly modulated about 3...12 pm (rms value) by noise modulation of the injection current at a bandwidth of up to 100 kHz. An interferometer serves as a coherent detector. The average beat frequency of the photodetector output is a measure for the absolute distance to the object. Combining the natural phase noise of the laser diode with an additional technical frequency-modulation process, sensitivity can be drastically improved, particularly in the short distance range. Due to the technical current modulation the coherent detection can be used well beyond the coherence length of the laser diode. For small target ranges the absolute distance resolution increases. This measurement behavior is advantageous for approach/docking applications, for example. The measurement procedure was verified experimentally in a distance range up to 4 m.
The frequency response of force-measuring microlaser sensors (Nd:YAG) is tested in a special opto-mechatronic test setup. In this setup a piezo translator/frame configuration generates sinusoidal forces in the range from DC up to 100 kHz and higher. The force amplitude covers a range of approximately eight decades (1 N - 10 nN). Amplitude and phase response of the test setup including the laser sensor under test are measured. Because of the frequency-analogue output signal of the laser sensor, dynamic frequency measurements based on frequency/period counting, Bessel spectrum analyzing and FM-demodulation have to be performed. To conclude on the dynamic response of the microlaser itself the mechanical part of the test setup and the laser sensor is modeled mathematically. The theoretical response of the test setup is in good agreement with its measured frequency response, which means that modeling of the force-to-frequency conversion by the laser sensor is realistic. Based on static and dynamic measurement data, we conclude an excellent proportional response of the laser sensor for modulation frequencies up to 100 kHz. In this frequency range, the characteristics of the force-to-frequency conversion are strictly linear over approximately nine decades.
Surface measurement is an important tool for quality control In our contribution we describe a novel application of reflection ellipsometry to profile measurement and material detection. The ellipsometric parameters (Delta) and (Psi) are measured by using a PSA-ellipsometer arrangement and a 4- zone-intensity-ellipsometric algorithm. To achieve a high lateral resolution we focus the laser beam on the surface through a microscope objective with high numerical aperture. The spot diameter is in the order to 1 micrometers . The focus adjustment is carried out by an integrated autofocus system. From the ellipsometric parameters we conclude to the refractive index of the local surface material and the local surface gradients. The height profile is calculated by an integrating and filtering algorithm. The material is ascertained form the refractive index. The feasibility of our novel microellipsometric measurement system is demonstrated by several tests. Measurements of material transitions and the height profile of a glass surface standard are presented.
A novel transmission ellipsometric measurement method with the component under test inside an active laser is presented. Analyzing frequency and polarization property of the laser radiation we can detect five ellipsometric parameters of the component under test. For the first time it is possible to distinguish between reciprocal and nonreciprocal optical rotation. The polarization properties of the laser are changed in a defined manner by two novel optical modulators. Our experiments using a diode-pumped Nd:YAG laser demonstrate the high accuracy and the short measurement time of the measurement method.
In our contribution we describe a novel application of reflection ellipsometry to surface measurement. Not only the refractive index of the surface material but also the deterministic surface structure is concluded from the ellipsometric parameters (Delta) , (Psi) , and p. To achieve this, we here proposed two novel procedures: (1) the ellipsometric differential topometry and (2) the ellipsometric reference film method. The required measurement setup is described. To measure the ellipsometric parameters we developed a novel measurement procedure. Results of preliminary experiments are presented.
We are reporting about a novel ellipsometric measurement procedure which enables us to detect forces and force- related sizes at high resolution by use of the laserinternal photoelastic effect. The measurement procedure is based on the intracavity transmission ellipsometry developed by us. Relative retardation (Delta) and orientation of the main axis p of the component under test can be concluded from the polarization state and from the beat frequency of the orthogonally polarized modes of an active laser. Our measurements of weight forces show a very good linearity in a measurement range of seven decades. The Nd:YAG-laser technology enables us to develop very small sensors at high accuracy.
Precise optical wavelength measurements can be performed by use of a novel wavelength sensor. This low-cost sensor consists of a wavelength-sensitive photodiode and a high-precision log ratio amplifier. The center wavelength of monochromatic or narrowband light
sources such as semiconductor lasers in the visible or near-infrared wavelength range can be determined. By simultaneously testing the properties of monomode laser diodes with a standard wavemeter, the feasibility and precision of the wavelength sensor are demonstrated. Wavelength variations less than 0.1 nm are detected clearly.
KEYWORDS: Semiconductor lasers, Modulation, Interferometers, Signal detection, Sensors, Photodetectors, Interferometry, Signal processing, Frequency modulation, Linear filtering
Theory, experimental performance, and application of the noise-modulated interferometer are presented. This novel two-beam interferometer applies (1st) a laser diode source that is stochastically modulated in its frequency and (2nd) a common photodetector in the output arm followed by an averaging frequency counter. It is demonstrated that homodyne detecting of the two interferometer beams and counting the averaged beat frequency can be understood as a correlation process. By applying Horton's Anticorrelation' function to the signal processing channel of the noise-modulated interferometer, the range R <EQ 2 m of reflecting target is determined with high precision.
An optical minimum configuration of a laser interferometer for noncooperative scattering objects is investigated. An optimization of the SNR (signal-to-noise ratio) of the homodyne receiver is achieved by beam design. Scattering measurement object surfaces generate a speckle pattern on the detector surface which influences the SNR. Within the whole operating range of the optics, the transmitted light spot on the surface under investigation should therefore be small. The receiving optics should image the light spot onto the detector surface. The detection of a noncooperative object in a distance of 26 m is proven experimentally.
KEYWORDS: Modulators, Polarizers, Optical signal processing, Polymethylmethacrylate, Phased array optics, Signal processing, Analog electronics, Polarization, Signal detection, Sensors
Novel applications of PMMA-modulator-polarizer configurations in analog optical processing are described. The PMMA system, consisting of two polarization modulators and two crossed polarizers, can perform addition as well as subtraction and multiplication. Experimental results obtained with a laboratory assembly made up of two photoelastic modulators and a pair of Glan-Thompson prisms confirm the theory. It is concluded that complex structures consisting of a multitude of programmable PMMA processors having high processing speeds can be realized by fiber and integrated optics technology.
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