Optical Distance Measurement

Fig. 1: Shape measurement of an object using laser triangulation

Introduction
As a result of ever shorter production cycles the demand for industrial metrology and the demands on manufacturing quality increase simultaneously. Optical metrology plays an important role as it has many advantages in comparison to conventional methods: contact-free and quick measurements at a high measurement resolution, simultaneous registration of large surfaces, and it is usable in difficult environments (EMV, temperature, …).

Fig 2a: Development of a laser distance sensor for industrial metrology applications, based upon a novel measurement technique, lab set-up

However, several optical measurement methods are based upon different measurement principles (triangulation, imaging processes, processing time, autofocus, interferometry, spectroscopy, fiber sensors). Every method has its specific properties and thus specific advantages for different applications.

Triangulation for instance (see figure 1) has become a standard method for distance and shape measurements. However, triangulation is, due to shadowing effects, not generally suitable for the measurement of edges or holes. Therefore, the appropriate measurement method must always be identified, optimized and customized for the specific application.

Aims
The metrology group develops and examines laser-optical sensors for measurement applications. The focus is on the use of novel sensor principles and on the utilization of not yet exploited aspects of conventional sensors.
The aim is to develop sensors with optimized measurement properties for specific applications or innovative measurement property combinations that have not yet been used for other sensors. That way new measurement tasks can be opened up that could not yet be solved so far.

Fig. 2b: Development of a laser distance sensor for industrial metrology applications, based upon a novel measurement technique - adaptation of the sensor to an Nd:YAG laser for measuring the focus position during laser material processing

Projects
Several current research projects of the metrology group are presented in the following:

Distance measurement
Development of a laser distance sensor within the scope of a cooperative project sponsored by the BMBF (German Federal Ministry of Education and Research). Project partners: TEM Messtechnik GmbH (Hannover), Sacher Lasertechnik GmbH (Marburg)

Different from conventional distance measurement methods the novel method employed here directly integrates the measurement object in the generation of the laser light. The surface of the measurement object acts as external resonator of an antireflection coated laser diode. Due to the high amplification by the laser diode scattering losses in the measurement object are compensated so that stationary laser operation with excitation of longitudinal modes is possible. By modulating the laser diode current synchronously to the rotational frequency of the light in the resonator, a resonance effect can be exploited that enables a higher measurement resolution than passive methods (e.g. transit time sensors).

Fig. 3: Integrated optical sensor for incremental path measurement

The principle promises a sensor with micrometer resolution at a measurement rate in the kilohertz range for measurements on rough surfaces. It is thus interesting for industrial applications such as in-process control for turning, milling and polishing processes or monitoring of laser material processing. Figure 2 shows a laboratory setup for examining the basic properties of the sensor (a) and the sensor adaptation to the head of a Nd:YAG laser material processing system for in-process control (b).

Integrated optical distance measurement technique
Within the scope of the SFB 615, subproject C5, and in cooperation with the PTB (Physikalisch-Technische Bundesanstalt) a modular integrable distance measurement sensor for active micro measurement systems is developed.
By means of direct laser processing optical fiber structures are generated in an optical substrate in order to implement an optical fiber coupler with dimensions in the submillimeter range. These structures are several orders smaller than those generated by conventional fused fiber couplers. By integrating additional optical elements, such as laser diodes, photo detectors or microlenses, an integrated optical interferometer with dimensions in the submillimeter range is created.

Fig. 4a: Characterization of a miniature flow sensor for space applications, sensor head in lab set-up

Diffraction of the laser radiation at an incremental length scale and interference of the diffracted light in the fiber coupler generates modulated electrical signals in the photodiodes if the interferometer is moved contrary to the length scale. These signals enable displacement measurements with resolutions in the submillimeter range and direction recognition.

Fig. 4: Characterization of a miniature flow sensor for space applications - measurement signal

Miniaturized fluid sensor for space research
A miniaturized fluid sensor for space applications is constructed and examined in cooperation with ZARM in Bremen.

The measurement system is based upon contact-free flow speed measurement using laser Doppler anemometry (LDA). The optics of the sensor head utilize a, to a large extent, monolithic setup and are miniaturized to such an extent that qualify for flow measurement tests under microgravitation conditions at the international space station (ISS).

Figure 4a) shows the miniature measurement head with coupled fiber optics for laser light input to illuminate the measurement volume in front of the sensor head, and for the output of the detected measurement signal. The measurement head was adjusted and examined at LZH. Figure 4b) shows the detected measurement signal and accompanying modulation. Based on the frequency of the modulation the flow speed of the medium flowing through the measurement volume can be determined.

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