Klaus Doerschel

Electrostatically driven micromirrors for a miniaturized confocal laser scanning microscope

A compact two-mirror microscanner has been fabricated to build the central part of a miniaturized confocal laser scanning microscope. This microscope shall be mounted at the tip of an endoscope to provide high resolution imaging for medical diagnostics. In order to achieve a resolution of 500 X 500 image elements large scan angles and also large mirror dimensions have to be realized within a spatially strong limited housing. While bulk silicon technology on the one hand enables fabrication of micromirrors with nearly ideal elastical behavior, those actuators on the other hand often are too fragile for a lot of applications. This paper describes the design, fabrication and assembling of electrostatically driven torsional micromirrors that meet the requirements of fast two-dimensional scanning with high angular precision over large scan angles, compact design and also high shock resistance. This is achieved with the combination of bulk silicon technology with metal surface micromachining. Besides medical diagnostics these microscanners can be used in a wider range of applications such as displays, two-dimensional barcode scanning, multiplexing of fiber optics, etc.

Optical properties of circulating human blood

We investigated the optical properties (mu) a, (mu) s, and g of human blood under flow conditions using integrating sphere measurements and inverse Monte-Carlo-simulations. The experiments were conducted at 633 nm with regard to the influence of the most important physiological and biochemical blood parameters. In addition, a spectrum of all three parameters was measured in the wavelength range 400 to 2500 nm for oxygenated and deoxygenated blood.

Experimental set-up and Monte-Carlo model for the determination of optical tissue properties in the wavelength range 330 to 1100 nm

The optical properties of different types of tissue were measured in the wavelength range 330 - 1100 nm. The measurements were carried out in native as well as in coagulated tissues. We used the double integrating sphere technique to provide reflection and transmission measurements and a special homogenizing technique to prepare the tissue. The optical properties were evaluated using an inverse Monte-Carlo simulation, considering the geometry of the experimental set-up. All tissues show characteristic absorption bands at 420 nm and 550 nm, related to the strong absorption of hemoglobin. After coagulation the scattering increases drastically while absorption remains nearly unchanged. The anisotropy factor g increases with increasing wavelength and drops down slightly after coagulation.

Confocal microscanner technique for endoscopic vision

The intention and first results of a research project called DELAS (diagnostic endo laser scanner) are presented. This paper is focused on the technology development of a scanning imaging method for endoscopy. This work is granted by the Bundesministerium fur Bildung, Wissenschaft, Forschung und Technologie (BMBF, FKZ 13 N 6796). The current endoscopic imaging technique is restricted to the visible spectral range. An extension of the usable wavelength range leads to new application fields of endoscopes, such as the use of fluorescence or other spectroscopic methods, e.g. to perform diagnostic investigations. The aim of the project is to investigate the feasibility of a scanning imaging technique on the basis of microsystem technology. The optical components are realized by using lithographic methods in addition to single fibers and microscanning mirrors being applied to avoid the spectral limitations of conventional image guides. The integration of the microscanning device into an imaging setup of an endoscope is planned in a further project part. Initially, an experimental setup using conventional components has to be realized in order to simulate a situation considering the requirements of an endoscopic utilization. Using this setup the verification of the imaging method regarding potential applications and an improvement of the imaging parameters are performed. These investigations lead to specific requirements of a microscanning device for a future prototype manufactured by using silicon technologies. First results of the investigations are discussed in terms of lateral and depth resolution, image distortion, and non linear behavior of the scanning mirrors. They are compared to the results obtained by conventional microscopy and evaluated with regard to the requirements of diagnostic applications.

PC-integrated laser Doppler blood flow measurements in skin

For a quantitative calculation of a flow spectrum from the Doppler frequencies the velocity-, the irradiation- and scattering-directions must be well known. With the assumption of a random distribution of the velocity and irradiation a realistic flow spectrum can be calculated. Furthermore the flow spectrum depends of the scattering phase function of the moving particles. A flow meter was developed, consisting of a PC and a control card for two Doppler sensors to be fixed on skin. A Pascal program shows the flows on the display with resolution in time greater than 150 Hz. Four different flow domains can be shown simultaneously and the frequency span can be set independently. Flows corresponding to low and high Doppler frequencies are different and provide the possibility to distinguish between the flow in the micro-capillaries and larger vessels.

High-resolution coherent tomography

From a cw-laser beam transmitted to strongly scattering tissue most photons have been scattered and therefore a complicated unknown path in tissue and decoding of the optical tissue parameter from the intensity of the scattered photons is very difficult. But some of the transmitted photons could undergo the absorption and scattering. From its intensity the integral sum of the absorption and scattering coefficient can be calculated easily for its well known straight way. With a focused laser beam by scanning the sample in x, y and z-direction the optical parameters can be calculated with high lateral resolution. To distinguish between scattered and non-scattered photons, a Mach-Zehnder interferometer can be employed.

Feasability of fiber optic delivery in medicine (Invited Paper)

Fiber optics are used in medicine in power transmission for therapeutic use as well as in signal transmission for diagnostics. Fibers are the key component of the applicators which find a wide and varied use. Applicators with optical fibers are part of the standard equipment used with the various continuous and pulsed medical laser systems. Recently, optical fiber applicators for the Er:YAG, CO, and CO2 laser have become available. A combination of therapeutic and diagnostic fibers promises, for example in lithotripsy and angioplasty, not only to improve the safety level, but also to ensure the accuracy of the dosage and the precision of the laser energy applied. For the design of commercial fiber optic components a practical definition of the engineering damage threshold including a reasonable safety factor is given. With respect to this engineering damage threshold an overview of existing fiber technology with respect to the fields of application is presented and two examples of feedback systems to increase safety are discussed.

Flexible cables for IR laser power delivery

Overview of the development in crystalline infrared fibera and hollow waveguides is made for the practical use of the last resialts in applications of the cables based on these fibers and waveguideB.

Main problems and new results on dosimetry in laser medicine

Lasers as a therapeutic tool have been used in medicine since 1961, but there are only a few papers on Dosimetry1. Considering the question how to measure the effective dose for a given medical procedure it is not sufficient to monitor the laser output precisely as a source of electromagnetic radiation but in addition one has to take into account the optical and thermal properties of human tissue as a target for the impinging laser radiation as well as the biological reaction of living tissue. Fig. 1 shows a sketch of the spectral behaviour of human skin. In the UV, the penetration depth turns out to be nearly zero, the maximum is at 780-800 urn and in the mid JR at 10 m the penetration depth becomes shallow again. The penetration depth for most tissue varies from jm in the Uv and mid JR to about one centimeter in the visible red and near JR region. The differentpenetration depth and the different interaction times from picoseconds to minutes give rise to the various effects of laser tissue interaction2. This is quite different from the situation for ionizing radiation.