Rainer Engelbrecht

Time-of-Flight 3-D Endoscopy

This paper describes the first accomplishment of the Time-of-Flight (ToF) measurement principle via endoscope optics. The applicability of the approach is verified by in-vitro experiments. Off-the-shelf ToF camera sensors enable the per-pixel, on-chip, real-time, marker-less acquisition of distance information. The transfer of the emerging ToF measurement technique to endoscope optics is the basis for a new generation of ToF rigid or flexible 3-D endoscopes. No modification of the endoscope optic itself is necessary as only an enhancement of illumination unit and image sensors is necessary. The major contribution of this paper is threefold: First, the accomplishment of the ToF measurement principle via endoscope optics; second, the development and validation of a complete calibration and post-processing routine; third, accomplishment of extensive in-vitro experiments. Currently, a depth measurement precision of 0.89 mm at 20 fps with 3072 3-D points is achieved.

Field simulation of dipole antennas for interstitial microwave hyperthermia

The electromagnetic field of dipole antennas for interstitial microwave hyperthermia is investigated using a finite integration algorithm program. The numerical method is applied to conventional, clinically used applicators and is also used for the improvement and optimization of sophisticated applicators, e.g., in a triaxial technique. Simulations of the frequency dependent impedance match, the E-field and the specific absorption rate (SAR) distribution of different applicators immersed in a muscle phantom are presented and compared with measurements. Moreover, results for arrays of two and four applicators are given. The field simulation allows one to study the effects at the various discontinuities of the applicator-catheter-tissue system and gives a better understanding of known phenomena.

Broadband time-domain absorption spectroscopy with a ns-pulse supercontinuum source

A Q-switched laser based system for broadband absorption spectroscopy in the range of 1390-1740 nm (7200-5750 cm(-1)) has been developed and tested. In the spectrometer the 1064 nm light of a 25 kHz repetition-rate micro-chip Nd:YAG laser is directed into a photonic crystal fiber to produce a short (about 2 ns) pulse of radiation in a wide spectral range. This radiation is passed through a 25 km long dispersive single-mode fiber in order to spread the respective wavelengths over a time interval of about 140 ns at the fiber output. This fast swept-wavelength light source allows to record gas absorption spectra by temporally-resolved detection of the transmitted light power. The realized spectral resolution is about 2 cm(-1). Examples of spectra recorded in a cell with CO(2):CH(4):N(2) gas mixtures are presented. An algorithm employed for the evaluation of molar concentrations of different species from the spectra with non-overlapping absorption bands of mixture components is described. The uncertainties of the concentration values retrieved at different acquisition times due to the required averaging are evaluated. As an example, spectra with a signal-to-noise ratio large enough to provide species concentrations with a relative error of 5% can be obtained in real time at a millisecond time scale. Potentials and limitations of this technique are discussed.

Numerical Modeling of Intracavity Spectral Broadening of Raman Fiber Lasers

Raman fiber lasers (RFLs) are efficient light sources at frequencies where no other comparable all-solid-state sources are available. Especially if fiber Bragg gratings (FBGs) with narrow bandwidths are used, the bandwidth of the Stokes light is strongly broadened by Kerr nonlinearities like four-wave mixing (FWM), and self- and cross-phase modulation (SPM, XPM). In this letter, we discuss an exact numerical model to calculate the spectral behaviour of RFLs and show its application to determine the effective reflectivity of the FBGs. The model is based on a combination of the nonlinear Schroedinger equation including dispersion, FWM, SPM, and XPM with a shooting method to solve the power steady-state equations for RFLs. Numerical results are in good agreement with measurements.

SBS shaping and suppression by arbitrary strain distributions realized by a fiber coiling machine

Experimental results of coiled fibers with a permanent arbitrary distribution of longitudinal strain along the fiber in order to shape or to suppress significantly the gain spectrum of Stimulated Brillouin Scattering (SBS) are presented.

Analysis of SBS Gain Shaping and Threshold Increase by Arbitrary Strain Distributions

Fibers with a longitudinal strain distribution can be used to shape the stimulated Brillouin scattering (SBS) gain spectrum or to increase the SBS threshold power. Theoretical analysis and experimental results on the SBS gain spectrum of coiled fibers with a continuous and arbitrary distribution of longitudinal strain are presented. The concept of a critical gain factor for determining the SBS threshold power with a simple formula is reviewed for fibers with different parameters and spectral shapes. A fiber coiling machine for high coiling forces is described for realizing permanent strain distributions with a maximum strain of more than 3%. SBS spectra were broadened to a spectral width of 1.7 GHz. Measurements verify a significant SBS gain suppression by a factor of 40, which is in excellent agreement with the theoretical analysis. Finally, synthetic triangular and broadened Lorentzian SBS spectral shapes with a spectral width of 550 MHz are demonstrated. The results can be used to suppress SBS in high-power fiber lasers and amplifiers or to tailor almost any arbitrary SBS spectral shapes, which could be useful for slow-light or active optical filter applications.

3D surface reconstruction for laparoscopic computer-assisted interventions: comparison of state-of-the-art methods

One of the main challenges related to computer-assisted laparoscopic surgery is the accurate registration of pre-operative planning images with patients anatomy. One popular approach for achieving this involves intraoperative 3D reconstruction of the target organs surface with methods based on multiple view geometry. The latter, however, require robust and fast algorithms for establishing correspondences between multiple images of the same scene. Recently, the first endoscope based on Time-of-Flight (ToF) camera technique was introduced. It generates dense range images with high update rates by continuously measuring the run-time of intensity modulated light. While this approach yielded promising results in initial experiments, the endoscopic ToF camera has not yet been evaluated in the context of related work. The aim of this paper was therefore to compare its performance with different state-of-the-art surface reconstruction methods on identical objects. For this purpose, surface data from a set of porcine organs as well as organ phantoms was acquired with four different cameras: a novel Time-of-Flight (ToF) endoscope, a standard ToF camera, a stereoscope, and a High Definition Television (HDTV) endoscope. The resulting reconstructed partial organ surfaces were then compared to corresponding ground truth shapes extracted from computed tomography (CT) data using a set of local and global distance metrics. The evaluation suggests that the ToF technique has high potential as means for intraoperative endoscopic surface registration.

SBS-suppression in variably strained fibers for fiber-amplifiers and fiber-lasers with a high spectral power density

Stimulated Brillouin-Scattering (SBS) limits the spectral power density of fiber amplifiers and lasers. Theory and measurements of SBS in fibers with a defined strain distribution are presented. SBS threshold was increased by a factor of 20.

Short 17-cm DBR Raman Fiber Laser With a Narrow Spectrum

We present experimental results on the lasing of a short distributed Bragg reflector (DBR) Raman fiber laser (RFL) with short cavity length. The high finesse laser cavity with an effective length of 17 cm was realized by direct inscription of two Bragg gratings into a short piece of highly nonlinear polarization maintaining Raman fiber. It is pumped by an Ytterbium fiber laser at 1100 nm with up to 8 W pump power. The Stokes wave at 1151 nm is generated within the DBR RFL with 700-mW output power and a narrow spectral full-width at half-maximum bandwidth of 60 pm. Due to the short cavity length, the number of oscillating longitudinal modes is strongly reduced to less than 24 modes. The short DBR RFL has a low lasing threshold of 4.1 W. Its spectral properties and temporal stability are investigated. Distinct longitudinal cavity modes could be observed within the radio-frequency intermode-beat spectrum detected with a fast photodiode. The measured mode separation of 607 MHz validates the short effective length of 17 cm.

Quasi-Distributed Fiber Bragg Grating Sensing Using Stepped Incoherent Optical Frequency Domain Reflectometry

We examine the quasi-distributed interrogation of fiber Bragg grating arrays (FBGA) with the method of stepped incoherent optical frequency domain reflectometry (IOFDR) combined with wavelength scanning of a tunable laser source. The technique of IOFDR provides some advantages over traditional time division multiplexing schemes, because continuous wave modulation of the light intensity instead of short and low energy pulses is used, maximizing the reflected signal received from the sensor fiber. Moreover, an electrical heterodyne demodulation by a microwave vector network analyzer provides a low-noise detection, while achieving high spatial resolutions down to the centimeter range. A quasi-distributed temperature measurement with ten FBGs of 0.5% reflectivity is demonstrated. In the experiment, a root mean square temperature error of 0.3 K and a maximum error of 1 K is observed, using a fiber-coupled power of only -12 dBm. A spatial two-point resolution of 2.14 cm is achieved, which enabled a successful interrogation of FBGAs with 20 and 30 cm spacings. The potential multiplexing capability of the given setup could reach more than 50 gratings in a 30 cm spacing at a measurement rate of 50 s/nm.