Christopher Taudt

Experimental study of supercontinuum generation in an amplifier based on an Yb3+ doped nonlinear photonic crystal fiber

The use of supercontinuum light sources in different optical measurement methods, like microscopy or optical coherence tomography, has increased significantly compared to classical wideband light sources. The development of various optical measurement techniques benefits from the high brightness and bandwidth, as well as the spatial coherence of these sources. For some applications, only a portion of the broad spectral range can be used. Therefore, an increase of the spectral power density in limited spectral regions would provide a clear advantage over spectral filtering. This study describes a method to increase the spectral power density of supercontinuum sources by amplifying the excitation wavelength inside a nonlinear photonic crystal fiber (PCF). An ytterbium doped photonic crystal fiber was manufactured by a sol-gel process and used in a fiber amplifier setup as the nonlinear fiber medium. In order to characterize the fiber’s optimum operational characteristics, group-velocity dispersion (GVD) measurements were performed on the fiber during the amplification process. For this purpose, a notch-pass mirror was used to launch the radiation of a stabilized laser diode at 976 nm into the fiber sample for pumping. The performance of the fiber was compared with a conventional PCF. Finally, the system as a whole was characterized in reference to common solid state-laser-based photonic supercontinuum light sources. An improvement of the power density up to 7.2 times was observed between 1100 nm to 1380 nm wavelengths.

A new small-package super-continuum light source for optical coherence tomography

Broadband light sources provide a significant benefit for optical coherence tomography (OCT) imaging concerning the axial resolution. Light sources with bandwidths over 200 nm result in an axial resolution up to 2 microns. Such broad band OCT imaging can be achieved utilizing super continuum (SC) light sources. The main important disadvantage of commercial SC light sources is the overall size and the high costs. Therefore, the use of SC light sources in small OCT setups and applications is limited. We present a new small housing and costeffective light source, which is suitable for OCT imaging. The used light source has dimensions of 110 x 160 x 60 mm and covers a wavelength range from 390 nm up to 2500 nm. The light source was coupled in a dual band OCT system. The light is guided into the interferometer and split in reference and sample beam. The superimposed signal is guided to the spectrometer unit, which consists of two spectrometers. This spectrometer system separates the light. One band centered at 800 nm with a full bandwidth of 176 nm and a second band centered at 1250 nm with a full spectral width of 300 nm was extracted. The 800 nm interference signal is detected by a silicon line scan camera and the 1250 nm signal by an indium gallium arsenide linear image sensor. In this test measurement a plastic foil was used as a sample, which is composed of several plastic film layers. Three dimensional images were acquired simultaneous with the dual band OCT setup. The images were acquired at an A-scan rate of 1 kHz. The 1 kHz A-line rate was chosen because so far the optical power of the light source is not optimal for high speed OCT imaging. The source provides 2 mW in the range of 390 nm to 800 nm and 25 mW in the range from 390 nm to 1650 nm. Furthermore, we coupled the light source by a 50:50 optical fiber coupler, which also reduces the overall optical power of the light source within the OCT setup. Nevertheless, we demonstrated that this new small-package and cost-effective light source is very suitable to carry out OCT imaging. The use of this light source can open up new OCT applications, which require OCT setups with very high axial resolution and small footprint.

All-fiber supercontinuum source with flat, high power spectral density in the range between 1.1 µm to 1.4 µm based on an Yb3+ doped nonlinear photonic crystal fiber

Supercontinuum light sources provide a high power spectral density with a high spatial coherence. Coherent octavespanning supercontinuum can be generated in photonic crystal fibers (PCFs) by launching short pulses into the fiber. In the field of optical metrology, these light sources are very interesting. For most applications, only a small part of the entire spectrum can be utilized. In biological tissue scattering, absorption and fluorescence limits the usable spectral range. Therefore, an increase of the spectral power density in limited spectral regions would provide a clear advantage over spectral filtering. This study describes a method to increase the spectral power density of supercontinuum sources by amplifying the excitation wavelength inside a nonlinear photonic crystal fiber (PCF). An all-fiber-based setup enables higher output power and power stability. An ytterbium-doped photonic crystal fiber was manufactured by a nanopowder process (drawn by the fiberware GmbH, Germany) and used in a fiber amplifier setup as the nonlinear fiber medium. In order to characterize the fiber’s optimum operational characteristics, group-velocity dispersion (GVD) measurements were performed. The performance of the fiber-based setup was compared with a free space setup. Finally, the system as a whole was characterized in reference to common solid state-laser-based supercontinuum light sources. An improvement of the power density was observed in the spectral range between 1100 nm to 1400 nm.

Holographic characterization and laser structuring of a microphone membrane

Microphone membranes tthat have theeir frequency response uniiquely tailoreed to specificc applications are typicallyy produced in an unautomated and expensive manuufacturing prrocess. A combination off holographic stroboscopyy, numerical simulation and laser structuriing is appliedd to shift the resonant frequencies of thee membrane too their desired values given an unknown tension across the microphone membrane due to manuufacturing tollerances. Thee uncharacterized microphone membrane is driven thrrough physical contact withh a piezo. Thhe piezo is swept through a range of frequencies and the full surface profile oscillations are recorded using stroboscopic digitaal holographyy techniques. These resonant displacemennt maps will bee used, in combination with finite differeence eigenvaluue simulationss and perturbattion theory, to determine the preloadedd tension profile across the membrane. Given a desired responsee function of the membrane,, a new membrane mass-density profile can be tailoreed to match the current meembrane to thee requirements of the micropphone. A 20 WW ns Q-switchhed laser steerred by galvanometer mirrorrs will be usedd to restructuree the mass density of the membrane to meeet the design requirements of the micropphone.

Dispersion-controlled low-coherent interferometry for thin-film characterization

In this work, a novel experimental procedure for thin-film characterization and its data analysis is described. The presented technique is based on low-coherence interferometry and resolves thicknesses with nm-precision. An element with known dispersion is placed in the interferometers sample arm and delivers a controlled phase variation in relation to the wavelength. This phase variation is dependent on the thickness of the material and its wavelength-dependent refractive index. Furthermore, the phase variation is characterized by a stationary point, the so called equalization wavelength. Changes in the thickness of the material under test will shift the equalization wavelength and transform its interference amplitude. In combination with an imaging spectrometer thin films are spatially resolved with a spatial resolution of 4 μm within a single acquisition. This makes data acquisition fast. The advantage over other conventionally used methods, like reflectometry and ellipsometry, is that signal processing is greatly simplified and therefore much faster.

Experimental measurement and numerical analysis of group velocity dispersion in cladding modes of an endlessly single-mode photonic crystal fiber

The optical properties of the guided modes in the core of photonic crystal fibers (PCFs) can be easily manipulated by changing the air-hole structure in the cladding. Special properties can be achieved in this case such as endless singlemode operation. Endlessly single-mode fibers, which enable single-mode guidance over a wide spectral range, are indispensable in the field of fiber technology. A two-dimensional photonic crystal with a silica central core and a micrometer-spaced hexagonal array of air holes is an established method to achieve endless single-mode properties. In addition to the guidance of light in the core, different cladding modes occur. The coupling between the core and the cladding modes can affect the endlessly single-mode guides. There are two possible ways to determine the dispersion: measurement and calculation. We calculate the group velocity dispersion (GVD) of different cladding modes based on the measurement of the fiber structure parameters, the hole diameter and the pitch of a presumed homogeneous hexagonal array. Based on the scanning electron image, a calculation was made of the optical guiding properties of the microstructured cladding. We compare the calculation with a method to measure the wavelength-dependent time delay. We measure the time delay of defined cladding modes with a homemade supercontinuum light source in a white light interferometric setup. To measure the dispersion of cladding modes of optical fibers with high accuracy, a time-domain white-light interferometer based on a Mach-Zehnder interferometer is used. The experimental setup allows the determination of the wavelengthdependent differential group delay of light travelling through a thirty centimeter piece of test fiber in the wavelength range from VIS to NIR. The determination of the GVD using different methods enables the evaluation of the individual methods for characterizing the cladding modes of an endlessly single-mode fiber.

Determination of biological activity from fluorescence-lifetime measurements in Saccharomyces cerevisiae

The importance of fluorescence lifetime measurement as an optical analysis tool is growing. Many applications already exist in order to determine the fluorescence lifetime, but the majority of these require the addition of fluorescence-active substances to enable measurements. Every usage of such foreign materials has an associated risk. This paper investigates the use of auto-fluorescing substances in Saccharomyces cerevisiae (Baker’s yeast) as a risk free alternative to fluorescence-active substance enabled measurements. The experimental setup uses a nitrogen laser with a pulse length of 350 ps and a wavelength of 337 nm. The excited sample emits light due to fluorescence of NADH/NADPH and collagen. A fast photodiode collects the light at the output of an appropriate high-pass edge-filter at 400 nm. Fluorescence lifetimes can be determined from the decay of the measurement signals, which in turn characterizes the individual materials and their surrounding environment. Information about the quantity of the fluorescence active substances can also be measured based on the received signal intensity. The correlation between the fluorescence lifetime and the metabolic state of Saccharomyces cerevisiae was investigated and is presented here.

Robust fiber optic flexure sensor exploiting mode coupling in few-mode fiber

Few-mode fiber (FMF) has become very popular for use in multiplexing telecommunications data over fiber optics. The simplicity of producing FMF and the relative robustness of the optical modes, coupled with the simplicity of reading out the information make this fiber a natural choice for communications. However, little work has been done to take advantage of this type of fiber for sensors. Here, we demonstrate the feasibility of using FMF properties as a mechanism for detecting flexure by exploiting mode coupling between modes when the cylindrical symmetry of the fiber is perturbed. The theoretical calculations shown here are used to understand the coupling between the lowest order linearly polarized mode (LP01) and the next higher mode (LP11x or LP11y) under the action of bending. Twisting is also evaluated as a means to detect flexure and was determined to be the most reliable and effective method when observing the LP21 mode. Experimental results of twisted fiber and observations of the LP21 mode are presented here. These types of fiber flexure sensors are practical in high voltage, high magnetic field, or high temperature medical or industrial environments where typical electronic flexure sensors would normally fail. Other types of flexure measurement systems that utilize fiber, such as Rayleigh back-scattering [1], are complicated and expensive and often provide a higher-than necessary sensitivity for the task at hand.

Experimental measurement of group velocity dispersion during operation in cladding-pumped large-mode-area Yb-doped fibers

Ever higher output power of fiber lasers leads to growing requirements on the fibers used. The increasing intensity of light in the fiber core is compensated by enlargement of the area of the doped core. The guiding properties of the optical fiber are significantly affected and must be very accurately defined. This high intensity leads to significant deviations of the guiding properties of the optical fiber during laser operation due to absorption and thermal effects. In particular, information about the change in group-velocity dispersion(GVD) during operation is important for the development of high power fiber laser systems. In an effort to gather such knowledge, this paper presents the results of dispersion characterization measurements using ytterbium-doped large-mode-area double-clad fibers which are cladding pumped at a wavelength of 975 nm. The direct measurement of the GVD in the emission range of the ytterbium-doped fibers (1000 nm to 1150 nm) is shown. GVD characteristics of two different large-mode-area double-clad fibers with defined launching pump laser power level were systematically analyzed. The dispersion parameters for different fiber designs and various doping levels are investigated over a broad spectral range in the emission area of Yb-doped fiber samples for controlled sets of operating parameters. The experiment utilizes a supercontinuum source developed within this laboratory as well as a Mach-Zehnder interferometer with a dual-channel spectral-detection system sensitive to wavelengths from 0.95 μm to 1.75 μm. Temporally resolved spectrograms recorded at distinct delay positions enable the detection of interference fringes for the equalization wavelength. By applying a Sellmeier polynomial fit to the wavelength dependent differential group-delay function, the GVD can be derived. The measured Yb-doped large-mode-area fibers show a variation of the doping concentration between 0.7 mass percent to 3 mass percent of ytterbium. The measurement of the Yb-doped large-mode-area fiber with or without optical load on the sample during the measurement was examined.

Fast alternative method for measuring the wavefront of lithography exposure systems

The work presented here describes the analysis of problems of the realization of alternative wavefront measurement methods for lithography exposure machines. The measurement method that is introduced is a redesign of a conventional Shack-Hartmann wavefront sensor and was based on the usage of a single pinhole and the lithography machine itself. Such a redesign is useful because it increases the degrees of freedom. Therefore, there is more freedom to analyze the problems of designing and planning measurements. A group of hardware prototypes under laboratory conditions with the required angular and lateral resolution were realized. For the approximation of the measurement results, numerical simulations are implemented using the Monte-Carlo method in order to statistically design the experiments themselves.