Biomedical subjects

Marija Strojnik

Publications and source records attributed to Marija Strojnik.

6 recordsLinked to original sources

PubMed2006

Characterization of upconversion coefficient in erbium-doped materials.

We describe a new method for determining the homogeneous upconversion coefficient by using only three measurements: two fluorescence decays as a function of time and the relationship of normalized fluorescence with excitation power. This technique results in increased accuracy upon employment of a smaller number of measurements. Specifically, the intermediate steps of finding the absorption cross section of the doped material, the emission spectrum of the pumping source, and its spatial power distribution are not necessary. Our technique employs only one experimental setup: changing the pumping conditions. Furthermore, it incorporates dual-function fitting to reduce the uncertainty and error propagation. This method will find ample applications in the study and characterization of erbium-doped materials used in lasers and optical amplifiers, where precise knowledge of efficiency and losses is of uppermost importance.

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PubMed2004

Effects of misalignment errors on the optical transfer functions of synthetic aperture telescopes.

A segmented or diluted aperture optical system will undergo phase errors as a result of errors in the positioning of the segments. The errors associated with a segmented primary mirror limit the image quality obtainable with synthetic aperture telescopes. Here we study the effects of segmentation errors on image quality, considering both the phase angle and the amplitude of the optical transfer function (OTF). We show that, in these kinds of telescope, phasing and alignment errors among segments reduce the amplitude and distort the phase angle of the OTF.

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PubMed2003

Phase recovery from a single undersampled interferogram.

A new method of phase determination from a single undersampled interferogram is described. Two low-fringe-density synthetic interferograms corresponding to the phase differences along orthogonal directions are obtained from neighboring pixels of the aliased measured data. The only assumption is that the illumination background, the modulation intensity, and the searched phase are smooth and continuous functions. The synthetic interferograms are demodulated by use of either standard frequency or spatial-domain procedures to obtain the phase differences. The phase is then recovered by integration of the phase differences with a least-squares method. The proposed method is demonstrated to be noise tolerant.

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PubMed2003

Comparison of linear and rotationally shearing interferometric layouts for extrasolar planet detection from space.

We use the point spread function and the modulation transfer function (MTF) as two figures-of-merit to evaluate the performance of the multiaperture interferometric configurations for the detection of a faint planet in the vicinity of its bright star. We design nonredundant interferometric layouts that provide satisfactory coverage of the spatial frequencies of interest. We propose a design incorporating a rotating, rotationally shearing interferometer in a gravity-free environment and compare its performance with the Earth-based, fixed, linear configurations. The side peak of its MTF may be centered on the coordinate associated with a likely planet spatial frequency, resulting in planet signal enhancement and isolation.

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PubMed2003

Dove prism with increased throughput for implementation in a rotational-shearing interferometer.

An analytical expression is derived for the tilt introduced into a wave front by a Dove prism with manufacturing errors in the prism's base angles and pyramidal angle. We found that the tilt decreases when the base angles are increased above the values of traditional design. The increase in the length-aperture ratio of a prism is detrimental to the prism's performance. However, a Dove prism with a widened aperture increases throughput and maintains a manageable prism weight for implementation in a rotational shearing interferometer. Thus we propose a Dove prism designed with a widened aperture to increase throughput in a rotational shearing interferometer and with larger base angles to minimize the wave-front tilt introduced by manufacturing errors. Experimental results implemented in a rotational shearing interferometer demonstrate the feasibility of this design.

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PubMed2003

Erbium-doped optical fiber fluorescence temperature sensor with enhanced sensitivity, a high signal-to-noise ratio, and a power ratio in the 520-530- and 550-560-nm bands.

We analyze and predict the performance of a fiber-optic temperature sensor from the measured fluorescence spectrum to optimize its design. We apply this analysis to an erbium-doped silica fiber by employing the power-ratio technique. We develop expressions for the signal-to-noise ratio in a band to optimize sensor performance in each spectral channel. We improve the signal-to-noise ratio by a factor of 5 for each channel, compared with earlier results. We evaluate the analytical expression for the sensor sensitivity and predict it to be approximately 0.02 degrees C(-1) for the temperature interval from room temperature to above 200 degrees C, increasing from 0.01 degrees C(-1) at the edges of the interval to 0.03 degrees C(-1) at the center, at 100-130 degrees C. The sensitivity again increases at temperatures higher than 300 degrees C, delineating its useful temperature intervals.

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