The inverse problems in fluorescence molecular tomography involve finding stable and meaningful solutions to under-determined and ill-posed linear systems of equations. A promising approach consists of minimizing an objective function, which includes a quadratic data-fidelity term combined with a nonquadratic and nonsmooth convex regularizer. Choosing ℓ1-norm as an example of this regularizer, our paper proposes three proximal splitting algorithms for the regularized output least-squares formulation of these problems. We evaluate costs of parameter initialization and reconstruction speeds of the three algorithms that split the original problem in different ways, including two types of techniques based on the forward-backward method and the primal-dual method, respectively. Extensive numerical experiments show that in a wide range of test cases (covering different distances between fluorescent targets, noise levels and numbers of sources), all of the three algorithms perform well and produce very nearly the same reconstructed images, with the primal-dual-type algorithm being significantly faster (in terms of CPU time taken by both initialization and reconstruction) than two variants of the forward-backward-type one, as well as two algorithms with publicly available implementations.
We report the efficient coupling of optical whispering gallery modes (WGMs) in liquid crystal microdroplets suspended in immiscible aqueous environment. Individual nematic liquid crystal (NLC) microdroplet is confined at the tip of a microcapillary used to generate the microdroplets and coupled through a tapered optical fiber waveguide positioned in the vicinity of the microdroplets. Efficient coupling of WGMs is observed in the NLC microdroplets with a diameter of 50–150 μm. In addition, the wavelengths of the WGMs can be tuned by temperature, making such NLC microdroplets suitable for thermal sensors. A temperature sensitivity of 0.244 nm/°C is achieved in a 75-μm-diameter microdroplet. The estimated thermal resolution of the microdroplet sensor is 8.2 × 10<sup>-2</sup> °C.
Large-core fibers are wildly used in astronomical applications. For multi-object fiber spectroscopic telescopes, for example the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST), the aiming accuracy is important. The fibers in these telescopes have two ends; one input end is in the focal plane to collect the star image while another output end is inside a spectrometer. If the input end doesn't exactly match the image of the star, the emitting spot from the output end could not be a central-peak circular spot. If the misalignment is serious, the output would be a ring. The ring spot would lead to wrong spectrum analysis result. To obtain the relationship of the input position and the output spot, we designed a scanning experiment. We chose a single-mode fiber for 650nm, which core diameter is 4 μm, to scan a large-core astronomical fiber FBP320385415, which is used in LAMOST. The experimental results show that if the input point is 122μm away the center of the fiber, the output spot will be a ring spot instead of the supposed circular pattern. The nearer of the input position is to the edge of fiber, the closer the output spot to a ring. This experimental result is important for designing and optimizing the fiber-end adjusting devices of multi-object fiber spectroscopic telescopes. It can also apply for various fields of astronomical spectroscopy observation.
When we use laser to measure the focal ratio degradation of astronomical fibers, we have to reduce the speckle contrast to fit the output spot to a 2-D Gaussian-like function. The origin speckle contrast is near to 100%. The simple average method doesn’t work because the speckle patterns are stable. We tried several ways to disturb randomly the transmission phase of the light modes inside the fiber to be tested. Both non-contact fiber-disturb-mode device (NCFDMD) and contact fiber-disturb-mode device (CFDMD) were established and tested. The NCFDMD is to set a vibrating phase plate against the output end of the tested fiber. The CFDMD is to set the vibrating device in the middle of the fiber. Under different vibration frequency we compared the contrast of speckle patterns. We set different exposure time of the CCD camera to check the effects. For NCFDMD, the exposure time should be long enough, for example 30 ms, to guarantee enough different patterns could be collected to suppress the contrast of the speckle and get good Gauss-like pattern. For CFDMD, we compared three kinds of fibers with different core-diameters. We found that 65-70 Hz is the optimized vibration frequency for all fibers and 30 ms is the best exposure time. The introduction of the phase modulation could dramatically suppress the speckle under coherent illumination. The measurement accuracy could be enhanced according to the speckle suppression
Recently, spectral measurement becomes an important tool in astronomy to find exoplanets etc. The fibers are used
to transfer light from the focal plate to spectrometers. To get high-resolution spectrum, the input slits of the
spectrometers should be as narrow as possible. In opposite, the light spots from the fibers are circle, which diameters are
clearly wider than the width of the spectrometer slits. To reduce the energy loss of the fiber-guide star light, many kinds
of image slicers were designed and fabricated to transform light spot from circle to linear. Some different setup of fiber
slicers are introduced by different research groups around the world. The photonic lanterns are candidates of fiber slicers.
Photonic lantern includes three parts: inserted fibers, preform or tubing, taped part of the preform or tubing. Usually the
optical fields concentrate in the former-core area, so the light spots are not uniform from the tapered end of the lantern.
We designed, fabricated and tested a special kind of photonic lantern. The special fibers consist polymer cladding and
doped high-index core. The polymer cladding could be easily removed using acetone bath, while the fiber core remains
in good condition. We inserted the pure high-index cores into a pure silica tubing and tapered it. During the tapering
process, the gaps between the inserted fibers disappeared. Finally we can get a uniform tapered multimode fiber end. The
simulation results show that the longer the taper is, the lower the loss is. The shape of the taper should be controlled
carefully. A large-zone moving-flame taper machine was fabricated to make the special photonic lantern. Three samples
of photonic lanterns were fabricated and tested. The lanterns with cladding-removable fibers guide light uniform in the
tapered ends that means these lanterns could collect more light from those ends.
Two kinds of full-filled photonic crystal fibers (PCFs) with different air hole size were investigated by experiments. All
the air holes were filled with liquid crystal (LC). The full-filled part was heated to different temperatures to research the
transmission character. In different temperature conditions, below or over 65°C, the two kinds full-filled LC-PCFs had
their own performance. Mode coupling analysis and LC phase theory were used to explain their different performance.
By researching their output light spots and transmission spectra, we demonstrated that LC-PCFs could be applied in
optical switching, filter, attenuator or other optical devices.
By changing pumping light for cholesteric liquid crystal based laser, the laser efficiency measurement of laser is done.
To gain laser of high power and high efficiency, some improvement measures are proposed and tested, such as align
treatment and temperature condition which also affects the wavelength of exit light. For the helical structures of
cholesteric liquid crystal and its temperature-tunable characteristics, the impact of temperature is also discussed.
Proc. SPIE. 8202, 2011 International Conference on Optical Instruments and Technology: Solid State Lighting and Display Technologies, Holography, Speckle Pattern Interferometry, and Micro/Nano Manufacturing and Metrology
KEYWORDS: Photonic crystal fibers, Sun, Polarization, Birefringence, Fiber optic gyroscopes, Cladding, Dispersion, Fiber optics sensors, Finite element methods, Analytical research
We design a new kind of high birefringence photonic crystal fibers with square-lattice cladding.
Through changing the size of only four air holes in the x and y direction respectively, the birefringence is highly increased. It reaches to 1.643×10<sup>-3</sup> when the wavelength is 1.8μm. Based on the finite element
method, the mode fields, dispersion and confinement loss of this kind of photonic crystal fibers are all analyzed.
We analyze the nature of modal cutoff in photonic crystal fibers which core is filled with liquid crystal. The radius of
effective modal fields is used to determine the cutoff wavelength and fiber dimension between the regimes with
single-mode and multi-mode operations in the photonic liquid crystal fibers (PLCFs). After calculated the normalized
frequency <i>V</i>, we establish the parameter subspace in photonic liquid crystal fibers (PLCFs) are single mode.