Fares Alhassen | University of California, Irvine (original) (raw)

Papers by Fares Alhassen

Research paper thumbnail of An All-fiber Tunable Polarization-Dependent Loss Element

OFC/NFOEC 2007 - 2007 Conference on Optical Fiber Communication and the National Fiber Optic Engineers Conference, 2007

We describe the implementation of a wavelength-tunable polarization-dependent-loss element on pol... more We describe the implementation of a wavelength-tunable polarization-dependent-loss element on polarization-maintaining fiber using two independently-controlled acoustic gratings. Continuous attenuation in both fast and slow axes at the same wavelength is demonstrated.

Research paper thumbnail of <title>A preclinical SPECT camera with depth-of-interaction compensation using a focused-cut scintillator</title>

Medical Imaging 2011: Physics of Medical Imaging, 2011

Preclinical SPECT offers a powerful means to understand the molecular pathways of metabolic activ... more Preclinical SPECT offers a powerful means to understand the molecular pathways of metabolic activity in animals. SPECT cameras using pinhole collimators offer high resolution that is needed for visualizing small structures in laboratory animals. One of the limitations of pinhole geometries is that increased magnification causes some rays to travel through the scintillator detector at steep angles, introducing parallax errors

Research paper thumbnail of Ultrafast multipinhole single photon emission computed tomography iterative reconstruction using CUDA

2011 IEEE Nuclear Science Symposium Conference Record, 2011

We have developed an ultrafast statistical iterative reconstruction method for multipinhole singl... more We have developed an ultrafast statistical iterative reconstruction method for multipinhole single photon emission computed tomography using high performance graphics processing unit computing and have demonstrated a significant performance improvement in reconstruction using computer-generated and experimental sinogram data.

Research paper thumbnail of A fiber acousto-optic tunable bandpass filter on high-polarization-splitting fiber

OFC/NFOEC 2008 - 2008 Conference on Optical Fiber Communication/National Fiber Optic Engineers Conference, 2008

Abstract: We report on the acousto-optic interaction in a newly developed high-polarization-split... more Abstract: We report on the acousto-optic interaction in a newly developed high-polarization-splitting fiber designed to lift the degeneracy of its cladding modes and on an all-fiber acousto-optic tunable bandpass filters based on this fiber. © 2008 Optical Society of America ... OCIS codes: (060.2310) Fiber optics; (230.1040) Acousto-optical devices ... Recent demonstration of all-fiber devices based on acoustic-optic interaction such as polarization controllers and wavelength monitors [1, 2], like other types of microbend-induced gratings (MIGs), have been limited ...

Research paper thumbnail of Sensing of Dynamic Strain via Acousto-Optic Coupling in Single-Mode Fiber

IEEE Sensors, 2005., 2005

We demonstrate dynamic strain sensing using a fiber acousto-optic tunable filter (AOTF). The sens... more We demonstrate dynamic strain sensing using a fiber acousto-optic tunable filter (AOTF). The sensing mechanism is based on the shift of resonant wavelength of the AOTF filter in the presence of mechanical strain on the fiber. An input laser signal can thus be modulated by a dynamic strain. The modulation due to a low-frequency (~10 Hz) dynamic strain applied to

Research paper thumbnail of Energy response of a room-temperature cadmium telluride (CdTe) photon-counting detector for simultaneous and sequential CT and SPECT

2012 IEEE Nuclear Science Symposium and Medical Imaging Conference Record (NSS/MIC), 2012

Semiconductor radiation detectors are a promising technology for single photon counting detectors... more Semiconductor radiation detectors are a promising technology for single photon counting detectors. Unlike scintillator-based indirect radiation detectors, semiconductor detectors directly convert absorbed photons to charge, thereby reducing energy spreading and signal loss that can occur in scintillator-based detectors. Cadmium telluride (CdTe) has attracted research interest due to its higher stopping power compared to other semiconductors such as germanium and silicon as well as its room-temperature operating capability. A CdTe photon-counting, energy-resolving detector system has been developed for x-ray and radionuclide imaging, and is capable of discriminating incident photon energy using multiple voltage thresholds per detector element with fast signal formation. Using this CdTe detector, a reproducible method of generating energy spectra was developed for a given incident radiation source. The energy resolution of the CdTe detector was determined by generating the energy spectra of three radiation sources.

Research paper thumbnail of Ultra-Compact Long-Period Fiber Grating and Grating Pair Fabrication using a Modulation-Scanned CO<inf>2</inf> Laser

OFC/NFOEC 2007 - 2007 Conference on Optical Fiber Communication and the National Fiber Optic Engineers Conference, 2007

Page 1. Ultra-Compact Long-Period Fiber Grating and Grating Pair Fabrication using a Modulation-S... more Page 1. Ultra-Compact Long-Period Fiber Grating and Grating Pair Fabrication using a Modulation-Scanned CO2 Laser Hon M. Chan, Eric Perez, Fares Alhassen, Ivan V. Tomov, and Henry P. Lee Department of Electrical Engineering ...

Research paper thumbnail of Multi-material decomposition using low-current X-ray and a photon-counting CZT detector

2011 IEEE Nuclear Science Symposium Conference Record, 2011

We developed and evaluated an x-ray photon-counting imaging system using an energy-resolving cadm... more We developed and evaluated an x-ray photon-counting imaging system using an energy-resolving cadmium zinc telluride (CZT) detector coupled with application specific integrated circuit (ASIC) readouts. This x-ray imaging system can be used to identify different materials inside the object. The CZT detector has a large active area (5×5 array of 25 CZT modules, each with 16×16 pixels, cover a total area of 200 mm × 200 mm), high stopping efficiency for x-ray photons (~ 100 % at 60 keV and 5 mm thickness). We explored the performance of this system by applying different energy windows around the absorption edges of target materials, silver and indium, in order to distinguish one material from another. The photon-counting CZT-based x-ray imaging system was able to distinguish between the materials, demonstrating its capability as a radiation-spectroscopic decomposition system.

Research paper thumbnail of Myocardial blood flow measurement with a conventional dual-head SPECT/CT with spatiotemporal iterative reconstructions - a clinical feasibility study

American journal of nuclear medicine and molecular imaging, 2013

Cardiac single photon emission computed tomography (SPECT) cameras typically rotate too slowly ar... more Cardiac single photon emission computed tomography (SPECT) cameras typically rotate too slowly around a patient to capture changes in the blood pool activity distribution and provide accurate kinetic parameters. A spatiotemporal iterative reconstruction method to overcome these limitations was investigated. Dynamic rest/stress (99m)Tc-methoxyisobutylisonitrile ((99m)Tc-MIBI) SPECT/CT was performed along with reference standard rest/stress dynamic positron emission tomography (PET/CT) (13)N-NH3 in five patients. The SPECT data were reconstructed using conventional and spatiotemporal iterative reconstruction methods. The spatiotemporal reconstruction yielded improved image quality, defined here as a statistically significant (p<0.01) 50% contrast enhancement. We did not observe a statistically significant difference between the correlations of the conventional and spatiotemporal SPECT myocardial uptake K 1 values with PET K 1 values (r=0.25, 0.88, respectively) (p<0.17). These r...

Research paper thumbnail of Observation of Orbital Angular Momentum Transfer between Acoustic and Optical Vortices in Optical Fiber

Physical Review Letters, 2006

Acousto-optic interaction in optical fiber is examined from the perspective of copropagating opti... more Acousto-optic interaction in optical fiber is examined from the perspective of copropagating optical and acoustic vortex modes. Calculation of the acousto-optic coupling coefficient between different optical modes leads to independent conservation of spin and orbital angular momentum of the interacting photons and phonons. We show that the orbital angular momentum of the acoustic vortex can be transferred to a circularly polarized fundamental optical mode to form a stable optical vortex in the fiber carrying orbital angular momentum. The technique provides a useful way of generating stable optical vortices in the fiber medium.

Research paper thumbnail of All-fiber acousto-optic polarization monitor

Optics Letters, 2007

We describe the operational principle of and experimentally demonstrate a narrowband, wavelength-... more We describe the operational principle of and experimentally demonstrate a narrowband, wavelength-tunable polarization monitor based on a fiber acousto-optic tunable filter. Two orthogonally vibrating acoustic waves are used to create a variable polarizer that can be used to measure the state of polarization of an incident narrowband light source. The accuracy of the polarization monitor is measured at two different wavelengths in reference to a commercial polarimeter and is shown to be within 5%.

Research paper thumbnail of Depth-of-Interaction Compensation Using a Focused-Cut Scintillator for a Pinhole Gamma Camera

IEEE Transactions on Nuclear Science, 2000

Preclinical SPECT offers a powerful means to understand the molecular pathways of drug interactio... more Preclinical SPECT offers a powerful means to understand the molecular pathways of drug interactions in animal models by discovering and testing new pharmaceuticals and therapies for potential clinical applications. A combination of high spatial resolution and sensitivity are required in order to map radiotracer uptake within small animals. Pinhole collimators have been investigated, as they offer high resolution by means of image magnification. One of the limitations of pinhole geometries is that increased magnification causes some rays to travel through the detection scintillator at steep angles, introducing parallax errors due to variable depth-ofinteraction in scintillator material, especially towards the edges of the detector field of view. These parallax errors ultimately limit the resolution of pinhole preclinical SPECT systems, especially for higher energy isotopes that can easily penetrate through millimeters of scintillator material. A pixellated, focused-cut (FC) scintillator, with its pixels laser-cut so that they are collinear with incoming rays, can potentially compensate for these parallax errors and thus improve the system resolution. We performed the first experimental evaluation of a newly developed focused-cut scintillator. We scanned a Tc-99m source across the field of view of pinhole gamma camera with a continuous scintillator, a conventional "straight-cut" (SC) pixellated scintillator, and a focused-cut scintillator, each coupled to an electron-multiplying charge coupled device (EMCCD) detector by a fiber-optic taper, and compared the measured full-width half-maximum (FWHM) values. We show that the FWHMs of the focused-cut scintillator projections are comparable to the FWHMs of the thinner SC scintillator, indicating the effectiveness of the focused-cut scintillator in compensating parallax errors.

Research paper thumbnail of Temperature Dependent Operation of PSAPD-Based Compact Gamma Camera for SPECT Imaging

IEEE Transactions on Nuclear Science, 2000

We investigated the dependence of image quality on the temperature of a position sensitive avalan... more We investigated the dependence of image quality on the temperature of a position sensitive avalanche photodiode (PSAPD)-based small animal single photon emission computed tomography (SPECT) gamma camera with a CsI:Tl scintillator. Currently, nitrogen gas cooling is preferred to operate PSAPDs in order to minimize the dark current shot noise. Being able to operate a PSAPD at a relatively high temperature (e.g., 5 °C) would allow a more compact and simple cooling system for the PSAPD. In our investigation, the temperature of the PSAPD was controlled by varying the flow of cold nitrogen gas through the PSAPD module and varied from -40 °C to 20 °C. Three experiments were performed to demonstrate the performance variation over this temperature range. The point spread function (PSF) of the gamma camera was measured at various temperatures, showing variation of full-width-half-maximum (FWHM) of the PSF. In addition, a (99m)Tc-pertechnetate (140 keV) flood source was imaged and the visibility of the scintillator segmentation (16×16 array, 8 mm × 8 mm area, 400 μm pixel size) at different temperatures was evaluated. Comparison of image quality was made at -25 °C and 5 °C using a mouse heart phantom filled with an aqueous solution of (99m)Tc-pertechnetate and imaged using a 0.5 mm pinhole collimator made of tungsten. The reconstructed image quality of the mouse heart phantom at 5 °C degraded in comparision to the reconstructed image quality at -25 °C. However, the defect and structure of the mouse heart phantom were clearly observed, showing the feasibility of operating PSAPDs for SPECT imaging at 5 °C, a temperature that would not need the nitrogen cooling. All PSAPD evaluations were conducted with an applied bias voltage that allowed the highest gain at a given temperature.

Research paper thumbnail of Phantom Experiments on a PSAPD-Based Compact Gamma Camera With Submillimeter Spatial Resolution for Small Animal SPECT

IEEE Transactions on Nuclear Science, 2000

We demonstrate a position sensitive avalanche photodiode (PSAPD) based compact gamma camera for t... more We demonstrate a position sensitive avalanche photodiode (PSAPD) based compact gamma camera for the application of small animal single photon emission computed tomography (SPECT). The silicon PSAPD with a two-dimensional resistive layer and four readout channels is implemented as a gamma ray detector to record the energy and position of radiation events from a radionuclide source. A 2 mm thick monolithic CsI:Tl scintillator is optically coupled to a PSAPD with a 8mm×8mm active area, providing submillimeter intrinsic spatial resolution, high energy resolution (16% full-width half maximum at 140 keV) and high gain. A mouse heart phantom filled with an aqueous solution of 370 MBq 99m Tc-pertechnetate (140 keV) was imaged using the PSAPD detector module and a tungsten knife-edge pinhole collimator with a 0.5 mm diameter aperture. The PSAPD detector module was cooled with cold nitrogen gas to suppress dark current shot noise. For each projection image of the mouse heart phantom, a rotated diagonal readout algorithm was used to calculate the position of radiation events and correct for pincushion distortion. The reconstructed image of the mouse heart phantom demonstrated reproducible image quality with submillimeter spatial resolution (0.7 mm), showing the feasibility of using the compact PSAPD-based gamma camera for a small animal SPECT system.

Research paper thumbnail of A Compact All-Fiber LPG-AOTF Frequency Shifter on Single-Mode Fiber and Its Application to Vibration Measurement

IEEE Photonics Technology Letters, 2000

We demonstrate a compact, all-fiber frequency shifter by cascading a CO 2 -written long-period gr... more We demonstrate a compact, all-fiber frequency shifter by cascading a CO 2 -written long-period grating with a flexural acoustic-wave-induced microbending grating on a single-mode fiber. Carrier and image sideband suppressions of 30.0 and 28.1 dB, respectively, are measured. An all-fiber vibrometer based on this frequency shifter capable of nanometer resolution at frequencies up to megahertz is also demonstrated.

Research paper thumbnail of Fabrication and Mode Identification of Compact Long-Period Gratings Written by CO<formula formulatype="inline"><tex>$_{2}$</tex></formula> Laser

IEEE Photonics Technology Letters, 2000

We report highly compact (1.2 cm) long-period fiber gratings (LPGs) exhibiting strong grating eff... more We report highly compact (1.2 cm) long-period fiber gratings (LPGs) exhibiting strong grating effect (>30-dB loss) and low insertion loss (<0.25 dB) inscribed by a continuously scanned CO 2 laser. The LPG resonances are also probed using traveling acoustic gratings with variable period. Our results show that these CO 2 laser side-exposed LPGs predominantly couple to antisymmetrical cladding modes.

Research paper thumbnail of A highly efficient thermally controlled loss-tunable long-period fiber grating on corrugated metal substrate

IEEE Photonics Technology Letters, 2000

We demonstrate a new approach for fabricating a compact loss-tunable long-period fiber grating (L... more We demonstrate a new approach for fabricating a compact loss-tunable long-period fiber grating (LPFG) by gluing a fiber onto a corrugated metal substrate filled with ultraviolet-cured epoxy. The strain caused by the difference in thermal expansion among the adhesive, the substrate, and the silica fiber induces a periodic microbending along the fiber when the device temperature is changed. The LPFG has an initial flat spectrum at room temperature. With 35 grating periods, a tuning range of 21 dB is achieved through core-cladding coupling for a temperature change from room temperature to 4 C. A theoretical model is presented to explain the high tuning efficiency and the operating mechanisms of the device.

Research paper thumbnail of A compact all-fiber PDL-compensated acoustooptic wavelength monitor

IEEE Photonics Technology Letters, 2000

A compact all-fiber acoustooptic wavelength monitor with passive polarization-dependent loss (PDL... more A compact all-fiber acoustooptic wavelength monitor with passive polarization-dependent loss (PDL) compensation is demonstrated. A Sagnac loop interferometer is used to compensate the intrinsic PDL, and hence, the polarization-dependent wavelength shift of the fiber acoustooptic tunable filter (AOTF). This increases the extrinsic wavelength accuracy of the device to 0.05 nm, a three-fold improvement from a previously reported uncompensated device. The relaxation in the PDL specifications for the AOTF also results in a reduction of fiber length from 38 to 14 cm.

Research paper thumbnail of Variable and stable dual-wavelength erbium-doped fiber laser based on fiber acousto-optic variable filter and highly nonlinear fiber

ABSTRACT A dual-wavelength erbium-doped fiber (EDF) variable ring laser using a fiber acousto-opt... more ABSTRACT A dual-wavelength erbium-doped fiber (EDF) variable ring laser using a fiber acousto-optic tunable filter (AOTF) and highly nonlinear fiber (HNLF) is demonstrated. Stable and variable lasing wavelengths were achieved by electronically adjusting the AOTF settings.

Research paper thumbnail of Acoustooptic Coherent Mode Coupling in Polarization-Maintaining Fiber and Its Application as a Variable-Polarization-Dependent Loss Element

IEEE Photonics Technology Letters, 2000

In this letter, we investigate coherent acoustooptic coherent mode coupling from the LP 01 core m... more In this letter, we investigate coherent acoustooptic coherent mode coupling from the LP 01 core mode to LP 1 cladding mode of a polarization-maintaining fiber (PMF) induced by two acoustic gratings. We show narrowband variable attenuation along either birefringent axis of the PMF by varying the relative phase between the two acoustic gratings. By launching two acoustic gratings at different frequencies, variable attenuation and low crosstalk along either birefringent axis within the acoustooptic coupling bandwidth are demonstrated.

Research paper thumbnail of An All-fiber Tunable Polarization-Dependent Loss Element

OFC/NFOEC 2007 - 2007 Conference on Optical Fiber Communication and the National Fiber Optic Engineers Conference, 2007

We describe the implementation of a wavelength-tunable polarization-dependent-loss element on pol... more We describe the implementation of a wavelength-tunable polarization-dependent-loss element on polarization-maintaining fiber using two independently-controlled acoustic gratings. Continuous attenuation in both fast and slow axes at the same wavelength is demonstrated.

Research paper thumbnail of <title>A preclinical SPECT camera with depth-of-interaction compensation using a focused-cut scintillator</title>

Medical Imaging 2011: Physics of Medical Imaging, 2011

Preclinical SPECT offers a powerful means to understand the molecular pathways of metabolic activ... more Preclinical SPECT offers a powerful means to understand the molecular pathways of metabolic activity in animals. SPECT cameras using pinhole collimators offer high resolution that is needed for visualizing small structures in laboratory animals. One of the limitations of pinhole geometries is that increased magnification causes some rays to travel through the scintillator detector at steep angles, introducing parallax errors

Research paper thumbnail of Ultrafast multipinhole single photon emission computed tomography iterative reconstruction using CUDA

2011 IEEE Nuclear Science Symposium Conference Record, 2011

We have developed an ultrafast statistical iterative reconstruction method for multipinhole singl... more We have developed an ultrafast statistical iterative reconstruction method for multipinhole single photon emission computed tomography using high performance graphics processing unit computing and have demonstrated a significant performance improvement in reconstruction using computer-generated and experimental sinogram data.

Research paper thumbnail of A fiber acousto-optic tunable bandpass filter on high-polarization-splitting fiber

OFC/NFOEC 2008 - 2008 Conference on Optical Fiber Communication/National Fiber Optic Engineers Conference, 2008

Abstract: We report on the acousto-optic interaction in a newly developed high-polarization-split... more Abstract: We report on the acousto-optic interaction in a newly developed high-polarization-splitting fiber designed to lift the degeneracy of its cladding modes and on an all-fiber acousto-optic tunable bandpass filters based on this fiber. © 2008 Optical Society of America ... OCIS codes: (060.2310) Fiber optics; (230.1040) Acousto-optical devices ... Recent demonstration of all-fiber devices based on acoustic-optic interaction such as polarization controllers and wavelength monitors [1, 2], like other types of microbend-induced gratings (MIGs), have been limited ...

Research paper thumbnail of Sensing of Dynamic Strain via Acousto-Optic Coupling in Single-Mode Fiber

IEEE Sensors, 2005., 2005

We demonstrate dynamic strain sensing using a fiber acousto-optic tunable filter (AOTF). The sens... more We demonstrate dynamic strain sensing using a fiber acousto-optic tunable filter (AOTF). The sensing mechanism is based on the shift of resonant wavelength of the AOTF filter in the presence of mechanical strain on the fiber. An input laser signal can thus be modulated by a dynamic strain. The modulation due to a low-frequency (~10 Hz) dynamic strain applied to

Research paper thumbnail of Energy response of a room-temperature cadmium telluride (CdTe) photon-counting detector for simultaneous and sequential CT and SPECT

2012 IEEE Nuclear Science Symposium and Medical Imaging Conference Record (NSS/MIC), 2012

Semiconductor radiation detectors are a promising technology for single photon counting detectors... more Semiconductor radiation detectors are a promising technology for single photon counting detectors. Unlike scintillator-based indirect radiation detectors, semiconductor detectors directly convert absorbed photons to charge, thereby reducing energy spreading and signal loss that can occur in scintillator-based detectors. Cadmium telluride (CdTe) has attracted research interest due to its higher stopping power compared to other semiconductors such as germanium and silicon as well as its room-temperature operating capability. A CdTe photon-counting, energy-resolving detector system has been developed for x-ray and radionuclide imaging, and is capable of discriminating incident photon energy using multiple voltage thresholds per detector element with fast signal formation. Using this CdTe detector, a reproducible method of generating energy spectra was developed for a given incident radiation source. The energy resolution of the CdTe detector was determined by generating the energy spectra of three radiation sources.

Research paper thumbnail of Ultra-Compact Long-Period Fiber Grating and Grating Pair Fabrication using a Modulation-Scanned CO<inf>2</inf> Laser

OFC/NFOEC 2007 - 2007 Conference on Optical Fiber Communication and the National Fiber Optic Engineers Conference, 2007

Page 1. Ultra-Compact Long-Period Fiber Grating and Grating Pair Fabrication using a Modulation-S... more Page 1. Ultra-Compact Long-Period Fiber Grating and Grating Pair Fabrication using a Modulation-Scanned CO2 Laser Hon M. Chan, Eric Perez, Fares Alhassen, Ivan V. Tomov, and Henry P. Lee Department of Electrical Engineering ...

Research paper thumbnail of Multi-material decomposition using low-current X-ray and a photon-counting CZT detector

2011 IEEE Nuclear Science Symposium Conference Record, 2011

We developed and evaluated an x-ray photon-counting imaging system using an energy-resolving cadm... more We developed and evaluated an x-ray photon-counting imaging system using an energy-resolving cadmium zinc telluride (CZT) detector coupled with application specific integrated circuit (ASIC) readouts. This x-ray imaging system can be used to identify different materials inside the object. The CZT detector has a large active area (5×5 array of 25 CZT modules, each with 16×16 pixels, cover a total area of 200 mm × 200 mm), high stopping efficiency for x-ray photons (~ 100 % at 60 keV and 5 mm thickness). We explored the performance of this system by applying different energy windows around the absorption edges of target materials, silver and indium, in order to distinguish one material from another. The photon-counting CZT-based x-ray imaging system was able to distinguish between the materials, demonstrating its capability as a radiation-spectroscopic decomposition system.

Research paper thumbnail of Myocardial blood flow measurement with a conventional dual-head SPECT/CT with spatiotemporal iterative reconstructions - a clinical feasibility study

American journal of nuclear medicine and molecular imaging, 2013

Cardiac single photon emission computed tomography (SPECT) cameras typically rotate too slowly ar... more Cardiac single photon emission computed tomography (SPECT) cameras typically rotate too slowly around a patient to capture changes in the blood pool activity distribution and provide accurate kinetic parameters. A spatiotemporal iterative reconstruction method to overcome these limitations was investigated. Dynamic rest/stress (99m)Tc-methoxyisobutylisonitrile ((99m)Tc-MIBI) SPECT/CT was performed along with reference standard rest/stress dynamic positron emission tomography (PET/CT) (13)N-NH3 in five patients. The SPECT data were reconstructed using conventional and spatiotemporal iterative reconstruction methods. The spatiotemporal reconstruction yielded improved image quality, defined here as a statistically significant (p<0.01) 50% contrast enhancement. We did not observe a statistically significant difference between the correlations of the conventional and spatiotemporal SPECT myocardial uptake K 1 values with PET K 1 values (r=0.25, 0.88, respectively) (p<0.17). These r...

Research paper thumbnail of Observation of Orbital Angular Momentum Transfer between Acoustic and Optical Vortices in Optical Fiber

Physical Review Letters, 2006

Acousto-optic interaction in optical fiber is examined from the perspective of copropagating opti... more Acousto-optic interaction in optical fiber is examined from the perspective of copropagating optical and acoustic vortex modes. Calculation of the acousto-optic coupling coefficient between different optical modes leads to independent conservation of spin and orbital angular momentum of the interacting photons and phonons. We show that the orbital angular momentum of the acoustic vortex can be transferred to a circularly polarized fundamental optical mode to form a stable optical vortex in the fiber carrying orbital angular momentum. The technique provides a useful way of generating stable optical vortices in the fiber medium.

Research paper thumbnail of All-fiber acousto-optic polarization monitor

Optics Letters, 2007

We describe the operational principle of and experimentally demonstrate a narrowband, wavelength-... more We describe the operational principle of and experimentally demonstrate a narrowband, wavelength-tunable polarization monitor based on a fiber acousto-optic tunable filter. Two orthogonally vibrating acoustic waves are used to create a variable polarizer that can be used to measure the state of polarization of an incident narrowband light source. The accuracy of the polarization monitor is measured at two different wavelengths in reference to a commercial polarimeter and is shown to be within 5%.

Research paper thumbnail of Depth-of-Interaction Compensation Using a Focused-Cut Scintillator for a Pinhole Gamma Camera

IEEE Transactions on Nuclear Science, 2000

Preclinical SPECT offers a powerful means to understand the molecular pathways of drug interactio... more Preclinical SPECT offers a powerful means to understand the molecular pathways of drug interactions in animal models by discovering and testing new pharmaceuticals and therapies for potential clinical applications. A combination of high spatial resolution and sensitivity are required in order to map radiotracer uptake within small animals. Pinhole collimators have been investigated, as they offer high resolution by means of image magnification. One of the limitations of pinhole geometries is that increased magnification causes some rays to travel through the detection scintillator at steep angles, introducing parallax errors due to variable depth-ofinteraction in scintillator material, especially towards the edges of the detector field of view. These parallax errors ultimately limit the resolution of pinhole preclinical SPECT systems, especially for higher energy isotopes that can easily penetrate through millimeters of scintillator material. A pixellated, focused-cut (FC) scintillator, with its pixels laser-cut so that they are collinear with incoming rays, can potentially compensate for these parallax errors and thus improve the system resolution. We performed the first experimental evaluation of a newly developed focused-cut scintillator. We scanned a Tc-99m source across the field of view of pinhole gamma camera with a continuous scintillator, a conventional "straight-cut" (SC) pixellated scintillator, and a focused-cut scintillator, each coupled to an electron-multiplying charge coupled device (EMCCD) detector by a fiber-optic taper, and compared the measured full-width half-maximum (FWHM) values. We show that the FWHMs of the focused-cut scintillator projections are comparable to the FWHMs of the thinner SC scintillator, indicating the effectiveness of the focused-cut scintillator in compensating parallax errors.

Research paper thumbnail of Temperature Dependent Operation of PSAPD-Based Compact Gamma Camera for SPECT Imaging

IEEE Transactions on Nuclear Science, 2000

We investigated the dependence of image quality on the temperature of a position sensitive avalan... more We investigated the dependence of image quality on the temperature of a position sensitive avalanche photodiode (PSAPD)-based small animal single photon emission computed tomography (SPECT) gamma camera with a CsI:Tl scintillator. Currently, nitrogen gas cooling is preferred to operate PSAPDs in order to minimize the dark current shot noise. Being able to operate a PSAPD at a relatively high temperature (e.g., 5 °C) would allow a more compact and simple cooling system for the PSAPD. In our investigation, the temperature of the PSAPD was controlled by varying the flow of cold nitrogen gas through the PSAPD module and varied from -40 °C to 20 °C. Three experiments were performed to demonstrate the performance variation over this temperature range. The point spread function (PSF) of the gamma camera was measured at various temperatures, showing variation of full-width-half-maximum (FWHM) of the PSF. In addition, a (99m)Tc-pertechnetate (140 keV) flood source was imaged and the visibility of the scintillator segmentation (16×16 array, 8 mm × 8 mm area, 400 μm pixel size) at different temperatures was evaluated. Comparison of image quality was made at -25 °C and 5 °C using a mouse heart phantom filled with an aqueous solution of (99m)Tc-pertechnetate and imaged using a 0.5 mm pinhole collimator made of tungsten. The reconstructed image quality of the mouse heart phantom at 5 °C degraded in comparision to the reconstructed image quality at -25 °C. However, the defect and structure of the mouse heart phantom were clearly observed, showing the feasibility of operating PSAPDs for SPECT imaging at 5 °C, a temperature that would not need the nitrogen cooling. All PSAPD evaluations were conducted with an applied bias voltage that allowed the highest gain at a given temperature.

Research paper thumbnail of Phantom Experiments on a PSAPD-Based Compact Gamma Camera With Submillimeter Spatial Resolution for Small Animal SPECT

IEEE Transactions on Nuclear Science, 2000

We demonstrate a position sensitive avalanche photodiode (PSAPD) based compact gamma camera for t... more We demonstrate a position sensitive avalanche photodiode (PSAPD) based compact gamma camera for the application of small animal single photon emission computed tomography (SPECT). The silicon PSAPD with a two-dimensional resistive layer and four readout channels is implemented as a gamma ray detector to record the energy and position of radiation events from a radionuclide source. A 2 mm thick monolithic CsI:Tl scintillator is optically coupled to a PSAPD with a 8mm×8mm active area, providing submillimeter intrinsic spatial resolution, high energy resolution (16% full-width half maximum at 140 keV) and high gain. A mouse heart phantom filled with an aqueous solution of 370 MBq 99m Tc-pertechnetate (140 keV) was imaged using the PSAPD detector module and a tungsten knife-edge pinhole collimator with a 0.5 mm diameter aperture. The PSAPD detector module was cooled with cold nitrogen gas to suppress dark current shot noise. For each projection image of the mouse heart phantom, a rotated diagonal readout algorithm was used to calculate the position of radiation events and correct for pincushion distortion. The reconstructed image of the mouse heart phantom demonstrated reproducible image quality with submillimeter spatial resolution (0.7 mm), showing the feasibility of using the compact PSAPD-based gamma camera for a small animal SPECT system.

Research paper thumbnail of A Compact All-Fiber LPG-AOTF Frequency Shifter on Single-Mode Fiber and Its Application to Vibration Measurement

IEEE Photonics Technology Letters, 2000

We demonstrate a compact, all-fiber frequency shifter by cascading a CO 2 -written long-period gr... more We demonstrate a compact, all-fiber frequency shifter by cascading a CO 2 -written long-period grating with a flexural acoustic-wave-induced microbending grating on a single-mode fiber. Carrier and image sideband suppressions of 30.0 and 28.1 dB, respectively, are measured. An all-fiber vibrometer based on this frequency shifter capable of nanometer resolution at frequencies up to megahertz is also demonstrated.

Research paper thumbnail of Fabrication and Mode Identification of Compact Long-Period Gratings Written by CO<formula formulatype="inline"><tex>$_{2}$</tex></formula> Laser

IEEE Photonics Technology Letters, 2000

We report highly compact (1.2 cm) long-period fiber gratings (LPGs) exhibiting strong grating eff... more We report highly compact (1.2 cm) long-period fiber gratings (LPGs) exhibiting strong grating effect (>30-dB loss) and low insertion loss (<0.25 dB) inscribed by a continuously scanned CO 2 laser. The LPG resonances are also probed using traveling acoustic gratings with variable period. Our results show that these CO 2 laser side-exposed LPGs predominantly couple to antisymmetrical cladding modes.

Research paper thumbnail of A highly efficient thermally controlled loss-tunable long-period fiber grating on corrugated metal substrate

IEEE Photonics Technology Letters, 2000

We demonstrate a new approach for fabricating a compact loss-tunable long-period fiber grating (L... more We demonstrate a new approach for fabricating a compact loss-tunable long-period fiber grating (LPFG) by gluing a fiber onto a corrugated metal substrate filled with ultraviolet-cured epoxy. The strain caused by the difference in thermal expansion among the adhesive, the substrate, and the silica fiber induces a periodic microbending along the fiber when the device temperature is changed. The LPFG has an initial flat spectrum at room temperature. With 35 grating periods, a tuning range of 21 dB is achieved through core-cladding coupling for a temperature change from room temperature to 4 C. A theoretical model is presented to explain the high tuning efficiency and the operating mechanisms of the device.

Research paper thumbnail of A compact all-fiber PDL-compensated acoustooptic wavelength monitor

IEEE Photonics Technology Letters, 2000

A compact all-fiber acoustooptic wavelength monitor with passive polarization-dependent loss (PDL... more A compact all-fiber acoustooptic wavelength monitor with passive polarization-dependent loss (PDL) compensation is demonstrated. A Sagnac loop interferometer is used to compensate the intrinsic PDL, and hence, the polarization-dependent wavelength shift of the fiber acoustooptic tunable filter (AOTF). This increases the extrinsic wavelength accuracy of the device to 0.05 nm, a three-fold improvement from a previously reported uncompensated device. The relaxation in the PDL specifications for the AOTF also results in a reduction of fiber length from 38 to 14 cm.

Research paper thumbnail of Variable and stable dual-wavelength erbium-doped fiber laser based on fiber acousto-optic variable filter and highly nonlinear fiber

ABSTRACT A dual-wavelength erbium-doped fiber (EDF) variable ring laser using a fiber acousto-opt... more ABSTRACT A dual-wavelength erbium-doped fiber (EDF) variable ring laser using a fiber acousto-optic tunable filter (AOTF) and highly nonlinear fiber (HNLF) is demonstrated. Stable and variable lasing wavelengths were achieved by electronically adjusting the AOTF settings.

Research paper thumbnail of Acoustooptic Coherent Mode Coupling in Polarization-Maintaining Fiber and Its Application as a Variable-Polarization-Dependent Loss Element

IEEE Photonics Technology Letters, 2000

In this letter, we investigate coherent acoustooptic coherent mode coupling from the LP 01 core m... more In this letter, we investigate coherent acoustooptic coherent mode coupling from the LP 01 core mode to LP 1 cladding mode of a polarization-maintaining fiber (PMF) induced by two acoustic gratings. We show narrowband variable attenuation along either birefringent axis of the PMF by varying the relative phase between the two acoustic gratings. By launching two acoustic gratings at different frequencies, variable attenuation and low crosstalk along either birefringent axis within the acoustooptic coupling bandwidth are demonstrated.