Peter Adany

Chirped Lidar Using Simplified Homodyne Detection

A simplified homodyne detection scheme for linear FM modulated lidar is presented in which pulse dechirping is performed in the optical domain. This method provides quantum limited detection sensitivity with much less receiver complexity compared to heterodyne detection systems. Another advantage of this approach is the reduced bandwidth requirement for the photodetector. This removes the limit on the chirp bandwidth, and enables the use of more efficient photodiodes with larger detector area. A field trial using a 5-in aperture diameter commercial telescope and a 370-m target range verified the sensitivity estimation and demonstrated the feasibility of this technique.

Tunable excitation source for coherent Raman spectroscopy based on a single fiber laser

We demonstrate a wavelength tunable optical excitation source for coherent Raman scattering (CRS) spectroscopy based on a single femtosecond fiber laser. Electrically controlled wavelength tuning of Stokes optical pulses was achieved with soliton self frequency shift in an optical fiber, and linear frequency chirping was applied to both the pump and the Stokes waves to significantly improve the spectral resolution. The coherent anti-Stokes Raman scattering (CARS) spectrum of cyclohexane was measured and vibrational resonant Raman peaks separated by 70 cm(-1) were clearly resolved. Single laser-based tunable excitation may greatly simplify CRS measurements and extend the practicality of CRS microscopy.

In vivo evidence of oxidative stress in brains of patients with progressive multiple sclerosis

Background: The oxidative stress hypothesis links neurodegeneration in the later, progressive stages of multiple sclerosis (MS) to the loss of a major brain antioxidant, glutathione (GSH). Objective: We measured GSH concentrations among major MS subtypes and examined the relationships with other indices of disease status including physical disability and magnetic resonance imaging (MRI) measures. Methods: GSH mapping was performed on the fronto-parietal region of patients with relapsing-remitting multiple sclerosis (RRMS, n = 21), primary progressive multiple sclerosis (PPMS, n = 20), secondary progressive multiple sclerosis (SPMS, n = 20), and controls (n = 28) using GSH chemical shift imaging. Between-group comparisons were performed on all variables (GSH, T2-lesion, atrophy, Expanded Disability Status Scale (EDSS)). Results: Patients with MS had substantially lower GSH concentrations than controls, and GSH was lower in progressive MS (PPMS and SPMS) compared with RRMS. GSH concentrations were not significantly different between PPMS and SPMS, or between RRMS and controls. Brain atrophy was significant in both RRMS and progressive MS compared with controls. Conclusion: Markedly lower GSH in progressive MS than RRMS indicates more prominent involvement of oxidative stress in the progressive stage of MS than the inflammatory stage. The association between GSH and brain atrophy suggests the important role of oxidative stress contributing to neurodegeneration in progressive MS, as suggested in other neurodegenerative diseases.

Switching of 800 nm femtosecond laser pulses using a compact PMN-PT modulator

A voltage-controlled birefringent cell based on ceramic PMN-PT material is used to enable fast intensity modulation of femtosecond laser pulses in the 800 nm wavelength window. The birefringent cell based on a PMN-PT compound has comparatively high electro-optic response, allowing for a short interaction length of 3 mm and thus very small size, low attenuation of 0.16 dB, and negligible broadening for 100 fs optical pulses. As an application example, agile wavelength tuning of optical pulses is demonstrated using the soliton self-frequency shift in a photonic crystal fiber. By dynamically controlling the optical power into the fiber, this system switches the wavelength of 100 fs pulses from 900 nm to beyond 1120 nm with less than 5 micros time. In addition, a feedback system stabilizes the wavelength drift against external conditions resulting in high wavelength stability.

B0-adjusted and sensitivity-encoded spectral localization by imaging (BASE-SLIM) in the human brain in vivo.

Spectral localization by imaging (SLIM) based magnetic resonance spectroscopy (MRS) provides a framework that overcomes major limitations of current MRS techniques, which allow only rectangular voxel shapes that do not conform to the shapes of brain structures or lesions. However, the restrictive assumption of compartmental homogeneity in SLIM can lead to localization errors, thus its applications have been very limited to date. SLIM-based localization is subject to errors due to inhomogeneous B0 and B1 fields, particularly in organs with complex compartmental geometry including the human brain. The limitations of SLIM were overcome through the development and implementation of B0-adjusted and sensitivity-encoded SLIM (BASE-SLIM) that includes corrections for inhomogeneities of both B0 and B1 fields throughout the volume of interest. In this study, we demonstrate significantly improved localization accuracy in compartments with arbitrary shapes and reliable quantification of metabolite concentrations in gray and white matter of the human brain using the BASE-SLIM technique.

Fiber laser based two-photon fret measurement of calmodulin and mcherry-E0GFP proteins

The speed and accuracy of Förster resonance energy transfer (FRET) measurements can be improved by rapidly alternating excitation wavelengths between the donor and acceptor fluorophore. We demonstrate FRET efficiency measurements based on a fiber laser and photonic crystal fiber as the source for two‐photon excitation (TPE). This system offers the potential for rapid wavelength switching with the benefits of axial optical sectioning and improved penetration depth provided by TPE. Correction of FRET signals for cross excitation and cross emission was achieved by switching the excitation wavelength with an electrically controlled modulator. Measurement speed was primarily limited by integration times required to measure fluorescence. Using this system, we measured the FRET efficiency of calmodulin labeled with Alexa Fluor 488 and Texas Red dyes. In addition, we measured two‐photon induced FRET in an E0GFP‐mCherry protein construct. Results from one‐photon and two‐photon excitation are compared to validate the rapid wavelength switched two‐photon measurements. Microsc. Res. Tech. 75:837–843, 2012.

Simplified Homodyne Detection for Linear FM Lidar

A fiber based lidar system is developed which simplifies the processing of linear FM pulses by using a modulated local oscillator power in the coherent detector. Experiments were conducted on lidar systems with direct, heterodyne and simplified homodyne detection to compare receiver sensitivity. A field experiment using the homodyne system verified the sensitivity estimation on a building target at 370-m range.

Simplified Coherent Detection Scheme for FM Chirped Laser Radar

A simplified optical homodyne detection scheme for FM chirped lidar is proposed where dechirping is performed within the photodetector. The concept is validated by experiment and a comparison between coherent and direct detection is presented.