Ofer Levi

Optical parametric oscillation in quasi-phase-matched GaAs

We demonstrate a GaAs based OPO. The singly resonant, pulsed OPO utilized an all-epitaxially-grown orientation-patterned GaAs (OP-GaAs) crystal. By tuning either near-IR pump wavelength or OP-GaAs temperature, we achieved broad tunability between 2 and 10 microns.

Improved dispersion relations for GaAs and applications to nonlinear optics

The refractive index of GaAs has been measured in the wavelength range from 0.97 to 17 μm, which covers nearly the entire transmission range of the material. Linear and quadratic temperature coefficients of the refractive index have been fitted to data measured between room temperature and 95u200a°C. In the midinfrared, the refractive index and temperature dependence are obtained from analysis of etalon fringes measured by Fourier-transform spectroscopy in undoped GaAs wafers. In the near infrared, the refractive index is deduced from the quasiphasematching (QPM) wavelengths of second-harmonic generation in orientation-patterned GaAs crystals. Two alternative empirical expressions are fitted to the data to give the refractive index as a function of wavelength and temperature. These dispersion relations agree with observed QPM conditions for midinfrared difference-frequency generation and second-harmonic generation. Predictions for various nonlinear optical interactions are presented, including tuning curves f...

Measurement of the nonlinear coefficient of orientation-patterned GaAs and demonstration of highly efficient second-harmonic generation

Quasi-phase-matched (QPM) GaAs structures, 0.5 mm thick, 10 mm long, and with 61-mum grating periods, were grown by a combination of molecular-beam epitaxy and hydride vapor phase epitaxy. These were characterized by use of mid-IR second-harmonic generation (SHG) with a ZnGeP(2) (ZGP) optical parametric oscillator as a pump source. The SHG efficiencies of QPM GaAs and QPM LiNbO(3) were directly compared, and a ratio of nonlinear coefficients d(14)(GaAs)/d(33) (LiNbO(3))=5.01+/-0.3 was found at 4.1-mum fundamental wavelength. For input pulse energies as low as 50muJ and approximately 60-ns pulse duration, an internal SHG conversion efficiency of 33% was measured in QPM GaAs.

Integrated semiconductor vertical-cavity surface-emitting lasers and PIN photodetectors for biomedical fluorescence sensing

Vertical-cavity surface-emitting lasers (VCSELs), optical emission filters, and PIN photodetectors were fabricated as part of a monolithically integrated near-infrared fluorescence detection system. The integration of these micro-fabricated components with micro-arrays, flow channel arrays, and biochips can drastically reduce cost and enable parallel sensing architectures. An optoelectronic design is presented that integrates VCSELs, optical filters, and photodetectors through a modification to a typical VCSEL structure. System designs were simulated and compared, leading to several innovative approaches for integrated sensors. The laser and detector modules were characterized independently and subsequently integrated to form a complete sensor. VCSELs with oxidation apertures measuring 4, 7, 14, and 20 /spl mu/m showed a lasing wavelength of /spl lambda/=773 nm, threshold current densities from 6400 to 1300 A/spl middot/cm/sup -2/, and maximum output powers of 0.6-4 mW, with transverse single-mode and multimode operation. PIN photodetectors were fabricated with integrated emission filters. Quantum efficiencies above 85% were observed with a dark current of 500 fA/(mm detector diameter). Complete sensor units were tested and near-infrared fluorescent molecules (IR-800) were detected. A theoretical detection limit of 10/sup 5/ fluorophores//spl mu/m/sup 2/ was determined. The compact parallel architecture, high-power laser, and low-noise photodetector make this sensor a good candidate for biomedical fluorescence-based sensing applications.

Difference frequency generation of 8-µm radiation in orientation-patterned GaAs

First-order quasi-phase-matched difference frequency generation of narrowband tunable mid-infrared light is demonstrated in orientation-patterned GaAs. The all-epitaxial orientation-patterned crystal is fabricated by a combination of molecular beam epitaxy and hydride vapor phase epitaxy. Lasers at 1.3 and 1.55 microm were mixed to give an idler output at 8 microm, with power and wavelength tuning consistent with theoretical estimates, indicating excellent material uniformity over the 19-mm-long and 500-microm-thick device.

Integrated bio-fluorescence sensor.

Due to the recent explosion in optoelectronics for telecommunication applications, novel optoelectronic sensing structures can now be realized. In this work, we explore the integration of optoelectronic components towards miniature and portable fluorescence sensors. The integration of these micro-fabricated sensors with microfluidics and capillary networks may reduce the cost and complexity of current research instruments and open up a world of new applications in portable biological analysis systems. A novel optoelectronic design that capitalizes on current vertical-cavity surface-emitting laser (VCSEL) technology is explored. Specifically, VCSELs, optical emission filters and PIN photodetectors are fabricated as part of a monolithically integrated near-infrared fluorescence detection system. High-performance lasers and photodetectors have been characterized and integrated to form a complete sensor. Experimental results show that sensor sensitivity is limited by laser background. The laser background is caused by spontaneous emission emitted from the side of the VCSEL excitation source. Laser background will limit sensitivity in most integrated sensing designs due to locating excitation sources and photodetectors in such close proximity, and methods are proposed to reduce the laser background in such designs so that practical fluorescent detection limits can be achieved.

Detection of Green Apples in Hyperspectral Images of Apple-Tree Foliage Using Machine Vision

It is important for orchard owners to be able to estimate the quantity of fruit on the trees at the various growth stages, because a tree that bears too many fruits will yield small fruits. Thus, if growers are interested in controlling the fruit size, knowing in advance that there are too many developing fruits will give them the opportunity to treat the tree. This study proposes a machine vision-based method of automating the yield estimation of apples on trees at different stages of their growth. Since one of the most difficult aspects of apple yield estimation is distinguishing between green varieties of apples or those that are green in the first stages of growth, and the green leaves that surround them, this investigation concentrates on estimating the yield of green varieties of apples. Hyperspectral imaging was used, because it is capable of giving a wealth of information both in the visible and the near-infrared (NIR) regions and thus offers the potential to provide useful results. A multistage algorithm was developed that uses several techniques, such as principle components analysis (PCA) and extraction and classification of homogenous objects (ECHO) for analyzing hyperspectral data, as well as machine vision techniques such as morphological operations, watershed, and blob analysis. The method developed was tested on images taken in a Golden Delicious apple orchard in the Golan Heights, Israel, in two sessions: one during the first stages of growth, and the second just before harvest. The overall correct detection rate was 88.1%, with an overall error rate of 14.1%.

Integrated semiconductor fluorescent detection system for biochip and biomedical applications

As biological analysis systems scale to smaller dimensions, the realization of small and portable biosensors becomes increasingly important. We present the monolithic integration of vertical cavity surface emitting lasers, PIN photodetectors and optical emission filters to be used as a fluorescence sensor. The integration will drastically reduce cost and size of fluorescence detection systems. Also, parallel sensing architectures of more than one hundred channels will be possible. PIN heterostructure photodetectors have been fabricated and tested. Photodetector experiments show extremely low dark current of less than 500 fA/mm, quantum efficiency greater than 85 percent and linear detector response. Optical simulations have been preformed to evaluate the performance of a proximity sensing architecture. These simulations predict a detection sensitivity lower than 10000 fluorescent molecules in a detection area of 10/sup 4/ /spl mu/m/sup 2/.

Laplace inversion of low-resolution NMR relaxometry data using sparse representation methods

Low-resolution nuclear magnetic resonance (LR-NMR) relaxometry is a powerful tool that can be harnessed for characterizing constituents in complex materials. Conversion of the relaxation signal into a continuous distribution of relaxation components is an ill-posed inverse Laplace transform problem. The most common numerical method implemented today for dealing with this kind of problem is based on L2-norm regularization. However, sparse representation methods via L1 regularization and convex optimization are a relatively new approach for effective analysis and processing of digital images and signals. In this article, a numerical optimization method for analyzing LR-NMR data by including non-negativity constraints and L1 regularization and by applying a convex optimization solver PDCO, a primal-dual interior method for convex objectives, that allows general linear constraints to be treated as linear operators is presented. The integrated approach includes validation of analyses by simulations, testing repeatability of experiments, and validation of the model and its statistical assumptions. The proposed method provides better resolved and more accurate solutions when compared with those suggested by existing tools.

Optical compressive change and motion detection

Localization information of moving and changing objects, as commonly extracted from video sequences, is typically very sparse with respect to the full data frames, thus fulfilling one of the basic conditions of compressive sensing theory. Motivated by this observation, we developed an optical compressive change and motion-sensing technique that detects the location of moving objects by using a significantly fewer samples than conventionally taken. We present examples of motion detection and motion tracking with over two orders of magnitude fewer samples than required with conventional systems.