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We present experimental and theoretical results obtained from the study of the effects of optical feedback in low-cost Fabry-Perot laser diodes due to the presence of an external cavity created by an external reflective or diffusive vibrating target. Experimental results show that a change in the length of the external cavity produces the well-known amplitude modulation of the optical output power and, depending on the amount of optical feedback, a subperiodicity appears in the amplitude modulation of the output power. The experiments show that the subperiodicity appears independently of the length of the external cavity and is due to mode hopping between different longitudinal laser modes. Numerical analysis focused on the effects observed support that the mode hop occurs between modes whose round-trip phase delay along the external cavity is out of phase, thus producing a subperiodicity of the total amplitude modulation. OCIS codes: 140.5960, 120.3180, 120.7280, 280.3420.

We describe the concept of a vision system based on an optoelectronic hardware neural processor. The proposed system is composed of a pulse coupled neural network (PCNN) preprocessor stage that converts an input image into a temporal pulsed pattern. These pulses are inputs to the optical broadcast neural network (OBNN) processor, which classifies the input pattern between a set of reference patterns based on a pattern matching strategy. The PCNN is to provide immunity to the scale, rotation, and translation of objects in the image. The OBNN provides high parallelism and a high speed hardware neural processor. OCIS codes: 100.0100, 200.4700.

The origin of intermittent bursts appearing in the output power of a laser diode is examined.

The importance of period-tripling behavior in various experimental studies made on directly modulated laser diodes is presented. The numerical analysis conducted indicate that the period-three solution coexist with the chaotic attractor created by a period-doubling cascade, a situation that results in a hysteresis loop. The important role of noise in achieving agreement between the experimental results and the numerical studies is further supported by the obtained route to chaos.

A study was conducted on the route to chaos produced by directly modulated semiconductor lasers subjected to a varied modulation index to determine the truncation of Feigenbaum sequence in these types of lasers. Results showed a coexistence between the period-doubling route to chaos and a period-three solution for certain values of modulation index. Moreover, noise fluctuations were found to be responsible for the effective truncation of the period doubling sequence in the route to chaos.

Optoacoustic imaging (OAI) is an emerging biomedical technique that allows visualization of in-depth tissues by using ultrasonic signals generated by short laser pulses. In this work, the authors combine the optical power of several pulsed high-power diode lasers (HPDLs) at 870 nm and 905 nm to a 7-to-1 675-µm fiber bundle to generate optoacoustic (OA) signals from different mixtures of two gold nanorods solutions with absorbance peak at ~ 860 nm and ~ 900 nm, respectively. The pulses produced to generate OA signals are alternated between the two wavelengths by a microcontroller circuit with fast switching (0.5ms). From the amplitude of the OA signals, the concentrations of the nanoparticles solutions are easily estimated with high accuracy using a fluence model. The results achieved with the proposed system show very good agreement between the concentrations of gold nanorods estimated from measurements and the expected values. [ABSTRACT FROM AUTHOR]

We determined the electrical sheet conductivity of silver nanowire-graphene hybrid films from transmission and reflection terahertz time-domain spectroscopy measurements. The sheet resistance extracted from noncontact terahertz measurement is in good agreement with one measured with a classical 4-point-probe technique. The conductivity is well described by a Drude-Smith model and is calculated to peak around 10 THz. [ABSTRACT FROM AUTHOR]

This work presents an experimental and theoretical study on the improvements to pulsed diode laser gain-switched (GS) optical sources. Simultaneous compression and reshaping of the limited quality pulses obtained with this technique are reported using a highly nonlinear optical loop mirror (HNOLM) directly coupled to the diode laser source, without any previous pulse conditioning. The HNOLM is based on the use of a microstructured optical fiber and a highly nonlinear semiconductor optical amplifier; it is compact and offers benefits in terms of reduced system complexity. The experimental observations are compared to a theoretical model of the system, and excellent agreement is observed. [ABSTRACT FROM PUBLISHER]

We present a non-metallic interferometric silica optical fiber ultrasonic wideband sensor for optoacoustic imaging applications. The ultrasonic sensitivity of this sensor has been characterized over the frequency range from 1 to 10 MHz. A comparative analysis has been carried out between this sensor and an array of piezoelectric transducers using optoacoustic signals generated from an optical absorbent embedded in a tissue mimicking phantom. Also, a two dimensional reconstructed image of the phantom using the fiber interferometric sensor is presented and compared to the image obtained using the Laser Optoacoustic Imaging System, LOIS-64B. The feasibility of our fiber optic based sensor for wideband ultrasonic detection is demonstrated. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

The nonlinear and chaotic phenomena in lateral coupled diode lasers have been widely studied theoretically. In this work an experimental analysis of the complex nonlinear and chaotic dynamic regimes experimentally observed in these devices and the mechanisms that produces it is made. This analysis is set by means of the RIN electrical spectrum and of the high resolution Fabry-Perot optical spectrum. A mapping of the nonlinear occurrences in these devices is obtained and a study of the relation between the relaxation oscillation frequency and the lateral locking frequency is achieved. [ABSTRACT FROM AUTHOR]

A high spatial resolution two-color (CO2, λ=10.6 μm, He–Ne, λ=633 nm) interferometer for density profile measurements in the TJ-II stellarator is under development and installation, based in the currently operational single channel two-color heterodyne interferometer. To achieve the objectives of 32 channels, with 4–5 mm lateral separation between plasma chords, careful design and calibration of the interferometric waveforms for both the measurement and vibration compensation wavelengths are undertaken. The first step has been to set up in our laboratories an expanded-beam heterodyne/homodyne interferometer to evaluate the quality of both interferometric wavefronts, a reported source of poor vibration compensation and thus low resolution in the density profile measurements. This novel interferometric setup has allowed us to calibrate the spatial resolution in the profile measurements resulting in ∼2 mm lateral resolution in the reconstruction of the interferometric wavefront. [ABSTRACT FROM AUTHOR]

In this article we describe the setup, installation, calibration, and operation of the two-color (CO2–HeNe) laser heterodyne interferometer installed in the TJ-II Stellarator. This diagnostic has been routinely working during the last TJ-II experimental campaign (Autumn 2003) in low-density [electron cyclotron resonance (ECRH) heated, ne≅1019 m-3] and medium density [neutral beam injection (NBI) heated, ne≅5×1019 m-3] plasmas with line integral error better than 2×1017 m-2. This work demonstrates the possibility of using low-cost two wavelength laser interferometric diagnostics to measure low and medium-density plasmas instead of the complicated far infrared (FIR) interferometric systems that also suffer from fringe-jumps errors. The issues of low sensitivity to plasma density and mechanical vibration subtraction typical of these laser interferometers are solved through proper calibration of the interferometric system in the machine. First results on NBI heated plasmas measured by the laser interferometric system in the Stellarator TJ-II will be shown. [ABSTRACT FROM AUTHOR]

Laser two-color heterodyne interferometry is a proven method to measure electron density in fusion plasmas. Though only used in tokamaks with high electron densities, the idea of also using two-color laser interferometers for stellarators and small machines to replace far-infrared laser interferometers is being proposed. This will lead to low-cost, reliable, and easy to operate diagnostics for electron-density measurements. In this article, we present the interferometric experiments we have used for calibration of a two-color laser heterodyne system for electron-density measurements in the TJ-II stellarator. These experiments have been based on the use of a novel interferometric scheme: the heterodyne/homodyne interferometer. Finally, we describe the interferometer we have installed in the TJ-II stellarator and present the first results on mechanical-vibration subtraction and electron-density measurements with a two-color laser interferometer in a stellarator (TJ-II, Madrid, Spain). © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Optoacoustic biomedical imaging combines the high spatial resolution of the ultrasound imaging with the specificity of the optical absorption spectroscopy techniques. It is being used in various scenarios such as anatomical, functional and molecular imaging. Typically light sources for this imaging technique is based on solid state lasers since they can produce high energy short optical pulses. However, they are bulky, expensive and the imaging speed is limited because their low pulse repetition rate. High power diode lasers (HPDLs) are a promising alternative for imaging small volume absorbers as they are compact, affordable and allow high repetition rates. However, HPDLs provide relative low peak optical power compared to solid state lasers. Therefore, imaging systems based on diode lasers require much longer pulse duration resulting in lower in-depth resolution and optoacoustic conversion efficiency. HPDLs need dedicated fast electronics to generate short optical pulses. In this work, we have designed, built and test a pulsed diode laser driver based on RF power MOSFETs, specifically considering the optimization of the current pulse in order to maximize the optical peak power, achieving current pulses of more than 900 A with a duration of 50 ns. We have studied the operation of a low cost HPDL out of the manufacturers datasheet ratings without noticeable degradation at high current (> 250 A) and short pulse duration (< 60 ns). We have obtained an optical peak power of 750 W and a energy per pulse of 31.2 µJ at 40 ns optical pulse duration. The optoacoustic images obtained in this operation regime shown a clear enhancement respect to the ones obtained in standard operation of the HPDL.