Horacio Lamela

High-sensitivity ultrasound interferometric single-mode polymer optical fiber sensors for biomedical applications

This work describes the results of ultrasonic wideband sensors based on single-mode polymer optical fibers that may be used for biomedical applications. We have compared the ultrasonic sensitivities of two Mach-Zehnder interferometric intrinsic optical fiber sensors. One is based on a single-mode polymethylmethacrylate optical fiber and the second on single-mode silica optical fiber, both operating at 632.8 nm. At a frequency of 1 MHz these sensitivities are 13.1 and 0.85 mrad/kPa, respectively. The ultrasonic phase sensitivity of the polymer optical fiber is more than 12 times larger than that from the fused silica fiber in the 1-5 MHz range.

Optoacoustic imaging using fiber-optic interferometric sensors

An interferometric sensor based on nonmetallic silica optical fiber is presented as an ultrasonic wideband transducer for optoacoustic imaging applications. We have characterized the sensitivity of the optical fiber sensor by detecting optoacoustic signals from an optically absorbing object embedded in a tissue-mimicking phantom and have compared the signals recorded with those detected from the same phantom using an array of piezoelectric transducers. The optical fiber sensor was also scanned along the phantom surface in order to reconstruct two-dimensional optoacoustic images of the phantom. These images have been compared with images obtained using the Laser Optoacoustic Imaging System, LOIS-64B, demonstrating the feasibility of our fiber-optic sensor as a wideband ultrasonic transducer.

A low power laser rangefinder for autonomous robot applications

Autonomous robots require specific vision systems capable of providing accurate three-dimensional scene analysis. Direct time-of-flight laser rangefinders present themselves as an attractive alternative to other vision systems commonly employed in robotic vision systems, such as triangulation and ultrasound rangefinders, thanks to their superior radial and lateral accuracy. However, laser rangefinders present safety problems, due to the power level of the emitted beam, which condition their use in human environments. In the present work, the authors deal with the problem of safety, reducing the emitted power by employing different means to enhance receiver sensitivity. A first result of this work is presented in this paper, describing the development and preliminary experimental results of a direct detection laser rangefinder, which covers a maximum range of 7 metres with an emitted optical power of only 2 milliwatts. This prototype will be installed and tested on a RWI B21 mobile robot.

Experimental modulation bandwidth beyond the relaxation oscillation frequency in a monolithic twin-ridge laterally coupled diode laser based on lateral mode locking

Monolithic twin-ridge laterally coupled diode lasers emitting at 1.3microm are presented that have a small-signal modulation bandwidth beyond the relaxation oscillation frequency of a single ridge. Spectra and spectrally resolved far fields are presented for three bias conditions: only one ridge lasing, both ridges lasing just above threshold, and both ridges lasing at biases well above threshold. In the first two cases the spectrum has single-peaked longitudinal modes, whereas the third cases shows splitting to in-phase and out-of-phase modes. The splitting frequency of the optical spectrum is measured to be 7.7 GHz. Small-signal modulation measurements reveal a strong resonance at 7.7 GHz, demonstrating an effect of lateral mode locking. As a result of this effect, the twin-ridge laser can be made to have a -3-dB bandwidth beyond that associated with its relaxation oscillation frequency.

Period tripling and chaos in the dynamic behavior of directly modulated diode lasers

The paper presents the relevance of period-tripling behavior that has recently been found in different experimental studies of directly modulated laser diodes. Applying different numerical techniques to the rate equation model of the laser diode, among which we highlight the continuation method to calculate the unstable solutions of the system, we show that period-tripling behavior appears and disappears in two tangent bifurcations. Therefore, the period-three solutions form a closed bifurcation curve called isola. In between these two tangent bifurcations, the period-three solution coexists with the chaotic attractor reached by a period-doubling cascade, giving rise to a hysteresis loop in the deterministic case. Also, we have found that a boundary crisis might be behind the chaotic behavior that is observed for the highest values of the modulation index. The effects of random noise fluctuations in the laser diode dynamics are also studied. Langevin noise sources are included in the rate equation model and appropriate stochastic integration methods have been used. The route to chaos that we have obtained points out the relevant role that noise has in achieving agreement between numerical studies and experimental results that have been published. The introduction of noise has been proved to be of major importance in determining the system behavior in the regions of the coexistence of solutions.

Analysis of the dynamic behavior and short-pulse modulation scheme for laterally coupled diode lasers

A theoretical investigation of the high-speed coupling phenomena of two laterally coupled diode lasers (LCDL) is presented. The analysis is centered on the spatiotemporal dynamics of the LCDL when coupling between emitters is varied, We have obtained the dynamic behavior of these devices showing high resonance frequencies beyond the well-known resonance frequency of a single diode laser. In this paper, we have presented a new modulation scheme by asymmetric switching of the injected current in both stripes, showing short optical output pulse modulation with a high repetition rate.

Self-mixing technique for vibration measurements in a laser diode with multiple modes created by optical feedback

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.

Terahertz electrical conductivity and optical characterization of composite nonaligned single- and multiwalled carbon nanotubes

Abstract. We employed noncontact terahertz time-domain spectroscopy to investigate and compare the electrical/optical properties of nonaligned single-walled carbon nanotube and nonaligned multiwalled carbon nanotube thin films. Using Drude–Lorentz model together with the Maxwell Garnett effective medium theory, we determined the electrical conductivities from the extracted data of differential complex terahertz analysis in the frequency range of 0.1 to 2 THz. The results demonstrate that the conductivity of isotropic single-walled carbon nanotube thin film is almost two times larger compared to isotropic multiwalled carbon nanotube thin film due to the increased number of surface defects and the availability of mobile carriers. By using Drude–Lorentz model, the broadening optical density and conductivity can be studied at higher frequencies.

High resolution CO2 interferometry on the TJ-II stellarator by using an ADC-based phase meter

A 10 MSample/s analog-to-digital converter has been used to analyze the signals from the double wavelength heterodyne interferometer (CO2 10.6 μm, HeNe   0.63   μm) in the TJ-II stellarator. The phase difference between the two interferometers has been calculated using a new algorithm in Labview environment. A systematic study of the 1 MHz intermediate frequency has been done using our software and Labview tools. Different sources of noise and nonlinear effects have been analyzed. Crosstalk (from the reference path) has been detected and corrected by software. The complete system (single channel double pass) has been routinely working during the complete TJ-II autumn experimental campaign, providing the plasma line integral density in scenarious with average densities ranging from few 1018 m−3 to more than 5×1019 m−3. The results have been compared with those from a 2 mm interferometer, obtaining an excellent agreement. Working with a final bandwidth of 4 kHz, a one pass line integral error level about ±−...

Improvements to Long-Duration Low-Power Gain-Switching Diode Laser Pulses Using a Highly Nonlinear Optical Loop Mirror: Theory and Experiment

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.