Emil Smeu

Long range extensometer for civil structure monitoring using fibre Bragg gratings

We report on an extensometer based on fibre Bragg gratings (FBG) which offers a relatively large and high dynamic range for the monitoring of the relative displacement between two adjacent buildings with individual foundations located in an earthquake-prone area. The sensor consists of a sensing FBG subjected to strain and a reference FBG. The fibre containing the strain-sensing FBG is directly surface mounted onto the walls of both buildings, with the fixations ~1 m apart. A large range of ±20 mm is achieved by desensitization through a tilt of the fibre axis with respect to the displacement vector (~6° away from the normal). The sensor is illuminated by a broadband source (SLD with λ0 = 815 nm) and the reflected signal is monitored directly by a CCD-based optical spectrum analyser. The recorded spectra are curve-fitted to two Gaussian distribution functions on a PC. A precision of 36 µm displacement with an integration time of 3 ms has been achieved.

Letter Phase-mismatching effects in the internal second-harmonic generation in InGaAs quantum-well laser diodes

Abstract We have characterized the role of phase-mismatching in the spectral distribution of the internal second-harmonic generation (ISHG) in the CW operation of high-power InGaAs quantum-well laser diodes emitting at around 960 nm. The observed pairs of narrow blue-green peaks of ISHG have symmetrical spectral positions with respect to the central peak of the pure second harmonic at ∼480 nm, suggesting that their generation in the laser waveguide is most probably mediated by a mechanism of reciprocal cancellation of the respective phase-mismatch vectors. The relatively strong emission of the ISHG radiation at the output mirror facet indicates that this second-order nonlinear optical process is enhanced by waveguiding in the laser active region.

Modeling of optical amplification on dopant concentration in the bent Er-doped LiNbO waveguide of the Mach-Zehnder interferometer

In the last years the Er-doped LiNbO optical waveguides are widely used for the fabrication of integrated optical components. We report some original results concerning the evaluation of the spectral optical gain, spectral noise figure and spectral signal-to-noise ratio in the bent Er-doped LiNbO waveguide of the Mach-Zehnder interferometer pumped near 1484 nm using erfc, Gaussian and constant profile of the Er ions in LiNbO crystal. Our model is based on a quasi-two-level system and the small gain approximation in the unsaturated regime in order to simulate the optical amplification in Er-doped LiNbO straight and bent waveguide amplifiers and we demonstrated that rather high gains (~1.5 dB), low noise figures (~3.5 dB) and good signal-to-noise ratios (~56.5) at the end of the bent arm of the integrated Mach-Zehnder interferometer are achievable for 70 mW input pump power. The obtained results can be used for the design of the integrated optical circuits, Mach-Zehnder interferometers which can be utilized for example for the precise measurement of the displacements, seismic motions, etc.

Analysis of phase matching for the internal second-harmonic generation in InGaAs quantum-well laser diodes emitting around 980 nm

In this paper we report theoretical and experimental results concerning the realization of the phase-matching conditions for the internal second-harmonic generation in InGaAs quantum-well laser diodes emitting around 980 nm in order to enhance the conversion efficiency. The role of the phase-mismatching in the spectral distribution of the internal second-harmonic generation in the CW operation of the above-mentioned lasers is also analyzed. The emission of pairs of narrow blue-green peaks having perfectly symmetrical spectral positions with respect to the central peak of pure second-harmonic generation at ~ 480 nm is most probably enhanced by a mechanism of reciprocal cancellation of the respective phase-mismatch vectors. The theoretical results obtained by numerical integration of the coupled amplitude equations corresponding to the fundamental and second harmonic concerning the interaction length and generation efficiency are in good agreement with the experimental data. The obtained results are important for the assessment of the relationship between the structural parameters of the laser and the conditions which contribute to the stimulation of second-order optical nonlinearities in the laser active region.

Analysis of phase-matching conditions for internal second-harmonic generation in InGaAs quantum well laser diodes

In this paper we present an analysis of the phase-matching conditions for internal second-harmonic generation in InGaAs quantum-well laser diodes in order to enhance the conversion efficiency. We have characterized the role of phase- mismatching in the spectral distribution of the internal second-harmonic generation in the CW operation of these lasers. The emission of pairs of narrow blue-green peaks having perfectly symmetrical spectral positions with respect to the central peak of pure second-harmonic generation at approximately 480 nm is most probably enhanced by a mechanism of reciprocal cancellation of the respective phase-mismatch vectors. This study is important for the assessment of the relationship between the structural parameters of the laser and the conditions which contribute to the stimulation of second-order optical nonlinearities in the laser active region.

Medium-power stabilized laser diode

Power stabilized DLs represent today convenient sources for radiometric applications, as transfer laboratory standards. Stability measurements were carried out and reported by other authors for different time intervals, but only for low optical power levels (max. 16 mW). For calibrating usual optical radiometers, such low emitted power DLs are useless. This paper reports stability measurements carried out on several collimated DLs with (lambda) around 980 nm and emitted power up to 265 mW in near-field/150 mW in far-field. Stabilities of the order 1e-4 for short-time intervals (tens of seconds) and 1e-3 for medium-time intervals (1 hour) were found for a non-thermostated structure, having the control photodiode (PD) in the same enclosure with the DL (at the rear of the structure). The corresponding stabilities for an external control PD resulted of the order 1e-5 for both short and medium time intervals. The schematic of the optical power stabilizer is presented.

Enhanced internal second harmonic generation in InGaAs/GaAs/AlGaAs strained single-quantum-well buried-heterostructure laser diodes

In this paper we report an indirect method based on photomultiplier response calibration to measure the radiant power of the internal second harmonic generation (ISHG) from InGaAs/GaAs/AlGaAs strained single quantum well buried heterostructure laser diodes. We observed enhanced ISHG radiant power, of the order of magnitude of 10-8 W. This phenomenon represents a signature of the beginning of the process of catastrophic optical degradation of the LD mirror facet layers, where the nonlinear optical interaction occurs.

Enhanced internal second harmonic generation in InGaAs/GaAs/AlGaAs strained SQW BH laser diodes

In this paper we report an indirect method based on photomultiplier response calibration to measure the radiant power of the internal second harmonic generation (ISHG) from InGaAs/GaAs/AlGaAs strained single quantum well buried heterostructure laser diodes. We observed enhanced ISHG radiant power, of the order of magnitude of 10-8 W. This phenomenon represents a signature of the beginning of the process of catastrophic optical degradation of the LD mirror facet layers, where the nonlinear optical interaction occurs.

COMPACT-SIZE ELECTRONIC DEVICE FOR DIODE-LASER CHARACTERISTIC CURVE TRACING

In order to monitor the voltage-current (V-I ) and opticalpower– current (P-I ) characteristic curves of pulsed diode lasers, we have designed, built, and tested a compact-size, low-cost electronic device with the function of both supplying periodic variable-amplitude current pulses to the diode laser and transmitting Y-X (either V-I or P-I ) signals to an ordinary oscilloscope. The paper describes the features of the device and the results of its testing by characterizing two sample InGaAs/AlGaAs diode lasers.