J. Hegarty

Two-photon-induced photoconductivity enhancement in semiconductor microcavities: a theoretical investigation

We describe a detailed theoretical investigation of two-photon absorption photoconductivity in semiconductor microcavities. We show that high enhancement (by a factor of >10,000) of the nonlinear response can be obtained as a result of the microcavity effect. We discuss in detail the design and performance (dynamic range, speed) of such a device with the help of the example of an AlGaAs/GaAs microcavity operating at 900 nm. This device shows promise for low-intensity, fast autocorrelation and demultiplexing applications.

H2S and CO2 gas sensing using DFB laser diodes emitting at 1.57 μm

Using wavelength-modulation spectroscopy and harmonic detection, we demonstrate, for the first time, direct optical detection of H 2 S by probing the v 1 + v 2 + v 3 combination absorption band centred at a wavelength λ = 1.59 μm using a distributed feedback (DFB) laser diode. A detection limit of less than 10 ppm at atmospheric pressure over a 5 m path length for the system is inferred from the measured detectivity at higher H 2 S concentrations. This detection sensitivity is at a level below the accepted 10 ppm safe-exposure limit. In addition, we demonstrate the capability of DFB laser diodes for multiple gas sensing in the λ = 1.575 μm region with the same laser by probing the 2v 1 + 2v 2 + v 3 absorption band of CO 2 , achieving a detection limit of about 100 ppm. These results demonstrate the utility of near-infrared laser diodes for industrial applications in the area of toxic- and trace-gas monitoring.

Two-photon absorption photocurrent enhancement in bulk AlGaAs semiconductor microcavities

We report on two-photon absorption (TPA) photocurrent in semiconductor microcavities. We experimentally show a substantial increase in the TPA photocurrent generated, at resonance, in a GaAlAs/GaAs microcavity designed for TPA operation at ∼890 nm. An enhancement factor of ∼12u200a000 of the photocurrent is obtained via the microcavity effect, which could have an important impact on the use of TPA devices for high speed switching and sampling applications. Our results also show the implications of the cavity photon lifetime on autocorrelation traces measured using TPA in semiconductor microcavities.

All-optical timing extraction with frequency division using a twin-section laser diode

All-optical timing extraction from a coded line signal with division of the extracted clock frequency is demonstrated using a single twin-section self-pulsating laser diode. Either the bit-rate clock or a submultiple of the bit-rate clock can be extracted, depending only on the DC biases applied to the self-pulsating laser diode. Timing extraction is carried out for the first time without optical injection locking of the self-pulsating laser diode by the optical data source.<<ETX>>

A sampled grating distributed Bragg reflector laser diode for spectroscopic based multi-gas detection at 1.5 /spl mu/m

Summary form only given. We report the first use of a sampled grating distributed Bragg reflector (SGDBR) widely tunable laser (/spl Delta//spl lambda/=50 nm) to laser based gas detection. The laser was used to simultaneously measure the concentration of several gas species with widely separated absorption features. Such wide wavelength scanning is not possible with conventional DFB and DBR lasers and is in high demand for tunable laser diode absorption spectroscopy trace gas detection. The SGDBR consists of four independently biased sections; a front and back sampled grating mirror, a phase section and a gain section The device can access wavelengths over the range 1520 nm/spl les//spl lambda//spl les/1570 nm while maintaining a side mode suppression of >30 dB. By holding one grating at fixed bias and varying the refractive index of the other grating discontinuous tuning is achieved. Continuous tuning is obtained by simultaneously changing the refractive index in the two gratings. This combination of discontinuous and continuous tuning gives full wavelength coverage over 30 nm.

Temperature Dependence of the Self-Pulsating Characteristics of Compact Disk Laser Diodes

Extensive research has teen performed, with varying SUECCS. to develop red l a e r diodes which exhibit strong self-pulsation over an extended temperature range (0 80 C) for optical storage applications. As an approach to undersianding this problem we havs imertipated the dependence of self-pulsation characteristics in commercial CD laser diodes emitting at >. = 780 nm. when operated between I = 77 K ilnd T = 310 K. Self-pulsation in these devices was found to extinguish below about I = 100 K. Measuremmts of the ariation of self-pulsation frequency (vsp) with bias current in the temperature range 150 K to 3 I O K (Fig. 1) showed a more sensitive dependence on bias current as the hearsink temperature decreases. When this data was viewed (Fig. 2) in the more conventional form, vSp Yersus normalised bias current (I& 1). it i s clear that thc slope of these curves are not strongly temperature dependent. These results are interesting since it i s cammonlt assumed that the selfpulsation frequency in CD laser sttuctues occurs at the laser diode modulation resonance frequency (fin) u,hich predicts a quadratic dependence of normalised bias current on QR and a temperature dependence of the slope due 10 the well know tempermure depcndence of carrier life. neither o f which are apparent.

Optical Gain in ZnCdSe-ZnSe Quantum Well Structures

We have measured the gain spectrum of an optically pumped 40A ZnCdSe-ZnSe multiple quantum well. Our calculation, which includes many body effects such as Coulomb enhancement and spectral broadening due to carrier scattering, gives excellent agreement with the experimental gain measurements. We then show the importance of the inclusion of the Coulomb enhancement for the calculation of optical gain when predicting laser threshold currents. This is emphasised by using our gain calculation as a basis to theoretically optimise a simple ZnCdSe-ZnSe quantum well laser structure incorporating the leakage current over the p-type cladding.