Nature of the rain spectrum at a low latitude coastal station

The internal structure of rain falls in terms of rain rate during various instances of time can be determined by measuring rain amounts received during very short intervals of time. A system that can record the intensity of rain fall at every minute has been used for this study.

Two-photon absorption and self-phase modulation in InGaAsP/InP multi-quantum-well waveguides

We report the first measurement of two-photon absorption (TPA) and self-phase modulation in an InGaAsP/InP multi-quantum-well waveguide. The TPA coefficient, β2, was found to be 60±10 cm/GW at 1.55 μm. Despite operating at 200 nm from the band edge, self-phase modulation as high as 8±2 rad was observed for 30-ps optical pulses at 3.8-W peak input power. A theoretical calculation indicates that this enhanced phase modulation is primarily due to bandfilling in the quantum wells and the free-carrier plasma effect.

First demonstration of two photon absorption in a semiconductor waveguide pumped by a diode laser

For the first time, lasers have been used to induce a fast all-optical nonresonant nonlinearity at wavelengths well beyond the band edge in a GaAs/GaAlAs multiquantum well waveguide. Using a Q-switched diode laser, which gave optical pulses of 3.5 ps duration and 7 W peak power, an intensity-dependent transmission was recorded that was consistent with the presence of two photon absorption in the waveguide. The measured two photon absorption coefficient was 11 ± 2cm/GW.

Bias-dependent recovery time of all-optical resonant nonlinearity in InGaAsP/InGaAsP MQW waveguide

We have investigated a resonant refractive nonlinearity in a semiconductor waveguide by measuring intensity dependent phase shifts and bias-dependent recovery times. The measurements were performed on an optimized 750-μm-long AR coated buried heterostructure MQW p-i-n waveguide with a bandedge at 1.48 μm. Figure 1 shows the experimental arrangement. The mode-locked color center laser was tuned to 50 meV beyond the bandedge and 8 ps pulses with peak incident power up to 57 W were coupled into the waveguide. Some residual bandtail absorption remains at this wavelength and this is sufficient to cause carriers to be photogenerated and these give rise to a refractive nonlinearity, predominantly by plasma and bandfilling effects. A Fabry-Perot interferometer is used to measure the spectrum of the light which exits the waveguide. The nonlinearity within the guide causes self phase modulation (SPM) of the light and a study of the spectrum allows information to be recovered on the magnitude and recovery time of the nonlinear phase shift with a reasonable degree of accuracy. SPM spectra were recorded for a variety of pulse energies coupled into he unbiased waveguide. Figure 2 shows the resultant phase shift measured from the SPM spectra as a function of pulse energy. The relationship is a linear one, indicating that no saturation of the nonlinearity occurs for coupled pulse energies up to 230 pJ. A π phase shift, the minimum necessary for an all-optical switch, is obtained for a coupled pulse energy of 57 pJ while the maximum phase shift, 4 π, was measured for 230 pJ. The SPM spectra were highly asymmetric with pulse energy shifted to higher frequencies. Such spectra are characteristic of a slow, negative nonlinearity. This relatively slow speed is expected for the unbiased guide as the recovery time will be of the order of the recombination time of the photogenerated electrons, about 1 ns for InGaAsP material. In order to reduce the recovery time of the nonlinearity, it is necessary to remove the photogenerated carriers from the waveguide by a process other than recombination. One such technique is to apply a reverse bias to the waveguide in order to sweep the carriers out. Figure 3 shows the effect on the recovery time of the nonlinearity of applying reverse bias to the waveguide for 230 pJ coupled power. The recovery time was reduced from one much longer than the length of the pulse, estimated to be about 1 ns, at zero bias to 18 ± 3 ps for a bias voltage greater than -4 V. This compares with a value of 24 ps obtained in a bulk waveguide.

Compositional analysis of thin film amorphous semiconductors and insulators using LIMA

LIMA (Laser-induced Ion Mass Analysis) is a new technique capable of compositional analysis of thin films and surface regions. Under UHV conditions a focused laser beam evaporates and ionizes a microvolume of specimen material from which a mass spectrum is obtained. LIMA has been used to examine a range of thin film materials with applications in electronic devices. The neutral photon probe avoids charging problems, and low conductivity materials are examined without prior metallization. Analyses of insulating silicon oxides, nitrides, and oxynitrides confirm estimates of composition from infrared measurements. However, the hydrogen content of hydrogenated amorphous silicon (a-Si : H) found by LIMA shows no correlation with values given by infrared absorption analysis. Explanations are proposed and discussed. © 1985.