Kenichi Kasahara

Detuning characteristics of the linewidth enhancement factor of a midinfrared quantum cascade laser

The linewidth enhancement factor of a 5.2-mum distributed-feedback quantum cascade laser above the threshold current caused by detuning from the gain peak was investigated. We clearly observed different values for the linewidth enhancement factor.

Multiplication Noise Characterization of InAlAs-APD With Heterojunction

The excess noise and ionization coefficient ratio of InAlAs avalanche photodiodes (APDs) with heterojunctions has been measured. To obtain these values accurately, we used a differential amplifier and determined the optical current at which the avalanche multiplication equals 1. These made it possible to conduct measurements on excess noise including that in the low multiplication region. Measured ionization coefficient ratios at M=10 were 0.18 and 0.17 with multiplication layer widths of 0.2 and 0.7 ¿m , respectively. Excess noise was reduced with an increase in the optical input power. This tendency appeared more prominently in APDs with a thin multiplication layer.

Theoretical and Experimental Study on Waveguide Avalanche Photodiodes With an Undepleted Absorption Layer for 25-Gb/s Operation

Waveguide avalanche photodiodes (WG-APDs) have been developed for a 25-Gb/s operation. The waveguide structure was adopted for the APDs in order to achieve both high-speed performance and high responsivity. First, the dependence of the multiplication and absorption layer thickness on the 3-dB bandwidths of the WG-APDs was theoretically clarified. It was found that introducing an undepleted absorption layer was effective in improving the 3-dB bandwidth without reducing efficiency. The 3-dB bandwidth of the fabricated WG-APD based on the calculation results was over 20 GHz up to a multiplication factor of 7. The responsivities were 0.6 A/W for 1.31 m and 0.75 A/W for 1.55 m wavelength. High reliability of the WG-APDs was also demonstrated.

Impact of Two Competing Energy Transfer Mechanisms on Spectra in Organic Light Emitting Diodes

We have investigated the emission spectra and intensity of a-NPD/Alq3 organic light emitting diodes. It is shown that they are offspring affected by the plasmon absorption/scattering and Forster energy transfer.

First direct observation of self-imaging effect in active multimode-interference semiconductor laser diodes

We report the first direct observation of the self-imaging effect in active multimode-interference semiconductor laser diodes (MMI-SLDs). Interference patterns inside laser diode waveguides were obtained by using the electroluminescence (EL) method. To the best of our knowledge, this result is the first direct observation of the self-imaging effect in self-photon-emitting active waveguides such as laser diodes. The observed EL pattern confirms the adequateness of the design of the higher light output-aimed MMI-SLDs.

Influence of anisotropic Raman gain on four-wave mixing signals

Gain of Raman-amplified four-wave mixing (FWM) signals in optical fibers was found to be different from less than the third power of the signal gain at and near spectral positions of local minima when the spacing of two input signals is a few nm. The gain there was enhanced which was unaccountable with theoretical analyses. It is likely that wavelength regions corresponding to dips are sensitive to the distribution of local Raman gain and that increased Raman gain for FWM signals was caused by anisotropic Raman gain over a fiber.

Small optical feedback-induced noise of 6.1-μm quantum cascade lasers

Relative intensity noise of a quantum cascade laser under optical feedback was first evaluated. Noise smaller than that of a conventional laser diode was due to a small line enhancement factor and long cavity length.

Trap-states Influence on Transient Electroluminescence of CBP:Ir(ppy)3-based Organic Light Emitting Diodes

The overshoot of CBP:Ir(ppy)3-based phosphorescent organic light emitting diodes was large under a negative bias voltage, different from previously reported results. This was probably caused by trap states formed in carrier-injecting organic layers.

Analytical Model for Obtaining the Ionization Rate Ratio of Mesa InAlAs Avalanche Photodiodes

We proposed a new analytical model to extract the effective ionization rate ratio k from the measured multiplication noise of Avalanche photodiodes (APDs). In this model, k was expressed as a function of the multiplication factor M. We extracted the effective k of the mesa-structured InAlAs APDs using this model. A measurement system that consisted of a low-noise differential preamplifier was developed for this purpose, leading to the measurements of the noise data, particularly in the region where it was small. APD noise data could be fitted to the curve based on this new analytical model better than with other analytical models over the wide range of M. The effective k in the region where M was small became appropriate using this model. The quantum efficiency of the APDs was also found to be reasonable compared with the results obtained using the other analytical models.

Linewidth enhancement factor of 6.1-µm quantum cascade laser at near room-temperature

The linewidth enhancement factor (α) of a quantum cascade laser near room-temperature and its current dependence were investigated. The α, probably affected by the detuning, was still smaller than that of conventional semiconductor laser diodes.