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We report the generation of high-purity correlated photon-pairs and polarization entanglement in a 1.5 μm telecommunication wavelength-band using cascaded χ((2)):χ((2)) processes, second-harmonic generation (SHG) and the following spontaneous parametric down conversion (SPDC), in a periodically poled LiNbO(3) (PPLN) ridge-waveguide device. By using a PPLN module with 600%/W of the SHG efficiency, we have achieved a coincidence-to-accidental ratio (CAR) higher than 4000 at 7.45×10(-5) of the mean number of the photon-pair per pulse. We also demonstrated that the maximum reach of the CAR was truly dark-count-limited by the single-photon detectors used here. This indicates that the fake (noise) photons were negligibly small in this system, even though the photon-pairs, the Raman noise photons, and the pump photons were in the same wavelength band. Polarization entangled photon pairs were also generated by constructing a Sagnac-loop-type interferometer which included the PPLN module and an optical phase-difference compensator to observe maximum entanglement. We achieved two-photon interference visibilities of 99.6% in the H/V basis and 98.7% in the diagonal basis. The peak coincidence count rate was approximately 50 counts per second at 10(-3) of the mean number of the photon-pair per pulse.

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We report what is to our knowledge the first demonstration of terahertz-rate optical pulse generation by harmonic passive mode locking in a distributed-Bragg-reflector laser diode. Along with the fundamental repetition rate of 38.8 GHz, we observed 400-GHz, 800-GHz, and 1.54-THz harmonics, depending on the bias condition of gain section. The pulse envelope for 1.54-THz pulses was in good agreement with a calculation from the Fourier transformation of the optical spectrum, indicating that the output pulses are transform limited.

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The fluorescence spectra ranging from 400 to 1000 nm were investigated on highly Er(3+) -doped fiber pumped by a 1.48-microm laser diode. A strong green emission was observed. An investigation of the fluorescence intensity dependence on pump power revealed that the green emission is attributed to successive excited-state absorption through a three-step process and also that saturation of the atomic levels that contribute to the emission occurs.

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