Explosive growth in the internet traffic makes developments of integrated photonic devices important issue to actualize the next generation photonic systems. Enormous number of devices that will be needed in the systems implies that integrated devices are indispensable components to make the system with a reasonable cost and space. Starting from the simple integrated device containing a DFB laser and an EA-modulator, various kind of integrated photonic devices have been developed with hybrid and monolithic technologies. This paper reviews recent progress of semiconductor integrated light sources for DWDM communications systems.

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Our recent progress in wavelength-selectable light sources (WSLs) is reviewed. Our developed WSLs basically consist of a monolithically integrated DFB laser array, multi-mode interference optical coupler, and semiconductor optical amplifier. Device characteristics of two types WSLs, one of them integrated with an electro-absorption modulator, are described.

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Pump sources for optical fiber amplifiers, e.g., Erbium-doped fiber amplifiers or Raman amplifiers, are reviewed. Higher optical power is required for the optical fiber amplifiers in DWDM transmission. Recently published results regarding improvements of the optical power are introduced.

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Modulator integrated lightsources are among the essential components of today's DWDM systems because of their compactness, high-speed operation, low frequency chirp and high cost efficiency. A recent progress in a R&D on modulator integrated lightsources is reviewed focussing on the technical issues and approaches.

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The tunable mode-locked laser based on time-to-wavelength mapping is a relatively unexplored alternative to tunable sources that can be applied to transmission and wavelength switching in optical networks. These lasers have the advantages of a simple tuning mechanism, good wavelength reproducibility, and simple cavity design. The state of research is discussed.

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We generated 100-1000 channel optical carriers from a single supercontinuum source and confirmed that these optical carriers have a sufficient SNR to achieve Q-factors of more than 18 dB. The channel spacing is strictly determined by the microwave mode-locking frequency with accuracy in kilohertz. The generated optical carriers can be applied to DWDM systems and optical frequency standards.

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Light emission in high-power laser welding has been detected by a multi-photosensor system. This system can discriminate between the light intensities emitted from the plume, keyhole opening, and finger parts of a keyhole. Bead-on-plate welding of thick plates was performed with a 20 kW CO₂ laser at different laser powers and welding speeds. The results showed poor correlation between time-averaged light emission and experimental penetration depth. This is because the light emission from the keyhole opening is much larger than that from the finger part. The light emission's power spectrum indicated that light emission from the finger part was higher than 200 Hz, whereas the light emission from the plume and keyhole opening was less than 200 Hz. A simple formula has been derived to predict the penetration depth on the basis of frequency analysis, and this prediction agrees well with the experimental penetration depth at various laser powers and welding speeds.

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