Reduction of the intensity noise in semiconductor lasers is an important subject for the higher performance of an application. Simultaneous usage of the superposition of high frequency current and the electric negative feedback loop was proposed to suppress the noise for the higher power operation of semiconductor lasers. Effective noise reduction of more than 25 dB with 80 mW operation was experimentally demonstrated.

Reduction of the intensity noise in semiconductor lasers is important subject to extend application range of the device. Blue-violet InGaN laser reveals high quantum noise when the laser is operated with low output power. The authors proposed a new scheme of noise reduction both for the optical feedback noise and the quantum noise by applying electric feedback which is positive type at a high frequency and negative type for lower frequency range. Noise reduction effect down to a level lower than the quantum noise was experimentally confirmed even under the optical feedback.

Properties of the quantum noise and the optical feedback noise in blue-violet InGaN semiconductor lasers were measured in detail. We confirmed that the quantum noise in the blue-violet laser becomes higher than that in the near-infrared laser. This property is an intrinsic property basing on principle of the quantum mechanics, and is severe subject to apply the laser for optical disk with the small consuming power. The feedback noise was classified into two types of "low frequency type" and "flat type" basing on frequency spectrum of the noise. This classification was the same as that in the near infra-red lasers.

As an estimating method of the threshold current level on experimental data of the semiconductor injection laser, finding peak position on second order derivative of I-L (injection current vs. light output) characteristics is popularly in use. A theoretical proof of this method is given in this letter.

Operation of a newly proposed semiconductor optical modulator based on absorption control by electron depletion around a p-n junction is demonstrated, forming preliminary structures of waveguide-type as well as panel-type (or surface-illuminated type) devices. The optical absorption is occurred at the intrinsic energy levels in the band structure not at the extended state into the band-gap. Performance of 35 dB on-off extinction ratio for 4 V variation of the applied voltage was obtained in a waveguide type device. Validity of the proposed mechanism were confirmed by getting large change of the absorption coefficient of around 5000 cm-1 over wide wavelength range of 30 nm.