Nec Corporation

Yoshiharu Muroya

Takao Morimoto

In a distributed feedback type semiconductor laser device having a waveguide structure for outputting a signal light from a front side of the waveguide structure and a monitoring light from a rear side of the waveguide structure, an active layer is formed above a semiconductor substrate, and an optical guide layer having a diffraction grating is provided. A coupling coefficient of the waveguide structure is gradually increased from the front side of the waveguide structure to the rear side of the waveguide structure.

Distributed feedback type semiconductor laser device having gradually-changed coupling coefficient

In exposing a diffraction grating pattern on a resist, the diffraction grating pattern is exposed via EB on an active region and the adjacent thereto only and the region which is not exposed via EB is exposed via Deep UV light so that the resist may be left on the active region and the adjacent thereto only after the exposed resist is developed according to the process of forming a diffraction grating for a distributed feedback semiconductor laser of the present invention. In addition, the resist-coated area can be gradually reduced from the resist existing area to the non-resist existing area and the average height of the substrate on which said diffraction grating is formed can be gradually changed on the diffraction grating forming region and non-diffraction grating forming region which results in preventing the crystallinity of a semiconductor layer on the substrate from being deteriorated. This enables formation of an excellent crystallinity semiconductor layer on the substrate on which the diffraction grating is locally formed.

Method for manufacturing diffraction grating and method for manufacturing semiconductor laser

A stable single-mode oscillation semiconductor laser is obtained by flattening the distribution of electric field intensity in the direction of the resonator axis, and to provide a distributed-feedback semiconductor laser generator having a more satisfactory difference in threshold gain between the principal and subsidiary modes. The laser includes an active layer which radiates light as a result of the injection of electrons, a light guide layer which is next to this active layer guides light emitted from the active layer, a plurality of semiconductor layers between which the active layer and light guide layer are interposed, electrodes which permit electrons to be injected from these semiconductor layers into the active layer, a non-reflective coating provided on both ends of the active layer in the direction of oscillation, and a diffraction grating which is provided on the light guide layer for selecting the oscillatory wavelength of the light. The diffraction grating has a plurality of uniform pitch areas formed with a uniform and equal pitch, and a varying pitch area interposed between the uniform pitch areas and having a plurality of pitches longer and shorter than those of these uniform pitch areas, the latter serving to select the oscillatory wavelength of the light, where the absolute value of the sum-total of the amount of phase change in the varying pitch area (cumulative amount of phase change) in relation to the phase in the uniform pitch areas is between 3/2 and 11/2 times the pitch of the diffraction grating of the uniform pitch areas.

Distributed-feedback semiconductor laser

A method of forming a micro dot pattern by using an electron beam exposure apparatus which sets a minimum unit moving distance of an electron beam smaller than an electron beam diameter includes the steps of defining one of lattice points formed for every minimum unit moving distance of the electron beam as a reference position, and irradiating the electron beam on at least two lattice points within an area separated from the reference position by a distance smaller than the electron beam diameter in an equal exposure amount or different exposure amounts to form a pattern having a center at an exposure peak position in a sum of the plurality of exposure amounts, thereby forming the micro dot pattern having a center at a position other than the lattice points.

Method of forming micropattern

A method for fabricating an InP diffraction grating for a distributed feedback semiconductor laser includes the steps of applying an electron beam resist on a semiconductor substrate, giving electron beam exposure to the electron beam resist and controlling heights of resist patterns by using fixed electron beam diameters but by varying incident electron doses. The semiconductor substrate is dry-etched. The electron beam exposure is such that the incident electron doses are made larger at a center portion than at portions towards two sides of the diffraction grating. Due to the proximity effect, the resist patterns after development will have a lower height and a narrower width at portions at which the incident electron doses are increased and, conversely, a higher height and a wider width at portions at which the incident electron doses are decreased. In a method of fabricating a distributed feedback laser using a substrate of the InP diffraction grating fabricated as above, the method includes the step of sequentially growing on the substrate a waveguide layer, an active layer and a cladding layer. The method enables to fabricate a low distortion distributed feedback laser for analog modulation having non-uniform diffraction gratings.

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Tokyo, Japan

Yoshiharu Muroya