Semiconductor device manufacturing: process – Making device or circuit emissive of nonelectrical signal – Including integrally formed optical element
Reexamination Certificate
1993-12-21
2001-02-27
Bowers, Charles (Department: 2813)
Semiconductor device manufacturing: process
Making device or circuit emissive of nonelectrical signal
Including integrally formed optical element
C438S029000, C385S014000, C385S130000
Reexamination Certificate
active
06194240
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method for the preparation of DFB/DBR lasers including electro-optic gratings. More particularly, the present invention relates to a method for the preparation of a wavelength selective electro-optic grating for DFB/DBR lasers and related structures wherein etched corrugation wavelength gratings are replaced with a grating formed by periodically modulating the index of refraction of the cladding employed or the waveguide itself.
BACKGROUND OF THE INVENTION
During the past decade, rapid advances have been made in laser technology. These advances have led to the integration of lasers with other active optical elements such as detectors, optical amplifiers, optical modulators and switches on one semiconductor chip. Interconnections employed for this purpose are miniature, transparent, passive waveguides which pipe light from one device to the next and function in a manner similar to metals which carry electricity in conventional circuits. Further enhancements in this technology have been attained from advances in crystal growth and large area wafer processing.
Among the more recent developments in photonic integrated circuitry has been the corrugated-waveguide grating. The fabrication of these gratings became routine after the distributed feedback (DFB) laser was adopted worldwide as the preferred optical telecommunications source where high spectral purity is required. These gratings provide high quality on-chip resonators without encountering the restraints imposed by the usual cleaved-facet resonators employed in semiconductor lasers. Additionally, the gratings function as filters in certain receiver and amplifier applications.
Scanning electron micrographs in distributed feedback lasers reveal a corrugated interface between a higher index core material and a lower index cladding material. As a light beam propagates down such a waveguide, each bump reflects a small portion of the light. If the beam's wavelength is not close to the Bragg wavelength, all the reflections are out of phase and interfere destructively. At the Bragg wavelength, all reflections add in phase, so leading to a large cumulative reflection.
At the present time, the control of output wavelength of semiconductor lasers in distributed feedback (DFB) and tuning in distributed Bragg reflector (DBR) based lasers is achieved by means of the incorporation therein of the above-noted corrugated waveguide grating. In the DBR configuration, gratings of this type are typically installed by etching back a portion of the active layer and exposing the surface of an underlying passive waveguide. Alternatively, the grating may be installed in the substrate prior to the growth of the waveguide. In subsequent processing, an interference pattern formed by ultraviolet lasers is typically used to expose photoresist deposited on the exposed waveguide. The developed photoresist then forms the mask for etching the grating corrugation. The exposed portion of the structure is then regrown and contacts attached for operation. In the DFB configuration, the grating is usually installed prior to growth of the active layer or waveguide. This end is typically attained by using an interference pattern, as described above, to expose photoresist deposited on the exposed passive waveguide. The exposed photoresist forms a mask for etching the corrugation. The active layer is then grown over the grating and suitable contacts are added to effect operation.
SUMMARY OF THE INVENTION
In accordance with the present invention, the etched corrugation waveguide grating in the aforementioned structures is replaced with a grating formed by periodically modulating the index of refraction of either the cladding material or the waveguide. The modulated media chosen may be either bulk material or, more specifically, contain quantum well structures. In the case of the latter, it would effect large changes in the index of refraction by means of the quantum confined Stark effect or phase space absorption quenching with the application of an electric field. In the case of bulk material, current injection is the preferred mode of operation since the field induced change in the refractive index (Franz-Keldysh effect) is less efficient. The periodicity of the described device is established by means of conducting media, for example, metal, conductive polymer, etc. The conductive media is employed in the form of strip lines through which current is passed or an electric field is established through the media. Alternatively, periodicity may be established simply by the plasmon interaction of the periodically spaced conductive strip lines with the media.
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Chiu Tien-Heng
Williams Michael D.
Bowers Charles
Kielin Erik
Lucent Technologies - Inc.
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