Optical waveguides – Planar optical waveguide
Reexamination Certificate
1999-01-27
2001-12-04
Healy, Brian (Department: 2874)
Optical waveguides
Planar optical waveguide
C385S014000, C385S031000, C385S037000, C385S130000, C385S131000
Reexamination Certificate
active
06327413
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to optoelectronic devices which include electrodes and optical waveguides as essential constituent elements and, more particularly, to an electrode structure of laser diodes and their application technology.
2. Description of Related Art
There is a DFB (distributed feedback) laser as one type of laser diodes. This laser diode has a diffraction grating in which a refractive index is changed periodically along the waveguide. The laser diode can oscillate in a single longitudinal mode (SLM) (i.e., one oscillation line) around the Bragg wavelength determined by the period of the diffraction grating. For
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. In this mesa electrode structure, in order to reduce a junction area, the p-side electrode
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must be formed on the top of a mesa stripe
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surrounding the active layer and also must be connected to a bonding pad
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on the thick SiO2 film
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, while taking account of step-recoverage.
As described above, in the laser diode in the related art, several stages of the epitaxial crystal growth by the MOCVD (Metal Organic Chemical Vapor Deposition) technique must be needed (growing steps (1) to (4) in the above related art), and also step-recovering processes, etc. must be needed to form the electrode.
Like the above, in the laser diode having an optical waveguide structure according to the related art, a number of manufacturing steps are needed since the device requires a complicated structure. Especially, there are problems such that its yield becomes lower and its cost increases higher because a plurality of delicate manufacturing procedures like the epitaxial crystal growth have reliability problems at regrowth interfaces. In addition, in the device structure in the related art, restrictions are imposed upon design of the optical waveguide and the electrode, and therefore it is difficult to improve the performance of the device according to the related art.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an optoelectronic device, and laser diode device, which are able to achieve improvement in yield of the optoelectronic device, which has an optical waveguide and improvement in parameters by simplifying a device structure.
As a preferred embodiment, an optoelectronic device that comprises an optical waveguide structure and electrodes, and the optical waveguide contained in the optical waveguide structure and at least a part of the electrode is formed of common material.
As described above, since the electrode structure and the optical waveguide mechanism are formed in common, the device can be simplified in configuration rather than the device structure in the related art. Accordingly, since the number of manufacturing steps can be reduced, especially steps of troublesome complicated epitaxial crystal growth, etc. can be reduced, yield of the device can be greatly improved and thus cost of the device can be effectively reduced.
In the related art, the waveguide performance is affected only by crystal properties and the electrode performance only by metal properties respectively. However, margin in design of the optical waveguide and the electrode can be enhanced if the transparent electrode and the optical waveguide are formed in common like the present invention, so that improvement in the performances can be achieved further.
The transparent electrode can be formed directly on the corrugated surface of an optical waveguide layer, and play a part of waveguiding role. Also, it becomes easier to produce grating function by making the distribution of refractive index, waveguide loss, or conductivity in the transparent electrode to be periodic along a waveguide direction of the optical waveguide. Also, the distribution of refractive index, waveguide loss, or conductivity in the transparent electrode or periodicity of the diffraction grating is set to produce Bragg diffraction of second order or more relative to the guided light. Also, there is provided dielectric substances or semiconductors, which are formed under the transparent electrode to be aligned periodically along a direction of the optical waveguide and has refractive index, optical loss, or gain being different from the transparent electrode. Also, underlying material of the transparent electrode is formed of material of an upper cladding layer or an optical waveguide layer of a laser diode. Also, a shape of the impurity layer or the metal layer between the transparent electrode and the underlying waveguide material is formed differently along an axis direction or a lateral direction. Also, an area of the transparent electrode is covered with opaque electrode except for a window area from which an optical output is emitted. Also, another change in optical properties can be controlled to giving a phase shifting effect to the original periodic structure. As a result, the DFB laser can be realized at simple steps rather than the device structure in the related art.
In the event that the periodical structure of refractive index is formed in the transparent electrode in itself, the expensive semiconductor wafer can be reused by re-forming the transparent electrode portion. Also, crystal properties are damaged if the dry etching is applied to the semiconductor crystal, nevertheless the transparent electrode is less damaged by the dry etching because it is not formed of a single crystal. Further, such periodical structure of refractive index formed in the transparent electrode is suited for mass production.
As a preferred embodiment, the transparent electrode are formed of ITO (Indium Tin Oxide), the refractive index of the ITO is controlled by an amount of oxygen contained in the ITO, and conductivity and insulating characteristic of the ITO is controlled by composition of tin contained in the ITO. Therefore, optical parameters of the transparent electrode such as refractive index can be controlled by using such material, method, etc. set forth in the invention. In addition, the control range of the transparent electrode can be set wider than the semiconductor crystal to thus allow larger margin in electrode design.
As a preferred embodiment, if the transparent electrode is employed in combination with insulating material or absorbing material, a gain/loss coupling DFB laser can be fabricated more easily.
Also, the distribution of refractive index, waveguide loss, or periodicity of the diffraction grating is set to produce Bragg scattering of second order or more relative to a waveguide light. Also, an area of the transparent electrode except for a window area from which an optical output is emitted is covered with opaque electrode according to the related art. Therefore, if the transparent electrode is applied to GCSEL employing a second-order diffraction grating, the output can be emitted more effectively because the transparent electrode can perform a role as a window for surface radiation output.
The optical properties include refractive index, waveguide loss, or conductivity. And also, the optical properties include structural properties such as thickness of waveguide or width thereof. Furthermore, the optical properties include the change in period of periodical structure, gain of underlying layer, non-linear optical coefficient when the waveguide is formed of LiNbO3.
If a sectional shape of the optical waveguide is set to a semicircle or a semiellipse, this area can function as a cylindrical lens. Therefore, a wider beam divergence angle of the radiation mode output in a direction along the waveguide width can be narrowed.
If the device may be constructed such that another change in the refractive index of the transparent electrode provides an effective phase shift to the underlying periodic structure, the phase shift can be realized by adjusting only the electrode.
The DFB lasers can be arranged in an array fashion such that they can oscillate at different wavelengths as a WDM (wavelength division multiplex) light source. In other words, a WDM integrated DFB laser array can be obtained only by working
Healy Brian
Hogan & Hartson L.L.P.
Kabushiki Kaisha Toshiba
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