Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
1999-04-20
2001-12-11
Leung, Quyen (Department: 2881)
Coherent light generators
Particular active media
Semiconductor
C372S096000
Reexamination Certificate
active
06330265
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to various kinds of novel optical functional elements and optical transmission and receiving devices based on waveguide means having holograms which generate radiation modes.
More specifically, the invention relates to a distributed feedback (DFB) laser emitting radiation-mode light normal to the substrate surface and its manufacturing method.
The invention also relates to a waveguide type optical isolator, distributed feedback laser being compact, high in directional selectivity and good in optical coupling, and a monolithically integrated optical element containing them.
The invention further relates to optical functional elements such as optical amplifier, optical modulator, laser oscillator, and so forth, having a resonator means, for high-efficiency amplification of radiation modes emitted from the waveguide means, and optical transmission and receiving devices using them.
Optical functional elements having a waveguide for guiding light waves involve semiconductor lasers. Conventional semiconductor lasers utilize a single waveguide structure as a resonator. A semiconductor optical amplifier (SOA) similarly has a single waveguide structure. A grating-coupled surface-emitting laser (GCSEL) also utilizes a single waveguide structure sometimes with a single vertical reflector placed on the substrate side for recycling the radiation mode emitted towards substrate by changing its direction toward surface. That is, any of conventional optical functional elements was based on the concept of one resonator means utilizing one-way feedback.
Since this conventional concept is a common knowledge, here is omitted detail explanation on structures of conventional optical functional element with reference to drawings. A drawback of these conventional techniques lies in the lack of extensibility and flexibility because of using only one resonator means.
SUMMARY OF THE INVENTION
The present invention jumps far from the conventional concept. The invention employs a dual reflector mechanism with a waveguide generating a radiation mode in order to realize novel DFB lasers. More specifically, the invention disclosed herein provides a DFB laser comprising waveguide means having a hologram (
10
) capable of emitting radiation mode light in upper and lower directions; first reflection means (
2
) provided below said waveguide means for returning said radiation mode light back to said waveguide means; and second reflection means (
21
A,
21
B,
21
C,
21
D,
21
E) provided above said waveguide means for returning said radiation mode light back to said waveguide means, and means controlling the intensity profile of said radiation mode light along said waveguide means being.
The present invention also employs a holograms asymmetrical with respect to the light traveling direction, with reflector and/or optical amplifier in order to realize a novel optical isolator. More specifically, the present invention provides an optical isolator comprising: waveguide means (
104
) having a second-order or higher-order corrugations as holographic means (
110
,
110
′,
110
″) whose cross-sectional configuration is asymmetric with respect to the waveguide direction, selectively affecting each of radiation modes emitted towards the opposite (upper and lower) directions of said waveguide means, provided with different reflection means on the opposite sides of said waveguide means (
104
).
The present invention also employs a further advanced concept that another resonator structure is made in a different direction along the conventional resonator structure or a waveguide structure in order to realize various kinds of novel optical functional elements. More specifically, the present invention provides an optical functional element comprising: first waveguide means which guides light waves and has a gain or a loss, said waveguide means having a hologram (
202
) capable of coupling with said light waves guided by said waveguide means to generate radiation mode light; amplifier means (
203
) for amplifying said radiation mode light released from said first waveguide means and releasing it; and first reflection means (
204
) for reflecting said radiation mode light emitted from said amplifier means back to said first waveguide means.
The above-mentioned every aspect of the invention is based on the concept of making a resonator structure of a radiation mode radiated from a waveguide having a hologram. In the present specification, the term “hologram” is defined as a “periodic structure of a complex index of refraction or complex index of reflection (the part of imaginary numbers indicates a loss/gain) capable of generating a spatially controlled radiation mode”. “2nd- or higher-order diffraction gratings” employed in embodiments described below are not but some modes of “holograms” because a diffraction grating (corrugation) is a structure based on a periodic structure with a simplest refractive index in which, mainly, the part of real numbers is uniform. Thus being easy to understand, examples using diffraction gratings are taken to progress explanation.
On the other hand, as explained later in greater detail, delicately changing the period of diffraction gratings in the waveguide direction and making a phase shift, as well, are important embodiments of the invention. To collectively categorizing these modifications, the “hologram” is as defined above.
A feature of the invention is to provide a high-reflectivity structure on the side opposite from a substrate in addition to that conventionally provided only on the side nearer to the substrate in a DFB laser having 2nd-order diffraction gratings. Reflecting mirrors provided above and below a waveguide can control the radiation mode by the 2nd-order diffraction gratings. Additionally, the reflectance of each reflector, including the presence or absence of reflectors, is changed in the cavity longitudinal direction or in the waveguide transverse direction.
Also in a surface-emitting DFB laser, if radiation mode light is taken out only from an area with larger radiation mode intensities and not from the entirety of the resonator, a part of the waveguide outside a light take-out window region is sandwiched by high-reflectivity structures.
With the construction shown above, since the high-reflectivity structure is provided also above the waveguide, the radiation mode caused by the 2nd-order diffraction gratings does not go out externally. Therefore, the loss of the radiation mode can be reduced, and an increase in threshold value can be prevented. The use of the 2nd-order diffraction gratings also facilitates its fabrication.
The phase shift provided in the 2nd-order diffraction gratings enables control of spatial distribution of light and carriers. That is, if the reflection at both facets of the laser is small and coupling of diffraction gratings and the guided mode is large to a certain extent, an intensity profile making both the guided mode and radiation mode more intensive by the central phase shift is obtained, and by making a gain/loss profile along the optical axis direction of the cavity in this manner, unevenness of the carrier density can be compensated to prevent spatial holeburning
Moreover, the invention increases the flexibility of controlling the intensity of the guided mode in the axial or transverse direction by making an appropriate gain/loss spatial profile without changing the structure of the active layer itself or changing the electrode structure. As a result, asymmetry of edge outputs and a new gain-guided structure can be provided.
When an output window and a high-reflectivity structure are provided above the waveguide, the radiation mode returns to the waveguide. Therefore, the radiation loss is minimized, and a surface-emission type laser is prevented from increasing in threshold value.
When a high-reflectivity structure is provided only above the front facet, the loss of the radiation mode of 2nd diffraction gratings at the front facet decreases, and the optical intensity there becomes
Hogan & Hartson L.L.P.
Kabushiki Kaisha Toshiba
Leung Quyen
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