Optical waveguides – Planar optical waveguide
Reexamination Certificate
1999-09-07
2002-03-12
Epps, Georgia (Department: 2873)
Optical waveguides
Planar optical waveguide
C385S088000, C385S089000, C385S014000, C385S046000
Reexamination Certificate
active
06356692
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an optical module, a transmitter, a receiver, an optical switch, an optical communication unit, an add-and-drop multiplexing unit, and a method of manufacturing the optical module.
In recent years, measures have been considered in various circles for providing silica waveguides, as well as such optical integrated circuits as optical splatters, wavelength division multiplexers/demultiplexers, optical switches, and the like, which use a silica waveguide to enhance the functions of optical parts for communications, as well as to reduce the size and cost of those parts. Measures have also been considered for obtaining high performance optical modules to be realized by mounting a semiconductor optical device, such as laser diodes and photo-diodes, on a substrate with optical integrated circuits. Those optical modules, when used for a wavelength division multiplexing (WDM) transmission unit and an optical add-and-drop multiplexing (ADM) unit, can enhance the performance of the object communication system, as well as cut down the size and cost of the same significantly. Optical modules, in each of which a semiconductor optical element is mounted in a silica waveguide, are described collectively in the Technical Digest of Third Optoelectronics and Communications Conference, Makuhari, Japan, p. 370-371 (1988). In this document, a WDM light source module is realized with semiconductor amplifiers mounted on a substrate with a waveguide array having diffraction gratings on a substrate. And, a wavelength converter module is realized with a semiconductor optical amplifier mounted on a substrate with a
3
dB coupler circuit. Furthermore, a fast wavelength filter is realized with semiconductor optical amplifiers mounted between two arrayed waveguide grating (AWG) wavelength multiplexers/demultiplexers.
On the other hand, not only silica waveguides but also polymer waveguides have been under examination. A polymer waveguide is fabricated by coating a silicon (Si) substrate with varnish obtained by dissolving a polymer in a solution.
Consequently, when compared with the silica waveguide, the mass productivity is higher and the cost is lower. The polymer waveguide also has a large thermo-optical coefficient. If such a polymer waveguide is used, therefore, it is possible to compose an optical integrated circuit, such as a wavelength tunable filter and a digital optical switch, with enhanced functions which have never been realized in a silica waveguide. A wavelength tunable wavelength division multiplexer is described in, for example, IECE Transactions on Electronics, Vol. 7, p.1020-1026 (1026) and a digital optical switch is described in the Technical Digest of the Third Optoelectronics and Communications Conference, Makuhari, Japan, p.66-67 (1998). Just like the silica waveguide, mounting a semiconductor optical element in a polymer waveguide or in an optical integrated circuit which uses such a polymer waveguide will result in a high performance optical module.
SUMMARY OF THE INVENTION
In spite of the above-mentioned favorable characteristics of the polymer waveguide, there are still some problems which must be solved. For example, in order to make sure that a high optical coupling efficiency is obtained between a waveguide type semiconductor optical element mounted on a substrate and a polymer waveguide or a polymer optical integrated circuit fabricated on the same substrate, the height of the light axis of the polymer waveguide must be aligned with the height of the light axis of the semiconductor optical element. At this time, the refractive index difference between the core layer and the cladding layer of the polymer waveguide is usually set to 0.3 to 1% and the thickness of the core layer is set to 5 to 8 &mgr;m considering the loss of coupling with an optical fiber, the fabrication tolerance of the optical circuit, and the size of the optical circuit. If a polymer waveguide is formed on an Si substrate, a lower cladding layer must be formed at a thickness of 10 &mgr;m or more so as to suppress an increase of the propagation loss and the polarization dependent loss (PDL), which are caused by the Si substrate. Consequently, the height of the core layer in the center portion from the surface of the substrate becomes 13 &mgr;m or more. On the contrary, the height of the core layer of the semiconductor optical element in the center portion from the surface of the substrate is at most 5 to 12 &mgr;m when the optical element is flip-chip bonded on the substrate. The difference in height between those items becomes at least 1 &mgr;m, so that the optical coupling loss between the semiconductor element and the polymer waveguide becomes very large. In order to align both of those items in height, two methods have been proposed. According to the first method, a projection referred to as a terrace is formed at a portion of the Si substrate, where a semiconductor element is mounted. The method already has been applied to silica waveguides. In this case, however, the manufacturing will become difficult, since both polymer and Si must be polished and flattened simultaneously when this method is applied to forma polymer waveguide. The second method applicable to such a height alignment of the polymer waveguide is described in IEICE technical report, EMD98-55 (1988). According to this method, part of the lower cladding layer on which a semiconductor element is to be mounted is left as is, and then the element is mounted on the left-over lower cladding layer (referred to as a pedestal). This method, however, also creates problems in that the temperature characteristics of the laser diode are degraded, since the laser diode comes to be mounted on a polymer with a low thermal conductivity, and a metallic layer must be formed in the middle of the lower cladding layer so as to stop the etching at a predetermined height.
Under such circumstances, it is an object of the present invention to provide an optical module which solves the foregoing problems and may be produced at lower prices than conventional ones.
The first object of the present invention is therefore to propose a method and a structure for matching the height of the polymer waveguide to the height of the core layer of a semiconductor element with less degradation of the characteristics and with fewer fabrication processes in an optical module provided with a waveguide type semiconductor element mounted on its substrate provided with a polymer waveguide or an optical integrated circuit composed of such a polymer waveguide, thereby providing an optical module of higher performance and lower cost than conventional ones. The second object of the present invention is to provide an optical communication unit which uses an optical module so as to enhance the functions and reduce the cost to an extent greater than conventional ones.
The present invention is characterized by an optical module which is composed as follows: At first, a polymer waveguide is formed at a portion of a silicon substrate coated with an oxide silicon film thereon, so that
the relational conditions of d>&lgr;/2&pgr;·(n
core
2
−n
SiO2
2
)
½
) are satisfied when it is assumed that the thickness of the oxide silicon film is d, the refractive index of the oxide silicon film is n
SiO2
, the refractive index of the core layer is n
core
, and the wavelength of a light transmitted through the polymer waveguide is &lgr;. Then, a semiconductor optical element is provided at another portion of the silicon substrate, and an end face of the waveguide is coupled with an end face of the semiconductor optical element optically within a predetermined error range, and the thickness of the substrate where the waveguide is provided is practically the same as the thickness of the substrate where the semiconductor optical element is provided in a cross sectional view of the-substrate.
The first object of the present invention described above is achieved as follows: At first, a polymer waveguide provided with a lower cladding la
Ichikawa Hirokazu
Ido Tatemi
Kinoshita Taira
Kuwahara Akira
Nagara Takamitsu
Antonelli Terry Stout & Kraus LLP
Epps Georgia
Hitachi , Ltd.
Lucas Michael A.
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