Optical waveguides – Polarization without modulation
Reexamination Certificate
1999-11-30
2001-11-27
Schuberg, Darren (Department: 2872)
Optical waveguides
Polarization without modulation
C385S045000, C385S028000, C385S027000
Reexamination Certificate
active
06324312
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to a polarization splitter. More particularly, the present invention relates to a structure and a method for fabricating a wide-angle polarization splitter combining a straight waveguide and a branch waveguide.
2. Description of Related Art
A polarization splitter in a waveguide typically is fabricated on a LiNbO
3
crystal substrate. Generally, the polarization splitter is divided into two types. One type is called as a direction coupler type that is fabricated according to the modes with different polarization directions and their different coupling lengths. In this manner, the coupling length between two polarization modes needs to satisfy a special requirement in order to obtain the higher extinction ratio, and it therefore needs a small error tolerance in fabrication. Another type of polarization splitter is call a Y-branch type that has a Y-like structure with two different polarization properties, so that two polarization modes can be split into, such as TE and TM modes. The polarization splitter with the Y-branch type, for example, has been disclosed in U.S. Pat. No. 5, 436,992. For the conventional Y-like polarization splitter, if the branching angle is greater than 2 degrees, the propagation loss is very large. This is the reason why the branching angle of the conventional Y-like polarization splitter cannot be effectively reduced. Moreover, a conventional nickel diffusion manner for fabricating the Y-like polarization splitter needs high temperature and long diffusion time to obtain an ordinary polarized waveguide with a single polarization direction. This fabricating manner has a disadvantage with a poor optical confinement.
The fabrication method for the Y-like polarization splitter on the LiNbO
3
crystal substrate typically includes three technologies to have an input end that is a random polarization waveguide, one output end that is an extraordinary polarization waveguide with a single polarization direction, and another output end is an ordinary polarization wave guide. The extraordinary polarization waveguide can be fabricated by manners of magnesium-oxide diffusion or proton exchange, and the ordinary polarization waveguide cab be fabricated by nickel diffusion. Due to the property of single polarization direction, the splitter has a high optical extinction ratio between, for example, the TE mode and the TM mode. However, the technologies above cannot fabricate a Y-like structure with a branching angle greater than 2 degrees. This also indicates that the dimension of the polarization splitter cannot be effectively reduced for the conventional technologies. The nickel diffusion manner needs high temperature and long diffusion time to obtain an ordinary polarized waveguide with a single polarization direction. Further still, the optical confinement for the waveguide is also poor.
Recently, some waveguide products have been developed to have a wide branching angle with good optical transmission, such as a prism type or a substrate prism type with a large branching angle, in which the substrate prism type needs only once of photolithography process. However this technology has not been applied to a polarization splitter yet.
SUMMARY OF THE INVENTION
The invention has at least an objective to effectively reduce the area of a polarization splitter and to solve the conventional poor optical confinement for the polarization splitter having an ordinary polarization waveguide at a single polarization fabricated by nickel diffusion.
As embodied and broadly described herein, the invention provides a wide-angle polarization splitter with a Y-like structure that is a combination from a straight waveguide and a bent waveguide with a substrate-prism. The straight waveguide is formed by a nickel waveguide having a random polarization direction. The bent waveguide is formed by a manner of proton exchange. The two waveguides have an overlapping portion, where is called as a nickel-diffusion/proton-exchanged waveguide. An input portion of the straight waveguide of the Y-like structure needs a random polarization waveguide
A method for fabricating a Y-like waveguide provided by the invention needs only once of nickel diffusion and once of proton exchange. At a branching portion of the Y-like waveguide includes a substrate prism region. The design with the proton exchanged waveguide and the substrate prism region can only bend an optical component having extraordinary polarization. The light component having the ordinary polarization direction remains in the nickel waveguide. The polarization splitter thereby has a high optical extinction ratio. The device area can therefore be reduced, and the device integration is effectively increased. Further still, since the polarization splitter includes only the nickel-diffusion waveguide with random polarization, its output has a better optical confinement on the ordinary polarization waveguide.
The present invention has an objective to provide a method for fabricating a wide-angle polarization splitter. The polarization splitter includes a nickel waveguide allowing a random polarization input, and a proton exchanged waveguide with a wide bend angle. Thereby, a wide-angle polarization splitter is fabricated to have a high optical extinction ratio and the wide branching angle. The device area can be effectively reduced, resulting in the improvement of device integration. The optical confinement for the ordinary polarization is also effectively improved.
The method of the present invention needs only once of nickel diffusion and a proton exchange. A nickel diffusion process is performed on the nickel strip, formed on a substrate, at a temperature about 700° C.-1000° C. for a period of time about 10-600 minutes. A nickel waveguide with random polarization direction is formed on the crystal substrate. A photolithography process with a photomask, which is used for fabricating a wide-angle bent waveguide in a substrate-prism type, is performed to deposit a silicon mask on the crystal substrate. The silicon mask is used to resist benzoic acid (C
6
H
5
COOH) used later. The silicon mask can be replaced with an aluminum mask or a titanium mask. The benzoic acid can also be replaced by pyrophosphoric acid (H
4
P
2
O
7
). A proton exchange process with benzoic acid is to be performed on a portion of the crystal substrate exposed by the silicon mask. There is a portion of about 1 mm-5 mm of the nickel waveguide is exposed by the silicon mask, and is called as an overlapping portion. Preferably, the overlapping portion is about 4 mm. The crystal substrate is then dipped in the benzoic acid about 2-24 hours at a temperature of about 200° C.-240° C. for performing the proton exchange process. After the proton exchange process, the exposed portion of the crystal substrate, which typically has a strip with a bend angle from the straight waveguide, is the wide-angle bent waveguide, that is, a proton exchanged waveguide. The straight waveguide and the bent waveguide form together a Y-like polarization splitter. The silicon mask can be removed by HF acid.
The invention employs the single polarization property, and the property for the proton exchanged waveguide serving as an extraordinary polarization waveguide and the nickel waveguide serving as an ordinary polarization waveguide. Thereby, the optical extinction ratio is effectively improved, and the device area is effectively reduced. It is helpful to achieve high integration. Since the branching angle of the invention is greatly increased, this additionally allows the optical confinement is effectively improved to satisfy various practical applications.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.
REFERENCES:
patent: 5436992 (1995-07-01), Wang et al.
patent: 5475771 (1995-12-01), Hosoi
patent: 62-36608 (1987-02-01), None
Liao Yu-Pin
Lu Ruei-Chang
Wang Way-Seen
Assaf Fayez
Huang Jiawei
J.C. Patents
National Science Council
Schuberg Darren
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