Optical waveguides – Integrated optical circuit
Reexamination Certificate
2002-01-22
2004-11-23
Lee, John D (Department: 2874)
Optical waveguides
Integrated optical circuit
C385S041000, C385S042000, C385S130000
Reexamination Certificate
active
06823094
ABSTRACT:
This application is based on Patent Application No. 2001-017943 filed Jan. 26, 2001 in Japan, the content of which is incorporated hereinto by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an interferometer and its fabrication method, which enable planar optical waveguide circuits used in optical communication field to adjust optical path lengths (phases) independently in the transverse electric (TE)polarization mode and transverse magnetic (TM) polarization mode.
2. Description of the Related Art
Optical circuits employing single mode waveguides formed on a substrate are characterized by high integration and mass productivity, and hence they are essential to construct economical optical network nodes. In particular, optical circuits utilizing silica-based waveguides with SiO
2
as the main ingredient has favorable characteristics such as low loss, superiority in an affinity for silica-based optical fibers, and long-term stability. Thus, a large variety of optical components typified by arrayed waveguide gratings are put to practical use, and are applied to commercial systems.
These optical components are fabricated by combining a glass film deposition technique such as flame hydrolysis deposition (FHD) and chemical vapor deposition (CVD) with a microfabrication technique such as reactive ion etching (RIE). More specifically, a glass film is deposited on a substrate such as a silicon wafer to form an lower-cladding, followed by depositing a core layer with a refractive index higher than that of the cladding layer. Then, a core pattern is formed by the microfabrication technique to form an optical circuit, followed by depositing a glass film to form an over-cladding layer, thereby fabricating an optical circuit composed of embedded waveguides.
Usually, the FHD carries out annealing with high temperature to consolidate a glass film, and the CVD also performs annealing to increase the transparency of a glass film. The high temperature process causes thermal stress in the glass film constituting the waveguides, resulting in waveguide birefringence (B-value) in which the effective refractive index of the waveguide varies depending on the polarization state, thereby bringing about optical polarization dependency in the circuit characteristics. In addition, since the waveguide birefringence differs slightly in a wafer surface because of fabrication error, it is necessary to trim the waveguide birefringence locally for each optical circuit to achieve satisfactory optical circuit characteristics.
As a conventional local waveguide birefringence trimming technique, a method is proposed that utilizes a stress-applying film consisting of an amorphous silicon thin film (Japanese Patent Application Laying-open No. 1-77002 (1989). It exploits a phenomenon that an amorphous silicon thin film, which is placed on a waveguide, causes a strong tensile stress in the waveguide, thereby varying the effective refractive index of the waveguide through the photoelastic effect of the glass. Varying the profile of the amorphous silicon thin film enables the control of the stress distribution, that is, the waveguide birefringence. Besides, since the amorphous silicon thin film can be removed by an Ar laser or the like, fine trimming of the length of the amorphous silicon thin film in accordance with the optical circuit characteristics enables the effective refractive index of the waveguide to be adjusted including the waveguide birefringence.
The technique using the amorphous silicon stress-applying film is more actively applied to a constituent element of a polarization beam splitter (PBS) (for example, see, “Birefringence control of silica waveguides on Si and its application to a polarization-beam splitter/switch”, Journal of Lightwave Technology, Vol. 12, No. 4, Apr. 1994).
The PBS comprises a Mach-Zehnder interferometer (MZI), which includes two 3 dB optical couplers (50% optical couplers) consisting of silica-based waveguides and two waveguide arms formed on a substrate (silicon substrate), and on which three types of amorphous silicon stress-applying films with different width are placed.
One of the three types of the amorphous silicon stress-applying films with different width is 50 &mgr;m wide, and is provided to control the waveguide birefringence principally. The remaining two types are 90 &mgr;m and 100 &mgr;m wide, and are basically provided to control the effective refractive index of the waveguide polarization independently. The length of the amorphous silicon stress-applying film is trimmed by removing its part by an Ar laser, so that the optical path length difference between the two waveguide arms becomes zero for the transverse magnetic polarization mode, and &lgr;/2 for the transverse electric polarization mode, where &lgr; is the wavelength. Thus, according to a known interference principle, the transverse magnetic polarization mode of the light entering the input port is guided to the cross port, whereas the transverse electric polarization mode is guided to the bar port. Thus, the MZI functions as a PBS.
The waveguide birefringence trimming technique utilizing the amorphous silicon stress-applying film, however, has a problem of complicating the device configuration and increasing its cost because of the final trimming using a laser and of the need for aligning the position of the laser irradiation at an accuracy of a few tens of micrometers.
On the other hand, a local-heat trimming method (see, Japanese Patent Application Laying-open No. 3-267902 (1991), for example) is put into practice as a method of trimming the effective refractive index of a waveguide. This technique changes the effective refractive index of the waveguide permanently by annealing the waveguide at rather high power by using thin film heaters patterned on the waveguide, thereby trimming the optical path length (phase) of the optical circuit. Since the thin film heaters are formed by the microfabrication technique using a photomask, it is enough to flow current through the thin film heaters without the high accuracy alignment at the annealing. Thus, the trimming is carried out by a rather simple equipment, enabling its automatization rather easily. This method, however, is insufficient as a method of controlling the waveguide birefringence because the principle of the effective refractive index change and the control of the polarization dependency still remain to be elucidated.
SUMMARY OF THE INVENTION
The inventors of the present invention have pursued intensive research to find that the principle of the foregoing local-heat trimming method is that it mainly changes the quality of the cladding between the heaters and the core by the local annealing (heating), and particularly the cladding immediately under the heaters (in other words, the glass quality near the top surface of the cladding), thereby causing a stress to be applied on the waveguide. Then, we demonstrated experimentally that the polarization dependency was controllable substantially by changing the stress distribution by adjusting the width w of the local annealing (heating) region. More specifically, we found that when the width of the local annealing region was 1.4-2.6 times the distance d from the top surface of the over-cladding to the core center, that is, in a range of ±30% of w
o
, where w
o
was twice that distance d, the effective refractive index changed almost polarization independently, and that a local annealing region wider than w
o
made the transverse magnetic polarization mode more dominant in the refractive index change, whereas a local annealing region narrower than w
o
made the transverse electric polarization mode more dominant.
Thus, making the width of the local annealing region wider or narrower than w
o
, twice the distance from the top surface of the over-cladding to the core center, enables the permanent effective refractive index control of the optical waveguide with retaining the polarization dependency. In particular, the local annealing using at least two type
Abe Makoto
Goh Takashi
Inoue Yasuyuki
Okuno Masayuki
Saida Takashi
Fitch Even Tabin & Flannery
Lee John D
Lin Tina M
Nippon Telegraph and Telephone Corporation
LandOfFree
Interferometer and its fabrication method does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Interferometer and its fabrication method, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Interferometer and its fabrication method will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3275383