Optical waveguides – Planar optical waveguide
Utility Patent
1999-06-11
2001-01-02
Ullah, Akm E. (Department: 2874)
Optical waveguides
Planar optical waveguide
C385S010000
Utility Patent
active
06169838
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to an integrated athermal optical waveguide grating device, and more particularly to a wavelength multi/demultiplexer exhibiting little or no temperature sensitivity, or alternatively to an optical device that has a controlled amount of temperature sensitivity.
BACKGROUND OF THE INVENTION
Integrated wavelength multi/demultiplexers are important components for wavelength division multiplexing (WDM) optical communication systems. Integration offers the advantages of compactness, reliability, and reduced packaging costs. Further, implementation in a semiconductor material, particularly the InGaAsP/InP system important for optical fiber communications systems, would permit monolithic integration of these passive devices with active ones, such as lasers, modulators, optical switches, and detectors, resulting in sophisticated wavelength sensitive photonic integrated circuits with complex functionalities.
Heretofore, one of the problems in an integrated wavelength multi/demultiplexer is the temperature sensitivity of the device. Since the waveguide material usually has a temperature dependent refractive index, the channel wavelengths of multi/demultiplexer shift as the temperature varies. This shift is typically of the order of 0.01 nm/° C. in silica based devices and 0.1 nm/° C. in InP based devices. This wavelength shift would result in a loss of signal or crosstalk in the communication system. As WDM systems are being designed towards smaller and smaller channel spacing (from 1.6 nm to 0.4 nm or even less in the future), even a small temperature dependent wavelength shift (e.g. <0.1 nm) is of concern.
To accommodate the temperature variation due to weather conditions in the field, the waveguide multi/demultiplexer is usually packaged in a sealed package with a thermo-electric temperature controller in order to keep the temperature, and thus the channel wavelengths, at fixed values. This significantly increases the packaging cost. As dense DWM systems are moving from merely long-haul point-to-point transmission systems applications to metropolitan or local area networks, the cost of the components becomes a predominant issue. It is therefore highly desirable that the multi/demultiplexing devices are temperature insensitive.
Two types of integrated wavelength multi/demultiplexers that have been widely investigated are phased waveguide arrays and etched reflecting diffraction gratings.
Diffraction grating based devices require high quality, deeply etched grating facets. The optical loss of the device depends critically on the verticality and smoothness of the grating facets. However, the size of the grating device is usually much smaller than the phased array and the spectral finesse is much higher due to the fact that the number of teeth in the grating is much larger than the number of waveguides in the phased array. This allows the grating based device to have a larger number of channels available over its free spectral range (FSR) and consequently can be scaled-up easily to high density operation.
In waveguide array based devices, several approaches have been used to compensate for the temperature sensitivity. In one design, the input waveguide is eliminated and light from the input optical fiber is coupled directly into the slab section. Then by mounting the fiber on a temperature sensitive rod, the temperature modified change in the position of the launched light is designed to compensate the change in wavelength of the channel due to the temperature variation of the refractive index of the waveguide material. This method is described by G. Heise, H. W. Schneider, and P. C. Clemens, in a paper entitled “Optical phased array filter module with passively compensated temperature dependence”, Proceeding of the 24th European Conference on Optical Communication, 1998, pp. 319-320, 1998,
In another technique, a triangularly shaped region is created in the arrayed waveguide section which is then filled with a material possessing negative thermal coefficient (e.g., silicone rubber in the case of silica waveguides). By proper design of this filled region, the temperature dependence of the total optical path (comprising both the normal waveguide sections and the portion with negative thermal effects), due to refractive index changes, can be made to approach zero. The application of this technique in silica based arrayed waveguide devices is described by Y. Inoue, A. Kaneko, F. Hanawa, H. Takahashi, K. Hattori, and S. Sumida, in a paper entitled “Athermal silica-Based arrayed waveguide grating demultiplexers”,
Electron. Lett
. 33, PP. 1945-46, 1997. The same technique used in InP based arrayed waveguide grating is described by H. Tanobe, Y. Kondo, Y. Kadota, K. Okamoto, and Y. Yoshikuni, in a paper entitled “Temperature insensitive arrayed waveguide gratings on InP substrates”, OFC'97 Technical Digest, ThM4, pp. 298-299, 1997.
These techniques have many disadvantages. The cantilevered fiber approach is inherently difficult to fabricate and adjust and is unreliable. A major problem of the second approach is that of insertion loss and fabrication complexity. By replacing a large section of the arrayed waveguide section with a slab containing the material with negative thermal index, lateral waveguiding is lost in this portion, increasing optical loss and crosstalk. Attempts to reduce these effects by breaking the triangular region into alternate waveguiding and compensator regions, as described by A. Kaneko, S. Kamei, Y. Inoue, H. Takahashi, and A. Sugita, in a paper entitled “Athermal silica-Based arrayed waveguide grating (AWG) multiplexers with new low loss groove design”, Optical Fiber Communications Conference, 1999, Technical Digest, pp. 204-206, 1999, can decrease loss but at the expense of design and fabrication complexity.
It is an object of the invention to provide a compact, diffraction grating or phased array based optical multiplexer/demultiplexer that is substantially temperature insensitive and which overcomes many of the limitations of prior art devices.
SUMMARY OF THE INVENTION
In accordance with the invention, there is provided, a method of compensating for temperature sensitivity normally associated with a planar waveguide device having a slab waveguiding region having a first temperature coefficient of refractive index, comprising the step of:
providing within the slab waveguiding region a second region having a predetermined shape and predetermined dimensions, the second region having a second different temperature coefficient of refractive index than the first slab region for providing temperature compensation of the device.
In accordance with another aspect of the invention, there is provided an integrated temperature compensated optical waveguide diffraction grating device comprising:
an input region having an input port,
an output region having at least a plurality of predetermined light receiving locations,
a first slab waveguide region having a first temperature coefficient of refractive index, said slab waveguide region being optically coupled with the input and output regions for partially confining a beam of light launched from the input region between two substantially parallel planes,
a diffraction grating disposed to receive a beam of light launched from the input port through the slab waveguide region and to separate the beam into sub-beams of light of different wavelengths to the plurality of predetermined light receiving locations,
a second slab waveguide region adjacent to the first slab waveguide region having a predetermined shape and predetermined dimensions, said second slab waveguide region having a second different temperature coefficient of refractive index than the first slab waveguide region for providing temperature compensation of the device.
In accordance with the invention there is provided an optical planar waveguiding light-transmissive device comprising:
an input/output region having an input waveguide and a plurality of predetermined light receiving locations;
a slab wavegui
He Jian-Jun
Koteles Emil S.
Szereszewski Juliusz
Ullah Akm E.
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