Optical waveguides – Integrated optical circuit
Reexamination Certificate
2002-07-22
2003-08-19
Ullah, Akm E. (Department: 2874)
Optical waveguides
Integrated optical circuit
Reexamination Certificate
active
06608947
ABSTRACT:
FIELD OF THE INVENTION
This invention is in the field of Planar Lightwave Circuits (PLC), and relates to a multilayer integrated optical device and a method of fabrication thereof.
BACKGROUND OF THE INVENTION
Optical communications is the enabling technology for the information age, and the essential backbone for long haul communications. As this technology progresses, there is a tremendous interest in providing optical routes in the short haul, metropolitan and access networks, as well as in local area networks and cable TV networks. In all these networks, the best of breed solution for bandwidth expansion has been the adoption of wavelength division multiplexing (WDM), which entails the aggregation of many different information carrying light streams on the same optical fiber. Devices capable of accessing individual information steams are fundamentally required in current and future networks. These devices can also add information streams to the optical fiber, as well as impress information on an optical stream by optical modulation.
PLC technology is central in the creation of modern optical elements for communications systems. According to this technology, optical waveguides and additional functional structures are fabricated in a planar optical transparent medium in order to direct the passage of light and to implement coupling, filtering, switching, and additional processing functions as required for optical communications.
Existing examples of Planar Lightwave Circuits include optical switches and modulators based on Mach Zender Interferometer (MZI), in which interference is produced between phase coherent light waves that have traveled over different path lengths arrayed waveguide routers (AWG) used for combining and spreading multiple optical channels, namely multiplexers and demultiplexers. However, to achieve a good modulation performance with the MZI, the latter is typically designed with long interference arms. As a result, this device is not size-efficient in its implementation, and limits the scaling ability of complex optical circuits. Another feature of MZI-type devices, in their predominant implementation, is their frequency insensitivity over a desired frequency bandwidth. As a result; MZI-type devices cannot be used directly for wavelength routing.
An important driving force pushing PLC technology is the need for enhanced functionality in the optical domain. This need is hampered by the limitation of state of the art waveguide technology, which is two-dimensional (i.e., single-functional-layer architecture). Unlike the very large scale integrated electronic circuitry, where dimensions of the basic elements were reduced to sub-micrometer size, the optical PLC circuitry is inherently much larger, thus the exploitation of multi-layer architectures is much more crucial than in electronics.
In the implementation of PLC, there is a contradiction between the requirements of coupling to optical fibers and decreasing circuit size. Coupling to fibers is best obtained by using waveguides with modal fields similar to the fiber modes with a small refractive index difference with respect to the surrounding medium. The functionality of the optical circuits depends on the amount of optical elements in the circuit. By decreasing the circuit size, more optical circuits can be integrated and the attainable functionality increases. Smaller dimensions imply tighter control of the optical mode and smaller optical modes, hence, a high index contrast between the waveguide core and surrounding medium. It is of fundamental importance to provide a means of combining both elements in one functional optical circuit.
The importance of utilizing the vertical dimensions in creating complex optical circuits has been recognized and addressed in the past. This is associated with the fact that vertical fabrication tolerances are better than horizontal tolerances, and therefore such a vertical integrated optical device, (filter, switch, modulator) is simpler or cheaper to manufacture. Optical devices utilizing this approach are disclosed, for example, in the article “
Vertically Coupled Glass Microring Resonator Channel Dropping Filters”
, B. E. Little et al., IEEE Photonics technology Letters, Vol. 11, No. 2, February 1999. This approach is critical for the fabrication of optical circuits based on structures with very different indices of refraction such that the effective coupling region between the structures is very small, e.g., coupling between waveguides and ring micro-resonators. In this case, the vertical dimension, which is easier to control in conventional processes, can mediate the structure for accurate coupling as described in the aforementioned reference.
Recently developed integrated electo-optical devices utilize resonant rings to achieve frequency selective switching. Such a device is disclosed, for example, in WO 99/17151. The device comprises a ring resonator interconnected by linear waveguides to couple light from a first linear waveguide to the second one, when the frequency of the light passing through the first waveguide fulfils that of the resonance condition of the ring. By applying an electric field to the ring, its refractive index, and consequently, its resonance condition can be desirably adjusted, thereby preventing the passage of the previously coupled light, the device therefore acting as a switch. Alternatively, the loss of the ring waveguide can be changed. Adding loss to the ring diminishes its operation as a resonant cavity, and light cannot be coupled from the waveguide to waveguide.
To create two or more layers of interconnected waveguides with the prior art techniques, a planarization step has to be performed.
FIGS. 1A-1D
illustrate main sequential steps of the prior art technique employed for manufacturing a waveguide structure shown in
FIG. 1E
being generally designated
10
. Initially, a buffer layer
12
of SiO
2
is deposited on a silicon wafer
14
(FIG.
1
A). Then, a layer
16
of doped SiO
2
with a higher refractive index (SiO
2
+Ge), as compared to that of the buffer layer
12
, is deposited onto the buffer layer (FIG.
1
B). This layer
16
serves for the formation of a core
16
A of the optical waveguide, and its thickness is typically in the range of 4-12 &mgr;m. To form the waveguide core
16
A (FIG.
1
C), the waveguide, as well as other optical structures, are masked using photolithography followed by etching. A third layer of SiO
2
, or upper cladding layer
18
, is then deposited so as to bury the etched structure (FIG.
1
D).
This layer
18
retains to some extent the topography of the underlying structure, and thus requires planarization to allow for an additional overlaying waveguide structure to be deposited. Planarization can be implemented by Chemical Mechanical Polishing, reflow techniques, deposition of a very thick layer, selective etching or deposition techniques. As shown in
FIG. 1F
, after achieving a planar top layer, a second waveguide structure
20
can be fabricated on top of the structure
10
in the above-described manner.
Planarization is a difficult process step, which utilizes expensive equipment and is difficult to be accurately applied for large area wafers. Therefore, it would be desirable to eliminate this step in the fabrication of multi-layered optical waveguide structures.
SUMMARY OF THE INVENTION
There is accordingly a need in the art to facilitate the manufacturing of a three-dimensional (multi-layered) integrated optical device, by providing a novel method of fabricating such a device, and a novel integrated optical device based on an optical structure embodying different material systems. Such a device may be an optical frequency dependent switch, a modulator, an Optical Add Drop Multiplexer (OADM), a spectral analyzer, a sensor, etc.
The main idea of the present invention consists of utilizing a waveguide definition on several layers, enabling to combine low coupling loss waveguides with high confinement waveguides. The present invention opens new horizons for the functionality of optical devi
Margalit Moti
Orenstein Meir
Browdy and Neimark , P.L.L.C.
Lambda Corssing Ltd.
Ullah Akm E.
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