Microlenses for integrated optical devices

Details Microlenses for integrated optical devices Microlenses for integrated optical devices

2001-09-25

2004-09-07

Dang, Phuc T. (Department: 2818)

Active solid-state devices (e.g., transistors, solid-state diode

Responsive to non-electrical signal

Magnetic field

C257S443000

Reexamination Certificate

active

06787868

ABSTRACT:

TECHNICAL FIELD
This invention is in the field of integrated optical devices. The invention relates to apparatus for coupling light into channel waveguides. The invention relates particularly to integrated optical devices which incorporate one or more lenses situated to efficiently couple light into a channel waveguide. The invention has particular application to arrayed waveguide gratings but may be used in other optical devices. Another aspect of the invention relates to lenses for use in integrated optical circuits.
BACKGROUND
Integrated optical devices of various kinds can be made by combining optical elements, such as waveguides, lenses, arrayed waveguide gratings and others. Such devices are typically formed within an optical layer on a generally flat substrate and hence are described generically as planar lightwave circuits (PLCs).
PLCs typically comprise various combinations of planar waveguides and/or channel waveguides. Such waveguides are described by H. Kogelnik, “
Theory of Optical Waveguides
,” in
Guided
-
Wave Optoelectonics
T. Tamir ed., Springer-Verlag, Berlin, 1988, and also by H. Nishihara, M Haruna, and T Suhara,
Optical Integrated Circuits
, McGraw Hill, New York, 1987.
In a slab waveguide (sometimes referred to as a planar waveguide), light is restricted to propagate in a region that is thin (typically between 3 &mgr;m and 10 &mgr;m) in one dimension, referred to herein as the lateral dimension, and extended (typically between 1 mm and 100 mm) in the other two dimensions. A plane that is perpendicular to the lateral dimension of the PLC is defined as the plane of the PLC. The longitudinal direction is defined as the direction of propagation of light at any point on the PLC; the lateral direction is defined to be perpendicular to the plane of the PLC; the transverse direction is defined to be perpendicular to both the longitudinal and the lateral directions.
Light propagating along a channel waveguide in a PLC has an optical field that is substantially confined in both the lateral direction and the transverse direction. In a typical channel waveguide, the field is substantially confined within a region that extends between 3 &mgr;m and 10 &mgr;m in the lateral direction, herein referred to as the height, and extends between 3 &mgr;m and 100 &mgr;m in the transverse direction, herein referred to as the width.
Slab waveguides may have various constructions. Constructions which use doped-silica are usually preferred because such constructions have attractive properties including low cost, low loss, low birefringence, stability, and compatibility for coupling to fiber. A typical doped-silica slab waveguide comprises a core layer of silica glass lying between top and bottom cladding layers of silica glass. The layers are doped so that the core layer has a higher index of refraction than either the top or bottom cladding layers. When layers of silica glass are used for the optical layers, the layers are typically deposited on a silicon wafer.
Slab waveguides may also be made using materials other than silica glass. For example, a slab waveguide may comprise three or more layers of InGaAsP. In this example, adjacent layers have compositions with different percentages of the constituent elements In, P, Ga, and As. Slab waveguides may also be made using layers of optically transparent polymers or a layer of a material having a graded index such that the region of highest index of refraction is bounded by regions of lower indices of refraction.
Channel waveguides may have constructions similar to slab waveguides with the addition of side cladding material that limits the transverse extent of the waveguide. The side cladding material has an index of refraction that is lower than that of the core material. The side cladding layer and top cladding layer are usually made of the same material.
There are various optical devices in which light propagating in a free propagation region, such as a slab waveguide, is coupled into one or more channel waveguides. An example of such a device is an arrayed waveguide grating router (AWGR). In a typical AWGR, light from one or more input ports is coupled into a first slab waveguide. The first slab waveguide is, in turn, coupled to a second slab waveguide by an arrayed waveguide grating (AWG). Light propagates through the second slab waveguide. The second slab waveguide is coupled to at least one output port.
An AWG comprises a plurality of channel waveguides. The length of the i
th
waveguide in the AWG is denoted as L
i
. The angular dispersion that is provided by the AWG is determined in part by the difference in length between adjacent waveguides, L
i+1
−L
i
. Details of construction and operation of AWGRs are described in M. K. Smit and C. Van Dam,
PHASAR
-
Based WDM
-
Devices: Principles, Design, and Application
, IEEE Journal of Selected Topics in Quantum Electronics, vol. 2, no. 2, pp. 236-250 (1996); K. McGreer,
Arrayed Waveguide Gratings for Wavelength Routing
, IEEE Communication Magazine, vol. 36, no. 12, pp. 62-68 (1998); and K. Okamoto,
Fundamentals of Optical Waveguides
, pp. 346-381, Academic Press, San Diego, Calif., USA (2000). Each of the publications and patents referred to in this application is herein incorporated by reference in its entirety. The particular output port to which light entering the AWGR at a particular input port is most strongly coupled is wavelength-dependent. Possible applications of AWGRs include, but are not limited to demultiplexing, multiplexing, or providing N×N routing.
An important consideration in the design of PLC devices, such as AWGRs, is that the insertion loss provided by the device should be small. It is a challenge to realize an AWGR with an insertion loss that is less than an insertion loss specification for a particular application. Many factors contribute to insertion loss including:
coupling losses where optical fibers are coupled to the PLC;
losses associated with the diffraction of light into diffraction orders that are not coupled to any output channel waveguide;
coupling losses between the AWG region and the input slab waveguide; and,
coupling losses between the AWG region and the output slab waveguide.
In certain devices it is also very important to minimize any back-reflection of light.
Dragone, U.S. Pat. No. 5,002,350, discloses that the coupling loss between a slab waveguide region and an AWG region may be reduced by providing adjacent channel waveguides of the AWG with a separation that is sufficiently large to substantially prevent mutual optical coupling in at least one region of the AWG and by making the separation between adjacent channel waveguides of the AWG sufficiently small to provide substantial mutual optical coupling in the region where the channel waveguides are coupled to the slab waveguide. In this region, it is preferred that adjacent channel waveguides have a separation that is as small as possible within the limits imposed by the fabrication process.
Li, U.S. Pat. No. 5,745,618, discloses another construction for reducing the coupling loss between a slab waveguide region and an AWG region. L
i
provides a transition region between the grating region and the slab waveguide region. The transition region comprises silica paths that traverse the waveguides of the grating. By arranging the silica paths to have widths that increase as the slab region is approached, the mode transition is made more gradual. This can reduce transition loss.
AWGRs having vertically tapered waveguides are disclosed in A. Sugita, A Kaneko, K. Qkamoto, M. Itoh, A. Himeno, and Y. Ohmori, “
Very low insersion loss arrayed
-
waveguide grating with vertically tapered waveguide
,” IEEE Photon. Technol. Lett. Vol. 12, no. 9, Pp. 1180-1182 (2000) and J. C. Chen, and C. Dragone, “
A proposed design for ultra
-
low loss waveguide grating routers
,” IEEE Photon. Technol. Lett., Vol. 10, no., Pp. 379-381 (1998).
Despite the extensive research that has been carried out to date with a view to reducing the insertion loss of PLCs, there remains a need for alternative

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