Optical waveguides – With optical coupler – Particular coupling function
Reexamination Certificate
1999-11-23
2001-10-30
Kim, Ellen E. (Department: 2874)
Optical waveguides
With optical coupler
Particular coupling function
C385S039000, C385S043000
Reexamination Certificate
active
06310995
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention pertains to optical coupling, and in particular, to optical resonant coupling between two waveguides.
There is a need for monolithically integrating various active and passive optical devices to obtain highly functional optical modules. The platform technology to this monolithic integration should be as simple as possible to keep costs low. Currently, the technologies used for monolithic integration, like selective area growth, or regrowth, are not only very expensive but also do not allow for enough freedom in designing the various active and passive devices.
The modes of a laser and an optical fiber are poorly matched in size and shape, leading to poor coupling efficiency therebetween. By integrating a mode expander with a laser, it is possible to obtain efficient coupling to an optical fiber. The invention describes a general technique by which a mode can be coupled from a tightly confined waveguide to a loosely confined waveguide. In particular, the invention employs resonant coupling to achieve mode expansion over a relatively short distance to efficiently couple a tightly confined waveguide in an active region of a semiconductor laser/amplifier and a loosely confined mode in a passive waveguide such as an optical fiber.
Various mode expanders have been demonstrated based on a concept called adiabatic mode transformation. In this approach, there are two separate sections in the device: a section optimized for high gain; and a section optimized for maximum coupling to a fiber. The sections are linked through a mode expander region, which adiabatically transforms, i.e. expands, the mode from the first section to the second section. For minimum losses to occur, adiabatic mode transformation must take place gradually over a relatively long length, e.g. 500 microns. In an exemplary device, as shown in
FIGS. 1A and 1B
, the adiabatic mode expander is formed as an extension to the active device such as a laser.
Mode transformation can be achieved by means of a technique known as resonant coupling, sometimes hereinafter referred to as phase matching. When two waveguides are approximately matched in their refractive indices and dimensions, and are located in close proximity, there is a transfer of power between the two waveguides in an oscillatory fashion. In other words, the mode in one waveguide is coupled to the proximate waveguide. This phenomenon is illustrated in
FIGS. 1C and 1D
wherein two waveguides I & II, each 1 &mgr;m wide and each with a refractive index of 3.21 are proximately located in side by side relationship. Waveguide I may be an active device having an electrical current input, and waveguide II may be a passive device. The power transfer between the two waveguides is theoretically 100%. If the refractive index of waveguide I is changed, for example, to 3.23 keeping the width unchanged, weak coupling results as the waveguides are no longer phase matched. However, if the width of the waveguide I is changed, for example, to 2 &mgr;m, strong coupling is once again observed. This implies that the phase matching condition is re-established. Thus, the phase matching condition depends both on the refractive index and the dimensions of the waveguide. The device, shown in
FIGS. 1C and 1D
, comprises two rectangular waveguides: active waveguide I and coupling waveguide II. Waveguide I has a higher refractive index than waveguide II, but has a smaller size in both the horizontal (lateral) and vertical (transverse) directions. On the other hand, the waveguide II has a lower refractive index than waveguide I, but has a larger size in both dimensions. The two waveguides can be designed so that their effective refractive indices are nearly equal. This close match of the effective refractive indices forces the power to couple back and forth between the two waveguides in an oscillatory fashion over a characteristic overlapping distance along the length of the waveguides, known as the coupling length L
c
.
In principle, by cleaving the device within the coupling length L
c
at the exact point where the mode resides in the lower coupling waveguide II, i.e., where transfer takes place, a mode expander can be realized. However, the refractive indices of materials are not known with great precision. Further, the refractive indices also depend on the current injection level in the active device I. Given this uncertainty in the refractive indices, it is not possible to design the waveguides for optimum power transfer. In addition, the point where the mode in waveguide I couples to waveguide II is not known a priori. Therefore, the practical feasibility of the method has not been demonstrated so far. In addition, there is an oscillatory power transfer along the device between the two waveguides due to the phase-matched condition being met along the entire length of the device. Because, there is a power loss during each oscillatory cycle, the device length is usually limited to L
c
. This limits the length of the gain (active) region of waveguide I, which in turn, affects the performance of the device.
SUMMARY OF THE INVENTION
The invention pertains to an approach for resonant coupling from a waveguide to another waveguide positioned in the horizontal or vertical plane, using an horizontal or a vertical taper, or a combination of both. The optical coupling between the two waveguides occurs over a very short taper with low optical loss. The invention enables monolithic integration of various passive and active optical devices and allows for improved coupling efficiency and alignment tolerances between various waveguides such as a laser and an optical fiber.
Multiple waveguides can be placed in close proximity to each other. Each waveguide can be optimized, for a specific optical function (e.g. active waveguide optimized for gain, passive waveguide optimized for ease of coupling, passive waveguide optimized for splitting, directional coupling or other passive devices). Using a properly designed taper, the optical mode can be moved from one waveguide to another as many times as required, therefore, achieving monolithic integration of several optical devices performing different functions.
The above-mentioned limitations of existing approaches are overcome in the device according to the invention by using a mode transformer having a selected tapered geometry for the active waveguide I. The horizontal dimension of waveguide I is tapered from a initial large width (W
i
) to a small final width (W
f
). To have arbitrarily long gain sections integrated into the device, it is necessary to have a portion where the two waveguides are off resonance, ensuring that the mode is resident almost entirely in waveguide I. This condition is achieved by choosing a suitably large initial width W
i
. The tapering of waveguide I between two extreme widths ensures that the mode matching condition is met somewhere along the taper. When this condition is met, the dimensions of the waveguide I change slowly so that the optical power is transferred resonantly to waveguide II. It does not matter where exactly it occurs as long as it is in the range expected. In addition, the taper is such that waveguide I achieves a cutoff condition over a length smaller than the coupling length L
c
, thus forcing the mode to be resident in waveguide II without oscillation. This makes the point of cleaving non-critical. In addition, the characteristic lengths can be designed to be much smaller than the lengths needed for adiabatic mode transformers. These short characteristic lengths directly translate to shorter lengths for the overall mode expanders.
In an exemplary embodiment, the mode transformer for coupling modes between first and second waveguides includes a coupling region having first and second ends, a coupling length, and a variable tapered width, such that, a phase matching condition is met within the coupling length defined in said coupling region. A tapered inlet for each end of the coupling region transmits a mode between each waveguide and the corresponding end of the cou
Bartolo Robert Ernest
Dagenais Mario
Heim Peter J.
Saini Simarjeet S.
Vusirikala Vijayanand
Kim Ellen E.
University of Maryland
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