Tapered rib fiber coupler for semiconductor optical devices

Details Tapered rib fiber coupler for semiconductor optical devices Tapered rib fiber coupler for semiconductor optical devices

1997-10-06

2001-05-08

Lee, John D. (Department: 2874)

Optical waveguides

Planar optical waveguide

Thin film optical waveguide

C385S043000, C385S049000, C385S131000

Reexamination Certificate

active

06229947

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to semiconductor optical devices. More specifically it relates to a novel tapered rib waveguide structure useful in coupling the output of the semiconductor optical device into and out of an optical fiber. Even more specifically, it relates to a device that achieves two dimensional expansion or contraction of the output optical mode of single-transverse-mode semiconductor waveguide modulators and lasers.
High performance optoelectronic systems using optical fiber signal routing require low-cost and low-loss coupling between single-mode fibers and active or passive semiconductor waveguide devices and circuits. However, the 1-3 &mgr;m elliptical modal spot of typical semiconductor waveguides is poorly matched to the 8-9 &mgr;m circular modal spot of conventional single-mode optical fibers. This modal mismatch results in a 7-10 dB insertion loss when directly coupling light between these single-mode fibers and semiconductor waveguides. Non-integrated solutions that improve this coupling efficiency often do so at the cost of tight alignment requirements and are thus not well suited for low-cost package using passive alignment systems. Tapered waveguide transitions offer a monolithically-integrated means by which efficient coupling can be achieved with relaxed alignment requirements. Most of these approaches, however, require complex growth or processing steps such as multiple etch-regrowth sequences or lithographic patterning of extremely small radius waveguide tips in order to achieve the desired coupler performance.
One reference of interest is U.S. Pat. No. 5,574,742 for a “Tapered Beam Expander Waveguide Integrated with a Diode Laser.” Therein the structure analogous to the tapered rib in the present invention, therein called the first semiconductor waveguiding layer is etched in a multi-step process to form a very sharp (less than 500 Angstroms radius) edge. The light emanating from this edge then is coupled into the optical fiber. The process to form the edge is complicated, the sharpness of the edge is essential to prevent scattering of light, and the height of the structure is critical, given that an etch stop is necessary below the first semiconductor waveguiding layer. This structure does not force the light in this layer down into a lower layer, as is done in the present invention.
There remains in the art an unmet need for an improved waveguide coupler structure that can be made easily and yet couple the light between a semiconductor optical device, such as an optical modulator or a laser, and an optical fiber with very high efficiency.
BRIEF SUMMARY OF THE INVENTION
The various shortcomings and challenges in the prior art have been solved by the present invention which uses a monolithic tapered rib waveguide extending from the end of a small rib waveguide located on top of a thick lower cladding which is partially etched to form a mesa structure. When the small rib is sufficiently wide, the fundamental optical mode is confined to the small rib, and the device functions as an efficient modulator or laser with high confinement of light within the undoped GaAs layer. The tapered rib extends from the end of the small rib towards the output face of the mesa structure, tapering down in width from the small rib to a smaller width at the end of the tapered rib. The decrease in the width of the tapered rib causes the fundamental optical mode to drop down into the mesa structure and away from the tapered rib as its width decreases. The optical mode remains centered below the tapered rib and well-behaved within the mesa. When it exits the output face of the mesa structure, it has excellent divergence characteristics which match extremely well with single mode, long wavelength optical telecommunication fiber, resulting in coupling loss below 1.0 dB for a semiconductor laser. The device works equally well to couple light from a fiber into the optical device having the tapered rib waveguide. The tapered waveguide is particularly well suited for integration with both active and passive etched rib waveguide structures. Fabrication is relatively simple, requiring only patterning and etching of the tapered waveguide and uniform-width outer mesa waveguide without any epitaxial regrowth.
Described herein is a tapered rib waveguide and its application to both passive optical modulators at 1.3 &mgr;m wavelength and active diode lasers at 0.98 &mgr;m wavelength. All of these tapered rib structures achieve less than 1 dB coupling loss to single-mode fiber at the design wavelength and may be fabricated with a single epitaxial growth step followed by conventional lithography and etch steps. One of these devices has even been designed using 1 &mgr;m minimum feature sizes and fabricated using contact-print optical lithography. This is the first known use of standard contact-print photolithography in the successful fabrication of a laterally-tapered adiabatic mode expander.


REFERENCES:
patent: 5737474 (1998-04-01), Aoki et al.
patent: 5745630 (1998-04-01), Vawter et al.
patent: 5844929 (1998-12-01), Lealman et al.

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