Method and apparatus for compactly coupling an optical fiber and a planar optical waveguide

This invention relates to a method and apparatus for compactly and efficiently coupling light from an optical fiber into a planar waveguide and vice versa.

Planar waveguide devices are important components of optical fiber communication systems. Such systems typically comprise long lengths of fiber for transmission and often use planar waveguide devices to perform a variety of processes such as filtering, multiplexing signal channels, demultiplexing, compensating chromatic dispersion and compensating polarization dispersion.

An optical fiber is typically in the form of a thin strand of glass having a central core of circular cross section peripherally surrounded by concentric cladding glass. The core has a higher refractive index than the cladding so that the light is retained in the core by total internal reflection and propagates in a fiber mode. For long distance transmission the core dimensions are typically chosen so that the light propagates in a single circular mode.

A planar waveguide device, in contrast, is typically formed by thin layers of silica supported by a silicon substrate. The core is typically of rectangular cross section. The core region is formed, as by etching of a masked surface, into a patterned configuration that performs a desired function. In order to permit small radius curves, and thus compact functionality, the difference in refractive index of the planar waveguide core and the index of the cladding is typically substantially greater than the corresponding difference for optical fiber. The planar waveguide is said to be high delta where delta (Δ) is given by the core index less the cladding index, all divided by the core index.

Unfortunately there is a problem in coupling light from the transmission fiber into a planar waveguide. In addition to a mismatch in refractive indices, there is also a mismatch in core size. The core size for a typical optical fiber is significantly larger than the optimal core size for a single mode planar waveguide; therefore, their optical modes don't match because the field is more confined in the waveguide than in the fiber. As a consequence of these mismatches, direct coupling of a fiber to the planar waveguide would incur prohibitive insertion loss of the optical beam.

The conventional approach to this problem is to provide the planar waveguide with an enlarged end for receiving the fiber and to gradually (adiabatically) taper the waveguide core in the lateral direction to optimal size. The lateral taper is on the same plane as the waveguide optical circuit. This approach reduces insertion loss but unfortunately adiabatic lateral tapering is not efficient for high delta waveguides, and it requires substantial length.

A more process-demanding approach is to start with an expanded height waveguide at the fiber end, that is then vertically tapered down to the waveguide circuit level. This technique was proposed by Koch et al. (T. L. Koch, et. al., “ Tapered Waveguide InGaAa/InGaAsP Multiple - Quantum - Well Lasers ,” IEEE Photonics Letters Vol 2. No 2 February 1990; See also A. Mahapatra and J. M. Connors, “ Thermal tapering of ion - exchanged channel guides in glass ,” Opt. Letters, vol. 13, pp. 169-171, 1988, and Shani, et. al., “ Efficient coupling of semiconductor laser to an optical fiber by means of a tapered waveguide on silicon ” Applied physics Letters 55(23), December 1989). However, it requires a substantially large waveguide starting core height which takes a very long time to grow.

A 2-D tapered segmented waveguide was demonstrated by Z. Weissman and A. Hardy. This technique implements two-dimensional mode tapering by introducing gaps between the segments of the waveguides. (Weissman and A. Hardy, “ Modes if Periodically Segmented Waveguide ” IEEE Journal of Lightwave Technology 11: 1831-1838 (1993)). In effect, the total effective index of the guiding area is reduced. This approach is very effective in reducing the coupling loss. However, for high index waveguides, the core thickness is small compared to the fiber, therefore, mode matching via segmented tapes is hard to achieve, and a better matching with the fiber mode can still be achieved.

Accordingly there is a need for an improved arrangement to compactly and efficiently couple light propagating in an optical fiber into a planar waveguide.

In accordance with the invention, an optical fiber is coupled to a planar waveguide through 1) a series of planar waveguide segments of enlarged cross section and 2) a segment of tapered cross section. The combination of spaced segments and tapered segments provide efficient coupling over a much shorter length than required for conventional adiabatic coupling.

The advantages, nature and various additional features of the invention will appear more fully upon consideration of the illustrative embodiments now to be described in detail in connection with the accompanying drawings. In the drawings:

FIG. 1 is a schematic view of an optical fiber coupled to a planar waveguide device in accordance with the invention;

FIG. 2 schematically illustrates a first example of a FIG. 1 coupling arrangement;

FIG. 3 schematically illustrates a second example of a FIG. 1 coupling arrangement;

FIG. 4 schematically illustrates an example of a coupling arrangement wherein the tapered region is step-tapered rather than continuously tapered;

FIG. 5 schematically illustrates an exemplary side view of a coupling arrangement comprising two materials of different indices of refraction wherein segments are situated on a planar guide; and

FIG. 6 schematically illustrates an exemplary side view of a coupling arrangement wherein a tapered segmented section comprises two materials of different indices of refraction.

It is to be understood that these drawings are for purposes of illustrating the concepts of the invention and, except for the graphs, are not to scale.

Referring to the drawings, FIG. 1 is a schematic view of a device 9 comprising an optical fiber 10 coupled to a planar waveguide device 11 in accordance with one embodiment of the invention. Specifically, the device 9 includes at least optical fiber 10 having a core 12 of diameter d and at least one axially extending planar waveguide 11 having a core 13 of transverse dimensions (height h and width w) each less than the diameter of the fiber core 12 . The fiber core 12 is optically coupled to the smaller waveguide core 13 by a segmented waveguide section 14 of enlarged transverse cross section and a tapered section 15 . The segmented section 14 preferably has a transverse cross section approximately the same as the fiber core 12 and provides the initial coupling from the core 12 . The thickness of the segments and/or the spacing of the segments varies from the fiber to the planar waveguide so that the light leaving the lower index fiber core fiber encounters a gradually increasing effective core index approximately the higher index planar waveguide core. This permits coupling from segmented section 14 into the enlarged area end 16 of tapered section 15 with minimal index mismatch. The tapered section tapers down to approximate the smaller transverse area of the planar waveguide core 12 . The waveguide core 12 is configured, as by geometry, other cores, phase shifters, or ring resonators (not shown) to process the coupled light.

The coupling device of FIG. 1 can be implemented in the form of a segment of optical fiber or as part of an integrated optical device including the planar waveguide.

Similarly, light from a planar waveguide device can be efficiently and compactly coupled out into an optical fiber by passing from the planar waveguide to the fiber. The light would pass in the opposite direction through the tapered section 15 to the enlarged end 16 . Then it couples through the segmented section 14 to the lower effective end where it is matched with the end of an optical fiber.

The segmented part of the vertical taper can be fabricated using a standard technique used in silicon optical bench technology. First a lower cladding layer is formed, as by low-pressure steam oxidation followed by an anneal. Then a higher index core layer is deposited, as by Plasma Enhanced vapor deposition (PECVD) or low pressure Chemical vapor deposition (LPCVD), and then it is annealed to flow the glass. Then the segmented waveguide is pattern by photolithography with 1:1 projection using a chrome mask contact. The mask information is then patterned using reactive ion etching through the core. Poly Silicon composition or a hard baked photoresist are used for this process. Gap sizes in the range of 0.7 to 32.5 μm can be achieved. Next, the cladding layer is deposited in several steps with anneals to flow the glass.

Shadow or dither masks can be used to pattern the vertically tapered portion of the structure. These masks allow for variable transmission of light intensities. Areas where the photoresist is exposed to higher light intensities will be developed faster. The etching speed of the core material is related to the how far the resist is developed, therefore, areas with different heights can be fabricated.

In an alternative embodiment, the tapered section 15 can be step-tapered, as shown in FIG. 4 , instead of continuously tapered as shown in FIG. 1 . The advantage of this approach is ease of fabrication. However, higher order modes can be excited which can lead to increase in loss when small number of steps are used.

In another alternative embodiment, as shown in FIG. 5 , the coupler structure comprises two layers to couple from a fiber to a planar lightwave circuit (PLC) chip. Segments of index of refraction n 1 are situated in contact with planer waveguide 52 having index of refraction n 2 . Core 51 (n 1 ) is a thick low index material that efficiently couples light from the fiber to the coupler chip. Core 52 (n 2 ) is a high index core of the same material as the PLC device. Light can be efficiently coupled to the n 1 , n 2 cores where the low index core is segmented with gaps. The gap size increases towards the PLC chip, therefore the effective index of the low index core is slowly reduced. Consequently, light is adiabatically couple to the high index core. The end width (near the PLC chip) can be tapered down to the single mode size of the waveguide.

In yet another alternative embodiment, as shown in FIG. 6 , variable height segments are tapered from fiber 12 to planar waveguide 62 . Segments 61 comprise two materials, one of index of refraction n 1 compatible with the fiber and one of index of refraction n 2 , the index of the planer waveguide. The portion of the segment comprising index n 2 is gradually increased until it matches the dimensions of planar waveguide 62 .

In the following examples, the dimensions of the segments, and in particular segment thicknesses, were determined by use of Prometheus. Prometheus is a 3D finite difference beam propagation computer program that uses standard techniques for waveguide simulation and device optimization. The Prometheus package (formerly BBV Software, The Netherlands) is now part of “OlympIOs”. OlympIOs, a commercially available optics software package, comprises Prometheus, as well as a 2D effective index beam propagation method. (C2V, P.O. Box 318, 7500 AH, Enschede, The Netherlands). The invention can now be more clearly understood by consideration of the following specific examples: