OPTICAL WAVEGUIDE STACKS AND ASSEMBLIESThis invention relates to optical waveguide stacks and assemblies.
This invention uses assemblies of thin film waveguides to achieve displacement, splitting and transformation of optical images.
Simple, stacked, thin film waveguides enable images to be displaced, split and transformed in one dimension. By assembling combinations of stacks of thin film waveguides arbitrary two dimensional and three dimensional transformations can be achieved. Applications for such assemblies are seen in the internal interconnection of complex integrated circuits, in the connection of chip to chip, and in board to board connection.
The invention addresses the optical interconnection of one array of locations with another arbitrary array of locations. This function is important for the interconnection of electrical subsystems by optical means. Optical interconnection offers advantages of wide bandwidth, low crosstalk, weight, power etc., but alignment of light sources, detectors and imaging optics is difficult. This idea provides a solid interconnecting device which permits transformation and fan out of interconnecting lines with potentially low insertion loss.
According to the present invention there is provided an integrated thin film structure comprising a plurality of waveguides stacked together to form a three dimensional structure and wherein each of said waveguides is connected to a plurality of optoelectronic components to provide optical data paths thereto.
Further according to the invention there is provided a waveguide assembly comprising a plurality of planar integrated waveguide layers stacked together to form a three dimensional structure and wherein waveguides are connected to a plurality of opto-electronic components to provide optical data paths thereto.
Preferably the optoelectronic components are connected electrically and mechanically to the waveguide ends by flip-chip solder bonding. The optoelectronic components are connected electrically to a plurality of integrated circuits and said components and integrated circuits are connected electrically and mechanically by flip-chip solder bonding.
The optoelectronic components may include modulator array, detector arrays or surface emitting laser diode arrays. Each element in said arrays correspond to a waveguide path in the waveguide structure.
Preferably recessed grooves are formed to provide additional waveguide path ends at right angles to the existing waveguide ends and laser diodes or laser diode arrays are positioned in said grooves thereby coupling light into said said additional waveguide path ends.
The laser diode on laser diode arrays are connected to said structure by flip-chip solder bonds.
Further the invention provides a waveguide assembly of waveguide structures which are oriented perpendicularly with respect of one another. The invention also provides an assembly of two or more of said waveguide assemblies.
The invention will now be described further by way of example with reference to the accompanying drawings in which: Figure la and ib illustrate one dimensional waveguide assembly in side and plan view;Figure 2 illustrates a wave guide interconnection pattern for connecting two integrated circuits, stacked to form a waveguide assembly;Figure 3 illustrates a wave guide interconnection pattern for connection of four integrated circuits;Figure 4 illustrates a two dimensional waveguide assembly comprising two wave guide stacks attached orthogonally, integrated circuits and semiconductor light sources;Figure 5 illustrates a three dimensional wave guide assembly comprising three mutually orthogonal wave guide stacks; and,Figure 6 illustrates a waveguide stack assembly comprising a semiconductor light source attached to the waveguide assembly.
In the configuration shown in figures la and lb an array of channel waveguides 5 on a single substrate is used to connect optically a linear array of points on an integrated circuit 1 to a corresponding array of points on a second integrated circuit 2, with a regular interconnect pattern. Each waveguide is aligned to a single modulator element of an array of modulators, 3 at one end and to a single detector element of an array of detectors 4, where said detector and transmitter arrays are electrically connected in data input and output points on the integrated circuits 1 and 2. One means of achieving the electrical and mechanical connections is solder-bonds 6. The interconnection pattern achieved between the two arrays of points is determined by the waveguide pattern between them and can include branching for fan out and recombination for fan in.Arbitrary connection patterns are achieved by allowing the waveguides to cross (at large angles to prevent coupling between them). The use of modulators for data transducer elements requires the use of a continuous-wave (CW) laser 7 to illuminate them, and this is provided by an additional waveguide pattern in the array. Electrical connection between the circuits for power supplies is provided by an additional plane 8, connected and attached to the integrated circuits 1 and 2 by solder-bonds.
An example of a specific interconnection pattern is shown inFigure 2. The waveguide pattern 9 implements a connection pattern between the integrated circuits 10 and 11 for achieving arbitrary computations through regular interconnection patterns. Figure 3 shows the full implementation of this interconnection between four integrated circuits, 12, 13, 14 and 15, by an assembly of eight individual but identical waveguide planes 16, optically fed by a semiconductor light source array 17. The waveguide arrays now provide interconnection between the 64 points (8 x 8 array) on each of the four ICs. Additionally the provision of the solid block of waveguide arrays provides a firm structure onto which the optoelectronic and electronic components can be rigidly and accurately aligned.It should be noted that the implementation proposed for this interconnect pattern requires the use of bulk optical components, free-space propagation and considerable volume (typically a small optical test bed). The waveguide implementation suggested here is compact (10cm3), robust and stable.
Figure 4 shows how the one-dimensional optical data interconnections can be extended to provide two-dimensional interconnections. In this case two stacks of waveguides 18 and 19 are positioned orthogonally to each other. Stack 18 interfaces with the integrated circuits 20 and provide data paths in the x-direction, fed by semiconductor light source arrays 21. The waveguide patterns in stack 18 include straight through waveguide paths 22 that allow light to be coupled into the lower waveguide stack 19.
Stack 19 provides data paths in the y-direction. Using a two layer structure each layer comprising a stack of planar waveguides and each stack accurately aligned and orthogonal to each other, and optical circuit board analogous to the multi-layer electrical boards (which are now well known) can be envisaged. The optical board would enable power for interconnects to be reduced, a greater number of interconnects, higher speed interconnects and greater fan out.
Figure 5 shows how the stacked waveguide assembly can provide optical interconnection paths in three dimensions. Stacks 23 and 24 perform the function illustrated in Figure 4 as described previously. The waveguides in stack 23 have now additional optical pathways 25 to connect with a third waveguide stack 26. Stack 26 provides connection in the z-direction. The three-dimensional interconnect assembly provides an optical back plane function whereby data is carried from integrated circuits on one board to those on another.
Figure 6 shows one means by which light is coupled from the semiconductor light source 27 to the waveguide paths on the stack 28 using solder bonds 29. A recess 30 is fabricated in the stack forming an additional edge 31 at which a waveguide end 32 is positioned during waveguide definition. Solder bonds 29 attach the source 27 onto the recess 30 so as to align the light output beam with the waveguide end 32. This means of optical source mounting provides good thermal dissipation, accurate positioning and is compatible with conventional surface mounting practices.
Waveguide splitting factors and coupling to detectors and modulators will be critically dependent on launch conditions when laser diodes are employed as the semiconductor source, due to modal noise and speckle pattern generation. This can be overcome by using a laser which is pulsed at high frequency by external modulation or due to internal instability or to frequency modulate the laser source or to use lasers having several longitudinal modes to give a wide spectrum and therefore a reduced speckle effect.