Interconnect structure for coupling an electronic unit and an optical unit, and optoelectronic module

Abstract

An optoelectronic module is provide and includes an electronic unit, an optical unit, and an interconnect structure. The electronic unit is capable of outputting and/or receiving electric signals, while the optical unit is capable of converting the electric signals into optical signals. The interconnect structure connects the electronic unit and the optical unit, and includes an electrically conducting substrate and a pair of transmission leads connecting electronic unit and the optical unit. The pair of transmission leads includes a signal lead and a ground lead having lower impedance than the signal lead.

Claims

1. An optoelectronic module, comprising: an electronic unit outputting and/or receiving electric signals; an optical unit for converting the electric signals into optical signals; an interconnect structure connecting the electronic unit and the optical unit, the interconnect structure comprising an electrically conducting substrate and a pair of transmission leads connecting the electronic unit and the optical unit and having a signal lead and a ground lead having lower impedance than the signal lead; and a second electronic unit processing electric signals generated by the optical unit.

2. The optoelectronic module according to claim 1, wherein the ground lead has a width that is larger than a width of the signal lead.

3. The optoelectronic module according to claim 2, wherein the ground lead has a width that is at least five times larger than the width of the signal lead.

4. The optoelectronic module according to claim 2, wherein the interconnect structure includes a ground plane layer underlying the ground lead.

5. The optoelectronic module according to claim 4, wherein the ground lead connects to the ground plane layer with an electrical connection.

6. The optoelectronic module according to claim 5, wherein the electrical connection comprises a via.

7. The optoelectronic module according to claim 6, wherein an impedance of the signal lead is defined by the width and a distance to the ground plane layer.

8. The optoelectronic module according to claim 1, wherein the interconnect structure includes a pair of receiver leads connecting the optical unit and the second electronic unit to each other.

9. The optoelectronic module according to claim 1, wherein the electronic unit includes a driver circuit and the optical unit includes an optical transmitter.

10. The optoelectronic module according to claim 9, further comprising an optical receiver and an amplifying circuit for amplifying output signals of the optical receiver.

11. The optoelectronic module according to claim 10, wherein the optical receiver includes an array of PIN (positive intrinsic negative) photo diodes.

12. The optoelectronic module according to claim 11, wherein the amplifying circuit includes a transimpedance amplifier (TIA) array.

13. The optoelectronic module according to claim 12, wherein the optical unit includes an array of vertical cavity surface emitting lasers (VCSEL).

14. The optoelectronic module according to claim 13, further comprising a 12-channel VCSEL array mounted in the electrically conductive substrate.

15. An optoelectronic module, comprising: an electronic unit outputting and/or receiving electric signals; an optical unit for converting the electric signals into optical signals; and an interconnect structure connecting the electronic unit and the optical unit, the interconnect structure comprising an electrically conducting substrate and a pair of transmission leads connecting the electronic unit and the optical unit and having a signal lead and a ground lead having lower impedance than the signal lead; wherein the electronic unit includes a driver circuit and the optical unit includes an optical transmitter.

16. The optoelectronic module according to claim 15, further comprising an optical receiver and an amplifying circuit for amplifying output signals of the optical receiver.

17. The optoelectronic module according to claim 16, wherein the optical receiver includes an array of PIN (positive intrinsic negative) photo diodes.

18. The optoelectronic module according to claim 17, wherein the amplifying circuit includes a transimpedance amplifier (TIA) array.

19. The optoelectronic module according to claim 18, wherein the optical unit includes an array of vertical cavity surface emitting lasers (VCSEL).

20. The optoelectronic module according to claim 19, further comprising a 12-channel VCSEL array mounted in the electrically conductive substrate.

21. An optoelectronic module, comprising: an electronic unit outputting and/or receiving electric signals; an optical unit for converting the electric signals into optical signals; a second electronic unit processing electric signals generated by the optical unit; an interconnect structure connecting the electronic unit and the optical unit, the interconnect structure comprising an electrically conducting substrate and a pair of transmission leads connecting the electronic unit and the optical unit and having a signal lead and a ground lead having lower impedance than the signal lead; a ground plane layer underlying the ground lead, wherein the ground lead connects to the ground plane layer using a via; wherein an impedance of the signal lead is defined by the width and a distance to the ground plane layer, and the ground lead has a width that is larger than a width of the signal lead.

22. The optoelectronic module according to claim 21, wherein the interconnect structure includes a pair of receiver leads connecting the optical unit and the second electronic unit to each other.

Description

BRIEF DESCRIPTION OF THE EMBODIMENT(S)

(1) The accompanying drawings are incorporated into and form a part of the specification to illustrate several embodiments of the invention. These drawings together with the description serve to explain the principles of the invention. The drawings are merely for the purpose of illustrating the preferred and alternative examples of how the invention can be made and used, and are not to be construed as limiting the invention to only the illustrated and described embodiments. Furthermore, several aspects of the embodiments may form—individually or in different combinations—solutions according to the invention. Further features and advantages will become apparent from the following more particular description of the various embodiments of the invention, as illustrated in the accompanying drawings, in which like references refer to like elements, and wherein:

(2) FIG. 1 is perspective view of a known optoelectronic module with interconnects having essentially equal impedance;

(3) FIG. 2 is a close up perspective view of the known optoelectronic module of FIG. 1 showing cross talk effects;

(4) FIG. 3 is a perspective view of an optoelectronic module having an interconnect structure according to the invention;

(5) FIG. 4 is a close up top plan view of the optoelectronic module with the interconnect structure of FIG. 3;

(6) FIG. 5 is a schematic diagram showing s-parameter coupling coefficient vs. frequency curve corresponding to the strongest cross talk from driver output to the neighbouring channel VCSEL input obtained from the known optoelectronic module of FIG. 1;

(7) FIG. 6 is a schematic diagram showing s-parameter coupling coefficient vs. frequency curve corresponding to the strongest cross talk from driver output to the neighbouring channel VCSEL input obtained from the optoelectronic module with the interconnect structure of FIG. 3;

(8) FIG. 7 is a 25 Gbps EYE diagram for a Driver-VCSEL channel with all aggressor channels activated, for the known optoelectronic module of FIG. 1; and

(9) FIG. 8 is a 25 Gbps EYE diagram for a Driver-VCSEL channel with all aggressor channels activated, for the optoelectronic module with the interconnect structure of FIG. 3.

DETAILED DESCRIPTION OF THE EMBODIMENT(S)

(10) Referring now to FIG. 3, an optoelectronic module having an interconnect structure according to the invention will be described.

(11) In FIGS. 3 and 4, the electrically insulating parts of the circuit carrier are not shown in order to more clearly illustrate the electric connections between the conductive parts.

(12) However, of course also insulating layers are present and furthermore, the shown metallic layers do not have to be the only electrically conductive layers.

(13) As shown in the perspective view of FIG. 3, a VCSEL array 100 having an electrically conductive substrate 102 is connected to a ground plane layer 108. On an active chip surface of the VCSEL array 100, contact pads (not visible in this figure) are provided for contacting the anode and cathode of the individual laser diodes. In particular, each signal line 104 is connected to an anode of a VCSEL. On the other hand, each ground line 106 is connected to a cathode terminal of a VCSEL.

(14) According to the invention, the ground lines 106 have a significantly lower impedance than the signal lines 104. In the embodiment shown in FIG. 3, this is firstly achieved by choosing the dimensions of the leads in a way that the signal lines 104 are significantly narrower than the ground lines 106. Furthermore, each of the ground lines 106 is connected to the underlying ground plane layer by means of a via contact 118.

(15) A driver array 120 to which the ground lines 106 and the signal lines 104 are connected may symbolize either a directly soldered or press fitted integrated circuit (IC), or a connector for attaching respective wires.

(16) By means of the arrangement according to the invention, the signal line impedance is not influenced by the distance between the signal line 104 and the ground line 106, but only by the distance to the underlying ground plane layer. This distance may for instance be as small as 10 μm, whereas the distance between the signal line 104 and the ground line 106 may be 60 μm.

(17) Preferably, the ground lines 106 are at least five times wider than the signal lines 104. For instance, the signal lines may have a width of about 20 μm, whereas the ground lines have a width of 110 μm. The impedance value of the signal lines 104 is set by the width of the signal lines and their distance to the ground plane layer to a value of about 50 Ohm. Of course, these particular dimensions are only examples of the values that can be used to implement the idea according to the invention.

(18) By using this particular construction, the impedance for the signal return path is significantly reduced and thus a much lower amount of the aggressor return current passes through the VCSEL of the target channel. This is illustrated in FIG. 4, which is a detail of FIG. 3. As can be seen from this figure, the main part of the return signal is guided by the ground line 106 and only a smaller fraction of the energy returns via the adjacent signal path 104b.

(19) This effect is also mirrored by comparing the s-parameter vs. frequency curves shown in FIGS. 5 and 6 for the arrangement of FIGS. 1 and 3, respectively. Each of these s-parameter coupling coefficients are shown corresponding to the strongest cross talk from the driver output to the neighboring channel VCSEL input.

(20) FIGS. 7 and 8 show in a comparison the respective EYE diagrams for a 25 Gbps bit rate and a bit error rate of BER=10.sup.−12. As shown in FIG. 7, the arrangement of FIG. 1 generates about 4 ps or 0.1 UI jitter for a driver VCSEL channel with all aggressor channels activated, whereas according to FIG. 8 the cross talk generates below 1.8 ps or 0.05 UI jitter in the EYE diagram. The outlines 701 and 801, respectively, correspond to BER=10.sup.−12.

(21) Consequently, both the coupling between neighboring channels and the jitter due to the cross talk are significantly reduced when comparing the interconnect solution according to the invention to the case with equal impedances according to FIG. 1. Thus, the interconnect structure according to the invention reduces crosstalk level between the channels, allowing longer transmission links and yielding a better quality of the signal.

(22) The invention is based on the idea that by modifying the impedance of the return path for the driver-VCSEL interconnect in a way that it is significantly lower than that of the signal path, a significantly lower amount of the aggressor return current passes through the VCSEL of any target channel.

(23) According to the invention, an optoelectronic module based on the electrical interconnect structure has an optical unit, such as a VCSEL array, with an electrically conducting substrate that can be connected to a ground plane layer of the interconnect structure. This topology leads to a particularly efficient cross talk suppression by choosing the impedance ratio of the signal and ground line according to the invention.

(24) The foregoing illustrates some of the possibilities for practicing the invention. Many other embodiments are possible within the scope and spirit of the invention. Therefore, more or less of the aforementioned components can be used to conform to that particular purpose. It is, therefore, intended that the foregoing description be regarded as illustrative rather than limiting, and that the scope of the invention is given by the appended claims together with their full range of equivalents.