WO2008121075A1

WO2008121075A1 - Optical interconnect structure and method

Info

Publication number: WO2008121075A1
Application number: PCT/SG2007/000086
Authority: WO
Inventors: Sik Pong Bryan Lee; Venkata Ramana Pamidighantam; Jing Li; Haridas Kuruveettil
Original assignee: Agency For Science, Technology And Research
Priority date: 2007-03-30
Filing date: 2007-03-30
Publication date: 2008-10-09

Abstract

An optical interconnect structure and method, The interconnect structure comprises a waveguide formed on or embedded in a substrate; two openings formed in the waveguide such that a pair of an internal total reflection mirror and a free space mirror are formed at each opening; two transceiver modules mounted on the substrate, each transceiver module comprising a light source element and a photo detector element; wherein the light source elements of the respective transceiver modules are aligned with the free space mirrors at the respective openings and the photo detector elements of the respective transceiver modules are aligned with the internal total reflection mirrors at the respective openings.

Description

OPTICAL INTERCONNECT STRUCTURE AND METHOD

FIELD OF INVENTION

The present invention relates broadly to an optical interconnect structure, to an optical transceiver module, and to method of providing a bi-directional optical interconnect between optical transceiver modules.

BACKGROUND

The transfer of signals from one information-processing system to another typically occurs via what may be generally referred to as "interconnects" between the information-processing systems. Typically, the interconnects are over a relatively short distance, such as communications between chips on a circuit board or between circuit boards, for example.

While single chip information-processing systems are preferred due to their independence of off-chip bandwidth limitations, for a large digital system a single-chip design is typically too complex to be practically implemented. Rather, such digital systems are typically partitioned into several modules, which may consist of individual chips, multi-chip modules (MCMs) or entire printed- circuit boards. As processing capability of each module increases, the capacity of the interconnection that connects the module is also required to increase. Therefore, there is a need for an interconnect technology with high signal transfer capacity. However, there is an increasing gap between the off-chip bandwidth available using existing electrical interconnect techniques and the processing capability of integrated circuits. In existing electrical interconnect techniques, electrical interconnects are typically parallel to transfer large data rates, like PCI and ISA buses.

The performance of electrical interconnects is limited by a number of factors such as bandwidth limitation due to the line impedance, increased power consumption with increase in frequency, effects of cross talk and electromagnetic interference due to the inductance and capacitance of the interconnects, the effects of interconnect density on interconnect performance, restrictions due to signal path termination, and planar design constraints. As a result, a bandwidth bottleneck is predicted within the next few years for the electrical interconnections linking integrated circuits (ICs) in a printed circuit board (PCB) due to the requirements of increased processing power.

One potential solution is the use of optical interconnects as an alternative to electrical interconnects. The advantages of optical interconnects include parallelism, low input/output driving energy, capability to withstand electromagnetic interference, and low dispersion.

In one suggested optical interconnect technique, separate transmitter and receiver modules are provided as individual chips, and uni-directionally interconnected via a polymer optical waveguide embedded in the PCB. One disadvantage with that suggested technique is that for bi-directional interconnection, two waveguides and a total of four separate modules (two receiver modules, two transmitter modules) are required, which increases the size and cost associated with that interconnect technique.

In another optical interconnect technique, PCB-to-PCB optical interconnects are provided. Uni-directional transmission of optical signals occurs between the PCBs via a waveguide, and utilizing discrete optics components on both PCBs for coupling into and from the waveguide. One disadvantage associated with that technique is the requirement for discrete optics components, which adds to the implementation cost. Furthermore, only uni-directional transmission is facilitated for each backplane waveguide, thus requiring multiple waveguides and multiple discrete optics components for bidirectional interconnects between the PCBs.

In another optical interconnect technique, optical fiber pluggable transceiver modules are used. One disadvantage with that technique is that the use of optical fiber interconnects may not be suitable for on-board chip-to-chip interconnects. Furthermore, this technique may have limitations in terms of miniaturization of the mechanical components involved in connecting the optical fiber to the transceiver module.

In yet another suggested optical interconnect technique, 45° ended optical connection blocks in fiber and waveguide embedded PCBs are utilized for uni-directional transmission between separate transmitter and receiver modules. The 45° ended connection blocks with embedded optical fibers are utilized to couple light signals from polymeric waveguides embedded in the PCB, with the 45° ended connection blocks being disposed in via-holes in the PCB. One disadvantage of that technique is that discrete receiver and transmitter components are used for uni-directional transmission, thus requiring multiple waveguide and transmitter and receiver pairs for bi-directional interconnections. Furthermore, a high bonding accuracy is required between the laser diodes and the photodiodes of the transmitter and receiver component on the one hand, and the 45° ended connection blocks with embedded fibers. Furthermore, the fabrication of the 45° ended connection blocks with embedded fibers may not be suitable for mass production.

A need therefore exists to provide an optical interconnects design and technique that seeks to address at least one of the above mentioned problems.

SUMMARY

In accordance with a first aspect of the present invention there is provided an optical interconnect structure comprising a waveguide formed on or embedded in a substrate; two openings formed in the waveguide such that a pair of an internal total reflection mirror and a free space mirror are formed at each opening; two transceiver modules mounted on the substrate, each transceiver module comprising a light source element and a photo detector element; wherein the light source elements of the respective transceiver modules are aligned with the free space mirrors at the respective openings and the photo detector elements of the respective transceiver modules are aligned with the internal total reflection mirrors at the respective openings. The light source elements may comprise surface emitting semiconductor devices.

The surface emitting semiconductor devices may comprise VCSELs or LEDs.

The photo detector elements may comprise photo diodes.

Each transceiver module may further comprise a driver element for the light source element.

Each transceiver module may further comprise a Trans-impedance Amplifier (TIA) element for the photo detector element.

Each transceiver module may further comprise a collimating lens for collimating a light signal emitted from the light source element.

Each transceiver module may further comprise a focusing lens for focusing a received light signal onto the photo detector element.

The collimating lens and the focusing lens of each transceiver module may be provided in an integral lens structure.

The integral lens structures may be mounted onto the respective transceiver modules using a pillar structure.

The integral lens structures may be molded.

Each transceiver module may be mounted onto the substrate via a solder self aligning process.

In accordance with a second aspect of the present invention there is provided an optical transceiver module comprising a surface mountable package structure; a light source element mounted on the package structure; a photo detector element mounted on the package structure; a collimating lens mounted on the package structure for collimating a light signal emitted from the light source element for directing towards a free space mirror formed in a waveguide; and a focusing lens mounted on the package structure for focusing a light signal reflected from an internal total reflection space mirror formed in the waveguide onto the photo detector element.

The light source elements may comprise VCSELs.

The photo detector elements may comprise photo diodes.

The transceiver module may further comprise a driver element for the light source element.

The transceiver may further comprise a TIA element for the photo detector element.

The collimating lens and the focusing lens may be provided in an integral lens structure.

The integral lens structure may be mounted onto the package structure using a pillar structure.

The integral lens structure may be molded.

The transceiver module may further comprise solder elements formed on the package structure for mounting of the transceiver module via a self aligning process.

In accordance with a third aspect of the present invention there is provided a method of providing a bi-directional optical interconnect between optical transceiver modules, the method comprising providing a waveguide on or embedded in a substrate; forming two openings in the waveguide such that a pair of an internal total reflection mirror and a free space mirror are formed at each opening; and aligning light source elements of the respective transceiver modules with the free space mirrors at the respective openings and photo detector elements of the respective transceiver modules with the internal total reflection mirrors at the respective openings.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be better understood and readily apparent to one of ordinary skill in the art from the following written description, by way of example only, and in conjunction with the drawings, in which:

Figure 1a shows a schematic cross-sectional view of an optical interconnect structure according to an example embodiment.

Figure 1b shows a schematic cross-sectional view of a waveguide of the optical interconnect of Figure 1a.

Figure 1c shows a schematic cross-sectional view of another optical interconnect structure according to an example embodiment.

Figure 2a shows a schematic bottom view of an opto-electronic transceiver module of the system of Figure 1a.

Figure 2b shows a schematic cross-sectional view of the opto-electronic transceiver module of Figure 2a. Figure 2c shows a circuit schematic diagram of the opto-electronic transceiver module of Figure 2a.

Figure 3 shows a schematic drawing illustrating the optical paths in the system of Figure 1a.

Figure 4 shows a schematic top view illustrating the orientation of two identical transceiver modules in a bi-directional multi-waveguides interconnection.

Figure 5 shows a flowchart illustrating a method of providing a bidirectional optical interconnect between optical transceiver modules, according to an example embodiment. Figure 6 shows a design drawing of a module cavity and solder ball structure for placement and attachment in one implementation.

Figures 7a) and b) show respective schematic waveguide cross-sectional views illustrating dicing of the waveguide in one implementation.

Figure 8 shows a high frequency design drawings of a SMT optical module including a transmission line design in one implementation. Figure 9 shows a perspective view design drawing of a cover for the SMT optical module, viewed from inside the cavity, in one implementation. Figure 10 shows a detail of Figure 9.

Figure 11 shows a schematic test board layout design according to an example embodiment.

DETAILED DESCRIPTION

The described embodiments relate to an opto-electronic module in which elements such as a vertical cavity surface emitting laser (VCSEL) and a PIN photodiode are integrated into a single transceiver module, which can be used for transmitting and receiving optical signals through a single optical waveguide interconnect.

Figure 1a shows a schematic cross-sectional view of two opto-electronic transceiver modules 100, 102 mounted on a PCB 104. A single waveguide 106 is attached on the top layer of the PCB 104, creating an opto-electronic circuit board (OECB) 108, in which all electrical traces are buried in the inner layers of the PCB 104.

The cross sectional area of the waveguide 106 is an important parameter, which determines the total coupling efficiency and insertion loss. It is determined by the source beam profile, the gap between the modules 100, 102 and the waveguide 106 and the photo detector 120, 124 active aperture. Preferably, the waveguide 106 is optimized so as to balance the trade off between input and output coupling efficiency. A large cross sectional area helps to increase the input misalignment tolerance, hence improves the input coupling efficiency. The optical waveguide 106 is designed for multimode operation for good input coupling efficiency and easy fabrication. Inter-modal dispersion is not critical as the transmission distance is short in the described embodiment.

The optical waveguide 106 in the described example is designed to operate in the multimode with a constant core height of about 50μm and width of about

70μm. The cross sectional area in the example implementation is optimized for a VCSEL 118, 122 beam profile of a divergence angle of about 18° and a photodiode 120, 124 detecting area diameter of about 70μm. The optical signal is designed to couple from the top of the waveguide 106. The upper cladding layer of the waveguide 106 is about 10μm thick while the lower cladding is about 25μm thick. This difference, which offsets the core position in the Z direction, reduces the propagation distance and gives better coupling in the described example. It will be appreciated that the dimensions can vary between different embodiments, and waveguides with other dimensions such as 50, 70, 100 or 130 micrometers can be implemented.

The waveguide 106 is fabricated separately and diced, and then fitted to the PCB 104 using an adhesive.

With reference to Figures 7a) and b), the waveguide 106 is mounted on an adhesive film 700 prior to the dicing. The adhesive film 700 has wider and longer dimensions compared to the waveguide 106. Thus, after the dicing processing, illustrated in Figure 7b), the waveguide 106 together with the adhesive 700 can be transferred onto the PCB for mounting.

Returning to Figure 1a), a U-shape groove may be formed on the surface of the PCB 104 to facilitate positioning of the waveguide 106 on the PCB 104.

The location of the dicing channels or openings 110, 112 is identified from the

VCSEL 118, 122 pads (not shown) and photodiodes 120, 124 pads (not shown) on the PCB 104, with no additional markers required in the described example.

The respective waveguide ends at the openings 110, 112 are then cut at about 45° using excimer laser processing technology for creating one total internal reflection mirror 114a, b and one free space mirror 116a, b at each opening 110,

112 respectively. The openings 110, 112 are designed for alignment between the VCSEL e.g. 118 with the free space mirror 116a, and the photodiode e.g.

120 with the internal reflection mirror 114a for each transceiver module e.g. 100. Therefore, in the described example, no discrete optic elements are advantageously required for optical coupling between the waveguide 106 and the transceiver modules 100, 102.

The transceiver modules 100, 102 are surface mounted packages containing the VCSEL e.g. 118 and the PIN photodiode e.g. 120, with or without associated driver electronics in different embodiments. The modules 100, 102 are assembled to the OECB 108 using normal surface mount technology (SMT) soldering processes in the described example. As will be appreciated by a person skilled in the art, the SMT assembly process controls the precision of placement of the transceiver modules 100, 102. More particular, a number of solder bumps e.g. 103 are patterned on the transceiver modules 100, 102 using screen printing methods in the described example, for attachment on corresponding solder pads (not shown) on the PCB 104. In the described example, the number of solder bumps e.g. 103 is deliberately increased by adding dummy bumps in addition to actual connection bumps of the Ball Grid Arrays (BGAs) of the transceiver modules 100, 102 respectively, to improve placement accuracy. The ability to align the transceiver modules 100, 102 to the PCB 104 spontaneously is based on the so called self-aligning effect in the two dimensional in-plane space of the PCB 104. More particular, if the transceiver modules 100, 102 are misaligned during the assembly, the molten solder material moves the respective modules 100, 102 to the required position during the reflow process facilitated by the solder pads (not shown) on the PCB 104. As will be appreciated by the person skilled in the art, the achievable accuracy depends on a number of parameters like the size of the modules the bump diameter and pitch, and the bump geometry. Figure 6 shows a design drawing, in a perspective view, of a module cavity 600 for positioning the optical and electrical devices, showing the BGA e.g. 602, 604 and a recess 606 for receiving a lense cover, in one example implementation.

Returning to Figure 1a, in the described example, the transceiver modules 100, 102 are identical, with one of the transceiver modules e.g. 102 placed at 180° orientation with respect to the other transceiver module 100, at respective ends of the waveguide 106 as defined by the openings 110, 112 respectively. As a result, for both modules 100, 102, the VCSELs 118, 122 are aligned with the respective free space mirrors 116a, b respectively, whereas the photodiodes 120, 124 are aligned with the internal reflection mirrors 114a, b respectively. Thus, bi-directional communication within the same single waveguide 106 is advantageously enabled using reversed transmitting and receiving channels between the VCSEL 118 and the photodiode 124 on the one hand, and the photodiode 120 and the VCSEL 122 on the other hand. In the described example, the waveguide 106 consists of a high refractive index material sandwich between layers of low refractive index material. A cross-sectional schematic drawing of the waveguide 106 in one implementation is shown in Figure 1b. The light propagates within the waveguide core 126 of high refractive index material through the principle of total internal reflection. The lower layer 128 is referred to as "under clad", while the upper layer 130 is referred to as "upper clad". Both the under clad 128 and upper clad 130 extend along side portions 132, 134 of the core 126, thus completely embedding the core 126 for light propagation. The waveguide 106 can be fabricated using existing manufacturing techniques, including spin coating, lamination or molding. Once the waveguide 106 is formed, the entire waveguide 106 can be attached onto the PCB 104 (Figure 1a) as described above. However, it is noted that in different embodiments, the waveguide 106 may be embedded within a PCB structure, for example by sandwiching the waveguide 106 between two internal layers of a multi layer board. Figure 1c shows a schematic cross-sectional drawing of such an alternative embodiment, in which the waveguide 106 is sandwiched between two internal layers 150, 152 in a multi layer board 154. Openings 156, 158 for light transmission into or out of the waveguide 106, or both at the same time, are formed in the upper internal layer 152 using e.g. excimer laser processing technology.

Figures 2a and b show a schematic bottom and a schematic cross- sectional view respectively of one opto-electronic transceiver module 200 in the described example. The transceiver module 200 is implemented in a surface mounted package, more particular a cavity down BGA design in the described example. In the schematic drawings of Figure 2a and b, only a smaller number of bump of pads 210 for solder bumps 212 are shown for clarity. In the described example, out of the 48 bump pads 210, only 12 are active with the rest being chosen as dummy or ground pads for facilitating improved alignment accuracy as described above. The design in the described example has an overall size of about 16 x 16mm, a cavity size of about 12 x 12mm, and a cavity depth of 8 metal layers with a layer thickness of about 0.175mm. A substrate 201 of the module 200 can be ceramic, organic, high resistivity silicon or another high frequency material in example embodiments, to support high frequency transmission. A VCSEL 202, a PIN photodiode 203, a VCSEL driver 204 and a Trans-impedance Amplifier (TIA) 205 and a Limiting Amplifier (LA) 207 are mounted and wire bonded in a chip on board format on the substrate 201 , to reduce the form factor of the transceiver module 200. A collimating lens 206 for the VCSEL 202, and a focusing lens 208 for the photodiode 203 can be formed together as a single cover 209 using injection molding processes. The cover 209 containing lenses 206 and 208 is assembled to the transceiver module 200 using finely fabricated pillars e.g. 210 that are glued to the assembly in the example embodiment.

It will be appreciated that the placement of the VCSEL 202 relative to the photodiode 203 is preferably chosen so as to minimise optical cross-talk, to reduce electrical noise in the transceiver module 200. The dimensioning of the openings 110, 112 (Figure 1) will be accordingly. The use of the collimating lense 206 and the focusing lense 208 further facilitates reduction of optical cross talk between the VCSEL 202 and the photodiode 203. In the described example, the lenses 206, 208 are bi-convex lenses. A spacer element 214 is provided for light blocking between the VCSEL 202 and the PIN photodiode 203. In addition to the mounting post or pillars e.g. 210 for alignment, the cover 209 is received in a recess 216 formed at the internal perimeter of the cavity 218. Figure 9 shows a design drawing of a lense cover 900 for an SMT module in one implementation, with a particular arrangement of mounting posts e.g. 902, lense surfaces e.g. 904, and a spacer 906. A recess 908 is formed at the parameter of the cover 900, for mounting onto the SMT module cavity.

Figure 2c shows a circuit schematic diagram 210 of the transceiver module 200. The VCSEL driver 204 is coupled to the VCSEL 202, and receives a differential electrical input signal via microstrip transmission lines 212 from an external digital signal interface..

On the receiving path, the photo current from the photodiode 203 is fed to the TIA 205, for converting the photo current into a voltage signal for further transmission. The output from the TIA 205 is coupled to the LA 207, which functions similar to an automated gain control, to ensure the signal amplitude does not go beyond define levels by changing the amplification in the LA 207. On the other hand, the LA 207 can also be utilized to compensate for electrical losses, in particular at high frequencies where electrical losses can be high. By adding the LA 207 in the described example, this can ensure that the received signal can be transmitted as an output signal via microstrip transmission lines 214 from the transceiver module 200 to an external device 216.

Figure 8 shows a design drawing of a transceiver module 800 in one implementation. A differential input signal is received at solder pads 802, 804 of the module 800. Microstrip transmissions lines 806 are provided between the pads 802, 804, and a DC block capacitor 808. A further microstrip transmission line 810 is provided between the DC block capacitor 808 and a VCSEL driver 812. The VCSEL driver 812 is coupled to the VCSEL 814. The active signal from VCSEL driver 812 is preferably connected to the VCSEL 814 through a direct wire bond between the chips to reduce the impedance at high frequencies. However, if the spacing between VCSEL 814 and the driver 812 is large, then the interconnect is preferably a micro strip line with wire bonding from the driver 812 (and VCSEL 814) to the micro strip line.

On the output side, the voltage signal converted from the photo current of the photodiode 816 is fed from the TIA 818 to a DC block capacitor 820 via microstrip transmission lines 822. A further microstrip transmission line 824 is provided between the DC block capacitor 820 and a limiting amp LA 826. After the limiting amp LA 826, another microstrip transmission line 828 is provided to differential output pads 834, 836 of the module 800.

Figure 3 shows a schematic drawing illustrating the optical paths in the regions of the gaps 110, 112 in the waveguide 106. In Figure 3, the darker arrows indicate the transmit signal path, whereas the lighter arrows indicate the receiver signal passes relative to one of the transceiver modules. More particular, from the VCSEL 118, the transmit signal 300 is reflected at the free space mirror 116a, and proceeds in the plane of the waveguide 106 and through the internal reflection mirror 114a from the gap 110 into the waveguide 106. Reflection losses, indicated by arrow 302, from the transmit signal will be directed away from the PIN photodiode 120, thus eliminating or at least significantly reducing optical cross-talk from the VCSEL 118 to the photodiode 120. For the received signal 304, reflection at the internal total reflection mirror 114a directs the received signal towards the photodiode 120.

Similarly, from VCSEL 122, the transmit signal 306 is reflected at the free space mirror 116b, and proceeds in the plane of waveguide 106 and through the internal reflection 114b from the gap 112 into the waveguide 106. Reflection losses, indicated by arrow 308, from the transmit signal will be directed away from the PIN photodiode 124 thus eliminating or least significantly reducing optical cross-talk from the VCSEL 122 to the photodiode 124. For the received signal 310, reflection at the internal total reflection mirror 114b directs the received signal towards the photodiode 124.

The described example embodiment provides a surface mounted optoelectronic module which facilitate bi-directional communications via a single waveguide. However, it will be appreciated that the same module can be used for communication through multiple waveguides by placing the respective modules appropriately with respect to the multiple waveguides.

Figure 4 shows a schematic top view illustrating the orientation of two identical transceiver modules 400, 402 relative to two optical waveguides 404, 406 in a bi-directional, multi-waveguides interconnection. The transceiver module 400 is at 180° orientation with respect to the other transceiver module 402, and with the VCSEL e.g. 408 and the photodiode e.g. 410 of each transceiver module e.g. 400 aligned with respective ones of the optical waveguides 404, 406. As will be appreciated by a person skilled in the art, in this configuration, the location of the waveguides 404, 406 is determined by the spacing between the VSCEL e.g. 408, and the PIN photodiode 410 and should match that spacing within the assembly tolerances.

Figure 5 shows a flowchart 500 illustrating a method of providing a bidirectional optical interconnect between optical transceiver modules, according to an example embodiment. At step 502, a waveguide is provided on a substrate. At step 504, two openings are formed in the waveguide such that a pair of an internal total reflection mirror and a free space mirrors are formed at each opening. At step 506, light source elements of the respective transceiver modules are aligned with the free space mirrors at the respective openings and photo detector elements of the respective transceiver modules with the internal total reflection mirrors at the respective openings.

Figure 11 shows a test board layout design for IOGbps interconnect on a Frame Resistant 4 (FR4) printed circuit board (PCB). The design includes two transceiver modules 1000, 1002 according to the example embodiments described above. Each module 1000, 1002 has 10GB/s differential data input lines 1004, 1006 respectively, as well as 10GB/s differential data output lines 1008, 1010 respectively. Clock and data recovery ICs 1012, 1014 are coupled to the differential data output lines 1008, 1010 respectively, to digitize the output data. The ICs 1012, 1014 utilized a lower frequency input clock signal received from reference clock lines 1016, 1018 respectively, at 600MHz in the example implementation, to synchronize the data edge. The remaining circuit components general indicated at numerals 1020, 1022, 1024 and 1026 are capacitors or impedance matching resistors for the example implementation.

In the described optical interconnects, integrated waveguides optical interconnects are provided which are suitable for high bandwidth connectivity. Multi mode waveguides in the described examples are preferred since they offer relaxed alignment tolerances while still providing sufficient bandwidth for lengths of the order of about less than one meter transmission. Multi mode waveguides are optical waveguides that allow more than one bound mode to propagate. The polymer waveguides deposited onto a PCB or embedded into a PCB structure are a viable choice for short distances interconnects.

It will be appreciated by a person skilled in the art that numerous variations and/or modifications may be made to the present invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly describe. The present embodiments are, therefore, to be considered in all respects to be illustrative and not restrictive.

Claims

1. An optical interconnect structure comprising: a waveguide formed on or embedded in a substrate; two openings formed in the waveguide such that a pair of an internal total reflection mirror and a free space mirror are formed at each opening; two transceiver modules mounted on the substrate, each transceiver module comprising a light source element and a photo detector element; wherein the light source elements of the respective transceiver modules are aligned with the free space mirrors at the respective openings and the photo detector elements of the respective transceiver modules are aligned with the internal total reflection mirrors at the respective openings.

2. The optical interconnect structure as claimed in claim 1 , wherein the light source elements comprise surface emitting semiconductor devices.

3. The optical interconnect structure as claimed in claims 1 or 2, wherein the photo detector elements comprise photo diodes.

4. The optical interconnect structure as claimed in any one of the preceding claims, wherein each transceiver module further comprises a driver element for the light source element.

5. The optical interconnect structure as claimed in any one of the preceding claims, wherein each transceiver module further comprises a TIA element for the photo detector element.

6. The optical interconnect structure as claimed in any one of the preceding claims, wherein each transceiver module further comprises a collimating lens for collimating a light signal emitted from the light source element.

7. The optical interconnect structure as claimed in any one of the preceding claims, wherein each transceiver module further comprises a focusing lens for focusing a received light signal onto the photo detector element.

8. The optical interconnect structure as claimed in claims 6 and 7, wherein the collimating lens and the focusing lens of each transceiver module are provided in an integral lens structure.

9. The optical interconnect structure as claimed in claim 8, wherein the integral lens structures are mounted onto the respective transceiver modules using a pillar structure.

10. The optical interconnect structure as claimed in claims 8 or 9, wherein the integral lens structures are molded.

11. The optical interconnect structure as claimed in any one of the preceding claims, wherein each transceiver module is mounted onto the substrate via a solder self aligning process.

12. An optical transceiver module comprising: a surface mountable package structure; a light source element mounted on the package structure; a photo detector element mounted on the package structure; a collimating lens mounted on the package structure for collimating a light signal emitted from the light source element for directing towards a free space mirror formed in a waveguide; and a focusing lens mounted on the package structure for focusing a light signal reflected from an internal total reflection space mirror formed in the waveguide onto the photo detector element.

13. The transceiver module as claimed in claim 1 , wherein the light source elements comprise emitting semiconductor devices.

14. The transceiver module as claimed in claims 12 or 13, wherein the photo detector elements comprise photo diodes.

15. The transceiver module as claimed in any one of claims 12 to 14, further comprising a driver element for the light source element.

16. The transceiver module as claimed in any one of claims 12 to 15, further comprising a TiA element for the photo detector element.

17. The transceiver module as claimed in any one of claims 12 to 16, wherein the collimating lens and the focusing lens are provided in an integral lens structure.

18. The transceiver module as claimed in claim 17, wherein the integral lens structure is mounted onto the package structure using a pillar structure.

19. The transceiver module as claimed in claims 17 or 18, wherein the integral lens structure is molded.

20. The transceiver module as claimed in any one of claims 12 to 19, further comprising solder elements formed on the package structure for mounting of the transceiver module via a self aligning process.

21. A method of providing a bi-directional optical interconnect between optical transceiver modules, the method comprising: providing a waveguide on or embedded in a substrate; forming two openings in the waveguide such that a pair of an internal total reflection mirror and a free space mirror are formed at each opening; and aligning light source elements of the respective transceiver modules with the free space mirrors at the respective openings and photo detector elements of the respective transceiver modules with the internal total reflection mirrors at the respective openings.