A new structure for bit synchronization in a tera-bit/s optical interconnection network has been designed using micro-electro-mechanical system (MEMS) technique. Link multiplexing has been adopted to reduce data packet communication latency. To eliminate link set-up time, adjustable optical delay lines (AODLs) have been adopted to shift the phases of the distributed optical clock signals for bit synchronization. By changing the optical path distance of the optical clock signal, the phase of the clock signal can be shifted at a very high resolution. A phase-shift resolution of 0.1 ps can be easily achieved with 30-μm alternation of the optical path length in vacuum.
© 2002 Optical Society of America
With the fast development of semiconductor technology, the working frequency of the central processor unit (CPU) is increasing exponentially obeying the Moor law. But the input/output capacity of the computer has not been improved accordingly. For a computer cluster composed of hundreds of or even thousands of personal computers, the interconnection network has become a bottleneck to improve the system performance. Currently, computer clusters are mainly interconnected by local area networks (LANs) [1,2]. As LANs are not developed specially for network computing, the performance of computer clusters is limited by the large communication latency and communication overheads. Optical interconnection has the potential of becoming an attractive alternative to electrical interconnection. In most of the architectures of optical interconnection networks, wavelength-tunable devices and optical switches (OSWs) have been used [3–5]. With these devices, the interconnection network can support various topologies and have more efficient use of wavelengths. But these devices are usually expensive with present manufacturing process. Optical time division multiplexing (OTDM) is also a promising technology towards the realization of high-speed interconnection network, where optical fiber delay lines are adopted for phase-shifting of the optical pulses [6–8]. But these kinds of delay lines can only generate fixed delays for optical signals. Or in other words, the phase of the optical signals can not be continuously shifted.
In this paper, a new structure for bit synchronization in a tera-bit/s optical interconnection network has been designed using micro-electro-mechanical system (MEMS) technique. Link multiplexing has been adopted to reduce data packet communication latency in the network. This structure is also cost-effective compared to packet-switching based routers. Currently, the link set-up time for the establishment of a data transmission channel is on the order of hundreds of microseconds. Although it can be reduced to several microseconds, but the design complexity and system cost will increase. To eliminate link set-up time, adjustable optical delay lines (AODLs) have been adopted to shift the phases of the distributed optical clock signals for bit synchronization. By changing the optical path length of the optical clock signal, its phase can be shifted at a very high resolution. A phase-shift resolution of 0.1 ps in time domain is in accordance to the optical path difference of 30 μm in vacuum. This can be easily achieved via MEMS optical delay line. To enlarge the phase-shift range, a multi-reflection structure has been adopted in the AODLs.
2. Structure of the interconnection network
The tera-bit/s optical interconnection network is made up of a 1024 × 1024 crossbar-based switch core and a distributed optical clock network with adjustable optical delay lines for phase adjustment. The structure of the network is shown in Fig. 1. It can be used for the interconnection of 2048 node computers. The main part of the interconnection network is the switch core based on 32×32 crossbar switch module. Each input signal can be switched to any output port and each output port can only be connected to one input port. The maximum working frequency of the crossbar is 1.5 GHz with a switching delay of 10 ns. The total bandwidth of the switch core is 1.536 tera-bit/s. Since the switch core is made up of independent crossbar modules, it has high flexibility to meet different bandwidth requirements.
Link switching has been adopted at the tera-bit/s switch router. Although the switching delay at the router is only 10 ns, a large delay will be introduced at the giga-bit/s fiber links (GFLs), where phase-locked loop is implemented for synchronization. The typical link setup time for the GFL is on the order of hundreds of microseconds. To solve this problem, a distributed clock network is introduced at the router. A high-quality clock signal is generated at the clock generator and converted to optical signal. Through optical amplifiers and optical splitters, the optical clock signal is divided to a distributed clock network (DCN), where adjustable optical delay lines (AODLs) are adopted for phase match between the clock and data signals. By adopting the DCN in the router, the link setup time at the GFL can be saved, so the total communication latency for data packet transmission between the source and destination node computers can be reduced.
The AODLs can be implemented with bulk optical lenses that are mounted on a mechanical linear motion rail (MLMR). The input and output lenses are connected to the input and output fibers, respectively. By changing the distance between the input and output lenses, the optical clock signal transmission distance can be changed. So the phase of the clock signal can be modified to match that of the data signal. Another method to implement the AODL is using micro-opto-electro-mechanical system (MOEMS), which will be discussed in detail in Section III. The two techniques can be used in combination to implement the AODL, where MLMR is adopted for coarse phase shifting and MOEMS is adopted for fine phase shifting.
3. Principle of the multi-reflection MOEMS optical delay line
MOEMS technique can be adopted to implement the adjustable optical delay line. It has the advantages of large-scale integration and accordingly low cost. Since it is easy to implement mechanical AODL with an accuracy of 200 μm, so the phase-shift range of 200 μm is enough for the adjustable optical delay line based on MOEMS technique. If only one corner mirror is adopted for the AODL, as is shown in Fig. 2, the corner mirror moving distance should be at least 100 μm. A large driving voltage (typically more than 100 V) should applied to the actuator of the MOEMS optical delay line to generate such a displacement. Such a high voltage is not convenient to generate in a digital-circuit system whose power supply voltage is usually 5.0 V or 3.3 V. Meanwhile, MOEMS optical delay line array with 50 μm axial displacement for the actuators will have a relatively large size after packaging. Consequently the integration scale will be limited.
For a driving voltage of 20 V, the displacement of the actuator is usually less than 10 μm. Adopting the AODL structure in Fig. 2, the maximum phase-shift range of the optical clock signal is only 20 μm. Accordingly, the resolution requirement for the mechanical optical delay line will be less than 20 μm. Although this resolution requirement can be satisfied with micro-linear-motion rail (μLMR) driven by a stepping-motor, the total cost of the AODL will increase. Typically, the cost of the μLMR system will increase linearly or super-linearly at high range-to-resolution ratio. If the phase-shift range of the MOEMS optical delay line can be up to 200 μm at a relatively low driving voltage, the total cost of the AODL can be reduced dramatically. Under this condition, the resolution requirement for the mechanical optical delay line will be 200 μm. This requirement can be simply met by control the length of the fiber, and the mechanical delay line is no longer required by the system. To achieve a larger phase-shift range under a low driving voltage, a multi-reflection structure can be adopted for the MOEMS optical delay line, see Fig. 3. In this structure, two corner mirror arrays are implemented to generate multi-reflections of the optical signal.
4. Performance analysis of the optical delay line
The phase-shift range of the multi-reflection MOEMS optical delay line is decided by the maximum displacement of the movable reflection mirror array and the number of reflections. In application, the optical power loss of the AODL should also be considered carefully to meet the optical power budget of the interconnection network. The optical power attenuation is mainly caused by three kinds of losses. The first is the transmission loss due to the beam expanding in propagation. The second is the reflection loss at the reflection mirror arrays. And the third is the coupling loss when focusing the light beams in free space into the single mode fiber. The transmission loss is neglectable for a transmission distance on the order of millimeters. The coupling loss is generated by the axial deviation of the light beam and non-perpendicular injection of the light beam on the collimating lens. This can be minimized by carefully assembling of the optical devices and improving the MOEMS-processing accuracy. The reflection loss may become the main source of optical power attenuation in a multi-reflection optical delay line.
If the distance between the fixed reflection mirror array and the movable reflection mirror array is x (μm), and the total reflection times is n , then we can get the phase-shift of the AODL in terms of optical path in vacuum as follows
where y is the vertical transmission distance of the optical signal at the corner reflection mirror. Since the actuator of the MOEMS optical delay line only moves horizontally, the optical path difference (OPD) generated by the movable reflection mirror can be described by
where s is the displacement of the movable reflection mirror array. The relationship between OPD and s is shown in Fig. 4.
For a phase-shift range of 200 μm, the axial displacement of the actuator is only 25 μm with 14 times of reflection. So the driving voltage of the actuator can be reduced significantly compared to that for 100-μm displacement. Although the driving voltage can be further reduced by increasing the number of reflections in the MOEMS optical delay line, the loss of the optical power may be too high. Since the loss of the optical power in the MOEMS optical delay line is mainly due to the reflection loss, a compromise should be taken between the driving voltage and the optical power loss. The reflection loss of each mirror can be reduced to less than 5% if coated with high-reflection film. The total reflection loss can be described as
where α is the reflection loss of each mirror in per cent. If α is 5%, the total reflection loss may reach 6.7 dB for 30-times of reflections. Currently, the collimating lens with dimensions of 50 μm × 30 μm × 4 μm can be fabricated, and the coupling efficiency can reach 71% . Micro-lenses with larger dimensions are more easily to fabricate. The size of the reflection mirrors is dependent on the diameter of the light beam. If the light-beam diameter is expanded to 20 μm by the collimating lens, the dimensions of each reflection mirror should be 30-μm in height and 50-μm in length. Further reducing the dimensions of the reflection mirrors may induce the increase of optical power loss.
The multi-reflection structure can not only be used in optical interconnection network for phase matching between the data and clock signal, it can also be adopted in an optical micro-interferometer to generate the required optical path difference [10,11]. A typical application is a Michelson interferometer where a movable reflection mirror is adopted in the scanning arm. By adopting multi-reflection structure, the optical path difference generated by the scanning arm can be increased. Accordingly, the measurement range can be expanded.
A new structure for bit synchronization in a tera-bit/s optical interconnection network has been designed using MOEMS technique. Link multiplexing was adopted to reduce data packet communication latency in the network. This structure is also cost-effective compared to packet-switching based routers. To eliminate link set-up time, AODLs have been adopted to shift the phase of the distributed optical clock signals for bit synchronization. By changing the optical path distance of the optical clock signal, the phase of the clock signal can be shifted at a very high resolution. A phase-shift resolution of 0.1 ps in time domain can be easily achieved via MOEMS optical delay line. To enlarge the phase-shift range, a multi-reflection structure can be adopted. Using this technique, the driving voltage of the actuator can be significantly reduced for a fixed phase-shift range. But the optical power loss will increase with the number of reflections. A compromise should be taken between the driving voltage and the optical power loss.
This work was supported by the Natural Science Fund of Tianjin, and the Teaching and Research Award Program for Outstanding Young Teachers in Higher Education Institutions of MOE, China.
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