Introduction
With the rapid and comprehensive development of multimedia services, including telephone, cable TV (CATV), digital TV and Internet, the requirements for circuit bandwidth and capacity have increased dramatically. In the traditional field of electricity, the speed of signal transmission and switching has been limited.
Taking electronic computers as an example, the main frequency of its CPU has reached 2-2.9GHz, and the speed of transmitting code streams on the telecommunications trunk line has even reached dozens or even thousands of Gbits.
In contrast, the bus transmission of the computer still stays at 10-100M, which is no more than Gbit. Obviously, the speed of the computer’s internal bus connection and computer interconnection has become the bottleneck of the entire computer environment.
Light has long been talked about as an interconnect within computers (including inside circuit boards) and between computers. In principle, the transmission rate connected by wires is affected and limited by its parasitic parameters (parasitic resistance, inductance and parasitic capacitance). For example, the transmission rate of signals in commonly used FR-4 substrates is about 70% of the speed of light. Such a rate can no longer meet the demand in many fields.
Optical interconnects can overcome this situation. Photons have a large bandwidth and low transmission loss, free from crosstalk and magnetic interference, and when transmitting multiple wavelengths in the same optical medium, different wavelengths can pass in parallel. Therefore, the application of photons in the field of electronics has played an important role.
In this context, the concept of optoelectronic printed circuit board was put forward. Simply put, the photoelectric printed circuit board is the packaging substrate required for the new generation of high computing that integrates light and electricity, uses light for signal transmission, and uses electricity for computing. The upper layer of light guide layer. Therefore, the use of circuit boards has been developed from the current electrical connection technology to the field of optical transmission.
the development status of photoelectric printed circuit boards
According to reports, Ilovat University in Scotland has completed a research and development program called HOLMS. Through innovative optoelectronic technology, the traditional printed circuit board is integrated with its functions, thereby achieving a significant improvement in the assembly process of standard electronic equipment. The main achievement of this research is the successful combination of optical fiber, wireless technology and optical PCB to form a powerful optical interface and solve the technical bottleneck of the existing memory delay.
It is reported that memory delay is one of the primary obstacles faced by current computer systems. The main problem is that although the speed of computer processors is getting faster and faster, it still takes a while to access the data in the memory. According to the research results of Ilovat University, optoelectronics technology is the only solution that can solve the difference in processor speed and memory bandwidth, and the American Semiconductor Industry Association has also proved this point.
The research units participating in the HOLMS project introduce economical and practical high-speed optical circuit boards into information systems, with the goal of developing optoelectronic technology to make it compatible with standard electronic device assembly processes. Its main key technology is: the optical interface is loaded on a commercial parallel optical fiber array and a low-cost optical waveguide, so that it can be easily integrated with a traditional printed circuit board.
The HOLMS program concluded in September 2005 with the completion of two demonstration works demonstrating the functional aspects of the technology. The main universities involved in the research are the University of Illowat in Scotland and the University of Hagen in Germany, all of which are integrating related technologies and academic research.
A number of industry partners, including PCB manufacturer ILFA, Germany’s Siemens and France’s Thales, have introduced research results into product development. Among them, Thales is discussing how to apply HOLMS optoelectronic technology to ultra-high-speed defense embedded systems, and Siemens is also developing high-bandwidth optical waveguides. printed circuit boards and is expected to be available within two years.
DaimlerChrysler (Germany) Research Center is developing optical waveguide-based backplanes to link several computers on an aircraft or to transmit signals between several computers used in telecommunications systems. The photons are emitted by a vertical cavity surface emitting laser, and the waveguide is made of a polymer material, which is said to be easier to integrate into the system than optical fibers.
Primarion is developing a waveguide optical system that transmits signals at a rate of 10Gb/s over short distances, with the aim of keeping the signal all the way to the processor. Electrical signals travel from the circuit board to the laser driver chip and then into an array of 12 vertical cavity surface emitting lasers. The laser beam enters a similar device on another circuit board through an optical fiber, and the photodetector and receiving unit convert the signal back into an electrical signal. The company hopes to use this technology for computer optical input and output devices within two to three years.
In the 2003 Taiwan Circuit Board and Assembly Exhibition unveiled at the World Trade Center in Taiwan, China, the Institute of Electronics of the Taiwan Industrial Technology Research Institute, China, displayed the high-speed electrical signal transmission photoelectric printer jointly developed by the Institute, Huatong Computer and Jialianyi Technology. The achievement of manufacturing circuit boards and its key technology – organic optical waveguide soft film technology, this new construction technology will provide strong technical support for the GHz-level high-speed signal transmission environment required by computer broadband networks and services.
The dynamic display system provided by Electronics uses a computer with a built-in 1Gbps network interface to set up two ends, one of which first uses an electrical/optical conversion module to convert the electrical signal of the network card into an optical signal, and relies on the optical signal of the photoelectric printed circuit board. The soft film layer of the waveguide transmits the data, and the computer at the other end transmits the received optical signal through the optical/electrical conversion module and then converts it back to an electrical signal to be received by the network card. Through the above-mentioned transmission structure, a display of high-speed data flow transmission through the photoelectric printed circuit board is presented. At present, the photoelectric printed circuit board developed by the Electronics Institute of Taiwan Industrial Technology Research Institute has passed the actual transmission application and signal eye diagram test of 2.5GHz.
In addition, it has many advantages such as high density, multi-circuit, high integration, and suitable for mass production. Many PCB manufacturers and PCB substrate material manufacturers in Europe, America, Japan, South Korea, Taiwan, etc. have actively invested in the development of this new technology and market. The world’s upsurge of developing optoelectronic printed circuit boards has begun. In the past two years, the world PCB industry has continuously published research papers in this regard.
3. The principle of the optical interconnection structure of the board of the photoelectric printed circuit
The electrical signal generated by the large-scale integrated chip passes through the VSCEL laser transmitter driven by the core surface, and the laser beam is transmitted directly or through the lens to the polymer waveguide with a 45° mirror surface, reflected into the waveguide, and then transmitted to the PD through the mirror reflection of the other end of the waveguide Receive, and then convert it into an electrical signal through the receiving chip and transmit it to the large integrated chip. This enables high-speed communication between chips through the optical waveguide, thereby improving overall system performance.
The production of the PCB is compatible with the traditional PCB production process, but the polymer waveguide is regarded as the characteristic of copper in the PCB.
the advantages of optical PCB
As mentioned earlier, the data transmission rate of copper wiring is affected by its parasitic parameters resistance, inductance and capacitance. In the low frequency band, the series resistance and bypass capacitor of the circuit board have a great influence on the performance, which directly determines the transition time of the rising edge and the falling edge, thereby affecting the data transmission rate; Beyond the resistors, the end result is the same as series resistors and bypass capacitors, limiting the rate at which data can be transferred.
All these parasitic parameters are largely dependent on the geometry of the wire. The resistance is proportional to the length of the wire and inversely proportional to the cross-sectional area. Therefore, the longer and thinner the wire, the lower the data transmission rate. Existing space constraints will not allow thicker wires. Although a harder connection can be used to reduce the conversion time, it will increase noise and power consumption at the same time, and the increase in heat generation will be difficult to control.
Compared with electrical interconnection, optical interconnection has the following characteristics:
1. The speed of optical interconnection has nothing to do with the interconnection channel;
2. Optical signals can propagate independently in space without interfering with each other;
3. Optical signals can propagate in three-dimensional free space.
In addition, the optical interconnection can be appropriately changed by the spatial light modulator (SLM), and the optical signal is very easily converted into an electrical signal.
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