Silicon photonics technology is becoming more and more popular: experience the progress of silicon light emitting technology (Part 2)

Great progress has also been made in miniaturization

　　Trial production of components targeting optical transmission between chips has also begun. The optical transceiver IC Note 3) trial-produced by the research and development organization "Photonics Convergence System Basic Technology Development (PECST)" supported by the Cabinet Office of Japan has achieved the world's highest integration and transmission capacity density. In September 2012, PECST released a technology that can integrate 526 optical transceivers with a data transmission speed of 12.5Gbps on a 1cm2 silicon chip Note 4), with a data transmission capacity density equivalent to about 6.6Tbit/second/cm2. It is mainly used for optical adapter boards responsible for large-capacity data transmission between LSIs (Figure 4).

　　Figure 4: Inter-chip wiring enters the "optical highway"

　　This picture shows the outline of the optical adapter board for data transmission between LSIs developed by Arakawa Laboratory of the University of Tokyo and PECST. Except for the laser element as the light source, everything is integrated on the SOI substrate using CMOS compatible technology. The laser element can also be mounted on the chip using a common chip mounter. (Photo: PECST on the right)

　　Note 3) PECST is a research and development organization established with the goal of realizing a "data center on a chip," or realizing data center functions on silicon chips, by 2025. Research work began in March 2010.

　　Note 4) Each group of this optical transceiver occupies an area of ​​0.19mm2. All components except the laser element are implemented using CMOS compatible technology.

　　This release is of epoch-making significance. The technology solves the original problems of large size of each component, difficulty in achieving short-distance transmission and high-density integration. Some people often compare optical transmission to "aircraft" transportation, and electrical transmission to "railway" or "car" transportation. If it is a long-distance transportation across the sea, it is more appropriate to use an airplane, but if it is just to transport goods to a neighboring city a few kilometers away, it is not suitable to use an airplane. Because there is not only a fuel problem, but also the "airport" required for aircraft takeoff and landing is too large. The size of the optical transceiver equivalent to the "airport" in optical transmission is originally very large, several centimeters square, which is not suitable for transmission over a distance of 1 cm (Figure 5).

　　From PECST's prototype, we can see the possibility of integrating multiple optical transceiver ICs on a chip with an area of ​​1cm2. The miniaturization of optical transceiver ICs and constituent components is almost directly related to low power consumption quantization. Because if the component area is small, the component capacity is also small. By promoting the miniaturization of component size, the two issues of power consumption and integration of optical transmission are improved at one stroke.

　　Figure 5: Transmission capacity density of 10Tbit/s/cm2 is about to be achieved

　　This figure shows the miniaturization of optical transmission transceivers and the improvement of integration level that accompanies miniaturization. If the integration level is improved through miniaturization, the transmission capacity density will also increase. The current highest transmission capacity density is 6.6Tbit/sec/cm2 achieved by PECST. PECST expects to achieve 10Tbit/sec/cm2 in the first half of 2013.

　　Developing a unique core technology group

　　The realization of PECST's optical transceiver mainly relies on four core technologies (Figure 6), namely (1) laser array elements as light sources, (2) spot size converters (SSCs) connecting the light sources and silicon waveguides, (3) Mach-Zehnder type optical modulators*, and (4) germanium photosensors.

　　Figure 6: Key elements to achieve 6.6Tbit/s/cm2 transmission capacity density

　　This picture shows the key technical points of the Arakawa Laboratory of the University of Tokyo and PECST in achieving a transmission capacity density of 6.6Tbit/second/cm2. In terms of laser components, large-scale array technology has been developed; the loss of the spot size converter has been greatly reduced; the size of the optical modulator has been reduced to 1/4 of the original size; and the germanium photosensitive element has achieved a speed increase of more than 2 times. (Photo: PECST)

　　*Mach-Zehnder optical modulator = a type of optical interferometer that generally splits light from the same light source into two beams, performs phase control and other processing on one of the beams before coupling it with the other beam.

　　(1) Laser array element: Successfully arranged 13 channels of laser diodes (LD) at a pitch of about 30 μm. PECST claims that it has already produced an element with 104 channels.

　　(2) SSC has changed the conventional single tapered waveguide structure to a three-waveguide structure, which significantly reduces optical coupling loss. In addition, the positional alignment accuracy when mounting the laser array element on silicon has been greatly relaxed to 0.9μm.

　　Solved two issues of modulator

　　The biggest contribution to the miniaturization of optical transceivers is the development of (3) optical modulators. In the past, Mach-Zehnder optical modulators required a long path length to compensate for the low modulation efficiency. The original length was more than 1 cm, but it has recently been shortened to about 1 mm, and this time it has been greatly shortened to 250 μm. This is achieved by vertically arranging pin diodes on the silicon waveguide like comb teeth, increasing the modulation efficiency by 4 times.

　　The optical modulator developed by PECST changes the refractive index by changing the carrier density in the silicon waveguide and its vicinity. The challenge at this time is how to balance the light sealing in the waveguide and the control of increasing the carrier density within a range that does not hinder the light. This design solves this problem by making the carrier inlet and outlet as fine as comb teeth to prevent light from leaking out.

　　(4) The germanium photosensitive element has achieved more than twice the speed of operation by changing from the original pin-type structure to the MSM structure* with a smaller element capacity.

　　*MSM (metal-semiconductor-metal) structure is a type of photodiode (PD), which is a structure combining a semiconductor and two metal electrodes.

　　PECST has also found a way to increase transmission capacity density. Its main researcher, Professor Yasuhiko Arakawa of the Advanced Science and Technology Research Center of the University of Tokyo, improved the electrode design of the optical modulator in 2012, further reducing its occupied area to less than 1/5 of the original. Professor Arakawa said, "If it is used for optical transceiver IC integration, it is expected to achieve the target transmission capacity density of 10Tbit/second/cm2."