25G/40G VCSELs Driving Short-reach Optical Interconnects

Just a few years ago, laser designers were struggling with stability of their 10G VCSELs. But now, at least one, VI Systems GmbH, claims it will have production-ready 40G VCSELs within the next few years. The German start-up has developed two products it believes will take VCSELs beyond 10G applications - a directly-modulated (DM) device and an electro-optic modulated (EOM) DBR VCSEL. Both are short-wavelength (850nm) lasers.

In a recent press release, VI Systems explains that it “developed the VCSEL products at a wavelength of 850 nm along with a range of extremely fast integrated circuits based on the SiGe BiCMOS (silicon-germanium bipolar junction transistors in complementary metal-oxide-semiconductor) technology. The company uses a patent pending micro-assembly platform for the integration of the opto-electrical components and for alignment to a standard high performance multi-mode glass-based fiber.” The start-up has been presenting data supporting its claims of highly stable devices for more than a year now. It gets there by changing the laser active region material and structure to InAs quantum dot (QD).

Not only is VI Systems working on innovative laser structures, it has also developed new electro-optic integration methods to further reduce the cost of these devices.

I’ve noted in previous posts how VCSELs are the key to low-cost optical networks in the data center. These new VCSELs and packaging methods would bring an even more cost-effective “serial” solution for 40/100G. They could also be used for very short-reach optical connections like for chip-to-chip, on-board or board-to-board. Perhaps these inventive products will rival Avago’s MicroPOD and Luxtera’s OptoPHY (also in previous posts). Based on the presentations that VI Systems has released, it sure appears that its management completely understand the needs of both the data center and optical interconnect markets so could very well give incumbents in the industry some competition.

Quantum Dot Lasers Now Reality

In recent research I conducted for the Optical Interconnect report I wrote for CIR, I found some encouraging news on quantum dot lasers. Measuring 20nm in diameter, a quantum dot (QD) is defined as a semiconductor whose electrons are confined in all three spatial dimensions. As a result, it has properties that lie somewhere between those of bulk semiconductors and those of discrete molecules. QDs have been studied for a wide range of applications such as transistors, solar cells, displays, medical imaging, optical amplifiers, sensors, drug delivery and light emitters (both LEDs and diode lasers). QDs also could be used as the physical "incarnation" of qubits in quantum computing R&D and in quantum encryption systems. All of these applications are based on the fact that QDs are zero dimensional, which gives them superior transport and optical properties. They also need very little power.

QD Laser, a Japanese firm backed by Fujitsu Limited, Mitsui Ventures and Mizuho Capital Co., Ltd, announced what I believe is the first commercially viable QD laser in March 2009. Since then, it has added several products to its portfolio. They include FP/DFB laser chips, TO-can and TOSA and wide-band SOA butterfly components. The lasers have capability to run at data rates up to 10 Gbps. These devices are well suited for datacom and telecom equipment.

It seems that QD lasers may have future applications in chip-to-chip optical connections. They may also have applications outside of telecom in sensors and in future quantum encryption/quantum computing systems. In addition to QD Laser's devices, Taiwanese researchers have built tunable QD VCSELs. Also, VI Systems (VIS), a German-based start-up, is working on QD-enhanced VCSELs. This components company has recently received substantial funding and we note that at recent industry conferences VI Systems presented a paper on 25-Gbps VCSELs that were rendered temperature insensitive with the use of QDs. VIS has recently released a product catalog of TOSAs, ROSAs, VCSELs, PINs, arrays, TIAs, VCSEL drivers and high-speed test boards that utilize its technology. Its products are suitable for 850nm 25G and 40G operation. It seems as though this may be one of the only currently viable solutions for stable operation of VCSELs beyond 10G.