VCSEL-based 25-Gbps transmission at 850 nm nearing readiness

While module developers work to perfect 4x25-Gbps transmission over singlemode fiber for 100GBase-LR4 and -ER4 applications, technologists have already begun work on enabling similar transmission over multimode fiber. Recent demonstrations have shown progress on both the transmission and reception ends of the spectrum. The demonstrations indicate that 4x25-Gbps products could be readily available once standards are in place.

For example, Discovery Semiconductors displayed at OFC/NFOEC a module-based receiver with a photodiode based on InGaAs/InP for these applications. The company followed its OFC/NFOEC activity with more information in a paper at the subsequent “Enabling Photonics Technologies for Defense, Security, and Aerospace Applications” conference sponsored by the SPIE. The paper described three 25-Gbps, 850-nm receiver formats: linear photodiode (PIN), linear photoreceiver (PIN-TIA), and limiting photoreceiver (PIN-TIA-LIM). The paper describes the first as good for instrumentation applications, the second for low-power VCSEL-based 850-nm links that need boosted RF output power to drive the following RF electronics, and the third for on-off links that require sufficient gain to drive the following clock and data recover (CDR) or demultiplex modules often found in digital links. The PIN-TIA-LIM device in particular showed good performance (BER of 10^-12 at an OMA of -4 dBm) over 100 m of OM3 fiber.

Abhay Joshi, president and CEO of Discovery Semiconductors, told Lightwave that his company has supplied photoreceiver technology for 25-Gbps, 850-nm research to a variety of laboratories, universities, and others working on the next generation of multimode transmission. At least some of that research likely is directed at the complementary VCSEL technology. Joshi says that pretty much everyone who has a 10-Gbps capable VCSEL is working on a 25-Gbps, 850-nm version. He expects such devices to be commercially available from multiple sources in the 2013-2014 timeframe.

One such supplier could be Germany’s VI Systems GmbH. VI Systems and partner Draka Communications Inc. reported June 8 that they had demonstrated error-free data transmission at 25 Gbps over 300 m of Draka MaxCap-OM4 fiber using multimode VCSELs. They also reported error-free transmission over 500 m of OM4 fiber with a received optical power of -6 dBm using singlemode VCSELs.

Joshi expects the first applications for 4x25 Gbps over multimode to be either optical backplanes or active optical cables. However, in addition to the necessary technology, standards also must be in place for a market to develop, Joshi allowed. Those could come from the IEEE for Ethernet or from the InfiniBand and Fibre Channel communities. The OIF’s CEI-25/28G backplane work also could create demand for a single-channel 25-Gbps VCSEL-based capability.

The aerospace industry also has expressed interest in the technology, Joshi concluded.