Whenever I hear a claim about some new highest speed, shortest time, or other research result, I ask, "How did they measure it?" and "How do they know if the results are accurate?" So, when IBM announced that engineers had successfully transferred data at 64 Gbit/s just prior to OFC 2014, I just had to ask how they verified the result.
At an OFC panel session, I heard about three methods for achieving data rates at 56 Gbit/s per channel. In two of the three presentations, engineers claimed that NRZ (non-return-to-zero) modulation wasn't practical for 56 Gbit/s, never mind higher speeds.
In its February 25 announcement, IBM claimed to have reliably sent data at 64 Gbit/s over 57 m of optical fiber. To learn more about the test setup and results, I spoke with IBM's Dan Kuchta just prior to OFC. NRZ modulation is commonly used in VCSEL (vertical-cavity-surface-emitting-laser) components and multimode fiber for use in Ethernet, Fibre Channel, and InfiniBand links.
Kuchta used an SHF 12103A pattern generator to generate the test pattern and an SHF 11104A to receive the signals and calculate BER (bit-error rate). The 64 Gbit/s data rates exceed SHF's stated rates, but they do run at those speeds, according to Kuchta. The Figure shows the test setup: 10 cm of coax cable connected the pattern generator to the transmitter (TX in the Figure 1) while 25 cm of cable connected the receiver (RX) to the BER tester. The cables use Rosenberger 1.8 mm connectors.
The test setup consisted of a pattern generator, optical transmitter, optical receiver, and a BERT.
The test ran for 200 s with no errors. Thus, its BER is less than 10-12. Figure 2 shows an eye diagram of the signal at the receiver after it turns the optical signal back to electrical.
The eye diagram of the test signal -- in electrical form -- as it looked at the transmitter.
Kuchta noted that the research team was able to reliably send data over 57 m of fiber at 64 Gbit/s. Reducing the data rate to 50 Gbit/s resulted in reliable data transfer over 107 m of fiber. At 40 Gbit/s, the fiber length increased to 257 m. The engineers kept adding 50 m of fiber setup, which includes a jumper of fiber 7 m long.
"Last year," said Kuchta, "we were able to send data at 56 Gbit/s, and we reached 64 Gbit/s this year. We think there's a chance to get to 70 Gbit/s with this technology as well." By "technology," he's referring to the common directly modulated 850 nm light. Kuchta believes NRZ modulation should have two more generations before its maximum speed is reached.