September 2008

EEEL demonstrates linear optical sampling of high-speed optical signals with milliradian phase noise

The speed of optical communication networks continues to grow, and 40 Gb/s systems are being installed with plans for upgrade paths to 100 Gb/s. With higher data rates, the format of the data becomes more complicated as well. Phase shift-keying (PSK) allows increased spectral efficiency (data rate per spectral bandwidth) by encoding information on the optical phase of the transmitted light. As data rates grow beyond the speed of measurement electronics, there is a measurement challenge to be able to measure the phase and amplitude modulations at speeds beyond the reach of electronic detection alone. The solution is to employ optical sampling techniques to resolve the time-domain signals used in these fast PSK communication systems. One very useful technique is linear optical sampling (LOS), which uses short optical pulses to sample (in equivalent time) both the magnitude and phase of the transmitted electric field.

Researchers in the Optoelectronics Division have assembled a LOS system using a unique phase referencing technique to remove laser phase noise, allowing a low-phase noise measurement without cumbersome curve-fitting or the bit rate dependence of existing LOS techniques. This system was demonstrated by measuring a 10 GB/s differential phase shift keyed (DPSK) signal. Results include a low-jitter bit synchronous averaging approach which allows the optical phase transmission of the modulator to be characterized to 1.2 mrad. The system bandwidth is fundamentally limited only by the pulse-width of the sampling laser itself, potentially allowing bandwidths as large as 1 THz.

For more information contact: Paul Williams, phone 303-497-3805