Flexible multi-rate capabilities are attractive for communications over the dynamic free-space optical (FSO)
channel since it enables links to be efficiently established over a variety of distances, channel conditions,
aperture sizes, and transmitter power levels. Recently, multi-rate differential phase shift keying (DPSK)
modulation has been shown to provide good performance with modest complexity over a wide range of
data rates. For these reasons, multi-rate DPSK has become the high-rate baseline format for NASA’s Laser
Communication Relay Demonstration (LCRD) and other space-based FSO systems currently in
development, operating at channel rates from 72 Mbit/s to 2.88 Gbit/s.
Here, we report WDM-scalable power-efficient optical-transceiver advances that can extend single-channel
rates by a factor of four to 11.52 Gbit/s using LCRD-compatible waveforms. Transmitter waveforms are
implemented using time-frequency-windowed directly modulated lasers (DMLs) that can generate burstmode
2-DPSK and 4-DPSK waveforms at the standard LCRD 2.88 GHz symbol rate and twice the rate at
5.76 GHz. At the receiver, standard 2.88 GHz delay-line-interferometer (DLI) based demodulation is
implemented for 2-DPSK. For 4-DPSK, a second 2.88 GHz DLI is required to demodulate the 2-bit-persymbol
format, with the pair of DLIs biased at ±/4. With proper precoding of the transmitter waveforms,
the double-rate 5.76 GHz symbols may be demodulated using the same 2.88 GHz DLIs using non-adjacent
DPSK demodulation. In this manner, a common DPSK receiver platform may be used to demodulate
standard multi-rate LCRD waveforms as well as 4-DSPK waveforms at 1x and 2x the standard symbol rate.
|