Time Stretch Analog-to-digital Converter - Impulse Response of The Photonic Time-stretch (PTS) System

Impulse Response of The Photonic Time-stretch (PTS) System

The PTS processor is based on specialized analog optical (or microwave photonic) fiber links such as those used in cable TV distribution. While the dispersion of fiber is a nuisance in conventional analog optical links, time-stretch technique exploits it to slow down the electrical waveform in the optical domain. In the cable TV link, the light source is a continuous-wave (CW) laser. In PTS, the source is a chirped pulse laser.

In a conventional analog optical link, dispersion causes the upper and lower modulation sidebands, f_optical ± f_electrical, to slip in relative phase. At certain frequencies, their beats with the optical carrier interfere destructively, creating nulls in the frequency response of the system. For practical systems the first null is at tens of GHz, which is sufficient for handling most electrical signals of interest. Although it may seem that the dispersion penalty places a fundamental limit on the impulse response (or the bandwidth) of the time-stretch system, it can be eliminated. The dispersion penalty vanishes with single-sideband modulation. Alternatively, one can use the modulator’s secondary (inverse) output port to eliminate the dispersion penalty, in much the same way as two antennas can eliminate spatial nulls in wireless communication (hence the two antennas on top of a WiFi access point). This configuration is termed phase-diversity. For illustration, two calculated complementary transfer functions from a typical phase-diverse time-stretch configuration are plotted in Fig. 4. Combining the complementary outputs using a maximal ratio combining (MRC) algorithm results in a transfer function with a flat response in the frequency domain. Thus, the impulse response (bandwidth) of a time-stretch system is limited only by the bandwidth of the electro-optic modulator, which is about 120 GHz—a value that is adequate for capturing most electrical waveforms of interest.

Extremely large stretch factors can be obtained using long lengths of fiber, but at the cost of larger loss—a problem that has been overcome by employing Raman amplification within the dispersive fiber itself, leading to the world’s fastest real-time digitizer, as shown in Fig. 3. Also, using PTS, capture of very high frequency signals with a world record resolution in 10-GHz bandwidth range has been achieved.