In optical communications systems, it is necessary to characterize the phase and amplitude of optical pulses as accurately as possible in order to predict and mitigate signal degradation. For example, in long distance wavelength-division multiplexed (WDM) systems optical signals generate nonlinear effects such as self-phase modulation or cross-phase modulation, which degrade the transmission properties of an optical signal. Additionally, characterizing the performance of a temporal modulator in an optical communication system can be advantageous in assessing the effectiveness of the communication system. Such characterizations of the effect of the distortions on a propagating optical signal and the performance of a temporal modulator can assist in determining corrective measures for an optical communications system.
As the need for information increases, though, so does the demand for higher speed and higher capacity communication systems. Higher speed communication systems mean shorter optical pulses for transmission at higher bit rates (e.g., approximately 8 ps pulses for 40 Gb/s systems), and fast (even ultrafast) optical components to process higher bit rate optical signals. Techniques for the time-frequency analysis of the electrical field of a short optical pulse typically require a non-stationary filter element capable of modulating the amplitude and phase of the pulse on a time scale of the order of its duration. In the domain of femtosecond pulses, these techniques are generally realized using the nonlinear interaction of the short pulse to be characterized with one or several other short pulses in a quasi-instantaneous nonlinear medium. These nonlinear interactions require nonlinear optics which require fairly intense pulses. As such, these nonlinear techniques are impractical for low power applications such as telecommunication systems, which typically have peak powers as low as 0.1 mW or less.
In regard to characterizing optical modulators, time-frequency analysis can also be used to measure the dynamic switching characteristics of optical modulators, used for example for pulse carving and all-optical signal processing. Typically, both the amplitude and the phase of the temporal response are needed to characterize the action of these modulators on light. Similar to the time-frequency analysis of the electrical field of a short optical pulse, the amplitude and the phase of the temporal response of an optical modulator is difficult to obtain with picosecond resolution implementing current analytical techniques.