Optical arbitrary waveform generation

nature photonics | VOL 4 | NOVEMBER 2010 | www.nature.com/naturephotonics Although the electromagnetic spectrum is a continuum, a strong distinction is typically drawn between electronics and optics. This distinction is due, in part, to the very different technologies used for electronics and optics. A further distinction is that electronic devices typically generate and measure electric field waveforms, whereas optical devices can generate and measure only intensity waveforms because the frequencies involved are much higher. For any intensity waveform there is a continuous family of electric field waveforms that differ by the overall phase. Furthermore, in optics there have been significant limits on what waveforms can be generated, whereas electronic devices have been used to generate arbitrary waveforms for several decades. Over the past decade, advances in optics have allowed the phase of an electric field, with respect to the corresponding intensity waveform, to be measured and controlled. These techniques are referred to as ‘optical frequency combs’ because the evolution of the electric field phase in a train of pulses causes a shift in the corresponding frequency spectrum, which is a comb of equally spaced sharp spectral lines1. Optical frequency comb technology has revolutionized optical frequency metrology and optical atomic clocks2, and has enabled the production of attosecond pulses3. The development of comb technology has also spurred recent work on the generation of arbitrary waveforms at optical frequencies. Techniques for shaping ultrashort optical pulses have existed for around two decades4. However, these techniques have significant limitations. First, the duration of the output pulses is limited by the spectral resolution of the pulse shaper and is less than the time between pulses. Thus, the output pulse train does not fill time; that is, there are gaps between pulses. Second, typically the pulses are all identical. An arbitrary waveform cannot be produced within these limitations. Producing a waveform that fills time requires a pulse shaper with a spectral resolution that matches the spacing of the comb lines of the input pulse train. Such high resolution is only useful if the frequency comb is stable and aligned with the pulse shaper5 — hence the need for frequency comb technology. Sometimes known as ‘line-by-line pulse shaping’, pulse shaping with a resolution matching the comb was demonstrated a few years ago5–7. However, the phase and/or amplitude of the individual comb lines were manipulated by ‘masks’ that are essentially static. To produce truly arbitrary optical waveforms requires that the pulse shaper can be updated for each pulse — a goal that has not yet been reached, but is being actively pursued in many laboratories around the world. optical arbitrary waveform generation