Measuring Ultrafast Supercontinuum on a Single Shot
 Supercontinuum generation is a remarkable process by which a laser pulse efficiently evolves into a white pulse. It has a wide range of applications, including microscopy, tomography, metrology, and laser-phase stabilization. Supercontinuum pulses are extremely temporally complex, and trains of them are inherently highly unstable. As a result, it has never been possible to measure the temporal intensity and phase of a single supercontinuum pulse. Such measurements have been possible if averaged over many--typically 100 billion--pulses, but at best they yield only an estimate of a typical supercontinuum pulse.
Continuum generation: An input narrowband laser pulse turns white inside a fiber.
An oceanic rogue wave can sink a ship with little or no warning.
Supercontinuum measurement recently acquired increased urgency when it was noted that it can give rise to optical rogue waves, mathematically equivalent to oceanic rogue waves that sink dozens of ships every year. While the measurement of an oceanic rogue wave is straightforward (the trick is surviving it!), its intentional generation is difficult--and ill-advised! On the other hand, the generation of an optical rogue wave is simple, routine, and safe, but its single-shot measurement has remained impossible. Measurements of optical rogue waves could lead to insight into, and eventually to the prediction of, their destructive and difficult-to-simulate oceanic counterparts.
 Our XFROG apparatus for measuring continuum on a single shot is simple. It uses the well-known polarization-gating beam geometry, which works for ("phase-matches") all wavelengths simultaneously. It also uses an amplified gate pulse, so the device sensitivity is high, as is necessary. Finally, we use a tilted gate pulse, which allows us to achieve a large delay range (in order to measure such long pulses). The tilted pulse also allows us to compensate for geometrical distortions that had been another serious problem preventing single-shot measurement in the past. This combination of innovations has allowed us to solve this seemingly very difficult measurement problem.
Our apparatus for making single-shot continuum measurements. Although not shown in the above figure, tilting the reference pulse solves two problems: 1) achieving the required large delay range and 2) avoiding the so-called delay smearing, which would have washed out the trace structure, preventing an accurate measurement of the continuum. The result is a device that solves all the problems in single-shot measurement of continuum.
Example of a single-shot continuum measurement using our new XFROG device. Note the excellent agreement between the measured and retrieved spectrograms (the top row), indicative of an excellent measurement. Note also the good (although not perfect) agreement between the measured spectrum (bottom row, right) using our XFROG and also using a separate spectrometer. The XFROG measurement is actually correct. The spectrometer measurement was contaminated by some additional light leaking into it; the XFROG measurement is immune to such contamination.
Read the paper now (Wong, et al.). In future work, we hope to use our new technique for measuring actual optical rogue waves. It can measure even more complex pulses than that shown here.