Measuring and Understanding the Most Complex Ultrashort Pulse Ever Generated (link to Full Paper)

One of the most exciting recent developments in optics has been the generation of ultrabroadband supercontinuum, accomplished simply by injecting readily available low-power ultrashort pulses into microstructure or tapered fibers [2-4]. These new fibers are nearly dispersion free at the Ti:Sapphire laser wavelength, so the pulse remains short for a much longer distance than in conventional fibers (many cm compared with a few hundred microns), drastically increasing nonlinear-optical effects. The resulting continuum's spectrum encompasses the entire visible and much of the infrared ranges, and it is also spatially highly coherent. Many applications have been proposed and demonstrated for this exotic light, from metrology to medical imaging, but a detailed measurement and understanding of this most complex pulse ever generated have both eluded researchers. Indeed, the incredible complexity of the supercontinuum provides a particular challenge to both models and measurement techniques.

In this work, we combined powerful new modeling capabilities and new measurement techniques to accurately model and measure the continuum's complete intensity and phase vs. time, revealing several complete surprises and important new strategies for using this fiber in the future.

In our simulations, for example, we found, quite surprisingly, that the extreme spectral width is actually achieved in the first cm of fiber. Also surprisingly, the strong spectral broadening in the first cm is also accompanied by strong pulse temporal compression. With further propagation, the already broad spectrum only develops asymmetry and complex temporal features, such as temporal pulse break-up, oscillations, and distinct soliton pulses, which separate from the residual input pulse due to group velocity walk-off. Also, because continuum contains such a broad spectrum that dispersion in the spectral wings becomes important, further propagation actually broadens the continuum significantly in time, rather than frequency. All these latter features, which only occur for longer (>1 cm) fiber lengths, are generally undesirable.

Previous measurements of the continuum had been limited to simple multi-shot spectral measurements, which have always yielded a broad, smooth, and stable spectrum. In contrast, we performed much more powerful cross-correlation frequency-resolved-optical-gating (XFROG) measurements with a newly developed angle-dithered-crystal technique to achieve the huge bandwidth necessary to measure continuum. These measurements generate a spectrogram of the continuum, which nicely illustrates the continuum's time and frequency characteristics, as well as yielding the first complete intensity and phase measurement of continuum. Our measurements nicely revealed the continuum intensity and color vs. time, and yielded some very surprising conclusions. First, in strong contrast to previous simple multi-shot spectrometer measurements, they showed a very complex and unstable spectrum, which agreed nicely both with our theory and also with single-shot spectral measurements we made to confirm these unintuitive results. Thus, we found that the continuum spectrum is broad, but neither smooth nor stable! Also, they showed that the continuum pulse generated in long (many-cm) fibers is quite long-several picoseconds-in agreement with our simulations.

The figure shows the theoretical and experimental continuum spectrograms, revealing the complex structure and the typical spectral oscillations that occur with a long (16 cm) fiber. The oscillation frequency varies across the profile, with the mean frequency of about 115 THz.

These simulations and measurements clearly showed that, while the input pulse can propagate large distances in these fibers without distortion, the continuum cannot. Thus- the optimal approach to supercontinuum generation is to use a short, ~1 cm, fiber. Indeed, using such a fiber, we have recently succeeded in generating a supercontinuum pulse only 25 fs long-considerably shorter than the 40-fs pulse that created it-and also much smoother and much more stable. This short-fiber continuum is not only a nearly ideal pulse for most broadband applications, but it is also potentially compressible to a few fs.

Figure caption. Left: Simulated supercontinuum spectrogram, showing the pulse break up and complex spectrum after 16 cm. Right: Measured and retrieved (a check on the measurement) continuum spectrograms, showing the predicted complex behavior. In both cases, the intensity is shown below and the spectrum at right. The original Optics Express paper contained a movie, showing the continuum spectrogram vs. distance along the fiber.

References
1. John M. Dudley, Xun Gu, Lin Xu, Mark Kimmel, Erik Zeek, Patrick O'Shea, Rick Trebino, Stèphane Coen, and Robert S. Windeler, "Cross-correlation frequency resolved optical gating analysis of broadband continuum generation in photonic crystal fiber: simulations and experiments," Opt. Expr., 10, 1215 (2002).

2. J. K. Ranka, R. S. Windeler, A. J. Stentz, "Visible continuum generation in air-silica microstructure optical fibers with anomalous dispersion at 800 nm" Opt. Lett. 25, 25-27 (2000).

3. J. H. V. Price, W. Belardi, T. M. Monro, A. Malinowski, A. Piper, D. J. Richardson, "Soliton transmission and supercontinuum generation in holey fiber using a diode pumped Ytterbium fiber source," Opt. Express 10, 382-387 (2002).

4. T. A. Birks, W. J. Wadsworth, P. St. J. Russell, "Supercontinuum generation in tapered fibres," Opt. Lett. 25, 1415-1417 (2000).