We compensated for chirp of optical pulses with an over-one-octave bandwidth (495 -1090 nm; center wavelength of 655.4 nm) produced by self-phase modulation in a single argon-filled hollow fiber and generated 3.4-fs, 1.56 optical-cycle pulses (500 nJ, 1-kHz repetition rate). This was achieved with a feedback system combined with only one 4-f phase compensator with a spatial light modulator and a significantly improved phase characterizer based on modified spectral phase interferometry for direct electric-field reconstruction. To the best of our knowledge, this is the shortest pulse in the visible-to-infrared region.

We experimentally demonstrate the generation of 5.0-fs optical pulses (2.5 J, 1-kHz repetition rate), using only a spatial light modulator for phase compensation. Pulse compression of the broadband pulse (500-1000 nm) from an argon-filled capillary fiber is achieved with a liquid-crystal spatial light modulator without any prechirp compensation. The output pulse width is found to be 4.1 fs by a fringe-resolved autocorrelator fitted with a transform-limited pulse and to be 5.0 fs by second-harmonic generation frequency-resolved optical gating with marginal correction. It is to our knowledge the shortest pulse ever generated by use of only a spatial light modulator for phase compensation.

that results from the propagation of the unoptimized input, the structure of the pulse is substantial throughout its temporal profile.The optimized output pulse reveals, in contrast, no detectable wing structure over the same range, and a pulse shape approaching that of the original unshaped input pulse, especially on the leading edge. The peak intensity of the optimized output exceeds that of the unoptimized output pulse by a factor of two. By analyzing the optimized shaped input pulse temporal phase function and comparing it to the unshaped pulse, the nature and relative effect of the nonlinearities can be determined. Specifically, by fitting the recovered temporal phase function, a strong compensation effect can be detected on the cubic and quartic phase terms, identifying these nonlinearities as the dominant contributions to the distortion of the propagating pulse.These results demonstrate the feasibility of fiber delivery of energetic ultrashort pulses defeating the nonlinear propagation effects by appropriate preshaping of the input pulse through linear filtering combined with adaptive control. These results are of fundamental importance in both the applied and basic realms where delivery of energetic ultrashort pulses is needed and the successful application of this technique will impact many fields, such as telecommunications, nonlinear imaging, optical coherence tomography or materials processing, to name a few. References 1.2. 3.Optical pulses in the 5-fs region have been generated using chirped mirrors for chirp compensation.' Chirped mirrors have an advantage of the high throughput. However, the difficulty of obtaining the very large bandwidth, the inter-dependence of different phase orders, and the inability to fine-tune the phase in the experimental setup are disadvantages. On the other hand, the pulse shaping technique* using the liquid crystal spatial light modulator (SLM) for pulse compression has advantages of the large bandwidth (300-1500 nm) and the in-situ adaptive phase control. Recently it was used to compress the broadband pulses with the pre-chirp compensation by the prism pair to obtain sub 6-fs pulse^.^Here we demonstrate experimentally that the pulse shaper with the SLM can be used to compress broadband (500-1000 nm) pulses from the argon-filled capillary fiber without any pre-chirp compensation to generate 5.0-fs pulses. By not using any pre-chirp compensation optics, the optical throughput increases, the alignment becomes easier and the total spectral width is not cut. Also the important parameter of the phase pattern applied by the SLM for generating pulses close to the transform-limit is identified. To accurately evaluate the pulses thus generated, the second-harmonic frequency-resolved optical gating (SH-FROG) is used.The experimental setup is shown in Fig. 1. The SLM was used as a phase-only modulator and the applied phase by the SLM at the position x is given by the 4th-order polynomial of the form:where wo is the center angular frequency for the Taylor expansion and ~( x )is th...

Ultrabroadband optical pulses generated through self-phase and induced-phase modulation effects and ultrashort optical pulses whose phases were compensated for using a 4f pulse shaper with a spatial phase modulator were generated. Interferometric autocorrelation, frequency-resolved optical gating and spectral phase interferometry for direct electric-field reconstruction (SPIDER) measurements were made to characterize these pulses, and the results were compared. The generation of 5.0 fs (2.4 cycle) or shorter optical pulses was confirmed. For much shorter pulses, below-two-cycle or monocycle optical pulses, single-shot characterization excluding the errors due to the pulse-to-pulse fluctuation is essential. The sensitivity of SPIDER, which is the most advantageous characterization technique apart from its low sensitivity, was improved by a factor of about a hundred (~1 nJ/THz-bandwidth). Instead of a chirped reference pulse split from the pulse to be characterized, a powerful external pulse from a Ti:sapphire laser amplifier as a highly intensive chirped pulse was employed. By use of this modified SPIDER, the characterization of an over-one-octave ultrabroadband optical pulse was performed. This modified-SPIDER method is the most promising for characterization of monocycle optical pulses.

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