A continuum generated from highly nonlinear seed pulses (N/spl Gt/1) propagating in a medium with only self-phase modulation (SPM) or with SPM and anomalous dispersion is highly sensitive to the noise of the input pump pulse. The combination of SPM and normal dispersion improves the stability. However, more efficient spectral broadening schemes are desirable for generating a broad-band continuum at gigahertz rates. The adiabatic compression of weakly nonlinear pulses (N/spl sime/1) via the soliton effect efficiently generates a broad-band continuum that is robust against noise. Detailed characterization of continuum generation in several different fibers is reported.

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Fundamentals of stable continuum generation at high repetition rates

Supercontinuum generation in optical fibres pumped with high power pulse sources in the modulation instability regime is reviewed. The physical mechanisms and supercontinuum dynamics are described in detail. Routes to optimized output in terms of spectral flatness and particularly blue and ultraviolet spectral extent are presented, including the use of cascaded and tapered photonic crystal fibres.

http://iopscience.iop.org/article/10.1088/2040-8978/12/11/113001/pdf

Blue extension of optical fibre supercontinuum generation

We review optical pulse compression techniques based on optical fibers. After discussing the fundamental issues on pulse compression, we summarize and categorize various kinds of techniques for pulse compression. We then focus on the use of highly nonlinear fiber (HNLF) in a pulse compressor to improve the performance. Introducing a figure of merit, we discuss which kind of HNLF of those recently reported is the most suitable for applications to pulse compression. As a HNLF-based optical pulse compressor, we review the technologies of the comb-like profiled fiber (CPF) which consists of alternate concatenations of HNLF and single-mode fiber. Discussing the features of CPF, we show that CPF is a truly practical and flexible solution for optical pulse compression. Finally, we refer to a new class of pulse compression called “the stationary rescaled pulse (SRP)”, which is a characteristic nonlinear stationary pulse propagating through CPF. The propagation characteristics of the SRP provides a systematic and efficient design method of CPF as well as optical pulse compression that has never been achieved by conventional pulse compression schemes. As an example, a highly steep pulse compression in CPF is demonstrated, where a pulse is compressed by a factor of 2.1 per step of the CPF, and a 7.2 ps-width optical pulse is compressed to 0.38 ps with a four-step structure of CPF.

Pulse compression techniques using highly nonlinear fibers

9.953 GHz phase-modulated continuous-wave laser combs are spectrally broadened via nonlinear propagation in normal and anomalous dispersion media and experimentally characterized using a differential phase shift keying (DPSK) decoder. DPSK bit-error rate data are in qualitative agreement with radio-frequency spectrum analyzer measurements.

/pdf/nonlinearly-broadened-phase-modulated-continuous-wave-laser-4vvqw294zz.pdf

Nonlinearly broadened phase-modulated continuous-wave laser frequency combs characterized using DPSK decoding

A 10-GHz train of nearly transform-limited 54-fs soliton pulses was generated by adiabatic compression of the output of a mode-locked fiber laser with a polarization-maintaining dispersion-flattened dispersion-decreasing fiber. The peak-to-pedestal ratio exceeded 23 dB. At high input powers, the pulse width was reduced to as short as 43 fs, although the wings of the pulse were degraded and the jitter increased. The compression properties are different for the two polarization axes, owing to their different dispersion characteristics. The output polarization exhibits no drift.

54-fs, 10-GHz soliton generation from a polarization-maintaining dispersion-flattened dispersion-decreasing fiber pulse compressor.

The compression characteristics at femtosecond durations are compared for dispersion-decreasing fibers (DDF's) constructed out of standard step index fibers and fibers designed to have low third-order dispersion (dispersion-flattened fibers). It is shown that the dispersion slope of a standard fiber causes the compressed pulse spectrum to vary substantially with wavelength. Furthermore, the peak of the compressed pulse spectrum can appear in a different wavelength band than the input pulse. Both of these problems are substantially reduced by using a dispersion-flattened DDF. The compression of 3-3.6-ps pulses to durations less than 180 fs was achieved between 1533-1565 nm at a repetition rate of 10 GHz. The shortest pulses obtained were 98 fs with peak-to-pedestal ratios in excess of 20 dB.

H. Nakazawa

Papers

Femtosecond soliton generation over a 32-nm wavelength range using a dispersion-flattened dispersion-decreasing fiber