Alternating dispersion boosts supercontinuum bandwidth without need for more laser power

Supercontinua created by laser light in nonlinear optical materials are the basis for a variety of photonic devices and applications, from spectroscopy to lidar to tunable light sources for general use and many others. Most techniques for creating a supercontinuum, however, encounter problems related to the presence of dispersion, which can thwart the needed spectral broadening, requiring higher powers to further spectrally broaden the light. Scientists at the University of Twente (UT; Enschede, Netherlands) have come up with an approach to supercontinuum generation that counteracts spectral stagnation, allowing the use of lower-power input beams.¹

The effect of dispersion in a nonlinear optical material is to cause the spectral broadening to cease beyond a certain propagation length (which gets shorter with increased dispersion), thus eliminating the otherwise sensible solution of increasing the propagation length to allow more interaction of the light with the material. The need to then turn up the optical power is detrimental especially in waveguide-based supercontinuum generation, often implemented in integrated optics, and usually with very small cross-sections that could be damaged at high optical powers.

Alternating dispersion

The UT researchers overcome the spectral-stagnation problem by creating a nonlinear-optical path in which the sign of the group-velocity dispersion along the propagation path is repeatedly alternated (see figure). This has the additional benefit of disrupting the formation of solitons. In the experiment, which is based around a dispersion-alternated optical fiber, incident pulses with 74 fs full-width at half-maximum (FWHM) pulse duration at a 79.9 MHz repetition rate, 50 mW average power, and 1560 nm center wavelength, are coupled into the fiber with 34.6 mW average power entering the fiber. The fiber itself is assembled from segments of standard telecom fibers with alternating normal and anomalous dispersions.

As the researchers describe it, the first type of fiber material widens the spectrum as expected, whereas the second type of fiber gives the laser light a chance to “recover” and reach its peak power again. By doing so, the two types of fiber build a supercontinuum that is much more stable and efficient.

“There are several avenues that we would like to explore with our new SCG concept,” says Haider Zia, one of the researchers. “On the fundamental side, we would like to explore this concept experimentally in the integrated-photonics setting (along with continuing work in the fiber platform). In integrated photonics, due to the control we have over key parameters, we could significantly increase the bandwidth-to-input-peak-power ratio, potentially by several orders of magnitude through the unique power scaling laws associated with sign-alternation. We are also exploring our concept where segment lengths do not vary, such as in resonator schemes.”

Haider notes that the group would also like to explore sign-alternation for power-efficient and high-quality nonlinear pulse compression. “We find that the supercontinuum pulses produced in the sign-alternated structure demonstrate emergent phase profiles that can easily be compressed temporally, despite effects such as uncompensated higher-order dispersion,” he says.

“In terms of applications,” he adds, “since the reduced power dependence allows for a fully integrated high-quality (flat spectrum) supercontinuum-generating laser, we would like to pursue incorporating our supercontinuum-generation concept into handheld integrated photonics devices and fiber devices. These devices can be used as inexpensive and portable sensing devices for lidar or as sources for spectroscopy, to name a few examples.”

REFERENCE

1. H. Zia et al., Laser Photonics Rev. (2020); https://doi.org/10.1002/lpor.202000031.