Engineers at Mesophotonics (Southampton, England) have demonstrated what is claimed to be an effective new method for generating a white-light continuum. Based on a planar waveguide grown on a standard silicon wafer, the resulting devices produce at least a 600-nm-bandwidth output from low-power femtosecond ultrafast pulses; the output has a smooth output spectrum and no visible spectral noise (see figure). Mesophotonics was set up in 2001 as a spinout from the University of Southampton's Optoelectronics Research Centre and specializes in photonic-crystal technology.
A planar-waveguide-based supercontinuum source produces most of its light in the 600- to 1100-nm wavelength band (some additional light is produced at shorter wavelengths, seen here).
The waveguides are made by using radio-frequency sputtering to deposit layers of a high-refractive-index material on top of a standard silicon wafer. The process creates a planar waveguide measuring 10 mm long by 5 mm wide and 1 µm thick. According to Mesophotonics, the continuum-generating chips (CGCs) offer a combination of the best properties of fibers and bulk media, like sapphire crystals. The continuum is produced with pulse energies as low as 10 nJ, much lower than the threshold observed in bulk media (on the order of microjoules) and comparable to the threshold observed in fiber.
Damage testing has shown that the waveguides are capable of withstanding femtosecond pulses with energies of several picojoules and 455-nm light of several watts average power—far higher than fibers and close to the tolerance of bulk media such as sapphire. In the laboratory, the CGC is pumped with a Ti:sapphire regenerative amplifier that provides 150-fs pulses at 800 nm. These input pulses produce strong output light from 600 to 1100 nm in a continuum that is symmetrical about the central wavelength.
Low noise is a key feature of the CGC. It is achieved by designing the waveguides to operate far from the zero point of group-velocity dispersion, which occurs at a wavelength of 1.6 µm. Operating in this regime avoids competing nonlinear processes, such as four-wave mixing, that are present in the high-noise continuum generated in optical fibers.
The Mesophotonics CGCs should have an impact on applications in frequency metrology, optical coherence tomography, and spectroscopy, all of which require high bandwidth but have previously have been hampered by the lack of stability in continuum generation methods, says John Lincoln, the company's director of business development. "It may be possible to compress these pulses down to just a few femtoseconds given the uniformity and symmetry in generated spectra," he says. "We shan't do this in-house, but would be interested in talking to anyone who was keen to take the experiments further."