Photonic Crystal Fibers: the 2. Generation of Optical Fibers

(taken from Optics Express Vol. 15, pp 15365)


Charles K. Kao was awarded this year with the Nobel Prize for his fundamental contribution about optical fiber in 1966. Nowadays, applied in high speed data communication optical fibers have an enormous impact in public life. But, is this already everything from optical fibers? Definitely not! In 1996 the research group around Prof. Dr. Russell was able to create the first micro-structured optical fibers, later called photonic crystal fibers (PCF). Contrary to normal optical fibers which consist of a core surrounded by a cladding with a smaller refractive index for total internal reflection, a PCF doesn’t have a cladding in this sense but its core is periodically surrounded by several small “air tunnels” which acts like a cladding, and much more! This construction allows researcher to engineer different parameters of the fiber, like the zero dispersion wavelength (ZDW) and the single mode condition. In bulk silica the ZDW is approximately 1.3µm. Above 1.3µm the dispersion is positive and this wavelength regime is called anomalous regime because it allows new phenomena (solitons, supercontinuum) which are not possible in the normal dispersion regime (negative dispersion). Note, that the dispersion parameter D [ps/km*nm] is proportional to –B2, the group velocity dispersion. The design of photonic crystal fibers enables to shift down the ZDW to 600nm, thus in the working regime of Ti:Sa lasers (800nm). Doing so, solitons (= “self-guiding light bullets”) are created which cover together a very large spectral range, called supercontinuum. Often, the supercontinuum ranges from 530-1100nm but depending on the fiber material, design, length and input laser parameter, different spectral ranges can be covered. Right now, to my knowledge the record covers a wavelength range from 1-5µm using an 8mm telluride PCF.
During the last years supercontinuum spectrums have found applications in spectroscopy. But the most famous, and perhaps most important one, application is on the field of laser based precision spectroscopy, for precision measurements of atomic structures and optical frequencies. Thereby the optical frequency comb technique is applied, which is based too on fiber generated supercontinuum. In 2005, Theodor Hänsch (Max Planck Institute of Quantum Optics) was one of the Nobel Prize winners “for their development of laser based precision spectroscopy, that is, the determination of the colour of the light of atoms and molecules with extreme precision.” Without fiber generated supercontinuum, hence without photonic crystal fibers, these very accurate measurements wouldn’t have been possible. And public life is also profiting from these experiments, because e.g. it helps to improve GPS systems.
Today, many different kind of photonic crystal fibers exist with different properties (see picture). Some have instead of a core a whole, so called hollow core fiber, other ones are doped in the core or cladding with another material, mostly Ytterbium (Yb) or Erbium (Eb). These doped fibers are extensively used in fiber amplifiers and fiber lasers, probably the next generation of lasers.
Photonic crystal fibers are at the very beginning to enter scientific experiments in a broad application range, and because of their special properties we can expect many new techniques and exciting discoveries. Hence, it is a good time to consider PCFs in your experiment…and perhaps you may be awarded with the Nobel Prize some years later ;).

Review Articles:
Photonic Crystal-Fibers
by P. St. J. Russell
JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 24, NO. 12, DECEMBER 2006

Nonlinear Waveguides Optics and Photonic Crystal Fibers
by J. C. Knight
Optics Express Vol. 15, pp 15365

Research Groups:
Russell Group at Max Planck Institute for the Science of Light, Erlangen

Centre for Photonics & Photonics Material, University of Bath

Optoelectronics Research Centre, University of Southampton

Companies:
NKT Photonics
(former called Crystal-Fibre)
with the well-known distributors
Thorlabs and Newport

Nufern

nLIGHT
(former Liekki, acquired by nLIGHT)