Photonic crystal fibers (PCF's) are neat and exciting, but what are the best applications? These are optical fibers which confine lightly by both, refractive index contrast between a high-index core and a lower index cladding, and distributed bragg reflection due to the periodic arrangement of dielectric constant radially away from the core. A cross section picture of  a PCF made by Crystal Fibre is shown on the right. A really neat feature of this fiber, which generated a lot of excitement in the research community, is that the core is hollow and light propagates in air. This can in principle allow very low loss propagation, but in practice hollow core modes are extremely sensitive to fabrication imperfections and bending losses, with the lowest propagation loss of 13 dB/km [1], which is fairly high compared to the 0.2 dB/km loss figure of standard silica fibers. Furthermore, PCF's are fairly expensive. A meter of PCF fiber can be purchased from Thorlabs for only $500-$900 here , compared to $5-$10 for standard fiber.

PCF fiber from Crystal Fibre. The hollow core can be infiltrated with gasses and other media for novel devices. These fibers won't be supplying your internet any time soon though.

Supercontinuum Generation

A great application of PCF's, which has been readily commercialized, is supercontinuum generation. The PCF dispersion (relationship between frequency and propagation speed in the fiber) can be exploited to convert short optical pulses to a broad frequency distribution, thus providing a bright and extremely broadband source of light for spectroscopy applications. The spectrum of such a fiber, made by Crystal Fibre and available from Thorlabs is shown in Fig 2. (The details are given in the application note [2].)

The supercontinuum is generated by confining the pump pulse to a very small fiber core, and thus generating a very high field amplitude. The dispersion of the fiber is tuned in such a way that the pulse spreads widely and uniformly. 

Nonlinear Optics

 As seen with supercontinuum generation, PCF's confine light tightly to the core and can be tailored to address dispersion in a controlled manner. Furthermore, these fibers can be filled with gases, atomic vapors and nonlinear materials in order to enhance spectroscopy of gases, generate Raman signals more efficiently and frequency convert light. Additionally, a bragg grating can be added along the fiber and used to create a third dimension of confinement. Such devices based on high nonlinearity will undoubtedly find applications in optical technologies, sensing and research.

[1]  Russell_PCF_Review_Science_2003.pdf (624.26 KB) 

[2]  Supercontinuum - General.pdf (653.57 KB) 

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