In recent years, research and development of new optical fibers referred to as holey fibers, photonic crystal fibers (PCFs) or photonic band gap fibers (PBFs) have progressed at a dramatic pace. In conventional optical fibers, the light is confined to the core by a simple refractive index difference. In contrast, these new optical fibers are characterized by having a complex two-dimensional structure in their cross section.
In other words, the light is confined by means of establishing a refractive index difference between the cladding and the core by arranging holes in the cladding to reduce the effective refractive index in the cladding (holey fibers, PCFs), or of forming a photonic band gap with respect to the propagation light in the core by making the cladding of a photonic crystal (PBFs).
It is possible to change the characteristics of PCFs and PBFs considerably through their structure, so that applications such as “dispersion compensation optical fibers with increased wavelength dispersion”, “optical fibers with large non-linear optical effects” and “zero dispersion optical fibers with zero dispersion in the visible spectrum” have been proposed. Moreover, the complex two-dimensional structures can be fabricated, for example, by heating and stretching a large number of quartz glass pipes that are bundled together (see Masaharu Ohashi, “Latest Technological Trend of Communication Optical Fiber”, O plus E, 2001, vol. 23, No. 9, p.p. 1061-1066, for example). Also, fibers utilizing a photonic crystal as the core have been proposed recently (see J. C. Knight and three others, Optical Society of America Annual Meeting 2002, Conference Program, 2002, (US), WA 3, p. 94, for example).
In most of the PCFs and PBFs that have been proposed so far, single mode propagation with the 0-th mode is used for electromagnetic waves propagating through the core. Even though single mode propagation is a necessary condition to prevent wavelength dispersion due to multi-mode propagation, it also poses restrictions with regard to the core size and the optical fiber performance.
On the other hand, it has been well known that the electromagnetic waves propagating through the photonic crystal have characteristic properties of “very large wavelength dispersion due to the anomalous band structure” and “group velocity anomaly of propagation light.” However, the above-mentioned 0-th mode propagation light does not show such properties very strongly. Therefore, in order to achieve functions making use of these properties, it is necessary to extend the waveguide, causing problems of increasing production costs and propagation loss.
The inventors of the present invention have studied the propagation of electromagnetic waves inside the photonic crystals. For example, when plane wave light as incident electromagnetic wave is made to enter perpendicularly an end face of a one-dimensional photonic crystal having no periodicity in the propagation direction, propagation light in a plurality of photonic bands is generated depending on the frequency of the incident light. Among them, propagation light in the band that is not the lowest-order band (in the following, referred to as higher-order band propagation light) has the above-mentioned very large wavelength dispersion and group velocity anomaly, so that it can be applied to various optical elements.
However, regardless of the frequencies, a part of incident light energy always propagates as propagation light in the lowest-order band (corresponding to 0-th mode in a conventional optical fiber; in the following, referred to as first band propagation light). Since the first band propagation light has little effects of “very large wavelength dispersion” or “group velocity anomaly” described above, it merely is a noise in the case of utilizing the higher-order band propagation light. Consequently, the first band propagation light not only lowers the utilization efficiency of the incident light energy considerably but also causes a decrease in a S/N ratio of the waveguide as stray light.