Photonic crystal fibers

Details Photonic crystal fibers Photonic crystal fibers

2001-11-08

2003-10-07

Ullah, Akm Enayet (Department: 2874)

Optical waveguides

Optical fiber waveguide with cladding

Utilizing nonsolid core or cladding

Reexamination Certificate

active

06631234

ABSTRACT:

This invention relates to the field of photonic crystal fibers.
A photonic crystal fiber is a special form of optical fiber. Single-mode optical fibers are widely used in applications such as telecommunications and sensing. Such fibers are typically made entirely from solid transparent materials such as glass and each fiber typically has the same cross-sectional structure along its length. The transparent material in one part (usually the middle) of the cross-section has a higher refractive index than the rest and forms an optical core within which light is guided by total internal reflection. We refer to such a fiber as a standard fiber.
There are many well-established techniques and machines for handling and processing standard fibers; for example, cleavers use a hard knife-edge to break a fiber, giving a clean end-face, and fusion splicers use a hot electric arc to join two fibers together at their ends. One process, fusion-tapering, is used to make a variety of fiber devices for performing some function on light passing along a fiber. In that process, a fiber is locally heated until it softens and then it is stretched so as to narrow the fiber locally in the heated region. Light passing along the fiber is affected by the narrowness of the treated region. In a typical tapered single-mode fiber, the light spreads out from the core and occupies more of the surrounding cladding. If the fiber is sufficiently narrowed, the light spreads out from the core completely and is then guided by the outer boundary of the entire fiber. The fiber is typically heated by immersion in a gas flame, proximity to an electrical heater or exposure to an intense laser beam.
A tapered fiber that is cleaved at the narrowest point of the taper can act as a beam expander because the light wave has a greater cross-section at the cleave than it has in the untreated fiber. Such a beam expander can assist the launching of light into the fiber and extraction of light from the fiber.
A fiber that is tapered so that light is locally guided at its outer boundary can act as a local optical sensor. In the tapered region, the light is sensitive to the medium surrounding the fiber, whereas elsewhere it is insensitive because it is buried in the central core.
Two or more fibers that are tapered together in parallel contact can act as a fiber beam-splitter (or directional coupler), in which at least some of the light in one fiber transfers across to the other fiber in the narrowed region.
In the last few years a non-standard type of optical fiber has been demonstrated, called the photonic-crystal fiber (PCF). Typically, this is made from a single solid, and substantially transparent, material, such as fused silica glass, within which is embedded a periodic array of air holes, running parallel to the fiber axis and extending the full length of the fiber. A defect in the form of a single missing air hole within the regular array forms a region of raised refractive index, which acts as a waveguiding fiber core within which light is guided, in a manner analogous to total-internal-reflection guiding in standard fibers. Another mechanism for guiding light is based on photonic-band-gap effects rather than total internal reflection. Photonic-band-gap guidance can be obtained by suitable design of the array of air holes. Light of some propagation constants can be confined to the core and will propagate therein.
Photonic-crystal fiber can be fabricated by stacking glass capillaries and canes on a macroscopic scale into the required shape, and then holding them in place while fusing them together and drawing them down into a fiber.
The PCF has a number of technologically significant properties; for example, it can support a single-mode over a very broad range of wavelengths, it can have a large mode area and thus carry high optical powers, and it can have a large normal dispersion at the telecommunications wavelength of 1.55 microns. PCFs are typically not circularly symmetric, as a result of the stack-and-draw method typically used to make them.
Technological application of PCFs would be facilitated by handling and processing techniques parallel to those described above for standard fibers. Unfortunately, some of those techniques are not appropriate for PCFs; for example, an attempt to fusion splice two PCFs together causes the air inside them to expand explosively, destroying the fiber ends being joined.
It is an object of the invention to provide PCF optical devices analogous to standard fiber devices. It is another object of the invention to provide a process for producing such devices.
According to the invention there is provided a photonic crystal fiber including a plurality of longitudinal holes, in which at least some of the holes have a different cross-sectional area in a first region of the fiber, that region having been heat-treated after fabrication of the fiber, from their cross-sectional area in a second region of the fiber, wherein the optical properties of the fiber in the heat-treated region are altered by virtue of the change in cross-sectional area of the holes in that region.
The words “after fabrication” should be taken to mean any time after the fiber has been drawn.
The heat treatment processes that can be used are typically the same as those described above for fusion tapering of standard fibers. As with standard fibers, the heat treatment can be accompanied by stretching, to narrow the fiber down. In contrast to standard fibers, however, changes in optical properties can result without stretching the fiber at all; that is because heat treatment can allow some or all of the holes in the fiber to collapse partially or completely under the influence of surface tension. That can be achieved either with or without simultaneous stretching. Furthermore, if some of the holes are pressurized, they can be made to expand instead of collapse, and differential pressurization of the holes can, in principle, be used to create any pattern of hole collapse and expansion. As with the tapering of standard fibers, for most applications, the transitions between untreated fiber and the middle of the heat treated region must be sufficiently gradual that an acceptably small amount of light is lost along the transition—the so-called criterion for adiabaticity.
It may be that at least some of the holes have expanded in the heat-treated region.
It may be that at least some, or all, of the holes have collapsed at least partially, or completely, in the heat-treated region.
The pattern of hole collapse and/or expansions may be not circularly symmetric. The birefringence of the fiber may be altered by the lack of circular symmetry.
The fiber may have been narrowed in the heat-treated region.
The photonic crystal fiber may be included in an optical device.
The photonic crystal fiber may be included in a mode-field transformer, the transformer being arranged so that a guided mode propagating through the transformer will have its field distribution changed by propagation through the heat-treated region of the photonic crystal fiber. The shape and size of the field distribution of the guided mode in a PCF depends on the relative sizes of the air holes and their separations from each other. Thus a PCF that has been heat treated to change the sizes of the holes (or narrow down the entire fiber) can act as a mode field transformer.
The photonic crystal fiber may be included in a multi mode to single mode transformer or mode filter, in which the untreated regions of the fiber are multi mode and the heat-treated region is single mode for at least one wavelength of light. Light propagating through the treated region will be forced into a single mode and will remain substantially single mode when it passes into the untreated region, which is capable of supporting other modes; those other modes will, in an ideal fiber, remain unexcited.
The photonic crystal fiber may be included in a fiber input- or output-coupler, in which the photonic crystal fiber is cleaved in the heat-treated region. Such devices can be used to enhance the coupling of lig

Affiliated with

Also associated with

Rating

Photonic crystal fibers does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Photonic crystal fibers, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Photonic crystal fibers will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3149824