Dispersion compensating photonic crystal fiber

Details Dispersion compensating photonic crystal fiber Dispersion compensating photonic crystal fiber

2000-06-20

2002-09-03

Sanghavi, Hemang (Department: 2874)

Optical waveguides

Optical fiber waveguide with cladding

Utilizing nonsolid core or cladding

C385S123000

Reexamination Certificate

active

06445862

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates generally to a photonic crystal optical waveguide structure for an optical communication system. More particularly, the present invention is directed to an optical fiber micro-structure having photonic crystal characteristics for producing dispersion compensating properties.
TECHNICAL BACKGROUND
Optical waveguide fibers can be generally classified into single-mode fiber and multimode fiber. Both types of optical fiber rely on total internal reflection (TIR) for guiding the photons along the fiber core. Typically, the core diameter of single-mode fiber is relatively small, thus allowing only a single mode of light wavelengths to propagate along the waveguide. Single-mode fiber can generally provide higher bandwidth because the light pulses can be spaced closer together, and are less affected by dispersion along the fiber. Additionally, the rate of power attenuation for the propagating light is lower in a single-mode fiber. Optical fibers which maintain their single mode characteristics for all wavelengths are defined as endlessly single mode fibers.
Optical fibers having a larger core diameter are generally classified as multimode fibers, and allow multiple modes of light wavelengths to propagate along the waveguide. The multiple modes travel at different velocities. This difference in group velocities of the modes results in different travel times, causing a broadening of the light pulses propagating along the waveguide. This effect is referred to as modal dispersion, and limits the speed at which the pulses can be transmitted; in turn limiting the bandwidth of multimode fiber. Graded-index multimode fiber (as opposed to step-index multimode fiber) has been developed to limit the effects of modal dispersion. However, current multimode and graded-index multimode fiber designs still do not have the bandwidth capabilities of single-mode fiber.
Photonic crystals are another means by which photons (light modes) can be guided through an optical waveguide structure. Rather than guiding photons using TIR, photonic crystals rely on Bragg scattering for guiding the light. The characteristic defining a photonic crystal structure is the periodicity of dielectric material along one or more axes. Thus, photonic crystals can be one-dimensional, two-dimensional and three-dimensional. These crystals are designed to have photonic band gaps which prevent light from propagating in certain directions within the crystal structure. Generally, photonic crystals are formed from a periodic lattice of dielectric material. When the dielectric constants of the materials forming the lattice are different, and the material absorbs minimal light, the effects of scattering and Bragg diffraction at the lattice interfaces allow the photons to be guided along or through the photonic crystal structure.
An exemplary photonic crystal
10
which is periodic in two directions and homogeneous in a third is shown in FIG.
1
. More specifically, photonic crystal
10
comprises a triangular lattice of dielectric columns
12
, extending in the Z-axis direction, which are periodic in the X-axis and Y-axis directions (measured center to center). The photonic crystal
10
is assumed to be homogeneous in the Z-axis direction. It is also known that a defect can be introduced into the crystalline structure for altering the planar propagation characteristics and localizing the light modes. For example, photonic crystal
10
includes a central column
14
(shown as a solid black column) comprising a dielectric material that is different from the other periodic columns
12
. Additionally, the size and shape of central column
14
can be modified for perturbing the single lattice site.
The characteristics of the crystalline structure may be used for producing a photonic band gap. The defect in the crystalline structure created by central column
14
allows a path for light to travel through the crystal. In effect the central column
14
creates a central cavity which is surrounded by reflecting walls. Light propagating through the central column
14
(along the Z-axis direction) becomes trapped within the resulting photonic band gap and cannot escape into the surrounding periodic columns
12
. Thus it has been demonstrated that light, whether a pulse or continuous light, can also be guided through this type of photonic band gap crystal. These same structures can be used as effective index structures where the defect acts as a high index core region for guiding light by total internal reflection.
An optical waveguide fiber having a photonic crystal cladding region known within the prior art is shown in FIG.
2
. The photonic crystal fiber (PCF)
16
includes a porous clad layer
18
, containing an array of air voids
20
that serve to change the effective refractive index of the clad layer
18
. This in turn serves to change the properties of the fiber
16
such as the mode field diameter or total dispersion. The air voids
20
defining the clad layer
18
create a periodic matrix around the central fiber core
22
, usually formed from solid silica.
Optical fibers having photonic crystal structures can also be designed which provide unique dispersion characteristics. These characteristics include both positive and negative dispersion. For positive dispersion (D>0) a light pulse may be broadened by slowing the lower (red) frequency components forming the light pulse compared to the higher (blue) frequency components forming the light pulse. Such a light pulse is said to be negatively-chirped. Conversely, for negative dispersion (D<0) a light pulse may be broadened by slowing the higher (blue) frequency components compared to the lower (red) frequency components. Such a light pulse is said to be positively-chirped. Chirped pulses may be narrowed to their original width by transmission through an optical system which reverses the chirp. For example, a pulse which becomes negatively chirped after transmission through an optical fiber with D
1
>0 and length L
1
may be unchirped by transmission through an optical fiber with D
2
<0 and L
2
=−L
1
*D
1
/D
2
. In both cases, the pulse will appear to become broader. Such fibers have potential for use in dispersion compensating modules, a preferred component for upgrading older long haul communication networks. The dispersion compensating fiber within a dispersion compensating module compensates for the chromatic dispersion in an existing communication link, thereby allowing operation of the communication link at a different wavelength. Accordingly, an incentive exists for developing reliable and reproducible optical fiber for producing unique dispersion properties which can be used, for example, in dispersion compensating modules.
FIG. 3A
shows an exemplary index profile for a typical effective index optical fiber. The graph shows the relationship between the refractive index versus the position within the optical fiber. More specifically, the index profile shows that the optical fiber has a high index core region
24
which is surrounded by a low index cladding region
26
. The graph of
FIG. 3A
is generally representative of the index profile of PCF
16
shown in FIG.
2
.
FIG. 3A
is provided primarily for comparison with the index profiles of
FIGS. 3B and 3C
.
FIG. 3B
shows the index profile of an exemplary dispersion compensating optical fiber. The index profile graph shows a fiber having a high index core region
28
surrounded by a low index moat region
30
. The low index moat region
30
is then surrounded by an intermediate index cladding region
32
.
FIG. 3C
shows the index profile for another exemplary dispersion compensating fiber which is similar to that of FIG.
3
B. The fiber of
FIG. 3C
also includes a high index core region
28
, a low index moat region
30
, and an intermediate index cladding region
32
surrounding the moat region
30
. The fiber of
FIG. 3C
includes an additional higher index feature
34
surrounding the moat region
30
for shifting the cutoff wavelength of the

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