Optical waveguides – Optical fiber waveguide with cladding
Reexamination Certificate
1999-11-05
2001-06-05
Palmer, Phan T. H. (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
C385S124000, C385S126000
Reexamination Certificate
active
06243522
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to an optical waveguide fiber. In particular, the core to clad refractive index contrast in the waveguide fiber is achieved by incorporating a photonic-crystal-like structure into the fiber clad layer.
Waveguide fibers having a photonic crystal clad layer have been described in the literature. At present the photonic crystal fiber (PCF) includes a porous clad layer, i.e., a clad layer containing an array of voids that serves to change the effective refractive index of the clad layer, thereby changing the properties of the waveguide fiber such as mode field diameter or total dispersion. The distribution of light power across the waveguide (mode power distribution) effectively determines the properties of an optical waveguide. Changing the effective index of the clad layer changes the mode power distribution and thus the waveguide fiber properties.
In addition to the properties set forth above, the cut off wavelength is also affected by the clad layer structure is cut-off wavelength. An advantageous feature of a porous clad PCF is that a particular choice of pore size and pore distribution in the clad layer results in the fiber transmitting a single mode for signals having essentially any wavelength. That is the wavelength span of the cut off wavelength is large without bound. Such a PCF has been denoted “endlessly single mode”. An additional benefit afforded by the PCF is the availability of high contrast in refractive index between core and clad at dopant levels near to or lower than the levels in non-PCF waveguide fiber.
The manufacture of a porous clad PCF is difficult because the porosity volume and distribution must be controlled in the preform. Further, the control of the PCF clad porosity must be maintained during drawing of the preform down to the dimensions of a waveguide fiber. Higher speed drawing does reduce manufacturing cost, which means that present PCF drawing processes increases factory cost. The drawing step occurs at very high temperatures and the final fiber diameter is small, about 125 &mgr;m. The drawing step must therefore include the maintaining a precise balance of pressure within the pore against viscous forces of the material surrounding the pore under relatively extreme conditions.
It is expected that the porous clad PCF will be susceptible to OH
−
contamination because at least a portion of the light carrying area of the fiber has a relatively large surface area open to atmosphere after the OH
−
removal step. The OH
−
removal step, known in the art, usually includes treating the heated preform with a reactive gas such as chlorine. An example of OH
−
contamination is shown in curve
2
of FIG.
1
. The overall attenuation is high, being above about 20 dB/km over the wavelength range 800 nm to 1600 nm. In addition the OH
−
absorption peak
4
at 1250 nm, and, the local maxima
6
and
8
, which characterize the broad OH
—
maximum from about 1390 nm to 1450 nm, are unacceptably high and essentially render the waveguide useless except perhaps in very short length applications.
The endlessly single mode property is however of sufficient value to attract workers to address the problem of PCF manufacture. Another incentive to develop a reliable and reproducible process for the PCF is the possibility of achieving unusual dispersion properties which can be used for example in dispersion compensating fiber. The dispersion compensating fiber compensates the dispersion in an existing communication link, thereby allowing operation of the link at a different wavelength. Another PCF advantage is that the large contrast available between core and clad effective index can be used to provide large effective area, thereby mitigating non-linear effects on transmitted signal integrity.
The present waveguide fiber and waveguide fiber preform disclosed and described herein reduces the unsatisfactory OH
−
contamination and effectively overcomes the problems in the prior art.
DEFINITIONS
The effective refractive index of a two or more component glass object, such as the clad layer in the PCF preform and PCF drawn therefrom, having a matrix of a first glass containing rods of a second glass, is defined as,
n
eff
2
=
[
(
1
-
f
)
n
matrix
2
+
f
n
rod
2
]
-
∫
&LeftBracketingBar;
∇
Ψ
&RightBracketingBar;
2
ⅆ
A
k
2
∫
&LeftBracketingBar;
Ψ
&RightBracketingBar;
2
ⅆ
A
where &PSgr; is the solution of the scalar wave equation, k is the wave vector, f is fraction of the field in the rods and n
matrix
and n
rod
are the respective indices of the matrix and rod glass of the clad layer.
The scalar wave equation for light propagating in the z direction is: &dgr;
2
&PSgr;/&dgr;x
2
+&dgr;
2
&PSgr;/&dgr;y
2
+[(kn
1
)
2
−&bgr;
2
]&PSgr;=0, where &bgr; is the propagation constant, k the wave number and n
1
the core refractive index.
The effective V number is, V
eff
=2&pgr;L/&lgr;(n
matrix
2
−n
eff
2
)
½
, where L is the pitch of the rod pattern and &lgr; is wavelength.
SUMMARY OF THE INVENTION
The PCF disclosed and described herein is free of air filled pores in the clad layer. The clad layer of the present fiber includes a matrix material and at least one additional material. The matrix material and the at least one additional material each have a refractive index and the respective refractive indexes are different from each other. The additional material is embedded in the matrix material. The volume and spacing of the embedded material is adjusted to provide a waveguide fiber having a wavelength range of single mode operation that is large without bound. That is the waveguide is endlessly single mode.
Because both the matrix material and the at least one additional material must transmit light in a pre-selected wavelength range, glass materials are good choices. The refractive indexes of the glasses can be raised or lowered by using appropriate dopant materials.
Thus the waveguide fiber preform and the fiber drawn therefrom meets the need for a PCF fiber which has uniform and reproducible performance, especially with regard to low spectral attenuation and geometry control.
A first aspect of the invention is an optical waveguide fiber preform having a core body surrounded by a clad layer made up of a plurality of clad rods. The clad rods each have a central portion, and a layer surrounding the central portion. The central portion of the clad rods has a refractive index different, by a pre-selected amount, from the refractive index of the surrounding layer. This refractive index difference together with the relative dimensions of the central portion and surrounding layer determine the effective refractive index of the clad layer and so affect fiber properties, for example, mode field diameter, cut off wavelength, zero dispersion wavelength, and effective area. The effective index of the clad layer must be lower than the core body refractive index in order for the assembly to ultimately become a light guiding structure. The clad rod diameters are selected to provide a final light guiding structure that is free of porosity. A maximum cross sectional dimension of the clad rods, in the preform, in the range of about 1.5 mm to 3.0 mm provides for a porosity free PCF after the preform is drawn to target fiber dimensions.
An assembly of the clad rods and core body may be held together by inserting it into holder such as a glass tube. One embodiment of the invention employs a glass tube and has a refractive index lower than that of either the central portion or the surrounding layer of the clad rod. As an alternative, the assembly can be bundled by using a frit to weld the preform parts together or by heating the adjoining parts to cause them to adhere to one another. An optically transparent adhesive may be used in place of the frit. Another alternative is to clamp the ends of the preform assembly in proper alignment and deposit a layer of glass soot on the assembly. De
Allan Douglas Clippinger
Borrelli Nicholas Francis
Fajardo James Conrad
Fiacco Richard Michael
Hawtof Daniel Warren
Chervenak William J.
Corning Incorporated
Palmer Phan T. H.
LandOfFree
Photonic crystal fiber 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 fiber, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Photonic crystal fiber will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2495657