Limited mode dispersion compensating optical fiber

Optical waveguides – Optical fiber waveguide with cladding

Reexamination Certificate

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Reexamination Certificate

active

06442320

ABSTRACT:

FIELD OF THE INVENTION
The invention relates generally to limited mode optical fibers used in optical fiber communication systems and in particular to high order mode dispersion compensating optical fibers.
BACKGROUND OF THE INVENTION
One measure of the performance of optical communication systems is the maximum bandwidth; the highest bit rate supported in the communication system. The bit rate generally refers to the speed in which data is transferred from one place to another. High bit rates permit large quantities of data to be transferred in a short period of time. The bit rate is often limited by physical characteristics of the communication link. For example, optical links typically transfer data through an optical waveguide such as an optical fiber in the form of light pulses. As the pulse of light propagates through the fiber, different wavelengths travel at different velocities. This speed differential of the various wavelengths making up the pulse, referred to as chromatic dispersion, causes a short pulse input to one end of the fiber to emerge from the far end as a broader pulse. This limits the bit rate at which information can be carried through an optical fiber. The effect of chromatic dispersion on the optical signal becomes more critical as the bit rate increases. Chromatic dispersion in an optical fiber is the sum of material dispersion and the waveguide dispersion and is defined as the derivative of the group delay with respect to wavelength divided by the length of the fiber.
Dispersion slope is defined as the rate of change of the total chromatic dispersion of the fiber as the wavelength changes, that is, the derivative of the dispersion with respect to wavelength. It is also know as second order dispersion. Third order dispersion is defined as the rate of change of the dispersion slope with respect to wavelength.
In order to achieve the high performance required by today's communication systems with their demand for higher bit rates, it is necessary to reduce the effect of chromatic dispersion. Several possible solutions are known to the art, including both active and passive methods of compensating for chromatic dispersion. One typical passive method involves the use of dispersion compensating (DC) fibers. DC fiber has dispersion properties that compensate for the chromatic dispersion inherent in optical communication systems. DC fibers exist that are designed to operate on both the fundamental or lowest order mode (LP
01
), and on higher order modes.
One desired property of DC fiber is significant negative dispersion. Increasing the magnitude of negative dispersion reduces the length of fiber required to compensate for a large amount of positive dispersion. Another desired property of a DC fiber is low optical signal attenuation. Preferably a DC fiber compensates for chromatic dispersion and dispersion slope, and would be operative over the entire transmission bandwidth. The optical transmission bandwidth typically utilized is known as the “C” band, and is conventionally thought of as from 1525 nm-1565 nm. Longer wavelengths are also coming into usage, and are known as the “L” band, consisting of the wavelengths from 1565 nm-1610 nm.
Refractive index profiles that support desired higher order modes typically also support undesired higher order modes which can generate unwanted effects. Furthermore, periodic perturbations in the fiber such as periodic bending due to spooling create coupling between the desired high order mode and the undesired high order modes guided in the fiber. Modes having approximately the same propagation constants couple more than modes having significantly different propagation constants. The propagation constant &bgr; is a function of the refractive index n according to the formula &bgr;=2&pgr;n/&lgr;. Thus, in place of the propagation constant &bgr;, the effective refractive index or each mode n
eff
may be utilized for each wavelength to determine the strength of coupling between modes.
Typical dispersion compensating fibers are designed as single mode fibers which support only the fundamental or LP
01
mode at operating wavelengths. Such fibers are characterized by having relatively low negative dispersion, high optical loss, and small effective area A
eff
. These fibers typically have low tolerance for high power, exhibit poor macro-bending loss, and provide limited compensation of dispersion slope. Higher order spatial modes such as the LP
02
mode are typically not guided through the fiber.
U.S. Pat. No. 5,361,319 discloses a family of DC fibers that are capable of providing dispersion which is more negative than −20 ps
m·km and attenuation of less than 1 dB/km at wavelengths in the 1520 nm to 1565 nm range. Several of the disclosed DC fibers also exhibit a negative dispersion slope in this region. The refractive index profiles are typically designed to have a relatively large difference in refractive index between the central core region and the surrounding cladding when compared to a conventional step index single mode fiber. The fibers also typically exhibit a relatively narrow width for the central core region as compared with conventional step index single mode fibers. The maximum dispersion achievable by these fibers is approximately −100 ps
m·km with a dispersion slope of approximately 0.8 to 1.2 ps
m
2
·km. The profile is designed to operate in the LP
01
mode, and not to support other higher order modes.
U.S. Pat. No. 5,448,674 discloses an optical DC fiber, containing a power law core refractive index profile, a refractive index “depression” (i.e., trench) surrounding the core, and a refractive index “rise” (i.e., ridge) surrounding the trench. The refractive index profile is designed to support the LP
01
and LP
02
modes, but not the LP
11
mode at &lgr;
op
, the operating wavelength. Dispersion compensation is accomplished with the optical signal in the LP
01
mode. Any optical power which is transferred to the LP
02
mode is lost, thereby contributing to a greater system loss.
U.S. Pat. No. 5,802,234 discloses an optical DC fiber with a refractive index profile selected such that the fiber supports the LP
01
mode, the LP
02
mode, and typically at least one higher order mode. The dispersion is substantially all in the LP
02
mode. The total dispersion is more negative than −200 ps
m·km over a wide wavelength range. The refractive index profile exhibits an effective mode field diameter which increases with increasing wavelength as the mode energy expands to the refractive index “ring” area. Such a mode field diameter results in losses in the operating wavelength range of 1525 nm to 1560 nm as the LP
02
mode expands into the refractive index “ring” with increasing wavelength. The DC fiber is designed to be operated in the trough of the dispersion curve, (i.e. close to the cutoff wavelength for the mode). The profile is designed so that the dispersion curve in the operative wavelengths is relatively flat, and thus relatively insensitive to manufacturing variations. The third order dispersion in this region is positive, with the slope increasing, indicative of attenuation losses in the LP
02
mode.
A need exists for a dispersion compensating fiber which overcomes these and other drawbacks of the prior art.
SUMMARY OF THE INVENTION
The present invention relates in one aspect, to a refractive index profile designed to support higher order spatial modes, and in particular the LP
02
spatial mode in an optical waveguide. The waveguide exhibits negative dispersion and negative dispersion slope and negative third order dispersion over the operating wavelength. In one embodiment, the profile is designed with a reduced refractive index depression in the center core region, and is intended to enhance the properties of the dispersion compensating waveguide. In addition, the refractive index profile of the present invention supports the LP
02
mode.
A limited mode dispersion compensating optical waveguide according to the present invention includes a center core portion having a center co

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