The present invention relates to an all-fiber optical filter and, more particularly, to an all-fiber optical filter comprising selective arrangements of core and cladding material to provide for wavelength-selective filtering, helpful in reducing amplified spontaneous emission (ASE).
Optical fiber systems are known to be sensitive to a variety of different sources of “optical noise” (extraneous signals at wavelengths other than the desired wavelength(s)) that result in impairing the system performance. Various types of filtering arrangements have been proposed through the years to address this problem. Discrete filtering elements (incorporating multiple thin film layers) have been used to remove selected wavelengths from propagating along the fiber. While such discrete filters may be able to reduce the accumulated noise power, they may not sufficiently reduce other system impairments, such as power lost due to noise amplification. Going forward, a distributed, in-line fiber filter is considered to be a more desirable solution than the use of discrete devices, especially for amplifier applications where splice losses, power lost to noise amplification, etc. can effect overall amplifier performance.
Bragg gratings may be formed within the core region of the transmission fiber as one such type of in-line fiber filter to “reflect” selected wavelengths and prevent further propagation of undesirable signal components. See, for example, U.S. Pat. No. 5,717,799 issued to A. Robinson on Feb. 10, 1998, describing the use of a Bragg grating, where the grating is particularly configured to be chirped and apodized to improve the filter qualities. While various in-line arrangements have been successful in providing some filtering, reflection gratings are problematic for in-line filtering in amplifiers, since reflections can lead to unwanted oscillations or lasing at the noise wavelengths. Reflecting gratings can be used as discrete filters, but then do not give the advantages of distributed filtering that are important to the present invention. U.S. Pat. No. 6,141,142 issued to R. P. Espindola et al. on Oct. 31, 2000 discusses an example of a fiber amplifier employing distributed filtering. In this case, filtering is provided by tilted (“blazed”) gratings, instead of reflection Bragg gratings. While some distributed filter embodiments can provide effective filtering, this method requires additional processing steps in fiber fabrication, and restricts the dopant profile of the fiber to those with appropriate photosensitivity.
Thus, a need remains in the prior art for an arrangement that provides optical filtering with enhanced wavelength selectivity, preferably using an in-line, distributed, all-fiber arrangement that eliminates the need to include discrete devices in the optical communication system.
The need remaining in the prior art is addressed by the present invention, which relates to an all-fiber optical filter and, more particularly, to an all-fiber, distributed optical filter comprising selective arrangements of core and cladding material to provide for wavelength-selective filtering that reduces amplified spontaneous emission (ASE).
In accordance with the present invention, a core region of the transmission fiber is configured to have a “raised index” value (that is, the core region is formed to exhibit a refractive index greater than that of the adjacent clad region), while selected areas in the surrounding cladding area are configured to form “features” that also exhibit a raised refractive index (with respect to the majority of the remaining cladding material). The index of the core is raised to provide a difference in refractive index between the cladding and the core such that the selected (desired) wavelengths will remain confined within the core region. Additionally, by careful choice of the refractive index values and physical properties of the raised-index cladding features, selective wavelength filtering may be obtained. The unwanted (filtered) wavelengths will “leak” out of the core and then guided into the cladding, away from the propagating signal path.
It is an aspect of the present invention that the parameters of the core and the cladding may be individually tailored to provide the desired wavelength sensitivity (i.e., “decoupling” the core parameters from the cladding parameters). That is, the core diameter and index value may be determined to allow for optimum propagation of the desired wavelength(s). The cladding raised index features are separately defined, in terms of physical design, location, passband values, stopband values, etc. to provide the desired filtering properties.
The cladding raised index features may comprise any desired geometry, such as one or more rings, “holes”, etc. and are formed using any preferred techniques associated with the formation of optical fiber preforms. In some cases, microstructured optical fibers may be used. Alternatively, MCVD techniques can be used to form one or more concentric “rings” of cladding material exhibiting a higher refractive index.
Other and further features, advantages and embodiments of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings.
Referring now to the drawings,
As mentioned above, there are a variety of different optical systems that can be improved, in terms of performance, by the application of wavelength-selective filtering to the signals propagating along the system optical fiber. In particular, “optical noise” can be reduced by selectively removing signals at certain wavelengths from propagating along the fiber. In accordance with the present invention, an all-fiber optical filter has been developed that exhibits separate cladding modes and core modes in the pass band(s) of interest, but exhibits only mixed core-cladding modes in the stop band(s). Mode mixing leads to reduced overall transmission via reduced gain and/or increased losses within the core region as a result of the signal coupling into the cladding region. An all-fiber filter formed in accordance with the present invention is based upon the following design principles: (1) a raised-index core region that guides light (propagating at desired wavelength(s)) primarily by total internal reflection (TIR), and (2) the inclusion of raised-index features in the cladding that are particularly configured to “guide” selected cladding modes. At wavelengths where the cladding modes are sufficiently index matched with a core mode, the modes will mix and light will “spill” into the cladding instead of being well-contained within the core. Once in the cladding, this light may be further lost from the system by radiation, absorption, or other processes. These wavelengths are thus defined as the stop band(s) of the filter design and can be controlled by manipulating various properties of the raised-index cladding features, such as (but not limited to) the selected refractive index value, the number of included features, the size of the individual features, placement of the features, etc.
The distributed, all-fiber filter of the present invention is considered to be particularly well-suited for fiber amplifier applications. In conventional fiber amplifiers, splice losses and power lost to noise amplification have been found to affect amplifier performance. By having the ability to selectively filter (reduce gain/increase loss) of signals propagating at known “noise” wavelengths, the overall amplifier performance will improve, as evidenced by lower total noise power, improved noise figure, or better power efficiency in converting pump power to desired signal power. One measure of good filter performance is a high extinction ratio—the ratio of loss at the noise wavelength to loss at the desired signal wavelength. Indeed, it is possible to provide a loss at a noise wavelength that is at least a factor of five greater than the loss at the desired signal wavelength (with a difference between the noise and signal wavelengths being less than 20%). The core-guided mode can experience large positive gain, or negative dispersion, for wavelengths near an index crossing point, with the dispersion controlled by proper design of the fiber.
a)-(d) contain cross-sectional illustrations of four exemplary configurations for providing wavelength-selective filtering in accordance with the present invention, where a graph of the refractive index profile is shown in association with the arrangement of
b) illustrates a different geometry, where a plurality of filled “holes” 20 are formed in a cladding region 22 of an optical fiber 24 including a central core region 26. As with the embodiment of
d) illustrates a different embodiment, where a pair of longitudinally-extended cladding features 36 are formed within cladding region 38 of an optical fiber 40 including a core region 42. In this particular embodiment, polarization selectivity is also provided, where features 36 provide birefringence of the optical modes. It is to be understood that the exemplary arrangements as shown in
In accordance with the present invention, therefore, by careful design and choice of the cladding feature geometry, refractive index value, location, etc., the wavelengths that are removed by filtering can be “fine-tuned” to well-defined values. Indeed, it is possible to form a “notch” filter, with wavelengths on either side of a defined “noise” wavelength being allowed to propagate along the core of the fiber. When the effective index of the core mode comes close to the index of the cladding modes, index-matched mode mixing will occur, and light will spill out into the cladding, instead of being well-confined within the core.
It is to be understood that the filtering properties associated with the selective inclusion of high-index material in the cladding of the inventive fiber can be further enhanced by using well-known techniques such as (but not limited to) mechanical deformations in the form of macrobending the fiber, microbending, twisting the fiber, incorporating gratings in the fiber, as well as the inclusion of absorptive or scattering materials in the fiber.
It is to be understood that the above-described embodiments are illustrative of only a few of the many possible specific embodiments which can represent applications of the principles of the invention. Numerous and varied other arrangements can be devised by those skilled in the art without departing from the spirit and scope of the invention.
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