JP2002525645A - Multi-core / multi-mode dispersion control fiber - Google Patents

Abstract

  (57) [Summary] Optical paths along an optical fiber that include multiple cores or multiple modes are aligned with positive and negative dispersion characteristics. The coupling mechanism defines a relative length of transmission between optical paths having different dispersion characteristics. Therefore, the total dispersion of the combined optical paths approaches zero dispersion in the range of signal wavelengths to be transmitted.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD OF THE INVENTION

This application is based on provisional application Ser. No. 60 / 100,495, filed Sep. 16, 1998, which claims the above date as the priority date of the present application. Optical signals traveling in a typical fiber optic medium undergo slight changes, which can cause significant signal quality degradation over long distances. Chromatic dispersion is one such change. The dispersion control fiber has positive and negative dispersion characteristics, which are tuned to produce a length-weighted average near zero dispersion.

[0002]

BACKGROUND OF THE INVENTION

Chromatic dispersion varies along the waveguide as a function of the material and structure of the waveguide, as well as changes in signal wavelength. Zero dispersion can be specified at a particular wavelength, but zero dispersion is associated with "four-wave mixing", a well-known phenomenon that causes crosstalk between adjacent wavelength channels. Four-wave mixing is most pronounced at zero dispersion, but increases with optical power and reduces channel spacing.

[0003] Chromatic dispersion and four-wave mixing can be avoided by a dispersion control fiber that combines the lengths of the positive and negative dispersion fibers (evaluated at the wavelength to be transmitted). If only non-zero dispersion fiber is used, four-wave mixing is avoided. If the length-weighted mean of the positive and negative dispersion is near zero, chromatic dispersion is avoided. Such a method comprises a positive dispersion fiber for transmitting an optical signal over its entire length, and a dispersion compensation module that periodically cuts the positive dispersion fiber and includes a roll of negative dispersion fiber. , Reducing the average dispersion of the combined optical paths. However, the compensation module reduces the signal power without advancing the signal in the desired direction.

Optical signals can be more efficiently transmitted while reducing chromatic dispersion by splicing the lengths of the positive and negative dispersion fibers. However, there is a need for mapping to track the dispersion characteristics of the fibers being combined, and furthermore, two different types of fibers must be kept in stock. The dispersion control fiber is formed of a continuous length with alternating sections having different signs of dispersion at the intended transmission wavelength. In this case, only one fiber needs to be stocked, but the dispersion period (ie, the repetition length of the two sections) must be selected at the time of manufacture and cannot be changed later. Also, if the fibers become contaminated at the interfaces between the sections, they must be individually polished and assembled before being drawn into the final shape.

[0005] The dispersion control cable includes a set of one or more fibers having different dispersion codes. Sections of the cable are spliced together such that the positive dispersion fiber of one section is connected to the negative dispersion fiber of an adjacent section. In addition, dispersion mapping is required to keep the section length information alive and the individual fiber design is limited because the dispersion of the different codes must be set to be equal at the transmission wavelength. It is done.

[0006]

Summary of the Invention

The present invention includes many embodiments of fiber optic systems that compensate for chromatic dispersion and avoid four-wave mixing. On the other hand, fiber inventory is minimized, giving more flexibility in fiber design and performance. Also, the dispersion period (ie, the length over which the dispersion change is repeated) can be shorter without complicating the fabrication and without requiring additional dispersion options after the fabrication.

One embodiment is a dispersion compensating fiber optical system that includes a single optical fiber having a plurality of continuous optical paths having different dispersion characteristics that carry an optical signal. One of the optical paths exhibits a positive dispersion at the center wavelength of the optical signal and the other of the optical paths has
It shows negative dispersion at the center wavelength of the optical signal. The coupling mechanism shifts the optical signal between the advancing portions of the two optical paths that produces a length-weighted average dispersion near zero at the center wavelength of the optical signal. Preferably, the dispersion and dispersion slope are adapted at the center wavelength (eg, of equal magnitude but opposite sign) such that the average dispersion over a range of wavelengths is closest to zero dispersion.

[0008] Successive optical paths extend parallel or concentric to one another, shifting the optical signal between the optical paths without breaking any optical paths without any additional coupling structures being present. For example, one fiber includes a plurality of cores surrounded by a clad. Each core forms one of the optical paths having different dispersion characteristics.

Signals can be actively shifted between cores by forming a coupling mechanism, such as one or more long-period gratings. Coupling mechanisms can also be formed as a result of core spacing by placing cores close enough together to support signal transfer. The latter mode of coupling requires symmetric dispersion characteristics between the cores and has a dispersion period equal to the coupling length between the cores. The former combined mode allows more flexibility in the dispersion characteristics of the core by shifting the signal between cores at unequal intervals. Regardless of the coupling mode, the dispersion gradients of the positive and negative dispersion cores are preferably adapted (eg, small magnitude or opposite sign) and the resulting average dispersion is zero over the intended signal wavelength range. Maintained near.

Another embodiment includes a fiber segment having one or more sets of cores having opposite sign dispersion characteristics. The segments are spliced end-to-end and the positive distribution core of one segment is aligned with the negative distribution core of the other segment. The segment length is chosen to achieve a length-weighted average close to zero variance. By equalizing the magnitudes of the absolute values of the positive and negative variances, and having both positive and negative divergent cores face-to-face in adjacent segments, the two cores in each set are parallel. Can be used to send signals. Multiple sets of positive and negative distributed cores may be separately arranged to support transmission of multiple bit rates or applications. More flexibility of dispersion mapping is also possible by controlling the angles between adjacent sections to align various combinations of cores with different dispersion characteristics.

The one fiber may be formed as a multimode fiber having different dispersion values and having a fundamental mode path and a higher-order mode path so as to form concentric optical paths having different dispersion characteristics. Can also. The coupling mechanism of this further embodiment includes one or more mode couplers and may be formed as a tapered coupling or long-period grating to shift the optical signal between the fundamental and higher order mode paths.

The fundamental modes may be arranged to exhibit a positive variance, and higher order modes may be arranged to exhibit a higher magnitude of the negative variance. Therefore, the mode couplers in this arrangement are arranged to shift the optical signal to the fundamental mode with a longer interval than the higher order modes. However, by appropriate core distribution design and selection of the normalized frequency, the dispersion and dispersion slope of the fundamental and second-order modes can be equal but different signs. Also, better signal limiting is possible with normalized frequency values away from the mode cutoff value.

A variety of connectors can be used to pass signals sent on multi-core or multi-mode fiber through a single-mode single-core fiber. For example, an optical grating may transmit an optical signal from one core to another core aligned with a single-mode single-core fiber, or from a higher mode to a fundamental mode, for further transmission by a single-mode single-core fiber. Can be used to shift. Tapered couplings also
It can be used to energize signals to single core or fundamental mode. In addition, a separate single-mode single-core fiber is connected to the different cores of the multi-core fiber, and the switch sends additional signals from one of the connected fibers to a common single-mode single-core fiber. Can be used to send.

[0014] Multimode fibers can be manufactured by conventional processes that focus on the dispersion characteristics of the modes. Multi-core fibers can be formed by incorporating two or more core canes into a preform prior to a conventional fiber drawing step. Various arrangements of tubes or rods can be used to align and separate the core canes, and the soot overcladding is consolidated around the core structure to seal the structure inside the preform. Stop.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION

As shown in FIG. 1A, a multi-core optical fiber 10 has a positive dispersion core 12 and a negative dispersion core 14 surrounded by a common cladding 16. Two cores 12 and 14
Is referred to with respect to the center wavelength (generally corresponding to an erbium expansion window) of the wavelength range intended to be transmitted by the fiber 10. 1530nm
In the case of dispersion control over the wavelength range from to 1560 nm, the positive core may have a design equivalent to SMF1528 fiber. Also, the negative core may have a design equivalent to a 1585LS fiber or series of products. The two fibers are from Corning Incorporated, Corning, Corning, NY.
New York). As described below, for other wavelength ranges, different types of known core designs can be used.

The two cores 12 and 14 extend in parallel by a distance “S” by the optical axis 18 of the fiber 10 and are adjusted to suppress or promote automatic coupling between the cores 12 and 14. Can be done. As shown, this distance "S" is large enough to suppress automatic coupling. During the manufacturing process, core canes having a core-cladding ratio of at least 0.4 are spaced sufficiently apart to provide the required spacing. fiber
An optional notch 20 on the surface of the fiber 10 provides a reference point for angularly grading the segment length of the fiber 10.

In FIG. 2, the originally continuous fiber 10 is divided into two lengths 10A and 10B.
Are axially aligned and relatively rotated about light distribution axes 18A and 18B before being spliced together, such as a splicer designed as a polarization maintaining fiber. The amount of rotation depends on the negative dispersion core 14B of the fiber segment 10B and the fiber segment 10B.
A positive distribution core 12A is selected to be aligned. In addition, by designing symmetrically, the positive dispersion core 12B of the fiber segment 10B and the fiber segment 10B
A negative dispersion core 14A can also be aligned at the same time. Segments 10A and 10B
Is adjusted in length so that the average variance along the combined length of the two segments 10A and 10B, as well as all connecting segment pairs, is close to zero variance.

If the two cores 12A, 14A and 12B, 14B of each of the segments 10A and 10B are used to carry different signals, the positive and negative dispersion of the two cores must be of equal magnitude Instead, the two segments 10A and 10B must be of equal length. However, in the case where one of each core of the segment (eg, core 12A of segment 10A and core 14B of segment 10B) carries a signal,
The variances of the two cores can be optimized at different magnitudes and different length segments can be combined to achieve an average variance close to zero. Also, the dispersion slope of the two cores is preferably adapted to maintain an average dispersion near zero over the wavelength range in which transmission is desired.

Instead of splicing the segment length of the fiber 10 to alternate the optical paths of the positive and negative dispersion cores 12 and 14, a passive or active coupling is provided between the running lengths of the cores 12 and 14. Can be Passive coupling can be achieved by reducing the distance S between the cores. That is, power transfer occurs between the cores with a desired distribution period, which is equal to the coupling length. The positive and negative dispersion of cores 12 and 14 must be symmetric (ie, equal in magnitude) about the center wavelength. That is, the signal is transmitted to cores 12 and 14
Use half of each time. Also, two isolated cores 12 and 14
Must be close enough to support a more complete transfer of power. The coupling length is determined by the difference between the propagation constants of the two lowest order supermodes of the composite waveguide and can be designed to be anti-chromatic or chromatic.

FIGS. 3A and 3B illustrate the refractive index distribution of the positive and negative dispersion cores 12 and 14 modified to consider the effective refractive index n (eff) between the two cores 12 and 14 equivalently. The positive dispersion core 12 has a simple step distribution (GeO ₂ —SiO ₂ core with SiO ₂ cladding) and has an effective refractive index n (eff) that is sized between the core and cladding values. The negative dispersion core 14 is adapted to match the effective refractive index n (eff) of the positive dispersion core 12,
It has a "w-type" or segment core (SEGCOR) distribution design with some updoping in the cladding. For example, the added cladding may consist of GeO ₂ -SiO ₂ or TiO ₂ -SiO _2. (Note: the dashed line in FIG. 3B represents the refractive index level of the outer silica cladding 16.) In general, more complex distribution types produce a negative dispersion and dispersion gradient opposite to a core having a positive dispersion. I need. Four more embodiments are shown in FIGS. 3C-3F. They can have negative dispersion without significantly compromising other optical properties such as effective area, mode field diameter, bending and microbending. Arrows across the distribution lines indicate the design freedom to change individual line segments of the distribution.

The distribution of FIG. 3C can be used to achieve positive or negative dispersion with a positive or negative dispersion slope. The design of FIG. 3D is particularly useful in achieving positive or negative dispersion with a relatively large effective area. The latter two designs, FIG. 3E and
3F also takes into account dispersion control in a low loss manufacturing method. As shown in FIG. 4, active coupling between cores 12 and 14 may be achieved by forming one or more long-period gratings 24 between the cores. Since the coupling function is localized, the two cores 12 and 14 can be designed independently. For example, the magnitude of the core dispersion between the cores 12 and 14 and the propagation constant can be changed. The spacing between gratings 24 may be adjusted to compensate for different magnitudes of the core variance, such that the length-weighted average approaches zero variance. Taper can be used with long period grating 24 to increase the coupling function.

The long-period grating 24 may be formed of a photosensitive core material that is exposed to a photochemical radiation pattern to cause a refractive index perturbation in the fiber 10. The cladding region between cores 12 and 14 can also be formed photorefractively to enhance the coupling function. The long period grating 24 can be written by a powerful excimer laser during the drawing operation of the fiber. Since the coupling mechanism does not add contamination to the fiber 10, the dispersion period can be very small and large.

The grating accuracy is not very critical. That is, the required spectral response band is very wide, and typically long-period gratings have periods on the order of hundreds of microns. Also, the magnitude of the refractive index perturbation can be very low (to eliminate the need for a hydrogen supply). That is, the long period grating 24 can occupy a relatively large distance (eg, 1-2 meters) along the fiber 10 without detrimental effects. Refractive index perturbations may be written in one spot at a time or several spots at a time, especially at high drawing speeds. Also, the index of refraction or curvature perturbation may be drawn by a powerful CO ₂ laser during the drawing operation to achieve a similar coupling function. Other perturbations form similar gratings, including stress-induced fluctuations caused by squeezing the fiber periodically or path length fluctuations due to periodic microbending.

For further information on long-period gratings and mode couplers, see Vengsarkar et al., Jan. 1996, Journal of Lightwave Technology, Vol. 14, No. 1, pp. 58-65. Long-Period Fiber Gratings as Band-R
ejection filters), and in May 1991, Journal of Lightwave Technology, Vol. 9, No. 5, pp. 598-604, by "Polar, et al.," Spiral gratings, two-mode fibers, spatial mode couplers. (Helical-Grating Tw
o-Mode Fiber Spatial-Mode Coupler). Both documents are
It shall be cited in the specification of the present application.

As shown in FIG. 5, more than one core may be formed in a single fiber. The two positive dispersion cores 32 and 34 and the two negative dispersion cores 36 and 38 of the fiber 30 are surrounded by a common cladding 40. Cores 32 to 38 are composed of positive and negative dispersion cores (for example, 32 and
36 and 34 and 38). Such pairs of cores may be optimized for individual bit rates or applications. Alternatively, fiber 30
In variance mapping by changing the angle landmark between adjacent sections of a pair, the pairing can be changed to give more flexibility. In other words, similar fibers 30 can be used to support multiple different dispersion maps. The dummy core 44 provides a mark for calibrating the angle of the fiber 30 around the optical axis 46.

As with the cores 12 and 14 of the fiber 10, the cores 32, 34, 36 and 38 of the fiber 30
It is offset from the optical axis 46, which can cause polarization mode dispersion problems. The periodic twist or continuous rotation of fiber 10 or 30
Can be used to reduce this problem. The cores 12 and 14 of the fiber 10 or the cores 32, 34, 36 and 38 of the fiber 30 may be surrounded by individual cladding zones to optimize the properties of separate optical paths through the fibers 10 and 30.

At the location of the in-line amplifier or at the link end, for example, when connecting to a conventional single-mode fiber or similar waveguide structure, each pair 2
One core is associated with a single core of a conventional waveguide. FIG. 6 shows a tapered coupling 60 connecting the dispersion control fiber 10 to a conventional single mode fiber 70.
Represents The two waveguides 62 and 64 are aligned with the positive and negative dispersion cores 12 and 14 of the fiber 10, and only the waveguide 64 is aligned with a single core 66 of the normal fiber 70. In this coupling position, the two waveguides 62 and 64 are tapered to each other to transfer power to the waveguide 64.

As shown in FIG. 7, the long period grating 24 can also be used in place of the tapered coupling 60, and a suitable core (eg, Guide the optical signal to the core 12). Normal fiber 70 core
66 is aligned with the core 14 of the dispersion control fiber 10 for receiving an optical signal propagating along the dispersion control fiber 10. V-groove substrate 72 supports the desired arrangement between fibers 10 and 70.

The normal fiber 70 can be tuned to either the core 12 or 14 of the dispersion control fiber 10. Alternatively, each of the cores 12 and 14 can be fitted with a separate regular fiber. In the latter case, an optical switch (not shown) can be used instead of connecting one regular fiber to a separate regular fiber. With sensors that detect the presence or absence of a signal on any of the separate regular fibers,
A switch can be controlled.

FIG. 1B shows two similar cores 12 ′ and 14 embedded in a common cladding 16 ′.
Represents another fiber 10 having '. In contrast to the core 12 of the fiber 10 (see FIG. 1A), the core 12 'is centered along the optical axis 18' of the fiber 10 '. The other core 14 'is offset from axis 18'. The centrally located core 12 'is easy to align with the core of a typical fiber. However, end-to-end splicing of sections of the fiber 10 'to shift signals between the centrally located core 12' and the offset core 14 'is more complicated. Therefore, the signal transfer between cores 12 'and 14'
Preferably, this is done by a lateral connection. Prior to any end-to-end connection, the signal is preferably shifted to the central core 12 'by tapering the fiber 10' or using a tapered coupling, as shown in FIG.

FIG. 1C shows that instead of centering only one of the cores 12 ′ around the optical axis 18 ′,
Shown is another fiber 10 "having two cores 12" and 14 "centered concentrically about 18". Lateral couplers are used to shift the signal between concentric cores 12 "and 14". Fiber 10 "having concentric cores 12" and 14 "exhibits less birefringence and is simpler than manufacturing methods using conventional manufacturing techniques. Also, concentric cores in combination with additional concentric cores or offset cores can be used.

Another method of controlling dispersion of an optical fiber is shown in FIG. Multimode optical fiber 80 has a central core 82 and an outer cladding 84 designed to support multiple modes of optical transmission. One mode (eg basic mode)
Exhibits a positive variance, and other modes (eg, quadratic modes) exhibit a negative variance.
The long-period grating 86 is drawn on the fiber 80 along the optical axis 88 with a repeating pattern that regulates the relative duration of the optical signal in each mode, such that the length-weighted average dispersion approaches zero. .

In conjunction with the plot of FIG. 8, the negative variance of the secondary mode has a greater magnitude than the positive variance of the fundamental mode. Accordingly, the interval L _F obtained by measuring the length of progression between the grating 86 of fundamental mode is greater than the distance L _H to the length of the progress was measured between the grating 86 in the secondary mode. Other combinations are possible, including variances of different codes of equal magnitude between the two modes of operation. Equally spaced gratings can be used to evenly distribute the optical travel length between two different modes. Also, higher order modes than the second order can be used to reduce polarization mode dispersion. Gratings can shift the signal to third and higher order modes, but unintended losses occurring between modes can limit the utility of higher order modes.

FIGS. 9 and 10 show a multimode fiber 80 considered with a stepped index core.
Is shown graphically. In FIG. 9, the normalized propagation constant b _n is
It is plotted against the normalized frequency V. This is then defined mathematically as follows:

(Equation 1) Where β is the propagation constant, n ₁ is the refractive index of the core, n ₂ is the refractive index of the cladding, λ is the center wavelength of a certain range, k is a constant of 2π / λ, and a is the core radius of the waveguide. . n (ef
The normalized propagation constant for f) varies between 0 and 1, where 0 indicates complete propagation in the cladding and 1 indicates complete propagation in the core. The propagation that occurs in the core is more tightly constrained than the propagation that occurs in the cladding. The normalized frequency value V has a relationship of the reciprocal of the center wavelength λ.

The curves LP ₀₁ , LP ₁₁ and LP ₀₂ represent the fundamental, second and third order modes, respectively. According to the exemplary graph of FIG. 9, a normalized frequency greater than 2.4 needs to support multiple modes, but even higher values around 3.5 are needed for most practical applications. Is necessary to limit the signal to

As shown in FIG. 10, in about 3.5 normalized frequency, giving a standard influencing for good waveguide dispersion D _n of opposite sign in the operating area. This is empirically defined as follows:

(Equation 2)

A normalized frequency of less than 3.5 gives a much higher dispersion potential, but the second-mode signal limit is reduced. Operation away from the secondary mode cutoff (ie,
At V = 2.4), bending loss, micro bending loss and polarization split are reduced as well. A periodic or continuous twist of the fiber 80 can also be used to reduce polarization mode dispersion.

The waveguide dispersion D measured in the unit of ps / km-nm is calculated by the following equation.

(Equation 3) Here, Δ is a relative refractive index difference. Waveguide dispersion D has a sign opposite to the normalized wavelength dispersion D _n, the waveguide dispersion of the fundamental mode LP ₀₁ positive waveguide dispersion of the secondary mode LP ₁₁ is negative.

At a normalized frequency greater than 2.4, the fundamental mode LP ₀₁ of the step index fiber
Is very small, so most of all substantial chromatic dispersion is due to material dispersion. In the 1550 nm window, the chromatic dispersion of the fundamental mode in the step index fiber is limited to about 17 to 20 ps / km-nm. However, more complex core distribution designs, including segmented cores and annular distributions, can be used to produce higher variance values. The core design between the positive and negative dispersion cores is selected with the dispersion slope appropriately related so that the average dispersion is kept close to zero over the entire range of signal wavelengths. For example, the dispersion gradients are of equal magnitude, but of opposite sign, and of small absolute magnitude.

Multimode and multicore designs are also combined within individual fibers to provide additional control over dispersion while optimizing other design criteria. For example, one or both of the cores 12 and 14 of the fiber 10 shown in FIG. 1A or the other cores shown in FIGS. 1B and 1C may be formed as multimode cores. Also, the dispersion requirements may be separated by mode and core.

Conventional manufacturing techniques can be used to form multimode fiber 80.
Also, similar photorefractive techniques for multi-core fibers as described above can be used to image the grating. While long period grating 86 is preferred for mode coupling, other patterns and types of perturbations, including diameter variations, can also be used to shift signals between different modes.

The remaining FIGS. 11 to 14 show preforms (or “preforms”) for manufacturing multi-core optical fibers using various rod-in-tube or OVD (optical vapor deposition) techniques for improving polarization maintaining fiber. Blank "). The fiber preform 90 shown in FIG. 11 has two glass core canes 94 and 96.
Has a rod 92 drilled to receive the. The rod 92 is made of a clad material. The two glass core canes 94 and 96 include core and cladding materials that are sequentially formed by optical vapor deposition into a refractive index profile that produces different dispersion characteristics. A glass soot 98 of cladding material is applied to the outside of the rod and is consolidated to form a preform 90. A multi-core fiber drawn from a preform has at least two cores extending parallel to each other and exhibiting different dispersion characteristics.

The fiber preform 100 shown in FIG. 12 has two glass core canes 102, 104 and two filler rods 106 and 108 fixed in a tube 110, and a tube 11.
It has a consolidated soot 112 surrounding 0. The two core canes 102 and 104 have a core and cladding distribution that support different dispersion characteristics. The filler rods 106 and 108, the tube 110, and the soot 112 are
All made of clad material that is melted with 4.

FIG. 13 shows a preform 120 that includes a specially shaped rod 114 of cladding material that supports two core canes 116 and 118 having different dispersion characteristics.
A consolidated soot 122 of clad material comprises a rod 114 and two core canes 116.
And 118. The two core canes 116 and 118 are secured by a rod 114 that supports the two core canes in a desired relative position until the preform 120 is consolidated.

As shown in FIG. 14, the preforms 130 are fixed to each other and
, Surrounded by two core canes 132 and 134. Core canes 132 and 134
Have different dispersion properties and refractive index profiles that can promote or prevent automatic bonding between the final cores drawn from the preform 130. Soot over cladding 98, 112, 122 and 136 are consolidate
Shrinking during the step produces both axial and radial forces that seal components within preforms 90, 100, 120 and 130. During the subsequent drawing step, photorefractive techniques can be used to create bonds between the cores,
Furthermore, polarization mode reduction techniques can be used to compensate for asymmetry of the cladding surrounding the core. An index mark is formed around the preform or is provided at a location where a filler rod made of a material distinguishable from the cladding appears to provide a reference angle.

The pairs of core canes 94 and 96, 102 and 104, 116 and 118 and 132 in the four preforms
And 134 have variances of opposite signs having equal or unequal absolute magnitudes. Periodic or continuous coupling can be used to achieve a length-weighted average variance close to zero with equal magnitude variance. However, unequal magnitude variances are required to achieve the same length-weighted average variance near zero in unequal length periodic couplings traveling through different distribution cores.

[Brief description of the drawings]

FIG. 1A is an enlarged cross-sectional view of a multi-core optical fiber having two offset cores having different dispersion characteristics.

FIG. 1B is an enlarged cross-sectional view of another multi-core optical fiber having one central core and one offset core having different dispersion characteristics.

FIG. 1C is an enlarged cross-sectional view of another multi-core optical fiber having two concentric cores having different dispersion characteristics.

FIG. 2 is an enlarged side view illustrating two segment lengths of a multi-core fiber spun relative to each other.

FIG. 3A shows a core radius r for a refractive index distribution of two cores of a multi-core fiber.
FIG. 4 is a plot of the refractive index as a function of.

FIG. 3B shows a core radius r with respect to a refractive index distribution of two cores of a multi-core fiber.
FIG. 4 is a plot of the refractive index as a function of.

FIG. 3C is an illustration of another refractive index profile that is particularly suitable for achieving negative dispersion.

FIG. 3D is a diagram of another refractive index distribution that is particularly suited to achieving negative dispersion.

FIG. 3E is a diagram of another refractive index distribution that is particularly suitable for achieving negative dispersion.

FIG. 3F is an illustration of another refractive index distribution that is particularly suited to achieving negative dispersion.

FIG. 4 is another side view of a multi-core fiber including a long-period grating that optically couples two cores.

FIG. 5 is an enlarged cross-sectional view of a multi-core fiber having four cores having two positive dispersion characteristics and two negative dispersion characteristics.

FIG. 6 is a side view of a taper coupling for connecting two cores of the multi-core fiber of FIG. 1A to a conventional single-core fiber.

FIG. 7 is a side view of a coupling for shifting a signal between two cores of a multi-core fiber and a connector for joining a core of a conventional single-core fiber and one of two cores of a multi-core fiber.

FIG. 8 is a side view of a multimode fiber having a continuation of a long period grating for shifting a signal between modes having different dispersion characteristics.

FIG. 9 is a graph of normalized propagation constant b _n versus normalized frequency V of a multimode fiber as exemplified by a step distribution core design.

10 is a graph of normalized wavelength dispersion d _n with respect to the normalized frequency V of the waveguide in the same step distribution core design.

FIG. 11 is an enlarged cross-sectional view of a preform supporting two core canes in a perforated rod.

FIG. 12 is an enlarged cross-sectional view of a preform supporting two core canes in a tube.

FIG. 13 is an enlarged sectional view of two core canes secured by a specially shaped rod.

FIG. 14 is an enlarged cross-sectional view of the combined two core canes before melting the preform around the two cores.

[Explanation of symbols]

 10 Multicore optical fiber 12 Positive dispersion core 14 Negative dispersion core 16,40 Cladding 18,46,88 Optical axis 20 Notch 24 Grating 30,70 Fiber 32,34,36,38 Dispersion core 44 Dummy core 60 Tapered coupling 62 , 64 waveguide 66 core 80 multimode optical fiber 82 central core 84 outer cladding 90,100 preform 92 rod 94,96,102,104,116,118 glass core cane 98,112,122 glass soot 106,108 filler rod 110 tube 114 rod

[Procedure amendment]

[Submission Date] October 26, 2001 (2001.10.26)

[Procedure amendment 1]

[Document name to be amended] Drawing

[Correction target item name] All figures

[Correction method] Change

[Correction contents]

FIG. 1A

FIG. 1B

FIG. 1C

FIG. 2

FIG. 3A

FIG. 3B

FIG. 3C

FIG. 3D

FIG. 3E

FIG. 3F

FIG. 4

FIG. 5

FIG. 6

FIG. 7

FIG. 8

FIG. 9

FIG. 10

FIG. 11

FIG.

FIG. 13

FIG. 14

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Claims (84)

[Claims] 1. A multi-core fiber for controlling chromatic dispersion composed of a plurality of cores surrounded by a clad, wherein the core has an index of refraction different from that of the clad to carry an optical signal, and the plurality of cores Characterized in that there is a difference in dispersion value between the multi-core fibers. 2. The fiber of claim 1, wherein one of said cores exhibits a positive dispersion value and another of said cores exhibits a negative dispersion value. 3. The fiber of claim 2, wherein said core exhibits a positive and a negative dispersion gradient. 4. The fiber according to claim 2, wherein the positive dispersion value and the negative dispersion value are symmetric about a center wavelength of the optical signal. 5. The fiber of claim 4, wherein said cores are located sufficiently close to each other to support a bond between said cores. 6. The fiber according to claim 5, wherein propagation constants between said cores are substantially the same. 7. The fiber according to claim 1, wherein one of said cores is a multi-mode core. 8. The fiber according to claim 7, wherein said multimode core includes a first mode exhibiting positive dispersion and a second mode exhibiting negative dispersion. 9. The fiber of claim 1, wherein the cores are spaced sufficiently to avoid coupling between the cores. 10. The fiber of claim 9, wherein the fiber includes a tapered portion to facilitate transfer of the optical signal between the core. 11. The fiber of claim 9, wherein a perturbation is formed in the core to facilitate transfer of an optical signal between the core. 12. The fiber of claim 11, wherein the perturbations are formed in a cladding region between the cores to facilitate coupling between the cores. 13. The fiber of claim 11, wherein the perturbations are arranged to form a plurality of optical gratings that transfer optical power between the cores. 14. The method of claim 14, wherein the perturbation is arranged to shift the optical signal between the cores in a pattern that produces a length-weighted average dispersion near zero at a center wavelength of the optical signal. 12. The fiber according to claim 11, wherein: 15. A tapered fiber is used with said perturbation,
The fiber of claim 11, wherein the fiber facilitates transfer of the optical signal between the cores. 16. The fiber is split into two lengths that are rotated relative to each other and spliced together at relative rotational positions to align a first pair of the cores exhibiting different dispersion values. The fiber according to claim 1, wherein the fiber is used. 17. The fiber of claim 2, wherein the two lengths of the fiber are rotatable relative to another angular position that aligns a second pair of the cores exhibiting different dispersion values. The fiber according to item 16. 18. The method of claim 18, wherein the first pair of cores is optimized at a first bit rate, and the second pair of cores is optimized at a second bit rate. 18. The fiber according to claim 17, wherein: 19. The method according to claim 19, further comprising a fiber interface for connecting a core of the additional fiber disposed at a center in the cladding and one of the plurality of cores disposed at a position other than the center of the cladding. The fiber of claim 1, wherein: 20. The fiber of claim 1, wherein the fiber is twisted at least periodically to avoid polarization mode dispersion. 21. The apparatus of claim 19, further comprising a coupling mechanism for transferring the optical signal between in-progress portions of the core that provides a length-weighted average dispersion near zero at a center wavelength of the optical signal. Item 7. The fiber according to Item 1. 22. The core according to claim 21, wherein the core exhibits a dispersion gradient relatively adapted such that the length-weighted average dispersion is close to zero in the wavelength range of the optical signal.
The fiber as described. 23. A core surrounded by a cladding that supports multi-mode transmission of light along an optical axis, a first mode path along the optical axis having a first dispersion value, and a second dispersion. A second mode path along the optical axis having a value, different first and second dispersion values from each other, and coupling light back and forth between the first and second mode paths. And a plurality of mode couplers along the optical axis. 24. The fiber according to claim 23, wherein the first mode path is a fundamental mode path having a positive dispersion value. 25. The fiber of claim 24, wherein the second mode path is a higher order mode path having a negative dispersion value. 26. The fiber of claim 23, wherein the dispersion values between the first and second mode paths have different signs and unequal magnitude absolute values. 27. The light coupling between the first and second mode paths occurs with unequal path lengths to compensate for unequal magnitude absolute values of the variance. 27. The fiber of claim 26. 28. The fiber according to claim 23, wherein the dispersion values between the first and second mode paths have different signs and substantially equal magnitudes. 29. The fiber of claim 23, wherein the mode coupler is formed by a refractive index perturbation. 30. The fiber of claim 29, wherein the refractive index perturbation forms a long period grating. 31. The fiber of claim 23, wherein the mode coupler is formed in at least a portion near a tapered portion of the fiber. 32. The mode coupler has a central wavelength of the light transmission and a length close to zero.
-Are spaced apart so as to generate a weighted average variance.
Fiber described in 3. 33. The first and second mode paths have a length in a transmission wavelength range of the light.
33. The fiber of claim 32, wherein the fiber exhibits a dispersion gradient that is relatively adapted so that the weighted average dispersion approaches zero. 34. A first multi-core fiber section having a first core exhibiting a positive dispersion value and a second core exhibiting a negative dispersion value, a first core exhibiting a positive dispersion value, and a negative dispersion. A second multi-core fiber section having a second core exhibiting a value, controlling an average dispersion along a combined length of said fiber sections, wherein said second core and said second core of said second fiber section are controlled. An optical interface between said first and second multi-core fiber sections in which said first core of one fiber section is aligned with said first core. 35. The first and second cores of each of the first and second fiber sections are sufficiently separated to avoid unwanted coupling between the first and second cores. 35. The optical system according to claim 34, wherein the optical system is disposed. 36. The method of claim 36, wherein the second core of the first fiber section is aligned with the first core of the second fiber section to control the average dispersion. The optical system of claim 34. 37. The second fiber section of the first fiber section and the first core of the second fiber section, wherein the fiber section has a wavelength range transmitted through the combined fiber section. 37. The optical system of claim 36, wherein the optical system exhibits a dispersion gradient adapted such that the average dispersion along the combined length of is zero. 38. The first and second multi-core fiber sections further include a third core exhibiting a positive dispersion value and a fourth core exhibiting a negative dispersion value. The optical system of claim 34. 39. The first core of the first fiber section is aligned with the second core of the second fiber section and optimized to be transmitted at a first bit rate. Forming a first optical path between said fiber sections;
The third core of the first fiber section is free to align with the fourth core of the second fiber section and transmits optical signals at a second bit rate. Item 39. The optical system according to Item 38. 40. The first and third cores of the first fiber section being separately alignable with the second core of the second fiber section to provide different dispersion compensation. 39. The optical system according to claim 38, wherein 41. The first and third cores of the first fiber section are separately alignable with the fourth core of the second fiber section to provide more for dispersion compensation. 41. The optical system of claim 40, wherein the selection is provided. 42. The optical system of claim 34, wherein an index mark is provided on the fiber section to assist in representing a desired angle between the fiber sections at the optical interface. 43. The optical system of claim 34, wherein a splicer designed for a polarization maintaining fiber is provided at the optical interface. 44. The optical system of claim 34, wherein said first and second fiber sections are substantially individual sections cut from the same fiber. 45. A multi-mode fiber having a fundamental mode path having a different dispersion value and a higher-order mode path, and the fiber for shifting an optical signal back and forth between the fundamental and the higher-order mode paths. A plurality of mode couplers along an optical axis; an optical system component aligned with the optical axis of the fiber for further carrying an optical signal; and one of the mode paths at an interface with the system component. A dispersion-compensating fiber optical system, comprising: one of the mode couplers arranged to direct an optical signal. 46. The system of claim 45, wherein the fundamental mode path has a positive variance and the higher order mode path has a negative variance. 47. The system of claim 46, wherein the negative variance has a higher magnitude than the positive variance. 48. The mode coupler of claim 47, wherein the mode coupler shifts the optical signal between the fundamental mode path and the higher order mode path at unequal intervals between the mode paths. System. 49. The system of claim 45, wherein said one mode coupler directs said optical signal on said fundamental mode path prior to said optical system component. 50. The optical system component of claim 45, wherein the optical system component is an optical amplifier and the one mode coupler directs the optical signal to the higher order mode path following the optical amplifier. system. 51. The system of claim 45, wherein the mode coupler includes an index perturbation along the optical axis to transfer the optical signal during the mode path. 52. An optical fiber having a plurality of continuous optical paths having different dispersion characteristics for carrying an optical signal, a first of said optical paths exhibiting a positive dispersion at a central wavelength of said optical signal, A second portion of the optical path exhibiting a negative dispersion at the center wavelength, and a length-weighted average dispersion near zero at the center wavelength of the optical signal. A dispersion compensating fiber optical system, comprising: a coupling mechanism for shifting an optical signal. 53. The first and second optical paths extend parallel to each other, and the coupling mechanism shifts the optical signal between parallel portions of the first and second optical paths. Clause 52. The system of clause 52. 54. The first and second optical paths include first and second optical paths surrounded by a cladding.
54. The system of claim 53, wherein the system is formed by: 55. The optical fiber according to claim 55, wherein the one optical fiber has a central axis, the first core is aligned with the central axis, and the second core is offset from the central axis. The system of claim 54, wherein 56. The optical fiber according to claim 54, wherein the one optical fiber has a central axis, and both the first and second cores are arranged offset from the central axis.
The described system. 57. The multi-mode core according to claim 54, wherein the first core is a multi-mode core having a first mode exhibiting positive dispersion and a second mode exhibiting negative dispersion. System. 58. The system of claim 54, wherein the coupling mechanism includes a plurality of couplers that actively shift the optical signal back and forth between the first and second cores. 59. The first and second cores exhibit different sign dispersions of different magnitudes at a center wavelength of the optical signal, and the couplers are arranged at unequal intervals. 59. The system of claim 58, wherein the system is arranged to shift the signal between. 60. The coupling mechanism is formed to ultimately position the first and second cores sufficiently close to each other to support transfer of the optical signal between the cores. 55. The system of claim 54, wherein: 61. The system of claim 60, wherein the first and second cores exhibit substantially uniform magnitudes of opposite sign dispersion at a center wavelength of the optical signal. 62. The first and second optical paths extend concentrically with each other, and the coupling mechanism shifts the optical signal between concentric portions of the first and second optical paths. 53. The system of claim 52, wherein the system comprises: 63. The one fiber is a multi-mode fiber, wherein the first and second optical paths are a fundamental mode path and a higher-order mode path having different dispersion values. 63. The system of claim 62. 64. The system of claim 63, wherein the coupling mechanism includes a plurality of mode couplers that actively shift the optical signal back and forth between the fundamental and higher order mode paths. 65. The system of claim 63, wherein the first and second cores exhibit substantially equal magnitude dispersion of the sign at the center wavelength of the optical signal. 66. The system of claim 52, wherein the coupling mechanism is formed by a perturbation in the optical path. 67. The system according to claim 66, wherein said coupling mechanism includes a plurality of optical gratings. 68. The system of claim 52, including third and fourth optical paths having different dispersion characteristics for carrying the optical signal. 69. A coupler that shifts the optical signal between the first and second optical paths at a first dispersion cycle, and a coupler that shifts the optical signal between the third and fourth optical paths at a second dispersion cycle. 69. The system of claim 68, comprising: a coupler for shifting the optical signal at. 70. The system of claim 52, wherein the first optical path exhibits a positive dispersion gradient and the second optical path exhibits a negative dispersion gradient. 71. The system of claim 52, wherein the first and second optical paths exhibit a dispersion gradient near zero. 72. A method of manufacturing a multi-core fiber for transmitting an optical signal with reduced chromatic dispersion, comprising: arranging at least two glass core canes having different refractive index profiles from each other; Enclosing the glass core cane; fusing the enclosing cladding material to the core cane to form a glass preform; Drawing a multi-core fiber from the reform. 73. The method of claim 72, wherein at the center wavelength of the optical signal, a first of the cores exhibits a positive dispersion and a second of the cores exhibits a negative dispersion. 74. The method of claim 72, wherein said surrounding step further comprises providing said cladding material as an optical soot. 75. The method of claim 72, wherein the aligning step includes aligning the two core canes on a glass rod having substantially the same reference index of refraction as the surrounding cladding material. 76. The method according to claim 75, wherein said glass rod supports said two core canes. 77. The method of claim 72, further comprising the step of securing the two core canes together prior to surrounding them with the glass clad material. 78. The method of claim 72, further comprising forming a bond between the two cores. 79. The method of claim 72, wherein a mark is provided on the fiber to provide a reference angle point. 80. A method for compensating for chromatic dispersion induced in an optical signal propagating along an optical fiber, the method comprising: aligning two parallel optical paths having dispersion characteristics of opposite signs at a center wavelength of the signal. Sending the signal along the disposed fiber; coupling the signal back and forth between successive portions of the two parallel paths of the fiber such that the signal exhibits an average dispersion close to zero dispersion; A method comprising: 81. The method of claim 80, wherein the coupling step comprises shifting the signal between different cores of a multi-core fiber. 82. The method of claim 80, wherein said coupling comprises shifting said signal between different modes of a multimode fiber. 83. The method of claim 80, wherein said combining step comprises the step of actively shifting said signal between two parallel paths at unequal intervals between said optical paths. 84. The method according to claim 80, wherein said combining step comprises arranging parallel paths carrying said signals with equal duration.