KR20070023693A - Optical fiber - Google Patents

Abstract

The optical fiber 200 of the present invention includes an optical axis AA, a core 205 extending along the axis and made of a undoped first material having a first refractive index, and having the same width as the core. A surrounding cladding 210 is provided. The cladding is formed with a background matrix 220 made of the first material, formed in the background matrix, extending parallel to the optical fiber axis, arranged substantially around the core in a concentric ring, and having a lower than the first index of refraction. And a plurality of holes 215 filled with a second material having a lower second refractive index. The number of rings of holes is two or three, the average distance Λ of the holes is at least about 6 μm, and the ratio between the effective radius r _eff of the core and the average distance between the holes must be large 1 And the ratio between the average dimension d of the holes and the average distance Λ between the holes is at least 0.5.

Microstructured Optical Fiber, Optical Communication System, Optical Cable

Description

Microstructured Optical Fiber

FIELD OF THE INVENTION The present invention relates generally to the field of optical fibers, and more particularly to optical fibers of microstructure.

Fiber optics are mainly used in optical communication systems.

In many applications, optical transmission loss is an important aspect. For example, fiber loss is a major constraint on the structure and associated costs of long distance optical transmission systems.

Silica-based transmission optical fibers typically exhibit a transmission loss of about 0.2 dB / km in a broad spectral window of tens of nanometers centered at about 1550 nm. Due to the fiber loss (and also the material inherent nonlinearity), the maximum fiber span length is generally at most 100 km, and the power loss at this length can be 20 dB or more.

Splitting the resulting long distance link into shorter intermediate spans contributes to the overall system complexity and cost. Thus, even a slight reduction in fiber loss can result in a large economic benefit overall. Applications with shorter links requiring single spans can also benefit, because the requirements for the optical components at the end of the link can be mitigated due to an increase in the power budget.

Currently, silica based transmission optical fibers generally include germanium doping in the optical fiber core to achieve a certain refractive index difference between the core and the cladding.

Some studies have shown that in low loss silica-based fiber optics, the loss is due to four major contributions: Rayleigh scattering due to density variations in the core material, Rayleigh scattering due to variations in dopant concentrations, and waveguide defects. Due to loss, and infrared absorption loss. These conclusions are, for example, S. Rayleigh scattering in silica glass with heat treatment, S. Sakaguchi et al., Journal of Non-Crystalline Solids 220, 1997, pp. 178-186; K. K. Tsujikawa et al., Rayleigh Scattering Reduction Method for Silica-Based Optical Fiber, Journal of Lightwave Technology 18, pp. 1528-1532; egg. Reported in R. Le Parc et al., "Thermal annealing and density fluctuations in silica glass', Journal of Non-Crystalline Solids 293-295, 2001, pp. 295-366; and US Patent No. 6,404,965. The first and third contributions to fiber loss can be optimized through proper profile design and optimization of the fiber take-out process, in particular the preform take-out rate and temperature, draw pull and annealing blast furnace temperature. Can be lowered and thus the first and third contributions to the recited optical fiber loss can be reduced.

However, Rayleigh scattering losses due to concentration variations in the dopant cannot be prevented by any of these optimization techniques. In particular, when the appropriate amount of GeO ₂ is used in the optical fiber to provide the necessary up-doping to the optical fiber core, the " Optical properties of GeO ₂ glass and optical fibers ", Applied Optics of Sakaguchi et al. , Vol. 36, 1997, p. As reported in 6809-6814, the presence of a dopant generally results in a loss of at least 0.02 dB / km.

Pure silica core (ie undoped core) optical fibers may be characterized by lower losses than optical fibers, possibly with germanium doped cores. In pure silica core optical fibers, cladding doped with a lower refractive index than the core must be provided to trap light in the core. Since only a small portion of the transmitted light passes to the cladding, pure silica core optical fibers can possibly maintain pure silica low loss.

A known approach to making pure silica cores is to dope silica cladding with fluorine to achieve a suitable refractive index profile. However, Applicants have observed that defects in this type of optical fiber are more expensive to manufacture because the overall cladding is doped with flow, thus the amount of material to be doped is greater than in the core.

A third solution to prevent either up-doping of the fiber core or low-doping of the cladding is to provide a microstructured fiber, ie holes filled with air (usually materials with a lower refractive index than silica). Provided by an optical fiber having a cladding of microstructures running along an optical fiber parallel to the axis. A brief description of these optical fibers, also known as "Photonic Crytal Fibers (PCFs)" or "Photonic BandGap fibers (PBG)" or "holey fibers", is available. A. Bjarklev et al., 'Photonic crystal fibers-The state-of-the-art', 28th European Conference on Optical Communication ECOC 02, Copenhagen, Denmark, 2002.

Microstructured optical fibers can be manufactured in several different ways. One method known as “stack-and-draw” is to stack silica capillary tubes in a tight spatial arrangement in a hollow glass cylinder, join the tubes together and then finalize them by conventional fiber optic drawing methods. And withdrawing the produced base material.

Although the microstructured cladding may be combined with an undoped core, microstructured optical fibers generally have a core made of a low loss pure (ie, undoped) material, such as pure silica, and of the microstructured cladding The provision ensures light guiding in the core. In practice, in general, the pattern of air filled holes effectively reduces the refractive index. Wherever possible, the losses in these optical fibers are instead optimized for all of the drawout conditions discussed above without any penalty for the appropriate amount of chemical dopant used in conventional optical fibers, ie pure silica core optical fibers, including low doped cladding. You can benefit from

However, the proposed microstructured optical fiber is still affected by the loss problem.

L. In L. Farr et al. 'Low loss photonic fibers', 28th European Conference on Optical Communication ECOC 02, Copenhagen, Denmark, 2002, the outer diameter is 170 μm as shown in FIGS. 1A and 1B. A microstructure with a pure undoped silica core having at least four shells or six cells of holes with a pitch Λ of 4.2 μm between holes (hole spacing) and an air hole diameter of 1.85 μm A loss of 0.58 dB / km at 1550 nm was reported for the fiber. This loss value is much greater than that obtained with conventional (ie non-microstructured) optical fibers. Excessive losses were due to absorption due to significant increases in hydroxyl groups, metal impurities, and Rayleigh scattering.

K. K. Tajma et al., 'Low-loss photonic crystal fibers' OFC 2002, ThS3, p. 523-524 features two types of PCF, six cores on one ring around the core to shift the core and cladding structure (GeO ₂ -doped silica core and pure silica cladding) and zero dispersion wavelengths. The first type having is compared with the second type made of pure silica glass and having six holes in the cladding.

All of Tasma et al. Have a loss problem. GeO ₂ -doped silica cores exhibited the Rayleigh scattering problem described above, while optical fibers with many holes have large Rayleigh scattering due to silica-air hole defects (such as the roughness of the hole structure) and OH contamination. In addition to contamination and surface scattering problems that are fatal to fiber loss, optical fibers with a large number of holes are also more difficult to manufacture due to the complexity of making holes, and possibly due to the large voids in the structure during and during the manufacturing process. All suffer mechanical strength problems. In particular, the final shape of the hole, the strength and the splicing properties of the optical fiber resulting from the fiber base material withdrawal are all adversely associated with the total number.

Microstructured optical fiber with small number of holes. D. Asnaghi et al., "Fabrication of a large-effective-area microstructured plastic optical fiber: design and transmission tests", ECOC 2003. The optical fiber has two cells of holes made of plastic and distributed over a hexagonal pattern without an inner cell of a central hole and holes. The large effective core diameter r _eff is obtained with a relatively small hole-to-hole distance Λ. In particular, r _eff = 1.44 Λ. The applicant has observed that the optical fiber described by Asunagi et al. Is a multimode optical fiber which is unsuitable for long distance optical communication.

Another study of microstructured fiber has proved to be disadvantageous in terms of transmission loss by reducing the number of holes.

For example, T.P. T.P. White et al., 'Confinement losses in microstructured optical fibers', Optics Letters, vol 26, 2001, p. 1660-1662, and d. D. Ferrarini et al., 'Leakage Losses in Photonic crystal Fibers', F15, OFC 2003, vol 2. p.699-700 shows radiation losses when the number of rings with holes in the cladding is small. losses are reported to be very large because the transmitted radiation leaks through the thin microstructured cladding. Both of these papers show that in terms of attenuation, fiber performance gradually improves from one ring of holes to eight rings. In addition, in these papers, Λ / λ (where Λ is the hole spacing and λ is the wavelength of the traveling light), d / Λ (where d is the hole diameter) and cladding leakage as a function of the number of rings N of the holes in the cladding The dependence of fiber loss is discussed.

Applicant hereby referred to T.P. The quantitative results reported in the paper by White et al. Have the opinion that the electromagnetic field, which is a vector quantity, is treated as a scalar quantity, which is particularly inaccurate. This simplification according to the applicant is limited in accuracy when large refractive index steps are in the structure being analyzed, such as refractive index steps between the air in the hole and the background matrix at the hole surface. Moreover, counterfeit absorption in the background matrix is introduced in the prior art reference under consideration, which may limit the application of the method if very low losses are calculated, such as the losses required for the transmission optical fiber.

According to the Applicant, the vector solution of the more accurate and complete Maxwell's equations can be solved without a defect in fake background absorption. Calculated in the paper cited above by Fellarini et al.

More complex microstructured optical fibers are also described in the art. In US Pat. No. 5,802,236, a microstructured optical fiber is described as having an inner cladding region having a first cladding form essentially arranged in a hexagonal shape, surrounded by an outer cladding region having at least four layers of a second cladding form. It is. Two embodiments of the optical fiber are discussed in the patent: In the first example, seven layers of features (cladding voids) have triangular units with a feature array pitch of 2 μm. Arranged around the central defect with the cell; In a second example, the core is a silica rod surrounded by an inner layer of six features, surrounded by four or more layers of smaller features having a feature pitch of 0.925 μm. Applicant has observed that the large number of features included in the optical fiber described in this patent cause the aforementioned loss problem.

International patent application WO 02/39159 describes a microstructured optical fiber with a specially designed cladding that provides single-mode waveguidance and low sensitivity to bending losses. The optical fiber has an inner cladding and an outer cladding, each having elongated features. The inner cladding feature normalizes the dimension in the range of 0.35 to 0.50 and the outer cladding feature normalizes the dimension in the range of 0.5 to 0.9, and the normalization factor is a general feature spacing. The optical fiber is characterized by feature spacing of the inner cladding greater than 2.0 microns.

The Applicant has found that the optical fibers described in WO 02/39159 require different d / Λ for the inner and outer cladding holes, and the same reference is given before holding for these optical fibers.

Therefore, in view of the state of the art summarized above, it may be desirable to provide a microstructured optical fiber, particularly a single mode microstructured optical fiber with very low transmission loss, suitable for long distance transmission systems. According to the applicant, very low losses can only be achieved if all possible attenuation contributions are taken into account in the fiber design, including losses due to contamination and defects in the holes and losses due to leakage through the cladding.

It may also be necessary to provide such an optical fiber that can be easily manufactured and exhibits reasonable manufacturing toughness and that ensures the optical performance regardless of the small geometrical defects that always occur in manufacturing, especially when it comes to the dimensions and regularity of the microstructures.

Applicant radiates through Rayleigh scattering and cladding (due to water absorption and surface defects) by appropriately selecting the geometry of the silica microstructured optical fiber, in particular the number of hole rings, the dimensions of the holes and cores and the spacing between the holes. It has been found that an optimized balance between leaks results in microstructured optical fibers that are particularly suitable for long distance communication with reduced signal loss and easy to manufacture with large mechanical strength. The proposed fiber is advantageously monomode over a wide wavelength band.

The Applicant in particular has said longitudinal requirements having longitudinal holes forming two or three rings around the core and made of a single undoped (and thus low loss) material matrix, preferably pure silica. It was found to be implemented by microstructured optical fibers. Here, the average distance Λ between holes is at least 6 μm, the ratio between the effective radius of the core and the average distance between holes is almost 1, and the ratio of the diameter d of the holes to the average distance between holes is at least about 0.5. to be.

In the optical fiber thus designed, in particular, due to the relatively large average distance between the holes and the relatively large dimension of the hole with respect to the average distance between the holes, radiation leakage through the cladding can be kept small. Moreover, due to the relatively small number of holes, surface defects Due to Rayleigh scattering and attenuation problems due to absorption of impurities (including water), optical fibers are easier to manufacture and have greater mechanical strength. Applicants have also verified that contributing to reducing Rayleigh scattering and impurity absorption problems by increasing the dimensions of the hole relative to the distance between the holes. Other contributions to Rayleigh scattering, as induced by the dopant concentration variations in core doped optical fibers, are prevented due to the provision of pure silica cores. Moreover, the optical fiber is single mode due to the relatively small dimensions of the core relative to the average distance between holes.

Thus, according to a first aspect of the present invention, the present invention has a central axis, extends along the axis, and is made of an undoped first material having a first index of refraction, and having the same width as the core. A cladding surrounding the cladding, the cladding having a background matrix made of the first material and a plurality of holes formed in the background matrix, the holes extending parallel to the optical fiber axis and substantially extending into a concentric ring. Filled with a second material arranged around the core and having a second index of refraction lower than the first index of refraction, the number of rings of holes being two or three, with an average distance of at least about 6 μm The ratio between the effective radius of the core and the average distance between the holes is at most 1, and the ratio between the average dimension of the holes and the average distance between the holes is Even it shows an optical fiber of 0.5, microstructure.

Preferably, the holes are arranged in a regular pattern. In particular, the holes are preferably arranged in a triangular pattern. When the number of rings of holes is three, the plurality of holes preferably includes at most 36 holes. If the number of rings of holes is two, the plurality of holes preferably includes at most eighteen holes.

The first material is preferably silica. The second material is preferably air.

In a preferred embodiment of the invention, the optical fiber is a single mode optical fiber in the wavelength range between at least 1400 nm and 1650 nm.

Preferably, the ratio between the effective radius of the core and the average distance between the holes is at most 0.8.

In a preferred embodiment of the invention, the holes may all have the same dimensions: the number of rings of the holes is preferably three.

Preferably, the average distance between the holes is at least about 9 μm.

The ratio between the average diameter of the holes and the average distance between the holes is at least about 0.55, more preferably at least about 0.6.

Another aspect of the invention includes an optical communication system comprising an optical fiber implemented according to any of the preceding claims.

These and other features and advantages of the present invention will be apparent from the following detailed description of some embodiments, which are provided by way of example only and not by reference to the accompanying drawings.

1 is a very schematic illustration of a long distance optical transmission system in which a low loss fiber is advantageously used in accordance with an embodiment of the present invention.

2 schematically illustrates a segment of a low loss optical transmission optical fiber of the long distance transmission system of FIG. 1 according to an embodiment of the present invention.

3 schematically illustrates a cross-sectional view of the low loss optical fiber of FIG. 2 in an enlarged size.

Fig. 4 shows a cell with holes in optical fiber cladding for a certain fixed ratio d / Λ (d: average diameter of cladding holes; Λ: hole pitch) and another Λ / λ ratio (λ: wavelength of light transmitted by the optical fiber). Or a diagram showing optical fiber attenuation (vertical coordinates dB / km) as a function of the number of rings N (horizontal coordinate).

FIG. 5 is a plot showing the attenuation (vertical coordinates, dB / km) of an optical fiber with two rings (N = 2) of holes in the cladding as a function of the d / Λ ratio for another Λ / λ ratio.

Fig. 6 shows the attenuation of the optical fiber as a function of Λ / λ ratio for a given fixed ratio d / Λ and another number of rings N (N = 2,3,4,5) of the holes in the cladding (vertical coordinates, dB / km). This is a chart.

FIG. 7 is a plot (vertical coordinates, dB / km) showing the ratio of the standard deviation to the mean of the probability distribution of fiber loss as a function of the d / Λ ratio to other values of the Λ / λ ratio.

FIG. 8 is a plot showing optical fiber attenuation due to macro bending loss (vertical coordinates, dB / km) per turn on a 6 cm diameter major axis.

In FIG. 1, a long distance optical transmission system 100 is schematically illustrated having a transmitting station (“TX STATION”) 105 and a receiving station (“RX STATION”) 110 located very far from each other. The distance may be thousands of kilometers (the transmitting and receiving stations 105,110 may be located on each side of the Atlantic for example).

The two countries 105 and 110 are optically coupled to each other by an optical fiber optical cable 115 composed of a plurality of cable spans 115a, 115b, ..., 115n, and each of the spans 115a, 115b, ..., 115n It has a maximum length that is substantially damaged by the overall damping introduced by the optical cables that make up the cable. For example, in a conventional up-doped core silica optical fiber, the maximum span length, characterized by a loss of 0.2 dB / km, is about 100 km (which corresponds to an overall attenuation of about 20 dB experienced by the optical signal traveling along the span).

Between the intermediate cable spans 115a, 115b,..., 115n, amplifiers (possibly both optical or mostly conventional electrical, if desired by the optoelectronic and electrooptical converters) and possibly the preceding cable A regeneration station ("REG") 120 is provided that includes additional elements for reproducing an optical signal that has been attenuated in propagation through the span.

With reference to FIG. 2, a segment of an optical fiber 200 according to an embodiment of the invention, for example used in system 100, is shown schematically: the same optical fiber 200 is shown in a cross-sectional view in enlarged size in FIG. 3. It is.

In particular, the optical fiber 200 is an optical fiber core 205 made of a pure material surrounded by a plurality of microstructures extending along the axis AA of the optical fiber 200 or simply a cladding 210 of microstructure having a hole 215. It is a microstructured optical fiber having a). The array of holes 215 is formed in a background matrix 220 formed in particular of the same pure and undoped material as the core 205, the holes having a refractive index lower than that of the core 205 (solid, liquid or Gas, possibly only air) material.

The optical fiber 200 is preferably made of silica glass that has a very low attenuation from nearly 800 nm to almost 1700 nm and allows transmission. A minimum loss of less than 0.2 dB / km at a wavelength of about 1550 nm can possibly be obtained. However, other low loss materials such as modified silica glass or other oxide or chalcogenide glass may also be considered.

The microstructured cladding 210 is surrounded by an outer cladding 225 of the same material as the background matrix 220 and the hole voids. The overall (including outer cladding 225) optical fiber diameter is denoted D.

Due to the provision of the holes 215 filled with a material having a lower refractive index than the optical fiber core 205, even if the optical fiber 200 is made of a single material, the effective refractive index of the cladding 210 suffered by light beam propagation in the optical fiber is reduced. It is lower than the index of refraction of the core 205 and thus the light beam is substantially confined within the core 205.

The holes 215 are preferably arranged along a regular pattern, such as a triangular pattern as illustratively shown in the figures: In particular, in the exemplary embodiment contemplated herein, the holes 215 are circular, They have substantially the same diameter d and are spaced apart from each other at substantially equidistant Λ (center to center spacing).

The core 205 is preferably made without one hole in the hole array, possibly more than one hole in the hole array, so that a "defect" of the microstructured grating is formed. However, the present invention is also applicable to optical fibers having cores 205 that are not formed by lattice "defects" such as solid core regions surrounded by concentric arrays of holes. Moving radially outward from the fiber axis A-A, successive concentric rings of the hole 215 are encountered. In the following description of the invention, the number of holes is denoted by N. According to the invention, the number of rings is two or more, preferably three. In the example shown, the number N of rings is three.

The holes 215 are preferably arranged along a triangular array, but it will be appreciated that other hole arrangements are possible, such as circular, hexagonal and square patterns.

Preferably, the holes are the same in diameter, but the holes in the other rings are found to have hole arrangements with different diameters. In addition, the hole may have a different shape from the circular shape.

The effective diameter of the core 205 denoted by d _eff corresponds to the diameter of the circle inscribed in the inner ring of the hole. When the core is formed without a central hole, it is shown that d _eff = 2Λ-2d / 2 = 2Λ-d. Thus, the effective radius of the core 205, denoted r _eff (r _eff = d _eff / 2), is Λ-d / 2.

The optical fiber of the present invention has an effective radius r _{eff such} that r _eff / Λ is about 1, preferably about 0.8 or less. Applicant has verified that this condition, in addition to the conditions for the ratio d / Λ ratio introduced below, is applied to the optical fiber and operates as a single mode optical fiber in the wavelength range of about 1300 nm to about 1650 nm.

Microstructured optical fibers can be manufactured in many different ways.

Examples include, for example, US Pat. One of the most common techniques for producing microstructured optical fibers described in US Pat. No. 5,802,236 is the above-mentioned "stacking and drawing". According to this technique, a large number of solid silica rods and hollow silica rods (silica capillary tubes) are stacked in a tight array of spaces in a hollow glass cylinder, replicating the arrangement of holes obtained in the final optical fiber. Thereafter, the laminated silica rods are bonded together and the final optical fiber base material is transferred to the conventional drawing blast furnace and drawn out by the conventional base material drawing method.

Another known technique for producing microstructured optical fibers, for example described in EP 1 172 339 A1, uses the sol-gel method. Generally a cylindrical mold is provided with a number of elongated elements extending therethrough: a silica containing sol is then introduced into the mold and the sol is gelled or allowed. The resulting gel body is removed from the mold and the elongated elements are removed from the gel body (by mechanical extraction or possibly by chemical action or pyrolysis, depending on the nature of the elongated elements). The gel body is then dried and sintered. Finally, the microstructured optical fiber is obtained by drawing out the sintered body.

Microstructured optical fibers may also be formed using extrusion-based techniques of the type described, for example, in US Pat. No. 5,774,779.

It has been found that other techniques for making microstructured optical fibers are known in the art.

In order to be suitable for communication applications and particularly for use in long distance communication systems such as the system 100 of FIG. 1, the optical fiber 200 should have a very low loss to minimize attenuation of the light beam propagating through the optical fiber. . This, among other things, makes the fiber span longer, thus reducing the relay station.

It has been found that the advantage of an optical fiber according to the exemplary embodiment described herein is the provision of an optical core 205 of pure, undoped material (silica) through which light travels: as discussed at the beginning of this specification. Likewise, it is ensured that light loss due to Rayleigh scattering due to variations in the concentration of the dopant is prevented.

However, this is in itself insufficient to ensure that the overall fiber loss is low. Indeed, the cladding 210 of the fiber optic microstructure has a finite thickness and light can leak through it; This phenomenon increases the light attenuation. Without proper cladding, it has been pointed out that leakage through the cladding is a significant limiting factor in signal transmission unless very many holes are used.

As mentioned at the beginning of this specification, the reduction in fiber loss as the number of rings of holes increases, results in a ratio of hole pitch to advancing light beam wavelength (Λ / λ) and hole diameter to hole pitch (d / Λ). Along with the dependence of the loss on the value of the ratio, it is already reported in the literature. Thus, the conclusion drawn from these conventional references is that the number of hole rings must be increased to reduce light loss.

However, as discussed above, Applicants note that if the number of holes is increased to further reduce the loss, another problem is actually increased due to the greater absorption by hydroxyl groups and metal impurities, the roughness of the hole surface. It was observed that scattering, increased manufacturing complexity, and increased optical fiber weakness were encountered. In order to limit these problems, the hole number should be kept as low as possible.

The Applicant then needs to have a large number of rings (as opposed to the need to limit the last mentioned problem (which would require having a small number of rings, as stated above). It has been found that the selection should be made in consideration of all the need to limit light leakage across the cladding.

In the discussion that follows, a criterion is presented to find the minimum number of microstructures needed to ensure that the losses are below a certain level.

The above-mentioned paper 'Leakage Losses in Photonic crystal Fibers' using a silica refractive index close to 1.44 at 1550 nm and Ferrarini et al., FI5, OFC 2003, vol. By adopting an intrinsically accurate solution to the Maxwell's equation shown in 2, p. 699-700, leakage through finite thickness cladding under various conditions was calculated. The results for silica optical fibers are reported below (the results are weakly dependent on the refractive index of the optical fiber background matrix 220, but the results indicate that they remain at least qualitatively valid for matrix materials other than silica as well). Is).

The geometric distribution of the holes in the cladding of several rings of air filled holes 215 of diameter d is arranged along a triangular lattice of pitch Λ around the fiber core 205. The optical fiber core 205 is the central lattice "defect" of the hole array of the same material as the background matrix 220 (ie, pure undoped silica) (ie, the center region of the array characterized by the absence of one hole). It is assumed to be.

Referring to FIG. 3, an optical fiber having two rings (N = 3) and three rings (N = 3) of holes 215 is particularly arranged in a triangular pattern meaning 18 and 36 holes, respectively. Considered.

The total diameter D of the optical fiber was 125 μm, the grating pitch Λ was 10 μm, and it was considered that the ratio d / Λ = 0.5 (where d is the average hole diameter if the holes diameter or the holes were not all the same diameter; If these are not circular, the diameter d can be replaced by a parameter describing the average dimension of the hole, such as the effective diameter).

It has been observed that the triangular distribution of holes is characterized by uniformly spaced holes to take a better advantage. Moreover, for a given cladding thickness, the triangular distribution features the smallest number of holes compared to other periodic or aperiodic distributions of holes.

In a series of measurements, we compared the performance of an optical fiber with two or three rings of holes with a larger number of holes for different combinations of dimensions and distances of the holes.

As shown in the figure of FIG. 4, which plots the optical fiber attenuation at logarithmic scale in logarithmic dimensions and the number of rings N of holes 215 at cladding 210 in abscissa, of Λ / λ (and d / Λ of 0.5). For values equal to the ratio, it was found that the radiation leakage through the cladding decreased exponentially with the number of rings N of the holes.

This behavior can be quantitatively understood using a simple tunneling model of limited core radiation through the cladding barrier, and leakage through the barrier is exponentially reduced with its thickness. In the following, the " cladding thickness " L _clad is used to indicate the area occupied by the hole, ie the radial dimension of the cladding 210 of the microstructure.

It can therefore be assumed that by increasing the ring number N of the holes, radiation leakage through the cladding can be ignored.

However, the number of holes increases to N ² , which is a great drawback in terms of optical fiber manufacturing and transmission loss as described above.

Thus, we investigated the dependence of the radiation leakage through the cladding according to the other two parameters: the pitch Λ and the hold diameter d are again arranged in a triangular grating on the optical fiber.

The diagram of FIG. 5 shows logarithmic fiber attenuation (dB / km) in ordinate and d / Λ ratio values in abscissa for an optical fiber with two rings of holes. As shown in the diagram above, the radiation leakage through the cladding decreases as the value of the d / Λ ratio increases at the value of the fixed Λ / λ (λ is the radiation wavelength) ratio.

This behavior can be quantitatively understood as the effective refractive index of the cladding decreases as the value of the d / Λ ratio provided to increase the barrier to light escape increases (air prevails relative to the background matrix).

6 shows logarithmic fiber attenuation (dB / km) in ordinate and Λ / λ ratio values in abscissa for two, three, four and five rings of holes. It is shown. Radiation leakage through the cladding decreases with increasing value of the Λ / λ ratio at a fixed d / Λ ratio. This behavior decreases as the height of the barrier to light escape increases with Λ / λ, but the effective refractive index of the cladding is closer to the refractive index of the background material and the refractive index of the limited mode, so that the increase in the thickness of the cladding layer becomes more pronounced and the The result is less tunneling effect of light and less radiation leakage.

Leak values (dB / km) limited to N = 2 and N = 3 in graphical form in FIGS. 5 and 6 are summarized in Table 1 below.

N = 2 d / Λ Λ / λ 0.2 0.3 0.4 0.5 0.6 0.7 30 6.11E + 03 7.28E + 02 4.69E + 01 1.22E + 00 7.57E-03 1.10E-05 15 2.82E + 04 3.50E + 03 2.38E + 02 6.77E + 00 4.70E-02 6.65E-05 12 4.74E + 04 5.99E + 03 4.19E + 02 1.24E + 01 9.26E-02 1.41E-04 6 2.73E + 05 3.78E + 04 3.01E + 03 1.09E + 02 1.15E + 00 1.87E-03 3 2.37E + 06 3.86E + 05 3.87E + 04 2.05E + 03 4.10E + 01 1.77E-01 1.5 3.20E + 07 9.15E + 06 1.63E + 06 1.75E + 05 1.07E + 04 3.22E + 02 1.2 7.17E + 07 2.70E + 07 6.82E + 06 1.08E + 06 1.08E + 05 6.71E + 03 One 1.33E + 08 6.14E + 07 2.12E + 07 4.89E + 06 7.55E + 05 8.32E + 04

N = 3 d / Λ Λ / λ 0.2 0.3 0.4 0.5 0.6 0.7 30 3.14E + 02 5.89E + 00 6.57E-02 2.69E-04 1.20E-07 1.64E-12 12 1.49E + 03 3.00E + 01 3.84E-01 1.78E-03 1.18E-06 3.30E-11 6 9.51E + 03 1.96E + 02 2.88E + 00 1.71E-02 1.88E-05 1.28E-09 3 3.85E + 05 6.72E + 03 1.25E + 02 1.23E + 00 3.44E-03 1.20E-06

Table 1: Cladding leakage as a function of Λ / λ and d / Λ for a microstructured fiber with two rings of holes (top) and three rings of holes (bottom)

Applicant has identified appropriate parameters to describe the behavior of the electromagnetic field only in the cladding of the microstructure. This parameter is the "in-plane wavelength" λ _p of light in the cladding.

For example A. In the earlier literature on microstructured fiber, a paper already cited by Bzarkrev et al, 'Photonic cyrstal fibers-The state-of-the-art', 28th European Conference on Optical Communication ECOC 02, Copenhagen, Denmark, 2002. Given the effective cladding refractive index n _eff (calculated as the largest refractive index of the corresponding photonic crystal) and the mode refractive index n _{m (} calculated from the solution of the propagation equation for the structure under consideration), as already demonstrated, the cladding Where the "in-plane wavelenght" λ _{p of light} is as follows:

Where λ is the wavelength of light traveling through the optical fiber. The 2π factor accounts for the fact that the electromagnetic field has a radial decay behavior in the cladding; In practice, the wavelength λ _p is easily read out as the logarithmic coordinate of the electromagnetic field strength along the radius of twice the inverse of the slope in the cladding region.

As discussed in more detail below, in order to keep the radiation leakage through the cladding low despite a relatively small number of holes, the average distance between the innermost and outermost holes is the optical wavelength of the plane perpendicular to the fiber axis. It should be large enough for.

Preferably, the average distance between the holes should not be too large for the in-plane wavelengths; In particular, Λ is preferably less than 6λ _p . This allows the radiation leakage through the cladding to remain finite and to have the optical fiber in control against geometrical inaccuracies such as uneven hole distance and diameter.

In addition, the in-plane wavelength should preferably not be too large for the light wavelength over which the signal is transmitted: preferably λ _p is less than 3λ. This results in an optical fiber with a small macrobanding loss.

By carefully selecting the distribution and size of the holes, it is possible to obtain an optical fiber with other advantageous optical properties such as cutoff wavelength and proper group velocity dispersion.

For optical fibers made of silica and intended for use in long distance optical communication systems, it may be desirable to achieve radiation leakage of less than 0.01 dB / km through cladding. Extensive numerical simulations and interpretations using experimental tunneling models have shown that this requirement is achieved for the thickness L _clad of the microstructured cladding 210 at least 10 times larger than the in-plane wavelength of light λ _p .

Considering a triangular hole grating, the cladding thickness L _clad is related to both the grating pitch Λ and the number of rings N of holes, and basically due to the effective core dimension, multiplying by about 0.1Λ, apart from the small decrease of the first ring. Is given by NΛ.

For other geometric distributions of the holes 215, when the hole distribution is sufficiently uniform and the cladding is circular, the cladding thickness L _clad is the average mutual distance Λ _ave of the microstructure 215 by the following equation, microstructure (hole) Can be related to the number N _m of 215 and the average distance L _avg of the microstructure from the fiber axis AA:

이고, d_ij는 i번째 및 j번째 마이크로구조(215)의 중심 간의 거리이며, 첨자 i 및 j는 전 마이크로구조에 대해 이어지고,

이고, r_i는 광섬유 축과 i번째 마이크로구조의 중심 사이의 거리이다.here,

D _ij is the distance between the centers of the i-th and j-th microstructures 215, the subscripts i and j continue for the entire microstructure,

And r _i is the distance between the optical fiber axis and the center of the i-th microstructure.

For a triangular lattice, to provide a L _clad = 2.64Λ for the L _clad = 1.82Λ for the expression of two rings (N = 2) shown above, and the three rings (N = 3), this simple measure for NΛ Similar to: The mismatch arises due to the use of the same circular cladding to define the effective cladding, while for triangular lattice this form is circular.

In Table 2 below, for the triangular lattice cladding under consideration, the calculated values of λ _p / λ and Λ / λ _p for other values of the calculated Λ / λ and d / Λ ratios are shown.

λ _p / λ d / Λ Λ / λ 0.2 0.3 0.4 0.5 0.6 0.7 30 21.32 14.17 10.27 7.69 5.68 4.12 12 9.18 5.99 4.31 3.21 2.38 1.73 6 5.20 3.25 2.31 1.71 1.27 0.93 3 3.71 1.94 1.32 0.97 0.72 0.53 1.5 1.76 0.96 0.65 0.48 0.36 One 1.14 0.66 0.46 0.34

Λ / λ _p d / Λ Λ / λ 0.2 0.3 0.4 0.5 0.6 0.7 30 1.41 2.12 2.92 3.90 5.28 7.28 12 1.31 2.00 2.78 3.74 5.05 6.29 6 1.15 1.85 2.60 3.52 4.74 6.45 3 0.81 1.54 2.27 3.10 4.17 5.61 1.5 0.85 1.56 2.29 3.13 4.14 One 0.88 1.52 2.18 2.90

Table 2: λ _p / λ (top) and Λ / λ _p (bottom) calculated for different Λ / λ and d / Λ

In conjunction with the results in Table 2, the negligible leakage is two rings with holes for larger values of the Λ / λ and d / Λ ratios (N = 2) and the smaller of the Λ / λ and d / Λ ratios. It was found that this can be achieved using three rings of holes (N = 3) for the values.

In particular, for communications applications in the wavelength range from about 1400 nm to about 1650 nm, the requirements are that the pitch Λ is greater than 6 μm, preferably greater than 9 μm, and the ratio of hole diameter to pitch d / Λ Greater than 0.5, preferably greater than 0.6.

Optical fibers characterized by thick cladding (i.e. cladding with a relatively large value of L _clad ) through larger values of the Λ / λ ratio (ie, larger hole pitch values relative to the wavelength of the light) have an effective refractive index difference Δn (light It has been found that the difference in refractive index between the core 205 and the microstructured cladding 210 is relatively small, and the mode field diameter may consequently have a large problem. These conditions represent possible problems for macro bending and micro bending loss. These losses also depend on other design, manufacturing and application factors such as coating softness, cable type, and splice joints, respectively. However, a test for tightly wound bobbins is a standard that features macro bending: for example, a critical test is to consider an optical fiber wound on a bobbin of 12 cm diameter: the optical fiber has a dB / m loss induced by bending. If it does not exceed the ratio of, it is considered robust for communication applications.

Applicants have also theoretically investigated the effect of the d / Λ ratio on the light loss generated at the hole surface. These light losses include Rayleigh scattering losses due to surface roughness and light absorption induced by exogenous impurities on the hole surface. All of the light losses are substantially proportional to the field strength at the hole surface. The Applicant has found that the following dimensionless parameter R can be characterized by these loss effects at wavelength λ:

은 광섬유 축에 수직한 면에 있는 홀들을 한정하는 원주를 따르는 좌표이며,

는 광섬유 축에 수직한 면의 좌표이고,

는 전기장이다. 따라서, 선적분이 광섬유 축에 수직한 면에 있는 홀 경계 위로 행해지고, 면적분은 전체면에 걸쳐 행해진다. 면적분은 기본적으로 광섬유에 의해 전송되는 전력속(power flux)에 비례하며 적절한 정규화를 제공한다.Where θ is between 0 and 2π,

Is the coordinate along the circumference that defines the holes in the plane perpendicular to the fiber axis,

Is the coordinate of the plane perpendicular to the fiber axis,

Is the electric field. Thus, the shipment is done over the hole boundary on the plane perpendicular to the optical fiber axis, and the area distribution is over the entire surface. The area fraction is basically proportional to the power flux transmitted by the fiber and provides proper normalization.

The factor R allows for a useful comparison for optical fibers characterized by different diameters to pitch ratios produced at the same process conditions. In practice, the conditions considered at the hole surface (ie the presence and roughness of the exogenous impurities) are basically independent of the hole diameter d, while the diameter d is micrometers, while the structural roughness is sub-nanometer and The diffusion of incoming metal impurities is sub-micrometer.

It is known to those skilled in the art that the transmission of the electric field into the hole (and thus the electric field strength to the surface) is greatly suppressed due to the large difference between the air (or vacuum) refractive index and the silica glass refractive index. Applicants have found that the larger the hole for a given wavelength, the higher this suppression. Furthermore, the Applicant has found that by increasing the hole dimension, smaller numbers of holes are not only required for good field confinement, but only the holes closest to the core of the optical fiber make a significant contribution to the factor R. Alternatively, when the holes are small, the suppression of electric field transmission into the holes is lowered and a large number of holes are required to be used due to the reduced field confinement in the core. Thus, a large number of holes contribute to the factor R.

Using the above formula, we compared the behavior of microstructured optical fibers according to the present invention and microstructured optical fibers with much lower diameters for pitch ratios. In particular, the Applicant considered a first optical fiber with d / Λ = 0.55 and Λ = 12 μm and a second optical fiber with d / Λ = 0.2 and Λ = 7.4 μm, the two optical fibers being substantially the same mode of about 150 μm ² . It has an effective area. The calculated value was R = 1.7 × 10 ⁻³ at λ = 1.55 μm for the first optical fiber and R = 6.5 × 10 ⁻³ at λ = 1.55 μm for the second optical fiber. It can be seen that the R value representing the loss generated at the hole surface is almost four times higher for the second optical fiber than for the first optical fiber.

Applicant also found that D. D. Marcuse's 'Curvature loss formula for optical fibers' Journal of the Optical Society of America, vol. 66, pages 216-220, 1976, using a widely used formula for step index fiber, to form the same step refractive fiber. Microstructured optical fibers also described in the already cited papers by 'Photonic cyrstal fibers-The state-of-the-art', 28 ^th European Conference on Optical Communication ECOC 02, Copenhagen, Denmark, 2002. The macrobanding loss was investigated using the effective refractive index model. The results are shown in Table 3 below. This approach is described, for example, in Electronic Letters vol. 37, pages 287-289 (2001).

d / Λ Λ / λ 0.2 0.3 0.4 0.5 0.6 0.7 30 8.78E + 02 1.76E + 03 3.54E + 03 6.24E + 03 7.72E + 03 2.62E + 03 12 8.19E + 02 6.34E + 02 1.18E + 02 8.56E-01 -0 -0 6 1.40E + 02 2.48E-01 -0 -0 -0 -0 3 1.91E + 00 -0 -0 -0 -0 -0

Table 3: Macrobanding losses measured in dB / km for different Λ / λ and d / Λ for optical fibers wound around 12cm diameter bobbins

Since the exponent of the above-cited formula for stepped refractive fibers is directly proportional to λ _p / λ, this ratio is resistant to macrobanding losses, as in optical fibers with values of λ _p / λ ratios less than 3. It can be used to characterize an optical fiber.

In addition, problems have been investigated in the fabrication of microstructured optical fibers where the implemented geometry exhibits a certain deviation from the geometry of the designed target.

In particular, the Applicant used a Gaussian probability distribution, characterized by the standard deviation σ _T over Λ, to investigate the effect on hole position and diameter variation in the microstructured optical fiber around the exactly predicted mean value.

Disorders resulting from these deviations have always been found to increase cladding radiation leakage through cladding and eventually to distort the probability distribution of the leakage to large values using standard deviations larger than average for large abnormalities. These probability distributions are of no interest when trying to produce these fibers with larger sizes (with standard deviations greater than the mean).

In particular, it has been found that optical fibers with large values of Λ / λ and d / Λ ratios are more sensitive to this problem. As an example, the diagram of FIG. 7 shows in ordinate the ratio of the mean to standard deviation of the probability distribution of cladding leaks over other values of the Λ / λ ratio as a function of the d / Λ ratio value. In addition, the simple parameter λ _p can be used to characterize an optical fiber that is quite robust against manufacturing inaccuracies. In particular, the Applicant has found from the extensive numerical calculation that good production yield is ensured for relative σ _T of up to 6% when the average distance Λ _avg between holes is Λ _avg <λ _p .

In Table 2, the ratio Λ / λ _p for the triangular lattice cladding case is shown. It was pointed out that the above limitation on the average distance Λ _avg between holes could be relaxed for relative σ _T less than 6%.

Two preferred examples obtained by numerical simulations are shown here below.

Preferred Embodiment

As a first example, an application of the present invention to an optical fiber for ultra-low loss applications in which a predetermined cladding leakage is set below 0.01 dB / km (ie, more stringent conditions) is considered. This is generally the case for optical fibers for use in long distance optical communication systems such as the system of FIG.

Applicant contemplates an optical fiber with three rings of holes, in particular a 36-hole triangle pattern with Λ = 12 μm, d / Λ = 0.55 (ie d = 6.6 μm) and r _eff /Λ=0.725 dimensions Are arranged according to.

This fiber is basically single-mode because the higher-order mode suffers from very large cladding leakage and large bending losses of more than 600dB / km. The proper length of a few tens of meters of straight fiber acts substantially like a single-mode fiber because the power transmitted in higher modes can be effectively ignored after proceeding over this length. Thus, radiation leakage through the cladding is used for filtering higher order modes for convenience.

For single mode transmission at 1550 nm, we measured cladding leakage of less than 0.01 dB / km for the optical fiber, although dependent on manufacturing tolerances.

Filtering through cladding leaks cooperates with the useful filtering that occurs in cable fiber provided by the loss of higher-order mode when the fiber is cabled and wound over the standard spindle in the splice box. This implementation defines the cable cutoff wavelength λ _cc . Cable cutoff wavelength, because the higher order macrobanding (MB) loss over the basic bobbin with 0.6 cm diameter bobbin is always more than the loss for the basic mode over the transparent window front of the silica glass, as shown in the table of FIG. λ _cc is certainly beneath this transparent window, which is required for long distance transmission fibers. In particular, macrobanding losses (lower than 0.01 dB for 100 turns on an axis of 60 mm diameter) for the fundamental mode in the 1400 nm to 1625 nm band can be neglected, whereas for the higher order mode. The basic mode has a significant effective area of 150 μm ² at 1550 nm. This large effective area, combined with the smaller nonlinearity of pure undoped silica compared to the effective area of doped silica, also provides a significantly lower nonlinear solution for ultra long distance applications. Moreover, large group velocity dispersion of 26 ps / (nm km) at 1550 nm further helps to limit the nonlinear wavelength mixing effect.

The variance value can be tuned by slightly varying one or more geometric parameters, such as hole diameter d.

The optical fiber structure according to the present invention is particularly suitable for long distance communication applications, but may also provide several advantages for short distance communication applications such as in terrestrial systems. Fiber optics for these applications have a large effective area to have low splice losses that are delivered with lower attenuation overall for optical links, advantageously for terrestrial applications that feature one splice about every 2 km. .

Short-range systems may include uncompensated and non-amplified links that are lost from the metro-ring or portions thereof and progress up to 10 Gbit / s from a few kilometers to tens of kilometers. For the lowest system cost, transmission of a single wavelength around 1310 nm or at most several wavelengths is desirable for these distances.

If the loss of silica is about 0.3 dB / km at 1310 nm, a clad leak of 0.1 dB / km (and thus a magnitude larger loss than in the previous example) can be tolerated.

Compared to the loss of a standard single mode fiber of about 0.35 dB / km, this additional loss does not substantially increase the power budget and system cost over the considered length. Using Λ = 15 μm, two holes with holes and d / Λ = 0.6, the effective area of the guide mode is 200 μm ² or more, which considerably lowers the splicing loss and is 0.05 dB / km compared to the conventional standard mode fiber. Barely recovers larger losses. Group velocity dispersion is about 6 ps / (nm km) at 1310 nm. This prevents dispersion compensation over 50 km in length, regardless of the quality of the laser source used in the application, as in standard single-mode products. Higher-order modes propagate in the fiber, but the clad leakage is greater than 60 dB / km.

For the application under consideration, the fiber is effectively suitable for single mode and high bit rate transmission. The macrobanding loss for an axis of 60 mm diameter is 0.026 dB / km, which is below the IEC requirements.

Applicants have measured cladding leakage that is less than about 0.1 dB / km at 1310 nm for this optical fiber, depending on manufacturing tolerances.

While the present invention has been disclosed and described by some embodiments, it will be apparent to those skilled in the art that various modifications to the described embodiments and other embodiments of the invention are possible without departing from the scope of the invention as defined in the claims. It is obvious.

For example, if a microstructured optical fiber provided with air filled holes is a preferred embodiment of the present invention, but the above-described requirements for the geometry of the microstructured optical fiber are met, the microstructured optical fiber according to the present invention is also different. It may also be embodied as a type microstructure or as a solid rod disposed in the hole or a hole filled with a gaseous material other than liquid or air.

Included in the Detailed Description of the Invention.

Claims (14)

Optical fiber shaft (A-A), A core 205 extending along said axis and made of a undoped first material having a first index of refraction, It has the same width as the core and has a cladding 210 surrounding the core, The cladding comprises a background matrix 220 made of the first material, a first formed on the background matrix and extending parallel to the optical fiber axis and arranged around the core in a substantially concentric ring and lower than the first index of refraction. A plurality of holes 215 filled with a second material having a second refractive index, The number of rings of holes is two or three, The average distance Λ of the holes is at least 6 μm, The ratio between the effective radius r _eff of the core and the average distance between the holes is at most 1, And the ratio between the average dimension (d) of the holes and the average distance (Λ) between the holes is at least 0.5. The method of claim 1, And said holes are arranged in a regular pattern. The method of claim 2, The holes are microstructured optical fiber arranged in a triangular pattern. The method of claim 3, wherein Wherein the number of rings of holes is three and the plurality of holes comprises at most 36 holes. The method of claim 3, wherein Wherein the number of rings of holes is two and the plurality of holes comprises at most 18 holes. The method according to any one of claims 1 to 5, And the first material is silica. The method of claim 1, And the second material is air. The method of claim 1, Wherein said optical fiber is a single mode optical fiber in a wavelength range between at least 1400 nm and 1650 nm. The method of claim 1, And a ratio between the effective radius of the core (r _eff ) and the average distance between the holes at most 0.8 is a single-mode optical fiber of the microstructure. The method of claim 1, Wherein said holes are all single-mode optical fibers of the same dimension. The method of claim 1, And a single-mode optical fiber having a ring number of three holes. The method of claim 1, And wherein the average distance between the holes is a single mode optical fiber of at least 9 μm. The method of claim 1, And a single mode optical fiber having a ratio of at least 0.5 between the average dimension (d) of the holes and the average distance (Λ) between the holes. An optical communication system implemented according to any one of claims 1 to 13 comprising a microstructured optical fiber.

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