ES2357483T3 - OPTICAL TRANSMISSION LINK WITH AMPLIFIED FIBER RAMAN OF LOW PENDING. - Google Patents

Abstract

Fiber optic transmission link (10), comprising: a first optical fiber having a first effective area greater than 45 µm 2; and a second optical fiber (20) coupled after the first optical fiber, which has a second effective area smaller than the first optical fiber, an absolute value of dispersion between about 2 and 20 ps / nm / km at 1550 nm, a value absolute dispersion slope less than about 0.040 ps / nm 2 / km at 1550 nm, and an attenuation coefficient at 1550 nm less than about 0.24 dB / km at 1450 nm less than about 0.30 dB / km, characterized by the fact that the second optical fiber (20) comprises - a glass core that includes an inner core (22) that has a first difference in refractive index Δn1 between about 0.008 and 0.014 and a surrounding first layer (24) radially the inner core (22) along the length of the fiber and having a second difference in refractive index Δn2 of between about -0,0005 and -0.0030 and - a surrounding glass liner (29) the glass core and that has a difference in refractive index ΔnCL substantially equal to 0.

Description

Background of the invention

[0001] The present invention relates in general to a fiber optic link that allows the transmission of multiplexed optical signals by dense wavelength division over a broadband without significant degradation of non-linear effects, while also allows Raman amplification on the link. More particularly, the present invention relates to an optical fiber having a low effective area, with non-zero dispersion, and a low dispersion slope to perform Raman amplification after a high effective area fiber.

[0002] Optical transmission systems that will communicate multiplexed signals by wavelength division over longer distances (several thousand kilometers) without amplification or regeneration, or with limited amplification or regeneration as far as possible, are continually being sought, and will handle larger bandwidths. The current amplification band for optical systems is generally approximately 30 nm wide in the so-called C band of approximately 1550 nm, but developments are expanding this band to approximately 70 nm and include the so-called amplification band L of approximately 1580 nm.

[0003] Non-linear optical effects are known to degrade transmission quality along the standard transmission optical fiber in certain circumstances. These non-linear effects, which include four-wave mixing, auto-phase modulation and Raman and Brillouin dispersion, and cross-phase modulation, induce distortions in the transmitted signal in high-power systems, which degrades the quality of the transmission. In particular, non-linear effects can impair transmission quality using wavelength division multiplexing (WDM), which otherwise significantly increases the ability to transport the signal from the optical transmission fibers by increasing the number of transmission channels through which signals can be sent.

[0004] These non-linear effects, particularly the phenomenon of four-wave mixing (FWM), can be minimized or avoided using single-mode transmission fibers that have a large effective area and an absolute local dispersion value that is greater than zero. around operating wavelengths. In advanced WDM systems, such as dense wavelength division multiplexing (DWDM) (distance ≤ 0.8 nm) and hyper-dense wavelength division (HDWDM) multiplexing (distance ≤ 0.4 nm), where the transmission channels are closely packed together, the value of the local dispersion must have a minimum value to maintain the quality of the signals. On the other hand, if the fiber dispersion value becomes too large, the signals are distorted during transmission, unless dispersion correction devices are included in the transmission line. Thus, for an optical fiber to be effective in a WDM system, the fiber must have a minimum dispersion, but the value of the dispersion must also be less than a maximum value.

[0005] In general, the increase in the effective area and the maintenance of low losses and a good behavior of the macrocurvature / microcurvature loss causes an increase in the dispersion slope. As the bandwidth of WDM communications expands, however, a flat dispersion slope becomes increasingly important to avoid varying the dispersion between channels. In particular, the combination of a high dispersion slope, wide WDM band, and long distance offers large differences in cumulative dispersion of the lateral wavelengths of the WDM band. Even in the presence of exact dispersion compensation in a particular position of the WDM band (for example, the central channel), the channels separated from that particular position (for example, the side channels) will accumulate large amounts of chromatic dispersion. The interaction between the dispersion and the non-linear effects deteriorates these peripheral signals in such a way that prevents the receiver from recovering them, even in the presence of an optimal previous compensation channel by channel on the transmitter side and / or after compensation in the receiver side Consequently, a low dispersion slope across an operating bandwidth is important for the efficient transmission of the WDM channels in the C and L bands.

[0006] Fibers are known that have a low dispersion slope. For example, US Patent No. 5,553,185 to Antos et al. describes a non-zero dispersion fiber that is characterized by a series of central regions that have different profiles and widths of the refractive index. The shape of the refractive index profiles, in terms of the difference in the refractive index and the radius, of each region can be adjusted to have custom properties for a high-performance telecommunications system. In particular, one of the regions has a depressed refractive index difference. The dispersion slope of the described fiber is less than 0.05 ps / nm2 / km and the absolute value of the total dispersion is between 0.5 and 3.5 ps / nm / km in the preselected transmission range, which covers approximately 1,490-1,575 nm. In addition, the zero dispersion wavelength is approximately 1593 nm, which is outside the preselected transmission range.

[0007] Another fiber for a high performance communication system is described in Y. Akasaka et al., "Enlargement of Effective Core Area on Dispersion-Flattened Fiber and Its Low Non-Linearity", OFC'98 Technical Digest, pages 302 -304. This fiber is also characterized by a series of central regions that have differences in refractive indices and varying radii. One of the central regions also has a difference in the depressed refractive index. The described fiber has a smaller dispersion slope than the standard single mode fiber in the transmission window.

[0008] EP 0 877 496 refers to an optical transmission line suitable for long-distance transmission and large-capacity transmission of signal light. EP 0 877 496 describes an optical transmission line comprising a first optical fiber having a first effective area and a first dispersion slope at 1550 nm and a second optical fiber, optically connected to the first optical fiber, which has in 1550 nm a second effective area smaller than the first effective area and a second dispersion slope smaller than the first dispersion slope.

[0009] Patent 5 905 838 describes a silica-based fiber, called "MetroWaveFiber" (MWF), which allows the operation of a double window (1310 and 1550 nm) WDM in short and medium distance systems, for example, in metropolitan systems.

[0010] Patent 5 838 867 describes a dispersion compensation fiber for long distance and high bit rate optical communications systems. The dispersion compensation fiber has, in 1550 nm, a chromatic dispersion in the range of -40 to 0 ps / Km / nm, a dispersion slope in the range of -0.5 to -0.1 ps / Km / nm2, a transmission loss of not more than 0.5 dB / Km and a modal field diameter in the range of 4.5 to 6.6 µm. Patent 5 838 867 indicates that the signal light can be amplified by Raman effect on the dispersion compensation fiber.

[0011] Raman amplification distributed on the transmission line can be used, for example, to increase the transmission distance between amplifiers doped with erbium-, and / or reduce the loss of separation that would result from an increase in the number of channels WDM or a decrease in power and cost reduction of amplifiers doped with erbium. Stimulated Raman dispersion is a nonlinear process that can cause an optical fiber to amplify an optical signal. The Raman dispersion converts a small fraction of the incident power from a pumped optical beam to another optical beam at a frequency reduced by a certain amount by means of the vibrating modes of the mean. The usual configuration of the distributed Raman amplifier uses the transmission line as the medium where a pump, which typically goes against propagation, causes a stimulated Raman dispersion in the wavelength of the signal. Two “index equations” govern the dynamics of the signal (PS) and pump (PP) powers in the optical fiber in the case of a pump that is against propagation:

image 1

where αS and αP are the attenuation coefficients of the optical fiber in the signal and the wavelength of the pump, respectively, ωSy ω P are optical frequencies of the wavelength of the signal and the pump, respectively, aS and aP are the effective central areas of the wavelength of the optical fiber in the signal and the pump, respectively, and gR is the Raman gain coefficient of the fiber material of a given pair of optical frequencies. See Agrawal, "Nonlinear Fiber Optics", 2nd edition, chap. 8 (1995).

[0012] In general, distributed Raman amplification operates in an unsaturated regime, where the signal power levels are kept well below the pump power levels. Under this assumption, one can ignore the depletion of the bomb, that is, the first term on the right side (2). Thus, equations (1) and (2) can be easily solved to obtain the signal strength:

image 1

Where L is the length of the average fiber, PS (0) is the power of the input signal, PP (L) is the power of the input pump (opposite propagation) at the opposite end of the fiber and Leff is the Effective interaction length in the wavelength of the pump given by:

image 1

[0013] A Raman amplifier is characterized in terms of amplifier gain (linear) or ON / OFF ratio, which is defined by the following:

image 1

Typical PP (L) values vary from 100 mW to 1000 mW and more. Moderate pump power values are considered to be less than 500 mW.

[0014] From equation (5) it can be determined that an efficient fiber in terms of distributed Raman amplification has the following properties:

-small effective area (less than 45 µm2) in the pump wavelength (usually between about 1400 nm and 1510 nm approximately, preferably between 1440 nm and 1490 nm) or equivalently a small field mode diameter in length pump wave; Y

- Long effective length in the wavelength of the pump, which is equivalent, for a fixed length L of the fiber medium, to a low loss αP in the wavelength of the pump.

These observations also agree with the experimental results shown in the article by Fludger et al., "An analysis of the improvements in OSNR from distributed Raman amplifiers using modem transmission fibers", OFC2000, FF2-1, pages 100-102 (2000).

[0015] Next, we will refer to an example pump wavelength of approximately 1450 nm, but the invention contemplates the use of pump wavelengths of approximately 1,350 nm to 1510 nm. In general, a fiber that has a small effective area at 1450 nm also exhibits a small effective area in the conventional transmission of C and L bands. This requirement conflicts with the need for a fiber with a high effective area to avoid nonlinear effects in dense WDM communications.

[0016] In addition to the low effective area, it is well known that other important conditions must be taken into account to achieve acceptable Raman amplification. The most important are the double backscatter of the amplified signal and the simple backscatter of the spontaneous emission of the pump signal, as explained in Nissov et al. "Rayleigh crosstalk in long cascades of distributed unsaturared Raman amplifiers", Electronics Letters, vol. 35, No. 12, pages 997-998 (June 10, 1999); Wan et al. "Impact of Double Rayleigh Backscatter Noise on Digital and Analog Fiber Systems," Journal of Lightwave Tech., Vol. 14, no. 3, pages 288-297 (March 1996), and Hansen et al., "Rayleigh Scattering Limitations in Distributed Raman Pre-Amplifiers", IEEE Photonics Tech. Letters, vol. 10, no. 1, pages 159 to 161 (January 1998).

[0017] In general, when a system uses distributed Raman amplification, there is an optimum value of the power of the pump above which the performance is degraded by the damaging effects due to double and simple backscatter, as shown in Garrett et al., "Field demonstration of distributed Raman amplification with 3.8 dB Q-improvement for 5x120 km transmission", OFC2000, PD42-1 to PD42-3 (2000).

[0018] In conclusion, a fiber suitable for a system that has distributed Raman amplification is characterized by:

- Small effective area in the wavelength of the pump, to have a high efficiency of Raman gain;

-small loss in the wavelength of the pump, to increase the length of the interaction between the pump and the signal;

-small loss in the wavelength of the signal, according to Equation (3);

- Low backscatter in the signal wavelength, to reduce noise, and

-Simple mode performance on the wavelength of the pump when it is in cable form, to limit the power of the pump in the fiber core.

[0019] Fiber loss is the sum of different attenuation mechanisms (such as infrared absorption, OH absorption, UV absorption), among which they play an important role depending on the wavelength of the Rayleigh dispersion and other dispersion mechanisms. Backscatter is the combination of an elastic dispersion process along the fiber (typically, but not restrictively, Rayleigh backscatter) and the mechanism of recapture the light in the backward direction. In general, for common transmission fibers, a high loss implies a high backscatter, because the dominant term of the loss is the elastic scattering of light.

[0020] A few publications are directed to a combination of optical fibers in a transmission link that includes Raman amplification. WO99 / 57822, for example, describes a unitary managed dispersion optical fiber having a total dispersion that changes from positive to negative along the length of the fiber in a transmission wavelength range. The fiber may include distributed amplification by stimulated emission of a rare earth dopant, Raman effects, or both. An embodiment in WO99 / 57822 contemplates shorter sections of the unit fiber that have greater loss, especially when the shorter sections are codopated with alumina and have a smaller effective area for better superposition of the pump / erbium light. In this embodiment, the longer sections will be designed to have less loss and a greater effective area. In a preferred embodiment, sections that have negative dispersion also have a negative dispersion slope. Applicants have noted that the latter generally causes a high loss due to the high content of fluoride dopant.

[0021] WO99 / 66607 describes an apparatus and method for combining optical amplification and dispersion compensation in a Raman amplifier. A device called Raman dispersion management amplifier (DMRA) combines Raman amplification and has its length and dispersion selected to balance the dispersion of a link. It is used as a dispersion compensator that aims to reduce the total dispersion to zero, the DMRA has a very negative dispersion and a low effective area, which allows Raman amplification.

[0022] US Patent No. 5,191,631 describes a hybrid optical fiber having a desirable fiber dispersion characteristic and a relatively large effective area. Hybrid fibers are constructed by splicing together a fiber with a large effective area, but typically unsatisfactory fiber dispersion characteristics and a fiber that has excellent dispersion characteristics, but a smaller effective area. The patent, however, does not contemplate having Raman amplification distributed through the second fiber, which has a low dispersion slope in the second fiber, or obtaining a low dispersion slope through the link for WDM applications.

[0023] Applicants have noted the need for an optical transmission link that allows the transmission of dense WDM channels through a wavelength band that includes the C band, and can also include the L band, without destructive FWM and which includes the amplification distributed in the line.

Description of the invention

[0024] Applicants have observed that an optical fiber link having a first non-zero dispersion optical fiber of a high effective area, coupled directly before a second optical fiber having a low effective area, non-zero dispersion, and a Low absolute value of dispersion slope allows efficient Raman amplification through the posterior fiber optic and the transmission of dense and hyper-dense WDM channels without a significant mixture of four waves and other harmful non-linear effects in a broadband.

[0025] Applicants have also observed that this optical link can achieve Raman amplification through the pumping of the subsequent fiber, both co-directionally and counter-directionally to an optical signal without significant signal degradation from single or double backscatter. .

[0026] In one aspect, the invention relates to a fiber optic transmission link according to claim 1, which includes a first optical fiber having a first effective area in 1550 nm greater than 45 µm2, and a second fiber subsequently coupled from the first fiber optic. The second fiber has a second effective area smaller than the first optical fiber, an absolute value of dispersion between approximately 2 and 20 ps / nm / km at 1550 nm, an absolute value of dispersion slope less than approximately 0.040 ps / nm2 / km at 1550 nm, and an attenuation coefficient at 1550 nm of less than about 0.24 dB / km and at 1450 nm of less than about 0.30 dB / km. In addition, the fiber advantageously shows a cut when it is in the form of a cable less than or equal to 1450 nm. The first optical fiber can be a single mode standard fiber, a non-zero positive dispersion fiber, or a non-zero negative dispersion fiber.

[0027] Preferably, the dispersion of the second optical fiber at 1550 nm is between -2 and -20 ps / km / nm, more preferably between -3 and -8 ps / km / nm. Preferably, the dispersion of the second optical fiber at 1625 nm is less than or equal to about -0.1 ps / km / nm. Preferably, the absolute value of the dispersion slope of the second optical fiber at 1550 nm is less than about 0.035 ps / nm2 / km, preferably less than about 0.030 ps / nm2 / km.

[0028] Preferably, the diameter of the modal field (MFD) of the second optical fiber in 1550 is less than 7.6 µm, preferably less than 7.4 µm, even more preferably less than 7.2 µm.

[0029] Preferably, the wavelength of the cable cut from the next fiber (ITU-T G650 Rec.) Is between approximately 1250 nm and 1450 nm.

[0030] Preferably, the attenuation of the macrocurvature of the second fiber for a loop on a 32 mm mandrel is less than about 0.5 dB at 1550 nm and less than about 1.0 dB at 1625 nm. More preferably, the macrocurvature attenuation is less than about 0.2 dB at 1550 nm and less than about 0.5 dB at 1625 nm.

[0031] Preferably, the second fiber has a microcurvability sensitivity of less than about 8 (dB / km) / (g / mm) at 1550 nm and less than about 12 (dB / km) / (g / mm) at 1625 nm . More preferably, the microcurvature sensitivity at 1550 nm is less than about 6 (dB / km) / (g / mm) at 1550 nm and less than about 10 (dB / km) / (g / mm) at 1625 nm.

[0032] Advantageously, the effective area at 1550 nm of the second fiber is less than about 45 µm2.

[0033] Preferably, the second fiber has one and only a zero dispersion wavelength between 1200 and 2000 nm.

[0034] In a second aspect, the invention relates to a method for transmitting a plurality of multiplexed optical wavelength division signals over a long distance according to claim 13.

[0035] In a third aspect, the invention relates to an optical fiber for improving Raman amplification and the transmission of wavelengths including the C band according to claim 14. The glass core has an internal core with a first difference of refractive index Δn1 of between about 0.008 and 0.014, a first layer radially around the inner core along the length of the fiber and having a second difference of the refractive index Δn2 between about -0,0005 and -0, 0030 The fiber can have a second layer radially around the first layer along the length of the fiber with a third difference in refractive index Δn3 greater than 0. In addition, the fiber can have a third layer radially around the second layer. along the length of the fiber with a fourth difference of the refractive index Δn4 not exceeding 0. The glass lining surrounding the glass core has a difference of the refractive index ΔnCL substantially equal to 0.

[0036] The optical fiber according to the third aspect of the invention has a modal field diameter at 1550 nm less than about 7.6 µm, an attenuation coefficient at 1550 nm less than about 0.24 dB / km at 1450 nm smaller of approximately 0.30 dB / km, an absolute dispersion value at 1550 nm between 2 and 20 ps / nm / km, and an absolute value of the dispersion slope at 1550 nm less than approximately 0.040 ps / nm2 / km.

[0037] Preferably, Δn1 of the optical fiber is between about 0.009 and 0.012, Δn2 is between about -0.0010 and -0.0025, and Δn3 is between 0 and 0.006. Preferably, the diameter of the modal field is less than about 7.4 µm at 1550 nm and less than about 6.8 µm at 1450 nm. More preferably, the diameter of the modal field is less than about 7.2 µm at 1550 nm and less than about 6.6 µm at 1450 nm.

[0038] Preferably, the optical fiber has an absolute value of the dispersion slope at 1550 nm of less than 0.035 ps / nm2 / km, preferably less than about 0.030 ps / nm2 / km.

[0039] Preferably, the optical fiber has one and only a zero dispersion wavelength between 1200 and 2000 nm.

[0040] In a fourth aspect, the invention relates to a fiber optic preform for the preparation of an optical fiber to improve Raman amplification and the transmission of wavelengths that include the C band, according to claim 20, comprising a glass core that includes an inner core that has a first width and a first difference in refractive index Δn1 between about 0.008 and 0.014 and a first layer radially around the inner core along the length of the fiber and that has a second width and a second difference of the index of refraction Δn2 between approximately -0,0005 and -0,0030. The preform also comprises a glass liner surrounding the glass core and having a third width and a difference in refractive index ΔnCL substantially equal to 0. The core is selected so that the fiber made has a modal field diameter a 1550 nm less than about 7.6 µm, an attenuation coefficient at 1550 nm less than about 0.24 dB / km at 1450 nm less than about 0.30 dB / km, an absolute value of the dispersion at about 1550 nm between 2 and 20 ps / nm / km, and an absolute value of the dispersion slope at 1550 nm less approximately 0.040 ps / nm2 / km.

[0041] It should be understood that both the above general description and the following detailed description are by way of example and explanation and are only intended to provide a more detailed explanation of the claimed invention. The following description, as well as the practice of the invention, set forth and propose additional advantages and purposes of the invention.

Brief description of the drawings

[0042] The accompanying drawings, which are incorporated and form part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the advantages and principles of the invention.

Figure 1 is a schematic diagram of an optical transmission link in accordance with the present invention;

Figure 2 is a cross section of a rear optical fiber having a low effective area in accordance with the present invention;

Figure 3 is a profile of the index of refraction of the fiber optic radius in Figure 2 according to an embodiment of the invention;

Figure 4 is a profile of the index of refraction of the fiber optic radius in Figure 2 according to a second embodiment of the invention;

Figure 5 is a profile of the index of refraction of the fiber optic radius in Figure 2 according to a third embodiment of the invention;

Figure 6 is a profile of the index of refraction of the fiber optic radius in Figure 2 according to a fourth embodiment of the invention;

Figure 7 is a profile of the index of refraction of the fiber optic radius in Figure 2 according to a fifth embodiment of the invention;

Figure 8 is a profile of the index of refraction of the fiber optic radius in Figure 2 according to a sixth embodiment of the invention;

Figure 9 is a profile of the index of refraction of the fiber optic radius in Figure 2 according to a seventh embodiment of the present invention; Y

Figure 10 is a profile of the index of refraction of the fiber optic radius A10 as described in example 3.

Detailed description of the preferred embodiments

[0043] Reference will now be made to the various embodiments according to this invention, examples of which are shown in the accompanying drawings and will be obvious from the description of the invention. In the drawings, the same reference numbers that represent the same or similar elements in the different drawings whenever possible.

[0044] Throughout this description reference is made to profiles of the refractive index of the optical fibers. The refractive index profiles comprise various radially arranged sections. Reference is made in the present description or has been represented in the figures to precise geometric shapes for these sections, such as step or Gaussian type. It is clear that the refractive index profiles obtained in practice may differ from the previous idealized profiles. It has been shown in the literature, however, that these differences do not change the characteristics of the fiber if they are kept under control.

[0045] In general, a section of the refractive index profile (or segment) has an associated section of the equivalent refractive index profile that is different in shape and dimensions. An equivalent refractive index profile or profile section can be replaced by its associated refractive index profile or profile section without altering the performance of the waveguide. For example, see “Single Mode Fiber Optics,” Luc B. Jeunhomme, Marcel Dekker Inc., 1990, page 32, section 1.3.2. It will be understood that the disclosure and claim of a particular refractive index profile form and its dimensions include the associated equivalents in the description and in the claims.

[0046] In accordance with the principles of the present invention, and as generally referred to as 10 in Figure 1, an optical fiber transmission link comprises a first single mode optical fiber A and a second optical fiber in simple mode B. Both fibers A and B are preferably glass optical fibers capable of carrying optical signals individually or by wavelength division multiplexing over long distances with low attenuation. To the left of fiber A in Figure 1, a signal is received and introduced into the

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fiber for transmission after a signal output at the end of the link. The signal input can be derived directly from a transmitter or from other parts (not shown) of an optical transmission system prior to link 10, such as a drop / addition multiplexer, a dispersion compensation unit, an optical amplifier, or Similar. Similarly, the signal output after link 10 can be fed directly to a receiver or other parts (not shown) of an optical transmission system, such as a drop / addition multiplexer, a dispersion compensation unit , or the like.

[0047] Transmission link 10 may include couplers such as 12 and 14 to add pump light on fiber B. The pump signals may be derived from one or more pump sources (not shown), such as laser diodes The pump light from a source is fed through the coupler 12 in the fiber B to pass codirectionally with signal light passing along the fiber link 10. In addition, or alternatively, the same or a signal of the different pump can be fed through the coupler 14 in the fiber B to pass directionally with the signal light passing along the fiber link 10. Preferably, the couplers 12 and 14 are conventional dichroic couplers.

[0048] The present invention contemplates the arrangement of link 10 in a hybrid configuration where a first section A is made of optical fiber having a relatively large effective area and a second section B is made of optical fiber having a smaller effective area , an absolute value of low dispersion slope and low attenuation. The lower characteristic effective area and the low attenuation are applied to the B segment in the signal wavelength at least in the C band and in the pump wavelength, which is typically approximately 1450 nm. Unlike the optical flattened dispersion fibers, the second stretch of fiber B has a zero dispersion in a single wavelength through the wavelength band of approximately 12002000 nm.

[0049] Based on these characteristics, fiber B is configured to provide distributed Raman amplification to the optical signals in the link due to the pump signals injected through one or both of the couplers 12 and 14. The amplification of the discrete signal through a dedicated local device such as an erbium-doped fiber amplifier 16 is separated from the coupling region of fibers A and B so that the power of an optical signal that is released into fiber B is substantially less than the power at the input of fiber A due to the attenuation of the signal as it passes to the point of fiber B input. Typically, the power of an optical signal released to fiber B is not significantly different from the power of the optical signal emanating from fiber A.

[0050] The large effective area fiber is known to provide a mixture of four favorable waves and non-linear effect performance for WDM channels. In the context of the present invention, large effective area is considered to be greater than about 45 µm2 at 1550 nm (ie, a modal field diameter greater than about 7.7 µm). Consequently, fiber A allows the transmission of dense WDM channels with a relatively high signal strength. Preferably, fiber A has optical characteristics that include non-zero local dispersion, preferably less than -0.1 ps / nm / km at 1625 nm, a relatively flat dispersion slope over the entire operating bandwidth 1530-1625, a Large effective area, preferably between about 45 µm2 and 90 µm2 at 1550 nm, a low total optical loss, and a low fluoride content (to improve industrial control of optical properties and to reduce the loss and divide the loss). Fiber A typically has a length such that an optical signal that travels through it is attenuated from its input power and needs some amplification.

[0051] Table 1 shows a list of the optical characteristics of the example fibers A1-A10 that could be used in a single bond or as the first fiber of section A in a hybrid configuration, as described in more detail below. . Throughout the present description, the characteristics of the fiber refer to a wavelength of 1550 nm if the opposite is not specified. The A1-A9 fibers are conventional, as described below, the A10 fiber is described in detail below.

Table 1

First stretch of the fiber

Dispersion (ps / nm-km) Slope of dispersion (ps / nm2 -km) Effective area at 1450 nm (µm2) Effective area (µm2) Attenuation (dB / km)

A1

17 0.06 74 80 0.19

A2

4.1 0.08 58 70 0.20

A3

4,5 0.05 47 55 0.20

A4

8.0 0.06 56 65 0.20

TO 5

-3.1 0.07 Four. Five 55 0.20

A6

-3.5 0.11 53 70 0.21

A7

-3.0 0.05 44 fifty 0.21

A8

-3.2 0.08 53 68 0.21

A9

-3.5 0.05 44 fifty 0.21

A10

-4.5 0.045 43 fifty 0.21

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55

Each of these examples for fiber A has an effective area greater than 45 µm2 with low attenuation, making them desirable for the transmission of high-power WDM signals (for example, in the order of between approximately -10 dBm and +10 dBm per channel).

[0052] A large effective area, however, is detrimental to the efficiency of Raman amplification. The fiber section B has an effective area smaller than that of section A (preferably less than or equal to 45 µm2) to allow efficient Raman amplification of the optical signals. Section B is designed to allow, if desired, for pumping against directional and co-directional through couplers 12 and 14 at a Raman pump wavelength between approximately 1400 and 1500, preferably between 1450 nm and 1490 nm.

[0053] In a preferred embodiment, the second section of fiber B has optical characteristics that allow both Raman amplification and efficient transmission of WDM channels. In particular, applicants have found that fiber B must have a modal field diameter of less than 7.6 µm at 1550 nm and less than 7.0 µm at 1450 nm (preferably less than 7.4 µm at 1550 nm and less 6.8 µm at 1450 nm), an absolute value of dispersion between approximately 2 and 20 ps / nm / km at 1550 nm (preferably with a dispersion of between -3 and 8 ps / nm / km at 1550 nm and less from -0.1 ps / km / nm to 1625 nm), and an absolute value of the dispersion slope of less than about 0.040 ps / nm2 / km at 1550 nm (preferably less than about 0.035 ps / nm2 / km, preferably less of approximately 0.030 ps / nm2 / km). Fiber B also has an attenuation coefficient at 1550 nm less than about 0.24 dB / km and at 1450 nm less than about 0.30 dB / km. The macrocurvation attenuation measured for 1 loop on a 32 mm mandrel is less than about 0.5 dB at 1550 nm (preferably less than about 0.2 dB) and less than about 1.0 dB at 1625 nm (preferably less than approximately 0.5 dB). The microcurvature sensitivity is less than about 8 (dB / km) / (g / mm) at 1550 nm (preferably less than 6 (dB / km) / (g / mm)) and less than about 12 (dB / km) / (g / mm) at 1625 nm (preferably less than 10 (dB / km) / (g / mm)).

[0054] To achieve the advantages of the invention, fiber A must not have a low slope especially by itself. Using fiber B, which has a low dispersion slope, fiber A will reduce the slope of the joint fiber bond. Preferably, however, fiber A also has a relatively low slope. In particular, the preferred dispersion slope for fiber A is approximately 0.05 ps / nm2 / km or less.

[0055] A cross-section (not to scale) of optical fiber B according to a preferred embodiment of the present invention is schematically illustrated in Figure 2 and is generally designated by the reference number

20. In accordance with the present invention, a fiber optic transmission for use in a WDM transmission system includes a glass core with an internal core having a first difference in refractive index Δn1, greater than a refractive index of an outer sheath of substantially pure silica glass, and a first layer radially around the inner core along the length of the fiber and having a second difference in refractive index Δn2, less than a refractive index of an outer sheath of substantially pure silica glass. The fiber core may have a second layer radially around the first layer along the length of the fiber with a third difference in refractive index Δn3 greater than 0. In addition, the fiber core may have a third layer radially around the second layer along the length of the fiber with a fourth difference in refractive index Δn4 of less than 0. The glass coating surrounds the glass core and has a difference in refractive index ΔnCL substantially equal to zero .

[0056] The optical fiber 20 includes a core with a plurality of conductive layers of glass and a coating. The axial center of the fiber 20 is the inner core 22, which is made of doped glass. The inner core 22 has a first difference in refractive index Δn1 and a radius r1. The difference in the index of refraction refers to the difference in the index of refraction between a certain layer of glass and the coating glass. That is, for example, the difference in the refractive index Δn1 of the inner core 22 is equal to n1-nCL. The first difference in the refractive index Δn1 of the inner core 22 is between about 0.008 and 0.014, preferably between 0.009 and 0.012. Preferably, the inner core 22 is made of SiO2 doped with a substance that increases the refractive index of pure SiO2, such as GeO2. The inner core 22 may include a central bath generated during the manufacturing process.

[0057] A first glass layer 24 radially surrounds the inner core 22 along the length of the fiber 20. The first glass layer 24 extends from the outer radius r1 of the inner core to a radius r2 and has an index of refraction decreased Δn2 across its width compared to region 22. The refractive index Δn2 of the first glass layer 24 is depressed. A depressed refractive index occurs when the refractive index of a given glass layer is less than the refractive index of the coating layer, that is, Δn2 is less than 0. The second difference of the refractive index Δn2 of the First layer of glass 24 is between about -0,0005 and -0.0030. As is well known in the art, the depressed index is lower due to the addition of dopants in region 24, such as fluoride.

5

[0058] A second glass layer 26 may radially surround the first glass layer 24 along the length of the fiber 20. The second glass layer 26 extends from the outer radius r2 of the first glass layer in a radius r3 and has a refractive index Δn3, in its width, greater than 0 and less than 0.006. Preferably, the second glass layer 26 is made of SiO2, doped with a substance that increases the index

10 of refraction of pure SiO2, such as GeO2.

[0059] A third glass layer 28 may radially surround the second glass layer 26 along the length of the fiber 20. If used, the third glass layer 28 extends from the outer radius r3 of the second layer of glass 26 to an outer radius r4, and has a minimum negative refractive index Δn4. As described

Subsequently, a preferred embodiment of the present invention has a refractive index Δn4 for the third layer 28 that is greater than Δn2.

[0060] Finally, a light conductive coating 29 surrounds the core in a conventional manner to help propagate the guiding light along the fiber 20 axis. The coating 29 may comprise pure SiO2 glass

20 with a difference in refractive index substantially equal to zero or include a dopant that modifies the refractive index.

[0061] The optical fiber 20 can be manufactured by any known technique for depositing glass particles to form an optical preform. Common approaches easily known in the field include deposition

25 axial phase vapor (VAD), external vapor deposition (OVD), and modified chemical vapor deposition (MCVD). Applicants prefer MCVD from the embodiments described herein.

[0062] The optical preform includes selected profile segments to provide the transmission characteristics necessary for the realized fiber.

[0063] The combined bond 10 of fiber A and fiber B in Figure 1 has a local chromatic dispersion that is nonzero along the entire transmission band, preferably with an absolute dispersion value greater than 1 ps / nm / km throughout the band. The combined line also has a low total dispersion slope, a low total optical loss, high Raman efficiency and a moderate backscatter of the link section where the

35 Raman bomb is launched in the optical path.

[0064] An understanding of the advantages of the present invention can be achieved through a comparison between the different types of optical links. Table 2 shows the optical characteristics of different fibers. With the exception of prior art fibers B1, B5 and B6, the fibers in Table 2 are example fibers B for

40 explore the advantages of the invention when combined after fibers A of Table 1. The invention B2-B4 and B7-B12 fibers have different levels of effective area less than or equal to 40 µm2, low attenuation and dispersion, dispersion no zero at 1550 nm, and low absolute value of the dispersion slope.

Table 2

Four. Five

Second stretch of fiber

Dispersion (ps / nm-km) Slope of dispersion (ps / nm2-km) EA (µm2) [1450 nm] EA (µm2) [1550 nm] Attenuation (dB / km) [1450 nm] Attenuation (dB / km) [1550 nm]

B1 (prior art)

-17 -0.6 20-30 25-35 > 0.30 * 0.24 *

B2

4 0.03 35 40 0.28 * 0.21 *

B3

-18 0.02 28 35 0.30 * 0.23 *

B4

-8 0.02 29 35 0.29 * 0.22 *

B5 (prior art)

-3.1 0.07 Four. Five 55 0.28 0.21

B6 (prior art)

-3.0 0.05 44 fifty 0.28 0.21

B7

-3.2 0.03 3. 4 40 0.28 * 0.21 *

B8

-3.5 0.03 3. 4 40 0.28 * 0.21 *

B9

-3.5 0.03 30 35 0.28 * 0.21 *

B10

-3.5 0.02 3. 4 40 0.28 * 0.21 *

B11

-3.5 0.02 30 35 0.28 * 0.21 *

B12

-4.2 0.02 30 35 0.28 * 0.21 *

[0065] The asterisks * in table 2 designate that the indicated values were derived from computer simulations. Attenuation values refer to an average mass production of the fiber, rather than isolated quantities. Table 3 identifies the optical characteristics of an optical link such as 10 with the

5 combination of optical fibers described in more detail below. These combinations offer comparative examples of the present invention.

Table 3

Link

Dispersion (ps / nmkm) Slope of dispersion (ps / nm2-km) Attenuation (dB / km)

A1 fiber only

17 0.06 0.20

Fibers A1 → B1

0 0.0 0.22

A2 fiber only

4.1 0.08 0.21

A3 fiber only

4,5 0.05 0.21

A4 fiber only

8.0 0.06 0.21

A5 fiber only

-3.1 0.07 0.21

A6 fiber only

-3.5 0.11 0.21

Fibers A6 → B5

-3.3 0.09 0.21

A7 fiber only

-3.0 0.05 0.21

A8 fiber only

-3.2 0.08 0.21

Fiber A8 → B6

-3.1 0.065 0.21

[0066] Table 4 lists the optical characteristics of combinations of optical fibers A and B according to the present invention and are described in more detail below.

Table 4

fifteen

Link

Dispersion (ps / nm-km) Slope of dispersion (ps / nm2-km) Attenuation (dB / km)

Fibers A1 → B2

10 0.045 0.205

Fibers A1 → B3

0 0.04 0.215

A2 → B2 fibers

4 0.055 0.21

Fibers A3 → B2

4.3 0.04 0.21

A4 → B2 fibers

6 0.045 0.21

A4 → B4 fibers

0 0.04 0.215

Fibers A5 → B7

-3.1 0.05 0.21

Fibers A6 → B8

-3.5 0.07 0.21

Fibers A8 → B7

-3.2 0.055 0.21

Fibers A7 → B9

-3.3 0.04 0.21

Fibers A8 → B10

-3.3 0.05 0.21

Fibers A9 → B11

-3.5 0.035 0.21

image2

Example 1

[0067] As a first comparative example refers to tables 1-4, an optical link is formed by fiber A1, which is a conventional single-mode fiber according to ITU-T Rec. G652. Alternatively, the A1 fiber could be a pure silica core fiber, which generally has a greater positive dispersion. Tables 1 and 3 list the optical performance of fiber A1 when used as a single fiber in the link. This standard single-mode fiber provides a very high total dispersion that requires strong dispersion compensation. In addition, this fiber has a low Raman efficiency, which opposes distributed amplification.

[0068] Alternatively, fiber A1 can be coupled with fiber B1, which has an equal but opposite value of the dispersion and the dispersion slope. As shown in Table 3, the combination of these fibers A1 and B1 provides compensation for the dispersion in the link and a total slope along the link of approximately 0 ps / nm2 / km. However, to achieve the very negative dispersion slope in the B1 fiber, a high fluoride content must be used, which leads to the loss of splicing and a high flexural sensitivity for the B1 fiber (especially in the L-band) and a joint difficulty in the control of dispersion and the dispersion slope industrially. In addition, in case of distributed Raman amplification, the structure of the B1 fiber necessary to achieve the indicated optical characteristics causes a large loss of dispersion that will deplete the signal and the pump and will cause high levels of noise due to single and double backscatter.

[0069] On the contrary, the advantage of the present invention can be seen through the combination of fiber A1 with fibers B2 or B3. Table 4 lists the optical characteristics of these combined fibers. Fiber B3 has a similar dispersion and effective areas such as fiber B1, but a different dispersion slope. Since fiber B1 has a negative dispersion slope and fiber B3 has a positive slope, one skilled in the art will appreciate that fiber B1 has a fluorine content greater than fiber B3. Comparatively, fiber B1 will have a greater diffusion and greater noise than fiber B3.

[0070] The bond of A1 fibers with B2 or B3 fibers, despite a non-zero total dispersion slope, only provides a moderate dispersion loss of the second stretch of the fiber, but with high Raman efficiency. Therefore, the second fiber section can be pumped to achieve Raman amplification. Due to a moderate fluorine content of fiber type B, the fibers in the bond have a moderate loss of splicing. The combination of fiber A1 with fiber B2 has a moderate total dispersion and the ratio between the slope and the dispersion is approximately 0.004 nm-1, which can be easily compensated with separate dispersion compensation modules. The use of B3 fiber provides compensation for line dispersion and gives it a low total dispersion slope. Consequently, the combination of a standard single-mode fiber segment with a B2 or B3 fiber segment may allow Raman amplification in the second section and allow efficient transmission of WDM channels over long distances.

Example 2

[0071] A second example of the present invention is a multi-fiber link with a first stretch of non-zero positive dispersion fiber. Table 1 indicates the optical characteristics of conventional fibers, such as A2-A4 fibers. Examples of A2 fiber are Corning LEAF ™ and Pirelli ™ FreeLight fibers. An example of A3 fiber is Lucent TrueWave® RS fiber, and an example of A4 fiber is Alcatel TeraLight ™ fiber. When any of these fibers is used only in one link, the link has a relatively high dispersion slope and low Raman efficiency.

[0072] In contrast, the use of one of the B2 or B4 fibers as a second stretch in the link helps achieve the desired results in line with the present invention. Table 4 shows the optical characteristics achieved by combining these fibers. The values in Table 4 are for the solo link (assuming a 1: 1 section length ratio) and do not take into account the contribution of the dispersion compensation modules after or before the link. In general, the combined link can provide high Raman efficiency with a moderate noise from double and single backscatter, together with a moderate dispersion and easily compensated dispersion slope ratio with separate dispersion compensation modules. In addition, the link having one of the A2-A4 fibers coupled with B2 or B4 fibers will have a moderate splice loss.

[0073] Figure 3 shows the refractive index profile of an example of an optical fiber 20 in accordance with the present invention having positive dispersion characteristics, a low dispersion slope, a low effective area, and a low flexion of lost. This fiber resembles B2 fiber. Throughout this description radii and widths are defined in terms of the sigma of the Gaussian curves adopted for modeling the profile of the index. The model used for the distribution of the refractive index profile of this and

Other examples are a superposition of Gaussian-like curves, according to the following formula:

image 1

[0074] Where Δni are the maximum refractive index differences of the respective segments, σi are the radii or widths of the respective segments and:

g1 = 12 g2 = 3 g3 = 4 g4 = 4 rdep = 1.15 * (σ1 + σ2) rout = rdep + 1.15 * σ2 + 1.1 * σ3 routdep = rout + 1.15 * σ3 + 1.1 * σ4

[0075] As shown in Figure 3, the inner core 22 has a maximum refractive index difference Δn1 of about 0.00875 and extends over a radius r1 (ie, σ1) of about 2.6 µm before from passing to the first layer 24. The difference in the refractive index of the inner core 22 can be established by doping the width of the inner core with GeO2 or with any other known dopant that increases the refractive index. Although Figure 3 shows the inner core 22 as having the refractive index with a decreasing slope with radius, its profile may have other geometries determined by the selected manufacturing process.

[0076] The first glass layer 24 has a difference in the reduced refractive index Δn2 at its peak of approximately -0.0017. Layer 24 has a sigma (σ2) of approximately 2.11 µm, which extends at a radial distance of approximately 7.8 µm at the start of adjacent layer 26. The difference in refractive index of the first glass layer 24 it can be reduced by doping the width of the first layer of the core with fluoride or other known dopants, in a conventional manner. The first layer 24 is shown to have a profile similar to Gaussian, but other geometries can be selected or achieved for layer 24.

[0077] The second glass layer 26 surrounding the layer 24 has a refractive index difference greater than 0 and a maximum value of Δn3 of approximately 0.003. Layer 26 has a sigma (σ3) of about 1.09 µm, extending radially to about 10.6 µm, where the difference in refractive index returns to 0. The second layer 26 is shown to have a Gaussian-like profile, but other geometries can be selected or achieved for layer 26, for example, it can comprise a tail with a positive refractive index difference in its radially external section.

[0078] The third glass layer 28 reaches a maximum refractive index Δn4 of approximately -0,00076 and has a sigma (σ4) of approximately 1 µm. The third layer of glass 28 is surrounded by a coating 29 which has a refractive index difference of about 0 made of substantially pure silica.

[0079] The specific embodiment of the fiber 20 illustrated in Figure 3 has the following optical transmission characteristics, which have been generated through computer simulations:

Theoretical cut LP11 = 1270 nm Theoretical cut LP02 = 1560 nm

[0080] In general, the cable cut according to ITU-T Rec. G650 has between 200 and 400 nm less than the larger theoretical cut-off values.

Zero dispersion wavelength = 1450 nm Zero dispersion slope = 0.043 ps / nm2 / km Dispersion in 1550; 1625 nm = 3.8; 5.9 ps / nm / km Pending dispersion at 1550; 1625 nm = 0.031, 0.027 ps / nm2 / km Modal field diameter at 1450; 1550; 1625 nm = 6.9; 7.4; 7.83 µm Effective area at 1450, 1550, 1625 nm = 36.1, 41.2, 46.2 µm2 Coefficient of nonlinearity γ in 1450; 1550; 1625 nm = 2.87; 2.19; 1.76 W-1 km-1 Macrocurvature attenuation at 1550; 1625 nm ≤ 0.1; 0.5 dB for a 32 mm diameter loop

[0081] Sensitivity to microcurvature at 1550, 1625 nm ≤ 2.4 (dB / km) / (g / mm). The sensitivity to the microcurvature was determined by the expandable coil procedure, as described, for example, in G. Grasso and

F. Meli “Microbending losses of cabled single-mode fibers”, ECOC'88, page 526 and following, or in G. Grasso et al. “Microbending effects in single-mode optical cables”, International Wire and Cable Symposium, 1988, page 722 and following.

Example 3

[0082] A third example of the present invention is a multi-fiber link with a first stretch of non-zero negative dispersion fiber fiber. Table 1 shows the optical characteristics of these fibers, such as A5-A10 fibers. An example of a fiber in this category is Pirelli DeepLight ™ fiber. With any of these fibers used alone, or with hybrid fibers formed from A6 fiber and B5 fiber and from A8 fiber and B6 fiber as shown in Table 3, the link will have a relatively high dispersion slope and a Low Raman efficiency, so it is undesirable for Raman amplification. Most of these comparative examples have a zero dispersion wavelength below 1600 nm, which makes them unsuitable for the transmission of WDM channels in the L band. The example of fiber A7 does not have a zero dispersion wavelength below 1600 nm, but still has a low dispersion (approximately -1.0 ps / nm / km) at 1600 nm, which is not suitable for dense WDM channels in the L band. [0083] A fiber developed by the Applicants that can be advantageously used in an optical link adapted for the L band is the A10 fiber.

[0084] Figure 10 shows the refractive index profile of an example of the A10 optical fiber. In this example, the inner core has a refractive index difference of approximately 0.0087 and a radius of approximately 2.56 µm. The first layer outside the inner core, which is doped with fluoride, has a negative refractive index difference of approximately -0.0022 and a sigma of approximately 1.41 µm. The second layer is 0.92 µm wide and has a refractive index difference of 0.0045. Finally, the outer ring is slightly doped inferiorly to produce a refractive index difference of -0,0008 and has a sigma of approximately 1 µm. One skilled in the art will readily understand that numerous dopants can be used to increase the refractive index of the second layer, but GeO2 is preferred. In this example, the next layer is essentially the coating.

[0085] The specific embodiment of the A10 fiber illustrated in Figure 10 has the following optical transmission characteristics, which have been generated through computer simulations:

Theoretical cut LP11 = 1386 nm Theoretical cut LP02 = 1494 nm

[0086] In general, the cable cut according to G650 Rec. Has between 200 and 400 nm less than the larger theoretical cut-off values.

Zero dispersion slope = 0.043 ps / nm2 / km Dispersion in 1450; 1550; 1625 nm = -8.5; -4.3, -0.4 ps / nm / km Slope of dispersion in 1450; 1550; 1625 nm = 0.038, 0.046, 0.058 ps / nm2 / km Modal field diameter at 1450; 1550; 1625 nm = 7.2; 7.9; 8.5 µm Effective area at 1450, 1550, 1625 nm = 39.2, 48.1, 57.0 µm2 Coefficient of nonlinearity γ in 1450; 1550; 1625 nm = 2.63; 1.87; 1.42 W-1 km-1 Attenuation of the macrocurvature at 1550; 1625 nm ≤ 0.2; 1.0 dB for a 32 mm diameter loop Microcurvability sensitivity at 1550, 1625 nm ≤ 4, 8 (dB / km) / (g / mm).

[0087] As previously anticipated, the A10 fiber has characteristics of microcurvature, macrocurvature and non-zero dispersion in the L band that make it especially advantageous for WDM transmissions in (or that include) this band. In particular, this fiber has an absolute dispersion value of approximately 2.3 ps / nm / km over a wavelength of 1600 nm. Applicants have determined that the same optical characteristics can be achieved, in general, by a fiber having a profile of the core refractive index comprising an internal core with a refractive index difference between about 0.008 and 0.012 and a radius ( sigma) between approximately 2.4 and 2.8 µm; a first layer outside the inner core, which is doped with fluoride, which has a negative refractive index difference between approximately -0.0030 and -0.0020 and a sigma between approximately 1.0 and 1.6 µm; a second layer that is approximately 0.8 to 1.1 µm wide and has a refractive index difference between approximately 0.0030 and 0.0060. Finally, the fiber may comprise an outer ring doped down.

[0088] Table 4 shows the improved optical performance that can be achieved with a fiber with a low effective area that has a low dispersion slope, such as fibers B7-B12, which are listed in Table 2. These combinations that shown in Table 4 provide a low dispersion slope across the link, together with high Raman efficiency and a moderate double and single backscatter. In addition, these links have a moderate loss and a moderate loss of splicing. In the preferred combination of fiber A10 with fiber B12, the link is particularly suitable for WDM transmission in the L band, because the absolute value of the local dispersion is always above 1 ps / nm / km in the L band In addition, this link that allows hyper-dense WDM transmission in the C band, due to a relatively high dispersion value.

[0089] Figure 4 shows the refractive index profile of an example of an optical fiber 20 according to the present invention having a negative dispersion. The parameters of the difference in the refractive index and the radius of the glass layers 22, 24, 26 and 28 are shown in Figure 4. In this example, the core has a maximum difference refractive index of about 0.0098 and a radius of about 2.34 µm. The first layer outside the core, which is doped with fluoride, has a negative refractive index difference of approximately -0.0017 and a sigma of approximately 1,722 µm. The second layer outside the core is approximately 0.8315 µm wide and has a refractive index difference of 0.0028. One skilled in the art will readily understand that numerous dopants can be used to increase the refractive index of the second layer, but GeO2 is preferred. In this embodiment, the next layer is essentially the coating, since it has no dopants intended to change its refractive index of pure silica glass.

[0090] The specific embodiment of the fiber 20 shown in Figure 4 has the following optical transmission characteristics, which have been generated through computer simulations:

Theoretical cut LP11 = 1165 nm Theoretical cut LP02 = 1492 nm Dispersion in 1450; 1550; 1625 nm = -6.4; -3.1; -0.8 ps / nm / km Pending dispersion in 1450; 1550; 1625 nm = 0.036, 0.030, 0.032 ps / nm2 / km Modal field diameter at 1450; 1550; 1625 nm = 6.6; 7.2; 7.7 µm Efficient area in 1450, 1550, 1625 nm = 32.9, 38.7, 44.6 µm2 Coefficient of nonlinearity γ in 1450; 1550; 1625 nm = 3.18; 2; 35; 1.84 W-1 km-1 Macrocurvature attenuation in 1550; 1625 nm ≤ 0.1; 1.0 dB for a 32 mm diameter loop Microcurvature sensitivity at 1550, 1625 nm ≤ 2.4 (dB / km) / (g / mm).

[0091] Figure 5 shows the refractive index profile of another example of an optical fiber 20 according to the present invention. The parameters of the difference in refractive index and the radius of the glass layers 22, 24, 26 and 28 are shown in Figure 5. This optical fiber has a core with three doped layers surrounding it. The core has a radius of approximately 2.36 µm and with a maximum refractive index difference of approximately 0.0099. The first layer, which is poorly doped with fluoride or a similar dopant, has a maximum refractive index difference of approximately -0.0017 through a sigma of approximately 1,976. The second layer outside the core has a maximum refractive index difference of approximately 0.003 through a 1.25 µm sigma. Finally, the outer ring is doped slightly less to produce a refractive index difference of -0,0007 and has a sigma (σ4) of approximately 1 µm.

[0092] The specific embodiment of the fiber 20 illustrated in Figure 5 has the following optical transmission characteristics, which have been generated through computer simulations:

Theoretical cut LP11 = 1430 nm Theoretical cut LP02 = 1657 nm Dispersion in 1450; 1550; 1625 nm = -5.9; -3.5, -2.0 ps / nm / km Pending dispersion in 1450; 1550; 1625 nm = 0.0310; 0.0204, 0.0206 ps / nm2 / km Modal field diameter in 1450; 1550; 1625 nm = 6.6; 7.1; 7.6 µm Effective area at 1450, 1550, 1625 nm = 32.4, 38.0, 43.8 µm2 Coefficient of nonlinearity γ at 1450, 1550, 1625 nm = 3.23; 2.40; 1.88 W-1 km -1 Attenuation of macrocurvature at 1550; 1625 nm ≤ 0.1; 1.0 dB for a 32 mm diameter loop Microcurvature sensitivity at 1550, 1625 nm ≤ 2.4 (dB / km) / (g / mm).

[0093] Figure 6 shows the refractive index profile of another example of an optical fiber 20 according to the present invention. The parameters of the difference in the refractive index and the radius of the glass layers 22, 24, 26 and 28 are shown in Figure 6. This fiber, which resembles the B2 fiber, has a core and three doped layers that surround. The maximum refractive index values are 0.0104, -0.0017, 0.003, and -0,0007, respectively.

The core has a radius of approximately 2.27 µm, while the three surrounding layers are approximately 2,062 µm, 1,135 µm and 1 µm wide, respectively. This example is the preferred embodiment.

[0094] The specific embodiment of the fiber 20 shown in Figure 6 has the following optical transmission characteristics, which have been generated through computer simulations:

Theoretical cut LP11 = 1294 nm Theoretical cut LP02 = 1576 nm Dispersion in 1450; 1550; 1625 nm = -6.7; -4.2, -2.8 ps / nm / km Slope of dispersion at 1450; 1550; 1625 nm = 0.032, 0.020, 0.019 ps / nm2 / km Modal field diameter at 1450; 1550; 1625 nm = 6.4; 6.9; 7.4 µm Effective area at 1450, 1550, 1625 nm = 30.7, 36.0, 41.4 µm2 Coefficient of nonlinearity γ in 1450; 1550; 1625 nm = 3.42; 2.54; 1,996 W-1 km-1 Macrocurvature attenuation in 1550; 1625 nm ≤ 0.1; 1.0 dB for a 32 mm diameter loop Microcurvature Sensitivity at 1550, 1625 nm = ≤ 2, 3 (dB / km) / (g / mm).

[0095] Figure 7 shows the refractive index profile of another example of an optical fiber 20 according to the present invention having a negative dispersion. The parameters the difference in refractive index and the radius of the glass layers 22, 24, 26 and 28 are shown in Figure 7. In this embodiment, the fiber has a core of three doped layers surrounding it. The core has a refractive index difference of approximately 0.011, while the surrounding layers, respectively, have values of approximately -0.0017, 0.003, and 0.0007. The core has a radius of approximately 2,149 µm, while in the next three layers that surround it they have a sigma of about 1,849 µm, 1.20 µm and 1 µm, respectively.

[0096] The specific embodiment of fiber 20 illustrated in Figure 7 has the following optical transmission characteristics, which have been generated through computer simulations:

Theoretical cut LP11 = 1319 nm Theoretical cut LP02 = 1552 nm Dispersion in 1450; 1550; 1625 nm = -11.4; -9.4, -7.8 ps / nm / km Pending dispersion in 1450; 1550; 1625 nm = 0.0242; 0.0183, 0.0249 ps / nm2 / km Modal field diameter in 1450; 1550; 1625 nm = 6.27; 6.86; 7.39 µm Effective area at 1450, 1550, 1625 nm = 29.5, 35.4, 41.5 µm2 Coefficient of nonlinearity γ in 1450; 1550; 1625 nm = 3.58, 2.60, 2.00 W-1km -1 Macrocurvature attenuation in 1550; 1625 nm ≤ 0.1; 1.0 dB for a 32 mm diameter loop Microcurvature sensitivity at 1550, 1625 nm ≤ 2.4 (dB / km) / (g / mm).

[0097] Figure 8 shows the refractive index profile of another example of an optical fiber 20 according to the present invention. The parameters of the difference in the index of refraction and the radius of the glass layers 22, 24, 26 and 28 are shown in Figure 8. As shown in Figure 8, the fiber of this embodiment has a core with a difference in refractive index of approximately 0.0124 maximum over a radius of approximately 1,954 µm. The first layer surrounding the core has been doped in a reduced manner with a substance to produce a refractive index difference of approximately -0.0017 through a sigma of approximately 1,714 µm. The final two layers have maximum refractive index differences of approximately 0.003 and -0,0005, respectively, sigma of approximately 1,296 µm and 1 µm, respectively.

[0098] The specific embodiment of fiber 20 illustrated in Figure 8 has the following optical transmission characteristics, which have been generated through computer simulations:

Theoretical cut LP11 = 1453 nm Theoretical cut LP02 = 1619 nm Dispersion in 1450; 1550; 1625 nm = -19.2; -18.0; -16.2 ps / nm / km Pending dispersion in 1450; 1550; 1625 nm = 0.012, 0.016, 0.036 ps / nm2 / km Modal field diameter at 1450; 1550; 1625 nm = 6.06; 6.72; 7.33 µm Effective area at 1450, 1550, 1625 nm = 27.5, 34.2, 41.5 µm2 Coefficient of nonlinearity γ in 1450; 1550; 1625 nm = 3.88, 2.72, 2.02 W-1km-1 Macrocurvature attenuation in 1550; 1625 nm ≤ 0.1; 1.0 dB for a 32 mm diameter loop Microcurvature sensitivity at 1550, 1625 nm ≤ 2.4 dB / km) / (g / mm).

[0099] Figure 9 shows the refractive index profile of another example of an optical fiber 20 according to the present invention. The parameters of the difference in the refractive index and the radius of the glass layers 22, 24, 26 and 28 are shown in Figure 9. The optical fiber of this embodiment is a "pure" W profile with no positive or negative peak. external. In this fiber, the core is surrounded only by a stretch of negative refractive index, followed by a wide region of pure silica. In particular, the core has a maximum refractive index difference of about 0.0107 over a radius of about 2,164 µm. The section is approximately 1,448 µm wide, with a maximum refractive index difference of approximately 0.0017. [0100] The specific embodiment of fiber 20 illustrated in Figure 9 has the following optical transmission characteristics, which have been generated through computer simulations:

Theoretical cut LP11 = 906.4 nm Theoretical cut LP02 = 563.4 nm Dispersion in 1450; 1550; 1625 nm = -7.19; -3.29; -0.78 ps / nm / km Pending dispersion in 1450; 1550; 1625 nm = 0.043, 0.035, 0.032 ps / nm2 / km Modal field diameter at 1450; 1550; 1625 nm = 6.3; 6.8; 7.3 µm Effective area at 1450, 1550, 1625 nm = 29.9, 34.9, 39.6 µm2 Coefficient of nonlinearity γ in 1450; 1550; 1625 nm = 3.52, 2.63, 2.09 W-1 km -1 Macrocurvature attenuation in 1550; 1625 nm ≤ 0.5; 1.0 dB for a 32 mm diameter loop Microcurvature sensitivity at 1550, 1625 nm ≤ 1.3 (dB / km) / (g / mm).

[0101] The design of fiber B limits the harmful effects typically associated with Raman pumping as single or double backscatter. In addition, a moderate dispersion is obtained in the second stretch of fiber by limiting the amount of minor dopants, such as fluorine or the like. As a result, a profile of the fiber B refractive index is characterized by moderate refractive index differences between adjacent layers, which reduces dispersion. The dispersion can be further reduced by reducing the slope of index discontinuities between adjacent layers. For example, step-like discontinuities can be replaced by index of graduated index sections, to provide a fluid connection between adjacent profile segments. In a preferred embodiment, the refractive index profile of fiber B also has a moderate concentration of a higher dopant, such as GeO2 or the like. This solution corresponds to a chromatic dispersion at 1550 nm above approximately -8 ps / nm / km and further reduces the dispersion and accumulated dispersion. In addition, the limited depth of the lowered layer index in the refractive index profile helps reduce the criticality of industrial control of the refractive index parameters, such as the radius of the inner core. This solution is thus industrially advantageous, since it mitigates the requirements for industrial tolerances. Finally, the invention provides a moderate total bond attenuation, due to the fact that the second fiber segment has a moderate attenuation coefficient and that the splice loss is limited, when the second fiber segment is spliced to a first stretch of fiber chosen among those commonly used for optical transmission.

[0102] It will be apparent to those skilled in the art that various modifications and variations can be made in the process and product described without departing from the scope of the invention. Other embodiments of the invention will be apparent to those skilled in the art from consideration of the memory and practice of the invention described herein. It is intended that the memory and the examples be considered as exemplary only, with a true scope of the invention indicated in the following claims.

Claims (20)

1. Fiber optic transmission link (10), comprising: a first optical fiber having a first effective area greater than 45 µm2; Y a second optical fiber (20) coupled after the first optical fiber, which has a second effective area smaller than the first optical fiber, an absolute value of dispersion between about 2 and 20 ps / nm / km at 1550 nm, an absolute value of dispersion slope less than about 0.040 ps / nm2 / km at 1550 nm, and an attenuation coefficient at 1550 nm less than about 0.24 dB / km at 1450 nm less than about 0.30 dB / km, characterized by the fact that the second optical fiber (20) comprises - a glass core that includes an inner core (22) that has a first difference in refractive index Δn1 between about 0.008 and 0.014 and a first layer (24) that radially surrounds the inner core (22) along the length of the fiber and having a second difference in refractive index Δn2 of between -0,0005 and -0.0030 and -a glass lining (29) that surrounds the glass core and has a difference in refractive index ΔnCL substantially equal to 0.

2.

Fiber optic transmission link (10) according to claim 1, which also comprises:

a pumping source coupled to provide a pumping light through the second optical fiber 20 and the second pumping source forming a distributed Raman amplifier.

3.

Fiber optic transmission link (10) according to claim 2, wherein the pump source is coupled to the pump light on the link after the second optical fibers (20).

Four.

Fiber optic transmission link (10) according to claim 2, wherein the pump source is coupled with the pump light on the link before the second fiber optic (20).

5.

Fiber optic transmission link (10) according to claim 1, wherein the dispersion of the second optical fiber (20) at 1550 nm is between about -2 and -20 ps / km / nm.

6.

Fiber optic transmission link (10) according to claim 5, wherein the dispersion of the second optical fiber (20) at 1550 nm is between about -3 and -8 ps / km / nm.

7.

Fiber optic transmission link (10) according to claim 6, wherein the dispersion of the second optical fiber (20) in 1625 nm is less than or equal to about -0.1 ps / km / nm.

8.

Fiber optic transmission link (10) according to claim 1, wherein the second optical fiber (20) has a macrocurvature attenuation of a loop of more than 32 mm of mandrel less than about 0.5 dB at 1550 nm and less than about 1.0 dB at 1625 nm.

9.

Fiber optic transmission link (10) according to claim 1, wherein the second optical fiber (20) has a modal field diameter at 1550 nm of less than 7.6 µm.

10.

Fiber optic transmission link (10) according to claim 1, wherein the second optical fiber (20) has a cable wavelength cut between 1250 and 1450 nm.

eleven.

Optical fiber transmission link (10) according to claim 1, wherein the first optical fiber is a non-zero positive dispersion fiber.

12.

Fiber optic transmission link (10) according to claim 1, wherein the first optical fiber is a non-zero negative dispersion fiber.

13.

Method for transmitting a plurality of multiplexed optical signals of wavelength division over a long distance, comprising:

transmit the optical signals along a first optical fiber that has a first effective area greater than 45 µm2; transmitting the signals along a second optical fiber (20) with a second effective area smaller than the first optical fiber, an absolute dispersion value of between about 2 and 20 ps / nm / km at 1550 nm, an absolute value of the dispersion slope less than about 0.040 ps / nm2 / km at 1550 nm, and an attenuation coefficient at 1550 nm less than about 0.24 dB / km at 1450 nm less than about 0.30 dB / km; coupling a pumping light on the second fiber (20) to provide a distributed Raman amplification of said optical signals characterized by the fact that the second optical fiber (20) has - a glass core that includes an inner core (22) that has a first difference in refractive index Δn1 between about 0.008 and 0.014 and a first layer (24) that radially surrounds the inner core (22) along the length of the fiber and having a second difference in refractive index Δn2 of between -0,0005 and -0.0030, and -a glass lining (29) surrounding the glass core and having a difference in the index of refraction ΔnCL substantially equal to 0. 14. Optical fiber (20) for Raman amplification and enhanced wavelength transmission that includes the C band, the optical fiber (20) having a modal field diameter at 1550 nm less than about 7.6 um, an attenuation , a coefficient at 1550 nm less than about 0.24 dB / km at 1450 nm less than about 0.30 dB / km, an absolute dispersion value at 1550 nm between about 2 and 20 ps / nm / km, and a value absolute of the dispersion slope at 1550 nm less than about 0.040 ps / nm2 / km, characterized by the fact that it comprises: a glass core that includes: an inner core (22) having a first difference in refractive index Δn1 between about 0.008 and 0.014; and a first layer (24) radially surrounding the inner core (22) along the length of the fiber and having a second difference in refractive index Δn2 between approximately 0.0005 and -0.0030, and a glass coating (29) surrounding the glass core and having a difference in refractive index ΔnCL substantially equal to 0.

fifteen.

Optical fiber (20) according to claim 14, wherein Δn1 is between about 0.009 and 0.012.

16.

Optical fiber (20) according to claim 14, wherein Δn2 is between about 0.0010 and -0.0025.

17.

Optical fiber (20) according to claim 14, which also comprises a second layer (26) radially surrounding the first layer (24) along the length of the fiber (20) and having a third difference in refractive index Δn3 greater than 0.

18.

Optical fiber according to claim 17, wherein Δn3 is less than about 0.006.

19.

Optical fiber (20) according to claim 17, which also comprises a third layer (28) radially surrounding the second layer (26) along the length of the fiber (20) and having a fourth difference in refractive index Δn4 less than 0.

twenty.

Fiber optic preform to make an optical fiber (20) for Raman amplification and transmission of improved wavelengths including the C band, the preform comprising:

-a glass core that including: an inner core (22) having a first width and a first difference in refractive index Δn1 between about 0.008 and 0.014; and a first layer (24) radially surrounding the inner core (22) along the length of the fiber and having a second width and a second difference in refractive index Δn2 of between about -0,0005 and -0 , 0030, and - a glass lining (29) that surrounds the glass core and has a third width and a difference in refractive index ΔnCL substantially equal to 0; wherein the core is selected so that the fiber made (20) has a modal field diameter at 1550 nm less than 7.6 µm, an attenuation coefficient at 1550 nm less than about 0.24 dB / km at 1450 nm less than approximately 0.30 dB / km, an absolute value of dispersion at 1550 nm between approximately 2 and 20 ps / nm / km, and an absolute value of the dispersion slope at 1550 nm less than approximately 0.040 ps / nm2 / km .