Wavelength division multiplexing system

The present invention relates to single-mode optical fiber systems, especially wavelength division multiplexing (WDM) systems on such systems.

Dispersion refers to a phenomenon in which different wavelengths propagate at different speeds in a medium having a dispersion function such as glass. Since the modulated carrier signal contains many wavelengths, the optical signal radiated from the far end of the glass optical fiber becomes bleeding (dispersed) when it is incident on the near end. In the case of linear dispersion, this can be solved by providing periodic compensation for the fiber optic route, and the fewer steps this compensation is, the better.

Traditional single-mode fiber optic systems operate in the wavelength region between 1285 nm and 1335 nm and have a zero dispersion wavelength at 1310 nm. However, the optical fibers used in such systems are not well suited to transmit multiple carrier wavelengths in close proximity due to non-linear interactions and mixing between channels.

A limited form of such a non-linear phenomenon (4-photon mixing = 4PM) is described, for example, in Non-Patent Document 1. Briefly, 4PM manifests itself as a phenomenon in which gain or loss fluctuates due to constructive and destructive interference between different signal channels. The magnitude of 4PM depends on the power and can be reduced by reducing the radiated power.<nplcit num="1"><text>"Effect of Fiber Nonlinearity on Long-Distance Transmission" by D. Marcuse, A. Chraplyvy, R. Tkach (Journal of Lightwave Technology, vol. 9, No. 1, January 1991, pp.121-128)</text></nplcit>

The multi-channel optical system can use the optical fiber most efficiently, has a wavelength division multiplexing device, and this device has one single optical device in one transmission direction for multiple channels (wavelength regions) in close proximity to each other. It is coupled to the path and separates these channels from this optical path in other transmission directions. Traditional single-mode fiber optic systems can perform WDM operation in the 1.55 μm wavelength region, but have too much linear dispersion to compensate (eg, about 17 ps / nm-km). For example, it needs to be compensated every 50 to 100 km, which is too short in practice.

Possible uses of fiber optics include transmitting all types of digital and analog information separately or together. One use case involves transmitting data (eg, Internet traffic) and television (TV) signals, which typically utilize the degenerate-sideband (AM-VSB) modulation of amplitude modulation. Analog signals are inherently sensitive to noise, which can be easily observed in TV images. In particular, when multiple wavelengths such as WDM signals are transmitted by a single fiber, energy is propagated from the WDM signal to another wavelength region as long as 120 nm due to excited Raman scattering (SRS). At present, there is no system that transmits WDM signals and analog TV signals over the same optical fiber.

According to Non-Patent Document 2, a completely OH-free optical fiber (in the loss spectrum from ultraviolet rays to infrared rays, in which there is no loss peak due to OH ions at any wavelength) is required in the future. Optical fibers will play an important role as a transmission medium for WDM systems. However, the above literature does not disclose information about the dispersion characteristics of such optical fibers, nor does it describe the allocation of optical channels within the available loss spectrum of optical fibers.<nplcit num="2"><text>"Fabrication Of Completely OH-Free VAD Fiber" (in Electronics Letters, August 28, 1980 Vol. 16 No. 19)</text></nplcit>

<p> An object of the present invention is to be compatible with devices designed for conventional single-mode fiber systems, to enable WDM operation without 4PM interference between WDM signals, and SRS interference between WDM signals and analog TV signals. Is to provide an optical transmission system that avoids the above.</p>

<p> The present invention solves the drawbacks of conventional systems in optical communication systems configured to transmit multiple WDM channels in a wavelength region of 1.4 μm. The system of the present invention has low loss characteristics at 1385 nm and zero dispersion wavelength (λ) at 1310 nm.₀) Includes an optical fiber with a linear dispersion of 1.5-8.0 ps / nm-km in the 1.4 μm wavelength region.</p>

<p> This small but important dispersion reduces the effects of four-wave mixing. In the broader concept of the present invention, the present invention reflects a number of observations. That is, (1) four-wave mixing is a related mechanism that must be considered when designing a WDM system. (2) Excited Raman scattering from WDM channels adversely affects the transmission of analog signals at 1550 nm. (3) Preferably, the new WDM system must be replaceable and compatible with the equipment used in conventional single-mode fiber systems.</p><p> As an effect of the present invention, by arranging the WDM channel in the 1.4 μm wavelength region, the wavelength regions of 1310 nm and 1550 nm can be used by the conventional apparatus in these regions.</p><p> Another advantage of the present invention is that by arranging the WDM channel in the 1.4 μm wavelength region, the dispersion is 8 ps / nm-km or less, and dispersion compensation is required for optical transmission systems in most urban areas of 200 km or less. And disappear.</p><p> Another advantage of the present invention is that by arranging the WDM channel in the wavelength region of 1.4 μm, the buffer wavelength region (about 120 nm) for protecting the analog signal (for television broadcasting) operating at 1550 nm from the noise of SRS. ) Exists.</p><p> In one embodiment of the invention, a Raman amplifier is used to amplify WDM channels in the 1.4 μm region, with an erbium-doped amplifier being used to amplify channels operating in the 1.55 μm region.</p><p> Terminology A conventional single-mode glass fiber optic that features zero dispersion (0) at a wavelength of 1310 nm, minimal loss at a wavelength of 1550 nm, and high loss at 1385 nm. Is the absorption of optical energy caused by hydroxyl group (OH) ions. Dispersion-When used alone, the term means dye dispersion, a linear effect due to wavelength-dependent velocities within the carrier spectrum.</p><p> Span-means the length of an optical fiber without a regenerator. This length may include an optical amplifier, which is the distance between stations where the conversion of the signal into or from the electronic form takes place (usually between the closest signal players). Distance). This span defines the entire system and can be combined with yet another span. Average System Wavelength-means a specific wavelength determined by the arithmetic mean of the carrier frequencies of a group of WDM channels.</p><p> Wavelength region-An abbreviation for a specific region of wavelength. As used herein, the 1.3 μm wavelength region includes wavelengths between 1285 nm and 1335 nm. The 1.4 μm wavelength region includes a wavelength region between 1335 nm and 1435 nm. The 1.55 μm wavelength region includes wavelengths between 1500 nm and 1600 nm. WDM-Wavelength division multiplexing. It represents a situation in which multiple communication channels with different center wavelengths are combined into a single signal transmission path, such as an optical fiber.</p>

FIG. 1 shows the high-capacity optical fiber network 100 of the present invention. Importantly, multiple wavelength division multiplexing (WDM) channels operate in the 1.4 μm wavelength region (ie, 1335-1435 nm). Furthermore, this WDM channel shares fiber optics with analog CATV signals operating in the 1.3 μm wavelength region (ie, 1285-1335 nm) and other WDM channels operating in the 1.55 μm wavelength region (ie, 1500-1600 nm). are doing.

The figure shows the carrier wavelength (λ).₁₁, λ₁₂, λ₁₃, λ₁₄) Are shown, and each carrier is modulated by transmitter 111 at a data rate of 10 Gb / s. Such channels can transmit large numbers of digital signals, including telephones, data, and images. These channels are shown to have an intercenter channel space of 100 GHz centered at 1400 nm.

Although four channels are shown in the figure, more or less channels can be used. Furthermore, the channel space may be 100 GHz or higher or lower as determined by the network designer based on the bandwidth of the amplifier and the availability and / or price of equipment such as multiplexers and demultiplexers. Transmitter 111-1 receives input data at a rate of 10 Gb / s, modulates a coherent optical source, and its nominal wavelength λ.₁₁Is 1400 nm. This optical signal is then input to the optical multiplexer 112, whose task is to combine multiple inputs with different wavelengths onto a single output port 110.

Furthermore, 16 digital channels are shown in the 1.55 μm wavelength region, and each channel contains a carrier signal modulated in transmitter 121 at a rate of 2.5 Gb / s. This channel is for transmitting digital information including telephones and data and images. These channels are dispersed around 1550 nm and have a carrier space of 100 GHz corresponding to 0.8 nm at 1550 nm.

The figure shows 16 channels, but more or less channels can be used. Furthermore, the channel space may be 100 GHz or greater or less, as determined by the network designer, based on the bandwidth of the amplifier and the availability and / or price of equipment such as multiplexers and demultiplexers. The optical signal from the transmitter 121 is then input to the optical multiplexer 122, the task of which is to combine multiple inputs with different wavelengths onto a single output port 210.

Optical multiplexing and separation is carried out via a pair of star couplers connected to an optical grating (ie, a plurality of parallel waveguides differing in length by a predetermined amount from adjacent waveguides). Examples of such devices are disclosed in Patent Documents 1 to 3.<patcit num="1"><text>U.S. Pat. No. 5,002,350</text></patcit><patcit num="2"><text>U.S. Pat. No. 5,136,671</text></patcit><patcit num="3"><text>U.S. Pat. No. 5,412,744</text></patcit>

In the positional direction of the optical transmission, the multiplexer has multiple separate wavelengths (λ).₁₁, λ₁₂, Λ_1n) Can be used as a multiplexer such that it is incident on different input ports of one star coupler and emitted from one output port of another star coupler. In the other direction of optical transmission, the multiplexer is as a demultiplexer in which multiple different wavelengths are incident on one port of a star coupler and radiated from multiple ports of another star coupler according to each wavelength. Function.

Depending on the distance between the transmitter (101,111,121) and the receiver (105,115,125) in network 100, it may be necessary to amplify the optical signal. The optical amplifier is preferably a regenerator. The reason is that the regenerator directly amplifies the optical signal without converting the optical signal into an electrical signal and then converting it back into the optical signal for electronic amplification.

Preferably, Raman amplifiers 103 and 113 are used in the wavelength regions of 1.3 and 1.4 μm, and erbium amplifier 123 is used in the 1.55 μm region. However, Raman amplifiers can also be used at all wavelengths within network 100. Raman amplifiers have an available bandwidth of 25-30 nm (see Patent Document 4 for this), which is suitable for use in the present invention.<patcit num="4"><text>U.S. Pat. No. 5,623,508</text></patcit>

Furthermore, Raman amplifiers can be arranged in a parallel configuration that increases the bandwidth. In fact, considerable effort has been devoted to the design of multi-stage, very wide optical amplifiers. See Non-Patent Document 3. Semiconductor optical amplifiers can also be used in the present invention at low cost at all relevant wavelengths, but with reduced performance.<nplcit num="3"><text>Paper by M. Yamada et al (vol. 33, No. 8, Electronics Letters, on April 10, 1997, pp. 710-711)</text></nplcit>

Combining optical signals in three different wavelength regions can be done with coarse WDM (CWDM) 131. The "coarse" in this CWDM131 is for sorting from routers that accept adjacent channels of a set of WDM channels. The coarse WDM131 combines optical signals in the 1.3 μm region, 1.4 μm region, and 1.55 μm region into one optical fiber 130.

The Mach-Zehnder interference system is suitable for use in constructing the coarse WDM131. The optical fiber 130 includes a single-mode optical fiber, and its loss and dispersion characteristics are as shown in FIG. The structure and characteristics of the optical fiber 130 will be described later, but the optical fiber 130 can be extended by a length L without compensation for regeneration or dispersion.

Most optical devices are bidirectional, and the network 100 shown in Figure 1 is symmetric, so the behavior of the right half can be easily inferred. For example, the CWDM 132 directs a wavelength in the 1.3 μm region to the receiver 105, a wavelength in the 1.4 μm region to the demultiplexer 114, and a wavelength in the 1.55 μm region to the demultiplexer 124. Similarly, the demultiplexer 114 directs the optical signal on the input port 140 to a specific output port according to the wavelength.

This is the carrier wavelength λ₁₁Routed to one output port of a 10 Gb / s channel with carrier wavelength λ₁₂Routes another 10 Gb / s channel with to another output port. Each of the 10 Gb / s channels is then distributed to the receiver 115, where it demodulates and converts the optical signal into an electrical signal. The lower part of FIG. 1 deals with transmission in the 1.55 μm region, where 16 channels are transmitted from transmitter 121 to receiver 125. In this wavelength region, the erbium amplifier 123 is preferred.

The optical multiplexer 122 and demultiplexer 124 are functionally similar to the optical multiplexer 112 and demultiplexer 114 described above. In the figure, the transmitter 121 and the receiver 125 communicate digital information at a rate of 2.5 Gb / s. Transmitters (101,111,121), receivers (105,115,125), multiplexers (112,122), demultiplexers (114,124) are known to those of skill in the art.

It is a great advantage of the present invention that the span of the optical fiber 130 in the network 100 is as long as 200 km at a bit rate of 10 Gb / s without the need for dispersion compensation. The reason this is possible is that the dispersion of the optical fiber 130 is 8.0 ps / nm-km or less in the 1.4 μm wavelength region, and the data rate of the WDM channel in the 1.55 μm region is 2.5 Gb / s or less. Such distances are long enough to cover almost all of the city's network.

FIG. 2 shows a second embodiment of a high-capacity optical fiber network 200 having a WDM channel in the 1.4 μm wavelength region operating at a maximum speed of 100 Gb / s and an analog CATV channel in the 1.55 μm wavelength region. The elements used in the network 200 are almost the same as those used in the network 100. However, the transmitter 141 and the receiver 145 are suitable for transmitting and receiving TV broadcast signals in the 1.55 μm region using the degenerate sideband (AM-VSB) modulation of amplitude modulation.

AM-VSB analog modulation is sensitive to noise (weak to noise) in that the image of the TV picture deteriorates when an invalid signal is added. Figure 2 discloses a technique for transporting many digitally transmitted optical signals along with broadcast TV signals, in which the TV optical channel is from the Super Head End at a preferred wavelength of wide dispersion (ie, 1550 nm). is there.

When transmitting multiple wavelengths over a single optical fiber, there are several non-linear mechanisms that propagate signal energy from one wavelength to another. One of these mechanisms is excited Raman scattering, which brings energy to higher wavelengths, especially when there are multiple wavelengths carried over a single optical fiber. It is especially problematic to provide a means of propagation. The following discussion is an excerpt from Non-Patent Document 4.<nplcit num="4"><text>Kaminow and Koch's "Optical Fiber Telecommunications IIIA", pp. 139-248</text></nplcit>

Excited Raman scattering (SRS) is a non-linear parametric interaction between the vibrations of light and the vibrations of molecules. The light incident on one optical fiber is partially scattered and the frequency is downshifted. This change in optical frequency corresponds to the molecular vibrational frequency. Excited Raman scattering (SRS) is similar to excited Brilliant scattering (SBS), but occurs either forward or backward.

The Raman gain coefficient is about three orders of magnitude smaller than the Brilliant gain coefficient, so that in a single channel system, the SRS threshold is about three orders of magnitude higher than the SBS threshold. However, the gain bandwidth of SRS is on the order of 12 THz, that is, 120 nm, which is much larger than that of SBS. This SRS combines separate channels within the WDM system, resulting in crosstalk.

With reference to FIG. 7 of the same document, the figure shows the Raman gain in a fiber made of fused silica. In WDM systems, longer wavelength signals due to SRS are amplified by good shorter wavelength signals, which causes degradation at shorter signals and noise at longer wavelengths. The SRS channel wavelengths that are separated by up to 140nm binds Le significantly reduced if it exceeds 120 nm.

If there are signals that carry a large number of wavelengths, as the preferred method for high-density WDM transmission systems, and most efficiently, these signals are cumulatively carried over wavelengths up to 120 nm longer. Energize the signal to be produced. For AM-VSB signals propagating within the 1.55 μm region, as shown in Figure 2, this means that any signal on the wavelength between 1430 nm and 1550 nm will energize the AM-VSB signal and degrade it. means. As a result, on the one hand, broadcast TV signals must be separated as much as possible, and on the other hand, optical fibers for as many active wavelengths as possible need to be developed.

FIG. 3 shows the characteristics of linear dispersion (303) and transmission loss (301) of an optical fiber suitable for use in the present invention. The loss in the wavelength region shown here is mainly due to Rayleigh scattering and hydroxyl ion (OH) absorption. Rayleigh scattering is a phenomenon that results from variations in the density and composition of an optical fiber within a material. Rayleigh scattering is 1 / λ⁴Is proportional to. Here, λ is the wavelength of light.

It is important to set a lower limit for fiber optic loss. For example, the loss in the 1.4 μm region is determined by the number of OH ions present in the glass. Such losses result from light wave energies absorbed by OH ions at wavelengths associated with different vibration modes. Therefore, operation within the 1.4 μm region is excluded for long-range optical transmission (ie, 10 km or more) because of the loss due to energy absorption by the OH ions.

This loss is at the water peak 302, as shown in FIG. 3, which is that of a conventional glass fiber. Such losses do not appear in the glass fibers used in the present invention. The specifications of the optical fiber suitable for use in the present invention are shown. However, it does not define the overall range of optical fibers acceptable in the present invention, but is for illustration purposes only.

-------------------------------------------------- ------------- Typical optical fiber specifications ------------------------------ --------------------------------- Maximum attenuation: 1310nm 0.35-0.40dB / km 1385nm 0.31dB / km 1550nm 0.21 -0.25dB / km Mode field diameter: 1310nm 9.3 ± 0.5μm 1550nm 10.5 ± 1.0μm Core / clad layer non-concentricity &lt;0.6μm Clad layer diameter 125 ± 1.0μm Cutoff wavelength &lt;1260nm Zero dispersion wavelength 1300-1322nm dispersion&gt; 0.8ps / nm-km (1335) -1435nm) Dispersion gradient &lt;0.092ps / nm2-km (maximum) Microbending &lt;0.5dB at 1550nm (1 rotation at 32mm diameter) Microbending &lt;0.05dB at 1550nm (100 rotations at 75mm diameter) Coating layer diameter 245 ± 10μm Tensile strength 100kpsi ----------------------------------------------- ---------------- ----------------

The present invention uses a low-loss optical fiber in the 1.4 μm wavelength region. Here, the low loss means that the amount of attenuation received by the optical signal is significantly lower than that of the conventional single-mode optical fiber. A method for manufacturing such an optical fiber will be described below.

FIG. 4 shows the steps of a method for manufacturing an optical fiber having a low loss at 1385 nm. The first three steps (41-43) relate to the production of core rods with low OH content (ie, 0.8 ppb or less) coated with glass tubes. Therefore, the first three steps can be replaced by a single step of forming a core rod with a clad layer / core ratio of 7.5 or less and an OH content of 0.8 ppb or less. Preferably, the core rod is formed by the Vapor Axial Deposition (VAD) process shown in step 41, as described below.

Manufacture of Core Rods With reference to Figure 5, the VAD process in which glass particles or "suits" are deposited on silica starting rods is described. The core rod 50 has a core 51, and the refractive index of the core 51 is larger than that of the clad layer 52. Light bends in the direction of the region of high refractive index, which is the law of physics that guides light along the central axis of an optical fiber. Fuel (eg, oxygen and hydrogen) and raw materials (eg, GeCl) on the torch 501 to form a region of high index of refraction₄And SiCl₄) Is supplied, and the raw material that has become steam in the flame with the torch is flowed toward the center of the glass rod.

This frame causes the raw materials to react and deposit glass particles (suit = soot) on the core rod 50. The core rod extends vertically at the top of its initial deposit site. It then rotates and moves upwards in the vertical direction, resulting in the deposition of glass suits along and around the entire length direction. Another torch 502 is used to deposit the clad layer 52 on top of the core 51. The raw material used in the torch 502 to form the clad layer 52 is, for example, SiCl.₄Is. Ge doping of the core 51 is one way to form a core with a higher index of refraction than the clad layer.

Alternatively, SiCl is used to dope the deposited clad layer with fluorine to form a clad layer with a lower refractive index than the core.₄Is the raw material used to form the core 51. In such a situation, for example SF₆, CCl₂F₂, CF₄Fluoride such as SiCl in the torch 502 for the clad layer₄Mix with. Details of various optical fiber manufacturing methods are described in Non-Patent Document 5. In particular, Section 4.4.4 (pages 169-180) deals with the VAD process.<nplcit num="5"><text>Chapter 4 in "Optical Fiber Telecommunications II" (Academic Press, Inc., (C) 1988 AT &amp; T and Bell Communications Research, Inc.)</text></nplcit>

In the above VAD process, the diameter (D) of the deposited clad layer is 7.5 times or less the diameter (d) of the core. Since the formation of the core rod is an expensive process, saving time for the production of the core rod directly leads to the cost reduction of the optical fiber. The actual amount of VAD volume required for the core rod is (D / d)²Is proportional to. As the core rod D / d decreases, so does the need for purity of the overclad layer tube.

By reducing the D / d, the optical power in the optical fiber propagates in the overclad tube, and impurities such as OH ions further increase the absorption loss. This is because the OH ion is mobile and moves especially toward the core during the operation of pulling out the optical fiber. And to make matters worse, the OH ions break down into hydrogen, which is much more mobile than the OH itself, resulting in diffusion into the core of the fiber optic during the drawing operation.

OH ions are formed there by the reaction between hydrogen and atomic defects in the core of the optical fiber. Core rods with a clad layer / core ratio of 2.0 or less require an overclad tube with an unusually low OH content, which is currently not cost effective. Therefore, the practical range of the deposited clad layer / core ratio is 2.0 &lt;D / d &lt;7.5.

Step 42 of FIG. 4 shows that dehydrogenation is performed by placing the core rod in a chlorine or fluorine-containing atmosphere at a temperature of about 1200 ° C. In this step, the core rod is a porous soot body, for example, chlorine gas easily enters the photons of this soot body and replaces OH ions with chlorine ions, resulting in a soot body with almost no water content. This OH ion substitution rate is related to the flow rate of chlorine gas and the temperature of dehydrogenation.

Step 43 in FIG. 4 means coalescing and strengthening the core rods by placing them in a helium atmosphere at a temperature of 1500 ° C. The coalescence strengthening in this step is to convert the porous soot rod into a dense glass with no particle boundaries. Details of the dehydrogenation step and the coalescence strengthening step are disclosed in Patent Document 5.<patcit num="5"><text>U.S. Pat. No. 3,933,454</text></patcit>

Step 44 of FIG. 4 shows that the core rod is elongated using an oxygen-hydrogen torch. This is the most cost-effective way to supply the large amount of heat required for this step. Alternatively, this step is performed using a hydrogen-free plasma torch as described below. And this eliminates the need for etching (step 45). Core rods grown by the VAD process are usually too large to fit into an overclad tube of suitable size and are usually stretched to reduce their diameter before insertion.

Stretching is done on a glass lathe, and its structure is known. In order to rotate this core rod, it is mounted between the headstock and the tailstock on the lathe. As the core rod rotates, the torch moves at a constant speed in the direction of the headstock along the central axis. At the same time as moving the torch, the tailstock is moved away from the headstock, which stretches the core rod and reduces its diameter.

Explosive gases such as hydrogen and oxygen flow into the torch at flow rates of 30 liters / minute (lpm) and 15 lpm, respectively. The use of hydrogen is commercially common, which forms an OH layer on the surface of the core rod. Stretching of the core rod is conventionally known and is disclosed in, for example, Patent Document 6.<patcit num="6"><text>U.S. Pat. No. 4,578,101</text></patcit>

Etching the core rod Step 45 etches the elongated core rod with a hydrogen-free plasma torch. The details of the plasma etching process are described below. Other etching techniques can be used to remove OH ions from the surface of the core rod, but these other etching techniques include, for example, mechanical polishing, chemical etching.

Isothermal plasma can be used to rapidly remove (etch) silica and silicate glass from the outer surface of the glass rod (see Patent Document 7 for this). When using an isothermal plasma torch, the main mechanism of material removal is vaporization due to the high temperature of the plasma (usually reaching levels above 9000 ° C in the center of the plasma). By bringing the conductive fireball into contact with the surface of the refractory dielectric, energy is transferred to the surface and the surface temperature is raised above the vaporization temperature of the dielectric material.<patcit num="7"><text>U.S. Pat. No. 5,000,771</text></patcit>

The overall cost of the optical fiber can be reduced by using a larger overclad tube. Preferably the overclad tube contains synthetic silica, which has high purity / low damping / high tensile strength. The purity of the overclad tube determines how close the tube can be placed to the core. Step 46 is to coat the core rod with a glass tube that has a sufficiently low level of OH, which means that the lower the D / d value, the more pure the tube (ie, its OH). Content should be low). For example, the table below shows the various OH concentration levels in the overclad tube, and these are low enough to be used in the present invention.

-------------------------------------------------- ------------- D / d OH concentration -------------------------------- ------------------------------- 7.5 &lt;200ppm 5.2 &lt;1.0ppm 4.4 &lt;0.5ppm -------- -------------------------------------------------- ----- -----

Step 47 of FIG. 4 shows a cross section of the preform 60 formed by collapsing a glass tube onto the core rod. Collapsing is performed using a ring-shaped torch to heat the overclad tube 62, which surrounds the core rod 50 and extends vertically. Details of this process are disclosed in Patent Document 8. Finally, step 48 in FIG. 4 shows the process of pulling the optical fiber from the end of the heated (about 2000 ° C) preform.<patcit num="8"><text>U.S. Pat. No. 4,820,322</text></patcit>

The present invention can be applied to a multiplexing channel different from the examples shown in FIGS. 1 and 2. The use of semiconductor optical amplifiers other than erbium amplifiers or Raman amplifiers, non-uniform spacing of WDM channels, and operation at data rates other than those disclosed herein are also modifications of the present invention.

<figref num="1">First Example of a high capacity fiber optic network including WDM channels in the 1.4 μm wavelength region operating at speeds up to 10 Gb / s and WDM channels in the 1.55 μm wavelength region operating at speeds up to 2.5 Gb / s. It is a figure showing.</figref><figref num="2">FIG. 5 illustrates a second embodiment of a high capacity fiber optic network including WDM channels in the 1.4 μm wavelength region operating at speeds up to 10 Gb / s and analog CATV channels in the 1.55 μm wavelength region.</figref><figref num="3">It is a graph which shows the dye (linear) dispersion and transmission loss of the optical fiber used in this invention.</figref><figref num="4">It is a flowchart which shows the step of manufacturing the optical fiber used in this invention.</figref><figref num="5">It is a figure which shows the method of manufacturing a core rod by an axial deposition process.</figref><figref num="6">It is sectional drawing of the optical fiber preform used for drawing out the glass optical fiber used in this invention.</figref><figref num="7">It is a graph which shows the relationship of Raman gain coefficient vs. frequency shift for molten silica at a pump wavelength of 1500 nm.</figref>

Code description

50 Core Rod 51 Core 52 Clad Layer 501,502 Torch 60 Preform 62 Overclad Tube 100,200 High Capacity Fiber Optic Network 101,111,121,141 Transmitter 103,113,123 Amplifier 105,115,125,145 Receiver 110,210 Output Port 112,122 Optical Multiplexer 114,124 Demultiplexer 130 Fiber Optic 131,132 Coarse WDM (CWDM) 140,240 Input Port 301 Loss 302 Forter Peak 303 Dispersion