CN102193140B - Optical fiber and optical communication system comprising the same - Google Patents

Abstract

The invention relates to an optical fiber and the like, which can be applied to an optical communication system for Raman amplification, and can achieve both improvement of OSNR and suppression of bending loss. The optical fiber is a silica optical fiber, has a concave refractive index curve composed of at least a core, a low refractive index inner cladding, and an outer cladding, and has an effective cross-sectional area A _eff greater than or equal to 110 ^μm at a wavelength of 1550 nm, and Fiber cutoff wavelength λ _C greater than or equal to 1.3 μm and less than or equal to 1.53 μm. The concave refractive index curve is designed such that the ratio of the inner cladding diameter to the core diameter Ra (=2b/2a) is greater than or equal to 2.5 and less than or equal to 3.5, and the relative refractive index difference between the inner cladding and the outer cladding Δ ^- greater than or equal to the bending loss at the wavelength used is the smallest relative refractive index difference Δ ^- _min and less than or equal to Δ ^- _min + 0.06%.

Description

Optical fiber and optical communication system including optical fiber

technical field

The present invention relates to an optical fiber and an optical communication system using the optical fiber, which can improve the optical signal-to-noise ratio (hereinafter referred to as OSNR: Optical Signal) in a wavelength division multiplexing optical communication system with a long relay distance using Raman amplification. -to-Noise Ratio).

Background technique

In optical communication systems, techniques for compensating waveform distortion due to dispersion using digital signal processing (DSP) in a receiver have been developed in recent years, as typified by digital coherent reception techniques. Along with this, compared with improving the dispersion value in the optical transmission path, the demand for improvement of OSNR is rapidly increasing. In a communication system where the interval between repeaters is fixed, it is necessary to increase the OSNR in order to increase the transmission rate without deteriorating the signal quality. For example, in order to double the transmission speed, it is necessary to double the OSNR (increase by 3 dB). Also, when increasing the relay distance while maintaining the transmission speed, the OSNR must also be increased. For example, to increase the repeater spacing by 5km, the OSNR needs to be increased by 1dB.

For increasing the OSNR of an optical communication system, it is effective to increase the effective cross-sectional area A _eff of an optical fiber which is a transmission medium, and to decrease the transmission loss of the optical fiber. By increasing the effective cross-sectional area A _eff , even when a high-power signal light is transmitted through the optical fiber, the occurrence of nonlinear optical phenomena can be sufficiently suppressed. Therefore, the optical fiber with increased effective cross-sectional area A _eff can allow greater incident signal optical power. Since the optical power of the incident signal is relatively large, the optical power of the received signal is correspondingly large, so the OSNR can be increased.

In addition, if the transmission loss is low, even if the incident signal optical power is the same, a larger signal optical power can be received at the receiving end. In this case, OSNR can also be increased.

Japanese Patent No. 4293156 (Document 1) discloses an optical fiber having an effective cross-sectional area A _eff of 110 μm ² or more and a transmission loss of 0.180 dB/km or less. This optical fiber has a depressed clad (depressed clad) type refractive index curve, and is composed of a core, an inner clad, and an outer clad in order from the center of the optical axis. The core diameter 2a is 11.5-23.0 μm, the diameter ratio 2b/2a of the inner cladding to the core is 1.1-7, the relative refractive index difference Δ ⁺ of the core to the outer cladding is 0.15%-0.30%, and the inner The relative refractive index difference ^Δ- of the cladding layer relative to the outer cladding layer is -0.15%～-0.01%.

In M.Bigot-Astruc et al., "Trench-Assisted Profiles for Large-Effective-Area Single Mode Fibers", Mo.4.B.1, ECOC2008 (Document 2), discloses an optical fiber which effectively cuts The area A _eff was 120 μm ² , and the transmission loss was 0.183 dB/km. The fiber has a grooved index profile.

Contents of the invention

As a result of studying the above-mentioned prior art, the inventors found the following problems. That is, in an actual optical communication system, both ends of a transmission optical fiber are connected to equipment such as a repeater or a transmitter/receiver. Both ends of the device are composed of general-purpose single-mode fiber (hereinafter referred to as SMF: Single-Mode Optical Fiber) or non-zero dispersion-shifted fiber (hereinafter referred to as NZDSF: Non-Zero Dispersion-Shifted Optical Fiber). Connect to optical fiber for transmission by fusion splicing or connector connection. Alternatively, the optical fiber for transmission may be connected to another optical fiber for transmission of a different type in the middle of the transmission path. If the effective cross-sectional area A _eff of the optical fiber for transmission is too large compared with the optical fibers at both ends of the equipment or other optical fibers for transmission, the connection loss will increase, and thus the OSNR of the optical communication system as a whole will deteriorate.

In addition, distributed Raman amplification is often used in long-distance optical communication systems, but if the effective cross-sectional area A _eff of the optical fiber for transmission is large, the Raman amplification efficiency will decrease. Therefore, in order to obtain a desired gain, a huge pump light power is required. According to the above situation, the effective cross-sectional area A _eff of the optical fiber for transmission is not as large as possible, and must be set to an optimal value. In the prior art described above, there is no report of an optical fiber in which the effective cross-sectional area A _eff is optimized so that the OSNR can be improved in consideration of the connection loss.

In general, an increase in the effective cross-sectional area A _eff in an optical fiber is accompanied by an increase in bending loss. In the optical fiber disclosed in Document 1 above, a depressed cladding type refractive index profile is used to suppress bending loss, but care must be taken to design the profile so that leakage loss due to fundamental mode cutoff does not occur. The above-mentioned document 1 describes the following content, that is, "when the absolute values of the relative refractive index difference Δn ⁺ of the core with respect to the outer cladding and the relative refractive index difference Δn ^- of the inner cladding with respect to the outer cladding are equal to each other, In this case, the fundamental mode light does not propagate in the optical fiber" (see paragraph number "0047" of the above document 1). However, this is only so as not to cause fundamental mode cutoff at the wavelength used. Actually, at a wavelength shorter than the cutoff wavelength of the fundamental mode, the fundamental mode transmission light starts to leak, and the transmission loss increases. It is necessary to suppress leakage loss due to fundamental mode cut-off in the entire used wavelength band (1530nm to 1625nm).

The present invention was made to solve the above-mentioned problems, and an object of the present invention is to provide an optical communication system having a connection structure for connecting devices and other types of optical fibers and performing Raman amplification and optical fibers that can be used therefor. In particular, the target optical fiber is an optical fiber having a configuration for achieving OSNR improvement, having a depressed cladding type refractive index profile that can suppress bending loss while avoiding leakage loss due to fundamental mode cutoff. In addition, this optical communication system as a whole uses a technique in which the receiver compensates for waveform distortion due to dispersion, such as a digital coherent reception technique, so that improvement of the fiber dispersion value does not need to be considered.

The optical fiber involved in the present invention is a silica-based optical fiber, which has, as optical characteristics, a transmission loss of 0.19 dB/km or less at a wavelength of 1550 nm, an effective cross-sectional area A _eff of 110 μm ² or more at a wavelength of 1550 nm, and a value greater than or equal to Or the fiber cut-off wavelength λ _C equal to 1.3 μm but less than or equal to 1.53 μm. In addition, in order to realize a depressed cladding type refractive index profile, the optical fiber has: a core made of pure silica having a refractive index n ₁ and a diameter 2a; an inner cladding provided on the outer periphery of the core and having a refractive index n ₂ , a diameter 2b; and an outer cladding disposed on the outer periphery of the inner cladding and having a refractive index n ₃ .

In the optical fiber according to the present invention, the refractive index n ₃ preferably satisfies n ₁ >n ₃ >n ₂ (Condition 1). Preferably, the relative refractive index difference Δ ^- (=100×(n ₂ -n ₃ )/n ₃ ) of the inner cladding relative to the outer cladding satisfies -0.12% ≤ Δ ^- ≤ -0.06% (condition 2). In addition, it is preferable that the ratio Ra (=2b/2a) of the inner cladding diameter to the core diameter satisfies 2.5≦Ra≦3.5 (condition 3).

In addition, the optical fiber according to the present invention can be used in a medium for transmitting light having a wavelength of less than or equal to 1625 nm. The fiber has a low-loss pure silica core and has a depressed-cladding index profile that is excellent in terms of bend loss. For this optical fiber, leakage loss reduction is achieved by setting the fundamental mode cutoff wavelength λ _FC to be greater than or equal to 2400 nm.

The optical fiber according to the present invention may satisfy the following two conditions in addition to the above-mentioned condition 1 and condition 3 while having the above-mentioned optical characteristics and a depressed-cladding type refractive index curve. That is, when the fundamental mode cutoff wavelength at which the fundamental mode starts to leak at the upper limit of the wavelength used is _λFCup , the fundamental mode cutoff wavelength _λFC is set to a wavelength longer than _λFCup (condition 4). In addition, as for the relative refractive index difference Δ ^- (=100×(n ₂ -n ₃ )/n ₃ ) of the inner cladding layer with respect to the outer cladding layer, Δ ^- which minimizes the bending loss at the wavelength used is Δ ^- min, Δ ^- is designed to be greater than or equal to Δ ^- _min and less than or equal to Δ ^- _min + 0.006% (condition 5).

In addition, in the optical fiber according to the present invention, the effective cross-sectional area A _eff at a wavelength of 1550 nm may be equal to or less than 150 μm ² so that transmission up to 1625 nm can be performed as the upper limit wavelength. In the relay span of the optical communication system using the optical fiber, it is assumed that at least two or more points are connected, and it is assumed that Raman amplification is performed in the relay span, and the optical communication system as a whole ensures OSNR improvement greater than or equal to 1dB. In addition, in order to ensure better OSNR, the effective cross-sectional area A _eff at a wavelength of 1550 nm may be greater than or equal to 120 μm ² and less than or equal to 140 μm ² .

An optical communication system according to the present invention has an optical fiber (optical fiber according to the present invention) having the above-mentioned structure. In addition, the optical communication system performs Raman amplification on the transmitted light in the optical fiber. In this case, as a condition for single-mode transmission of excitation light for Raman amplification, it is preferable that the optical fiber has a fiber cutoff wavelength of 1.45 μm or less.

The optical communication system according to the present invention constitutes an optical communication system having a relay span (long relay span) of 80 km or more using a plurality of first optical fibers and a plurality of second optical fibers. In this case, the plurality of first optical fibers are installed at two or more positions in the relay span of 80 km or more. The plurality of second optical fibers are connected to the first optical fibers at four or more positions in the relay span. In addition, each of the plurality of second optical fibers has an effective cross-sectional area A _eff of 85 μm ² or less at a wavelength of 1550 nm. At least any one of the plurality of second optical fibers is installed at both ends of the relay span, or at one position in the relay span. That is, in this optical communication system, it is assumed that the first optical fiber is connected to the second optical fiber having a smaller effective cross-sectional area A _eff of less than or equal to 85 μm ² at more than or equal to 4 positions, since the first optical fiber has a 110 μm ² ~150μm ² large effective cross-sectional area A _{e ff} , so the input of excitation light for Raman amplification with higher power can be carried out. On the other hand, since the first optical fiber has a wavelength of less than or equal to 0.19dB/km The low transmission loss enables optical transmission over long relay spans. As a result, an OSNR improvement of 1 dB or more can be realized in the entire optical communication system.

Preferably, each of the first optical fibers has a transmission loss of 0.19 dB/km or less at a wavelength of 1550 nm, an effective cross-sectional area A _eff of 110 μm or ^more and 150 μm or ^less at a wavelength of 1550 nm, and 1.3 μm or more. Fiber cut-off wavelength λ _C less than or equal to 1.45 μm. In this case, the optical communication system performs Raman amplification on propagating light in each first optical fiber.

In order to realize a depressed-cladding type refractive index profile, each first optical fiber may also have: a core composed of pure silica having a refractive index n ₁ and a diameter 2a; an inner cladding disposed on the outer periphery of said core, And has a refractive index n ₂ , a diameter 2b; and an outer cladding provided on the outer periphery of the inner cladding and has a refractive index n ₃ . In addition, it is preferable that the refractive index n ₃ satisfies the above-mentioned condition 1. It is preferable that the ratio Ra of the inner cladding diameter to the core diameter satisfies the above-mentioned condition 3. Furthermore, it is preferable that each first optical fiber satisfies the above-mentioned condition 4 and condition 5 at the same time.

In addition, each of the first optical fibers used in the optical communication system according to the present invention is preferably a medium for transmitting light having a wavelength of 1625 nm or less, has the above-mentioned depressed-clad type refractive index curve, and satisfies the above-mentioned condition 1 , condition 2 and condition 3. In this case, the effective cross-sectional area A _eff of each of the first optical fibers is preferably greater than or equal to 120 μm ² and less than or equal to 140 μm ² .

Description of drawings

FIG. 1 is a graph showing the relationship between the effective cross-sectional area A _eff and the amount of improvement in OSNR.

2A and 2B are diagrams showing a cross-sectional structure of an embodiment of an optical fiber according to the present invention and its refractive index curve.

FIG. 3 is a graph showing the relationship between the wavelength at which leakage loss starts to occur and the fundamental mode cutoff wavelength.

Fig. 4 is a graph showing the relationship between the bending loss and the cutoff wavelength of the fundamental mode at a diameter of 20 mm.

FIG. 5 is a table summarizing structural parameters and optical characteristics of optical fibers according to the present embodiment (Examples 1 to 15) and optical fibers according to comparative examples.

FIG. 6 is a table summarizing the Young's modulus of the coating resin and the microbend loss when the effective cross-sectional area A _eff is changed for a plurality of samples of the optical fiber according to the present embodiment.

7A to 7C are diagrams showing the improvement of OSNR with respect to single-mode fiber (SMF) due to the difference in the connection state in the relay span, using the relationship between the effective cross-sectional area A _eff and the transmission loss.

FIG. 8 is a flowchart for explaining the optical fiber structure determination operation according to this embodiment.

9A and 9B are diagrams showing configurations of various embodiments of the optical communication system according to the present invention.

Detailed ways

Hereinafter, each embodiment of the optical fiber and the optical communication system according to the present invention will be described in detail using FIGS. In addition, in the description of the drawings, the same reference numerals are attached to the same elements, and overlapping descriptions are omitted.

First, the optimum value of the effective cross-sectional area A _eff of the optical fiber according to the present embodiment, which can be applied to a transmission optical fiber of an optical communication system, will be described. That is, from the viewpoint of the characteristics of an optical fiber, OSNR can be approximately represented by the following formula (1).

OSNR(dB)∝10log Aeff×α(1/km))-α _sp (dB)×N-α(dB/km)×L(km) (1)

Here, A _eff is the effective cross-sectional area of the optical fiber at the signal light wavelength, α is the transmission loss at the signal light wavelength, α _SP is the connection loss, N is the number of connection positions per 1 relay span, and L is the Fiber length for 1 trunk span.

The first term of the above formula (1) is related to the allowable incident light when the nonlinear phase shift generated by self-phase modulation (SPM: Self-Phase Modulation), which is one of the nonlinear optical phenomena generated in the optical fiber, is fixed. corresponding to the power. The nonlinear phase shift amount Φ _SPM is obtained according to the following formula (2).

${\mathrm{<span\; class="notranslate">\; \&phi;</span>}}_{\mathrm{<span\; class="notranslate">\; SPM</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}}{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}\frac{{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}{{\mathrm{<span\; class="notranslate">\; A</span>}}_{\mathrm{<span\; class="notranslate">\; eff</span>}}}<span\; class="notranslate">\; \&Center\; Dot;</span>{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; eff</span>}}<span\; class="notranslate">\; \&Center\; Dot;</span>{\mathrm{<span\; class="notranslate">\; P</span>}}_{\mathrm{<span\; class="notranslate">\; in</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$

Here, λ is the wavelength of the signal light, n ₂ is the nonlinear refractive index of the fiber, L _eff is the effective length of the fiber, and P _in is the incident light power.

As long as the fiber length is sufficiently long, for example greater than or equal to 50km, the effective length L _eff can be approximated as According to the above formula (2), when the nonlinear refractive index n ₂ is fixed and the nonlinear phase shift Φ _SPM is fixed, the allowable incident power increases in proportion to A _eff (μm ² )×α(1/km) .

The second term of the above formula (1) corresponds to the connection loss of the optical fiber. If only the mismatch of the mode field diameter (hereinafter referred to as MFD: Mode Field Diameter) between the two connected fibers is considered, the connection loss ( dB) can be estimated according to the following formula (3).

${\mathrm{<span\; class="notranslate">\; \&alpha;</span>}}_{\mathrm{<span\; class="notranslate">\; sp</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; dB</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 10</span>}\mathrm{<span\; class="notranslate">\; log</span>}<span\; class="notranslate">\; [</span>{<span\; class="notranslate">\; (</span>\frac{{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; W</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; W</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}{{\mathrm{<span\; class="notranslate">\; W</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; W</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; ]</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 3</span>}<span\; class="notranslate">\; )</span>$

Here, W ₁ is the MFD of the optical fiber according to this embodiment, and W ₂ is the MFD of the optical fiber at both ends of the equipment or other transmission optical fibers.

As another type of optical fiber for transmission, for example, a single-mode fiber (SMF: Single Mode Fiber) with A _eff =80 μm ² , MFD ₌ 10.1 μm at a wavelength of 1550 nm, or A _eff =50 to 70 μm ² , MFD = 8 ~10 μm dispersion-shifted fiber (DSF: Dispersion-Shifted Fiber) and non-zero dispersion-shifted fiber (NZDSF: Non-Zero Dispersion-Shifted Fiber). According to the second term of the above formula (1), if A _eff is larger than the optical fibers at both ends of the equipment or other transmission optical fibers, the MFD mismatch becomes larger and the connection loss becomes larger. In addition, the third term of the above formula (1) represents the transmission loss of the optical fiber. Hereinafter, the characteristics depending on the wavelength are assumed to be characteristics at a wavelength of 1550 nm unless the wavelength is particularly shown.

FIG. 1 is a graph showing the relationship between the effective cross-sectional area A _eff and the amount of improvement in OSNR. In addition, in FIG. 1, the graph which shows the OSNR of each connection structure in which the number N of connection positions in 1 relay span is 0, 2, and 4 is shown. In FIG. 1 , the vertical axis represents the improvement amount of OSNR compared to the case of using SMF (A _eff =80 μm ² , transmission loss = 0.190 dB/km) as the optical fiber for transmission. In addition, in FIG. 1 , in order to evaluate the influence of the effective cross-sectional area A _{ef f} on the OSNR improvement amount, the transmission loss is all made the same as that of the SMF. In addition, W ₂ in the above formula (3) is the MFD of the SMF, specifically, 10.1 μm.

According to FIG. 1 , in the case of no connection position (N=0), the larger the effective cross-sectional area A _eff can improve the OSNR. Then, when there are two connection positions at both ends of one relay span (N=2), it corresponds to the state of being connected to optical fibers in equipment such as repeaters at both ends of the transmission optical fiber (see FIG. 9A ). . In this case, when the effective cross-sectional area A _eff is greater than or equal to 150 μm ² , the amount _of improvement in OSNR is substantially saturated. Therefore, even if the effective cross-sectional area A _eff continues to increase, the OSNR improvement effect is small. In addition, in order to suppress the pump light power of distributed Raman amplification within a practical range (less than or equal to several watts), the effective cross-sectional area A _eff must be less than or equal to 150 μm ² . Therefore, in order to make the OSNR improvement amount greater than or equal to 1 dB, it is only necessary that the effective cross-sectional area A _eff falls within the range of 110-150 μm ² . In the case where there are four connection positions in one relay span, it is equivalent to that there are further connections between the transmission optical fiber and another transmission optical fiber at two positions in the middle of the relay span. situation (see Figure 9B). In this case, the amount of OSNR improvement becomes maximum when the effective cross-sectional area A _eff is 135 μm ² . In addition, the condition for making the OSNR improvement amount greater than or equal to 1 dB is that the effective cross-sectional area A _eff falls within the range of 110-150 μm ² . It falls within a range in which the pump light power for Raman amplification is at a practical level.

2A and 2B are diagrams showing a cross-sectional structure of an embodiment of an optical fiber according to the present invention and its refractive index curve. The optical fiber 100 according to this embodiment has, as shown in FIG. 2A , a core 110 extending along a predetermined axis and having a refractive index n ₁ and a diameter 2a; an inner cladding 120 provided on the outer periphery of the core 110 has a refractive index n ₂ (<n ₁ ), a diameter 2b; the outer cladding 130, which is provided on the outer periphery of the inner cladding 120, has a refractive index n ₃ (<n ₁ ,>n ₂ ); and a cladding resin 140, It is provided on the outer periphery of the outer cladding 130 . Furthermore, the coating resin 140 is composed of a first coating resin 141 provided on the outer periphery of the outer cladding layer 140 , and a second coating resin 142 provided on the outer periphery of the first coating resin 141 .

In addition, in FIG. 2B, a refractive index profile 150 of the glass region in the optical fiber 100 shown in FIG. 2A is shown. In this refractive index curve 150 , a region 151 represents the refractive index of the core 110 , a region 152 represents the refractive index of the inner cladding 120 , and a region 153 represents the refractive index of the outer cladding 130 .

As a feature of the above-mentioned refractive index curve 150, for optical fibers with the same effective cross-sectional area A _eff , the optical fiber _with the refractive index curve 150 can suppress the bending loss compared with the optical fiber with the step-type refractive index curve. lower. However, in an optical fiber having a refractive index curve 150, it is known that as the wavelength becomes longer, the fundamental mode light starts to leak from the core to the cladding, causing fundamental mode cutoff at a certain wavelength. In addition, there is no fundamental mode cut-off in fibers with a step-type index profile, but the bending loss is large.

3 is a graph showing the relationship between the wavelength λ _LK (nm) at which leakage loss starts to occur and the fundamental mode cutoff wavelength λ _FC (nm). Here, the wavelength λ _LK is defined as that at which the leakage loss of the fundamental mode is greater than or equal to 20% of the transmission loss in an optical fiber with a step-type refractive index profile and a central core of substantially pure silica wavelength. In order not to cause leakage loss in the entire band of C-band to L-band (1530-1625nm: the wavelength band used in the optical communication system according to the present embodiment) used in optical communication (making the wavelength λ _LK greater than or equal to 1625nm), according to Fig. 3, it is necessary to make the fundamental mode cut-off wavelength λ _FC at least greater than or equal to 2400nm. In addition, #1 shown in FIG. 3 represents a comparative example shown in Table 1 of FIG. 5 described later.

FIG. 4 is a graph showing the relationship between bending loss and fundamental mode cutoff wavelength at a diameter of 20 mm. Specifically, when the relative refractive index difference Δ ⁻ of the inner cladding 120 to the outer cladding 130 and the ratio (2b/2a) of the diameter of the inner cladding 120 to the diameter of the core 110 are changed, the fundamental mode The relationship between the cutoff wavelength λ _FC (nm) and the bending loss at a diameter of 20 mm. In FIG. 4 , the relative refractive index difference Δ ⁺ of the core 110 with respect to the inner cladding 120 and the diameter 2a of the core 110 are adjusted so that the effective cross-sectional area A _eff is 135 μm ² and the cutoff wavelength λ in the LP11 mode is _C is 1350nm.

In the case of Raman amplification, in order to realize a single mode even at the excitation light wavelength, it is preferable that the fiber cutoff wavelength is less than or equal to 1.45 μm. The bending loss is the smallest when the ratio 2b/2a is 3.0. In order to suppress the bending loss at or below the SMF, that is, less than or equal to 20dB, the ratio 2b/2a needs to be 2.5 to 3.5, so that the relative refractive index difference Δ ^- is less than or Equal to -0.06%. In addition, in order to make the fundamental mode cut-off wavelength λ _FC greater than or equal to 2400nm, it is necessary that the relative refractive index difference Δ ^- is greater than or equal to -0.12% when the ratio 2b/2a is 3.0.

FIG. 5 is a table summarizing structural parameters and optical characteristics of optical fibers according to the present embodiment (Examples 1 to 15) and optical fibers according to comparative examples. In the comparative example shown in the table of FIG. 5 , the fundamental mode cut-off wavelength λ _FC is 2263 nm, which is longer than the signal light wavelength, the leakage loss starts to occur at 1441 nm (= λ _LK ), and the transmission loss at a wavelength of 1550 nm is 0.32dB/km, also higher. On the other hand, none of Examples 1 to 15 caused leakage loss in the optical communication band. In addition, the effective cross-sectional area A _{ef f} of Example 1 was 134 μm ² , and the bending loss was 8.0 dB/m, which were good. Other Examples 2 to 15 also have more excellent bending loss than the Comparative Example.

In addition, when the effective cross-sectional area A _eff is increased, the microbending loss is also increased. FIG. 6 is a table summarizing the Young's modulus of the coating resin and the microbend loss when the effective cross-sectional area A _eff is changed for a plurality of samples of the optical fiber according to the present embodiment. In FIG. 6 , the relationship between the glass diameter (the diameter of the outer cladding 130 ), the Young's modulus and the diameter of the first coating resin 141 , the Young's modulus and the diameter of the second coating resin 142 , and the diameter of the optical fiber are shown. A _eff , λ _C relative microbending loss. Here, the microbending loss is represented by the amount of loss increase when an optical fiber is wound around a spool with a diameter of 400 mm at a tension of 80 g, and the surface of the spool is covered with a wire mesh with a diameter of 50 μm and an interval of 100 μm.

From the comparison between samples 1 to 3 and samples 4 to 6 shown in FIG. 6, it can be seen that by reducing the Young's modulus of the first coating resin 141, between optical fibers having the same effective cross-sectional area A _eff , The optical fibers of samples 4-6 can reduce the microbend loss. In addition, from the comparison between sample 5 and sample 7, it can be seen that by increasing the Young's modulus of the second coating resin 142, the optical fiber of sample 7 can reduce the microbending loss between optical fibers having the same effective cross-sectional area Aeff.

Generally, an optical fiber with a lower Young's modulus of the first coating resin 141 and a higher Young's modulus of the second coating resin 142 has lower microbend loss. Specifically, it is preferable to set the Young's modulus of the first coating resin 141 within a range of 0.3 to 0.6 MPa, and to set the Young's modulus of the second coating resin 142 within a range of 700 to 1500 MPa.

As a method of reducing the microbend loss, there is also a method of increasing the diameter of the glass or the diameter of the coating resin 140 (including the first coating resin 141 and the second coating resin 142), but since it is different from the generally used optical fiber (Glass diameter: 125 μm, cladding diameter: 245 μm) becomes large, so it is not practical. Therefore, in the optical fiber according to this embodiment, the diameter of the glass is set to 125±1 μm, and the diameter of the second coating resin 142 is set to 240 to 250 μm.

7A to 7C are diagrams showing the improvement amount of OSNR with respect to SMF due to the difference in the connection state in the relay span, using the relationship between the effective cross-sectional area A _eff and the transmission loss. In particular, Fig. 7A uses the relationship between the effective cross-sectional area A _eff (μm ² ) and the transmission loss (dB/km) to show that a relay span of 80 km is formed by a transmission optical fiber not connected to the SMF. Figure 7B uses the relationship between the effective cross-sectional area A _eff (μm ² ) and the transmission loss (dB/km) to show the amount of improvement in OSNR relative to the SMF in the distance structure, which includes the transmission optical fiber connected to the SMF at two positions. In the 80km relay span structure, relative to the OSNR improvement of the SMF, Figure 7C uses the relationship between the effective cross-sectional area A _eff (μm ² ) and the transmission loss (dB/km) to show The amount of improvement in OSNR compared to SMF in the 80km relay span structure of optical fiber for transmission.

In addition, as described above, FIGS. 7A to 7C show the effective cross-sectional area A _eff and transmission loss as contour lines between the transmission optical fiber and the SMF according to this embodiment in one relay span. Graph of OSNR improvement when there are 0, 2, and 4 connection positions. The length of one relay span is 80 km, and as W ₂ in the above formula (3), the MFD of the SMF is 10.1 μm. In addition, the amount of OSNR improvement is based on the case of using SMF as the optical fiber for transmission.

From Figures 7A to 7C, it can be seen that if the effective cross-sectional area A _eff is 110 to 150 μm ² and the transmission loss is less than or equal to 0.19 dB/km, then even if there are 0 to 4 connection positions in one relay span , and the OSNR improvement amount can also be greater than or equal to 1dB. In addition, if the effective cross-sectional area A _eff is 120-140 μm ² and the transmission loss is 0.18 dB/km, if there are 0 or 2 connection positions in one relay span, the OSNR improvement amount can be Greater than or equal to 2dB. Furthermore, if the effective cross-sectional area A _eff is 120-140μm ² and the transmission loss is less than or equal to 0.17dB/km, even if there are 4 connection locations in one relay span, the OSNR can be improved by Greater than or equal to 2dB.

In addition, it is preferable that the fiber cutoff wavelength λ _C falls within the range of 1.3 to 1.53 μm. If it is less than this range, the bending loss becomes large. Conversely, if it exceeds this range, the signal light cannot be single-mode in the C-band (1530 to 1565 nm). And, more preferably, the fiber cut-off wavelength λ _C is less than or equal to 1450 nm. The reason for this is that since the wavelength of the pump light used in the distributed Raman amplification is about 1450 nm, the pump light becomes single-mode, and as a result, the excitation efficiency does not deteriorate.

In the optical fiber 100 according to this embodiment, since the core 110 that transmits most of the signal light power is made of pure silica substantially free of impurities, the transmission loss can be reduced, which is more preferable. However, a small amount of additives may be contained in the core 110 to such an extent that the transmission loss is not increased. For example, a trace amount of halogen elements or alkali metal elements may be contained in the core 110 . Specifically, the fiber core 110 may also contain chlorine Cl of less than or equal to 2 mol%, fluorine F of less than or equal to 1 mol%, and potassium K of less than or equal to 0.1 mol%.

In addition, the fiber with a pure silica core has a lower nonlinear refractive index n ₂ , which can further increase the allowable incident power when the nonlinear phase offset Φ _SPM is fixed. Thereby, OSNR can be further increased. In addition, the nonlinear refractive index n ₂ of the SMF with Ge added to the core 110 is 2.35×10 ^-20 (m ² /W), as opposed to the nonlinear refractive index n 2 of an optical fiber with a pure silica core n ₂ is 2.20×10 ^-20 (m ² /W).

Next, the optical fiber structure determination operation according to this embodiment will be described in detail using FIG. 8 . In addition, FIG. 8 is a flowchart for explaining the optical fiber structure specification operation according to this embodiment.

In determining the structure of the optical fiber 100, first, the material of the core 110 is determined (step ST1). A core of substantially pure silica is more preferred. The inner cladding 120 and the outer cladding 130 are preferably doped with fluorine (in a pure silica core, the reduction in transmission loss is small and the nonlinear refractive index n ₂ is also low).

In step ST2, according to the above formula (1) and formula (2), the effective cross-sectional area A _eff for improving the desired OSNR is determined. In addition, in step ST3, λ _C is determined such that a single mode is formed in the used wavelength band and bending loss does not increase. In step ST4, the relative refractive index difference Δ ⁺ of the core 110 relative to the inner cladding 120 and the diameter 2a of the core 110 are determined to obtain the above determined A _eff and λ _C .

On the other hand, in step ST5 , λ _FC is determined in such a manner that λ _LK is longer than the used band in FIG. 3 . In addition, in step ST6, the target bending loss is determined. In step ST7, the relative refractive index difference ^Δ- and the ratio 2b/2a of the inner cladding layer 120 relative to the outer cladding layer 130 are determined according to _FIG . , the first coating resin 141 and the second coating resin 142 are determined so as to suppress the microbending loss to a low level.

9A and 9B are diagrams showing configurations of various embodiments of the optical communication system according to the present invention. In addition, FIG. 9A shows the configuration of an optical transmission system in which there are two connection positions with the SMF in one relay span. In addition, FIG. 9B shows the configuration of an optical transmission system in which there are four connection positions with the SMF in one relay span.

Specifically, the optical communication system 200A shown in FIG. 9A has a transmitter 210 that outputs signal light, and a receiver 220 that receives signal light. A plurality of Repeaters 230A, 230B. The relay span is an optical transmission section between these repeaters 230A and 230B, and in this optical communication system 200A, this relay span includes one transmission optical fiber 100 (optical fiber according to the present embodiment). Both ends of the transmission optical fiber 100 are respectively connected to another transmission optical fiber (not shown) at the ends of the repeaters 230A and 230B. In the optical communication system 200A of FIG. There are two connection positions A1 and A2 in .

On the other hand, the optical communication system 200B shown in FIG. 9B also has a transmitter 210 that outputs signal light and a receiver 220 that receives signal light, and multiple communication channels are arranged on the transmission path between these transmitters 210 and receivers 220. repeaters 230A, 230B. The relay span is the optical transmission section between these repeaters 230A, 230B. In the optical communication system 200B, further, the relay span includes two transmission optical fibers 100 (optical fibers involved in this embodiment ) and another optical fiber 300 for transmission. This relay span has four connection positions B1 to B4 including the connection positions B1 and B4 between the ends of the repeaters 230A and 230B. That is, in the optical communication system 200B of FIG. 9B , there are four connection positions B1 to B4 in one relay span.

In addition, in either optical communication system 200A, 200B, the optical fiber 231 at both ends of the repeater 230A, 230B is a standard SMF (usually, the effective cross-sectional area A _eff at a wavelength of 1550nm is 80 μm ² , but sometimes it is 85 μm ² ). The effective cross _- sectional area A _eff of another transmission optical fiber 300 is smaller than that of a standard SMF (for example, 85 μm ² ).

As described above, according to the present invention, it is possible to improve OSNR in an optical communication system that performs Raman amplification, and achieve both avoidance of leakage loss due to fundamental mode cutoff and suppression of bending loss. In other words, as the optical communication system as a whole, along with the improvement of the performance of the optical receiver, it becomes less affected by the increase of the dispersion value of the optical fiber itself as the optical transmission path. The purpose is not to improve the dispersion value, but to improve the OSNR and bend The optical fiber according to the present invention in which loss is suppressed becomes effective.

Claims (6)

1. an optical fiber, it is quartzy type optical fiber, have under wavelength 1550nm, be less than or equal to 0.19dB/km loss, under wavelength 1550nm, be more than or equal to 120 μm ²and be less than or equal to 150 μm ²net sectional area A _eff, and be more than or equal to 1.3 μm and be less than or equal to the fiber cut off wavelength λ of 1.53 μm _c,

It is characterized in that having:

A fibre core that () is made up of pure quartz, it has refractive index n ₁, diameter 2a;

B () inner side covering, it is arranged on the periphery of described fibre core, and has refractive index n ₂, diameter 2b; And

C () outside covering, it is arranged on the periphery of described inner side covering, and has refractive index n ₃,

(d) described refractive index n ₃be less than described refractive index n ₁and be greater than described refractive index n ₂,

E the diameter of () described inner side covering is more than or equal to 2.5 relative to the diameter ratio Ra of described fibre core and is less than or equal to 3.5, wherein, and Ra=2b/2a,

F fundamental mode cutoff wavelength that the basic mode used under the wavelength upper limit is starting when leaking by () is set to λ _fCuptime, by fundamental mode cutoff wavelength λ _fCbe set greater than λ _fCupwavelength,

G () described inner side covering is relative to the refractive index contrast Δ of described outside covering ^-be more than or equal to-0.12% and be less than or equal to-0.06%, wherein, Δ ^-=100 × (n ₂-n ₃)/n ₃.

2. optical fiber according to claim 1, is characterized in that,

Net sectional area A under wavelength 1550nm _effbe less than or equal to 140 μm ².

3. optical fiber according to claim 2, is characterized in that,

Described optical fiber has the fiber cut off wavelength being less than or equal to 1.45 μm.

4. an optical communication system, in this optical communication system, the transmission path as relaying span has the optical fiber described in claim 1 or 2, and the feature of this optical communication system is,

Use digital coherent reception technique, this digital coherent reception technique, by receiver acknowledge(ment) signal light, uses the digital signal processing in receiver, compensates the waveform distortion of the flashlight that dispersion causes.

5. optical communication system according to claim 4, is characterized in that,

In described optical fiber, conveying light is by Raman amplifiction.

6. an optical communication system, it has:

Multiple 1st optical fiber, it is erected to be more than or equal to and is more than or equal to 2 positions in the relaying span of 80km, has the structure identical with the optical fiber of claim 3 respectively; And

Multiple 2nd optical fiber, it is more than or equal to 4 positions and described 1st Fiber connection in described relaying span and two ends thereof, has respectively and be less than or equal to 85 μm under wavelength 1550nm ²net sectional area A _eff, and one of them of described multiple 2nd optical fiber is erected at the two ends of described relaying span, or be erected at 1 position in described relaying span.