JP2006154713A - Single mode optical fiber, transmission system and multiple-wavelength transmission system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical fiber capable of improving the threshold power of induction Brillouin scatter as compared with conventional optical fibers, and to provide a transmission system using the optical fiber and a multiple-wavelength transmission system. <P>SOLUTION: In a single mode optical fiber comprising a core and a clad, the core has a certain refractive index distribution and the clad has an almost fixed refractive index. When it is defined that a distance from a center is r, the specific refractive index of the core to the clad is Δ(r)% and the maximum value of Δ(r) is Max(Δ(r)), relation of Δ(0)<Max(Δ(r)) is formed, and when it is defined that the electric field distribution of a basic mode of light propagated into the optical fiber is E(r), the displacement quantity distribution of an acoustic mode is Y(r), diameters when E<SP>2</SP>(r) and Y(r) are maximum values are r<SB>maxE</SB>and r<SB>maxY</SB>, the r<SB>maxE</SB>and r<SB>maxY</SB>are respectively different in the single mode optical fiber. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a single mode optical fiber (hereinafter referred to as an optical fiber) capable of suppressing the occurrence of stimulated Brillouin scattering (hereinafter referred to as SBS) and capable of transmitting with a higher power signal.

  In recent years, a fiber to the home (hereinafter referred to as FTTH) service has been started in which an optical fiber is extended to each home and various information is exchanged using the optical fiber.

  As one form of FTTH for transmitting various information, there is a system for simultaneously transmitting a broadcast signal and other communication signals using different optical systems using a single optical fiber (ITUT Recommendation G. 983.3). In general, in this system, the broadcast signal is often an analog signal, a baseband signal, or an optical SCM signal.

The characteristics of the system from the viewpoint of an optical fiber as a transmission medium are as follows.
FTTH is usually a double star PON (Passive Optical Network), and has a large distribution loss (usually a maximum of 32 branches is assumed).
-Since analog signals, baseband signals, or optical SCM signals are transmitted, it is necessary to increase the CNR (Carrier Noise Ratio) in the receiver, and the required minimum signal light power in the light receiving unit is necessary for digital transmission used in communication. Bigger than that.

  Therefore, in this system, it is necessary to increase the required signal light power in the signal input unit. In particular, considering attenuation and distribution loss during transmission of signal light, higher power is required for longer-distance lines and more branched lines. As a matter of course, it is advantageous from various viewpoints (construction cost, maintainability, system design, etc.) that signals can be transmitted as far as possible and distributed to many subscribers at the same time.

Non-Patent Documents 1 to 4 and Patent Document 1 are given as conventional techniques related to the present invention.
ARCharaplyvy, J. Lightwave Technol., Vol.8, pp.1548-1557 (1990) Okamoto, “Fundamentals of optical waveguides”, Corona (1992) Japanese Patent No. 2584151 K. Shiraki, et al., J. Lightwave Technol., Vol. 14, pp. 50-57 (1996) Y. Koyamada, et al., J. Lightwave Technol., Vol.22, pp.631-639 (2004)

  However, in optical transmission using an optical fiber, even if light exceeding the power existing in the optical fiber is made incident by SBS, which is a kind of nonlinear phenomenon, only a certain amount of light (hereinafter referred to as SBS threshold power) is used. There is a case in which the incident cannot be made and the rest becomes backscattered light and returns to the incident light side, which may limit the signal light power at the input unit, which has been a problem (for example, non- (See Patent Document 1).

  The present invention has been made in view of the above circumstances, and an optical fiber capable of suppressing the generation of SBS and increasing the SBS threshold power as compared with a conventional optical fiber in order to make higher power light incident on the optical fiber. An object of the present invention is to provide a transmission system using the same and a wavelength division multiplexing transmission system.

To achieve the above object, the present invention provides a single mode optical fiber comprising a core and a cladding,
The core has a refractive index profile, and the clad has a substantially constant refractive index. The distance from the center is r, the relative refractive index of the core with respect to the clad is Δ (r)%, and the maximum value of Δ (r) is Max. If (Δ (r)), Δ (0) <Max (Δ (r)) is established,
The electric field distribution of the fundamental mode of light propagating in the optical fiber is E (r), the displacement distribution of the acoustic mode is Y (r), the radius at which E ² (r) and Y (r) are maximum values, respectively. when the r _MAXE and _{r maxY,} to provide an optical fiber, characterized in _{that r MAXE} and _{r maxY} different.

  In the optical fiber of the present invention, it is preferable that the optical mode field diameter and the acoustic mode field diameter defined by E (r) and Y (r) are different.

  In the optical fiber of the present invention, the field diameter (MFD) has the following formulas (A) and (B):

( _Where MFD _petII is the mode field diameter as defined by Petermann II, w _p is the intervening variable of the derivation bond, r is the distance from the center, F (r) is the distribution in r (E (r) and Y ( Each of r)) is preferably defined by

  In the optical fiber of the present invention, the ratio of (acoustic mode field diameter) / (optical mode field diameter) is preferably 0.5 or less.

  In the optical fiber of the present invention, the optical mode field diameter is preferably a value at a wavelength of 1550 nm.

  In the optical fiber of the present invention, the cutoff wavelength is preferably 1260 nm or less.

  In the optical fiber of the present invention, the optical mode field diameter at a wavelength of 1310 nm is preferably 8.0 μm or more and 10.0 μm or less.

  In the optical fiber of the present invention, the zero dispersion wavelength is preferably 1300 nm or more and 1324 nm or less.

In the optical fiber of the present invention, A _eff and MFD _petII at a certain wavelength are obtained by the following equation (C).

( _Where A _eff represents the effective core area)
, It is preferable that the k value is larger than 1 at wavelengths of 1310 nm and 1550 nm.

  In the optical fiber of the present invention, the k value at a wavelength of 1310 nm is preferably larger than 1.1.

Further, the present invention is a unimodal single mode optical fiber comprising a core and a clad, wherein the core and the clad have a substantially constant refractive index,
The single-mode optical fiber according to claim 1 and the unimodal single-mode optical fiber have substantially the same optical characteristics including cut-off wavelength, optical mode field diameter, chromatic dispersion value, and bending loss. A single mode characterized in that when the stimulated Brillouin scattering threshold power of the single-mode optical fiber is Pth1, and the stimulated Brillouin scattering threshold power of the single-peak single-mode optical fiber is Pth2, Pth1 is 3 dB or more larger than Pth2. An optical fiber is provided.

  The present invention also provides a transmission system configured to perform analog signal transmission, baseband transmission, or optical SCM transmission using the above-described optical fiber according to the present invention.

  Further, the present invention is characterized in that data transmission and / or voice transmission is performed together with analog signal transmission and / or baseband transmission or optical SCM transmission using the optical fiber according to the present invention. A wavelength division multiplexing transmission system is provided.

  According to the present invention, there are provided an optical fiber capable of suppressing the generation of SBS and transmitting with a higher power signal, a transmission system capable of multi-branching and long-distance transmission using the same, and a wavelength division multiplexing transmission system. can do.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
The optical fiber according to the present invention has a configuration in which the mode field diameter of the fundamental mode of the acoustic mode generated in the optical fiber is different from the mode field diameter of the light intensity distribution of the optical fiber, thereby suppressing the occurrence of SBS, It is characterized by enabling transmission with higher power signals.

SBS is a type of inelastic scattering that occurs due to the interaction between acoustic phonons in a medium such as quartz glass that constitutes an optical fiber and incident light. From the viewpoint of high power signal light input, SBS is
・ Threshold power is low compared to other inelastic scattering.
・ Most of Brillouin scattered light becomes backscattered light,
It has the characteristics.

FIG. 1 is a graph illustrating the SBS threshold power in a normal optical fiber.
The light intensity at which the generation efficiency (in this case, SBS efficiency) increases rapidly, and the threshold power is low, that is, SBS is generated at a lower incident light power than other inelastic scattering. Means that. Furthermore, most of the generated SBS light is scattered backward (incident side), and the SBS light does not propagate forward (receiving side), but only the incident signal light reduced by SBS propagates. Due to these two actions, even if high power light is intended to enter the optical fiber, light above the SBS threshold power does not propagate forward (see the curve indicating the power of transmitted light in FIG. 1).

  The SBS threshold power is approximately expressed by the following equation (1) when the spectral line width of the signal light is narrow.

In equation (1), P _th represents the SBS threshold power, A _eff represents the effective core area, and g _BO represents the Brillouin gain coefficient. L _eff is an effective action length and is obtained by the following equation (2).

  In Expression (2), α represents the loss of the optical fiber, and L represents the actual fiber length (line length).

As can be seen from Equation (1), in order to improve the SBS threshold power, the effective core area A _eff may be increased or the Brillouin gain coefficient g _BO may be decreased. However, since A _eff is closely related to other optical characteristics such as dispersion characteristics, for example, it is desirable to avoid selection of increasing A _eff as much as possible. Then, as a means that can be taken substantially, it can be considered to reduce the Brillouin gain coefficient.

  The Brillouin gain coefficient is expressed by the following equation (3) under the assumption that the Brillouin spectrum is Lorentz type when the spectral line width of the signal light is narrow.

In equation (3), n ₀ is the refractive index of the optical fiber medium (in this example, quartz glass), p ₁₂ is the Pockels photoelastic coefficient, c is the speed of light, λ is the signal light wavelength, and ρ ₀ is the optical fiber. The density of the medium (quartz glass in this example), v _A represents the speed of sound in the clad of the optical fiber, and Δν _B represents the full width at half maximum (FWHM) of the Brillouin spectrum. In addition, about the description of general SBS so far, it can refer to nonpatent literature 2, etc., for example.

Looking at the equation (3), parameters other than Δν _B are inherent to the material or the system, so it is difficult to change or improve them significantly. In other words, to increase the threshold power, one solution is to increase the Brillouin spectrum width.

  As a method of realizing the Brillouin spectral width expansion, by changing the optical characteristics, dopant concentration, residual stress in the longitudinal direction of the optical fiber, by changing the frequency (peak frequency) at which the Brillouin gain is maximized in the longitudinal direction, A method for expanding the Brillouin spectrum width of the entire optical fiber transmission line has been reported (see, for example, Patent Document 1 and Non-Patent Document 3). However, according to this conventional technique, the optical characteristics of the optical fiber inevitably change in the longitudinal direction, which is not preferable in practice.

  On the other hand, the Brillouin spectrum is expressed as the following expression (4), (5) as the sum of the overlap between the spatial spread of the interacting acoustic phonon mode and the spatial power distribution of the optical mode. (See Non-Patent Document 4, for example).

In equations (4) and (5), S _i (ν) is the Brillouin gain at the frequency ν of each acoustic mode, i is the acoustic mode number, ν ₀ is the peak frequency, f _i is the acoustic frequency of each acoustic mode, Γ _i represents an acoustic wave attenuation coefficient (reciprocal of attenuation relaxation time). G _i is obtained by the following equation (6).

In equation (6), p ₁₂ represents the Pockels photoelastic coefficient, ω ₀ represents the angular frequency of the incident light, and β _i represents the propagation constant of each acoustic mode. X (r) and Y _zi (r) are the electric field distribution of the propagation light in the optical fiber and the displacement distribution of the vibration in the axial (z) direction of each acoustic mode, respectively. Here, S (ν) (= S (ν ₀ )) at the peak frequency corresponds to g _B0 in equation (3).

According to equation (4) to (6), reducing the G _i, i.e., the electric field distribution and the axial (z) direction of the vibration displacement distribution of the acoustic modes of the propagating light, to reduce the overlap of the distribution The present inventors have found that the Brillouin gain can be reduced (= threshold power can be increased).
Therefore, the present inventors have intensively studied a specific method for reducing the overlap. The examination results are shown below.

  The structure of an existing optical fiber is illustrated in FIG. The optical fiber 1 includes a core 2 made of quartz glass having a high refractive index and a clad 3 made of quartz glass provided on the outer periphery thereof. A dopant (for example, germanium) for increasing the refractive index is added to the core 2 portion. The clad 3 is not intentionally added with a dopant. The structure of the optical fiber 1 is concentric as shown in FIG. 2, the refractive index of the central core 2 is relatively higher than that of the cladding 3, and light is confined in the core 2 and guided.

  On the other hand, the acoustic mode related to SBS also propagates in the optical fiber. For acoustic waves, the addition of foreign elements to quartz glass has the effect of lowering the sound speed (= confining acoustic waves). Therefore, in the case of the structure shown in FIG. 2, the acoustic wave is confined and guided in the core 2 portion in the same manner as the light.

  FIG. 3 shows an example of calculation results of the displacement distribution (field distribution) of the acoustic mode and the light power distribution (square distribution of the electric field distribution) in the structure of FIG. As exemplified here, in a conventional optical fiber not specifically intended to reduce SBS, the acoustic mode field distribution and the light power distribution have substantially the same distribution.

To reduce the overlap between the acoustic field distribution and the light power distribution, as shown in FIG.
(A) The acoustic field distribution center and the optical power distribution center are shifted (see FIG. 4A).
(B) The acoustic field distribution is wider than the optical power distribution (see FIG. 4B), or (c) The optical power distribution is wider than the acoustic field distribution (see FIG. 4C).
The method can be considered.

However, with regard to (a), the acoustic mode and the optical power distribution must be concentric due to the structure of the optical fiber as shown in FIG.
Therefore, according to the methods (b) and (c), it is possible to reduce the overlap of the acoustic field distribution and the light power distribution, and to suppress SBS and increase the SBS threshold power.

  In FIG. 4 (b), as another form of the method for expanding the acoustic field distribution more than the optical power distribution, as shown in FIG. 5, there is a form in which the acoustic field distribution and the peak position of the optical power distribution are shifted. Conceivable. By adopting such a form, it is possible to more efficiently reduce the overlap between the acoustic field and the optical power distribution as compared with FIG.

The field distribution as shown in FIG. 5 can be obtained by the refractive index distribution as shown in FIG. The refractive index distribution of FIG. 6 shows the following optical characteristics.
・ The fiber cutoff wavelength is 1.33 μm.
• Cable cutoff wavelength is 1.24 μm.
Mode field diameter (Petermann II) is 9.32 μm at a wavelength of 1310 nm and 10.57 μm at a wavelength of 1550 nm.
-Zero dispersion wavelength is 1321.1 nm.
The chromatic dispersion value is -1.03 ps / nm / km at a wavelength of 1310 nm and 16.83 ps / nm / km at a wavelength of 1550 nm.
The bending loss at a bending diameter of 20 mm is 0.2 dB / m at a wavelength of 1310 nm and 3 dB / m at a wavelength of 1550 nm.

  The mode field diameter is based on the definition formula of Petermann II known by the following formulas (7) and (8).

In equations (7) and (8), MFD _petII is the mode field diameter as defined by Petermann II, w _p is the intervening variable of the derived bond, r is the distance from the center, and E (r) is the electric field strength at r. Represent each.

  Similarly, the field diameter of the acoustic field is defined by the following equations (9) and (10).

In the above formulas (9) and (10), MFD _ac is the field diameter of the acoustic field, w _p-ac is the intervening variable of the derivation process, r is the distance from the center, and E (r) is the vibration of the acoustic mode at r. Represents the amount of displacement.

Based on the definitions of the formulas (9) and (10), MFD _ac = 2.80 μm in the optical fiber of FIG.

  As a comparison, consider the optical fiber shown in FIG. The characteristics of the optical fiber of FIG. 7 are shown below. This is because ITU-T G.I. A single mode optical fiber for 1.3 μm band categorized as 652.

・ The fiber cutoff wavelength is 1.27 μm.
• Cable cutoff wavelength is 1.22 μm.
Mode field diameter (Petermann II) is 9.42 μm at a wavelength of 1310 nm and 10.57 μm at a wavelength of 1550 nm.
-Zero dispersion wavelength is 1307.0 nm.
The chromatic dispersion value is 0.26 ps / nm / km at a wavelength of 1310 nm and 17.43 ps / nm / km at a wavelength of 1550 nm.
The bending loss at a bending diameter of 20 mm is 0.3 dB / m at a wavelength of 1310 nm and 3.2 dB / m at a wavelength of 1550 nm.

In terms of optical characteristics, it can be seen that the optical fiber shown in FIG. 6 and the optical fiber shown in FIG. 7 show substantially the same characteristics. FIG. 8 shows the optical power and acoustic field distribution of the optical fiber shown in FIG. It can be seen that the central values of the acoustic field and optical power are located in the center of the core and have substantially the same distribution. According to the formulas (9) and (10), MFD _ac = 6.00 μm.

FIG. 9 shows Brillouin spectra of the optical fiber of the present invention shown in FIG. 6 and the conventional optical fiber shown in FIG. It can be seen that the peak value of the Brillouin spectrum of the optical fiber of the present invention is significantly low, about 0.4, compared to the conventional optical fiber. This means that the Brillouin gain g _B0 in equation (3) is 0.4 times that of the conventional type. Therefore, the SBS threshold power defined by equation (1) is improved by about 4 dB.

Next, the transmission system according to the present invention will be described.
The advantage of using the optical fiber of the present invention described above is that higher power signal light can be introduced. Therefore, by carrying out analog transmission, baseband transmission or optical SCM transmission that requires relatively high power using the optical fiber of the present invention, it becomes possible to transmit more multi-branches and longer distances and enjoy the greatest benefits. it can. In particular, in the case of a system in which the transmission distance is 15 km or more and / or the number of branches is 32 or more, the greatest benefit can be obtained.

In addition to the above-described analog transmission, baseband transmission, or optical SCM transmission, the optical fiber according to the present invention can be used for wavelength division multiplexing transmission in which other transmissions are simultaneously performed. As wavelength multiplexing transmission, ITU-TG As one form of FTTH as shown in 983.3, CWDM or the like is conceivable.
Of course, the transmission system need not be limited to these applications. For example, it can be used not only for ordinary public data communication but also for digital long-distance repeaterless transmission systems, ITS, sensor applications, remote laser cutting systems, and the like.

  Finally, in this example, the acoustic mode has been described with reference to the basic mode. However, in practice, the acoustic mode is not necessarily a single mode, and there may be a plurality of modes. However, even in that case, the acoustic mode with the largest overlap of the equation (6) is almost always the fundamental mode because the light mode is the fundamental mode. Therefore, when considering SBS suppression, it should be noted that there is generally no problem even if the acoustic mode targets only the basic mode.

It is a graph explaining threshold power. It is an end view which shows the structure of a general optical fiber. It is a graph which shows the overlap state of optical power distribution and acoustic mode field distribution. It is an illustration of the optical power and acoustic mode field distribution which implement | achieve SBS suppression. It is a graph which shows the distribution example of the light intensity distribution of this invention, and an acoustic mode. It is a graph which illustrates the refractive index distribution of the optical fiber of this invention. It is a graph which illustrates the refractive index distribution of the conventional optical fiber. It is a graph which shows the light intensity distribution and the distribution example of an acoustic mode of the conventional optical fiber. It is a graph which shows the Brillouin spectrum of the optical fiber of this invention, and the conventional optical fiber.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Optical fiber, 2 ... Core, 3 ... Cladding.

Claims (13)

A single mode optical fiber comprising a core and a cladding,
The core has a refractive index profile, and the clad has a substantially constant refractive index. The distance from the center is r, the relative refractive index of the core with respect to the clad is Δ (r)%, and the maximum value of Δ (r) is Max. If (Δ (r)), Δ (0) <Max (Δ (r)) is established,
The electric field distribution of the fundamental mode of light propagating in the optical fiber is E (r), the displacement distribution of the acoustic mode is Y (r), the radius at which E ² (r) and Y (r) are maximum values, respectively. when the r _MAXE and _{r maxY,} single-mode optical fiber, characterized in _{that r MAXE} and _{r maxY} different.   The single mode optical fiber according to claim 1, wherein the optical mode field diameter and the acoustic mode field diameter defined by E (r) and Y (r) are different. Field diameter (MFD) is expressed by the following formulas (A) and (B)

( _Where MFD _petII is the mode field diameter as defined by Petermann II, w _p is the intervening variable of the derivation bond, r is the distance from the center, F (r) is the distribution in r (E (r) and Y ( r)) respectively.)
The single-mode optical fiber according to claim 2, defined by:   4. The single mode optical fiber according to claim 3, wherein a ratio of (acoustic mode field diameter) / (optical mode field diameter) is 0.5 or less.   The single mode optical fiber according to claim 4, wherein the optical mode field diameter is a value at a wavelength of 1550 nm.   The single mode optical fiber according to any one of claims 1 to 5, wherein a cutoff wavelength is 1260 nm or less.   The single mode optical fiber according to claim 1, wherein an optical mode field diameter at a wavelength of 1310 nm is 8.0 μm or more and 10.0 μm or less.   The single-mode optical fiber according to any one of claims 1 to 7, wherein a zero dispersion wavelength is 1300 nm or more and 1324 nm or less. A value obtained by the following equation (C) for A _eff and MFD _petII at a certain wavelength.

( _Where A _eff represents the effective core area)
The single mode optical fiber according to claim 1, wherein the k value is larger than 1 at wavelengths of 1310 nm and 1550 nm.   The single mode optical fiber according to claim 9, wherein the k value at a wavelength of 1310 nm is larger than 1.1. A unimodal single mode optical fiber comprising a core and a clad, wherein the core and the clad have a substantially constant refractive index,
The single-mode optical fiber according to claim 1 and the unimodal single-mode optical fiber have substantially the same optical characteristics including cut-off wavelength, optical mode field diameter, chromatic dispersion value, and bending loss. A single mode characterized in that when the stimulated Brillouin scattering threshold power of the single-mode optical fiber is Pth1, and the stimulated Brillouin scattering threshold power of the single-peak single-mode optical fiber is Pth2, Pth1 is 3 dB or more larger than Pth2. Optical fiber.   12. A transmission system configured to perform analog signal transmission, baseband transmission, or optical SCM transmission using the single mode optical fiber according to claim 1. A single-mode optical fiber according to any one of claims 1 to 11 is configured to perform data transmission and / or voice transmission together with analog signal transmission and / or baseband transmission or optical SCM transmission. Wavelength division multiplexing transmission system.

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