JP2010028710A - Optical amplification transmission system and design method of optical amplification transmission system - Google Patents

Abstract

An optical amplification transmission system capable of dealing with a wide wavelength band and an optical amplification transmission system design method capable of improving noise characteristics in optical amplification and transmission characteristic deterioration due to cumulative dispersion are provided.
In an optical amplification transmission system including a transmitter having a pre-chirp adjustment function and a lumped-constant Raman amplifier having a Raman amplification optical fiber, the transmitter is connected to the pre-chirp adjustment function. The chirp parameter was determined based on the dispersion of the Raman amplification optical fiber 22 in the lumped Raman amplifier 13.
[Selection] Figure 1

Description

  The present invention relates to an optical amplification transmission system using a lumped-constant Raman amplifier and an optical amplification transmission system design method.

  In photonic crystal fibers that have been studied recently, the core portion is formed of a pure quartz glass base material, and a plurality of holes are provided around the core portion to form an effective clad portion. By changing the diameter and arrangement of the holes, it is possible to design the theoretical cutoff wavelength, chromatic dispersion, and mode field diameter with a high degree of freedom. For this reason, for example, it is possible to perform optical communication in a wide wavelength band from visible to near infrared, such as wavelengths 658 to 1556 [nm].

A fiber Raman amplifier (see Non-Patent Document 1 below) is an optical amplifier that can cope with the wavelength band necessary for optical communication in such a wide wavelength band.
On the other hand, the optical pulse signal propagated through the optical fiber is distorted in the pulse waveform due to the accumulated dispersion in the optical fiber. For this reason, in the high-speed and long-distance optical transmission, it is indispensable to compensate the accumulated dispersion, and a dispersion compensating fiber (see Non-Patent Document 2 below) having a wavelength dispersion characteristic opposite to that of the optical transmission medium is used. .

JP-A-9-236781 M.M. N. Islam, “Raman Amplifiers for Telecommunications”, IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, Vol. 8, no. 3, May / June 2002, p. 548-559 L. G. Nielsen, 7 others, “Dispersion-Compensating Fibers”, JOURNAL OF LIGHTWAVE TECHNOLOGY, Vol. 12, no. 11, November 2005, p. 3566-3579 C. Fukai, five others, “Optimum Optical Fiber Design for a DRA-Based DWDM Transmission System”, JOURNAL OF LIGHTWAVE TECHNOLOGY, Vol. 23, no. 3, March 2005, p. 1232-1238 R. H. Stolen, “Nonlinearity in Fiber Transmission”, PROCEEDINGS OF THE IEEE, Vol. 68, no. 10, October 1980, p. 1232-1236 R. J. et al. Essembre, 3 others, “Design of Bidirectionally Pumped Fiber Amplifiers Generation Double Rayback Backscattering”, IEEE PHOTOTONICS TECHNOLOGY TECHNOLOGY. 14, no. 7, July 2002, p. 914-916 M.M. Ohashi, two others, “Optical Loss Property of Silica-Based Single-Mode Fibers”, JOURNAL OF LIGHTWAVE TECHNOLOGY, Vol. 10, no. 5, May 1992, p. 539-543 E. Brinkmeyer, “Analysis of the backscattering method for single-mode optical fibers”, J. Am. Opt. Soc. Am. Vol. 70, no. 8, August 1980, p. 1010-1012 G. P. Agrawar, translated by Takashi Odagaki, translated by Koichi Yamada, "Nonlinear Fiber Optics (Original Edition 2nd Edition)", 1st Edition, Yoshioka Shoten, May 25, 1997, p. 78-81 G. P. Agrawal, 1 other, “Effect of frequency shipping on the performance of optical communication systems”, OPTIC LETTERS, Vol. 11, no. 5, May 1986, p. 318-320 Shojiro Kawakami, Kazuo Shiraishi and Shoji Ohashi, “Advanced Electronics I-16 Optical Fiber and Fiber-Type Device”, Baifukan, July 10, 1996, p. 24-33, 70-73

  However, in the dispersion compensating fiber described in Non-Patent Document 2, it is difficult to achieve positive chromatic dispersion characteristics under a single mode condition, particularly in the wavelength band shorter than the wavelength 1.3 μm band. There has been a problem that dispersion compensation cannot be performed for an optical transmission medium having negative wavelength dispersion characteristics in the wavelength band.

  The present invention has been made in view of such problems, and an optical amplification transmission system capable of supporting a wide wavelength band and an optical signal capable of improving a noise figure in optical amplification and transmission characteristic deterioration due to cumulative dispersion. An object of the present invention is to provide a method for designing an amplification transmission system.

An optical amplification transmission system according to a first invention for solving the above-described problems is
A transmitter having a pre-chirp adjustment function;
In an optical amplification transmission system comprising a lumped constant type Raman amplifier having an optical fiber for Raman amplification,
The transmitter may determine a chirp parameter in the pre-chirp adjustment function based on dispersion of the Raman amplification optical fiber in the lumped constant Raman amplifier.

A method for designing an optical amplification transmission system according to the second invention for solving the above-described problems is
In a design method of an optical amplification system composed of a lumped Raman amplifier having an optical fiber for Raman amplification and a transmitter having a prechirp adjustment function,
A desired net gain in the lumped-constant Raman amplifier and a pumping light intensity of the lumped-constant Raman amplifier are set, and a Raman gain factor, an effective area, and a group velocity dispersion of the Raman amplification optical fiber are used. And determining a noise figure and a dispersion penalty.

  According to the present invention, it is possible to provide an optical amplification transmission system capable of dealing with a wide wavelength band and an optical amplification transmission system design method capable of improving transmission characteristic deterioration due to noise figure and cumulative dispersion in optical amplification. it can.

Embodiments of an optical amplification transmission system and an optical amplification transmission system design method according to the present invention will be described below with reference to the drawings.
FIG. 1 is a schematic diagram showing a configuration example of an optical amplification transmission system according to the present invention.
An optical amplification transmission system according to the present invention includes a transmitter 11 that generates signal light having a desired wavelength λ _S , a transmission optical fiber 12 having a cutoff wavelength characteristic shorter than the wavelength λ _S, and a lumped constant Raman amplifier. 13 and a receiver 14.

The transmitter 11 includes components necessary for transmitting an optical signal, such as a light source, a pulse generator, and a modulator. The transmitter 11 includes a prechirp adjustment function 15 in particular. For example, in a Z-cut LiNbO ₃ modulator, it can be realized by controlling the chirp parameter C, that is, by adjusting the applied voltage.

Here, the pre-chirp is a method in which the modulated signal light is subjected to frequency modulation corresponding to the light intensity, and pulse compression is caused by wavelength dispersion (see Patent Document 1). The chirp parameter C expressed using the optical phase φ and the light intensity I represents the degree of chirp. The chirp parameter C is expressed as in equation (1).

  The receiver 14 includes components for receiving optical signals such as an optical filter, a photoelectric converter, an electric amplifier, and an electric filter, converting the signals into electric signals, and reading the signal information. In the schematic diagram shown in FIG. 1, the lumped constant Raman amplifier 13 is connected to the output end of the transmission optical fiber 12, but the lumped constant Raman amplifier 13 is connected to the incident end of the transmission optical fiber 12. Alternatively, it may be configured to be connected to both the incident end and the exit end.

FIG. 2 is a schematic diagram showing a configuration example of the lumped constant Raman amplifier 13 constituting the optical amplification transmission system according to the present invention.
Lumped Raman amplifier 13 used in the optical amplification transmission system according to the present invention, an excitation light source 21 of the wavelength lambda _P, and Raman amplification optical fiber 22 having a shorter cutoff wavelength characteristics than the excitation wavelength lambda _P, the wavelength The coupler 23 is configured to couple the excitation light of λ _{P to} the Raman amplification optical fiber 22.

The wavelength λ _{P of the} excitation light source is preferably set to a wavelength at which the maximum gain is obtained by Raman amplification, that is, a wavelength shifted to 13.2 [THz] shorter wavelength side than the signal wavelength λ _S. For example, when the signal wavelength λ _S is 1.12 [μm], the excitation wavelength λ _P is preferably 1.06 [μm]. As an excitation light source having an output at the wavelength, for example, Nd: A YAG laser or an ytterbium-doped fiber laser can be used.

The coupler 23 has a function of coupling the signal light having the wavelength λ _S to the incident end side of the Raman amplification optical fiber 22 or the receiver 14 shown in FIG. In the schematic diagram shown in FIG. 2, the excitation light source 21 and the coupler 23 are connected to both ends of the Raman amplification optical fiber 22, but the incident end or the emission end side of the Raman amplification optical fiber 22. It may be configured to be connected to only one of these.

Therefore, by using the configuration example shown in FIGS. 1 and 2, it is possible to realize a single-mode optical amplification transmission system using the lumped-constant Raman amplifier 13 at a desired signal wavelength λ _S.

In lumped Raman amplifier 13 according to the present invention, the length of the Raman amplification optical fiber 22, a Raman gain factor, and a loss factor, each _{L [km], C R [} Wkm -1], α [dB / km And the pumping light intensity of the pumping light source 21 connected to the incident side and the emitting side of the Raman amplification optical fiber 22 is P _F [W] and P _B [W], respectively, the lumped constant type Raman amplifier The net gain G (0, L) from 13 fiber positions 0 to L is expressed as in equation (2).

Equation (2) is shown in the right column on page 914 of Non-Patent Document 5 where the net gain G (0, L) at the fiber positions z ₁ to z ₂ is “G (z ₁ , z ₂ ) = T _F ( z ₁ , z ₂ ) G _R (z ₁ , z ₂ ) ”, so that“ _TF (z ₁ , z ₂ ) = exp [−α _s (z ₂ −z ₁ )] ”and G _R (z ₁ , z ₂ ) represented by Equation (2) in Non-Patent Document 5 above. In the present application, “z ₁ = 0, z ₂ = L”.

ここで、ｈν、Ｂ_e及びＢ_oは、それぞれ光子エネルギー、電気フィルタ帯域幅、及び光フィルタ帯域幅を表し、Ｎ_ASE及びＰ_DRSは、それぞれ雑音成分の自然放出光による光パワー密度、及び二重レイリー散乱による雑音パワーを表す。 Further, the noise figure NF is expressed as in Expression (3) (see Expression (7) on page 915 of Non-Patent Document 5).

Here, hv, B _e and B _o are each photon energy, represents electrical filter bandwidth, and the optical filter bandwidth, N _ASE and P _DRS, the optical power density by the spontaneous emission of the respective noise components, and secondary Represents the noise power due to heavy Rayleigh scattering.

Further, the Raman gain factor C _R (λ) at the wavelength λ is expressed by the equation (4) using the relative refractive index difference Δ of the core of the Raman amplification optical fiber 22 and the effective area A _eff . In the present invention, λ _S and λ _P indicate a signal wavelength and an excitation wavelength, respectively, and λ indicates either a signal wavelength or an excitation wavelength.

Note that equation (4), in formula (6) excitation wavelengths according to 1233, left column of Non-Patent Document 3 represents the Raman gain factor C _R at the time of 1.45 .mu.m (see the page TABLE I), Assuming that a germanium-doped fiber is used as the optical fiber 22 for Raman amplification, g ₁ = 2.16 was substituted.

Further, in the right column on page 1232 of Non-Patent Document 4, the fourth line from the bottom “The Raman gain coefficient at the excitation wavelength λ is“ 1 / λ with respect to the Raman gain coefficient at the excitation wavelength 1 [μm] ”. Therefore, the Raman gain factor C _{R was} further multiplied by “(1.45 × 10 ⁻⁶ ) / λ _P ”.

なお、式（５）は、上記非特許文献６の５３９頁左欄に記載の損失α_Tを表す式（１）において、レイリー散乱損失係数０．８［ｄＢ／ｋｍ μｍ⁴］とラマン増幅用光ファイバ２２としてゲルマニウム添加ファイバを用いることを仮定して、「Ａ＝Ａ₀（１＋０．４４Δ）」を代入し、上記非特許文献６の５３９頁左欄に記載の赤外吸収損失α_IRを表す式（３）に上記非特許文献６の５４０頁左欄４行目に記載の吸収損失係数６．６５×１０¹²と５２．６２を代入することによって導出した。 Further, the loss coefficient α (λ) is expressed as in Expression (5) using the relative refractive index difference Δ of the core of the Raman amplification optical fiber 22 and the effective area A _eff .

In addition, the equation (5) is the same as the equation (1) representing the loss α _T described in the left column on page 539 of Non-Patent Document 6, and the Rayleigh scattering loss coefficient is 0.8 [dB / km μm ⁴ ] and is used for Raman amplification. Assuming that a germanium-doped fiber is used as the optical fiber 22, “A = A ₀ (1 + 0.44Δ)” is substituted, and the infrared absorption loss α _IR described in the left column on page 539 of Non-Patent Document 6 is calculated. It was derived by substituting the absorption loss coefficients 6.65 × 10 ¹² and 52.62 described in the fourth column of the left column of page 540 of Non-Patent Document 6 into Expression (3).

N _ASE (refer to Equation (1) on page 914 of Non-Patent Document 5 above) is obtained by using the relative refractive index difference Δ of the core of the Raman amplification optical fiber 22 and the effective area A _eff (6) ).

ここで、Ｐ_inは信号入力強度である。 Further, P _DRS (see Equation (10) on page 1233 of Non-Patent Document 3 above) is obtained by using the relative refractive index difference Δ of the core of the Raman amplification optical fiber 22 and the effective area A _eff (7) ).

Here, P _in is the signal input intensity.

G (z, L) is the net gain from the fiber position z to L, R (λ _S ) is the Rayleigh backscattering coefficient, and is expressed by the equation (8) using the refractive index n _core of the _core. .

Note that the first term on the right side of Equation (8) is the same as that in Equation (2) representing the Rayleigh scattering loss coefficient α _R described in the right column on page 539 of Non-Patent Document 6. (5) representing the Rayleigh scattering coefficient A in the germanium-doped fiber described in 1. and the Rayleigh scattering coefficient 0.8 [dB / km μm of pure silica glass described in the left column, page 540, page 540 of Non-Patent Document 6 above. ⁴ ] was derived by substituting.

Further, the second term on the right side of Equation (8) was derived from the equation representing the backscattering factor S in Equation (12) described in the right column on page 1011 of Non-Patent Document 7. In Non-Patent Document 7, n, ω ₀ , ω, and c represent the refractive index of the core, the mode field diameter, the angular frequency, and the speed of light, respectively. In the present application, since the core refractive index is n _core , “n = n _core ”.

The effective area A _eff used in the present application can be expressed as “A _eff = πω ₀ ² ” using the mode field diameter ω ₀ . Further, in general, the wave number k is k = ω / c, and “k = 2π / λ” when expressed using the wavelength λ. Therefore, for the backscatter factor S, equation (9) can be derived.

Accordingly, in the optical amplification transmission system using the lumped Raman amplifier 13 according to the present invention, the desired net gain G (0, L), and the excitation light intensity P _F and set the P _B, the Raman amplification optical fiber The dependence of the Raman gain factor C _R and the effective area A _eff on the relative refractive index difference Δ of the core in FIG. 22 and the noise in the optical amplification transmission system by using the equations (2) to (9) It becomes possible to design the characteristics of the exponent NF.

On the other hand, an optical signal pulse having a wavelength λ having a chirp characteristic passes through a dispersion medium having a length L [km] whose group velocity dispersion is β ₂ [ps / km], and a penalty DP [ dB] is expressed as in Expression (10) using the incident pulse width T ₀ and the chirp parameter C.

ここで、Γはガンマ関数を示す。また、ｍは、パルスの形状を関数として示すための補正関数であり、例えば、ガウス型パルス、及び超ガウス型パルスの場合、それぞれｍ＝１、ｍ＞１である（上記非特許文献８参照）。なお、ｍは、上記非特許文献９の式（５）に示されるように、パルスの半値幅σと立ち上がり時間τ_rから求めることができる。 Equation (10) is expressed in decibels from Equation (3.2.27) representing the pulse spreading factor described on page 81 of Non-Patent Document 8. In addition, z, T, T ₀ , i, and C in Non-Patent Document 8 are time “T = tz−z / measured in a coordinate system that travels with a pulse of time t at a distance and a group velocity ν _g , respectively. ν _g ”, incident pulse width, imaginary number, and chirp parameter C.

Here, Γ represents a gamma function. M is a correction function for indicating the shape of the pulse as a function. For example, in the case of a Gaussian pulse and a super Gaussian pulse, m = 1 and m> 1, respectively (see Non-Patent Document 8 above). ). Note that m can be obtained from the half-value width σ of the pulse and the rise time τ _r as shown in Equation (5) of Non-Patent Document 9.

ここで、ｃは光速を示す。 Further, the group velocity dispersion beta _2, the chromatic dispersion D at the wavelength λ of the dispersion medium [ps / nm · km] to have the relationship of Equation (11).

Here, c indicates the speed of light.

FIG. 3 is an example of the relationship between the product of the group velocity dispersion and the length, that is, the cumulative group velocity dispersion β ₂ L, and the penalty DP due to the dispersion, and is a diagram describing the chirp parameter C as a parameter. Here, 50 [ps] and 1.5 were used for T ₀ and m, respectively. From FIG. 3, when the product β ₂ L of group velocity dispersion and length is positive and negative, the penalty DP due to dispersion is controlled negative by setting the chirp parameter C to a negative value and a positive value, respectively. I understand that I can do it.

Therefore, by using the chromatic dispersion D at the signal wavelength λ _S of the Raman amplification optical fiber 22 and the equations (10) and (11), the lumped constant type constituting the optical amplification transmission system according to the present invention is used. It becomes possible to design a penalty DP for transmission characteristics due to dispersion in the Raman amplifier 13 and to control the penalty DP by the code of the chirp parameter C.

In the following, with respect to a design example of an optical amplification transmission system using the lumped constant Raman amplifier 13 according to the present invention, the signal wavelength λ _S is 1.12 [μm], the pumping wavelength λ _P is 1.06 [μm], The desired net gain G (0, L) in the optical amplification transmission system is 30 [dB], the ratio of the excitation light intensities P _F and P _B is 1: 1, and the total excitation light intensity “P _P = P _F + P _B ” A case of 0.5 to 1 [W] will be described with reference to the drawings. In this embodiment, the ratio of the excitation light intensities P _F and P _B is 1: 1, but the ratio of the excitation light intensity P _P can be arbitrarily set.

FIG. 4 is a conceptual diagram showing the refractive index distribution in the cross-sectional direction of the optical fiber 22 for Raman amplification used in the embodiment of the optical amplification transmission system according to the present invention.
In this example, a quartz fiber having germanium added to the core was used as the Raman amplification optical fiber 22. Here, Δ and a represent the relative refractive index difference between the core portion 41 and the cladding portion 42 and the core radius, respectively.

  The refractive index of the cladding part 42 was set to a level of pure quartz. In the present embodiment, the refractive index distribution of the Raman amplification optical fiber 22 is not limited to the structure having the step type refractive index distribution in which germanium is added to the core of the present embodiment, and has an arbitrary refractive index distribution. It may be a structure.

FIG. 5 shows that in the Raman amplification optical fiber 22 having the refractive index distribution shown in FIG. 4, the cutoff wavelength λ _C is 1.06 [μm], that is, the excitation wavelength λ _P or less in a desired optical amplification transmission system. FIG. 5 is a diagram showing the relationship between the core relative refractive index difference Δ and the core radius a.
In this embodiment, the relationship between the relative refractive index difference Δ and the core radius a that achieves a cutoff wavelength of 1.06 [μm] or less is obtained by numerical calculation using the multilayer division method (see Non-Patent Document 10 above). It was.

The solid line in FIG. 5 shows the relationship of the core radius a with respect to the relative refractive index difference Δ between the core part and the cladding part when the cutoff wavelength λ _C is 1.06 [μm], and is indicated by the oblique line below the solid line. By designing the core radius a for the relative refractive index difference Δ in the region, it becomes possible to realize a cutoff wavelength λ _C of 1.06 [μm] or less.

6, the cutoff wavelength lambda _C condition shown in FIG. 5, a diagram showing a dependency on the effective area A _eff and the Raman gain constant C relative refractive index difference _R of the core Δ in the wavelength 1.12 [[mu] m] is there.
In this example, when the cutoff wavelength λ _C is 1.06 [μm], the relationship between the relative refractive index difference Δ and the effective area A _{eff at} the wavelength of 1.12 [μm] is expressed by the multilayer division method ( It calculated | required by numerical calculation using the said nonpatent literature 10). Also, the relationship between the wavelength 1.12 relative refractive index difference at [[mu] m] delta and the Raman gain constant C _R calculated using Equation (4).

FIG. 7 is a diagram showing the dependence of the wavelength dispersion D and the group velocity dispersion β ₂ on the relative refractive index difference Δ of the core at a wavelength of 1.12 [μm] under the cutoff wavelength λ _C condition shown in FIG.
In this embodiment, the relationship between the relative refractive index difference Δ and the wavelength dispersion D at the wavelength of 1.12 [μm] when the cutoff wavelength λ _C is 1.06 [μm] is expressed by the multilayer division method (the above non-patent document). It was calculated | required by numerical calculation using literature 10). Further, the relationship between the relative refractive index difference Δ and the group velocity dispersion β ₂ at a wavelength of 1.12 [μm] was determined using the equation (11).

8, using a dependency on the effective area A _eff and Raman gain factor C relative refractive index difference _R of the core Δ shown in FIG. 6, the requirements of the net gain G (0, L) = 30 [dB] 8 is a diagram in which the equal NF characteristics satisfying the above are plotted as a function of the relative refractive index difference Δ of the core and the fiber length L of the optical fiber 22 for Raman amplification.

That is, at the plot points of the relative refractive index difference Δ (Raman gain factor C _R and loss factor α) and the fiber length L, the pumping light intensities P _F and P _B are changed little by little, and the net gain G (0, L) is changed. The pumping light intensities P _F and P _B to be 30 [dB] are obtained using the equation (2), and then each relative refractive index difference Δ (Raman gain factor C _R and loss factor α), fiber length L, net gain G (0, L), and that time by using the excitation light intensity P _F and P _B of a graph plotting calculated noise figure NF from equation (3). The transmission rate was 10 [Gbit / s], electrical filter bandwidth B _e, and the optical filter bandwidth B _o were respectively 7 [GHz] and 10 [GHz]. The signal input intensity P _in to the lumped constant Raman amplifier 13 was set to −30 [dBm].

The solid line and the broken line in FIG. 8 indicate the noise figure NF and the total excitation light intensity P _P , respectively. When the total excitation light intensity P _P is 0.5 to 1 [W], the relative refractive index difference Δ is about 0.6% or more, that is, the Raman gain factor C _R is about 2 [Wkm ⁻¹ ] or more, and the fiber. By setting the length L to 1 [km] or more, a net gain G (0, L) of 30 [dB] can be realized while maintaining a noise figure NF characteristic of 3.2 to 5 [dB]. I understand that.

In particular, when the total excitation light intensity P _P is 0.5 [W], the net gain G (0, L) of 30 [dB] is realized by setting the relative refractive index difference Δ to 1.1% or more, By setting the fiber length L to 7.5 [km] or less, it is more preferable because a noise figure NF characteristic of 4 [dB] or less can be realized.

FIG. 9 shows the characteristics of the cumulative group velocity dispersion β ₂ L with respect to the relative refractive index difference Δ of the core when the total excitation light intensity P _P is 0.5 and 1 [W] in the equal NF characteristics shown in FIG. FIG.
The accumulated group velocity dispersion β ₂ L at the relative refractive index difference Δ of each core was calculated by reading the accumulated group velocity dispersion β ₂ L and the fiber length L from FIGS. 7 and 8, respectively. When the total excitation light intensity P _P is 0.5 and 1 [W], the cumulative group velocity dispersion β ₂ L takes positive values of 25 and 50 [ps] or more, respectively, and the relative refractive index difference Δ decreases. It can be seen that the cumulative group velocity dispersion β ₂ L increases.

FIG. 10 is a diagram showing the dependence of the transmission characteristics due to dispersion on the relative refractive index difference Δ of the core in the cumulative group velocity dispersion β ₂ L characteristics shown in FIG.
Here, the dispersion penalty at each relative refractive index difference was obtained from equation (10) using a chirp parameter C of −0.7. The incident pulse width T ₀ and the pulse shape correction coefficient m were 50 [ps] and 1.5, respectively. As can be seen from FIG. 10, since the cumulative group velocity dispersion β ₂ L takes a positive value, the transmission characteristic penalty DP due to dispersion can be negatively controlled by setting the chirp parameter C negative.

FIG. 11 is a conceptual diagram showing the effect of reducing the noise figure NF and reducing the dispersion penalty DP in the code error rate BER characteristics.
In general, the signal-to-noise ratio characteristic can be improved by reducing the noise figure NF, and the code error rate BER at the same light receiving power _PREC can be reduced. Further, by dispersion penalty DP is reduced to negative, it is possible to reduce the light power P _REC to achieve a desired bit error rate BER.

That is, in the optical amplification transmission system having the code error rate BER characteristic indicated by the solid line in FIG. 11, when the noise figure NF or the dispersion penalty is reduced, the code error in the received light power P _REC indicated by the broken line in FIG. Since the value of the rate BER shifts in the direction indicated by each arrow, the code error rate BER characteristic indicated by the solid line in FIG. 11 can be improved to the code error rate BER characteristic indicated by the broken line.

Therefore, the desired net gain G (0, L) and the total pumping light intensity “P _P = P _F + P _B ” are set, and the Raman gain factor C _R and the effective area A _eff of the optical fiber 22 for Raman amplification are set. Using the dependence on the relative refractive index difference Δ of the core, the NF characteristic of the lumped Raman amplifier 13 is derived, and the relative refractive index of the core of the accumulated group velocity dispersion β ₂ L of the Raman amplification optical fiber 22 is derived. By using the dependence on the rate difference Δ to derive the dispersion penalty DP characteristic of the lumped constant Raman amplifier 13, the noise figure NF characteristic of the lumped constant Raman amplifier 13 and the transmission characteristic deterioration due to dispersion are improved. A one-mode optical amplification transmission system can be realized.

More specifically, when the total excitation light intensity P _P is 0.5 [W], the relative refractive index difference Δ and the fiber length L are set to 1.2% and 5.3 [km], respectively. Thus, a noise figure NF of 3.8 [dB] and a dispersion penalty DP of −0.25 [dB] can be realized. When the total excitation light intensity P _P is 1 [W], the noise index of 3.6 dB is obtained by setting the relative refractive index difference Δ and the fiber length L to 0.6% and 6 [km], respectively. NF and a dispersion penalty DP of −0.25 [dB] can be realized.

Next, a method for designing a single-mode optical amplification transmission system using a lumped constant type Raman amplifier described in the embodiment of the optical amplification transmission system according to the present invention will be described.
In the design of the optical amplification transmission system according to the present embodiment, specifically, the charts of FIGS. 8 and 10 are obtained through the setting of each parameter using the equations (2) to (11).
FIG. 8 and FIG. 10 obtained are compared, and the relationship between the relative refractive index difference Δ and the fiber length L (for example, relative refractive index difference 1...) That provides a low noise figure NF and good dispersion penalty DP characteristics. 1%, and a specific numerical value such as a fiber length of 7.5 [km] is found.
Then, by configuring an optical amplification system using the Raman amplification optical fiber 22 having the relative refractive index difference Δ and the fiber length L, the performance (noise figure NF and dispersion penalty DP) in the system is shown in FIG. It can be grasped from FIG.

Here, “the noise figure NF is low and the dispersion penalty DP characteristic is good” varies depending on the relationship between the received light power _PREC and the code error rate BER in the system to be used (see FIG. 11). That is, when the absolute value of the slope of the graph of the received light power P _REC and the code error rate BER is large, the larger the improvement of the dispersion penalty DP, the higher the effect. For example, when P _P = 0.5 [W] If so, it is desirable to set the relative refractive index difference Δ that maximizes the improvement of the dispersion penalty from FIG. 10 to 1.1%.
On the contrary, when the absolute value of the slope of the graph of the received light power P _REC and the code error rate BER is small, the smaller the noise figure NF, the higher the effect. For example, P _P = 0.5 [W] For example, it is desirable to set the relative refractive index difference Δ to 2% from FIG.

Next, the procedure of the design method of the single mode optical amplification transmission system using the lumped constant Raman amplifier 13 described in the embodiment of the optical amplification transmission system according to the present invention will be described.
FIG. 12 is a diagram showing a flowchart relating to a design method of a single mode optical amplification transmission system using the lumped-constant Raman amplifier 13 described in the embodiment of the optical amplification transmission system according to the present invention.
As shown in FIG. 12, first, in step S1, the desired net gain G (0, L) of the optical amplification transmission system, the pumping light intensities P _F and P _B , and the absolute value of the chirp parameter C of the pre-chirp adjustment function 15 Set.

Next, in step S2, the relationship between the Raman gain factor C _R of the Raman amplification optical fiber 22, the effective area A _eff , and the dependence of the group velocity dispersion on the relative refractive index difference Δ of the core is obtained. In the refractive index distribution of FIG. 4, for example, by using a multilayer division method, the relationship between the relative refractive index difference Δ satisfying the cutoff wavelength λ _C characteristic and the core radius a as shown in FIG. 5 is obtained. From this relationship, the effective area A _eff and chromatic dispersion D of FIGS. 6 and 7 can be obtained.

Next, in step S3, the noise figure NF characteristic of the lumped constant Raman amplifier 13 is derived. In steps up to step S2, net gain G (0, L), pump light intensities P _F and P _B , relative refractive index difference Δ, effective area A _eff , and Raman gain factor C _R are obtained. It is possible to calculate the noise figure NF using the equations (3) to (9). Thereby, the noise figure NF characteristic when the relative refractive index difference Δ and the fiber length L are changed as shown in FIG. 8 can be obtained. Incidentally, is set as the net gain G (0, L) is 30 [dB] of the excitation light intensity in FIG. 8 P _F and P _B to changes in the relative refractive index difference Δ and the fiber length L.

Next, in step S4, the dispersion penalty DP characteristic of the lumped constant Raman amplifier 13 is derived. Relationship from Figure 7 the relative refractive index difference Δ and the group velocity dispersion beta ₂ is specific excitation light intensity from FIG 8 P _F and P _B (e.g., 0.5 [W]) in the relative refractive index difference Δ and the fiber Since the relationship with the length L is obtained, the relationship between the relative refractive index difference Δ and the accumulated group velocity dispersion β ₂ L (product of the group velocity dispersion β ₂ and the fiber length L) as shown in FIG. 9 is obtained. Thus, by using the equation (10), the dispersion penalty DP characteristic shown in FIG. 10 can be obtained.

  Finally, in step S5, the relative refractive index difference Δ and the fiber length L of the Raman amplification optical fiber 22 and the sign of the chirp parameter C of the pre-chirp adjustment function 15 are determined. 8 and 10, the relationship between the relative refractive index difference Δ and the fiber length L, which has a low noise figure NF and good dispersion penalty DP characteristics, is found.

  As described above, according to the optical amplification transmission system according to the present invention, the lumped-constant Raman amplifier 13 having the Raman amplification optical fiber 22 and the transmitter 11 having the prechirp adjustment function 15 are provided. An optical amplification transmission system that can improve the dispersion penalty DP at an arbitrary signal wavelength can be realized.

Also, as the design method of the optical amplification transmission system according to the present invention, in the optical amplification transmission system, and the parameters desired net gain G (0, L), and the excitation light intensity P _F and P _B, of the Raman amplification medium Taking into account the relationship between the effective area A _eff , the Raman gain factor C _R , and the dependence of the group velocity dispersion β _{2 on} the relative refractive index difference Δ, the noise figure NF characteristics and dispersion penalty DP of the lumped Raman amplifier 13 Therefore, the transmission performance of the optical amplification transmission system can be improved.

  The present invention can be used, for example, in an optical amplification transmission system using a lumped-constant Raman amplifier and an optical amplification transmission system design method.

It is the schematic which shows the structural example of the optical amplification transmission system which concerns on this invention. It is the schematic which shows the structural example of the lumped-constant type Raman amplifier which comprises the optical amplification transmission system which concerns on this invention. It is an example of the relationship between the product of the group velocity dispersion and the length, that is, the accumulated group velocity dispersion β ₂ L and the penalty DP due to dispersion, and is a diagram describing the chirp parameter C as a parameter. It is a conceptual diagram which shows the refractive index distribution of the cross-sectional direction of the optical fiber for Raman amplification used in the Example of the optical amplification transmission system which concerns on this invention. In the Raman amplification optical fiber 22 having the refractive index distribution shown in FIG. 4, the core ratio is set such that the cutoff wavelength λ _C is 1.06 [μm], that is, the excitation wavelength λ _P or less in the desired optical amplification transmission system. It is a figure which shows the relationship between refractive index difference (DELTA) and the core radius a. In the cutoff wavelength lambda _C condition shown in FIG. 5 is a diagram showing a dependency on the effective area A _eff and the Raman gain constant C relative refractive index difference of the core of the _R delta at a wavelength 1.12 [μm]. In the cutoff wavelength lambda _C condition shown in FIG. 5 is a diagram showing the dependence on the relative refractive index difference of the core of the chromatic dispersion D and the group velocity dispersion beta ₂ delta at the wavelength 1.12 [μm]. Using the dependence of the effective area A _eff and the Raman gain factor C _R on the relative refractive index difference Δ of the core shown in FIG. 6, the NF satisfying the requirement of the net gain G (0, L) = 30 [dB] FIG. 6 is a graph plotting characteristics as a function of the relative refractive index difference Δ of the core and the length L of the optical fiber for Raman amplification. FIG. 9 is a diagram illustrating the characteristics of the cumulative group velocity dispersion β ₂ L with respect to the relative refractive index difference Δ of the core when the total excitation light intensity P _P is 0.5 and 1 [W] in the equal NF characteristics illustrated in FIG. 8. . In a cumulative group velocity dispersion beta ₂ L characteristic shown in FIG. 9 is a diagram showing the dependence on the relative refractive index difference of the core of the penalty DP delta of transmission characteristics due to dispersion. It is a conceptual diagram which shows the reduction effect of the noise figure NF in the code error rate BER characteristic, and the reduction effect of dispersion | distribution penalty DP. It is a figure regarding the flowchart regarding the design method of the single mode optical amplification transmission system using the lumped constant type Raman amplifier demonstrated in the Example of the optical amplification transmission system which concerns on this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 Transmitter 12 Transmission optical fiber 13 Lumped constant type Raman amplifier 14 Receiver 15 Prechirp adjustment function 21 Excitation light source 22 Raman amplification optical fiber 23 Coupler

Claims (2)

A transmitter having a pre-chirp adjustment function;
In an optical amplification transmission system comprising a lumped constant type Raman amplifier having an optical fiber for Raman amplification,
The transmitter determines a chirp parameter in the pre-chirp adjustment function based on dispersion of the optical fiber for Raman amplification in the lumped-constant Raman amplifier. In a design method of an optical amplification system composed of a lumped Raman amplifier having an optical fiber for Raman amplification and a transmitter having a prechirp adjustment function,
A desired net gain in the lumped-constant Raman amplifier and a pumping light intensity of the lumped-constant Raman amplifier are set, and a Raman gain factor, an effective area, and a group velocity dispersion of the Raman amplification optical fiber are used. A method for designing an optical amplification transmission system, characterized by obtaining a noise figure and a dispersion penalty.

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