JPH08171109A - Optical frequency arrangement method - Google Patents

Abstract

PURPOSE: To provide an optical frequency arrangement method capable of previously setting the frequency band occupied by the entire channel and lessening the influence of crosstalks by four light wave mixing with an optical transmission path subjected to optical frequency-division multiplexing into plural channels. CONSTITUTION: One combination of optical frequency shift quantity ΔX(t) is determined in a step A1. The sum PFr (L) of the light intensity of the four light wave mixed light is calculated in a step S2. The max. value among PFr (L) is selected as PFmax (L) in a step S3. Whether PFmax (L) is determined with all the combinations of ΔX(t) is judged in a step S4. The processing is advanced to the step S1 if the result of the judgment is 'no' and to a step S5 if 'yes'. The combination of ΔX(t) at which PFmax (L) is minimized is defined as ΔX0(t) in the step S5 and ΔX0(t) is outputted as the result of the processing in a step S6.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical frequency allocation method for reducing the influence of four-wave mixing light on the transmission speed and the transmission distance in an optical transmission apparatus in which optical frequency division multiplexing is performed.

[0002]

2. Description of the Related Art The low loss wavelength region of an optical fiber is 1.
In 5 μm band optical transmission, a dispersion shift fiber is used in order to reduce transmission waveform deterioration due to wavelength dispersion of the optical fiber. Further, in recent years, in order to expand the transmission capacity, a wavelength division multiplexing (WDM) / optical frequency division multiplexing (FDM) transmission system for multiplexing and transmitting optical signals of many wavelengths / frequencies has been developed. However, when light of a plurality of wavelengths is simultaneously transmitted in a region near the zero-dispersion wavelength of the optical fiber, the four-wave mixing phase matching condition, which is one of the nonlinear effects of the optical fiber, is likely to be established. Therefore, in the optical frequency division multiplex transmission using the dispersion shift fiber, crosstalk occurs between the optical frequency channels and the transmission characteristics deteriorate.

The crosstalk due to the four-wave mixing becomes the largest when the optical frequency arrangement is equidistant. for that reason,
An arrangement method has been proposed by F. Forghieri et al. (IEEE Photonics Technology Letter) in which optical frequencies are arranged at irregular intervals to reduce crosstalk due to four-wave mixing.
s, vol.6, No6, pp.754-756 (1994)). The method is shown below.

As shown in FIG. 4, when the channel spacing of the optical frequency is m ₁ , m ₂ , ..., _{M N-1} , for any j, k where 1 ≦ j <k ≦ N

[Equation 1] Are all different from each other, and

[Equation 2] _{M 1} , m ₂ , ...
Let it be the optical frequency allocation. In this method, s _jk is N
There are (N-1) / 2 types, and combinations of m _i that have different values are obtained, and the arrangement that minimizes the above value S is selected.

[0005]

In the above-mentioned conventional example, in order to prevent the crosstalk light due to the four-wave mixing from completely overlapping the signal light, the channel intervals m _i between arbitrary channels are all arranged differently. To do. Therefore, the frequency band occupied by all the channels is larger than the frequency band occupied when the channels are arranged at equal intervals, but it is difficult to accurately predict the amount of increase in advance. That is, the conventional optical frequency allocation method has a drawback that it is difficult to preset the frequency band occupied by all channels. Further, since the optical frequency interval obtained by the conventional optical frequency arrangement method has almost no periodicity, it is difficult to apply the periodic optical frequency filter that can be realized by a simple configuration.

Further, since the generation efficiency of four-wave mixing decreases as the distance from the zero-dispersion wavelength increases and decreases as the channel spacing increases, the four-wave mixing light does not always completely overlap the signal light for all combinations. do not have to. However, in the above-mentioned conventional example, the characteristics of the generation efficiency of the four-wave mixing described above are not taken into consideration, and there is a drawback that the occupied frequency band is excessively large to avoid the influence of the four-wave mixing light. It was

The present invention has been made under such a background, and it is possible to apply a periodic optical frequency filter in which the frequency band occupied by all channels can be set in advance and which can be realized by a simple structure. An object of the present invention is to provide an optical frequency allocating method which is possible and reduces the influence of crosstalk due to four-wave mixing in consideration of characteristics of generation efficiency of four-wave mixing.

[0008]

According to the first aspect of the present invention,
The reference optical frequency of the optical frequency division multiplexing by N channels is f _0, and the frequency interval between the N channels is Δf.
And an optical frequency shift amount of each channel is ΔX (i), an optical signal whose optical frequency is represented by f _i = f ₀ + i · Δf + ΔX (i) is In the optical frequency allocation method for inputting to the i-th channel,
Based on the generation efficiency of the four-wave mixed light, the four-wave mixed light having the maximum light intensity among the four-wave mixed light generated on the optical frequencies of the nth to Nth channels is minimized. It is characterized in that the optical frequency shift amount ΔX (i) of each channel is determined. The invention described in claim 2 is claim 1
In the optical frequency allocation method described, the optical frequency shift amount ΔX
The feature is that (i) is set to a value smaller than the frequency interval Δf.

[0009]

According to the first aspect of the invention, the i-th (i = 1, 2, ..., N) optical frequency of the N-channel optical frequency is set to f _i = f ₀ + iΔf + ΔX (i) (However, f ₀ is the reference optical frequency, Δf is the frequency interval between channels, ΔX
(I) is the optical frequency deviation of each channel)
Based on the generation efficiency of the four-wave mixed light, the four-wave mixed light having the maximum light intensity among the four-wave mixed lights generated on the Nth to Nth optical frequencies is minimized. The frequency shift amount ΔX (i) of the channel is determined. As a result, the frequency band occupied by all channels can be set in advance. Further, by performing the calculation based on the light intensity of the four-wave mixing light that is actually generated, it is possible to perform the optical frequency arrangement in consideration of the characteristics of the generation efficiency of the four-wave mixing. Claim 2
According to the described invention, since the optical frequency shift amount ΔX (i) is set to a value smaller than the frequency interval Δf, the optical frequencies f _i can be arranged almost periodically, and the periodic optical frequency filter is applied. Is possible.

[0010]

EXAMPLE The optical frequency f _i of the i-th (i = 1, 2, ..., N) channel of the N-channel optical frequencies as shown in FIG. 1 is expressed as f _i = f ₀ + iΔf + ΔX (i ) (1) Here, f ₀ is the reference optical frequency, Δf is the frequency interval between channels, and ΔX (i) is the optical frequency shift amount of each channel.

The input light that generates four-wave mixing is f _i , f _j ,
If f _k , the optical frequency f _ijk of the four-wave mixed light is f _ijk = f _i + f _j −f _k ··· (2) Become. However, it is assumed that i, j ≠ k. Substituting equation (1) into equation (2), f _ijk = {f ₀ + i · Δf + ΔX (i)} + {f ₀ + j · Δf + ΔX (j)} − {f ₀ + k · Δf + ΔX (k)} = {F ₀ + (i + j−k) · Δf} + {ΔX (i) + ΔX (j) −ΔX (k)} ... (2) ′.

Further, the optical frequency f _{i + jk} of the (i + j−k) th channel is calculated from the formula (1) as follows: f _{i + jk} = f ₀ + (i + j−k) · Δf + ΔX (i + jk)・ ・ ・ ・ (3) Comparing equation (2) ′ and equation (3), Δf> Δ
When X (i), the condition that the optical frequency f _{i + jk of} the (i + j−k) th channel and the optical frequency f _ijk of the four-wave mixing light do not overlap is ΔX (i + jk) ≠ ΔX (i ) + ΔX (j) -ΔX (k) ... (4) Therefore, for the combination of i, j, and k such that the left side and the right side of the equation (4) match, the light intensity of the four-wave mixed light is calculated by the following equations (5) to (10), and the respective frequencies are calculated. Optical frequency shift amount ΔX (i) that minimizes the maximum value of the sum of the optical intensities of the four-wave mixed light for each channel
Should be decided.

Letting P _i (0) and P _i (L) be the input light intensity and the output light intensity to the dispersion shift fiber (length L, attenuation factor α) of the i-th channel, respectively, P _i (L) = The relationship of P _i (0) e ^{-aL ...} (5) holds. In addition, the fiber output light intensity of the four-wave mixing light f _ijk by the input lights f _i , f _j , and f _k is P _Fijk (L)
Then

(Equation 3) Becomes Here, n is the refractive index of the fiber, λ is the wavelength, c is the optical velocity, X is the nonlinear constant, A _eff is the effective area of the fiber, D is the degeneracy coefficient, and when i = j ≠ k, D = 3. Yes, D = 6 when i ≠ j ≠ k.

In the equation (6), η (i, j, k) is the generation efficiency of four-wave mixing, and is given by the following equation (7).

[Equation 4] Here, Δβ (i, j, k) is the amount of phase mismatch, and can be given by the following equation (8) when the optical frequency matches or is very close to the zero dispersion wavelength of the fiber.

(Equation 5) However, f _zero is the optical frequency corresponding to the zero-dispersion wavelength, and D _c is the fiber dispersion.

Here, P _i (0) = P _j (0) = P _k (0) =
P ₀ ,

(Equation 6) Then, the fiber output light intensity of the four-wave mixing light f _ijk is given by the equation (6).

(Equation 7) Becomes Here, r = i + j−k (r = 1, 2, ...,
N), the sum P _Fr (L) of the optical intensities of the four-wave mixed light that overlaps f _r = f _{i + jk} is given by the formula (4) and the formula (9).

(Equation 8) Becomes Using the equation (10), the optical frequency shift amount ΔX (i) is determined so that the maximum value of P _Fr (L) at r = 1, 2, ..., N is minimized.

Next, the process of obtaining the optical frequency shift amount ΔX (i) using the above equation (10) will be described with reference to the flowchart shown in FIG. When the process is started, in step S1, the optical frequency shift amount ΔX (t) (t = 1,
2 ..., N: t is the channel number, N is the total number of channels)
, {ΔX (1), ΔX (2) ..., ΔX
One combination of (N)} is determined, and the process proceeds to step S2. In step S2, i ≦ i, j, k ≦ N, i,
For j and k, the sum P _Fr (L) of the light intensities of the four-wave mixed light is calculated using the equation (10), and the process proceeds to step S3.
However, here, r = i + j−k (r = 1, 2, ...,
N). In step S3, the obtained P _F1 (L),
The maximum value in P _F2 (L) ..., P _FN (L) is P
_{Set Fmax} (L), and proceed to step S4.

In step S4, {ΔX (1), ΔX
(2) ..., It is determined whether or not P _Fmax (L) has been calculated for all combinations of ΔX (N)}. If the result of this determination is "NO", the flow proceeds to step S1. on the other hand,
When the determination result of step S4 is "YES", the process proceeds to step S5. In step S5, {ΔX (1), ΔX
(2) ..., ΔX (N)} of all combinations of P _Fmax (L), the combination having the smallest P _Fmax (L) is set to ΔX0 (t), and the process proceeds to step S6.
In step S6, ΔX0 (t) is output as the processing result, and the example processing ends.

Next, a specific example of the optical frequency shift amount ΔX (i) obtained by the optical frequency allocation method of this embodiment will be shown.
The optical frequency shift amount ΔX (i) can take continuous values or arbitrary n discrete values. Here, assuming that the number of discrete values n = 3, the optical frequency shift amounts ΔX (1), ΔX
(2), ..., ΔX (N) are + δ, 0, −δ
Consider the case of taking any one of the three values of. An example of the result of obtaining the optimum optical frequency arrangement for the case where the number of channels N is N = 3 to N = 16 is shown below. However, + δ is expressed as 1, and −δ is expressed as −1.

When N = 3 0 1 0 When N = 4 0 1 0 0 When N = 5 0-1 1-1 0 When N = 6 0-1 1 1-1 0 When N = 7ー 1 ー 1 1 0 1 ー 1 1 When N = 8 ー 1 ー 1 1 1 0 1 ー 1 1 1 When N = 9 ー 1 ー 1 1 1 ー 1 0 ー 1 0 1 When N = 10- 1 -1 1 1 -1 0 -1 0 1 1 When N = 11 -1 1 1 -1 -1 1 0 1 0 1 1 When N = 12 -1 1 1 -1 -1 1 0 1 0 1 1 0 N = 13 1 -1 1 0 -1 -1 -1 1 -1 0 1 0 0 N = 14 1 -1 1 0 -1 -1 -1 1 -1 0 1 0 0 1 N = 15 1 1 -1 1 0 -1 -1 -1 1 -1 0 1 1 0 1 N = 16 1 1 -1 1 1 0 -1 -1 -1 1 1-
1 0 1 1 0 1 1 0

The parameters used in the calculation are shown below. α: 0.225 dB / km L: 40 km P
₀ : ⁻⁶ dBm k: 5.84 × 10 ⁻⁶ m ² W ⁻² dDc / dλ: 0.
07ps / km-nm-nm Δf: 80 GHz δ: 20 GHz f _zero = f ₀
+ Δf (N−1) / 2 FIG. 3 shows the light intensity of the four-wave mixed light generated by the arrangement of the optical frequencies based on the above specific example and the light intensity of the four-wave mixed light generated when the light is arranged at equal intervals. . As is clear from FIG. 3, when the optical frequency allocating method according to the present embodiment is used, the optical intensities of the four-wave mixing light are approximately 10 when the optical intensity is arranged at equal intervals.
It is possible to reduce it to less than a fraction. Although the embodiment of the present invention has been described in detail above with reference to the drawings, the specific configuration is not limited to this embodiment, and there are design changes and the like without departing from the scope of the invention. Also included in the present invention.

[0021]

As described above, according to the first aspect of the present invention, it is possible to design an optical frequency arrangement with good visibility using a pre-allocated occupied optical frequency band, and to generate four-wave mixing. It is possible to provide an optical frequency allocation method in consideration of efficiency characteristics. According to the invention described in claim 2, by periodically arranging the optical frequency within the determined optical frequency band (± ΔX), it becomes possible to apply the periodic optical frequency filter which can be configured simply. .

[Brief description of drawings]

FIG. 1 is a frequency allocation diagram of optical frequencies obtained by using an optical frequency allocation method according to an embodiment of the present invention.

FIG. 2 is a flowchart showing a flow of processing for obtaining an optical frequency shift amount that minimizes the light intensity of four-wave mixing light.

FIG. 3 is a graph showing the light intensity of four-wave mixing light generated by the arrangement of optical frequencies and the light intensity of four-wave mixing light generated when the light intensity is arranged at equal intervals, which is obtained in the same example.

FIG. 4 is a frequency allocation diagram of optical frequencies according to a conventional example.

Claims (2)

[Claims] 1. A reference optical frequency of optical frequency division multiplexing by N channels is f ₀ , a frequency interval between the N channels is Δf, and an optical frequency shift amount of each channel is Δ.
When X (i), the optical frequency is f _i = f ₀ + i · Δf
In the optical frequency allocation method of inputting the optical signal represented by + ΔX (i) to the i-th channel of the N channels, the optical signals are generated on the optical frequencies of the first to N-th channels. Based on the generation efficiency of the four-wave mixed light so that the light intensity of the four-wave mixed light having the maximum light intensity among the four-wave mixed light is minimized, the optical frequency shift amount ΔX (i) of each channel is calculated. An optical frequency allocation method, characterized in that. 2. The optical frequency allocating method according to claim 1, wherein the optical frequency shift amount ΔX (i) is set to a value smaller than the frequency interval Δf.