JP3473661B2 - Optical wavelength converter - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical wavelength conversion device for wavelength conversion utilizing four-wave mixing in an optical fiber.

[0002]

2. Description of the Related Art Four-wave mixing in an optical fiber is caused by third-order non-linear polarization, and a fourth light (frequency f ₄ = f ₁ + f ₂ −) is generated by three lights having frequencies f ₁ , f ₂ and f _3. f ₃ ) is a phenomenon that occurs. Currently, in the case of degenerate four-wave mixing with f ₁ = f ₂ , that is, two lights with frequencies f ₁ and f ₃ are incident on the optical fiber, and light with a frequency f ₄ (= 2f ₁ −f ₃ ) is newly generated. The application to the wavelength conversion of an optical signal is being examined in the case of making it.

By the way, in order for four-wave mixing to occur,
The phase matching condition (propagation constant difference = 0) needs to be satisfied. This propagation constant difference is determined by the wavelength dispersion D (= d ² β / dωdλ) of the optical fiber unless the pump light intensity is so high. In the case of generating converted light of frequency f _C (= 2f _P −f _S ) by four-wave mixing of pumping light of frequency f _P (wavelength λ _P ) and optical signal of frequency f _S (wavelength λ _S ), The propagation constant difference Δβ is the zero dispersion frequency of the optical fiber f
_{If 0} and the corresponding zero-dispersion wavelength are λ ₀ ,

[0004]

[Equation 2]

It is expressed as From equation (1), the pumping light wavelength λ
_{If P} is set to the zero-dispersion wavelength λ ₀ of the optical fiber (λ _P =
λ ₀ ), Δβ = 0 for all optical signal wavelengths λ _S , and it can be seen that the phase matching condition is satisfied. Therefore, when wavelength conversion is performed on an optical signal in the 1.55 μm band, a dispersion-shifted fiber having a zero dispersion wavelength near 1.55 μm and a small dispersion slope dD / dλ of the optical fiber is used as a four-wave mixing medium. The wavelength of the pumping light has been set to its zero-dispersion wavelength (Arai et al., “Development of Wide-Waveband FWM Optical Generation Fiber”, 1996 IEICE General Conference (March 1996), B-1137).

[0006]

By the way, in wavelength conversion by four-wave mixing, it is necessary to change the pumping light wavelength λ _{P in order} to change the conversion destination wavelength. However, when the pumping light wavelength λ _P deviates from the zero-dispersion wavelength λ ₀ of the optical fiber, the propagation constant difference Δβ deviates from 0, the phase matching condition is not satisfied, and the intensity of the converted light sharply decreases. .

According to the present invention, in the wavelength conversion by four-wave mixing, the converted light intensity does not decrease even if the pumping light wavelength λ _P is changed in the range from the zero dispersion wavelength λ ₀ of the optical fiber to the optical signal wavelength λ _S. An object is to provide a wavelength conversion device.

[0008]

The optical wavelength conversion device of the present invention uses, as a four-wave mixing medium, an optical fiber having a zero-dispersion wavelength λ ₀ on the short wavelength side of the optical signal wavelength λ _S (λ ₀
<Λ _S ). When the difference between the maximum value and the minimum value of the change curve of the propagation constant difference according to the pumping light wavelength λ _P is Δκ _PP , the pumping light intensity P _P incident on the optical fiber is set to Δκ _PP / 4γ or more. .

Note that γ = n ₂ ω _P / cA _eff (2) Here, n ₂ is the nonlinear refractive index coefficient, ω _P is the angular frequency of the excitation light, c is the optical velocity, and A _eff is the effective core area of the optical fiber. As a result, within the range from the zero-dispersion wavelength λ ₀ of the optical fiber to the optical signal wavelength λ _S , the pumping light wavelength λ
The converted light intensity when _P is changed (λ ₀ <λ _P <λ _S ) is the converted light intensity when the pumping light wavelength λ _P matches the zero-dispersion wavelength λ ₀ (λ _P = λ ₀ ). Does not fall below. The principle will be described below with reference to FIG.

The propagation constant difference Δβ depends only on the wavelength dispersion of the optical fiber, but when the power density of the pumping light in the optical fiber becomes sufficiently large, the propagation due to the change in the refractive index due to the Kerr effect of the pumping light occurs. Changes in constant difference cannot be ignored. The propagation constant difference κ in consideration of this is expressed as κ = Δβ + 2γP _P (3) Here, 2γP _P of the second term on the right side is a change in the propagation constant difference due to the influence of the Kerr effect of the excitation light. Conventionally, there is little influence of the excitation light intensity P _P small 2GanmaP _P, i.e. four-wave mixing at the 2γP _P «Δβ has been performed.

FIG. 1 schematically shows the relationship between the pumping light wavelength λ _P and the propagation constant difference κ. In the figure, (a) is the case where the zero-dispersion wavelength λ ₀ of the optical fiber is shorter than the optical signal wavelength λ _S (λ ₀ <λ _S ), and (b) is the zero-dispersion wavelength λ of the optical fiber.
_{When 0} is on the longer wavelength side than the optical signal wavelength λ _S (λ ₀ >
λ _S ). The curve indicated by the broken line is the case where the influence of the Kerr effect of the excitation light is not considered (2γP _P << Δβ), and the curve indicated by the solid line is the case where the influence is considered. If the effect of the Kerr effect of the excitation light is not considered, both (a) and (b) are λ _P
= Λ ₀ and λ _P = λ _S , the propagation constant difference κ becomes 0. When the excitation light intensity P _P is increased and the Kerr effect is exerted,
The curve indicated by the broken line increases by 2γP _{P in the} vertical axis direction to become the curve indicated by the solid line.

The difference Δκ _PP between the maximum value and the minimum value of the change curve of the propagation constant difference κ is calculated by using the equation (1),

[0013]

[Equation 3]

It is expressed as Below, λ shown in Fig. 1 (a)₀<
λ_SAnd λ shown in Fig. 1 (b).₀> Λ_SIn case of
Excitation light wavelength λ_PIs the zero-dispersion wavelength λ of the optical fiber₀From
Optical signal wavelength λ_SConverted light wavelength, shifting within the range ofλ
_C (= Λ _P ・ Λ _S / (2λ _S −λ _P ))Transformation when changing
The light intensity will be described.   λ₀<Λ_P<Λ_Sin the case of Change in propagation constant difference κ 2γP_PIs Δκ_PPGreater than / 2
Case, that is, the excitation light intensity P_PIs Δκ_PP/ 4g or more
, Λ₀<Λ_P<Λ_SThe absolute value of the propagation constant difference κ at is
As indicated by the thick line in the figure, λ_P= Λ₀Propagation constant difference in
Will always be smaller than. Therefore, the pumping light wavelength λ_PBut
λ₀<Λ_P<Λ_SThe converted light intensity when in the range of
λ_P= Λ₀It becomes larger than the converted light intensity at.

Here, Δ which determines the lower limit of the excitation light intensity P _P
κ _PP / 4γ is calculated by using equation (2) and equation (4).

[0016]

[Equation 4]

It can be rewritten as It should be noted that the four-wave mixing efficiency within the range where the pumping light wavelength λ _P is λ ₀ <λ _P <λ _S does not become smaller than the four-wave mixing efficiency at λ _P = λ ₀ as described above. The wavelength λ _P is shifted from the zero-dispersion wavelength λ ₀ and the converted light wavelength λ _C (= λ _P
• Even if λ _S / (2λ _S −λ _P )) is changed, the converted light intensity does not decrease. However, while the four-wave mixing efficiency increases as the pumping light intensity P _P increases, the four-wave mixing efficiency decreases as the propagation constant difference changes. Therefore, the upper limit of the pumping light intensity P _P exists at an appropriate point. To do. FIG. 1 (a) shows 2γP _P = 2Δκ _PP as an example of the upper limit.

[0018] λ _0> λ _P> λ regardless of the size of the case the excitation light intensity P _P of _{S, λ 0> λ P>} λ S
The propagation constant difference κ at is λ _{P as} shown by the thick line in the figure.
It is always larger than the propagation constant difference at = λ ₀ . Therefore, the converted light intensity when the pumping light wavelength λ _P is within the range of λ ₀ > λ _P > λ _S is smaller than the converted light intensity when λ _P = λ ₀ . That is, when λ ₀ > λ _S ,
When the pumping light wavelength λ _P is in the range of λ ₀ > λ _P > λ _S , the converted light intensity becomes further smaller and it cannot be used for wavelength conversion in which the pumping light wavelength is variable.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 2 shows an embodiment of the optical wavelength conversion device of the present invention. In the present embodiment, the wavelength (λ _S ) 157
Zero-dispersion wavelength (λ ₀ ) 1561.7 nm for optical signal pulse of 0.9 nm, pulse width 40 ps and repetition frequency 2.5 GHz
An optical fiber of (= λ _S -9.2 nm) is used as a four-wave mixing medium.

In the figure, the wavelength converter of the present embodiment multiplexes the wavelength tunable pumping light source 11 for generating a 2.5 GHz pumping light pulse train (pulse width of about 60 ps) synchronized with the optical signal, and the optical signal and the pumping light. The optical coupler 12 and the thin core fiber 13 (mode field diameter connected to the optical coupler 12
4.2 μm, zero dispersion wavelength 1561.7 nm, length 5 km). The optical signal incident on the device is the optical coupler 1
At 2, the light is combined with the excitation light and is incident on the small-diameter core fiber 13, and the converted light is newly generated by the four-wave mixing in the small-diameter core fiber 13. The average incident intensities of the optical signal and the excitation light on the small-diameter core fiber 13 are −20.0 dBm and +, respectively.
It was set to 11.0 dBm.

The mode field diameter of a general dispersion-shifted fiber is about 8 μm, whereas the mode field diameter is smaller than this, that is, the effective core area A
By using the small-diameter core fiber 13 with a small _eff, γ shown in the equation (2) becomes large and 2γ in the second term of the equation (3) becomes large.
The effect of P _P increases. This makes it possible to increase the change in the propagation constant difference κ with a low excitation light intensity P _P.

Further, the small-diameter core fiber 1 has a dispersion slope dD / dλ smaller than that of a general dispersion-shifted fiber.
If 3 is used, Δκ _PP shown in equation (4) becomes small, and the change in the propagation constant difference κ can be made small. This makes it possible to reduce the change in the converted light intensity depending on the excitation light wavelength. FIG. 3 shows the result of measuring the converted light intensity with respect to the excitation light wavelength λ _P.

The converted light intensity when an excitation light wavelength lambda _P was varied from zero dispersion wavelength lambda ₀ of the optical fiber to near optical signal wavelength lambda _S is the zero dispersion wavelength of the excitation light wavelength lambda _P is the optical fiber It can be seen that the converted light intensity does not fall below the case. Further, in that range, the converted light intensity is larger than the incident light signal intensity −20.0 dBm, and it can be seen that the function of optical amplification is fulfilled at the same time as wavelength conversion.

FIG. 4 shows the converted light with respect to the pumping light intensity when the pumping light wavelength λ _P is fixed on the wavelength side 1.2 nm longer than the zero dispersion wavelength λ ₀ of the optical fiber (λ _P = λ ₀ +1.2 nm). The result of measuring the strength is shown. Average excitation light intensity is +6 dB
From around m, the converted light intensity rapidly increases due to the change in the propagation constant difference due to the excitation light. For example, the excitation light average incident intensity was slightly increased from +6.5 dBm to +9.5 dBm.
When increased by 3.0 dB, the converted light intensity will change from -37.5 dBm to -1.
It can be seen that there is a sharp increase of 19.3 dBm to 8.2 dBm. This is due to the synergistic appearance of the result that the propagation constant difference κ approaches 0 due to the increase of the pump light intensity and the increase of the four-wave mixing efficiency (wavelength conversion gain) due to the increase of the pump light intensity.

[0025]

As described above, the optical wavelength conversion device of the present invention uses an optical fiber having a zero-dispersion wavelength λ ₀ on the shorter wavelength side than the optical signal wavelength λ _S as a four-wave mixing medium, and The intensity P _P is Δκ _PP / 4γ or more, that is,

[0026]

[Equation 5]

The above is done. As a result, the pumping light wavelength λ _P
It is possible to prevent the converted light intensity when the shift is within the range of λ ₀ <λ _P <λ _S from being lower than the converted light intensity when λ _P = λ ₀ . In such an optical wavelength conversion device, the wavelength range of the conversion destination of the optical signal can be expanded in the optical switching by the all-optical operation using the wavelength conversion, and it will be useful for the wavelength multiplexing transmission network in the future.

[Brief description of drawings]

FIG. 1 is a diagram schematically showing a relationship between a pumping light wavelength λ _P and a propagation constant difference κ.

FIG. 2 is a block diagram showing an embodiment of an optical wavelength conversion device of the present invention.

FIG. 3 is a diagram showing a result of measuring a converted light intensity with respect to a pumping light wavelength λ _P.

FIG. 4 is a diagram showing a result of measuring a converted light intensity with respect to an excitation light intensity when the excitation light wavelength λ _P is fixed to λ ₀ +1.2 nm.

[Explanation of symbols]

11 Tunable pump light source 12 Optical coupler 13 Thin core fiber

Front page continued (56) References Takashi Yamamoto et al., High-efficiency four-wave mixing in optical fiber considering phase matching condition depending on light intensity, IEICE technical report, Japan, Electronic Information Network Shinshin Gakkai, October 3, 1996, Vol. 96 No. 284, P61-66 (58) Fields investigated (Int. Cl. ⁷ , DB name) G02F 1/35

Claims (4)

(57) [Claims] 1. Four-wave mixing for generating a converted light of wavelength λ _C (= λ _P · λ _S / (2λ _S −λ _P )) by inputting an optical signal of wavelength λ _S and pumping light of wavelength λ _P As the medium, the optical signal wavelength λ _S
In an optical wavelength conversion device using an optical fiber having a zero-dispersion wavelength λ ₀ on the shorter wavelength side, the intensity of the pumping light is +6 dBm or more, and the pumping light wavelength λ _P is the zero-dispersion wavelength λ of the optical fiber. ₀
To the optical signal wavelength λ _S , an optical wavelength conversion device characterized by being set. 2. The optical fiber is a thin core fiber.
The optical wavelength conversion device according to claim 1, wherein
Place 3. The mode fiber diameter of the optical fiber
Is a small-diameter core fiber smaller than 8 μm.
The optical wavelength conversion device according to claim 1, which is a characteristic. 4. The mode field diameter of the optical fiber
Is a 4.2 μm thin core fiber
The optical wavelength conversion device according to claim 1.