CN100354745C - Wavelength converter - Google Patents

Abstract

To provide a wavelength converter having a structure capable of generating high-power converted light even when the difference between the excitation light wavelength and the zero-dispersion wavelength increases. The wavelength converter comprises, for example, an optical fiber having a dispersion slope with an absolute value ≦0.01 ps/nm ² /km for a wavelength of 1550 nm.

Description

wavelength converter

technical field

The present invention relates to a wavelength converter for generating second wavelength-converted light from input light of a first wavelength by utilizing nonlinear optical phenomena.

Background technique

In general, it is known that when high-power light propagates in a medium, various nonlinear optical phenomena occur due to nonlinear polarization in the medium. In this nonlinear optical phenomenon, four-wave mixing (FWM: Four-Wave Mixing) occurs due to a third-order nonlinear effect. Specifically, when three photons are input into a medium, a new photon is generated from them The phenomenon. When both the law of energy conservation and the rate of momentum conservation among a plurality of photons participating in such a nonlinear optical phenomenon hold, the nonlinear optical phenomenon occurs with maximum efficiency.

So far, nonlinear optical phenomena such as those described above have been positively generated in optical fibers, and studies on the use of such optical fibers for wavelength conversion and the like have been in the ascendant. For example, a wavelength converter is an optical device that produces from input light at a first wavelength a second wavelength converted light having the same information as the input light. Such wavelength converters are provided at nodes in an optical communication network in which a plurality of nodes are interconnected by an optical fiber transmission network. At this node, the wavelength converter outputs, as output light, converted light obtained by performing wavelength conversion on the wavelength of incoming input light.

In addition, as a highly nonlinear fiber in which the above-mentioned nonlinear optical phenomenon is likely to occur, for example, in Document 1: Jiro Hiroishi et al., "Dispersion slope controlled HNL-DSF with high γ25 W-1km-1 and bandconversion e4xperiment using this fiber" , ECOC2002, PD1.5, disclosed a highly nonlinear fiber that reduces the dispersion slope to 0.013ps/nm ² /km. In Document 2: Toshiaki Okuno et al., "Generation of Ultra-Broad-Band Supercontinuum By Dispersion-Flattened and Decreasing Fiber", IEEE PHOTONICS TEC.LETT., VOL.10, NO.1, JAN.1998, PP.72-74 , disclosing a high nonlinear dispersion flat fiber. In Document 3: KP Hansen et al., "Fully Dispersion Controlled Triangular-Core Nonlinear Photonic Crystal Fiber", OFC2003, PD2, discloses a dispersion flat highly nonlinear photonic crystal fiber with a short effective length due to large loss. In Document 4: Ju Han Lee et al., "Four-Wave Mixing Based10-Gb/s Tunable Wavelength Conversion Using a HoleyFiber With a High SBS Threshold", IEEE PHOTONICSTECH.LETT., VOL.15, NO.3, MAR.2003 , pp.440-442, discloses that the wavelength difference between the signal light and the excitation light is only about 10nm due to the large absolute value of the wavelength dispersion, but a wavelength converter using a fiber with holes is used. In Document 5: K. Inoue, "Arrangement of fiber pieces for a wide wavelength conversion range by fiber four-wave mixing", OPTICSLETTERS, VOL.19, NO.16, Aug.15, 1994, it is disclosed that the tandem connection is different The technology of expanding the frequency bandwidth to about 2THz by using multiple optical fibers with zero dispersion wavelengths. In addition, in Document 6: M. Onishi et al., "Highly Nonlinear Dispersion-Shifted Fibers and Their Application to BroadbandWavelength Converter", OPTICAL FIBER TECHNOLOGY, Examples of highly nonlinear fibers are disclosed in Vol. 4, 204-214 (1998).

Contents of the invention

As a result of research on the above-mentioned highly nonlinear fiber, the inventors of the present invention found the following problems. That is, in the wavelength converters using the highly nonlinear fibers disclosed in the above-mentioned documents 1 to 6, once the excitation light wavelength deviates from the zero-dispersion wavelength of the optical fiber used, the phase matching condition is not satisfied, so the power of the converted light is sharply reduced. decline. Therefore, in such a wavelength converter, it is difficult to realize variable wavelength conversion in which input signal light is converted to a desired wavelength using only one channel of excitation light.

In addition, in Document 7: Kyo Inoue, "Tunable and Selective Wavelength Conversion Using Fiber Foue-Wave Mixing with Two Pump Lights", IEEE PHOTONICS TECH.LETT., VOL.6, NO.12, DEC.1994, introduced the two The excitation light of the channel is supplied to the wavelength converter of the optical fiber. However, if the excitation light wavelength deviates from the zero-dispersion wavelength of the fiber, the power of the converted light still drops. The excitation light supplied to the two channels itself causes an increase in the manufacturing cost of the wavelength converter. Thus, even with the wavelength converter described in Document 7, it is difficult to perform efficient wavelength conversion in a wider wavelength band.

The present invention was made to solve such problems, and an object of the present invention is to provide a wavelength converter having a structure capable of generating high-power converted light even when the difference between the excitation light wavelength and the zero dispersion wavelength is increased.

The wavelength converter of the present invention is a wavelength converter using an optical fiber, which uses a nonlinear optical phenomenon to input light at a first wavelength to generate second wavelength-converted light different from the first wavelength.

The optical fiber suitable for the wavelength converter of the present invention preferably has a dispersion slope with an absolute value of 0.01 ps/nm ² /km or less ("below" means "≤", the same applies throughout) at a wavelength of 1550 nm. In this case, even if Detuning, which is the difference between the wavelength of light input into the optical fiber and the zero-dispersion wavelength of the optical fiber, increases, high-power converted light can be generated.

In addition, the optical fiber suitable for the wavelength converter of the present invention may have a dispersion slope with an absolute value of 0.01 ps/nm ² /km or less with respect to the wavelength of the pumping light to be supplied. This is because, in the wavelength converter using excitation light, the converted light can be extracted more efficiently by making the dispersion slope of the optical fiber that transmits the excitation light sufficiently small. Especially for excitation light with high optical power, high-power converted light can be generated even if Detuning, which is the difference between the excitation light and the zero-dispersion wavelength of the fiber, is increased by reducing the dispersion slope of the fiber.

The optical fiber suitable for the wavelength converter of the present invention may also have wavelength dispersion with an absolute value of 0.2 ps/nm/km or less in the wavelength range of at least 1530 nm to 1565 nm. Since the wavelength dispersion of this optical fiber can be sufficiently suppressed within the range of the C-band, wavelength conversion with a wider wavelength band can be performed. In addition, within this wavelength range, even if the wavelength of the pumping light is changed, the change in the obtained converted light power is small, so it is possible to generate high-power converted light in a wider wavelength band.

The optical fiber suitable for the wavelength converter of the present invention preferably has at least two zero-dispersion wavelengths within the wavelength range of 1300 nm to 1700 nm. By designing the optical fiber so that there are two or more zero-dispersion wavelengths, the wavelength range in which the absolute value of the wavelength dispersion is small can be expanded. As a result, four-wave mixing can occur efficiently over a wider range of wavelengths.

The wavelength converter of the present invention utilizes a nonlinear optical phenomenon to generate converted light of at least one channel whose wavelength has been converted from excitation light of at least one excitation channel and signal light of at least one signal channel. In this case, preferably, the wavelength converter is equipped with: an excitation light source with a variable wavelength for the ^excitation channel; Slope fiber. Since the dispersion slope of the wavelength of the excitation light is suppressed to be small in the structure in which the excitation light and the signal light are input, the converted light can be generated more efficiently. In addition, especially for excitation light with high optical power, even if the dispersion slope of the fiber is reduced to increase Detuning, which is the difference between the excitation light and the zero-dispersion wavelength of the fiber, high-power converted light can be generated. .

When the optical fiber with the above-mentioned structure has a wavelength of 1550nm, preferably, it has 8 (1/W/km) or more ("above" means "≥", the same throughout), more preferably, it has 10 (1/W/km) The above nonlinear constants. If the nonlinear constant is equal to or greater than this value, converted light can be efficiently generated using actual input optical power. In addition, even if the fiber length is shortened to less than 1 km, converted light with a sufficiently wide wavelength band and high power can be obtained.

In addition, the optical fiber preferably has a transmission loss of 1 dB/km or less at a wavelength of 1550 nm. This is because by keeping the transmission loss low, the effective fiber length for causing the nonlinear optical phenomenon can be sufficiently long, and converted light with higher power can be obtained. In other words, the effective length of the optical fiber can be maintained sufficiently long, and high-power converted light can be generated.

The above optical fiber preferably has a threshold of inductive Brillouin scattering of input excitation light of 10 dBm or more. This is because if the occurrence threshold is equal to or greater than 10 dBm, it is possible to avoid a decrease in the effective fiber length that would cause a nonlinear optical phenomenon, and it is possible to sufficiently divide the input excitation light into converted light. That is, if the generation threshold is equal to or greater than 10 dBm, converted light of high power that can be practically used is generated.

In addition, in the wavelength converter of the present invention, the allowable variable width of the wavelength of the converted light output from the optical fiber is 20 nm or more. By enabling the input signal light to be converted in a wavelength range above 20nm, it can be used as a very practical wavelength converter in an actual optical network.

In the wavelength converter of the present invention, at least for signal channels in the wavelength range (C-band) of 1530nm to 1565nm, the allowable variable width of the wavelength of the converted light output from the optical fiber is preferably 20nm or more. Because very practical wavelength conversion can be realized in the C-band. That is, it is possible to convert to any wavelength independently of the signal light wavelength.

It is preferable that the wavelength converter of the present invention further includes an optical member for shielding excitation light propagating through the optical fiber. The optical component is arranged on the side of the optical output end of the optical fiber. Using the optical component can avoid the impact on the subsequent transmission system caused by the output of high-power excitation light from the above-mentioned optical fiber.

In addition, each embodiment of the present invention can be understood more fully by the following detailed description and accompanying drawings. These examples are for illustration only, and do not constitute a limitation to the present invention.

In addition, further application ranges of the present invention will be clarified by the following detailed description. However, although the detailed description and the specific examples represent preferred embodiments of the present invention, they are only shown for example, and those skilled in the art will understand from the detailed description that they are within the spirit and scope of the present invention. Variations and improvements.

Description of drawings

1A and 1B are cross-sectional views showing the structure of a high nonlinear dispersion flat fiber used in the wavelength converter of the present invention, and their refractive index distributions.

Fig. 2 is a summary table of a plurality of samples (No. 1 to No. 7) that were trial-produced as the high nonlinear dispersion flat fiber shown in Figs. 1A and 1B.

3A and 3B are diagrams showing other distributions of the refractive index of the high nonlinear dispersion flat fiber used in the wavelength converter of the present invention.

Fig. 4 is a configuration diagram showing an evaluation system of an optical fiber sample applied to the wavelength converter of the present invention.

Fig. 5 is a summary table of a plurality of samples (No. 8, No. 9) and fibers to be compared that were trial-produced as evaluation objects of the evaluation system shown in Fig. 4 .

Fig. 6 is a graph showing wavelength dispersion characteristics of an optical fiber of sample No. 8 (high nonlinear dispersion flat fiber) and an optical fiber of sample No. 10 (normal high nonlinear fiber).

Fig. 7 is a graph showing measurement results of FWM optical power.

Fig. 8 is a graph showing a computer simulation of the wavelength dependence of the FWM band width while changing the wavelength dispersion value while the dispersion slope was fixed, based on the optical fiber (high nonlinear dispersion flat fiber) of sample No. 9. In addition, for comparison, simulation results in the case of an optical fiber (normal highly nonlinear fiber) as sample No. 10 are also described. In addition, the actual measured values of the optical fiber as sample No. 9 are also plotted.

Fig. 9 is a graph showing the relationship between wavelength dispersion and FWM band width.

10A to 10E are configuration diagrams showing a first embodiment of an optical communication system to which the wavelength converter of the present invention can be applied.

11A to 11E are configuration diagrams showing a second embodiment of an optical communication system to which the wavelength converter of the present invention can be applied.

Detailed ways

1A, 1B, 2, 3A, 3B, 4 to 9, and 10A to 11D, an embodiment of the wavelength converter of the present invention will be described in detail below. In addition, in the description of the drawings, the same elements are denoted by the same symbols, and overlapping descriptions are omitted.

First, the structure of an optical fiber suitable for the wavelength converter of the present invention will be described. 1A and 1B are cross-sectional views showing the structure of Highly Nonlinear Dispersion Flattened Fiber (HNL-DFF: Highly Nonlinear Dispersion Flattened Fiber), which is an optical fiber suitable for this wavelength converter, and a distribution diagram of its refractive index.

In FIG. 1A , an optical fiber 100 includes: a core 110 extending along a predetermined axis with an outer diameter 2a and a refractive index n1; The cladding region 120 is disposed on the outer periphery of the core region 110, and is equipped with: an inner cladding layer 121 having an outer diameter of 2b and a refractive index n2 (<n1); and a refractive index n3 disposed on the outer periphery of the inner cladding layer 121 (<n1,>n2) outer cladding layer 122 .

In addition, when the outer cladding layer 122 which is the outermost layer of the cladding region 120 is used as a reference region, relative to the outer cladding layer 122, the relative refractive index difference Δ ⁺ of the core region 110 and the relative refractive index difference of the inner cladding layer 121 Δ ^- are given by the following formulae, respectively.

Δ ⁺ ≈(n1-n3)/n1×100

Δ ^- ≈(n2-n3)/n2×100

Fig. 1B is the refractive index distribution 150 of the optical fiber 100 shown in Fig. 1A, in this refractive index distribution 150, the region 151 represents the refractive index of each part on the line L of the core region 110, and the region 152 represents the inner cladding layer 121 on the line L and the region 153 represents the refractive index of each portion of the outer cladding layer 122 on the line L. Such an optical fiber 100 has, for example, silica glass as its main component, GeO ₂ is added to the core 110 , and fluorine is added to the inner cladding 121 . The outer cladding 122 is made of chlorine-doped quartz glass made of pure quartz.

In addition, the optical fiber suitable for the wavelength converter of the present invention can have various refractive index distributions 160, 170 as shown in Figs. 3A and B. The refractive index profile 160 shown in FIG. 3A can be realized by providing an intermediate cladding between the inner cladding 121 and the outer cladding 122 of the optical fiber 100 shown in FIG. 1A . That is, in the refractive index distribution 160, the area 161 represents the refractive index of the core region with the refractive index n1 and the outer diameter 2a; is the refractive index of the inner cladding layer of 2b; region 163 represents the outer periphery of the inner cladding layer, the refractive index is n3 (>n2,<n1), and the outer diameter is the refractive index of the intermediate cladding layer of 2c; region 164 represents the refractive index of the intermediate cladding layer disposed on The outer periphery of the intermediate cladding, the refractive index of the outer cladding with a refractive index of n4 (<n3, >n2).

In addition, the refractive index profile 170 shown in FIG. 3B can be realized by providing two layers of intermediate cladding between the inner cladding 121 and the outer cladding 122 of the optical fiber 100 shown in FIG. 1A . That is, in the refractive index distribution 170, the area 171 represents the refractive index of the core region with the refractive index n1 and the outer diameter 2a; is the refractive index of the inner cladding layer of 2b; the region 173 represents the refractive index of the first intermediate cladding layer arranged on the outer periphery of the inner cladding layer, the refractive index is n3 (>n2,<n1), and the outer diameter is 2c; the region 174 represents Set on the outer periphery of the first intermediate cladding layer, the refractive index of the second intermediate cladding layer with a refractive index n4 (>n2, <n3) and an outer diameter of 2d; the area 175 indicates that it is arranged on the outer periphery of the second intermediate cladding layer, and the refractive index is The refractive index of the outer cladding layer whose index is n5 (<n3,>n4).

[Example 1]

Next, various embodiments of the high nonlinear dispersion flat fiber suitable for the wavelength converter of the present invention will be described. Fig. 2 is a summary table of a plurality of samples (No. 1 to No. 7) that were trial-produced as the high nonlinear dispersion flat fiber shown in Figs. 1A and 1B. In addition, the optical fibers of these samples No. 1 to No. 7 all have the cross-sectional structure and refractive index distribution shown in Fig. 1A and Fig. 1B.

(Sample No.1)

In the optical fiber of Sample No.1, the relative refractive index difference Δ ⁺ of the core region relative to the outer cladding serving as the reference region was 1.37%, and the specific refractive index difference ^Δ- of the inner cladding relative to the outer cladding was -0.82 %. In addition, the value of α for determining the distribution shape of the core region is 3.0. The outer diameter 2a of the core was 4.890 microns, and the ratio Ra (=a/b) of the outer diameter 2a of the core to the outer diameter 2b of the inner cladding was 0.52. As various characteristics at a wavelength of 1550 nm, the optical fiber of this sample No. 1 has a transmission loss of 0.48 dB/km, a wavelength dispersion of 0.063 ps/nm/km, and a dispersion slope of -0.0011 ps/nm ² /km. The cutoff wavelength is 989nm. In addition, as various characteristics at a wavelength of 1550 nm, the optical fiber of Sample No. 1 has an effective cross-sectional area A _eff of 16.4 μm ² , a nonlinear constant γ of 10.4 (1/W/km), and a mode field diameter MFD of 4.6 micron, and the polarization mode dispersion PMD is 0.05ps·km ^-1/2 .

(Sample No.2)

In the optical fiber of Sample No. 2, the relative refractive index difference Δ ⁺ of the core region relative to the outer cladding serving as the reference region was 1.37%, and the specific refractive index difference ^Δ- of the inner cladding relative to the outer cladding was -0.82 %. In addition, the value of α for determining the distribution shape of the core region is 3.0. The outer diameter 2a of the core was 4.908 microns, and the ratio Ra (=a/b) of the outer diameter 2a of the core to the outer diameter 2b of the inner cladding was 0.52. As various characteristics at a wavelength of 1550 nm, the optical fiber of this sample No. 2 has a transmission loss of 0.48 dB/km, a wavelength dispersion of 0.525 ps/nm/km, and a dispersion slope of 0.0006 ps/nm ² /km. The cutoff wavelength is 995nm. In addition, as various characteristics at a wavelength of 1550nm, the optical fiber of Sample No. 2 has an effective cross-sectional area A _eff of 16.5 μm ² , a nonlinear constant γ of 10.3 (1/W/km), and a mode field diameter of MFD of 4.6 micron, and the polarization mode dispersion PMD is 0.06ps·km ^-1/2 .

(Sample No.3)

In the optical fiber of Sample No. 3, the relative refractive index difference Δ ⁺ of the core region relative to the outer cladding serving as the reference region was 1.37%, and the relative refractive index difference ^Δ- of the inner cladding relative to the outer cladding was -0.82 %. In addition, the value of α for determining the distribution shape of the core region is 3.0. The outer diameter 2a of the core was 4.860 microns, and the ratio Ra (=a/b) of the outer diameter 2a of the core to the outer diameter 2b of the inner cladding was 0.52. As various characteristics with a wavelength of 1550nm, the optical fiber of this sample No.3 has: a transmission loss of 0.47dB/km, a wavelength dispersion of -0.771ps/nm/km, and a dispersion slope of -0.0045ps/nm ² / km. The cutoff wavelength is 980nm. In addition, as various characteristics at a wavelength of 1550nm, the optical fiber of Sample No. 3 has an effective cross-sectional area A _eff of 16.3 μm ² , a nonlinear constant γ of 10.5 (1/W/km), and a mode field diameter of MFD of 4.6 micron, and the polarization mode dispersion PMD is 0.02ps·km ^-1/2 .

(Sample No.4)

In the optical fiber of Sample No. 4, the relative refractive index difference Δ ⁺ of the core region relative to the outer cladding serving as the reference region was 1.37%, and the specific refractive index difference ^Δ- of the inner cladding relative to the outer cladding was -0.82 %. In addition, the value of α for determining the distribution shape of the core region is 3.0. The outer diameter 2a of the core was 4.892 micrometers, and the ratio Ra (=a/b) of the outer diameter 2a of the core to the outer diameter 2b of the inner cladding was 0.52. As various characteristics at a wavelength of 1550nm, the optical fiber of this sample No. 4 has: a transmission loss of 0.51dB/km, a wavelength dispersion of -0.097ps/nm/km, and a dispersion slope of -0.0015ps/nm ² /km . The cutoff wavelength is 987nm. In addition, as various characteristics at a wavelength of 1550nm, the optical fiber of Sample No. 4 has an effective cross-sectional area A _eff of 16.4 μm ² , a nonlinear constant γ of 10.4 (1/W/km), and a mode field diameter of MFD of 4.6 micron, and the polarization mode dispersion PMD is 0.03ps·km ^-1/2 .

(Sample No.5)

The optical fiber of sample No. 5 is a dispersion-managed fiber (DMF: Dispersion-Managed Fiber) whose wavelength dispersion changes along the longitudinal direction from one end (hereinafter referred to as end A) to the other end (hereinafter referred to as end B). In the optical fiber of sample No. 5, the relative refractive index difference Δ ⁺ of the core region relative to the outer cladding serving as a reference region is 1.37%, and the specific refractive index difference Δ ^- of the inner cladding relative to the outer cladding is - 0.82%. In addition, the value of α for determining the distribution shape of the core region is 3.0. The outer diameter 2a of the core is 4.88 micrometers on the A-end side and 5.36 micrometers on the B-end side. The ratio Ra (=a/b) of the outer diameter 2a of the core to the outer diameter 2b of the inner cladding was 0.52. As various characteristics at a wavelength of 1550nm, the optical fiber of this sample No. 5 has: the average value of transmission loss is 0.55dB/km, the average value of wavelength dispersion is 5.432ps/nm/km, and the average value of dispersion slope is 0.0168 ps/nm ² /km. In addition, the wavelength dispersion and the dispersion slope on the A side were -0.2 ps/nm/km and -0.002 ps/nm ² /km, respectively. On the other hand, the wavelength dispersion and dispersion slope at the B-terminal side were 9.0 ps/nm/km and 0.026 ps/nm ² /km, respectively. The cutoff wavelength is 987 nm on the A side and 1084 nm on the B side. In addition, as various characteristics at a wavelength of 1550 nm, the optical fiber of Sample No. 5 had a polarization mode dispersion PMD of 0.05 ps.km ^-1/2 on average. The effective cross-sectional area A _eff on the A side was 16.4 μm ² , and the effective cross-sectional area A _eff on the B side was 17.4 μm ² . The nonlinear constant γ on the A side is 10.4 (1/W/km), and the nonlinear constant γ on the B side is 9.8 (1/W/km). In addition, the mode field diameter MFD on the A side is 4.6 micrometers, and the mode field diameter MFD on the B side is 4.8 micrometers.

(Sample No.6)

In the optical fiber of sample No. 6, the relative refractive index difference Δ ⁺ of the core region relative to the outer cladding serving as the reference region was 1.30%, and the specific refractive index difference ^Δ- of the inner cladding relative to the outer cladding was -0.75 %. In addition, the value of α for determining the distribution shape of the core region is 2.8. The outer diameter 2a of the core was 5.288 micrometers, and the ratio Ra (=a/b) of the outer diameter 2a of the core to the outer diameter 2b of the inner cladding was 0.55. As various characteristics at a wavelength of 1550 nm, the optical fiber of this sample No. 6 has a transmission loss of 0.43 dB/km, a wavelength dispersion of 0.31 ps/nm/km, and a dispersion slope of 0.001 ps/nm ² /km. The cutoff wavelength is 948nm. In addition, as various characteristics at a wavelength of 1550nm, the optical fiber of sample No. 6 has an effective cross-sectional area A _eff of 18.2 μm ² , a nonlinear constant γ of 9.1 (1/W/km), and a mode field diameter of MFD of 4.9 micron, and the polarization mode dispersion PMD is 0.03ps·km ^-1/2 .

(Sample No.7)

In the optical fiber of Sample No. 7, the relative refractive index difference Δ ⁺ of the core region relative to the outer cladding serving as the reference region was 1.30%, and the specific refractive index difference ^Δ- of the inner cladding relative to the outer cladding was -0.75 %. In addition, the value of α for determining the distribution shape of the core region is 2.8. The outer diameter 2a of the core was 5.274 micrometers, and the ratio Ra (=a/b) of the outer diameter 2a of the core to the outer diameter 2b of the inner cladding was 0.55. As various characteristics at a wavelength of 1550nm, the optical fiber of this sample No. 7 has: a transmission loss of 0.40dB/km, a wavelength dispersion of -0.10ps/nm/km, and a dispersion slope of -0.001ps/nm ² /km . The cutoff wavelength is 944nm. In addition, as various characteristics at a wavelength of 1550nm, the optical fiber of sample No. 7 has an effective cross-sectional area A _eff of 18.2 μm ² , a nonlinear constant γ of 9.1 (1/W/km), and a mode field diameter of MFD of 4.9 micron, and the polarization mode dispersion PMD is 0.01ps·km ^-1/2 .

It can be known from the above embodiments: as the wavelength of 1550nm is various characteristics, the optical fiber suitable for the wavelength converter of the present invention has: the wavelength dispersion with an absolute value of 2 ps/nm/km or less, with an absolute value of 0.01 ps/nm ² / The dispersion slope of km, and the nonlinear constant γ of 8 (1/W/km) or more, preferably 10 (1/W/km) or more. In addition, preferably, the dispersion management fiber has a wavelength dispersion of +4 to +15 ps/nm/km on the A side, a dispersion slope with an absolute value of 0.04 ps/nm ² /km or less, and 8(1/W/km ) above the nonlinear constant γ, on the other hand, at the B terminal side, there are: wavelength dispersion of +2～-2ps/nm/km, absolute value of dispersion slope below 0.01ps/nm ² /km, and 8(1 /W/km) above the nonlinear constant γ. In addition, the effective cross-sectional area A _eff is 20 μm ² or less, preferably 17 μm ² or less, the polarization mode dispersion PMD is preferably 0.3 ps·km ^−1/2 or less, and the transmission loss is preferably 1.0 dB/km or less.

In order to obtain a preferable refractive index distribution shape, it is preferable that the relative refractive index difference Δ ⁺ of the core region based on the outer cladding is 1.2% or more, and the relative refractive index difference ^Δ- of the inner cladding is -0.6% or less. In addition, preferably, the value of α when the refractive index distribution of the core region is approximated to the power distribution is 2 or more, and the ratio Ra (=a/b) of the outer diameter 2a of the core region to the outer diameter 2b of the inner cladding is 0.30 ~0.70.

Next, the superiority of the high nonlinear dispersion flat fiber (HNL-DFF) suitable for the wavelength converter of the present invention is verified by comparing with the existing high nonlinear fiber (HNLF). Fig. 4 is a configuration diagram showing an evaluation system of an optical fiber sample applied to the wavelength converter of the present invention.

The evaluation system shown in FIG. 4 includes a dual-input-dual-output 3dB photocoupler 50 . A variable-length laser source (TLS: Tunable Laser Source) 10a for supplying probe light is optically connected to the first input end of the optical coupler 50, and a polarization control device is arranged between the optical coupler 50 and the TLS 10a. (PC: Polarization Controller) 20a, Erbium-Doped Fiber Amplifier (EDFA: Erbium-Doped Fiber Amplifier) 30a, and variable band-pass filter (BPS: Band Pass Filter) 40a. On the other hand, the TLS 10b for supplying excitation light is optically connected to the second input end of the optical coupler 50, and the PC 20b, the EDFA 30a, and the BPS 40a are disposed between the optical coupler 50 and the TLS 10b.

Optical Spectrum Analyzers (OSA: Optical Spectrum Analyzer) 70a, 70b are arranged on the first output end and the second output end of the optical coupler 50 respectively, and the evaluation object fiber 60 is arranged on the first output end of the optical coupler 50 and the OSA 70a Therefore, the OSA 70a is configured to monitor the output of the fiber 60 to be evaluated.

Fig. 5 is a summary table of a plurality of samples (No. 8, No. 9) and fibers to be compared that were trial-produced as evaluation objects of the evaluation system shown in Fig. 4 . In addition, the optical fibers of sample No. 8 and No. 9 are high nonlinear dispersion flat fibers (HNL-DFF: Highly Nonlinear Dispersion-Flattened Fiber) suitable for the wavelength converter of the present invention, and the optical fiber of sample No. 10 It is a conventional highly nonlinear fiber (HNLF: Highly Nonlinear Fiber), sample No. 11 is a dispersion flat fiber (DFF: Dispersion-Flattened Fiber) disclosed in the above-mentioned document 2, and sample No. 12 is disclosed in the above-mentioned document 3 Highly Nonlinear Dispersion-Flattened Photonic Crystal Fiber (HNL-DFPCF: Highly NonlinearDispersion-Flattened Photonic Crystal Fiber).

(Sample No.8)

The length of HNL-DFF of sample No.8 is 1000m, and various characteristics of wavelength 1550nm include: transmission loss 0.47dB/km, wavelength dispersion 0.42ps/nm/km, dispersion slope 0.0002ps/nm ² /km, and the nonlinear constant γ is 10.4 (1/W/km).

(Sample No.9)

The HNL-DFF of sample No.9 has a length of 500m, and various characteristics at a wavelength of 1550nm are: transmission loss of 0.62dB/km, wavelength dispersion of 0.063ps/nm/km, and dispersion slope of -0.0011ps/ nm ² /km, and the nonlinear constant γ is 10.4 (1/W/km).

(Sample No.10)

The length of the HNLF of sample No. 10 is 1000m, and the various characteristics at the wavelength of 1550nm are: transmission loss of 0.56dB/km, wavelength dispersion of -0.36ps/nm/km, and dispersion slope of 0.025ps/nm ² /km, and the nonlinear constant γ is 20.4 (1/W/km).

(Sample No.11)

The length of the DFF of sample No. 11 is 1000m, and the various characteristics of the wavelength of 1550nm are: transmission loss 0.22dB/km, wavelength dispersion 0.32ps/nm/km, dispersion slope 0.0036ps/nm ² / km, and the nonlinear constant γ is 5.1 (1/W/km).

(Sample No.12)

The length of the PCF of sample No. 12 is 500m, and as various characteristics at a wavelength of 1550nm, there are: a transmission loss greater than 9.9dB/km, a wavelength dispersion of -1ps/nm/km, and a dispersion slope of 0.001ps/nm ² /km, and the nonlinear constant γ is 11.2 (1/W/km).

6 is a graph showing the wavelength dispersion characteristics of the optical fiber of sample No. 8 (HNL-DFF) and the optical fiber of sample No. 10 (conventional HNLF). In FIG. 6 , curve 610 represents the wavelength dispersion characteristic of HNL-DFF, and curve G620 represents the wavelength dispersion characteristic of HNLF. As can be seen from FIG. 6 , the HNL-DFF has a small dispersion slope in a wider wavelength range and can perform effective wavelength conversion.

In addition, the inventors measured the optical power of the FWM converted light while changing the actual excitation light wavelength in the evaluation system shown in FIG. 4 . Fig. 7 is a graph showing measurement results of FWM optical power. In this measurement, HNL-DFF of the above-mentioned sample No. 9 was prepared. Furthermore, the FWM optical power corresponding to the wavelength of the probe light was measured when the input power of the excitation light and the probe light were each 16 dBm in a state where the wavelength of the excitation light was fixed at 1540 nm.

In this specification, the wavelength band that is 3dB lower than the peak value of the FWM optical power is defined as the FWM band width. In this case, it was found that a band width of 20 nm can be obtained by using the above-mentioned measurement method (see FIG. 7 ). The FWM band width is plotted for different excitation light wavelengths, and the result is curve G860 in FIG. 8 . It can be seen from FIG. 8 that a FWM band width of 20 nm can be ensured in the wavelength range of 1530 nm to 1565 nm. This fact indicates that the detuning of the excitation light wavelength is 30nm or more, which means that by adopting HNL-DFF, the wavelength band that can be converted can be greatly expanded than before. In addition, the conversion power is about -19dB, and with a fiber length of 500m, higher conversion efficiency than conventional dispersion flat fibers can be obtained, and a practical value can be realized. Therefore, the nonlinear constant γ is preferably 10 (1/W/km) or more.

Fig. 8 is based on the optical fiber (HNL-DFF) of sample No. 9, and under a certain dispersion slope, the curve of the wavelength dependence of the FWM band width when the peak dispersion value is shifted is computer-simulated. picture. In FIG. 8, curve G810 represents the FWM band width corresponding to the excitation light wavelength of HNLF (sample No. 10) for comparison, and curve G820 represents the HNL of sample No. 9 corresponding to wavelength dispersion (wavelength 1545 nm). -The original wavelength dispersion of DFF, the same below) is the FWM band width of the excitation light wavelength of HNL-DFF of 0.065ps/nm/km, and the curve G830 represents the excitation light corresponding to the HNL-DFF of 0ps/nm/km wavelength dispersion The FWM band width of the wavelength, the curve G840 represents the FWM band width corresponding to the excitation light wavelength of the HNL-DFF with a wavelength dispersion of -0.065ps/nm/km, and then the curve G850 represents the wavelength dispersion corresponding to +0.13ps/nm/km The FWM band width of the exciting light wavelength of the HNL-DFF in km. In addition, as described above, the curve G860 is the measurement result of plotting the width of the FWM band for different excitation light wavelengths. From this figure, it can be confirmed that by employing the HNL-DFF in the wavelength converter, even if the excitation light wavelength oscillates greatly, the sharp narrowing of the FWM band width can be avoided. In addition, it can be seen from the curve G810 that the existing HNLF needs to match the excitation light wavelength with the zero dispersion wavelength, and if the excitation light wavelength deviates from the zero dispersion wavelength, the conversion efficiency will drop sharply.

In addition, as shown in the table in FIG. 5 , regarding the transmission loss of the above optical fiber, a value sufficiently lower than 1 dB/km can be obtained. However, in the optical fiber applied to the wavelength converter of the present invention, if the nonlinear constant γ is 10 (1/W/km) or more, even if the transmission loss is 1 dB/km, the optical fiber with a fiber length of about 1 km (1000 m) can be used. Since sufficiently high conversion efficiency is obtained, it can be considered that there is no practical problem if the transmission loss is 1 dB/km or less.

In addition, regarding the induced Brillouin scattering, whether it will be found to be a problem under actual use conditions. Conversely, as an actual input condition, when the generation threshold of signal light and pumping light is 10dBm or less, the reduction in conversion efficiency is a problem, so this means that it is necessary to use optical fibers and Exciting light source.

In addition, FIG. 9 is a graph showing the relationship between chromatic dispersion and FWM band width at the excitation wavelength. Actually, if the minimum wavelength variable range is ±6nm (FWM band width=12nm), it is considered that a flexible optical network can be realized. It can be seen from the curve in Fig. 9 that the absolute value of the required wavelength dispersion is ±0.2 ps/nm/km or less. Therefore, in order to realize variable wavelength conversion in the entire range of the C-band (1530nm-1565nm), the absolute value of the wavelength dispersion must be less than 0.2 ps/nm/km in the wavelength range of 1530nm-1565nm.

Next, an optical communication system to which the wavelength converter of the present invention can be applied will be described. 10A to 10E are configuration diagrams showing a first embodiment of an optical communication system to which the wavelength converter of the present invention can be applied.

In the optical communication system shown in Figure 10A, on the main line of the transmission line, from the light transmitting unit (TX) 201 to the light receiving unit (RX) 202, the following configurations are arranged in sequence: EDFA 211, DMF 221, guiding light from the transmission line branch line Optical coupler 231, EDFA 212, DMF 222, variable attenuator 241 (ATT), EDFA 213, AWG 250 are used. A wavelength converter 200 (wavelength converter of the present invention) is provided on the branch line of the transmission line, and the excitation light output from the excitation light source 204 is input to the wavelength converter 200, and the excitation light output from the optical transmission unit (TX) 203 is sequentially transmitted to the EFDA 216 and the signal light transmitted in the transmission line fiber 224, the optical coupler 231 outputs the converted light of a new predetermined wavelength to the main line. The wavelength converter 200 is provided with an optical coupler 232, and the optical coupler 232 passes through the EDFA 214 and the variable BPF 261 successively after the excitation light output from the excitation light source 204 and the excitation light output from the transmission line fiber 224 and passes through the EDFA successively. 215. The signal light of the variable BPF 262 is combined, and the HNL-DFF 223 is connected to the output end of the optical coupler 232. In addition, variable BPF 263 and variable ATT 242 are arranged between HNL-DFF 223 and photocoupler 231.

Usually, since FWM is a high-speed development in the order of (femto) seconds, as a method of packet processing signal light, it can be mentioned that by appropriately modulating the excitation light used for conversion, adding the converted component to The method obtained by transforming light. The optical communication system shown in Fig. 10A is such a system: Assuming that the signal light from the branch line is added to the main line of the transmission line, the signal light transmitted in the main line is burst-switched, and the data from the branch line is carried in its idle time The so-called time division multiplexing system. In the experiment, TDM (Time Division Multiplexing) signals were received, the signal components from the main line and the signal components from the branch lines were studied separately, and it was confirmed that good optical transmission can be realized. In addition, at the subsequent stage of the wavelength converter 200, a variable BPF 263 is provided to remove excitation light (and input signal light).

In addition, Fig. 10B shows the main signal light component on the output port A of the EDFA 211 on the main line, Fig. 10C shows the additional signal light component on the output port B of the EDFA 215 on the branch line, and Fig. 10D shows the light component installed in the wavelength converter The wavelength-converted converted light component on the output port C of the variable ATT 242 in the rear stage of 200, and then, Fig. 10E shows the composite signal light component on the output port D of the EDFA 212 on the main line.

11A to 11E are configuration diagrams showing a second embodiment of an optical communication system to which the wavelength converter of the present invention can be applied.

In the optical communication system shown in FIG. 11A , along the propagation direction of the multiplexed signal light of multiple channels, on the transmission line main line, EDFA 301, transmission line fiber 311, and guiding light from the transmission line branch line are sequentially arranged. Used optical coupler 320, EDFA302, transmission line fiber 312, EDFA 303. The wavelength converter 300 is arranged on the branch line of the transmission line, and another signal light is guided into the wavelength converter 300 through the EDFA 304 and the transmission line fiber 313. Then, the converted light output from the wavelength converter 300 is introduced into the main line through the optical coupler 320 .

In the case of a flexible network, it is expected that the wavelength distribution of WDM (Wavelength Division Multiplexing) signals in the main line of the transmission line will change with time. Therefore, in order to improve the utilization efficiency of each signal channel and match the signal light merged from the branch line with the idle condition of the signal channel in the main line, it is necessary to properly tune the conversion wavelength. In this case, the wavelength converter of the present invention is suitable for generating converted light of a desired wavelength in a wide wavelength band as a variable wavelength converter, and it is easy to configure an optical communication system.

In addition, Fig. 11B represents the WDM signal light on the input port A of the EDFA 301 on the main line, Fig. 11C represents the signal light on the input port B of the EDFA 304 on the branch line, and Fig. 11D represents the output port C of the wavelength converter 300 The converted light on which the wavelength has been changed, and then, FIG. 11E shows the WDM signal light on the output port D of the EDFA 302 on the main line.

In addition, with the HNL-DFF of the present invention, it is possible to generate high-efficiency SC (Supercontinuum) light, realize a parametric amplifier of broadband light, and the like.

From the above description of the present invention, it can be understood that various modifications can be made to the present invention. Such modifications should not be regarded as departing from the spirit and scope of the present invention, and all improvements that are self-evident to those skilled in the art are included in the following claims.

Possibility of industrial use

According to the present invention, a wavelength converter is realized by utilizing a high nonlinear dispersion flat fiber having a small dispersion slope with respect to a high-power excitation light, even if the wavelength of the excitation light and the zero dispersion of the high nonlinear dispersion flat fiber The detuning of the wavelength difference increases, and high-power converted light can also be generated. In addition, even if the wavelength of the excitation light is changed over a wavelength range of about 35 nm, since the optical power of the wavelength-converted light corresponding to the wavelength of the excitation light can be maintained sufficiently, it is possible to obtain a variable wavelength that can realize wavelength conversion in a wider band. converter.

Claims (11)

1. A wavelength converter for generating converted light of a second wavelength different from the first wavelength from input light of a first wavelength by utilizing nonlinear optical phenomena, characterized in that: Comprising an optical fiber having a wavelength dispersion with an absolute value ≤ 0.2 ps/nm/km at least for the wavelength range of 1530 nm to 1565 nm. 2. The wavelength converter according to claim 1, characterized in that: The optical fiber above has a dispersion slope with an absolute value ≤0.01 ps/nm ² /km at a wavelength of 1550 nm. 3. The wavelength converter according to claim 1, characterized in that: The above-mentioned optical fiber has a dispersion slope with an absolute value ≤ 0.01 ps/nm ² /km for the wavelength of the excitation light supplied to the wavelength converter. 4. The wavelength converter according to claim 1, characterized in that: The wavelength converter uses nonlinear optical phenomena to perform wavelength conversion from the excitation light of at least one excitation channel and the signal light of at least one signal channel to generate converted light of at least one channel, The wavelength converter is equipped with: A wavelength-variable excitation light source for the above-mentioned excitation channel; and An optical fiber having a dispersion slope with an absolute value ≤ 0.01 ps/nm ² /km with respect to the wavelength of excitation light supplied from the above-mentioned excitation light source. 5. A wavelength converter for generating converted light of a wavelength-converted second wavelength different from the first wavelength from input light of a first wavelength by utilizing nonlinear optical phenomena, characterized in that: Contains optical fibers with at least two zero-dispersion wavelengths in the wavelength range from 1300nm to 1700nm, and For the input excitation light, there is an occurrence threshold of inductive Brillouin scattering of ≥10 dBm. 6. The wavelength converter according to claim 1 or 5, characterized in that: The above optical fiber has a nonlinear constant ≥ 10 (1/W/km) for a wavelength of 1550 nm. 7. A wavelength converter according to claim 1 or 5, characterized in that: The above optical fiber has a transmission loss of ≤1dB/km for a wavelength of 1550nm. 8. The wavelength converter according to claim 4, characterized in that: The allowable variable width of the wavelength of the converted light output from the above-mentioned optical fiber is ≥ 20nm. 9. The wavelength converter according to claim 4, characterized in that: At least for signal channels in the wavelength range of 1530nm-1565nm, the allowable variable width of the wavelength of the converted light output from the optical fiber is ≥ 20nm. 10. The wavelength converter according to claim 1 or 5, characterized in that: An optical component for shielding excitation light propagating through the optical fiber is also provided, which is arranged on the light output end side of the optical fiber. 11. The wavelength converter according to claim 1, characterized in that: This wavelength converter has an occurrence threshold of induced Brillouin scattering of ≥ 10 dBm for the input excitation light.

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