JP2004287382A - Wavelength converter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a wavelength converter of structure enabling generation of converted light with high power even with a large difference between the wavelength of pumping light and the zero-dispersion wavelength. <P>SOLUTION: The wavelength converter includes an optical fiber having a dispersion slope whose absolute value at the wavelength of, for example, 1,550 nm, is 0.01 ps/nm<SP>2</SP>/km or less. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a wavelength converter for generating converted light of a second wavelength from input light of a first wavelength using a nonlinear optical phenomenon.

  Generally, it is known that when high-power light propagates in a medium, various nonlinear optical phenomena occur due to nonlinear polarization in the medium. Among these nonlinear optical phenomena, four-wave mixing (FWM) is caused by a third-order nonlinear effect. Specifically, when three photons enter a medium, one new photon is generated from them. Is a phenomenon that occurs. When the law of conservation of energy and the law of conservation of momentum are satisfied between a plurality of photons involved in such a nonlinear optical phenomenon, the nonlinear optical phenomenon occurs with the highest efficiency.

  2. Description of the Related Art Conventionally, researches for actively generating the above-described nonlinear optical phenomenon in an optical fiber and using the optical fiber for wavelength conversion and the like have been actively conducted. For example, a wavelength converter is an optical device that generates, from input light of a first wavelength, converted light of a second wavelength having the same information as the input light. Such wavelength converters are provided at these nodes in an optical communication network in which a large number of nodes are interconnected by an optical fiber transmission network. At that node, the wavelength converter outputs converted light whose wavelength has been converted from the wavelength of the input light that has arrived, as output light.

For example, Non-Patent Document 1 discloses a highly nonlinear fiber in which the dispersion slope is reduced to 0.013 ps / nm ² / km as a highly nonlinear fiber in which the above-described nonlinear optical phenomenon easily occurs. Non-Patent Document 2 discloses a highly nonlinear dispersion flat fiber. Non-Patent Document 3 discloses a dispersion-flat highly nonlinear photonic crystal fiber having a short effective length due to a large loss. Non-Patent Document 4 discloses a wavelength converter using a holey fiber, although the absolute value of chromatic dispersion is large, so that the wavelength difference between signal light and pump light is only allowed to be about 10 nm. Non-Patent Document 5 discloses a technique in which a plurality of optical fibers having different zero-dispersion wavelengths are connected in cascade to expand the bandwidth to about 2 THz, and Non-Patent Document 6 discloses a highly nonlinear fiber. Are disclosed.
Jiro Hiroishi, et al., "Dispersion slope controlled HNL-DSF with high γ 25 W-1km-1 and band conversion e4xperiment using this fiber", ECOC2002, PD1.5 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. KP Hansen, et al., "Fully Dispersion Controlled Triangular-Core Nonlinear Photonic Crystal Fiber", OFC2003, PD2 Ju Han Lee, et al., "Four-Wave Mixing Based 10-Gb / s Tunable Wavelength Conversion Using a Holey Fiber With a High SBS Threshold", IEEE PHOTONICS TECH. LETT., VOL. 15, NO. 3, MAR. 2003, pp.440-442 K. Inoue, "Arrangement of fiber pieces for a wide wavelength conversion range by fiber four-wave mixing", OPTICS LETTERS, VOL. 19, NO.16, Aug. 15, 1994 M. Onishi, et al., "Highly Nonlinear Dispersion-Shifted Fibers and Their Application to Broadband Wavelength Converter", OPTICAL FIBER TECHNOLOGY, VOL. 4, 204-214 (1998) Kyo Inoue, "Tunable and Selective Wavelength Conversion Using Fiber Four-Wave Mixing with Two Pump Lights", IEEE PHOTONICS TECH.LETT., VOL. 6, NO.12, DEC. 1994

  As a result of studying the above-mentioned highly nonlinear fiber, the inventors have found the following problems. That is, in the wavelength converter using the highly nonlinear fiber disclosed in the above-mentioned Non-Patent Documents 1 to 6, the phase matching condition is not satisfied when the excitation light wavelength departs from the zero dispersion wavelength of the used optical fiber. The optical power of the converted light sharply decreases. Therefore, with such a wavelength converter, it is difficult to realize variable wavelength conversion for converting input signal light into a desired wavelength with pump light of only one channel.

  Non-Patent Document 7 introduces a wavelength converter that supplies two channels of pump light to an optical fiber. However, if the wavelength of the pumping light is separated from the zero dispersion wavelength of the optical fiber, the optical power of the converted light also decreases. Supplying two channels of pump light in the first place is a factor in increasing the manufacturing cost of the wavelength converter. Thus, even with the wavelength converter described in Non-Patent Document 7, it is difficult to efficiently perform wavelength conversion over a wider band.

  The present invention has been made to solve the above-described problems, and has a structure that enables generation of high-power converted light even when the difference between the pumping light wavelength and the zero-dispersion wavelength increases. It is intended to provide a wavelength converter.

  A wavelength converter according to the present invention is a wavelength converter using an optical fiber, wherein a second wavelength different from the first wavelength is converted from input light of a first wavelength by using a nonlinear optical phenomenon. Generates wavelength converted light.

The optical fiber applied to the wavelength converter according to the present invention preferably has a dispersion slope having an absolute value of 0.01 ps / nm ² / km or less at a wavelength of 1550 nm. In this case, even if Detuning, which is the difference between the wavelength of light input to the optical fiber and the zero-dispersion wavelength of the optical fiber, becomes large, it is possible to generate high-power converted light.

Further, the optical fiber applied to the wavelength converter according to the present invention may have a dispersion slope having an absolute value of 0.01 ps / nm ² / km or less at the wavelength of the supplied pump light. This is because, in the wavelength converter using the pump light, the converted light can be extracted more efficiently by sufficiently reducing the dispersion slope of the optical fiber through which the pump light propagates. In particular, by reducing the dispersion slope of the optical fiber with respect to the pumping light of high optical power, even if the detuning, which is the difference between the pumping light and the zero-dispersion wavelength of the optical fiber, increases, the conversion of the high-power converted light can be improved. Generation becomes possible.

  The optical fiber applied to the wavelength converter according to the present invention may have a chromatic dispersion having an absolute value of 0.2 ps / nm / km or less in at least a wavelength range of 1530 nm to 1565 nm. This is because the chromatic dispersion of the optical fiber is sufficiently suppressed in the range of the C band, thereby enabling wavelength conversion in a wider band. Further, in this wavelength range, even if the pumping light wavelength is changed, the fluctuation of the optical power of the converted light is small, so that converted light having a higher optical power in a wider band is generated.

  The optical fiber applied to the wavelength converter according to the present invention preferably has at least two zero dispersion wavelengths in a wavelength range of 1300 nm to 1700 nm. By designing the optical fiber such that two or more zero-dispersion wavelengths exist, the wavelength range where the absolute value of chromatic dispersion is small can be expanded. As a result, four-wave mixing can be efficiently generated over a wider wavelength band.

A wavelength converter according to the present invention generates at least one channel of converted light that has been wavelength-converted using a nonlinear optical phenomenon from at least one pump channel excitation light and at least one signal channel signal light. At this time, the wavelength converter provides a pump light source whose excitation channel wavelength is variable and a dispersion slope having an absolute value of 0.01 ps / nm ² / km or less at the wavelength of the pump light supplied from the pump light source. And an optical fiber having the same. This is because, in a configuration in which the pump light and the signal light are input, the converted light can be generated more efficiently by suppressing the dispersion slope at the pump light wavelength to be small. In addition, by reducing the dispersion slope of the fiber particularly for the pumping light of high optical power, even if the detuning, which is the difference between the pumping light and the zero-dispersion wavelength of the optical fiber, increases, the conversion power of the high-power converting light increases. Can be generated.

  The optical fiber having the above-mentioned structure preferably has a nonlinear constant of 8 (1 / W / km) or more, more preferably 10 (1 / W / km) or more at a wavelength of 1550 nm. If the nonlinear constant is equal to or more than such a value, it is possible to efficiently generate converted light with practical input light power. Even if the fiber length is reduced to 1 km or less, a sufficiently wide band and high power converted light can be obtained.

  Further, the optical fiber preferably has a transmission loss of 1 dB / km or less at a wavelength of 1550 nm. This is because, by suppressing the transmission loss, the effective fiber length at which the nonlinear optical phenomenon occurs can be made 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 is generated.

  Preferably, the threshold value of the stimulated Brillouin scattering of the optical fiber with respect to the input excitation light is 10 dBm or more. If the generation threshold is 10 dBm or more, it is possible to avoid a reduction in the effective fiber length in which the nonlinear optical phenomenon occurs, and it is possible to sufficiently distribute the input pump light to the converted light. That is, if this generation threshold is 10 dBm or more, practically usable high-power converted light is generated.

  Further, in the wavelength converter according to 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 input signal light to be converted within a wavelength range of 20 nm or more, it becomes possible to apply the wavelength converter to a practical level on an actual optical network.

  In the wavelength converter according to the present invention, it is preferable that an allowable variable width of the wavelength of the converted light output from the optical fiber is at least 20 nm for a signal channel in a wavelength range (C band) of at least 1530 nm to 1565 nm. . This is because a sufficiently practical wavelength conversion can be realized in the C band. That is, conversion to an arbitrary wavelength is possible without depending on the signal light wavelength.

  It is preferable that the wavelength converter according to the present invention further includes an optical component for blocking the excitation light propagating in the optical fiber. This optical component is arranged on the optical output end side of the optical fiber. With this optical component, it is possible to avoid the influence on the subsequent transmission system due to the output of the high-power pumping light from the optical fiber.

  As described above, according to the present invention, a wavelength converter is realized by using a highly nonlinear dispersion flat fiber having a small dispersion slope with respect to a high power pump light, so that the pump light wavelength and the highly nonlinear dispersion flat fiber are realized. Even if Detuning, which is the difference from the zero-dispersion wavelength, becomes large, it is possible to generate high-power converted light. Further, even if the wavelength of the excitation light is changed in a wavelength range of about 35 nm, the optical power of the converted light having a wavelength corresponding to the excitation light wavelength is sufficiently maintained, so that the variable wavelength conversion for achieving a wider band wavelength conversion is possible. A wavelength converter is obtained.

  Hereinafter, embodiments of the wavelength converter according to the present invention will be described in detail with reference to FIGS. In the description of the drawings, the same elements will be denoted by the same reference symbols, without redundant description.

  First, the structure of an optical fiber suitable for the wavelength converter according to the present invention will be described. FIG. 1 is a cross-sectional view showing the structure of a highly nonlinear dispersion flattened fiber (HNL-DFF) as an optical fiber suitable for the wavelength converter, and a refractive index profile thereof.

  In FIG. 1A, an optical fiber 100 includes a core region 110 having an outer diameter 2a and a refractive index n1 and extending along a predetermined axis, and a cladding region 120 provided on the outer periphery of the core region 110. The cladding region 120 is provided on the outer periphery of the core region 110, and has an inner cladding 121 having an outer diameter 2b and a refractive index n2 (<n1), and a refractive index n3 (<n1, > N2).

Incidentally, when the outer cladding 122 is the outermost layer of the cladding region 120 as a reference region, the relative refractive index difference delta ⁺ of the core region 110 with respect to the outer cladding 122, the relative refractive index difference delta of the inner cladding 121 ^- is less, respectively Given by

Δ ⁺ ≒ (n1-n3) / n1 × 100
^{Δ - ≒ (n2-n3)} / n2 × 100

FIG. 1B is a refractive index profile 150 of the optical fiber 100 shown in FIG. 1A. In the refractive index profile 150, the region 151 has a refractive index of each part on the line L of the core region 110. The region 152 shows the refractive index of each part on the line L of the inner cladding 121, and the region 153 shows the refractive index of each part on the line L of the outer cladding 122. Such an optical fiber 100 includes, for example, silica glass as a main component, GeO ₂ is added to the core region 110, and fluorine is added to the inner cladding 121. The outer cladding 122 is made of silica glass to which chlorine is added, which is made of pure silica.

  The optical fiber suitable for the wavelength converter according to the present invention may have various refractive index profiles 160 and 170 as shown in FIG. The refractive index profile 160 shown in FIG. 3A is realized by providing an intermediate cladding between the inner cladding 121 and the outer cladding 122 of the optical fiber 100 shown in FIG. That is, in the refractive index profile 160, the region 161 is provided with a refractive index n1 and a refractive index of a core region having an outer diameter 2a, and the region 162 is provided on the outer periphery of the core region, and has a refractive index n2 (<n1) and an outer diameter 2b. The region 163 is provided on the outer periphery of the inner cladding and has a refractive index n3 (> n2, <n1), a refractive index of the intermediate cladding having an outer diameter 2c, and the region 164 has Indicate the refractive indices of the outer claddings provided on the outer periphery of the outer cladding and having a refractive index n4 (<n3,> n2).

  Further, the refractive index profile 170 shown in FIG. 3B is realized by providing two intermediate claddings between the inner cladding 121 and the outer cladding 122 of the optical fiber 100 shown in FIG. You. That is, in the refractive index profile 170, the region 171 is provided with a refractive index n1 and a refractive index of a core region having an outer diameter 2a, and the region 172 is provided on the outer periphery of the core region, and has a refractive index n2 (<n1) and an outer diameter 2b. Is provided on the outer periphery of the inner cladding, and has a refractive index n3 (> n2, <n1) and a first intermediate cladding having an outer diameter 2c. The refractive index n4 (> n2, <n3), the refractive index of the second intermediate cladding having an outer diameter 2d, and the region 175 are provided on the outer periphery of the intermediate cladding, and the refractive index n5 ( The refractive index of the outer cladding having (n3,> n4) is shown.

  Next, embodiments of the highly nonlinear dispersion flat fiber suitable for the wavelength converter according to the present invention will be described. FIG. 2 is a table summarizing the specifications of a plurality of samples (No. 1 to No. 7) prototyped as the highly nonlinear dispersion flat fiber shown in FIG. Note that these sample Nos. 1 to No. Each of the optical fibers 7 has the cross-sectional structure and the refractive index profile shown in FIGS. 1 (a) and 1 (b).

(Sample No. 1)
Sample No. In the optical fiber No. 1, the relative refractive index difference Δ ⁺ of the core region with respect to the outer cladding as the reference region is 1.37%, and the relative refractive index difference Δ ⁻ of the inner cladding with respect to the outer cladding is -0.82%. The α value for determining the profile shape of the core region is 3.0. The outer diameter 2a of the core region is 4.890 μm, 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 0.52. This sample No. The optical fiber 1 has, as characteristics at a wavelength of 1550 nm, a transmission loss of 0.48 dB / km, a chromatic dispersion of 0.063 ps / nm / km, and a dispersion slope of −0.0011 ps / nm ² / km. The cutoff wavelength is 989 nm. Further, as various characteristics at a wavelength of 1550 nm, sample No. The optical fiber No. 1 has an effective area A _eff of 16.4 μm ² , a nonlinear constant γ of 10.4 (1 / W / km), a mode field diameter MFD of 4.6 μm, and 0.05 ps · km ^{−. It has} a polarization mode dispersion PMD of ^1/2 .

(Sample No. 2)
Sample No. In the optical fiber No. 2, the relative refractive index difference Δ ⁺ of the core region with respect to the outer cladding as the reference region is 1.37%, and the relative refractive index difference Δ ⁻ of the inner cladding with respect to the outer cladding is -0.82%. The α value for determining the profile shape of the core region is 3.0. The outer diameter 2a of the core region is 4.908 μm, 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 0.52. This sample No. The optical fiber No. 2 has, as characteristics at a wavelength of 1550 nm, a transmission loss of 0.48 dB / km, a chromatic dispersion of 0.525 ps / nm / km, and a dispersion slope of 0.0006 ps / nm ² / km. The cutoff wavelength is 995 nm. Further, as various characteristics at a wavelength of 1550 nm, sample No. 2 has an effective area A _eff of 16.5 μm ² , a nonlinear constant γ of 10.3 (1 / W / km), a mode field diameter MFD of 4.6 μm, and 0.06 ps · km ^{−. It has} a polarization mode dispersion PMD of ^1/2 .

(Sample No. 3)
Sample No. In the optical fiber No. 3, the relative refractive index difference Δ ⁺ of the core region with respect to the outer cladding as the reference region is 1.37%, and the relative refractive index difference Δ ⁻ of the inner cladding with respect to the outer cladding is -0.82%. The α value for determining the profile shape of the core region is 3.0. The outer diameter 2a of the core region is 4.860 μm, 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 0.52. This sample No. The optical fiber No. 3 has a transmission loss of 0.47 dB / km, a chromatic dispersion of −0.771 ps / nm / km, and a dispersion slope of −0.0045 ps / nm ² / km as characteristics at a wavelength of 1550 nm. . The cut-off wavelength is 980 nm. Further, as various characteristics at a wavelength of 1550 nm, sample No. The optical fiber No. ^{3 has} an effective area A _eff of 16.3 μm ² , a nonlinear constant γ of 10.5 (1 / W / km), a mode field diameter MFD of 4.6 μm, and 0.02 ps · km ^{−. It has} a polarization mode dispersion PMD of ^1/2 .

(Sample No. 4)
Sample No. In the optical fiber No. 4, the relative refractive index difference Δ ⁺ of the core region with respect to the outer cladding as the reference region is 1.37%, and the relative refractive index difference Δ ⁻ of the inner cladding with respect to the outer cladding is -0.82%. The α value for determining the profile shape of the core region is 3.0. The outer diameter 2a of the core region is 4.892 μm, 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 0.52. This sample No. The optical fiber No. 4 has a transmission loss of 0.51 dB / km, a chromatic dispersion of −0.097 ps / nm / km, and a dispersion slope of −0.0015 ps / nm ² / km as characteristics at a wavelength of 1550 nm. . The cutoff wavelength is 987 nm. Further, as various characteristics at a wavelength of 1550 nm, sample No. The optical fiber No. ^{4 has} an effective area A _eff of 16.4 μm ² , a nonlinear constant γ of 10.4 (1 / W / km), a mode field diameter MFD of 4.6 μm, and 0.03 ps · km ^{−. It has} a polarization mode dispersion PMD of ^1/2 .

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

(Sample No. 6)
Sample No. In the optical fiber No. 6, the relative refractive index difference Δ ⁺ of the core region with respect to the outer cladding as the reference region is 1.30%, and the relative refractive index difference Δ ⁻ of the inner cladding with respect to the outer cladding is -0.75%. The α value for determining the profile shape of the core region is 2.8. The outer diameter 2a of the core region is 5.288 μm, 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 0.55. This sample No. The optical fiber No. 6 has, as characteristics at a wavelength of 1550 nm, a transmission loss of 0.43 dB / km, a chromatic dispersion of 0.31 ps / nm / km, and a dispersion slope of 0.001 ps / nm ² / km. The cut-off wavelength is 948 nm. Further, as various characteristics at a wavelength of 1550 nm, sample No. The optical fiber No. 6 has an effective area A _eff of 18.2 μm ² , a nonlinear constant γ of 9.1 (1 / W / km), a mode field diameter MFD of 4.9 μm, and 0.03 ps · km ^{−. It has} a polarization mode dispersion PMD of ^1/2 .

(Sample No. 7)
Sample No. In the optical fiber No. 7, the relative refractive index difference Δ ⁺ of the core region with respect to the outer cladding as the reference region is 1.30%, and the relative refractive index difference Δ ⁻ of the inner cladding with respect to the outer cladding is -0.75%. The α value for determining the profile shape of the core region is 2.8. The outer diameter 2a of the core region is 5.274 μm, 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 0.55. This sample No. The optical fiber No. 7 has, as characteristics at a wavelength of 1550 nm, a transmission loss of 0.40 dB / km, a chromatic dispersion of −0.10 ps / nm / km, and a dispersion slope of −0.001 ps / nm ² / km. . The cut-off wavelength is 944 nm. Further, as various characteristics at a wavelength of 1550 nm, sample No. The optical fiber No. 7 has an effective area A _eff of 18.2 μm ² , a nonlinear constant γ of 9.1 (1 / W / km), a mode field diameter MFD of 4.9 μm, and 0.01 ps · km ^{−. It has} a polarization mode dispersion PMD of ^1/2 .

From the above embodiments, the optical fiber suitable for the wavelength converter according to the present invention has, as various characteristics at a wavelength of 1550 nm, a chromatic dispersion having an absolute value of 2 ps / nm / km or less and an absolute value of 0.01 ps / nm. ^{It has} a dispersion slope of ² / km and a nonlinear constant γ of 8 (1 / W / km) or more, preferably 10 (1 / W / km) or more. The dispersion management fiber has a chromatic dispersion of +4 to +15 ps / nm / km, a dispersion slope having an absolute value of 0.04 ps / nm ² / km or less, and a dispersion slope of 8 (1 / W / km) or more on the A-end side. At the B end, a chromatic dispersion of +2 to −2 ps / nm / km, a dispersion slope having an absolute value of 0.01 ps / nm ² / km or less, and 8 (1 / W / km) ) It is preferable to have the above nonlinear constant γ. Further, the effective area A _eff is, 20 [mu] m ² or less, preferably 17 .mu.m ² or less, the polarization mode dispersion PMD is, 0.3 ps · miles ^-1/2 or less, that the transmission loss is less than 1.0 dB / miles preferable.

In order to obtain a preferable refractive index profile shape, the relative refractive index difference Δ ⁺ of the core region with respect to the outer cladding is preferably 1.2% or more, and the relative refractive index difference Δ ⁻ of the inner cladding is preferably -0.6% or less. The α value when the refractive index profile of the core region is approximated by a 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 to 0. .70 is preferred.

  Subsequently, the superiority of the highly nonlinear dispersion flat fiber (HNL-DFF) suitable for the wavelength converter according to the present invention will be verified as compared with the conventional highly nonlinear fiber (HNLF). FIG. 4 is a diagram showing a configuration of an optical fiber sample evaluation system applied to the wavelength converter according to the present invention.

  The evaluation system shown in FIG. 4 includes a 2 dB × 2 output 3 dB optical coupler 50. A variable-length laser light source (TLS: Tunable Laser Source) 10a for supplying a probe light is optically connected to a first input end of the optical coupler 50, and between the optical coupler 50 and the TLS 10a. Is provided with a polarization controller (PC) 20a, an Er-doped fiber amplifier (EDFA) 30a, and a variable band-pass filter (BPS) 40a. On the other hand, a TLS 10b for supplying excitation light is optically connected to the second input end of the optical coupler 50, and a PC 20b, an EDFA 30a, and a BPS 40a are arranged between the optical coupler 50 and the TLS 10b. Have been.

  Optical spectrum analyzers (OSA: Optical Spectrum Analyzers) 70a and 70b are disposed at the first output terminal and the second output terminal of the optical coupler 50, respectively. By being disposed between the OSA 70a and the OSA 70a, the OSA 70a monitors the output of the evaluation target fiber 60.

  FIG. 5 is a table summarizing a plurality of samples (No. 8 and No. 9) prototyped as evaluation targets in the evaluation system shown in FIG. The sample No. 8 and No. The optical fiber of Sample No. 9 is a highly nonlinear dispersion flat fiber (HNL-DFF) suitable for the wavelength converter according to the present invention. The optical fiber of No. 10 is a conventional highly nonlinear fiber (HNLF: Highly Nonlinear Fiber); Reference numeral 11 denotes a dispersion flat fiber (DFF: Dispersion-Flattened Fiber) disclosed in Non-Patent Document 2; Reference numeral 12 denotes a highly nonlinear dispersion-flattened photonic crystal fiber (HNL-DFPCF) disclosed in Non-Patent Document 3.

(Sample No. 8)
Sample No. The HNL-DFF No. 8 has a length of 1000 m, and has various characteristics at a wavelength of 1550 nm, such as a transmission loss of 0.47 dB / km, a chromatic dispersion of 0.42 ps / nm / km, and 0.0002 ps / nm ^2. / Km and a nonlinear constant γ of 10.4 (1 / W / km).

(Sample No. 9)
Sample No. The HNL-DFF No. 9 has a length of 500 m, and has various characteristics at a wavelength of 1550 nm, a transmission loss of 0.62 dB / km, a chromatic dispersion of 0.063 ps / nm / km, and -0.0011 ps / nm. ^{It has} a dispersion slope of ² / km and a nonlinear constant γ of 10.4 (1 / W / km).

(Sample No. 10)
Sample No. The HNLF of No. 10 has a length of 1000 m, and has various characteristics at a wavelength of 1550 nm, such as a transmission loss of 0.56 dB / km, a chromatic dispersion of −0.36 ps / nm / km, and a 0.025 ps / nm ² /. km and a nonlinear constant γ of 20.4 (1 / W / km).

(Sample No. 11)
Sample No. The 11 DFF has a length of 1000 m, and as characteristics at a wavelength of 1550 nm, transmission loss of 0.22 dB / km, chromatic dispersion of 0.32 ps / nm / km, and 0.0036 ps / nm ² / km And a nonlinear constant γ of 5.1 (1 / W / km).

(Sample No. 12)
Sample No. Twelve PCFs have a length of 500 m, and have various characteristics at a wavelength of 1550 nm, such as a transmission loss of more than 9.9 dB / km, a chromatic dispersion of −1 ps / nm / km, and 0.001 ps / nm ² / km. And a nonlinear constant γ of 11.2 (1 / W / km).

  Note that FIG. No. 8 optical fiber (HNL-DFF) and sample no. 10 is a graph showing chromatic dispersion characteristics of ten optical fibers (conventional HNLF). 6, a graph 610 shows the chromatic dispersion characteristics of the HNL-DFF, and a graph G620 shows the chromatic dispersion characteristics of the HNLF. As can be seen from FIG. 6, the HNL-DFF has a small dispersion slope over a wider wavelength range and is capable of efficient wavelength conversion.

  Furthermore, the inventors measured the optical power of the FWM converted light while changing the actual excitation light wavelength in the evaluation system of FIG. FIG. 7 is a graph showing the measurement results of the FWM optical power. In this measurement, the above sample No. Nine HNL-DFFs were prepared. Then, the FWM light power for the probe light wavelength was measured when the pump light wavelength was fixed at 1540 nm and the input power of the pump light and the probe light was 16 dBm, respectively.

  In this specification, a wavelength band that is 3 dB lower than the peak of the FWM optical power is defined as an FWM bandwidth. In this case, it is understood that a bandwidth of 20 nm can be obtained according to the above-described measurement method (see FIG. 7). The result of plotting this FWM bandwidth with respect to different pump light wavelengths is a graph G860 in FIG. As can be seen from FIG. 8, a 20-nm FWM bandwidth can be secured in the wavelength range of 1530 nm to 1565 nm. This indicates that the detuning of the wavelength of the pump light is 30 nm or more, and that the wavelength band capable of wavelength conversion can be expanded far more than before by applying the HNL-DFF. . Further, the conversion efficiency is about -19 dB, and a conversion length higher than that of a conventional dispersion flat fiber is obtained with a fiber length of 500 m, and a practical value is realized. Therefore, the nonlinear constant γ is preferably 10 (1 / w / km) or more.

  FIG. 9 is a graph obtained by computer simulation of the wavelength dependency of the FWM bandwidth when the peak dispersion value is shifted while keeping the dispersion slope constant with reference to the optical fiber No. 9 (HNL-DFF). 8. In FIG. 8, a graph G810 indicates an FWM bandwidth with respect to an excitation light wavelength of HNLF (sample No. 10) for comparison, and a graph G820 indicates a chromatic dispersion of 0.065 ps / nm / km (sample No. 9 at a wavelength of 1545 nm). The FWM bandwidth with respect to the pumping light wavelength of the HNL-DFF having the original wavelength dispersion of the HNL-DFF; Graph G840 shows the FWM bandwidth with respect to the pumping light wavelength of the HNL-DFF having a chromatic dispersion of -0.065 ps / nm / km, and graph G850 shows the pumping of the HNL-DFF having a chromatic dispersion of +0.13 ps / nm / km. The FWM bandwidth with respect to the optical wavelength is shown. Note that the graph G860 is a measurement result obtained by plotting the FWM bandwidth with respect to different excitation light wavelengths, as described above. From this figure, it can be confirmed that by applying the HNL-DFF to the wavelength converter, it is possible to avoid a sharp narrowing of the FWM bandwidth even if the wavelength of the pump light is largely changed. As is clear from the graph G810, the conventional HNLF needs to adjust the pumping light wavelength to the zero-dispersion wavelength, and when the pumping light wavelength departs from the zero-dispersion wavelength, the conversion efficiency rapidly decreases.

  As shown in the table of FIG. 5, the transmission loss of the optical fiber is sufficiently lower than 1 dB / km. However, in the optical fiber suitable for the wavelength converter according to the present invention, if the nonlinear constant γ is 10 (1 / W / km) or more, the fiber length is about 1 km (1000 m) even if the transmission loss is 1 dB / km, and is sufficiently high. Since conversion efficiency is obtained, it is considered that there is no practical problem if the transmission loss is 1 dB / km or less.

  Also, regarding stimulated Brillouin scattering, it is a problem whether or not it occurs under actual use conditions. Conversely, if the threshold for generation of signal light or pump light is 10 dBm or less as a condition to be actually input, a reduction in conversion efficiency becomes a problem. Therefore, the generation threshold of at least 10 dBm is secured. This means that it is necessary to use an optical fiber or a pump light source.

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

  Next, an optical communication system to which the wavelength converter according to the present invention is applied will be described. FIG. 10 is a diagram showing a configuration of a first embodiment of an optical communication system to which the wavelength converter according to the present invention is applied.

  In the optical communication system shown in FIG. 10A, light from the EDFA 211, the DMF 221 and the transmission line branch line is transmitted from the optical transmission unit (TX) 201 to the optical reception unit (RX) 202 on the main transmission line. An optical coupler 231, an EDFA 212, a DMF 222, a variable attenuator 241 (ATT), an EDFA 213, and an AWG 250 for guiding are arranged in this order. To the transmission line branch line, the pump light output from the pump light source 204 and the signal light output from the optical transmission means (TX) 203 and sequentially transmitted through the EFDA 216 and the transmission line fiber 224 are input, and a new predetermined signal is input. A wavelength converter 200 (a wavelength converter according to the present invention) that outputs the converted wavelength light to the main line via the optical coupler 231 is provided. The wavelength converter 200 combines the pump light output from the pump light source 204 and passing through the EDFA 214 and the variable BPF 261 in order, and the signal light output from the transmission line fiber 224 and passing through the EDFA 215 and the variable BPF 262 in order. An optical coupler 232 is provided, and an HNL-DFF 223 is connected to an output terminal of the optical coupler 232. Further, a variable BPF 263 and a variable ATT 242 are arranged between the HNL-DFF 223 and the optical coupler 231.

  Normally, FWM is a high-speed phenomenon on the order of femtoseconds. Therefore, a method of adding a modulation component to the obtained converted light by appropriately modulating the pumping light used for conversion while processing the signal light in packets. No. The optical communication system shown in FIG. 10A assumes a case where signal light from a branch line is added to the main line of the transmission line. Since the signal light propagating through the main line is subjected to burst switching, it becomes empty. This is a so-called time division multiplex system in which data from a branch line is added to time. In the experiment, a TDM (Time Division Multiplexing) signal was received, and the signal component from the main line and the signal component from the branch line were examined. It was confirmed that good optical transmission could be realized. Note that a variable BPF 263 is provided downstream of the wavelength converter 200 in order to remove pump light (and input signal light).

  10B is a main signal light component at the output terminal A of the EDFA 211 located on the main line, and FIG. 10C is an additional signal light component at the output terminal B of the EDFA 215 located on the branch line. FIG. 10D shows the converted light component at the output end C of the variable ATT 242 provided at the subsequent stage of the wavelength converter 200, and FIG. 10E shows the output end D of the EDFA 212 located on the main line. The combined signal light components are shown.

  FIG. 11 is a diagram illustrating a configuration of a second embodiment of the optical communication system to which the wavelength converter according to the present invention is applied.

  In the optical communication system shown in FIG. 11A, light from the EDFA 301, the transmission line fiber 311, and the transmission line branch line is transmitted onto the transmission line along the traveling direction of the signal light in which a plurality of channels are multiplexed. An optical coupler 320, an EDFA 302, a transmission line fiber 312, and an EDFA 303 for guiding are arranged in this order. A wavelength converter 300 is disposed on the transmission line branch line, and another signal light passes through the EDFA 304 and the transmission line fiber 313 and is guided to the wavelength converter 300. Then, the converted light output from the wavelength converter 300 is guided to the main line via the optical coupler 320.

  In the case of a flexible network, it is expected that the wavelength distribution of a WDM (Wavelength Division Multiplexing) signal on the transmission path main line will change over time. Therefore, in order to increase the utilization efficiency of each signal channel, it may be necessary to appropriately tune the conversion wavelength of the signal light merging from the branch line according to the availability of the signal channel in the main line. In this case, the wavelength converter according to the present invention is suitable as a variable wavelength converter for generating converted light of a desired wavelength over a wide band, and the construction of an optical communication system is facilitated.

  11B shows the WDM signal light at the input end A of the EDFA 301 located on the main line, FIG. 11C shows the signal light at the input end B of the EDFA 304 located on the branch line, and FIG. Shows the converted light at the output end C of the wavelength converter 300, and FIG. 11E shows the WDM signal light at the output end D of the EDFA 302 located on the main line.

  Further, by using the HNL-DFF in the present invention, it is possible to generate a highly efficient SC (Supercontinuum) light and realize a wideband optical parametric amplifier and the like.

FIG. 1 is a cross-sectional view showing a structure of a highly nonlinear dispersion flat fiber suitable for a wavelength converter according to the present invention, and a refractive index profile thereof. 2 is a table summarizing the specifications of a plurality of samples (No. 1 to No. 7) prototyped as the highly nonlinear dispersion flat fiber shown in FIG. 9 is another refractive index profile of a highly nonlinear dispersion flat fiber suitable for the wavelength converter according to the present invention. FIG. 2 is a diagram showing a configuration of an optical fiber sample evaluation system applied to the wavelength converter according to the present invention. 5 is a table summarizing a plurality of samples (No. 8 and No. 9) prototyped as evaluation targets in the evaluation system shown in FIG. 4 and specifications of comparison target fibers. Sample No. 8 (highly nonlinear dispersion flat fiber) and sample No. 8 10 is a graph showing chromatic dispersion characteristics of ten optical fibers (ordinary highly nonlinear fibers). It is a graph which shows the measurement result of FWM optical power. Sample No. 9 is a graph obtained by performing a computer simulation of the wavelength dependency of the FWM bandwidth while changing the chromatic dispersion value with a fixed dispersion slope based on the optical fiber No. 9 (highly nonlinear dispersion flat fiber). Further, for comparison, the sample Nos. Simulation results for 10 optical fibers (normal highly nonlinear fibers) are also described. In addition, the sample No. The measured values of nine optical fibers are plotted. 4 is a graph showing a relationship between chromatic dispersion and FWM bandwidth. FIG. 1 is a diagram illustrating a configuration of a first embodiment of an optical communication system to which a wavelength converter according to the present invention is applied. FIG. 3 is a diagram illustrating a configuration of a second embodiment of the optical communication system to which the wavelength converter according to the present invention is applied.

Explanation of reference numerals

  100: highly nonlinear fiber; 200, 300: wavelength converter.

Claims (11)

A wavelength converter for generating converted light of a second wavelength different from the first wavelength, the wavelength of which is converted from the input light of the first wavelength using a nonlinear optical phenomenon,
A wavelength converter including an optical fiber having a dispersion slope having an absolute value of 0.01 ps / nm ² / km or less at a wavelength of 1550 nm. A wavelength converter for generating converted light of a second wavelength different from the first wavelength, the wavelength of which is converted from the input light of the first wavelength using a nonlinear optical phenomenon,
A wavelength converter including an optical fiber having a dispersion slope having an absolute value of 0.01 ps / nm ² / km or less at a wavelength of pump light separately supplied to the wavelength converter. A wavelength converter for generating converted light of a second wavelength different from the first wavelength, the wavelength of which is converted from the input light of the first wavelength using a nonlinear optical phenomenon,
A wavelength converter including an optical fiber having a chromatic dispersion having an absolute value of 0.2 ps / nm / km or less in a wavelength range of at least 1530 nm to 1565 nm. A wavelength converter for generating converted light of a second wavelength different from the first wavelength, the wavelength of which is converted from the input light of the first wavelength using a nonlinear optical phenomenon,
A wavelength converter including an optical fiber having at least two zero dispersion wavelengths in a wavelength range of 1300 nm to 1700 nm. A wavelength converter for generating converted light of at least one channel, which has been wavelength-converted using the nonlinear optical phenomenon from the pump light of at least one pump channel and the signal light of at least one signal channel,
An excitation light source in which the wavelength of the excitation channel is variable;
An optical fiber having a dispersion slope having an absolute value of 0.01 ps / nm ² / km or less at a wavelength of the pump light supplied from the pump light source.   The wavelength converter according to claim 1, wherein the optical fiber has a nonlinear constant of 10 (1 / W / km) or more at a wavelength of 1550 nm.   The wavelength converter according to any one of claims 1 to 5, wherein the optical fiber has a transmission loss of 1 dB / km or less at a wavelength of 1550 nm.   The wavelength converter according to any one of claims 1 to 5, wherein the wavelength converter has a threshold value of stimulated Brillouin scattering of 10 dBm or more for input excitation light.   The wavelength converter according to claim 5, wherein an allowable variable width of a wavelength of the converted light output from the optical fiber is 20 nm or more.   The wavelength converter according to claim 5, wherein an allowable variable width of a wavelength of the converted light output from the optical fiber is at least 20 nm for a signal channel having a wavelength range of at least 1530 nm to 1565 nm.   The optical component according to any one of claims 1 to 5, further comprising an optical component disposed on a light output end side of the optical fiber, for blocking excitation light propagating in the optical fiber. Wavelength converter.

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