Abstract
An optical fiber made of silica glass includes a core having a maximum refractive index n1, a depressed portion surrounding the core and having an average refractive index n2, and cladding surrounding the depressed portion and having an average refractive index n3. In the optical fiber, n1>n3>n2. The optical fiber has a local maximum value of chromatic dispersion within a wavelength range of 1530 nm to 1610 nm, and the local maximum value is 2 ps/nm/km or greater and below 0 ps/nm/km.
Claims
1. An optical fiber made of silica glass, comprising: a core having a maximum refractive index n1; a depressed portion surrounding the core and having an average refractive index n2; and cladding surrounding the depressed portion and having an average refractive index n3, wherein n1>n3>n2, wherein the optical fiber has a local maximum value of chromatic dispersion within a wavelength range of 1530 nm to 1610 nm, and the local maximum value is 2 ps/nm/km or greater and below 0 ps/nm/km, and wherein a dispersion curve at a wavelength at which chromatic dispersion becomes maximal within the wavelength range of 1530 nm to 1610 nm is 0.0003 ps/nm.sup.3/km or greater and 0 ps/nm.sup.3/km or smaller.
2. The optical fiber according to claim 1, wherein an effective area at a wavelength of 1550 nm is 18 m.sup.2 or smaller.
3. The optical fiber according to claim 1, wherein a nonlinear coefficient at the wavelength of 1550 nm is 9/W/km or larger.
4. The optical fiber according to claim 1, wherein a k value at the wavelength of 1550 nm is 1.01 or smaller.
5. The optical fiber according to claim 1, wherein a core area A [% m] is 2.2% .Math.m or greater, the core area A being defined as follows:
A =.sub.0.sup.a(r)dr,(1) where a denotes a radius [m] of the core, and (r) denotes a relative refractive-index difference [%] at a radial distance r [m] from a fiber axis.
6. The optical fiber according to claim 5, wherein a maximum relative refractive-index difference 1 of the core with respect to the cladding is 1.2% or higher and 3.0% or lower, and wherein a core-area ratio is 0.7 or greater and 1.0 or smaller, the core-area ratio being defined as follows:
=A/(1a)(2).
7. The optical fiber according to claim 1, wherein a diameter 2a of the core is 3.0 m or greater and 5.0 m or shorter.
8. The optical fiber according to claim 1, wherein a ratio 2b/2a of an outside diameter 2b of the depressed portion to the diameter 2a of the core is 1.6 or greater and 3.2 or smaller, and wherein an average relative refractive-index difference 2 of the depressed portion with respect to the cladding is 1.0% or higher and 0.5% or lower.
9. The optical fiber according to claim 7, wherein a ratio 2b/2a of an outside diameter 2b of the depressed portion to the diameter 2a of the core is 1.6 or greater and 3.2 or smaller, and wherein an average relative refractive-index difference 2 of the depressed portion with respect to the cladding is 1.0% or higher and 0.5% or lower.
10. A light source device comprising: a seed light source that emits seed light having four or less wavelength components with a center wavelength being within a range of 1530 nm to 1610 nm; and the optical fiber according to claim 1 that receives the seed light at an input end of the optical fiber, generates converted light having more wavelength components than the seed light with a nonlinear optical phenomenon that occurs in the optical fiber while the seed light is being guided in the optical fiber, and outputs the converted light from an output end of the optical fiber.
11. The optical fiber according to claim 1, wherein the dispersion curve at a wavelength at which chromatic dispersion becomes maximal within the wavelength range of 1530 nm to 1610 nm is 0.0002 ps/nm.sup.3/km or greater and 0 ps/nm.sup.3/km or smaller.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) FIG. 1 is a conceptual diagram illustrating a refractive-index distribution of an optical fiber according to an embodiment of the present invention.
(2) FIG. 2 is a conceptual diagram illustrating a relative refractive-index difference (r) [%] of the optical fiber illustrated in FIG. 1 with respect to a radial distance r [m] from a fiber axis.
(3) FIG. 3 is a graph illustrating a peak wavelength of the optical fiber illustrated in FIG. 1 for each of different values of 1 and a core-area ratio .
(4) FIG. 4 is a graph illustrating an effective area Aeff of the optical fiber illustrated in FIG. 1 at a wavelength of 1550 nm for each of different values of 1 and the core-area ratio .
(5) FIG. 5 is a graph illustrating the effective area Aeff of the optical fiber illustrated in FIG. 1 at the wavelength of 1550 nm for each of different values of 1 and a core area A.
(6) FIG. 6 is a graph illustrating a nonlinear coefficient of the optical fiber illustrated in FIG. 1 at the wavelength of 1550 nm for each of different values of 1 and the core-area ratio .
(7) FIG. 7 is a graph illustrating the nonlinear coefficient of the optical fiber illustrated in FIG. 1 at the wavelength of 1550 nm for each of different values of 1 and the core area A.
(8) FIG. 8 is a graph illustrating a dispersion curve of the optical fiber illustrated in FIG. 1 at a peak wavelength for each of different values of 1 and the core-area ratio .
(9) FIG. 9 is a graph illustrating the dispersion curve of the optical fiber illustrated in FIG. 1 at the peak wavelength for each of different values of 1 and the core area A.
(10) FIG. 10 is a graph illustrating a k value of the optical fiber illustrated in FIG. 1 at the wavelength of 1550 nm for each of different values of 1 and a core diameter 2a.
(11) FIG. 11 is a graph illustrating the peak wavelength of the optical fiber illustrated in FIG. 1 for each of different values of 1 and 2.
(12) FIG. 12 is a graph illustrating the peak wavelength of the optical fiber illustrated in FIG. 1 for each of different values of 1 and a ratio 2b/2a.
(13) FIG. 13 is a table that summarizes relevant specifications of dispersion-flattened HNLFs according to different examples.
(14) FIG. 14 is a table that summarizes other relevant specifications of the dispersion-flattened HNLFs according to the examples.
(15) FIG. 15 is a conceptual diagram illustrating a configuration of a light source device according to an embodiment of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
(16) Embodiments of the present invention will now be described in detail with reference to the accompanying drawings, wherein like elements are denoted by identical reference numerals, and redundant description of such elements is omitted. The present invention is not limited to the following embodiments. It is intended that the scope of the present invention be defined by the appended claims and encompasses all equivalents to the claims and all changes made to the claims within the scope thereof.
(17) In a dispersion-flattened HNLF, as the wavelength deviates from the peak wavelength toward the shorter wavelength side or the longer wavelength side, the value of chromatic dispersion deviates from the peak dispersion. Therefore, to generate a wideband optical frequency comb over either the C band (1530 nm to 1565 nm) or the L band (1565 nm to 1610 nm) or both the C band and the L band with a small absolute value of chromatic dispersion, the peak wavelength needs to be within the C band or the L band, rather than the chromatic dispersion at the wavelength of 1550 nm being 0 ps/nm/km.
(18) In the range of anomalous dispersion, a nonlinear optical phenomenon called modulation instability occurs, which causes distortion in the waveform transmitted. Therefore, it is desired that the dispersion be normal (the chromatic dispersion be negative) over the entirety of the wavelength range to be used. That is, the peak dispersion needs to be below zero.
(19) Furthermore, in the dispersion-flattened HNLF, a smaller Aeff and a greater nonlinear coefficient are preferable because FWM can be made to occur with higher efficiency. JP2005-331818A also refers to the benefit of a smaller Aeff that makes the nonlinear coefficient greater. If such a dispersion-flattened HNLF is employed as an element of a light source device, one end or both ends of the HNLF are connected to a standard single-mode fiber (SSMF). At the wavelength of 1550 nm, the SSMF has an Aeff of about 80 m.sup.2 and a mode-field diameter (MFD) of about 10.4 m.
(20) In general, the MFD of the dispersion-flattened HNLF is smaller than the MFD of the SSMF. It is known that a large difference in MFD between fibers that are connected to each other results in a large splicing loss. A large splicing loss substantially results in low optical power to be inputted to the dispersion-flattened HNLF. Therefore, the splicing loss in the connection to the SSMF needs to be small. Accordingly, to reduce the difference in MFD between the dispersion-flatted HNLF and the SSMF and thus reduce the splicing loss, the MFD of the dispersion-flatted HNLF is desired to be large. To increase the MFD without changing Aeff, a k value defined in the following expression,
Aeff=k(MFD/2).sup.2,(4)
needs to be small. However, consideration for increasing the MFD is not given by either Masaaki Hirano et al, Silica-based Highly Nonlinear Fiber Advances, OFC2016, Tu2E.4 (2016) or JP2005-331818A.
(21) FIG. 1 is a conceptual diagram illustrating a refractive-index distribution of an optical fiber 20 according to an embodiment of the present invention. The optical fiber 20 is made of silica glass and includes a core 30 having a maximum refractive index n1, a depressed portion 40 surrounding the core 30 and having an average refractive index n2, and cladding 50 surrounding the depressed portion 40 and having an average refractive index n3. The relationship among the refractive indices is expressed as n1>n3>n2. The core 30 has a diameter 2a. The depressed portion 40 has an outside diameter 2b. The maximum relative refractive-index difference of the core 30 with respect to the cladding 50 is denoted as 1 (=100(n1n3)/n1) [%]. The average relative refractive-index difference of the depressed portion 40 with respect to the cladding 50 is denoted as 2 (=100(n2n3)/n2) [%]. The core 30 is made of silica glass containing GeO.sub.2. The depressed portion 40 is made of silica glass containing F. The cladding 50 may be made of either pure silica glass or silica glass containing F or Cl.
(22) FIG. 2 is a conceptual diagram illustrating a relative refractive-index difference (r) [%] of the optical fiber 20 with respect to a radial distance r [m] from the fiber axis. The area of the hatched part is defined as core area [%.Math.m] and is defined as follows:
A=.sub.0.sup.a(r)dr.(5)
Furthermore, a core-area ratio is defined as follows:
=A/(1a),(6)
where a denotes the radius of the core 30 and corresponds to a distance from the fiber axis to a position where the refractive index becomes equal to the refractive index n3 of the cladding 50, or a distance from the fiber axis to a position where the differential value obtained by differentiating the refractive-index distribution of the core 30 with a variable r becomes maximum.
(23) FIGS. 3 to 10 described below each illustrate a property of the optical fiber 20 having 2=0.79% and 2b/2a=2.0. Furthermore, the core radius a of the optical fiber 20 is adjusted such that the peak dispersion becomes 0.5 ps/nm/km.
(24) FIG. 3 is a graph illustrating the peak wavelength for each of different values of 1 and the core-area ratio . As 1 becomes higher, the peak wavelength becomes longer. However, if is increased, the peak wavelength can be shifted toward the shorter wavelength side. To set the peak wavelength to a value within a range of 1530 nm to 1610 nm, it is preferable that 1 be within a range of 1.2% to 3.0% and be within a range of 0.7 to 1.0.
(25) FIG. 4 is a graph illustrating the effective area Aeff at the wavelength of 1550 nm for each of different values of 1 and the core-area ratio . FIG. 5 is a graph illustrating the effective area Aeff at the wavelength of 1550 nm for each of different values of 1 and the core area A. As 1 becomes higher, Aeff becomes smaller. As illustrated in FIG. 4, if 1 is fixed, a greater is preferable because Aeff becomes smaller. Consequently, the density of optical power is increased, whereby the nonlinear optical phenomenon can be produced efficiently. Furthermore, as illustrated in FIG. 5, a greater core area A is preferable because Aeff can be made smaller. To set Aeff to 18 m.sup.2 or smaller, it is preferable that 1 be 1.2% or higher, be 0.7 to 1.0, and A be 2.2%.Math.m or greater. It is more preferable to set Aeff to 15 m.sup.2 or smaller, with 1 being 1.4% or higher and A be 2.6%.Math.m or greater. It is most preferable to set Aeff to 12 m.sup.2 or smaller, with 1 being 2.0% or higher and A being 3.3%.Math.m or greater.
(26) FIG. 6 is a graph illustrating the nonlinear coefficient y at the wavelength of 1550 nm for each of different values of 1 and the core-area ratio . FIG. 7 is a graph illustrating the nonlinear coefficient at the wavelength of 1550 nm for each of different values of 1 and the core area A. As 1 becomes higher, becomes greater. As illustrated in FIG. 6, if 1 is fixed, a greater is preferable because becomes greater, whereby the nonlinear optical phenomenon can be produced efficiently. Furthermore, as illustrated in FIG. 7, a greater core area A is preferable because can be made greater. To set to 9/W/km or greater, it is preferable that 1 be 1.2% or higher, be 0.7 to 1.0, and A be 2.2%.Math.m or greater. It is more preferable to set to 11/W/km or greater, with 1 being 1.4% or higher and A being 2.6%.Math.m or greater. It is most preferable to set to 15/W/km or greater, with 1 being 2.0% or higher and A being 3.3%.Math.m or greater.
(27) FIG. 8 is a graph illustrating the dispersion curve at the peak wavelength for each of different values of 1 and the core-area ratio . FIG. 9 is a graph illustrating the dispersion curve at the peak wavelength for each of different values of 1 and the core area A. Herein, the dispersion curve [ps/nm.sup.3/km] refers to a value obtained by differentiating the dispersion slope [ps/nm.sup.2/km] with wavelength. As the absolute value of dispersion curve at the peak wavelength becomes smaller, the dispersion becomes flatter with respect to the wavelength. Therefore, to reduce the absolute value of dispersion over a wide waveband and thus generate a wideband optical frequency comb, the dispersion curve is preferably close to zero.
(28) As illustrated in FIG. 8, a higher 1 is preferable because the absolute value of dispersion curve can be made smaller. Furthermore, at a 1 of 2.1% or lower, if 1 is fixed, a greater is preferable because the absolute value of dispersion curve can be made smaller. Furthermore, as illustrated in FIG. 9, a greater A is preferable because the absolute value of dispersion curve can be made smaller. To set the dispersion curve at the peak wavelength to a value within a range of 0.0003 to 0 ps/nm.sup.3/km, it is preferable that 1 be 1.1% or higher, be 0.7 to 1.0, and the core area A be 2.2%.Math.m or greater. It is more preferable to set the dispersion curve at the peak wavelength to a value within a range of 0.0002 to 0 ps/nm.sup.3/km, with 1 being 1.3% or higher and the core area A being 2.6%.Math.m or greater.
(29) FIG. 10 is a graph illustrating the k value at the wavelength of 1550 nm for each of different values of 1 and the core diameter 2a. A smaller k value is preferable because the MFD can be increased without changing Aeff. Furthermore, a smaller 1 is preferable because the k value can be made smaller with a greater 2a. To set the k value to 1.01 or smaller, it is preferable that 1 be 3.0% or lower and 2a be 3.0 m or greater. It is more preferable to set the k value to 1.00 or smaller, with 1 being 2.4% or lower and 2a being 3.5 m or greater.
(30) FIG. 11 is a graph illustrating the peak wavelength of optical fibers having 2b/2a=2.0 and =0.83 for each of different values of 1 and 2. In this case, the core radius a is adjusted such that the peak dispersion is 0.5 ps/nm/km. As 2 becomes more negative, the peak wavelength can be made shorter. To set the peak wavelength to a value within a range of 1.53 m to 1.61 m, it is preferable that 2 be within a range of 1.0% to 0.5%.
(31) FIG. 12 is a graph illustrating the peak wavelength of optical fibers having 2=0.76% and =0.83 for each of different values of 1 and the ratio 2b/2a. In this case, the core radius a is adjusted such that the peak dispersion is 0.5 ps/nm/km. The peak wavelength is shortest at a ratio 2b/2a of around 2.0. If the ratio 2b/2a is smaller than or greater than around 2.0, the peak wavelength becomes longer. To set the peak wavelength to a value within a range of 1.53 m to 1.61 m, it is preferable that the ratio 2b/2a be within 1.6 to 3.2.
(32) FIGS. 13 and 14 are tables that summarize relevant specifications of dispersion-flattened HNLFs according to different examples. FIG. 13 summarizes the following specifications of Fibers 1 to 12: the maximum relative refractive-index difference 1 of the core 30, the average relative refractive-index difference 2 of the depressed portion 40, the outside-diameter ratio 2b/2a between the core 30 and the depressed portion 40, the diameter 2a of the core 30, the core area A, the core-area ratio , the chromatic dispersion (at the wavelength of 1550 nm), and the dispersion slope (at the wavelength of 1550 nm). FIG. 14 summarizes the following specifications of Fibers 1 to 12: the peak wavelength, the peak dispersion, the dispersion curve (at the peak wavelength), the mode-field diameter MFD (at the wavelength of 1550 nm), the effective area Aeff (at the wavelength of 1550 nm), the nonlinear coefficient (at the wavelength of 1550 nm), and the k value (at the wavelength of 1550 nm).
(33) Now, an exemplary method of manufacturing the optical fiber 20 will be described. A glass rod for a core having a core-area ratio of 0.7 to 1.0 is manufacturable by a method such as vapor-phase axial deposition (VAD), outside vapor deposition (OVD), or the like. The exponent a of the index profile of the core 30 is preferably greater because the core-area ratio can be made closer to 1.0. For example, =2.3 substantially corresponds to =0.7, and =3.0 substantially corresponds to =0.75.
(34) Alternatively, the core-area ratio can be increased to near 1.0 by removing a peripheral portion of a core glass rod having a small core-area ratio . However, if a portion of the core glass rod that is doped with a large amount of Ge is removed inclusively with the peripheral portion, the probability that the core glass rod may become fragile or be foamed tends to increase, in general. To prevent the portion doped with a large amount of Ge from being removed inclusively with the peripheral portion, the core-area ratio is preferably set to 0.7 to 0.9.
(35) The core glass rod obtained as above is wrapped with a glass layer serving as the depressed portion 40, and the resulting body is further wrapped with a glass layer serving as the cladding 50, whereby an optical fiber preform is obtained. By drawing the optical fiber preform, a dispersion-flattened HNLF according to the embodiment is obtained.
(36) FIG. 15 is a conceptual diagram illustrating a configuration of a light source device 1 according to an embodiment of the present invention. The light source device 1 includes a seed light source 10 and an optical fiber 20. The seed light source 10 emits light having four or less wavelength components with a center wavelength being within a range of 1530 nm to 1610 nm. The optical fiber 20 is the dispersion-flattened HNLF according to the embodiment. The optical fiber 20 receives, at the input end thereof, the light emitted from the seed light source 10 and allows the light to be guided there inside. While the light is being guided, a nonlinear optical phenomenon occurs. Consequently, light having more wavelength components than the light emitted from the seed light source 10 is generated and is outputted from the output end of the optical fiber 20. The wavelength of the light outputted from the optical fiber 20 preferably falls within a range of 1530 nm to 1610 nm. The wavelength components of the light outputted from the optical fiber 20 are preferably at regular intervals. The output end of the seed light source 10 may be connected to an SSMF, and the SSMF and the optical fiber 20 may be optically connected to each other by a connecting method such as fusion splicing, connection with connectors, or the like.
