Patent Application
20240402417
The present disclosure relates to an optical fiber. The present application claims priority to Japanese Patent Application No. 2021-168535 filed on Oct. 14, 2021, the content of which is incorporated herein by reference in its entirety.
The transmission loss of the optical fiber in a near-infrared region used as a communication wavelength band is greatly affected by Rayleigh scattering. Therefore, reduction of the transmission loss requires reduction of the Rayleigh scattering. The Rayleigh scattering occurs by reflecting non-uniformity of the glass structure. The non-uniformity of the glass structure is reduced by promoting structural relaxation from a glass state to a crystal state having a periodic uniform structure.
Patent Literature 1 discloses a method in which an alkali metal is added to a core portion of the optical fiber preform to increase fluidity of the core portion during drawing of the optical fiber and promote structural relaxation even at a lower temperature. Patent Literature 1 discloses a method of controlling an annealing time during drawing of the optical fiber by installation of an annealing furnace to promote structural relaxation.
An optical fiber according to a first aspect of the present disclosure includes: a core made of silica glass; and a cladding surrounding the core and made of silica glass, wherein a spectrum of a wavenumber derivative dR(k)/dk of a Raman scattering spectrum R(k) obtained by irradiating the core with pump light having a wavelength 532 nm passes through 0 twice or less in a range of wavenumber from 400 cm−1 to 550 cm−1.
An optical fiber according to a second aspect of the present disclosure includes: a core made of silica glass; and a cladding surrounding the core and made of silica glass, wherein in a Raman scattering spectrum R(k) obtained by irradiating the core with pump light of wavelength 532 nm, a ratio PD1/Pω3 of a maximum value PD1 of intensity of a Raman scattering light D1 caused by a four membered ring structure to a maximum value Pω3 of intensity of a Raman scattering light ω3 caused by one of vibration modes of Si—O vibration of a SiO4 structure is 5 or more.
The silica glass is glass mainly composed of SiO2. In the silica glass, SiO2 melted at a high temperature is rapidly cooled to freeze a liquid random structure at a high temperature. Therefore, silica glass has not only a six membered ring structure of quartz which is a crystal of SiO2, but also three membered ring and four membered ring Si—O bonding structures in which the six membered ring structure is collapsed. As a result, non-uniformity occurs in the glass structure and the Rayleigh scattering increases.
In the background art as described in Patent Literature 1, reduction of three membered ring structure and four membered ring structure of glass is mainly performed in order to reduce the non-uniformity of glass structure. However, there is a limit in suppressing the Rayleigh scattering by reducing the three membered ring structure and the four membered ring structure of glass from the viewpoint of compatibility with productivity. For example, in the method of reducing the viscosity by the additive element, SiO2 is easily transferred to the crystal state by the additive element. As a result, crystallization using the compound of the additive element as a crystal nucleus easily occurs, and the yield decreases. Further, the slow cooling time at the time of drawing the optical fiber becomes longer by lowering the drawing speed, but the productivity is lowered.
The purpose of the present disclosure is to provide an optical fiber having high productivity and low transmission loss.
According to the present disclosure, an optical fiber having high productivity and low transmission loss can be provided.
Embodiments of the present disclosure will first be listed and described. An optical fiber according to a first aspect of the present disclosure includes: a core made of silica glass; and a cladding surrounding the core and made of silica glass, wherein a spectrum of a wavenumber derivative dR(k)/dk of a Raman scattering spectrum R(k) obtained by irradiating the core with pump light having a wavelength 532 nm passes through 0 twice or less in a range of wavenumber from 400 cm−1 to 550 cm−1.
In the optical fiber according to the first aspect of the present disclosure, the fluctuation of the glass structure is suppressed by increasing the ratio of the four membered ring structure in the glass. Thus, the Rayleigh scattering is reduced. As a result, transmission loss may be reduced without reducing productivity.
An optical fiber according to a second aspect of the present disclosure includes: a core made of silica glass; and a cladding surrounding the core and made of silica glass, wherein in a Raman scattering spectrum R(k) obtained by irradiating the core with pump light of wavelength 532 nm, a ratio PD1/Pω3 of a maximum value PD1 of intensity of a Raman scattering light D1 caused by a four membered ring structure to a maximum value Pω3 of intensity of a Raman scattering light ω3 caused by one of vibration modes of Si—O vibration of a SiO4 structure is 5 or more.
In the optical fiber according to the second aspect of the present disclosure, the fluctuation of the glass structure is suppressed by increasing the ratio of the four membered ring structure in the glass. Thus, the Rayleigh scattering is reduced. As a result, transmission loss may be reduced without reducing productivity.
In the optical fiber according to the second aspect of the present disclosure, a spectrum of a wavenumber derivative dR(k)/dk of the Raman scattering spectrum R(k) may pass through 0 twice or less in a range of wavenumber from 400 cm−1 to 550 cm−1. In this case, transmission loss may be further reduced. [Details of Embodiments of Present Disclosure]
A specific example of optical fiber in the present disclosure will be described below with reference to drawing. It should be noted that the present invention is not limited to these examples, but is defined by the scope of claims and is intended to include all modifications within the meaning and scope equivalent to the scope of claims. In the description of drawing, the same elements are denoted by the same reference numerals, and overlapping descriptions are omitted.
Core 10 is made of silica glass and includes, for example, an alkali metal element such as Li, Na, and K and a halogen element. Core 10 may further include another element. Cladding 20 is made of silica glass and includes, for example, a halogen element. Cladding 20 may further include another element.
All additive elements added to core 10 and cladding 20 have the effect of reducing the viscosity of SiO2 glass. Therefore, if the additive concentration (mass fraction) is too high, the promotion of the four membered ring structure is suppressed during the application of high voltage and the drawing process in the manufacturing phase. The additive concentration may be 10000 ppm or less. On the other hand, when the viscosity increases, there is concern about an increase in transmission loss due to the occurrence of defects such as NBOHC (non-bridging oxygen hole center) during drawing. Thus, a modifying element may be added to core 10 and cladding 20. The modifying element is an element that can modify and repair defect portions such as NBOHC. The modifying element includes, for example, halogen elements such as fluorine and chlorine. Both core 10 and cladding 20 may include at least one type of halogen element in a mass fraction of 100 ppm or more, and may include two or more types of halogen elements.
In optical fiber 1 according to the embodiment, a spectrum of a wavenumber derivative dR(k)/dk of a Raman scattering spectrum R(k) obtained by irradiating the core 10 with pump light having a wavelength 532 nm passes through 0 twice or less in a range of wavenumber from 400 cm−1 to 550 cm−1. In addition, in the Raman scattering spectrum R(k), a ratio PD1/Pω3 of a maximum value PD1 of intensity of a Raman scattering light D1 caused by the four membered ring structure to a maximum value Pω3 of intensity of a Raman scattering light ω3 caused by one of vibration modes of Si—O vibration of a SiO4 structure is 5 or more.
During preform manufacture of optical fiber 1 according to the embodiment, high-pressure processing may be performed to facilitate generation of the four membered ring structure. Since a pressure is applied to the glass in the high-pressure processing, the promotion of a three membered ring structure or the four membered ring structure, which have smaller voids comparing to a six membered ring structure commonly found in an ordinary crystal, can be expected. By the high-pressure processing, a pressure which is, for example, 10−4 GPa or more and 10 GPa or less may be applied to the glass. As the high-pressure processing method, HIP (hot isostatic pressing) method may be used. The HIP method is a method of performing pressurization using gas as a high pressure applying medium. According to the HIP method, generation of defects due to contamination and pressure application unevenness may be suppressed as compared to a pressure application method in which a material having high hardness is directly pressed.
During drawing optical fiber 1 according to the embodiment, He atmosphere having a high thermal conductivity may be used in the furnaces and cooling units in order to promote the creation of four membered ring structures. Further, in order to rapidly cool optical fiber 1 after drawing, equipment for forcibly cooling optical fiber 1 after drawing may be used. The drawing speed is, for example, 500 m/min or more. This facilitates the creation of additional four membered ring structure. The drawing speed may be 1000 m/min or more, and it may also be 2000 m/min or more.
Optical fiber 1 according to an example was manufactured as follows. First, a core portion including chlorine or fluorine in a mass fraction of 100 ppm or more and other additive elements was prepared. Subsequently, after preforming, a pressure greater than 10−4 GPa and equal to or less than 10 GPa was applied to the glass by using the HIP method. Subsequently, drawing was performed at the drawing speed of 500 m/min or more, and rapid cooling was performed in the He atmosphere.
The optical fiber according to a comparative example was manufactured in the same manner as optical fiber 1 according to the example, except that none of the techniques described above for facilitating the generation of the four membered ring structure was employed. That is, in the manufacturing method of the optical fiber according to the comparative example, the high-pressure processing was not performed. In addition, He atmosphere was not used in the furnace and the cooling unit. In addition, equipment for forcibly cooling the optical fiber after drawing was not used. In addition, drawing was performed at the drawing speed of less than 500 m/min. The manufacturing method of optical fiber according to the comparative example corresponds to the technique of the background art that reduces transmission loss by equally reducing the four membered ring structure and the three membered ring structure.
Here, the Raman scattering spectrum will be described. In general, when a material is irradiated with light, Raman scattering light having a wavelength different from that of irradiation light is generated due to interaction between the light and the material (molecular vibration). The structure of the molecular level of the substance can be analyzed by a Raman scattering spectrum obtained by dispersing the Raman scattering light. In the Raman scattering spectrum, a plurality of peaks occurs according to the number of vibration modes of atom coupling in the substance. For glass that does not have a long-range ordered structure, the Raman scattering spectrum is one of a few techniques for confirming the presence ratio of three membered ring structures and four membered ring structures in the glass structure.
The Raman scattering spectrum of optical fiber 1 is measured, for example, using microscopic Raman spectroscopy similar to that in Patent Literature 2. That is, the laser light of the wavelength 532 nm output from the semiconductor laser device is condensed to have a spot diameter of about 2 μm and is irradiated to an end face of the optical fiber. Exposure is performed twice for 30 seconds in total. The intensity of the laser light is an oscillation output 1 W (about 100 mW in the end face of the optical fiber). Then, the laser light is vertically irradiated to the end face of the optical fiber to measure the Raman scattering spectrum by backscattering geometry.
First, the optical fibers according to the example and the comparative example were used to compare the relationship between a ratio ID2/Iω3 and transmission loss reported in Patent Literature 2.
The area intensity ID2 is represented by an area of a region sandwiched between a baseline drawn in the range of the wavenumber from 565 cm−1 to 640 cm−1 in the Raman scattering spectrum and the Raman scattering spectrum. The area intensity Iω3 is represented by an area of a region sandwiched between a baseline drawn in the range of the wavenumber from 750 cm−1 to 875 cm−1 in the Raman scattering spectrum and the Raman scattering spectrum.
In the optical fiber according to the comparative example, the four membered ring structure and the three membered ring structure are equally reduced. The transmission loss of the optical fiber according to the comparative example decreases as the ratio ID2/Iω3 decreases. On the other hand, in the optical fiber according to the example, since only the four membered ring structure is increased, the presence ratio of the three membered ring structure and the six membered ring structure in the glass structure decreases at the same degree. Therefore, the transmission loss of the optical fiber according to the example varies even if the ratio ID2/Iω3 is substantially constant. Therefore, the reduction effect of transmission loss of the optical fiber according to the example cannot be explained by the ratio ID2/Iω3.
With reference to
In the above-mentioned the wavenumber range, if the measured noise is small enough to be ignored, the number of the extremum values should be within three. Since the measured Raman scattering spectrum includes measured noise, the extremum values exceeding three points may exist due to the noise. In this case, the moving average may be performed in the wavenumber range in which the number of the extremum values is three or less. The wavenumber range in which the moving average is performed may be, for example, a range of a wavenumber k≤10 cm−1.
From the relationship shown in
The number of the extremum values defined above is an index indicating how many six membered ring structures and four membered ring structures coexist. In the conventional glass, six membered ring structures and four membered ring structures usually coexist, and the intensities of the peaks are approximately the same. Since there are two peaks in the above wavenumber range, the number of the extremum values is three in total, the extremum values including the local maximum value of the peak of the Raman scattering light ω1 caused by the six membered ring structure, the local maximum value of the peak of Raman scattering light D1 caused by the four membered ring structure, and the local minimum value located at the intersection of the skirts of these two peaks.
The state in which the number of the extremum values is two or less occurs because the peak of Raman scattering light D1 increases with respect to the peak of Raman scattering light ω1 as a result of the change from six membered ring structure to the four membered ring structure. That is, by unifying the glass structure into one type, the fluctuation of the glass structure (fluctuation of density) is suppressed and the Rayleigh scattering is reduced. Consequently, the state in which the number of the extremum values is two or less is obtained. The number of extremum values is an index indicating how much the glass structure is unified into the four membered ring structure. Therefore, the number of extremum values may be an important parameter affecting transmission loss.
Subsequently, the relationship between the ratio PD1/Pω3 of the maximum value of the core portion and the transmission loss was compared.
As illustrated in
In the comparative example, a trend opposite to that of the embodiment is observed. That is, in the optical fiber according to the comparative example, the transmission loss increases as the ratio PD1/Pω3 increases. As described above, the optical fiber according to the comparative example is manufactured by the technique of the background art that controls the fluctuation of the glass structure by reducing the three membered ring structure and the four membered ring structure. Actually, as the ratio PD1/Pω3 decreases, the transmission loss of the optical fiber according to the comparative example decreases. According to the optical fiber according to the example, it is possible to control the fluctuation of the structure of the glass by increasing only the four membered ring structure so that the four membered ring structure occupies most of the glass structure. Actually, in the optical fiber according to the example, the ratio PD1/Pω3 itself is increased as compared with the optical fiber according to the comparative example. The reason why the opposite tendency to the comparative example is observed in the example is presumed to be that only the four membered ring structure is increased.
The voids contained in the four membered ring structure are smaller than those contained in the six membered ring structure based on the SiO4 tetrahedral structure found in quartz crystal. Thus, as the four membered ring structure increases, the density per unit volume increases. Consequently, in the optical fiber according to the example, the absolute refractive index is increased when compared to the optical fiber according to the comparative example. That is, the increase or decrease in the refractive index reflects the increase or decrease in the four membered ring structure. Accordingly, the increase in the refractive index may be a parameter indicating the reduction of transmission loss caused by the increase in the four membered ring structure.
As the absolute refractive index of the core increases, the absolute refractive index of the cladding may also increase. In general, the larger the relative refractive index difference between the core and cladding, the easier it is to confine light. Therefore, the absolute refractive index of the cladding must be somewhat smaller than that of the core. In the present example, the absolute refractive index of the core can be increased. Therefore, even if the absolute refractive index of the cladding can be increased with respect to the comparative example with the background art, the light confinement amount can be secured. For example, the absolute refractive index of the cladding may be greater than 1.42, greater than 1.44, greater than 1.46, or greater than 1.49, for example.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2021-168535 | Oct 2021 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2022/034075 | 9/12/2022 | WO |
