optical fiber

Abstract

An optical fiber comprises a core (10) made of silica glass and a cladding (20) surrounding the core (10) and also made of silica glass. The center wavenumber k of the Raman scattered light ω4(TO) peak in the Raman scattering spectrum obtained by irradiating the core with excitation light of wavelength 532 nm is... ω4 With half-value half-width k H4 The area is 38000 cm² ‑2 the following.

Description

Technical Field

[0001] This disclosure relates to optical fibers.

[0002] This application claims priority based on Japanese Application No. 2021-001521, filed on January 7, 2020, and invokes all the contents set forth in that Japanese application. Background Technology

[0003] As optical fibers exhibiting low Rayleigh scattering and low transmission loss, optical fibers with a core containing an alkali metal element are known (e.g., Patent Document 1). When the core of the optical fiber matrix contains an alkali metal element, the viscosity of the core can be reduced during fiber drawing, thereby mitigating the mesh structure of the quartz glass. Consequently, the hypothetical temperature of the glass within the optical fiber decreases, thus reducing the transmission loss of the optical fiber.

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: Japanese Patent Publication No. 2005-537210

[0007] Patent Document 2: Japanese Patent Application Publication No. 2016-130786

[0008] Non-patent literature

[0009] Non-patent literature 1: RPWang et al., "Fluorine-doping concentration and fictivetemperature dependence of self-trapped holes in glasses", J.Appl.Phys.98,023701(2005)

[0010] Non-patent document 2: DJLittle et al., "Femtosecond laser modification of fusedsilica: the effect of writing polarization on Si-O ring structure", Opt. Express16, 24, 20029, (2008) Summary of the Invention

[0011] The optical fiber disclosed herein comprises a core made of silica glass and a cladding made of silica glass surrounding the core. The center wavenumber k of the Raman scattered light ω4(TO) obtained by irradiating the core with excitation light of wavelength 532 nm is... ω4 With half-value half-width k H4The area is 38000 cm² -2 the following. Attached Figure Description

[0012] [ Figure 1 ] Figure 1 This is a cross-sectional view of the optical fiber involved in the implementation method.

[0013] [ Figure 2 ] Figure 2 The figure shows the Raman scattering spectrum of silica glass.

[0014] [ Figure 3 ] Figure 3 To show the center wavenumber k ω4 With half-value half-width k H4 A diagram showing the relationships between them.

[0015] [ Figure 4 ] Figure 4 To show the product k ω4 ×k H4 Compared with strength ratio I ratio A diagram showing the relationships between them.

[0016] [ Figure 5 ] Figure 5 To illustrate the relationship between transmission loss and product k ω4 ×k H4 A diagram showing the relationships between them. Detailed Implementation

[0017] [The problem this disclosure aims to solve]

[0018] Besides Rayleigh scattering, infrared absorption and OH-group-based absorption also contribute to increased transmission loss in optical fibers in the near-infrared region, which is used as a communication band. Transmission loss caused by infrared absorption increases with the shift of the infrared absorption peak towards shorter wavelengths and the increase of the half-width at half-maximum (WHM) of the infrared absorption peak. Non-Patent Literature 1 describes wavenumber shifts in the infrared absorption peak due to different additive elements. The position and WHM of the infrared absorption peak are greatly affected by the strength and bond angle of the Si-O bond, and therefore react sensitively to the type and amount of additive elements, as well as the crystallization transformation of the glass. Therefore, even at hypothetical low temperatures, partial crystallization and the distribution of additive elements may not necessarily reduce transmission loss caused by infrared absorption, and the transmission loss may deteriorate.

[0019] The purpose of this disclosure is to provide optical fibers with low transmission loss.

[0020] [The Effects of This Disclosure]

[0021] According to this disclosure, it is possible to provide optical fibers with low transmission loss.

[0022] [Description of embodiments of this disclosure]

[0023] First, embodiments of this disclosure are listed and described. One embodiment involves an optical fiber comprising a core made of silica glass and a cladding surrounding the core and also made of silica glass. The center wavenumber k of the Raman scattered light ω4(TO) obtained by irradiating the core with excitation light of wavelength 532 nm is... ω4 With half-value half-width k H4 The area is 38000 cm² -2 the following.

[0024] In the aforementioned optical fibers, transmission loss can be reduced.

[0025] In the aforementioned optical fiber, the half-value half-width k H4 It can be 36cm -1 Below. In this case, the center wavenumber k of the Raman scattered light ω4(TO) peak is... ω4 With half-value half-width k H4 The area is easily 38000 cm² -2 the following.

[0026] In the aforementioned optical fiber, the center wavenumber k ω4 It can be 1070cm -1 Below. In this case, the center wavenumber k of the Raman scattered light ω4(TO) peak is... ω4 With half-value half-width k H4 The area is easily 38000 cm² -2 the following.

[0027] In the aforementioned optical fibers, the core may contain alkali metal elements. In this case, transmission loss can be further reduced due to the suppression of Rayleigh scattering.

[0028] [Details of the embodiments disclosed herein]

[0029] Specific examples of the optical fiber of this disclosure will now be described with reference to the accompanying drawings. It should be noted that the invention is not limited to these examples, but is defined by the claims, and is intended to include all modifications within the meaning and scope equivalent to the claims. In the description of the drawings, the same elements are labeled with the same reference numerals, and repeated descriptions are omitted.

[0030] Figure 1 This is a cross-sectional view of the optical fiber involved in the implementation method. For example... Figure 1As shown, the optical fiber 1 according to the embodiment includes a core 10 and a cladding 20. The core 10 is made of silica glass and contains, for example, alkali metal elements such as potassium, fluorine, or chlorine. By containing alkali metal elements, Rayleigh scattering can be suppressed in the core 10, thereby reducing transmission loss. The core 10 is substantially free of Ge, and the Ge mass fraction is 0.1% or less. The diameter (core diameter) of the core 10 is, for example, 6 μm or more and 18 μm or less. The cladding 20 surrounds the core 10. The cladding 20 is made of silica glass and contains, for example, fluorine or chlorine. The cladding 20 has a refractive index lower than that of the core 10.

[0031] In the optical fiber 1 according to the embodiment, the center wavenumber k of the Raman scattered light ω4(TO) obtained by irradiating the core 10 with excitation light of wavelength 532nm is... ω4 With half-value half-width k H4 The area is 38000 cm² -2 The following is a preferred volume: 37500 cm³. -2 The following, or more preferably, is 33000cm -2 Below. Half-value half-width k H4 For example, 36cm -1 Below. Center wavenumber k ω4 For example, 1070cm -1 the following.

[0032] Here, Raman scattering spectroscopy is explained. Typically, when light shines on a substance, the interaction between the light and the substance (molecular vibrations) produces Raman scattered light with a wavelength different from the wavelength of the shining light. By dispersing this Raman scattered light, the Raman scattering spectrum can be obtained, allowing analysis of the substance's molecular-level structure. In Raman scattering spectra, multiple peaks are generated based on the number of vibrational modes of the atomic bonds within the substance.

[0033] Figure 2 This diagram illustrates the Raman scattering spectrum obtained by irradiating quartz glass (silica glass) with a laser beam of wavelength 532 nm. Figure 2 In the diagram, the horizontal axis represents the Raman displacement (cm). -1 The vertical axis represents intensity. Figure 2 In the Raman scattering spectrum shown, it can be confirmed that the peak of ω0 in the Raman scattering light of calcium fluoride caused by the sample stage is at wavenumber 300 cm⁻¹. -1 Above 350cm -1 Within the following range, it can be confirmed that the peak of Raman scattering ω3 caused by Si-O stretching vibration is at wavenumber 750 cm⁻¹. -1 Above 875cm -1 Within the following range, the peak of Raman scattering light D2, which belongs to the silicon dioxide three-membered ring structure, can be confirmed at a wavenumber of 565 cm⁻¹. -1 Above 640cm -1Within the following range, it can be confirmed that the peak of the Raman scattered light ω4(TO) induced by Si-O stretching vibration is at a wavenumber of 1000 cm⁻¹. -1 Above 1100cm -1 Within the following range, the peak of the Raman scattered light ω4(TO) originates from the scattered light of the transverse wave generated by the asymmetric stretching vibration of Si-O in the vibrational mode of the Si-O bond (Non-Patent Document 2).

[0034] The wavenumber positions of these peaks (i.e., the central wavenumber) are significantly influenced by the strength and bond angles of the Si-O bonds constituting the silica glass. Their bonding states produce several wavenumber shifts depending on the type and amount of added elements, or partial crystallization of the glass. These wavenumber shifts reflect how the bond strength and bond angles of the atomic bonds change with the added elements.

[0035] Similar to Raman scattering, infrared absorption is also a phenomenon related to the interaction between atomic bond vibrations and light. Therefore, under the same conditions, a wavenumber shift in the same direction as the Raman scattered light ω₄(TO) will occur in infrared absorption. However, in infrared absorption spectra, multiple peaks tend to overlap due to the combination of Si-O vibrational modes, making peak discrimination difficult. Furthermore, the intensity ratio I described later... ratio From a comparative perspective, the Raman scattered light ω4(TO) of the Raman spectrum is considered more suitable for quantitatively evaluating wavenumber shift and half-width. Therefore, in the optical fiber according to this embodiment, the change in transmission loss caused by infrared absorption is evaluated by Raman scattering spectroscopy.

[0036] To suppress transmission loss caused by infrared absorption, the optical fiber involved in the embodiment is manufactured, for example, by adjusting the drawing conditions. Specifically, to suppress the crystallization transformation of SiO2 and the precipitation of additives, the cooling rate immediately after fiberization is increased and rapid cooling is performed so that the optical fiber does not remain in the temperature range where crystallization is easy (above 1200°C and below 1700°C) for an extended period. Rapid cooling is performed, for example, in a temperature range of 2000°C to 1200°C. Helium (He) can be used as the atmosphere gas during rapid cooling. After rapid cooling, the optical fiber is exposed to atmospheric temperature once, for example, and then slowly cooled by a heating mechanism (slow cooling furnace) at around 900°C to 1200°C. To improve the heat retention of the heating mechanism and lower the hypothetical temperature, nitrogen (N2) can be used as the atmosphere gas during slow cooling, for example.

[0037] The Raman scattering spectrum of the optical fiber was measured, for example, by microscopic Raman spectrometry, using the same method as in Patent Document 2. Specifically, a 532 nm wavelength laser beam from a semiconductor laser device was focused and irradiated onto the end face of the optical fiber with a spot diameter of approximately 2 μm. The exposure was set to two exposures totaling 30 seconds. The laser intensity was an oscillating power of 1 W (approximately 100 mW at the end face of the optical fiber). The laser was then irradiated perpendicularly onto the end face of the optical fiber, and the Raman scattering spectrum was measured using a backscattering configuration.

[0038] Next, the method for quantitatively deriving the wavenumber shift in Raman scattering spectra will be explained.

[0039] 1. Correction of wavenumber deviation in Raman scattering spectra

[0040] Due to measurement conditions, artifacts may occur in the wavenumber position of the Raman scattering spectrum. Therefore, the artifacts generated during measurement are first corrected by defining the peak position of the Raman scattered light ω0. Specifically, the peak of the Raman scattered light ω0 is fitted with the following Gaussian function, and the center wavenumber k of the peak of the Raman scattered light ω0 is obtained through fitting. ω0 With 321cm -1 Consistent (k) ω0 =321cm -1 ).

[0041] I=I0exp(-α0(kk ω0 )^2)

[0042] Here, I represents intensity, k represents wavenumber, and I0 represents the maximum intensity of the Raman scattered light ω0 (center wavenumber k). ω0 (Intensity), where α0 is a coefficient.

[0043] 2. Determination of the central wavenumber of Raman scattered light ω4(TO)

[0044] Next, at a wavenumber of 1000 cm⁻¹ -1 Above 1100cm -1 In the following region, the Raman scattering spectrum is fitted using the following Gaussian function. Since the baseline effect is small in this wavenumber region, baseline correction is not performed, but it is permissible.

[0045] I=I1exp(-α4(kk ω4 )^2)

[0046] Here, I represents intensity, k represents wavenumber, and I1 represents maximum intensity (center wavenumber k). ω4 (Intensity), α4 is a coefficient. The center value of the Gaussian function obtained by fitting is defined as the center wavenumber k. ω4 The half-value half-width of the Gaussian function is taken as the half-value half-width k. H4 .

[0047] Figure 3 To show the center wavenumber k ω4 With half-value half-width k H4 A diagram showing the relationships. In Figure 3 In the middle, the horizontal axis represents the half-value and half-width k. H4 (cm -1 The vertical axis represents the center wavenumber k. ω4 (cm -1 Here, multiple optical fibers are fabricated by changing the cooling conditions immediately after fiberization (presence or absence of rapid cooling, temperature of the slow cooling furnace, slow cooling time, and atmospheric gas, etc.), and for each optical fiber, the center wavenumber k of the Raman scattered light ω4(TO) peak is determined using the method described above. ω4 and half-value half-width k H4 .exist Figure 3 The diagram distinguishes between optical fibers manufactured by rapid cooling immediately after fiberization (referred to as "rapidly cooled optical fibers") and optical fibers manufactured without rapid cooling immediately after fiberization (referred to as "unrapidly cooled optical fibers"). It is particularly desirable to use manufacturing conditions in which the atmosphere gas is altered during rapid cooling and gradual cooling.

[0048] center wavenumber k ω4 and half-value half-width k H4 The larger the value, the more adverse the impact on transmission loss in the near-infrared communication band. For example... Figure 3 As shown, when the center wave number k ω4 When it increases, half the value and half the width k H4 The tendency is to decrease. In optical fibers with rapid cooling, compared to optical fibers without rapid cooling, the center wavenumber k is reduced relative to the same degree of cooling. ω4 This can reduce the half-width k. H4 In optical fibers with rapid cooling, the half-value and half-width are both 36cm. -1 Below. In the fiber optic cables without rapid cooling, all but one half-value and half-width are greater than 36cm. -1 It is believed that this difference is due to the fact that rapid cooling inhibits the crystallization transformation of SiO2.

[0049] Figure 4 To show the product k ω4 ×k H4 Compared with strength ratio I ratio A diagram showing the relationships. In Figure 4 In the middle, the horizontal axis represents the strength ratio I. ratio The vertical axis represents the center wavenumber k. ω4 (cm -1 ) and half-value half-width k H4 (cm -1 The product of k) ω4 ×k H4 (cm -2 Strength ratio Iratio The intensity I of the Raman scattered light ω3 ω3 The intensity I of the Raman scattered light D2 D2 The ratio of I D2 / I ω3 Patent document 2 describes that in optical fibers with silica glass as the main component, the strength is higher than that of I. ratio The smaller the value, the more homogeneous the silica glass becomes, thus reducing Rayleigh scattering and lowering transmission loss.

[0050] like Figure 4 As shown, product k ω4 ×k H4 Compared with strength ratio I ratio There is no strong correlation, and the deviation is relatively large. It is believed that this is because the central wavenumber and half-width at half-maximum (WWHM) of the Raman scattered light vary greatly depending on the degree of crystallization, the amount of added elements, and the type of added elements. In optical fibers with rapid cooling, compared to optical fibers without rapid cooling, by controlling the WWHM k... H4 and center wavenumber k ω4 It is possible to intentionally reduce the product k. ω4 ×k H4 The value of . It is believed that this can reduce the half-value half-width k in optical fibers with rapid cooling. H4 The result was obtained. The product k was obtained. ω4 ×k H4 30000cm -2 Fiber optic cable.

[0051] Figure 5 To illustrate the relationship between transmission loss and product k ω4 ×k H4 A diagram showing the relationships. In Figure 5 In the diagram, the horizontal axis represents the center wavenumber k. ω4 (cm -1 ) and half-value half-width k H4 (cm -1 The product of k) ω4 ×k H4 (cm -2 The vertical axis represents transmission loss (dB / km). For example... Figure 5 As shown, with product k ω4 ×k H4 As the value of k decreases, transmission loss tends to decrease as well. Transmission loss can also increase or decrease due to various factors such as impurities, among which the product k... ω4 ×k H4 This represents the factors that cause changes in the intrinsic loss due to variations in the structure of silica glass. In the product k... ω4 ×k H4 38000cm -2The following optical fibers achieve a transmission loss of less than 0.15 dB / km. More preferably, the fiber length is 37500 cm. -2 The following, and more preferably, is 33000cm. -2 Therefore, the transmission loss becomes 0.149 dB / km or less, more preferably 0.146 dB / km or less. In optical fibers with rapid cooling, the product k ω4 ×k H4 Both are 38000cm -2 Below, a transmission loss of less than 0.15 dB / km was achieved. In the fiber without rapid cooling, all but one fiber had a half-width of more than 36 cm. -1 The transmission loss is higher than 0.15dB / km.

[0052] Explanation of symbols

[0053] 1… Optical fiber

[0054] 10…core

[0055] 20…cladding

Claims

1. An optical fiber, comprising: The core and the core are made of silica glass. A cladding composed of silica glass surrounds the core. The center wavenumber k of the Raman scattered light ω4(TO) obtained by irradiating the core with excitation light of wavelength 532 nm is... ω4 With half-value half-width k H4 The area is 38000 cm² -2 the following, The half-value half-width k H4 36cm -1 the following.

2. The optical fiber according to claim 1, wherein, The central wave number k ω4 1070cm -1 the following.

3. The optical fiber according to claim 1 or claim 2, wherein, The core contains alkali metal elements.

4. The optical fiber according to claim 1 or claim 2, wherein, The transmission loss is below 0.15 dB / km.

5. The optical fiber according to claim 1 or claim 2, wherein, The volume is 37500 cm³ -2 the following.

6. The optical fiber according to claim 1 or claim 2, wherein, The transmission loss is below 0.149 dB / km.

7. The optical fiber according to claim 1 or claim 2, wherein, The volume is 33000 cm³ -2 the following.

8. The optical fiber according to claim 1 or claim 2, wherein, The transmission loss is below 0.146 dB / km.

9. The optical fiber according to claim 1 or claim 2, wherein, The Ge mass fraction of the core is less than 0.1%.

10. The optical fiber according to claim 1 or claim 2, wherein, The diameter of the core is between 6 μm and 18 μm.

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