fiber optic

Abstract

The present invention comprises: a core (10) formed from silica glass; and a cladding (20) which is formed from silica glass and surrounds the core (10). The product of the central wavenumber kω4 and the half width at half maximum kH4 of the peak of the Raman scattering light ω4 (T0) in the Raman scattering spectrum obtained by irradiating the core with excitation light having a wavelength of 532 nm is 38000 cm-2 or less.

Description

[Technical Field]

[0001] This disclosure relates to optical fibers. This application claims priority under Japanese application No. 2021-001521, filed on January 7, 2020, and incorporates all the provisions of the said Japanese application. [Background technology]

[0002] Optical fibers with a core containing alkali metal elements are known to exhibit low Rayli scattering and low transmission loss (for example, Patent Document 1). When the core portion of the optical fiber matrix contains alkali metal elements, the viscosity of the core portion can be reduced when drawing the optical fiber matrix, and the relaxation of the quartz glass network structure progresses. As a result, the virtual temperature of the glass within the optical fiber decreases, making it possible to reduce the transmission loss of the optical fiber. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Special Publication No. 2005-537210 [Patent Document 2] Japanese Patent Publication No. 2016-130786 [Non-patent literature]

[0004] [Non-Patent Document 1] RP Wang et al., “Fluorine-doping concentration and fictive temperature dependence of self-trapped holes in glasses”, J. Appl. Phys. 98, 023701 (2005) [Non-Patent Document 2] DJ Little et al., “Femtosecond laser modification of fused silica: the effect of writing polarization on Si-O ring structure”, Opt. Express 16, 24, 20029, (2008) [Overview of the project]

[0005] The optical fiber of this disclosure comprises a core made of silica glass and a cladding made of silica glass surrounding the core, and the Raman scattered light ω4(T) obtained by irradiating the core with excitation light of wavelength 532 nm O The central wavenumber of the peak of the ) ω4 and half width k H4 The product of these is 38,000 cm². -2 The following applies: [Brief explanation of the drawing]

[0006] [Figure 1] Figure 1 is a cross-sectional view of an optical fiber according to an embodiment. [Figure 2] Figure 2 shows the Raman scattering spectrum of silica glass. [Figure 3] Figure 3 is a graph showing the relationship between the central wave number kω4 and the half-width kH4. [Figure 4] Figure 4 is a graph showing the relationship between the product kω⁴ × kH⁴ and the intensity ratio (Iratio). [Figure 5] Figure 5 is a graph showing the relationship between transmission loss and the product kω⁴ × kH⁴. [Modes for carrying out the invention]

[0007] [Issues this disclosure aims to address] In addition to Rayleigh scattering, infrared absorption and absorption by OH groups contribute to the increase in transmission loss of optical fibers in the near-infrared region used as a communication band. The transmission loss due to infrared absorption increases due to the shift of the infrared absorption peak to the shorter wavelength side and the increase in the full width at half maximum of the infrared absorption peak. Non-Patent Document 1 describes that a wavenumber shift of the infrared absorption peak occurs due to a difference in added elements. Since the position and full width at half maximum of the infrared absorption peak are greatly affected by the strength and bond angle of the Si-O bond, they are sensitive to the type and amount of added elements and the crystallization transition of the glass. Therefore, even when the virtual temperature is low, depending on partial crystallization and the profile of the added elements, the transmission loss due to infrared absorption is not necessarily reduced, and the transmission loss may deteriorate.

[0008] An object of the present disclosure is to provide an optical fiber with low transmission loss.

[0009] [Effects of the Present Disclosure] According to the present disclosure, an optical fiber with low transmission loss can be provided.

[0010] [Description of Embodiments of the Present Disclosure] First, embodiments of the present disclosure will be listed and described. An optical fiber according to one embodiment includes a core made of silica glass and a cladding surrounding the core and made of silica glass, and the product of the central frequency k O of the peak of the Raman scattered light ω4(T ω4 and the full width at half maximum k H4 is 38000 cm -2 or less, obtained by irradiating the core with excitation light having a wavelength of 532 nm.

[0011] In the above optical fiber, the transmission loss can be reduced.

[0012] In the above optical fiber, the full width at half maximum k H4 may be 36 cm -1 or less. In this case, the product of the central frequency k O of the peak of the Raman scattered light ω4(T ω4 and the full width at half maximum k H4 is 38000 cm-2 The following is easier.

[0013] In the optical fiber described above, the central wavenumber k ω4 1070cm -1 The following is also acceptable. In this case, the Raman scattered light ω4(T O The central wavenumber of the peak of the ) ω4 and half width k H4 The product of these is 38,000 cm -2 The following is easier.

[0014] In the optical fiber described above, the core may contain alkali metal elements. In this case, Rayli scattering is suppressed, which further reduces transmission loss.

[0015] [Details of the embodiments of this disclosure] Specific examples of the optical fibers of this disclosure will be described below with reference to the drawings. However, the present invention is not limited to these examples, and is intended to include all modifications within the meaning and scope of the claims, as defined by the claims. In the description of the drawings, identical elements are denoted by the same reference numerals, and redundant descriptions are omitted.

[0016] Figure 1 is a cross-sectional view of an optical fiber according to an embodiment. As shown in Figure 1, the optical fiber 1 according to the embodiment comprises a core 10 and a cladding 20. The core 10 is made of silica glass and contains, for example, an alkali metal element such as potassium, fluorine, or chlorine. By containing an alkali metal element in the core 10, Rayli scattering can be suppressed and transmission loss can be reduced. 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 and chlorine. The cladding 20 has a refractive index lower than that of the core 10.

[0017] In the optical fiber 1 according to this embodiment, Raman scattered light ω4(T) is obtained by irradiating the core 10 with excitation light of wavelength 532 nm. OThe central wavenumber of the peak of the ) ω4 and half width k H4 The product of these is 38,000 cm². -2 The product is as follows: 37500 cm -2 Preferably, it is 33,000 cm². -2 The following is more preferable: Half width at half maximum k H4 For example, 36cm -1 The following applies: Central wavenumber k ω4 For example, 1070cm -1 The following applies:

[0018] Here, we will explain Raman scattering spectra. Generally, when light is shone on a substance, the interaction between the light and the substance (molecular vibrations) generates Raman scattered light with a different wavelength from the light source. By spectrally analyzing this Raman scattered light, the Raman scattering spectrum obtained allows for the analysis of the molecular-level structure of the substance. In a Raman scattering spectrum, multiple peaks are generated depending on the number of vibrational modes of atomic bonds within the substance.

[0019] Figure 2 shows the Raman scattering spectrum obtained by irradiating a quartz-based glass (silica glass) with laser light at a wavelength of 532 nm. In Figure 2, the horizontal axis is the Raman shift (cm²). -1 The graph shows the intensity, with the vertical axis representing the intensity. In the Raman scattering spectrum shown in Figure 2, the peak of the Raman scattered light ω0 from calcium fluoride, which originates from the sample stage, is at wavenumber 300 cm⁻¹. -1 More than 350cm -1 The following range is observed: The peak of Raman scattered light ω3 due to Si-O stretching vibration is at wavenumber 750 cm⁻¹. -1 More than 875cm -1 The following peaks are observed: The Raman scattered light D2 peak attributed to the silica three-membered ring structure is found at wavenumber 565 cm⁻¹. -1 More than 640cm -1 The following range is observed: The peak of Raman scattered light ω4(TO) due to Si-O stretching vibration is at wavenumber 1000 cm⁻¹. - 1 More than 1100cm -1The peak in the following range is observed. The Raman scattered light ω4(TO) peak originates from the scattered light of the transverse wave generated by the asymmetric stretching vibration of Si-O, which is one of the vibrational modes of the Si-O bond (Non-Patent Literature 2).

[0020] The wavenumber positions (i.e., central wavenumbers) of these peaks are greatly influenced by the strength and bond angles of the Si-O bonds that make up silica glass. These bonding states undergo slight wavenumber shifts depending on the type and amount of added elements, or the partial crystallization of the glass. This wavenumber shift reflects the changes in the strength and bond angles of the atomic bonds caused by the added elements.

[0021] Infrared absorption, like Raman scattering, is a phenomenon related to the interaction between atomic bond vibrations and light. Therefore, under the same conditions, a wavenumber shift occurs in the same direction as Raman scattered light ω4(TO) in infrared absorption. However, in infrared absorption spectra, multiple peaks tend to overlap due to the coupling of Si-O vibrational modes, making it difficult to distinguish peaks. Furthermore, as described later, the intensity ratio I ratio From a comparative perspective, the Raman scattered light ω4(TO) of the Raman spectrum is more suitable for quantitatively evaluating wavenumber shift and full width at half maximum. Therefore, in the optical fiber according to this embodiment, the change in transmission loss due to infrared absorption is evaluated by the Raman scattering spectrum.

[0022] The optical fiber according to the embodiment is manufactured by adjusting the drawing conditions, for example, in order to suppress transmission loss due to infrared absorption. Specifically, in order to suppress the crystallization transition of SiO2 and the deposition of additives, the optical fiber is rapidly cooled by increasing the cooling rate immediately after fiberization so that it does not remain in the temperature range in which crystallization is easily formed (1200°C to 1700°C) for a long time. Rapid cooling is performed, for example, in the temperature range of 2000°C to 1200°C. For example, helium (He) gas can be used as the atmospheric gas during rapid cooling. After rapid cooling, the optical fiber is slowly cooled by, for example, being exposed to ambient temperature once and then passing through a heating mechanism (slow cooling furnace) at a temperature of about 900°C to 1200°C. In order to improve the heat retention by the heating mechanism and reduce the virtual temperature, for example, nitrogen (N2) gas can be used as the atmospheric gas during slow cooling.

[0023] The Raman scattering spectrum of an optical fiber is measured, for example, by microscopic Raman spectroscopy similar to that described in Patent Document 2. Specifically, a laser beam with a wavelength of 532 nm output from a semiconductor laser device is focused to create a spot with a diameter of approximately 2 μm and irradiated onto the end face of the optical fiber. Exposure is performed twice for an integrated total of 30 seconds. The laser beam intensity is 1 W (approximately 100 mW at the end face of the optical fiber). The laser beam is then irradiated perpendicularly to the end face of the optical fiber, and the Raman scattering spectrum is measured by backscattering.

[0024] Next, we will explain a method for quantitatively deriving the wavenumber shift in the Raman scattering spectrum.

[0025] 1. Correction of wavenumber shift in Raman scattering spectra Due to measurement conditions, artifacts such as shifts in the wavenumber position of the Raman scattering spectrum may occur. Therefore, artifacts generated during measurement are corrected by first 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 central wavenumber k of the Raman scattered light ω0 peak obtained by fitting is... ω0 321cm -1 Align to (k ω0 = 321cm -1 ). I=I0exp(-α0(kk ω0 )^2) Here, I is intensity, k is wavenumber, and I0 is the maximum intensity of the Raman scattered light ω0 (center wavenumber k). ω0 The intensity in α0 is a coefficient.

[0026] 2. Determination of the central wavenumber of Raman scattered light ω4(TO) Next, wave number 1000cm -1 More than 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 may be done if desired. I = I1 exp(-α4(kk) ω4 )^2) Here, I is intensity, k is wavenumber, and I1 is maximum intensity (center wavenumber k). ω4 The intensity at α4 is a coefficient. The center value of the Gaussian function obtained by fitting is the center wavenumber k ω4 Defined as follows, the half-width at half maximum of the Gaussian function is half-width k H4 Let's assume that.

[0027] Figure 3 shows the central wavenumber k ω4 and half width k H4 This graph shows the relationship. In Figure 3, the horizontal axis is the half-width at half maximum k. H4 (cm -1 ) is shown, with the vertical axis representing the central wavenumber k. ω4 (cm -1 This shows the following. Here, multiple optical fibers are manufactured 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 Raman scattered light ω4(T) is obtained using the method described above. O The central wavenumber of the peak of the ) ω4 and half width k H4 The following was determined. Figure 3 shows a distinction 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 atmospheric gas is changed during rapid cooling and slow cooling.

[0028] center wave number k ω4 and half width at half maximum k H4 In all cases, a larger value of k negatively impacts transmission loss in the near-infrared communication band. As shown in Figure 3, the central wavenumber k ω4 As it increases, the half width at half maximum k H4 It tends to become smaller. In optical fibers with rapid cooling, the central wavenumber k is similar to that of optical fibers without rapid cooling. ω4 For this, the half-width k H4 This can be made smaller. In optical fibers with rapid cooling, the half-width at half maximum is 36 cm in all cases. -1 The following applies to optical fibers without rapid cooling: All but one have a half-width at half maximum of 36 cm. -1 It is larger than that. This difference is thought to be due to the suppression of the crystallization transition of SiO2 by rapid cooling.

[0029] Figure 4 shows the product k ω4 ×k H4 and intensity ratio I ratio This graph shows the relationship. In Figure 4, the horizontal axis is the intensity ratio I ratio This shows the central wavenumber k on the vertical axis. ω4 (cm -1 ) and half width at half maximum k H4 (cm -1 ) and the product k ω4 ×k H4 (cm -2 ) indicates. Intensity ratio I ratio This is the intensity I of the Raman scattered light ω3. ω3 and the intensity of Raman scattered light D2 I D2 Ratio I D2 / I ω3 Patent Document 2 states that in an optical fiber mainly composed of silica glass, the intensity ratio is ratio It has been stated that the smaller the value, the more uniform the silica glass becomes, reducing Rayli scattering and thus lowering transmission loss.

[0030] As shown in Figure 4, product k ω4 ×k H4 and intensity ratio I ratioIt has no strong correlation and relatively large variations. This is presumably because the central frequency and full width at half maximum of the Raman scattered light vary significantly depending on the degree of crystallization, the amount of additive elements, and the type of additive elements. In the optical fiber with quenching, compared with the optical fiber without quenching, the product k H4 and the central frequency k ω4 can be significantly reduced by controlling k ω4 ×k H4 . This can be said to be the result of being able to reduce the full width at half maximum k H4 in the optical fiber with quenching. An optical fiber with a product k ω4 ×k H4 of 30000 cm -2 has been obtained.

[0031] <000025'7>Figure 5 is a graph showing the relationship between the transmission loss and the product k ω4 ×k H4 . In Figure 5, the horizontal axis represents the product k of the central frequency k ω4 (cm -1 ) and the full width at half maximum k H4 (cm -1 ), and the vertical axis represents the transmission loss (dB / km). As shown in Figure 5, as the value of the product k ω4 ×k H4 (cm -2 ) decreases, the transmission loss also tends to decrease. Among the various factors that increase or decrease the transmission loss, such as impurities, the product k ω4 ×k H4 represents the essential loss change factor that varies due to the structure of silica glass. In optical fibers with a product k ω4 ×k H4 of 38000 cm ω4 ×k H4 or less, a transmission loss of 0.15 dB / km or less has been achieved. More preferably, it is 37500 cm -2 or less, and even more preferably, it is 33000 cm -2 or less. Thereby, the transmission loss becomes 0.149 dB / km or less, and more preferably 0.146 dB / km or less. In the optical fiber with quenching, in all cases, the product k -2 ×k ω4 ×k H4 is 38000 cm -2The following results were achieved, with a transmission loss of 0.15 dB / km or less. For optical fibers without rapid cooling, all but one had a half-width at half maximum of 36 cm. -1 It is larger than that, and the transmission loss is higher than 0.15 dB / km. [Explanation of Symbols]

[0032] 1… Optical fiber 10... Cores 20... Clad

Claims

1. A core made of silica glass, The aforementioned core is surrounded by a cladding made of silica glass, Raman scattered light ω obtained by irradiating the core with excitation light of wavelength 532 nm ４ The central wavenumber k of the peak of (TO) ω４ and half value half width k Ｈ４ The product of these is 38,000 cm². －２ The following: The aforementioned half-width kH4 is 36 cm - 1 or less. The Ge mass fraction of the core is 0.1% or less. Optical fiber.

2. The aforementioned central wavenumber k ω４ It is 1070cm －１ The following is: The optical fiber according to claim 1.

3. The aforementioned core contains an alkali metal element. The optical fiber according to claim 1 or claim 2.

4. The transmission loss is 0.15 dB / km or less. The optical fiber according to any one of claims 1 to 3.

5. The aforementioned volume is 37,500 cm³. －２ The following is: The optical fiber according to any one of claims 1 to 4.

6. The transmission loss is 0.149 dB / km or less. The optical fiber according to any one of claims 1 to 5.

7. The aforementioned volume is 33,000 cm³. －２ The following is: The optical fiber according to any one of claims 1 to 6.

8. The transmission loss is 0.146 dB / km or less. The optical fiber according to any one of claims 1 to 7.

9. The diameter of the core is 6 μm or more and 18 μm or less. The optical fiber according to any one of claims 1 to 8.

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