Optical communication system

Abstract

This optical communication system comprises: an optical transmission unit that includes a light source for outputting laser light having a spectral width of 0.1 nm or greater in an oscillation spectrum and a signal generator for applying a modulation signal of 25 GBaud or greater to the light source, the optical transmission unit outputting a modulated optical signal in which the laser light is modulated by the modulation signal; and an optical fiber that is composed of a plastic which contains fluorine atoms in at least the core thereof, the optical fiber being such that the absolute value of material dispersion at a wavelength within the spectral width of the modulated optical signal is less than the absolute value of material dispersion in quartz glass, having a length of 100 m or less, having a GI-type refractive index distribution, and propagating the modulated optical signal.

Description

Optical communication system

[0001] This invention relates to an optical communication system.

[0002] Conventionally, optical communication systems are known that use a light source that emits laser light, such as a VCSEL (Vertical-Cavity Surface-Emitting Laser), to generate a modulated optical signal modulated with a modulation signal such as PAM (Pulse Amplitude Modulation), and transmit it through a plastic optical fiber (Patent Document 1).

[0003] Multimode optical fibers, which propagate light in multiple modes, possess a characteristic called modal dispersion, where the propagation speed of light varies depending on the mode of propagation. Modal dispersion can limit the modulation speed in optical communication systems and degrade the quality of modulated optical signals. One method known to reduce modal dispersion is to make the refractive index distribution in the optical fiber core a Graded Index (GI) type (Non-Patent Literature 1).

[0004] International Publication No. 2022 / 215366

[0005] Y. Koike, “Fundamentals of Plastic Optical Fibers”, Wiley-VCH, 2015.

[0006] In light sources such as VCSELs, which have multiple oscillation peaks due to multimode oscillation in the oscillation spectrum of the output laser light, the spectral width of the oscillation spectrum can be as wide as 0.1 nm or more. When using laser light with such a wide spectral width, even if mode dispersion is reduced, chromatic dispersion may limit the modulation speed or degrade the quality of the modulated optical signal. Wavelength dispersion refers to the characteristic in optical fibers where the propagation speed of light varies depending on the wavelength of the light. In particular, material dispersion, which is one of the factors of chromatic dispersion, is a characteristic that depends on the constituent material of the optical fiber, and therefore cannot be reduced by controlling the refractive index distribution in the optical fiber core alone.

[0007] The present invention has been made in view of the above, and aims to provide an optical communication system that can suppress limitations on the modulation speed and degradation of quality in modulated optical signals.

[0008] To solve the above-mentioned problems and achieve the objective, one aspect of the present invention is an optical communication system comprising: a light source that outputs laser light with a spectral width of 0.1 nm or more in its oscillation spectrum; a signal generator that applies a modulation signal of 25 GBA ud or more to the light source, and an optical transmission unit that outputs a modulated optical signal in which the laser light is modulated by the modulation signal; and an optical fiber that transmits the modulated optical signal, the optical fiber being made of a plastic containing fluorine atoms in at least its core, having a length of 100 m or less, a GI-type refractive index distribution, and having an absolute value of material dispersion at wavelengths within the spectral width of the modulated optical signal smaller than the absolute value of material dispersion in quartz glass.

[0009] According to the present invention, it is possible to suppress limitations on the modulation speed and degradation of quality in modulated optical signals.

[0010] Figure 1 is a schematic diagram of the optical communication system according to Embodiment 1. Figure 2 is a diagram showing an example of the oscillation spectrum of a light source. Figure 3 is a diagram showing an example of the relationship between the injection current into the light source and the center wavelength. Figure 4 is a diagram showing an example of the relationship between the injection current into the light source and the spectral width. Figure 5 is a diagram showing an example of the specific configuration of the optical coupling section. Figure 6 is a diagram showing an example of NFP and FFP of the modulated optical signal on the end face of an optical fiber. Figure 7 is a diagram showing an example of the relationship between the g value and the -3 dBO bandwidth. Figure 8 is a diagram showing an example of the relationship between the Abbe number and the refractive index. Figure 9 is a diagram showing an example of the relationship between the molar concentration of fluorine and the refractive index. Figure 10 is a diagram showing an example of the relationship between the molar concentration of fluorine and the Abbe number. Figure 11 is a diagram showing an example of material dispersion. Figure 12 is a diagram showing an example of frequency response. Figure 13 is a diagram showing an example of frequency response. Figure 14 is a diagram showing an example of frequency response. Figure 15 is a diagram showing an example of the relationship between spectral width and the -1.5 dBO bandwidth. Figure 16 is a diagram showing an example of the relationship between spectral width and the -3 dBO bandwidth. Figure 17 is a diagram showing an example of frequency response. Figure 18 is a diagram showing an example of the frequency response. Figure 19 is a diagram showing an example of the frequency response. Figure 20 is a diagram showing an example of the eye pattern. Figure 21 is a diagram showing an example of the relationship between fiber length and TDECQ. Figure 22 is a diagram showing an example of the eye pattern. Figure 23 is a diagram showing an example of the relationship between fiber length and TDECQ. Figure 24 is a diagram showing an example of the eye pattern. Figure 25 is a diagram showing an example of the relationship between fiber length and TDECQ. Figure 26 is a diagram showing an example of the eye pattern. Figure 27 is a diagram showing an example of the relationship between fiber length and TDECQ. Figure 28 is a diagram showing an example of the eye pattern. Figure 29 is a diagram showing an example of the relationship between fiber length and TDECQ. Figure 30 is a diagram showing an example of the eye pattern. Figure 31 is a diagram showing an example of the relationship between fiber length and TDECQ. Figure 32 is a diagram showing the wavelength dependence of the -3dBo bandwidth. Figure 33 is a schematic configuration diagram of the optical communication system according to Embodiment 2. Figure 34 is a diagram showing an example of the frequency response. Figure 35 is a diagram showing an example of the frequency response. Figure 36 is a diagram showing an example of the frequency response. Figure 37 shows an example of a frequency response.Figure 38 shows an example of the relationship between the center wavelength and the -1.5 dBo bandwidth.

[0011] Embodiments of the present invention will be described in detail below with reference to the drawings. However, the present invention is not limited to these embodiments. Furthermore, in each drawing, the same or corresponding elements are denoted by the same reference numerals as appropriate, and redundant explanations are omitted as appropriate. It should also be noted that the drawings are schematic, and the dimensional relationships of each element may differ from those in reality. Even between drawings, there may be parts where the dimensional relationships and ratios differ.

[0012] Traditionally, in multimode optical fibers, mode dispersion has been considered dominant, with the contribution of material dispersion being relatively small, and its effects not becoming apparent at practical communication distances. In particular, material dispersion was not considered a problem unless the optical fiber was several hundred meters or longer, and it was thought that the influence of material dispersion on waveform quality in multimode optical fibers, regardless of whether they were silica-based or plastic-based, could be ignored.

[0013] However, contrary to this conventional wisdom, the inventors have discovered that under new transmission conditions of high speed (high baud rate) and short distance, such as 25 GBaud, 50 GBaud or higher, and even exceeding 100 GBaud, the influence of material dispersion when using a wide spectral width light source can become dominant. They then conceived that plastic multimode optical fibers with low material dispersion can achieve superior waveform quality compared to silica-based multimode optical fibers. Furthermore, this not only enables higher quality optical transmission when using existing light sources, but also relaxes the requirements for the spectral width of the light source, thereby increasing the flexibility of light source design. For example, even if the spectral width of the oscillation spectrum widens as a result of designing the light source to obtain stable modulation speed and noise characteristics, the low material dispersion of plastic multimode optical fibers allows for the maintenance of good transmission characteristics. This is an advantage that cannot be obtained from designs based on the material dispersion of silica-based multimode optical fibers (MMF), and it can provide new guidelines for the design of optical communication systems.

[0014] <Embodiment 1> Figure 1 is a schematic diagram of an optical communication system according to Embodiment 1. The optical communication system 100 includes an optical transmission unit 10, an optical coupling unit 20, an optical fiber 30 which is an optical transmission path, and an optical receiving unit 40.

[0015] (Configuration of the optical transmitting unit) The optical transmitting unit 10 includes a signal generator 11, a DC current source 12, a bias T 13, and a light source 14.

[0016] The signal generator 11 generates a modulated signal of 25 GBaud or higher to be applied to the light source 14. The modulated signal is, for example, a multi-level modulated signal, and the multi-level modulated signal is, for example, a PAM4 signal.

[0017] The DC current source 12 generates a bias current to be injected into the light source 14.

[0018] The bias T13 outputs a superimposed modulated signal from the signal generator 11 and a bias current from the DC current source 12.

[0019] (Characteristics of the light source) The light source 14 outputs, for example, laser light having a plurality of oscillation peaks due to multi-mode oscillation in the oscillation spectrum. In the present embodiment, the light source 14 is a VCSEL. The light source 14 outputs a modulated optical signal that is output from the bias T13 and modulated by a modulation signal with a bias current superimposed thereon.

[0020] FIG. 2 is a diagram showing an example of the oscillation spectrum of the light source 14. As shown in FIG. 2, the laser light output from the light source 14 has a spectrum width of 0.1 nm or more in the oscillation spectrum. Also, the wavelength of the laser light belongs to the 850 nm wavelength band. Here, the 850 nm wavelength band is meant to refer to a wavelength range of approximately 844 nm to 863 nm in accordance with customary usage.

[0021] The light source 14 is a VCSEL developed, for example, for 100 Gbps PAM4 transmission (that is, 50 GBaud PAM4 transmission). When the injection current (drive current) is 9 mA, the modulation bandwidth is about 27.5 GHz, and the power of the output laser light is 3.5 mW. Note that FIG. 2 shows the case where the injection current is 15 nm. Also, the spectrum in FIG. 2 is data obtained by setting the measurement resolution to 0.07 nm, the video bandwidth (VBW) to 100 Hz, and performing an average process of 10 measurements in an optical spectrum analyzer (manufactured by Anritsu Corporation, MS9740B).

[0022] FIG. 3 is a diagram showing an example of the relationship between the injection current into the light source and the center wavelength. Also, FIG. 4 is a diagram showing an example of the relationship between the injection current into the light source and the spectrum width. Both the center wavelength and the spectrum width were obtained by a method conforming to IEC 61280-1-3. Specifically, the center wavelength was calculated as the centroid (first moment) of the oscillation spectrum intensity distribution, and the spectrum width was calculated as the RMS (Root Mean Square) value obtained from its standard deviation (second moment). Components 20 dB or more lower than the peak value of the intensity of the oscillation spectrum were removed as noise components.

[0023] As shown in FIGS. 3 and 4, it was confirmed that both the central wavelength and the spectral width increase as the injection current increases. Note that this is an example of the oscillation characteristics of a VCSEL, and depending on the structure design and operating environment of the VCSEL, the central wavelength and the spectral width may not monotonically increase with the increase of the injection current, and may show discontinuous or complex changes.

[0024] In addition, general communication semiconductor lasers, such as DFB (Distributed Feedback) lasers, operate in single mode and show a single peak on the oscillation spectrum. At this time, the spectral width of the oscillation mode is extremely narrow, usually less than 0.10 nm, and in some cases may be 0.05 nm or less. For example, the spectral width of a DFB laser oscillating in the 850 nm band is, as an example, about 0.03 nm, which is very narrow.

[0025] On the other hand, a VCSEL has a plurality of resonance modes in the transverse mode and the longitudinal mode, and generally oscillates in multi-mode. As a result, two or more oscillation peaks appear on the oscillation spectrum, and the spectral width (usually defined by the RMS value) of the entire oscillation mode becomes wide. With this wide spectral width, the modulated optical signal causes an interaction with material dispersion during optical fiber transmission, which is a factor that greatly degrades the transmission band. Note that depending on the structure of the VCSEL, the oscillation peaks do not necessarily appear separated into a plurality, and there are cases where a plurality of modes overlap and are observed as a single peak, or cases where it actually oscillates in single mode. However, even in such cases, if the spectral width is wide, it becomes a factor that degrades the transmission signal due to the interaction with material dispersion.

[0026] In the oscillation spectrum of a VCSEL for high-speed communication, the spectral width is, for example, in the range of 0.20 nm to 0.90 nm. Furthermore, it is known that the oscillation spectrum of a VCSEL changes depending on driving conditions such as injection current and environmental temperature. In the actual use environment, it is general for the spectral width to be in the range of 0.10 nm to 1.20 nm, and in some cases, it may vary from 0.05 nm to 1.50 nm.

[0027] Furthermore, standardization bodies such as IEEE 802.3 have established regulations recommending operating conditions such as a VCSEL spectral width of 0.60 nm or less in short-distance optical interconnects using VCSELs, in order to suppress signal waveform degradation due to material dispersion. This spectral width value is set assuming transmission using silica-based multimode optical fibers.

[0028] (Configuration of the optical coupling unit) Figure 5 shows an example of the specific configuration of the optical coupling unit 20. The optical coupling unit 20 includes a lens 21, a dichroic mirror 22, an optical attenuation filter 23, a lens 24, and a CCD camera 25. The optical attenuation filter 23 and the CCD camera 25 are provided in a removable manner. The optical attenuation filter 23 and the CCD camera 25 may be removed while optical communication is in progress, and may be attached during the construction or testing of the optical communication system.

[0029] Lens 21 is, for example, an aspherical lens, and collimates the modulated optical signal SL output from the light source 14. The dichroic mirror 22 reflects the modulated optical signal SL, which has been collimated by lens 21, toward lens 24. Lens 24 is, for example, an aspherical lens, and focuses the modulated optical signal SL from the dichroic mirror 22 toward the end face 31 of the optical fiber 30. The end face 31 of the optical fiber 30 is tilted at an angle of 8 degrees with respect to a plane perpendicular to the optical axis in order to prevent degradation of the light source and output fluctuations due to reflected light. The modulated optical signal SL focused toward the end face 31 is optically coupled to the optical fiber 30 and transmitted through the optical fiber 30.

[0030] Furthermore, the return light RL that comes back from the optical fiber 30 to the optical coupling section 20 is collimated by the lens 24, passes sequentially through the attached optical attenuation filter 23 and dichroic mirror 22, and then reaches the attached CCD camera 25. At this time, using microscopic observation with the CCD camera 25, the modulated optical signal SL is precisely focused onto the end face 31 of the optical fiber 30 and input to the core center of the optical fiber 30. This is precisely centered using an optical stage in three orthogonal directions in space (vertical, horizontal, and height directions) and in the rotational direction around each axis. This allows evaluation under central excitation conditions for the optical fiber 30. The optical attenuation filter 23 is installed to attenuate the return light RL input to the light source 14 and the CCD camera 25. The attenuation amount by the optical attenuation filter 23 is, for example, 3 dB.

[0031] Figure 6 shows an example of the Near-Field Pattern (NFP) and Far-Field Pattern (FFP) of the modulated optical signal SL on the end face 31 of the optical fiber 30. From these results, the beam diameter of the modulated optical signal SL on the end face 31 and the divergence angle (total angle) when the modulated optical signal SL is input to the optical fiber 30 were calculated using the second-order moment method in accordance with ISO 11146-1. In the example shown in Figure 6 (corresponding to Example 1 described later), the beam diameter is 12.3 μm and the divergence angle (total angle) is 9.3 degrees. The beam diameter of the input beam is smaller than the core diameter of the optical fiber, and the divergence angle of the input beam is also smaller than the allowable angle corresponding to the numerical aperture (NA) of the optical fiber, confirming that the conditions for limited-mode excitation are met.

[0032] (Composition of optical fiber) The optical fiber 30 is an optical fiber with a length of 100 m or less, made of plastic containing fluorine atoms in at least the core. Specifically, the optical fiber 30 is made of a fully fluorinated resin material or a partially fluorinated resin material that is transparent in the wavelength band of the light source used.

[0033] As for all-fluorine resin materials, products such as TEFRON-AF (DuPont), HyflonAD (Solvay), and CYTOP (Asahi Glass Co., Ltd.) can generally be used. Alternatively, all-fluorine polymers copolymerized with tetrafluoroethylene or the like in the main ring structure of these materials may be used. All-fluorine polymers having a dioxolene skeleton can also be used.

[0034] As partially fluorinated resin materials, polymers of acrylic monomers such as HFIP 2-FA and TFEMA shown in Table 3, or polymers in which hydrogen atoms of the benzene ring are substituted with fluorine, can be used.

[0035] Furthermore, the optical fiber 30 has a GI-type refractive index distribution. Examples of dopants added to form such a refractive index distribution include oligomers of CTFE (chlorotrifluoroethylene). Other examples include low molecular weight compounds or compounds in which hydrogen atoms present in these compounds are replaced with fluorine atoms. Examples of low molecular weight compounds with high refractive indices include diphenyl sulfone (DPSO) and diphenyl sulfone derivatives (e.g., chlorinated diphenyl sulfones such as 4,4'-dichlorodiphenyl sulfone and 3,3',4,4'-tetrachlorodiphenyl sulfone), sulfur compounds such as diphenyl sulfide (DPS), diphenyl sulfoxide, dibenzothiophene, and dithiane derivatives; phosphoric acid compounds such as triphenyl phosphate (TPP) and tricresyl phosphate; benzyl benzoate; benzyl n-butyl phthalate; diphenyl phthalate; biphenyl; and diphenylmethane. Compounds in which some of the hydrogen atoms in these low molecular weight compounds are replaced with fluorine are also candidates. These may be used individually or in combination of two or more.

[0036] Furthermore, the refractive index distribution coefficient is a parameter used to specify the refractive index distribution of a GI type. The refractive index distribution is expressed using the refractive index distribution coefficient as shown in the following equation. Here, n １ n is the refractive index at the center of the core, ２is the refractive index of the cladding, Δ is the relative refractive index difference of the refractive index at the center of the core with respect to the refractive index of the cladding, g is the refractive index distribution coefficient (also called the g value), r is the distance in the radial direction from the center of the core, a is the radius of the core, and n(r) is the refractive index of the core at the distance r. n(r) = n １ [1 - 2Δ(r / a) ｇ １／２ 0 ≤ r ≤ a n ２ r > a

[0037] The refractive index distribution coefficient in the optical fiber 30 is preferably set to a value that suppresses mode dispersion in the optical fiber 30.

[0038] The inventor performed transmission band analysis by the WKB (Wentzel-Kramers-Brillouin) method regarding the refractive index distribution. Calculations were performed assuming that the center wavelength of the modulated optical signal was 850 nm, the spectral width was 0.60 nm, and the fiber length was 100 m.

[0039] As the optical fiber 30, a GI type plastic optical fiber (GI POF) of Example 1 manufactured by the melt extrusion method using PFMMD (Poly(perfluoro-2-methylene-4-methyl-1,3-dioxolane)), which is a fully fluorinated polymer, was assumed. As the configuration, a PFMMD homopolymer was used for the cladding material, and PFMMD added with a high refractive index dopant was used for the core material. The high refractive index dopant added to the core material is an oligomer of CTFE, and the addition amount of the dopant was adjusted so that the numerical aperture (NA) defined by the following formula at 850 nm would be 0.18. NA = √(n １ ２ - n ２ ２ )

[0040] ​Furthermore, calculations were also performed for a GI-type silica-based multimode optical fiber (SMMF) as Comparative Example 1, which used pure silica glass (SiO2) as the cladding and silica glass doped with germanium dioxide (GeO2) as the core. In the SMMF, the amount of GeO2 added was adjusted so that the NA at 850 nm was 0.2.

[0041] Figure 7 shows an example of the relationship between the g value and the -3 dBo bandwidth (transmission bandwidth). As shown in Figure 7, in SMMF, even if the g value is set to the optimal value of approximately 2.03, the transmission bandwidth is limited to about 30 GHz. In contrast, in the case of GI POF, it was confirmed that a transmission bandwidth of about 50 GHz can be obtained with a g value of approximately 2.06. This is due to the fact that the material dispersion in GI POF is smaller than that in SMMF, and it shows that GI POF significantly outperforms SMMF in terms of the final transmission performance obtained when the refractive index distribution is optimized.

[0042] Furthermore, Figure 7 shows that when the g value is set to a range of 2.03 to 2.10 in GI POF, mode dispersion is effectively suppressed, and the transmission bandwidth becomes 18 GHz or higher.

[0043] Here, the value of 18 GHz corresponds to the worst-case bandwidth of a glass MMF assumed in IEEE 802.3db-2022 when transmitting a 53.125 Gbps PAM signal over a distance of 100 m in the 850 nm band, and is considered one of the practical standards for short-distance high-speed transmission. Therefore, by setting the refractive index distribution coefficient g to the range of 2.03 to 2.10 and ensuring that the GI POF satisfies this standard, transmission performance equivalent to or better than that of SMMF can be achieved. In other words, it is preferable that the refractive index distribution of the optical fiber 30 is designed such that, under the conditions of a predetermined fiber length and a predetermined wavelength and spectral width of the modulated optical signal, the -3 dBo bandwidth is 18 GHz or higher.

[0044] In actual use, the spectral width of the modulated optical signal may be narrower than 0.60 nm (e.g., 0.30 nm, 0.20 nm, 0.10 nm, etc.), and the fiber length may be less than 100 m (e.g., 50 m, 30 m, 20 m, 10 m, etc.), which increases the design flexibility of the refractive index distribution coefficient g. Therefore, in practical terms, a sufficient bandwidth can be obtained if the refractive index distribution coefficient g is in the range of 1.70 to 2.50. The range of the refractive index distribution coefficient g is more preferably 1.85 to 2.30, more preferably 1.95 to 2.20, and particularly preferably 2.00 to 2.15.

[0045] (Material dispersion in optical fibers) Next, the material dispersion of optical fiber 30 will be explained. Material dispersion is a physical quantity with units of [ns / nm·km], and it represents the time difference in ns (nanoseconds) in the arrival time at the other end when light with wavelengths differing by 1 nm is simultaneously input to one end of an optical fiber and transmitted over a distance of 1 km. If the refractive index of the constituent material of the optical fiber is constant and does not depend on the wavelength, the speed of light propagating through the optical fiber will be the same regardless of the wavelength, so the arrival time of light that has propagated over a distance of 1 km will be the same, and therefore the material dispersion will be zero.

[0046] However, the refractive index of polymer materials generally tends to increase monotonically as the wavelength decreases from the near-infrared region to shorter wavelengths. In particular, polymers containing benzene rings in their molecules, such as polystyrene and polycarbonate, contain aromatic double bonds, which increases the polarizability of π electrons, especially in the ultraviolet region, so the refractive index increases as the wavelength decreases.

[0047] The inventors have been diligently researching and developing polymer materials with low material dispersion, i.e., low refractive index wavelength dependence, by using polymers that contain as few functional groups with high electronic polarizability as possible.

[0048] The Abbe number ν is a useful physical property for demonstrating the wavelength dependence of this refractive index. Ｄ There is an Abbe number ν. Ｄ This is shown by the following equation. Here, n Ｆ ,n Ｄ ,n ｃThese are the refractive indices of the polymer at wavelengths of 486 nm, 589 nm, and 656 nm, respectively. Ｄ = (n Ｄ -1) / (n Ｆ -n ｃ )

[0049] As mentioned above, the refractive index of a polymer increases monotonically as the wavelength of the transmitted light becomes shorter, therefore ν Ｄ The denominator is (n Ｆ -n ｃ ) is always positive, and (n Ｆ -n ｃ The larger the value of (n), the greater the wavelength dependence of the refractive index. A polymer in which the refractive index is constant regardless of the wavelength of the transmitted light, that is, a polymer with zero material dispersion, is (n Ｆ -n ｃ Since ) = 0, the Abbe number ν Ｄ The Abbe number becomes infinite (∞). In contrast, the Abbe numbers of polystyrene (PSt), polycarbonate (PC), and polymethyl methacrylate (PMMA), which exhibit wavelength dependence of refractive index, are 31, 30, and 55, respectively. Therefore, increasing the Abbe number is important as a guideline for obtaining polymers with low material dispersion, that is, polymers with low refractive index wavelength dependence.

[0050] The wavelengths of light emitted by LEDs and VCSELs, which are thought to be applicable to optical communication systems using plastic optical fibers, fall within the range of visible light to near-infrared light. To examine the wavelength dependence of the refractive index when using these light sources, the n in the definition of the Abbe number described above can be used. Ｆ ,n Ｄ ,n ｃ Therefore, for example, by using the Sellmeyer approximation and expressing the wavelength dependence of the refractive index as a series, the wavelength dependence of the refractive index near any given wavelength can be determined.

[0051] Figure 8 shows the Abbe number ν in various polymers investigated by the inventor. Ｄ and refractive index n Ｄ This figure shows an example of the relationship. Also, the structural formula of the polymer and refractive index n shown in Figure 8. Ｄ , Abbe number ν ＤThese are shown in Tables 1 to 4. Note that the data points in the area enclosed by the dashed line in Figure 8 represent the data points showing the properties of the perfluoropolymer shown in Tables 1 and 2.

[0052] In Figure 8, the Abbe number on the horizontal axis follows the usual conventions used in academic journals, with a larger Abbe number (i.e., lower material dispersion) moving to the left. From Figure 8, it can be seen that polymers tend to have a larger Abbe number as their refractive index decreases. Therefore, a guideline for obtaining polymers with low material dispersion, i.e., high Abbe numbers, is to further reduce the refractive index.

[0053] As can be seen from Figure 8, the refractive index n of polymers with small material dispersion and an Abbe number of 60 or higher Ｄ The Abbe number is 1.45 or less, and it can be seen that these are polymers listed in Tables 1 to 3 that contain fluorine atoms F in the polymer molecule. Conversely, when the Abbe number is 60 or less, n Ｄ Polymers with a value of 1.45 or higher are those listed in Table 4 that do not contain fluorine atoms in their polymer molecules.

[0054] The polymers listed in Tables 1 to 3 are preferred as polymers constituting the optical fiber 30. However, the polymers are not limited to those listed in Tables 1 to 3, and include polymers with an Abbe number of 60 or higher or a refractive index n Ｄ The value is 1.45 or less, preferably the Abbe number is 70 or more or the refractive index n Ｄ The value is 1.40 or less, more preferably the Abbe number is 80 or more or the refractive index n Ｄ Polymers with a molecular weight of 1.35 or less are exemplified as constituent materials for the optical fiber 30.

[0055] Next, according to the Lorentz-Lorentz equation, the following relationship holds between the refractive index n of the polymer, the atomic refraction [R] of the atomic group constituting the polymer, and the molecular volume V: (n ２ -1) / (n ２ +2) = [R] / V

[0056] Since the refractive index of the polymer is approximately 1.3 < n < 1.7, as n increases, (n２ -1) / (n ２ +2) also increases. Therefore, in order to reduce the refractive index n of a polymer, it is sufficient to reduce the polymer's [R] / V ratio. Thus, in order to efficiently reduce the refractive index, it is sufficient to introduce an atomic group or atom with a low atomic refraction [R] and a large molecular volume V. The inventors diligently investigated the most efficient method for reducing the refractive index and came up with the idea of ​​introducing fluorine atoms F into the polymer molecule. The van der Waals radius of a fluorine atom is 1.47 Å, which is larger than that of a hydrogen atom with a van der Waals radius of 1.20 Å. Despite this, the atomic refraction [R] of a fluorine atom is relatively small at 0.810 (for comparison, for example, the atomic refraction of a hydrogen atom is 1.100, and the atomic refraction of an oxygen atom (ether group) is 1.643). For this reason, the introduction of fluorine atoms can reduce the refractive index significantly compared to ordinary polymers (such as acrylic polymer derivatives) composed of other hydrocarbons, oxygen, etc. Therefore, as described above, the wavelength dependence of the refractive index can be greatly reduced.

[0057] Figure 9 shows an example of the relationship between the molar concentration of fluorine (F molar concentration) and the refractive index in various polymers investigated by the inventor. Figure 10 shows an example of the relationship between the molar concentration of fluorine and the Abbe number. The F molar concentration can be defined as the number of moles of fluorine atoms in a unit volume of the polymer. As can be seen from Figures 9 and 10, in fluorine-containing polymers, due to the extremely small [R] / V effect of the fluorine atoms, the refractive index n is substantially independent of the polymer structure. Ｄ and Abbe number ν Ｄ However, it was found that this can be correlated with the molar concentration of fluorine atoms in the polymer in approximately unique terms. Therefore, the molar concentration of fluorine atoms in the polymer is 0.02 mol / cm³. ３ Preferably, the concentration is 0.05 mol / cm³. ３ More preferably 0.06 mol / cm³ ３ The polymers described above are exemplified as constituent materials for the optical fiber 30.

[0058] Conventionally, plastic optical fibers using fluorine-containing polymers as constituent materials have been developed, with the aim of reducing absorption loss due to C-H stretching vibrations in the near-infrared wavelength range by replacing it with C-F bonds. However, as described above, the inventors have found that extremely low material dispersion can be obtained by introducing fluorine atoms into the polymer, and that this improves bandwidth even for optical signals with relatively wide spectral widths, contributing to improved transmission signal quality in high-baud rate communications.

[0059] A preferred range of material dispersion is one in which the absolute value of the material dispersion at wavelengths within the spectral width of the modulated optical signal is smaller than the absolute value of the material dispersion in quartz glass.

[0060] Figure 11 shows an example of material dispersion. In the legend, "PFMMD" corresponds to the cladding material of GI POF in Example 1 described above, and "PFMMD + dopant" corresponds to the core material. Also, "SiO2" corresponds to the cladding material of SMMF in Comparative Example 1, and "SiO2 + GeO2" corresponds to the core material. From Figure 11, it can be seen that the wavelength within the spectral width of the modulated optical signal is preferably 1200 nm or less.

[0061] (Configuration of the optical receiver) Returning to Figure 1, the optical receiver 40 receives the modulated optical signal transmitted by the optical fiber 30. The optical receiver 40 includes, for example, an O / E converter that converts the received modulated optical signal into an electrical signal.

[0062] (Measurement of Optical Fiber Response Characteristics 1) Next, the measurement results of the response characteristics of the optical fiber 30 in the optical communication system 100 will be described. In this measurement, the GI POF of Example 1 and the SMMF of Comparative Example 1 were used as the optical fiber 30. The length of both optical fibers was 100 m. In Example 1, the core diameter was 50 μm, the NA was 0.18, the transmission loss was 65 dB / km at a wavelength of 850 nm, and the g value was approximately 2.06. Comparative Example 1 is an optical fiber conforming to the OM5 specification as defined in IEC 60793-2-10. In Comparative Example 1, the core diameter was 50 μm, the NA was 0.20, and the transmission loss was 2.5 dB / km or less at a wavelength of 850 nm and 1.8 dB / km or less at a wavelength of 953 nm. Furthermore, regarding the transmission bandwidth, the minimum effective modal bandwidth (EMB) is 4700 MHz·km or more at a wavelength of 850 nm and 2470 MHz·km or more at a wavelength of 953 nm.

[0063] As the light source, a VCSEL developed for 100 Gbps PAM4 transmission, whose characteristics are also shown in Figures 2-4, was used. For measuring the response characteristics, a Lightwave Component Analyzer (LCA) (Keysight, N4376E) was used. The LCA consists of a vector network analyzer (VNA) and optical conversion modules (E / O and O / E), and can evaluate the frequency characteristics of various optical devices used in optical communication with high accuracy.

[0064] For the measurement, a high-frequency (RF) signal from a VNA was applied to the VCSEL via a bias T (Anritsu, V251). Additionally, a bias current from a DC current source (Newport, LDX-3220) was injected into the VCSEL via the same bias T.

[0065] Then, the modulated optical signal output from the VCSEL was input to the optical fiber under measurement, and the modulated optical signal propagated through the optical fiber was input to the LCA. This allowed the frequency response (S21) of the optical fiber under measurement to be determined.

[0066] In LCA (Life Cycle Assessment), the frequency response is measured by sweeping the frequency of the RF signal. In this measurement, the range from 10 MHz to 50 GHz was swept at 10 MHz intervals, and a total of 5000 data points were acquired. The bandwidth of the intermediate frequency (IF) filter was set to 10 kHz, and the RF signal intensity was set to -1 dBm. The frequency sweep was repeated 30 times, and averaging was performed to obtain a highly accurate frequency response.

[0067] Furthermore, the injection current (bias current) to the VCSEL was set to 11 mA, 13 mA, or 15 mA, respectively, according to the relationship shown in Figure 4, so that the spectral widths would be 0.51 nm, 0.61 nm, or 0.68 nm.

[0068] Figure 12 shows an example of the frequency response when the spectral width is 0.51 nm. Figure 13 shows an example of the frequency response when the spectral width is 0.61 nm. Figure 14 shows an example of the frequency response when the spectral width is 0.68 nm. Here, "Measured" is an example of the measurement result, and "Smoothed" is the result of smoothing the measurement result. Generally, the transmission bandwidth is defined as the frequency at which the signal gain drops by 1.5 dB or 3 dB relative to DC (0 Hz), and in this measurement, the bandwidth was evaluated according to this definition. Note that these measurement results are power ratios (dBe) in the electrical domain, and the values ​​are twice as large when compared to gains (dBo) in the optical domain. That is, -3 dBe corresponds to -1.5 dBo, and -6 dBe corresponds to -3 dBo.

[0069] As can be seen from Figures 12-14, in SMMF, the attenuation of the frequency response was large, and the degradation of the response became particularly pronounced as the spectral width of the modulated optical signal widened. This indicates that the effect of material dispersion is strongly evident. On the other hand, in GI POF, which has low material dispersion, the attenuation of the frequency response was suppressed, and an improvement in transmission bandwidth was confirmed. In other words, the results of this measurement show that the low material dispersion of GI POF suppresses the degradation of the frequency response and contributes to the improvement of transmission characteristics.

[0070] Figure 15 shows an example of the relationship between spectral width and -1.5 dBo bandwidth, including the results from Figures 12 to 14. Figure 16 shows an example of the relationship between spectral width and -3 dBo bandwidth, including the results from Figures 12 to 14. However, since the upper frequency limit of the measuring device in this measurement is 50 GHz, data points were plotted at 50 GHz when the bandwidth exceeded 50 GHz.

[0071] As can be seen from Figure 15, in the case of SMMF, the bandwidth of -1.5 dBo decreases from 50 GHz as the spectral width increases from approximately 0.35 nm, but in the case of GI POF, the bandwidth is above 50 GHz from approximately 0.35 nm to 0.56 nm, and even at spectral widths above that, the bandwidth is always larger than that of SMMF.

[0072] Furthermore, as can be seen from Figure 16, in the case of SMMF, the -3dBo bandwidth becomes less than 50 GHz when the spectral width exceeds 0.51 nm, but in the case of GI POF, the bandwidth remained above 50 GHz even when the spectral width increased to 0.68 nm.

[0073] (Measurement of optical fiber response characteristics 2) Next, as Measurement 2, the spectral width of the modulated optical signal was fixed at 0.61 nm, and the length of the optical fiber to be measured (fiber length) was set to 50 m, 70 m, or 100 m, and the same measurement as in Measurement 1 was performed.

[0074] Figure 17 shows an example of the frequency response when the fiber length is 50 m. Figure 18 shows an example of the frequency response when the fiber length is 70 m. Figure 19 shows an example of the frequency response when the fiber length is 100 m.

[0075] As can be seen from Figures 17-19, the difference in characteristics between SMMF and GI POF becomes more pronounced as the fiber length increases. Based on these results, a fiber length of 50m or more is preferable.

[0076] However, measurements 1 and 2 were performed using equipment with a frequency limit of 50 GHz. In principle, as the frequency increases, even with short fiber lengths of 50 m or less, the effect of material dispersion becomes more apparent, and the difference in characteristics between SMMF and GI POF becomes more pronounced. In other words, the higher the signal frequency, the clearer the superiority of GI POF becomes.

[0077] (Measurement of Transmission Characteristics) Next, an optical communication system 100 was constructed and its transmission characteristics were measured. In this measurement as well, as in the case of optical fiber response characteristics measurement 2, SMMF or GI POF with fiber lengths of 50m, 70m, or 100m was used as the optical fiber. Also, as in the cases of measurements 1 and 2, a VCSEL developed for 100Gbps PAM4 transmission was used as the light source. A pulse pattern generator (Keysight, M8042A) was used as the signal generator 11. A bias T (SHF, SHF BT65R-A) and a DC current source (Newport, LDX-3220) were used for the superposition of bias current. An oscilloscope (Keysight, N1092C) was used as the optical receiver 40. However, considering the measurement sensitivity and linearity of the oscilloscope, a variable attenuator (Keysight, N7768A) was inserted between the optical fiber 30 and the optical receiver 40, and adjusted so that the input optical power to the oscilloscope was 0 dBm.

[0078] The data rate of the PAM4 signal was set to 106.25 Gb / s (53.125 Gbaud) or 128 Gb / s (64 Gbaud). A Short Stress Pattern Random Quaternary (SSPRQ) was used as the test pattern. In addition, a 7-tap Feed-Forward Equalizer (FFE) was applied within the pulse pattern generator, and the modulation amplitude was adjusted to achieve an appropriate extinction ratio (ER).

[0079] Furthermore, the injection current (bias current) into the VCSEL was set to 12 mA, 14 mA, or 16 mA, so that the spectral width of the multi-level modulated optical signal was 0.54 nm, 0.61 nm, or 0.70 nm.

[0080] Furthermore, for acquiring the eye pattern under Back-to-Back (BTB) conditions, an optical fiber of the same type as SMMF, with a length of 2m, was used. Since the fiber length of this optical fiber is sufficiently short, the effect of dispersion is negligible, so the configuration using this optical fiber instead of optical fiber 30 was considered to be the BTB condition.

[0081] For acquiring the eye pattern, a SIRC filter with a frequency equivalent to half the baud rate was applied in accordance with IEEE 802.3dB. Specifically, a 26.5625 GHz SIRC filter was used for a 106.25 Gb / s (53.125 Gbaud) signal. On the other hand, for a 128 Gb / s (64 Gbaud) signal, although IEEE 802.3dB is not applicable, a 32 GHz SIRC filter was applied based on a similar evaluation method. Furthermore, a 9-tap FFE was added to calculate the TDECQ (Transmitter and Dispersion Eye Closure Quaternary). TDECQ is an index that quantitatively evaluates the waveform quality of the PAM4 signal, expressing the degree of eye closure in dB compared to an ideal waveform. A smaller TDECQ value indicates less noise and distortion, and better signal quality.

[0082] Figure 20 shows an example of an eye pattern. The measurement conditions in Figure 20 are when the spectral width of the multi-level modulated optical signal is 0.54 nm and the modulation frequency is 106.25 Gb / s. In this case, the TDECQ at BTB was 1.31 dB and the ER was 2.56 dB. Note that the waveform shown in Figure 20 is the waveform after applying a SIRC filter. Figure 21 shows an example of the relationship between fiber length and TDECQ, including the results shown in Figure 20.

[0083] As shown in Figures 20 and 21, in both SMMF and GI POF, there was a tendency for TDECQ to worsen with increasing fiber length. This is because dispersion accumulates as fiber length increases, degrading the quality of the transmitted signal due to waveform distortion. However, in the case of GI POF, at all fiber lengths, the TDECQ was lower compared to SMMF, and the degradation of TDECQ with increasing fiber length was also suppressed. Specifically, in GI POF, the amount of TDECQ degradation compared to BTB was 0.43 dB or less for fiber lengths of 100 m or less. This is because GI POF has less material dispersion than SMMF, so waveform distortion is suppressed and the quality of the transmitted waveform is maintained. In other words, it was shown that the low material dispersion characteristics of GI POF contribute to improving the waveform quality of PAM signals.

[0084] Next, Figure 22 shows an example of an eye pattern. The measurement conditions in Figure 22 are the same as in Figure 20, with a modulation frequency of 106.25 Gb / s, but the spectral width of the multi-level modulated optical signal is 0.61 nm. In this case, the TDECQ at BTB was 1.17 dB and the ER was 2.54 dB. Figure 23 shows an example of the relationship between fiber length and TDECQ, including the results shown in Figure 22.

[0085] In Figures 22 and 23, the spectral width is larger than in Figures 20 and 21, so the TDECQ value is also relatively larger. This is thought to be because the effect of material dispersion is more strongly expressed. However, even in this case, in the case of GI POF, the TDECQ is lower compared to the case of SMMF, and the degradation of TDECQ with increasing fiber length is also suppressed. Specifically, in GI POF, if the fiber length is 100m or less, the amount of TDECQ degradation is 0.65 dB or less compared to BTB.

[0086] Next, Figure 24 shows an example of an eye pattern. The measurement conditions in Figure 24 are the same as in Figure 20, with a modulation frequency of 106.25 Gb / s, but the spectral width of the multi-level modulated optical signal is 0.70 nm. In this case, the TDECQ at BTB was 1.12 dB and the ER was 2.25 dB. Figure 25 shows an example of the relationship between fiber length and TDECQ, including the results shown in Figure 24.

[0087] In the cases of Figures 24 and 25, the spectral width is even larger than in the cases of Figures 20-23, so the TDECQ value is also relatively larger. However, even in these cases, the TDECQ is lower in the case of GI POF compared to SMMF, and the degradation of TDECQ with increasing fiber length is also suppressed. Specifically, in GI POF, if the fiber length is 100m or less, the amount of TDECQ degradation is 1.21dB or less compared to BTB.

[0088] As can be seen in Figures 21, 23, and 25, increasing the injection current of the VCSEL widens the spectral width of the modulated optical signal, but the TDECQ of the VCSEL itself in the BTB actually decreases. Furthermore, as the spectral width increases, the difference in TDECQ between GI POF and SMMF increases. One reason for this is that increasing the injection current expands the modulation bandwidth of the VCSEL, improving signal quality. Therefore, in conventional system designs based on silica-based multimode fibers, it was necessary to impose restrictions on the driving conditions and modulation speed of the VCSEL in order to suppress transmission degradation due to wavelength dispersion associated with the widening of the spectral width. On the other hand, by using GI POF as the transmission line, the impact of transmission degradation due to spectral width is relatively small, suggesting that it becomes possible to set driving conditions that more actively extract the performance of the VCSEL and optimize the laser structure.

[0089] Next, Figure 26 shows an example of an eye pattern. The measurement conditions in Figure 26 are when the spectral width of the multi-level modulated optical signal is 0.54 nm and the modulation frequency is 128 Gb / s. In this case, the TDECQ at BTB was 2.78 dB and the ER was 2.35 dB. Furthermore, Figure 27 shows an example of the relationship between fiber length and TDECQ, including the results shown in Figure 26.

[0090] In Figures 26 and 27, the modulation frequency is higher than in Figures 20 and 21, so the TDECQ value is also relatively larger. This is thought to be because the effect of material dispersion is more pronounced. However, even in this case, in the case of GI POF, the TDECQ is lower compared to the case of SMMF, and the degradation of TDECQ with increasing fiber length is also suppressed. Specifically, in GI POF, if the fiber length is 100m or less, the amount of TDECQ degradation is 0.94 dB or less compared to BTB.

[0091] Figure 28 shows an example of an eye pattern. The measurement conditions in Figure 28 are the same as in Figure 26, with a modulation frequency of 128 Gb / s, but the spectral width of the multi-level modulated optical signal is 0.61 nm. In this case, the TDECQ at BTB was 2.32 dB and the ER was 2.02 dB. Figure 27 shows an example of the relationship between fiber length and TDECQ, including the results shown in Figure 26.

[0092] In Figures 28 and 29, the TDECQ was lower in the case of GI POF compared to the case of SMMF, and the degradation of TDECQ with increasing fiber length was also suppressed. Specifically, in GI POF, if the fiber length is 100m or less, the amount of TDECQ degradation is 1.18 dB or less compared to BTB.

[0093] Figure 30 shows an example of an eye pattern. The measurement conditions in Figure 30 are the same as in Figure 26, with a modulation frequency of 128 Gb / s, but the spectral width of the multi-level modulated optical signal is 0.70 nm. In this case, the TDECQ at BTB was 2.42 dB and the ER was 1.78 dB. Figure 31 shows an example of the relationship between fiber length and TDECQ, including the results shown in Figure 30.

[0094] In the cases shown in Figures 30 and 31, the TDECQ was lower in the GI POF case compared to the SMMF case, and the degradation of TDECQ with increasing fiber length was also suppressed. In particular, when the fiber length was 100m, the TDECQ was unmeasurable in the SMMF case, but it was suppressed to 4.99dB in the GI POF case. Specifically, in the GI POF case, the amount of TDECQ degradation is 2.57dB or less compared to BTB when the fiber length is 100m or less.

[0095] Standardization bodies such as IEEE 802.3 set upper limits on the TDECQ value to ensure good communication quality. For example, for 26.5625 GBaud PAM4 transmission, the TDECQ is specified to be 4.9 dB or less, and for 53.125 GBaud PAM4 transmission, it is specified to be 4.4 dB or less. This TDECQ value is an indicator that combines the performance of the transmitter and the influence of the transmission path, and the acceptable range of TDECQ degradation due to the transmission path depends on the performance of the transmitter. It is preferable that the amount of TDECQ degradation due to the transmission path be 4 dB or less, more preferably 3 dB or less, and even more preferably 2.5 dB or less.

[0096] (Wavelength Division Multiplexing Transmission Method) The low material dispersion characteristics of GI POF are advantageous not only in transmission using light sources with a wide spectral width such as VCSELs, but also in wavelength division multiplexing transmission methods using multiple wavelengths.

[0097] Wavelength Division Multiplexing (WDM) is a technology that expands transmission capacity by simultaneously transmitting optical signals of different wavelengths through a single optical fiber. Traditionally, it has been widely used in long-distance trunk line communications, mainly using single-mode fiber. However, in recent years, WDM has been increasingly adopted as a means of improving the transmission capacity per optical fiber even in short-distance, high-density communication environments, such as server-to-server connections within data centers.

[0098] For short-distance applications, methods are being considered that use VCSELs with different wavelengths, such as 850 nm, 880 nm, 910 nm, and 940 nm, as light sources to simultaneously transmit optical signals at multiple wavelengths. However, with silica-based MMFs, the material dispersion changes significantly with wavelength, which presents a challenge as it easily leads to variations in the transmission bandwidth at each wavelength. Furthermore, because the refractive index of silica glass is highly wavelength-dependent, even if the refractive index distribution is optimized for a specific wavelength, it will deviate from the optimal conditions at other wavelengths, resulting in a significant degradation of the transmission bandwidth.

[0099] Figure 32 shows the wavelength dependence of the -3 dBo bandwidth for the GI POF of Example 1 and the SMMF of Comparative Example 1, when the refractive index distribution is optimized for a wavelength of 850 nm. The calculation was performed based on the WKB method, assuming a spectral width of 0.60 nm for the modulated optical signal and a fiber length of 100 m. In the silica-based MMF, the bandwidth decreases sharply as the wavelength moves away from the design wavelength, confirming that there are large differences in characteristics for each wavelength.

[0100] On the other hand, GI POF, by using a fully fluorinated polymer as its base material, exhibits low material dispersion and low wavelength dependence of refractive index. Therefore, it can ensure a high transmission bandwidth over a wide wavelength range. As a result, it can stably exhibit broadband characteristics even in WDM transmission spanning multiple wavelengths.

[0101] <Embodiment 2> Figure 33 is a schematic diagram of the optical communication system according to Embodiment 2. The optical communication system 100A includes an optical transmitting unit 10A, an optical coupling unit 20A, an optical fiber 30, and an optical receiving unit 40A. The optical communication system 100A is configured to enable WDM transmission.

[0102] The optical transmitting unit 10A comprises a plurality of optical transmitting units 10 according to Embodiment 1. These optical transmitting units 10 output modulated optical signals of different wavelengths.

[0103] The optical coupling unit 20A wavelength-multiplexes the modulated optical signals from multiple optical transmitting units 10 and outputs them as a WDM optical signal into a single optical fiber. The optical coupling unit 20A is configured to include, for example, a WDM coupler.

[0104] The optical receiver 40A receives the WDM optical signal transmitted by the optical fiber 30. The optical receiver 40A, for example, wavelength-divides the received WDM optical signal into individual modulated optical signals and converts each of them into an electrical signal using an O / E converter.

[0105] (Measurement of optical fiber response characteristics 3) Next, the measurement results of the response characteristics of the optical fiber 30 in the optical communication system 100A will be described. In this measurement, the optical fiber used was either the SMMF of Comparative Example 1 or the GI POF of Example 1, both with a fiber length of 100 m.

[0106] Furthermore, four VCSELs (850nm-VCSEL, 880nm-VCSEL, 910nm-VCSEL, and 940nm-VCSEL) developed for 100Gbps PAM4 transmission were used as light sources. The center wavelengths and spectral widths of the four VCSELs were 862nm, 0.62nm, 885.9nm, 0.61nm, 908.1nm, 0.61nm, 943.3nm, and 0.59nm, respectively.

[0107] The measurement equipment and conditions for this measurement 3 are the same as those for measurements 1 and 2 of the optical fiber response characteristics.

[0108] Figure 34 shows an example of the frequency response in the case of 850 nm-VCSEL. Figure 35 shows an example of the frequency response in the case of 880 nm-VCSEL. Figure 36 shows an example of the frequency response in the case of 910 nm-VCSEL. Figure 37 shows an example of the frequency response in the case of 940 nm-VCSEL.

[0109] As can be seen from Figures 34-37, in VCSELs of any wavelength, SMMF exhibits significant attenuation of the frequency response, limiting the transmission bandwidth. On the other hand, GI POF suppresses attenuation of the frequency response, and an improvement in transmission bandwidth is confirmed. In other words, the results of this measurement 3 demonstrate that, even in WDM transmission, the low material dispersion of GI POF suppresses the degradation of the frequency response and contributes to the improvement of transmission characteristics.

[0110] Figure 38 shows an example of the relationship between the center wavelength and the -1.5 dBo bandwidth, obtained from the results in Figures 34-37. In the case of SMMF, the maximum transmission bandwidth is obtained around 880 nm, but the transmission bandwidth decreases at other wavelengths. This is because, in SMMF, the refractive index distribution is optimized around 880 nm, and the effect of dispersion becomes significant at other wavelengths.

[0111] On the other hand, in the case of GI POF, a wider transmission bandwidth is obtained at all measured wavelengths than in the case of SMMF. Note that in this measurement, the upper limit of measurement was set to 40 GHz, and sensitivity decreases significantly at frequencies exceeding this. Therefore, in Figure 38, even when the transmission bandwidth exceeds 40 GHz, the value is plotted as 40 GHz. Consequently, the actual transmission bandwidth in GI POF is even wider than the value shown.

[0112] The results above demonstrate that GI POF can maintain broadband transmission characteristics over a wide wavelength range, supporting its high suitability for short-distance WDM transmission.

[0113] In the above embodiment, the modulated signal is a multi-level modulated signal such as PAM4, but it may also be a modulated signal such as an NRZ (Non Return to Zero) signal.

[0114] Furthermore, the present invention is not limited by the embodiments described above. Configurations that appropriately combine the above-described components are also included in the present invention. Moreover, further effects and modifications can be easily derived by those skilled in the art. Therefore, broader aspects of the present invention are not limited to the embodiments described above, and various modifications are possible.

[0115] 10, 10A: Optical transmitter 11: Signal generator 12: DC current source 13: Bias T 14: Light source 20, 20A: Optical coupling unit 21, 24: Lens 22: Dichroic mirror 23: Optical attenuation filter 25: CCD camera 30: Optical fiber 31: End face 40, 40A: Optical receiver 100, 100A: Optical communication system

Claims

1. An optical communication system comprising: a light source that outputs laser light with a spectral width of 0.1 nm or more in its oscillation spectrum; a signal generator that applies a modulation signal of 25 GBaud or more to the light source, and an optical transmission unit that outputs a modulated optical signal in which the laser light is modulated by the modulation signal; and an optical fiber that transmits the modulated optical signal, the optical fiber being made of a plastic containing fluorine atoms in at least its core, having a material dispersion absolute value at wavelengths within the spectral width of the modulated optical signal smaller than the material dispersion absolute value in quartz glass, a length of 100 m or less, and having a GI type refractive index distribution.

2. The optical communication system according to claim 1, wherein the light source performs multimode oscillation having a plurality of oscillation peaks in the oscillation spectrum.

3. The optical communication system according to claim 1, wherein the refractive index distribution of the optical fiber is designed such that, under the conditions that the optical fiber has a predetermined fiber length and the modulated optical signal has a predetermined wavelength and spectral width, the bandwidth of the -3 dBO is 18 GHz or higher.

4. The optical communication system according to claim 1, wherein the refractive index distribution coefficient (g value) of the optical fiber is 1.70 or more and 2.50 or less.

5. The optical communication system according to claim 1, wherein the degradation of the TDECQ (Transmitter and Dispersion Eye Closure Quaternary) in the optical fiber is 4 dB or less.

6. The optical communication system according to claim 1, wherein the modulated signal is a signal of 50 GBaud or higher.

7. The optical communication system according to claim 1, wherein the modulated signal is a PAM4 signal of 50 GBaud or higher.

8. The optical communication system according to claim 1, wherein the Abbe number of the material constituting the core of the optical fiber is 60 or more.

9. The refractive index n of the material constituting the core of the optical fiber at a wavelength of 589 nm Ｄ The optical communication system according to claim 1, wherein is 1.45 or less.

10. The molar concentration of fluorine atoms in the polymer of the material constituting the core of the optical fiber is 0.02 mol / cm³. ３ The optical communication system according to claim 1, wherein the above is true.

11. The optical communication system according to claim 1, wherein the difference between the TDECQ (Transmitter and Dispersion Eye Closure Quaternary) in the optical fiber and the TDECQ of the optical fiber made of silica glass tends to increase as the spectral width of the light source increases.

12. The optical communication system according to claim 1, wherein the optical transmitting unit has a plurality of light sources.

Images