Optical transmission system and optical transmission method

Abstract

An optical transmission system (1) converts a first electrical signal to an optical signal, transmits the converted optical signal, and converts the transmitted optical signal to a second electrical signal. The optical transmission system (1) comprises an electro-optical conversion device (2), an optical transmission path (3), and an opto-electrical conversion device (4). The first electrical signal includes a high frequency communication signal which exceeds 9MHz and is equal to or less than 10GHz. The optical transmission system (1) further comprises an additional signal generation device (6) that generates an additional signal of a low frequency between 1Hz and 9MHz. The electro-optical conversion device (2) converts, to an optical signal, the first electrical signal which includes the communication signal and the additional signal generated by the additional signal generation device (6).

Description

[Technical Field]

[0001] The present invention relates to an optical transmission system and an optical transmission method. [Background technology]

[0002] An optical transmission system is known that includes a laser chip, an optical fiber, and an optical isolator interposed between them (see, for example, Patent Document 1 below). In the optical transmission system described in Patent Document 1, light emitted from the laser chip passes through the optical isolator and then enters the optical fiber. The optical isolator transmits only light traveling in the forward direction, while blocking light traveling in the reverse direction. Therefore, in the optical transmission system, the optical isolator attenuates the returning light from the optical fiber. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2003-14992 Summary of the Invention [Problem to be solved by the invention]

[0004] Optical transmission systems are required to suppress fluctuations in signal quality over time, depending on the application and purpose. Signal quality includes CNR (carrier-to-noise ratio). Signal quality can also be referred to as signal characteristics. The optical transmission system of Patent Document 1 suppresses the above-mentioned fluctuations by attenuating the above-mentioned return light using an optical isolator. However, optical isolators are expensive and have the disadvantage of complicating the configuration of the optical transmission system.

[0005] The present invention provides an optical transmission system and an optical transmission method that can suppress fluctuations in signal quality over time, have a simple configuration, and are low cost. [Means for solving the problem]

[0006] The present invention (1) is an optical transmission system that converts a first electrical signal into an optical signal, transmits the converted optical signal, and converts the transmitted optical signal into a second electrical signal, comprising: an electro-optical conversion device that converts the first electrical signal into the optical signal; an optical transmission line that transmits the optical signal converted by the electro-optical conversion device; and an opto-electrical conversion device that converts the optical signal transmitted from the optical transmission line into the second electrical signal, wherein the first electrical signal includes a high-frequency communication signal exceeding 9 MHz and not more than 10 GHz, and further comprising an additional signal generating device that generates a low-frequency additional signal having a frequency of not less than 1 Hz and not more than 9 MHz, and wherein the electro-optical conversion device converts the additional signal generated by the additional signal generating device and the first electrical signal including the communication signal into the optical signal.

[0007] In this optical transmission system, a first electrical signal, which is generated by an additional signal generating device and includes a low-frequency additional signal having a frequency of 1 Hz or more and 9 MHz or less, is converted into an optical signal, thereby suppressing fluctuations in signal quality over time.

[0008] Moreover, this optical transmission system does not need to include an optical isolator as in Patent Document 1, but only needs to include an additional signal generating device that generates an additional signal of a specific low frequency, so the configuration is simple and low cost.

[0009] The present invention (2) includes the optical transmission system according to (1), wherein the intensity of the additional signal is 30 dBμV or more.

[0010] In this optical transmission system, the strength of the additional signal is 30 dBμV or more, so fluctuations in signal quality over time can be further suppressed.

[0011] The present invention (3) includes the optical transmission system according to (1) or (2), further comprising a communication signal generating device that generates the communication signal.

[0012] The present invention (4) includes an optical transmission method for converting a first electrical signal into an optical signal, transmitting the converted optical signal, and converting the transmitted optical signal into a second electrical signal, wherein the first electrical signal includes a high-frequency communication signal exceeding 9 MHz and not exceeding 10 GHz, the method comprising a first step of converting the first electrical signal into the optical signal, a second step of transmitting the optical signal converted in the first step, and a third step of converting the optical signal transmitted in the second step into the second electrical signal, and further comprising a fourth step of generating a low-frequency additional signal having a frequency not less than 1 Hz and not more than 9 MHz, wherein the first step converts the additional signal generated in the fourth step and the first electrical signal including the communication signal into the optical signal.

[0013] In the first step of this optical transmission method, a first electrical signal including a low-frequency additional signal having a frequency of 1 Hz or more and 9 MHz or less is converted into an optical signal, so that fluctuations in signal quality over time can be suppressed.

[0014] Moreover, the optical transmission method has a simple configuration and low cost because an additional signal with a specific low frequency is generated in the fourth step.

[0015] The present invention (5) includes the optical transmission method according to (4), wherein the intensity of the additional signal is 30 dBμV or more.

[0016] In this optical transmission method, the strength of the additional signal is 30 dBμV or more, so fluctuations in signal quality over time can be further suppressed.

[0017] The present invention (6) includes the optical transmission method according to (4) or (5), further comprising a fifth step of generating the communication signal. [Effects of the Invention]

[0018] The optical transmission system and temporary transmission method of the present invention can suppress fluctuations in signal quality over time, while having a simple configuration and being low cost. [Brief explanation of the drawings]

[0019] [Figure 1] FIG. 1 shows an embodiment of an optical transmission system according to the present invention. [Figure 2] FIG. 2 shows a modified optical transmission system. [Figure 3] FIG. 3 shows a modified optical transmission system. DETAILED DESCRIPTION OF THE INVENTION

[0020] <One embodiment of an optical transmission system> An embodiment of an optical transmission system of the present invention will be described with reference to Fig. 1. This optical transmission system 1 converts a first electrical signal into an optical signal, transmits the converted optical signal, and converts the transmitted optical signal into a second electrical signal. Each signal will be described below.

[0021] <First electrical signal> In the present invention, the first electrical signal includes an additional signal.

[0022] <Additional signal> The additional signal is always included in the first electrical signal regardless of the passage of time, that is, the first electrical signal includes the additional signal at any timing.

[0023] The additional signal is a low-frequency signal. The frequency of the additional signal is 1 Hz or more and 9 MHz or less. If the frequency of the additional signal is less than 1 Hz or more than 9 MHz, fluctuations in signal quality, which will be described later, cannot be sufficiently suppressed. The frequency of the additional signal is preferably 10 Hz or more, more preferably 100 Hz or more, even more preferably 1 kHz or more, and particularly preferably 10 kHz or more. Furthermore, the frequency of the additional signal is preferably 1 MHz or less, more preferably 300 kHz or less.

[0024] The strength of the additional signal is, for example, 10.0 dBμV or more, preferably 30.0 dBμV or more, and more preferably 75.0 dBμV or more. If the strength of the additional signal is equal to or greater than the above-mentioned lower limit, fluctuations in signal quality over time can be further suppressed. On the other hand, the upper limit of the strength of the additional signal is not limited. The upper limit of the strength of the additional signal is, for example, 1000 dBμV or 100.0 dBμV.

[0025] <Communication signal> The first electrical signal further includes a communication signal containing information to be communicated. The communication signal is a high-frequency signal. The communication signal is added to or superimposed on the aforementioned additional signal. Note that the additional signal is sometimes called a "non-communication signal" because it does not contain information to be communicated. The communication signal is transmitted over time. That is, there are times when the communication signal is transmitted and times when it is not transmitted. Specifically, in the optical transmission system 1, when the communication signal is not transmitted (when the communication signal is OFF), the first electrical signal does not include the communication signal and only includes the additional signal. On the other hand, in the optical transmission system 1, when the communication signal is transmitted (when the communication signal is ON), the first electrical signal includes the communication signal and the additional signal.

[0026] Examples of communication signals include analog signals and digital signals. Examples of analog signals include RF signals. RF signals include electromagnetic waves having a frequency band used for wireless communication. RF signals are transmitted, for example, using frequency division multiplexing. Frequency division multiplexing is a method of modulating communication signals, multiplexing the modulated communication signals in parallel on the frequency axis, and transmitting them over multiple channels. Examples of RF signals include multi-wave CW signals (multiple single-frequency signals) and single-wave CW signals (one single-frequency signal). The above-mentioned RF signals and methods are described, for example, in JP 2020-096363 A.

[0027] The frequency of the communication signal is, for example, higher than the frequency of the additional signal. The frequency of the communication signal is, for example, greater than 9 MHz, preferably 10 MHz or higher, more preferably 20 MHz or higher, and even more preferably 50 MHz or higher. The frequency of the communication signal is 10 GHz or lower.

[0028] The strength of the communication signal is not limited and is set appropriately depending on the use and purpose of the optical transmission system 1.

[0029] <Optical signal and second electrical signal> The optical signal is a signal obtained by converting the first electrical signal described above. The second electrical signal is a signal obtained by converting the optical signal described above. The optical signal and the second electrical signal include at least the additional signal described above.

[0030] <Configuration of Optical Transmission System 1> 1, the optical transmission system 1 includes an electrical-optical converting device 2, an optical transmission line 3, and an optical-electrical converting device 4. The optical transmission system 1 also includes a communication signal generating device 5, an additional signal generating device 6, and a combining device 7.

[0031] <Electro-optical conversion device 2> The electro-optical conversion device 2 can convert the first electrical signal into an optical signal. The electro-optical conversion device 2 is not limited. An example of the electro-optical conversion device 2 is a TOSA. The TOSA is a transmitter optical subassembly. The electro-optical conversion device 2 includes, for example, a light source. An example of the light source is a laser diode. An example of the laser diode is a vertical cavity surface emitting laser (VCSEL). The electro-optical conversion device 2 is also connected to a direct current generating device 12. An example of the direct current generating device 12 is a direct current source.

[0032] <Optical transmission path 3> The optical transmission line 3 is capable of transmitting an optical signal converted by the electro-optical converting device 2. The optical transmission line 3 extends in the transmission direction. The upstream end of the optical transmission line 3 in the transmission direction is connected to the electro-optical converting device 2. The optical transmission line 3 is not limited. For example, the optical transmission line 3 may be an optical fiber. For example, the optical fiber may be a plastic optical fiber or a glass optical fiber. For example, the optical transmission line 3 may be a multimode or a single mode.

[0033] <Optical-electrical conversion device 4> The opto-electrical conversion device 4 can convert an optical signal transmitted from the optical transmission line 3 into a second electric signal. The opto-electrical conversion device 4 is connected to the downstream end of the optical transmission line 3 in the transmission direction. The opto-electrical conversion device 4 is not limited. For example, an ROSA can be used as the opto-electrical conversion device 4. The ROSA is a receiver optical subassembly. The opto-electrical conversion device 4 includes, for example, a photodiode (PD).

[0034] <Communication signal generating device 5> The communication signal generating device 5 is capable of generating the above-described communication signal. The communication signal generating device 5 is connected to the electro-optical converting device 2 via a combining device 7, which will be described later. A communication line 8 is wired between the communication signal generating device 5 and the combining device 7. The communication signal generating device 5 is not limited. For example, the communication signal generating device 5 includes an antenna and an antenna board capable of receiving an RF signal. Further, for example, the communication signal generating device 5 includes a multi-signal generating device.

[0035] <Additional signal generating device 6> The additional signal generating device 6 is capable of generating the above-mentioned additional signal. The additional signal generating device 6 is connected to the electro-optical converting device 2 and the communication signal generating device 5 via a combining device 7, which will be described later. An additional line 9 is wired between the additional signal generating device 6 and the combining device 7. The downstream portion of the additional line 9 in the transmission direction is common to the downstream portion of the communication line 8 in the transmission direction. The additional signal generating device 6 is not limited. An example of the additional signal generating device 6 is a low-frequency signal generating device. An example of the low-frequency signal generating device is a multi-signal generating device.

[0036] <Synthetic Device 7> The combining device 7 is capable of combining (superimposing) the communication signal generated by the communication signal generating device 5 and the additional signal generated by the additional signal generating device 6. The combining device 7 is connected to the communication signal generating device 5 via a communication line 8. The combining device 7 is also connected to the additional signal generating device 6 via an additional line 9. A connection line 10 is wired between the combining device 7 and the electro-optical converting device 2. Therefore, the combining device 7 is connected to the electro-optical converting device 2 via the connection line 10.

[0037] <Optical transmission by optical transmission system 1 (optical transmission method)> Next, the optical transmission method according to the optical transmission system 1 will be described.

[0038] <Operation when communication signal is OFF> In this optical transmission system 1, the communication signal generating device 5 does not generate a communication signal at all times, i.e., the communication signal is OFF. On the other hand, in this optical transmission system 1, the additional signal generating device 6 always generates the above-mentioned additional signal. In other words, the fourth step is performed.

[0039] Then, the additional signal generated by the additional signal generating device 6 is input to the electro-optical converting device 2 via the additional line 9, the combining device 7, and the connection line 10. When passing through the combining device 7, the additional signal is processed so as to be included in the first electrical signal. In other words, the electro-optical converting device 2 receives the first electrical signal including the additional signal.

[0040] The electro-optical converting device 2 converts the first electrical signal into an optical signal. That is, the first step is performed. The electro-optical converting device 2 converts the first electrical signal, including the low-frequency additional signal, into light. At this time, the electro-optical converting device 2 uses the direct current input from the direct current generating device 12. Then, the electro-optical converting device 2 inputs the converted optical signal into the optical transmission line 3.

[0041] The input optical signal is transmitted through the optical transmission line 3 and input to the opto-electric conversion device 4. That is, the second step is performed. Examples of optical signal transmission methods include a multimode method and a single mode method.

[0042] The opto-electrical conversion device 4 converts the optical signal input from the optical transmission line 3 into a second electrical signal. That is, the third step is performed. The second electrical signal may be the same as or different from the first electrical signal. The second electrical signal is input to an external device 11 shown by a phantom line. An example of the external device 11 is an image display device. An example of the image display device is a television or a recorder.

[0043] <Operation when communication signal is ON> When the communication signal is ON, the communication signal generating device 5 generates the communication signal. That is, the fifth step is performed. Even at this time, the additional signal generating device 6 continuously generates the additional signal. That is, the fourth step is performed.

[0044] The communication signal generated by the communication signal generating device 5 and the additional signal generated by the additional signal generating device 6 are input to the combining device 7 via the communication line 8 and the additional line 9, respectively. The combining device 7 combines the communication signal and the additional signal into a first electrical signal containing them. That is, the combining device 7 adds (superimposes) the additional signal on the communication signal to combine them into a first electrical signal.

[0045] The first electrical signal combined by the combining device 7 is input to the electro-optical conversion device 2 via the connection line 10 .

[0046] When the communication signal is ON, the conversion from the first electrical signal to an optical signal by the electro-optical converting device 2 (first step), the transmission of the optical signal through the optical transmission line 3 (second step), and the conversion from the optical signal to a second electrical signal by the opto-electrical converting device 4 (third step) are the same as those when the communication signal is OFF as described above. However, the first electrical signal in the first step includes an additional signal and the communication signal.

[0047] <Effects of one embodiment> In this optical transmission system 1, the first electrical signal generated by the additional signal generating device 6 and including a low-frequency additional signal having a frequency of 1 Hz or more and 9 MHz or less is converted into an optical signal, thereby suppressing fluctuations in signal quality over time.

[0048] Moreover, the optical transmission system 1 does not need to include an optical isolator as in Patent Document 1, but only needs to include an additional signal generating device 6 that generates an additional signal of a specific low frequency, so the configuration is simple and low cost.

[0049] In this optical transmission system 1, if the strength of the additional signal is 30 dBμV or more, fluctuations in signal quality over time can be further suppressed.

[0050] In the first step of the optical transmission method of one embodiment, a first electrical signal including a low-frequency additional signal having a frequency of 1 Hz to 9 MHz is converted into an optical signal, thereby suppressing fluctuations in signal quality over time.

[0051] Moreover, the optical transmission method has a simple configuration and low cost because an additional signal with a specific low frequency is generated in the fourth step.

[0052] In this optical transmission method, if the strength of the additional signal is 30 dBμV or more, fluctuations in signal quality over time can be further suppressed.

[0053] <Modification> In the modified example, the same components and steps as those in the first embodiment are denoted by the same reference numerals, and detailed descriptions thereof will be omitted. Furthermore, the modified example can achieve the same effects as those in the first embodiment unless otherwise specified. Furthermore, the first embodiment and its modified example can be combined as appropriate.

[0054] 2, the optical transmission system 1 does not include a separate combining device 7, but instead includes the electro-optical converting device 2 as the combining device 7. In other words, the electro-optical converting device 2 also functions as the combining device. The communication signal generating device 5 is connected to the electro-optical converting device 2 via a communication line 8. The additional signal generating device 6 is connected to the electro-optical converting device 2 via an additional line 9.

[0055] As shown in Fig. 3, the optical transmission system 1 does not necessarily have to include the communication signal generating device 5. In the modification of Fig. 3, a communication signal is input from the outside to the combining device 7 via a communication line 8 indicated by a virtual line. The combining device 7 combines the above-mentioned communication signal and the additional signal input from the additional signal generating device 6 to generate the above-mentioned first electrical signal. [Example]

[0056] The present invention will be described in more detail below with reference to examples and comparative examples. It should be noted that the present invention is in no way limited to these examples and comparative examples. The specific numerical values ​​of the blending ratios (content ratios), physical property values, parameters, etc. used in the following description can be replaced with the corresponding upper limit values ​​(numeric values ​​defined as "equal to or less than") or lower limit values ​​(numeric values ​​defined as "equal to or greater than" or "exceeding") of the blending ratios (content ratios), physical property values, parameters, etc. described in the "Description of the Invention" above.

[0057] <Examples 1 to 5, and Comparative Examples 1 and 2> <High frequency signal frequency 100MHz, intensity 80dBμV>

[0058] Example 1 As Example 1, an optical transmission system 1 including the following devices shown in FIG. 1 was prepared.

[0059] Electrical-optical conversion device 2: TOSA using VCSEL with a central wavelength of 850 nm Optical transmission line 3: Multimode optical fiber Optical-electrical conversion device 4: ROSA using PD Communication signal generator device 5: Agilent multi-signal generator with model number N5183A Additional signal generating device 6: Model number WF1973, multi-signal generator manufactured by NF Corporation External device 11: A spectrum analyzer manufactured by Keysight, model number N9010B DC current generating device 12: Model number 2400 Source Meter, DC current source manufactured by KEITHLEY

[0060] In Example 1, the communication signal generating device 5 generated a high-frequency signal, which was a single CW wave, with a frequency of 100 MHz and an intensity of 80 dBμV. At the same time, the DC current generating device 12 inputted a DC current of 8 mA to the electro-optical converting device 2, and the additional signal generating device 6 generated an additional signal with a frequency of 1 kHz and an intensity of 85.0 dBμV. Subsequently, the external device 11 measured the fluctuation in the quality of the second electrical signal in the optical-electrical converting device 4 for 30 minutes. The fluctuation was calculated as the difference between the maximum and minimum values ​​of the signal quality measured by the external device 11. The results are shown in Table 1.

[0061] <Example 2> The fluctuation was measured in the same manner as in Example 1, except that the frequency of the additional signal was changed from 1 kHz to 10 kHz. The fluctuation results are shown in Table 1.

[0062] Example 3 The fluctuation was measured in the same manner as in Example 1, except that the frequency of the additional signal was changed from 1 kHz to 100 kHz. The fluctuation results are shown in Tables 1 and 2.

[0063] Example 4 The fluctuation was measured in the same manner as in Example 1, except that the frequency of the additional signal was changed from 1 kHz to 200 kHz. The fluctuation results are shown in Table 1.

[0064] <Example 5> The fluctuation was measured in the same manner as in Example 1, except that the frequency of the additional signal was changed from 1 kHz to 500 kHz. The fluctuation results are shown in Table 1.

[0065] <Comparative Example 1> The fluctuation was measured in the same manner as in Example 1, except that the frequency of the additional signal was changed from 1 kHz to 10 MHz.

[0066] <Comparative Example 2> The fluctuation was measured in the same manner as in Example 1, except that no additional signal was generated by the additional signal generating device 6. The fluctuation results are shown in Table 1.

[0067] <Examples 6 to 12> <High frequency signal frequency: 100MHz>

[0068] < reference Example 6 The fluctuation was measured in the same manner as in Example 3. However, in the additional signal generating device 6, the strength of the additional signal was changed from 85.0 dBμV to 10.0 dBμV. The fluctuation results are shown in Table 2.

[0069] Example 7 The fluctuation was measured in the same manner as in Example 3. However, in the additional signal generating device 6, the strength of the additional signal was changed from 85.0 dBμV to 30.0 dBμV. The fluctuation results are shown in Table 2.

[0070] Example 8 The fluctuation was measured in the same manner as in Example 3. However, in the additional signal generating device 6, the strength of the additional signal was changed from 85.0 dBμV to 50.0 dBμV. The fluctuation results are shown in Table 2.

[0071] Example 9 The fluctuation was measured in the same manner as in Example 3. However, the strength of the additional signal in the additional signal generating device 6 was changed from 85.0 dBμV to 70.0 dBμV. The fluctuation results are shown in Table 2.

[0072] Example 10 The fluctuation was measured in the same manner as in Example 3. However, the strength of the additional signal in the additional signal generating device 6 was changed from 85.0 dBμV to 80.0 dBμV. The fluctuation results are shown in Table 2.

[0073] Example 11 The fluctuation was measured in the same manner as in Example 3. However, the strength of the additional signal in the additional signal generating device 6 was changed from 85.0 dBμV to 90.0 dBμV. The fluctuation results are shown in Table 2.

[0074] Example 12 The fluctuation was measured in the same manner as in Example 3. However, in the additional signal generating device 6, the strength of the additional signal was changed from 85.0 dBμV to 100.0 dBμV. The fluctuation results are shown in Table 2.

[0075] Example 13 In Example 13, a direct current of 8 mA was generated from the direct current generating device 12. The additional signal generating device 6 generated an additional signal with a frequency of 10 kHz and an intensity of 85 dBμV. However, no communication signal was generated by the communication signal generating device 5. Next, the external device 11 measured the fluctuation of the second electrical signal in the photoelectric conversion device 4 for 30 minutes. The fluctuation was calculated as the difference between the maximum and minimum values ​​of the noise intensity at 100 MHz measured by the external device 11. The results are shown in Table 3.

[0076] The fluctuations were then measured, and the results are shown in Table 3.

[0077] <Comparative Example 3> The fluctuation was measured in the same manner as in Example 13. However, no additional signal was generated by the additional signal generating device 6. In other words, no communication signal was generated by the communication signal generating device 5, and no additional signal was generated by the additional signal generating device 6. Next, the fluctuation of the second electrical signal in the photoelectric converting device 4 was measured for 30 minutes by the external device 11. The fluctuation was calculated as the difference between the maximum and minimum values ​​of the noise intensity at 100 MHz measured by the external device 11. The results are shown in Table 3.

[0078] <Examples 14 to 18 and Comparative Examples 4 to 8> <Additional signal frequency 1kHz, intensity 70dBμV> The fluctuation was measured in the same manner as in Example 1. However, the strength of the additional signal was changed from 85.0 dBμV to 70.0 dBμV. The frequency of the high-frequency signal, which is the communication signal, was changed as shown in Table 4. In all of Comparative Examples 4 to 8, the additional signal generating device 6 did not generate an additional signal. The results are shown in Table 4.

[0079] Example 14 The frequency of the high frequency signal was set to 50 MHz.

[0080] Example 15 The frequency of the high frequency signal was set to 100 MHz.

[0081] Example 16 The frequency of the high frequency signal was set to 1 GHz.

[0082] Example 17 The frequency of the high frequency signal was set to 5GHz.

[0083] Example 18 The frequency of the high frequency signal was set to 10 GHz.

[0084] <Comparative Example 4> The additional signal generating device 6 did not generate an additional signal, and the frequency of the high frequency signal was set to 50 MHz.

[0085] <Comparative Example 5> The additional signal generating device 6 did not generate an additional signal, and the frequency of the high frequency signal was set to 100 MHz.

[0086] <Comparative Example 6> The additional signal generating device 6 did not generate an additional signal, and the frequency of the high frequency signal was set to 1 GHz.

[0087] <Comparative Example 7> The additional signal generating device 6 did not generate an additional signal, and the frequency of the high frequency signal was set to 5 GHz.

[0088] <Comparative Example 8> The additional signal generating device 6 did not generate an additional signal, and the frequency of the high frequency signal was set to 10 GHz.

[0089] <Examples 19 to 23 and Comparative Examples 9 to 13> <Additional signal frequency 10kHz, intensity 70dBμV> Fluctuations were measured in the same manner as in Example 1. However, the frequency of the additional signal was changed from 1 kHz to 10 kHz. The intensity of the additional signal was changed from 85.0 dBμV to 70.0 dBμV. The frequency of the high-frequency signal, which is the communication signal, was changed as shown in Table 5. In all of Comparative Examples 9 to 13, the additional signal generating device 6 did not generate an additional signal. The results are shown in Table 5.

[0090] Example 19 The frequency of the high frequency signal was set to 50 MHz.

[0091] Example 20 The frequency of the high frequency signal was set to 100 MHz.

[0092] <Example 21> The frequency of the high frequency signal was set to 1 GHz.

[0093] Example 22 The frequency of the high frequency signal was set to 5GHz.

[0094] Example 23 The frequency of the high frequency signal was set to 10 GHz.

[0095] <Comparative Example 9> The additional signal generating device 6 did not generate an additional signal, and the frequency of the high frequency signal was set to 50 MHz.

[0096] <Comparative Example 10> The additional signal generating device 6 did not generate an additional signal, and the frequency of the high frequency signal was set to 100 MHz.

[0097] <Comparative Example 11> The additional signal generating device 6 did not generate an additional signal, and the frequency of the high frequency signal was set to 1 GHz.

[0098] <Comparative Example 12> The additional signal generating device 6 did not generate an additional signal, and the frequency of the high frequency signal was set to 5 GHz.

[0099] <Comparative Example 13> The additional signal generating device 6 did not generate an additional signal, and the frequency of the high frequency signal was set to 10 GHz.

[0100] <Examples 24 to 28, and Comparative Examples 14 to 18> <Additional signal frequency 100kHz, intensity 70dBμV> Fluctuations were measured in the same manner as in Example 1. However, the frequency of the additional signal was changed from 1 kHz to 100 kHz. The intensity of the additional signal was changed from 85.0 dBμV to 70.0 dBμV. The frequency of the high-frequency signal, which is the communication signal, was changed as shown in Table 6. In all of Comparative Examples 14 to 18, the additional signal generating device 6 did not generate an additional signal. The results are shown in Table 6.

[0101] Example 24 The frequency of the high frequency signal was set to 50 MHz.

[0102] Example 25 The frequency of the high frequency signal was set to 100 MHz.

[0103] Example 26 The frequency of the high frequency signal was set to 1 GHz.

[0104] Example 27 The frequency of the high frequency signal was set to 5GHz.

[0105] Example 28 The frequency of the high frequency signal was set to 10 GHz.

[0106] <Comparative Example 14> The additional signal generating device 6 did not generate an additional signal, and the frequency of the high frequency signal was set to 50 MHz.

[0107] <Comparative Example 15> The additional signal generating device 6 did not generate an additional signal, and the frequency of the high frequency signal was set to 100 MHz.

[0108] <Comparative Example 16> The additional signal generating device 6 did not generate an additional signal, and the frequency of the high frequency signal was set to 1 GHz.

[0109] <Comparative Example 17> The additional signal generating device 6 did not generate an additional signal, and the frequency of the high frequency signal was set to 5 GHz.

[0110] <Comparative Example 18> The additional signal generating device 6 did not generate an additional signal, and the frequency of the high frequency signal was set to 10 GHz.

[0111] <Examples 29 to 33 and Comparative Examples 19 to 23> <Additional signal frequency 300kHz, intensity 70dBμV> Fluctuations were measured in the same manner as in Example 1. However, the frequency of the additional signal was changed from 1 kHz to 300 kHz. The intensity of the additional signal was changed from 85.0 dBμV to 70.0 dBμV. The frequency of the high-frequency signal, which is the communication signal, was changed as shown in Table 7. In all of Comparative Examples 19 to 23, the additional signal generating device 6 did not generate an additional signal. The results are shown in Table 7.

[0112] Example 29 The frequency of the high frequency signal was set to 50 MHz.

[0113] Example 30 The frequency of the high frequency signal was set to 100 MHz.

[0114] Example 31 The frequency of the high frequency signal was set to 1 GHz.

[0115] Example 32 The frequency of the high frequency signal was set to 5GHz.

[0116] Example 33 The frequency of the high frequency signal was set to 10 GHz.

[0117] <Comparative Example 19> The additional signal generating device 6 did not generate an additional signal, and the frequency of the high frequency signal was set to 50 MHz.

[0118] <Comparative Example 20> The additional signal generating device 6 did not generate an additional signal, and the frequency of the high frequency signal was set to 100 MHz.

[0119] <Comparative Example 21> The additional signal generating device 6 did not generate an additional signal, and the frequency of the high frequency signal was set to 1 GHz.

[0120] <Comparative Example 22> The additional signal generating device 6 did not generate an additional signal, and the frequency of the high frequency signal was set to 5 GHz.

[0121] <Comparative Example 23> The additional signal generating device 6 did not generate an additional signal, and the frequency of the high frequency signal was set to 10 GHz.

[0122] <Example 34 and Comparative Example 24> <Additional signal frequency 500kHz, intensity 81.5dBμV> Fluctuations were measured in the same manner as in Example 1. However, the frequency of the additional signal was changed from 1 kHz to 500 kHz. The intensity of the additional signal was changed from 85.0 dBμV to 81.5 dBμV. The frequency of the high-frequency signal, which is the communication signal, was changed as shown in Table 8. In Comparative Example 24, the additional signal generating device 6 did not generate an additional signal. The results are shown in Table 8.

[0123] Example 34 The frequency of the high frequency signal was set to 5GHz.

[0124] <Comparative Example 24> The additional signal generating device 6 did not generate an additional signal, and the frequency of the high frequency signal was set to 5 GHz.

[0125] < reference Example 35 and Comparative Example 25> <Additional signal frequency 10MHz, intensity 81.5dBμV> Fluctuations were measured in the same manner as in Example 1. However, the frequency of the additional signal was changed from 1 kHz to 10 MHz. The intensity of the additional signal was changed from 85.0 dBμV to 90.0 dBμV. The frequency of the high-frequency signal, which is the communication signal, was changed as shown in Table 9. In Comparative Example 25, the additional signal generating device 6 did not generate an additional signal. The results are shown in Table 9.

[0126] < reference Example 35> The frequency of the high frequency signal was set to 5GHz.

[0127] <Comparative Example 25> The additional signal generating device 6 did not generate an additional signal, and the frequency of the high frequency signal was set to 5 GHz. [Table 1]

[0128] [Table 2]

[0129] [Table 3]

[0130] [Table 4]

[0131] [Table 5]

[0132] [Table 6]

[0133] [Table 7]

[0134] [Table 8]

[0135] [Table 9]

[0136] The above invention is provided as an exemplary embodiment of the present invention, but it is merely an example and should not be interpreted as being limiting. Modifications of the present invention that are obvious to those skilled in the art are included in the scope of the following claims. [Industrial Applicability]

[0137] Optical transmission systems are used to transmit electrical signals using light. [Explanation of symbols]

[0138] 1 Optical transmission system 2. Electro-optical conversion devices 3 Optical transmission line 4. Photoelectric conversion devices 5. Communication signal generating device 6 Additional signal generating device

Claims

1. 1. An optical transmission system that converts a first electrical signal into an optical signal, transmits the converted optical signal, and converts the transmitted optical signal into a second electrical signal, an electro-optical conversion device that converts the first electrical signal into the optical signal; an optical transmission line for transmitting the optical signal converted by the electro-optical conversion device; an optical-electrical conversion device that converts the optical signal transmitted from the optical transmission line into the second electrical signal; the first electrical signal includes a high-frequency communication signal that is greater than 9 MHz and less than or equal to 10 GHz; further comprising an additional signal generating device that generates a low-frequency additional signal having a frequency of 1 Hz or more and 9 MHz or less; the electrical-optical conversion device converts the additional signal generated by the additional signal generating device and the first electrical signal including the communication signal into the optical signal; An optical transmission system, wherein the strength of the additional signal is 30 dBμV or more.

2. 10. The optical transmission system of claim 1, further comprising a communication signal generating device for generating said communication signal.

3. 1. An optical transmission method comprising: converting a first electrical signal into an optical signal; transmitting the converted optical signal; and converting the transmitted optical signal into a second electrical signal, the first electrical signal includes a high-frequency communication signal that is greater than 9 MHz and less than or equal to 10 GHz; a first step of converting the first electrical signal into the optical signal; a second step of transmitting the optical signal converted in the first step; a third step of converting the optical signal transmitted in the second step into the second electrical signal, A fourth step of generating a low-frequency additional signal having a frequency of 1 Hz or more and 9 MHz or less is further included, In the first step, the first electrical signal including the additional signal and the communication signal generated in the fourth step is converted into the optical signal; An optical transmission method, wherein the intensity of the additional signal is 30 dBμV or more.

4. 4. The optical transmission method according to claim 3, further comprising a fifth step of generating said communication signal.

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