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, the frequency being more than 10GHz but not greater than 100GHz. The optical transmission system (1) further comprises an additional signal generation device (6) that generates an additional signal of a low frequency of 1Hz-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 Art

[0002] An optical transmission system including a laser chip, an optical fiber, and an optical isolator interposed therebetween is known (see, for example, Patent Document 1 below). In the optical transmission system described in Patent Document 1, the light emitted from the laser chip is input to the optical fiber after passing through the optical isolator. The optical isolator transmits only the light traveling in the forward direction while blocking the light traveling in the reverse direction. Therefore, in the optical transmission system, the optical isolator attenuates the return light from the optical fiber.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] Depending on the application and purpose, an optical transmission system is required to suppress fluctuations in signal quality over time. The signal quality includes the CNR (carrier-to-noise ratio). The signal quality can be referred to as signal characteristics. The optical transmission system of Patent Document 1 suppresses the above-described fluctuations by attenuating the return light by the optical isolator. However, the optical isolator is expensive and has a problem that the configuration of the optical transmission system becomes complicated.

[0005] The present invention provides an optical transmission system and an optical transmission method that are simple in configuration and low in cost while suppressing fluctuations in signal quality over time.

Means for Solving the Problems

[0006] (1) The present invention provides 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 an optical signal; an optical transmission path that transmits the optical signal converted by the electro-optical conversion device; and an optical-electrical conversion device that converts the optical signal transmitted from the optical transmission path into a second electrical signal, wherein the first electrical signal includes a high-frequency communication signal having a frequency of over 10 GHz and 100 GHz or less, and further comprises 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, and the electro-optical conversion device converts the first electrical signal, which includes the additional signal generated by the additional signal generating device and the communication signal, into an optical signal.

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

[0008] Moreover, this optical transmission system does not require an optical isolator like the one in Patent Document 1, and only requires an additional signal generation device that generates an additional low-frequency signal of a specific frequency, thus resulting in a simple configuration and low cost.

[0009] The present invention (2) includes the optical transmission system described in (1), wherein the intensity of the additional signal is 70 dBμV or less.

[0010] In this optical transmission system, since the intensity of the additional signal is 70 dBμV or less, 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 for generating the communication signal.

[0012] The present invention (4) is 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 having a frequency of over 10 GHz and 100 GHz or less, and comprises a first step of converting the first electrical signal into an 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 a second electrical signal, further comprising a fourth step of generating a low-frequency additional signal having a frequency of 1 Hz or more and 9 MHz or less, wherein the first step converts the first electrical signal, which includes the additional signal generated in the fourth step and the communication signal, into an optical signal.

[0013] In the first step of this optical transmission method, a first electrical signal including a low-frequency additional signal with a frequency of 1 Hz or higher and 9 MHz or lower is converted into an optical signal, thereby suppressing fluctuations in signal quality over time.

[0014] Moreover, since the optical transmission method generates an additional low-frequency signal of a specific frequency in the fourth step, its configuration is simple and low-cost.

[0015] The present invention (5) includes the optical transmission method described in (4), wherein the intensity of the additional signal is 70 dBμV or less.

[0016] In this optical transmission method, since the intensity of the additional signal is 70 dBμV or less, 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 being simple in configuration and low in cost. [Brief explanation of the drawing]

[0019] [Figure 1] Figure 1 shows an embodiment of the optical transmission system of the present invention. [Figure 2] Figure 2 shows a modified optical transmission system. [Figure 3] Figure 3 shows a modified optical transmission system.

Embodiments for Carrying Out the Invention

[0020] <An Embodiment of the Optical Transmission System> An embodiment of the 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. Hereinafter, each signal will be described.

[0021] <The 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, at any timing, the first electrical signal includes the additional signal.

[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 exceeds 9 MHz, fluctuations in the signal quality described below cannot be sufficiently suppressed. The frequency of the additional signal is preferably 10 Hz or more, more preferably 100 Hz or more, still more preferably 1 kHz or more, and particularly preferably 10 kHz or more. Also, the frequency of the additional signal is preferably 1 MHz or less, more preferably 500 kHz or less, still more preferably 300 kHz or less.

[0024] The intensity of the additional signal is, for example, 1000 dBμV or less, preferably 100 dBμV or less, more preferably 80 dBμV or less, and even more preferably 70 dBμV or less. The intensity of the additional signal is, for example, 10 dBμV or more, preferably 40 dBμV or more. If the intensity of the additional signal is above the lower limit and below the upper limit mentioned above, fluctuations in signal quality over time can be further suppressed.

[0025] <Communication Signals> The first electrical signal further includes a communication signal containing the 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 does not contain the information to be communicated, and is therefore sometimes referred to as a "non-communication signal." The communication signal is transmitted over time. That is, there are times when the communication signal is transmitted and times when it is not. Specifically, in the optical transmission system 1, when the communication signal is not being transmitted (communication signal OFF), the first electrical signal does not include the communication signal and includes only the additional signal. On the other hand, in the optical transmission system 1, when the communication signal is being transmitted (communication signal ON), the first electrical signal includes both 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, by frequency division multiplexing. Frequency division multiplexing is a method of transmitting multiple channels by modulating a communication signal and then multiplexing the modulated communication signals in parallel on the frequency axis. Examples of RF signals include multi-wave CW waves (multiple single-frequency signals) and single-wave CW waves (one single-frequency signal). The above-mentioned RF signals and methods are described, for example, in Japanese Patent Application Publication No. 2020-096363.

[0027] The frequency of the communication signal is, for example, greater than 10 GHz, preferably 11 GHz or higher, more preferably 12 GHz or higher, even more preferably 15 GHz or higher, and particularly preferably 20 GHz or higher. Alternatively, the frequency of the communication signal may be 100 GHz or lower, preferably 50 GHz or lower, and more preferably 40 GHz or lower. The frequency of the communication signal is higher than the frequency of the additional signal described above.

[0028] The ratio of the frequency of the communication signal to the frequency of the additional signal is, for example, 1.1 × 10⁻⁶. 4 Preferably, 5 × 10 4 That's all, and also, for example, 10 8 Preferably, 5 × 10 7 The following applies:

[0029] The strength of the communication signal is not limited. The strength of the communication signal is set appropriately according to the application and purpose of the optical transmission system 1.

[0030] <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.

[0031] <Configuration of Optical Transmission System 1> As shown in Figure 1, the optical transmission system 1 comprises an electro-to-optical conversion device 2, an optical transmission path 3, and an optical-to-electrical conversion device 4. Furthermore, the optical transmission system 1 includes a communication signal generation device 5, an additional signal generation device 6, and a synthesis device 7.

[0032] <Electro-optical conversion device 2> The electro-optical conversion device 2 is capable of converting the first electrical signal described above 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. A TOSA is a Transmitter Optical Subassembly. The electro-optical conversion device 2 described above includes, for example, a light source. An example of a light source is a laser diode. An example of a laser diode is a vertical-cavity surface-emitting laser (VCSEL). The electro-optical conversion device 2 is also connected to a DC current generating device 12. An example of a DC current generating device 12 is a DC current source.

[0033] <Optical transmission path 3> The optical transmission path 3 is capable of transmitting the optical signal converted by the electro-optical conversion device 2. The optical transmission path 3 extends in the transmission direction. The upstream end of the optical transmission path 3 in the transmission direction is connected to the electro-optical conversion device 2. The optical transmission path 3 is not limited. Examples of optical transmission paths 3 include optical fibers. Examples of optical fibers include plastic optical fibers and glass optical fibers. Examples of embodiments of the optical transmission path 3 include multimode and single-mode.

[0034] <Photoelectric conversion device 4> The photoelectric conversion device 4 is capable of converting an optical signal transmitted from the optical transmission path 3 into a second electrical signal. The photoelectric conversion device 4 is connected to the downstream end of the optical transmission path 3 in the transmission direction. The photoelectric conversion device 4 is not limited. An example of the photoelectric conversion device 4 is a ROSA, which is a Receiver Optical Subassembly. The photoelectric conversion device 4 described above also includes, for example, a photodiode (PD).

[0035] <Communication signal generating device 5> The communication signal generating device 5 is capable of generating the communication signals described above. The communication signal generating device 5 is connected to the electro-optical conversion 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 antenna substrate capable of receiving RF signals. Another example of the communication signal generating device 5 is a multi-signal generating device.

[0036] <Additional signal generating device 6> The additional signal generation device 6 is capable of generating the additional signals described above. The additional signal generation device 6 is connected to the electro-optical conversion device 2 and the communication signal generation device 5 via a combining device 7, which will be described later. An additional line 9 is wired between the additional signal generation 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 generation device 6 is not limited. An example of the additional signal generation device 6 is a low-frequency signal generation device. An example of a low-frequency signal generation device is a multi-signal generation device.

[0037] <Synthetic Device 7> The synthesis device 7 can combine (superimpose) the communication signal generated by the communication signal generation device 5 and the additional signal generated by the additional signal generation device 6. The synthesis device 7 is connected to the communication signal generation device 5 via the communication line 8. The synthesis device 7 is also connected to the additional signal generation device 6 via the additional line 9. A connection line 10 is wired between the synthesis device 7 and the electro-optical conversion device 2. Therefore, the synthesis device 7 is connected to the electro-optical conversion device 2 via the connection line 10.

[0038] <Optical transmission using optical transmission system 1 (optical transmission method)> Next, we will explain the optical transmission method using optical transmission system 1.

[0039] <Operation when communication signal is OFF> In this optical transmission system 1, the communication signal generating device 5 is normally not generating a communication signal; in other words, the communication signal is OFF. On the other hand, in this optical transmission system 1, the additional signal generating device 6 is normally generating the additional signal described above. In other words, the fourth step is performed.

[0040] The additional signal generated by the additional signal generation device 6 is then input to the electro-optical conversion device 2 via the additional line 9, the combining device 7, and the connecting line 10. When it passes through the combining device 7, it is processed so that it is included in the first electrical signal. In other words, the first electrical signal, which includes the additional signal, is input to the electro-optical conversion device 2.

[0041] The electro-optical conversion device 2 converts the first electrical signal described above into an optical signal. In other words, the first step is performed. The electro-optical conversion device 2 converts the first electrical signal, which includes the low-frequency additional signal described above, into light. At this time, the electro-optical conversion device 2 uses the DC current input from the DC current generation device 12. Subsequently, the electro-optical conversion device 2 inputs the converted optical signal into the optical transmission path 3.

[0042] In the optical transmission path 3, the input optical signal is transmitted and input to the photoelectric conversion device 4. In other words, the second step is performed. Examples of optical signal transmission methods include multimode and single-mode methods.

[0043] The optical-electrical conversion device 4 converts the optical signal input from the optical transmission path 3 into a second electrical signal. In other words, the third step is performed. The second electrical signal may be the same as or different in phase from the first electrical signal described above. The second electrical signal is input to an external device 11, indicated by a dashed line. An example of the external device 11 is an image display device. Examples of image display devices include a television and a recorder.

[0044] <Operation when communication signal is ON> When the communication signal is turned ON, the communication signal generating device 5 generates the communication signal. In other words, the fifth step is performed. Even at this time, the additional signal generating device 6 continuously generates the additional signal described above. In other words, the fourth step is performed.

[0045] 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 to form a first electrical signal that includes them. In other words, the combining device 7 adds (superimposes) the additional signal onto the communication signal to form the first electrical signal.

[0046] The first electrical signal synthesized by the synthesis device 7 is input to the electro-optical conversion device 2 via the connection line 10.

[0047] The first step (conversion from the first electrical signal to an optical signal by the electro-optical conversion device 2), the second step (transmission of the optical signal by the optical transmission path 3), and the third step (conversion from the optical signal to a second electrical signal by the optical-electrical conversion device 4) when the communication signal is ON are the same as those when the communication signal is OFF, as described above. However, the first electrical signal in the first step includes both an additional signal and a communication signal.

[0048] <Effects of one embodiment> In this optical transmission system 1, a first electrical signal, which includes a low-frequency additional signal generated by the additional signal generation device 6 and having a frequency of 1 Hz or higher and 9 MHz or lower, is converted into an optical signal, thereby suppressing fluctuations in signal quality over time.

[0049] Moreover, the optical transmission system 1 does not need to include an optical isolator like the one in Patent Document 1, and only requires an additional signal generation device 6 that generates an additional low-frequency signal of a specific frequency, thus resulting in a simple configuration and low cost.

[0050] In this optical transmission system 1, if the intensity of the additional signal is 70 dBμV or less, fluctuations in signal quality over time can be further suppressed.

[0051] In the first step of the optical transmission method according to one embodiment, 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, thereby suppressing fluctuations in signal quality with respect to time.

[0052] Moreover, since the optical transmission method generates an additional low-frequency signal of a specific frequency in the fourth step, its configuration is simple and low-cost.

[0053] In this optical transmission method, if the intensity of the additional signal is 70 dBμV or less, fluctuations in signal quality over time can be further suppressed.

[0054] <Variation> In the modified examples, components and processes similar to those in the first embodiment are given the same reference numerals, and their detailed descriptions are omitted. Furthermore, the modified examples can achieve the same effects and advantages as the first embodiment, unless otherwise specified. Moreover, the first embodiment and its modified examples can be combined as appropriate.

[0055] As shown in Figure 2, the optical transmission system 1 does not have a separate combining device 7, but rather the electro-optical conversion device 2 has the function of the combining device 7. In other words, the electro-optical conversion device 2 also functions as the combining device. The communication signal generation device 5 is connected to the electro-optical conversion device 2 via the communication line 8. The additional signal generation device 6 is connected to the electro-optical conversion device 2 via the additional line 9.

[0056] As shown in Figure 3, the optical transmission system 1 does not necessarily have to include the communication signal generation device 5. In the modified example shown in Figure 3, the combining device 7 receives a communication signal from an external source via a communication line 8, which is shown as a dashed line. The combining device 7 combines the above-mentioned communication signal with the additional signal input from the additional signal generation device 6 to produce the first electrical signal described above. [Examples]

[0057] The present invention will be further described below with reference to examples and comparative examples. However, the present invention is not limited to the examples and comparative examples. Furthermore, specific numerical values ​​such as blending ratios (content ratios), physical properties, and parameters used in the following description may be replaced with the corresponding upper limits (numerical values ​​defined as "less than or equal to" or "less than") or lower limits (numerical values ​​defined as "greater than or equal to" or "greater than or equal to") of the blending ratios (content ratios), physical properties, and parameters described in the "Modes for Carrying Out the Invention" above.

[0058] <High-frequency signal with frequency 28 GHz and intensity 80 dBμV>

[0059] <Example 1> As Example 1, an optical transmission system 1 was prepared, which includes the following devices shown in Figure 1.

[0060] Electro-optical conversion device 2: TOSA optical transmission line using a VCSEL with a center wavelength of 850 nm; Optical transmission line 3: Multimode optical fiber Photoelectric conversion device 4: ROSA using PD Communication signal generation device 5: Model number N5183A, a multi-signal generator manufactured by Agilent. Additional signal generation device 6: Model number WF1973, a multi-signal generator manufactured by NF Circuit Design Block Co., Ltd. External device 11: Model number N9010B, a spectrum analyzer manufactured by Keysight. DC current generating device 12: Model number 2400 Source Meter, a DC current source manufactured by KEITHLEY.

[0061] In Example 1, a high-frequency signal with a frequency of 28 GHz and an intensity of 80 dBμV, consisting of a single CW wave, was generated from the communication signal generation device 5. Simultaneously, a DC current of 8 mA was input from the DC current generation device 12 to the electro-to-optical conversion device 2, and an additional signal generation device 6 generated an additional signal with a frequency of 1 kHz and an intensity of 70 dBμV. Subsequently, the fluctuation in the quality of the second electrical signal in the optical-to-electrical conversion device 4 was measured for 30 minutes using an external device 11. The fluctuation was determined 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.

[0062] <Example 2> The fluctuations were 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 results of the fluctuations are shown in Table 1.

[0063] <Example 3> The fluctuations were 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 results of the fluctuations are shown in Tables 1 and 2.

[0064] <Example 4> The fluctuations were measured in the same manner as in Example 1, except that the frequency of the additional signal was changed from 1 kHz to 300 kHz. The results of the fluctuations are shown in Table 1.

[0065] <Example 5> The fluctuations were 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 results of the fluctuations are shown in Table 1.

[0066] <Comparative Example 1> The fluctuations were measured in the same manner as in Example 1. However, the additional signal was not generated by the additional signal generating device 6. The results of the fluctuations are shown in Table 1.

[0067] <Comparative Example 2> The fluctuations were 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.

[0068] <Examples 6-9> <High-frequency signal frequency: 28 GHz>

[0069] <Example 6> The fluctuations were measured in the same manner as in Example 3. However, in the additional signal generation device 6, the intensity of the additional signal was changed from 70 dBμV to 30 dBμV. The results of the fluctuations are shown in Table 2.

[0070] <Example 7> The fluctuations were measured in the same manner as in Example 3. However, in the additional signal generation device 6, the intensity of the additional signal was changed from 70 dBμV to 50 dBμV. The results of the fluctuations are shown in Table 2.

[0071] <Example 8> The fluctuations were measured in the same manner as in Example 3. However, in the additional signal generation device 6, the intensity of the additional signal was changed from 70 dBμV to 85 dBμV. The results of the fluctuations are shown in Table 2.

[0072] <Example 9> The fluctuations were measured in the same manner as in Example 3. However, in the additional signal generation device 6, the intensity of the additional signal was changed from 70 dBμV to 90 dBμV. The results of the fluctuations are shown in Table 2.

[0073] < reference Example 10> reference In Example 10, a DC current of 8 mA was generated from the DC current generation device 12. Additionally, the additional signal generation device 6 generated an additional signal with a frequency of 10 kHz and an intensity of 70 dBμV. However, the communication signal generation device 5 did not generate a communication signal. Next, the fluctuation of the second electrical signal in the optical-electrical conversion device 4 was measured for 30 minutes using an external device 11. The fluctuation was determined as the difference between the maximum and minimum noise intensity values ​​at 28 GHz measured by the external device 11. The results are shown in Table 3.

[0074] Subsequently, the fluctuations were measured. The results of the fluctuations are shown in Table 3.

[0075] <Comparative Example 3> reference The fluctuations were measured in the same manner as in Example 10. However, the additional signal was not generated by the additional signal generating device 6. That is, the communication signal was not generated by the communication signal generating device 5, and the additional signal was not generated by the additional signal generating device 6. Subsequently, the fluctuations of the second electrical signal in the optical-electrical conversion device 4 were measured for 30 minutes using an external device 11. The fluctuations were determined as the difference between the maximum and minimum values ​​of the noise intensity at 28 GHz measured by the external device 11. The results are shown in Table 3.

[0076] [Table 2]

[0077] [Table 3]

[0078] The above invention is provided as an illustrative embodiment of the present invention, but this is merely illustrative and should not be interpreted restrictively. Modifications of the present invention that are obvious to those skilled in the art are included in the claims below. [Industrial applicability]

[0079] Optical transmission systems are used for optical transmission. [Explanation of Symbols]

[0080] 1. Optical transmission system 2. Electric-to-optical conversion device 3 Optical transmission path 4. Photoelectric conversion devices 5. Communication signal generating device 6. Additional signal generation device

Claims

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 path for transmitting the optical signal converted by the aforementioned electro-optical conversion device, The system comprises an optical-electrical conversion device that converts the optical signal transmitted from the optical transmission path into the second electrical signal, The first electrical signal includes a high-frequency communication signal that is above 10 GHz and below 100 GHz. The system further includes an additional signal generating device that generates low-frequency additional signals with frequencies between 1 Hz and 9 MHz. Without an optical isolator between the electro-optical conversion device and the optical transmission path, the ratio of the frequency of the communication signal to the frequency of the additional signal is set to 1.1 × 10⁻¹⁰ to suppress fluctuations in signal quality due to the return light to the electro-optical conversion device. ４ The above 10 ８ The following applies: An optical transmission system in which the electro-to-optical conversion device converts the first electrical signal, which includes the additional signal generated by the additional signal generation device and the communication signal, into an optical signal.

2. The optical transmission system according to claim 1, wherein the intensity of the additional signal is 70 dBμV or less.

3. The optical transmission system according to claim 1 or claim 2, further comprising a communication signal generating device for generating the aforementioned communication signal.

4. 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 above 10 GHz and below 100 GHz. A first step of converting the first electrical signal into the optical signal, A second step involves transmitting the optical signal converted in the first step, The process comprises a third step of converting the optical signal transmitted in the second step into a second electrical signal, The system further includes a fourth step of generating an additional low-frequency signal with a frequency of 1 Hz or higher and 9 MHz or lower. Without an optical isolator between the electro-optical conversion device and the optical transmission path, the ratio of the frequency of the communication signal to the frequency of the additional signal is set to 1.1 × 10⁻¹⁰ to suppress fluctuations in signal quality due to the return light to the electro-optical conversion device. ４ The above 10 ８ The following applies: An optical transmission method comprising, in the first step, converting the first electrical signal, which includes the additional signal generated in the fourth step and the communication signal, into an optical signal.

5. The optical transmission method according to claim 4, wherein the intensity of the additional signal is 70 dBμV or less.

6. The optical transmission method according to claim 4 or claim 5, further comprising a fifth step of generating the communication signal.

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