Optical Transmission Device and Optical Transmission System

Abstract

To provide an optical transmission device capable of increasing the total electric power of a signal to be transmitted and also maintaining signal quality in optical transmission using a multi-carrier modulation system.SOLUTION: An optical transmission device is used for an optical transmission system which uses a multi-carrier modulation system, and comprises a light emission part 110 which emits laser light, a plurality of external modulation parts 130, and a coupling part. An external modulation part 130 inputs the laser light and an electric control signal, and performs modulation processing on the laser light based upon the electric control signal to generate a first light signal. The coupling part couples a plurality of first light signals modulated by the plurality of external modulation parts 130 into one band to generate a second light signal. Electric control signals input to the plurality of external modulation parts 130 are signals of mutually different frequency bands, and also previously OFDM (Orthogonal Frequency Division Multiplexing)-modulated signals.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to an optical transmission device and an optical transmission system.

Background Art

[0002] A technique of applying an orthogonal frequency division multiplexing demodulation technique (OFDM demodulation technique, Orthogonal Frequency Division Multiplexing) used in wireless communication or the like to a modulation means for optical transmission has been proposed. Patent Document 1 discloses an optical transmission system using the OFDM demodulation technique. The optical transmission system disclosed in Patent Document 1 uses the OFDM demodulation technique as a modulation means to relax the distance limitation and bandwidth limitation caused by wavelength dispersion and mode dispersion.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In a multi-carrier modulation method such as OFDM, the more the number of sub-carriers to be transmitted and the wider its bandwidth, the more information can be transmitted. On the other hand, in the multi-carrier modulation method, when the total power of the signal to be transmitted is equal, the wider the bandwidth of the sub-carrier, the smaller the SNR (Signal-to-Noise Ratio), and the transmission quality deteriorates. Therefore, in the multi-carrier modulation method, in order to maintain the SNR, more power is required. Further, in order to expand this SNR, if the total power of the signal to be transmitted is increased, the SNR is conversely reduced by the third-order intermodulation distortion caused by the non-linearity of the light-emitting element and the modulator, and the signal quality deteriorates. Therefore, when transmitting an optical signal by a multi-carrier modulation method such as OFDM, an optical transmission technique capable of increasing the total power of the signal and maintaining the signal quality is required.

[0005] The present invention has been made in view of the problems of such prior art. And an object of the present invention is to provide an optical transmission apparatus capable of increasing the total power of a signal to be transmitted and maintaining the signal quality in optical transmission using a multi-carrier modulation method.

Means for Solving the Problems

[0006] An optical transmission apparatus according to an aspect of the present invention is an optical transmission apparatus used in an optical transmission system using a multi-carrier modulation method, and includes a light emitting unit that emits laser light, and inputs the laser light and an electrical control signal, and based on the electrical control signal, performs modulation processing of the laser light to generate a first optical signal. A plurality of external modulation units, and a combining unit that combines the first signals modulated by the plurality of external modulation units to generate a second optical signal integrated into one band. The electrical control signals input to the plurality of external modulation units are signals in different frequency bands from each other and are signals that have been pre-modulated by OFDM (Orthogonal Frequency Division Multiplexing).

[0007] An optical transmission system according to another aspect of the present invention includes an optical transmission unit that is the above-described optical transmission apparatus, and an optical reception unit that inputs a second optical signal. The optical reception unit includes a photoelectric conversion unit that converts the input second optical signal into a first electrical signal, and a first amplifier that amplifies the power of the first electrical signal to generate a second electrical signal.

Effects of the Invention

[0008] According to the present invention, in optical transmission using a multi-carrier modulation method, it is possible to provide an optical transmission apparatus capable of increasing the total power of a signal to be transmitted and maintaining the signal quality.

Brief Description of the Drawings

[0009]

Figure 1

Figure 2A

Figure 2B

Figure 3

Figure 4

Figure 5

Figure 6A

Figure 6B

Figure 6C

Figure 6D

Figure 7A

Figure 7B

Figure 7C

Figure 7D

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Embodiments for Carrying Out the Invention

[0010] Hereinafter, the optical transmission system 10 and the optical transmission device according to the present embodiment will be described in detail with reference to the drawings. Note that the dimensional ratios in the drawings are exaggerated for convenience of explanation and may be different from the actual ratios. Also, in the following description of the drawings, the same or similar parts are denoted by the same or similar reference numerals.

[0011] (First Embodiment) FIG. 1 is a diagram showing the configuration of an optical transmission system 10 according to the first embodiment. The optical transmission system 10 includes an optical transmission unit 100 and an optical reception unit 200 connected to the optical transmission unit 100 via an optical fiber 300. In the first embodiment, the optical transmission unit 100 corresponds to an optical transmission device.

[0012] In the first embodiment, the optical transmission system 10 transmits an optical signal generated by the optical transmission unit 100 to the optical reception unit 200 via the optical fiber 300. Also, the optical transmission system 10 is a multi-carrier modulation in which data is loaded on a number of carriers (sub-carriers), and transmits an optical signal multiplexed by an OFDM modulation method in which these sub-carriers are orthogonal to each other.

[0013] Here, the relationship between frequency and power in a multi-carrier modulation method such as OFDM will be described. FIGS. 2A and 2B are diagrams for explaining the relationship between frequency and power in the OFDM modulation method, and are diagrams schematically showing the waveform of an OFDM-modulated signal.

[0014] In the OFDM modulation method, the larger the number of subcarriers to be transmitted and their bandwidth, the more information can be transmitted. However, when the total signal power is equal, the SNR (Signal-to-Noise Ratio) per subcarrier becomes smaller. For example, assume that the total signal power in FIGS. 2A and 2B is the same. Also, the waveform shown in FIG. 2A has a narrower frequency bandwidth compared to the waveform shown in FIG. 2B. Further, the waveform shown in FIG. 2B is a diagram of the case where the frequency bandwidth is wide when the total signal power is the same as that of the waveform shown in FIG. 2A. That is, as shown in FIGS. 2A and 2B, the power in a certain frequency band is larger and the SNR is larger in the example shown in FIG. 2A compared to the example shown in FIG. 2B. That is, when the total signal power is the same, the narrower the frequency bandwidth, the larger the SNR and the higher the quality.

[0015] When the frequency bandwidth is wide, more power is required to maintain the SNR. If the total signal power is increased to expand the SNR, the SNR is conversely reduced by the third-order intermodulation distortion caused by the non-linearity of the light-emitting element or the modulator, and the signal quality deteriorates. FIGS. 3 and 4 are diagrams showing waveforms when more power is used to expand the SNR in the case of the signal characteristics shown in FIG. 2B. As shown in FIG. 3, when more power is used, although the power increases, the SNR is reduced by the third-order intermodulation distortion (distortion component in FIG. 3) caused by the non-linearity of the light-emitting element or the modulator, and the signal quality deteriorates.

[0016] FIG. 4 is a diagram showing the relationship between, for example, the input power to the optical receiver 200 and the EVM (Error Vector Magnitude) of the signal at the electrical signal coupling unit 250 in the optical receiver 200. The horizontal axis in FIG. 4 indicates the magnitude of the input power, and the vertical axis in FIG. 4 indicates the EVM. The EVM is used as an index indicating the quality of the modulation signal. As shown in FIG. 4, as the input power increases, the EVM tends to increase due to the distortion caused by the non-linearity of the light-emitting element and the modulator. Therefore, in the case of transmitting an optical signal by a multi-carrier modulation method such as OFDM, an optical transmission technology capable of maintaining the signal quality even when the total power of the signal is increased is required.

[0017] (Schematic Configuration of Optical Transmitter 100) In the first embodiment, the optical transmitter 100 includes a light-emitting unit 110 that is a light source of an optical signal, a first optical waveguide 120, a plurality of external modulation units 130, a second optical waveguide 140, and a connector 150. An end portion of the first optical waveguide 120 on the light-emitting element side is provided with a spot size conversion structure, and the light emitted from the light-emitting element is coupled to the first optical waveguide 120 with low loss.

[0018] The light-emitting unit 110 is a light-emitting element (LD: Laser Diode) that continuously emits laser light based on the power supplied from a DC power source (not shown) that is a DC power source for driving the light-emitting unit 110.

[0019] The light-emitting unit 110 is composed of, for example, a VCSEL (Vertical Cavity Surface Emitting Laser). Note that the configuration of the light-emitting unit 110 does not limit the embodiment. For example, the light-emitting unit 110 may be composed of a distributed feedback (DFB) laser or an LED (Light Emitting Diode).

[0020] FIG. 5 is a diagram showing an example of the laser light output from the light-emitting unit 110. As shown in FIG. 5, the laser light output from the light-emitting unit 110 is constant at a predetermined power.

[0021] As shown in FIG. 1, the first optical waveguide 120 is an optical waveguide that branches the laser light output from the light emitting unit 110. The branching of the laser light in the first optical waveguide 120 is performed by, for example, a Y-branch waveguide. In the first embodiment, the laser light output from the light emitting unit 110 is first branched into two by a Y-branch waveguide, and each laser light is further branched into two by a Y-branch waveguide. That is, in the first embodiment, the laser light output from the light emitting unit 110 is branched into four by a Y-branch waveguide and input to four external modulation units 130 described later.

[0022] The four laser lights branched by the first optical waveguide 120 are input to the external modulation unit 130. An electrical control signal is also input to the external modulation unit 130 via the electrical signal line 400. The external modulation unit 130 inputs the laser light and the electrical control signal, and performs modulation processing of the laser light based on the electrical control signal. In the present embodiment, the external modulation unit 130 is composed of a general Mach-Zehnder modulator or an EA modulator (Electro-Absorption). In the present embodiment, the laser light modulated by the external modulation unit 130 is referred to as the first optical signal.

[0023] Also, in the example shown in FIG. 1, four external modulation units 130a, 130b, 130c, and 130d are shown as the external modulation unit 130. Four laser lights branched by the above-described first optical waveguide 120 are input to these external modulation units 130a to 130d. Further, different electrical control signals are input to the external modulation units 130a to 130d via the electrical signal line 400. In FIG. 1, an example in which the electrical signal line 400 is connected only to the external modulation unit 130a is schematically shown, but the electrical signal line 400 is also connected to the external modulation units 130b to 130d in the same manner as the external modulation unit 130a. That is, it is assumed that electrical control signals via the electrical signal line 400 are input to the external modulation units 130b to 130d in the same manner as the external modulation unit 130a. Hereinafter, when it is not necessary to distinguish and explain each of the external modulation units 130a to 130d, it is simply referred to as "external modulation unit 130".

[0024] The electrical signal line 400 is connected to an external device or an antenna provided outside the optical transmission system 10. That is, an electrical control signal is transmitted from the external device or the antenna to the optical transmission unit 100 via the electrical signal line 400. Note that the electrical control signal input to the external modulation unit 130 in the present embodiment is a signal that has been OFDM-modulated in advance in an external device or the like. Further, the electrical control signals input to the external modulation units 130a to 130d are signals in different frequency bands from each other.

[0025] Figures 6A to 6D are diagrams schematically showing waveforms of the electrical control signals input to the external modulation units 130a to 130d respectively. In the examples shown in Figures 6A to 6D, examples of frequencies with a bandwidth of 400 MHz are shown respectively. For example, in the example shown in FIG. 1, an OFDM-modulated electrical control signal with a frequency bandwidth of 27.0 GHz to 27.4 GHz as shown in FIG. 6A is used to modulate the laser light in the external modulation unit 130a. Similarly, an OFDM-modulated electrical control signal with a frequency bandwidth of 27.4 GHz to 27.8 GHz as shown in FIG. 6B is used to modulate the laser light in the external modulation unit 130b. Also, an OFDM-modulated electrical control signal with a frequency bandwidth of 27.8 GHz to 28.2 GHz as shown in FIG. 6C is used to modulate the laser light in the external modulation unit 130c. Further, an OFDM-modulated electrical control signal with a frequency bandwidth of 28.2 GHz to 28.6 GHz as shown in FIG. 6D is used to modulate the laser light in the external modulation unit 130d.

[0026] Figures 7A to 7D show examples schematically showing the first optical signals output from the external modulation units 130a to 130d. As shown in FIGS. 7 to 7D, the first optical signals output from the external modulation units 130a to 130d become signals with different frequency bands, similar to the electrical control signals shown in FIGS. 6A to 6D above. In the examples shown in Figures 7A to 7D, the first optical signals output from the external modulation units 130a to 130d have waveforms obtained by adding the center frequencies of the respective modulation signals to the center wavelength λc of the laser light. When the wavelength of the laser light is 1310 nm, the frequency of λc is approximately 228.85 THz.

[0027] As shown in FIG. 1, the second optical waveguide 140 couples the first optical signal, which is the optical signal output from the external modulation unit 130, by means of a Y-branched waveguide or the like. The second optical waveguide 140 corresponds to the coupling part. Also, the optical signal coupled by the second optical waveguide 140 corresponds to the second optical signal. That is, in the present embodiment, the second optical signal is an optical signal obtained by gathering the first optical signal into one band by means of a Y-branched waveguide or the like. The second optical signal coupled by the second optical waveguide 140 is sent to the connector 150.

[0028] FIG. 8 is a diagram schematically showing an example of the waveform of the second optical signal, which is an optical signal gathered into one band. As shown in FIG. 8, the signal (second optical signal) output from the optical transmission unit 100 according to the first embodiment has a large power as the total power and has no distortion. That is, from the optical transmission unit 100, even if the total power of the signal is increased, an optical signal with a large SNR and maintained quality is output.

[0029] The connector 150 is connected to the optical fiber 300 and transmits the second optical signal to the optical reception unit 200 via the optical fiber 300.

[0030] The optical reception unit 200 includes a photoelectric conversion unit 210 and a first amplifier 220. The photoelectric conversion unit 210 converts the second optical signal transmitted from the optical transmission unit 100 into an electrical signal. The photoelectric conversion unit 210 is constituted by, for example, a general photodiode (PD). Also, the electrical signal converted by the photoelectric conversion unit 210 corresponds to the first electrical signal.

[0031] The first amplifier 220 amplifies and outputs the power of the electrical signal converted by the photoelectric conversion unit 210 according to the purpose of use. Note that the electrical signal amplified by the first amplifier 220 corresponds to the second electrical signal.

[0032] As described above, the optical transmission unit 100 according to the first embodiment is an optical transmission device used in an optical transmission system using a multi-carrier modulation method, and includes a light emitting unit 110 that emits laser light, a plurality of external modulation units 130, and a second optical waveguide 140. The external modulation unit 130 inputs laser light and an electrical control signal, performs modulation processing on the laser light based on the electrical control signal, and generates a first optical signal that is a modulated optical signal. The second optical waveguide 140 combines a plurality of first optical signals modulated by the plurality of external modulation units 130 and generates a second optical signal grouped into one band. The electrical control signals input to the plurality of external modulation units 130 are signals in different frequency bands and are signals that have been pre-modulated by OFDM (Orthogonal Frequency Division Multiplexing). Note that the second optical waveguide 140 corresponds to a combining unit.

[0033] With this configuration, each of the plurality of external modulation units 130 generates a first optical signal with high power and a large SNR. Further, the second optical waveguide 140 generates a plurality of second optical signals with a large SNR by combining the first optical signals. Therefore, the optical transmission unit 100 outputs an optical signal with a large SNR and maintained quality. As a result, the optical transmission unit 100 can increase the total power of the signal to be transmitted and maintain the signal quality in optical transmission using the multi-carrier modulation method.

[0034] (Second Embodiment) As described above, one specific embodiment has been described, but the above-described embodiments are examples and do not limit the embodiments. For example, in the above-described embodiment, a form in which laser light is modulated in different frequency bands in the optical transmission unit 100 is exemplified. Here, further, an optical transmission system 10 according to a second embodiment in which an electrical signal is amplified in different frequency bands in the optical reception unit 200 will be described with respect to configurations different from those of the first embodiment. In the following description, the same content as that described in the description of the above first embodiment will be omitted or described in a simplified manner.

[0035] The optical receiver 200 of the optical transmission system 10 according to the second embodiment is different from the optical transmission system 10 according to the first embodiment in that it includes a plurality of filter units 230 and a plurality of second amplifiers 240.

[0036] In the second embodiment, the plurality of filter units 230 generate electrical signals that pass an electrical signal amplified by the first amplifier 220 in a predetermined frequency band. In this specification, the electrical signal passed by the filter unit 230 in a predetermined frequency band corresponds to the third electrical signal. That is, the filter unit 230 divides the second electrical signal into different frequency bands by passing the electrical signal at a predetermined frequency. The filter unit 230 divides the second electrical signal into the frequency bands shown in FIGS. 6A to 6D, for example, and generates the third electrical signal.

[0037] Also, the plurality of second amplifiers 240 further amplify the third electrical signal that has passed through the plurality of filter units 230 and generate a fourth electrical signal.

[0038] With this configuration, the optical receiver 200 in the second embodiment amplifies the weak signal from the optoelectronic conversion unit 210 to a level that is not affected by external influences in the first amplifier 220. Further, in the optical receiver 200, since the frequencies of the signals are divided into different frequency bands by the plurality of filter units 230, the bandwidth to be processed by each of the plurality of second amplifiers 240 is reduced. As a result, similar to the optical transmitter 100 in the first embodiment, the optical receiver 200 in the second embodiment can transmit signals with high quality (high SNR) without being affected by distortion.

[0039] (Other Embodiments) Although the embodiments have been described in detail with reference to the drawings, the present embodiment is not limited by the content described in the above embodiments. Further, the components described above include those that can be easily assumed by those skilled in the art and those that are substantially the same. Furthermore, the configurations described above can be combined as appropriate. Also, various omissions, substitutions, or changes in the configuration can be made without departing from the gist of the embodiment.

[0040] In the above-described second embodiment, an example in which the optical receiving unit 200 includes a plurality of filter units 230 and a plurality of second amplifiers 240 has been shown. However, the optical receiving unit 200 may further be configured to include an electrical signal combining unit 250. FIG. 10 shows a configuration diagram of the optical receiving unit 200 according to the second embodiment when the electrical signal combining unit 250 is included and the antenna 260 is further included in the optical transmission system 10. The electrical signal combining unit 250 is realized, for example, by a Y-branch waveguide. In this way, at the electrical stage after photoelectric conversion by the photoelectric conversion unit 210, by multiplexing (combining) the fourth electrical signals amplified for each frequency band by the plurality of filter units 230, it becomes possible to radiate a broadband and high-SNR signal from the antenna 260. Note that the antenna 260 may be configured to be mounted on the same substrate as the optical receiving unit 200.

[0041] Also, in the above-described embodiment, a configuration in which the optical transmission unit 100 includes one light emitting unit 110 has been shown. However, the optical transmission unit 100 may include a plurality of light emitting units 110. FIG. 11 shows an example of the optical transmission unit 100 including two light emitting units 110. In the configuration shown in FIG. 11, the two light emitting units 110 each emit laser light with the same level of power. Also, the two light emitting units 110 each send the laser light branched once to the external modulation unit 130 through different first optical waveguides 120. Thereby, the optical transmission unit 100 shown in FIG. 11 can reduce the number of branchings and prevent power loss due to branching of the laser light in the optical waveguide by providing a plurality of light emitting units 110 that emit laser light with the same level of power. That is, the optical transmission unit 100 shown in FIG. 11 can obtain a high SNR by transmitting an optical signal with a higher power, and can maintain the quality in the optical transmission system 10.

[0042] Also, in the above-described second embodiment, an example where the number of the filter units 230 and the second amplifiers 240 of the optical receiver unit 200 is the same as the number of the external modulation units 130, which is four, was shown. The embodiment is not limited to this configuration, and the filter units 230 and / or the second amplifiers 240 of the optical receiver unit 200 may be implemented with a number different from the number of the external modulation units 130. FIG. 12 shows a case where the number of the filter units 230 is two. For example, when the distortion characteristics of the second amplifiers 240 are sufficiently better than those of the external modulation units 130, even with the configuration shown in FIG. 12, the influence of signal degradation due to distortion is reduced. That is, when the distortion characteristics of the second amplifiers 240 are good, even if the number of the filter units 230 and / or the second amplifiers 240 is less than the number of the external modulation units 130, it is possible to maintain high signal transmission quality. Thereby, the optical transmission system 10 can reduce the circuit and cost of the filter units 230.

[0043] The features of the optical transmission device and the optical transmission system 10 will be described below.

[0044] The optical transmission device according to the first aspect is an optical transmission device used in an optical transmission system using a multi-carrier modulation method. The optical transmission device includes a light emitting unit 110 that emits laser light, and a plurality of external modulation units 130 that input the laser light and an electrical control signal, perform modulation processing of the laser light based on the electrical control signal, and generate a first optical signal. The optical transmission device also includes a combining unit that combines the plurality of first optical signals modulated by the plurality of external modulation units 130 and generates a second optical signal integrated into one band. The electrical control signals input to the plurality of external modulation units 130 are signals in different frequency bands from each other and are signals that have been subjected to OFDM (Orthogonal Frequency Division Multiplexing) modulation in advance.

[0045] According to the above configuration, the plurality of external modulation units 130 each generate a first optical signal with high power and a large SNR. Further, the coupling unit corresponding to the second optical waveguide 140 couples the plurality of first optical signals with a large SNR to generate a second optical signal. Therefore, the optical transmission unit 100 outputs an optical signal (second optical signal) with a large SNR and maintained quality. As a result, the optical transmission device corresponding to the optical transmission unit 100 can increase the total power of the signal to be transmitted and maintain the signal quality in optical transmission using a multi-carrier modulation method.

[0046] The optical transmission device according to the second aspect may include a plurality of light emitting units 110. Further, the plurality of light emitting units 110 of the optical transmission device may each emit laser light with the same level of power.

[0047] According to the above configuration, the optical transmission device can reduce the number of branching times and prevent power loss due to branching of laser light in the optical waveguide by providing a plurality of light emitting units 110 that emit laser light with the same level of power. That is, the optical transmission device corresponding to the optical transmission unit 100 can obtain a high SNR by transmitting an optical signal with higher power, and maintain the quality in the optical transmission system 10.

[0048] The optical transmission system 10 according to the third aspect includes an optical transmission unit 100 that is an optical transmission device, and an optical reception unit 200 that inputs a second optical signal. The optical reception unit 200 includes a photoelectric conversion unit 210 that converts the input second optical signal into a first electrical signal, and a first amplifier 220 that amplifies the power of the first electrical signal to generate a second electrical signal.

[0049] According to the above configuration, the plurality of external modulation units 130 of the optical transmission device each generate a first optical signal with high power and a large SNR. In addition, the coupling unit corresponding to the second optical waveguide 140 of the optical transmission device couples the plurality of first optical signals with a large SNR. Therefore, the optical transmission unit 100 outputs an optical signal with a large SNR and maintained quality. As a result, the optical transmission system 10 including the optical transmission device and the optical reception unit 200 can increase the total power of the signal to be transmitted and maintain the signal quality in optical transmission using a multi-carrier modulation method.

[0050] The optical reception unit 200 of the optical transmission system 10 according to the fourth aspect may include a plurality of filter units 230 that pass the second electrical signal amplified by the first amplifier 220 in a predetermined frequency band to generate a third electrical signal. In addition, the optical reception unit 200 may include a plurality of second amplifiers 240 that further amplify the third electrical signal that has passed through the plurality of filter units 230 to generate a fourth electrical signal. The plurality of filter units 230 may generate a third electrical signal by passing the second electrical signal in mutually different frequency bands.

[0051] According to the above configuration, in the optical reception unit 200 of the optical transmission system 10, the first amplifier 220 amplifies the weak signal from the optoelectronic conversion unit 210 to a level that is not affected by external influences. Further, the optical reception unit 200 can generate an electrical signal with a small influence of distortion in the first amplifier 220 by amplifying electrical signals with different frequency band widths by the second amplifier 240. In addition, in the optical reception unit 200, since the frequencies of the signals are separated into different frequency bands by the plurality of filter units 230, the bandwidth processed by each of the plurality of second amplifiers 240 becomes narrow. As a result, the optical transmission system 10 including the optical reception unit 200 can transmit signals with high quality (high SNR) without being affected by distortion.

[0052] The optical reception unit 200 of the optical transmission system 10 according to the fifth aspect may further include an electrical signal coupling unit 250 that couples the plurality of fourth electrical signals amplified by the plurality of second amplifiers 240.

[0053] According to the above configuration, the electrical signal combining unit 250 combines (couples) the fourth electrical signals amplified for each frequency band by the plurality of filter units 230 at the electrical stage after being photoelectrically converted by the photoelectric conversion unit 210. As a result, the optical transmission system 10 including the electrical signal combining unit 250 can radiate a broadband and high-SNR signal from the antenna 260.

Explanation of Reference Numerals

[0054] 10 Optical transmission system 100 Optical transmitter 110 Light emitting unit 120 First optical waveguide 140 Second optical waveguide 130, 130a to 130d External modulation unit 150 Connector 200 Optical receiver 210 Photoelectric conversion unit 220 First amplifier 230 Filter unit 240 Second amplifier 300 Optical fiber 400 Electrical signal line

Claims

1. An optical transmission device used in an optical transmission system using a multi-carrier modulation method, comprising: a light emitting unit that emits laser light; a first external modulation unit that receives the laser light and a first electrical control signal, performs modulation processing on the laser light based on the first electrical control signal, and generates a first first optical signal having a waveform obtained by adding the center frequency of the first electrical control signal to the frequency corresponding to the center wavelength of the laser light; a second external modulation unit that receives the laser light and a second electrical control signal, performs modulation processing on the laser light based on the second electrical control signal, and generates a second first optical signal having a waveform obtained by adding the center frequency of the second electrical control signal to the frequency corresponding to the center wavelength of the laser light; a third external modulation unit that receives the laser light and a third electrical control signal, performs modulation processing on the laser light based on the third electrical control signal, and generates a third first optical signal having a waveform obtained by adding the center frequency of the third electrical control signal to the frequency corresponding to the center wavelength of the laser light; a fourth external modulation unit that receives the laser light and a fourth electrical control signal, performs modulation processing on the laser light based on the fourth electrical control signal, and generates a fourth first optical signal having a waveform obtained by adding the center frequency of the fourth electrical control signal to the frequency corresponding to the center wavelength of the laser light; a combining unit that combines the first first optical signal, the second first optical signal, the third first optical signal, and the fourth first optical signal modulated by the first external modulation unit, the second external modulation unit, the third external modulation unit, and the fourth external modulation unit, and generates a second optical signal integrated into one band; wherein the first electrical control signal, the second electrical control signal, the third electrical control signal, and the fourth electrical control signal are signals in different frequency bands and are signals that have been subjected to OFDM (Orthogonal Frequency Division Multiplexing) modulation in advance; and the first first optical signal, the second first optical signal, the third first optical signal, and the fourth first optical signal are signals in different frequency bands. An optical transmission device.

2. Comprising a first light emitting unit and a second light emitting unit as a plurality of the light emitting units, the first light emitting unit emits the laser light with the same level of power to the first external modulation unit and the second external modulation unit. The second light emitting unit emits laser light with the same level of power as the laser light emitted from the first light emitting unit to the third external modulation unit and the fourth external modulation unit. The optical transmission device according to claim 1.

3. An optical transmission unit that is the optical transmission device according to claim 1 or 2, and an optical reception unit that inputs the second optical signal. The optical reception unit includes one photoelectric conversion unit that converts the input second optical signal into a first electrical signal, and a first amplifier that amplifies the power of the first electrical signal to generate a second electrical signal. An optical transmission system comprising the above.

4. The optical reception unit is connected in series with the first amplifier, and is a filter that allows the second electrical signal amplified by the first amplifier to pass through in a predetermined frequency band to generate a third electrical signal. A plurality of filter units that input signals obtained by branching the second electrical signal in parallel; a second amplifier that is connected in series with the filter unit and further amplifies the third electrical signal that has passed through the filter unit to generate a fourth electrical signal; and an electrical signal combining unit that combines the plurality of fourth electrical signals amplified by the plurality of second amplifiers. The plurality of filter units generate a plurality of third electrical signals with different frequency bands by passing the plurality of second electrical signals through different frequency bands. The optical transmission system according to claim 3.

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