JP7776121B2 - Transmitter and multi-level transmission method - Google Patents

Description

The present invention relates to a transmitter and a multi-level transmission method using the transmitter.

Conventionally, optical signal transmission methods using optical solitons are known. In an ideal optical transmission line, optical solitons are transmitted over long distances without changing their waveform. Therefore, long-distance transmission of optical signals is possible by utilizing optical solitons. However, in actual optical transmission lines, the properties of optical solitons can be lost due to optical loss associated with long-distance transmission. Non-Patent Documents 1 and 2 disclose technology for such long-distance optical soliton transmission, in which the output of optical solitons is amplified using an erbium-doped fiber amplifier (EDFA) before the soliton properties are lost.

Conventionally, multilevel transmission methods have been known in which the amplitude or phase of an optical pulse is multilevel-modulated to transmit multilevel (more than one bit) digital information per optical pulse. This type of multilevel transmission method can transmit more than one bit of digital information by transmitting a single optical pulse, thereby improving the efficiency of optical communications. In particular, when the amplitude of an optical pulse is multilevel-modulated, the receiver can be configured simply.

Masataka Nakazawa, "Optical Soliton Communication", Laser Research, Vol. 19, Supplement, pp. 237-240, 1991. Kazunori Suzuki et al., "Soliton Communication Using Erbium-Fed Optical Fiber Amplifiers," Laser Research, Vol. 20, No. 8, pp. 662-672, 1992.

However, when modulating the amplitude of an optical soliton in optical communications using amplitude modulation, the optical pulse whose amplitude is changed by modulation no longer satisfies the conditions for an optical soliton. Therefore, when an optical signal containing such an optical pulse is transmitted over a long distance, the waveform of the optical pulse becomes significantly distorted. In other words, when optical communications using amplitude modulation using optical solitons are performed, there is a problem in that stable long-distance transmission cannot be achieved simply by modulating the amplitude of the optical soliton.

In order to solve the above problem, a transmitter according to one aspect of the present invention comprises a laser light generation unit that generates laser light, and an optical signal generation unit that generates an optical signal by modulating the laser light, wherein the optical signal has a predetermined amplitude corresponding to the data value of a data signal and includes a plurality of optical pulses whose pulse widths corresponding to one data value vary depending on the predetermined amplitude.

The system may also include an amplifier that amplifies the intensity of the optical signal generated by the optical signal generation unit to shape the multiple optical pulses contained in the optical signal into multiple optical solitons.

Furthermore, the plurality of optical solitons may include a first optical soliton and a second optical soliton, the amplitude of the second optical soliton being smaller than the amplitude of the first optical soliton, and the pulse width of the second optical soliton being larger than the pulse width of the first optical soliton.

Furthermore, the pulse widths of the multiple optical solitons may be approximately inversely proportional to the predetermined amplitude.

The optical signal generating unit may also include an arbitrary waveform generator that generates an electrical signal including multiple voltage pulses corresponding to the waveforms of the multiple optical pulses in accordance with the data signal, and an intensity modulator that intensity-modulates the light from the laser light generating unit in accordance with the electrical signal.

The optical signal generating unit may also include a plurality of waveform generators that generate a plurality of voltage pulses corresponding to the waveforms of the plurality of optical pulses, a signal selector that selects a voltage pulse corresponding to the data signal from the plurality of voltage pulses to generate an electrical signal, and an intensity modulator that intensity-modulates the light from the laser light generating unit in accordance with the electrical signal.

The optical signal generating unit may also include an optical pulse generating unit that generates the plurality of optical pulses, and an optical selecting unit that selects an optical pulse corresponding to the data signal from the plurality of optical pulses.

The optical pulse generation unit may also include an intensity modulator that intensity-modulates the light from the laser light generation unit to generate a first optical pulse having a pulse width corresponding to a first optical soliton; a demultiplexer that splits the first optical pulse into multiple optical paths including at least a first optical path and a second optical path; and a band-limiting unit that is provided in at least the second optical path and converts the first optical pulse into a second optical pulse having a pulse width corresponding to a second optical soliton that differs in amplitude and pulse width from the first optical soliton.

The optical pulse generating unit may also include an attenuator provided at least on the second optical path that attenuates the amplitude of the second optical pulse so that the ratio of the amplitude of the second optical pulse incident on the optical selecting unit to the amplitude of the first optical pulse incident on the optical selecting unit substantially matches the ratio of the amplitude of the second optical soliton to the amplitude of the first optical soliton.

Furthermore, the optical selection unit may include a plurality of optical switches provided on each of the plurality of optical paths, and an optical switch provided on an optical path transmitting an optical pulse corresponding to the data signal may select the optical pulse corresponding to the data signal by passing the optical pulse.

In order to solve the above problem, a multilevel transmission method according to one aspect of the present invention includes a first optical soliton generation step of generating a first optical soliton having a first amplitude corresponding to a first data value of a data signal; a second optical soliton generation step of generating a second optical soliton having a second amplitude different from the first amplitude and corresponding to a second data value of the data signal, the second optical soliton having a different pulse width from the first optical soliton; and an optical signal transmission step of transmitting an optical signal including the first optical soliton and the second optical soliton.

In order to solve the above problems, a multilevel transmission method according to one aspect of the present invention performs communication using an optical signal including a first optical soliton having a first amplitude corresponding to a first data value of a data signal, and a second optical soliton having a second amplitude different from the first amplitude and corresponding to a second data value of the data signal, the second optical soliton having a pulse width different from that of the first optical soliton.

According to one aspect of the present invention, stable long-distance transmission can be achieved even when optical communications are performed using amplitude multi-level modulation.

1 is a block diagram showing a schematic configuration of an optical communication system according to a first embodiment of the present invention. 10A and 10B are diagrams showing four types of amplitude-modulated optical pulses according to a comparative example. 2 is a diagram showing an example of a waveform of an optical signal shaped by an amplifier 2 of the optical communication system. FIG. FIG. 2 is a block diagram showing an example of the configuration of the optical signal generating unit. FIG. 10 is a block diagram showing an example of the configuration of an optical signal generating unit according to a second embodiment of the present invention. FIG. 10 is a block diagram showing an example of the configuration of an optical signal generating unit according to a third embodiment of the present invention. FIG. 10 is a block diagram showing an example of the configuration of an optical signal generating unit according to a fourth embodiment of the present invention. 10A and 10B are diagrams illustrating an example of arrangement of signal points according to amplitude and phase in an optical signal generated by the optical signal generating unit. 10 is a graph showing the transmission limit distance versus pulse intensity according to a comparative example and an example.

[Embodiment 1]
(Schematic configuration of optical communication system 100)
Fig. 1 is a block diagram showing a schematic configuration of an optical communication system 100 according to a first embodiment of the present invention. As shown in Fig. 1, the optical communication system 100 includes a transmitter 1, an optical fiber 3 (optical transmission path), and a receiver 4. The transmitter 1 includes a laser light generator 5, an optical signal generator 10, and an amplifier 2. In the optical communication system 100, the optical fiber 3 functions as an optical transmission path connecting the transmitter 1 and the receiver 4.

Transmitter 1 is a device that emits an optical signal modulated based on a data signal into optical fiber 3. Receiver 4 is a device that receives the optical signal from transmitter 1 and demodulates the optical signal into a data signal using a known detection method. In other words, the optical signal emitted from transmitter 1 is transmitted to receiver 4 via optical fiber 3.

The laser light generation unit 5 is a light source that generates laser light. The laser light generation unit 5 is, for example, a known light-emitting element such as a laser diode (LD). The optical signal generation unit 10 generates an optical signal by modulating the laser light emitted from the laser light generation unit 5. Here, the optical signal generated by the optical signal generation unit 10 has a predetermined amplitude corresponding to the data value of the data signal and includes multiple optical pulses whose pulse widths vary depending on the predetermined amplitude. Note that the pulse width corresponds to one data value. The detailed configuration of the optical signal generation unit 10 will be described later with reference to Figure 4. The amplifier 2 amplifies the intensity of the optical signal generated by the optical signal generation unit 10. This forms the multiple optical pulses included in the optical signal into multiple optical solitons. The amplifier 2 is, for example, a known optical amplifier such as an erbium optical fiber amplifier.

An optical soliton is an optical pulse whose waveform is appropriately shaped so that the pulse broadening due to chromatic dispersion in the optical transmission path (optical fiber 3) and the pulse compression due to self-phase modulation cancel each other out.

Figure 2 shows waveforms of four types of amplitude-modulated optical pulses according to a comparative example. As shown in Figure 2, optical pulse P1 is an optical pulse having a first amplitude A1 corresponding to a first data value of "10" (binary notation). Optical pulse P2 is an optical pulse having a second amplitude A2 corresponding to a second data value of "11" (binary notation) and smaller than the first amplitude A1. Optical pulse P3 is an optical pulse having a third amplitude A3 corresponding to a third data value of "01" (binary notation) and smaller than the second amplitude A2. Optical pulse P4 is an optical pulse with an amplitude of 0, corresponding to a fourth data value of "00" (binary notation). By transmitting an optical signal containing these four types of optical pulses, four levels (two bits) of digital information can be transmitted per optical pulse. In this specification, the pulse width of an optical pulse refers to the full width at half maximum of the optical pulse.

In the comparative example shown in Figure 2, optical pulse P1 has a pulse width corresponding to the first amplitude A1, which satisfies the conditions for an optical soliton. Here, optical pulses P2 and P3 are generated by modulating the amplitude of optical pulse P1. That is, the pulse widths of optical pulses P2 and P3 are equal to the pulse width of optical pulse P1. Therefore, optical pulses P2 and P3 do not have pulse widths corresponding to the second amplitudes A2 and A3, respectively, which satisfies the conditions for an optical soliton. In other words, optical pulses P2 and P3 generated by modulating the amplitude of optical pulse P1 do not satisfy the conditions for an optical soliton. Therefore, when an optical signal containing four types of optical pulses according to the comparative example is transmitted over a long distance, the waveforms of optical pulses P2 and P3 are significantly distorted.

Figure 3 shows an example of the waveform of an optical signal shaped by amplifier 2. As shown in Figure 3, the optical signal includes a first optical soliton S1 and a second optical soliton S2. The first optical soliton S1 has a first amplitude A1 corresponding to a first data value. The first optical soliton S1 also has a pulse width Δt1 corresponding to the first amplitude A1, which satisfies the conditions for an optical soliton. The first optical soliton S1 has a waveform similar to that of the optical pulse P1 in Figure 2.

The second optical soliton S2 has a second amplitude A2 corresponding to the second data value. The second optical soliton S2 also has a pulse width Δt2 corresponding to the second amplitude A2, which satisfies the conditions for an optical soliton. The pulse width Δt2 is a different value from the pulse width Δt1. In other words, the second optical soliton S2, which is generated by modulating the amplitude of the first optical soliton S1 and then adjusting the pulse width, satisfies the conditions for an optical soliton, just like the first optical soliton S1.

That is, the optical signal generation unit 10 (see FIG. 1) generates a first optical soliton S1 having a first amplitude A1 corresponding to the first data value "10" of the data signal (first optical soliton generation step). The optical signal generation unit 10 also generates a second optical soliton S2 having a second amplitude A2 different from the first amplitude A1 and corresponding to the second data value "11" of the data signal, and having a pulse width different from that of the first optical soliton S1 (second optical soliton generation step). Furthermore, the transmitter 1 transmits an optical signal including the first optical soliton S1 and the second optical soliton S2, as shown in FIG. 3, to the optical fiber 3 (optical signal transmission step). Using this multilevel transmission method, the transmitter 1 transmits an optical signal on which a data signal is superimposed to the receiver 4.

Furthermore, the optical communication system 100 (see Figure 1) communicates using an optical signal including the first optical soliton S1 and the second optical soliton S2 described above. This multilevel transmission method enables stable long-distance transmission.

Note that, below, a transmission method using four types of amplitude-modulated optical solitons is referred to as the 4PAM (Pulse Amplitude Modulation) soliton method. When optical communication is performed using the 4PAM soliton method, the optical signal also includes a third optical soliton S3 having a third amplitude A3 corresponding to a third data value and a pulse width Δt3 corresponding to the third amplitude A3 that satisfies the conditions for an optical soliton. Similarly, the optical signal also includes a waveform with an amplitude of 0 corresponding to a fourth data value.

In other words, the optical signal generated by the optical signal generation unit 10 has a predetermined amplitude corresponding to the data value of the data signal, and includes multiple optical pulses whose pulse widths vary according to the predetermined amplitude. Note that this pulse width corresponds to one data value. This allows multiple amplitude-modulated optical solitons to be formed by varying the pulse width in addition to modulating the amplitude in optical communications using amplitude modulation. Therefore, by using an optical signal containing multiple optical solitons, stable long-distance transmission can be achieved even in optical communications using amplitude modulation. A specific method for generating optical solitons with adjusted pulse widths will be described later with reference to Figures 4 to 8.

Furthermore, the amplifier 2 may amplify the intensity of the optical signal generated by the optical signal generation unit 10, thereby shaping the multiple optical pulses contained in the optical signal into multiple optical solitons. In other words, the optical signal generated by the optical signal generation unit 10 includes multiple optical pulses (hereinafter referred to as adjusted optical pulses) whose amplitudes and pulse widths have been adjusted so that they become multiple optical solitons when amplified by the amplifier 2.

Furthermore, the pulse width Δt2 of the second optical soliton S2 is greater than the pulse width Δt1 of the first optical soliton S1. This makes it possible to generate a waveform for the second optical soliton S2 in which the pulse broadening due to chromatic dispersion in the optical transmission path (optical fiber 3) and the pulse compression due to self-phase modulation cancel each other out.

Furthermore, the pulse widths of the multiple optical solitons included in the optical signal may be approximately inversely proportional to a predetermined amplitude. For example, the second amplitude A2 of the second optical soliton S2 is approximately 0.98 times the first amplitude A1 of the first optical soliton S1. In this case, the pulse width Δt2 of the second optical soliton S2 is approximately 1/0.98 times the pulse width Δt2 of the first optical soliton S1. Note that the second amplitude A2 of the second optical soliton S2 is set to a value that allows the second optical soliton S2 to have a pulse width Δt2 that does not interfere with adjacent optical pulses.

(Schematic configuration of optical signal generating unit 10)
Fig. 4 is a block diagram showing the configuration of the optical signal generating unit 10. As shown in Fig. 4, the transmitter 1 includes a laser light generating unit 5, an optical signal generating unit 10, and an amplifier 2. The optical signal generating unit 10 includes a control unit 11, an arbitrary waveform generator 12, and an intensity modulator 13.

The control unit 11 outputs a data signal input from an external device to the arbitrary waveform generator 12. The data signal is converted into an optical signal and transmitted from the transmitter 1 to the receiver 4 (see Figure 1). The control unit 11 is composed of, for example, an electrical circuit.

The arbitrary waveform generator 12 generates an electrical signal including multiple voltage pulses corresponding to the waveforms of multiple adjusted optical pulses in accordance with the data signal output from the control unit 11. For example, if the data signal includes a data value of "10," the arbitrary waveform generator 12 generates an electrical signal including voltage pulses corresponding to the waveform of a first optical soliton S1 having a first amplitude A1 and a pulse width Δt1. The arbitrary waveform generator 12 outputs such an electrical signal to the intensity modulator 13.

The intensity modulator 13 is an optical modulator that intensity-modulates the light from the laser light generating unit 5 in accordance with the electrical signal output from the arbitrary waveform generator 12. The intensity modulator 13 generates an optical signal including multiple modulated optical pulses.

With this configuration, amplitude multilevel modulated optical solitons can be generated with a simple configuration in which the intensity modulator 13 modulates the light from the laser light generating unit 5 in accordance with an electrical signal serving as an arbitrary waveform signal output by the arbitrary waveform generator 12. Furthermore, by using the arbitrary waveform generator 12, optical solitons of any type (number of levels) can be generated.

[Embodiment 2]
Other embodiments of the present invention will be described below. For ease of explanation, the same reference numerals will be used to designate components having the same functions as those described in the above embodiment, and the description thereof will not be repeated.

(Schematic configuration of optical signal generating unit 20)
Fig. 5 is a block diagram showing the configuration of an optical signal generating unit 20 according to a second embodiment of the present invention. As shown in Fig. 5, the optical signal generating unit 20 includes a plurality of waveform generators 21, a control unit 22, a signal selector 23, and an intensity modulator 24. In the optical communication system 100 shown in Fig. 1, the optical signal generating unit 20 according to this embodiment may be applied instead of the optical signal generating unit 10 according to the first embodiment.

The multiple waveform generators 21 each generate multiple voltage pulses corresponding to the waveforms of the multiple adjusted optical pulses. The multiple waveform generators 21 output the multiple voltage pulses to the signal selector 23. For example, the multiple waveform generators 21 include waveform generator 21a and waveform generator 21b. Waveform generators 21a and 21b generate voltage pulses corresponding to the first optical soliton S1 and the second optical soliton S2, respectively.

When optical communications are performed using N types of optical pulses, the multiple waveform generators 21 need only include N-1 waveform generators that generate N-1 types of voltage pulses corresponding to the waveforms of the N-1 types of optical solitons. For simplicity, only two waveform generators, 21a and 21b, are shown in Figure 5.

The control unit 22 receives a data signal that is superimposed on an optical signal and transmitted from the transmitter 1 to the receiver 4 (see Figure 1). The control unit 22 outputs the data signal to the signal selector 23.

The signal selector 23 selects a voltage pulse corresponding to the data signal output from the control unit 22 from the multiple voltage pulses and generates an electrical signal. The signal selector 23 outputs the electrical signal to the intensity modulator 24. For example, when the signal selector 23 receives a data signal containing a first data value "10", it selects a voltage pulse corresponding to the first optical soliton S1 from the multiple voltage pulses and outputs the voltage pulse to the intensity modulator 24. Furthermore, when the signal selector 23 receives a data signal containing a fourth data value "00", it does not select a voltage pulse from the multiple voltage pulses and outputs a voltage pulse with an amplitude of 0 to the intensity modulator 24.

The intensity modulator 24 intensity-modulates the light from the laser light generator 5 in accordance with the electrical signal output from the signal selector 23. As a result, the intensity modulator 24 generates an optical signal including multiple modulated optical pulses.

With this configuration, a voltage pulse corresponding to a data signal is selected from multiple pre-generated voltage pulses, making it possible to generate a high-speed multilevel optical soliton signal train.

[Embodiment 3]
Other embodiments of the present invention will be described below. For ease of explanation, the same reference numerals will be used to designate components having the same functions as those described in the above embodiment, and the description thereof will not be repeated.

(Schematic configuration of optical signal generating unit 30)
Fig. 6 is a block diagram showing the configuration of an optical signal generating unit 30 according to a third embodiment of the present invention. As shown in Fig. 6, the optical signal generating unit 30 includes an optical pulse generating unit 36, a control unit 34, and an optical selecting unit 35. In the optical communication system 100 shown in Fig. 1, the optical signal generating unit 30 according to this embodiment may be applied instead of the optical signal generating unit 10 according to the first embodiment.

The optical pulse generation unit 36 includes an intensity modulator 31, a branching filter 32, a band limiting unit 33, and attenuators 371 and 372. The optical pulse generation unit 36 generates a plurality of adjusted optical pulses and outputs the adjusted optical pulses to the optical selection unit 35.

The intensity modulator 31 intensity-modulates the light from the laser light generation unit 5 to generate a first optical pulse having a pulse width corresponding to the first optical soliton S1.

The splitter 32 splits the first optical pulse output from the intensity modulator 31 into multiple optical paths including a first optical path T1 and a second optical path T2. When optical signal transmission in the optical signal generation unit 30 is configured using a spatial optical system, the splitter 32 may be, for example, a beam splitter. Also, when optical signal transmission in the optical signal generation unit 30 is configured using optical fiber components or optical waveguides, the splitter 32 may be, for example, a 3 dB coupler.

The band-limiting unit 33 is provided at least on the second optical path T2. The band-limiting unit 33 narrows the spectral width (widens the pulse width) of the first optical pulse. That is, the band-limiting unit 33 changes the first optical pulse into a second optical pulse having a pulse width corresponding to a second optical soliton S2, which has a different amplitude and pulse width from the first optical soliton S1. The band-limiting unit 33 includes a band-limiting filter, such as a low-pass filter (LPF) or a band-pass filter (BPF).

Attenuators 371 and 372 are provided on the first optical path T1 and the second optical path T2, respectively. Attenuators 371 and 372 attenuate the amplitudes of the first optical pulse and the second optical pulse, respectively, so that the ratio of the amplitude of the second optical pulse incident on the optical selection unit 35 to the amplitude of the first optical pulse incident on the optical selection unit 35 approximately matches the ratio of the amplitude of the second optical soliton S2 to the amplitude of the first optical soliton S1. This causes multiple adjusted optical pulses to be emitted from attenuators 371 and 372. Note that an attenuator may be provided on at least one of the first optical path T1 and the second optical path T2. When there are three or more optical paths, an attenuator may be provided on each optical path, or an attenuator may be provided on all but one of the multiple optical paths.

When optical communication is performed using N types of optical pulses, the demultiplexer 32 simply divides the first optical pulse into N-1 optical paths. Furthermore, the band-limiting units 33 may be provided in at least N-2 optical paths. The band-limiting units 33 in the at least N-2 optical paths each include a different type of band-limiting filter and/or a different number of band-limiting filters. This allows the optical pulse generating unit 36 to generate N-1 types of adjusted optical pulses. For simplicity, only two optical paths are shown in Figure 6.

The control unit 34 generates a data signal that is superimposed on the optical signal and transmitted from the transmitter 1 to the receiver 4 (see Figure 1). Based on the data signal, the control unit 34 controls the optical switches 351 and 352 (described below).

The optical selection unit 35 includes optical switches 351 and 352 and a multiplexer 353. The optical selection unit 35 selects an adjusted optical pulse corresponding to the data signal from the multiple adjusted optical pulses output from the optical pulse generation unit 36.

Optical switches 351 and 352 are provided on the first optical path T1 and the second optical path T2, respectively. The control unit 34 outputs an instruction to pass the adjusted optical pulse to the optical switch provided on the optical path that transmits the adjusted optical pulse corresponding to the data signal. Optical switches 351 and 352 are optical shutters that use the electro-optic effect, such as a Kerr shutter or an optical intensity modulator.

For example, when the control unit 34 receives a data signal containing the first data value "10," it outputs an instruction to the optical switch 351 provided on the first optical path T1 that transmits the adjusted optical pulse corresponding to the first optical soliton S1 to pass the adjusted optical pulse. This causes the optical selection unit 35 to select the adjusted optical pulse corresponding to the first data value "10." Furthermore, when the control unit 34 receives a data signal containing the fourth data value "00," it outputs an instruction to all optical switches not to pass the adjusted optical pulse. This causes the optical selection unit 35 to generate a waveform with an amplitude of 0 that corresponds to the fourth data value "00."

The multiplexer 353 multiplexes the adjusted optical pulses emitted from the optical switches 351 and 352 and transmitted through the first optical path T1 and the second optical path T2. When optical signal transmission in the optical signal generation unit 30 is configured using a spatial optical system, the multiplexer 353 may be, for example, a combination of a half mirror and a mirror. Furthermore, when optical signal transmission in the optical signal generation unit 30 is configured using optical fiber components or optical waveguides, the multiplexer 353 may be, for example, a 3 dB coupler.

With this configuration, it is possible to generate the multilevel optical soliton signal train according to this embodiment through optical signal processing.

[Embodiment 4]
Other embodiments of the present invention will be described below. For ease of explanation, the same reference numerals will be used to designate components having the same functions as those described in the above embodiment, and the description thereof will not be repeated.

(Schematic configuration of optical signal generating unit 40)
Fig. 7 is a block diagram showing the configuration of an optical signal generating unit 40 according to a fourth embodiment of the present invention. Fig. 8 is a diagram showing an example of the arrangement of signal points according to the amplitude and phase in an optical signal generated by the optical signal generating unit 40. The optical signal generating unit 40 according to this embodiment differs from the optical signal generating unit 30 according to the third embodiment in that it includes a control unit 41 instead of the control unit 34 and in that it further includes a phase modulator 42. As shown in Figs. 7 and 8, the optical signal generating unit 40 generates an amplitude-phase-modulated optical signal by further performing phase modulation on the amplitude-modulated optical signal.

The control unit 41 operates either the optical switches 351 or 352 based on the data signal. This causes the optical signal generation unit 40 to generate an adjusted optical pulse corresponding to the first optical soliton S1 or the second optical soliton S2. The control unit 41 also outputs the data signal to the phase modulator 42. The phase modulator 42 is an optical modulator that performs phase modulation on the amplitude-modulated optical signal output from the optical selection unit 35 based on the data signal. The optical signal generation unit 40 generates optical signals with phase differences of, for example, π/4. As a result, the optical signal generation unit 40 generates an amplitude-phase-modulated optical signal that can transmit 16 values (4 bits) of digital information per optical pulse, as shown in Figure 8, for example.

With this configuration, it is possible to transmit a larger number of bits of digital information per optical pulse compared to embodiments 1 to 3.

In the optical communication system 100 shown in FIG. 1, the optical signal generation unit 10 according to the first embodiment may be replaced with the optical signal generation unit 40 according to this embodiment. In this case, the receiver 4 shown in FIG. 1 includes a known detection circuit for demodulating the phase-modulated optical signal.

[Embodiment 5]
FIG. 9 is a graph showing the transmission distance limit Z [km] versus pulse intensity I for the comparative example and the example. Here, pulse intensity I represents the ratio of the amplitude of an optical pulse corresponding to one data item to the maximum amplitude of another data item. Furthermore, transmission distance limit Z represents the distance over which an optical signal can be properly transmitted. The graph labeled "Comparative Example" in FIG. 9 shows the transmission distance limit Z when an optical signal including multiple optical pulses with only amplitude modulation, as shown in FIG. 2, is transmitted. The graph labeled "Example" in FIG. 9 shows the transmission distance limit Z when an optical signal including multiple optical solitons with pulse widths that are modulated in addition to amplitude modulation, as shown in FIG. 3, is transmitted. As shown in FIG. 9, by using multiple optical solitons with pulse widths that are modulated in addition to amplitude modulation, the transmission distance limit Z can be increased compared to when only amplitude modulation is performed.

This configuration can contribute to ultra-long-distance communications, the next step after 5G communications. In other words, the above-mentioned optical communications system can promote the development of communications infrastructure, thereby contributing to the achievement of the Sustainable Development Goals (SDGs).

The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present invention.

1 Transmitter 2 Amplifier 3 Optical fiber (optical transmission path)
4 Receiver 5 Laser light generation unit 10, 20, 30, 40 Optical signal generation unit 11, 22, 34, 41 Control unit 12 Arbitrary waveform generator 13, 24, 31 Intensity modulator 21, 21a, 21b Waveform generator 23 Signal selector 32 Demultiplexer 33 Bandwidth limiter 35 Light selection unit 36 Optical pulse generation unit 351, 352 Optical switches 371, 372 Attenuator A1 First amplitude A2 Second amplitude S1 First optical soliton S2 Second optical soliton T1 First optical path T2 Second optical path

Claims (11)

a laser light generating unit that generates a laser light;
an optical signal generating unit that generates an optical signal by modulating the laser light , the optical signal including a plurality of optical pulses each having a different predetermined amplitude corresponding to a data value of a data signal, and each pulse width corresponding to one data value varying according to the predetermined amplitude;
an amplifier that amplifies the intensity of the optical signal generated by the optical signal generation unit to shape the plurality of optical pulses included in the optical signal into a plurality of optical solitons . the plurality of optical solitons include a first optical soliton and a second optical soliton;
2. The transmitter according to claim 1 , wherein the amplitude of said second optical soliton is smaller than the amplitude of said first optical soliton, and the pulse width of said second optical soliton is larger than the pulse width of said first optical soliton. 3. The transmitter of claim 2 , wherein pulse widths of the plurality of optical solitons are approximately inversely proportional to the predetermined amplitude. The optical signal generation unit
an arbitrary waveform generator that generates an electrical signal including a plurality of voltage pulses corresponding to the waveforms of the plurality of optical pulses in accordance with the data signal;
The transmitter according to claim 1 , further comprising: an intensity modulator that intensity-modulates the light from the laser light generating unit in accordance with the electrical signal. The optical signal generation unit
a plurality of waveform generators each generating a plurality of voltage pulses corresponding to the waveforms of the plurality of optical pulses;
a signal selector that selects a voltage pulse corresponding to the data signal from the plurality of voltage pulses and generates an electrical signal;
The transmitter according to claim 1 , further comprising: an intensity modulator that intensity-modulates the light from the laser light generating unit in accordance with the electrical signal. The optical signal generation unit
an optical pulse generating unit that generates the plurality of optical pulses;
The transmitter according to claim 1 , further comprising: an optical selection unit that selects an optical pulse corresponding to the data signal from the plurality of optical pulses. a laser light generating unit that generates a laser light;
an optical signal generating unit that generates an optical signal by modulating the laser light,
the optical signal includes a plurality of optical pulses having a predetermined amplitude corresponding to a data value of a data signal, the pulse width corresponding to one data value being different depending on the predetermined amplitude;
The optical signal generation unit
an optical pulse generating unit that generates the plurality of optical pulses;
a light selection unit that selects an optical pulse corresponding to the data signal from the plurality of optical pulses,
The optical pulse generating unit
an intensity modulator that intensity-modulates the light from the laser light generating unit to generate a first optical pulse having a pulse width corresponding to a first optical soliton;
a splitter that splits the first optical pulse into a plurality of optical paths including at least a first optical path and a second optical path;
a band limiting unit provided at least in the second optical path, which changes the first optical pulse into a second optical pulse having a pulse width corresponding to a second optical soliton having an amplitude and pulse width different from those of the first optical soliton. 8. The transmitter according to claim 7, wherein the optical pulse generating unit comprises an attenuator provided at least on the second optical path, which attenuates the amplitude of the second optical pulse so that a ratio of an amplitude of the second optical pulse incident on the optical selecting unit to an amplitude of the first optical pulse incident on the optical selecting unit substantially matches a ratio of an amplitude of the second optical soliton to an amplitude of the first optical soliton. 9. The transmitter according to claim 7, wherein the optical selection unit includes a plurality of optical switches respectively provided on the plurality of optical paths, and an optical switch among the plurality of optical switches provided on an optical path transmitting an optical pulse corresponding to the data signal selects the optical pulse corresponding to the data signal by passing the optical pulse . a first optical soliton generating step of generating a first optical soliton having a first amplitude corresponding to a first data value of the data signal;
a second optical soliton generating step of generating a second optical soliton having a second amplitude different from the first amplitude, the second optical soliton corresponding to a second data value of the data signal, and having a pulse width different from that of the first optical soliton;
an optical signal transmitting step of transmitting an optical signal including the first optical soliton and the second optical soliton. a first optical soliton having a first amplitude corresponding to a first data value of the data signal;
A multi-value transmission method for communicating using an optical signal including a second optical soliton having a second amplitude different from the first amplitude and corresponding to a second data value of the data signal, the second optical soliton having a pulse width different from that of the first optical soliton.