Modulated optical signal generator and transmission module

Abstract

The modulated optical signal generating device (33) of the present invention is a device for generating a multilevel modulated optical signal transmitted and received between a transmitter and a receiver, and comprises an input branching section (333), an output multiplexing section (334), and one waveguide (331) and the other waveguide (332) connecting the input branching section and the output multiplexing section, the input optical pulse is branched at the input branching section, one optical pulse propagates through one waveguide, and the other optical pulse propagates through the other waveguide and is delayed by an electrical modulation signal, the one optical pulse and the other optical pulse are combined at the output multiplexing section, the modulated optical signal has one optical pulse and the other optical pulse for each transmission baud, the input data is mapped to the time difference between one optical pulse and the other optical pulse caused by the delay, and the modulation format has a different time difference for each transmission baud. As a result, the present invention can provide a modulated optical signal generating device that can increase the number of symbols and reduce power consumption.

Description

[Technical Field]

[0001] The present invention relates to a generator and a transmitter module for a modulated optical signal used for transmitting and receiving an optical multilevel signal. [Background technology]

[0002] In recent years, the amount of data processed by computers has increased, and there has been interest in new computer architectures to improve computer processing capabilities.

[0003] High performance computer architectures require high bandwidth and low latency data transmission, where transmitting data over optical links allows for high data rates and reduces the transmitted energy per bit.

[0004] In optical links, the bit rate per lane can be increased by using multi-level signals to adapt a large number of symbols to their respective baud durations.

[0005] Also, the increase in data flow size leads to an increase in the number of packets, which in turn increases the probability of packet collisions. Furthermore, optical data transmission at high data rates requires error detection and correction upon reception. [Prior art documents] [Non-patent literature]

[0006] [Non-Patent Document 1] 1. S. Tancock, E. Arabul and N. Dahnoun, "A Review of New Time-to-Digital Conversion Techniques," in IEEE Transactions on Instrumentation and Measurement, vol. 68, no. 10, pp. 3406-3417, Oct. 2019. Summary of the Invention [Problem to be solved by the invention]

[0007] However, a problem with multi-level signals is that the number of levels is limited by the available energy per baud.

[0008] Optical switching also makes it possible to avoid packet collisions, but requires clock recovery of input data in the optical link and synchronization between connected units.

[0009] Furthermore, data transmission at high data rates in computer architectures is problematic because error detection and correction takes time, increasing latency. [Means for solving the problem]

[0010] In order to solve the above-mentioned problems, the apparatus for generating a modulated optical signal according to the present invention is an apparatus for generating a multilevel modulated optical signal transmitted and received between a transmitter and a receiver, and comprises an input branching section, an output multiplexing section, and one waveguide and the other waveguide connecting the input branching section and the output multiplexing section, wherein an input optical pulse is branched at the input branching section, one optical pulse propagates through one waveguide, and the other optical pulse propagates through the other waveguide and is delayed by an electrical modulation signal applied to the other waveguide, wherein the one optical pulse and the other optical pulse are combined at the output multiplexing section, the modulated optical signal has the one optical pulse and the other optical pulse for each transmission baud, input data is mapped to the time difference between the one optical pulse and the other optical pulse caused by the delay, and the modulation format has a different time difference for each transmission baud.

[0011] Furthermore, a transmission module according to the present invention is a transmission module for a multilevel modulated optical signal transmitted and received between a transmitter and a receiver, and comprises two light sources that generate optical pulses, an electric pulse generator that generates an electric pulse and outputs the electric pulse to one of the two light sources, an electric pulse delay unit that delays the electric pulse input from the electric pulse generator and outputs the delayed electric pulse to the other light source, and a passive coupler that couples one optical pulse output from one light source with the other optical pulse output from the other light source, wherein the modulated optical signal has one optical pulse and the other optical pulse for each transmission baud, input data is mapped to the time difference between the one optical pulse and the other optical pulse, and the time difference has a modulation format that differs for each transmission baud. [Effects of the Invention]

[0012] According to the present invention, it is possible to provide a modulated optical signal generator and a transmission module that can increase the number of symbols and reduce power consumption. Furthermore, according to the present invention, synchronization can be easily achieved and latency can be reduced. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 1 is a schematic diagram showing a modulation format in a modulated optical signal according to a first embodiment of the present invention. [Figure 2] FIG. 2 is a diagram for explaining a method for generating a modulation format in a modulated optical signal according to the first embodiment of the present invention. [Figure 3A] FIG. 3A is a diagram for explaining the effect of the modulated optical signal according to the first embodiment of the present invention. [Figure 3B] FIG. 3B is a diagram for explaining the effect of the modulated optical signal according to the first embodiment of the present invention. [Figure 4A] FIG. 4A is a diagram for explaining the effect of the modulated optical signal according to the first embodiment of the present invention. [Figure 4B] FIG. 4B is a diagram for explaining the effect of the modulated optical signal according to the first embodiment of the present invention. [Figure 5A] FIG. 5A is a diagram for explaining a conventional method for generating a modulated optical signal. [Figure 5B] FIG. 5B is a diagram for explaining a conventional method for generating a modulated optical signal. [Figure 6] FIG. 6 is a block diagram showing the configuration of a transmission module according to the first embodiment of the present invention. [Figure 7] FIG. 7 is a block diagram showing the configuration of a transmission module according to the second embodiment of the present invention. [Figure 8] FIG. 8 is a block diagram showing the configuration of a transmission module according to the third embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0014] First Embodiment A first embodiment of the present invention will be described with reference to FIGS. 1 to 4B.

[0015] <Configuration of modulated optical signal> The modulated optical signal according to this embodiment has the optical modulation format shown in Fig. 1. In the optical modulation format, baud (symbol) duration T b Each transmitted baud contains two ultrashort light pulses 1 and 2.

[0016] The timing of one pulse (first pulse) 1 is used as a time reference. Hereafter, this pulse will be referred to as the "reference pulse." The start of reference pulse 1 is a clock pulse that is positioned and fixed at the edge of the transmit baud.

[0017] The timing of the other pulse (second pulse) 2 is delayed relative to the reference pulse 1 based on the value of the input data.

[0018] The delay time Δt consists of multiple (m) steps with time interval ΔT, and the initial inter-pulse time (T opt ) then mΔT+T opt It is expressed as:

[0019] Here, the value of the input data (modulation data) is a symbol to be transmitted and is mapped into the time difference Δt between the two pulses.

[0020] As described above, the modulated optical signal according to this embodiment has two optical pulses for each transmission baud, and has a modulation format in which input data is mapped to the time difference between the two optical pulses, where the time difference between the two optical pulses differs for each transmission baud.

[0021] <Optical modulation format generation method> A method for generating an optical modulation format for a modulated optical signal according to this embodiment will be described below.

[0022] The arrangement of data pulses for different transmission symbols is shown in Figure 2. Here, for example, assume optical pulses with a duration of 100 ps baud, which corresponds to a baud rate of 10 GHz, and equally spaced 10 ps apart.

[0023] Initially, for the first symbol, which corresponds to the lowest value of the input, zero, only a reference pulse 1 is required, ie no data pulse is used (step S1).

[0024] Next (the next step), the initial inter-pulse time (T opt ) is introduced to separate the reference pulse 1 from the data pulse (corresponding to the input value "1") 2 of the second symbol (step S2). opt is set to 10 ps, data pulse 2 of the second symbol starts 20 ps from the start of reference pulse 1.

[0025] Next (next step), for a step of 2 ps chosen as the time difference between each two consecutive symbols, data pulse 3 of the third symbol (corresponding to input value "2") is positioned 22 ps from reference pulse 1 (step S3).

[0026] The above steps are repeated to place a data pulse for each transmitted symbol.

[0027] Finally, the data pulse N of the last symbol (corresponding to the input data of the highest value) is placed (step SN). At this time, this symbol and the duration T b The time between the end of grd )

[0028] Guard Time T grd is the time for the transmitter to prepare for the transmission of the next baud and for the receiver to avoid collisions in recognizing the reference pulse 1 of the next input baud. For example, T int and 20ps T grd For a 2 ps step in the symbol space, the number of symbols accommodated in 1 baud is 23 (including zero), which corresponds to 4.52 bits per baud.

[0029] Furthermore, the initial inter-pulse time T opt , reducing (or eliminating) the time step separating adjacent symbols, or the guard time T grd The reduction in the number of symbols allows for an increase in the number of symbols.

[0030] Here, the increase in the number of symbols depends on the time resolution between optical pulses, and does not depend on the power (intensity) of the transmitted optical pulses.

[0031] One example of a conventional receiving method is the use of a time-to-digital converter (TDC) (Non-Patent Document 1). A TDC is an electronic circuit used to detect the time difference between two input pulses. While a TDC has a detection resolution of 0.5 ps, it is limited to operating at low receiving speeds.

[0032] Additionally, data transmitted at high data rates is subject to errors detected at the receiver, some of which are caused by timing problems such as clock imperfections.

[0033] On the other hand, the modulation scheme in this embodiment allows for self-synchronized reception of transmitted data.

[0034] In self-synchronizing operation, each transmit baud has a clock pulse (reference pulse) 1 and a data pulse (delayed pulse 2, etc.). The clock pulse provides the receiver with the start time of the incoming baud. This ensures that the incoming baud begins with reference pulse 1, which acts as the clock pulse at the receiver. Subsequently, the data pulses are received within the expected time without the need for a CDR or external synchronization.

[0035] Thus, in self-synchronizing operation, the receiver can continuously monitor the timing and order of every incoming data pulse for any duration of an incoming burst, with the start times of the incoming bauds provided by the clock pulses.

[0036] This makes the modulation scheme self-synchronizing reception of transmitted data and therefore immune to timing problems.

[0037] Furthermore, this modulation scheme offers a new degree of freedom to reduce reception errors beyond the usual parameter of received power: for any given implementation, different timing parameters (duration, pulse width, guard time, separation step between successive symbols) can be adjusted to minimize reception errors.

[0038] This allows optical signals to be transmitted at high bit rates with low FEC, which requires short processing times, and reduces the overhead latency of optical links.

[0039] The ultrashort optical pulse in this embodiment will be described below with reference to FIGS. 3A and 3B.

[0040] The optical waveguides over which data is transmitted using this modulation scheme must not distort the waveform of the ultrashort optical pulses used in the modulation format, a requirement that is met by short-distance optical links of a few meters to interconnect computing resources.

[0041] For short optical pulses propagating through optical fibers, the group delay dispersion of the quartz material (-22 fs at a wavelength of 1500 nm) 2 / mm) causes optical dispersion. The pulse width broadening from τ0 (before propagation) to τ (after propagation) is approximated by equation (1).

[0042]

number

[0043] Here, D2 is a dispersion parameter and is expressed by equation (2).

[0044]

number

[0045] where λ is the wavelength, k is the wave number, and ω is the angular frequency.

[0046] At a wavelength of 1550 nm, the dispersion parameters of the standard fiber and the dispersion-shifted fiber are 18.4 ps / (nm km) and 8.9 ps / (nm km), respectively.

[0047] Figures 3A and 3B show the calculation results for the relationship between pulse width before and after propagation in an optical fiber. Figure 3A is for a standard fiber, and Figure 3B is for a dispersion-shifted fiber. The wavelength of the propagating light was set to 1550 nm, and the lengths of the respective fibers were calculated as 1 m (solid line), 10 m (dotted line), and 100 m (dashed line).

[0048] As shown in Figures 3A and 3B, for standard fiber and dispersion-shifted fiber, when the input pulse width (before propagation) is 1 ps at lengths of 1 m to 10 m, the pulse width after propagation is approximately 1 ps. Here, when the input pulse width (before propagation) is approximately 0.7 ps to 1.7 ps, the pulse width after propagation increases or decreases by 50%. Thus, to suppress the increase in pulse width after propagation, an input pulse width (before propagation) of approximately 0.7 ps to 1.7 ps is desirable.

[0049] Therefore, the input pulse width (before propagation) used in this embodiment is set to about 1 ps, and may also be about 0.7 ps to 1.7 ps.

[0050] In this manner, in this embodiment, the pulse widths of the reference pulse and the data pulse are maintained without widening until they reach the receiving side by using an optical waveguide structure such as an optical fiber.

[0051] To facilitate identification on the receiving side, the reference pulse and data pulse of the same baud may be generated with different characteristics. For example, the two pulses may be generated with orthogonal polarizations, or may have different wavelengths. Alternatively, the reference pulse may be generated with a sufficiently higher optical energy than the data pulse.

[0052] The application of this embodiment to burst mode transmission of modulated optical signals will now be described with reference to FIGS. 4A and 4B.

[0053] 4A and 4B show clock data recovery for continuous and burst mode transmissions, respectively.

[0054] In continuous transmission, the link is point-to-point, the connection is limited to a single hop, and is implemented using electrical switching. In this case, a clock signal embedded in the incoming data is monitored at the receiving end using a Phase Locked Loop (PLL).

[0055] Here, as shown in FIG. 4A, when packet 11_1 is transmitted followed by packet 11_2, labels 12_1 and 12_2 are set to maintain the clock mechanism, and dummy data 13_1 and 13_2 are transmitted to fill the time interval between packet 11_1 and packet 11_2 where no actual data is transmitted.

[0056] When using optical switch 15, burst mode reception is required due to multi-hop transmission, as shown in Figure 4B, where the transmitted data propagates over different paths, resulting in time intervals on multiple links (physical parts) that are not occupied by any data transmission.

[0057] To address this issue, conventional methods perform clock data recovery (CDR) from the incoming burst-mode data. However, as shown in Figure 4B, when clock recovery is performed using preamble bits 14_1 and 14_2, overhead data is required, which reduces link utilization efficiency.

[0058] The modulation scheme of this embodiment is directly compatible with burst mode reception since each transmitted bit is self-synchronized.

[0059] <Effects> In the modulated optical signal according to this embodiment, modulation data is mapped to the time difference between two optical pulses, rather than to the intensity change of a single transmitted pulse (normal intensity modulation).

[0060] For example, with a binary transmitted signal, the receiver used in the optical link requires high receiver sensitivity to accurately distinguish high level input signals from low level (zero level) signals.

[0061] In the case of normal multi-level intensity modulation, the transmission power increases by an amount equivalent to the receiving sensitivity for each increase in the number of symbols per baud in the transmission pulse.

[0062] In contrast, the modulated optical signal according to the present invention uses a modulation scheme based on time differences, so that the transmitted data is mapped to time differences rather than power variations, and as a result, increasing the number of symbols per baud does not increase the transmitted power.

[0063] As described above, the modulated optical signal according to this embodiment allows the number of symbols to be increased without being limited by the output level of the transmission signal, and also reduces power consumption in transmitting and receiving signals.

[0064] Furthermore, in the modulated optical signal according to this embodiment, the time difference between short optical pulses is short.

[0065] The total transmission bit rate is the product of the baud rate and the number of symbols per baud, so for a high transmission bit rate, a high baud rate, i.e., a short baud duration, is necessary.

[0066] The modulated optical signal according to this embodiment has a short duration, which allows an increase in the number of symbols and an increase in bit rate. Furthermore, unlike electrical pulses, short optical pulses can propagate over long distances without degradation.

[0067] Furthermore, the modulated optical signal according to this embodiment eliminates the need for clock recovery of input data and synchronization between connected units, and also reduces latency.

[0068] <First Example> A transmission module according to a first embodiment of the present invention will be described with reference to FIGS. 5A to 6. FIG.

[0069] First, a light source that generates an optical signal will be described.

[0070] The modulation scheme in this embodiment requires short optical pulses. As shown in Figure 5A, in a method of generating an optical waveform by modulating a continuous optical signal from a conventional CW (Continuous Wave) laser 21 with a modulator 22, it is difficult to generate the desired short optical pulses (10 ps or less) due to limitations in the modulation bandwidth.

[0071] 5B, in the periodic pulse generator 23, short optical pulses can be generated by transmitting or blocking the optical output using optical pulse waveform shaping and an optical gate 24. In the figure, optical pulses 25 and 26 represent the outputs in the on and off states, respectively.

[0072] For example, a mode-locked laser (MLL) is a periodic light source. Monolithically integrated MLLs can realize compact transmitter modules for new modulation schemes. Monolithically integrated MLLs have the following advantages (Michael L. Davenport, Songtao Liu, and John E. Bowers, "Integrated heterogeneous silicon / III-V mode-locked lasers," Photon. Res. 6, 468-478 (2018)).

[0073] 1. The period of the MLL generating the optical pulses is chosen to match the duration of the transmitted baud.

[0074] 2. Optical pulses with a sufficiently short bandwidth (about 10 ps) are generated.

[0075] 3. Light pulses of sufficient light energy are available and can be increased by special design.

[0076] 4. MLL can be implemented on silicon.

[0077] FIG. 6 shows an example of the transmission module 30 according to this embodiment.

[0078] The transmission module 30 includes a light source (for example, an MLL) 31, a pulse control unit 32, and an optical pulse (modulated optical signal) generating device 33.

[0079] The optical pulse generating device 33 includes an input branching section 333 , two waveguides 331 and 332 branched by the input branching section 333 , and an output multiplexing section 334 .

[0080] Here, one of the two waveguides 331 and 332, the waveguide 332, is a passive optical waveguide, and the other waveguide 331 has an electro-optic modulation structure.

[0081] An optical pulse 1 (S31 in the figure) output from a light source 41 is input to an input branching unit 333 of an optical pulse generating device 33, and is branched into two waveguides 331 and 332. Here, the optical pulse 1 is divided into two waveguides having a duration T b is a short optical pulse having the following structure:

[0082] In a predetermined region (modulation region) 335 of the other waveguide 331, the effective refractive index changes based on the modulation electrical signal from pulse control section 32. As a result, the optical pulse propagating through modulation region 335 is delayed.

[0083] At this time, the delay time of the optical pulse is changed by changing the amplitude voltage of the modulated electrical signal.

[0084] For example, an amplitude voltage V1 is applied to delay the light pulse by ΔT1, and an amplitude voltage V2 is applied to delay the light pulse by ΔT2.

[0085] In this way, delayed optical pulses 2 and 3 are generated (S32 in the figure).

[0086] In one waveguide 332, reference pulse 1 propagates (S33 in the figure).

[0087] The reference pulse and the delayed optical pulse are combined in the output combining section (passive coupler) 334, and a modulation format consisting of two ultrashort optical pulses is generated (S34 in the figure). For example, a modulation format consisting of reference pulse 1 and delayed optical pulse 2, and a modulation format consisting of reference pulse 1 and delayed optical pulse 3 are generated.

[0088] In this embodiment, the other waveguide 331 has a guard time (T grd ) and the optical pulse is propagated under the changed refractive index. After the optical pulse is propagated, a new guard time (T grd ) and change the refractive index between them. These steps are repeated to generate sequential light pulses with different time intervals from the reference pulse.

[0089] If the refractive index then changes while the optical pulse propagates through the modulation region 335 in the other waveguide 331, different parts of the optical pulse will be subjected to different values of refractive index and the pulse will be deformed.

[0090] Therefore, in order to propagate and delay the optical pulse through the other waveguide 331 without deforming it, it is desirable to complete the change in refractive index within the guard time, i.e., it is desirable that the time required for the refractive index change be equal to or shorter than the guard time.

[0091] The transmitter module of this embodiment can generate a modulation format consisting of two ultrashort optical pulses.

[0092] <Second Example> A transmission module according to a second embodiment of the present invention will be described with reference to FIG.

[0093] In the first embodiment, as described above, it is desirable to set the modulation rate so that the change time of the refractive index is equal to or shorter than the guard time. Therefore, the modulation rate of the electro-optic modulation structure (the other waveguide) used in the transmitter module and the selected T grd This requires a trade-off with the duration of the

[0094] The transmission module 40 of this embodiment includes a light source (e.g., MLL) 41, an electrical control unit 421, a pulse control unit 422, an optical splitter 45, two optical gates 461, 462, two optical pulse generating devices 43, 44, and an optical multiplexer 47.

[0095] The two optical gates 461 and 462 are connected to two optical pulse generators 43 and 44, respectively.

[0096] The two optical pulse generators 43 and 44 have the same configuration as in the first embodiment, and include passive optical waveguides (one waveguide) 432 and 442 and waveguides (the other waveguide) 431 and 441 of an electro-optic modulation structure.

[0097] The outputs of the two optical pulse generators 43 and 44 are connected to an optical multiplexer 47 .

[0098] Of the two optical pulse generators 43 and 44, the optical pulse generator 43 generates optical pulses for even bauds, and the other optical pulse generator 44 generates optical pulses for odd bauds.

[0099] Optical gates 461 and 462, which are placed in front of the two optical pulse generators 43 and 44, respectively, are controlled by an electrical control unit 421 and block (intercept) optical pulses from the light source 41 that are input to the optical pulse generator that is not operating.

[0100] In this configuration, optical pulses are generated by two optical pulse generators 43 and 44, which increases the time required to generate a pulse per optical pulse generator, and therefore there is no need to perform modulation in one optical pulse generator at a higher speed than in the first embodiment.

[0101] In this way, by interleaving the generation of the optical signal between the two optical pulse generators 43, 44, the modulation rate of the modulator and the selected T grd This can mitigate the trade-off with the duration of the

[0102] <Third Example> A transmission module according to a third embodiment of the present invention will be described with reference to FIG.

[0103] As shown in FIG. 8, the transmission module 50 of this embodiment includes two light sources (e.g., MLLs) 511 and 512, a pulse control unit (electrical domain) 52 having an electrical pulse generator 521 and an electrical pulse delay unit 522, and a passive coupler 53.

[0104] Of the two light sources 511 and 512, one light source 512 is controlled by an electric pulse generator 521 to generate a reference pulse.

[0105] Furthermore, the electrical pulse generated by the electrical pulse generator 521 is delayed in accordance with the transmission symbol by the electrical pulse delay unit 522. The delayed electrical pulse is used to generate an optical pulse from the other light source 511.

[0106] A reference pulse from one light source 512 and a delayed light pulse from another light source 511 are combined in a passive coupler 53 to generate two ultrashort light pulses.

[0107] The transmission module according to this embodiment can easily generate two ultrashort optical pulses.

[0108] In the embodiments of the present invention, examples of the structure, dimensions, materials, etc. of each component in the configuration of the modulated optical signal generator and transmission module are shown, but the present invention is not limited to these examples. Anything that can demonstrate the functions and effects of the modulated optical signal generator and transmission module may be used. [Industrial Applicability]

[0109] The present invention can be applied to optical data transmission and reception systems and computers. [Explanation of symbols]

[0110] 33 Optical pulse (modulated optical signal) generator 331 The other waveguide 332 One of the waveguides 333 Input branch 334 Output multiplexer

Claims

1. A device for generating a multi-level modulated optical signal transmitted and received between a transmitter and a receiver, comprising: an input branch; an output multiplexing unit; one waveguide and another waveguide connecting the input branching section and the output multiplexing section; Equipped with The input branching unit branches the input optical pulse, One optical pulse propagates through one waveguide, the other optical pulse propagates through the other waveguide and is delayed by an electrical modulation signal applied to the other waveguide; the one optical pulse and the other optical pulse are combined in the output multiplexing section, The modulated optical signal has the one optical pulse and the other optical pulse for each transmission baud, input data is mapped to a time difference between the one optical pulse and the other optical pulse caused by the delay, and the time difference has a modulation format that differs for each transmission baud.

1. A device for generating a modulated optical signal.

2. The one light pulse has different characteristics from the other light pulse.

2. The apparatus for generating a modulated optical signal according to claim 1.

3. The pulse width of the one optical pulse and the other optical pulse is 0.7 ps or more and 1.7 ps or less.

2. The apparatus for generating a modulated optical signal according to claim 1.

4. the one optical pulse is a clock pulse, the other optical pulse is a data pulse, The one light pulse provides the receiver with the start time of the transmit baud.

2. The apparatus for generating a modulated optical signal according to claim 1.

5. The apparatus for generating a modulated optical signal according to claim 1; a light source that generates the input light pulse; a pulse control unit that generates the electrical modulation signal; A transmitting module comprising:

6. a light source that generates light pulses; two optical gates connected in parallel to the light source; two modulated optical signal generating devices according to claim 1 connected to the two optical gates respectively; an optical multiplexer to which the outputs of the two modulated optical signal generators are connected; a pulse control unit that generates the electrical modulation signal; an electrical control unit connected to the optical gate and the pulse control unit; Equipped with One of the two modulated optical signal generators generates optical pulses for even bauds; The other modulated optical signal generator generates optical pulses for odd bauds. A transmission module comprising:

7. A transmission module for a multi-level modulated optical signal transmitted and received between a transmitter and a receiver, comprising: Two light sources that generate light pulses; an electric pulse generator that generates an electric pulse and outputs the electric pulse to one of the two light sources; an electric pulse delay unit that delays the electric pulse input from the electric pulse generator and outputs the delayed electric pulse to the other light source; a passive coupler that couples one optical pulse output from the one light source with the other optical pulse output from the other light source; Equipped with the modulated optical signal has one optical pulse and the other optical pulse for each transmission baud, input data is mapped to the time difference between the one optical pulse and the other optical pulse, and the time difference has a modulation format that differs for each transmission baud A transmission module comprising:

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