WO2024038543A1

WO2024038543A1 - Optically modulated signal generation device and transmission module

Info

Publication number: WO2024038543A1
Application number: PCT/JP2022/031222
Authority: WO
Inventors: Ibrahim Salah; Toshikazu Hashimoto; Takeshi Sakamoto
Original assignee: Nippon Telegraph And Telephone Corporation
Priority date: 2022-08-18
Filing date: 2022-08-18
Publication date: 2024-02-22

Abstract

A generation device according to this invention for a multilevel optically modulated signal transmitted/received between a transmitter and a receiver includes an input branch, an output multiplexer, and one waveguide and another waveguide that connect the input branch and the output multiplexer, wherein the input branch branches an input optical pulse, one optical pulse propagates through the one waveguide, the other optical pulse propagates through the other waveguide and is delayed by an electrical modulated signal applied to the other waveguide, the output multiplexer multiplexes the one optical pulse and the other optical pulse, and the optically modulated signal has a modulation format in which the optically modulated signal has the one optical pulse and the other optical pulse for each transmission baud, input data is mapped in a time difference between the one optical pulse and the other optical pulse that is generated by the delay, and the time difference is different for each transmission baud.

Description

OPTICALLY MODULATED SIGNAL GENERATION DEVICE AND TRANSMISSION MODULE

The present invention relates to an optically modulated signal generation device and transmission module used for transmission/reception of a multilevel optical signal.

Recently, the data amount is increasing in computer processing. To improve the processing capability of computers, a new computer architecture is attracting attention.

For higher performance of the computer architecture, data transmission with high bandwidth and low latency is necessary. Transmission data on an optical link can be transmitted at a high data rate, reducing the transmission energy per bit.

On the optical link, the bit rate per lane can be increased by adapting a large amount of symbols to the baud duration using a multilevel signal.

The number of packets increases due to an increase in data flow size. As a result, the probability of packet collisions rises. Further, in optical data transmission at a high data rate, an error needs to be detected and corrected at the time of reception.

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.

However, when a multilevel signal is used, the number of multilevels is restricted by available energy per baud.

Collision between packets can be avoided by optical switching, but clock recovery of input data on the optical link and synchronization between connection units are necessary.

Further, in data transmission at a high data rate in the computer architecture, error detection and correction take time, increasing the latency.

To solve the above-described problems, according to the present invention, there is provided a generation device for a multilevel optically modulated signal transmitted/received between a transmitter and a receiver, comprising an input branch, an output multiplexer, and one waveguide and another waveguide that connect the input branch and the output multiplexer, wherein the input branch branches an input optical pulse, one optical pulse propagates through the one waveguide, the other optical pulse propagates through the other waveguide and is delayed by an electrical modulated signal applied to the other waveguide, the output multiplexer multiplexes the one optical pulse and the other optical pulse, and the optically modulated signal has a modulation format in which the optically modulated signal has the one optical pulse and the other optical pulse for each transmission baud, input data is mapped in a time difference between the one optical pulse and the other optical pulse that is generated by the delay, and the time difference is different for each transmission baud.

According to the present invention, there is provided a transmission module for a multilevel optically modulated signal transmitted/received between a transmitter and a receiver, comprising two light sources configured to generate an optical pulse, an electrical pulse generator configured to generate an electrical pulse and output the electrical pulse to the one light source out of the two light sources, an electrical pulse delay unit configured to delay the electrical pulse input from the electrical pulse generator and output the electrical pulse to the other light source, and a passive coupler configured to couple one optical pulse output from the one light source and the other optical pulse output from the other light source, wherein the optically modulated signal has a modulation format in which the optically modulated signal has the one optical pulse and the other optical pulse for each transmission baud, input data is mapped in a time difference between the one optical pulse and the other optical pulse, and the time difference is different for each transmission baud.

The present invention can provide an optically modulated signal generation device and transmission module capable of increasing the number of symbols and reducing the power consumption. According to the present invention, synchronization can be easily established and the latency can be reduced.

Fig. 1 is a chart showing a modulation format in an optically modulated signal according to the first embodiment of the present invention; Fig. 2 shows charts for explaining a method of generating the modulation format in the optically modulated signal according to the first embodiment of the present invention; Fig. 3A is a graph for explaining the effects of the optically modulated signal according to the first embodiment of the present invention; Fig. 3B is a graph for explaining the effects of the optically modulated signal according to the first embodiment of the present invention; Fig. 4A is a chart for explaining the effects of the optically modulated signal according to the first embodiment of the present invention; Fig. 4B is a chart for explaining the effects of the optically modulated signal according to the first embodiment of the present invention; Fig. 5A is a chart for explaining a conventional optically modulated signal generation method; Fig. 5B is a chart for explaining the conventional optically modulated signal generation method; Fig. 6 is a block diagram showing the arrangement of a transmission module according to the Example 1 of the present invention; Fig. 7 is a block diagram showing the arrangement of a transmission module according to the Example 2 of the present invention; and Fig. 8 is a block diagram showing the arrangement of a transmission module according to the Example 3 of the present invention.

(First Embodiment)
The first embodiment according to the present invention will be described with reference to Figs. 1 to 4B.

(Format of Optically modulated signal)
An optically modulated signal according to this embodiment has an optical modulation format shown in Fig. 1. The optical modulation format has ultrashort

optical pulses

1 and 2 with two transmission bauds in a baud (symbol) duration T_b.

The timing of one pulse (first pulse) 1 is used as the time base. This pulse will be referred to as a "reference pulse". The reference pulse 1 is a clock pulse whose start is set and fixed at the edge of a transmission baud.

The timing of the other pulse (second pulse) 2 delays compared to the reference pulse 1 based on the value of input data.

The optically modulated signal according to this embodiment has a modulation format in which the optically modulated signal has two optical pulses for each transmission baud and input data is mapped in the time difference between the two optical pulses. The time difference between two optical pulses is different for each transmission baud.

(Optical Modulation Format Generation Method)
A method of generating an optical modulation format in an optically modulated signal according to this embodiment will be described below.

Fig. 2 shows the arrangement of data pulses with respect to different transmission symbols. For example, a 100-ps baud duration equivalent to a 10-GHz baud rate and optical pulses at 10-ps equal intervals are assumed.

First, only the reference pulse 1 is necessary for an initial symbol equivalent to a minimum value "0 (zero)" of an input, that is, no data pulse is used (step S1).

Next, an initial pulse interval (T_opt) is introduced to separate the reference pulse 1 and the data pulse (equivalent to an input value "1") 2 of the second symbol (step S2). When T_opt is set to be 10 ps, the data pulse of the second symbol starts at 20 ps after the start of the reference pulse 1.

Next, a data pulse 3 of the third symbol (equivalent to an input value "2") is arranged at 22 ps after the reference pulse 1 with respect to a 2-ps step selected as the time difference between two successive symbols (step S3).

The above-described steps are repeated to arrange data pulses for respective transmission symbols.

Finally, a data pulse N of the final symbol (equivalent to input data of a maximum value) is arranged (step SN). At this time, the time between this symbol and the termination of the baud duration T_b is a guard time (T_grd).

The guard time T_grd is a time for preparing for transmission of the next baud in a transmitter, and is a time for avoiding collision in recognition of the reference pulse 1 of the next input baud in a receiver. For example, the number of symbols accommodated in one baud is 23 symbols (including 0) for a step of 10-ps T_int, 20-ps T_grd, and 2-ps symbol space, and is equivalent to 4.52 bits per baud.

The number of symbols can be increased by reducing (or eliminating) the initial pulse interval T_opt, reducing the time step for separating adjacent symbols, or reducing the guard time T_grd.

The increase in the number of symbols depends on the time resolution between optical pulses, and not on the power (intensity) of a transmission optical pulse.

A conventional reception method uses, for example, a time to digital converter (TDC) (non-patent literature 1). The TDC is an electronic circuit and is used to detect a time difference between two input pulses. The TDC has a 0.5-ps detection resolution, but is restricted to an operation at a low reception rate.

Data transmitted at a high data rate is influenced by an error detected in reception. Part of the error arises from the timing problem such as imperfection of a clock.

To the contrary, the modulation scheme in this embodiment enables self-synchronous reception of transmission data.

In the self-synchronous operation, each transmission baud has the clock pulse (reference pulse) 1 and a data pulse (for example, delayed pulse 2). The clock pulse supplies the start time of an input baud to a receiver. In response to this, the input baud starts at the reference pulse 1 functioning as a clock pulse on the receiving side. Subsequently, a data pulse is received in an expected time without requiring the CDR or external synchronization.

In the self-synchronous operation, the receiver can continuously monitor the timing and order of each input data pulse for an input burst of any duration by using the start time of an input baud provided by a clock pulse.

The modulation scheme enables self-synchronous reception of transmission data and is not influenced by the problem regarding the timing.

Further, the modulation scheme provides a new degree of freedom for reducing a reception error, in addition to a normal parameter of reception power. Different timing parameters (baud duration, pulse width, guard time, separation step of successive symbols) are adjusted to minimize a reception error with respect to an arbitrary implementation.

An optical signal can be transmitted at a high bit rate in low FEC requiring a short processing time, and the overhead latency of an optical link can be reduced.

An ultrashort optical pulse in this embodiment will be described with reference to Figs. 3A and 3B.

It is necessary for an optical waveguide through which data is transmitted using this modulation scheme, not to deform (distort) an ultrashort optical pulse used in the modulation format. This condition is satisfied by an optical link with a short distance of several m in interconnection with a computing resource.

where D₂ is the dispersion parameter given by equation (2):

The dispersion parameters of a standard fiber and dispersion-shifted fiber at a wavelength of 1,550 nm are 18.4 ps/(nm km) and 8.9 ps/(nm km), respectively.

Figs. 3A and 3B show the calculation results of the relationship of the pulse width before and after propagation in the optical fiber. Fig. 3A shows the calculation result in the case of the standard fiber, and Fig. 3B shows the calculation result in the case of the dispersion-shifted fiber. The calculation was made on the assumption that the wavelength of propagating light was 1,550 nm and each fiber had lengths of 1 m (solid line), 10 m (dotted line), and 100 m (broken line).

As shown in Figs. 3A and 3B, the pulse width after propagation is about 1 ps at a length of 1 m to 10 m and an input pulse width (before propagation) of 1 ps in the standard fiber and the dispersion-shifted fiber. When the input pulse width (before propagation) is about 0.7 ps to 1.7 ps, the pulse width after propagation increases/decreases by 50%. To suppress the increase in pulse width after propagation, an input pulse width (before propagation) of about 0.7 ps to 1.7 ps is desirable.

From this, an input pulse width (before propagation) used in this embodiment is set to be about 1 ps. The input pulse width may be about 0.7 ps to 1.7 ps.

In this embodiment, an optical waveguide structure such as an optical fiber is used to maintain the pulse widths of the reference pulse and data pulse without spreading toward the receiving side.

To facilitate identification on the receiving side, a reference pulse and a data pulse in the same baud may be generated with different characteristics. For example, two pulses may be generated by orthogonal polarization. Alternatively, two pulses may have different wavelengths. Alternatively, a reference pulse may also be processed and generated with sufficiently high light energy, compared to a data pulse and the like.

Application of an optically modulated signal to burst mode transmission in this embodiment will be explained with reference to Figs. 4A and 4B.

Figs. 4A and 4B show states of recovery of clock data in continuous transmission and burst mode transmission, respectively.

In continuous transmission, a link is a connection between two points, and the connection is restricted by a single hop. The continuous transmission is performed with an electrical switch. In this case, a clock signal embedded in input data is confirmed (monitored) on the receiving side by using a phase locked loop (PLL).

As shown in Fig. 4A, when a packet 11_1 is transmitted and subsequently a packet 11_2 is transmitted, labels 12_1, 12_2 are set to maintain the clock mechanism, and dummy data 13_1, 13_2 are transmitted while filling a time interval between the packets 11_1, 11_2 at which there is no actual data to be transmitted.

With an optical switch 15, as shown in Fig. 4B, burst mode reception is necessary because multihop transmission is performed. In this case, transmission data propagates through a different path. As a result, a time interval not occupied by any data transmission is generated in a plurality of links (physical parts).

To cope with this, clock data recover (CDR) is executed from input burst mode data in the conventional method. However, for example, when clock recover is executed using a preamble bit 14, as shown in Fig. 4B, the data amount of an overhead is necessary, decreasing the use efficiency of the link.

The modulation scheme in this embodiment can directly cope with burst mode reception because respective transmission bits are self-synchronized.

(Effects)
In an optically modulated signal according to this embodiment, modulated data is mapped not in the intensity change (normal intensity modulation) of a single transmission pulse, but in the time difference between two optical pulses.

For example, for a binary transmission signal, a receiver used in an optical link requires a high reception sensitivity to accurately identify a high-level input signal from a low-level (zero-level) signal.

In the case of normal multilevel intensity modulation, transmission power increases by power equal to the reception sensitivity for every number of symbols increased per baud in a transmission pulse.

In contrast, an optically modulated signal according to this embodiment uses the modulation scheme based on the time difference, so transmission data is mapped not in the power change but in the time difference. Even if the number of symbols per baud is increased, no transmission power increases.

The optically modulated signal according to this embodiment can increase the number of symbols without any restriction by the output level of a transmission signal. Also, the optically modulated signal according to this embodiment can reduce power consumption in transmission/reception of a signal.

In the optically modulated signal according to this embodiment, the time difference between short optical pulses is short.

The total transmission bit rate is the product of the baud rate and the number of symbols per baud. For a high transmission bit rate, a high baud rate, that is, a short baud duration is necessary.

The optically modulated signal according to this embodiment can increase the number of symbols because of a short baud duration, and increase the bit rate. Unlike an electrical pulse, a short optical pulse propagates through a long distance without deterioration.

The optically modulated signal according to this embodiment does not require clock recovery of input data and synchronization between connection units. The optically modulated signal according to this embodiment can reduce the latency.

(Example 1)
A transmission module according to the Example 1 of the present invention will be described with reference to Figs. 5A to 6.

First, a light source that generates an optical signal will be explained.

A modulation scheme according to Example 1 requires a short optical pulse. As shown in Fig. 5A, it is difficult to generate a desired short optical pulse (10 ps or less) due to the limitation of the modulation bandwidth in a method of modulating, by a modulator 22, a continuous optical signal by a normal CW laser 21 to generate a light waveform.

To the contrary, as shown in Fig. 5B, a source 23 of periodic short optical pulses can generate a short optical pulse by transmitting/cutting off an optical output by optical pulse waveform shaping and an optical gate 24.

Optical pulses

25, 26 in Fig. 5B show outputs of ON state and OFF state, respectively.

An example of the periodic light source is a mode locked laser (MLL). A monolithic integrated MML can implement a compact transmission module for a new modulation scheme. The monolithic integrated MML is excellent in the following points (Michael L. Davenport, Songtao Liu, and John E. Bowers, "Integrated heterogeneous silicon/III-V mode-locked lasers," Photon. Res. 6, 468 - 478 (2018)):

1. The period of the MML that generates an optical pulse is so selected as to coincide with the duration of a transmission baud.

2. An optical pulse of a sufficiently short bandwidth (about 10 ps) is generated.

3. An optical pulse of sufficient optical energy can be used and increased by a special design.

4. The MLL can be mounted on silicon.

Fig. 6 shows an example of a transmission module 30 according to Example 1.

The transmission module 30 includes a light source (for example, MLL) 31, a pulse controller 32, and an optical pulse (optically modulated signal) generation device 33.

The optical pulse generation device 33 includes an input branch 333, two

waveguides

331 and 332 branched by the input branch 333, and an output multiplexer 334.

Of the two

waveguides

331 and 332, one waveguide 332 is a passive optical waveguide. The other waveguide 331 has an electrooptical modulation structure.

An optical pulse 1 (S31 in Fig. 6) output from the light source 31 enters the input branch 333 of the optical pulse generation device 33, and is branched to the two

waveguides

331 and 332. The optical pulse 1 is a short optical pulse having a baud duration T_b.

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

At this time, the delay time of the optical pulse is changed by a change of the amplitude voltage of the modulated electrical signal.

In this manner, delayed

optical pulses

2 and 3 are generated (S32 in Fig. 6).

The reference pulse 1 propagates through one waveguide 332 (S33 in Fig. 6).

The output multiplexer (passive coupler) 334 multiplexes the reference pulse and the delayed optical pulses, generating a modulation format formed from two ultrashort optical pulses (S34 in Fig. 6). For example, a modulation format formed from the reference pulse 1 and the delayed optical pulse 2, and a modulation format formed from the reference pulse 1 and the delayed optical pulse 3 are generated.

In Example 1, the refractive index is changed during a guard time (T_grd) and an optical pulse propagates through the other waveguide 331 at the changed refractive index. After the propagation of the optical pulse, the refractive index is newly changed during the guard time (T_grd) for delay of a pulse of the next baud. These steps are repeated to sequentially generate optical pulses different in time interval from the reference pulse.

At this time, if the refractive index changes while an optical pulse propagates through the modulation region 335 in the other waveguide 331, different parts of the optical pulse are influenced by different values of the refractive index, deforming the pulse.

To propagate an optical pulse through the other waveguide 331 and delay it without deforming the optical pulse, it is desirable to complete the change of the refractive index within the guard time. That is, the change time of the refractive index is desirably equal to or shorter than the guard time.

The transmission module according to Example 1 can generate a modulation format formed from two ultrashort optical pulses.

(Example 2)
A transmission module according to the Example 2 of the present invention will be described with reference to Fig. 7.

In the Example 1, it is desirable to set the modulation rate so that the change time of the refractive index becomes equal to or shorter than the guard time, as described above. It is therefore necessary to trade off the modulation rate of the electrooptical modulation structure (other waveguide) used in the transmission module and the duration of the selected guard time T_grd.

A transmission module 40 according to Example 2 includes a light source (for example, MLL) 41, an electric controller 421, a pulse controller 422, a beam splitter 45, two

optical gates

461 and 462, two optical

pulse generation devices

43 and 44, and an optical multiplexer 47.

The two

optical gates

461 and 462 are connected to the two optical

pulse generation devices

43 and 44, respectively.

The two optical

pulse generation devices

43 and 44 have an arrangement similar to that of the Example 1, and include passive light waveguides (one waveguide) 432 and 442 and waveguides (other waveguide) 431 and 441 having the electrooptical modulation structure.

The outputs of the two optical

pulse generation devices

43 and 44 are connected to the optical multiplexer 47.

Of the two optical

pulse generation devices

43 and 44, one optical pulse generation device 43 generates an optical pulse for an even-numbered baud. The other optical pulse generation device 44 generates an optical pulse for an odd-numbered baud.

The

optical gates

461 and 462 respectively arranged on the former stage of the two optical

pulse generation devices

43 and 44 are controlled by the electric controller 421, and block (cut off) an input pulse to the inactive optical pulse generation device among optical pulses from the light source 41.

In this arrangement, the two optical

pulse generation devices

43 and 44 generate an optical pulse, and the time taken to generate a pulse by each optical pulse generation device can be prolonged. Modulation by one optical pulse generation device need not be executed at high speed, compared to the Example 1.

In this way, generation of an optical signal between the two optical

pulse generation devices

43 and 44 is interleaved, so the tradeoff between the modulation rate of the modulator and the duration of the selected guard time T_grd can be relieved.

(Example 3)
A transmission module according to the Example 3 of the present invention will be described with reference to Fig. 8.

As shown in Fig. 8, a transmission module 50 according to Example 3 includes two light sources (for example, MLLs) 511 and 512, a pulse controller (electrical domain) 52 having an electrical pulse generator 521 and an electrical pulse delay unit 522, and a passive coupler 53.

Of the two

light sources

511 and 512, one light source 512 is controlled by the electrical pulse generator 521 to generate a reference pulse.

An electrical pulse by the electrical pulse generator 521 is delayed by the electrical pulse delay unit 522 in accordance with a transmission symbol. By using the delayed electrical pulse, an optical pulse is generated from the other light source 511.

The passive coupler 53 couples the reference pulse from one light source 512 and the delayed optical pulse from the other light source 511, generating two ultrashort optical pulses.

The transmission module according to Example 3 can easily generate two ultrashort optical pulses.

The structures, sizes, materials, and the like of the respective components have been exemplified in the arrangements and the like of the optically modulated signal generation device and transmission module according to the embodiment of the present invention, but are not limited to them as long as the functions of the optically modulated signal generation device and transmission module are implemented and exert their effect.

The present invention can be applied to an optical data transmission/reception system and a computer.

33: optical pulse (optically modulated signal) generation device
331: other waveguide
332: one waveguide
333: input branch
334: output multiplexer

Claims

A generation device for a multilevel optically modulated signal transmitted/received between a transmitter and a receiver, comprising:
an input branch;
an output multiplexer; and
one waveguide and another waveguide that connect the input branch and the output multiplexer,
wherein the input branch branches an input optical pulse,
one optical pulse propagates through the one waveguide,
the other optical pulse propagates through the other waveguide and is delayed by an electrical modulated signal applied to the other waveguide,
the output multiplexer multiplexes the one optical pulse and the other optical pulse, and
the optically modulated signal has a modulation format in which the optically modulated signal has the one optical pulse and the other optical pulse for each transmission baud, input data is mapped in a time difference between the one optical pulse and the other optical pulse that is generated by the delay, and the time difference is different for each transmission baud.
The optically modulated signal generation device according to claim 1, wherein the one optical pulse has a characteristic different from a characteristic of the other optical pulse.
The optically modulated signal generation device according to claim 1, wherein the one optical pulse and the other optical pulse have a pulse width of not less than 0.7 ps and not more than 1.7 ps.
The optically modulated signal generation device according to claim 1, wherein the one optical pulse is a clock pulse,
the other optical pulse is a data pulse, and
the one optical pulse supplies a start time of the transmission baud to the receiver.
A transmission module comprising:
an optically modulated signal generation device according to claim 1;
a light source configured to generate the input optical pulse; and
a pulse controller configured to generate the electrically modulated signal.
A transmission module comprising:
a light source configured to generate an optical pulse;
two optical gates parallel-connected to the light source;
two optically modulated signal generation devices according to claim 1 that are connected to the two optical gates, respectively;
an optical multiplexer connected to outputs of the two optically modulated signal generation devices;
a pulse controller configured to generate the electrical modulated signal; and
an electric controller connected to the optical gates and the pulse controller,
wherein of the two optically modulated signal generation devices, one optically modulated signal generation device generates an optical pulse for an even-numbered baud, and
the other optically modulated signal generation device generates an optical pulse for an odd-numbered baud.
A transmission module for a multilevel optically modulated signal transmitted/received between a transmitter and a receiver, comprising:
two light sources configured to generate an optical pulse;
an electrical pulse generator configured to generate an electrical pulse and output the electrical pulse to the one light source out of the two light sources;
an electrical pulse delay unit configured to delay the electrical pulse input from the electrical pulse generator and output the electrical pulse to the other light source; and
a passive coupler configured to couple one optical pulse output from the one light source and the other optical pulse output from the other light source,
wherein the optically modulated signal has a modulation format in which the optically modulated signal has the one optical pulse and the other optical pulse for each transmission baud, input data is mapped in a time difference between the one optical pulse and the other optical pulse, and the time difference is different for each transmission baud.