Optical pulse generation device and optical pulse generation method

Abstract

An optical pulse generation device comprises: a light source (1); a plurality of light intensity modulators (5) that each modulate the intensity of an optical signal outputted from the light source (1) centered about an operation bias point; an electrical signal source (2) that generates an RF signal having a desired frequency; a voltage control unit (4) that generates a drive signal by controlling the amplitude of the RF signal outputted from the electrical signal source (2) and supplies the drive signal to each of the light intensity modulators (5); a bias control unit (6) that sets the operation bias point for driving each of the light intensity modulators (5) on the basis of the drive signal; and a delay control unit (3) that controls a relative delay of the drive signal to be supplied to each of the light intensity modulators (5).

Description

Optical pulse generating device and optical pulse generating method

[0001] The present disclosure relates to an optical pulse generating device and an optical pulse generating method.

[0002] Quantum-related technologies such as quantum computers and quantum key distribution involve quantum communication, such as photon transmission. Photon sources used in photon transmission include attenuated light, photon pair generation, and quantum dots.

[0003] In quantum communication, the probability of confusion between photons of adjacent optical pulses can be reduced by shaping the pulse into one with a time width sufficiently short relative to the frequency of the optical pulse. Non-Patent Document 1 discloses that by making the time width sufficiently short relative to the period of the optical pulse, the probability of a photon included in one pulse being confused with a photon included in another adjacent pulse and being detected can be reduced.

[0004] As a method for shortening the time width of an optical pulse, Patent Document 1 discloses pulsing continuous wave (CW) light using a Mach-Zehnder intensity modulator. Shortening optical pulses may also be applied to high-speed optical communications, fiber sensors, optical measurement, and the like, in addition to quantum communications.

[0005] Japanese Patent Application Laid-Open No. 2000-89176

[0006] A. Taniguchi et. al., “20-GHz quantumkey distribution using Mach-Zehnder intensity modulation and low jittersuperconducting single photondetectors” CLEO 2022, paperCThA7E_03 (2022).

[0007] However, although the technology disclosed in Patent Document 1 can shorten the time width relative to the period of the optical pulse, it uses a single optical intensity modulator (Mach-Zehnder intensity modulator), and therefore the effect of shortening the time width is low, and there has been a growing demand for an even shorter time width.

[0008] The present disclosure has been made in consideration of the above circumstances, and its purpose is to provide an optical pulse generating device and an optical pulse generating method that are capable of shortening the time width of an optical pulse and lengthening the interval between optical pulses.

[0009] An optical pulse generating device according to one aspect of the present disclosure includes a light source, a plurality of optical intensity modulators that modulate the intensity of an optical signal output from the light source around an operating bias point, an electric signal source that generates electric signals of a plurality of frequencies, a voltage control unit that controls the amplitude of the electric signal output from the electric signal source to generate drive signals and supplies the drive signals to each of the optical intensity modulators, a bias control unit that sets an operating bias point for each of the optical intensity modulators to be driven by the drive signals, and a delay control unit that controls the relative delay of the drive signals supplied to each of the optical intensity modulators.

[0010] An optical pulse generation method according to one aspect of the present disclosure includes: an electric signal source generating electric signals of a plurality of frequencies; a voltage control unit controlling the amplitude of the electric signal output from the electric signal source to generate drive signals to be supplied to a plurality of optical intensity modulators; a bias control unit setting, for each optical intensity modulator, an operating bias point to be driven by the drive signals; a delay control unit controlling the relative delay of the drive signals to be supplied to each optical intensity modulator; and each optical intensity modulator modulating the amplitude of an optical signal output from a light source around the operating bias point using the drive signals.

[0011] According to the present disclosure, it is possible to shorten the time width of the light pulse and lengthen the interval between the light pulses.

[0012] FIG. 1 is a block diagram showing the configuration of an optical pulse generating device according to an embodiment. FIG. 2A is a diagram showing waveforms when an optical pulse signal is generated by supplying a drive signal with an amplitude of 2Vπ and a frequency of 4ω to an optical intensity modulator. FIG. 2B is a diagram showing waveforms when an optical pulse signal is generated by supplying a drive signal with an amplitude of Vπ and a frequency of 4ω to an optical intensity modulator. FIG. 3(a) is an explanatory diagram showing the waveforms of optical pulse signals output from each optical intensity modulator, and FIG. 3(b) is an explanatory diagram showing the waveforms of an optical pulse signal output from an optical pulse generating device and an optical pulse signal generated by a conventional method. FIG. 4 is a flowchart showing the processing procedure for setting the operating bias point in each optical intensity modulator. FIG. 5(a) is a graph showing a waveform with a frequency of 7ω and a square wave generated by combining sine waves that are odd multiples of the frequency ω, and FIG. 5(b) is an enlarged view of a main portion. FIG. 6 is a block diagram showing the configuration of an optical communication system equipped with an optical pulse generating device according to an embodiment. FIG. 7 is a block diagram showing the hardware configuration of this embodiment.

[0013] Hereinafter, an embodiment will be described with reference to the drawings. Fig. 1 is a block diagram showing the configuration of an optical pulse generating device according to an embodiment. In this embodiment, an example will be described in which an optical pulse generating device generates an optical pulse signal suitable for quantum communication.

[0014] As shown in FIG. 1 , the optical pulse generating device 100 according to this embodiment includes a light source 1, an electrical signal source 2, a delay control unit 3, a voltage control unit 4, multiple (four in this example) optical intensity modulators 5 (5-1 to 5-4), and a bias control unit 6. Hereinafter, when one of the optical intensity modulators is specifically referred to, it will be referred to with a suffix such as "optical intensity modulator 5-1." When not specifically referred to or when referring collectively, it will be referred to without a suffix, such as "optical intensity modulator 5." The optical intensity modulators 5 are connected in series in the order of optical intensity modulator 5-1, 5-2, 5-3, and 5-4 from the output side of the light source 1. The number of optical intensity modulators 5 is not limited to four, and may be two, three, five, or more.

[0015] The above-mentioned components, i.e., the light source 1, the electrical signal source 2, the delay control unit 3, the voltage control unit 4, the optical intensity modulators 5 (5-1 to 5-4), and the bias control unit 6, may be configured using existing devices that have been conventionally used.

[0016] The light source 1 is, for example, a semiconductor laser, and outputs continuous light to the first-stage optical intensity modulator 5-1.

[0017] The electric signal source 2 generates and outputs RF (Radio Frequency) signals at a desired frequency ω and frequencies 2ω and 4ω that are even multiples of the frequency ω. The electric signal source 2 outputs a DC voltage. That is, the electric signal source 2 generates an electric signal (RF signal) at a desired frequency. The desired frequency ω can be set to, for example, a frequency used in quantum communication. Note that in this embodiment, an example is described in which 2ω and 4ω are used as frequencies that are even multiples of the frequency ω, but frequencies such as 6ω, 8ω, ... may also be used. That is, the electric signal source 2 generates electric signals at an arbitrary frequency ω and frequencies 2ω and 4ω that are even multiples of the arbitrary frequency ω. The electric signal source 2 generates electric signals at a plurality of frequencies.

[0018] The voltage control unit 4 controls the amplitude of the RF signal output from the electrical signal source 2. The voltage control unit 4 controls the voltage of the RF signal so that the amplitude of the drive signal supplied to the first-stage optical intensity modulator 5-1 is 2Vπ, and the amplitude of the drive signals supplied to the second-stage and subsequent optical intensity modulators 5-2 to 5-4 is Vπ. "2Vπ" is the voltage corresponding to the distance from the minimum point to the next minimum point in the transmitted light intensity of the continuous light (see FIG. 2A, described later). "Vπ" is the voltage corresponding to the distance from the minimum point to the maximum point in the transmitted light intensity of the continuous light (see FIG. 2B, described later).

[0019] The voltage control unit 4 outputs a drive signal, with the amplitude of the RF signal having a frequency of 4ω output from the electric signal source 2, set to 2Vπ, to the first-stage optical intensity modulator 5-1. The voltage control unit 4 outputs a drive signal, with the amplitude of the RF signal having a frequency of ω set to Vπ, to the second-stage optical intensity modulator 5-2, a drive signal, with the amplitude of the RF signal having a frequency of 2ω set to Vπ, to the third-stage optical intensity modulator 5-3, and a drive signal, with the amplitude of the RF signal having a frequency of 4ω set to Vπ, to the fourth-stage optical intensity modulator 5-4. That is, the voltage control unit 4 controls the amplitude of the electric signal output from the electric signal source 2 to generate drive signals to be supplied to each optical intensity modulator 5. The voltage control unit 4 supplies a drive signal with an amplitude of 2Vπ to one of the multiple optical intensity modulators 5, and supplies drive signals with an amplitude of Vπ to the other optical intensity modulators 5-2 to 5-4.

[0020] The bias control unit 6 outputs a bias voltage to each optical intensity modulator 5 so that the operating point of the drive signal of the first-stage optical intensity modulator 5-1 coincides with the point at which the transmitted light intensity of the continuous light output from the light source 1 becomes maximum.

[0021] 2A is a diagram showing waveforms when a drive signal of 2Vπ is supplied to the optical intensity modulator 5-1 to generate a pulse signal, and FIG. 2B is a diagram showing waveforms when a drive signal of Vπ is supplied to the optical intensity modulators 5-2 to 5-4 to generate pulse signals. The bias control unit 6 outputs a bias voltage to each optical intensity modulator 5 so that the operating point of the drive signal in the first-stage optical intensity modulator 5-1 is the midpoint between two minimum points in the transmitted light intensity of the continuous light output from the light source 1. Specifically, the bias control unit 6 outputs a bias voltage to the optical intensity modulator 5-1 so that the operating point is point p1 in the continuous light q1 shown in FIG. 2A.

[0022] The bias control unit 6 outputs bias voltages to each optical intensity modulator 5 so that the operating points of the drive signals in the second and subsequent optical intensity modulators 5-2 to 5-4 are midpoints between the maximum and minimum points of the transmitted light intensity of the continuous light output from the light source 1. Specifically, the bias control unit 6 outputs bias voltages to the optical intensity modulators 5-2 to 5-4 so that the operating point is point p2 in the continuous light q1 shown in Figure 2B. That is, the bias control unit 6 sets an operating bias point for each optical intensity modulator 5 to be driven by the drive signal.

[0023] Therefore, a drive signal s11 (see FIG. 2A) with an amplitude of 2Vπ and a frequency of 4ω is applied to the optical intensity modulator 5-1, with the center being the operating bias point p1 of the continuous light q1. Also, a drive signal s12 (see FIG. 2B) with an amplitude of Vπ and a frequency of 4ω is applied to the optical intensity modulator 5-4, with the center being the operating bias point p2 of the continuous light q1.

[0024] Similarly, a drive signal with an amplitude of Vπ and a frequency ω is applied to the optical intensity modulator 5-2 with the operating bias point p2 of the continuous light q1 as its center, and a drive signal with an amplitude of Vπ and a frequency 2ω is applied to the optical intensity modulator 5-3 with the operating bias point p2 of the continuous light q1 as its center.

[0025] The optical intensity modulators 5 (5-1 to 5-4) generate optical pulses by intensity-modulating continuous light using a drive signal (amplitude-controlled RF signal) output from the voltage control unit 4, with the operating bias point adjusted by the bias control unit 6 as the center. That is, each optical intensity modulator 5 modulates the intensity of the optical signal output from the light source 1 with the operating bias point as the center.

[0026] For example, a Mach-Zehnder intensity modulator can be used as the optical intensity modulator 5. As is well known, a Mach-Zehnder intensity modulator splits an input optical signal into two waveguides, modulates the optical signal passing through one or both of the waveguides, and then combines the two waveguides to modulate the intensity of the optical signal. Each optical intensity modulator 5 may be configured to include multiple Mach-Zehnder interferometers.

[0027] FIG. 3(a) is a graph showing the optical pulse waveforms output from each optical intensity modulator 5 when the frequency ω is set to 10 GHz, and waveforms s1 to s4 represent the optical pulse signals output when the optical intensity modulators 5-1 to 5-4 are operated independently.

[0028] Fig. 3(b) is a graph showing an optical pulse waveform generated by the optical pulse generating device 100 of this embodiment and an optical pulse waveform generated by another method. Waveform s6 in Fig. 3(b) shows an optical pulse waveform obtained by each of waveforms s1 to s4 shown in Fig. 4A. Waveform s7 shows an optical pulse waveform obtained when a drive signal with a sine wave of frequency 0.5ω and amplitude 2Vπ is output to a single optical intensity modulator. Waveform s8 shows an optical pulse waveform obtained when a drive signal with an amplitude of 2Vπ is output to a single optical intensity modulator using a square wave generated by synthesizing sine waves of frequencies 0.5ω, 1.5ω, 2.5ω, and 3.5ω.

[0029] As described above, a drive signal with an amplitude of 2Vπ and a frequency of 4ω is input to the optical intensity modulator 5-1. The output signal of the optical intensity modulator 5-1 is a pulse signal with a frequency of 4ω, as shown by waveform r1 in Fig. 2A. This pulse signal corresponds to waveform s1 in Fig. 3(a).

[0030] A drive signal with an amplitude of Vπ and a frequency ω is input to the optical intensity modulator 5-2. The optical transmittance of the optical intensity modulator 5-2 has a waveform with a frequency ω, as shown by waveform s4 in FIG.

[0031] A drive signal with an amplitude of Vπ and a frequency of 2ω is input to the optical intensity modulator 5-3. The optical transmittance of the optical intensity modulator 5-3 is a waveform with a frequency of 2ω, as shown by waveform s3 in FIG.

[0032] A drive signal with an amplitude of Vπ and a frequency of 4ω is input to the optical intensity modulator 5-4. The optical transmittance of the optical intensity modulator 5-4 is a square wave with a frequency of 4ω, as shown by waveform r2 in Fig. 2B. This waveform corresponds to waveform s2 in Fig. 3(a).

[0033] Therefore, the optical pulse signal that has passed through each of the four optical intensity modulators 5-1 to 5-4 has a waveform with a frequency ω and a short pulse width, as shown by the waveform s6 in FIG. 3(b).

[0034] Returning to Fig. 1, after the bias level control by the bias control unit 6 and the voltage level control by the voltage control unit 4, the delay control unit 3 measures the optical pulses output from each optical intensity modulator 5 using a sampling oscilloscope, a high-speed photodetector, an optical switch, etc., and adjusts the delay amount when the voltage control unit 4 outputs a drive signal to each optical intensity modulator 5 so that the peak of the optical pulse is maximized. In other words, the delay control unit 3 controls the relative delay of the drive signal supplied to each optical intensity modulator 5. Note that the delay control unit 3 and the voltage control unit 4 may be integrated with the electric signal source 2 or may be separated into multiple units. The order in which the optical intensity modulators 5-1 to 5-4 are arranged is not limited to the example shown in Fig. 1, and may be reversed.

[0035] Next, the operation of the optical pulse generating device 100 according to this embodiment will be described. In the optical pulse generating device 100 according to this embodiment, the electric signal source 2 and the voltage control unit 4 generate drive signals to be supplied to each optical intensity modulator 5. Specifically, an RF signal with a frequency of 4ω is generated as the drive signal to be supplied to the optical intensity modulator 5-1, an RF signal with a frequency of ω is generated as the drive signal to be supplied to the optical intensity modulator 5-2, an RF signal with a frequency of 2ω is generated as the drive signal to be supplied to the optical intensity modulator 5-3, and an RF signal with a frequency of 4ω is generated as the drive signal to be supplied to the optical intensity modulator 5-4.

[0036] Furthermore, the voltage control unit 4 controls the amplitude of each RF signal to generate a drive signal to be supplied to each optical intensity modulator 5. Specifically, the amplitude of the RF signal with frequency 4ω is controlled to be 2Vπ, and this is used as the drive signal to be supplied to the optical intensity modulator 5-1. The amplitude of the RF signal with frequency ω is controlled to be Vπ, and this is used as the drive signal to be supplied to the optical intensity modulator 5-2. The amplitude of the RF signal with frequency 2ω is controlled to be Vπ, and this is used as the drive signal to be supplied to the optical intensity modulator 5-3. The amplitude of the RF signal with frequency 4ω is controlled to be Vπ, and this is used as the drive signal to be supplied to the optical intensity modulator 5-4.

[0037] The bias control unit 6 controls the bias operating point of each optical intensity modulator 5. Specifically, as shown in Fig. 2A, a bias voltage is applied so that the bias operating point of the optical intensity modulator 5-1 is the maximum point (p1) of the transmitted light intensity of the continuous light. Also, as shown in Fig. 2B, a bias voltage is applied so that the bias operating point of the optical intensity modulators 5-1 to 5-4 is the midpoint (p2) between the maximum and minimum points of the transmitted light intensity of the continuous light.

[0038] The procedure for setting the operating bias point of each optical intensity modulator 5 will be described below with reference to the flowchart shown in Fig. 4. The process shown in Fig. 4 is executed by the bias control unit 6 shown in Fig. 1.

[0039] 4, the bias control unit 6 determines whether or not there is any optical intensity modulator 5 to which a bias voltage is not being applied among the plurality of optical intensity modulators 5. If there is any optical intensity modulator 5 to which a bias voltage is not being applied (S11; YES), the process proceeds to step S12; otherwise (S11; NO), the process ends.

[0040] In step S12, the bias control unit 6 determines whether the optical intensity modulator 5 is the first stage (i.e., the optical intensity modulator 5-1). If it is the first stage (S12; YES), the process proceeds to step S13. If it is the second stage or later (i.e., the optical intensity modulators 5-2 to 5-4) (S12; NO), the process proceeds to step S16.

[0041] In step S13, the bias control unit 6 increases the bias voltage supplied to the first-stage optical intensity modulator 5-1.

[0042] In step S14, the bias control unit 6 determines whether the transmitted light intensity (light transmittance) of the continuous light is at a maximum. If it is at a maximum (S14; YES), the process proceeds to step S15. If not (S14; NO), the process returns to step S13.

[0043] In step S15, the bias control unit 6 sets the point where the transmittance is maximized as the operating bias point. As a result, the operating bias point (maximum point p1) shown in FIG. 2A is set. Then, the process returns to step S11.

[0044] In step S16, the bias control unit 6 increases the bias voltage supplied to the optical intensity modulator 5-2 in the Nth stage (N=2).

[0045] In step S17, the bias control unit 6 determines whether the transmitted light intensity (light transmittance) of the continuous light is at a maximum. If it is at a maximum (S17; YES), the process proceeds to step S18; if not (S17; NO), the process returns to step S16.

[0046] In step S18, the bias control unit 6 increases the bias voltage of the optical intensity modulator 5-2.

[0047] In step S19, the bias control unit 6 determines whether the transmitted light intensity (light transmittance) of the continuous light is minimal. If it is minimal (S19; YES), the process proceeds to step S20. If not (S19; NO), the process returns to step S18.

[0048] In step S20, the bias control unit 6 sets the midpoint between the maximum and minimum transmittance points as the operating bias point for the optical intensity modulator 5-2. Then, the process returns to step S11. Similarly, steps S16 to S20 are performed for the optical intensity modulators 5-3 and 5-4. That is, the above processes are performed with N=3 and 4. For each of the optical intensity modulators 5-2 to 5-4 in the second and subsequent stages, the operating bias point (midpoint p2) shown in FIG. 2B is set.

[0049] As a result, it becomes possible to set the operating bias point with high precision in each optical intensity modulator 5 and supply a drive signal to each optical intensity modulator 5 .

[0050] Next, a comparison between the optical pulse generating device 100 according to this embodiment and a comparative example will be described with reference to the graphs shown in Fig. 5. Fig. 5(a) is a graph showing a square wave generated by combining a sine wave with a frequency of 7ω and a sine wave with a frequency that is an odd multiple of ω, and Fig. 5(b) is a graph showing an expanded time interval near the zero point.

[0051] For example, to generate a high-frequency square wave of about 10 GHz, a conventional method involves adding sine waves of frequency ω and odd-number multiples of ω, i.e., 3ω, 5ω, and 7ω, where ω is the frequency of the square wave. For example, if a square wave of frequency ω is synthesized using a signal generator whose maximum output frequency is 7ω, the resulting waveform will be like waveform s22 shown in Figure 5(a).

[0052] On the other hand, a sine wave with a frequency of 7ω appears as waveform s21, and when comparing the slopes near the zero point, it is steeper than the square wave. That is, near the zero point, as shown in FIG. 5B, waveform s21 changes more steeply than waveform s22. Therefore, using a sine wave with the maximum frequency (7ω in this case) that can be output from a signal generator is advantageous for generating short pulses. That is, to shorten the pulse width of an optical pulse signal, it is desirable to use a sine wave with a higher frequency. However, on the other hand, it is extremely difficult to lengthen the pulse interval by adjusting the interval between the rising and falling edges, as with a square wave, using only a high-frequency sine wave.

[0053] In the conventional method using a single optical intensity modulator, it is difficult to simultaneously achieve the two effects of shortening the pulse width of an optical pulse signal and lengthening the pulse interval. In contrast, in this embodiment, multiple optical intensity modulators 5 are installed, and the first-stage optical intensity modulator 5-1 uses a drive signal with an amplitude of 2Vπ and a frequency of 4ω. Furthermore, the second-stage and subsequent optical intensity modulators 5-2 to 5-4 use drive signals with amplitudes of Vπ and frequencies ω, 2ω, and 4ω, i.e., frequencies that are even multiples of the frequency ω.

[0054] Specifically, the optical pulse signal shown in the waveform s6 in Fig. 3(b) is obtained. That is, as shown in the waveform s6, an optical pulse signal having a short pulse width and a long pulse interval is obtained. It is possible to shorten the pulse width of the optical pulse signal and lengthen the interval of the optical pulse signal at the same time.

[0055] Furthermore, compared with waveforms s7 and s8 in Fig. 3(b), an optical pulse signal having a short pulse width and a long pulse interval is obtained. That is, as shown in waveform s7 in Fig. 3(b), when a single optical intensity modulator is used with a drive signal having an amplitude of 2Vπ and a frequency of 0.5ω, a waveform of frequency ω is generated, but the pulse width is wider than that of waveform s6. As shown in waveform s8, when a single optical intensity modulator is used with a drive signal of 2Vπ using a rectangular wave generated by synthesizing sine waves with frequencies of 0.5ω, 1.5ω, 2.5ω, and 3.5ω, the pulse width is also wider than that of waveform s6. It can be seen that the optical pulse signal generated using the optical pulse generating device 100 according to this embodiment can achieve both a short pulse width and a wide pulse interval.

[0056] Fig. 6 is a block diagram showing the configuration of a communication system 200 equipped with an optical pulse generating device 100 according to this embodiment. As shown in Fig. 6, the communication system 200 includes a transmitting device 31, a receiving device 32, and a transmission path 33 that connects the transmitting device 31 and the receiving device 32 via an optical fiber cable. The transmission path 33 is not limited to an optical fiber cable, and may be space (i.e., wireless communication).

[0057] The transmitting device 31 includes the optical pulse generating device 100 shown in FIG.

[0058] The pattern generating section 311 generates a pattern of a transmission signal to be transmitted to the receiving device 32 , and outputs it to the coding section 312 and the optical pulse generating device 100 .

[0059] The encoding unit 312 encodes the optical pulse signal output from the optical pulse generating device 100 so that it matches the pattern output from the pattern generating unit 311. The encoding unit 312 transmits the encoded optical pulse signal to the receiving device 32 via the transmission path 33.

[0060] The receiving device 32 includes a decoding unit 321 , a photon detection unit 322 , and a time measurement unit 323 .

[0061] The decoding unit 321 decodes the optical pulse signal transmitted from the transmitting device 31 and outputs it to the photon detection unit 322 .

[0062] The photon detector 322 detects photons from the optical pulse signal decoded by the decoder 321. The photon detector 322 converts the detected photons into an electrical signal.

[0063] The time measurement unit 323 is synchronized with the clock of the pattern generation unit 311, and measures the arrival time of the photon detected by the photon detection unit 322. In this way, quantum communication between the transmission device 31 and the reception device 32 becomes possible.

[0064] As described above, the optical pulse generating device 100 according to this embodiment includes a light source 1, a plurality of optical intensity modulators 5 that modulate the intensity of an optical signal output from the light source 1 around an operating bias point, an electric signal source 2 that generates RF signals (electrical signals) of a plurality of frequencies, a voltage control unit 4 that controls the amplitude of the RF signal output from the electric signal source 2 to generate drive signals and supplies these to each optical intensity modulator 5, a bias control unit 6 that sets an operating bias point for each optical intensity modulator 5 to drive it with the drive signal, and a delay control unit 3 that controls the relative delay of the drive signals supplied to each optical intensity modulator 5.

[0065] In this embodiment, it is possible to shorten the time width of the optical pulse signal and lengthen the interval between optical pulses, which enables photon transmission with high accuracy, reduces the error rate in quantum communication, and improves communication quality.

[0066] As shown in Fig. 1, the optical pulse generating device 100 according to this embodiment is provided with a plurality of optical intensity modulators 5. Existing devices can be used for each optical intensity modulator 5 and the other components, namely the light source 1, the electrical signal source 2, the delay control unit 3, the voltage control unit 4, and the bias control unit 6. Therefore, the optical pulse generating device 100 according to this embodiment can be configured by making minor changes to an existing optical communication system. This prevents problems such as requiring major design changes.

[0067] In this embodiment, the electrical signal source 2 generates RF signals (electrical signals) at an arbitrary frequency ω and its even multiples, 2ω and 4ω, which makes it possible to make the rise and fall of the optical pulse signal steeper and further shorten the time width of the optical pulse signal.

[0068] In this embodiment, the voltage control unit 4 supplies a drive signal with an amplitude of 2Vπ to one of the multiple optical intensity modulators 5 (for example, the optical intensity modulator 5-1), and supplies a drive signal with an amplitude of Vπ to the other optical intensity modulators 5 (for example, the optical intensity modulators 5-2 to 5-4). This makes it possible to make the rise and fall of the optical pulse signal steeper, and further shorten the time width of the optical pulse signal.

[0069] In the above-described embodiment, an example has been described in which the optical pulse generating device 100 generates an optical pulse signal for use in quantum communication. However, the present invention can also be applied to optical communication, a light source for measurement to identify the location of a fault in an optical fiber, a light source for measuring fluorescence lifetime, and the like, in addition to quantum communication.

[0070] The optical pulse generating device 100 of the present embodiment described above can be, for example, a general-purpose computer system including a CPU (Central Processing Unit, processor) 901, a memory 902, a storage 903 (HDD: Hard Disk Drive, SSD: Solid State Drive), a communication device 904, an input device 905, and an output device 906, as shown in Fig. 7. The memory 902 and the storage 903 are storage devices. In this computer system, the CPU 901 executes a predetermined program loaded on the memory 902, thereby realizing each function of the optical pulse generating device 100.

[0071] The optical pulse generating device 100 may be implemented by one computer or by multiple computers. Furthermore, the optical pulse generating device 100 may be a virtual machine implemented on a computer.

[0072] The program for the optical pulse generating device 100 can be stored in a computer-readable recording medium such as a HDD, SSD, USB (Universal Serial Bus) memory, CD (Compact Disc), DVD (Digital Versatile Disc), or can be distributed via a network. The computer-readable recording medium is, for example, a non-transitory recording medium.

[0073] The present disclosure is not limited to the above-described embodiments, and various modifications are possible within the scope of the present disclosure.

[0074] REFERENCE SIGNS LIST 1 Light source 2 Electric signal source 3 Delay control section 4 Voltage control section 5 (5-1 to 5-4) Optical intensity modulator 6 Bias control section 31 Transmitting device 32 Receiving device 33 Transmission path 100 Optical pulse generating device 200 Communication system 311 Pattern generating section 312 Encoding section 321 Decoding section 322 Photon detecting section 323 Time measuring section

Claims

1. An optical pulse generating device comprising: a light source; a plurality of optical intensity modulators which modulate the intensity of an optical signal output from the light source around an operating bias point; an electric signal source which generates electric signals of a plurality of frequencies; a voltage control unit which controls the amplitude of the electric signal output from the electric signal source to generate drive signals and supplies them to each optical intensity modulator; a bias control unit which sets, for each optical intensity modulator, an operating bias point for driving the optical intensity modulator with the drive signals; and a delay control unit which controls the relative delay of the drive signals supplied to each optical intensity modulator.

2. An optical pulse generating device as described in claim 1, wherein the electrical signal source generates an electrical signal of an arbitrary frequency and a frequency that is an even multiple of the arbitrary frequency.

3. An optical pulse generating device as claimed in claim 1 or 2, wherein the voltage control section supplies a drive signal having an amplitude of 2Vπ to one of the plurality of optical intensity modulators, and supplies a drive signal having an amplitude of Vπ to the other optical intensity modulators.

4. A method for generating optical pulses, comprising: an electric signal source generating electric signals of a plurality of frequencies; a voltage control unit controlling the amplitude of the electric signal output from the electric signal source to generate drive signals to be supplied to a plurality of optical intensity modulators; a bias control unit setting, for each optical intensity modulator, an operating bias point for driving the optical intensity modulator with the drive signals; a delay control unit controlling the relative delay of the drive signals supplied to each optical intensity modulator; and each optical intensity modulator modulating the amplitude of an optical signal output from a light source with the drive signals, centered around the operating bias point.