JP7257103B2 - Optical transmission device and optical transmission method - Google Patents

Description

The present invention relates to an optical transmission apparatus and an optical transmission method, and more particularly, to an optical signal suitable for improving encryption strength and increasing transmission distance when data is modulated by multi-level intensity modulation and transmitted as an optical signal. The present invention relates to a transmission device and an optical transmission method.

Yuen quantum cryptography, also called optical communication quantum cryptography (Y-00) communication, is a communication technology that diffuses the quantum fluctuations of light (quantum shot noise) by modulation to prevent an eavesdropper from accurately receiving the optical signal. Its application to symmetric-key quantum cryptography has been proposed. In this common key quantum cryptography, one set (referred to as a base) is a binary optical signal that carries binary transmission data, and a plurality of M bases are prepared. It is randomly determined by pseudo-random numbers according to the cryptographic key. In reality, optical M-value signals are designed so that the inter-signal distance is so small that they cannot be identified due to quantum fluctuations.

Since the optical modulator/demodulator of the authorized sender and receiver communicates by switching between M binary bases according to a common pseudo-random number, the authorized receiver can read data by binary signal judgment with a large inter-signal distance. can be done. Therefore, errors due to quantum fluctuations can be ignored, and accurate communication is possible between authorized senders and receivers. Ciphers based on this optical modulation method are called Yuen quantum cryptography based on the Yuen-2000 cryptographic communication protocol (abbreviated as Y-00 protocol).

In such a Y-00 protocol, as a method of increasing the cryptographic strength, it is common to increase the number of multiple values to make it difficult to analyze the signal pattern and make it difficult to guess the encryption key required for decryption. .

A DAC (Digital Analog Converter) is generally used to generate this multilevel signal. As another mechanism for generating a multilevel signal, Patent Document 1 discloses an optical transmission device in which the signal level of a light source output from a light source generator is made variable to generate an optical signal having two or more optical signal levels. A technique is disclosed for generating an optical signal with multiple stages of optical signal levels by synthesizing signals with variable optical signal levels from light sources in an optical modulation unit.

JP 2016-116119 A

In the conventional technology described above, in an optical transmission device that uses a DAC to generate a multilevel signal, it is necessary to change the DAC, that is, to redesign the optical transmission device in order to increase the number of multilevel signals. there is a problem in terms of In addition, there may be cases where there is no DAC that satisfies both the required multilevel number and transmission speed, and it may not be possible to easily realize an increase in the multilevel number.
Further, in the optical transmission device described in Patent Document 1, it is necessary to change the design of the light source control section and the light source modulation section in the optical transmission device.

In any of the conventional techniques, as a method of increasing the encryption strength, the method of increasing the number of multilevel signals requires updating not only the optical transmitter but also the optical receiver that receives the optical signal. As a transmission/reception system, a drastic change is required.

SUMMARY OF THE INVENTION It is an object of the present invention to improve encryption strength without increasing the number of multilevel signals and by simply modifying the hardware of an optical transmitter.
Another object of the present invention is to extend the transmission distance while maintaining encryption strength in optical transmission and reception.

The optical transmission device of the present invention is preferably an optical transmission device that modulates data into an optical signal and transmits it using optical communication quantum cryptography based on multilevel intensity modulation, wherein quantum noise is added to the optical signal. The noise power adjustment circuit includes an attenuator that attenuates the optical signal and a first optical amplifier that adds quantum noise by ASE (Amplified Spontaneous Emission) light. , an attenuator and a second optical amplifier connected to the first optical amplifier for amplifying the optical signal.

According to the present invention, it is possible to improve the encryption strength without increasing the number of multilevel signals and by simply changing the hardware of the optical transmission device. In addition, in optical transmission and reception, the transmission distance can be extended while ensuring encryption strength.

FIG. 2 is a diagram showing the relationship between the appearance of quantum noise and signals in general optical communication; FIG. 10 is a diagram showing the relationship between the appearance of quantum noise and signals in the Y-00 protocol; FIG. 10 is a diagram showing the relationship between the state of quantum noise and the signal when the amount of quantum noise increases. FIG. 10 is a diagram showing the relationship between a signal and how quantum noise overlaps when the amount of quantum noise increases. 1 is a block diagram showing the configuration of an optical transmission device according to Embodiment 1; FIG. 1 is a diagram showing the configuration of an optical transmission device according to Embodiment 1 broken down to the device level; FIG. FIG. 10 is a diagram showing simulation set values; FIG. 10 is a diagram showing a graph of PowerPenalty of a legitimate receiver (Legitimate User) of Optical Amp2; FIG. 11 is a graph showing the PowerPenalty of an Eavesdropper of Optical Amp2; FIG. 3 is a diagram showing calculation conditions of an optical transmitter and an optical receiver; FIG. 10 is a diagram showing extinction ratios required for making the receiving sensitivity of an eavesdropper equal to the receiving sensitivity of a legitimate receiver when using the conventional technology, for each multilevel number; FIG. 4 is a diagram showing calculation results of reception sensitivity of a legitimate receiver when using the optical transmission device of Embodiment 1; 2 is a block diagram showing the configuration of an optical transmission device according to Embodiment 2; FIG. FIG. 12 is a block diagram showing the configuration of an optical transmission device according to Embodiment 3; 1 is a configuration diagram of an optical communication system based on a general Y-00 protocol; FIG.

Hereinafter, each embodiment according to the present invention will be described with reference to FIGS. 1 to 15. FIG.

[Configuration of optical communication system]
First, the configuration of an optical communication system based on the general Y-00 protocol will be described with reference to FIG.
An optical communication system based on the Y-00 protocol is a system that converts an electrical signal into an optical signal, encrypts a transmission line such as an optical fiber between an optical transmitter 100 and an optical receiver 200, and transmits the signal. Although not shown in FIG. 15, the actual system includes a data transmission device for inputting transmission data as an electrical signal to the optical transmission device 100, and a data transmission device for inputting the transmission data output from the optical reception device 200. A receiving device is connected.

The optical transmitter 100 comprises a running key generator 104, a multilevel signal generator 106, a light source generator 108, and an optical modulator 110, as shown in FIG.
The shared key 102 is an encryption key shared between the optical transmitting device 100 and the optical receiving device 200 (shared key 202), and is digital data that is the basis for determining the transition pattern of the multilevel signal.

The running key generation unit 104 uses the shared key 102 as original data to generate a running key having the property of a pseudo-random number (random pattern). This is a one-time key that is generated to increase cryptographic strength.
The multilevel signal generator 106 converts the transmission data (digital data) into a multilevel signal according to the running key.

On the other hand, the light source generator 108 is composed of, for example, a photodiode, and generates light having a certain level of intensity. Then, the optical modulation section 110 modulates the output light of the light source according to the multilevel signal to generate an optical multilevel signal. For example, in the case of intensity modulation, the optical multilevel signal is represented by differences in light intensity.
This encrypted optical multilevel signal is sent to the optical receiver 200 via the transmission path.

Next, as shown in FIG. 15, the optical receiver 200 comprises a running key generator 204, a threshold generator 206, an O/E (Optical/Electrical) converter 208, and a discriminator 210. FIG.

In the optical receiver 200, the optical multilevel signal sent via the transmission line is converted into an electrical multilevel signal by the O/E converter 208. FIG.
Here, although details are omitted, a common running key is used by the synchronization process between the optical transmission device 100 and the optical reception device 200, and the threshold value to be input to the discriminator 210 is generated by the threshold generation unit 206. be.

The discriminator 210 uses the input threshold value to discriminate the electrical multilevel signal output from the O/E conversion unit and restores the transmission data.

[Principle of quantum noise]
Next, the relationship between encryption and quantum noise in the Y-00 protocol will be described with reference to FIGS. 1 to 4. FIG.
In general optical communication, as shown in FIG. 1, signals 0 and 1 with different intensities are determined by a threshold value th provided therebetween. In general, signal 0 and signal 1 are sufficiently far apart in light level even in the presence of noise, and using a constant threshold signal makes it possible to accurately discriminate between the two signals. Here, in the graph of FIG. 1, the horizontal axis represents the intensity of noise around signal 0 and signal 1. In FIG.

On the other hand, FIG. 2 shows the effects of each signal and quantum noise in the Y-00 protocol. Unlike general optical communication, the Y-00 protocol uses multi-level modulation, so it has two or more signal levels (in FIG. 2, it has five levels, and signals 0 to 4 are do). At this time, if the light level difference between each signal is narrowed, noise overlaps (overlapping portion of olp0 to olp3 by signal 0 to signal 4 in FIG. 2), and for an eavesdropper who does not know the threshold, any It becomes impossible to determine whether it is a level signal or not. This amount of overlap guarantees encryption strength in the Y-00 protocol.

To increase the amount of overlap, it is possible to further reduce the level difference between signals (the idea that this increases the number of multiple values), or to increase the amount of quantum noise as shown in FIG. 3 (noise intensity g1→noise intensity g2).

In the present invention, the latter method of increasing the amount of quantum noise is selected, and as shown in FIG. 4, the amount of overlap between signals is increased by increasing the amount of quantum noise (signal 0 to signal 4 The overlapping portion of olp0 to olp3 due to ), which makes signal discrimination even more difficult than before the noise increase.
Specific means for increasing quantum noise will be described in detail below.

[Configuration of Optical Transmission Device (Embodiment 1)]
Next, the configuration of the optical transmitter according to the first embodiment will be described with reference to FIGS. 5 and 6. FIG.
In the optical transmission device of this embodiment, an increase in quantum noise is realized by using an optical amplifier (hereinafter referred to as "optical amplifier").

The optical Amp outputs ASE (Amplified Spontaneous Emission) light when used without input light. The ASE light is light (spontaneous emission light) having a wavelength different from that of the excitation light emitted by rare earth elements or the like absorbing the excitation light. This ASE light is known as a main noise factor of optical ASE having a broadband spectrum, and quantum noise is generated during photoelectric conversion. The optical transmitter of this embodiment is intended to increase the encryption strength by adding this ASE light to the conventional multilevel optical signal.

As shown in FIG. 5, the optical transmission device 300 of this embodiment includes a running key generation circuit (a circuit corresponding to the running key generation unit 104 in FIG. 15; illustration is omitted), a multilevel signal generation circuit 301, a CW (Continuous Wave (continuous wave) light source 303 , LN (Lithium Niobate: linear) modulator 304 , and noise power adjustment circuit 400 .

A multilevel signal generation circuit 301 corresponds to the multilevel signal generation unit 106 in FIG. 15, and is a circuit that receives a running key and generates a multilevel signal according to the running key. A multilevel signal generation circuit 301 includes a DAC 302 . The DAC 302 digital-to-analog converts the N transmission signals to generate a 2 ^N −1-bit electrical multilevel signal.
A CW light source 303 is a circuit corresponding to the light source generator 108 in FIG. 15 and generates transmission light.
The LN modulator 304 modulates the transmission light from the CW light source 303 into an optical multilevel signal according to the electrical multilevel signal.

The noise power adjustment circuit 400 is a circuit that forms a characteristic part of the present invention for improving encryption strength, and has the function of increasing the quantum noise of the optical signal and adjusting the optical power.
The noise power adjustment circuit 400 comprises an ATT (ATtenuaTor: attenuator) 401, an optical Amp1 (402), and an optical Amp2 (403).

Optical Amp1 (402) is a device that generates ASE light for increasing quantum noise.
ATT 401 is a circuit that attenuates the optical multilevel signal generated by LN modulator 304 .
If the optical power of the optical multilevel signal is too large for the ASE light generated by the optical Amp1 (402), the quantum noise added by the ASE light is buried in the optical multilevel signal, making no sense. Sometimes. For this reason, it is necessary to attenuate the optical multilevel signal to an optical power at which noise due to ASE light cannot be ignored.

The optical Amp2 (403) is a circuit for amplifying the attenuated optical multilevel signal in the ATT401 and securing the transmission distance.

Breaking down the optical transmission device 300 shown in FIG. 5 further to the circuit level, it becomes as shown in FIG.
ATT 401 in FIG. 5 corresponds to VOA (Variable Optical Attenuator) 501 in FIG. 6, Amp 1 (402) in FIG. 5 corresponds to SOA (Silicon Optical Amplifier) 502 in FIG. of light Amp2 (403) corresponds to the SOA 503 in FIG. Also, an optical waveguide (or an OMUX (Optical MUltipleXer: optical multiplexer)) 510 is interposed in the transmission path of the ATT 401, Amp1 (402), and the optical Amp2 (403) in FIG.

In this device configuration, the VOA 501 is used to attenuate the optical power of the output light from the LN modulator 304, and the SOA 502 is used to add ASE noise generated. At this time, an optical waveguide (or OMUX) 510 is used to add both signals. Finally, the SOA 503 is separately used for amplifying to normal transmission light power.

It is also possible to mount the VOA 501, SOA 502, optical waveguide (or OMUX) 510, and SOA 503 as one waveguide device. In this case, there are merits that the apparatus can be small-sized and can be realized at low cost.

[Improved encryption strength]
Next, the effect of improving the encryption strength in the optical communication device according to the first embodiment will be described with reference to FIGS. 7 to 9. FIG.
In the following, the optical transmission device and the optical reception device are set with the settings shown in FIG. 7, and a simulation is performed for improvement of encryption strength. In the figure, EDFA (Erbium Doped Optical Fiber Amplifier) is an amplifier using an optical fiber doped with a rare earth element Er: Erbium. Also, in the simulation, the number of multiple values is reduced to four values in order to make it easier to understand the effect of the present invention.

Here, the results of simulating the reception sensitivity difference (Power Penalty) (in units of dB) between the Y-00 device designed according to the prior art and the Y-00 device designed according to the present invention will be compared and explained.
FIG. 8 shows the PowerPenalty of 4 dB gain of optical Amp2 for a Legitimate User at a Bit Error Rate of 1E-12.
Here, the horizontal axis indicates the gain (G1) of the optical Amp1, and the vertical axis indicates the reception sensitivity difference from the prior art. According to this graph, it can be seen that the power penalty increases by increasing the gain (G1) of the light Amp1. Increasing the gain also affects the reception sensitivity of legitimate receivers because it leads to an increase in noise.

Next, a simulation result of PowerPenalty for an unauthorized recipient (eavesdropper) will be described.
FIG. 9 shows the Power Penalty of 4 dB gain of optical Amp 2 for an Eavesdropper at a Bit Error Rate of 1E-12.
According to FIG. 9, for example, when G1=4 dB, the authorized receiver has a power penalty of 1 dB or less, while the eavesdropper has a reception sensitivity difference of about 3 dB. If G1 is further increased and set to 6 dB, the Power Penalty of an eavesdropper diverges and a reception sensitivity difference of 10 dB or more is produced, whereas the normal receiver is about 1 dB. As described above, the effects of the optical Amp1 and the optical Amp2 added according to the present embodiment can increase the encryption strength without changing the multilevel number and without significantly deteriorating the reception sensitivity of the authorized recipient. .

[Improved transmission distance]
According to the optical transmitter of this embodiment, an improvement in transmission distance can be expected.
The effect of improving the transmission distance will be described below with reference to FIGS. 10 to 12. FIG.
The transmission distance is compared with the Y-00 device designed according to the prior art. In the following, the result of calculating how much the transmission distance is extended when using the Y-00 device of the present invention while maintaining the same level of safety as the Y-00 device of the prior art will be described.

The calculation procedure is as follows.
(1) Calculate the extinction ratio at which the receiving sensitivity of an eavesdropper when using the present invention is equal to that of a legitimate receiver when using conventional technology. (2) Use the present invention with the extinction ratio setting of (1). (3) Calculate the difference (PowerPenalty) between the minimum reception sensitivity of (2) and the reception sensitivity of legitimate recipients when using conventional technology (4) PowerPenalty of (3) Calculate the transmission distance from The calculation conditions are as shown in FIG.

Here, the reception sensitivities of the authorized receiver and the eavesdropper when using the conventional Y-00 device are as follows.
Regular receiver: BER=ErrorFree(1E-12) @Pin=-13.4dBm (=minimum reception sensitivity)
Eavesdropper: BER = 0.954 @ Pin = 0dBm

Next, the receiving sensitivity of the eavesdropper when using the Y-00 device of the present invention is equal to the receiving sensitivity of the legitimate receiver when using the Y-00 device of the prior art (-13.4@1E-12). Find the extinction ratio necessary for The extinction ratio for each multilevel number at this time is as shown in FIG. According to FIG. 11, it can be seen that the encryption strength increases and the necessary extinction ratio increases as the number of multiple values increases.

Next, as shown in FIG. 11, the calculation result of the receiving sensitivity of the authorized receiver when using the Y-00 apparatus of the present invention with the multi-level number and the extinction ratio set will be described with reference to FIG. .
It can be seen from FIG. 12 that the receiving sensitivity (Prmin) of the legitimate receiver improves as the extinction ratio increases.

At this time, the difference (PowerPenalty) from the receiving sensitivity (-13.4 dB) of the regular receiver using the Y-00 device of the prior art also increases, and there is no margin in the input light power required to obtain the same receiving sensitivity. As a result, the transmission distance becomes longer.

[Configuration of Optical Transmission Device (Embodiment 2)]
Next, the configuration of the optical transmission device according to the second embodiment will be described with reference to FIG.
In the optical transmitter 300 of the second embodiment, a plurality of optical amplifiers are provided in place of the optical amplifier 1 (402) of the first embodiment, and as shown in FIG. The optical Amp13 (4023) is connected in parallel to the transmission line connecting the ATT401 and the optical Amp2 (403).

The advantage of doing so is that the gain of the optical Amp can be easily adjusted, the degree of freedom is increased, and the quantum noise to be added can be increased.

[Configuration of Optical Transmission Device (Embodiment 3)]
Next, the configuration of the optical transmission device according to the third embodiment will be described using FIG.
In the optical transmitter 300 of the third embodiment, a plurality of optical amplifiers are provided in place of the optical amplifier 1 (402) of the first embodiment, and as shown in FIG. The optical Amp13 (4023) is connected in series to the transmission line connecting the ATT401 and the optical Amp2 (403).

The advantage of doing so is that, as in the case of the second embodiment, the gain of the optical Amp can be easily adjusted, the degree of freedom increases, and the quantum noise to be added can be increased.

DESCRIPTION OF SYMBOLS 100, 300... Optical transmitting apparatus 200... Optical receiving apparatus 102... Shared key (transmitting apparatus side) 202... Shared key (receiving apparatus side) 104... Running key generation part (transmitting apparatus side) 204... Running key Generation unit (receiving device side) 106 Multilevel signal generation unit 108 Light source generation unit 110 Optical modulation unit 206 Threshold generation unit 208 O/E (Optical/Electrical) conversion unit 210 Identification vessel,
301... Multilevel signal generation circuit 302... DAC (Digital Analog Converter) 303... CW (Continuous Wave: continuous wave) light source 304... LN (Lithium Niobate: linear) modulator 400... Noise power adjustment circuit 401... ATT (ATtenuaTor: attenuator), 402... Optical Amp1, 403... Optical Amp2

Claims (5)

An optical transmission device that performs optical communication quantum cryptography by multilevel intensity modulation and transmits an encrypted optical signal,
a noise power adjustment circuit that inputs an optical multilevel signal, adds quantum noise to the optical multilevel signal, and adjusts optical power;
The noise power adjustment circuit is
an attenuator that attenuates the optical multilevel signal;
a first optical amplifier for generating ASE (Amplified Spontaneous Emission) light for adding quantum noise to the optical multilevel signal attenuated by the attenuator ;
and a second optical amplifier connected to the attenuator and the first optical amplifier for amplifying the optical multilevel signal to which the quantum noise is added to generate the encrypted optical signal. optical transmitter. 2. The optical transmitter according to claim 1, comprising a plurality of said first optical amplifiers. 3. The optical transmitter according to claim 2, wherein said first optical amplifier is connected in parallel with a transmission line connecting said attenuator and said second optical amplifier. 3. The optical transmitter according to claim 2, wherein said first optical amplifier is connected in series with a transmission line connecting said attenuator and said second optical amplifier. An optical transmission method for transmitting an encrypted optical signal by applying optical communication quantum cryptography by multilevel intensity modulation,
attenuating the optical multilevel signal with an attenuator;
a first optical amplifier generating ASE (Amplified Spontaneous Emission) light for adding quantum noise to the optical multilevel signal attenuated by the attenuator ;
a second optical amplifier connected to the attenuator and the first optical amplifier amplifies the optical multilevel signal to which the quantum noise is added to generate the encrypted optical signal. An optical transmission method characterized by:

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